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Ci a ion: Delgado-To es, A.M.;
Ga cía-Rod íguez, L. Sola
Desalina ion D i en by O ganic
Rankine Cycles (O c) and
Supe c i ical CO2Powe Cycles: An
Upda e. P ocesses 2022,10, 153.
h ps://doi.o g/10.3390/p 10010153
Academic Edi o : Chiing-Chang
Chen
Recei ed: 31 Oc obe 2021
Accep ed: 11 Janua y 2022
Published: 13 Janua y 2022
Publishe ’s No e: MDPI s ays neu al
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Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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A ibu ion (CC BY) license (h ps://
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4.0/).
p ocesses
Re iew
Sola Desalina ion D i en by O ganic Rankine Cycles (O c) and
Supe c i ical CO2Powe Cycles: An Upda e
Agus ín M. Delgado-To es 1,* and Lou des Ga cía-Rod íguez 2
1Depa amen o de Ingenie ía Indus ial, Escuela Supe io de Ingenie ía y Tecnología (ESIT),
Uni e sidad de La Laguna (ULL), 38200 La Laguna, Spain
2Depa amen o de Ingenie ía Ene gé ica, Escuela Técnica Supe io de Ingenie ía (ETSI),
Uni e sidad de Se illa, 41092 Se illa, Spain; [email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +34-922-84-60-45
Abs ac :
In he ield o desalina ion powe ed by enewable ene gies, he use o sola powe cycles
exhibi s some a o able cha ac e is ics, such as he possibili y o implemen ing he mal ene gy s o age
sys ems o a mul i-gene a ion scheme (e.g., elec ici y, wa e , cooling, hyd ogen). This a icle p esen s
a e iew o he la es design p oposals in which wo powe cycles o g ea po en ial a e conside ed:
he o ganic Rankine cycle and he supe c i ical CO
2
powe cycle, he la e o g owing in e es in
ecen yea s. The designs ound in he li e a u e a e g ouped in o h ee main ypes o sys ems. In
he case o sola ORC-based sys ems, he op ion o e e se osmosis as a desalina ion echnology
is conside ed in medium- empe a u e sola sys ems wi h s o age bu also wi h low- empe a u e
using sola ponds. In he i s case, i is also common o inco po a e single-e ec abso p ion sys ems
o cooling p oduc ion. The use o he mal desalina ion p ocesses is also ound in many p oposals
based on sola ORC. In his case, he usual con igu a ion implies he cycle’s cooling by he own
desalina ion p ocess. This op ion is also common in sys ems based on he supe c i ical CO
2
powe
cycle whe e MED echnology is usually selec ed. Designs p oposals a e e iewed and assessed o
poin ou design ecommenda ions.
Keywo ds:
sola ORC desalina ion; sola supe c i ical CO
2
; sola desalina ion; sola e e se osmosis
1. In oduc ion
The need o change he global ene gy model and he inc ease in d inking wa e
demand in impo an a eas o he plane make desalina ion powe ed by enewable ene gies
(REs) pa o he solu ion o he u u e challenge o he ene gy–wa e nexus. Acco ding o
In e na ional Ene gy Agency (IEA) da a, almos 81% o he wo ld’s ene gy supply had i s
o igin in ossil uels (coal, na u al gas and oil) in 2019 [
1
]. I we ocus on elec ical ene gy,
he pe cen age was 63% in he same yea . These alues explain he s ill-g owing end o
global g eenhouse gas emission le els. In addi ion o his, he e is he p oblem o access o
wa e . Acco ding o Wo ld Bank da a, in e nal enewable eshwa e esou ces pe capi a
ha e allen by 36% and 25% be ween 2002 and 2017 in a eas such as Sub-Saha an A ica o
he MENA (Middle Eas and No h A ica) egion, espec i ely [
2
]. These educ ions a e
la gely due o popula ion inc ease, which is he expec ed end in he coming decades.
RE-powe ed desalina ion o e s ad an ages and bene i s compa ed o he ossil- uels-
d i en op ion, such as he coincidence o enewable esou ce a ailabili y—mainly sola —
and wa e sca ci y. In addi ion, he en i onmen al impac s associa ed wi h he ene gy
consump ion o he p ocesses a e educed [
3
]. In his sense, he Li e Cycle Analysis
(LCA) o desalina ion echnologies sys ema ically indica es ha he en i onmen al impac s
associa ed wi h he ene gy consump ion o he p ocess a e mainly a ibu able o he
ope a ion and main enance phase in compa ison o cons uc ion and decommissioning
phases [
4
]. The e o e, he use o enewables as he ene gy inpu o hese p ocesses esul s in
a signi ican educ ion o he en i onmen al impac s o eshwa e p oduc ion. On a e age,
P ocesses 2022,10, 153. h ps://doi.o g/10.3390/p 10010153 h ps://www.mdpi.com/jou nal/p ocesses
P ocesses 2022,10, 153 2 o 23
he global wa ming po en ial educ ion is a ound 90% o con en ional he mal desalina ion
echnologies—mul i-s age lash (MSF) and mul i-e ec dis illa ion (MED)—and 80% o
e e se osmosis (RO) [4].
The abo e ad an ages jus i y he end in scien i ic ac i i y conce ning RE-powe ed
desalina ion shown in he bibliome ic analysis o he scien i ic li e a u e published on
desalina ion be ween 2000 and 2020, as p esen ed by Zapa a-Sie a e al. [
5
]. Among o he
esul s, he clus e ing o publica ions is highligh ed; he “Renewable ene gy desalina ion”
clus e is he second mos impo an o he eigh clus e s, de ined wi h a weigh o 26.1%,
jus one pe cen age poin below he i s ‘’Re e se osmosis” clus e ).
Wi hin his ame, his wo k deals wi h sola -powe ed desalina ion by means o sola
he mo-mechanic con e sion h ough a powe cycle, hus allowing he use o elec ici y
and/o hea o d i e desalina ion p ocesses. The main aim o he pape is he e iew and
assessmen o ecen design p oposals exis ing in he li e a u e o gi e design ecommenda-
ions on such sola desalina ion sys ems.
The la es gene al e iew o RE-desalina ion concludes ha he echno-economic
op imiza ion o some aspec s is s ill necessa y o o e s able solu ions in he long e m [
6
].
Rega ding he sola he mal-powe ed op ion, he need o i s applica ion o bo h small
and indus ial scales and he impo ance o he mal ene gy s o age a e emphasized. The
need o expand he implemen a ion o o -g id sys ems is also indica ed in gene al. In he
ield o sola he mal-powe ed desalina ion, i is possible o combine his esou ce wi h
all exis ing desalina ion echnologies (see Figu e 1). Only he me hods associa ed wi h
he use o powe cycles o he he mo-mechanical con e sion o sola he mal ene gy a e
shown since his is he ou e explo ed in his a icle. The e o e, he use o he elec ici y
gene a ed and/o he hea ejec ed by a sola powe cycle a e he op ions e iewed ins ead
o he di ec coupling o he mal desalina ion p ocesses o a sola he mal collec o ield.
Conce ning di ec coupling, we e e o Buena en u a Pouy acon and Ga cía-Rod íguez [
7
].
P ocesses 2022, 10, x FOR PEER REVIEW 2 o 23
esul s in a signi ican educ ion o he en i onmen al impac s o eshwa e p oduc ion.
On a e age, he global wa ming po en ial educ ion is a ound 90% o con en ional he -
mal desalina ion echnologies—mul i-s age lash (MSF) and mul i-e ec dis illa ion
(MED)—and 80% o e e se osmosis (RO) [4].
