Ac kno wle d gm e nt Cryogenic Energy Storage Systems: An Exergy-based Evaluation and Optimization vorgelegt von M.Sc. Sarah Hamdy ORCID: 0000-0002-7577-9595 von der Fakultät III – Prozessw issenschaften der Technischen Universität Berl in zur Erlangung des akademis chen Grades Doktorin der Inge nieurwissenschaften -Dr.-Ing.- genehmigte Dissertation Promot ionsaus schuss Vors itzender: Prof. Dr.-Ing. Fe lix Ziegler Gutac hte rin: Prof. Dr . -Ing. Tetyana Morozyuk Gutac hte r: Pro f. Dr.- I ng. Aaron Praktiknjo Gutac hter: Pro f. Dr.- Ing . Geo rge T satsa ronis Tag der wissenschaftl ich en Aussprache: 12.07.2019 Berlin 2 019 Ac kno wle d gm e nt II Acknowledgment III Acknowledgment First and f oremost, I would like to express my sincere g ratitud e to my supervisor Prof. Tetya na Morozyu k, for her immense eng agemen t, trust, gui dance, and cont inuous support not only with regards to scien tific research but also c oncerni ng adminis trative, managerial and other academic assignments. I would also like to thank Prof. G eorge Tsatsaroni s fo r his insp iring lectures and resea rch on exergy-b ased methods, his valuable feedback , and his encourag ement. My g enuine thanks to Prof. Aar on Pr aktiknjo fo r his willingness to be the external eva luator of this thesis, his in v a l u a b l e a d v i c e , a n d a l w a y s sympathetic ear. Without the remarkable assistance, p atience, a nd reinforcemen t of my advis ors, I would not have been able to comple te this t hesis in par allel to ful fill m y r ole as a study coordinator. Successfully m anaging this challenge with the help a nd s upport of my advis ors has g reatly benefitted both my professi onal and person al growth. My g rati tude also goes to Prof. Feli x Zie gler, not onl y for chair ing my PhD d efense but also for the enrichin g discussi ons a nd his inspira tion as a teache r. This work was condu cted during my period as a research associat e at the Department o f Energy Engineering of the Ze ntralinstit ut El Gouna at the Techn ische Universität Berlin. My positio n in the Transnatio nal Education (TNB ) Project (ID 57 1 28418) was supported by the G erman Academic Exchange Servi ce ( DAAD) with fu nds from the F ederal Minist ry of Educ ation and Researc h (BMBF). I am deeply grateful for havi ng had the opport unity to teach, coor dinate, and sh ape two Mast er degree programs a t t he TU Berlin Campus El Gouna. I app reciated the opportun ities giv en and the challeng es faced in the p roject. Enab ling a unique journe y for very inspiring s tudents has been a truly enriching experie nce. There fore, I would like to express the deepest apprecia tion to t hose who made this except ional aca demic enviro nment poss ible. Moreover, I offer my gratitude t o all the colleagues fro m the d epart ment, an d b oth chairs Exergy -base d methods of refr igeration sys tems and Energy Eng ineering and Environmental Protection . In part icular, my sincere th anks to Stefanie Tesch, Timo Blumberg, Johanne s Wellmann, Sara AlAhmed, Mohamed Noaman, Saee d Sayadi, Sebastian Spieker , Jing Luo and Elisa Papadis for the knowl edge excha nge, the great motivation, and the fruitful scientific discussions. My special thanks also to Dr. Daniel Wolf, who initiated the id e a o f m y m a s t e r a n d l a t e r PhD rese arch tog ether with Prof. Mor ozyuk, as well as D r. Marti n Robi nius for his honest feedb ack, and encourag ement. I would also like to thank the stu dents who assisted in my work and research: Tala Rifka, Jim ena Ince r Valve rde, Bianc a As torga, Francisco Moser, and Baha a Alha ddad. I as wel l owe very importa nt debt to my frie nds and fam ily for their t remendous spir itual support. In particu lar, I am grateful to my husband Louay, for the pr eciou s co mmit ment, patie nce, and proofreadi ng as well as my pare nts and my broth er I b r a h i m f o r t h e e n d l e s s support. Moreover, I am deeply thank ful for the patience and st rengt h that God rewar ded me to pursue this goal. IV Abstract V Abstract Grid-scale en ergy storage sy stems a re cap able of p roviding th e needed flexib ility to th e power g rid operators in o rder to ensure a secure power supp ly w ith increas ing sh ares of highl y interm ittent elec tricity generati on from renewable energ y sources. Cryogenic energy storage (CES) i s a g rid-scal e en ergy storage concept in whi ch electric ity is sto red i n t h e f o r m o f l i q u e f i e d g a s e n a b l i n g a r e m a r k a b l y h i g h e r e x e r g y density than com peting technologies such as pumped hydro st orage and compressed air en ergy storage and frees the techno logy of common geogr aphical re striction s. CES h as rec ently receiv ed mu ch attention in re search and application due to its advantageous characteristics, e.g ., th e lon g cycle life, l ow stora ge losses, and being c ompose d of m ature c o mp on e n ts . The present work aims to identify and apply meas ures for thermo dynamic per forman ce enhance ment and cost redu ction of C ES sy stems with the aid of e xerg y-based methods. Vario us system configur ations we re desi gned and sim ulate d in As pe n Plus® and e valuate d with ene rgetic, exerge tic, econom ic, and exerg oeconom ic analysi s . M o r e o v e r , i t e r a t i v e exergoe conom i c optimizatio n was perform ed. Six liquefac tion sys tems (charging process) with and without “cold” (low-temperature exergy) storage were a ssessed. Based on th e cost- optim al and t he most ef ficie nt li quefa ction pr ocess, t wo a diabatic CES systems (base cases) with an install ed discharge power/ene rgy capacit y of 100 MW/400 MWh were evalua te d and optim ized. The cost- optim ized syst em (optimize d c ase) was subjected to the integr ation of exter nal heat sources and sinks (integrat ed syst ems) to an alyze the e ffect o f system inte grati on on the ther modyna mic and the econom ic p erfor mance of CES sys tems. The resul ts showed that th e addition o f c old stora ge increases the rou ndtrip effic iency by 60-80 % and reduces the specific costs of the liquefactio n (cha rgi ng) proce ss by 50 %. The selection of th e l iquefaction pro cess was also r evealed to be o f signi ficance. The liquefacti on process causes the m ajority o f the bare module cos ts and more than 60 % o f t h e e x e r g y d e s t r u c t i o n i n t h e b a s e c a s e s y s t e m s . I n t h e e x e r g o e conom ic optim izati on, the leveli zed cost of d ischarg ed electricity was redu ced f rom 2 67 € /MWh to 19 5 €/MWh a t the expense of a reduction in t he roundtrip efficiency from 47 to 4 0 %. The specific investm ent cost o f the cost-op timal adiabatic CES sy stem reached 1,200 €/k W, similar to competing technologies but relatively high concerning the low roundtrip e fficien cies. The i ntegra tion of waste heat, combus tion and/or the regasifi cation of LNG to C ES is a v iable option reaching roundtrip efficiencies higher than 70 % and further re ducing the leveli zed cost of dischar ged e lect ricity to 13 0-170 € /MWh. Apart fro m the costs; th e operation hours and the respon se ti me were i dentified a s the constraint to the economic feasibility. Potential energy storag e applications suitable for CES were ide ntified acco rdingly. However, the mone tary val ue of the i dentified applications is too low to recover the revenue required of the CES sy stems . As a r esult, CES is only economica lly viable if further reve nue streams are identified and fi nancial incentives for the investment in ES are p rovided, or th e CES co sts ar e redu ced significantly. VI Zusammenfassung VII Zusammenfassung Mittels leistung sstarker Energiesp eichers ystem e kann d ie notwe n dige Flexibilität bereitge stellt werden, um ein e sich ere Stromv ersorgung bei st ei gender Einsp eisung ho ch flukt uiere nder Str omerzeu gung a us erne uerbar en Ener gieque llen z u gewährleisten. In dieser Arbei t werde n kryogene Energie speic hers ysteme (CES) unte rsuc ht, welc he groß e Mengen a n Üb erschu ssstrom in Form e ines Flüssiggases zwischen s peich ern könne n . Dies erm öglicht eine deutlic h höh ere Exergie dic hte, welche im Verhäl tn is z u anderen Netzs peiche ranlage n, wie Pum pspeiche rkraft werken oder Druc kluf t speichern, eine gleich zeitige Un abhängi gkeit von geografischen Einschränkungen erlaub t. Aufgrund seiner vorteil hafte n Eigenschaf ten, wie z.B. einer langen Leben s daue r, geringe n Speiche rverlus ten und der Zusamm ensetz ung aus ausgere ifte n Komp on enten, hat d as Inter esse a n CES-S ystemen in den verg angene n Jahre n i n der Indu strie und der Forschung stark z ugeno mmen. Die vorliege nde Arbeit befasst sich m it der therm odynamischen L e istung ssteigerung und der Kostensenk ung kryogener Energies peic hersystem e mit Hilfe ex ergiebasierter Met hod en . In d ie se m Zu sa mme nha ng wur den v ers ch iede ne Sy st emk onf iguratione n mit der Simulati onssof tware Aspen Plus® entworfen, simulier t und mi ttel s energetischer, exergetischer, wirtsch aftlicher un d exergoökono mischer Analysem ethode n bewe rtet. Darüber hinaus, wurde eine iterative exergoökono mische Op timier ung durchgeführ t. Sechs Luft verflü ssigungs prozesse (Einspeic herung) m it und ohne "Kältespeicher" (Niedertem peraturspeicher) wu rden betrac htet. Auf Gru ndla ge des koste ngüns tigs ten und des effizientesten Verflü ssigungsp rozesse s wurden zwei adiabate C E S - S y s t e m e ( b a s e cases) mit einer installierten Au sspeicherl eistung/Speicherkapa zität von jeweils 100 MW/400 MWh entwickelt, b ewertet und optim iert. Das optimier te Syste m (optim ized case) w urde ansc hließend m it verschie denen Ko nzepten d er Inte gr ati on ext erner Wärmeq uelle n und - senken erweitert (integrated system s) um die Auswirkungen d er System integra tion auf die therm o dynam ische und wirtschaftl iche Leistungsst eigerung von CES-Systemen zu untersuchen. Die Auswe rtung der Erge bnisse zeigt, d ass die Integra tion des K ält esp ei ch ers ein e Effizienzsteigerung um 60-80 % ermöglicht und gleichzeitig die Kosten des Verflüssigung s- bzw. Ladeprozesses um d ie Hälfte reduziert . Hi erbei ist auch die Wahl des Verflü ssigungspro zesses vo n Bed eutung. Der Verflü ssigungspr ozess verursach t den Großteil der Anlageninvestitionskos ten und der Exergi evernichtu ng in den Base-Case- Systemen. Die exergoökonomische Optimierung e rmöglicht die Mind erung der Stromgestehu ngskosten der Entladung von 26 7 €/MWh auf 195 €/MWh , welche eine Reduzierung des Gesamtwirkungsg rades von 47 auf 40 % nach s ich zieht. Das kostenop timale adi abate CES-Sy stem erreich t spezifisc he Investi tionskosten von 1.20 0 €/kW, was im Vergleich zu ande ren Netzspeic herte chnolog ie n k onkurrenzfähig i st. In Anbetr acht des niedri gen Gesam twirku ngsgra d fallen die spez i fis ch en Invest itions kosten jedoc h rela tiv hoch a us. Zusammenfassung VIII Die Integration von industrieller Abwärme, von Verbrennungsproz essen und/oder von der Wiedervergasung von Flüssig erdgas (LNG) erhöht d en Gesamtwirkun gsgra d auf über 70 % un d senkt die Stro mgestehungskosten der En tladung weiter a uf 130-17 0 €/ MWh. Neben den reinen Investiti onsko sten wur den auch die Betriebs stu nden und d ie Re akt ion szei ten d es C ES al s Hin d erni ss e fü r ei ne wi rts ch aft lich e Umsetz ung deutli ch. Darauf basierend wurden potentielle Anwendungen für CES-Sy steme hervo rgehoben. Die mit den Energiespeicheranwendung en verb undenen potent iellen Ein nahm en, ermöglic hen jedoch nicht die Rückzahlung d er gesamten Investitionskosten der dargelegten CES- System e. Daher sin d CES-System e nur dann wirtschaft lich, we nn w eitere Einnahmequellen identifiziert, finanzielle Anreize für die Inve stition in Energies peiche rsyste me ge geben oder die Inves titions koste n deu t lich reduziert werden. Nomenclatur e IX Nomenclature Symbol Explanation Unit A area m 2 c specific cost of power €/kW 𝐶 cost € 𝐶 cost r ate €/s, €/cy cle 𝑒 specific exergy J/kg E exergy J 𝐸 exergy rate MWh/cy cle, W 𝑓 factor , exer goecon omi c fact or - ℎ specific enthalpy J/kg H enthalpy J 𝐻 enthalpy rate W 𝑘 overal l hea t trans fer co effic ient W/m 2 K 𝑚 mas s kg 𝑚 mass flow rate kg/s 𝑛 econ omic lif e year s 𝑝 pressure bar 𝑄 heat t ransfer rate MWh/cycle , W 𝑟 (splitting) ratio, r elative cost di fference (exergoeconomic analy sis) - 𝑠 specific entropy J/kgK 𝑇 temperatur e °C, K 𝑊 power W 𝑋 equipment cost € 𝑍 cost ra te €/s, €/cycle Greek sy mbols 𝛼 scaling componen t - 𝛾 rate, ratio , yield -, % Δ differe nce - 𝜀 exerge tic eff icie ncy % 𝜂 energe tic (or ther mal) effici ency % 𝜂 isentropic ef ficiency % 𝜏 dura tion h Nomenclatur e X Sup er sc rip ts CH chemical KN kinetic M m echanical PH ph ysic al PT pot enti al T thermal Subscripts 0 ambient , restricte d dead state , refere nce A state po int at T 0 and p char charge CC combustion cham ber CI capital investmen t CP cryog enic pum p CS cold storage cs carbon stee l CM compr essor/co mpressi on el electri city D destruction d design dis disc harge EX expander/expansion F fuel HE heat excha nger HS heat storage i, j running index inst ins talled k k-th component L loss (exergetic analy sis), le veliz ed (e conom ic a nalysi s) l liquid m m ateria l ma metal al loys Mix mixer OM operation and maintenance P pr od uct p pressure PE purchased equipme nt Q associated with heat transfer RH reheater SP splitter ss stainless st eel ST storage tank sys system T turbine tot total Nomenclatur e XI Abbreviation s ASU air separatio n unit BMC bare module co st CAES compres sed air ener gy storage CAPEX capital expend iture CC carry ing ch arges CELF constant escalati on levelization factor CEPCI chem ical enginee ring plant co st index CES cryogenic energy storage CRF capital-r ecov ery factor EES engin eerin g eq uati on sol ver ES energy storage FC fuel cost FCI fixed ca pital investment FES flywheel ener gy storage FLH full load hours FT flash tank IC Int erc ool er LA lead aci d battery LAES liquid air energy storage LCOE levelized cost of electricit y LMTD log mean temperature difference LNG liquefied natural gas Li-Ion lithium ion b attery MHE main heat exchan ger NaS sodium sulfur battery NA not avai lable OMC operation and maintenance costs OPEX operational expenditure ORC organ ic Ran kine cy cle PBCS pack ed- bed co ld s tora ge PHS pumped hydro storage RE renewable energy RTE roundtrip effic iency SNG synth etic na tural g as SOFC solid oxide fuel cell SPECO specific exergy costing TCI total capita l investm ent TIT turbine inlet temperature TRL technology readiness level TRR total revenue requirement TV throttli ng valve T&D transmission and distribution UPS uninterruptible power su pply VRB vana diu m red ox f low ba tter y WH waste heat Nomenclatur e XII Contents XIII Contents Acknowle dgment ....... ........... ........ ........ .......... .... .... ........... ........ ........... ........ .......... ........ ... III Abstract ....... .......... ........ ........... ........ ........... ........ .......... ........ ........... ........ ........... ...... V Zusammenfassung ........... ........... ........ ........... ...... .. .......... ........ ........... ........ ........... ........ ... V II Nomencla ture ......... ........ ........... ........ ........... ... ..... ........ ........ .......... ........ ........... ........ ..... .. IX Chapter 1: Introduction ........ ........... .......... .......... ......... ........ ........... ........ .......... ........ ...21 Chapter 2: State of the art ......... ........... ........ ....... .... ........ ........ .......... ........ ........... ........ 23 2.1. Histor ical dev elopmen t of cry ogenic energy storag e .. ........... ............. 23 2.2. Adiabatic cryogenic energy storage systems ..... ............... ........... ......... 26 2.2.1. Princi ple of ope ration ............. ........ ........ .... .... .......... ........ ........... 26 2.2.2. System param eters .......... ........ .......... ........ .. ...... ........ ........... ........ 27 2.2.3. Charg ing processes . .......... ........ ........ ........ .. ......... ........ ........... .....28 2.2.4. Discharging processe s . ........... ........ .......... .... .... ........ ........... ........ 30 2.3. Syste m integra tion: waste h eat, waste cold, and comb ustion ............ 31 2.4. Classifica tion, charac teristic s a nd benc hmarki ng ....................... ......... 33 2.4.1. Classifica tion and c ompeting tec hnologies .... .......... ........ ........... 33 2.4.2. Characteris tics a nd benchmarki ng ...... ........... ...... .......... ........ .....34 2.5. Potential a pplicatio n and ec onom ic benef it .................. ............... ......... 39 2.5.1. Applicati ons suitable for CES systems ............ ...... .......... ........ ...39 2.5.2. Economic benefits and val ue propositions ............. .. ........ ........... 41 2.6. Summary of the literature review ....................... ........... ............... ......... 45 Chapter 3: Methodology .............. ........... .......... ..... ... ........ ........... ........ ........... ........ .....47 3.1. Ene rgetic analys is ............... ........... ............... ........... .......... .. ............... ..... 47 3.2. Exe rgetic analys is ............... ........... ............... ........... .......... .. ............... ..... 48 3.3. Econo mic an alysis ...... ............... ............ ........... ............... ... ........ ............. 51 3.3.1. Cost estimat ion ............... ........ .......... ....... ......... ........ ........... ........ 51 3.3.2. Assum ptions made ......... ........ .......... ........ .... .. .......... ........ ........... 53 3.3.3. TRR m ethod ................ ........ ........ ........... .. ...... .......... ........ ........... 54 3.3.4. Economic sensit ivity analysis ................ ........ .. ...... .......... ........ ...55 3.3.5. Determ ination of cost rates .............. ........ ...... ........ .......... ........ ...56 3.4. Exe rgoeconomic an alysis and optimizat ion ............. ............... ............ . 57 3.4.1. Exergy costing .......... ........ ........ ........... ........ ........... ........ .......... ...58 3.4.2. E xergoeconom ic optimization .......... ........ .......... ...... ........... ........ 59 3.5. Summary of the methodology .......... ............... ........... ........... ............... .. 60 Contents XIV Chapter 4: Design and simulation ........... .......... ......... ....... ........... ........ ........... ........ .....61 4.1. Simu lation and data manage ment software .................. ............... ......... 62 4.2. Gen era l assump t ion s mad e in t he s imul at ion ................... ............... ..... 62 4.3. Adiabatic cry ogenic energy storage sy stems ........ ............... ............ ..... 64 4.3.1. Charging unit ....... ........ ........ ........... ........ ........ ........ .......... ........ ...65 4.3.2. Storage unit ......... ........... ........ .......... ..... ... ........ ........... ........ ........70 4.3.3. Discharging unit ....... ........... ........ ........... .. ...... ........ .......... ........ ...72 4.4. Integrated system s ................... ............... ........... ........... ...... ......... ............ . 75 4.4.1. Waste heat integration .............. ........... ........ ........ ........ ........... .....77 4.4.2. Diabatic CE S with com bustion ................ ........ ...... .......... ........ ...77 4.4.3. Integration of LNG low-tem perature exe rgy ... ........... ........ ........ 78 4.5. Summary of the design and simulation ..................... ............... ............ . 80 Chapter 5: Results and discuss ion ............ ........ ......... .......... ........ ........... ........ .......... ...81 5.1. Evaluati on of different c harging p roce sses for adiabatic CES .......... 81 5.1.1. Ene rgetic a nd exe rgetic evalua tion of th e li quefact ion p rocess es ..81 5.1.2. Ec onomic evaluat ion of the C laude -base d liquef action p ro cesses .85 5.2. Analysis a nd optim ization of two a diabatic CES system s .... ............. 86 5.2.1. Exerg etic a nalysis ..... ........ ........ ........... ..... ........ ........ ........... ........ 86 5.2.2. Economic analysis .......... .......... ........... ....... . ........... ........ .......... ...90 5.2.3. Exergoeconom ic analysis and optim izatio n ........ ....... ...... ........... 91 5.3. Exergy-based evaluation of CES system integration ................. ....... 100 5.3.1. Com parative e nergetic and e xerge tic anal ysis ......... .. ...... .........100 5.3.2. Comparati ve economic anal ysis .......... ........... ..... ........ ........... ...102 5.4. Economic viabi lity of CES sy stems ....................... ........... ........... ....... 107 5.4.1. Econom ic sensit ivity a nalysis of the a-CES syste m ...... ......... ...107 5.4.2. Econom ic sensit ivity a nalysis of the integrated CES s yst ems ..10 9 5.4.3. Validation and asses sment of results ........ ........... ................ ......112 5.5. Summ ary of the res ults and di scussi on .............. ............... ........... ....... 116 Chapter 6: Conclusion and outlook .......... ........... ........ ........... ........ .......... ........ .........119 6.1. Summary of the main results ................... ............ ............... ........... ..... .. 120 6.2. Scope of t he pres ent work ............. ........... ............ ............... ........... ....... 121 6.3. Limitations ........ ............... ........... ............... ............ . .......... ............... ....... 123 6.4. Sum mary of pot ent ial f utur e wor k .............. ............... ............ .............. 123 Refere nces ....... .......... ........ ........... ........ ........... ........ .......... ........ ........... ........ ........... ...1 25 Appendix A ............ ........... ........ .......... ........... ........ ........ ........... ........ .......... ........ .........1 37 Appendix B ....... .......... ........... ........ ........... ..... ... .......... ........ ........... ........ ........... ........ ...1 41 Appendix C ....... .......... ........... ........ ........... ..... ... .......... ........ ........... ........ ........... ........ ...1 55 Appendix D ............ ........... ........ .......... ........... ........ ........ ........... ........ .......... ........ .........1 63 List of figures XV List of figures Figure 2. 1: His torical devel opme nt of cryoge nic ene rgy st orage. ........ .......... ........... ....... 23 Figure 2. 2: Countr y of origin of revie wed artic les and patents on CES. ........... ........... .... 24 Figure 2.3: Princi ple of operat ion of adiabatic C ES. .......... ....... ........... .......... ........ .......... 26 Figure 2.4 : Integ ration of in ternal/extern al heat and ”cold ” so urces to a CE S system. .... 31 Figure 2.5: Rated power, energy capacity and discharge durati on of ES techno log ies.. . . 33 Figure 2.6 : Comparison of sel ecte d weighted characteri stics of CES, CAES, and PHS. . 35 Figure 2.7 : CES specific costs p er kW installed capaci ty report ed in the literat ure. ........ 36 Figure 2.8 : C APEX an d sp ecific costs ove r CES sy stem ca pac ity. .............. ........ .......... .. 37 Figure 2.9 : Discharge period and power rating of various energy storage appl ications. .. 39 Figure 2.1 0: “Lif e cycle” val ue prop osition of E S applicatio ns. ............ ........... ........ ....... 42 Figure 2.11 : Potential primary (“anchor”) and secondary service s of CES. .......... ........ .. 43 Figure 2. 12: Value pr opositi on for s tacke d benef its s uitable f o r CES. ................ .......... .. 44 Figure 3.1: The exergy transfer rate associated with the h eat t ra nsfer over th e temperatur e difference. .. ........... ........ .......... ........ ........... . .......... ........ .......... ........ ........... ........ ........ ... .... 50 Figure 3.2 : Logic flowch art for the ex ergoecono mic optimiza tion applied i n this work. 57 Figure 4. 1: Schem atic of the system conf igurati ons conside red f or this work. ............ .... 61 Figure 4.2 : Flow sheet of t he stand-al one ad iabatic CES sy stem ( Base Cases A a nd B). . 64 Figure 4.3 : Flowsh eet of t he ga s cleanin g and purificat ion unit ................ ........ ........... .... 65 Figure 4.4: Minim u m specif ic wor k and the m aximum liquid yiel d over th e pre ssure. ... 66 Figure 4.5 : Flowsheets of th e Linde-based liqu efaction pro cesse s with cold recycle. ..... 68 Figure 4.6: Flowsheets of the Cl aude-based liquefaction p rocess es with cold recycle. ... 69 Figure 4.7 : T, ∆ H -diagram of the main heat exchanger in the di scharge process. . ........ .. 72 Figure 4. 8: RT E over pumpi ng pre ssure s for Ba se Cas e B. ....... .......... .......... ........ .......... 73 Figure 4. 9: Spec ific dischar ge w ork per kg of liqui d air, RT E, and η over the TIT . ........ 74 Figure 4.10: F lowshee ts of the integrated s ystems. ...... ...... .......... .......... ........ ........... ....... 75 Figure 4.1 1: RTE and exerget ic effi ciency over the specif ic mas s flow of LNG. ........ .... 79 Figure 5.1: Results of th e energet ic and exergetic eva luation f or the liquefaction processes with/without integrated col d stora ge. ......... ........... ..... ........ ........ ........... ........ .......... .......... 82 Figure 5. 2: Sensiti vity a nalysis results f or the Claude -based l iquefaction processes. ...... 83 Figure 5.3: Minim u m specif ic wor k and m aximum liqu id yi eld o ver t he split ting ra tio. . 84 Figure 5.4 : Exergeti c efficien cy of se lected co mponents o f the two base case systems. . 86 Figure 5. 5: Grassm ann diagram of the ex ergy flo w in the B ase Ca s e A. .......... ........ ....... 87 Figure 5. 6: Grassm ann diagram of the ex ergy flo w in the B ase Ca s e B. .......... ........ ....... 88 Figure 5.7 : Breakdo wn of th e exergy destru ction o f the overall system s. .............. ........ .. 89 Figure 5.8 : Cost breakdown of the LCOE dis for the two base case systems ........... .......... 90 Figure 5.9 : Bare mod ule cost s of the B ase Cases A and B. . ..... ......... ........ ........... ........ .... 91 Figure 5 .10: Exe rgoeconomic analy sis result s for th e five compo n e n t s w i t h t h e h i g h e s t t o t a l cost rate. .......... ........... ........... ........ .......... .. ......... ........... ........ .......... ........ ........... ...... .. ....... 92 List of figures XVI Figure 5. 11: Com ponent cost ra tes of s elected compone nts of the Base Ca se B. ............ 95 Figure 5.12: RTE over LCOE dis for the optimization s teps for the Base Ca se A. ........ .... 9 6 Figure 5.13: RTE over LCOE dis for the optimization s teps for the Base Ca se B. .......... .. 9 6 Figure 5. 14: Norm alized capital investm ent c ost ove r the rela ti ve exergetic efficiency. . 97 Figure 5.15 : Exergo economic an alysis resu lts for the base case s and optimized cases. .. 98 Figure 5. 16: Exerg etic ana lysis results for the integrated CES s ystem configu rations. . 100 Figure 5.17 : Bare module costs o f th e components of the integra ted s ystems. .............. 102 Figure 5.18 : Specifi c invest ment costs over RTE of the ten CES system s........... .......... 104 Figure 5.19 : Specifi c investment costs and LCOE dis o ver the sp ec. mas s flo w o f LNG. 105 Figure 5.20 : The levelized cost of ele ctric ity of th e ten CES s y stem conf igurat ions. .... 106 Figure 5.21 : Economic sensi tivit y analys is results of the a-CES system. ......... ........... .. 107 Figure 5.22 : LCOE dis over th e FLH dis f or the ten consi dered sys tems. .......... ........... ..... 109 Figure 5.23 : LCOE dis over the price of ele ctricity f or selected CES s ystems. ...... .......... 111 Figure 5.24 : LCOE dis of PHS, d-C AES, a-CAES, and s elected CES s ystems. ............ .. 112 Figure 5. 25: Box-pl ot di agram of the spe cific c osts of PHS, CAE S, and CE S. ........... .. 113 Figure 5.26 : LCOE dis of se lected CES sy stems c ompare d to values from the literature . 114 Figure A.1: Patents and resear ch articles published from 2013 t o 2017. .......... ............. 138 Figure A.2: Compati bility of occasi onal a nd freq uent use E S app licati on ........... .......... 140 Figure B.1 : Day-a head marke t clearing price over t he hours in t he y ea r in Ge r man y . . . 1 4 4 Figure B. 2: His tori cal devel opment o f the ann ual a verage val ue of the CE PCI. ........... 1 44 Figure B.3: Histo rical data for th e conversion rate of €/USD a n d €/GBP. ........... .......... 145 Figure B.4 : Printe d cir cuit hea t exc hanger by He atric used by H ighview Ltd. [ 161] .... 14 8 Figure C.1 : Sensiti vity a nalysis results f or the isentro pic eff iciency of the expander. ... 157 Figure C.2 : Flows heet of the cold stor age inte gration i nto the CES system . ................. 160 Figure C.3: R TE and exergetic effici ency over ma ss f low rat e o f t h e f u e l f o r t h e d - C E S system ............. ........... ........ ........ ........... ..... ... ........ .......... ........ ........... ........ ........... .... ...... 161 Figure D.1: T, ∆ H -diagram s of the M HE1 i n the Clau de-base d liquef action p rocesse s. 163 Figure D .2: Co mpa rison of the compos ite curv es of the MHE1 f or th e Claud e and the Kapitza process. ...... ........... ........ ........... ........ ........ .......... ........ ........... ........ ........... ........ . . 164 Figure D.3: Max . exergeti c efficiency and min. specific work r equi red for liquefaction over the liquefaction pressures for the Claud e process and the Heylandt process. ........ 165 Figure D.4 : Maxim um liqui d yield a nd li quefact ion press ure ove r the splitting ratio. . 165 Figure D.5: Exergy destructio n rati o of selec ted com ponents fo r the base cases. ......... 167 Figure D.6: Effect of the increase d mass flow rate of the heat storage medi a on the RTE over the L COE dis f or the base cases. ....... ........ ........... ........ ........ ........... .......... ........ ........ 1 68 Figure D.7: Specific exergy destru ction and losses over the cos t o f t h e f i n a l p r o d u c t fo r th e optimized cases a fter parametri c changes on Base Cases A and B. ............... ........ ........ 1 68 List of figures XVII Figure D.8: LCOE dis ove r the ma ss flow of LNG in t he inte grate d systems. ........ ...... .. 169 Figure D.9: LCOE dis over the mass flow of fuel suppli ed to the d-CES sy stem ...... ...... 169 Figure D.10: LCOE dis over FLH for all conside red systems with consi deration of CO 2 emission prices. ............. ........ .......... ........ ........ ........... ........ ........... ........ .......... ........ ...... .. 170 Figure D.11: Box-p lot diagram of th e sp ec. costs o f di fferen t b ulk ES t echnolo gies. ... 17 2 Figure D.12: Box-p lot diagram of the LCOE dis of PHS, CAES, and CES. ......... .......... 172 XVIII List of tables XIX List of tables Table 2.1: Para meters of CES s ystems p resented i n litera ture. . ..... ........ ........... ........... .... 27 Table 3.1: Cost f unction s develope d for t he estimation of the B MC of th e CES syst em. 53 Table 3.2: Assum ptio ns made in the eco nomic a nalysis. ......... .... .......... ........ ........... ....... 54 Table 4 .1: Isentropi c efficiencie s presumed for the tu rbomachin ery. .................. ........ .... 62 Table 4.2: G eneral assum ptions a nd des ign para meter for the hea t e xchangers. ...... ....... 63 Table 4.3: R efrigera nt pro perties co mpared to air [3 7, 56] . ... ...... ........ .......... ........ .......... 71 Table 4.4: Ene rgy de nsities a nd eff icienci es rep orted in t he li terature. ............. ........... .... 74 Table 4 .5: Power and heat capacities in MW for indicated work a nd heat flows. .......... .. 76 Table 5.1: Desi gn par ameters f or the t hree Cla ude-base d CES s ys tems com p ared. . ....... 85 Table 5.2: Resu lts of th e econom ic ana lysis for the Cla ude-base d liquefacti on units ...... 85 Table 5.3: R esults ob taine d from the exe rgetic e valuat ion of th e two base case s ystem s. 86 Table 5.4: R esults obt ained in exer goecon omic a nalysis f or t he two base case systems. 92 Table 5.5: Decis ion varia bles for exer goecon omic optim izati on. ............. ........... ........ .... 93 Table 5.6: P arameters and r esults of the optim izat ion ste ps for the Case A. ................ .... 94 Table 5.7: P arameters and r esults of the optim izat ion ste ps for the Case B.. ......... .......... 94 Table 5.8: RTE, energe tic and exergetic e fficien cy of the te n c onsidered systems. ...... . 101 Table 5 .9: Re sults o f economic an alysis for the ten CES system con figurations. .......... 1 03 Table A. 1: Comparis on of CES chara cteristi cs to other bulk ES t echnologies .............. 137 Table A. 2: B ulk ener gy applicatio ns com patible with CES chara ct er istics . .......... ........ 1 38 Table A.3: Anc illary services co mpatible with CES charact eristi cs ............ .......... ........ 139 Table A.4: Frequen cy regulation services compatible with CES ch aract eristics . .......... 139 Table A. 5: Re newable energ y inte grati on applic ations com patible with CES ............. .. 139 Table A. 6: T&D sup port ap plicatio ns com pati ble wit h CES c haract eristics ................ .. 139 Table A.7: R esults o f t he li terature review on the economic val ue of pote ntial CES applications. ............ ........ ........... ........... ....... ......... ........ .......... ........ ........... ........ ......... .. .. 140 Table B.1: Defin ition of the exe rgetic and energeti c efficienci es f or CES s ystems ....... 141 Table B.2: Defi nition of exer gy of fuel a nd exe rgy of pro duct o f selec ted components 1 42 Table B.3: Definition of exergy of fuel and exerg y of product o f pu mps and turb ines fo r different cases of the entering and the exiting tempe rature. .. ......... ........ ........... ........... .. 141 List of tables XX Table C.1: S tream values of t he Base Case A. ............. .......... ........ ........... ........ ........... .. 155 Table C.2: S tream values of t he Base Case B. .............. .... ...... ........... ........ ........... ........ .. 156 Table C.3: Strea m values fo r the states indi cated in the flowsh eets in Figure 4.6 .... ..... 158 Table C.4: Strea m values fo r the states indi cated in the flowsh eets in Figure 4.5 .... ..... 159 Table C.5: St ream v alues for th e ad iabatic CES sy stems w ith wa s te heat in tegration. .. 160 Table C.6: Strea m values fo r the CES sy stems with s ingle and do uble combu stion . ..... 161 Table C.7 : Stream value s for f or the CES s ystem s with/witho ut i ntegration of LNG. ... 162 Table D.1: Exer gy a nalysis re sult s on c ompo nent le vel fo r Ba se Case A. ............. ........ 1 66 Table D.2: Exer gy a nalysis re sult s on c ompo nent le vel fo r Ba se Case B. ................ ..... 167 Table D.3: Econom ic anal ysis resu l ts for Base Cases A and B. .. .............. ........ ........... .. 