ene gies
A icle
Decision Suppo Sys em o Inno a i e High-Tempe a u e
La en Hea S o age o Was e Hea Reco e y in he
Ene gy-In ensi e Indus y
Pa icia Royo 1,2,* , Luis Ace edo 1,2,Ál a o J. A nal 1,* , Ma yo i Diaz-Ramí ez 1,2 ,
Ta iana Ga cía-A mingol 1,2, Vic o J. Fe ei a 1,2, Ge mán Fe ei a 2and Ana M. López-Sabi ón1,2


Ci a ion: Royo, P.; Ace edo, L.;
A nal, Á.J.; Diaz-Ramí ez, M.;
Ga cía-A mingol, T.; Fe ei a, V.J.;
Fe ei a, G.; López-Sabi ón, A.M.
Decision Suppo Sys em o
Inno a i e High-Tempe a u e La en
Hea S o age o Was e Hea
Reco e y in he Ene gy-In ensi e
Indus y. Ene gies 2021,14, 365.
h ps://doi.o g/10.3390/en14020365
Recei ed: 11 Decembe 2020
Accep ed: 8 Janua y 2021
Published: 11 Janua y 2021
Publishe ’s No e: MDPI s ays neu-
al wi h ega d o ju isdic ional clai-
ms in published maps and ins i u io-
nal a ilia ions.
Copy igh : © 2021 by he au ho s. Li-
censee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and con-
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ibu ion (CC BY) license (h ps://
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1Fundacion CIRCE—(Resea ch Cen e o Ene gy Resou ces and Consump ion), A enida Ranillas,
Edi icio Dinamiza 3D, 50018 Za agoza, Spain; [email p o ec ed] (L.A.); mdiaz@ ci ce.es (M.D.-R.);
ga cia@ ci ce.es (T.G.-A.); j e ei a@ ci ce.es (V.J.F.); amlopez@ ci ce.es (A.M.L.-S.)
2Ins i u o Uni e si a io de In es igación Mix o CIRCE—(Fundacion CIRCE—Uni e sidad de Za agoza),
Pa que Emp esa ial Dinamiza, A enida Ranillas 3D, 50018 Za agoza, Spain; ge man @uniza .es
*Co espondence: [email p o ec ed] (P.R.); aja nal@ ci ce.es (Á.J.A.)
Abs ac :
Reduc ions in ene gy consump ion, ca bon oo p in , equipmen size, and cos a e key
objec i es o he o hcoming ene gy-in ensi e indus ies oadmaps. In his sense, solu ions such as
was e hea eco e y, which can be eplica ed in o di e en sec o s (e.g., ce amics, conc e e, glass, s eel,
aluminium, pulp, and pape ) a e highly p omo ed. In his line, la en hea he mal ene gy s o age
(TES) con ibu es as an inno a i e echnology solu ion o imp o e he o e all sys em e iciency by
eco e ing and s o ing indus ial was e hea . To his end, phase-change ma e ial (PCM) selec ion is
assis ed h ough a decision-suppo sys em (DSS). A simpli ied ool based on he MATLAB
®
model,
based on co ela ions among he mos ele an sys em pa ame e s, was de eloped o p o e he
easibili y o a c oss-sec o ial app oach. The esea ch wo k conduc ed a pa ame ic analysis o assess
he echno-economic pe o mance o he PCM-TES solu ion unde di e en wo king condi ions
and sec o s. Addi ionally, a mul ic i e ia assessmen was pe o med compa ing he ool ou pu s
om me al alloys and ino ganic hyd a ed PCM sal s. O e all, he ino ganic PCMs p esen ed highe
ne economic and ene gy sa ings (up o 25,000
€
/y ; 480 MWh/y ), while me al alloys in ol ed
p omising esul s, sho e cycles, and compe i i e economic a ios; i s comme cial de elopmen is
s ill limi ed.
Keywo ds:
decision suppo sys em; he mal ene gy s o age; was e hea eco e y; phase change
ma e ials; ene gy-in ensi e indus y; sys em in eg a ion; mul ic i e ia analysis
1. In oduc ion
Imp o ing e iciency as an o e all concep plays a c ucial ole in he de elopmen
o sus ainable ene gy policies ega ding indus ial sec o s and clima e change mi iga ion
ac ions [
1
]. Speci ically, he objec i es o he Ene gy Union (EU) [
2
] a e mean o achie e an
in eg a ed ene gy ma ke wi h inc easing ene gy in e connec ion boos ing compe i ion and
e icien use o esou ces. Among hese ac ions, he SET-Plan has con i med i s ole as he
key EU ene gy esea ch and inno a ion ini ia i e aim o achie e he Eu opean ansi ion o
clima e neu ali y by 2050. In o de o achie e he objec i es men ioned abo e, a se ies o
policies and measu es g ouped in o i e dimensions a e p oposed: deca bonisa ion, ene gy
e iciency, ene gy secu i y, inno a ion and compe i i eness, and in e nal ene gy ma ke
and esea ch [3].