The abo e ad an ages jus i y he end in scien i ic ac i i y conce ning RE-powe ed
desalina ion shown in he bibliome ic analysis o he scien i ic li e a u e published on
desalina ion be ween 2000 and 2020, as p esen ed by Zapa a-Sie a e al. [5]. Among o he
esul s, he clus e ing o publica ions is highligh ed; he “Renewable ene gy desalina ion”
clus e is he second mos impo an o he eigh clus e s, de ined wi h a weigh o 26.1%,
jus one pe cen age poin below he i s ‘’Re e se osmosis’’ clus e ).
Wi hin his ame, his wo k deals wi h sola -powe ed desalina ion by means o sola
he mo-mechanic con e sion h ough a powe cycle, hus allowing he use o elec ici y
and/o hea o d i e desalina ion p ocesses. The main aim o he pape is he e iew and
assessmen o ecen design p oposals exis ing in he li e a u e o gi e design ecommen-
da ions on such sola desalina ion sys ems.
The la es gene al e iew o RE-desalina ion concludes ha he echno-economic op-
imiza ion o some aspec s is s ill necessa y o o e s able solu ions in he long e m [6].
Rega ding he sola he mal-powe ed op ion, he need o i s applica ion o bo h small
and indus ial scales and he impo ance o he mal ene gy s o age a e emphasized. The
need o expand he implemen a ion o o -g id sys ems is also indica ed in gene al. In he
ield o sola he mal-powe ed desalina ion, i is possible o combine his esou ce wi h
all exis ing desalina ion echnologies (see Figu e 1). Only he me hods associa ed wi h he
use o powe cycles o he he mo-mechanical con e sion o sola he mal ene gy a e
shown since his is he ou e explo ed in his a icle. The e o e, he use o he elec ici y
gene a ed and/o he hea ejec ed by a sola powe cycle a e he op ions e iewed ins ead
o he di ec coupling o he mal desalina ion p ocesses o a sola he mal collec o ield.
Conce ning di ec coupling, we e e o Buena en u a Pouy acon and Ga cía-Rod íguez
[7].
Figu e 1. Possible indi ec sola desalina ion op ions when a powe con e sion uni (powe cycle)
is conside ed as he solely inal ene gy supply uni . MSF: mul i-s age lash; MED: mul i-e ec dis-
illa ion; MD: memb ane dis illa ion; HDH: humidi ica ion dehumidi ica ion; MVC: mechanical a-
po comp ession; RO: e e se osmosis; ED/EDR: elec odialysis/elec odialysis e e sal; CDI: capac-
i i e deioniza ion; EDI: elec odeioniza ion. Adap ed om [6].
The he mal p ocesses shown in Figu e 1 equi e a hea supply in he ange o 50–120
°C [6]. Tempe a u es om 50 o 60 °C a e only sui able o humidi ica ion dehumidi ica-
ion (HDH), app op ia e only o small-scale use due o i s ela i ely low ene gy e iciency
[8]. Mo eo e , memb ane dis illa ion (MD) sys ems equi e 75–95 °C o achie e easona-
ble speci ic he mal consump ion, up o 237 and 170 kJ/kg, espec i ely [7]. Howe e ,
he e a e only small capaci y uni s, hus limi ing hei applica ion a an indus ial scale.
Figu e 1.
Possible indi ec sola desalina ion op ions when a powe con e sion uni (powe cycle)
is conside ed as he solely inal ene gy supply uni . MSF: mul i-s age lash; MED: mul i-e ec
dis illa ion; MD: memb ane dis illa ion; HDH: humidi ica ion dehumidi ica ion; MVC: mechanical
apo comp ession; RO: e e se osmosis; ED/EDR: elec odialysis/elec odialysis e e sal; CDI:
capaci i e deioniza ion; EDI: elec odeioniza ion. Adap ed om [6].
The he mal p ocesses shown in Figu e 1 equi e a hea supply in he ange o
50–120 ◦C
[
6
]. Tempe a u es om 50 o 60
◦
C a e only sui able o humidi ica ion de-
humidi ica ion (HDH), app op ia e only o small-scale use due o i s ela i ely low ene gy
e iciency [
8
]. Mo eo e , memb ane dis illa ion (MD) sys ems equi e 75–95
◦
C o achie e
easonable speci ic he mal consump ion, up o 237 and 170 kJ/kg, espec i ely [
7
]. How-
e e , he e a e only small capaci y uni s, hus limi ing hei applica ion a an indus ial scale.
Mul i-e ec dis illa ion (MED) has been supe io o mul i-s age lash (MSF) dis illa ion
since he end o he 20 h cen u y due o i s lowe main and auxilia y ene gy consump ion
P ocesses 2022,10, 153 3 o 23
along wi h lowe he mal consump ion a he same op empe a u e. Fo he sake o com-
pa ison, he MED p ocess achie es 230 kJ/kg o he mal ene gy consump ion wi h a op
empe a u e o 67
◦
C, whe eas he MSF would equi e a leas 105
◦
C o allow he said
main ene gy consump ion. In addi ion, he auxilia y ene gy equi ed by he MSF p ocess is
much highe han ha o MED, since he o me equi es b ine eci cula ion. The indus ial
s anda d o con en ional MED echnology consis s o a MED uni coupled o a he mal
apo comp ession p ocess (MED-TVC). The MED uni is d i en by he ou le low o he
TVC, which is ed by a s eam ex ac ion o he s eam u bine o a Rankine cycle (mo i e
s eam) and s eam gene a ed wi hin one o he las e ec s o he MED plan .
Rega ding desalina ion p ocesses ha consume sha powe o elec ici y, only me-
chanical apo comp ession (MVC) and e e se osmosis (RO) a e sui able o seawa e
desalina ion applica ions. This is due o he signi ican inc ease in ene gy consump ion in
elec odialysis (ED), elec odialysis e e sal (EDR), capaci i e deioniza ion (CDI), and elec-
odeioniza ion (EDI) wi h sal concen a ion. The e o e, hey do no compe e wi h RO o
salini ies wi hin he ange o seawa e , abou 0.035 kg o sea sal s pe kg o saline solu ion.
In addi ion o con en ional phase-change echnologies, a MED uni can be coupled o
a double-e ec abso p ion hea pump (DEAHP). This echnology has been de eloped a
he Spanish esea ch cen e Pla a o ma Sola de Alme ía-CIEMAT [
9
–
11
]. Fo a gi en low
and he modynamic condi ions o he ex e nal s eam sou ce, MED-DEAHP echnology
exhibi s lowe main and auxilia y ene gy consump ions han MED-TVC, hus esul ing in
highe he modynamic e iciency o he in eg a ed powe and wa e p oduc ion. The e o e,
MED-DEAHP is supe io o MED-TVC, al hough he o me is no comme cially a ailable.
Ano he ad anced MED concep is a high- empe a u e MED p ocess by using nano il a ion
as seawa e p e ea men . This was p oposed and de eloped by Hassan [
12
–
14
]. No only
he concep bu also he expe imen al assessmen ha e been epo ed in he li e a u e [
15
].
A ep esen a i e indus ial plan in which nano il a ion and dis illa ion p ocesses a e
in eg a ed is he Sha jah plan , based on MSF dis illa ion [16].