168 Chapt er 1 : Intr oduction 21 Chapter 1: Introduction There is no doubt that the chall enges in the power sector are n owadays of great er impor tanc e than earlier, resulting in majo r political, social, economic, a nd ecological consequenc es [1, 2]. The rapid devel opment towards a h igher pen etrati on of electrici ty g ener ation fr om r enewab le energy sources introduces new cha llenges to the electricity gri ds [2]. The highly fluctuat ing genera tion pattern, as well as t he low power capacity fa ctor of renewa ble ene rgy generators, offer a weak predict ion for pow er availability; leave power gri d operators with l ittle means to sustain th e supply-demand equilib rium and secure uninterrupted power supply [3]. Especially w i n d a n d s o l a r p o w e r g e n e r a t i o n , w h i c h a r e s u b j e c t t o s t r o n g i n termittenc y, have recently faced a significant boost globally [2 , 3, 4]. The power supply should b e able to cover the dem and at any point in time. The infrast ruct ure and services o f the exist ing electricity grid are not optimized to sustain reliable f uncti on while accomm odatin g a hi gh amoun t o f electricity from renewa ble ene rgy s ources [5, 6, 7] . So far, grid operators avoid system failures, which would cause power cutbacks and would harm equipm ent, during a possi ble mism atch between demand and g eneration, by introducing flexibilit y to the grid [8] . Ener gy storage (ES) and particular ly grid-scale electricity storage i s widel y regarded to be the most v aluable option to advance power grid flexibility while facing extensi ve renewable energy incor poration [6, 9, 10, 11]. Mid an d long-term electricity storag e provide flexi bility on multiple layers by (a) leveling the high pen etration of r enewable energies and capturing surplus electric ity at low demand ; (b) providing peak shaving, and (c) reducing necessary reserv e capacity [12]. Grid-scale energy storage technologies vary in their energy capacities, power capacities, dis charge periods, roundtrip eff icien cies (AC/AC effici ency), response tim es, and capita l costs [6]. Grid balanci ng services entail a storage device with reasonabl e effici ency (> 50 %) bu t low costs at a significan t s cale. These requirements are consiste nt wit h the c oncept of c ryogenic energy s torage [13, 14 ]. The working principle of Cryogenics-based energy storage (CES) i s classified as thermal or thermo-electric energy storage ( ES). CES charges excess electri city in an energy -inten se air liquefaction process. T he liquefied gas is stored at ambient pr essure and a cryogenic temperature in an insula ted stora ge vessel. During discharge; t he liquid air is pumped to high pressure s, evaporat ed, superheate d, and finally expanded partia lly r ecovering th e electrici ty charge d. CES is the only grid-sc ale ES technology without geogr aphical constr aints, a remarkably higher volumetric ener gy d ensity, very l o w s t o r a g e l osses, and a long cycle life. CES int egration with oth er pro cesses such as the recov ery o f “w aste heat” or “waste cold” increase the technologies attrac tiveness as well as the efficie ncy [15]. CES consists of components with well-known industrial-sca le applications [2], a llowing great scale-up excessing present s upply infrastructure [16, 17, 18]. Hence, CE S is expected com paratively fast progress towards commercializatio n [ 19]. Being a pre-com mercial , technology CES still has to prove its favorable c haracteristics [3]. Chapt er 1 : Intr oduction 22 The reduction of the energy requ irement in the l iquefaction pro cess and the increase of the specific power output of the disc harge process were identifie d as two main research and development ob jectives, that need to be met to p repare CES tech nology for the market [20]. Apart from the enhancement of t he roundtrip effici ency, the red uction in costs is a major milestone to be accom p lished for the techno logy t o reach m aturi ty. With this background, the present work on cryogeni c energy stor age aims t o eval uate the potentials of the technology and the associated challenges review ing and validat ing C ES character istics stated in literatu re (e.g., exergy density, roundtrip effi ciency, specific cos ts), drawing a com pa r ison to its competing technologies and evalua ting its potential application . quantify th e impact of the different cha rge, storage and discha rge process config urati ons on the thermodynamic and economic perf orm ance of a diabat ic CES systems. revea l the relation of thermodynamic inefficiencies a n d c o s t s i n t h e C E S system and identify a m easure for cost reduction . identify the cost-optimal system des ign of adiabatic CES systems. eval uate the performance enhancement throug h the integration of liquid natura l gas ( LNG), i nternal c ombust ion and wa ste heat to CE S sy stems. evaluate CES economic viability using sensitiv ity analysis: identifying the limiting factor and revealing p otential r evenue streams. These objectives are achieved wit h the application of exergy-ba sed methods after the design and simula tion of several CES configurati ons in Aspen Plus® bas ed on an inten se literatu re review. In Cha p ter 2 of this thesis, the background t o the CES principle of operati on, history, and maturity is given. CES core char acteristics are reviewed and b e nchm arked tow ards c om petin g e n e r g y s t o r a g e t echnologies. Moreover, po tential energy storage applications for CES are identified along w ith potential revenue stream s. The a ssumpt ion s made and m ethods applied in the analy sis are subject to Cha pt er 3 . In Chapter 4 , the considered C ES system con figurations are prese nted, the design decisi ons made for the system configu rations are justifie d, and the assump tions made in the si mula tion are elaborated . The results from the analysis of the different system configurations, their se nsitivity, validation, and discu ssion are presented in Chap ter 5 . While Cha pte r 6 , places the main findings i n a holistic context. The conclusio n s a r e s t a t e d , a n d the limitations to the present w or k are assesse d with the prosp ect for future work. Chapt er 2 : State of t he ar t 23 Chapter 2: State of the art In this c hapter, cryogenic energy storage history, principle of operation, most significa nt system param eters and sys tem configurat ions are introduced. In particu lar, different charge and discharge unit configu rations are reviewed, and the potential f or sys tem integra tion with external heat sources and sinks i s discussed. Furt her, the CES i s a d d r e s s e d i n a b r o a d e r c o n t e x t . ES technologies are classified, and competing technologies are identified. Fu rther CES charact eristics are comp ared to those of compe ting techno logy. Fina lly, the pote ntial applications for CES and their e conomic benefit are outlayed. 2.1. Historical devel opment o f cry ogenic energy storage The concept of storing e lectricity in the form of liquefied air has been firstly published by S mith et al. [ 21] from the University of Newcastle in 1977. The stora g e c o n c e p t w a s c l a i m e d t o e n a b l e roundtrip efficiencies (RTE), see equation (3.5), up to 72 % through adiabatic compression and expansion [21]. The associated p atents were obtained by several research groups in France, Japan, a nd Germany (1981-1983) [15 ]. In particular , Hitachi Ltd [22, 23, 24, 25] and Mitsubishi Hi tach i Po wer Sys te ms Lt d [26 , 2 7, 28] engaged i n the technolog y’s furthe r de velopm ent. [3] The first pilot plant was built in 1995 ( Mitsubis hi Hitachi Pow e r S y s t e m s L t d , [ 2 7 ] ) b y extending an existin g air liquefaction plant with a 2.6 MW cryo genic power recovery unit (dischar ge unit). The pi lot plant was argue d to conf irm the s ys tem via bilit y [27] despi te a ve ry low RTE [29]. The technology’s h istorical advan cem ent is shown i n Figure 2.1. Figure 2.1: Historical development of cryogenic energy storage [ 3] adopted fro m [30]. 1977 System firstly proposed University of Newcastle, UK (Smith, 1977) 1995 – 1997 Power recove ry unit (2.6 MW) by Mitsubishi, J apan (Kishimoto et al., 1998) 2008 – 201 1 Pilot scale plant (350 kW/2.5 MWh) by Univers ity of Leeds and Highview , UK (Mor gan et al., 2015) 2016 – 2018 Demonstrator plant (5 MW/15 MW h) Highview and project partners, UK (https://www .highviewpower .com/) CONCEPT LAB BENC H PILO T DEMONSTRA TION COMM ERCI AL Future Giga plant (200 MW/1.2 GWh ) Chapt er 2 : State of t he ar t 24 The f irst integrate d pil ot CES pla nt ( 350 kW/2.5 MWh) was the r esul t of a joint rese arch project of the Universit y of Leeds and Highview Power Storag e Ltd. (fur ther “Highview Ltd.”), a privatel y owned company [31]. The dischar ge unit was built in 2009. The pilot plant was collocate d to a biom ass-fired power p lant providing waste heat to the system in Slough, UK. In 2010, the system w as connected to the electricity grid. Only in 2011, the recovery unit was extended with an air liqu efier. First results from t his pilot p lant were published in 2015 by Morgan et al. [16 ] 1 . The system reached a R TE of 7-11 % [32]. Provision of grid ba lancing services such as loa d follow was found tec hnically realizable. The technologies e fficiency and costs w ere predict ed as comp etiti ve once the system would reach commercial scale. The plant was later moved to the U niversity of Birm i ngham for further tes ting [29] . Figure 2.2: Country of orig in of reviewed articles and patents on CES [3]. In 2014, funding of approximate ly € 9 Million by the UK Departm ent of Energy and Climate Change was granted to Highview Ltd . and project pa rtners for th e constr ucti on of the demonstration plant (5 MW/15 MWh ). In April 2018, the demonstra tion plant started oper ation n e x t t o a l a n d f i l l f a c i l i t y i n B u r y , G r e a t e r M a n c h e s t e r . T h e p l ant provid es selected b alancing services such as peak shaving and short term operating r eserve [31]. The “G iga p lant” developed by Highview Ltd. is planned to be located in Dallas, T e x a s , U S A , a t a t o t a l c o s t of € 230 Million and rea ch a RTE of 60 %. [3] 1 In this work, the names of the authors are only given for key publications. Chapt er 2 : State of t he ar t 25 In 2013, the Liquid Air Energy Network (LAEN) was formed as a r esult of a joint research project between UK universities and the Centre for Low C arbon F utures [8]. The LAEN aims to investi gate the poten tial role of liquid air as an energy ve ct or in the futur e energy and transportation sectors [30]. The number of publications and pat ents on cry ogenics-based or liquid-air energy stor age increased since 2013 (the number of publications can be extracted from Figure A.1 in Appendix A). Figure 2.2 shows the origin of reviewed papers and patents on cryogenic and liquid air energy stora ge. Leading compani es i n the field of industrial gases, e.g., Air Liquide, and Linde AG obtained p atents in 2012 and 20 14, respectively. The largest number of patents and articles came from Chinese and UK experts . [3] The large majority of UK pate nts relat e to th e activ ities of Highview Ltd. [31] . Maturity of the technology CES consists of mature componen ts with well-known industrial-sc ale applicatio ns in the field of industrial gases and chemical processes as well as power gen eration ; allowing scale-up excessing pr esent suppl y infrastr ucture [15, 16, 17, 1 8]. For t he same reason, de velope rs expec t comparatively fast progress towa rds commercialization [ 19]. Acc ording to several case studies, e.g. [20] , CES is expected to reach maturity in less than five y ears. The European Association for Storage of Energy evaluated CES in 2017 with a Technology R eadiness Level (TRL) of approximately TRL 8 (TRL 9 de noting a mature technology) [33] . T h e T R L 8 r e f e r s t o t h e system b eing in the end phase of the d evelopment for the majori ty of components. CES commercial design has been compl eted and the technology was p ro ven in testing and demonst ration [ 34, 3 5]. For example , the char ging system of CES is in parts similar to convent ional air separation un its (ASU). Commercial air liquefaction plants are about ten times s m aller than the liquef acti on capacities needed for g rid-scale CES. Thus a technological gap still exists [15] while it could be adapted from commercial-scale natural gas liquefaction. Avai lable indust rial cryogen storage vessels based on lique fied natural gas (LNG) technology enable capacities of 200,000 m³, w hich would enable 17-23 GWh of stor age capacity [1 8] . The discharge unit is a l s o a s s e m b l e d o f c o m p o n e n t s c o m m o n i n t h e p o w e r a n d p r o c e s s s e ctor (e. g., tur bines, compressors, pumps, and electric motors). Potential vendors and supply chains for C ES key compon ents were ev alua ted by S trahan et al. [8 ]. Li mitation s to t h e s c a l e o f t h e s y s t e m s w e r e identi fied by B rett and Barnett [18]. Scales comparable with a plant size of up to 100 MW are currently achiev able [ 12, 16, 18]. Being a p re-comm ercial tech nology, CES still has to prove its expected advanta geous character istics [3] , e.g., its specif i c cost and its efficiency (sect ion 2.4. 2). The operati on and design of CES systems are subject to th e fo llowing sub section 2 .2 and 2. 3. Chapt er 2 : State of t he ar t 26 2.2. Adiabatic cryogeni c energy storage systems 2.2.1. Principle of operation In CES systems, large quantities of e lectricity are sto red in t he form of liquefied gas at cryoge nic temperature (< - 150 °C). C ES is frequently named liquid air ene r gy storage (L AES) as air, or air constituents are com monl y utilize d as the worki ng and stora ge media. The integrated methods of operat ion (charging, storage, and dischargi ng) of an adiabatic CES system are displa yed in Figure 2. 3. An ener gy- intensi ve liq uefacti on process form s the charging process o f C E S . I n t h e liquefaction process, the gas i s p re-t reated, co mpressed , coole d and expande d until it reaches its due point. The liquefied gas (cryogen) is stored in a site- independent thermally insulated storage vessel at approximately a mbie nt pressur e and very low t emperatures (e .g., - 194 °C). The com pression proces s of the li quefacti on is prese nted separa tely, as in th e adiabatic C ES th e heat of compressi on is recovere d a nd store d to be used i n the d ischarge process. In the discharge process, the liquefied gas is pumped to supercritical pressure (e.g., 1 50 bar) in a cryogenic pump, evap o rated and sup erheated . The thermal energ y supplied to the gas in the superheating process ca n be provide d by t he envir onme nt (e.g. , 15 °C), the intern al heat storage (e.g., 200 °C), a comb ustion process (e.g., > 800 °C) or a wast e heat source (e. g., 350 °C). The high-temperature high-pressure g a s i s su p pl i e d t o a s e r i es o f e xpanders regaini ng a part of the electricity charged to the system. Figure 2.3: Principle of operation of adiabatic CES systems. T,s-diagram for the charging and t he discharging process [36] . Storage Cold storage Hea t stor age STOR AGE Liquef. Compr. CHARGE Pump. Expansion Evap. DISCHARG E Air Liqu id air Heat Cold Electricity -225 -150 -75 0 75 150 225 -4000 - 2000 0 2000 s, k J/kg K T, °C -225 -150 -75 0 75 150 225 -4000 -2000 0 2000 s, kJ/kg K T, °C Chapt er 2 : State of t he ar t 27 The low-temperature ex e r g y ( o r “ c o l d ” ) r e j e c ted during the evap oration process can be recov ered and sto red. When addit ional low -temper ature exergy is supplied to the liquefaction process (charging), the amount of air that is liquefied is incr eased. Withou t co ld storag e and recovery, CES sy stems would reac h roundtrip efficiencies of les s than 30 %. Stand-alone adiabatic CES systems reach a r oundtrip efficien cy of 40-50 % [ 37, 38] . The integration of thermal energy through external heat and cold sources can signi ficantly increase the RTE to above 70 %. The potential fo r system i nteg ration is reviewed in sect ion 2 .3. The state of the art o f differen t parameters, charging and disc har ging conf igurat ions of adiaba tic CES systems are discussed in the following subsections 2.2.2 to 2.2.4. The system design and simulati on of the CES systems evaluated in this work are discus sed in greater detail in chapter 4. 2.2.2. System parameters Recently, several publications ha ve d iscussed the thermodynamic p e r f o r m a n c e o f C E S . A number of adiabatic CES system c onfigurations hav e been propose d. The different liquef action processes, cha rge and disc harge press ures ( 𝑝 , 𝑝 ), li quid yields ( 𝛾 ), cold storage configurations and r oundtrip effi ciencies ( RTE ) o f t h e a d i a b a t i c C E S s y s t e m s d i s c u s s e d i n s e l e c te d l it er a t u r e a re gi v en i n T a bl e 2 . 1. T he se l e c t e d s ys te m parameters varied s ignificantly; the liquefaction pressures rang e from 20 to 180 bar and dischar ge pressures vary between 50 and 150 bar . Table 2.1: Parameters of CES systems presented in the literatur e. Ref. Liquefaction process 𝑝 , bar 𝛾 , - Cold storage configuration 𝑝 , bar RT E , % [39 ] Linde-Hampson ~130 0.44-0.74 fluid tanks (CH 4 O, R218) 112-120 28-37 Expander cycle N A N A fluid tanks (CH 4 O, R218) N A 40-46 [10 ] Linde-Hampson 20 0.70 direct integration (ideal) 100 20-50 [40 ] Linde-Hampson 120 0.83 fluid tanks (CH 4 O, C 3 H 8 ) 50 50-60 [41 ] Integr. Linde-Hampson 90 0.60 fluid tanks (CH 4 O, C 3 H 8 ) 120 60 [42 ] modified Claude 180 0.86 p acked bed gravel (air) 75 48.5 [14 ] 4 turbine Claude 56.8 0.551 p acked bed gravel (air) 190 > 50 [16 ] 2 turbine Claude/Collins 54 N A p acked bed gravel (air) 150 47 [2] Heylandt 180 0.61 fluid tanks (CH 4 O, R218) 150 41 [43 ] Linde-Hampson 180 0.842 fluid tanks (CH 4 O, C 3 H 8 ) 65 50 [44 ] Linde-Hampson 140 N A N A 70 47.2 The liquid yield 𝛾 refers to th e ratio of th e mass flow of th e air liqu efied in t he liqu efaction process to the mass flow o f the compressed air, equation (3.4). The liquid yield is an indicat ion of the cha rging- unit perform ance. The values fo r the liqui d yie ld vary strongly (0.44-0.86), Chapt er 2 : State of t he ar t 28 while the RT E are wit hin the same ra nge m ostly b etween 40 % and 55 %. Theref ore, no optimal CES desi gn configur ation coul d be ident ified [3]. Different assum ptions were made in s i m u l a t i o n , e . g . , i d e a l d r y a i r w a s a s s u m e d , h e a t a n d p r e s s u r e losses in most compone nts as well a s h e a t l os s e s i n t h e c o l d r e c o v e r y a n d t he a s s o c i a t e d h e a t e x c hangers were negl ected [40] , or assumed lower th an 8 % [14]. Moreover, the reviewed publications rarely u sed exergy-based me thods on the component level. N e i t h e r t h e “ f u e l a n d p r o d u c t a pproach” nor the “splitting of p h y s i c a l e x e r g y ” w a s t a k e n i n t o consi derati on. T he origin a nd m argi n of the rmodynam ic ineff icie ncies i n the s ystem have thus not thoroughly been iden tified. The role of cold storage and he a t r e c o v e r y w a s e m p h a s i z e d i n all paper s. Neve rtheless , the effect of heat and c old rec overy a nd stor age was not quant ifie d in terms of exergy. A com parison of the sim ulation and analysis re sults while neglecting the various assum ptions would resul t in misleading conclusions. In order to identify an optim al system design, the different parameters and configurations must be analyzed under common assumptions. In this w ork, a cost- optim al system design of adia batic CES system s is identified with the aid of ex ergy -based methods. In the following s ubsecti ons the different air liquefaction processes, cold storage configurati ons and potential discharge processes are assessed in the context of CES. 2.2.3. Charging processes The significance of th e liquefaction process to the CES’s p erfo rmance was addressed in [39] as “the ke y part” of the s yste m as the dischar ge unit is relativel y s im ple a nd ef fic ie nt . D es pite a ir liquefaction being co mmercial since the 1940 s [45], the chargi ng process of CES s ystems s till h a s r o o m f o r i m p r o v e m e n t [ 4 6 ] . W hen additional low-temperature ex ergy i s avail able, e.g . through cold storage, t he performance of th e liquefaction proce ss is c onsidera bly improve d [2, 15]. On e o f the vital find ings from th e te sting of the fi rst pi lot plant was the increa se of system efficiency though cold recovery a nd storage [14] . Thus, the cho ice of the liquefa ction process for CES infl uences the therm odynam ic and economic perform ance o f the overall system . This work aims to q uantif y this impro vement and identif y the most su itable liquefaction process wit h cold storage a nd recovery. (a) Liquefaction processes Nowadays, various lique faction processes are i n operation. The first-industrialized and simplest confi gurat ion is t he Linde pr oce ss. The pur ifie d air is t here by com presse d, cool ed a nd fe d to a throttling valve to undergo an is enthal pic expans ion, bringin g the air to its due point [47] . Gas liquefaction is at the present ti me achieved in more complex pr ocesses [10]. The Claude p rocess and i ts modi ficatio ns a re the most comm only applied in c omm erci al a ir l iquefac tion plants d ue to the ir elevate d efficie ncy [48] . I n t h e C l a u d e p r o c e s s , t h e c o m p r e s s e d a i r i s c o o l e d b y a c o l d r ecycle stream – a share of the pressurized air that underwent a n isentropic exp ansion in one o r m o r e c o l d t u r b i n e s [ 1 4 ] . T h e Heylandt pr ocess im plied t he elim ination of the f irst heat exch anger in t he Claude s ystem [48] and the Kapitza proce ss dates back to 1939 w hen the inventor su ggeste d the use of centrif ugal expansio n turbines in the Claude process [47] . Most modern liqu efiers use expan sion tu rbines proposed by Kapitza [ 49] and mos t high-pressure air liquefactio n plant operate with the Heylandt process. [46] Chapt er 2 : State of t he ar t 29 A comparative evaluation of air liquefaction processes in the c o n t e x t o f C E S s y s t e m s w a s presented by Li et al . [39], Borri et al. [ 50] and Abdo et al. [51 ]: Borri et al. [50] compared three air liqu efaction processes for application in a micr o-scale CES: the Linde-Ham pson, the Claude, and the Collins process. The Cla ude process was identified as the most suitable air liquefactio n process for application in CES. The Linde-Hampson process operating with a Joule-T homson v a l v e w a s c l a i m e d t o b e i n f e r i o r . Also, a second cold expander used in the Collins process was found not economically feasible . Borri et al. [50] thereby did not take into account the effect o f th e in tegration of cold rec overy and storage on the perform ance of t he li quef action processes. L i [ 3 9 ] o n l y c o n s i d e r e d t h e i n t eg ration of a co ld expander ins tea d of a throttling valve in the Linde process apart fro m a n “ e x p a n d e r p r o c e s s ” w h i c h e m p l o y s a refrigeration process with Helium as working fluid. Li [39 ] came to the same conclusion t h a n B o r r i e t a l . [ 5 0 ] , t h a t t h e throttle-valve-based Linde-Hampson process is not applicable fo r CES. Abdo et al. [51] compared a CE S s ystem design (patente d by Chen e t al. [52]) based on a sim ple L i n d e - H a m p s o n p r o c e s s t o t w o a l t e r n a t i v e s y s t e m s : t h e C o l l i n s a n d t h e C l a u d e p r o c e s s . T h e heat of compression w as accounted for in the analysis, but the cold storage was not considered. The Claude and Collins process showed similar therm odynamic per formance and reached greater RTE than the Linde-based process. The more efficient Claude-based system was evaluated as the bes t option, although the Linde-Hampson achiev ed the lowest specific costs [51] . None of the three co mparative an alyses reviewed assessed the im pact of cold storag e on the selecti on an d perform ance of the liquefac tion de spite discussin g specif ica lly t he applicati on in CES systems. The present work aims to compare several air lique faction processes with integrat ed co ld storag e to iden tify th e most efficien t and econ omic liquefaction process for implem entation in CES sys tems. The differe nt liquefacti on proce sses that w ere considered are descri bed in sectio n 4.3.1. The resul ts from the com parative en ergetic, exergetic, and econom ic evalua tion a re prese nted in secti on 5. 1. (b) Cold storage Without cold recovery and storage , adiabatic CES efficiency wou ld drop bel ow 35 % [15]. The temperature in the cold storage cyclically varie s between cryog enic temperature (discharging process) a nd envir onmental temperature ( chargi ng proces s) [53] . For the effective reco very and storage of the low-tem perature exergy from the dischar ging proc ess, the charact eristics o f the storage medium play a vital role [42]. From the literature revi ewed (T able 2.1), two kinds of a cold storage configuratio n can be extract ed: 1) q u a r t z i t e g ra v e l ba s e d p a c ke d be d st o r a ge w i t h d r y a i r a s t h e s econdary working fluid, and 2) two- tank fluid stora ge with metha nol and propane (or R218) as s econdary working fluids and stora ge med ia on two d iffer ent t empera tur e lev els. The latter cold storage configu ration (2) dates back to 1997 wh en Hitachi, Ltd. developed and patent ed the first conce pt for cold recove ry and storage [ 22]. The concept was afterward adopted for a solid-packed-bed of pebbles by Highview Power Sto rage Ltd. [52] . The high densit y and specific heat capacity o f the pebbles in packed bed cold storage (PBCS) enables higher e nergy de nsit y. The P BCS can be des igne d with a w ide ra n ge of storage m aterials. Hd- Chapt er 2 : State of t he ar t 30 polyethylene and polyp ropylene were evalua ted suitable for appl icati on in P BCS w hile m etals were found unfitting b y [53]. The dynamic perform ance evaluatio n of PBCS reveale d that the dynamic cycli ng between the chargi ng and the dis chargi ng proces s increases the work required for the li quefacti on pr ocess by a n undesire d 25 % in com pariso n to steady-state analysis [42]. Due to a large amount of low-tem perature exergy to be stored, s afe (non-toxic and non- flammable) and inexpensive cold sto rage media are preferab le [3 9, 22]. The cold storage configuration was the subject of many publications. The cold st orage performance (e.g., t e m p e r a t u r e , t h e m i n i m u m p i n c h t e m p e r a t u r e a n d t h e p r e s s u r e d r o p i n the CS) has a high impac t on the RTE of the overall system independent of the st orage material [ 43, 52, 22]. On a system l e v e l , t h e r e c o v e r y r a t e o f t h e l o w - t e m p e r a t u r e e x e r g y a n d t h e c o l d s t o r a g e c o s t s a r e o f impor tance. Thus, t he cold storage config uration itself is not ana lyzed in this work. S olely, one CES configur ation was considered (a pair of thermal fluids). Th e considered C S design is explained i n m ore detai l in secti on 4. 3.2. 2.2.4. Discharging processes Power cy cles are driven by the t emperature gradient between two thermal reservoirs: the heat sink and the h eat source [2]. The electricity “charged” to a cr yogen (liquid gas) while liquefaction may be p artially r eg ained in a c old power genera ti on c y cl e wh er e th e cr yo g en a ct s as the heat sink. Di verse hea t sources can be util ized: ambie nt heat, low/high-grade waste heat or heat of com bustion [ 45, 54]. V arious methods for extracting low-temperature exergy stored in a cryogen have b ee n publi shed in China, USA, and Belgium [10 , 17, 37, 55, 56, 57]. Altogether three different method s for power generatio n and t he ir com bination can be identif ied [2]: 1) The ‘Direct Expansion’ refers to power generation wh ile expanding the beforehand pressur ized, vaporize d and superhe ated cryogen, in a direct man ner [8, 45, 55]. The cryogen thereby serv es as the only working fluid o f the Rank ine cycle. 2) Methods for extracting cold exer gy from a cryog en employin g a s econd working fluid in addition to the c ryogen are referred to as ‘indirect’: a. The ‘Indirect Rankine’ cycl e i s th e most commo nly considered ind irect cold power g eneration method [10, 37, 45, 58]. The cryogenic fluid t hereby acts as the heat sink of the cycle, wh ile the heat f or eva porat io n of t he working f lui d is provided by the ambient o r another heat source. b. Within the ‘Indirect Brayton’ cycle the cryogen cools down a gas to decrease compression work, the dense gas is further heated and expanded generati ng shaft work in a gas tu rbine [54, 58]. 3) Combination s of th e three me thods can be referred to as ‘Com bined Met hods’ [ 45, 54]. Methods for extracting cold exergy in power cycles is most comm only discussed in the context of the regasificat ion of LNG. The low-tem perature exergy vented in the LNG regasificat ion usua lly ser ves no f urther purpos e. The co ld exergy r eleased in the evapor ation process in CES system s, in c ontras t, coul d be stored a nd s upplied for the subs equent c hargi ng pr ocess. F or CES the indirect methods (2.-3.) are thus in direct competition wit h the cold recovery and storage within the system. For CES only th e direct expansi on of the liq uid air wa s considered. Chapt er 2 : State of t he ar t 31 2.3. System integration: waste h eat, waste cold, and combustion Cryogenic energy storage RTE can be increased by 40-75 % whe n internal and/or external heat sources or heat sinks (“cold” sou rces) are integrated. Examples for the integration o f external heat and cold sources to an adia batic CES system are shown in F igure 2.4. Four different system configur atio ns can be diff erentiated [ 59]: the adiaba tic system, waste heat recovery, LNG waste cold recovery, and the inte gration of i nterna l combus tion. In adiabatic systems, the heat o f compr ession in the charging p rocess can be recovered and stored in the heat stora ge subsystem. The heat stora ge is commo nly realized with pressurize d water or therm al oil. The store d thermal energy is later used t o increase the turbine inlet temperature (TIT) and power output o f the discharge process. Th e s pecific power output of the discharge unit increases linearly with the increase in TIT [2]. The tempera ture of the captured thermal energy increases with fewer com pression stages and high er pressures. Figure 2.4: Integration of internal and external heat and ”cold” sources to a CES system [59]. The recovery of “waste heat” is seen as one of the main potenti als of CES [10, 8, 16, 37]. Waste heat refers to heat transfer streams from energy and manufacturing processes that serve no furthe r purp ose and m a y occ ur at diff erent tem peratur e leve ls [ 60] . For waste-heat recovery in CHARG E STORAGE DISCHARGE Air Liquid air Heat Cold Electricity ADIABATIC SYSTEM Stor ag e Cold sto rag e Heat storage Liquefaction Comp res sion Pum pi ng Expansio n Reg asif icat ion LNG deli very LNG por t LNGregasific ation Gaspip eline Indust ry/ pow erplan t WASTE COLDRECOVERY WASTE HEAT RECO VERY CO‐FIR ING Combustion Chapt er 2 : State of t he ar t 32 cryogenic energy stora ge, only high-temperature (> 300 °C) wast e heat is of int erest. The heat of com pression recovered in a dia batic CES systems easily reache s temperatures o f 200 °C. Waste heat recovery would necess itate c olocation of the storage to an existing waste heat source setting up geogra phical lim itations. As an alte rnative, a CES c an con tain a combustion process in which fuel, e.g., natural gas, is burned to supply additional thermal energy, which increases the temperature of the high-pres sure gas before expansi on. Natu ral-gas-fire d systems may reac h RTE s of 55-6 0 % [15, 43] . I n t h e d i s c h a r g e p r o c e s s of a d i a b a t i c CE S s y s t e m s , t h e p r e s s u r i zed liquid g as i s evaporated in heat e xcha nge to the col d stora ge m edia t o rec over and store th e low-tem perature exergy. When the low-temperature exergy is s uppli ed to the subsequent liquef action process, a higher share of compressed air is liquefied, and the efficien cy o f th e ch arg ing process significantly increas es [2] , section 2.2. 3. Low-temperature exergy may also be supplied from LNG r egasifica tion plants or industrial processes, e.g., air separation units [61]. Low-tem perature exe rgy that would else be released to the environment is often refer red to as “waste cold”. The wa ste cold can either be integrated instead of or in additi on to the cold stora ge. Es peciall y in th e cont ext of e nvironm ental impact, as we ll as e nergy an d c ost savin gs waste col d recove ry from the LNG re-ga sificati on proce sses has attracted great interest in t he past decade. LNG integrated C E S s y s t e m s w e r e r e p o r t e d t o reach RTE s of 63-70 % [8, 62, 63]. Exe rgetic effi ciencies for LNG i ntegr ated systems p resented by [ 62], [63] and [64] reached 33-43 %. Integrating both LNG co ld recuperation and comb ustion, t he s ystem may reac h energetic effici encies above 7 0 % [43] and exerget ic efficiencies of 60-70 % [64] . Other CES sy stem in tegra tions hav e be en suggest ed, e.g . into ga s-fired power plants [8, 39], with a secondary organi c Rankine cycle (ORC ) driv en by the heat of com pression [ 41, 63] , with an absorpti on refrigera tion machine [65] , or with a pumped ther m al energy storage ope rating as a topping cycle [66]. Several sources mention the favou rable integration of CES with other processes. The influence of system integrati on on cost was rarely discusse d in the revie wed literat ure. Diff erent system configurations were not compare d on common ground (neither ther modynamically nor economically) to quantify the p erf orm ance enhancement of integr atin g different heat sour ces and sinks. The only comp arison o f CES RTE s was given by Stöver et al. [15] . Stöver et al. emphas ized the unavaila bility of reliable data and presente d on ly rough estimates for RTE s of differe nt system configurations [15]. For this reason, the inte grat ion of CES in this work aims to qua ntif y the ef fect of system integrati on on CES therm odynam ic performance and economics, see sect ion 4.3 a nd 5.3 . Chapt er 2 : State of t he ar t 33 2.4. Classification, char acteristics and benchmarking 2.4.1. Classification and competing technologies G r ea t i n t er e s t i n e ne r gy s t or a ge s ys t e m s ha s e m e r g e d i n r ec e n t yea rs [67], and a large number of energy storage (ES) technologies exist. ES technologies can be classified according to: the principl e of operatio n or type of energy store d (mechanical , chemical electrom agnetic/el ectrical, therm al); the rated power, the energy c apacity, and the freq uency a nd durat ion of discharge [ 3]. The working principle of CES systems is classified as thermal ( or thermo- electr ic) ES [7] . The rated power, energy capacit y and discharge duratio n of most com mon ES technologies are shown in Figure 2.5. When classifying ES technologies accor ding to the rated power of the discharge unit, sm all (≤ 1 MW), me dium (10-100 MW) and l arge-sc ale ( ≥ 100 M W) ES syst ems are diff erent iated [ 68]. Figure 2.5: Rated power, energy capa city and discharge duration of E S technologies [3 ] based on Guney and Tepe [69 ] and Yoon et al. [70 ] . 1,000 100 10 1 1 m in 1 hr 1 day LA NaS 1 MWh 10 MWh 100 MW h 1 GWh 10 GWh 100 GWh FES Li-Ion VRB CAES PHS H2 SNG rated power , MW ener gy capacity dischar ge duration 1 m onth CES CES cry ogenic energy storage Li-Ion Li-Ion battery FES flywheel energy storage PHS pum p ed hydr o com pressed air energy storage CAES NaS sodium sulphur batter y H2 hydrogen storage SNG Synthetic natural gas LA lead acid battery VRB vanadium redox flow battery Chapt er 2 : State of t he ar t 34 Being anticipated to achieve hundr eds of MW in power rating and energy capacitie s larger than 10 GWh, CES is categorized as gr id-scale energy storage. As a n alterna tive, ene rgy stor age can be classified accordi ng t o the a p plication [3]. Technologies wi th similar c haracteristics com pete for the sam e services (or applic ations) that can be provided to various stakeholders in the power system ( e.g., transmission or distribut ion s ystem operators) [7 1]. The services an ES t echnology can provide are dependent on a number of character istics. The p ower and energy capaci ty, as well as the response time, are o f particular importance. In the f ol l ow in g s ec ti o n, a c o m par is o n i s d r a w n b e t w e e n t h e c h a r a c t e r i s t i c s o f C E S a n d t h e c h a r a c t e r i s tics of the com peti ng technologies. Potential ES applications suitable f or CES storag e are assessed i n section 2.5. Being chara cterize d by high power ra tings and ener gy capac ity a long with response times o f up to several m inutes CES competes with other bulk ES technologies for energy management applications [7, 37, 72]. Cryogenics-based energy stora ge is in d irect co mpetition with oth er grid-scale ES t echnologi es such as pumped hy dro storage (PHS) a nd com presse d-air energ y storage (CAES), as well as selec te d batter y-base d and hydrogen- based ES technologies [3] . 