Su ace p ocesses op imisa ion in a no el CO
2
-based elec o he mal ene gy
and geological s o age igene a ion sys em
☆
A. Ca o
a,*
, C. O iz
b
, S. Unge
c
, A. S oikos
d,e
, A.-S. Ky iakides
d
, I.N. Tsimpanogiannis
d
,
J.A. Bece a
a,
, S. Vou e akis
d
, U. Hampel
c
, R. Chaca egui
a,
a
Dp o. Ingenie ía Ene g´
e ica, Uni e sidad de Se illa, Camino de los Descub imien os s/n, 41092 Se illa, Spain
b
Ma e ials and Sus ainabili y G oup, Depa men o Enginee ing, Uni e sidad Loyola Andalucía, A da. De las Uni e sidades s/n, 41704 Dos He manas, Se ille, Spain
c
Helmhol z-Zen um D esden-Rossendo , Ins i u e o Fluid Dynamics, Bau zne Lands . 400, 01328 D esden, Ge many
d
Chemical P ocess and Ene gy Resou ces Ins i u e, Cen e o Resea ch and Technology-Hellas, 60361, 57001 The mi, Thessaloniki, G eece
e
Depa men o Mechanical Enginee ing, A is o le Uni e si y o Thessaloniki, 54124 Thessaloniki, G eece
Uni e sidad de Se illa, Labo a o y o Enginee ing o Ene gy and En i onmen al Sus ainabili y, 41092 Se ille, Spain
HIGHLIGHTS
•Op imised CO2 and ene gy s o age sys em enhances e iciency and ex ends applica ions.
•Sequen ial hea ans e imp o es he mal in eg a ion and b oadens demand co e age.
•Reducing phase change empe a u e o −40 ◦C inc eases e iciency ange o 56.6–67.7 %.
•Recupe a i e cycle and mul i-s age p ocesses imp o e e iciency & educe equi emen s.
•Al e na i e con igu a ions enable sys em o co e he mal demands om −50 o 270 ◦C.
ARTICLE INFO
Keywo ds:
Renewable ene gy s o age
Elec o he mal ene gy s o age
T ansc i ical CO
2
Geological s o age
CO
2
T igene a ion
ABSTRACT
Elec o he mal ene gy s o age is a p omising echnology o high pene a ion o enewable ene gy. In ecen
yea s, he in eg a ion o his ene gy s o age sys em wi h geological CO
2
s o age has been in oduced. The sys em
consis s o a e e sible hea pump o med by ansc i ical CO
2
cycles wi h he mal s o age a wo empe a u e
le els, enabling he simul aneous ope a ion o geological CO
2
s o age and he s o age/p oduc ion o enewable
elec ical ene gy. This wo k ocuses on s udying high and low- empe a u e he mal ene gy s o age. S ep hea ing
on he high- empe a u e side allows o be e in eg a ion o he supe c i ical and subc i ical empe a u e p o iles
o he CO
2
and he he mal s o age luid. The mal s o age a di e en empe a u e le els p o ides a highe
u bine inle empe a u e, imp o ing he e iciency o he powe p oduc ion cycle and inc easing hea ing ap-
plica ions such as dis ic hea ing o domes ic ho wa e . Conside ing ou high- empe a u e anks, ound- ip
e iciency inc eases om 52.8 o 55.4 %. I p esen s a he mal demand co e age ange o abou 20–150 ◦C,
wi h empe a u e inc eases o app oxima ely 30 ◦C. The phase change empe a u e shi on he low- empe a u e
side di ec ly impac s elec ic powe p oduc ion and enables new cooling applica ions. The sys em's e iciency
inc eases as he low- empe a u e phase change empe a u e dec eases, eaching 58.7 % a −30 ◦C. Using
al e na i e con igu a ions in he ansc i ical CO
2
cycle, such as he ecupe a i e cycle and mul i-s age
comp ession and expansion, high-e iciency alues can be main ained wi h lowe sys em equi emen s.
1. In oduc ion
The ansi ion owa ds 100 % enewable ene gy sys ems has eached
ad anced le els in di e en coun ies a ound he wo ld [1,2]. The in-
c ease in enewable elec ici y global capaci y eached 507 GW in 2023,
nea ly 50 % mo e han in 2022, ma king a signi ican shi in he global
g ow h end [3,4]. In 2024–2030, enewable ene gy capaci y
☆
This a icle is pa o a Special issue en i led: ‘SDEWES2024(B.N.)’ published in Applied Ene gy.
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (A. Ca o).
Con en s lis s a ailable a ScienceDi ec
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jou nal homepage: www.else ie .com/loca e/apene gy
h ps://doi.o g/10.1016/j.apene gy.2025.126165
Recei ed 9 Janua y 2025; Recei ed in e ised o m 27 Ap il 2025; Accep ed 16 May 2025
Applied Ene gy 395 (2025) 126165
A ailable online 31 May 2025
0306-2619/© 2025 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY license ( h p://c ea i ecommons.o g/licenses/by/4.0/ ).
inc emen s will con inue o ise, wi h gene a ion cos s lowe han hose
o ossil and non- ossil al e na i es in mos coun ies, suppo ed by po-
li ical ini ia i es [3]. The inc ease in enewable ene gy's sha e o he
elec ici y demand p esen s challenges in ene gy sys ems, depending on
he ex en o he elec ici y demand co e age [4]. In he Eu opean
Union, i is p ojec ed ha he po en ial pene a ion o enewable ene gy
in 2028 will exceed 50 % in nine coun ies, wi h o e 90 % o Denma k's
elec ici y gene a ion coming om wind and sola pho o ol aic sys ems
[5]. One phase behind a e he UK, Spain, Ge many, and I eland, whe e
a iable enewable ene gy (VRE) mee s nea ly all demand in some pe-
iods [6,7]. The inc easing p opo ion o enewable ene gies, especially
in coun ies wi h high pene a ion o wind and pho o ol aic sola en-
e gy, poses signi ican challenges ela ed o ene gy s o age [8,9].
Elec ici y s o age solu ions can mi iga e enewable ene gy in eg a-
ion issues in o he g id [10]. Cu en ly, wo complemen a y echnolo-
gies a e capable o p o iding cos -e ec i e s o age: s a iona y ba e ies
o 1–4 h [11] and pumped hyd oelec ic powe plan s (PHS) o 4–15 h
[12], depending on ese oi size. Suppo i e policies and he decline in
s a iona y ba e y cos s o e he las decade ha e spu ed in es men
be ween 2018 and 2023 in Eu ope, he Uni ed S a es, China, and
Aus alia [6]. They aim o capi alise on ma ke a bi age oppo uni ies
du ing peak hou s bu canno co e he daily o seasonal misma ch
be ween enewable esou ce a ailabili y and ene gy demand [13].
Long- e m s o age is key o achie ing e ec i e enewable deploymen in
ene gy sys ems [14]. I s deploymen emains limi ed compa ed o s a-
iona y ba e ies, especially whe e i is mos needed: ma ke s o ecas ed
o exceed a 50 % sha e o enewable-o igin elec ici y by 2028. Al hough
exis ing long- e m s o age capaci y in Eu ope, he Uni ed S a es, and
Japan will con ibu e o sys em in eg a ion, long- e m s o age needs a e
g owing apidly, equi ing in es men decisions oday o plan s ha
will be needed in 6–10 yea s [15].