Analyses o in eg a ing desalina ion p ocesses wi hin con en ional sola powe plan s
based on Rankine cycles wi h wa e as wo king luid ha e been equen ly epo ed in he
li e a u e, conside ing mos ly RO, MED, and MED-TVC p ocesses. Some exempla y pape s
a e [
17
,
18
], among o he s. Rega ding he in eg a ion o dis illa ion p ocesses, he main
issue is he a ailabili y o ope a ing models p ope ly alida ed o simula e he dis illa ion
subsys em a pa -load ope a ion. Conce ning MED-TVC plan s, a pe o mance model [
19
]
has been de eloped o calcula e he e iciency and wa e p oduc ion o he MED-TVC
dis illa ion p ocess as a whole wi hin a sui able ange o he ex e nal s eam sou ce. Some
selec ed p e ious pape s a e Hana i e al. [
20
], who desc ibed a ho ough model o a
he mocomp esso o he said applica ion, and Ame i e al. [
21
] and Mazini e al. [
22
], who
p esen ed use ul models o MED-TVC plan s. Addi ionally, he pa ame ic analysis needed
o plan design can be ound in wo ks by Kouhikamali e al. [
23
] and Es ahani e al. [
24
].
Finally, expe imen al da a use ul o alida ing models a e p o ided by Temse e al. [
25
]
and Al-Mu az and Wazee [
26
]. On he o he hand, bo h MED and nano il a ion-MED
p ocesses can be modeled based on he a o emen ioned li e a u e. Addi ionally, ope a ion
ou o nominal condi ions o a MED uni has been expe imen ally assessed by Fe nandez-
Izquie do e al. [27].
Recen p oposals o sola desalina ion sys em designs ha i he scheme o Figu e 1
a e p esen ed h oughou his pape . Mino inno a ions ha e been ound conce ning
coupling con en ional sola Rankine cycles and desalina ion. Indeed, inno a i e p oposals
ely on he o ganic Rankine cycle (ORC) and he supe c i ical CO
2
(sCO
2
) cycle as powe
con e sion uni s.
Medium- empe a u e echnology o sola he mal collec o s consis s o sys ems wi h
op ope a ing empe a u es wi hin he ange o 150–400
◦
C, hus equi ing one-axis sun
acking o concen a e he sola i adiance on a linea ocus in which he abso be ube
is placed. Sola pa abolic ough collec o s (PTCs) and linea F esnel concen a o s co e-
spond o medium- empe a u e collec o s. Since con en ional Rankine cycles ope a ed wi h
P ocesses 2022,10, 153 4 o 23
wa e exhibi qui e limi ed e iciency a ela i ely low empe a u es, signi ican esea ch
ac i i y has been ocused on o ganic Rankine cycles (ORCs), sui able e en o he lowes
empe a u es wi hin he said ange. Indeed, ORCs ope a ed wi h low- empe a u e sola
collec o s—i.e., s a iona y collec o s—ha e also been de eloped. The i s sola ORC desali-
na ion sys ems da e back o he la e 1970s and ea ly 1980s, all o hem using e e se osmosis
as he desalina ion echnology. Due o he high speci ic ene gy consump ion o he p ocess
a ha ime, hese i s expe iences we e de eloped o b ackish wa e desalina ion [
28
–
30
].
A e iew o s a e-o - he-a sola he mal e e se osmosis desalina ion sys ems up o 2009
can be ound in [
31
]. I s conclusions indica e, among o he issues, ha he wa e –elec ici y
cogene a ion op ion has no been su icien ly explo ed. Design ecommenda ions based
on he wo k de eloped up o 2012 is epo ed in [
32
], and he upda e o he s a e o he
a can be ound in [
33
]. The exis ing sys ems and p oposals a e classi ied in he la e
acco ding o he ORC con igu a ion in o single s age ( e e ing o simple cycles), wo-s age
( e e ing o double cascade cycles), and low- empe a u e sys ems. Mo e ecen ly, and
co e ing a wide scope, he e alua ion o sola he mal-powe ed desalina ion echnologies
has been conduc ed by [
7
] o iden i y ma ke oppo uni ies o hese echnologies. Among
he op ions o g owing in e es is e e se osmosis powe ed by sola ORCs, which p esen s
he possibili y o inco po a ing he mal ene gy s o age sys ems (TES) ins ead o ba e -
ies. In low demand scena ios (less han 100 m
3
/d), con en ional dis illa ion echnologies
(MED and MSF) and RO powe ed by sola ORCs a e disca ded agains he pho o ol aic
(PV)–RO op ion, while in in e media e demand scena ios (100 o 25,000 m
3
/d), he sola
ORCs wi h PTCs o F esnel is one o he echnologies wi h ma ke oppo uni ies. The
ollowing sec ions p esen he mos ecen p oposals ound in he scien i ic li e a u e on
sola desalina ion sys ems based on ORC as he main ene gy con e sion uni s. As will be
seen, he app oaches based on poly- o mul i-gene a ion a e dominan .
Las ly, sCO
2
B ay on cycles ha e been ecen ly de eloped in o de o achie e much
highe ene gy e iciency, wi h op empe a u es echnically achie able wi h he cu en
echnology o sola owe powe plan s. Helios a s concen a e he sola adia ion wi hin
a ocal poin on he op o a owe whe e he sola ecei e is placed. Speci ically, ene gy
e iciency a ound 45% co esponds o he op empe a u e o 550
◦
C wi h a B ay on cycle
wi h egene a ion and ecomp ession. In he nea u u e, 52% could be eliable wi h 700
◦
C
o op empe a u e [34].
2. Sola ORC (O ganic Rankine Cycle)-D i en Desalina ion Sys ems: An Upda e
2.1. In eg a ion o Re e se Osmosis (RO) as Desalina ion Technology
Re e se osmosis desalina ion equi es elec ici y o powe he main pump o p essu ize
he saline wa e eed abo e he osmo ic p essu e o he concen a e ou pu low. P essu ized
eedwa e low ci cula es in pa allel wi h he RO memb ane su ace, whe eas pa o he
sol en passes h ough he memb ane along wi h a mino po ion o sal s, hus gene a ing
he pe mea e low (p oduc ). The emaining eed s eam wi h inc eased salini y becomes
he concen a e low. Since his concen a e exi s a a p essu e sligh ly below he eed
p essu e, ene gy eco e y de ices a e needed in seawa e desalina ion plan s. Recen
p oposals o sola RO desalina ion sys ems wi h ORC uni s as he p ime mo e gene ally
co espond o he con igu a ions desc ibed in he ollowing subsec ions, dealing wi h
medium- and low- empe a u e sola sys ems, espec i ely.
2.1.1. Medium-Tempe a u e Sola Sys ems
The scheme shown in Figu e 2desc ibes he la es inno a i e p oposals o medium-
empe a u e sola ORC sys ems. As can be seen, a sola he mal plan wi h he mal ene gy
s o age (TES) d i es a single-e ec abso p ion e ige a ion sys em by he hea ejec ed
a he condense . This scheme would he e o e espond o a poly- o mul i-gene a ional
sys em i no he en i e elec ical ene gy ou pu o he ORC is consumed by he RO uni .
Addi ionally, pa o his he mal ene gy ejec ed p ehea s he eed wa e o he RO sys em.
This exploi a ion has wo ad an ages: (1) he use o saline wa e as a cooling medium
P ocesses 2022,10, 153 5 o 23
makes an addi ional cooling low unnecessa y; (2) he pe meabili y o he RO memb anes
ises wi h eed wa e empe a u e, ha is o say, he p oduc i i y o he plan is imp o ed
al hough his occu s a he expense o an inc ease in he salini y o he p oduc .
P ocesses 2022, 10, x FOR PEER REVIEW 5 o 23
s o age (TES) d i es a single-e ec abso p ion e ige a ion sys em by he hea ejec ed a
he condense . This scheme would he e o e espond o a poly- o mul i-gene a ional sys-
em i no he en i e elec ical ene gy ou pu o he ORC is consumed by he RO uni .