Ene gy-in ensi e indus ies (EII) accoun o 80% o he o al indus ial ene gy con-
sump ion, becoming a majo ocus o ene gy and en i onmen al e iciency s a egies [
4
].
Among he speci ic measu es o he was e ene gy in he indus ial sec o , he p io i y
measu es con empla ed a e he eco e y o esidual ene gy (bo h hea and cold) and he
Ene gies 2021,14, 365. h ps://doi.o g/10.3390/en14020365 h ps://www.mdpi.com/jou nal/ene gies
Ene gies 2021,14, 365 2 o 13
in eg a ion o sys ems h ough p ocess op imisa ion and indus ial symbiosis [
3
]. The
SET-Plan also iden i ied he EII o i on and s eel p oduc ion as well as he chemical and he
pha maceu ical sec o s as he sec o s wi h he mos signi ican po en ial o ene gy sa ings
and high socio-economic impo ance [
5
] due o he gene a ion o alue-added p oduc s
and he associa ed job c ea ion. Fu he mo e, he ce amic sec o is also a ele an example
wi hin he manu ac u ing indus y because o he high uel consump ion equi ed in i ing,
d ying, and sp ay d ying p ocesses [6].
In he indus ial sec o , a subs an ial pa o he ene gy is con e ed in o was e hea
due o ine iciencies, o which 50% co esponds o empe a u es abo e 250
◦
C [
7
]. Aligned
o he aising awa eness o global wa ming e ec s and inc emen o uel p ices, was e hea
eco e y and u nace e o i ing ha e a di ec and bene icial impac on he e iciency o
he p ocess and, consequen ly, on he educ ion o consump ion, en i onmen al pollu ion,
and size and cos o equipmen . Especially in he EII sec o s [
8
], he e exis s a g ea po-
en ial o ake ad an age o he exhaus s eam o gases, especially a high empe a u es.
Jouha a e al. [9]
comp ehensi ely e iewed was e hea eco e y me hodologies and ech-
nologies used o indus ial p ocesses in s eel and i on, ood, and ce amic sec o s, poin ing
ou he applicabili y o echnologies based on sensible hea exchange such as ecupe a o s,
egene a o s, passi e ai p ehea e s, hea exchange based on la pla es, economise s, and
uni s such as was e hea boile s and un a ound coil. O e he o al was e hea po en ial in
he EU (300 TWh/y ), he ep esen a i i y sha e co esponds o low-g ade below 200
◦
C
(33%), medium-g ade (25%), and high-g ade (33%) was e hea abo e 500 ◦C [6].
Among he ene gy e iciency al e na i es, i is wo h men ioning he ele ance o he -
mal s o age sys ems (TES) o inc ease he sys em lexibili y and o mi iga e he decoupling
be ween ene gy gene a ion and demand. Se e al success ul cases ha e been ound in o he
ields, such as he in eg a ion o s o age solu ions in buildings and enewable ene gies
p oduc ion owa ds sus ainable ene gy [
10
]. Rega ding i s applica ion on indus ial en i-
onmen s, Gibb, e al. [
11
] poin ed ou ha a majo challenge is iden i ying he pe o mance
ac o s ha make a TES sui able and consequen ly ma ching he mos bene icial s o age
sys ems wi h an app op ia e p ocess. I needs a p ecise me hod and an e alua ion p oce-
du e o TES sys ems in eg a ed in o di e en applica ions, such as he one de eloped in
Annex 30 o he In e na ional Ene gy Agency (IEA) echnology collabo a ion p og amme
o Ene gy Conse a ion h ough Ene gy S o age (ECES) [
12
]. This me hodology e alua es
TES sys ems in eg a ed in o p ocesses and de ines some p ocess analysis guidelines o
do so. Howe e , his me hodology would need o be u he adap ed o he EII sec o
applica ion and in eg a ion mo e speci ically.
Recen ly he main challenge is o ocus on was e hea eco e y (WHR) and TES sys-
ems wo king a high empe a u es. In his scena io, he e is a majo limi a ion in e ms o
ma e ial a ailabili y and ope a ing condi ions [
13
]. The e o e, he selec ion o sui able ma e-
ial is a c ucial aspec o he PCM-TES design [
14
]. In his sense, Fe nandes e al. [
15
] ca ied
ou an in-dep h analysis o high- empe a u e TES ma e ials. Main esul s highligh ed he
me al oams as a p omising al e na i e o ino ganic sal s o imp o e he he mo-mechanical
p ope ies. A medium empe a u e was e hea quali y, Fe ei a e al. [
16
] analysed he
en i onmen al beha iou o wen y indus ial applica ions combining ou PCM-TES sys-
ems a ying he ype o sal (PCM) inco po a ed and ob aining e y p omising esul s.