E ec i e in eg a ion o enewables also equi es conside a ion o he
co e age o he mal ene gy demand [16]. Hea accoun ed o nea ly hal
o o al inal ene gy consump ion and 38 % o ene gy- ela ed CO
2
emissions in 2022 [6]. U ban hea ing ne wo ks o e conside able po-
en ial o enewable hea in eg a ion. They co e ed nea ly 7 % o global
hea demand om he building and indus y sec o s in 2022 [17]. Lowe
ope a ing empe a u es, in eg a ed he mal s o age, ad anced mea-
su emen , con ol, and op imisa ion s a egies o ou h and i h-
gene a ion u ban hea ing and cooling ne wo ks and local ambien
hea ci cui s can u he acili a e enewable ene gy in eg a ion [18]. In
he building sec o , he ins alla ion o hea pumps has played a c ucial
ole in mee ing he mal demand, esul ing in inc eased consump ion o
bo h elec ici y and hea o space and wa e hea ing [19]. La ge-scale
hea pump echnologies and sola he mal sys ems a e gene a ing
inc easing in e es in China and se e al Eu opean coun ies, wi h
400–500 MW
h
p ojec s unde de elopmen in 2023 [20]. The inc eased
elec ici y use o p ocess hea aises enewable hea consump ion bu is
insu icien o cu b ossil uel use. Acco ding o IEA [7] da a, he global
indus ial hea demand is p ojec ed o inc ease by 16 % be ween 2023
and 2028. I ossil uel use is no con ained, he hea ing sec o alone
could consume o e one- i h o he emaining ca bon budge o limi
global wa ming o 1.5 ◦C (86 G o CO
2
) in he pe iod 2023–2028 [6].
In his con ex o an u gen need o de elop new pho o ol aic and
wind ene gy s o age sys ems and co e age o elec ici y and hea de-
mands h ough enewable ene gy, elec o he mal ene gy s o age (ETES)
s ands ou . Recen ad ancemen s in pumped he mal ene gy s o age
(PTES) and liquid ai ene gy s o age (LAES) o e cos -e ec i e, scalable
al e na i es o PHS and CAES wi hou geog aphical limi s [21]. As
Ca no ba e ies (CB), hey in ol e a e e sible con e sion o elec ical
ene gy in o he mal ene gy [22], and co e a b oade ange o appli-
ca ions, which a e no accessible o ba e ies and pumped hyd o, such as
in eg a ion in o hea ing and cooling ne wo ks, opening up mo e
po en ially a ou able business cases [23]. A e iew o CB p ojec s and
li e a u e [23] e eals di e ences in layou s and scales, especially o
Rankine and B ay on PTES. PTES (in bo h Rankine and B ay on con-
igu a ions) seem o achie e highe e iciency han LAES, bu B ay on
PTES also ha e he highes speci ic cos . They ocus on di e en
ins alla ion sizes: Rankine PTES a 1–10 MW, B ay on 5–50 MW and
LAES o mo e han 50 MW. Rankine-based PTES s udies include s eam
cycles (and s eam-ammonia cascade cycles) [24], o ganic Rankine cy-
cles (ORC) [25] and ansc i ical CO
2
cycles [26]. The ound- ip e i-
ciency o he Rankine-based PTES cycle usually anges om [25] o 70 %
[24]. Me cang¨
oz and Mo andin [26,27] se he basis o elec o he mal
ene gy s o age sys ems using ansc i ical CO
2
cycles, demons a ing
a ound 50–60 % elec ic- o-elec ic con e sion e iciencies. This allows
o lexible elec ici y p oduc ion and di ec co e age o he mal ene gy
demands (hea ing and cooling) by s o ing he mal ene gy a wo em-
pe a u e le els and le elised cos o elec ici y (LCOE) o 70–140 USD/
MWh [28]. In hese sys ems, ansc i ical CO
2
cycles and he mal ene gy
s o age in wa e and ice (low-cos ma e ials wi h high a ailabili y and
easy access, low en i onmen al impac , and good he modynamic
p ope ies) play a p ominen ole [26].
One o he mos in e es ing applica ions o ansc i ical CO
2
cycles as
elec o he mal ene gy s o age sys ems includes CO
2
s o age wi hin un-
de g ound geological o ma ions, de eloped by Ca o e al. [29,30]. The
use o CO
2
as a wo king luid and he ope a ing condi ions o an-
sc i ical cycles allow he sys em o in eg a e he s o age o CO
2
in
Nomencla u e
CAES Comp essed Ai Ene gy S o age
CB Ca no Ba e ies
CC Ca bon Cap u e
CCS Ca bon Cap u e and S o age
CEEGS No el CO
2
-based Elec o he mal Ene gy and Geological
S o age
CO
2
Ca bon dioxide
ECE Elec ic- o-elec ic Con e sion E iciency
ETES Elec o he mal Ene gy S o age
EOR Enhanced Oil Reco e y
GS Geological S o age
h Speci ic en halpy (kJ/kg)
HE Hea Engine
HP Hea Pump
HT High Tempe a u e
hx Hea exchange
IEA In e na ional Ene gy Agency
LAES Liquid Ai Ene gy S o age
LCOE Le elised Cos o Elec ici y
LT Low Tempe a u e
M pa Mega- onnes pe annum
ORC O ganic Rankine Cycle
P P essu e (ba )
PHS Pumped Hyd o S o age
PTES Pumped The mal Ene gy S o age
Q Hea
Rec Recupe a i e
s Speci ic en opy (kJ/kg-K)
T Tempe a u e (◦C)
TIT Tu bine Inle Tempe a u e
TRL Technology Readiness Le el
TES The mal Ene gy S o age
VRE Va iable Renewable Ene gy
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
2
geological o ma ions such as po ous media (saline aqui e s, deple ed
oil/gas ese oi s) o sal ca e ns [31,32]. The bene i s o including
geological s o age in he ETES sys em include he mal and mechanical
ene gy s o age, geo he mal gains, la e al mig a ion, o pa ial seques-
a ion o s o ed CO
2
[33]. Wi h elec ic- o-elec ic con e sion e i-
ciencies exceeding 50 %, he sys em could achie e +1 M pa CO
2
conside ing 20 MW plan s [29]. These s udies o med he basis o he
“No el CO
2
-based Elec o he mal Ene gy and Geological S o age Sys-
em” (CEEGS) p ojec [34], cu en ly in p og ess, unded by he Eu o-
pean Commission h ough Ho izon, he EU F amewo k P og amme o
Resea ch and Inno a ion, unde G an Ag eemen No. 101084376. A
Technology Readiness Le el (TRL) 2, he sys em s ill has signi ican
de elopmen gaps, such as he op imal in eg a ion be ween he an-
sc i ical su ace cycles and he unde g ound CO
2
s o age.
This wo k p esen s a no el s udy o he p ocesses in ol ed in he
su ace ene gy con e sion cycles o he CEEGS sys em, ocusing on
op imising he mal s o age a bo h empe a u e le els and app oaching
al e na i e con igu a ions o he hea pump and he powe p oduc ion
cycles. The analysis o new scena ios is p esen ed, such as he ecupe -
a i e con igu a ion o he cha ging phase cycle and he use o s aged
comp ession and expansion wi h hea eco e y. The s udy o s aged
sensible hea ans e on he HT side is in oduced, p o iding be e
in eg a ion o supe c i ical and subc i ical empe a u e p o iles o he
CO
2
and he he mal s o age luid and es ablishing di e en empe a u e
le els o he mal ene gy demand co e age. The impac o phase change
empe a u e on he LT-TES side is also in es iga ed. The phase change
empe a u e in luences he shape o ansc i ical CO
2
cycles and is he
mos impo an ac o in sys em cooling applica ions. This s udy dem-
ons a es how enewable ene gy s o age and lexible powe gene a ion
sys ems, wi h he s o age o CO
2
cap u ed in geological o ma ions,
acqui e he po en ial o lexible co e age o many high- and low-
empe a u e he mal ene gy demands. The CEEGS sys em expands he
empe a u e ange o he mal ene gy in he o m o sensible hea , om a
single empe a u e limi ed o abou 140–150 ◦C [29,30] o a ious
empe a u es, anging om −30 ◦C o 275 ◦C. The a ailabili y o he -
mal ene gy in he o m o la en hea also expands i s possibili ies,
allowing co e age in a ange o −50 ◦C o 20 ◦C, depending on he phase
change empe a u e. The elec ic- o-elec ic e iciency is used as a
e e ence indica o , anging om 58.54 o 71.09 % when he phase
change empe a u e is −50 ◦C.