Addi ionally, pa o his he mal ene gy ejec ed p ehea s he eed wa e o he RO sys-
em. This exploi a ion has wo ad an ages: (1) he use o saline wa e as a cooling medium
makes an addi ional cooling low unnecessa y; (2) he pe meabili y o he RO memb anes
ises wi h eed wa e empe a u e, ha is o say, he p oduc i i y o he plan is imp o ed
al hough his occu s a he expense o an inc ease in he salini y o he p oduc .
Figu e 2. Gene al scheme o a sola he mal-d i en RO desalina ion and cooling sys em.
A sola poly-gene a ion sys em wi h a ecupe a i e ORC uni wi h n-oc ane as he
wo king luid o elec ici y p oduc ion is p esen ed in [35]. The sola sys em conside ed
consis s o a pa abolic ough collec o ield ope a ed wi h The minol-66 as he hea ans-
e luid (HTF) and a wo- ank he mal ene gy s o age (TES) sys em wi h comme cial mol-
en sal s. Hi ec XL is conside ed in o de o ake ad an age o i s ela i ely low eezing
poin (120 °C) and cos . A ac ion o he hea ans e ed in he ORC’s ecupe a o is ex-
ploi ed o he p oduc ion o domes ic ho wa e . Mo eo e , he hea ejec ion o he ORC
is used in he gene a o o a single-e ec abso p ion e ige a ion cycle. The ORC’s elec-
ici y ou pu is consumed by an elec olyze o hyd ogen p oduc ion and by he sea-
wa e RO uni , whe e a Pel on u bine is conside ed as he ene gy eco e y sys em. The
wa e needed in he elec olyze is p ehea ed by he he mal oil low a he ou le o he
ORC’s sola hea exchange . The p oposed sys em is analyzed o p oduce be ween 200
and 500 kW elec ic and 450 ppm eshwa e om 35,000 ppm eed wa e . The nominal
capaci y o he seawa e e e se osmosis plan (SWRO) is 40 kg/s, and he abso p ion e-
ige a ion sys em would ha e a cooling capaci y o 500–800 kW. Bo h ene gy and exe gy
e iciency o he whole sys em is calcula ed, and echno-economic op imiza ion is also
pe o med using e olu iona y algo i hms.
Ano he con igu a ion o pa abolic ough sola collec o s wi h TES and The minol
VP-1 as HTF and s o age medium is p oposed by [36]. A con en ional Rankine cycle is
he mally d i en by his sola ene gy sys em. The s eam u bine’s ou pu low, a e being
pa ially expanded, ac s as he p ima y low o a s eam ejec o . The discha ge mixed low
is he hea sou ce o he ORC uni . A e hea ing he ORC sys em, his s eam is spli in o
wo lows. One o hem comple es he Rankine cycle and he second low is used o a
p elimina y p ehea ing o he RO eedwa e and subsequen ly o cooling p oduc ion. The
cooling o he ORC is also ca ied ou wi h he eedwa e o he desalina ion uni o ob ain
Figu e 2. Gene al scheme o a sola he mal-d i en RO desalina ion and cooling sys em.
A sola poly-gene a ion sys em wi h a ecupe a i e ORC uni wi h n-oc ane as he
wo king luid o elec ici y p oduc ion is p esen ed in [
35
]. The sola sys em conside ed
consis s o a pa abolic ough collec o ield ope a ed wi h The minol-66 as he hea ans e
luid (HTF) and a wo- ank he mal ene gy s o age (TES) sys em wi h comme cial mol en
sal s. Hi ec XL is conside ed in o de o ake ad an age o i s ela i ely low eezing poin
(120
◦
C) and cos . A ac ion o he hea ans e ed in he ORC’s ecupe a o is exploi ed
o he p oduc ion o domes ic ho wa e . Mo eo e , he hea ejec ion o he ORC is used in
he gene a o o a single-e ec abso p ion e ige a ion cycle. The ORC’s elec ici y ou pu
is consumed by an elec olyze o hyd ogen p oduc ion and by he seawa e RO uni ,
whe e a Pel on u bine is conside ed as he ene gy eco e y sys em. The wa e needed
in he elec olyze is p ehea ed by he he mal oil low a he ou le o he ORC’s sola
hea exchange . The p oposed sys em is analyzed o p oduce be ween 200 and 500 kW
elec ic and 450 ppm eshwa e om 35,000 ppm eed wa e . The nominal capaci y o he
seawa e e e se osmosis plan (SWRO) is 40 kg/s, and he abso p ion e ige a ion sys em
would ha e a cooling capaci y o 500–800 kW. Bo h ene gy and exe gy e iciency o he
whole sys em is calcula ed, and echno-economic op imiza ion is also pe o med using
e olu iona y algo i hms.
Ano he con igu a ion o pa abolic ough sola collec o s wi h TES and The minol
VP-1 as HTF and s o age medium is p oposed by [
36
]. A con en ional Rankine cycle is
he mally d i en by his sola ene gy sys em. The s eam u bine’s ou pu low, a e being
pa ially expanded, ac s as he p ima y low o a s eam ejec o . The discha ge mixed low
is he hea sou ce o he ORC uni . A e hea ing he ORC sys em, his s eam is spli in o
wo lows. One o hem comple es he Rankine cycle and he second low is used o a
p elimina y p ehea ing o he RO eedwa e and subsequen ly o cooling p oduc ion. The
cooling o he ORC is also ca ied ou wi h he eedwa e o he desalina ion uni o ob ain
addi ional hea ing. The e o e, in his p oposal, he ORC is no di ec ly hea ed by he sola
he mal plan .
2.1.2. Low-Tempe a u e Sola Sys ems
The o he op ion commonly add essed in ecen s udies is he combina ion o sola
ponds (SPs) and a low- empe a u e ORC uni (see Figu e 3) o powe a RO desalina ion

P ocesses 2022,10, 153 6 o 23
sys em. The concep is no new; wo o such sys ems we e implemen ed in he USA—
Los Baños (Cali o nia) and El Paso (Texas) [
37
]. A sola pond is in eg a ed in he same
de ice o bo h sola – he mal ene gy con e sion and long- e m hea s o age. Wi h ade-
qua e p ocedu es o c ea ion and main enance, h ee zones wi h di e en salini ies emain
h oughou he li e ime o he SP. Thanks o he sola hea ing, he mal ene gy eaches he
lowes laye (lowe con ec i e zone), consis ing o high salini y solu ion. On op, a laye
wi h an app op ia e salini y g adien is c ea ed, so-called he non-con ec i e zone (NCZ).
Due o he co esponding densi y p o ile, i s salini y g adien a oids con ec ion in o de
o maximize he he mal insula ion o he LCZ. The uppe laye (uppe con ec i e zone,
UCZ) p o ides insula ion om he a mosphe ic phenomena. As Figu e 3depic s, sola
hea is ex ac ed om he lowe con ec i e zone (LCZ) o he SP, whe eas he hea ejec ed
by he ORC is e-injec ed in o he uppe con ec i e zone (UCZ).
P ocesses 2022, 10, x FOR PEER REVIEW 6 o 23
addi ional hea ing. The e o e, in his p oposal, he ORC is no di ec ly hea ed by he sola
he mal plan .