2.4.2. Characterist ics and benchmarking In order to identify suitable ES applications, it is necessar y to evalu ate th e characteristics of the respective ES technology . In p articular, the rated po wer , the energy capacity , the RTE , the response time , and the discharge time influence the technologie s viability for various applications. The characteristic s of CES and competing technolo gie s are revie wed in this section. T he gat hered data are given in Table A.1 in Appendi x A . The capacity of the ch a rg e a nd t he dis cha rg e un it of C ES sys tem s can be sized independently, contrary to other bulk ES technologies. The power rating o f CES sy stems commonly r efers to the installed capacity of the discharge unit. CES power ra tings o f 10 to 500+ MW are economically a nd ecologically di fficult to achieve by hydrogen- based and battery-based energy storage [7, 16, 17, 29, 72]. The volume o f t he sto rage tank and the specif ic work output of the discharge unit de termine the energy capa cit y of CES s ystems. Energy capacities larger than 25 GWh [ 39] are realiza ble with avai lable industrial cryogen tanks , ena bling storage losses bel ow 0.2 % Vol per day, and come at low associated costs (appr oximately 160 € /k W dis [18]) [12, 16]. Selected characteristics of bulk -energy technologies (PHS, CAES , and CES) are compare d on a weighte d scale in Figure 2.6. Energy capac ities of m ultip le h undreds of MWh and power ratings of 50-200 MW are expected to be achieved by CES systems in the near future [ 8]. One of the greatest advantages of CES systems is the high volumetric energy dens ity . CES energy density may rea ch v alues of 90 Wh/l [18, 73], or even 16 0-200 Wh/l [10, 17, 37] , while, other bulk energy storage technologies such as PH S (0.5-1 .5 Wh/ l) and CAES (3-6 Wh/l) reach by the order of two magnitudes sm aller energy densities [8, 15, 16, 29, 37] . The high volum etric density enables CES “site-free” or site-independent storage. On ly hydrogen-based energy storag e achieves a higher vo lume tric energy density than cryoge ns, regardl ess of the storage form [74]. The storage of hydroge n, however, entails po tential safety hazards due to extreme flammability a t high pressures [ 29, 74]. Liquid air can be stor ed with low technical requirements at ambient pressur e [3]. Fo r a d a il y cy c l i n g C E S s y s t e m, t h e s t a r t - u p o f t h e l i q u e f a c ti o n proce ss takes up t o 20 m inutes. While ramp up after a d ay at standby may take one hour [18]. Th e response time of the discharge Chapt er 2 : State of t he ar t 35 unit of the pilot plant o f fewer than 150 seconds was reported by Alyami and Williams [75] . For comparison, CAES systems tak e 10-15 m inutes to start operat ion [ 76]. The discharge duration i s s o l e l y l i m i t e d b y t h e c h a r g e - t o - d i s c ha r g e r a t i o a n d t h e s t o rag e tank size and m ay be adjuste d according to the aim ed applica tion of the storage. The charge-to- dischar ge ratio of CES system s is usually an ticipa ted betwe en t w o a n d s e v e n [ 18, 31, 77]. Several minutes to days [7], and even longer discharge duration s e . g . s e a s o n a b l e s t o r a g e a r e technically realizable. The standing losses of CES systems are r a t h e r l o w a n d a r e c o m p a r a b l e to that of battery-bas ed ES technologies [18]. A discharge dura tion of multiple hours at daily operat ion is m ost comm o nl y report ed [ 7, 11, 37]. Regarding the response tim e of the chargi ng process (< 20 min [18] ), chargin g time should exceed 2-4 hours. Figure 2.6: Comparison of weighted characteristic s of CES, CAES, and PHS [36] adopted fr om [68 ] , based on data extracted from literature (see Table A.1). The roundtr ip efficie ncy o f s t a n d - a l o n e C E S i s r e p o r t e d t o r e a c h 4 0 - 5 0 % [ 3 7 , 3 8 ] . T h e specific power output of the discharge un it can be increased by the reco very of waste heat e.g., from industrial processe s [10, 16]. The RTE may reach values of up to 55 % with the integra tion of waste heat [12]. The integr ation of “waste cold“ (low-tempe ratu re exergy) provided by e.g., re- gasif ying LNG can als o in crease the RTE o f C E S s y s t e m s . T h e s i g n i f i c a n t r e d u c t i o n i n t h e energy demand for liquefactio n process may increase the RTE of CES sy stems to 63 -70 % according to [8, 62, 63]. The int egrat ion of CES sy stems with e xte rnal heat/c old sources is discusse d i n sect ion 2.3 an d secti on 4. 4. PHS has currently the highest installed ener gy storage capacity globally. As a mature technology, the RTE s typically range from 70 to 85 % [33, 78, 79, 80 ]. The perform ance of CAES syste ms depend on the respe ctive system configuration. CAE S syst ems i n opera tion achieve RTE s of 40-54 % [33] and are e xpecte d to inc rease t o 60 % [76, 81] . Adiabat ic CAES systems with heat storage as well as isotherm al small-scale CAE S sy stems are expected to reach spe cific ener gy [Wh/ kg] energy densi ty [kWh/ m³] round-trip e fficienc y [%] lifesp an [ye ars] scale [MW] specific investm ent costs [€/kW] technical maturity (TRL ) CAES PHS CES Chapt er 2 : State of t he ar t 36 RTE s of 70 % and higher [33]. These efficiencies are only approach able for CES sy stems with the integra tion of waste cold or a com bustion process. Cryogens (liquid air and nitroge n) were com pared t o hydrogen as an energy carrier and storag e media by Ding et al. [74]. The RTE s of both ES t echnologies w ere found comparable, while CES was eval uated and regarde d to be more environm entall y frien dl y. A num ber of char ging and discharging technologies are possible for hy drogen-based ES . F or the generati on of hydrogen, water electrolysis is t he most common process, reachi ng e fficienci es o f up to 70 %. The dis charge process e.g. fuel cells reaches an efficiency of 40-70 %. Hence, hydrogen-based ES reach RTE s as low as 30- 50 % [7 4, 82] . The RTE o f CES systems does not decrease during the lifecycle, unlike battery -based ES. Th e lifetime of CES system component s is expecte d to be more than 25 years and up to 60 years [29, 37, 38], which is m ultiple times higher than for the compo ne nts of batteri es or hydrogen- base d ES system s (5-15 years [37] , < 30 ye ars [8 3, 84]) [29, 82 ] . In contrast to a limite d cycle life of batteries, the cycle fre quency has little effect on CES syste m performanc e. The low- temperature energy supp lied by the cold storage i s limited, whi ch is why the cold storage perform ance limits the “depth of dis charge” of CES systems . The p erformance of industrial air liquefaction plants strongly increase s with the size (installed capacity) [74] . Hence, the scale of the CES system also dete rmines the achieva ble RTE [3]. Due to the low RTE , the specific investme nt cost of CES is of great importa nce to CES applicability, section 2.5.1, in particular, due to the rather low RTE . The specific capital cost (per kW o f in stalled capacity ) for CES syst ems r eported in the literature vary strongly, Figure 2.7. CES technologies sp ecific costs r educe with size, accordin g to the “economy of scale”, Figure 2. 8. The specif ic costs of CES are expecte d to decrease by 24.2 % whe n the power capacity is doubled [32]. The larg e variation in t he considered CES power capacity m ay justify the larg e cost r ange r eporte d in the literature [3]. Figure 2.7: CES specific cost per kW installed capacity reported in literature (a: [85], b: [64], c: [33], d: [72] , e: [1 6] , f : [18] , g: [1 5] , h: [8] , i: [3 2] , j : [38 ] and k : [3 7] ). 0 1000 200 0 300 0 4000 source k, year 2 009 source j, y ear 2 010 source i, y ear 2 012 source h, year 2 013 source g, year 2 013 sourc e f, year 201 4 sour ce e, year 2015 source d, year 2 015 sour ce c, year 2016 source b, year 2 018 sour ce a, year 2018 specific investm ent cost of CES, € 2017 /kW inst Chapt er 2 : State of t he ar t 37 High c apital investment costs als o characterize competing techn ologies: Compressed air e nergy storage has a capital cost p er k W installed of 500 €/kW to 2,20 0 €/kW [33] . The investment cost for PHS can be lower, about 350 t o 1,500 €/kW [33], but m a y reach value s up to 4,300 €/kW [86] depending on the site. In the industrialized co untries, economically feasible sites for PHS are widely diminished. The specific capital cost o f C E S s y s t e m s i s r e p o r t e d t o reach 500-2,400 €/kW and be c ompara ble with CAES, PHS, and flow batteries [ 29]. When CES technology reaches matu rity, costs are exp ecte d to dec rease fur ther [3] . Hydrogen-based ES lower investment cost limit is approxim ately 2,000 €/kW, accounting 520 €/kW for hydrogen st orage in salt cavern s, 500 €/kW for the e l e c t r o l y z e r , a nd 920 €/kW or 1500 €/kW for polymer e lectrolyte membrane fuel cells (PEMS) or solid oxide fuel cell (SOFC), respectively. The specifi c investm ent cos ts of hydroge n are thus significantly high er than that of CES, as well as other bulk en ergy storage technolo gies CAES, and PHS [87]. Figure 2.8: Capital expenditure (CAPEX), specific co sts (per unit of power and unit of energy stored) over CES system capacity based on cost estim ations published by Highview Ltd. [ 88]. Anothe r base for the compariso n of ES technologies can be the s pecific cost of storage (per kWh of installe d ener gy capacity). In partic ular, batter y-base d ES technologies are commonly compared based on th e specific capital cost of storage . Yet, for grid-scale ES technologies, the comparison based on th e cost pe r unit of installed energy ca pac ity can be misleading due to the dissimilar specific cost of the st orage unit. Larg e-scale reser voirs necessary for PHS and CAES make up for a significant share o f the ov erall investment costs , whereas, cryoge nic tanks have low specific costs in comparison to other CES equipm ent. The v a lues r eported for PHS fo r example range from 5- 100 €/kWh [37] , 250-430 €/kWh [ 86] or 10-2 0 €/kWh [83] according to different sources , Table A .1. The construction lead-time of CES projects is expected to be on ly one or two years, which is vastly faster than for CAES and PHS projects (5-10 yea rs) [ 15, 76]. Fast comm issioning paired with an extensive economic life (2 5-60 years [20, 29, 37, 38]) i s i n f a v o r o f C E S e c o n o m i c perf ormanc e. 0 20 40 60 80 100 120 140 160 180 0 200 400 600 800 1000 50 100 150 200 250 300 capital expendi ture, Mio € specific costs, € /kW or €/kWh installed di schar ge capa city , MW inst specific power cost [€/kW] specific e nergy cost [€/kWh] CAPEX [Mio €] Chapt er 2 : State of t he ar t 38 The en vironmental impact of ES technologies is as well regarded as an important decisio n criterion. The m ajor n egative im pacts of ES technologies are, e .g. the occupation of large areas for PHS [37 ], the use of scarce and toxic chemical materials in batteries [72 ], th e air p ollution b y e x h a u s t g a s e s o f d i a b a t i c C A E S. In adiabatic CES systems, no chemical reactions take place. T h u s , C E S i s o f t e n c l a s s i f i e d a s “low-carbon” or environmentall y friendly technology [45]. The recovery of energy that else wou ld h ave been vented to the envi ronment as well as the gas cleaning process in CES systems can be vi ewed as a positive env ironm ental impact [7 4]. The site-independe nt installation of the system s frees CES proj ects of geological risks that o c c u r i n P H S a n d C A E S p r o j e c t s . A l s o , t h e h i g h - p r e s s u r e s t o r a g e of hydrogen im poses potentia l safety hazard s [45] . High-pressure CAES systems in cavities face challenges, e .g., uplift failure [89] or gas enric hment and i gnition of resi dual hydrocarbons [90]. The storage of cryogens implies fewer safety issues. CES i mplies common health h azards alike in cryogenic ASUs or power plants, which may b e avoi ded by protective measur es, e.g., clothing and equipm ent [18, 9 1]. Chapt er 2 : State of t he ar t 39 2.5. Potential application and economic benefit 2.5.1. Applications suitable for CES systems E n e r g y s t o r a g e s y s t e m s a r e c a p a b l e o f s u p p l y i n g b o t h p o s i t i v e ( supply -side) and negat ive (consumption-sid e) control power to the grid. Therefore, ES tec hnologies h ave a broad range of applications. ES applications can b e classified according to multi ple criteria. In Figur e 2.9 the classification according to th e capacity and discharg e du ra tion of E S systems is given. Three types of applications can be differentiated: uninterruptible power supply (UPS) , grid support , and energy management applications . To provide power quality a pplications or UPS , the ES sys tem ne eds to be capab le to supply one-fourth of its power capacity ( less than 1 MW) wit hin m illiseconds. To e nsure grid support , which i s also r eferred to as bridging power or ride-through capability , the rated power is required to exceed 100 k W and the response time of the storage system may be up to one second. For energy management applicatio n a slower res ponse of severa l minut es is acceptable but capacities above 100 MW are necessary [ 3, 72]. D u e t o t h e s l o w s t a r t - u p t i m e , C E S s y s t e m s a r e n o t s u i t a b l e t o supply UPS o r ride-t hrou gh capability applications. Energy management is, therefore, the only ca tegory o f applications, which is suitable for CES systems [7, 37] . Figure 2.9: Discharge period and pow er rating of va rious energy storage applications [3] based on [72, 92, 93, 9 4]. Renewables forecast / hedging Ti m e s h i f t hours minutes seconds power rating, [ MW] 0.1 1 10 100 1000 0.01 Energy Managem ent Grid support UPS V oltage Support Power qual ity UPS Frequency control V oltage support T ransmission s tability Peak shaving Primar y reser ve Secondary reserve Blacksta rt Load levelling/f ollowing Load f actor incre ase Capacity deferral Bulk ener gy trading Arbritrage Frequency excursion suppression Angular stability V oltage st ability Fluctuation suppression Short duration power quality Dischar ge duration Chapt er 2 : State of t he ar t 40 The chara cteristics of the selected ES applications suita ble fo r CES are given in Table A .2 through Table A.5 in Appendix A. Bulk e nergy applic ations , selected ancillary services , transmission and dist ribution support as well as facilitating the integrati on of renew able ener gy (RE) sources are among the energy m anagement applications relevant for CES. Bulk e nergy appli cation r e f e r s t o t h e a p p l i c a t i o n s t h a t m a k e u s e o f t h e e l e c t r i c i t y p r ice difference (trading) by electrici ty time shifting b etween the low-pri ced electr icity charg ed and the discharge during high -price hours [95]. Both peak shaving and energy arbitrage a r e b u l k energy applications su itable for the response time of CES sy ste ms (> 100 seconds). Peak shaving is commonly instal led at the consumer end, whe reas , energy arbi trage ra ther takes place at the supply side . The principle of energy arbit rage application is char acterized by the economic benefit of electricity time shifting [3]. The main goa l of peak shaving is to supply energy during peak load hours by leveling out the typical fluct uati on o f the demand curve [78] . Peak shaving thereby does not h av e a primary economic target bu t improves the outlay of the overall power system by red ucing t he requi red n omina l insta lled capacity [92] . Ancillary services, also referred to as “grid operat ional support”, enable the m ai nte nance of the power reliability and quality by facilitati ng the sm ooth operat ion of the power grid as well as reducing efficiency lo sses [93]. CES has the potential to suppl y; reserve generating capacity, load follo wing, tertia ry frequency regulation, and black start [3]. The presence of reserve generating capacity in the power system is indispensab le to compensate for sudden changes in t he load curve or loss of oper ating generators. Reserve generating capacity accounts as instal led capac ity but i s not u sed in norm al oper ation [ 92]. For reserve generat ing capacit y; “spinni ng” reserves – imm ediately accessible, and “ non-spinning” reserves – a vailable within 10-30 minutes [96] are differentiat ed. Only the non-s pinni ng reserve application is suitable for CES systems. For non-spinning reser ve application, the CES systems must enable discharge periods larger than one hour in order to bridge the power generatio n until the nominal output is reinitiated, e.g. ,by the bac kup power sy stem [3, 92]. Load following assists in the maintenanc e of t he real-time balance b etween el ectricity supply and dem and, also called regu lat io n cont rol . Preventing a s upply-demand mism atch at all times brings along noteworthy technical and economic benefits [93] . T he c apability of load follow was a key f indin g of t he first i ntegrat ed CES pil ot pl ant [1 6]. The frequenc y of the power grid is required to be continua lly p reserved within th e tolerated l i m i t ( e . g . , 0 . 1 H z o r 0 . 2 H z , N o r t h o r M i d d l e E u r o p e [ 9 2 ] ) . E S systems can operate as frequency regulators by amending a n y d e v i a t i o n a n d k e e p the freque ncy wit hin the regulatory rang e of operation [71] . CES is appropriat e for tertiary frequenc y contr o l a p p l i c a t i o n [ 3 ] , w h i c h i s manual ly cont rolle d by the system operator within 15 minutes (o r hours) subseq uent to a frequenc y di sr uptive incident [71, 92]. When po wer is injected to a power system that suffere d a blacko ut, it is referred to as black start capa city . Black start capacity facilitate s the start-up o f l arge pow er plants or re-energizes the distribution lines [96]. CES response time and capacity ran ge are suita ble to pro vide this applicatio n, yet the storage unit is require d to be capable to start operation autonomously. During the discharge process of CES systems, pumping power is r equired before electricity can be discharge d. To provide black start capacity, CES systems nee d to be ext ended , e.g., with batter y stora ge [3, 92]. Chapt er 2 : State of t he ar t 41 The integra tion of hi ghly fluc tuati ng RE gener ation to the grid is associated with technical issues such as frequency and vo ltage irregularities [1]. The su c cessful integration of inte rmittent electricity generation from RE s ources embodies the g reatest po tentia l of ES s ystems [95]. The application of ES in fir ming, shifting, and smoothi ng the elect ricity generation fro m RE sou rces may enable a higher shar e of RE generator s in the power syste m without endangering the supply security [96]. Renewabl e ener gy capacity fir ming applicat ion, i n essence, aims to smooth both the voltage and the power output of RE generators. This increas es the capacity credit of the overall power system, which allows a higher penetratio n of RE g enerators and avoids the cost of additional backup power [92, 95]. [3] The dela y and at times entire avoidance of investment in upgrad es of the transm ission and distribu tion sy stem is referred to as transmission and distri bution (T&D) support [97]. Such upgrades include, e.g ., congestion relief, infrastructural upgr ades, and avoided load-shedding due to access generation that ca nnot be incorporated by the inf rastruct ure of the T&D system [3, 93]. To ensure a secu re and re liabl e power syste m op eration, congestion management i s o f g r e a t impor tance [71]. T&D capacity extens ions are widely unable to k eep p ace with the r apid increase in peak dem and. By eith er lowering the peak dem and or excess electricity gen eration, ES systems facilitate the reduction, postponing or even waiving of congestion-related costs, e.g., additional tran smission capacity ( transmission curtailment or upgrade deferral ) [96, 97]. Moreover, this application preve nts congested transmission line s and substati ons and avoids the undesirable shut down of ex cess RE gener ation [78]. I n congestion management application, the ES unit is required to enabl e a disc harge duration of m ulti ple hours. CAES h as been identified to be compatib le with congestion management application. CES is expected to be suitable as well, as the systems operate very sim ilarly [3, 78] . The objective of load shifting ap plicati on is the avoida nce o f a number of technical operation problem s of the T&D system that occur in case of a demand-suppl y mism atch and entail economic drawbacks [1]. In operation, this applicatio n is v ery sim ilar to peak s having , energy arbitrage, and renewable time shift . The key difference is that the serv ices are provi ded to differe nt stakeholders. Thus, the econom ic value of sim ilar app lications vary, and must be distinguished [3]. 2.5.2. Economic benefits and value propositions The applications that CES may pot entially supply h ave diverse m one tary com pensat ions. The moneta ry compensa tion for providing a specific service is refer red to as “benefit” or “value proposition”. The benefit of a p articular application is either d ete rmine d by the market rate (e.g., ancillary service or elect ricity market clearing price) or the cost of the alter native solution (e.g., grid extension). The econo mic benefits presented in this section are based on the US (and Canada ) market due to t he availa bilit y of da ta [3]. In Figure 2.10, the potential “life cycle” value propositions o f multiple applic ations is shown with the c orresponding discharge d uration. The life cycle value proposition of th e services that a specific ES system can provide is the key driver for investm e nt in the technology, system or project [97]. The applica tions within the dischar ge time range of CES (2-6 hours) enable the highest potential e conomic benefit. Chapt er 2 : State of t he ar t 42 The specific value proposition (in €/kW) o f the applications su itable for CES are given in Table A.7 in Appendix A [3]. The poten t i a l v a l u e a n d t h e v a l u e p r o p o s itions are calculated over an ES l ifetime of 10 ye ars. V arious services are tr aded in markets and are acknowledged as products, whereas the monetary val ue of other applications is h arder to assess. Therefore, the value propositions in th e literature vary [98], Table A.7. The larg est un certainty in the value proposition is associated with t ransmission curtailment and dis tribution upgrade deferral. Reason for this is that the monetary value of the application i s determine d by the avoided investment and may reach values of up to 1000 €/kW. Reserve cap acity reaches a rather low value proposition of less t han 200 €/kW. RE integration (time s hift and capacity firm ing), as well as peak shaving and energy a rbitrage, are more consistent and econom ically viable applications for CES. Figure 2.10: “Life cycle” value proposition of ES a pplications for various di scharge durations [3 ] adapted from [99 ] based on [97 ] . T h e i n v e s t m e n t i n a n E S s y s t e m i s o n l y f a v o r a b l e i f t h e v a l u e p roposition o f the potential applications excee ds the cost as sociated with the ES system by a sign ificant margin. In o rder to maxim ize the market value o f an ES technology, a number of serv ices that can be provided simultaneously or successively are to be identified. The aim of i ncreasing the value proposition Chapt er 2 : State of t he ar t 43 of an ES by bundling multiple app lications is referred to as “b enefit aggregation” or “value stacking” [100, 101]. The compatibility of different E S applicati ons is shown in Figu re A.2 in Appendix A. CES potent iall y low spec ific cost per unit of ener gy and high energ y density make the technology most sui table f or en ergy applications. Frequent energy applications, e.g., supply capacity, RE time shift or energy arbitra ge, as well as infrequent energy applications such as T&D support, are easily coupled with other app lications. For stacking b enefi ts, the main application is referred to as the “anchor service” (or “anchor application”). The prima ry service should have a value proposition at least as high as 25-50 % of the co sts associated with the ES [102 ]. Poten tial anchor and secondary s ervices suitable for CES are shown in Fig ure 2. 11 [3] . When an ES supplies a specific se rvice, e.g., with p ositive rea ctive power, for an extende d amount o f time, not sufficien t ES capacity is available for fur ther discharge to supply another application. ES applicati ons tha t require unlimited availabilit y o f t h e s t o r a g e c a p a c i t y f o r sudden d ischarge, e.g., spinnin g reserve and black start, are, therefore, hard to combine with other applications. While a numb er of other services can easily be provided simultaneously. To evaluate the compatibility of di fferent ES applications energy system modeling is necessary to account for the time series and st ate of charge required for different applications [3]. Moreover, the combina tion of pote ntially appr opriate applica tions is requ ired to be mode led to i dentif y th e stacked value pr oposition [103]. Figure 2.11: Potential primary (“anchor”) and secondary service s of CES [3] based on reviewed literature [37, 72, 78, 84, 9 2, 93, 102, 103, 104, 105] The value propositions (Table A. 7) are not additive. The aggreg ation of benefits is difficult as there is not su fficient exp erience with value stacking [3] . There are various reasons for bene fit aggregation not being com mon practice: technical a nd ope ratio nal c onflicts , the la ck of engineeri ng standa rds and regulatory fram ework, unproven and ne w technology, weak or missing price signals , and m ultiple stakehol ders t hat m ust to b e coordinated [ 3, 97]. Disregarding the still unfavorable market s ituation, potential value ranges for stacked value propositions of selected ES app lications are approximated and s hown in Figure 2.12 based on Primary (anchor) service Load following T&D upgrade defer ral RE ti m e shift RE cap acity f irm ing RE capacity firm ing Electric energy tim e shift Electric supply capacity Electr ic supply reserve capaci ty Electric supply capacity RE capacity firm ing Electr ic supply reserve capaci ty RE tim e shift Secondary serv ice Chapt er 2 : State of t he ar t 44 Table A.7 in the Appendix A. To d etermine the exact stacked val ue p roposition an energy system model is necessary [103]. A s compatible app lications hav e nearly additive stacked value, Figure 2.12 show s the additive value propositions for se lected C ES app lication s. With the exception of renewable energy time shifting applicatio n co mbin ed with eith er p eak shavin g or ene rgy arbit rage, t he additive v alue of the value pr opositions of most o f th e poten tial anchor a nd secondary s ervices for CES syste ms are higher than 1 ,000 € /kW reac hi ng va lues o f up to 1,600 €/kW. Figure 2.12: Value proposition for stacked benefits suitable fo r CES based on data presented in reviewed literature [97, 106, 107, 108, 109] The application-specific value propositions indicate that CES c osts should be lower than the potential stacked value proposi tion. When reducing CES costs be low th e equivalent of 1,000 €/kW (10 years), C ES could cove r i ts investm ent by provid ing the proposed services. The limiting factors to C ES economic v iability is thus, not only th e comparatively low RTE a n d th e long response time but also the relat ively h igh specific co sts. 0 200 400 600 800 100 0 1200 1 400 160 0 1800 Loa d follow + Renewable capaci t y firmi n g T & D up g r ad e de fe rr al + E n e r gy a rb i t ra ge T & D up g r ad e de fe rr al + Pea k s hav i ng R e ne wa bl e time sh ift + Ene r gy arbitrage R e ne wa bl e time sh ift + Pe a k s hav i ng Renewable capaci t y firmi n g + Pe a k s ha v i ng Renewable capaci t y firmi n g + RE t i m e s h i f t stacked value propositio n, €/k W Load fo ll ow + Ren ewable cap acit y fi rming T&D upgrade deferral + Ener gy arbitrage T&D upgrade deferral + Peak shaving Renewable tim e shift + Ener gy arbitrage Renewable tim e shift + Peak shaving Renewable capacity firming + Peak shaving Renewable capacity firming + RE tim e shift Chapt er 2 : State of t he ar t 45 2.6. Summary of the literature review Cryogenic energy storage is a th ermal bulk-electricity storage technology that has attracted great inte rest in r es earch and app lication sin ce the in stallati on of the fi rst int e grated pilo t-scale plant in 2011. CES is composed o f mature components with well-k nown applicatio n in power genera tion, indus trial g ases, and LNG value chain. Thus rapid commercialization of CES is expected. The thermodynamic performance of CES systems has been subj ect t o m any publicat ions in the past three years; an optimal CES system configuration could not be derived. The charg ing process of CES is an inverse ther modynam ic cycle in which a gas , e.g., air, is liquefied. A number of air liquefaction processes are in comme rcial operation, and different liquefaction processes have b een considered for implem entation in CES system s. Despite, the co ld storage bei ng the ke y com pone nt of the c harging system, the impact of c old storag e integ ration on the selectio n and perform ance of the liquefacti on pr ocess has not b een subject to tho rough a nalysis. The cold storage design and in particular safe and low-co s t s t o r a g e m a t e r i a l t h a t a l l o ws high energy recovery i s subject to sev eral research pr ojects, especi ally in jo int research with industry . For the overall system, only the low-temperature exergy recover ed, and the costs of the cold storag e are of relev ance. For the discharge process , thus far, only the di rect expansion method – a liquid air Rankine cycle has been considered for CES system s in literature and application. Alternatives of using a secondary working fluid or combined methods are possible, similar to the low-temperature exergy rec overy from re-gasifying LNG. The integration of external he at sour ces and sink s (e.g., industrial w aste heat, low-tem perature exergy from re-gasifying LNG or combustion) into CES sy stems wa s frequently suggested, but no comparat ive analysis quantif y ing and comparing the effect on CES thermodynam ic and economic perform ance was presented. Due to its high power ratings and energy capacities (> 100 MW/1 0 GWh), CES is suita ble for energy managem ent applications a nd prima rily competes with comp r e s s e d a i r e n e r g y s t o r a g e (CAES), pumped hydro storage (PHS ) and hydrogen-based ES. CES m ost significant advantage s are its hi gh volumetri c energy density, site-free st orage, low s torage losses, and long lifetime. The specific investment costs of CES are comparable w ith PHS and CAES (500 – 2,400 €/kW) but are h igh c oncerning the rather low RTE of only 40-60 %. The slow response time of CES limits its application to energy arbitrage and peak shaving, selected ancillary and T&D support services. A single app lication cannot cover CES pro jected investment costs. Providing the p roposed combined applications, e.g., load follow a n d R E c a p a c i t y f i r m i n g o r T&D upgrade deferral and peak sh aving, a revenue o f up to 1,600 €/kW can b e retur ned ov er 10 years. Chapt er 2 : State of t he ar t 46 Chapte r 3: Met hodolog y 47 Chapter 3: Methodology This section aims to introduce the methods applied to achie ve t he objectives presented in Chapter 1 . Different CES system config urati ons have been evaluate d and e nhanced in the present work. The design and sim ulation of the anal yzed system s a r e d i s c u s s e d i n Cha pter 4 . The methods applied for evaluati on are energetic and exergetic analysis, economic analysis, exergoeconomic analysis, and optim ization. 3.1. Energetic analysis For energetic analysis, the princi ple of energy and mass conservation is applied. For an open system (control-volume), the global mass and energy balance are given in equati on (3.1) and (3.2). Only stati onary processes are evaluate d, and the changes in kinetic and potential energy of the s ystem are neglec ted. 𝑚 , 𝑚 , (3.1) 0𝑄 𝑊 𝑚 , ∙ℎ , 𝑚 , ∙ℎ , (3.2) The net rate of energy transporte d by mass (enthalpy flowrates) over the sy stem boundaries is equivale nt to the net rate at which energy is transferre d by he at trans fer 𝑄 and by power 𝑊 [110] . For the air li quefacti on process in CES system s, the liq ui d yield 𝛾 is a measure of perform ance [48] . The liqui d yiel d refers to the ratio of the mass flow that is liquefied 𝑚 relative to the mass flow enter ing the high-pressure compresso r in the liquefaction unit 𝑚 . 𝑚 𝑚 ∙ 𝑚 ∙ (3.3) 𝛾 𝑚 𝑚 (3.4) The overall performa nce of an energ y conversion sys tem is defin ed by the ratio of the positive effect (in terms o f energy, 𝑊 o r 𝑄 ) over the driving energy. For s im ple electricity storage, both the positive effect and the drivi ng energy are electricity. In more compl ex ES conf igurat ions, the supplied f uel or t hermal ene rgy needs to be accounted as pa rt of the driving energy. In Table B.1 in Appendix B, the energetic efficiencies are defined for t h e d i f f e r e n t C E S s y s t e m s evalua ted i n t his work. Chapte r 3: Met hodolog y 48 The overall perform ance of an ener gy (electrici ty) stora ge syst em can be evaluated with its roundtrip efficiency ( RT E) . The RTE i s d e f i n e d a s t h e r a t i o b e t w e e n t h e e l e c t r i c i t y c h a r g e d t o the system and the electricity dischar ged by the s ystem : 𝑅𝑇𝐸 𝑊 𝑊 ∙ 𝜏 𝜏 (3.5) The charging duration 𝜏 and the discharge duration 𝜏 need to be accounted for a s the charge -to-di sc harge ratio ( 𝜏 𝜏 ) may be unequal to one. 3.2. Exergetic analysis Exergy is a measurem ent for the quanti ty and quality o r “true ther mod ynamic value” of energy [111, 112] . Exergy is de fined as the m axim um useful work t hat w ould theoreticall y be obtained when a given thermodynam ic syste m would be b rought into complet e thermodynamic equilibrium with the environm ent wi th sole inte raction w ith the environment [113]. Maxim um theoretical useful work is, for example, shaf t work or electric al work, w hile the heat has a lower exergy content. When comparing s ystems operating at different t emperatures and integrating different energy sources (e.g., fu el, heat or low-temperature e nergy), exergy is of greater economic value as well as the best ground for analysis, optim iz atio n, and com parison of energy conversion systems [112]. The total exergy of a system consists of four components when m agnetic, nuclear, surface tension, and electrical effects are absent or negligible. The o verall exergy of a system 𝐸 can be describe d as the sum of the chemical exer gy 𝐸 , the physical exergy 𝐸 , the potential exergy 𝐸 , and the kinetic exergy 𝐸 . 𝐸 𝐸 𝐸 𝐸 𝐸 (3.6) The specif ic exer gy on a mass ba sis is expressed with a lower c ase 𝑒 : 𝑒 𝐸 𝑚 ⁄ 𝑒 𝑒 𝑒 𝑒 (3.7) The assumption of the system bei ng at rest in relation to the e nvironment ( 𝑒 𝑒 0 ) is applicable for many engi neering appl ications [112] . In this wor k, the change s in kinetic and potential exergy are not consi dered. The physical exergy of a system o r stream is defined by t he sys tems deviation to the restricted dead s tate , when the temperat ure and pressure are equival ent to those of the environment ( 𝑇 𝑇 , 𝑝 𝑝 ). At a given state 𝑗 , the specific physical exerg y can be expr essed by: 𝑒 ℎ ℎ 𝑇 ∙ 𝑠 𝑠 (3.8) Where ℎ , 𝑇 , and 𝑠 denote, respectively, the ent halpy, temperatu re and entropy of t h e s y s t e m . The subs cript 0 denotes the prop ertie s when the system is at the restricted de ad state [112] . The physical exergy of a materi al stream can further be split i nto the thermal exergy 𝑒 (its temperature related part) and the mechanical exergy 𝑒 (its pressure relate d part) [112] : Chapte r 3: Met hodolog y 49 𝑒 ℎ ℎ , 𝑇 ∙ 𝑠 𝑠 , ℎ , ℎ 𝑇 ∙ 𝑠 , 𝑠 (3.9) Thermal and mechanical exergie s are calculated through consider ation of an a dditi onal stream at state 𝐴 . State 𝐴 has the same pressure as the 𝑗 -th material stream but ambient temperature 𝑇 [114]. A s systems analy zed in this work partially operate below ( o r c r o s s i n g ) t h e t e m p e r a t u r e of the environm ent, a distinctio n between pressure relat ed and temperature related physical exergy becomes valuable. The chemical exergy is defined b y the deviation of the che mical compositi on of the material stream or system from that of th e thermodynam ic environment [11 3]. In other words, the chemical e xergy is t he maxim um useful work that can be obta ined w h e n a s y s t e m t h a t i s a l r e a d y a t t h e r m a l e q u i l i b r i u m ( 𝑇 𝑇 , 𝑝 𝑝 ) is brought into chemical equilibrium with the environm ent [111] . The chem ical com positi on of the the rm odynami c envi ronment nee ds to be define d (exergy- refer ence e nviro nment ) [112] . The chem ical exe r gies in this wor k are based o n Szargut’s standard model for chemical exergies [115] . The exer getic anal ysis appl ied in t his wor k is based on Beja n e t al. [112] and fo llows the “fuel and product” approach. Exergetic analysis aims to identify the cause, the magnitude and the location of thermodynamic ineffici encies in a thermodynamic sys tem [111] . In cont rast to ener gy analys is, exe rgy is not conserved but c an be dest royed [ 112] . Exergy destruction refers to the true thermodynamic ineffi ci ency that occu rs within a sy s tem due to i rrevers ibili ties [111]. Under steady-state condition ( 𝑑𝐸 𝑑𝑡 ⁄ = 0), the exer gy balance f or a controlle d volum e (o pen syst em) is exp ressed as: 0 𝐸 , 𝑊 𝑚 ∙𝑒 𝑚 ∙𝑒 𝐸 (3.10) The rate a t whic h exergy is des troye d in the s ystem (or contr ol led vo lume) 𝐸 d u e t o irreversibilities is the dif ference between the rate at which e xergy is transf erred into and the rate at which exergy is trans ferred out of the system. Exer gy can be transferred over the system boundaries by matter, heat 𝐸 , o r w o r k 𝑊 (oth er than flow work ) [112]. The exergy associated with heat transfer is given by 𝐸 , 1𝑇 𝑇 ⁄ ∙𝑄 (3.11) The temperature 𝑇 is the thermo dynamic average temperature at w h i c h t h e h e a t i s supplied. The exergy tr ansfer rate associ ated with the heat tran sfer ( 𝐸 , 𝑄 ⁄ ) is displayed as a function of the temperat ure dif ferenc e 𝑇 𝑇 i n Figu re 3 .1. At the same temperature differe nce to am bient tem perature | 𝑇 𝑇 | , the exergy transfer rate associat ed w ith th e h eat tran sfer ( 𝐸 , 𝑄 ⁄ ) is higher at a therm odynamic m ean temperature below the environmental temperature ( 𝑇 𝑇 ) than above the environm ent ( 𝑇 𝑇 ) . Hence, the exergy conten t of thermal en ergy stored below the ambient t em perature is higher t han above the environmental tem perature at the same ∆𝑇 . Chapte r 3: Met hodolog y 50 Figure 3.1: The exergy transfer rate a ssociated with the heat transfer ( 𝐸 , 𝑄 ⁄ ) over the temperature difference 𝑇 𝑇 , based on [112 ] ad opted from [39 ] The thermodynamic performance of a n y g i v e n e n e r g y conversion sy stem or system component can be evaluated with the exe rgetic efficien cy 𝜀 . Th e d efinit ion o f th e exergetic efficien cy 𝜀 necessitates the def inition of the fuel (expended resources) and the product (the desi red eff ect) of the r especti ve system (or c ompone nt) expr essed in term s of e xergy [112]. 