A high- empe a u e anges, Royo e al. [
17
] p oposed PCM-TES con igu a ions wo king
a high empe a u es, namely, a shell and ube s uc u e wi h PCM con ained in o double
concen ic ubes, a PCM-TES sys em o med by wo hea exchange (HX) modules and
a hea ans e luid (HTF), a c oss low sys em in a double HX chambe illed wi h PCM
ubes, and an in e changeable c oss low wi h inned PCM ubes. O e all, ino ganic mol en
sal s and me al-based alloys could be used as PCM a high empe a u es.
All hings conside ed, his pape p oposes a decision-suppo sys em (DSS) o ind
sui able PCMs wo king a high empe a u es. The me hodology was based on a echno-
economic analysis and en i onmen al assessmen o de e mine he po en ial o imp o ing
e iciency, educing en i onmen al impac , and cos sa ings. Di e en indica o s we e
Ene gies 2021,14, 365 3 o 13
selec ed wi h he objec i e o e alua e and cha ac e ise he pe o mance o TES, especially
in he EII in eg a ion. In his sense, a case-s udy conside ing wo di e en ypes and
na u es o PCMs was conside ed, namely, ino ganic mol en sal s and wo di e en me al-
based PCMs we e assessed. As an ul ima e pu pose, he p esen wo k aims a boos ing
and demons a ing he easibili y o PCM-TES sys ems a indus ial scale o eco e ing
was ed ene gy om EIIs and o e coming he cu en lack o in o ma ion, especially a
high empe a u es.
2. Ma e ials and Me hods
2.1. Me hodology o he PCM Feasibili y Tool
The DSS ool aims a e alua ing he easibili y and he eplica ion po en ial o a PCM-
sys em in eg a ion in an EII plan , cen ed on he e o i ing o u naces o imp o ing
hei ene gy e iciency along he alue chain o o he plan s. I is based on a co e algo i hm
implemen ed in MATLAB
®
so wa e c ea ed using co ela ions ob ained om modelling
nume ous s udy cases. The diag am in Figu e 1desc ibes he me hodology s eps ollowed,
s a ing wi h he in oduc ion o he inpu da a om an indus ial plan o he alida ion o
he PCM-TES sys em con igu a ion.
Figu e 1. Diag am o me hodology, inpu s, and ou pu s o he easibili y ool.
Fi s ly, he use willing o ins all a PCM-TES is equi ed o en e he inpu da a o
he plan p ocesses. The mos common was e hea s eams include exhaus gases, la ing
gases, ho ai , o s eam. Addi ionally, i migh be possible eco e ing hea e en om ho oil,
wa e a e e ige a ion p ocesses, and ho was e and p oduc s a e he mal p ocessing.
Typical hea sou ces examples in he EII a e mel ing and hea ing u naces o kilns, boile s,
incine a o s, he mal ea men s, and s eam ne wo ks.
Based on ha , he easibili y ool can choose an app op ia e PCM adap ed o he
plan needs (sea ching in he PCM da abase, which also p o ides in o ma ion ega ding
mel ing empe a u e, cos s, and o he p ope ies). In unc ion o he inpu and he PCM
p ope ies, ele an pa ame e s o he PCM-TES sys em con igu a ion a e calcula ed (mass,
olume, capaci y, cycle pe iod, and in es men o he s o age sys em). Was e ene gy
om he p e iously men ioned sou ces can be eco e ed and eused in ano he plan
p ocess o e en in o he plan s (inc easing he syne gy o indus ial pa ks) o di e en
Ene gies 2021,14, 365 4 o 13
pu poses depending on he hea quali y. The mos common applica ions a e p ehea ing o
combus ion ai o loads be o e en e ing he u nace and o o he ups eam and downs eam
p ocesses (e.g., d ying). In his case, he p ehea ing o combus ion ai o inc ease u nace
e iciency was conside ed.
The ool ou pu s allow analysing he PCM-TES sys em beha iou om an o e all
pe spec i e (mul ic i e ia analysis), including echnical, economic, and en i onmen al
impac s. Se e al key indica o pa ame e s (KPI) we e chosen wi h he pu pose o assessing
he TES. Some o hem we e based on he indica ions o he me hodology p oposed by
IEA ECES-Annex 30 [
12
] bu adap ed o indus ial applica ions. I i esul s in a easible
p ojec acco ding o hose indica o s, an EII plan could implemen he PCM-TES in e-
g a ed in o i s p oduc ion p ocess. In his case, i would be ad isable o moni o he key
indica o s om he PCM-TES ope a ion and pe o mance, hus esul ing in an i e a i e
e o eeding p ocess.
Finally, he esul s coming om he implemen a ion o he sys em in he EII plan
could be in oduced as eedback o he ool o an e ol ing op imisa ion and ine- uning
p ocess. Thus, he co e algo i hm o he ool can be con inuously upda ed wi h his new
inpu da a. In his sense, he ool esul s could be applicable and alida ed in a b oad ange
o indus ies and wo king unde di e en ope a ional condi ions.