2.1.2. Low-Tempe a u e Sola Sys ems
The o he op ion commonly add essed in ecen s udies is he combina ion o sola
ponds (SPs) and a low- empe a u e ORC uni (see Figu e 3) o powe a RO desalina ion
sys em. The concep is no new; wo o such sys ems we e implemen ed in he USA—Los
Baños (Cali o nia) and El Paso (Texas) [37]. A sola pond is in eg a ed in he same de ice
o bo h sola – he mal ene gy con e sion and long- e m hea s o age. Wi h adequa e p o-
cedu es o c ea ion and main enance, h ee zones wi h di e en salini ies emain h ough-
ou he li e ime o he SP. Thanks o he sola hea ing, he mal ene gy eaches he lowes
laye (lowe con ec i e zone), consis ing o high salini y solu ion. On op, a laye wi h an
app op ia e salini y g adien is c ea ed, so-called he non-con ec i e zone (NCZ). Due o
he co esponding densi y p o ile, i s salini y g adien a oids con ec ion in o de o max-
imize he he mal insula ion o he LCZ. The uppe laye (uppe con ec i e zone, UCZ)
p o ides insula ion om he a mosphe ic phenomena. As Figu e 3 depic s, sola hea is
ex ac ed om he lowe con ec i e zone (LCZ) o he SP, whe eas he hea ejec ed by
he ORC is e-injec ed in o he uppe con ec i e zone (UCZ).
Figu e 3. Gene al scheme o sola -pond-d i en RO desalina ion p oposals.
The in es iga ion o a sola RO desalina ion sys em using an SP ha p o ides he mal
ene gy o a ecupe a i e ORC uni is p esen ed by [38]. Fo his pu pose, an HTF is used
o ex ac ene gy om he lowe con ec i e zone. This HTF is ci cula ed h ough he ORC
e apo a o . As o he desalina ion sys em, he eed wa e is i s p ehea ed in he ORC
condense and hen ci cula ed in o he uppe con ec i e zone o he sola pond be o e
en e ing he desalina ion uni . The b ine s eam om he RO uni is pa ially e apo a ed
and a po ion is subsequen ly injec ed in o he lowe con ec i e zone in o de o eplenish
sal s losses due o hei di usion o he uppe laye s. The seawa e RO uni is elec ically
d i en by he ORC.
A RO desalina ion sys em based on a sola ORC powe ed by an SP is also p esen ed
in [39]. This con igu a ion is p oposed oge he wi h he analogous sys em, in which he
ORC is eplaced by a Kalina cycle. In he sola ORC con igu a ion, an ex ac ed luid
s eam o ms he LCZ and is ci cula ed o he ORC e apo a o , which inco po a es ecu-
pe a ion. Pa o he elec ical powe p oduced is used in he desalina ion uni while he
es is injec ed in o he g id. I he use o he hea om he ORC condense is also con em-
pla ed o a he moelec ic gene a ion (TEG) sys em. A low ex ac ed om he uppe con-
ec i e zone is used as a cold sink o he TEG and e-injec ed in o he uppe con ec i e
zone a e being hea ed. Se en di e en wo king luids a e conside ed o he ORC, e-
sul ing in an o e all sys em exe gy e iciency o up o 46.4% when using R227ea.
Figu e 3. Gene al scheme o sola -pond-d i en RO desalina ion p oposals.
The in es iga ion o a sola RO desalina ion sys em using an SP ha p o ides he mal
ene gy o a ecupe a i e ORC uni is p esen ed by [
38
]. Fo his pu pose, an HTF is used o
ex ac ene gy om he lowe con ec i e zone. This HTF is ci cula ed h ough he ORC
e apo a o . As o he desalina ion sys em, he eed wa e is i s p ehea ed in he ORC
condense and hen ci cula ed in o he uppe con ec i e zone o he sola pond be o e
en e ing he desalina ion uni . The b ine s eam om he RO uni is pa ially e apo a ed
and a po ion is subsequen ly injec ed in o he lowe con ec i e zone in o de o eplenish
sal s losses due o hei di usion o he uppe laye s. The seawa e RO uni is elec ically
d i en by he ORC.
A RO desalina ion sys em based on a sola ORC powe ed by an SP is also p esen ed
in [
39
]. This con igu a ion is p oposed oge he wi h he analogous sys em, in which he
ORC is eplaced by a Kalina cycle. In he sola ORC con igu a ion, an ex ac ed luid s eam
o ms he LCZ and is ci cula ed o he ORC e apo a o , which inco po a es ecupe a ion.
Pa o he elec ical powe p oduced is used in he desalina ion uni while he es is
injec ed in o he g id. I he use o he hea om he ORC condense is also con empla ed
o a he moelec ic gene a ion (TEG) sys em. A low ex ac ed om he uppe con ec i e
zone is used as a cold sink o he TEG and e-injec ed in o he uppe con ec i e zone a e
being hea ed. Se en di e en wo king luids a e conside ed o he ORC, esul ing in an
o e all sys em exe gy e iciency o up o 46.4% when using R227ea.
The double cascade concep is conside ed in he h ee designs in es iga ed by [
40
],
one o hem wi hou ORC, whe e an SP ac s as he hea sou ce o he opping cycle. The
he modynamic and he moeconomic analysis o h ee con igu a ions is pe o med. In wo
o hem, a single ORC is he mally d i en wi h ene gy ex ac ed om he LCZ o he SP. The
elec ici y ou pu o his ORC is consumed by he RO uni . The hea ejec ion o he opping
cycle is used as hea abso bed by he bo oming cycle, which, in one o he con igu a ions,
P ocesses 2022,10, 153 7 o 23
is a Kalina cycle, and in he o he , a simple ORC. In bo h cases, he cooling o he bo om
cycle’s condense is ca ied ou wi h wa e om he UCZ o he SP. A he modynamic
and he moeconomic analysis is pe o med, assuming a SP su ace a ea o 10,000 m
2
, a
empe a u e o 90
◦
C in he LCZ, and a RO uni con e sion o 30% o seawa e salini y o
42,485 g/kg.
2.2. In eg a ion o The mal P ocesses as Desalina ion Technology
When a he mal desalina ion p ocess is conside ed o be in eg a ed wi h a sola ORC
sys em, ecen p oposals ha e mos ly ma ched he layou shown in Figu e 4. As can be
seen, he in eg a ion implies he use o he hea ejec ed by he ORC o d i e he he mal
desalina ion p ocess, which, in mos o he p oposals, in ol es a MED o MED-TVC uni .
The e a e also some p oposals wi h HDH o an MSF uni . Those desalina ion p ocesses a e
desc ibed below.
P ocesses 2022, 10, x FOR PEER REVIEW 7 o 23
The double cascade concep is conside ed in he h ee designs in es iga ed by [40],
one o hem wi hou ORC, whe e an SP ac s as he hea sou ce o he opping cycle. The
he modynamic and he moeconomic analysis o h ee con igu a ions is pe o med. In
wo o hem, a single ORC is he mally d i en wi h ene gy ex ac ed om he LCZ o he
SP. The elec ici y ou pu o his ORC is consumed by he RO uni . The hea ejec ion o
he opping cycle is used as hea abso bed by he bo oming cycle, which, in one o he
con igu a ions, is a Kalina cycle, and in he o he , a simple ORC. In bo h cases, he cooling
o he bo om cycle’s condense is ca ied ou wi h wa e om he UCZ o he SP. A he -
modynamic and he moeconomic analysis is pe o med, assuming a SP su ace a ea o
10,000 m2, a empe a u e o 90 °C in he LCZ, and a RO uni con e sion o 30% o seawa e
salini y o 42,485 g/kg.
2.2. In eg a ion o The mal P ocesses as Desalina ion Technology
When a he mal desalina ion p ocess is conside ed o be in eg a ed wi h a sola ORC
sys em, ecen p oposals ha e mos ly ma ched he layou shown in Figu e 4. As can be
seen, he in eg a ion implies he use o he hea ejec ed by he ORC o d i e he he mal
desalina ion p ocess, which, in mos o he p oposals, in ol es a MED o MED-TVC uni .
The e a e also some p oposals wi h HDH o an MSF uni . Those desalina ion p ocesses
a e desc ibed below.
Figu e 4. In eg a ion o he mal desalina ion p ocesses in sola -ORC-based poly-gene a ion sys-
ems.