2. Me hods
2.1. Resou ces
Fo he s udy, nume ical modelling was conduc ed using he Py hon
p og amming language along wi h a ious specialised lib a ies. In
implemen ing he model, he CoolP op lib a y [35] was used o calcu-
la e he di e en he modynamic p ope ies o he subs ances in ol ed
in he s udy [36]. This combina ion o ools enabled a de ailed and ac-
cu a e sys em ep esen a ion, acili a ing he analysis o i s pe o mance
unde a ious ope a ional condi ions.
2.2. No el CO
2
-based elec o he mal ene gy and geological s o age sys em
as igene a ion echnology
The sys em unde s udy is he no el in eg a ed elec o he mal ene gy
s o age and CO
2
s o age sys em in geological o ma ions. The concep
o igina es om he combina ion o wo echnologies wi h di e en ob-
jec i es. On he one hand, a e e sible hea pump o Ca no ba e y is
used o enewable ene gy s o age and subsequen elec ici y p oduc ion
h ough elec ical- o- he mal and ice e sa ene gy con e sion [37]. On
he o he hand, he injec ion and p oduc ion o CO
2
in o geological
o ma ions o CO
2
s o age [38] om cap u e plan s (CCS) o o
enhanced oil eco e y (EOR) [39], among o he applica ions.
The common denomina o is using CO
2
as he wo king luid [40].
The Ca no ba e y, composed o ansc i ical CO
2
cycles, o e s in e-
g a ion possibili ies wi h geological s o age. Fa ou able condi ions o
CO
2
in eg a ion wi h injec ion and p oduc ion p ocesses om geological
o ma ions occu du ing he cycles. The concep was i s p esen ed in
2020–2021 by Ca o e al. [29,30] and con inues o ad ance in i s
de elopmen phases, now p opelled by he CEEGS p ojec [34].
The CEEGS sys em o e s a wide ange o po en ial applica ions.
Fa ou able CO
2
injec ion o p oduc ion condi ions in geological s o age
p ocesses allow he CEEGS sys em join in eg a ions wi h CO
2
cap u e
plan s. Geological s o age p o ides se e al po en ial ad an ages, such as
geo he mal gains o esidual and/o dissol ed CO
2
apping unde -
g ound in he sho e m, while chemical eac ions can ap CO
2
in he
o m o mine als in he long e m. As a igene a ion sys em, i adds
hea ing o cooling applica ions o lexible elec ici y gene a ion. The
empe a u e ange o he ansc i ical CO
2
cycle allows o he use o ho
wa e as a he mal s o age medium on he high- empe a u e (HT) side o
ice slu y on he low- empe a u e (LT) side [27,29].
2.2.1. P inciple o ope a ion. Ene gy and geological s o age
As a e e sible hea pump o Ca no ba e y, he elec o he mal en-
e gy s o age sys em is based on con e ing elec ical ene gy o he mal
ene gy, s o age in he o m o he mal ene gy, and subsequen con e -
sion back o elec ical ene gy. The sys em s o es he mal ene gy a wo
empe a u e le els, hus gaining he abili y o mee he mal demand
di ec ly.
The e e sible hea pump u ilises ansc i ical cycles, wi h CO
2
as he
wo king luid. The condi ions o ansc i ical CO
2
cycles a e compa ible
wi h hose necessa y o CO
2
injec ion/p oduc ion in unde g ound
geological o ma ions. I allows he elec o he mal ene gy s o age sys-
em o include geological CO
2
s o age, expanding in eg a ion possibil-
i ies wi h CO
2
cap u e, u ilisa ion, and s o age applica ions. See Fig. 1.
Du ing he cha ging phase, whe e elec ical ( enewable) ene gy is
con e ed in o he mal ene gy, he sys em can injec a ce ain low a e
o p e iously cap u ed CO
2
in o an unde g ound geological o ma ion.
The he mal ene gy s o ed a wo empe a u e le els could be used
di ec ly in hea ing/cooling applica ions. Du ing he discha ge phase, he
s o ed he mal ene gy would be lexibly con e ed back in o elec ical
ene gy. The discha ge cycle can be coupled wi h he p ocesses o CO
2
p oduc ion om unde g ound geological o ma ion and subsequen e-
injec ion, allowing o exploi ing phenomena such as geo he mal gains.
2.2.2. The mal ene gy s o age and powe p oduc ion. Elec ic- o-elec ic
con e sion
Fig. 2 shows a schema ic o he componen s and p ocesses o he
sys em ha pe o m enewable ene gy (pho o ol aic o wind) s o age
and lexible elec ici y p oduc ion. Du ing he cha ging phase (1-2-3-4),
a comp esso is used as he main equipmen o he ansc i ical CO
2
cycle. I ans e s hea unde supe c i ical condi ions o he HT s o age
and ecei es he mal ene gy o comple e i s e apo a ion om he LT
s o age. Du ing he discha ge phase (5-6-7-8), he he mal ene gy s o ed
a high and low empe a u es is used o hea he CO
2
unde supe c i ical
condi ions and condense i a e he u bine's expansion.
One c i ical aspec o ansc i ical CO
2
cycles as an elec o he mal
ene gy s o age sys em is he in eg a ion in o he empe a u e p o iles o
hea exchanges wi h he mal s o age. P ope in eg a ion o empe a u e
p o iles allows he cha ging and discha ging cycles o ha e a simila
shape, inc easing elec ical con e sion e iciency. Due o he cha ac-
e is ics o ansc i ical CO
2
cycles, wi h supe c i ical hea ing/cooling
p ocesses on he HT side and e apo a ion/condensa ion on he LT side,
good in eg a ion equi es sensible hea ans e on he HT side and la en
hea ans e on he LT side.
The empe a u e-en opy CO
2
cycle e olu ion cons ains he hea
ans e ed o he he mal s o age anks in he cha ging and discha ging
phases. The sys em may equi e balancing depending on he a io be-
ween he hea ans e ed and he hea equi ed a each empe a u e
le el. Balancing would be aimed a u ilising he he mal ene gy s o ed in
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
3
he su plus ese oi when he limi ing ese oi becomes deple ed.
Fig. 2 shows an addi ional simple comp ession e ige a ion cycle con-
nec ed o LT-TES, which would allow balancing o he simul aneous
discha ge using enewable ene gy su plus du ing he cha ging phase.
2.2.3. CO
2
s o age in geological o ma ions
The empe a u e and p essu e condi ions o CO
2
in he ansc i ical
cycles o he elec o he mal ene gy s o age sys em a e compa ible wi h
hose o CO
2
injec ion and p oduc ion in geological o ma ions. The
echnique has al eady been used in geological CO
2
s o age applica ions
o educe CO
2
emissions in o he a mosphe e and comba clima e change
[41]. This echnique has been adi ionally used in o he applica ions,
such as Enhanced Oil Reco e y (EOR) [42]. I in ol es injec ing CO
2
in o oil ields o inc ease p essu e and, in a subsequen s ep, acili a ing
addi ional hyd oca bon ex ac ion ia e ia y miscible oil displace-
men . Once he oil is eco e ed and he dissol ed CO
2
is sepa a ed om
he oil, he CO
2
can be einjec ed in o he geological o ma ion o
u he use as an enhanced oil eco e y agen . Wo ldwide, he mass o
CO
2
injec ed annually exceeds 40 M on [43].