𝜀 𝐸 𝐸 (3.12) The overall performance of the CES system is evaluated with the RTE , the en ergetic efficiency 𝜂 a n d t h e e x e r g e t i c e f f i c i e n c y 𝜀 , in ord er to assess v alues given for the CE S efficien cy in the literatur e. The definiti on of 𝜀 of the analyzed system s are given in Table B.1. The definitions for the exergetic effi ciency on the comp onen t level can be derived fro m Table B.3 . The summ ation of the exergy destr ucti on 𝐸 , the exergy o f the product 𝐸 and the exe rg y loss 𝐸 am ou n t t o t he e x ergy o f fuel 𝐸 of the system. The definition o f fuel and p roduct is further discusse d in section 3.4 and definiti ons on th e c om ponent level are given in Table B.2 to Table B.3 i n Appe ndix B. 𝐸 𝐸 𝐸 𝐸 (3.13) The exergy loss 𝐸 refers to the exergy transfer t o the environment [111, 112]. F or exerge tic analysis on the component leve l, the com ponent boundaries are s et to environm ental conditio ns, the exergy loss 𝐸 , o f t h e 𝑘 -th component thus amounts to ze ro. To identify the components with g reater s ignifi cance to the ov erall sy stem e nhan cement, th e exergy destruction ratio 𝛾 , and the exergy d es truction rate 𝛾 , ∗ can be c alculat ed. 𝛾 , 𝐸 , 𝐸 , (3.14) 𝛾 , ∗ 𝐸 , 𝐸 , (3.15) 0 1 2 3 -200 -100 0 100 20 0 300 𝐸 , 𝑄 ⁄ 𝐸 , 𝑄 ⁄ Exe r gy tr ansfe r rate 𝐸 , 𝑄 ⁄ , - T emperatu r e differ ence 𝑇 𝑇 , K Chapte r 3: Met hodolog y 51 3.3. Economic analysis The economic analysis of an energ y conversion system in the d es ign phase serves three main purposes: the evaluation and im provement of the overall project profitability , the evaluation of various desi gn opt ions and the optimization of the system param eters [116] . The economic analysis p resented in this wor k was performed acco rding to the Total Revenue Requirement (TRR) m ethod [111, 112 ]. The TRR met hod originated from procedures adopted b y t h e E l e c t r i c P o w e r R e s e a r c h I n s t i t u t e [ 1 1 7 ] . F o r t h e c o s t e v aluat ion and optimizati on of an energy conversion system the lev elized annual values for the ca pital e xpenditures, the f uel costs and the operati on and m aintena nce need to be c ompute d and com pa red. The TRR m ethod consists of three main steps: the estim ation of t he tot al capita l investm ent (se ction 3.3.1), the determination of p arameters necessary for the detailed cost calcu lation (sectio n 3.3. 2), the calculat ion of the tot al revenue required and the levelize d cost of the final product (sectio n 3.3. 3). An additional step i s necessary to provide the input value to e xergoeconom ic analysis, the calculation of the component cost rates (section 3 .3.5). In ord er to evaluate and compare the results gained in econom ic analys is, the param eters set i n sect ion 3. 3.3 are var ied i n se nsiti vity analys is to determ ine the cos t range of the le velized co st of t he final product (section 3.3.4). 3.3.1. Cost estimation F o r t h e e s t i m a t i o n o f t h e t o t a l c a p i t a l i n v e s t m e n t , t h e s y s t e m design, parameters, and components need to be known first. W ith the help o f the system design parameters, the bare module costs ( BMC) of the compon ents can be es timated. Estimation of bare module costs c an be base d on the foll owing o ptions: vendors quotations, past purchase orders, c ost estim ating charts, or cost estimatin g equations. The optimal estim ate of BMC can be acquired through quotations from vendors [111, 11 2]. As cryogenics-based energy storage is composed o f ma ture components with w ell-known industrial-scale applications in the LNG, industri a l g a s a n d p o w e r s e c t o r [ 1 5 , 1 6, 17, 18] , several sources can be accessed for estimation of the BMC [2] . Cost estimati ng char t s f o r p r o c e s s a n d c h e m i c a l engineering such as [118, 119], c ost estimating equations and c apital cost corre latio ns [39, 112, 116], as well as past purchase orders for CES [14, 32, 120], we re considered. Potential supply cha ins for CES key components were evaluated i n [8]. Despite being based on mature and commercially availabl e technology [15] li mits to the scale of single components were identifie d [18]. Key components for CES sys tems of up to 1 00 MW are available from power and process industry supply chains [16]. Consequently, th e base case system size was kept to 100 MW dis . The scale was an important fact or for deciding the method of BMC estimation fo r each component ty pe. When available, preference was given to past purchase orders. Chapte r 3: Met hodolog y 52 In o rder to estimate th e component co sts based on the known cos ts of a similar com ponent of d i f f e r e n t s i z e [ 1 1 2 ] , t h e e f f e c t of size of equipment on costs can be accounted f or with the following equation 𝐶 , 𝐶 , ∙ X X (3.16) where 𝐶 , is the known cost of the equipment of the size 𝑋 e x t r a c t e d f r o m l i t e r a t u r e a n d 𝐶 , is the desired BMC at the respective size 𝑋 . The equipment “size” 𝑋 i s thereby a prim ary design variable depe ndent on the equipm ent type. For he at exchangers, the primary design v ariable i s th e heat exchang er area 𝐴 i n m ² , w h i l e f o r t u r b o m a c h i n e r y i t i s t y p i c a l l y the power capacity 𝑊 in kW [118]. The scaling factor 𝛼 , is also dependent on the size and type of the equipment. For very sma ll sized equipm ent 𝛼 is close to zero while at large scale when transport and assem bly of the compon ent b ecome very cost ly, th e scaling factor 𝛼 approaches one [111]. As for equipment types, the B MC of heat exchange rs increases rather linearly ( 𝛼 ≈ 0 . 1 6 … 0 . 6 6 ) w h i l e turbomachinery (e.g ., compressors, turbines) follow the economy -of-scale ( 𝛼 ≈ 0.60…0.95) [112] . The average value for 𝛼 across the chemical industry is about 0.6 [121] , which implies a cost reduction o f 24.2 % at twice the size. The same value was rec ommende d to be used in the absence of other values for 𝛼 [112] and was used by Highvie w Power Storage Ltd. [32] for estimating the com pon ent c osts for t he demo nstratio n plant. In cost estimating charts pu rchased -equipmen t base costs are ty pically p lotted versus the equipment size on a graph that us es logarithmic scales on both axes (log-log plot) [111, 118, 119] and is based on equations si milar to equati on (3 .16) . Ap ar t f r o m t h e s c a l e , t h e p r e s s u r e and temperature ranges were also t a k e n i n t o a c c o u n t , w i t h t h e c hoice of the material (e.g., stainless steel or aluminum) and equipment type ( e.g., cryogeni c pump, fin plate heat exchangers) . For consiste nt com parabilit y of the econom ic analysis, cost fun cti ons were develo ped to estimate the BMC of the compone nts in t he evalua ted CES system s . The cost functions, design variab les, assum ptions, and references are given for the most i m portant component types in Table 3.1. The detailed information on t he com ponent design, range of operation, construction material, potential s upply chain, and lim ita tions considered f o r the cost est imation a re give n in detail i n Ap pendix B. The reference cost data is extracted from a variety of sources w i t h d i f f e r e n t r e f e r e n c e y e a r s a n d currencies. By using cost indice s, the data can be b rought to a comm on bas is wit h the fol lowi ng equation. 𝐶 , 𝐶 , ∙ Index Index (3.17) The Chemical Engineeri ng Plant Cost Index CEPCI was used, being particularly suitable for the type of equipment incorporate d in CES systems [116]. The Ch emical Engineering Magazine regularly publishes the annual and monthly cost index [111]. Th e c o s t s i n t h i s w o r k a r e g i v e n in € and are adjuste d to the CEPCI of the year 2017. T he histor ical d evelopmen t o f the C EPCI is shown in Figure B.2 in Appendi x B. The conversion rates used for the conversion o f the currencies a re gi ven i n Figure B. 3 in A ppendi x B. Chapte r 3: Met hodolog y 53 Table 3.1: Cost functions devel oped for t he estimation of the B MC of the CES system components. Compon ent Desi gn variabl e C ost func tion ′000 € Assumpti ons Adopted from Compressors ca pacity , 𝑊 M W 795 ∙ 𝑊 8 𝑀𝑊 . Centri fugal [32], [119] Expand er capacity , 𝑊 M W 𝑓 ∙ 1,795 ∙ 𝑊 0.001 M W . 𝑓 , = 5.0, 𝑓 , = 3.0 [118] Turbines capacity , 𝑊 M W max 45 MW [8] 𝑓 ∙ 3,945 ∙ 𝑊 0.0001 . 𝑓 , = 3.5, 𝑓 , = 6.0, 𝑓 , = 8.0, axial [118] Combus tio n chamb ers Outlet te mperat ure, 𝑇 K , mass flow, 𝑚 kg/s 67.49 0.995 𝑝 𝑝 ∙𝑚 ∙ 1𝑒 .∙ . [112] Cryopu mps cap acity, 𝑚 kg/s max 50 kg/s 644 ∙ 𝑚 23 kg/s . recipr ocating pump [32] Storag e tanks capacity , 𝑉 m³ 50,000 m³ 𝑓 ∙ 𝑓 ∙ 0.0458 ∙ 𝑉 117.80 𝑓 , = 2.0…3.0, 𝑓 , = 1.0 [119] Inte rcoolers, Reheat ers HE area, 𝐴 m² 𝑓 ∙ 1.3 1.88 ∙ 57.64 ∙ 𝐴 1,000 𝑓 = 1…1.3, shell/tu be, cs/cs [118] Cryogeni c heat excha nger s (MHE ) MHE area, 𝐴 m² 91.28 45.64 𝑓 𝑓 ∙ 𝐴 2,000 plate- fin, 𝑓 , =2.3, 𝑓 = 1.2…1.3 [118] cs carbon steel, ss stainless steel 3.3.2. Assumptions made For the d etailed cost c alculation, the economic parameters need to be determined first. The assumptions made for econom ic an alysis are summ arized i n Table 3.2. CES indust rial p rojects are expected to have a relatively fast construction lead-tim e o f only one or tw o yea rs [76] . The economic life of CES projects is expected to be higher than tha t of other ene rgy pr ojec ts. CES economic life is expected to reach more than 25 years and up to 60 years [29, 37, 38]. Both faster comm issioning and a long economic life favor the economi c perform ance of CES syst ems. All system s analyzed are daily cycling uni ts, designed to suppl y electricity for u p to four hours of peak d emand at a charge-to-d ischarge ratio of 2/1 and 1460 a nnual hours at full load dischar ge operation. The operation and mai ntenance costs (OMC) are accoun ted for as 4 % o f the fixe d capita l inves tm ent (FCI) per annum. The compa ny Air P roducts and Chemicals, Inc. rates the CES OPEX at approximat ely 105 €/MW [20] annually, whi le [8] estimated OMC at 1.5 % to 3 % of the purcha se costs of CES and the compa ny Expan sion Energy claimed CES OMC lower than those of natural g as-fired power plants [20] . The system is assumed to operate at low electricity prices. The price of electricity of 20 € /MWh at the beginning o f the fi rst year of operation was set based on the assumption that the CES system charges at average day-ahead market price in Chapte r 3: Met hodolog y 54 Germany for the 2,920 lowest pr iced hours in the year 2016-2018 , see Figu re B.1. In th is work, ES system s are assum ed to be exem pted from the taxes on electr i c i t y , R E a c t l e v y ( E E G Umlage) as well as distribution and grid charges. The plant ope ration is schedu led for 01.01.2021, and the co nstruction and comm issioning tim e is two years. The changes in the prices, e.g., electricity and f uel, were accounte d for in ele va te d char ges for the cont inge ncy of 15 % of BMC. Contingencies refer to the add itional costs added to the project budget taking into account the potential v ariation from the initial cost esti mate, as all cost estimates are uncertain. As CES system param et ers and associated cos ts are st ill to be proven, a higher val ue was assumed [121]. Table 3.2: Assumptions made in the economic analysis. Parame ter Assumpti on Effective inter est r ate 10 % Plant econom ic life 40 yea rs Averag e gen eral in flati on ra te 2.5 % Daily char ging/dis charg ing duration, 𝜏 /𝜏 8 h/4 h Annua l full -load disc harging opera tion, 𝜏 1,460 h/a Base electr icit y pr ice, 𝑐 , 20 €/MWh Natural g as price, 𝑐 , 262 €/ton Servi ce faciliti es and architec tural wo rk 30 % of B MC Conti ngencie s 15 % of BMC Annual OMC 4 % of FCI 3.3.3. Total revenue requirement method The sum of sales of products in a year needs to cover the annua l exp enditures. This annual revenue requirem ent (TRR) of a system consists of carryi ng char ges and expenses. The carrying charges 𝐶𝐶 represent the indebtedness associated with the initial capital investment, a ccounting for the cap ital recove ry (capital lent from investors), the ret urn on investment (debt, stock, and equity) as well as t axes and i nsuran ces. 𝑇𝑅𝑅 𝐶 𝐶 𝐹 𝐶 𝑂 𝑀 𝐶 (3.18) The fuel costs 𝐹𝐶 a n d t h e operation and main tenance costs OMC a r e t h e m o s t p r o m i n e n t examples of expenditures that als o need to be estim ated over th e economic life of the system [ 1 1 1 ] . T h e s e r i e s o f a n n u a l e x p e nditures (FC and OMC) and costs associated with carrying charge s CC is not uniform. Al l costs are thus levelized to con s tant annuities over the systems economic lif e, e.g., w ith the help of the ca pital re covery f act or (CRF). 𝐶𝐶 𝑇 𝐶 𝐼 ∙ 𝐶 𝑅 𝐹 (3.19) The total capital invest ment (TCI) is gained from the fixed cap ital investment (FCI), which represe nts the direc t and indirect c ost associ ated wit h the des ign, constr uction, a nd installat ion of the plant and the alterat ions needed for the preparati on of the plant site [121] . The direct costs are on- and offsite costs associated with the permanent e xpenditu res such as the BMC, Chapte r 3: Met hodolog y 55 the installation of equipment, t h e l a n d , a n d o t h e r r e s o u r c e s a n d labor costs required for the system. The indirect costs ref er to the costs as soci ated w ith t he desi gn and cons truc tion phas e, e.g., engineering and supervisio n, cons tructi on costs, a nd c ont inge ncies. The TC I is the sum of the FCI, and other outlays such a s the allow ance for the funds nee ded during constr uction time, the working capital, the licensi ng costs, and the start-up cost s. The exp enditures (FC and OMC) are escalated und er th e assu mptio n of a constant escalation levelization factor CELF . 𝐹𝐶 𝐹 𝐶 ∙𝐶 𝐸 𝐿 𝐹 𝐹 𝐶 ∙𝐶 𝑅 𝐹 ∙ 𝑘 1 𝑘 1 𝑘 3.20 T h e f u e l c o s t a t t h e b e g i n n i n g o f t h e f i r s t y e a r 𝐹𝐶 is converted in to c onstant annuities accounti ng f or both the cost o f m oney a nd the c onsta nt esca lati on. T he OMC are escalated and levelized in the sam e manner. Finall y, when the T RR is determine d, the pr oduct costs can b e c al c u l a t e d , an d t h e f e a s i b i li t y o f the investm ent can be ev aluated. 3.3.4. Economic sensitivity analysis The fin al product of the CES systems i s th e electricity gener at ed in the dischar ge process and fed back into the grid or supplie d to a custom er. The levelized c o s t o f e l e c t r i c i t y d i s c h a r g e d 𝐿𝐶𝑂𝐸 can directly be calcu lated from the annual TRR and the annuall y generated electricity 𝐸 . 𝐿𝐶𝑂𝐸 𝑇𝑅𝑅 𝐸 𝐶𝐶 𝐹 𝐶 𝑂 𝑀 𝐶 𝐸 (3.21) An economic sensitivity analysis was conduct ed to evaluate the economic param e ters stated in literature and identif y the depe ndency o f economic viability on economic parameters independent of the system design. For th e economic sen sitivity analysis, the most significant par ameters were identified and varied. The economic life, th e discharge capacity, the RTE , the FLH of the discharge unit, the OMC as sha re of the T CI, the interest rate, a nd the BMC were va ried in a range o f 20-60 years, 25-200 MW, 31.5-46.5 % , 720-2920 h/a, 1-8 % TCI, 5-15 % and € 6 0-130 m illion, respectively. In o rder to compar e the weighted effect of parame tric changes on the overall economic viability measured by the LCOE dis , the parametric changes are normalized to the initial value of the respec tive base case param eter. Finally, t he LCOE dis is compared to LCOE dis of other bulk ES technologies re ported in the literature under similar assumptions for the economic param eters. Chapte r 3: Met hodolog y 56 3.3.5. Determinati on of cost rates When the economic anal ysis is conducted as part of an exergoeco nomic an alysi s, the lev elized cost rate 𝑍 o f each component k and the spec ific cos t per uni t of e xer gy of t he f uel 𝑐 need to be determined. The com ponent cost rate considers the contributi on of each respective compone nt to the costs assoc iate d with the capital investm ent a nd the operation and mainte nance cos ts. The c omponent c ost rate i s calcula ted as 𝑍 𝑍 𝑍 𝐵𝑀𝐶 ∑ 𝐵𝑀𝐶 ∙ 𝐶𝐶 𝑂 𝑀 𝐶 𝜏 (3.22) 𝜏 refers to the annual operatio n time of the k -th compon ent, 𝐵𝑀𝐶 to the compon ents investment costs and ∑ 𝐵𝑀𝐶 t o t h e s u m o f t h e b a r e m o d u l e c o s t s o f a l l c o m p o n e n t s i n t h e systems. The c omponent cost rate asso ciated with the initial ca pi tal investm ent 𝑍 is calculated over the levelized carrying charges 𝐶𝐶 . The component cost rate ass ociated with the operation and mainte nance costs 𝑍 , respectively, over the 𝑂𝑀𝐶 . The specific costs of all stream s entering the system need to b e ident ified fo r th e exergoeconomic analysis. The speci fic costs associated with the exerg y rate of the fue l to the system are given by: 𝐶 𝐹𝐶 𝜏 (3.23) Chapte r 3: Met hodolog y 57 3.4. Exergoeconomic analysis and optimi zation In this work, exer go economic optimization is applied to deter mi ne t h e e f fe ct o f pa r a met r ic a nd conception al ch anges on the cost- effectiv eness of CES systems t o minim ize the c ost of the fina l product. Exergoeconomic optimizat ion follows the approach o f Be j a n e t a l . [ 1 1 2 ] . T h e l o g i c flowchart for the methodology f ollowed is shown in Figure 3.2. The optimization is performed in several iterations. Figure 3.2: Logic flowchart for the exergoeconomic optimization applied in this work [36 ] . After the definition o f the init ial design parameters of the ba se case sy stem, th e system is s i m u l a t e d i n A s p e n P l u s ® . I n t h e s i m u l a t i o n , t h e m a s s a n d e n e r g y balances are fulfilled, and the en thalpy and the entr opy v al ues at all given states are cal c ulated. The exergy values are calculate d wit h the help of an integrat ed Fortran routin e. For exergetic analysis, the exergy o f the fu el 𝐸 , , the exergy o f the product 𝐸 , , the exergy destruction 𝐸 , a nd the exergeti c effici ency 𝜀 are determined on the com ponent and the system level. The exergy of the losses 𝐸 , of the overal l system are calculated. Com p on ent a nd stre am va lu es 𝑠, ℎ , 𝑒 𝑒 + 𝑒 Exerge tic analysis EES 𝜀 ,𝐸 , ,𝐸 , , 𝐸 , , 𝐸 , 𝑇𝑅𝑅 , 𝑇𝐶𝐼 →𝑍 ,𝑍 Economic ana ly sis Excel Exer goeco nomic analy sis EES 𝑓 ,𝑍 ,𝐶 , ,𝑐 , , yes no yes Change des ign parameter to 𝜀 ↑ Stop 𝑐 , , 𝑐 , , no no yes Change system design Change d esign parame ter to 𝑍 ↓ Start Sim ulation ASPEN Plus 𝑖 𝑖 1 𝑖 𝑖 1 Design p ara meters 𝑝 ,𝑇 ,𝑚 𝑖 𝑖 1 𝑓 ≫𝑓 , 𝑓 ≪𝑓 , Chapte r 3: Met hodolog y 58 The cost rate of the com ponents 𝑍 are determined in economic analysis. Exergoeconomic analysis corresponds to the combination o f the exergetic and co st analysis and discloses the costs associated with thermodynam ic inefficiencies ( exergy dest ruction) 𝐶 , . The exergoeconomic factor 𝑓 is the decision variable u sed to determine whet her to adj ust t he design param eters to: increase the ex ergetic effi ciency 𝜀 (reduce the exergy destruction 𝐸 , ), or decrease t he investm ent cost 𝑍 of the respective com ponent. After eac h change in the design param eters, a new itera tion (si mulation, exergetic, economic, and exergoeconomic analysis) is conducted. As l ong as the cost o f t h e f in a l p ro d u ct i s r ed u c ed , the changes are continue d ( 𝑐 , , 𝑐 , , ). When parametric changes do not reduce the cost of the final product any furt her, changes in the system de s i g n a r e a p p l i e d . If changes in system design do reduce the cost f urther and the exergoeconomic factor of the co mponent s is close to the optimum , the optimizati on is disc ontinued. The fin a l p r o d u c t o f t h e C E S s y s t e m s 𝑐 , i s t h e d i s ch a r g e d e l e c t r i c i ty . A s a r e s u l t , t h e t e rm s average cost of the final product 𝑐 , and levelized co st of discharged electricity LCOE dis are used interchange ably i n this work. 3.4.1. Exergy costing The exergoeconomic analysis aims to determine; the costs associ ated with thermodynamic ineffici encies, th e mai n c ontribut ing comp onents to th e co sts o f t h e o v e r a l l s y s t e m , t h e c o s t - effective ness of a system, and the potential for reduction in c os ts. This correlati on between thermodynam ic performance and costs is ach ieve d by applying exergy costing [112]. In exergy costing, each stream w ith an ass ociate d exergy transf er rate 𝐸 , e.g., matter, work or heat , is denoted an av erage cost per un it of ex ergy 𝑐 , e.g. 𝑐 (3.24) 𝑐 (3.25) 𝑐 (3.26) The specific costs are determ ined in cost balances . For each com ponent 𝑘 , the differe nce of the sum of the cost rates 𝐶 ( associated with the 𝑚 exiting and 𝑛 entering stream s of the matter) is equal to the sum of the cos t rates associate d with the heat su p pl ied to the component 𝐶 , and the work done by the com ponent ( 𝐶 , ) as wel l as the cost rate of the respective com ponent 𝑍 . 𝐶 , 𝐶 , 𝐶 , 𝐶 , 𝑍 (3.27) All costs associat ed with t he streams entering the systems n eed to be known. Auxili ary equations ar e necessar y when m o re than one st rea m exits the com ponent. For t he defini tion of fuel and product, the SPECO approa ch [122] was follo wed. Follo w ing the “fuel and product” Chapte r 3: Met hodolog y 59 approach the cos t balance can also be expressed with t he cos t r ate associated with the fuel 𝐶 , and the product 𝐶 , of the component. 𝐶 , 𝐶 , 𝑍 , and (3.28a) 𝐶 , 𝐶 , 𝐶 , ∑ 𝑍 (3.28b)(3.28a) The cost balances for the co mponent s in th e evaluated sy stems a re given in Appendix B. On s y s t e m l e v e l t h e c o s t r a t e a s s o c i a t e d w i t h t h e e x e r g y l o s s e s 𝐶 , n eeds to be consi dered as well. The costs associated with th e ex ergy destr uction in the 𝑘 -th comp onen t ar e determined by the average cost per uni t of exergy of the fuel 𝑐 , : 𝐶 , 𝑐 , ∙𝐸 , (3.29) The cost rate associated with the exergy losses 𝐶 , is calculated similar to equation (3.29). The exergoecono mic factor 𝑓 a nd the relative cost d ifference 𝑟 should be used for evaluation of the c ost-e ffective ness of the 𝑘 -t h com ponent: 𝑓 𝑍 𝑍 𝐶 , (3.30) 𝑟 𝑐 , 𝑐 , 𝑐 , (3.31) 3.4.2. Exergoeconomic optimization After the e xergoec onomic a nalys is is perf orm ed on the Base Ca se system, the fi rst it erat ion of the exergoeconom ic optimization is conducted based on [112]. A i m of the exe rgoe conomic optim ization is t he si gnificant reduc tion of the average cost of exer gy of the produ ct 𝑐 , . The exergoeconomic optimization does n ot intend to identify a singl e mathematical optimum b ut operates with a knowle dge-based iterative a pproach to limit sys tem com plexity and account for availabi lity and safety . The op tim ization can be broken down in three main steps : 1. Identif y the componen ts with the highest cost im portanc e ( 𝑍 𝐶 , ) 2. Ide ntify the potential source of the high cost s in the respecti ve components by reviewing: a. the relative cost difference 𝑟 , b. the exergoeconom ic factor 𝑓 and c. syst em design . 3. A pply cha nges to t he system design: a. conception al changes o r b. pa rametric c hanges . By ranking the com ponents accordi ng to the sum of costs associa ted with t he initial investment of the c ompone nt Z and the costs a ssoc iated wit h the exerg y des tructi on C , prior ity is give n to the components with the highest contribution to the cost of the exergy of th e product of the sy stem. Chapte r 3: Met hodolog y 60 The cause of the high costs cause d by a componen t or component g roup can b e id entifi ed by review ing the exergo economic parameters ( 𝑟 , 𝑓 ) o f t he co mp one nt ( s ) a nd the overall system design. The relati ve cost diffe rence 𝑟 indicates whet her the com ponent is the ori gin of t he high costs associated with th e exergy destruction 𝐶 , or whether the average cost of the exergy of the fuel to the comp onen t 𝑐 , is relatively high . If th e 𝑐 , i s relatively high, the comp onents prior to this component and t he system design shou ld be inspect ed first. Reviewing the exergoeconomic factor 𝑓 two recomme ndations can be drawn: when 𝑓 is elevat ed, th e cost s asso ciat ed w ith the initial investment of the component 𝑍 dominate and shoul d be reduced despite a potential reduction i n efficien cy 𝜀 , and when 𝑓 is low, the costs associate d with the exergy destructi on 𝐶 , dominate and the exerget ic efficiency 𝜀 should be increased (exergy des truction 𝐸 , ↓ ). In general, the initial i nvestm ent of th e compon ent Z and the cos ts assoc iated with t he exergy destruction C , should be in the sam e m agnitude ( 𝑓 → 0 . 5 ) . H o w ev e r , t h e “optimal” value for the exergoeconomic factor ( 𝑓 , in Figure 3.2) differs according to the type of component. For turbomachinery fo r instance an elevate d exergoeconomic fact or is characteristic, e.g., 𝑓 = 0.6-0. 7. Apart from identifyi ng parametric changes on com pone nt level, changes in system design should also be considered . If t he effect of the parametr ic change is positiv e, it can be continued until no eff ect can be identified. 3.5. Summary of th e methodology For the ev aluation o f cryogenic ener gy storage sy stems presente d in this work, exergy-based methods based on Bejan et al. [ 112] were applied. The “fuel and p roduct” approach was u sed, and the physical exergy was split into its pressure related ( me chanical exergy 𝑒 ) and temperature related (therm al exergy 𝑒 ) pa rt. Thes e m ethods were a pplied f or severa l reasons : CES systems p artially operate bel ow the environmental tem peratu re. For th e analysis of the in tegrated CES systems, the qu antity an d quality of differe nt energy sources need to b e considered . The relationship between thermodyn amics and costs should be ana lyzed, and a cost- optima l C ES syst em des ig n shou ld b e iden tified. The econom ic a nalysis follows t he Tot al Re venue Re quirem ent (T R R) method [111, 112]. Cost estimati on is based on past purchase orders from previous CES p rojects and potential supply chains from LNG, industrial g as , and power industry. Lim itation s for commercial availabilit y of components were ac knowledged. For this reason, the design si ze o f the evaluated CES systems was kept to 100 MW of discharge capacity. The exergoeco nom ic optimization present ed in this work operates with a knowledge-b ased iterativ e appr oach to limit system complexity and acc ount for availabili ty and safety. Chapte r 4: Design and sim ulation 61 Chapter 4: Design and simulation The methods described in Cha pte r 3 were applie d to different CES system configur ations. A number o f design confi gurations are p ossible for the charge, th e s torage, and the disch arge unit of a cryogenic energy stor age syste m. Moreo ver, CES syste ms hav e g reat po tenti al for system integrati on with internal and ex t e r n a l h e a t s o u r c e s a n d s i n k s ( “cold sources”). In this section, the assumptions made in simulatio n, the process design, the sel ectio n of com ponents, w orking fluids, and parameters are descr i bed. The design opt ions t hat were considered and a pplie d for the pre-design , the base cases, and the integrated systems are shown in F igure 4. 1. Figure 4.1: Schematic of the system con figurations considered for this work. In the pre-desi gn phase, different design options were reviewed and compared in simulation. The design options that are indicated in white color in Figure 4.1 were consider ed in the pre- design phase but were finall y not implem ented in the Base Case systems for var i ous reasons, Cryogenic ener gy storage Charge Liquefaction process Claude-based Li nde-ba se d Cold source Cold storage LNG integration Storage Cold storage Se nsible st ora ge La tent and sensib le Heat storage Compressed water Therm al oil Discharge Discharge process Direct expansion Secondary power cycle Heat source Hea t storag e Com bustion W aste heat Le gend: Im plemented in Base cases Ap plied in sy stem integrat io n Pre-design phase Chapte r 4: Design and sim ulation 62 e.g., the systems thermodynam ic performance, or economic feasib ility. The design of the charge, the storage, and the di scharge unit of the stand-alone adiabatic CES are described in section 4.3.1, 4.3.2 and 4.3.3, respectively. The integrated systems are based o n the adiab atic CES syst em de sign but were extended w ith e x t e r n a l h e a t s o u r c e s a n d s i n k s . F o r s y s t e m i n t e g r a t i o n , w a s t e heat and internal combustion were considered as e xter nal hea t s ources. LNG as well as cold s torage were considered as heat sinks. The design of the integrated system s is discus sed in se c tion 4.4. 4.1. Simulation and data management software Aspen Plus® [123] was c hosen as a suit able software for process simu lation. Aspen Plus softwar e i s ch aracte rized by several advantag es amo ngst other i ts years of experience and close cooperation with the chemical i ndustry, its p ractical user inte rface w ith appropriate computa tional basis, an extens ive model library, and the possib ility for integration of u ser- specific modules, e.g ., in FORTRAN, Excel o r V BA. With the aid of the simulation software, all mass and energy balances are fulfilled, and the specific en thalpy and entropy values of all streams and substances are calculated. The Peng-Robinson eq uation of state was selected, and the simulation was performed und er steady-state conditions. FOR TRAN-based user property subroutines 2 were integrated with Asp en Plu s to calculate ex ergy values as input for the exerge tic analysis. F or the e xecution of e xergy-ba sed analysis, the Engineeri ng Equatio n Solver (EES) [124] was u sed. The cor e ben efit of the softw are p ackage EES is its built-in, extensive, and accurate thermodynam ic database. 4.2. General assumptions made in the simulation The ambient temperature is assum ed at 15 °C and a mbient pressur e at 1.01325 bar according to the Inter national Sta ndard Atm osphere [125]. Both liquid ni t rogen and liquid air are w idely consi dere d as w orkin g fl uid i n CE S systems as their thermo-phys ical properties are similar [8, 37]. The inlet air to the compression process is modele d with a molar fraction of 0.79 for nitroge n and 0.21 for oxygen. The real opera tion of the turbo ma chinery was assum ed with isentropic efficiencies; the v alu es were chosen according to th e literature reviewed, see Table 4.1. Table 4.1: Isentropic efficiencies presumed for the turbomachin ery. Isentro pic efficie ncies Value Ref. Compr essors 0.85 [42] Cold expander 0.84 [57, 126] Cryogenic p ump 0.75 [57] Turbi nes 0.90 [39] The i sentropic efficiency of the cold expander in the liquefact ion process was reported to have efficiencies as low as 0 .60-0.78 [126] , while recent enhancemen t i n t h e d e s i g n o f c r y o g e n i c 2 Developed and revised in the Cha ir for Energy a nd Environmenta l Protection at Technisc he Universität Berlin Chapte r 4: Design and sim ulation 63 turbines enabled isentropic efficiencies as high as 0.88 in t es t ing [5 7]. Sensit ivity analysis showed that varying the isentropic efficiency 𝜂 , between 0.78 and 0.88 has no significant effect on the performa nce of the system . While when 𝜂 , is reduced furthe r, the roundtrip efficiency ( RTE ) is reduced signi ficantly, Figure C. 1 i n A ppendi x C. The he at excha ngers in the sys te m s can be divide d into two type s of heat ex changers: the cryogenic m ulti-stream heat e xchangers – the main h eat exch ange rs (MHE1, MHE2) of the charge and the discharge unit, and two-stream heat excha ngers, e.g., intercool ers and reheaters th at op erate abov e 𝑇 . The assumptions made for the pinch temperat ure differe nce ∆𝑇 , the press ure drop ∆𝑝 and the design parameters for the he at exchangers are given in Tabl e 4. 2. Two Cases are differe ntiate d: the Base Case and the Optimized Case . Th e Base Case refers to the in itial design param eters of the two system con figurations for the stand-alone a diabatic CES: the Ba se Case A and the Base C ase B. After perfo rming exergoeconomic optimizati on on th e two base case system s, the Optimized Case was derived. The integrated systems a r e b a s e d o n t h e d e s i g n param eters of the Optimized Case. Table 4.2: General assumpti ons a nd design parameter for the heat exchangers. Compon ent Unit Ba se Case Optimized Case Inte rcool ers Hot end exit temperature, 𝑇 , °C 18 18 Pinch temperature difference, ∆𝑇 , K 2 3 Relative pressure drop (liquid/gas), ∆𝑝 % / K 0.02 0.02 Reheat ers Hot end tempe rature approach, ∆𝑇 , K 2 6 Pinch temp diffe rence, ∆𝑇 , K 2 6 Pressure drop (liquid/ga s), ∆𝑝 % / K 0.02 0.02 Main heat exchanger 1 Hot end exit temperature, 𝑇 ° C - 17 7 - 17 7 Pinch temperature difference, ∆𝑇 , K 1 3 Pressure drop (e vaporation/liquefac tion), ∆𝑝 %/K 0.04 0.04 Main heat exchanger 2 Pinch temperature difference, ∆𝑇 , K 1 .3 4 Pressure drop (e vaporation/liquefac tion), ∆𝑝 % / K 0.04 0.04 For the cryogenic h eat- exchangers, a min imu m approa ch tempe ratu re ∆𝑇 , o f 1 K i s realizable. In the MHE1 and the MHE2 of the base case systems, the ∆𝑇 , w a s approache d. The pressure drop in a given sys tem is affected by various factors, in particula r, the type of flow (e.g ., laminar or turbulent ), the den sity of t he working fluid, and the geom etry of the section in heat exchangers. The pressure drop assum ed in the heat exch angers accounts for the type o f heat e xchanger (two-stream or multi-s tream) and w h e t h e r p h a s e - c h a n g e d o c c u r s , adopted from [127]. Chapte r 4: Design and sim ulation 64 4.3. Adiabatic cryogeni c energy storage systems In this sec tion, t he desi gn and sim ulatio n of the overal l stand - alone a diaba tic CES sys tem s are discusse d. The schemati c flowsh eet of the adiabatic CES (a-CES) syste m is shown in Figure 4.2; the d iffere nt process ste ps are denot ed wit h (a) to (g). T h e d e c i s i o n s m a d e i n t h e p r e - d e s i g n phase for the simulati on of the charge, the storage, and the di scharge unit are specified in the subsecti ons 4.3.1 to 4.3.3. The properties of the streams displ a y e d i n F i g u r e 4 . 