The indus ial s anda d o he MED p ocess comp ises a MED uni coupled o a he -
mocomp esso , as ollows:
• MED uni . A se o se e al chambe s (so-called e ec s) a e kep unde acuum con-
di ions wi h dec easing p essu es, co esponding o he sa u a ion p essu e a de-
c easing empe a u es om 50 o 67 °C, up o 45–35 °C. No mally he e is a single
hea exchange consis ing o a ho izon al ube bundle and he e apo a o . An ex e -
nal low, p o iding he ex e nal hea sou ce o he MED p ocess, ci cula es wi hin he
e apo a o ubes o he i s e ec . Seawa e eed wi h sligh p ehea ing is sp ayed
on he su ace o he ube bundle, hus esul ing in a hin ilm ha is pa ially e ap-
o a ed. The emaining b ine is discha ged by he bo om o he e ec , whe eas he
s eam gene a ed is sen o ci cula e inside he ubes o he e apo a o o he nex e -
ec . The s eam gene a ed in he las e ec is condensed in he end condense . A sea-
wa e low ci cula es h ough he end condense ; pa o his low (seawa e cooling)
is discha ged back o he sea and he es o he low is he p ehea ed seawa e ha
en e s all e ec s in pa allel. Indus ial plan s no mally ha e 8–12 e ec s in MED uni s
and 4–8 e ec s in MED-TVC, al hough designs wi h 14 e ec s a e easible, wi h an
Figu e 4.
In eg a ion o he mal desalina ion p ocesses in sola -ORC-based poly-gene a ion sys ems.
The indus ial s anda d o he MED p ocess comp ises a MED uni coupled o a
he mocomp esso , as ollows:
•
MED uni . A se o se e al chambe s (so-called e ec s) a e kep unde acuum
condi ions wi h dec easing p essu es, co esponding o he sa u a ion p essu e a
dec easing empe a u es om 50 o 67
◦
C, up o 45–35
◦
C. No mally he e is a single
hea exchange consis ing o a ho izon al ube bundle and he e apo a o . An ex e nal
low, p o iding he ex e nal hea sou ce o he MED p ocess, ci cula es wi hin he
e apo a o ubes o he i s e ec . Seawa e eed wi h sligh p ehea ing is sp ayed on
he su ace o he ube bundle, hus esul ing in a hin ilm ha is pa ially e apo a ed.
The emaining b ine is discha ged by he bo om o he e ec , whe eas he s eam
gene a ed is sen o ci cula e inside he ubes o he e apo a o o he nex e ec . The
s eam gene a ed in he las e ec is condensed in he end condense . A seawa e
low ci cula es h ough he end condense ; pa o his low (seawa e cooling) is
discha ged back o he sea and he es o he low is he p ehea ed seawa e ha en e s
all e ec s in pa allel. Indus ial plan s no mally ha e 8–12 e ec s in MED uni s and
4–8 e ec s in MED-TVC, al hough designs wi h 14 e ec s a e easible, wi h an a e age
o a ound 2.5
◦
C o empe a u e g adien be ween adjacen e ec s. Wi h 14 e ec s, he
he mal ene gy consump ion would be abou 230 kJ/kg in a MED uni and 166 kJ/kg
in MED-TVC.
•
Te mocomp esso . A high-p essu e s eam low (mo i e s eam) is mixed wi h a low-
p essu e s eam low, hus esul ing in a s eam low wi h in e media e p essu e and
empe a u e. This ou le s eam low d i es he MED p ocess: he low-p essu e low
is he s eam gene a ed a he las e ec o one o he las e ec s, whe eas he mo i e
P ocesses 2022,10, 153 8 o 23
s eam is gene a ed by he a ailable hea sou ce. In con en ional MED-TVC plan s,
he mo i e s eam is a u bine ex ac ion. S eam a abou 140–225
◦
C is equi ed in
MED-TVC plan s. Coupling he he mocomp esso o he MED plan a he las e ec
makes he end condense unneccesa y. This esul s in a oiding he seawa e cooling
low ha equi es signi ican elec ici y consump ion, along wi h inc eased capi al cos
a ibu able o seawa e in ake in as uc u e and pumps.
Mo eo e , MSF plan s ely on s eam gene a ion due o he quick educ ion o he
p essu e ( lash) o he saline solu ion hea ed by an ex e nal hea sou ce. An MSF plan
consis s o many lash chambe s, he so-called s ages, connec ed in se ies wi h a ube bundle
(condense ) a he op. S eam gene a ed in he p ocess p ehea s he seawa e eed as i
ci cula es wi hin he condense ubes. A e being p ehea ed, seawa e en e s an ex e nal
hea exchange (b ine hea e ) d i en by he ex e nal hea sou ce. Indus ial plan s include
b ine ecycling since only a small po ion o s eam is gene a ed. Then, b ine, ins ead o
seawa e , su e s successi e lash p ocesses wi hin he lash chambe s, coupled in se ies.
Condense s o se e al s ages a he ail a e cooled by an ex e nal seawa e cooling low.
MSF plan s exhibi e y high auxilia y ene gy consump ion, a ibu able o he cooling
low, he acuum sys em, and he la ge low o b ine ci cula ing h ough he lash s ages.
Finally, he HDH p ocess is based on inc easing he ela i e humidi y o he ai by
means o sp aying wa e wi hin an ai low a he humidi ie . A leas one o hose lows,
wa e and/o ai , should be p e iously hea ed by an ex e nal hea sou ce. Finally, a he
dehumidi ie , pa o he s eam wi hin he ho mois ai is condensed on he ou e su ace
o colling ubes, hanks o he ci cula ion o a cooling low. Wa e and ai ci cui s can be
ei he open o closed. Mo eo e , hei ci cula ion can be o ced o na u al.
A sola poly-gene a ion scheme is p esen ed in [
41
]. The ORC uni wi h o-Xylene
as he wo king luid is powe ed by a sola he mal plan wi h PTCs (Sky T ough model)
and a sensible TES. In bo h sys ems, The minol VP-1 is used as he he mal luid. The
hea ejec ed by he ORC d i es he MED uni and a single-e ec abso p ion e ige a ion
sys em. The sys em is s udied om ene ge ic and exe ge ic poin s o iew.
The use o a ecupe a i e ORC uni d i en by a sola he mal plan wi h PTC and
TES, bo h wi h he mal oil as he he mal luid, is p oposed by [
42
], among o he op ions.
This con igu a ion is compa ed wi h he analogous sys em comp ised o a con en ional
Rankine cycle and mol en sal s as HTF and s o age medium. In bo h cases, a MED plan
is conside ed.
A sys em based on a sola he mal plan wi h PTCs and TES wi h he mal oil as he HTF
and s o age medium o d i e an ORC uni is analyzed in [
43
,
44
]. N-oc ane is he wo king
luid. The elec ici y p oduced by he ORC is consumed in an elec olyze o hyd ogen
p oduc ion. The hea ejec ed by he ORC is used in an HDH desalina ion uni and also o
p ehea he desalina ed wa e ou pu , which will be consumed by he elec olyze .
A sola poly-gene a ion sys em o mee he cooling and wa e demands o a g een-
house is s udied in [
45
]. The sola he mal plan wi h PTCs and TES p o ides he mal
ene gy o an ORC uni and an abso p ion cooling sys em. Pa o he elec ici y p oduced
by he ORC—wi h oluene as he wo king luid—is used in an elec olyze , while he hea
ejec ed is used o d i e an MSF plan . The desalina ed wa e is hen used bo h in he
elec olyze and as a supply o he g eenhouse. The oxygen and hyd ogen p oduced a e
s o ed o p oduce backup he mal ene gy du ing pe iods o no sola adia ion using a
hyd o-oxy combus o .