CO
2
is injec ed in o sui able unde g ound geological o ma ions o
long- e m s o age. In CCS, he shape and sp ead o he plumes a e o
conce n o sa e y easons. Common geological o ma ions al eady used
o his pu pose include deep saline aqui e s, deple ed oil and gas ields,
and coal seams. These o ma ions mus ha e speci ic geological cha -
ac e is ics o ensu e CO
2
e en ion capaci y, such as adequa e po osi y
and pe meabili y [44]. Iden i ying he cha ac e is ics o ese oi s ha
a ou he ea ly de elopmen o a su icien ly la ge plume is impo an
o unde s anding he limi a ions imposed by geological en i onmen s
on he CEEGS echnology. Cu en ly, h ough he CEEGS p ojec , po ous
media such as deep saline aqui e s, deple ed hyd oca bon ese oi s,
and sal ca e ns [33] a e being in es iga ed. Po ous media a e cha ac-
e ised by hei high s o age capaci y and associa ed CO
2
ixa ion
mechanisms, such as la e al mig a ion, esidual apping, dissolu ion o
mine alisa ion [32]. Sal ca i ies, gene ally wi h a lowe le el o s o age
capaci y and associa ed ixa ion mechanisms, p esen be e capabili ies
o mechanical ene gy s o age, as he allowable p essu es a e consid-
e ably highe han hose o po ous media [33].
2.2.4. In eg a ed ene gy and CO
2
s o age sys em
The CEEGS sys em ea u es wo dis inc ope a ing modes: closed-
loop ope a ion and open-loop ope a ion, wi h a cha ging and dis-
cha ging phase in each. In he closed-loop ope a ion mode, CO
2
ci cu-
la es only h ough he powe equipmen , con e ing elec ical ene gy o
he mal ene gy du ing he cha ging phase and he mal ene gy o elec-
ical ene gy du ing he discha ging phase in a closed ci cui . See Fig. 3a.
This mode o ope a ion co esponds o ha o he adi ional elec o-
he mal s o age sys em, in which he CEEGS sys em ac s only as an
elec o he mal ene gy s o age/p oduc ion sys em. In he open-loop
ope a ion mode, geological s o age capaci y is included. A s eam o
CO
2
om a cap u e plan (CC) is in oduced in o he ansc i ical cycle
du ing he cha ging phase a he inle o he e apo a o (LT-hx) on he
low- empe a u e side. A e passing h ough he elec ical- o- he mal
ene gy con e sion p ocess equipmen , he CO
2
in a supe c i ical s a e
wi h a ela i ely low empe a u e a he high- empe a u e side hea
exchange (HT-hx) ou le is injec ed and s o ed wi hin he geological
o ma ion (GS). The en i e p ocess co esponds o he sequence CC-4-1-
2-3-GS in Fig. 3b. In he discha ging phase, a e he p oduc ion p ocess
om he geological o ma ion, he CO
2
is in oduced in o he powe
p oduc ion cycle, ecei ing he mal ene gy om HT-TES and using LT-
TES as a condense , o be inally einjec ed in a second injec ion p ocess
sequence GS-6-7-8-5-GS in Fig. 3b. In he open-cycle ope a ion mode,
he CEEGS sys em ac s simul aneously as an elec ici y s o age/p o-
duc ion sys em and a CO
2
geological s o age sys em. Depending on he
Fig. 1. Ope a ing p inciple o he CEEGS igene a ion sys em.
Fig. 2. Scheme o he CEEGS sys em.
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
4
cha ac e is ics o he geological o ma ion, such as dep h, e ain, and
he geo he mal g adien , addi ional equipmen is equi ed o pe ec ly
adap o he CO
2
condi ions in he injec ion/p oduc ion p ocesses and
he ansc i ical cycle, such as auxilia y pumps and condense s o
cleaning equipmen .
The p essu e (P) and empe a u e (T) condi ions in he ese oi and
he a ia ion o P-T be ween wellheads and bo om holes a ec he e -
iciency o he CEEGS sys em wi h unde g ound injec ion and CO
2
e o-
p oduc ion. These condi ions depend p ima ily on he low a es and
diame e s o he wells and hei leng h, i.e., he dep h o he ese oi . P-
T changes wi hin he well a e especially impo an o CO
2
e o-
p oduc ion om he ese oi o ensu e ha he CO
2
sa u a ion line is
no eached and ha no wo-phase low occu s in he p oducing well.
These addi ional p ocesses de i ed om CO
2
injec ion/p oduc ion in
geological o ma ions po en ially bene i he al eady high ela i e e i-
ciency o he elec o he mal s o age sys em. Conside ing he wo modes
o ope a ion, he ene gy s o age/p oduc ion capaci y o he CEEGS
sys em is independen o he cap u ed CO
2
s o age equi emen s in he
geological o ma ion.
The mos c i ical poin in he e iciency o he CEEGS sys em is he
in eg a ion be ween ansc i ical CO
2
cycles and he mal s o age, bo h
a high and low empe a u es. The ene gy s o age p ocesses o he
cha ging phase and powe p oduc ion o he discha ging phase a e
p esen in bo h ope a ing modes. By imp o ing he in eg a ion o em-
pe a u e p o iles and explo ing all op ions o e ed by he mal s o age,
he ound- ip e iciency o elec o he mal con e sion can be imp o ed,
and addi ional applica ions in he igene a ion sys em can also be
ound.
2.3. Main assump ions and indica o s
The componen s a e modelled using a lumped olume app oach,
e alua ing inle and ou le s eams en halpies. Hea ans e in hea
exchange s ( ˙
Qhx) and powe exchange ( ˙
W) in comp esso s, pumps and
u bines a e e alua ed h ough he gene al Eqs. (1)–(2).
˙
Qhx =˙
mho •(hho
in −hho
ou )= − ˙
mcold •(hcold
in −hcold
ou )(1)
˙
W=˙
m• (hin −hou ) = ˙
m•Δh(2)
Being ( ˙
m) he mass low a e and he subsc ip s ‘in’ and ‘ou ’ he inle
and ou le condi ions ha de ine he speci ic en halpy. E olu ions in
u bines, comp esso s and pumps a e closed h ough hei isen opic
e iciencies, Eqs. (3)–(4), using e e ence alues om simila s udies
[27,29].
η
exp
s=hin −hou
hin −hs
ou
(3)
η
comp
s=hs
ou −hin
hou −hin
(4)
whe e
η
s is isen opic e iciency, ‘exp’ and ‘comp’ e e o expansion and
comp ession p ocesses, h is he speci ic en halpy and hs is he en halpy in
an isen opic p ocess, a he inle (‘in’) and ou le (‘ou ’) o he equip-
men unde conside a ion. Hea exchange s a e modelled by se ing a
minimum empe a u e di e ence (dTmin) be ween he empe a u e
p o iles. P essu e losses in hea exchange s a e neglec ed in his s udy,
conside ing hey will be compa a i ely small compa ed o he absolu e
p essu e and e ec s on he injec ion/ex ac ion p ocesses. The main
assump ions a e summa ised in Table 1.
The CO
2
p essu e on he LT side depends on he phase change em-
pe a u e o CO
2
(e apo a ion du ing cha ging and condensa ion du ing
discha ging), acco ding o he phase change empe a u e o he LT-TES.
See Eqs. (5)–(6). The inle condi ions in he comp esso (sa u a ed gas)
and he pump (sa u a ed liquid) a LT side p essu e es ablish he i s
e e ence poin in cons uc ing he ansc i ical CO
2
cycles o he
cha ging and discha ging phases, espec i ely. The ou le condi ions in
he comp ession p ocesses a e es ablished om he isen opic e iciency
o each uni and he p essu e on he HT side. The comp ession line
du ing cha ging, om sa u a ed gas condi ions o he p essu e on he HT
side, ma ks he highes empe a u e o he sys em. The e apo a ion, also
Fig. 3. Ope a ing modes o he CEEGS sys em: a) Closed-cycle ope a ion, b) Open-cycle ope a ion.