2 a r e g i v e n i n Table C.1, and Table C.2 in the Appe ndix C. The a-CES system s a re designed for a power capacity o f the discharge unit of 100 MW el and an energy capacity o f the storage unit o f 400 MWh el . The systems are daily cycling units, enabling four hours of d aily disch arge at full load cap acity during p eak dem and at a charge-t o-disc harge rati o of two (8h/4h). The charging process is a conventional air l iquefaction process cons isti ng of the gas cleanin g and purification unit, the compre ssion and the final expansion and liquefaction process (a)-(c ) in Figure 4.2. In the charging p rocess, the pre-treated air is compressed in three compression steps with inter -stage cooling – (b) compression process . The liquefaction process is based on the Kapitza process for Base Cas e A (str eam 7a -13 in Figure 4 .2 ) and th e Heyl andt process fo r Base C ase B (stream 7b-13) w ith one cold expander – (c) liquefaction process . T h e h e a t rejected in the intercoolers is recovere d and stor ed to enha nce the performance of the discharge unit – (f) heat recove ry and storage . Figure 4.2: Flowsheet of the stand-alone adiabatic CES system (Base Cases A and B). (e) expansion process 1 𝑊 𝑊 23 45 6 37 38 33 34 MHE2 ST 𝑊 18 19 P HE IC IC IC RH RH RH RH 21 22 23 24 25 26 27 28 29 30 CM CM CM TTTT HS (b) compre ssion process 𝑊 7b 8 9 11 10a 12 13 15 14 16 17 9a 8a 10b 7a 35 36 31 32 MHE1 FL TV EX (c) liquefaction process (d) pressurization and evaporation (f) heat rec ove ry an d st ora ge (g) cold sto ra ge 20 DISCHARGE ST ORAGE CHARGE ambient air air HS m edia R218 m ethanol (a) gas cleaning Chapte r 4: Design and sim ulation 65 I n t h e d i s c h a r g e p r o c e s s , t h e e x e r g y s t o r e d i n t h e l i q u i d a i r i s recovered in a simple direc t Rankin e cycle. The liquid air is pumped to super critical pressu re and supplied to the second main heat exchanger (MHE2). In t he MHE2 the liquid air is evapo rated and heated in heat e x c h a n g e w i t h t h e c o l d s t o r a g e m e d i a – (d) pressurization and evaporation . The low- temperature exergy released duri ng e vaporati on of the pr essuriz ed liquid air is recovered in the cold storage. The cold storage is realized thr ough two circulat ing wor king fluids in l iquid sta te – (g) co ld storag e . The recovered cold is supplied to the first main heat exchang er (MHE1) to i n c r e a s e t h e s h a r e o f l i q u e f i e d a ir i n the liquefaction process of the subseq uent char ging process. The slightly subcooled liquid air is stored in a simp le insu lat ed storage vessel at near ambient pressur e (1.1 bar) and a temperat ure of - 194 °C. The mass flo w in the d ischarge process is twice as large ( 𝑚 = 2 ∙𝑚 ) as in the char ging process. The high-pr essure air is super hea te d and expanded in four gas turbine stages with reheating – (e) expansion block . The different proc ess steps ( a) to (g) are d es cribed in mo re de tail in the fo llowing subsections. In a c omparati ve a nalysis, t he syste ms were eva luated wi th unif orm design parameters. Either, the comp re ssion p ressu re and mass flow rate 𝑚 in the compr essors ( 𝑊 = const.) or the discharge power 𝑊 ( 𝑊 𝑊 ) and the energy stored 𝐸 ( 𝑊 ∙ 𝜏 ) were kept c onsistent f or all system s in com parative analys is. 4.3.1. Charging unit Gas cleaning and purification unit (block (a) in Figure 4.2) Prior to compression and liquef action, any contaminants and unw anted components of the inlet air, e .g., hydrocarbons and elemen t s w h i c h m a y d i s r u p t t h e l i q u efaction process, such as freezing water and CO 2 , are removed in the gas cleanin g and pu rification p rocess (Fig ure 4.3) [18, 75]. In cr yogenic air separation units, thi s i s c omm only r ealized by feeding filtered ambie nt air at slightly elevated pressure (e.g., 1 .03 bar) to a direct c o n t a c t c o o l e r a n d a v e s s e l w i t h a fixed bed of ads orbent, e.g., molecu lar sieve unit [128] or ac t ivated alumina [91 ]. After all trace contaminants have been removed th e purified air i s fed to t he c om pressors . Figure 4.3: Flowsheet of the gas cleaning and purification unit adopted from [128 ] . Filter Compressor Direct contact cooler Adsorbent unit Ambient air Purified air Chapte r 4: Design and sim ulation 66 The air pu rification c ontributes to th e energy requi rement of t he li quefact ion unit. The therm al energy required for the regenera tion of the absorbe nt used to r emove the undesirable components contributes about 10 % of the energy requirement in the air liquefaction process [129]. This process step cannot b e avoided. The air purificatio n process does not need to be simulate d fo r the dif fere nt s ystem conf igura tion, b ut t he energ y c onsum ption a nd c osts nee d to be accounted. The air exiting the gas cleaning and purification p r o c e s s i s e s t i m a t e d w i t h a m o l a r fraction of 0.79 for nitrogen an d 0.21 for oxygen at ambient te m perature and press ure. Compression process (block (b) in Figure 4.2) I n t h e l i q u e f a c t i o n p r o c e s s , t h e a i r i s c o m p r e s s e d t o h i g h p r e s sure, cooled a nd expanded to achieve low temperatu res (< - 192 ° C) and reach the due-point. The com pression process is considered separately from the liqu efaction pro cess in th is the sis due to its role in heat recovery and stora ge. In adiabatic CES systems, the heat of compression that is reje cted i n the intercoole rs (IC) i s rec overed an d stor ed to be supplied to the d ischarging proce ss, superheating the air at the inlet of the turbines. The lower the number of c ompress ion-s teps and the higher the com pressio n-press ure ( 𝑝 ), the higher the temperat ure rec overed in the IC as well as th e higher the turbin e inlet temperat ure (TIT) in the discharging p rocess. For the base cases, a three-stage com pression process with inte rcooling was selected. After each compression step, the air is coo led to 18 °C. An isentropic eff iciency of η is,CM = 85 % [ 42] was selected for the com pressors. Lower values f or η is,CM increase the temperature at the com pressor outlet. The compression pressure was s elected according to cond u cted sensitivity analy sis of the maxim um liquid yiel d 𝛾 achieved and th e minim um specif ic work require d for the liquefact ion pr ocess for dif ferent c harging press ures (70 – 140 bar), Figure 4.4. Figure 4.4: Minimum specific work requirement 𝑤 and maximum liquid yield 𝛾 of the charging process over the compression pressure 𝑝 for the Base Cases A and B. The specific work requirement ref ers to the work required to li quef y one kg of air 𝑤 . The liquid yield 𝛾 is define d as the ratio between the mass flow of the air lique fied and the mass 900 950 1000 1050 1100 1150 1200 1250 0,3 0 0,3 5 0,4 0 0,4 5 0,5 0 0,5 5 0,6 0 70 80 90 100 110 12 0 130 140 w_m i n B a se C ase A w _ m in Base Case B max y B a s e C a se A max y Ba s e Ca s e B 1,200 950 1,000 Specific work, 𝑤 𝑘𝐽/𝑘𝑔 1,250 1,050 1,100 1,150 900 Liquid y ield, 𝛾 Press ure, bar 0.60 0.35 0.40 0.45 0.50 0.55 0.30 𝛾 – Base Case A 𝛾 – Base Ca se B 𝑤 – Base Case B 𝑤 – Base Case A Chapte r 4: Design and sim ulation 67 flow exiting the last compression step 𝑚 ( Eq. (3.4) ). B oth the spec ific work 𝑤 and the liquid yield 𝛾 a r e i n d i c a t o r s f o r t h e p e r f o r m a n ce of the liquef action process . The two base cases use different liquef action processes and thus achieve their bes t performance ( 𝑤 , 𝛾 ) at differe nt pressures. The operation pressures were chosen at 85 bar and 120 bar, for the Base Cases A and B, respectiv ely, when the specif ic work required to liquefy a kg of air reaches its min i mu m 𝑤 . Liquefaction processes (block (c) in Figure 4.2) The performance of the liquefact ion process has a significant i nflu ence on th e overall perform ance and cost-ef fectivenes s o f t h e C E S s y s t e m . H e n c e , a number of air liquefaction processes w ere consi dered and evaluated f or appl ication in a dia batic CES sy ste ms. Most c ommerci al air li quefacti o n plants opera te with th e Clau de proc ess and its m odificati ons due to their high effi ciency [48], section 2. 2.3 (a). The lea di ng CES developer , Highview Ltd., base their charging station on the Claude process, relying on t he matur ity of the proces s and the trouble-free scale-up [18]. Despite Claude-based sys tems lead ing in industrial air liquefac tion, other liquef action processes are of interest for implementati on in adiabatic CES sy stems. Th e Linde process, for example, has ofte n be en propose d for CE S, see Table 2. 1 in Cha pter 2. In the com parati ve exergy-base d analysis, three liquef acti on pr ocess based on the Linde process, and three Claude-based processes were evaluated: the simple Linde process, the pr ecooled Linde pro cess, the dual-pressure Linde process, the simple Claude process, the Kapitza process, and the Heylandt process. The simulated systems are shown in Figure 4.5 and Figure 4.6, t he stream values are reporte d i n T a b l e C .3 a n d T a b l e C . 4 i n A p p e ndix C. A second bypass turbi ne was found to be redundant regard ing RTE , liquid yield and costs in a pr evious study. The Collins p roce ss was, therefore, not fur ther i nvestig ated i n this work [ 51]. At first, the liquefaction processes were m anually optim ized an d later modifi ed to accommoda te the cold sto rage. The operation p ressures for th e different liq uefaction processes deviate [48]. For com parison, the liquefac tion pressure was kept to p max,CM = 2 0 0 b a r [ 4 8 ] f i r s t a n d l a t e r p max,CM was varied in a sensitivity ana lysis (80-200 bar). The system s w ere simulated with and without cold storage. The modeling and assumption for the cold st orage and recove ry are described in s ection 4.3.2. Chapte r 4: Design and sim ulation 68 Linde-Hampson Consisti ng of only four sets of compone nts: the compressor(s ), the main heat ex changer (MHE), the throttli ng valve (TV) and the flash tank (FT ), the Linde-Ha mps on process is the most straightf orward of all liquefacti on processes. In the Linde pro cess, F igure 4.5 (a), purifie d com pres se d air is c oo le d ( 1- 2) a nd undergoes isenthalpic expans i on ( 2-3) in a throt tlin g valve, bein g brought to its dew point by the Joule-Thom son effect [47] . The tempe rature of the hig h- pressure air is reduced to a val ue b elow - 100 ℃ in heat exc han ge with the recurring stream (5- 6). The recurring stream is the gaseous air that exits the flas h tank after b eing separat ed fr om the liquefied air (3-4-5). Precooled Linde The tempe rature of the air at the inlet of the throttlin g valve (2) strongly influences the efficiency of the Linde-H ampson proce ss. By reduci ng this tempe rature with th e help o f a secondary refr igeration cy cle, th e precooled Linde p r o c e s s ( F i g u r e 4 . 5 ( b ) ) a i m s t o a c h i e v e higher liquid y ields. Carbon diox ide, ammonia, o r Freon compoun ds are comm only used for the compres sion refri geration process [ 48]. In t his work, R32 ( Difluormethane) was used. Dual-pressure Linde By introducing a second p ressur e level, the h eat transfer in th e MHE1 is improved. In the dual- pressure Linde process, the air is compressed to an intermediate-pressure bef ore entering the liquefaction process (1). Further, the pressure of the air is e le vated to the high-press ure level (2-3) afte r being mixed with the recurring gaseous stream exiti ng the intermediate-pressure flash tank (1-12-2). The air is cooled in the M HE1 before under going isenthalpic expansion to the interm ediate-pre ssure level (4-5). The l iquefied air exiting the interm ediate-press ure tank i s fed to th e second pressure-stag e (6 -8). In the dual-pressure Linde p rocess, the sp ecific wo rk required to liquefy the air is r educe d a t the ex pense of t he l i quid yield, with resp ect to the simp le Linde process. Figure 4.5: Flowsheets of the Linde-based ai r liquefaction processes with cold recycle [46]. Liquid air Air Work Methanol R218 R32 (a) Simple Linde ‐ Hampso n (b) Dual ‐ pressure Linde (c) Precooled Linde 2 1 3 4 5 6 3 4 5 6 2 1 2 1 3 9 4 5 6 7 11 8 10 12 MHE1 MHE1 MHE1 TV TV TV TV FT FT FT Chapte r 4: Design and sim ulation 69 Simple Cl aude The Claude process was develope d to reduce the liquefaction pre ssure. The Claude process proposes a solution with two exp ansion mechanism s, a throttling v alve, and a cold expander. The cold expander is located al ong a bypass [51] . The pressuriz e d a i r t h a t u n d e r w e n t t h e isentropic expan sion in the cold expander is used to provide a low-temperature cold recycle stream ( 𝑚 ) to further reduce the tem perature before the thrott ling proce ss [14]. The expander thus does not replace the throttl ing valv e (4 -5) be fore the fla sh tank. The additi on of a col d expander a voids part of the exer gy de structi on in t he thr ottlin g process and reduces the required power for liquefaction by the p ower output of the expander ( 𝑊 ∑ 𝑊 𝑊 ) while allowing lower worki ng pressures than in t he Linde process. Figure 4.6: Flowsheets of the Claude-based ai r liquefaction processes with cold recycle [46]. Kapitza The Kapitz a process i s adopted f rom the Claude pr ocess, yet, th e low -temp erature h eat excha nger is elimina ted. In other words, stream 7 is not fed ba ck to the MHE1 before being mixed with stream 10, a voiding the third partit ion of the MHE1. The dif fere nce in the perform ance of the Claude and the Kapitza process is comm only l ittle, due to the rather small temperature difference of the t wo mix ing stre ams (7 and 10 ). Heylandt The Heylandt p rocess can either be seen as analogous to t he Cla ude process or as a variation of the precooled Linde-Hampson proce ss using air as a refrigerant. In the Heylandt p rocess, the first parti tion of the MHE1 in the Claude syst em is eliminate d. A p a r t i t i o n o f t h e a i r i s f e d t o the cold expander directly (at ambient temperature) instead of being fed to the MHE1 first to be prec ooled. The split ting of the c ompresse d air before enter i ng the MHE1 impr oves the heat transfer process in the MHE1 [48] . (b) Kapitza (c) Heylandt (a) Sim ple Claude 2 1 3 4 5 6 7 8 9 10 11 12 2 1 3 4 5 6 8 9 10 12 7 2 1 3 4 5 7 8 9 Liquid air Air Work Methanol R218 MHE1 M HE1 MHE1 FT FT TV EX EX EX TV TV FT 6 10 Chapte r 4: Design and sim ulation 70 Splitting r atio r for Cl aude- based sys tems The perform ance of the C laude- based process es is dependent on t he splitting ratio 𝑟 . The splitting ratio is defined as t he mass flow t hrough the expande r 𝑚 over the mass flow exiting the com pression proce ss 𝑚 . 𝑟 𝑚 𝑚 (4.1) The splittin g ratio, therefore, influe nces the share of air liq uefied in the liquefactio n process. W h e n a s m a l l e r s h a r e o f a i r b y p a s s e s t h e M H E 1 a n d i s f e d t o t h e e x p a n d e r , t h e t e m p e r a t u r e differe nce in the MHE1 is decreased, and a larger share of air is fed to the throttling process and liquefie d. The splitti ng rati o is constrai ned by the minim um pinch temperature of the MHE1 ( ∆𝑇 , → 1 K ) . W h e n t h e s p l i t t i n g r a t i o i s r e d u c e d t o i t s m i n i m u m , t h e liqui d yield is max i miz ed ( 𝛾 ) and the specif ic liquef action w ork is m inimized ( 𝑤 ). 4.3.2. Storage unit Heat storage (block (f) in Figure 4.2) Aim of the r ecovery and storage of the com pression heat is to s upply additional thermal energy t o t h e d i s c h a r g e p r o c e s s – i n c r e a s i n g t h e T I T a n d t h e p o w e r g e n erated i n the expansi on process. The h eat r ejected in the intercooling process is recover ed wi th a s i n g l e h e a t t r a n s f e r a n d s t o r a g e medium [18] . Pressurized wate r or therm al oil can be employed a s working media [ 12]. In the sim ulatio n, press urized wate r (25 bar) was utilize d as h e a t t r a n s f e r , a n d s t o r a g e m e d i a f o r low-medium temperatures (T ≤ 200 °C) and therm al oil (6 bar, Do wth-01) was used for medium -high temperatures (200 °C < T ≤ 500 °C). The heat losses in the heat st orage were accounte d for with 5 K /c ycle. The mass flow of the heat transfe r fluid is ada pted t o reduc e t he temperature after each compressio n step to 18 °C. Accordingly, t h e m a s s f l o w o f t h e f l u i d i s determ ined by the com pression pressure 𝑝 and the mass flow rate of the air 𝑚 . Cold storage (block (g) in Figure 4.2) The “col d” (low -tempe rature exer gy) re jected in the evapora tion proc ess of the liquid air Rankine cycl e (discharge pro cess) can be recovered and stored. Supplying this low-tem perature exergy to the liquefaction proc ess significantly decreases the sp ecific work required for liquefaction and increases the RTE . Two di fferent sto rage configura tions have been d iscussed in the context of CES. Either, gravel- based pack ed bed storage (PBC S) or fluid storage with methanol and propane (or R218) were considere d, see Table 2.1. The cold storage of the Base Case CE S systems w as mo del ed with two fl uid ta nks, a s suggested in [ 39 , 40, 41, 43]. Reason for this is that the cold storage (CS) media are kept in a liquid state in contrast to dry air being the secondary working fluid in the PBCS. The work req uired in the cold storage is expected to be much lower whe n p umping the liqui d w orkin g fl uids in com parison to the work necessar y to overcome the pressure drop in the PBCS configur ati on. Moreover, the use of d ry Chapte r 4: Design and sim ulation 71 gas, e.g., nitrogen would require gas storage, which would decr ease the volumetric energy densit y of the storage – the main advantage t owards com peting tec hnologies, see sectio n 2.4. 2. Several refrigerants were reviewe d and evalua ted concerning: th eir boiling and freezing temperatures, their toxicity and flammability and comm ercial us e, Table 4.3 . Being neith er toxic nor flammable, R218 was f ound to be more suitable to reco ver the high-grade cold as Propane. Pr opane was sugges ted by [ 22, 39, 43] . For recupe ratin g the low-g rade cold M ethanol is more appropriate as its boiling point is higher than the ambient temperature. The same conclusion was derived by [39]. The cold recovery is thus reali zed with these working fluids circulating on two t emperature levels : Perfluoropropane C 3 F 8 (R218), between -180 to -61 °C, 2 b ar Methanol (CH 3 OH), betwee n -21 t o -59 °C, 5 ba r T h e p r e s s u r e o f t h e c o l d s t o r a g e m e d i a i s s l i g h t l y e l e v a t e d t o avoid air le akage and ensure improve d heat exchange. The amount of lo w-te mperature ener gy re covered is d eterm ined by the amount of air liquefied i n the li quefac tio n proce ss. The ma ss flow rates of the c old s tora ge media are therefore determined by a share of the mass flow rate of the liquefied air 𝑚 : 𝑚 2 . 2 9 ∙ 𝑚 (4.2) 𝑚 0 . 4 6 ∙ 𝑚 (4.3) The ratio is adjusted to th e op t i m a l h e a t t r a n s f e r b e t w e e n t h e evaporating liquid air and the cold storage media. According to [14] t he e f f i c i e n c y o f the cold sto r a g e m a y r e a c h v a l u e s o f 𝜀 = 9 5 % [ 1 4 ] . S t ö v e r e t a l . [ 1 2 ] e s t i m a t e d t h e C S e f f i c i e n c i e s a t 85-95 %. Thermal losses in the cold storage were accounted for with 4 K/cy cle and 2 % ma s s of boil-o ff losses. The integ ration of the cold storage w ith the two main heat exchangers is di spla yed in Figure C.2. The T, ∆ H - d i a g r a m o f t h e e v a p o r a t i n g l i q u i d a i r a n d t h e c u m u l a t i v e c u r v e of the CS media in the MHE2 is displayed in Figure 4.7. The pressure of the liqui d air was varied from subcritic al pressur es of 20- 30 bar to supe rcritica l pressures of 40-150 bar . Air reaches the c ritical point at - 140.5 °C an d 37.4 bar. Table 4.3: Refrigerant properties compared to air [ 39, 58]. Refrige rant no. Name Chemical formula Boilin g point at 1 bar Freezi ng po int at 1 bar R-218 Octafluoropro pane C 3 F 8 - 38 - 183 R-29 0 Propane C 3 H 8 - 42 - 190 - Methanol CH 3 OH 65 - 97 R-729 Air - - 196 - 21 0 R-732 Oxy gen O 2 - 196 - 216 R-728 Nitr ogen N 2 - 183 - 218 Chapte r 4: Design and sim ulation 72 Liquid air storage The li quefie d air is store d in a sim ple ins ulate d storage t ank as standa rd for bul k stora ge in the industrial gas and the LNG indust ry ( e.g. double-wall flat-bott om tank) [15] . At a size larger than 1,000 tons, liquid gases ar e stored at about ambient press ur e [32] . Storage vess els for LN G can contain m ore than 5 ,000 t ons of liquid air [15]. According to reported values, boil-off losses in low- pressure tanks range from 0.05 % up to 0.2 % by volume p er day d ecreasing with tank size [12, 16, 18, 39, 130]. The storage vessel simulated h as a pressure of 1 .1 bar and temp erature of - 192 °C with boil- off losses o f 0 .2 % Vol per cycle. Being physically indepe ndent, the charge and the di scharge unit may be sized inde pende ntly [75]. The st orage tank size is depe ndent on the amount of air liquefie d in the charging process ( 𝑚 ∙𝜏 ) . W i t h a n e s t i m a t e d c h a r g i n g d u r a t i o n o f 8 hours, the tank size amounts t o approximately 3,000 tons for all considered system s. Figure 4.7: T, ∆ 𝐻 -diagram of the main heat exchanger in the discharge process (MHE2) for different pressures of the liquid air evaporating in heat exchan ge to the cold storage media (cumulative curve). 4.3.3. Discharging unit Pressurizing and evaporation (block (d) in Figure 4.2) During discharge, the liquid air undergoes a Rankine cycle. At first, t he subcooled l iqui d air is brought to high pressures in a cry ogenic pump before b eing evap orat ed, superheated, and expanded for electricity generati on. In the s imula tion, the liq uid air enters the c ryopump slightly subcooled (1. 1 bar, -192 °C) to av oid cavitation. A cryopump is a vacuum pump that seizes liquid gas by surfaces cooled to cryogenic temperatures (< 120 K) [131] . Bot h centrifugal and positive dis placement pumps may operate as cryogenic compression pumps, and at large-scale, high-pressu re positive dis placement -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 0 0 ,2 0 , 4 0 ,6 0 , 8 1 1 ,2 tem perature, T [°C] enthalpy rate, ΔH [MW] Co ld st o r a g 150 b ar 130 b ar 120 b ar 100 b ar 90 bar 70 bar 60 bar 50 bar 40 bar 30 bar 20 bar 0.0 Liquid air at CS m edia 0.4 0.2 0.6 0.8 1.0 1.2 . Chapte r 4: Design and sim ulation 73 pumps operate at pressures h ighe r t h a n 2 0 0 b a r [ 4 7 ] . A m u l t i - s t age centrif ugal pump out of aluminum and stainless steel with copper windings in the m otor was suggested for commercial- scale CES systems to enable disc harge pressures of up to 200 ba r and mass flowrate s of up to 300 kg/s [8]. Brett and Barnett [ 18] stated that 200 bar and ma ss flow rates necessary for commercial size are th e long -term ta rgets for CES further dev el opmen t, while a single cryogenic pum p can already achieve flowrates greater than neces s a r y f o r 2 0 M W ( 5 0 k g / s ) dischar ge capacit y and pressur es > 100 bar, e.g., applied in L N G regas ification termina ls. The power output of the discharge unit 𝑊 increases with the pumping pressure 𝑝 until a threshold is reached above which the additional power generated in the turbines ∑ ∆𝑊 counterparts the additional pumpi ng power necessary to increase t h e p r e s s u r e ∆𝑊 . The threshold depends on the dischar ge c onfiguration. Additionally, the am ount of c old recycled in the evaporation proce ss reduces w ith increas ing pressure, reduc ing the effectiven ess of the charging process [16] , Figure 4 .7. In Figure 4.8, the roundtrip efficie ncy is displaye d over the pumping pressu re (< 200 bar). Figure 4.8: RTE over pumping pressures for Base Case B. Ameel et al. [10] stated that the enhancement in power output f or pressure increase between 100 and 500 bar is insignificant. Morgan e t al. [16] identified the threshold b etween 150-200 bar. Several other feed p ressures were suggested in the literat u r e , T a b l e 2 . 1 . I n t h e e v a l u a t e d system, this power threshold i s reached at a pressure above 200 bars. The feed pressure o f the Base Case was set to 150 bar, an d the isen tropic efficien cy i s assumed with 0.75 [57]. The isentropic efficie ncy was estimated low in comparison to values s u g g e s t e d i n t h e l i t e r a t u r e (0.75-0.80 [16, 42, 39, 51] ) bas ed on a recommendation from the industry. The pressurized liquid air is sub sequently fed to the second ma in heat exchanger (MHE 2) and re-gasifie d. The heat exchange i n the MHE2 is shown in Figure 4 .7 for different pumping pressur es. The air is fu rther heated in heat excha nge with the stream exiting the e xpansio n line (HE4) an d supe rheate d in heat e xcha nge wit h the hea t stora ge me dia (HE 5), Figure 4.2. 20 25 30 35 40 45 50 10 35 60 85 110 135 160 185 R T E, % pumping pressure, bar Chapte r 4: Design and sim ulation 74 Expansion process (block (e) in Figure 4.2) The high-pressure high-temperatu re g as is expanded in four g as turb ine stag es with reh eating. The superheating occur s w i t h t h e a i d o f t h e h e a t s u p p l i e d b y t h e heat storage. The isentropic efficiency of the turb ines is es timated at 90 %. Values propose d in the literature on CES system s reached 90- 92 % [14, 57] . The spec ific e nergy gene rate d per kg of liquid air increases linearly with the turbine inlet tempe rature (T IT), Figure 4.9. Figure 4.9: Specific work of the discharge unit per kg of liquid air, RTE, and η over the TIT based on the assumptions made for the integrated systems (section 4.4, 𝑤 = 1,200 kJ/kg). For an adiabatic system with heat recover y and storage (TIT 200 °C), the specific energy is approximately 475 kJ/ kg while with the integrat ion of waste hea t at 350-450 °C values of 650- 750 k J/kg are reached. The integra tion of heat sources a nd sink s is further discussed in section 4.4. 1. Energy densities and efficiencies reported in th e literature are give n i n Ta ble 4. 4. The values range from 360 to 900 kJ/kg, whic h are realizable wi th waste h eat integration and TIT of 175-600°C. The RTE accordingly would range from 37 to 70 %, [17] and [18] claim t o reach even higher v alues. In o rder to reach higher RTE at en ergy densi ties that ar e simi lar to those achieved in adiab atic CES sys tems or with waste heat inte gration are only realizable with integrat ed systems, e.g ., with LNG wa ste cold. In tegrated syste ms are sub ject to the following sectio n 4. 4. Table 4.4: CES energy densities and efficiencies reported in the literature. Energy density Effici ency S ource [kJ/k g] [ %] 540-900 40-60 [37] 360-720 70 + [17] 360-515 60, 70+ [18] 540 47-57 [ 16] 0 10 20 30 40 50 60 70 80 90 100 0 100 200 300 400 500 600 700 800 900 150 200 250 300 350 400 45 0 500 550 efficiency , % specif ic work w dis , kJ/kg TI T , ° C spec work, kJ/kg RTE , % ƞ , % Chapte r 4: Design and sim ulation 75 4.4. Integrated systems The reduction of the energy requ irement in the l iquefaction pro cess and the increase of the specific power output of the disc harge process were identi fied a s t w o p r i m a r y r e s e a r c h a n d development ob jectives, that need to be met to p repare CES tech nology for the market [20]. The integration of CES sy stem s with internal/external thermal e nergy sources aim s for such enhancement in performance. Aim of the exergy-ba sed evaluation of the “integrat ed sys tems” is to ident ify the pote ntial of inte grate d CES syste ms concer ni ng thermodynam ic and economic perform ance . In this sectio n, the desig n and simulat ion of the system s with the in tegration o f d ifferent h eat sources and sin ks is discusse d. A total of ten syst em confi gura tions were assesse d i n this w ork (Figure 4.10). Waste heat (TIT 350-450°C) and in ternal combusti on are considered an alternative to heat storage in th e a-CES system. The integratio n of LNG waste cold instead of or in combin ation with cold st orage is also consi dere d. Two adi abatic CES systems, one without cold stora ge (a-CES w/ o CS) and one with CS (a-C ES). The a-CES system s were compared to two systems with waste h eat inte gration at 350 °C and 450 °C, f our systems with LNG waste cold integration and two diab atic CES sy stems with combustion . Figure 4.10: Flowsheets of the integrated systems (a) based on the adiabatic CES system and (b) based on the diabatic CES system. Adiabatic system s: a-CES w/o CS: (a) 𝑚 = 0, 𝑄 = 0, 𝑚 = 𝑚 = 0 a-CES: (a) 𝑚 =0 , 𝑄 =0 (a) Sys tems based o n the a-C ES a2 a1 a3 a4 a5 NG 1 𝑊 𝑊 𝑊 LN G 2 LN G 1 a6 NG 2 a2 𝑊 a1 a3 a4 a5 𝑊 𝑊 𝑄 𝑊 LNG 2 LNG 1 a6 (b) Systems based on the d-CES Internal Combustion: d-CE S: (b) 𝑚 =0 , 𝑚 =0 d-C ES 2: (b ) 𝑚 =0 Air HS media R218 Methanol LNG W aste heat integratio n: WH 35 0, WH 450 : (a) 𝑚 =0 , 𝑄 >0 LNG integrated systems : LNG w/o CS: 𝑄 = 0, 𝑚 = 𝑚 = 0 LNG integration: (a) 𝑄 = 0 LNG and WH integration: (a) 𝑄 > 0 LNG d-CES: (b) 𝑚 = 0 Chapte r 4: Design and sim ulation 76 For exerge tic analysis the compression pressure and mass flow r ate 𝑚 i n t h e c o m p r e s s o r s (com pression w ork 𝑊 ) were kept uniform for all s yst em configurations. In economic analysis, the r esults for a) a comm on mass f low rate 𝑚 = 200 kg/s, and b) a c omm on disc harge capacity of 𝑊 = 100 MW, were compare d. All considered syste ms are b ased on the design p aram eters of the Optimize d Case (Table 4.2) and the general assum ptions made for the isent ropic efficien cies of th e turbomachinery (Table 4.1). T h e s t r e a m v a l u e s o f t h e s y s t e m s p r e s e n t e d i n F i g u r e 4 . 1 0 a r e g iven in Table C.5 and Table C. 6 in Appendix C. The power and hea t capacities of the work and he at flows indicated in the Figure 4. 10 are g iven in Table 4.5. The system s are descri bed i n d etail in the follo wing subsecti ons. Table 4.5: Power and heat capacities in MW for all work and hea t flows indicated in Fi gure 4.10 for the ten systems considered in comparative analysis. System Abbrev. 𝑊 . 𝑊 𝑾 𝒄𝒉𝒂𝒓 𝑄 𝑊 . 𝑊 𝑾 𝒅𝒊𝒔 Adiabatic CES systems without cold storage a-C ES w /o CS 138.2 21.2 117.0 - 59.1 2.9 56. 1 with cold storage a-CES 138.2 11.2 127.0 - 105.2 5.2 100.0 Waste h eat in tegra tion with TIT of 350 °C WH 350 138.2 11.2 127.0 222.1 141.0 5.2 1 3 5 . 8 with TIT of 450 °C WH 450 138.2 11.2 127.0 329.7 164.0 5.2 1 5 8 . 8 LNG integrat ion without cold storage LNG w/o CS 138.2 20.1 118.0 - 64.6 3.2 61.4 with cold storage LNG 138.2 2.8 135.4 - 144.1 7.1 137.0 with waste heat LNG + WH 138.2 2.8 135.4 290.0 224.7 7.1 2 17.6 with combustion LNG d-CES 138.2 2.8 135.4 - 297.4 4.1 293.4 Diabatic C ES systems with single combustion d -CES 138.2 11. 2 127.0 - 21 7.4 3.0 214.4 with double combustion d-CES 2 138.2 11.2 127.0 - 188.7 3.0 185.8 Chapte r 4: Design and sim ulation 77 4.4.1. Waste heat integration R e c o v e r y o f “ wa s te h e a t ” w as i de n t i f i e d a s o n e o f t he c o r e p o t e ntials of CES (section 2.3) [8, 10, 14, 37]. As the heat of com pression rec overed in a-CES syst e m s , m a y e x c e e d T I T o f 2 0 0 ° C and waste heat (WH) integration entails the colocation to th e w aste h eat source, only high- temperature WH higher than 300 ° C is considerable for CES syste m s. The sources for high-temperature waste heat are limited. The qu ality of waste heat from electricity production was found i nsufficient in terms of exergy content and temperature level (< 100 °C). The European annual waste heat potential in the tem perature range of 200–500 °C was estimate d higher than 75 TWh [60] . In t he industry and tran sportation sector s, h igh-quality waste heat (> 300 °C) is disp osed of [132]. For application in CES, only industrial waste heat is of potential. Apart from the quality of the waste heat, the thermal carrier of the waste heat is also of im p ortance. T he heat reject ed f rom industria l processe s is contained i n various therm al carriers, e.g., exhaust gases, low-qua lity steam, hot oil, cool ing water, or comm oditie s such as hot steel [60]. Among the numero us industrial processes and sec tors, the quantity o f waste heat varies strongly [133] . The i ndust rial sectors with the highest pot ential of hi gh-grade waste heat (200-500 °C) are iron and steel, non-m etallic m ineral, and chem ical indu stry [60]. In Germany alone, the high-grade waste heat potential of iron and steel in dustr y is estimat ed larger than 45 TWh annual ly [60]. In the simulation, high-grade waste heat nece ssary to e levate t he TIT to 350-450 °C was taken into conside ration. The power output of the discharge unit incr eases linearly with increased T IT , F ig ure 4. 9. Th e f lo ws he e t f or CES wi t h w as te he at i s s im il ar to that of the ad iabatic C ES system except for an additional heat rate 𝑄 , that is supplied to the heat storage, Figure 4.10 (a). R e a s o n f o r t h e w a s t e h e a t b e i n g s u p p l i e d t o t h e h e a t s t o r a g e i s that the waste heat is disposed of continuous industri al processes, which h ave operation hours independent from that of the dischar ge process of the CES system. Due t o the high tempera tur es, the heat transfer and stora ge media of the sys tem with waste heat integrati on is therm al oi l (Dowth- 01) a t a pressure of 6 bar. The power and heat capaci ties o f t he sy stem s w ith waste heat in tegration (WH 350, WH 450) are compared to the adiabatic CES system in Table 4.5. The stre am valu es indicated in Figure 4.10 (a) are given in Ta ble C.5 in A ppendix C. 4.4.2. Diabatic CES with combustion As an alternative, a CES can contain a combustion process in wh ich fuel is burned to supply additional heat, increasing the t empe rature of the high-pressur e gas before the expansion process [ 12, 134]. The c ombustion of natura l gas ( NG) causes th e le ast specific GHG e missions and pollution. Being the relativ ely “clean est” convention al fue l, NG is used in the combustion of the e valuate d d-CES system s. The NG was simulated with CH 4 an d a l o w e r h e a t i n g va l u e o f 50 MJ/kg. T w o d i a b a t i c C E S s y s t e m s w e r e c o nsi dered in the analys is: d-CES , with a single combustio n c h a m b e r , a n d d - C E S 2 , w i t h t w o c ombustion chambers. The flowshe et of the diabat ic CES systems are shown in Figure 4.10 (b), the corresponding stream v a l u e s a r e g i v e n i n T a b l e C . 6 in Appendix C. The pres sure drop in the com bustion chamber (CC) is esti mated w ith 4-8 % of the inlet pressure adopted from [127], depending on the size of the CC. Chapte r 4: Design and sim ulation 78 In the CES sys tems tha t employ c ombus tion, heat st orage ca n be avoided. As a result, a higher mass f low rate of pr essurized wat er at ambie nt tem perature is u se d as a cool ing m edium in the intercoole rs. With increasing m ass fl ow o f the fuel, th e RTE and the exergetic efficiency of the d -CES system rise, Figure C.3. The mass flow rate of the fuel was adjusted t o the max imu m o per ating temperature of the construction m aterial of the turbines (metal alloys, < 1100 °C [118] ) to avoid excessively expensive e quipment. The total mass flow of the fue l for both systems w as kept consiste nt for c ompari son of the sys tem s performanc e and c ost-e f f e c t i v e n e s s . W i t h a m a s s f l o w rate of 𝑚 = 4.5 kg/s the temperature at the inlet of the subsequent turbi ne is augmented to 1,100 °C in the d-CES 1 system. I n the d-CES 2 sy stem, the TIT of the two turbines is 720 °C and 730 °C. The reason for introducing a second combustion cham ber is to use ch eaper materials for t he turbines wh ile achieving comparable power cap acity of the disc harge unit. The power a nd heat capa cities of the d- CES s ystem s (for 𝑚 = 200 kg/s) are com pared to t he a-CES syste m in Table 4.5. The liqui d air is pumped to 80 bar instead of 150 bar i n the d-CES systems, reducing the energy req uire ment of the cryogenic pum p. The d-CES sys tems achieve approximately twice th e power capacity and RTE than the a-CES. The performance o f the d- CES 2 system i s by 13.4 % lower. 4.4.3. Integration of LN G low-temperature exergy The most p rom inent waste col d source is the regasif ication proc ess of LNG. Natural gas is globally transported by ship in th e form o f LNG in c ryogenic st orage vessels at about a t m o s p h e r i c p r e s s u r e a n d a t e m p e rature of approximately - 160°C [ 1 3 5 ] . T h e N G i s f e d b a c k to the transmission grid at press ures between 30-70 bar [136]. For the transmission grid, a p r e s s u r e o f 7 0 b a r i s r e q u i r e d , while local distribution requir es lower pressures. The mass flowrates in LN G terminals may ex ceed values of 1 50 kg/s [137]. In the conside red sys tem s, the LNG is fed to the MHE1 of the li que factio n proc ess with a pressure o f 32 bar and a tempera ture o f - 158 °C. Four differen t systems were simulated and evalua ted in this work. L NG regas ificatio n was i ntegrate d into: an a-CES system without col d storage (LNG w/o CS), an a-CES system with c old stor age (LNG), a CES system with waste heat int egration at 450 °C (LNG + WH), and a d-CES system with NG com busti on (L NG d-CES), s ee Figu re 4 .1(a ) and (b). The effect of integrating LNG to t h e a d i a b a t i c C E S s y s t e m i s d e picted in Figure 4.11. With a higher mass flow of LNG fed to the system , the splitting ratio can be reduce d, and a higher share of the air m ass f low is fed to the MHE 1 an d li quefied. T h e splitting ratio and liquid yield are limited by the minim um temperature difference in the MHE1, section 4.3.1(c). The RTE and the exergetic effici ency o f th e sy stem ɛ tot i ncrease linearly at first. When the mass flow of the LNG reaches approximately 9 % of the mass flow of the co mpr e s s e d a i r , t h e m a x i m u m value for th e RTE and the ɛ tot is reached. Chapte r 4: Design and sim ulation 79 Figure 4.11: RTE and exergetic efficiency over the specific mass flow of LNG. W i t h t h e fu r t h e r s u p p l y o f L N G , t h e RTE st a y s co n s t an t . T h e e x e rg e t i c e f f i c i e n c y d e c l i n e s w it h higher mass flows 𝑚 , as the exergy of fuel–the low- tem perature exergy supplied by the LNG fed to the system –increas es fur ther, while the exer gy of th e p r o d u c t s t a y s c o n s t a n t . F o r the LNG systems, the mass flow of the LNG was set to 8.8 % of t he mass flow of the air to a c h i e v e ma x i m u m e x e r g e t i c e f f i c i e n c y f o r a l l s y s t e m s w i t h t h e e xception of the sys tem without c o l d s t o r a g e ( L N G w / o C S ) . F o r t h e L N G w / o C S s y s t e m , t h e m i n i m um splitti ng ratio and maximu m effi ciency is achiev ed at a mass flo w rat io ( 𝑚 /𝑚 ) o f 2.8 %. The stream v alues for the states i n d i c a t e d i n F i g u r e 4 . 1 0 a r e g i v e n f o r t h e L N G i n t e g r a t e d systems in Table C.7. The LNG leaves the MHE at a temperature o f – 4 °C and a pressure o f 30 bar, to be fed to the local d istribution grid [136] . The sha re o f th e liquefied air is increased b y 1 0 % ( w / o C S ) a n d b y 4 0 % i n r e s p e c t t o t h e a - C E S s y s t e m s a f ter the integration of LNG waste cold. Th e resul ts of the comp arative analys is o f the inte g r a t e d C E S s y s t e m s a r e d i s c u s s e d in Chapter 5.3. 