2.2. PCM Da abase
The mel ing empe a u e o he PCM mus be app op ia ely selec ed o ake as much
ad an age as possible om he was ed hea . The selec ion o he PCM was made e alua ing
he he mophysical p ope ies o di e en compounds ha accomplish he equi emen o
he applica ion sys em, ha is, mel ing empe a u e, la en hea o usion, densi y, he mal
conduc i i y, cos s, ma e ial composi ion, and comme cial ma u i y [
18
,
19
]. This selec ion
a ec s he he mal capaci y o he sys em, he quali y o hea accumula ed, he empe a u e
o he ou le s eams, and he sizing o he PCM-TES.
When he PCM was chosen, i s echnical p ope ies de ined in he da abase we e
cha ged o pe o m he calcula ions. The cu en da abase includes a lis o comme cially
a ailable PCM [
20
] and o he po en ial ma e ials o wo king a high empe a u es [
21
].
They ha e been classi ied in o ou ca ego ies in unc ion o he composi ion and he
empe a u e ange:
•o ganic PCM a low empe a u e
•ino ganic sal hyd a es a low empe a u e
•ino ganic sal hyd a es and eu ec ics a high and medium empe a u e
•me al alloys PCM a medium and high empe a u e
I is essen ial o men ion ha he majo limi a ion o ino ganic sal s is hei e y low
he mal conduc i i y. Hence, he PCM-TES sys em equi es cha ging/discha ging pe iods
ha a e oo long, and i does no allow one o quickly ans e all hea o he combus ion ai .
On he con a y, a mo e apid cha ging/discha ging pe iod would enhance i s adap abili y
o a wide a ie y o p ocess in eg a ion, inc easing he eplica ion in o he EII sec o s
and he o e all sys em lexibili y. The combus ion ai speed and he HX ans e ence a ea
should be maximised, hence inc easing he con ec ion and he hea ans e coe icien [
22
].
In his ega d, he discha ging pe iod can be educed wi hou comp omising he g ea
anges eached a he combus ion ai ou le . To do so, he in eg a ion o me allic ins [
23
]
and he c ea ion o composi es mixed wi h PCM [
24
] ha e become a majo ocus o he e-
sea che s. O he he mal conduc i i y enhancemen echniques/me hods a e, o example,
he inco po a ion o po ous me allic oam, s uc u es, o ad anced nanoma e ial and he
conside a ion o encapsula ing he PCM ma e ial a mic o o mac o scales [25].
Despi e he low s o ed ene gy a e by mass uni in compa ison wi h ino ganic sal s,
me al alloys a e an a ac i e and up-and-coming al e na i e [
26
], especially a he high-
empe a u e ange, since hey o e in e es ing he mo-physical p ope ies o be used as
PCMs in TES. Fo ins ance, hese compounds usually p esen high alues o he mal con-
duc i i y, which signi ican ly educes mel ing and solidi ica ion pe iods and, subsequen ly,
Ene gies 2021,14, 365 5 o 13
cha ging and discha ging o he sys em. Howe e , he e is a lack o unde s anding o he
impac o me allu gical issues, mainly ela ed o non-desi ed eac ions and issues in he ma-
e ials’ phase- ansi ion beha iou ( apou p essu e, unde cooling, co osion, seg ega ion,
changes in composi ion and mic os uc u e) unde he mal cycling a high empe a u es [
21
].
2.3. Equa ions and Co ela ions
Rega ding PCM sys ems, he hea ans e de ini ion du ing he mel ing and he
solidi ica ion p ocesses is he basic p oblem ha de e mines he sys em e iciency [
27
]. One
o he pa icula i ies in he sizing is he de ini ion o i s la en hea s o age capaci y (
QLHS
),
which is used o size he PCM mass (mPCM) in he sys em acco ding o Equa ion (1):
mPCM =QLHS
H usion
(1)
being he a io o he
QLHS
and he la en hea o usion o he selec ed PCM (
H usion)
. The
PCM-TES size is de e mined by conside ing he hea demand co e ed only by la en hea
(
QLHS
) and by assuming ha he sys em ope a es wi hin he phase- ansi ion ange. Then,
he olume o he PCM needed is calcula ed (
VPCM
) conside ing he s a us phase (solid o
liquid) wi h he lowes densi y.