A sola poly-gene a ion sys em based on an ORC uni in eg a ing a s eam ejec o
placed a he u bine’s ou le is analyzed in [
46
]. The discha ge mixed low o he ejec o is
condensed wi h seawa e , while he seconda y low o he ejec o is p e iously used o
cooling p oduc ion. A MED plan ed wi h he apo om an ex ac ion a he ORC u bine
is p oposed as he desalina ion uni . Seawa e is conside ed as he hea ans e luid in he
sola he mal plan .
In addi ion o he abo e, he e a e o he ecen p oposals in he li e a u e ha do no
ma ch he scheme o Figu e 4bu inco po a e ORC uni s o supply he ene gy demanded
P ocesses 2022,10, 153 9 o 23
by a he mal desalina ion p ocess. Examples o his a e he p oposals whe e he was e hea
eco e ed om he exhaus gas o a gas u bine is used o supply hea o he ORC [47,48].
A comp essed ai ene gy s o age (CAES) sys em o consume elec ici y du ing he o -
peak hou s o elec ical demand is p oposed by [
47
]. An in e cooled wo-s age comp ession
p ocess is conside ed, and a e cooling p io o s o age also akes place. The hea eco e ed
in bo h p ocesses is used in he p oduc ion o domes ic ho wa e . Du ing peak demand
hou s, he s o ed comp essed ai is used o p oduce elec ici y by means o a gas u bine.
P io o i s combus ion chambe , he ai is p ehea ed in a sola dish sys em. The exhaus
gases om he u bine a e used o supply he hea demanded by he simple cycle ORC uni ,
wi h oluene as he wo king luid. The condense o his uni is cooled wi h a MED plan .
A 10 m diame e sola dish and a 6-e ec MED plan wi h a op b ine empe a u e o 70
◦
C
and a eed o 40,000 ppm a e analyzed. The esul s indica e an ORC ou pu o 17.4 kW
and 2.5 m
3
/day o eshwa e p oduc ion wi h an o e all exe gy e iciency o 41.7%. As
a s o age sys em, he calcula ed con igu a ion has a ound ip e iciency o 65.2%. Two
mul i-objec i e op imiza ions wi h e olu iona y algo i hms a e also pe o med by [
47
],
esul ing in op imal exe gy e iciencies o a ound 51%.
A poly-gene a ion sys em composed o a sola / ossil hyb id ecupe a i e gas u bine
cycle wi h an in e cooled wo-s age comp ession sys em is p oposed in [
48
]. The ai coming
om he ecupe a o is sola -hea ed be o e en e ing he combus ion chambe . The exhaus
gases om he gas u bine a e used o d i e he ORC uni . Mo eo e , he e is an abso p ion
e ige a ion sys em and a desalina ion uni , which consis s o a single-s age lash p ocess.
The seawa e eed is p ehea ed in he ORC condense and hen wi hin he condense o he
desalina ion uni . A e being p ehea ed, he seawa e low is sepa a ed in o wo s eams.
One o hem is di ec ly sen o he condense o he e ige a ion sys em. The second ac s
as he cooling low o he wo-s age comp ession be o e en e ing he e ige a ion sys em,
passing h ough he gene a o . The la e inally en e s he desalina ion uni as eed wa e .
A he ou le o he lash uni , he b ine is ai -cooled o low-g ade hea p oduc ion. Fo
he he modynamic analysis, concen a ing sola collec o s a e conside ed, while R134a is
he wo king luid in he simple ORC. The p oposed mul i-gene a ion con igu a ion has
an exe gy e iciency o 27%. This con igu a ion is compa ed wi h ano he in which he
sola hyb id gas cycle con igu a ion is main ained bu he ORC, cooling, and desalina ion
sys ems a e eplaced by a Kalina cycle.
2.3. P oposals wi h In eg a ion o Hyb id RO/The mal Desalina ion
Recen hyb id desalina ion con igu a ions wi h a sola -d i en ORC uni can also be
ound in speci ic li e a u e.
A sola –wind hyb id sys em, in which a simple sola ORC ope a ed wi h oluene is
connec ed o a sola pa abolic ough ield wi hou TES, is p esen ed in [
49
]. In addi ion o
he ORC, elec ici y p oduc ion om a wind u bine is also conside ed. The o al elec ici y
ou pu d i es a seawa e RO desalina ion plan and he su plus is sold o he g id. The
ejec ed hea o he ORC is also used in a MED-TVC plan .
Jaube e al. [
50
] p esen he analysis o h ee RO-MED hyb id sola desalina ion
sys ems powe ed by a PTC ield wi hou TES. The minol VP-1 is selec ed as he hea
ans e luid. Pa o he RO b ine low is used in he MED plan . As o he ORC, wo
possible wo king luids a e conside ed in his wo k. On one hand, he e is isopen ane, a
luid usually conside ed in ORC s udies, and on he o he hand, e hyl bu y a e, which
ep esen s a case no s udied so a and, acco ding o he au ho s, shows p omising esul s.
In he h ee p oposed con igu a ions, he hea ejec ed by he ORC p ehea s he eed wa e
o he e e se osmosis uni and he b ine low o he e e se osmosis uni be o e i en e s
he MED plan . As o he ORC a chi ec u es, he simple cycle, simple egene a i e and
simple egene a i e, and sola ehea ing o he apo a e conside ed.
P ocesses 2022,10, 153 16 o 23
whe e
TSF
is he a e age inle /ou le hea ing empe a u e o he HTF, and a
1
and a
2
a e
he hea losses coe icien s. Values o hese coe icien s a e de i ed om he in o ma ion
a ailable a [
69
]. Pa ame e s selec ed o he e alua ion o he sola ield pe o mance
a design condi ions a e gi en in Table 3. Two ex eme alues a e conside ed o he a
1
coe icien o quan i y he in luence o he ORC’s condensa ion empe a u e in scena ios o
di e en sola - o- he mal ene gy con e sion e iciency alues. The wo esul ing cu es a e
depic ed in Figu e 9.
Table 3. Loca ion and design condi ions o he e iciency compu a ion o he PTC sola ield.
Loca ion Fue e en u a (Cana y Islands, Spain)
La i ude, longi ude 28.317–14.071◦
Design condi ions
Ambien empe a u e, Tamb 23 ◦C
Di ec no mal i adiance, Gb,N 850 W/m2
Peak op ical pe o mance, ηop ,0075
Cleanliness index, Fe0.97
A ailabili y, a SF 0.95
Design poin Sola noon, 21 June
O ien a ion No h-Sou h (Eas -Wes acking)
Incidence angle, θ4.87◦
Op ical e iciency, η0,SF 0.67
Hea losses coe icien , a1(low–high alue) 0.02–0.4 W/m2×K
Tempe a u e dependence o he hea loss
coe icien , a20.001 W/m2×K2
P ocesses 2022, 10, x FOR PEER REVIEW 16 o 23
Fo he compa ison p esen ed in his sec ion, i is assumed ha a sola ield is com-
posed o PTCs. In ha case, he op ical e iciency is compu ed by aking in o accoun he
shadow (ηshadow) and end losses (ηendloss), he cleanliness o he sola collec o s (Fe), he a ail-
abili y o he sola ield (a SF), and he in luence o he incidence angle (
θ
) on i s peak op-
ical pe o mance (ηop ,0) ia he incidence angle modi ie (K):
𝜂, =𝜂,∙𝐾∙𝐹
∙𝜂 ∙𝜂∙𝑎𝑣 (3)
On he o he hand, he mal losses a e modeled as:
𝑄󰇗, =󰇟𝑎∙󰇛𝑇
 −𝑇)+𝑎∙󰇛𝑇
 −𝑇)󰇠∙
𝐴
 (4)
whe e 𝑇
 is he a e age inle /ou le hea ing empe a u e o he HTF, and a1 and a2 a e he
hea losses coe icien s. Values o hese coe icien s a e de i ed om he in o ma ion a ail-
able a [70]. Pa ame e s selec ed o he e alua ion o he sola ield pe o mance a design
condi ions a e gi en in Table 3. Two ex eme alues a e conside ed o he a1 coe icien o
quan i y he in luence o he ORC’s condensa ion empe a u e in scena ios o di e en
sola - o- he mal ene gy con e sion e iciency alues. The wo esul ing cu es a e de-
pic ed in Figu e 9.