0.0 38 41 43 46 48 51 53 ef ficiency , % roundt rip efficie ncy ( RTE) exergetic effi ciency ɛ 0.2 0.1 0.3 0.4 𝑚 𝑚 ⁄ , - 0.1 Chapte r 4: Design and sim ulation 80 4.5. Summary of the de sign and si mulation Several design options exist f or t he cha rge, stora ge, an d disch arge process of C ES systems. In the pre-design phase, different d esign options were assessed an d are discusse d in this chapter. Two types of systems can be differentiated: the adiab atic stand -alone C ES systems, and the integrate d CES system s. Two sets of assumptions are presented for the Base Case(s) a n d t h e Optimized Case(s) . The B a s e c a s e s r e f e r to th e t wo st a n d - a l o n e a d i a b a t i c C E S sy s t e ms t hat are undertaken comparative exergetic and economic analysis f ollowed by exergoeconomic anal ysis and optimization. The integrated system s are adopted from the final design of the Optim ized Case configur atio n and are extended with extern al h eat sources and sinks. Waste heat and internal combustion were integrated as heat sources. As heat sink, the integration of LN G instead o f o r in combin ation with c old st orage wa s consi dered. For correct comparability, the capacities were kept consistent i n t h e c o m p a r a t i v e a n a l y s i s . Either the compression work 𝑊 , or the discharge power 𝑊 were kept constant as co mmon ground. The processes simulation was undertaken in Aspen Plus® software, while the evaluation was commenc ed in the Engineering Equation S olver (EE S). Chapter 5: Resul ts and dis cussion 81 Chapter 5: Results and discu ssion 5.1. Evaluation of different cha rging proc esses for adiabatic C ES This section presen ts the results from the evaluation of differ ent charging process configur ations in a diabatic CES system s with exergy-based m etho ds. At first, the performance of six different liquef action pr ocesses is compare d using ener g e ti c a n d e xe rg et ic a na ly s is wi th and without the integration of co ld storage. Subs equently, the effect of cold storage on di fferent system param eters of the liquefact ion pr ocesses is q uantifie d. Concern ing the three liquefaction processes with the highest exerg etic efficien cy, fu rther sens it ivity analyses are und ertaken to identify the optimal system pressure. The three systems are com pared, applying an economic analys is at optim al system p ressur e. 5.1.1. Energetic and exergetic evaluati on of the liquefaction processes T h e r e s u l t s o f t h e e n e r g e t i c a n d e x e r g e t i c a n a l y s e s o f t h e s i x analyze d lique facti on conf igur ati on before and af ter the integrati on of cold stora ge are presented in Fi gure 5.1. The int egration of cold storage significantl y increa ses the liquid yield 𝛾 (Figure 5.1 (a)). The liquid yield of the simple Linde and the dual-pr essure Linde is tripled. The liquid y ield in the Claud e-based systems is increased by 80-90 % . The exe rgy of t he product incr eases correspondingly. With an increase in the mass fl o w r a t e o f t h e l i q u e f i e d a i r 𝑚 , the low-temperature exergy supplied to the liq uefacti o n process by the cold storage 𝐸 increases proportionally. With the addit ion of cold stora g e, also a substantial reduction of the specific work required to produce one kg o f liquid air ( 𝑤 ), by 30 to 70 %, is obser ved for all processes. Thus, the cold storage augmen ts the exerge tic efficiency c onsiderably by up to 200 %, as s hown in Figure 5.1 (b ) and (c). Despite th e more significant redu c t i o n i n t h e s p e c i f i c w o r k r e q uirem ent in the Linde- based configurations, th eir overall p erformance cannot co mpet e with t hat of the Claude -base d configur ations. The simple Claud e process, the H eylandt process , a nd the Kapitza process reach the highest exergetic efficiencies (69-72 %), have t he lowest s pec ific work requirem ent (1,435- 1,533 kJ/kg) and reach the highest li quid yield s (0.566-0.601). For the most efficient liquefaction configurations – t h e C l a u d e -based processes – a sensitivity analysis was conducted. The compression (liquefaction) pressure w a s v a r i e d ( 𝑝 = 70- 140 bar), and the s plitting ratio 𝑟 was reduced to its absol ute minim um value. The effect of these variations on the exergetic efficiency 𝜀 can be seen in Figure 5.2. The exerget ic efficiencies f or the res pective li quefa ction press ure are give n over t he spli tting rati o for (a) t he Claude and the Kapit za process and (b) the Heylandt proces s. Chapter 5: Resul ts and dis cussion 82 Figure 5.1: Results of the energetic and exergetic eva luation for the liquefaction processes with/without integrated cold storage (CS). With increasing liquefaction pres sure, lo wer splitting ratio s a re achievable (Figure 5.2). The share of air liquefied in the process incr eases with a reductio n in the value of th e splitting ratio ( 𝑟 𝑚 𝑚 ⁄ ), as a greater mass flow enters the MHE1 and the throttling pr ocess. The temperature difference in the M HE1 decreases with a reduction i n “cold feed” ( 𝑚 ) and a simultaneous increase in “hot feed” ( 𝑚 𝑚 ) . T h e m i n i m u m s p l i t t i n g r a t i o i s t h e r e f o r e restricted by the minim um pinch temperature ( ∆𝑇 , → 1 K). By minimizing th e sp litting ratio the maximum liquid y ield 𝛾 , the ma xi mum e ffi ci ency 𝜀 , and the minim um specific liquefaction work 𝑤 are obtained f or the res pective pressure. The m aximum exergetic efficiency curve s are indicated i n F i g u r e 5 . 2 w i t h a s o l i d black line. The thermodynamic pe rform ances of the Claude and Ka pitza processes are non- d i f f e r e n t i a b l e . R e a s o n f o r t h i s i s t h e t e m p e r a t u r e d i f f e r e n c e o f only 2 K o f the two mixing streams (Figure 4.6) and that the minimum pinch temperature in t h e M H E 1 w as s e t t o 1 K f or all processes. The T, ∆ H -diagrams of the MHE1 for all proces ses are given in Figure D.1 and Figure D.2 in A ppendi x D. 0% 20% 40% 60% 80% (c) exergetic efficiency Sim ple L inde + sto rage Ka pitza + storag e Dual -pr. Linde + storag e Precooled Linde + storag e Sim ple Cla ude + storag e Heylandt + sto rage 80 60 40 20 0 exergetic efficiency 𝜀 % 0 400 0 800 0 1 200 0 (b) specific liquefaction work 4,000 0 8,000 specific work 𝑤 𝑘𝐽/𝑘𝑔 Sim ple L inde + storag e Kap it za + sto rage Dual-pr. L ind e + storag e Precooled Linde + stor age Sim ple Cla ude + storag e Heylandt + sto rage 0,000 0,200 0,400 0,600 (a) liqu id yield 0.60 0.40 0.20 0.00 liquid yiel d 𝛾 without CS with CS Sim ple L inde + storage Kapitza + sto rage Dual-pr. Linde + storag e Prec ooled Linde + storage Sim ple Cla ude + sto rage Heylandt + sto rage 12,000 Chapter 5: Resul ts and dis cussion 83 Figure 5.2: Sensitivity analysis results for (a) the Claude and the Kapitza proc ess and b) the Heylandt process: the exergetic efficiency 𝜀 over splitting ratio r, for va rious liquefaction pressures. The Heylandt process reaches lower splitting ratios (< 0.3) and higher exergetic efficie ncies (> 8 0.5 %) t han t he Cla ude and t he Kapitz a process ( 𝑟 > 0.35, 𝜀 < 78.5 %), F igure 5.2. The processes reach their maxim um ex erget ic efficien cy 𝜀 at different splitting r atios and pressur es. This confirm s that comparing the s y s t e m s a t a s i n g l e p r e s s u r e l evel is not sufficien t. The Heylandt process reac hes its maximum exergeti c efficie ncy 𝜀 o f 80.9 % at a sp litting ratio o f 0 . 3 a n d a p re ss u re o f 1 2 0 b a r while the Claude and the Kapit za process reach 𝜀 = 7 8 .3 % a t a s p l i t t i n g r a t i o o f 0 . 3 9 7 a n d a p r e s s u r e o f 8 5 b a r . I n F i g u re 5 .3, the change in the minim al specif ic w ork r equire d for liquefac tion 𝑤 and t he maxim um liquid yield 𝛾 a r e g i v e n o v e r the splitting ratio 𝑟 for the Claude and t he Heylandt process. 0,735 0,745 0,755 0,765 0,775 0,785 0,795 0,25 0,3 0 ,35 0 ,4 0,45 0,5 0 ,55 exer getic ef fi ci enc y , 𝜀 [−] splitt ing rati o r , [−] max exerg e t i c 7 0 bar - C l a u 7 5 bar - C l a u 8 0 bar - C l a u 8 5 bar - C l a u 9 0 bar - C l a u 9 5 bar - C l a u 100 ba r - Cl a u 110 ba r - Cl a u 120 ba r - Cl a u 130 ba r - Cl a u 140 ba r - Cl a u (a) Claude and Kapitza process (b) Hey landt process 0,7 5 5 0,7 6 5 0,7 7 5 0,7 8 5 0,7 9 5 0,8 0 5 0,8 1 5 0 , 25 0,3 0 ,35 0 ,4 0 , 45 0,5 0 ,55 exer getic e f fi ci e n c y , 𝜀 [−] splitt ing r atio r , [−] max ex erg e tic 100 ba r - H e y 105 ba r - H e y 110 ba r - H e y 115 ba r - H e y 120 ba r - H e y 130 ba r - H e y 140 ba r - H e y 150 ba r - H e y 160 ba r - H e y 160 bar 140 bar 120 bar 100 bar 90 bar 110 bar 140 bar 120 bar 100 bar 85 bar 80 bar 90 bar 110 bar 130 bar 70 bar 75 bar . . 𝜀 , 𝜀 , 0.785 0.735 0.745 0.795 0.755 0.765 0.775 0.785 0.795 0.755 0.765 0.775 0.805 0.815 exer getic efficiency 𝜀 exer getic efficiency 𝜀 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.25 0.30 0.35 0.40 0.45 0.50 0.55 Chapter 5: Resul ts and dis cussion 84 Figure 5.3: Minimum specific work required for liquefaction 𝑤 and maximum liquid yield 𝛾 over the splitting ratio 𝑟 of the Cl aude/Kapitza process and the Heylandt process. In Figure 5.3, the liquef action pressures are indicated w ith a secondary x-axis. The reduction in the splitting ratio 𝑟 at higher pressures is less significant. The maxim um liquid yi eld 𝛾 increases linearly with the re ducti on in the split ting ratio 𝑟 . Thus, at higher pressures the increase in the m aximum liquid yi eld 𝛾 is less substa ntial. As a res ult, the minim al specifi c work required for liquefaction 𝑤 i ncreases a t higher pressures as the additional compression power outweighs the increase in air liquefied ( 𝛾 ). The minim al specific liquefac tion work 𝑤 thus reaches a minim um at 85 ba r for the Claude/Kapitza proces s and at 120 b ar for the Hey lan dt p roc ess . Th e pro ces s par amet er s at the min imu m wo rk r e quirement are consistent with that of the m axim um efficienc y 𝜀 . The minim um work req uired to liquefy one kg of air in the Heyla ndt process amounts to 967 kJ/kg (120 bar). T he Claude /Kapi tza pr ocess requires more specific liquefaction work 𝑤 = 984 k J/kg (85 bar). For liquefacti on pressures lower than 95 bar, the Claude/Kapitza process reaches lower values for 𝑤 and higher val ues for 𝛾 and 𝜀 than the Heylandt process. While, for variable and higher pressure s , t h e a p p l i c a t ion of the Heyla ndt process in CES systems is prefer able in term s of thermodynam ic performanc e , Figure D.3 in Appendix D. 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 90 0 95 0 10 0 0 10 5 0 11 0 0 11 5 0 12 0 0 12 5 0 min spec work - Heyland t min spe c work - Cla ude & Kap itza ma x l iq uid y ie ld - H ey lan dt max liq uid yield - C laude & Kapi tza 0.30 0. 35 0.40 0.45 0.50 0.55 splitting ratio, 𝑟 1,200 950 1,000 1,250 1,050 1,100 1,150 900 0.6 0.1 0.2 0.7 0.3 0.4 0.5 0 liquid y ield, 𝛾 spec ific work, 𝑤 𝑘𝐽/𝑘𝑔 120 b ar 140 120 100 90 80 70 pressure, 𝑝 𝑏𝑎𝑟 Cl aude and Kapi tza 140 120 110 100 90 80 70 Heyland t 85 bar 110 bar 140 bar 90 bar 80 ba r 70 ba r 100 bar 120 bar 140 bar Chapter 5: Resul ts and dis cussion 85 5.1.2. Economic evaluat ion of the Claude-based liquefaction processes The economic analysis was conduct ed for the optimal design of e ach of the three Claude -based liquefact ion units. The desig n parameters are given in Table 5. 1. The liquefacti on capac ity o f all processes was adjusted to supply a 100 MW/400 MWh a-CES sys te m. The results of the economic anal ysis a re presente d i n T able 5.2. E xc e pt f o r t h e hi gher charging pressure 𝑝 o f t h e Heyl andt system , all design parameters o f the Claude and the Ka pitza process-based systems are similar. The liquid yields 𝛾 and specific work requirement of the liquef action processes co rrespond to a RTE of ap p r o x im at el y 44.4-46.8 %, wh ich is within the expected ra n ge of 40 -60 % for st and- alone CE S system s [37, 38]. The results of the economic analy sis for the Claude a nd the Kapitza process differ des pite their identical des ign parameters a nd similar perf ormance in the ener getic and th e ex ergetic analy sis. T h e s m a l l d i f f e r e n c e i n t h e s i z e o f t h e M H E 1 r e s u l t s i n a n o t a b le difference in com ponent costs. The specific investm ent costs and the production costs per kg o f liquid air are lowest for the Kapitza liquefaction pr ocess, Table 5.2. When com paring the results of en ergetic and economic analysis o f the liquefac tion processes with integration o f co ld sto rage to valu es for stand-alon e air liquefaction plants, the effect of c o l d s t o r a g e b e c o m e s e v e n m o r e a pparent. The work required to p roduce one ton of liquid air is reduce d to the half (~ 290 kWh /ton), the values reported in literature range from 5 20 to 760 kWh/ton [8, 50]. Th e production costs of liquid air are als o significantl y reduced, amount ing to only 14-17 €/t on inst ead of 37-48 €/to n [8] withou t cold storage. The specific costs associated with the initial investment for the liquefac ti on unit are also reduced by half, compared to values repor ted in the literature f o r the Claude pr ocess of 1.640 €/kW [51]. Table 5.1: Design parameters for the three Claude-based CES sys tems compared in economic analysis. Param eter Unit Claude process Heylandt pr ocess Kapitza pro cess Liquefact ion pr essure bar 85 120 85 Chargi ng capa city MW 112 107 113 Liquefact ion capaci ty tons/day 9,256 8,985 9,256 Liquid yield - 0.503 0.559 0.501 Table 5.2: Results of the econom ic analysis performed on the th ree Claude-based liquefaction units. Parame ter Unit Claud e process Heylandt process Kapitza process Specific invest ment costs €/kW 8 07 959 803 Specif ic work r equireme nt 𝑤 kWh/ton 291 286 292 Product ion costs of liquid air €/ton 13 .8 1 6.4 13.7 The TRR of the liquefaction un its amounts to € 21.9, € 22.8 and € 20.8 million for the Claude, t h e H e yl a n dt a n d t he Ka pi t z a pr o cess respectively. The most cos t-efficient system is based on the Kapitza process with a specific production cost o f the liqu id air o f 𝑐 = 13.7 €/t on. The highest-efficiency system, w hich is based on the Heylandt p rocess performed less well in economic analysis and reached b y 20 % higher specific product c osts of 𝑐 = 16. 4 €/ ton. Chapter 5: Resul ts and dis cussion 86 5.2. Analysis and optimizati on of two adiabatic CES systems In this section, two c omplete adiabatic CES syste ms based on th e cost-optimal and the highest- efficiency lique faction pr ocess are evalu ated and optimized wit h exergy-based methods. The system employing the Kapitza pro cess is henceforth r eferred to as Base Case A and the Heylandt- based system as Ba se Ca se B . The two system configura tions are compared in the exergetic, economic, and exergoe conomic analysis. The base case systems are further optimized in iterative exergoeconomic analysis, and an optimal system con f igurat ion is i dentifie d. 5.2.1. Exergetic analysis Both base case syst ems compar ed in this section were designe d f or the same exergy o f the product 𝐸 , – a d i s c h a r g e c a p a c i t y o f 1 0 0 M W . T h e r e s u l t s o f t h e e x e r g e t i c analysis on the syst em level: the exergy of the fuel 𝐸 , the exer gy destructi on 𝐸 , , the exergy los ses 𝐸 , a s w e ll a s t h e e xe r ge t i c e f f i c ie n c y 𝜀 are given in Table 5.3. The exerg y values are given in MWh per cycle, to avoid mislea ding results caused by the differ ent oper ating hours of t he charge , storag e and d ischarge uni t. E ach d aily cycle entail s ei ght charging hours and four discharging hours at full capacity . The exergetic efficiencies of selecte d com ponents ar e given in Figure 5.4. Further results of t h e e x e r g e t i c a n a l y s i s o n t h e com ponent le vel are given in Ta ble D.1 an d Table D. 2 in Ap pendix D. Table 5.3: Results obtained from the exergetic evaluation of th e two base case systems. 𝐸 𝐹,𝑡𝑜𝑡 [MW h/cycle ] 𝐸 𝑃,𝑡𝑜𝑡 [MW h/cycle ] 𝐸 ,𝑡𝑜𝑡 [MW h/cycle ] 𝐸 𝐿,𝑡𝑜𝑡 [MW h/cycle ] 𝜀 [-] Base Ca se A 900.9 400. 0 439.7 61.1 0.444 Base Case B 855.2 400 .0 393.9 61.3 0.468 Figure 5.4: Exergetic efficiency 𝜀 𝑘 of selected components of the two CES base case systems. 0 , 60 0 0 , 6 5 0 0,7 0 0 0 ,75 0 0,8 0 0 0 ,85 0 0, 90 0 0 ,9 5 0 1 , 00 0 Co mpre ssors Intercoolers Expa nder Mixer MHE1 Throttling valve Storage ta nk Cryogenic pump MHE2 Super -heaters Turbines Heat storage Cold sto rage Base Case A Base Case B 0.60 0. 65 0.70 0.75 0.80 0.85 0.90 0. 95 1.00 exerge tic efficienc y 𝜀 , Chapter 5: Results and discussion 87 The exergy of the fuel 𝐸 , to the Base Case A is 45 MWh hi gher than that o f the Base Case B, d u e t o t h e l o w e r e x e r g e t i c e f f i c i ency o f 44 .5 % in contr ast t o 46.8 %, Table 5.3. The exergy losses 𝐸 , in both systems are relatively low (7 % 𝐸 , ). All exiting streams and the heat vented from the heat s torage after dischar ge are accounted as l osses, while h eat losses and l e a k a g e s o n c o m p o n e n t l e v e l , e . g . , i n t h e c o l d s t o r a g e , a r e c o n sidere d as exergy destructi on. The exergy flow through the b ase case systems over the p eriod o f one cycl e i s p resented Grassmann diagrams for the Base Case A and the Base Case B, in Figure 5.5 and Figure 5 .6, respectively. Figure 5.5: Grassmann diagram of the e xergy flow in the Base Case A system throughout one cycle. The Base Case B system shows higher exergetic efficiencies in m ost comp onents excep t th e expander, the throttl ing valve, a nd the m ixer in the liquefacti on process, Figure 5.4. The h igher compression pressure o f the Base Case B not only improves the t herm odynamic perform ance of t he c ompress ors and the heat exchange rs in the char ging unit (MHE1, IC) but also increases the TIT and thermodynam ic performanc e of the turbi nes and the s uperheaters in the discharge unit and the heat storage. The high ex ergetic effi ciency in the MHE 1 in the Base Case B re sults from th e lower logarithmic mean temperature difference (LMTD), Figure D.1. In the Base Case A, the high- Chapter 5: Results and discussion 88 pressur e air is fed to the MHE1 to reduce the temperature to ap proximately - 40 °C before a fractio n of the m ass flow is fed to the expa nder. In the Base C a s e B , t h e a i r i s s p l i t b e f o r e passi ng the MHE1, and the air enters the expande r at about ambi ent temperature resulting in the lower exergetic efficiency o f the expander. The stream exit ing the expander is mixed with the recurring gas stream from th e flash tank to be fed b ack to the MHE1. In the B ase Case A, the air exits the expander at approximately the same te mperatur e than the recurring stream (- 191°C), almost completely avoiding exergy d estruc tion in the mixi ng process ( 𝜀 , = 99.9 % ). In the expa nsion process of the Base Case B, the air i s cooled down to only - 171°C causing avoidable exergy destruc tion in the mixing process caus ed by the temperature differe nce of the t wo mixi ng str eams. Figure 5.6: Grassmann diagram of t he exergy flow in the Base Case B system throughout one cycle [36] . The cold expander serves two pu r poses in t he liquefaction proce ss. On the one hand, additional low-tem perature exer gy is supplied to the MHE1. On the other ha nd, the exergy of the fu el to the system 𝐸 , is reduced by the electric ity ge nerated in the expa nsion pr oce ss . In both cases, the expander supplies approximately 8 % of the 𝐸 , despite the by 4.4 percentage points lower exergeti c efficiency of th e expand er in the Ba se Case B ( 𝜀 , = 72.5 %). Chapter 5: Resul ts and dis cussion 89 The low-temperatu re exergy recovered from the evapor ating liqui d air i n the MHE 2 (discharge process) and supplied to the lique faction process amounts to a significan t share of the exer gy of the liqui d air of approxim ately 35 % (Fi gure 5.5 and Figure 5.6). The effect o f the heat recove ry is also of ma gnitude t o the perf ormance of t he overall sys tem, am ounting to 37-39 % of the exergy the fina l product of t he system 𝐸 , . The major contributor to the exergy losses are the 61 MWh/cyc le of thermal ex ergy vented from the he at st orage t o the environm ent after the dischar ge pr ocess . Rea son fo r this i s that the heat storage medium must be s upplied to the inter coolers at amb ient tem perature to re duce the work requir ement in the compr ession process. The therm al exergy could be utilized in an additional ORC, as reported in [41] . Figure 5.7: Breakdown of the exergy destruction of the overall system 𝐸 , shown by the exergy destruction rates 𝛾 , ∗ , , for Base Cases A and B. The exergy destructi on breakdown i nto the exergy destruction ra tes 𝛾 , ∗ of selected compone nts of the base cases are depicted in Figure 5.7. In both base case s , t h e c o m p r e s s o r s c a u s e t h e highest share of the total exergy destruction in the system, ap proximately 23-24 %, despite high exerget ic efficiency ( 𝜀 = 9 0 % ) . R e a s o n f o r t h i s i s t h e l a r g e e x e r g y o f t h e f u e l e n t e r ing th e compression process of 850-900 M Wh/cycle. For the Base Case A, the second highest contributi on to the exergy destructio n ori ginates from the M HE1 (14 % 𝐸 , ) followed by the expander (13 % 𝐸 , ) and the throttling v alve (10 % 𝐸 , ). In the Base Case B, the exergy CM 24% MHE1 7% EX, 13% TV , 14% IC, 6 % T, 1 1 % SH, 1 1% CP , 5% MHE2, 4% CS, 4% HS, 2% other , 1% CM, 23% MHE1, 14% EX, 13% TV , 10% IC, 5 % T , 10% SH, 10 % CP , 4% MHE2, 4% CS, 3% HS, 3% o ther , 1 % a) Exer gy destruction in the Base Case A 𝐸 D,tot = 435.62 MWh/cycle b) Exer gy destruction in the Base Case B 𝐸 D,tot = 393.92 MWh/cycle Cha r ge uni t CM the compressors MHE1 the m ain heat exch anger 1 EX the ex pander TV the th rottling v alve IC the interco olers Discha r ge unit T the turbines SH the s uperh eaters CP th e cryogen ic pump MHE2 the main heat exchan ger 2 Storage unit CS the cold storag e HS the he at st orage Chapter 5: Resul ts and dis cussion 90 destruc tion in the MHE1 is less dominant (7 % 𝐸 , ), while the exergy destruction in the throttl ing va lve (14 % 𝐸 , ) is more significant. The higher exer gy destr uction in the TV of the Base Case B is a r e s u l t o f t h e l o w e r e x e r g e t i c efficiency o f 85.7 % ( 𝜀 , = 8 8 . 0 % ) a n d t h e l o w e r 𝐸 , . The abs olute values of the exerg y destruc tion in the com ponents of the discharge unit (e.g., the turbin es, the superheaters, the cryogenic pump, the MHE 2) are smaller in the Base Case B despite the higher e xergy destr uctio n r atio 𝛾 , , Figure D.5 in t he Appendix D. I n b ot h B a s e C a s e s A a n d B , t he highest share of exergy destruc tion is caused in the ch arging process (64-66 %) while the exergy destructi on in the d isc harge system amounts to only 28- 30 % of the exer gy destruction in the ov erall system. The high exergy destruction in the charging process under lines the importanc e of the selecti on of th e liquefaction process to the overal l exer getic effi ciency. The sha re of t he e xergy des tructi on in the storage unit amounts to o n l y 6 % o f 𝐸 , , indicating that CES systems allo w long storage durations at l ow losses. Further results of the exergetic analys is on the compone nt leve l are given in Table D.1, Table D.2 an d Figure D. 5 , in Appendix D. 5.2.2. Economic analys is R e su l t s o f t h e e c o n o mi c a n a l y s i s r e v e a l e d t h at s i m i l a r t o t h e e conomic analysis on the charging system s the TRR of the Base Case A is lower than that of the B a s e C a s e B . T h e T R R f o r t h e base cases amount to 37.3 ˑ 10 6 € and 39.0 ˑ 10 6 €, respectivel y. Moreover, despite the lower exerget ic effi ciency and RTE of th e Base Case A, a lower LCOE dis o f 2 5 5 € / M W h w a s a c h i e v e d , compared to 267 €/MWh in the Ba se Case B. In Figure 5.8, the LCOE dis of both base cases is broken down into the cost associa ted with the levelize d carryin g charges of the char ging, storage and discharg ing uni ts as wel l as the levelized O&M c ost s a nd the le velized f uel cos ts. Figure 5.8: Cost breakdown of the LCOE dis into levelized carrying charges (CC), lev elized fuel costs (FC) and levelized operation and maintenance costs (OMC) for the two base case systems. Figure 5.8 shows that the l evel ized CC represen t around 53.3 % and 54.3 % of the LCOE dis of the Base Cases A and B . The costs of the com ponents in the char ging system dom in ate in both base case systems, while the cos ts assoc iated wi th the storage unit are rather insignific ant. - 50 100 150 200 250 300 Bas e C ase B Base Case A levelized c osts, [€/MW h] charging u nit storage unit discharge unit Levelized OMC Levelized FC 59 56 64 60 80 81 5 5 59 51 L evelized CC Chapter 5: Resul ts and dis cussion 91 The fuel to the systems refers t o th e electrici ty charged durin g the liquefac tion process. Varying the price of e lectricity in sensitivity a nalysis showed that th e LCOE dis of the Base Case A is smaller than that of t he Base Case B for electricity prices sma ller than 100 €/M Wh. For hig her electricity prices, the Base Cas e B becom es more econom ically f easible t han the Base Case A. The distribution of the bare modu l e c o s t s ( B M C ) o f t he m a i n s y s tem components is sh own in Figure 5.9. The heat exchangers a re responsible fo r 62-64 % of the investment costs in both Base Case system s. In the Base Case B, the two main heat exchan gers al one m ake up f or 51 % of the total compone nt costs. The MHE 1 is the major reason for the cos t differenc e betwee n both systems being twice as expensive in the Base Case B than i n the Base C ase A. The intercoole rs, in contrast, are less costly in the Base Case B. The costs of all other co mponents are similar for both base case system s. The s pecific investment cos ts am ount to 2,089 €/kW a nd 1,942 €/kW of instal led disch arge capac ity whi ch is wi thin the range for cost estimates suggested by [16 , 18, 33, 64, 72]. Figure 5.9: Bare module costs of the Base Cases A and B with indicated cost shares of selected components. 5.2.3. Exergoeconomic analysis and optimization In the exergoec onomic anal ysis, the cost rate associated with t he exergy of the fuel 𝐶 , , the exergy of the product 𝐶 , and the ther modynam ic ineff iciencies – the exe rgy destruction 𝐶 , , and the exergy losses 𝐶 , – a re ob t ai ne d. Th e r es ul ts o f the exergoeconom ic analysis of the two base case sys tems are given in Table 5.4. The relati ve cost dif ference 𝑟 b e t w e e n the specific costs pe r unit of exergy of the product and the ex ergy of the fuel is large. T he c ost per exer gy unit of the produc t of the system ( LCOE dis ) is appr oximately ten times as high as the cost per exe rgy unit of the fue l to the system. The exergoecono mic factor 𝑓 is elevated ( ≫ 0.5) which indicates that the cost rate associated with the in itial investment and the OMC outsizes the costs asso ciated with the exer gy destruction in th e syst em. 0 20000 40000 60000 80000 100000 120000 140000 Base Case A Base Case B other EX CP RH CM MHE1 IC T MHE2 123.4 Mio € 1 15.2 Mio € 120 20 40 140 60 80 100 0 bare module costs [10 6 €] other components the expander the cry ogeni c pump th e re-/ su per-heat ers th e com pre sso rs the main heat exchanger 1 the intercoolers the turbines the main heat exchanger 2 Chapter 5: Resul ts and dis cussion 92 Table 5.4: Results obtained in e xergoeconomic analysis for the two base case syste ms. 𝑍 𝐶 , 𝐶 , 𝐶 , 𝐶 , 𝑐 , 𝑟 𝑓 [ €/cy cle] [ €/cy cle] [ €/cy cle] [ €/cy cle] [ €/cy cle] [ €/MW h] [-] [-] Base Case A 7 8.513 23,422 102 ,197 11,433 1,590 255 8.8 0.814 Base Cas e B 84.3 17 22,235 106,803 10,242 1,593 267 9.3 0.8 28 With the aim of identifying m eans to reduce the co st of th e fin al product, the components with the highest associated total cost rate 𝑍 𝐶 , are prioritized in exer goeconomic analysis and optimization. Five sets of components: the main heat exchangers , the turbines , the intercoolers , the co mp ressors and the reheater s , m a k e u p m o r e t h a n 8 0 % o f t h e s u m o f c o m p o n e n t c o s t r a t e s in both bas e case systems. In Figure 5.10, the component co s t r a te s of t he f iv e p r i or i t i z e d components are display ed. For the majority of the components, the costs associated with the i nitial investment and OMC 𝑍 dominate, which is al so indicated in an e levated exergoeconom ic factor 𝑓 . Figure 5.10: Exergoeconomic analysis results for the five components with the hi ghest total cost rate 𝑍 𝐶 , . O n l y i n t h e M H E 1 a n d t h e r e h e a t e rs o f the Base Case A the costs associate d with the exergy destr uction in the com ponents 𝐶 , cause more than 50 % of the total cos t rate of the res pecti ve c o m p o n e n t . F o r t u r b o m a c h i n e r y s u c h a s t h e t u r b i n e s a n d t h e c o m p ressors an elevate d exergoeconomic factor e .g., 0 .6-0.8 is to b e expected, while in heat exc hangers the costs associated with the exergy des truction commonl y takes over ( 𝑓 < 0.5). - 10.000 20.000 30.000 Base Case B Main HE2 - B a se Ca se A Base Case B Main HE1 - B a se Ca se A Base Case B Turbines - Base Case A Base Case B Intercool ers - Base Case A Base Case B Compressors - Base C ase A Base Case B Re-heaters - B ase Case A Ba se Case B 𝑓 = 0 . 41 𝑓 = 0.48 𝑓 = 0.55 𝑓 = 0.71 𝑓 = 0.71 𝑓 = 0 .94 𝑓 = 0.93 𝑓 = 0.66 𝑓 = 0.65 𝑓 = 0.46 𝑓 = 0.76 𝑓 = 0.90 𝑓 = 0.91 𝑍 [€/cy cle] 𝐶 , [€/c y cle] cost rat es [€/ cy cl e] Chapter 5: Resul ts and dis cussion 93 The objective for the priorit ized c o m p o n e n t s i s t o d e c r e a s e t h e com ponent cost rate 𝑍 w h i l e accepting a lower t herm odynamic perform ance of the respective c omponent in order to reduce the LCOE dis . The decision variables that we r e s e l e c t e d t o r e d u c e t h e i n i t i al inves tment cost of the respective component are given in Table 5.5. W i t h t h e ex ce p t io n o f t h e M H E 1 i n the Base Case A, the same cha nges to t he selecte d decis ion varia bles were applie d to bot h sy stem s. The ite rati on in w hich the par ameters w ere chang ed is given for both systems. The turbi nes and the com pressors were not listed despite being the third and 5 th most cost-inte nse com ponents. R eason for this is that the redu ction i n component costs through param etri c changes is not applicable. The investm ent costs of the turbomachinery are determined by the power capac ity. The isentr opic efficiency of the purchas ed m achinery is fixed. Reducing costs at the expense o f performance is theref ore practicall y not possible in that sense unless components are purchased secondhand. For the com pr essors, the component cost rate can only be red uced w hen th e RTE i s i n cr ea se d , a nd l es s c o mp re ss io n ca p ac it y is r e qu ir e d. The capacity of the turbines i s fixed by the boundary condition s of a fixed dis charge capacity for all syst em s of 100 MW. Table 5.5: Decision variables for exergoeconomic optim ization o f selected components. Compo nent Decis ion vari able P arametr ic c hange Bas e Cas e A Base Case B MHE1 𝐿𝑀𝑇𝐷 ↑ 𝑝 ↑ - 1 st itera tion MHE2 𝐿𝑀𝑇𝐷 ↑ 𝑚 , 𝑚 ↓ 2 nd iter ation 2 nd iter ation Inte rcool ers 𝐿𝑀𝑇𝐷 ↑ 𝑚 ↑ 3 r d iteratio n 3 r d iteratio n Reheat ers 𝐿𝑀𝑇𝐷 ↑ 𝑇 , ↓ 4 th iterati on 4 th iter ation The investm ent costs o f the heat exchangers are a function of t he heat exchanger design, working fluid pr operties (e.g., pressure , temperature, heat cap acity), materials and heat exchanger area 𝐴 . For cost reduction in t he heat exchangers, the 𝐴 can be reduced by increasing the LMTD. Four dis tinct iterations were applied: 1 st iteration : To reduce the costs and increase the LMTD between the cumulati ve curves of the MHE1 of the Base Case B, the compression pressure was g r ad ually increased. The splitting ratio was k ept to its minimum. 2 nd iteration : The LMTD in the MHE2 is increase d by reducing the mass flow o f the cold storage med ia ( 𝑚 ↓ , 𝑚 ↓ ). 3 rd iteration: By incre asing t he mass f low of the heat s torage m edia 𝑚 the L MTD of the intercoolers is i ncreased, decreasing the heat exchanger ar ea 𝐴 and 𝑍 . 4 th iteration: For cost redu ction in the reh eaters, a lower exit t emperatur e 𝑇 , ( o r TIT) is accepted to in crease the logarithm ic m ean temperature d ifference. The parametric change and the e ffect of all performed optimization steps on the TRR, the LCOE dis, and the exerge tic efficie ncy are g ive n in Table 5.6 and Table 5.7. The effect o f the param etric changes on the RTE and LCOE dis a r e i n d i c a t e d i n F i g u r e 5 .12 and Figure 5.13 for the Base Cases A and B, r espe ctively. Chapter 5: Resul ts and dis cussion 94 Table 5.6: Parameters and results of the optimization steps for the Case A. Param eter Unit Base Case A 1 st 2 nd 3 rd iteration 𝑝 bar 85 85 85 85 𝑚 𝑚 ⁄ - 2.28 0 2.08 3 2.08 3 2.083 𝑚 𝑚 ⁄ - 0.450 0 .414 0 .414 0 .414 ∆𝑇 K 2 2 3 3 ∆𝑇 K 2 2 2 7 TRR 10 6 € 37.3 32.3 30.7 30.4 LCOE dis €/MWh 255 221 210 208 RTE - 0.444 0.397 0.386 0.381 Table 5.7: Parameters and results of the optimization steps for the Case B. Param eter Unit Base Case B 1 st 2 n d 3 r d 4 th iteration 𝑝 bar 120 180 180 180 180 𝑚 𝑚 ⁄ - 2.28 0 2.28 0 2 .000 2.000 2.000 𝑚 𝑚 ⁄ - 0.450 0.450 0.40 0 0.400 0.400 ∆𝑇 K 2 2 2 3 3 ∆𝑇 K 2 2 2 2 6 TRR 10 6 € 39.0 33.6 28.8 28.6 28. 5 LCOE dis €/MWh 267 230 197 196 195 RTE - 0.468 0.449 0.415 0.399 0.394 In the 1 st iteration of the exergoeconomic optimization the pressure 𝑝 of th e comp ression ratio of the compressors in the Base Case B, was gradually incr eased to reach a higher absolute pressure in the liquefaction p rocess. The component cost rates of the Base Case B (120 bar) and the cases with increased p ressure are shown in Figure 5.11. The a i m e d r e d u c t i o n i n t h e investment costs of the MHE1 and the intercoolers was achieved while t he compone nt cost rate of the MHE2 onl y decreased sli ghtly. The iteration was stopped at 180 bar to avoid more costl y equipm ent and m aterials of subsequent co mponents. The cost of the M HE1 was reduced by almost 7 0 % and the exergoe conomic factor of previously 0.75 was reduced to 0. 42, due to the i ncrease in LMT D from 3 K to 9 K. The increased co mpression ratio also lead s to a 60 K higher temp era tu re at th e outlet o f the compress ors 𝑇 that can be recovere d in heat stora ge. The turbine inlet tempe rature TIT increases, improving the ex ergeti c efficien cy 𝜀 and reducing the cost rate asso ciated with th e exergy destruction in the tu rbines 𝐶 , . The temperature level in the reheaters rises, which at constant mass flow would decreas e their heat exchanger area. Ho wever , a t incr eased 𝑝 the RTE drops and a highe r mass flow in the discharge unit becomes n ec essary, retaining the co st rate of the reheaters. By increasing 𝑝 to 180 bar, the LCOE dis o f t h e B a s e C a s e B w a s significantly red uced from 267 €/MWh to 230 €/MWh. Chapter 5: Resul ts and dis cussion 95 Figure 5.11: Component cost rates 𝑍 of selected co mponents of the Base Case B (with increased compression pressure from 120 to 180 bar). The 2 nd iteration that is applied to bot h base case system s entails th e reduction of 𝑍 . The reduction of the mass fl ow rate of the cold storage working flu i d r e d u c e s t h e a m o u n t o f l o w - temperature exergy that is suppli e d t o t h e M H E 1 . A t a c o n s t a n t splitting ratio, this leads to a reduction in the LMTD and increa ses co sts of th e MHE1. Therefor e, the splitting ratio 𝑟 nee ds to be increased to meet t he min imum temp erature difference ∆𝑇 , = 1 K a n d a v o i d a r i s e in the inves tm ent costs of the MHE1. When the s plit ting rati o 𝑟 is inc reased, the l iqui d yield 𝛾 is redu ced and the exerget ic efficiency 𝜀 of the liquefaction process and the overall system is reduced. The mass flow rates 𝑚 and 𝑚 a r e r e d u c e d u n t i l t h e e x e r g e t i c e f f i c i e n c y o f t h e charging systems drops so low, that the additional investment i n lar ger equipment in the chargi ng unit exceeds the cost r educti on in the M HE2. For the B ase Case A system, the LCOE dis wa s min i miz ed at 𝑚 /𝑚 = 2.083 and 𝑚 /𝑚 = 0.414. The ratios reached lower values in Base Case B, Table 5.7. The LCOE dis of both system s is reduced by 13-14 % to 221 €/MWh and 197 €/MWh . A s th e spl itting ratio n eed ed to be increased from 0.39 to 0.46 ( 0.31 to 0.33), th e liquid yield of the system dec reases from 0.501 t o 0.445 (0.559 t o 0 . 5 3 3 ) i n t h e B a s e C a s e A ( B ) a n d t h e e x e r g e t i c e f f i c i e n c y 𝜀 ( RTE ) drops accordingly, Figure 5.12 and Figure 5.13. T he reduction in performanc e i n t h e Base Case A is more significa nt (10. 6 %) tha n in the Base Case B (7.6 % ). By increasing the mass flow of the heat storage media 𝑚 in th e 3 rd iteration , th e temp erature diff erenc e withi n the i nterc oolers ∆𝑇 is inc rease d to a m axim um of 3 K. I ncreasi ng t he 𝑚 further in creases the LMTD while reducing the LCOE dis further. With an in creased mass flow of the heat transfer media, the TIT decreases, reducing the exe rgetic e fficien cy of the tu rbine and the reheaters and according ly, th e specific power output of t h e d i s c h a r g e u n i t . Conseq uently, a minim um LCOE dis is reached when the additional investm ent in large r equipm ent and fuel costs outweighs the cost reducti on in the in terc oolers. The LCOE dis i s minim ized at a LMTD of the interc oolers of ap proxim ately 7 K (9 K) for Base Case A (B). 