The cha ging (
CH
) and he discha ging (
DCH
) iming a e calcula ed using
Equa ions (2) and (3), espec i ely:
CH =QLHS
QCH
(2)
DCH =QLHS
QDCH
(3)
conside ing he s o age capaci y o he sys em (
QLHS
) and he hea gene a ed by he
hea sou ce du ing cha ging (
QCH
) o he hea demand o ai p ehea ing applica ion
du ing discha ging (
QDCH
). The cha ge pe iod is conside ed while he empe a u e o
ho lue gases is g ea e han he PCM phase-change empe a u e un il he maximum
s o age capaci y is achie ed. The discha ge pe iod is main ained while he expec ed ou le
empe a u e is eached. Howe e , his calcula ion is heo e ical, hence, o adap i o he
eal pe o mance and he sys em ou pu s, co ec ion ac o s a e needed o include he
in luence o he sys em e iciency and he hea ans e ence o he pa icula PCM-TES
design and con igu a ion used.
The co e algo i hm is based on he ou pu s ob ained om de ailed simula ion s udies
pe o med unde a a ie y o wo king ope a ion condi ions a high empe a u es and using
di e en PCM ma e ials o s o ing he was ed hea in se e al EII. The ob ained esul s
we e analysed and used o eed he ool co e and ind ep esen a i e co ela ions be ween
he PCM-TES pe o mance pa ame e s. Fo mo e in o ma ion, a de ailed example o he
simula ion me hodology and esul s can be ound in Royo, e al. [28].
In his line, and in o de o ob ain he sys em ou pu s, di e en ac o combina ions
we e applied o he PCM-TES con igu a ion, namely:
•Sizing o he PCM-TES (mass and olume)
•Numbe o concen ic ubes inside he shell
•PCM al e na i es as s o age ma e ial
•Was e hea condi ions (composi ion, empe a u e, and mass low o lue gases)
•Wo king ope a ion condi ions ( empe a u e and mass low o combus ion ai )
•EII sec o s wi h p ocesses in he ange om 500 o 1500 ◦C (ce amic and s eel)
Consequen ly, he mos ele an pa ame e s (MRP) o ope a ion a e iden i ied and
analysed unde hose condi ions. To do so, a pa ame ic analysis and a sensi i i y analysis
we e conduc ed. As a esul , he mal conduc i i y, olume, and s o age capaci y o he PCM

Ene gies 2021,14, 365 6 o 13
a ose as he mos in luencing pa ame e s, and hey we e conside ed o he co ela ions
and he co ec ion ac o s.
Then, he ou pu s om he simpli ied compu a ional ool (X, Y) in ol e a close connec-
ion o some echnical and design pa ame e s (PCM mass, sys em olume, empe a u es o
lue gases ou le , empe a u e o he ai s eam ou le , cha ging and discha ging pe iods,
in es men cos s). These ela ionships may be linea , polynomial, loga i hmic, po en ial,
e c. The selec ion o p ope co ela ions consis s o inding a s ong co ela ion, ha is, high
squa ed-co ela ion coe icien (R
2
), and minimising he de ia ion o he ela ionship calcu-
la ed compa ed wi h he de ailed simula ion esul s. An example o a possible co ela ion
is p esen ed in Equa ion (4):
Xi=a·MRPb·xi(4)
whe e aand ba e he co ela ion coe icien s and he MRP. Fo ins ance, in o de o
de e mine he solidi ica ion and he mel ing pe iods, he MRPs chosen we e he sys em
s o age capaci y and he PCM he mal conduc i i y.
O he co ela ions ollow he s uc u e ep esen ed in Equa ion (5):
Yi=ci+ MRPi·di(5)
c
i
and d
i
being coe icien s o he linea equa ion and he MRP
i
applicable o he speci ic “i”
ou pu equa ion. Fo ins ance, his co ela ion ype was applied o calcula e he in es men
cos o he PCM-TES sys em in unc ion o he
VPCM
ha needed o be in oduced in he
PCM-TES and he ope a ing empe a u e om which he hea was going o be eco e ed.
2.4. Mul ic i e ia App oach
A mul ic i e ia app oach was applied o e alua e ene gy, economic, and en i onmen al
sa ings. F om he ene gy pe spec i e, he hea ha was eco e ed and able o be eused
was he mos aluable inpu , along wi h he PCM-TES sys em pe o mance indica o s
de ined in he abo e sec ion. The modelling esul s conside ha he hea sou ces we e
a ailable o ull cha ging o discha ging o he PCM-TES sys em.
In he economic assessmen , he echno-economic indica o s and cos s incu ed o a
u nace including a PCM-TES sys em we e calcula ed o he iabili y assessmen . Namely,
he ne sa ings (k
€
/y ) we e conside ed o calcula e he ne sa ings, simila o a ne p esen
alue (NPV) wi h no in e es a e, and he a io
€
sa ed/
€
in es ed p esen s an es ima ion
o he p o i abili y o he PCM-TES as WHR s a egy. The in es men cos o he PCM-TES
sys em was calcula ed conside ing bo h he cos s o he s o age co e ma e ial ( he PCM
i sel ) and he cos s o he s o age sys em in as uc u e. This cos ollowed he linea
co ela ion in Equa ion (5) as a ixed cos (c
i
) in addi ion o a a io in unc ion o he
PCM olume o be s o ed (
VPCM
) as one o he MRP. This a io (d
i
) was highe when he
ope a ional empe a u e o he lue gases and he PCM mel ing empe a u e we e a e y
high le els (>700
◦
C), since he ma e ial cos s ise conside ably. Wi h his in o ma ion,
i was also possible o calcula e he la en s o age capaci y cos (
€
/kWh) o he di e en
PCM-TES sys ems analysed as he a io o he o al in es men cos s o a sys em and
he o al amoun o he mal ene gy [kWh] ha his sys em s o es and eco e s du ing
i s li e ime.