Table 1
Main s a ing assump ions in he modelling o he sys em.
Re e ence Equipmen Value Uni
E iciency CO
2
comp esso 0.89 –
CO
2
u bine 0.91 –
CO
2
pump 0.86 –
CO
2
expande 0.88 –
Minimum empe a u e di e ence HT-hx 4 ◦C
LT-hx 4 ◦C
Phase change empe a u e LT-TES 0 ◦C
Numbe o s o age anks HT-TES 1 –
Cha ging cycle CO
2
HT-P essu e 200 ba
Discha ging cycle CO
2
HT-P essu e 200 ba
CO
2
low a e Cycle 30 kg/s
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
5
om he cha ging phase, es ablishes he lowes empe a u e. These
ac o s di ec ly in luence he hea ing and cooling applica ions o he
igene a ion sys em.
PCO2
cha ge,LT =PCO2
sa (T=TLT−TES
phase −dTLT−hx
min )(5)
PCO2
discha ge,LT =PCO2
sa (T=TLT−TES
phase +dTLT−hx
min )(6)
The ou le empe a u e o he comp ession p ocesses, pump and
comp esso , and he minimum empe a u e di e ence, se he limi s o
he empe a u e ange in HT-TES. The ou le empe a u e o he
comp esso condi ions he uppe limi . The pump ou le empe a u e
condi ions he lowe limi . The hea ans e is modelled in sequen ial
mode: i s , he cooling o he cha ging phase and hen he hea ing o he
discha ging phase. The pinch poin o he supe c i ical cooling is placed
a he CO
2
ou le , se ing he inle empe a u e a he expansion o he
cha ging phase. The empe a u e p o ile o he supe c i ical CO
2
cooling
is calcula ed as a cons an p essu e cooling be ween he comp esso 's
ou le empe a u e and he expansion's inle empe a u e. The hea ex-
change is op imised by adjus ing he slope o he empe a u e p o ile
(s aigh ) o he subc i ical hea ing o he HT-TES luid. In his e e ence
case, p essu ised wa e . This is achie ed h ough he mass low a e o
he wa e in he cha ging phase. See Eq. (1). The HT-hx ou le empe -
a u e depends on he in eg a ion be ween he supe c i ical p o ile's
cu a u e and he subc i ical p o ile's s aigh line. In he supe c i ical
CO
2
hea ing o he discha ging phase, he empe a u e p o ile o he HT-
TES luid is he same as in he cha ging phase, wi h he pinch poin a he
CO
2
inle . On his occasion, he supe c i ical empe a u e p o ile is
adjus ed o he s aigh line o he subc i ical p o ile, op imising he
second hea exchange. This adjus men is condi ioned by he mass low
a e o wa e in he discha ging phase (Eq. (1)). The CO
2
ou le em-
pe a u e se s he u bine inle empe a u e. The u bine inle empe a-
u e depends on he in eg a ion be ween he p o iles o supe c i ical
hea ing and cooling o CO
2
on he HT side and he empe a u e change
p o ile o he HT-TES. Op imising he in eg a ion o empe a u e p o iles
inc eases he u bine inle empe a u e and sys em's e iciency.
The ou le condi ions o he expansion equipmen , expande in
cha ging and u bine in discha ging, a e de e mined om he inle
condi ions and he isen opic e iciency (Eqs. (3)–(4)) o each equip-
men . The ou le condi ions o he expansion a e he inle condi ions a
LT-hx, cha ging, and discha ging. The hea exchanges in LT-hx close he
cycles, and a e modelled as hea ans e s a cons an p essu e ha
condi ion he mass low a e o he s o age medium in LT-TES, in his
e e ence case, ice slu y. The u bine inle empe a u e and he
condensa ion condi ions on he LT side ma k he inc ease in en halpy
du ing expansion. En halpy inc eases will be nega i e o comp ession
equipmen and hea ans e in hea exchanges. This inc eases in
en halpy de e mine he e iciency o he sys em, de ined based on he
indica o s o Eqs. (7)–(8).
η
ele
LT =(ΔhT
HE +ΔhP
HE)•ΔhLT−TES
HP
(ΔhC
HP +ΔhHydT
HP )•ΔhLT−TES
HE
(7)
η
ele
HT =(ΔhT
HE +ΔhP
HE)•ΔhHT−TES
HP
(ΔhC
HP +ΔhHydT
HP )•ΔhHT−TES
HE
(8)
Eqs. (7)–(8) indica e he elec ic- o-elec ic con e sion e iciency
(ECE), di e en ia ing be ween he HT and LT sides. Whe e ΔhTES
HP and Δ
hTES
HE e e o he en halpy d op in he high o low- empe a u e exchanges
o each phase, cha ging and discha ging. ΔhX
HP,HE is he en halpy d op
de eloped by he comp ession and expansion equipmen , comp esso
(C), pump (P), hyd aulic u bine (HydT) and gas u bine (T) in cha ging
(HP) o discha ging (HE) phases. This di e ence in e iciency in he
o e all con e sion p ocess es ablishes a ange depending on he he mal
ene gy s o ed a each empe a u e le el.
ECE quan i ies he con e sion o elec ical o elec ical powe om
he poin o iew o he wo empe a u e le els o TES. The indica o is
based on he a io be ween he elec ical ene gy gene a ed in he
discha ge phase and he elec ical ene gy consumed du ing he cha ging
phase. The de elopmen o he e iciency indica o , which depends on
en halpy d ops, has been made possible by conside ing ha all he en-
e gy s o ed du ing he cha ging phase is used in he powe p oduc ion o
he discha ging phase [∅HT,LT−TES
HP =∅HT,LT−TES
HE ], a each empe a u e
le el. ECE depends only on he shape o he CO
2
ansc i ical cha ge-
discha ge cycles. This hypo hesis implies es ablishing a ange in which
he sys em e iciency is loca ed, a limi ing ese oi and an excess
ese oi . The ank on he side (LT o HT) ha ma ks he lowe limi o
he e iciency ange will ac as he ‘limi ing’ ank and will be discha ged
be o e he o he , which will ma k he uppe limi . The sys em's ound-
ip e iciency will ul ima ely depend on using his su plus ene gy.
The su plus he mal ene gy in HT could be used in powe p oduc ion
when hea is gi en up o he en i onmen in he condensa ion o he
discha ge phase, o in an abso p ion cooling p ocess, ans e ing ene gy
om HT o LT, b inging he e iciency close o he uppe limi . In he
case o using an addi ional simple comp ession e ige a ion cycle du -
ing he cha ging phase, he ound ip e iciency (RdE ), whe e he
sys em e u ns o i s ini ial s a e, would also depend on he comp esso
powe o he e ige a ion cycle and he addi ional he mal powe on he
LT-side, as shown in he Eq. (9).
RdE =(˙
WT
HE +˙
WP
HE)•(˙
QLT−TES
HP +
˙
QLT−TES
Re )
(˙
WC
HP +
˙
WHydT
HP +
˙
WC
Re )•
˙
QLT−TES
HE
=(˙
WT
HE +˙
WP
HE)•
˙
QHT−TES
HP
(˙
WC
HP +
˙
WHydT
HP +
˙
WC
Re )•
˙
QHT−TES
HE
(9)
whe e ˙
QLT−TES
Re ep esen s he addi ional he mal powe in LT-TES du ing
he cha ging phase, and ˙
WC
Re is comp esso powe o he addi ional
e ige a ion cycle. Conside ing he empe a u e ange equi ed in he
e apo a o o he comp ession e ige a ion cycle, imposed by LT-TES,
and ha o he condense , which e acua es hea o he en i onmen ,
an ammonia-based cycle sui s he empe a u e p o iles. This op ion was
conside ed in [27], in a hea pump unde simila condi ions. The
ammonia comp ession e ige a ion cycle would ope a e be ween 3.67
ba (e apo a o ) and 11.32 ba (condense ), when he CEEGS sys em
ope a es a he condi ions in he Table 1. Unde hese design condi ions,
i would ha e a COP o 6.4. The cycle is sized based on he hea exchange
in he e apo a o ( ˙
QLT−TES
Re ), which will ope a e du ing he cha ging
phase, balancing he wo phases. The equi ed he mal powe depends
on he he mal powe o he LT and HT exchanges in he cha ging and
discha ging phases, and is de ined by Eq. (10).