0 50 00 10 000 15 000 20 000 25 000 CM I C EX MHE1 ST CP MHE2 RH T 120 bar 130 bar 140 bar 15 0 ba r 16 0 ba r 170 bar 180 bar 5,000 10,000 15,000 20,000 25,000 𝑍 , [€/cycle] CM the com pressors IC the intercoolers EX the expander MHE1 the m ain HE 1 ST the storage tank CP the cryogenic pum p MHE2 the main HE 2 RH the reh eater s T the turbine s Chapter 5: Resul ts and dis cussion 96 The effect of cost red uction when increasi ng 𝑚 is more significant in the Base Case A. Nevert heless , Base Case B achieves a lower LCOE dis at considerably higher RTE , Figure D.6 in Appendix D. Figure 5.12: RTE over LCOE dis for the optimization steps perf ormed on the Base Case A with an indication of the changes applied to the d ecision variables in the respective iteration. Figure 5.13: RTE over LCOE dis for the optimization steps perf ormed on the Base Case B with an indication of the changes applied to the d ecision variables in the respective iteration. In the final iteration perfo rmed on both systems, th e ou tlet tempe rature o f the rehea ters was reduced res ulting i n a higher temperature difference ∆𝑇 betw e en the he at st orage m edi a an d the TIT (2 K → 10 K). Th e minimum LCOE dis o f the base cases was a chieve d at 6-7 K, Table 5.6 and Tabl e 5.7 – 4 th iteration. The reduction in RTE in both cases is comp arable, 14 .1 % a nd 15.4 % for the Optimized Cases A and B , respectively. However, the cost reducti on is much more drastic in the Optim ized Case B. 34 36 38 40 42 44 204 214 224 234 244 25 4 R TE, [%] LC O E dis , €/MWh 2nd itera t io n 3rd it erat ion 4th iteration m ↓ , m ↓ T , ↓ m ↑ Base Case A p ↑ m ↓ , m ↓ T , ↓ 37 39 41 43 45 47 185 195 205 215 225 235 245 255 265 R TE, [ %] LC O E dis , €/MWh 1s t it era ti on 2nd itera tion 3rd iteration 4th iteration m ↑ Base Case B Chapter 5: Resul ts and dis cussion 97 The effect of the parametric changes in the iterative exergoeco nomic optimization on the RTE and the LCOE dis is depict ed in F igure 5. 12 an d Figur e 5.13. In each of the ite ration, a sensitivity analys is of the RTE and the LCOE dis while gradually changing the respective decision variable was conducted. The operation ste ps are p erformed successively . The param etric changes o f the subseq uent iteration were app lied to the “best case” (lowest LCOE dis ) of the res pectiv e iteration. The reduction of the cost of the final product in the system is more significa n t when adopting changes to the c omponents with h igher cost-im porta nce (higher 𝑍 𝐶 , ). When the liquefaction pressure or the mass flow o f th e co ld storag e medi um is gradually change d to reduce the compone nt cost rates of the MHE1 and MHE 2, the reduc tio n in the LCOE dis i s m u c h more noticeable than when changing the parameters of the less c ost- intensive co mponents such as the in tercoolers and the reh eaters. For this reason, the cos t redu ction when applying furth er changes is expected to be insignificant. In Figure 5.14, the sum of the compone nt cost rates normali zed to the unit of p roduct exergy ( ∑ 𝑍 𝐸 , is given over the sum of exergy destructi on and exergy losses per unit of product exergy ( 𝐸 , 𝐸 , 𝐸 , 1 𝜀 𝜀 ⁄ ) fo r the intermediate results of the exergoeconomic optimization step s performed on the base cases. For both cases, the levelize d costs are reduced at the expense o f a r e d u c t i o n i n e x e r g e t i c e f ficiency of the systems. Despite lower levelized investment costs and LCOE dis of the B ase Case A, the Optimized Case B achieves lower leve lized costs and the sum of exergy destruc tio n and losses per unit of product exergy throughout th e iterative op timization. This underlines a gain the im porta nce of exer gy- based m ethods w hen evaluati ng the system s in rega rds to cost -ef fectiveness. Figure 5.14: Normalized capital investment cost (per unit of exergy of product) for the Cases A and B during the iterative exergoeconomic optimiza tion over the relative exergetic efficiency. 130 150 170 190 210 230 250 1,0 1 , 2 1, 4 1 , 6 1, 8 2 , 0 Ca se A Ca se B Base Case B Optim ized Case B Optim ized Case A Base Case A 1.8 1.6 1.4 1.2 1.0 2.0 ∑ Z E , , €/MWh E , E , E , 1 𝜀 𝜀 ⁄ , - Chapter 5: Resul ts and dis cussion 98 The exergoeconomic analysis resu l t s o n t h e c o m p o n e n t l e v e l o f t he optim ized cases a re compared to those of the base cas es in Figure 5 .15. The objecti ve of reducing the costs associated with the initial investment 𝑍 w as ac hie ve d f or a l l of th e s el e c te d c om p on e nt s. Th e met objective is indicated in t he reduction of the exergoeconom ic factor. Figure 5.15: Exergoeconomic analysis results for the base cases and the optimized cases. Due to the cos t re ductio n i n the overall s ystem , the spec ific c ost of t he e xergy of the fuel to the turbines 𝑐 was r educed. Con sequently , the co sts associ ated with the exe rg y destruction in the turbines 𝐶 , were decreased. Nevertheless, a f ter the exe rgoec onomic optim iz ation is complete d, the turbines becom e the com ponents with the highes t associated total cos t rate. - 10.000 20.000 30.000 Opt. Case B Base Case B Opt. C ase A Main HE2 - Ba se Case A Opt. Case B Base Case B Opt. C ase A Main HE1 - Ba se Case A Opt. Case B Base Case B Opt. C ase A Turbines - Base Case A Opt. Case B Base Case B Opt. C ase A Intercoolers - Base Case A Opt. Case B Base Case B Opt. C ase A Compresso rs - Base Ca se A Opt. Case B Base Case B Opt. C ase A Re-h eate rs - Base Case A Opt. Case B Base Case B Opt. C ase A Other components - Base Case A cost rate assoc iated with the compone nt, €/cy cle 𝑓 = 0. 43 𝑓 = 0.41 𝑓 = 0. 48 𝑓 = 0.55 𝑓 = 0.71 𝑓 = 0.71 𝑓 = 0.94 𝑓 = 0. 93 𝑓 = 0.66 𝑓 = 0.65 𝑓 = 0.46 𝑓 = 0.76 𝑓 = 0.90 𝑓 = 0. 91 𝑓 = 0.46 𝑓 = 0.46 𝑓 = 0.43 𝑓 = 0.40 𝑓 = 0.71 𝑓 = 0.72 𝑓 = 0.85 𝑓 = 0.79 𝑓 = 0. 72 𝑓 = 0.75 𝑓 = 0.37 𝑓 = 0.60 𝑓 = 0.65 𝑓 = 0.73 𝑍 [€/cy cle ] 𝐶 , [€/cy cle] Chapter 5: Resul ts and dis cussion 99 In the optim ized cases, t he BMC of t he tur bines am ount to a ppro xim ately 30% of the BM C of the total system s. A cost re duc tion of the tur bines coul d thus reduce the LCOE dis considerably. To compe nsate for the reduction in RTE in the optimized cases, th e compression power needs to be augm ent ed, w hich res ult s in a n increase d tot al cost ra te of the com pressors. T he cha nge s made to the parameters o f the Ba se Case A, reduce the exergoeco nomic factor of the MHE1 further. In both systems, the interrelat ion of the com ponents is very st rong. An examp le o f this i s the intercoole rs. Despite the exer goeconom ic factor suggesting a fu rther reduction in investm ent costs at the exp ense of exerg etic effi ciency , any furth er reduc tion in 𝜀 w o u l d c a u s e t h e c o s t of the final product to increase. Reason for this is, t hat in o rder to increase th e LMTD in the IC, either the temperature at the outlet of the IC would need to be raised or the mass flow in the h e a t s t o r a g e w o u l d n e e d t o b e i n creased which would reduce the TIT. In both cases, the work requirem ent of the com pression process would be augmented and t he additional investm ent costs for larger compressors woul d outwe igh the reduct ion in co sts of the intercooler s. The objective of cost reduc tion of the costs associat ed with th e reheaters was also achieved. Moreover, the sum of the total c ost rates of all othe r system c om ponents was reduced. The slight increase in the exergoeconomic f actor o f the “other com ponents” to 0.46 indicated that the cos t reduction mainly results fro m reduced costs asso ciated wi th the exergy destruction in the componen ts. A s the comp onen t cost rat es of th e majo rity o f cost -intense com ponents are reduce d, the specific cost per unit of exer gy of the fuel to th e subsequent components is reduced, decreasing 𝐶 , . Chapter 5: Resul ts and dis cussion 100 5.3. Exergy-based evaluation of C ES system integration In this secti on, the r esults from the exergy-b ased evaluation o f sev eral C ES sy ste m configur atio ns are presented, and the enhancement in the thermo dynamic and economic perform ance of CES system s with integrati on of heat sour ces and sinks is quantified. The integration of re-gasifying LNG was considered as a heat sink f o r C ES s y s te ms ins te a d of an d in com bina tion with cold stora ge. Waste h eat at 350-450 °C and natural gas combustion were considered as heat sources. 5.3.1. Comparative energet ic and exergetic analysis Two adiabatic CES sy stem s and ei ght sy stems with integration of external heat and/or cold sources were compared u si ng energe tic and e xergetic anal ysis on t he sys tem level. T he exe rgy of t he f uel 𝐸 , the exergy o f the product 𝐸 and the exergeti c effici ency 𝜀 of sel ected systems are represented in Figure 5.16. The exergy values are given in MWh over the duration of one daily cycle ( 𝜏 = 8 h, 𝜏 = 4 h). The mass flow rate of the inlet air was s et to 200 kg/s fo r a l l system s. As a result, t he compres sion wor k 𝑊 of 1 ,105 MWh is equal for all systems and is indicated in the fig ure for com parison. Figure 5.16: Exergetic analysis results for th e integrated CES system configurations. The produ ct of all sy stems is disch arged electrici ty. The fuel to the system s is the sum of the electricity charged ( 𝑊 𝑊 ) and the low-temperature exergy 𝐸 , (for LNG integration) and/or t he exe rgy of the s upplied f uel 𝐸 (for com bustion) and/ or the high-te mperatur e exe rgy 𝐸 , (fo r waste h eat integra tion), Ta ble B.1. The results show that the integration of waste heat does not have a positive effect on the exergetic efficienc y d e s p i t e t h e d e s i r e d i n c r e a s e i n the specific power output of the discharge unit. The exergy con tent of the heat that needs to be supplied in order to elevate the TIT to 350 °C and 450 °C, r esp ectively, is m ore significant than the in crease in the dis charge po wer. 24 39 33 29 26 47 44 38 - 10 20 30 40 50 0 500 1000 1500 2000 2500 Adiabti c w o cold st o ra ge Adiaba tic sys t em W a s t e h e at 35 0 Wa st e hea t 45 0 LNG wi t hou t CS LNG with C S C om bus ti on C o m bus ti on V2 E_F E_ P Eff 𝜀 % exergy rate, 𝐸 𝑀𝑊ℎ/𝑐𝑦𝑐𝑙𝑒 exer getic efficiency 𝜀 , % simple 350 ° C 450 ° C LNG integration Combus tion sim ple with CS W aste heat Adiabatic with CS single double 𝑊 𝑀𝑊ℎ/𝑐𝑦𝑐𝑙 𝑒 𝐸 𝑀𝑊ℎ/𝑐𝑦𝑐𝑙 𝑒 𝐸 𝑀𝑊ℎ/𝑐𝑦𝑐𝑙𝑒 𝑊 𝑀𝑊ℎ /𝑐𝑦𝑐𝑙 𝑒 Chapter 5: Resul ts and dis cussion 101 The el ectricity charged to the LN G integr ated systems is slight ly higher than in the adiab atic CES systems. Due to t he additi ona l low-temperature exergy, less a i r n e e d s t o b e f e d t o t h e c o l d expander in the liquefaction pro cess, and the electricity gener at ed in th e exp ander is redu ced ( 𝑊 ↓ ). T he LNG i ntegra ted sys tem with CS re aches the highest 𝜀 of 47 %. Reason f or this is that the cont ribution of the low-tempe rature exergy 𝐸 , to the exergy of the fuel 𝐸 in t h e sys te m s wi th L NG int eg rat ion is re la ti ve ly s m all in c om par is on t o its s ig nif ic ant im pr ove m ent of the yield of the li quefaction process. The roundtrip, energetic, and ex ergetic efficiency of the ten e v a l u a t e d s y s t e m s a r e g i v e n i n Table 5.8. The RTE gives the ratio between the electricity disch arged and th e ele ctricity charged 𝑊 𝑊 ⁄ ).The values of the RTE , conse quentl y, m ay e xcee d 1. The va lues of the RTE , the energetic and the exerge tic efficien cy of the a diabatic sys tems are the same as no ex ternal h eat source or sink, needs t o be accounted for in the energetic or exergetic evaluatio n (Table B. 1). Table 5.8: Roundtrip, energetic, and exergetic efficiency of th e ten considered system configurations. System Abbreviati on RTE 𝜂 𝜀 Adiabatic CES without c old storage a-CES w/o CS 0.240 0.240 0.2 40 Adiab atic CES w ith c old storage a-CES 0.394 0.394 0.394 Waste h eat in tegra tion w ith TIT o f 350 °C W H 350 0.535 0.234 0. 332 Waste h eat in tegra tion w ith TIT o f 450 °C W H 450 0.625 0.194 0. 291 Diabatic CES wit h a singl e combusti on chambe r d-CES 0.845 0.448 0.438 Diabati c CES with two combustion chambers d- CES 2 0.732 0.388 0 .380 LNG inte gration withou t cold sto rage LNG w /o CS 0.260 0.251 0.2 54 LNG integration with col d storag e LNG a-CES 0.506 0.452 0.470 LNG integration and waste heat LNG + WH 0.804 0.253 0.376 LNG integrati on and comb ustion LNG d- CES 1 .084 0.480 0.47 7 The adiabatic CES syste m without cold storage (a-CES w/o CS) re aches efficiencies by 40 % l o w e r t h a n t h e a - C E S s y s t e m . W i t h t h e i n t e g r a t i o n o f L N G , t h e RTE of the a-CES w/o CS increases by only 2 % to 26 % and the 𝜀 increase s by 1.4 %. Even when the mass flow of the LNG is increased, the RTE does not increase any fu rther due to the fixed ∆𝑇 , , while the 𝜀 is reduced (Figure 4.11). In con t r a s t , t h e a d d i t i o n o f c o l d s t orag e in creases the RTE by 64 %. Reason for this is, that the LNG enters the MHE1 at a tem perature of - 158 °C while the high-gr ade co ld s torage suppl ies low-te mperatu re exergy at temp eratures as low as - 180 °C. As a result, the int egration of re-gasifying LNG cannot replace the cold storage. Integrating waste heat to the a- CES sy stem was found favorable for the RTE , incr easing fro m 39.4 % to 62.5 %, whil e the energetic and t he exergetic efficie nc y of the system s drop by 50 % and 26 %, respectively. The low er energetic and exergetic effic ienc y can be explained by the excess heat vented after the expansi on process. With waste heat integration, the amount and temperature; and accordingl y, the exer gy conte nt of the vented he at is elev ated c ausing the exergy losses of the system to increase. The integrat ion of a combustion process augments all three effi ciencies. The RTE is doubled and the energetic and exergetic efficiency increase by 5.4 and 4.4 percentag e points, respectively. The valu es achieved for the RTE of 73 -85 % are significa ntly higher than values stated in l iterature (55-60 % [ 15, 43]), which can be explai ned by a larger mass flow rate of the Chapter 5: Resul ts and dis cussion 102 fuel, see Figure C.3. The effi ciency of the double combustion c onfiguration is slightly lower than the single combustion confi guration (d-CES 1 ). Due to decr eased TIT, t he RTE , ener getic and ex ergetic efficien cy drop by approximately 15 % at the same total mass flow rate of the fuel 𝑚 . The introducti on of two smaller sized separa te combus tion cha m bers is expected to lower the specific costs du e to lower maxim um TIT. The integration of LNG to the a-CES system increases the RTE by 28 % and the 𝜀 by 6 percent age points. Re-gasif ying LNG is thus a therm odynam ically feasib le low- temperature exergy sou rce when cold storage is p resent. The sys tem s with simple integration o f l i q u e f i e d n a t u r a l g a s ( L N G , L N G w / o C S ) r e a c h e x e r g e t i c e f f i ciencies of 25-47 %, similar to the v alue range proposed in litera ture of 33-43 % [62, 63, 6 4]. The RTE , in contrast, are lower than antic ipated, instea d of reaching expected values of 63-70 % [8, 62, 63] the RTE o f the LNG system reaches only 51 %. With additional waste heat in te gration, the sys tem reaches a RTE of 84 %. While the CES sy stem with integration of LNG and comb ustion achie ves the highest exergetic efficiency of 47.7 % and the highest RTE of 108 % under the gi ven conditions. 5.3.2. Comparativ e economic analysis The effect of the integration of h e a t s o u r c e s a n d s i n k s o n t h e b a r e m o d u l e c o s t s ( B M C ) o f selected components of the system s can be seen in Figure 5.17. All systems op erate with th e same mass flow of inlet air and liquefac tion pressur e. The BMC of the com press ors are therefore consta nt for a ll consi dered systems. Figure 5.17: Bare module costs of the components of the integrated systems (C M – compressors, IC – intercoolers, EX – expander, MHE – main heat exchan ger, ST – storage tank, CP – cryogen pump, RH – reheaters, CC – combustion chamber(s), T – turbines, HS – heat storage, CS – cold storage and other components). The costs associ ated with the int ercoolers and the reh eaters ar e reduced in the systems with waste heat int egration a s thermal oil is used as heat transf er and s torage medium and a smaller heat exchanger area is necessary. The cost of heat storage incr eases accordingly. 0 10.0 0 0 20.0 0 0 30.0 0 0 40.0 0 0 50.0 0 0 60.0 0 0 70.0 0 0 CM IC EX MHE1 ST CP MHE2 RH CC T other a-CES w/o CS a-CES WH 350 WH 450 LN G w/o C S LNG LNG + WH LNG d-CES d-CES d-CE S 2 50 40 30 20 10 0 60 70 bare m odule cost s [10 3 €] Chapter 5: Resul ts and dis cussion 103 For the systems that do not cont ain cold recovery and storage, environm ental air is used as a heat sour ce fo r th e ev aporation of t he l iquid ai r (d ischarge) . The LMTD of the MHE2 is increased significantly , reducing the compon e n t s B M C t o o n e - t h i rd of th at o f th e a-C ES system. The LMTD of the MHE1, in contrast, decreases without the low-te mperature exergy supplie d from cold storage, increasi ng th e needed h eat exchanger area an d costs by 40% while a significantly smaller share of air is liquefied. Due to the red uced liquid yield, the costs of all components in the disc harge unit are reduced in the absence of c o l d s t o r a g e i n c o m p a r i s o n t o all oth er CES sy stems. With t he int egratio n of L NG to t he MHE1 of t he CES system , the 𝐴 a n d t h e B M C i n c r e a s e t o m o r e t h a n t w i c e w i t h r e s p e c t t o t h e a - C E S s y s t e m . A s t h e a m o unt of air liquefied is increas ed by up to 40%, the investm ent cost of the components in the disc harge unit increase alongside. In order to account for both the effect on the economic and the thermodynam ic performance, the levelized cos ts were calc ulate d in com parative economic ana lysis. The TCI, the spec ific investment costs 𝑐 , and the LCOE dis of th e two adiabatic and eigh t integrated CES system configurat ions are given in Table 5.9. T h e assumptions made in the analys is are d escrib ed in secti on 3.2. The economic analys is wa s performed on two system designs: a) a constant mass flow rate of t he inlet air of 200 kg/s, and b) a discharge capac ity of 100 MW. The discharge capacitie s are given for the case a), as a refere n c e . T h e a d i a b a t i c C E S s y s t e m with cold storage ( a-CES) reach es a discharg e cap acity o f 100 M W at a mass flow at the inlet of the compression process of approxim ately 200 kg/s. Some syst ems of the system s reach dischar ge capacities of two to three times h igher than the a-CE S s y s t e m w h i c h r e s u l t s i n l o w specific inves tment cos ts 𝑐 and LCOE dis . Table 5.9: Results of economic analysis for the ten CES system configurations. System 𝑚 = 200 kg/s 𝑊 = 100 MW TCI, 10 6 € 𝑐 , €/kW LCOE, €/MWh 𝑊 , MW 𝑐 , €/kW LCOE, €/MWh a-CES w/o CS 61.2 1,835 295 56 1,719 283 a-CES 75.4 1, 269 195 100 1,269 195 WH 3 50 80.1 993 1 50 136 1 ,040 154 WH 4 50 83.4 884 1 31 159 9 50 138 d-CES 105 .7 830 1 28 214 9 77 140 d-CES 2 93.0 8 43 136 186 954 143 LNG w/o CS 65.4 1,735 274 63 1,647 265 LNG a -CES 104.1 1,278 181 137 1,32 3 186 LNG + WH 112.3 869 121 218 966 130 LNG d-CES 134.1 77 0 1 15 293 95 9 1 31 The specific investm ent costs 𝑐 are determined by the total capital investment (TCI) of the syst em ov er th e in stalled discha rge capacity. The specific co st s o f C ES s y st em s de c r e as e w it h s i z e . T h e s c a l i n g f a c t o r f o r t h e s y s t e m s w a s f o u n d t o b e b e t w e e n 0.81-0.92, which refers to a decrease i n cos ts by 10-25 % w hen the capac ity of the sys tem is doubled. Chapter 5: Resul ts and dis cussion 104 The in tegra tion of cold stora ge reduc ed the LCOE dis by 32-34 % despite an additional investment of approximately € 15 million. The integration of w a ste heat reduces the LCOE dis furthe r, by 21-30 % compare d to the a-CES. The CES systems with n atur al gas firing reached the lowest s pecif ic i nvestm ent costs 𝑐 whe n t he m ass flow is ke pt consiste nt and f o r an i nstalle d dischar ge capacity of 100 MW. The lowest LCOE dis at th e g iven assu mp tion was achieved by the C ES syste m w ith both w aste h eat and waste c old integration of 130 €/MWh, followed by t h e d i a b a t i c C E S s y s t e m s a n d t h e C E S s y s t e m w i t h w a s t e h e a t i n t egration at 450 °C of 131- 143 €/MWh. The addition al costs assoc iated with the waste heat, combust ion a nd/or waste cold integration are rather low r elative to the increase in RTE , resulting in a sign ificantly lower LCOE dis of the respective systems. Despite the sim ilar thermodynam ic pe rforma nce of t he tw o d-CE S systems, t he lev elized cost s of the d-CE S 2 with t wo com bustion chambers a re lower. Re ason f or t his is the tem peratur e at the outlet of the combustion cha mbers. The material of the TIT is selected according to this temperature. To achieve the s ame power capacit y in the single c om bustion d- CES s ystem than in the double combustion d-CES 2. The TIT of the d-CEs system w ill accordingly exceed that of the d-CES 2 system and necessitate more expensi ve turbine ma terials wh ile th e additional investment associa ted with the second combus tion chamber is rat her low in compar ison. The sensitivity of the LCOE dis o f t h e d - C E S s y s t e m o v e r t h e m a s s f l o w r a t e o f t h e f u e l t o t h e s y s t e m is given in Figure D.9 in Appendi x D . D e s p i t e t h e l o w e r i n v e s t m ent costs of the d-CES 2 system; the d-CES system achieves slightly lower LCOE dis . Reason for this is the by 15 % higher RTE . The specific investment costs 𝑐 of all systems are represented over the RTE i n Figure 5.18. Figure 5.18: Specific investment costs over RTE of the ten CES systems for 𝑊 = 100 MW. The sy stems wi th high er RTE reach significantly l ower specific inve stm ent costs and LCOE dis . Reason for th is is that the cha rging unit makes up for the high est share in the investment costs and higher RTE significantl y reduce t he size o f the charging un it necessary t o suppl y 100 MW of discharge power. 7 50 9 50 1 .150 1 .350 1 .550 1 .750 0 , 20 , 4 0 , 60 , 8 1 1 , 2 1 , 4 100 80 120 60 40 20 140 a-CES w/o C S LNG w/o C S a-CES LN G WH 350 d-CES WH 450 L NG + WH 450 LN G d- C ES d-CES 2 1,150 R TE [%] 1,350 1,550 1,750 750 950 specific inv estm ent costs, c i [€/kW] 𝑚 𝜀 𝑚 2 · 𝑚 𝜀 Chapter 5: Resul ts and dis cussion 105 F o r t h e s y s t e m s w i t h L N G i n t e g r a t i o n , a r a n g e f o r t h e LCOE dis i s i n d i c a t e d i n F i g u r e 5 . 1 8 . Reason f or this is that sp ecific costs of the s ys tem s are a fun ction of the mass f low of the LN G supplied. For exerget ic and preliminary economic analysis the m ass flow of the LNG was adjusted to the value of h i ghest exerget ic efficien cy 𝜀 as describe d in section 4.4.3. The sensitivit y o f the specific investment costs and the LCOE dis o ver the mass flow of LNG supplie d is shown in Figure 5.19. For small mass flowrates of the LNG (< 9 % o f 𝑚 ) , t h e s p l i t t i n g ratio is gradually reduced, and the RTE increases linearly, Figure 4.11. The ∆𝑇 , i s thereby fixed but the LM TD cha nges. When a s plitti ng ratio of r = 0.08 is reached, the splitting ratio cannot be reduced further without violating the constrain t for th e ∆𝑇 , . As a result, the RTE does not increase further w ith a n a m p l i f i e d m a s s f l o w r a t e o f the LNG. Still, the additional low-temperatu re exergy increases the LMTD, hence, th e BMC of the MHE1 are reduced. A s the MHE1 is the sec o n d m o s t c o s t l y c o m p o n e n t , t h e r eduction in BMC leads to significantly lower specif ic investment costs. The LCOE dis reach es v alues lower th an 170 €/MWh at doubled ma ss flow, while the RTE s tays con stant at 51 % and th e exerg etic efficiency 𝜀 d r o p s b e l o w 3 8 % . I n F i g u r e 5 . 1 8 , t h e s p e c i f i c i n v e s t m e n t c o s t s of the LNG integrated systems are given for the maximum efficiency case ( 𝑚 𝜀 ) and the case o f doubled flowrate. For further comp arison the results for the sy stems with twice as large mass flow 𝑚 than necessary t o reac h 𝜀 are considered. Figure 5.19: Specific investment costs and LCOE di s over the specific mass flow of LNG. In Figure 5.20, the LCOE dis is broken down into the cost shares associate d with the fuel c osts , the cost of electricity charged, the operational expenditure (O PEX) and the capit al expenditure (CAPEX) related to the charging, t he s to ra ge, an d the dis chargi ng un it. The a- CE S wi th wast e heat inte gration reached LCOE dis comparable with the CES system with com bustion. Also, their specific costs are co mpetitive despite th e lower RTE , Figu re 5.18 . The waste heat integration ma y becom e more promisin g whe n taki ng t he depende ncy on fuel p r i c e s a n d t h e e n v i r o n m e n t a l impact of the combusti on gases into account. 160 165 170 175 180 185 190 195 200 1.000 1.100 1.200 1.300 1.400 0 0 ,1 0 , 2 0 ,3 0 , 4 LCOE dis [€/MWh ] specific investm ent costs, c i [€/ kW] m L NG, k g / s specific i nvestment cos ts, €/k W leve lized c ost of ele ctric ity , €/MWh 𝑚 𝑚 ⁄ , [-] 0.4 0.0 0.2 0.1 0.3 Chapter 5: Resul ts and dis cussion 106 Figure 5.20: The levelized cost of electricity of the ten CES system configuration with indicate d shares of the costs associated with the CAPEX, the OPEX, th e cost of electricity charged and the fuel costs at 𝑊 = 100 MW and 𝑚 𝑚 𝜀 . With increased 𝑚 , the system s with LNG integrati on became even more competiti ve . To draw a c omparis on bet ween t he diff erent CE S sys tem s at a single e l e c t r i c i t y p r i c e a n d N G p r i c e is misleading. The LCOE dis o f the different syste ms is compa red for vario us electrici ty p rices, FLH dis , natural gas prices and CO 2 emission prices in section 5.4.2. T he LCOE dis is highly depende nt on t he i nitial assumpti ons made i n t he economic a naly sis. Thus, a sen sitivity analysis was performed to evaluate the wei ghted effect of changes applie d to the initial economic param eters. The results from economic sensitivity analysis of t he adiab atic CES system are discusse d in section 5.4.1. In sec tion 5 .4.3 , th e results for C ES economic assessm ent are compared to values repor ted in the literat ure an d val ues propos ed for com peting technologies. 0 50 100 150 200 250 300 a-CES w/o CS a-CES WH 350 WH 450 d-CES d-CES 2 LNG w/o CS LNG LN G + WH LNG d-CES LCOE dis , [ €/ MWh ] Fuel costs Cost of electrici ty OPEX CAPEX discharge CAPEX storage CAPEX charge 283 125 195 154 138 140 143 237 173 122 Chapter 5: Resul ts and dis cussion 107 5.4. Economic viab ility of CES systems In this section, the economic vi ab ility of C ES sy stems is furth er assessed. The results from economic sen sitivity analy sis pe rformed on the adia batic a nd th e integ rated syste ms ar e presented. The parameters which affect the e conomic feasibility o f the systems are revealed. The most suitable applications a re identified accordingl y. The results obtained in econom ic analys is discusse d in secti on 5.2. 2 and section 5.3.2 are compa red to values prese nted in the literature. Finally, CES econom ic performance is com pared to ot her bulk ES technologies. 5.4.1. Economic sensitivity analysis of the a-CES system The weighted effect of change s i n t he ec onomic param eters on th e le velized cost of disc harged electricity of the adiabatic CES system is shown in Figure 5.21 . The LCOE dis changes proportionally to changes in the b are module costs (BMC), the O M C a n d t h e p r i c e o f electricity 𝑐 . The annual full-load hour s of t he discharge unit FLH dis , the RTE , the capacity of the dischar ge unit and the econom ic life have an inver se and no n-linear relationship to the LCOE dis . Changes in the FLH dis have the stronges t influence on the LCOE dis . Figure 5.21: Economic sensitivity analysis results of the a-CES system. 100 150 200 250 300 350 0% 50% 100% 15 0% 200% LCOE dis , [€ /MWh] param etric change, [-] econom ic life i nterest rate BMC OMC FLH capacity RTE electricity price economic lif e 20 y ears 60 y ears FLH dis 720 h/a 2920 h/a capacity 25 MW 200 MW BMC 60 Mio € 130 Mio € RT E 31.5% 46.5% OMC 1% of TCI 8% of T CI interest rate 5 % 15 % FLH dis Chapter 5: Resul ts and dis cussion 108 In principle, ES sys tems have significantl y lower op eration hou rs t han m ost pow er plants. The CES systems assessed in this w or k have a c harge -to-disc harge ra t i o o f t w o , w h i c h r e f e r s t o t w o charging hours for every hour of discharge. Consequently, even if the storage durat ion is zero (when the dischar ge process takes place imm ediatel y after the c harging process) , the maximum number of FLH of the discharge unit is 2,920 hours annually. W h e n t h e F L H dis are further decreased in relation to the base case assum ption (1,460 h/a), the LCOE dis increases significantly, exceed ing values of 250 €/MWh. Alread y a 10 % reduc tion of the FLH dis , increases the LCOE dis by 8 %. Thus, only applicatio ns which comprise a freque nt and extensive operation are econo mically feasible for CES s yste ms . Potential applications were discussed in section 2.5, and th e characteristics (cy cles p er a nnum and discharge duration) o f considered applicati ons are given in Table A.2 through Table A. 5. The ES applications whic h were found to have annual operatio n hours s uitable for CES are; load s heddi ng or s hiftin g (720- 3,000 h/a [93]) or peak s having (up to 2,500 h/a [72, 37, 93] ), energy arbitra ge or RE tim e shift (up to 3,000 h/a [103] ) and capacity f irming (1,200- 2,000 h/a [ 37]), secti on 2.5. T he operati on as a reserve in standby, in contr ast, is economically not viabl e for C ES sy stems, desp ite thei r low standby losses. Reason for t his is not only t he low num ber of operation hours, but also the economic value for reserve capac ity and b lack start application is r at h e r l o w i n c om p a r i s o n t o other applications (Tabl e A.7) [97, 106, 107]. T he majority of T&D support services are not suitable for CES operation, due t o low operation hours and diff icult prediction of the revenue streams. The second strong est weighte d cha nge of the LCOE dis is ca used by cha nges in the B MC of the system (Figure 5.21). The bare m odule costs (BMC) were varied t o show th e effe ct o f chang es in the initial investment costs of the system on the LCOE dis . Investment costs commo nly tend to be higher than in the initial cost estimation. For this reas on, the BMC were varied fr om 80 % to 180 %. When the BMC are almos t twice as large, the LCOE dis reaches values higher than 290 € /MWh. Th e r e duction of investment co st is th erefore o f gre at importa nce to CES economic feasibility. The validation of the c ost estimates i s o f im portance to t he concl usivenes s and significance of economic valu es presente d in this work. The specific investm ent costs are furthe r dis cussed i n secti on 5. 4.3. The RTE al s o ha s a s tr o n g i n fl ue nc e o n t he LCOE dis , yet not as strong as the investment costs. An i mprovemen t o f th e sy stem effici ency a t the expen se of an eq ually hig h (in percentile ) investment is conseque ntly not v iable. As an increase in RTE commonly is coupled w ith an increase i n the investment costs, the RTE was varied only by +/- 20 %. A red uction of the RTE also increas es the dependenc y on the p rice of electricity charg ed to the system. The effect on the LCOE dis caused by changes in the electricit y price is only indicated w ith a dotted lin e in Figure 5.21, as a reference in co m parison to t he ot her param ete rs. The behavior o f the LCOE dis o f d i f f e r e n t C E S s y s t e m s w i t h v a r i o u s RTE for varying electricity prices and FLH dis is discussed in the f ollow ing sec ti on 5.3.2 and can b e seen in Figure 5 .22 a nd Figure 5.23. The specific costs o f CES systems are decreased by approximatel y 23 % with doubled capacity ( 𝛼 = 0 .8 2 ). The LCOE dis decreases accordingly with incre ased capacity (Figure 5.21). W ith the assumptions made, a 200 MW a-CES system reaches a LCOE dis of 180 €/MWh. A change in t h e in te r e s t r a t e a t w h i c h t h e a l l o w a n c e fo r f u n d s i s l e n t i s o f sim ilar significance than a change in the RTE . When the interest rate is reduced by 20 %, the LCOE dis is reduced by approximately 13 %. At an interest rate of 5 %, the LCOE dis becomes as low as 165 €/MWh. Chapter 5: Resul ts and dis cussion 109 T h e O M C o f t h e C E S s y s t e m s a r e a s s u me d a s a s h a r e o f t h e T C I . A ccording to l iterature, OMC for CES system s can be as low as 1.5- 3 % of the e quipm ent costs [8 ] o r e v e n l o we r [2 0 ] . W h e n varyi ng the shar e from 1-8 % of TCI, the LCOE dis changes linearly w ith an increase/decrease in O M C . OM C a s lo w as 1 % of TC I wo u ld a l lo w th e LCOE dis to drop below 170 €/MWh. T he economic life o f the CES sy stem was found to have less influenc e on the LCOE dis , despite its large value range. 5.4.2. Economic sensitivity analysis of the integrated CES sy stems Electrici ty is trad ed in di ffere nt markets, and the electricity price changes significantly over ti me , s ee F i gu r e B .1 a . Th e n u mb e r of h ou r s in a y e ar a t w h i ch e lectricity is available at a given price is limited. To evaluate th e econom ic perform ance of the v a r i o us C E S s ys t e m s p r e s e n t e d in s ection 5.3 , t he electricity price is given as a function of FLH of discharge. In Figure 5.22, the LCOE dis o f t h e d i f f e r e n t s y s t e m s i s c o m p a r e d t o i n c r e a s i n g FLH dis . The price of electricit y is given as a functi on of the operati on hours of t h e c ha rg in g s ys te m ba se d o n t he German day-ahead market in 2018 [138]. The systems are assumed to operate in the hou rs of the lowest electricity prices (see Figure B.1c). F or the natura l gas price, an average of the Henry Hub natural gas spot prices in the year 2018 was used (c NG = 126 €/ton NG [139]). Figure 5.22: LCOE dis over the FLH dis for the ten considered systems. The higher the operation hours of the storage, the lower the LCOE dis of th e sy stem. With increased FLH dis , the average price at which the e l ec t ri c it y i s c ha r g e d t o th e system increases. For this reason, systems with higher RTE become more compe titive at higher FLH dis , despite higher i nitial in vestm ent costs. 18 22 26 30 34 38 75 12 5 17 5 22 5 27 5 730 1230 1730 2230 27 30 LCOE dis , [€ /MWh] FLH dis , h/a a-CES w/o CS a-CES WH 350 WH 450 LNG w/o CS LNG d-C ES d-C ES 2 LNG + WH LNG d- CES c el price of electricity c el , €/MWh Chapter 5: Resul ts and dis cussion 110 For FLH dis higher than approxima tely 1,730 h/a, the LCOE dis of the d- CES d ecrease below the LCOE dis o f the a-CES system with waste heat integration at 450°C, the latter of whic h is mor e competitive at lower FLH dis . The LNG systems with waste heat integra tion and with internal combust ion also intersect but at higher FLH (2, 150 h/a). Despi t e the small difference in LCOE dis of the two di abatic CES system s, the d-CE S 2 sys tem cannot c omp ete due to its inferior RTE . The ten system s were also compared, taking into account CO 2 emission prices of up to 40 € /ton CO2 , Figure D.10. The CO 2 em issions amount to 0 .208 and 0.239 kg CO2 /k Wh dis for the d-CES and the d-CES 2 system, respectivel y, similar to the CO 2 emissions of natural gas power plants ( 0.2 kg CO2 /k Wh dis [ 1 4 0 ] ) . A l r e a d y a t a n e m i s s i on price of only 2.5 €/ ton CO2 , the attractiveness of diabatic CES systems decreases s ignificantly as the LCOE dis o f t h e d - C E S system s becom es highe r than that of the WH 450 sys tem. At highe r emission prices of 20- 40 € /ton CO2, even the WH 350 system becomes more economically feasible than the d-CES syst em. Electricity prices and natural gas prices are im po ssible to pre dict over 40 year s to c ome. Whe n comparing th e sy stems at fixed daily operation hours ( 𝜏 /𝜏 = 8h/4h), the natural gas prices and the electricity prices can b e varied more strongly. In Figu re 5.23 (a) and (b), the LCOE dis of selected system s is compared o ver the price of the electrici t y c h a r g e d t o t h e s y s t e m s . T h e price of elec tricity is onl y depicte d unt il 80 €/M Wh to make th e inters ections m ore visible. F or higher electricity prices, the tr ends for the graphs stay consi stent. The cost-optim al system (lowest LCOE dis ) is indicated with a black line for the di fferent electricity price intervals. For negative el ectricity prices (< - 15 € /MWh) the LNG integrat ed CES system without CS is most competitive, see Figure 5.23 (a). For positive electricity prices, only the two LNG integrated systems with either waste heat or combustion compete f o r t h e l o w e s t LCOE dis . At lower ele ctricity prices, the LN G + WH system is mo re comp etiti ve; the LNG d-CES becom es more economical at high er electricity prices. With a higher nat ur al g as p ri c e o r CO 2 equivalent emission prices, the intersection of the LCOE dis g raphs is shifted towar ds higher electricity prices. For the given cases of 𝑐 =126 €/ton NG (average 2018) and twice as l arge costs of 𝑐 = 252 €/ton NG (black dotted line in Figure 5.23), the LNG d-CES system become s the cost- o p t i m a l s y s t e m f o r e l e c t r i city prices higher than 𝑐 = 27 €/MWh and 𝑐 = 4 9 € / M W h , respectively. Both, the potential for int egration of LN G and the r ecovery of waste heat are site dependent. Especially th e availab ility of LN G waste cold is very limited. Only 23 large-scale land-based LNG receiving terminals exist in Europe. As an alternative, onl y the system s without LNG integrati on were compare d in Figur e 5.23 (b). When only the CES system s withou t LNG in tegration are compared , th e a -CES withou t cold storage is the cost-optim al CES sys tem for electricity prices l ower tha n - 23 €/MWh, followe d by the waste heat integrate d system WH 450 and the d-CES system . Depending on the fuel costs (126-252 €/ton NG ) the waste heat int egration (450°C) is m ore economically feasi ble than the d-CES system until the price of electricity exceeds 20-46 €/MWh. The a-CES, in contrast, is in none of the given scenari os as the cost-optim al solution. A s a r e s u l t , t h e w a s t e h e a t integration is evaluated as the most suitable option, taking i nto account increasing CO 2 emission prices and the dependency on fue l prices. Whereas, the d-CES sy stem is the best alterna tive when a low-cost low-emission fuel becomes available. Chapter 5: Resul ts and dis cussion 111 (a) (b) Figure 5.23: LCOE dis over the price of electricity for (a) all ten analyzed systems, and (b) the systems without LNG integration. 