Du ing cha ging, he e was no any sa ing in compa ison o he ini ial ope a ing mode
o he u nace. Howe e , signi ican ossil uel sa ings (NG
sa ed
in economic e ms) we e
ob ained annually when he PCM-TES sys em discha ged he accumula ed hea acco ding
o Equa ion (6).
NGsa ed =Esa ed ·nºcycles·ndays ·LHVNG ·ρNG ·cNG (6)
E
sa ed
being he ene gy sa ed pe one cycle o ope a ion, n
ºcycles
he numbe o daily cycles
ha he PCM-TES could pe o m as maximum depending on he discha ge+cha ge pe iods,
and n
days
is he numbe o wo king days conside ed pe yea (260 days). The conside a-
Ene gies 2021,14, 365 7 o 13
ions assumed o he na u al gas (NG) a e: lowe hea ing alue (LHV) = 15.75 kWh/kg;
densi y (
ρNG
) = 0.743 kg/Nm
3
, and p ice (
cNG
) = 0.052
€
/kWh [
29
] o non-household con-
sume s wi hin he Eu opean Union. Besides, he dep ecia ion cos o he PCM-TES sys em
a ibu able o each yea o ope a ion, a li espan o 20 yea s, was conside ed. Finally, he
global wa ming indica o was selec ed as he mos ele an en i onmen al indica o due o
he aising awa eness o clima e change. The emission ac o s belonging o ReCiPe me hod
e alua ed wi h SIMAPRO
®
so wa e we e used o calcula e he equi alen CO
2
emissions.
3. Resul s o a S udy-Case
3.1. S udy-Case Desc ip ion
The PCM-TES con igu a ion unde s udy (mo e design de ails in e e ence [
28
]) o e s
se e al oppo uni ies o in eg a ion in an indus ial plan . In he p esen applica ion,
he sys em is loca ed close o he mel ing u nace o eco e hea om he exhaus gases
a e combus ion. The hea s o age could se e o many pu poses, such as p ehea ing
he combus ion ai going o he u nace inle , inc easing he load empe a u e, and d ying
p ocesses, among many o he ups eam and downs eam p ocesses. In his case, he main
objec i e o he PCM con igu a ion was p ehea ing he combus ion ai o imp o e he
e iciency o he o e all hea ing p ocess.
The ool was ed by he pa ame e s illus a ed in Table 1, conside ing hose as ep-
esen a i e da a o an EII indus y. Fo he p esen design, he wo s eam- lows (was e
gases and ai ) ci cula ed sepa a ely, a oiding undesi able mixing o con aminan and
ouling compounds, e y common, o example, in exhaus gases om ce amic plan s. As a
concep ual idea and p elimina y calcula ion pu poses, i was assumed he PCM sys em
could elease a cons an combus ion ai empe a u e. Mo eo e , negligible ans e ence
losses in PCM solu ion we e conside ed.
Table 1. Pa ame e s o he hea - eco e y and phase-change ma e ial (PCM) solu ion con igu a ion.
Inpu s Value Ou pu s Value
Flue gases low en e ing o PCM solu ion (Nm3/h) 1800 Ho combus ion ai o he PCM solu ion (Nm3/h) 1800
Tempe a u e o lue gases en e ing PCM solu ion (◦C) 1000 Tempe a u e o combus ion ai en e ing PCM solu ion (◦C) 25
Ra io o lue gas low o he PCM solu ion (% o gas s eam)
90
Desi ed empe a u e o combus ion ai lea ing PCM solu ion (
◦
C)
600
3.2. Selec ed PCM Al e na i es as S o age Ma e ial
The eby, a PCM wi h a phase-change empe a u e o e 600
◦
C was selec ed as he
co e ma e ial o he TES sys em o achie e he desi ed empe a u e speci ied in
Table 1
.
In addi ion, he selec ed PCMs should (i) abso b/ elease conside able ene gy du ing
mel ing/solidi ica ion, espec i ely; (ii) p esen well-de ined and s able phase change
empe a u e; (iii) a oid supe cooling; (i ) be s able o e eeze/mel cycles; ( ) be non-
haza dous and non-co osi e; and ( i) be p o i able o he applica ion [28].