˙
QLT−TES
Re =
˙
QLT−TES
HP −
˙
QLT−TES
HE •
˙
QHT−TES
HP
˙
QHT−TES
HE
(10)
The geological o ma ions conside ed in he model a e po ous media
and sal ca i ies. In po ous media, he downhole equi emen s a e
cha ac e ised by he hyd os a ic p essu e [Phyd o =
ρ
wa e •g•z], de ined
by he p essu e exe ed by he wa e column (Pa) and whe e
ρ
is he
densi y o wa e (kg/m
3
), g is he g a i y cons an (m/s
2
), and z is he
ese oi dep h (m) [29]. The limi is conside ed o be 20 % o he
accumula ed p essu e. In sal ca e ns, he maximum s o age p essu e is
limi ed by he minimum p incipal s ess in he sal ock ac ing on he
ca e n oo , he li hos a ic p essu e [45]. The li hos a ic p essu e (MPa)
is calcula ed as a unc ion o he dep h (m) o he op as [Pli h =
0.022 •z], and he allowable ange ollows he maximum and minimum
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
6
p essu e alues used in he CAES indus y [31,45]: 30–80 % o he
li hos a ic p essu e. The geological o ma ion dep h ange is be ween
he minimum alue equi ed o main ain he supe c i ical s a e o he
s o ed CO
2
, and he maximum ecommended o CAES o na u al gas
s o age, which is 2500 [31]. The equi ed injec ion condi ions a he
wellhead a e de e mined using he wellbo e low model o Adams e al.
[33,46], in which he a e age alues wi hin he ese oi and he
p essu e loss in he p oducing well a e de e mined. The low a e and he
diame e o he well play an impo an ole in he empe a u es and
p essu es equi ed a he injec ion wellhead.
2.4. Valida ion me hods
Me hods and esou ces such as he CoolP op lib a y and he Py hon
p og amming language ha e been used and alida ed in nume ical
modelling. The open-access CoolP op lib a y has been alida ed [35]
and is inc easingly used in a a ie y o echnical ields. The main limi-
a ion o his lib a y is ha i does no include he mophysical p ope ies
o mix u es, which does no in luence his s udy. The he modynamic
cycles comp ise basic comp ession/expansion p ocesses and hea ex-
changes, sol ed wi h mass and ene gy balances, simila o hose used in
o he ela ed wo ks [27,29]. The isen opic e iciency conside ed in he
equipmen and he empe a u e di e ences in he hea exchanges a e
also iden ical o hose used in o he s udies o he same ype. Fig. 4a
shows he T-s diag ams o he ansc i ical cycles du ing he cha ging
and discha ging o he CEEGS sys em, and Fig. 4b ep esen s he T-s
diag ams o he addi ional Ammonia comp ession e ige a ion cycle,
acco ding o he ini ial condi ions lis ed in Table 1.
The phase change empe a u e o LT-TES and he minimum es ab-
lished empe a u e condi ion o he p essu es o CO
2
on he LT side
de e mine he cha ac e is ics o e apo a ion du ing cha ging and
condensa ion du ing discha ging, see Fig. 4c. On he HT side, he p es-
su es o he ansc i ical cycles and he in eg a ion in he empe a u e
p o iles in he hea exchange de e mine he e olu ion in HT-TES, see
Fig. 4d. The e olu ion o empe a u e p o iles in he hea exchange di-
ag ams T-Q co esponds o hose o he T-s and P-h diag ams o CO
2
.
Hea ans e is consis en a bo h empe a u e le els. The minimum
empe a u e di e ence is es ablished in LT-hx. In he case o HT-hx, i is
sligh ly exceeded due o adjus men wi h he ansc i ical cycles, bu no
c ossings occu in any case. The ECE ange o 48.3–56.7 % is also
consis en wi h simila sys ems s udied in he wo ks men ioned abo e
[27,29]. The ound ip e iciency using he addi ional ammonia e ig-
e a ion cycle would be 52.76 %.
Fig. 4. Diag ams o he CEEGS sys em ope a ing in closed-cycle mode: a) T-s CO
2
ansc i ical cycles, b) T-s ammonia e ige a ion cycle, c) T-Q o he exchange wi h
LT-TES, and d) T-Q o he exchange wi h HT-TES.
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
7
2.5. Scena ios unde analysis
The s udy on he op imisa ion o su ace p ocesses and al e na i e
cycle con igu a ions in he CEEGS sys em pe o ms se e al analyses,
anging om he adap a ion o CO
2
ansc i ical hea ing/cooling p o-
iles wi h HT-TES and he impac o phase change empe a u e in LT-
TES, ecupe a i e cycles, mul i-comp ession, hea eco e y in he in-
e media e ehea and mul i-expansion.
The sys em uses wa e as he he mal s o age medium in he e e -
ence case. The empe a u e anges o he ini ial condi ions allow o he
use o wa e as a s o age medium a bo h empe a u e le els. In he case
o HT-TES, he 22–140 ◦C ange could be co e ed wi h sligh ly p es-
su ised liquid wa e (less han 8 ba ). On he low- empe a u e side, LT-
TES could u ilise solid-liquid phase change, employing ice slu y ech-
nology o acili a e hea ans e . The di e en op imised con igu a ions
and p ocesses a e compa ed wi h he e iciency o elec ical ene gy
con e sion, and he new hea ing/cooling op ions ha he sys em can
o e a e desc ibed quali a i ely.
2.5.1. S ep hea ing/cooling on he HT-TES
Fi s , he bene i s o in eg a ing a s epped sensible hea ans e on
he HT-TES side a e e alua ed. I aims o in eg a e be e he supe -
c i ical and subc i ical empe a u e p o iles o CO
2
and he he mal
s o age luid. See Fig. 5.
A g ea e numbe o s o age anks on he HT-TES side and be e
in eg a ion in empe a u e p o iles allow o he expansion o he pos-
sibili y o mee ing he mal demand. This would o e he oppo uni y o
co e a ious empe a u e anges, di e en om he maximum and
minimum empe a u es p o ided by he single-s ep sensible hea
ans e con igu a ion. The numbe o s ages inc eases wi h he numbe
o anks. In o de o ha e a simila hea ans e in each s age, he di-
ision is made acco ding o he hea ans e ed, and he slope o he
subc i ical empe a u e p o ile is adjus ed o ha o he supe c i ical
empe a u e p o ile. This equi es sligh ly a ying he low a e in each
hea exchange sec ion o he he mal s o age luid, wa e in his case,
coun e ing he e ec s o he a iable hea capaci y o CO
2
a high
p essu e. Inc easing he low a e can achie e a lowe slope in he
empe a u e p o ile a subc i ical hea ing/cooling and ice e sa. The
addi ional he mal s o age anks manage he low a ia ions in he
di e en sec ions, so he size is much smalle han ha o he main anks.
Fu he mo e, he s o ed luid in each ank could ha e di e en cha ac-
e is ics. A s udy ange o 1–10 s epwise hea ans e con igu a ions is
conside ed.