0 20 40 60 80 100 120 140 160 180 200 - 4 0 - 2 0 0 2 04 06 08 0 LCOE dis [€/MW h] price of electrici ty char ged [€/MW h] a-CES w/o CS a-CES WH 350 WH LNG w/o CS LNG d-CES d-CES 2 LNG + WH LNG d-CE S LNG w/o CS LNG + WH LNG d-CES 𝑐 ↑ 𝑐 = 126 €/ton 𝑐 = 252 €/to n 0 20 40 60 80 100 120 140 160 180 200 - 4 0 - 2 0 0 2 04 06 08 0 LCOE dis [€/MWh] price of electricity char ged [€/MWh] a-CES w/o CS a-CES WH 350 WH d-CES d-CES 2 WH 4 5 0 a-CES w/o CS WH 4 5 0 d-CES 2 𝑐 ↑ 𝑐 = 126 €/ton 𝑐 = 252 €/ton Chapter 5: Resul ts and dis cussion 112 5.4.3. Validation and a ssess ment of results The LCOE dis o f s e l e c t e d C E S s y s t e m s w a s c o m p a r e d w i t h t h e LCOE dis of othe r bul k ES technologies to evaluate the compet itiveness of the CES sys tems presented in this work, see Figure 5.24. The LCOE dis o f t h e p u m p e d h y d r o s t o r a g e ( P H S ) a n d t h e a d i a b a t i c a n d t h e d i ab atic compres sed air ene rgy st orage ( C AES ) are based on t he data and the ass umptions presente d in [87]. The same assumptions were a lso adopted for the calculatio n of the leveli zed cos ts of the C E S s y s t e m s . H e n c e , t h e r e s u l t s s l i g h t l y d i f f e r f r o m t h e r e s u l t s presente d in Figure 5.20. The values from the literature were co mpar ed to the a-CES sy stem a n d th e CE S syst ems wi th the integration of waste heat (WH 450), combustion (d-CES) and re-g asif ying LNG d iscusse d in sectio n 5.3. Costs are very sensitive to the assumptions made in the estimat i o n o f t h e B M C a n d i n t h e economic analysis (sect ion 5.4. 1). When comparing different g ri d-scale el ectricity stor age technologies, the economic analysi s needs to be performed with the same method and a s s u m p t i o n s . F o r t h e r e s u l t s s h o w n i n F i g u r e 5 . 2 4 , t h e i n t e r e s t rate wa s set to 8 %, and daily - cycl ing op eration was ant icipated . Natur al ga s costs of 3.5 € ct / kW h a n d a C O 2 emission p rice of 5 €/t CO2 w ere consi dered f or all system s [87] . Figure 5.24: LCOE dis of PHS, d-CAES, a-CAES adopted from [ 87] and selected CES systems presented in this work. The levelized costs associated w ith the initial investm ent (CAP EX) and the operation and mainte nance (OPEX) of the adiaba tic CES system is comparable to t h e a - C A E S s y s t e m . However, the LCOE dis o f t h e a - C E S i s 2 8 % h i g h e r d u e t o t h e l o w R T E , d e s p i t e t h e r e l atively low pr ice of electricity of 30 €/MW h. As a r esult, only the int e grated system s reach competitive LCOE dis under the given assump tions . In pa rticul ar , the waste heat int egrated s ystem reaches a LCOE dis similar to the d -CAES sy stem. The in itial inve stment of th e CES systems p resented in thi s wor k are based on cost estimation from various sources, s ee section 3.3.1. Hence, it is necessary to valida te the specif ic investm ent c o s t s w i t h t h e a i d o f t h e v a l u e s f o r t h e s p e c i f i c i n v e s t m e n t c o sts reported in th e literature. In Figure 5.25, the spe cific investment costs obtained in the econ om ic analysis of the base case systems, the optimized systems and the i ntegrate d s ystems (CE S (res ults)) are c o mpared to the - 40 80 120 160 200 PHS d-CAES a-CAES a-CES WH 450 d-CES LNG LCOE dis , [€ /MWh ] Cost of electr icity OPEX CAPEX dischar ge CAPE X stora ge CAPE X charge Chapter 5: Results and discussion 113 specific investm ent costs reporte d in the literature for PHS an d CAES. The data for CES specific investm ent costs given in the literature (Figure 2. 7 i n secti on 2.4. 2) is als o show n as a reference in Figure 5. 25 (CES (literature)). The data is presented in a box-pl ot diagram; differentiating th e lower, middle, and upper quartile with a box containing 50 % of the data points. The middle quart ile or “median” indicates that half o f th e values are less or e qual (and half are higher or eq ual) to the value. The arithmetic mean value is also indicate d in Figure 5.25. The data for the P HS and the CAES are extracted from an e xtensi ve indepe ndent re view on ES technologies [ 68]. Figure 5.25: Box-plot diagram of the specific cost s of PHS, CAES [68 ] , and CES extracted from literature compared to values achieved in the analysis. The values reported on PHS specif ic investm ent cos ts vary more strongly than for CAES and CES, w hich can b e e xpla ined by the lar ge num ber of P HS p roj ects installed w orldwi de, ra nging in size and economic conditions. O n l y a f e w C A E S s y s t e m s e x i s t [ 76] and the inve stm ent costs reported in the literature are r ather ba sed on e stimates than a ct ual plants. The value s for t he spec ific investm ent cos ts of CE S system s det ermined in this thesis are within t h e c o s t r a n g e r e p o r t e d i n t h e l iterature [6, 13-14, 16, 30-31, 34-35, 60, 68, 79] . The investment costs eval uated in this work vary less significa ntly tha n the v alues suggested in the literature. The majority of the systems in t his thesis were designed for either 200 kg/s or for 100 MW of dischar ge power, and the specific investment costs of CES syste ms reduce with increased capacity (economy of scale). Hen ce, the deviation of the values g iven in the literature from the specific investment cost s comp uted in th is work can be exp laine d by the larger capacit y range of CES sys tem s from 10 to 500 MW dis [7, 16, 17, 29, 72] . The appr oach for the est imation of the BMC a nd econom ic assessm ent of CES s ystem s that has been u s ed is therefor e valida ted. Despite PHS reaching significantly higher va l ues f or the s pecif ic investment costs, the m edian value of PHS specifi c investm ent costs is compara ble with that o f C E S ( F i g u r e 5 . 2 5 ) . D u e t o the similar cha racteristics, CES is s uitable for simila r applic ations of PHS and CAES. Chapter 5: Resul ts and dis cussion 114 The specific investm ent costs of approximately 1, 000 €/kW (1,27 0 €/kW for the a-CES) are comparable with the ma jority o f value propo sitions introduced i n secti on 2.5. 2. Howe ver, the identified value propositions do not return the total revenue r equired, which is necessary to cover also the OMC, and the cost of the charged electricity of the CES systems. The total revenue requirement for the CES syste ms presen ted in this work range from 1 ,900 -2,850 €/kW over 10 years while the additive value propositions for the sam e period reach a maximum of 1,600 €/kW [97, 106, 107, 108, 109]. With reg ard to the resu lts of th e economic sensitivity anal ysis , the specific investment costs, and the stac ked value propositi on, tw o sets ES appl ications can be identified to have the highes t potential for CES: load follow tog ether with ren ewab le ene rgy ca pac ity firming, or rene wable en ergy cap acity fir ming in parallel with pe ak sh aving (Figure 2.12). In Figure 5.26, the LCOE dis obtained in the analysis is compared t o competing tec hnologies for an installed capacity o f 100 MW/ 800MWh. The values presented a re adopted from a broad study on energy storage costs [141], which was extended with CE S by Highview Ltd. [142]. The comparison shows that batter y-bas ed technol ogies (e.g., Li- I o n , Z i n c ) c a n n o t c o m p e t e a t large capacities. PHS reaches a higher LCOE dis than in the previous com parison as the econom ic life was kept to 20 years, while in the previous study 80 years were estimated for the PHS system. The higher value for the LCOE dis o f th e PHS system in [141 ] (compared to [87]) can also be explained by t he lar ge value range f or the specific cos ts of PHS sy stems (Figure 5.25). Figure 5.26: LCOE dis of selected CES systems compared to values from the literature [141, 142 ] . In general, a large number o f studies investigating the LCOE dis ( o r l e v e l i z e d c o s t o f s t o r a g e LCOS) of different ES technologie s have been publi shed lately [ 8 7 ] . D i f f e r e n t s t u d i e s a p p l y different r esearch approaches and various assumptions, e.g., in t e r m s o f E S s y s t e m d e s i g n , performance, and size, or economic parameters. The LCOE dis reported for PHS range from 120 to 185 €/MWh. LCOE dis value ranges of 100-175 € /MWh a r e a n t i c i p a t e d f o r C A E S a n d 5 0 - 500 €/MWh ( or 200-700 € /MWh [143]) for battery technologies [10 2 , 1 4 3] . T h e v a l u e r a n g e s 100 150 200 250 300 350 CES-LN G d-CE S CES-WH a-CES CAES PHS CES Zinc Li -Ion this work literature LCOE dis , [€/MWh] Chapter 5: Resul ts and dis cussion 115 of the LCOE dis of PHS and CAES based on review ed literature are given in a bo x diagram in comparison to CES in Figure D.12. Despite the deviatio n of the resul ts for the LCOE dis in the diverse studie s, general concl usions can be m ade that were c onsisten t i ndepende nt of the study pe rfo rmed and can also be obse rve d in Fi gure 5. 26: the stand-alone adiabatic CES ca nnot compete with CAES and PHS i n t e r m s o f t h e LCOE dis . the integrate d CES system s achieve LCOE dis values com parable w ith PHS and CAES, battery-based and hydro gen-based ES cannot c ompete at large sca les and reach significantly highe r va lues for the LCOE dis than CES. Due to the absence of geogra phical cons traints and the specifi c investm ent costs being similar to other bulk ES technologies, the CES technology m ay still be competitive. This is particularly t h e c a s e w h e n e i t h e r t h e i n v e s t m e n t c o s t s a r e r e d u c e d f u r t h e r o r the RTE is increased, which can be achieved by the integration of exter nal heat s ources and sinks to CES systems. Thus far, CES systems are expected to be able to compete for th e selected frequ ent ES applications (with high discharge durations and a large number of c ycles per annum) and creat e revenues tha t cover t he in itial investment costs. Chapter 5: Resul ts and dis cussion 116 5.5. Summary of the resu lts and di scussion Evaluation of the charging processes Three Claude-based and three Linde-bas ed air liquefactio n proce sses are compared in exergy- based a nalys is, and two c harging c onf igurations were ide ntified : the most cost-efficient liquefaction process, the Kapitza proce ss, and the proc ess with the bes t therm odynamic perf ormance, t he Heyla n dt process. The perf ormance enhancem ent of the s ix l iquef actio n processes t hrough the integration of cold storage w as quantif ied: the l iqui d yield 𝛾 was significantly increased by 80-200 %, the specific power requirement 𝑤 was reduced by 30-70 % and the exergetic efficiency 𝜀 of all liquefaction processe s assessed was considerably improved by up to 200 %. The Claude-based syst ems reach ed th e h ighest ex er getic efficienci es, the lowest specific work requirement and the highest li quid yi elds. S ensitivit y anal ysis showed tha t for lower liq uefacti on pressure s (< 95 bar) the Claude and the Kapitza pr ocess are superior to the Hey landt process. The minim um work requir ed to l iquef y one kg of air in t he Heyl andt pr ocess amounts to 967 kJ/kg at 120 bar, reaching an exergetic efficiency of 81 %. The Claude /Kapi tza pr ocess required more liquefaction work ( 𝑤 = 984 kJ/kg, 85 ba r), and reached 𝜀 of only 78.5 %. T he econom ic analysis revealed t hat the Kapit za proce ss-ba sed sy stem has b ot h the lowest specific investm ent cost and total r evenue requirement of a ll three sy stems under t he given cond itions. Evaluation of the adiabatic CES systems Two stand- alone adiabatic CES systems (100 MW/400 MWh) based on t h e c o s t - o p t i m a l liquefaction process ( Base C ase A ) and the highest-efficiency liquefaction process ( Base Case B ) w e r e e v a l u a t e d a n d o p t i m i z e d w ith ex ergy-based metho ds. The b ase case systems reached exergetic e fficiencies o f 44.5 % and 46.8 %. The exergy d estruction in the charging process dominated with a sha re o f 64-66 % of the exerg y des troy ed in the overall system, which puts emphasis on the importa nce of the selecti on and performanc e of the liquefactio n process. The discharge system and the storage system made up only 28-30 % and 6 % of the overal l exergy destructi on, respec tivel y. The exergy losses are caused almost exclusively by the thermal exergy vented from the heat storage to t he envir onm ent after the discharge process. The effect of cold storage and heat storage were found significant to the performance of the overall sy stem. The total revenue requi rement for the Base Cases A and B amount ed to € 37.3 million and € 39.0 m illion annually, respectively. The heat exchangers were found to cause 6 2–64 % o f the investment costs in both b ase case system s. The higher costs of t h e Ba s e C a s e B w e r e m a i n l y attributed to the cost of MHE1, which is tw ice exp ensive if c om pared to that of Base Case A. Desp ite the low er exerg etic efficien cy and RTE of the Base Case A, a lower LCOE dis o f 255 €/MW h was ac hieve d, com pared to 267 €/MWh in the Base Case B . T h e B a s e C a s e A w a s expected to stay more economically v i a b l e t h a n t h e B a s e C a s e B , under the given condition and for electricity prices l ow er than 100 €/ MWh. In the iterative exergoec onomic analys is, the five sets of com p one nt s wh ic h c au se th e maj o r ity of costs in both systems w ere identified: the main he at exchangers , the turbin es , th e intercooler s , the compressors, and the re heat er s . T h e c o s t s a s s o c i a t e d w i t h t h e i n i t i a l i n v e s t m e n t a n d t h e O M C a r e s i g n i f i c a n t l y h i g h e r t h a n t h e c o s t s associated with the thermodynam ic ineffici encies (exergy destruc tion) in the m ajori ty o f components. The decision Chapter 5: Resul ts and dis cussion 117 variab les necessar y to be change d for cost reduction of the inv estment costs were ide ntified an d applied in four iterations. In each of the iterations, a sensit ivity an alysis of the RTE a n d t h e LCOE dis was conducted. In both systems, t he interrelation of the compo nents was found to be very strong. Changing parameters w i t h t h e i n t e n t i o n o f c o s t r e d uction in a single com ponent had a great influence on the perform ance and costs of other com ponents. The objective of reducing the costs a ssociated with the i nitial investment Z was achieved for all of the selecte d components. The Optimized Case B achieved low e r l e v e l i z e d c o s t s a n d h i g h e r exergetic efficiencies throughout the iterativ e optimization. The LCOE dis o f the Base Case A was reduced from 255 €/MWh to 208 €/MWh at the expe nse of a reduc tion in the exe rgetic effi ciency from 44.4 % to 38. 1 %. The LCOE dis of the p rior less cost-effective Base Case B was reduced to 19 5 €/MWh ( 𝜀 = 39.6 %). Evaluation of th e integration of heat sources and sinks to CES Eight sys tem s with i ntegrat ion of externa l heat a nd/or c old sou rces were compared in energetic, exerg etic and econo mic analy sis to two adi abatic CES syst ems. T he inte grat io n of LNG wast e cold as an alternativ e to cold storage was found therm odynamica lly infeasib le. The low- temperature exergy suppli ed by th e LN G in creased 𝜀 by only 1.4 %, while cold stora ge increases 𝜀 by more than 60 %. When introduc ing LNG in addition to cold st orage, the amount of air liquefied increases b y another 40 %. Waste heat i ntegration was found to be o nly benefi cial to t he RTE (39.4 % → 62.5 %). The energetic and ex ergetic efficien cies decrease by up to 50 %. Desp ite the additional invest ment , the integrated s y stems reach significantly l ower specific investm ent costs and LCOE dis due to the increas ed RTE concerning the a-CES system. The CES s ystem wit h both waste he at and w aste col d inte grati on achieved the lowest LCOE dis of 130 €/ MWh . The d iabat ic C ES syst ems a nd th e sy stem w ith wast e heat integration (450 °C) reached comparably low LCOE dis o f 131-143 €/MWh. CO 2 e m i s s i o n p r i c e s o f only 2 .5 € /ton woul d make the d-CES inferior to the waste heat integrate d s yst em. The costs of the LNG integrated system s were found to decrease with an increased mas s f l o w o f L N G , d e s p i t e n o further increase in RTE (51 %). With an increase of the 𝑚 from 10-20 % o f 𝑚 , the LCOE dis is reduced by 10 % and reaches values l ower than 122-170 €/MWh . Economic sensiti vity analysis Changes in the full load hours o f the discharge unit were revea led to have the strongest influence on t he LCOE di s . Therefore onl y fre quent ene rgy applicati ons were eval uated to be econom ically vi ab le fo r CE S s yst em s, e.g . , l o ad shedding, energy time shift or capa cit y firm ing. Des pite the low storage losses, the operatio n as a reserve is uns uitable fo r CES s ystem s. As changes i n the investment costs also have a s t r o n g i n f l u e n c e o n t h e LCOE dis , the specific investment cost obtained in economic analysis w as valida ted in c omparis on to va l ues re ported i n the l iteratu re. CES s ystems are a ble t o achie ve c ompeti tive specif ic investm ent cos ts ( 1,000 €/kW) sim ilar to PHS and can compete for the same applications. The adiabatic CE S was found inferior to comp ressed a ir energy stor age (C AES) and pumped hydro storage ( PHS ) in terms of the LCOE dis due to t he lower RTE . The integrated systems, in cont rast, achieved competitive val ue s for the LCOE dis but entail site de pendency. Potential m ethods of int egrati ng he at sources and/or sinks to CES systems require a more thorough investigation in t erms of av ailabi lity and costs. None of the stacke d value propositions identif ied in section 2. 5.2 based on the reviewe d literature cover the to tal revenue required f o r t h e C E S s y s t e m s presented in this work (1,900- 2,850 €/kW). Chapter 5: Resul ts and dis cussion 118 Chapte r 6: Con clusi on and outl ook 119 Chapter 6: Conclusion and outlook Cryogeni c energy stor age is a grid-scal e energy storage con cept with pro mising characte ristics such as; b eing based on mature technology, high volumetric ener gy den sity, ab sence of geograp hical constra ints, long cyc le life, and low storage loss es. The largest drawback of the technology that limits its commercialization and successful app lica tion (e.g., for grid balancin g) is the high specific c ost relative to its low roundtrip efficie ncy (40-60 %). The integration of heat sources and sinks have benefits for both CES efficiency an d costs. However, so far, no highest-ef ficienc y or cost-o ptim al system configurat ion for CES has been identified, nor the effect of the integration on the economic and thermodynamic performance has b een thoroughly investigated. This wor k aimed to ident ify m easures for cost reduction and the rmodynam ic performance enhancem ent of CES s ystem s wit h the aid of exe rgy-ba sed methods . Under steady- state operati on, vari ous s yste m conf igurati ons we re desi gned i n Aspe n Plus® based on an extensiv e literature review and sensitivity analysis. Cryogenics-based en ergy storage s tate o f the art was assessed and benchm ar ked towards competing technologies. Potent ial applications and challe nges to CES comm ercializatio n were assessed. The most com petitive charging process configur atio ns with regards to costs and therm odynamic perform a nc e were ident ified. The effect of cold storage on the efficiency and costs was measured . Two base ca se syst ems we re presented and evalua ted in exergetic, ec onomic, and exergoecono mic analysis. In the exergoeconomic analysis, the cos t of the therm odynamic inefficiencies in the system was quantif ied, and measures for cost reduction were identif ied. In exergoec onomic optim ization, two optimized cases were obtain e d . E i g h t s y s t e m s w i t h t h e i n t e g ration of waste heat, combust ion, and LNG waste cold were designed based on the optim ized CES configuration. The inte grated s ystem s were furthe r eval uated i n compar ative ex ergetic and econo mic analysis. In an econom ic sensitivity analysis, the econom ic viability of C ES was assessed and the resu lts from the economic analyses conduc ted in this work were validate d with res pect to values reported in lite rature. Chapte r 6: Con clusi on and outl ook 120 6.1. Summary of the main results The extended summary of the main results is given in section 5. 5. The main findings identified in this thesis are: The inte gration of cold s torage si gnifica ntly inc reases the the rm odynamic pe rformanc e and reduce the costs of the liquefac tion (c harging) proces ses i n CES systems. The Li nde-base d liquef action processes are not suitable for imp lem entatio n in CES syst ems. The cost s associ ated with the initial inv estme nt do minate in th e maj ority o f the compon en ts of th e C ES syst ems, w hile th e co sts as soci ated with exergy destruction are minor. A cost -optimal ad iabatic CES s ystem design was obtained; the LCOE dis o f t h e 100 MW/400 MWh system was reduce d from 267 € /MWh t o 1 9 5 € / M W h a t the expense of a reduction in the r oundtrip efficien cy from 47 to 4 0 %. With the integrat ion of waste heat, combusti on and/or re-gasif y ing LN G, the LCOE dis are reduced to 122-170 €/MWh. The integration of CES was f ound a viable option reaching e ffic ie ncies lar ger than 70 %. The LCOE dis when rec overi ng waste heat a t 450 °C is competitive w ith d-CES systems at higher fuel or CO 2 emi ssi on p rice s. The integ ration of LNG is not an altern ativ e to cold sto rage an d is only benefici al to CES thermodynam ic performance in combination with cold storage. The operation hours of the discha rge unit, the specific costs, the response time and the RTE are the lim iting factors for CES econom ic feasibility. Only frequent bulk ES applicati ons with long disc harge duration are suita ble for CES syst ems. CES speci fic costs r eached a media n value of approxim ately 1,00 0 €/kW, similar to PHS. The stacked value proposition when providing multiple ES applic ations was found to potentially reach values o f 1,600 €/kW. The combined applications that ar e m ost feasible for CES system s are load follow with RE capacity firming and RE capacity firming w i t h peak shaving apart fro m oth er frequent bulk e ner gy applications. The stacked value propositions ide ntified for the combined appl ications adopted from the reviewed lite rature do not recover the revenue required for the CES system s under the give n as sumptions. Chapte r 6: Con clusi on and outl ook 121 6.2. Scope of the present work The benchmark analysis presente d reveale d the advantages of CES towards other bulk ES technologies. When assessing pot ential applications, this work showed that m eans to reduce the high specific investment costs a nd increase the RTE need to be implem ented before CES becomes competitive. The cost-optimal design configuration for the adiabatic CES system present ed in this work as well as the quantifie d enhancem ent in the thermodynam ic and the economic performance of CES wit h system integration should be c onsidered for future process design and analysis. The charging p rocess was shown to cause both the majority o f ex ergy destruction and the majorit y of the investment costs in the CES systems. The choice o f the liquefaction process is therefore of great significance to th e economic and the thermod ynamic viability of the sy stems. The Linde-based processes that w ere presented in v arious public ations and p atents [10, 40, 41, 43, 44, 52] are not via ble for implementation in CES systems. I nstead of the Linde- process, the Heylandt process is recom mended as the c harging process for CES sy st ems wi t h c ol d r e co ve ry and stora ge. Despite the cold storag e configur ation itself not being subject t o t h i s t h e s i s , t h e n e c e s s i t y f o r furthe r development of the cold storage was confirmed. The low- tem perature exergy recovery is of crucial importance to the RTE and the cost -effecti veness of the entire CES system. The liquefaction work and the levelize d costs of the liquefied air is shown to be reduced to half, despite the additional investm ent costs associate d with the col d storage. Eve n when an addition al heat sink, e .g. “wa ste cold” fr om LNG r egasifi cation is availa ble, the c old s torage is still necessary to enable reasonab le RTE and LCOE dis . Without the high-gr ade cold (- 180 °C to - 160 °C) provide d by the inte rnal cold recover y and storage , the CES with int egration of re- gasif ying LNG would r each a RTE of only 26 %. T hus, in part ic ular l ow-cost m aterial tha t can realize the efficient recovery o f the high-gr ade cold is an imp ort ant subj ect fo r furth er rese arch and essentia l for CES commercialization. In the exergoec onomic analysis, the r eduction in efficiency was found to be stronger than expected when chan ging the param eters of selected components. T he operatio n of selected components was f ound to have a s ignificant influence on the per formance of oth er compon ents. The strong correlat ion between the com ponents limits the cost-s aving potential of CES sy stems, which is why the endogenous and exogenous part of the exergy de struct ion of the c ompone nts in CES s ystem s should be ident ified i n an adva nced exer getic an alysis. CES is based on mature compone nts commonl y used in the LNG valu e chain, for industrial gases and power g eneration. Due to the worldwide increasing LNG capacity , a redu ction in the costs of large-scale cryogenic equipment is plausible, which wo uld have a posit ive ef fect on CES commercialization. A lso, the further developm ent of gas tur bines for low-tem perature heat recovery application could be o f remarkable benefit to CES syst e m s . A f t e r t h e e x e r g o e c o n o m i c optimization was performed, t he t urbines remained the component s with the h ighest cost- impor tance. Thus, a cos t reduct io n of the turbines could reduce t h e LCOE dis o f the CES sy ste ms nota bly. For syst em integr ation, several findings wer e of signi ficance. The specific costs of the CES system w ere found to be reduced with larger mass flow rates of LNG, despite no further increase in the RTE. Thus, mass flow rat es of the LNG higher than necess ary to increase the RTE are recomm ended. The inte grat ion of L N G t o a dia ba ti c CE S s ys te m s wi th cold storage was found Chapte r 6: Con clusi on and outl ook 122 to increase the share of air li quefied by another 27 %. The RTE of 63-70 % as p roposed in [8, 62, 63], h owever, are not achieva ble with LNG integration alone . Only sy stems with internal comb ustion of natural gas or the combinat ion of bot h waste heat an d c o l d a c h i e v e R T E o f 7 0 % and higher. The integration of LNG shows gr eat potenti al and sh ould be subject to further investigati on. In particular, the applicabilit y of integrati ng LNG to the main heat exchange r in the CES sys tem shoul d be assesse d with r egards to associat ed co sts and r isks. The recovery o f waste heat was c laimed a core advantage of CES sy ste ms in v ariou s publications [8, 10, 14, 74]. The recovery of waste h eat was pr ov en b eneficia l fo r the RTE i n t h i s w o r k . T h e e n e r g e t i c a n d e x e r g e t i c e f f i c i e n c y , i n c o n t r a s t , were shown to d ecrease due to the heat which is not used in the discharge but vented before t he intercool ing process. In particula r, for CES systems with waste heat integration, an add itional ORC re covering the surplus heat is strongly recomme nded. Moreover, further investi gation of the availability of waste heat s ources with suitable temperatures (> 300 °C) and su itable energy transfer materials is necessary to evaluate t he econom ic feasibility. The efficiencies proposed in t he literature are only achievab le at significantly higher costs than suggested or w ith the integratio n of external heat sources or s inks. For a 100 MW/400 MWh stand-al one adiabatic CES system, RTE s of 40-47 % at specific i nvestment costs of 1,270- 2,090 €/kW are realizable, reaching LCOE dis as low as 195 €/MWh. The LCOE dis of the C ES system is only co mpetitive to PH S and C AES with th e i n t e g r a t i o n of external heat sources or sinks. W ith the integration of waste h e a t o r L N G r e g a s i f i c a t i o n , C E S loses the com petitive-edge towards PHS and CAES of site-indepen dent storage. Whereas, a waste heat s ource may be m ore easily accessible than an undergr ound salt cavern for CAES or an ele vate d wate r rese rvoir with significa nt sca le f or PHS. Wit h the in tegrati on of com bustio n, CES loses its advantage of bei ng classified as a “carbon-neutra l” or “low-carbon” technology. This work showed that the integration of CES systems is of sign ificance to both costs and thermodynam ic perform ance whic h is why a thorough assessm ent o f the a vaila bility and hidde n costs of heat sources and sinks is another potential research f ield in the contex t of CES. T h e f i n d i n g s o f t h i s r e s e a r c h a r e a l s o a p p l i c a b l e t o o t h e r f i e l ds. The advances made can be applied to cold power generation cycles integra ted in LNG rega s ifica tion units or in the field of introducing flexible loads, e.g., using LNG term inals or ASU as variable loads. T h e s l o w r e s p o n s e t i m e o f C E S s y s t e m s w a s i d e n t i f i e d t o b e o n e of the main lim iting factors for CES to provide selected ES a pplications. T his obstac le coul d be overcom e by coupling CES syst ems wi th a high power ES t ec hnology, e.g., flyw heel o r batt e ry- based storage. Reduci ng the response or “ramp up” time of CES would significantly incre ase the number of applications CES could supply. More over, this work showed that for greater r evenue, the stor age needs to provide multi ple applications. With a faster response time, CES would be suitable for more easily combinable ES application s, which would significantly in crease the monetary value of the technology. The ident ified stacked val ue proposition s for the comb ined ES a pplications were found to c over the costs associated with the total capital investment of the C E S s y s t e m s b u t w e r e u n a b l e t o recover the total rev enue requi red. Either additional revenue s treams need t o be i dentified, and fina ncial incenti ves for the investment in ES need to be g iven, or the CES costs need to be significantly reduced, in order for CES to becom e economically viable. Chapte r 6: Con clusi on and outl ook 123 6.3. Limitations In th is wo rk, th e sy stems we re s imulated und er ste ady-state con ditions. The thermodynamic perform ance of the system may differ significantl y duri ng the s tart- up, the shut-dow n or part load operat ion of the CES system . Not only the constraint of th e response time but also the efficiency have a significant e ffect on CES economic viability. T h u s , a d y n a m i c s i m u l a t i o n investigating the d rop in the efficiency and associated costs s hould be undertaken. For the economic evalua tion, the bare module costs are estimate d base d on an existing sys tem design. The cost estimates are a lways exposed to larg e uncertai nties and can be a strong point of weakness. Whereas i n this work the assum ptions m ade and sour ces accessed are thoroughly unfolde d and enable further improvem ent of the cost estim ates. Moreover, the costs were partial ly based on past purchase orders and costs published by the technologies’ m anufacturers and devel opers which reduce unce rtainties. Finally, the specifi c costs were validated wi th values stated in liter ature and undertaken s ensitivity analysis. For the monetary eva luation of different E S applications, value propositions give n for selecte d ES app lications in literatu re wer e assessed. The monet ary val ue proposition of a specific application can only be g ained in detailed simulation and analy sis (energ y system model ing), taking into account the storage confi guration and the market co nditions. In this work, CES was benchmar ked towards other bulk ES technol ogies that com pete for the same applications. Fle xibility could also b e supplied to power grids by o ther mea ns, e.g., grid interconnec tion, demand-si de mana gem ent, or flexible generation . A comparison should be drawn between the a pplication of b ulk ES a nd othe r flexi bility options for CES feasibility. 6.4. Summary of pot ential future work This work addresse d several chall enges of CES, and many interes ting results were obtained. Nonetheless, further investigat ion is necessary to analyze and optimize CES and fac ilitate the furthe r developm ent and applicati on of the technology. Based on the main findings, scope, and limitations of this re search, the following potential resear ch paths wer e ide ntified: Dynamic sim ulation of the propose d cost-optimal adiabatic CES s ystem. Applicat ion of an ad vanced e xergoec onomic anal ysis o n the propo sed sy stem. Assessment of the market potential of CES system s and m onetary val ue propositi on of CES applications through energy system and ES application model ing. Feasibility study on the in tegration o f waste heat, ORC, and LN G in reg ards to availabi lity, ri sks, costs. Assessment of low-c ost and high-ef ficient c old stora ge materi al and geom etry. Eval uation of a ir separation or liquefac tion units and LNG term i nals as poten tial variable loads or ES. Experim ental studies on com ponents of CES system s to ide ntify t echnological diffic ulties in the process design and valida te the behavior, e .g., the cryogenic pump, the high-pressure cry ogenic HE, cold expande r or the c old st ora ge and recovery. Chapte r 6: Con clusi on and outl ook 124 Refere nces 125 Refere nces [1] T. K ousksou, P. Bruel, A. Jamil, T. ElRhafiki and Y. Zeraouli, "Energy storage: Applications and challenges," So lar Energy Materia ls & Solar Cells , 120:59-80, 2014. [2] S . H a m d y , T . M o r o s u k a n d G . Tsatsaronis, "Cryogenics- b ased energy storag e: Evaluat ion of cold e xer gy recovery cycles," Energ y, 138:1069-1080, 2017. [3] S. Hamdy, T. M orosuk and G. Tsats aronis, "Cryogenic Energy Stor age: Characteristics, Potential App lications and Economic Benefit," in R ecent Developments in Cr yogenics Research , New Yo rk, Nova Science Publishers, Inc., 2019, pp. 277-310. [4] M . B e a u d i n , H . Z a r e i p o u r , A . S c h e l l e n b e r g a n d W . R o s e h a r t , " C h a p t e r 1 – E n e r g y Storage for Mitigating th e Variab ility of Renewable Electricity Sources," in En e rgy Storage for Smart Grids - Pl anning and Operation for Renewable and Variable E nergy Resources (VERs) , Academic Press Inc, 2015, pp. 1-33. [5] A . S o l o m o n , D . M . K a m m e n a a n d D . C a l l a w a y , " T h e r o l e o f l a r g e - s cale energy storage design and dispatch in the power g r i d : A s t u d y o f v e r y h i g h g r i d pene trati on of va riable renewable resources," A pplied E nergy, 134:75-89, 2014. [6] A . C a s t i l l o a n d D . F . 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A m e e l , C . T ' J o e n , K . D e K e r p e l, P. De Jaeger, H . Huisseune a nd M. Van Belleghem, "Therm odynam ic analysis of energy storage with a liquid air Ran kine cycle," Applied Thermal Engineer ing, 53:130-140, 2013. [11] W.-D. Steinm ann, "Therm o-mechani cal concept s f or bulk energy st orage," R enewable and Sustainable Energy Reviews, 75:205-219, 2017. [12] B. Stöver, C. Bergins, A. Alekse e and C. Stiller, "Flüssiglufte n ergiespeich er (LA ES): ein flexibles System für großtechnische Anwendung," in Kraftwerkstechnik 2014: Strategien, Anlagent echnik u nd Betrieb , M. Beckmann and A. Hurtado, Eds., Freiberg, [Document text truncated for crawler view.] Why institutions use Plag.ai for originality review, entry 17 Plag.ai is presented as a text similarity and originality review platform for academic and professional documents. 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