He ea e , i is p esen ed a selec ion and an assessmen o ou di e en PCMs as
po en ial hea eco e y and s o age ma e ials wi hin he desi ed ange. Two ino ganic
hyd a ed sal s and wo me als, whose p ope ies a e ga he ed in Table 2, we e chosen as
al e na i es. The pa ame ic analysis allows compa ing he pe o mance o ypes o PCM,
e y di e en in na u e, composi ion, cos s, and he mo-physical p ope ies, in ol ing
adap abili y o hea ing and p oduc ion p ocesses.
As men ioned in Sec ion 2.2, he DSS ool includes a PCM da abase wi h hei espec-
i e p ope ies and cos s acco ding o manu ac u e s and p o ide s ha could go up o
25
€
/kg. In he compa ison, di e en al e na i es we e selec ed o wo k as me al alloys
and ino ganic sal s; om he a ailable op ion a he empe a u e ange o ope a ion o he
s udy case, he mos cos -e ec i e op ions we e chosen. A his poin , i was impo an o
choose an a o dable op ion by sligh ly a ying he selec ion o he phase-change ange
o ind a sui able op ion wi hou s ongly a ec ing he sys em pe o mance, esul ing in a
mo e economic sys em. A p esen , he wide a ailabili y o ino ganic sal s allows selec ing
mo e cos -e icien ma e ials.
Ene gies 2021,14, 365 8 o 13
Table 2. Main p ope ies o he PCM selec ed o he pa ame ic analysis. Da a om [20,21,30].
PCM Type Composi ion Mel ing
Poin (◦C)
La en Hea
(kJ/kg)
Densi y
(kg/m3)
The mal Conduc i i y
(W/m K)
Cos
(€/kg)
Mg Comme cial pu i y 648 365 1740 157 2.5
Al Comme cial pu i y 661 388 2700 236 2.3
Hyd a ed sal
(HS1) Na2CO3640 338 2380 0.559 0.25
Hyd a ed sal
(HS2) Na2CO3+ K2CO3+ Li2CO3687 300 2450 0.557 0.25
3.3. PCM-TES Sizing and MRP Pe o mance
The ool is capable o de e mining he mos ele an sizing pa ame e s acco ding o
he me hodology p e iously p esen ed, aking in o accoun he mo-physical and economic
p ope ies o he selec ed PCMs shown in Table 2. The pa ame e s we e calcula ed conside -
ing size speci ica ions de ailed in he sec ion abo e based on a shell-and- ube con igu a ion
wi h he PCM encapsula ed in double concen ic ubes and o ul il an ene gy demand o
393 kWh/h by he PCM-TES.
In Table 3, he pa ame ic analysis esul s ega ding design, cos , and pe o mance
o he applica ion o a PCM-TES equipmen o eco e and s o e a e p esen ed and we e
conduc ed by he p e- easibili y and eplica ion ool.
Table 3. Main design, pe o mance, and cos s pa ame e s om he pa ame ic analysis.
Design Pa ame e PCM-Mg PCM-Al PCM-HS1 PCM-HS2
Mass o PCM (kg) 3877 3647 4186 4716
Volume o PCM (m3)2.23 1.35 1.76 1.93
Cos o he PCM (€) 9691 8388 1047 1179
Discha ging+cha ging cycle (h) 0.07 0.06 7.5 8.4
Discha ging/cha ging a io 0.21 0.19 0.27 0.22
nºcycles—Daily cycles (cycles/day) 303 407 3.20 2.85
P ehea ed combus ion ai empe a u e (◦C) 589 601 582 625
Esa ed—Ene gy sa ed (MJ/cycle) 17.8 12.7 2079 2114
I was obse ed ha me als and alloys equi ed less mass due o i s high s o age
capaci y. The esul ing PCM olumes we e all be ween 1.35 and 2.23 m
3
; he aluminium
p esen ed he lowes olume due o i s high densi y, which would in ol e much mo e
compac PCM-TES sys em (60% olume educ ion). This ac would allow a educ ion
in cos s, while mo e signi ican olumes a e usually ela ed o highe cons uc ion and
ma e ial cos in es men . Rega ding he PCM cos s, he me al alloys a e much mo e
expensi e han he hyd a ed sal s ma e ials.
Con e sely, he hyd a ed sal s equi ed much longe cha ging and discha ging s ages
(app oxima ely 5–10 h pe cycle) o co e he ene gy demand in his case s udy. The
maximum cycles ha he PCM-TES sys em could unde go e e y day we e e y g a ed
due o he sho pe iods in which he me al alloys we e in ol ed, mainly hanks o hese
ma e ials’ g ea he mal conduc i i y. Ano he pa ame e was he a io be ween he
discha ging and he cha ging ime as a ep esen a ion o how long he PCM bene i s could
las , he hyd a ed sal s showing an o e all highe a io. Thus, he cycle ime and he
combus ion ai empe a u e achie ed a he PCM-TES sys em’s ou le (which depended
on he mel ing empe a u e o he selec ed PCM) s ongly in luenced he ene gy sa ed
pe cycle and day. The ene gy sa ed pe cycle was much highe in he hyd a ed sal s,
while he PCMs based on me al and alloys could cha ge and discha ge as amoun s o
hea in sho pe iods. Thus, he possible numbe o cycles pe o med pe day was o e
300 imes. This p ope y is also ela ed o he sys ems’ lexibili y po en ial, being mo e
adap a i e in he me allic PCMs. In his sense, he ype o PCM should be adequa ely
Ene gies 2021,14, 365 9 o 13
chosen o adap he discha ging and cha ging cycle ex ension in unc ion o he p oduc ion
p ocesses, conside ing i hey a e in e mi en , con inuous, o pe ba ches [31].