2.5.2. Phase change empe a u e change on he LT-TES
The phase change empe a u e on he LT-TES side is he s a ing
poin o cons uc ing he ansc i ical cycles, es ablishing he lowe
limi on he en halpy inc emen s o he expansion/comp ession p o-
cesses. The LT-TES phase change empe a u e ul ima ely de e mines he
shape o he ansc i ical cycles and, he e o e, he sys em's e iciency.
The LT-TES phase change empe a u e is he key ac o in he sys-
em's cooling applica ions. This s udy conside s a empe a u e ange
om −40 o 20 ◦C, co e ing cooling op ions abo e and below he 0 ◦C
ini ial assump ions. Once he impac o he phase change empe a u e in
LT-TES is conside ed, di e en in eg a ion possibili ies wi h luids ha
can p o ide he equi ed condi ions a e also conside ed.
2.5.3. Recupe a i e cycle and mul i-s age comp ession
The e a e di e en oppo uni ies o op imisa ion in he p ocesses
ha o m he ansc i ical CO
2
cycles. In he cha ging cycle, he ex eme
empe a u e condi ions in he sys em a e eached: he highes empe -
a u e a e comp ession and he lowes empe a u e a e expansion.
Due o he cha ac e is ics o he ansc i ical cycle, he comp esso inle
empe a u e is he e apo a ion empe a u e below he expande inle
empe a u e. A ecupe a i e cycle con igu a ion seems app op ia e in
his case. In his s udy, he impac o he ecupe a i e cycle on he
cha ging phase is analysed by di e ing pa o he CO
2
lea ing he low-
empe a u e hea exchange (LT-hx) o a hea exchange wi h he CO
2
lea ing he high- empe a u e hea exchange (HT-hx), as shown in
Fig. 6a. In he case o he discha ging phase, he ecupe a i e con igu-
a ion would only make sense i he u bine ou le empe a u e we e
highe han he pump ou le empe a u e.
In addi ion o he ecupe a i e cycle, he cases o mul i-s age
comp ession in he comp esso in he cha ging phase and mul i-s age
expansion in he u bine in he discha ging phase a e s udied. In he
case o he comp esso , he use o a s aged comp ession wi h in e me-
dia e cooling educes he highes empe a u e o he sys em, which
implies a educ ion o he u bine inle empe a u e (TIT) o he dis-
cha ging phase. Howe e , using a s aged expansion unde condi ions
simila o hose o comp ession, he hea e acua ed du ing he cooling o
he mul i-s age comp ession could be used in an in e media e ehea ing
in he mul i-s age expansion, Fig. 6b. Using 1–3 in e media e comp es-
sion s eps is conside ed. Fo cooling, i is conside ed ha a maximum o
20 % o he o al hea a ailable is used in each s ep. When he a ailable
hea is no eco e ed in he in e media e ehea o he u bine, i is
e acua ed o he en i onmen . In he expansion, be ween 1-n
C
s eps a e
conside ed, whe e n
C
is he numbe o comp esso s eps, and all he
eco e ed hea is used.
3. Resul s and discussion
This sec ion p esen s he analysis esul s o he di e en op imisa ion
scena ios conside ed in his s udy.
3.1. S ep hea ing/cooling on he HT-TES
The nume ical simula ion yields good esul s in e ms o in eg a ion
and e iciency. Fig. 7 shows he empe a u e hea exchange p o iles
be ween he CO
2
and he HT-TES o di e en hea eco e y sequen ial
s ages.
The sequen ial supe c i ical and subc i ical empe a u e p o iles
in eg a e be e . The mul i-s age hea exchange allows he empe a u e
p o iles o he CO
2
and he s o age luid o be be e adap ed, wi h close
empe a u e p o iles and main aining he minimum empe a u e di -
e ence in he ope a ion ange. The maximum empe a u e o HT-TES
inc eases wi h he numbe o s ages in he sensible hea ans e , along
wi h he Tu bine Inle Tempe a u e (TIT). I inc eases he e iciency o
he sys em.
Addi ionally, he sys em gains signi ican po en ial o he mal de-
mand co e age applica ions. The HT-TES side ansi ions om ha ing a
single high- empe a u e ese oi o mul iple ese oi s wi h di e en
in e media e empe a u es, allowing o he di ec co e age o a ious
empe a u e anges and o he he mal ene gy demands. Sequen ial
hea ing no only leads o an inc ease in TIT, which esul s in imp o ed
elec ic- o-elec ic con e sion e iciency bu also es ablishes mul iple
empe a u e anges o he di ec co e age o he mal demand. U ban
Fig. 5. S ep sensible hea exchange in HT-TES.
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
8
hea ing ne wo ks o e conside able po en ial o he in eg a ion o
enewable hea .
Thus, he CEEGS sys em expands i s possibili ies o hea demand
co e age and p o ides ano he a enue o enewable ene gy o mee
he mal demand. Fig. 8 shows he sys em's e iciency ange and he main
empe a u es on he HT-TES side o he CEEGS sys em, acco ding o he
numbe o s ages in he HT-TES.
The sys em e iciency inc eases wi h he u bine inle empe a u e,
which inc eases wi h be e in eg a ion in he empe a u e p o iles.
Al hough a mono onic and asymp o ic a ia ion in he e iciency alues
migh be expec ed, he esul s show small oscilla ions. The di ision o
he hea exchange in o sec ions acco ding o hea ans e allows he
subc i ical empe a u e p o ile o be be e ma ched o he supe c i ical
p o ile by using a ia ions in he slope o he empe a u e p o ile in each
sec ion. Howe e , depending on he ela i e posi ion o he pinch wi h
espec o he supe c i ical p o ile cu e (depending on he numbe o
exchange s ages), he in eg a ion may be be e o wo se. A cu a u e
zone ha in e e es wi h he s aigh p ojec ion o he limi s in he
empe a u e p o ile es ablished by he sec ion can cause a educ ion in
he po en ial slope o ha sec ion, which esul s in a lowe ou le em-
pe a u e in he hea exchange. This phenomenon occu s when going
om one o wo s ages when ECE, acco ding o HT-TES, dec eases om
56.5 % o 56 %. As he numbe o s ages inc eases, he beha iou o he
sys em is as expec ed, and he in eg a ion in he empe a u e p o iles
ends o imp o e, bu always subjec o his adjus men , which depends
on he posi ion o he pinch and he cu a u e o he empe a u e p o-
iles, which can cause small oscilla ions. While i ises apidly passing
om wo o ou HT anks, i emains s able, a ound 59–60 % o hose
cases in which mo e han ou HT anks a e used. The empe a u e ange
o he HT-TES unde goes no signi ican changes excep o an inc ease in
he uppe limi due o be e in eg a ion in he empe a u e p o iles.
S ill, i does no ha e a no able in luence. The minimum empe a u e in
HT-TES emains cons an ega dless o he s aged sensible hea ans e .
The scena io wi h ou anks on he HT-TES side places he e iciency
ange a 51–59 % and es ablishes empe a u e anges o 21.9-53.1-79.6-
109.7-147 ◦C o po en ial he mal demand co e age applica ions.
When wa e is used as he s o age medium, only he las wo s ages
would need p essu ising, wi h p essu es o 1.97 and 5.72 ba o ensu e
he liquid s a e. The ound- ip e iciency o he sys em using he
comp ession e ige a ion cycle would go om 52.8 o 55.4 %. The
addi ional cos s a e ela ed o a highe in es men in he hea exchange
and addi ional anks. Howe e , he o al hea exchanged and he ex-
change su ace do no ha e signi ican a ia ions, so he inc ease in he
o al in es men cos should be low. As o he addi ional anks, i is
wo h men ioning ha hey a e only used o egula e he di e ences in
low a es esul ing om adjus men s in he slope o he empe a u e
p o iles in he hea exchange. The size o hese anks is much smalle
compa ed o he main anks. Al hough he e is an ini ial cos in ol ed,
he use o s aged hea ans e on he HT side should p o ide po en ial
long- e m sa ings in ene gy e iciency and g ea e ope a ional lexibili y
o adap o a ia ions in he mal demand.
3.2. Phase change empe a u e change on he LT-TES
Nex , he esul s o he LT he mal s o age analysis a e p esen ed.
The phase change empe a u e is one o he co ne s ones upon which he
CO
2
ansc i ical cycles a e buil . Addi ionally, i is he undamen al
ac o upon which he sys em's cooling applica ions depend. Fig. 9
shows he co esponding T-s diag ams o di e en LT-TES phase change
empe a u e cases.
The shape o he ansc i ical CO
2
cycles on he T-s diag am changes
no iceably. The change in he phase change empe a u e o LT-TES
di ec ly a ec s he condensa ion/e apo a ion lines o CO
2
. As he em-
pe a u e dec eases, comp ession in bo h he cha ging and discha ging
phases occu s u he away om he c i ical poin . Highe empe a u es
a e eached a he comp esso ou le , exceeding 200 ◦C in cases o lowe
phase change empe a u es. Con e sely, empe a u es a he pump
ou le dec ease, d opping below 0 ◦C. The empe a u e ange in HT-TES
is di ec ly a ec ed by he phase change empe a u e in LT-TES. Fig. 10
illus a es he end o he e iciency ange and he empe a u es on he
HT side when changing he phase change empe a u e on he LT side.
The e iciency ange inc eases as he phase change empe a u e in
LT-TES dec eases. When conside ing a empe a u e o 0 ◦C, he e i-
ciency was wi hin a ela i ely high ange o 48–56 %, and i eached
56.6–67.7 % when conside ing −40 ◦C. Con e sely, inc easing he
empe a u e o he phase change esul s in a signi ican penal y on e -
iciency, apidly dec easing as he empe a u e inc eases. I alls wi hin
he 38.6–45.3 % ange when he empe a u e o he phase change in LT-
Fig. 6. Scheme o he CEEGS sys em: a) Recupe a i e cycle con igu a ion, and b) Mul i-s age comp ession and hea eco e y.
A. Ca o e al.
Applied Ene gy 395 (2025) 126165
9
empe a u e anges o 20–150 ◦C (s eps o app oxima ely 30 ◦C).
Wi h wa e as he s o age medium, only he las wo s agge s need o
be p essu ised, wi h p essu es below 6 ba in he las one, o ensu e a
liquid s a e. Round- ip e iciency inc eases om 52.8 o 55.4 %.
•The LT-TES phase change empe a u e is he undamen al ac o on
which he sys em's cooling applica ions depend. The sys em's e i-
ciency ange inc eases as he LT-TES phase change empe a u e de-
c eases, eaching alues in he ange o 56.6–67.7 % a −40 ◦C. This
esul s in highe empe a u es a he comp esso ou le , abo e
200 ◦C, and lowe empe a u es a he pump ou le , below 0 ◦C,
di ec ly condi ioning he empe a u e ange in HT-TES. An inc ease
in he phase change empe a u e penalises he e iciency ange,
dec easing o 38.6–45.3 % a 20 ◦C. Howe e , i could be used o
u ilise su plus he mal ene gy, balancing he sys em wi hou using
he addi ional e ige a ion cycle. The ound- ip e iciency wi h he
ammonia e ige a ion cycle would be 44 % using a empe a u e o
20 ◦C and could each 58.7 % using −30 ◦C in LT-TES.
•Using he ecupe a i e con igu a ion in he cha ging cycle, a highe
empe a u e is achie ed a he comp esso ou le and u bine inle .
This esul s in high alues o he maximum empe a u e o he HT-
TES, which may inc ease he sys em's equi emen s, such as he
wa e p essu e le el. The e iciency is in he ange o 52.6–61.4 %
when he en i e CO
2
mass low en e ing he comp esso passes
h ough he ecupe a o . This also educes he inle empe a u e in
he expande and he apo quali y o he CO
2
a he e apo a o inle ,
which inc eases he hea s o ed in he LT-TES. Round- ip e iciency
inc eases om 55.4 o 57.9 %.
•Mul i-s age comp ession wi h in e cooling in he cha ging phase al-
lows o a educ ion in he comp esso ou le empe a u e. I is
necessa y o eco e he hea e acua ed in an in e media e ehea ing
o he mul i-s age expansion in he discha ge phase o main ain he
e iciency ange a high alues. Using wo-s age comp ession wi h
in e media e cooling educes he maximum empe a u e o HT-TES
om 186.6 ◦C a e single-s age comp ession o 146.6 ◦C. Using
wo s ages in he expansion o he discha ge phase and eco e ing
he e acua ed hea in he in e media e ehea , he e iciency ange is
51.4–59.5 % and ound- ip e iciency is 55.9 %, close o he case
wi h he ecupe a i e cycle and single-s age comp ession and
expansion, bu wi h a maximum empe a u e in HT-TES o 40 ◦C less.
Combining he di e en op imisa ion mechanisms desc ibed in his
s udy, he e iciency can be imp o ed o e y high alues, such as hose
p o ided by a e y low phase change empe a u e. Conside ing al e -
na i e e ige a ion luids, such as IsoBu ane, CycloP opane, o n-P o-
pane wi h e y low empe a u es phase changes, elec ic- o-elec ic
e iciency alues in he ange o 58.9–71.2 % can be achie ed using one-
s ep comp ession and expansion. This p esen s addi ional challenges,
such as a nega i e s e ch in he HT-TES empe a u e ange o exces-
si ely high alues in he maximum empe a u e. This e ec can be
educed using o he con igu a ions, such as wo-s age comp ession,
expansion, and hea eco e y. The e iciency ange would be 58.5–71.1
%, wi h lowe equi emen s associa ed wi h high empe a u es. The
ound- ip e iciency could each 59.8 % when he empe a u e a LT-
TES is −30 ◦C and a wo-s age comp ession and expansion wi h hea
eco e y in he in e media e ehea o he u bine is employed. The
analysis highligh s he impo ance o he mal s o age cha ac e is ics and
cycle con igu a ion in he CEEGS sys em. Key conside a ions ega ding
in eg a ion wi h ansc i ical CO
2
cycles and ope a ing condi ions a e
shown o maximise i s e iciency and po en ial in enewable ene gy-
based igene a ion he mal demand co e age.
CRediT au ho ship con ibu ion s a emen
A. Ca o: W i ing – o iginal d a , Visualiza ion, Valida ion, Supe -
ision, So wa e, Me hodology, In es iga ion, Fo mal analysis,
Concep ualiza ion. C. O iz: W i ing – e iew & edi ing, Fo mal
analysis. S. Unge : W i ing – e iew & edi ing, Valida ion, Fo mal
analysis. A. S oikos: Valida ion, Fo mal analysis. A.-S. Ky iakides:
Valida ion, Fo mal analysis. I.N. Tsimpanogiannis: W i ing – e iew &
edi ing, Supe ision. J.A. Bece a: Funding acquisi ion. S. Vou e akis:
Supe ision, Funding acquisi ion. U. Hampel: Supe ision, Funding
acquisi ion. R. Chaca egui: W i ing – e iew & edi ing, Supe ision,
P ojec adminis a ion, Funding acquisi ion, Fo mal analysis,
Concep ualiza ion.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Acknowledgemen s
This wo k has been pa ially unded by he p ojec ‘CEEGS: No el
CO
2
-based Elec o he mal Ene gy and Geological S o age’, om he
Eu opean Commission h ough Ho izon, he EU F amewo k P og amme
o Resea ch and Inno a ion, unde G an Ag eemen No. 101084376.
Da a a ailabili y
Da a will be made a ailable on eques .
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