3.4. Mul ic i e ia and KPI Assessmen
Based on he p e ious esul s, ele an KPI a e calcula ed in Table 4 om echnical
(ene gy and uel sa ings), economic (cos in es men s and sa ings), and en i onmen al
(e alua ion o PCM by means o i s na u e and he na u al gas sa ed) pe spec i es.
Table 4. Key indica o pa ame e (KPI) e alua ion esul s.
KPI Resul s PCM-Mg PCM-Al PCM-HS1 PCM-HS2
Ene gy sa ed (MWh/y ) 389 373 480 435
Ne economic sa ing (k€/y ) 20.2 19.4 25.0 22.6
La en s o age capaci y cos (€/MWh) 28.0 20.1 18.1 21.4
NGsa ed / In es men cos (€/€) 1.86 2.59 2.88 2.43
To al en i onmen al impac o PCM ( CO
2
eq)
138 97 9 11
En i onmen al impac NG sa ed ( CO
2
eq/y )
−88 −86 −116 −105
Gi en ha he e is no a ia ion in he u nace p ocess as a esul o he inco po a ion
o PCM-TES, all he abo e scena ios p esen he same ene gy demand. The e o e, he
highe he combus ion ai empe a u e was, he sho e he PCM cha ging pe iod was,
consequen ly lowe ing he na u al gas ha mus be consumed in he mel ing p ocess. In his
sense, PCM-HS1 was he ma e ial able o sa e mo e ene gy in he p esen ed case. E en
hough he o he PCMs could p ehea he combus ion ai up o g ea e empe a u es, i had
he highes discha ging/cha ging a io (0.27), and his compensa ed he ai empe a u e
di e ence. A he opposi e end o he spec um, he lowes ene gy-sa ing ook place
when PCM-Al was selec ed. Despi e i s excellen he mophysical p ope ies as PCM, he
pe o mance was less ad an ageous, mainly due o i s low a io be ween discha ging and
cha ging pe iods (0.19).
F om he economic iewpoin , he wo ac o s which de e mine he new economic
sa ings a e he educ ion in NG consump ion and he PCM-TES in es men cos . Unde
his app oach, using me als as PCM is abou 10 imes mo e expensi e han using sal
hyd a es. Despi e achie ing mo e ene gy sa ings, he ini ial in es men o a PCM-Mg
sys em migh no compensa e o he ope a ional cos sa ings and, consequen ly, his
con igu a ion p esen s he lowes ne economic sa ings. On he o he hand, he mos
sui able op ion om he economic pe spec i e is he use o PCM-HS1, al e na i e wi h he
lowes in es men cos (a ound 1000
€
). Besides, i p o ides he g ea es na u al gas sa ings
(480 MWh/y and 25 k
€
/y ). In line wi h he abo e, he a ion ela ing
€
NG sa ed du ing
he PCM-TES li espan pe
€
o in es men is be e when PCM-HS1 is chosen as s o age
ma e ial (2.88
€
/
€
), ollowed by he use o PCM-Al. Simila ly, ega ding he la en s o age
capaci y cos , hyd a ed sal s showed good esul s, HS1 being he lowes wi h 18
€
/MWh
and he aluminium me al PCM wi h a alue nea ly o 20
€
/MWh. O e all, he in es men
cos o he in as uc u e needed o manu ac u e he PCM-TES sys ems e alua ed in he
p esen wo k had an es ima ed in es men cos in he ange be ween 140 and 210 k€.
I he compa ison is made om an en i onmen al pe spec i e compa ing he en i-
onmen al impac associa ed wi h each PCM ( alues on he igh in Figu e 2), PCM-Mg
p oduc ion has he highes ca bon oo p in (138 CO
2
eq.), ollowed by he emissions
incu ed in he p oduc ion o PMC-Al (97 CO
2
eq.). In ac , he GHG emissions o his
ma e ial a e up o 14 imes g ea e han he hyd a ed sal p oduc ion (abou 10 CO
2
eq.).
These alues we e calcula ed conside ing he impac o manu ac u ing and he amoun
o PCM equi ed acco ding o he di e en PCM-TES con igu a ion (based on hei la en
hea s o age capaci y).