Table 3. Loca ion and design condi ions o he e iciency compu a ion o he PTC sola ield.
Loca ion Fue e en u a (Cana y Islands,
Spain)
La i ude, longi ude 28.317°–14.071°
Design condi ions
Ambien empe a u e, Tamb 23 °C
Di ec no mal i adiance, Gb,N 850 W/m2
Peak op ical pe o mance, ηop ,0 075
Cleanliness index, Fe 0.97
A ailabili y, a SF 0.95
Design poin Sola noon, 21 June
O ien a ion No h-Sou h (Eas -Wes acking)
Incidence angle,
θ
4.87°
Op ical e iciency,
η
0,SF 0.67
Hea losses coe icien , a1 (low–high alue) 0.02–0.4 W/m2×K
Tempe a u e dependence o he hea loss coe i-
cien , a2 0.001 W/m2×K2
Figu e 9. Sola ield’s e iciency acco ding o he pa ame e s ixed in Table 3.
Figu e 9. Sola ield’s e iciency acco ding o he pa ame e s ixed in Table 3.
Figu e 10 shows he CSP
−
ORC + (MED + RO) con igu a ion o which a condensa ion
empe a u e o 70
◦
C is s udied. To assess only he desalina ion applica ion, he emaining
ne powe ou pu (
.
Wne
) no consumed as auxilia y by he MED plan (Speci ic Elec ici y
Consump ion, (
SECMED
) is dedica ed o an RO uni (
SECRO
). Unde hese assump ions,
he olume ic low a e o eshwa e ( .
Vp) is compu ed wi h he ollowing exp essions:
.
Vp,MED =.
Qin·(1−ηORC)
STECMED
(5)
.
Vp,RO =
.
Wne
SECRO
−
.
Vp,MED·SECMED
SECRO (6)

P ocesses 2022,10, 153 17 o 23
This con igu a ion is compa ed wi h he one shown on he le side, o which a
condensing empe a u e o 40
◦
C is assumed and whose ne powe p oduced is ully
consumed by an RO uni . In his case,
.
Vp,RO
can also be compu ed wi h he exp essions
abo e, making
.
Vp,MED =
0. Table 4shows he nume ical esul s o he compa ison,
assuming he same o al desalina ion capaci y in bo h cases.
P ocesses 2022, 10, x FOR PEER REVIEW 17 o 23
Figu e 10 shows he CSP − ORC + (MED + RO) con igu a ion o which a condensa-
ion empe a u e o 70 °C is s udied. To assess only he desalina ion applica ion, he e-
maining ne powe ou pu (𝑊󰇗) no consumed as auxilia y by he MED plan (Speci ic
Elec ici y Consump ion, 𝑆𝐸𝐶) is dedica ed o an RO uni (𝑆𝐸𝐶). Unde hese as-
sump ions, he olume ic low a e o eshwa e (𝑉󰇗) is compu ed wi h he ollowing
exp essions:
𝑉󰇗, =𝑄
󰇗 ∙󰇛1−𝜂)
𝑆𝑇𝐸𝐶 (5)
𝑉󰇗, =𝑊󰇗
𝑆𝐸𝐶 − 𝑉󰇗, ∙𝑆𝐸𝐶
𝑆𝐸𝐶  (6)
This con igu a ion is compa ed wi h he one shown on he le side, o which a con-
densing empe a u e o 40 °C is assumed and whose ne powe p oduced is ully con-
sumed by an RO uni . In his case, 𝑉󰇗, can also be compu ed wi h he exp essions abo e,
making 𝑉󰇗, =0. Table 4 shows he nume ical esul s o he compa ison, assuming he
same o al desalina ion capaci y in bo h cases.
Figu e 10. Con igu a ions conside ed o be compa ed: CSP − ORC + RO (le ) and CSP − ORC + (RO
+ MED) ( igh ).
As can be seen, inc easing he condensing empe a u e o 70 °C o include he MED
uni equi es an ORC uni wi h less ne powe due o he u iliza ion o he hea ejec ed
by he cycle o eshwa e p oduc ion ou pu (118 kW ins ead o 125 kW pe 1000 m3/day
o capaci y, 6% less). Howe e , he nega i e e ec o he abo e is clea ly e lec ed in he
sola ield a ea and s o age olume equi emen s. In he i s case, 11% mo e a ea would
be equi ed o sola mul iple (SM) = 1 due o he highe he mal ene gy inpu o he ORC
uni , while in he second case, he inc ease would be 24%.
The con igu a ions o be compa ed when he hea ejec ed by he cycle is consumed
in he gene a o o an abso p ion cooling sys em a e shown in Figu e 11. To make he
compa ison consis en , a mechanical apo comp ession (VC) e ige a ion sys em is an-
alyzed when he condense ’s hea is no used o his pu pose. Fo he CSP − ORC + (RO
+ ARS) con igu a ion, he auxilia y elec ical consump ion o he abso p ion e ige a ion
uni is conside ed negligible, in which case, bo h he cooling capaci y (𝑄󰇗,) and he de-
salina ed wa e low a e (𝑉󰇗,) a e easily compu ed as:
G
M edium
em pe a u e
ORC
Condense
Tu bine
Pump
Recupe a o
E apo a o -
supe hea e
P ehea e
1
2
3
y
x
4
B ine
F eshw a e
Elec ici y
Mul i-E ec
Dis illa ion uni
Feedw a e
B ine
F eshw a e
RO
uni
G
M edium
e m p e a u e
ORC
Condense
Tu bine
Pump
Recupe a o
E apo a o -
supe hea e
P ehea e
1
2
3
4
y
x
Elec ici y
B ine
F eshw a e
RO
uni
Feedw a e
Figu e 10.
Con igu a ions conside ed o be compa ed: CSP
−
ORC + RO (
le
) and CSP
−
ORC +
(RO + MED) ( igh ).
Table 4.
Compa ison o he CSP
−
ORC con igu a ion o desalina ion (same desalina ion capaci y).
SECRO = 3 kWh/m3;SECMED = 2 kWh/m3; PR = 10. Di ec TES wi h he mal oil The minol VP-1.
CSP −ORC + RO CSP −ORC +
(MED + RO)
ORC’s condensa ion empe a u e, Tcond 40 ◦C 70 ◦C
ORC’s e apo a ion empe a u e, Te ap 270 ◦C 270 ◦C
ORC’s he mal e iciency, ηORC [%] 24.98 21.37
HTF’s ou le empe a u e, THEX,o 226.5 234.5
A e age sola ield empe a u e [◦C] 263 267
Sola ield’s e iciency, ηSF (low a1/high a1) 0.608 0.501 0.606 0.500
S o age medium olume/s o ed ene gy
[m3/MWh] 25.9 29.1
S o age medium olume/equi alen elec ic
s o ed ene gy [m3/MWhe]103.7 136.1
ORC he mal inpu / o al desalina ion capaci y
[kW/(1000 m3/day)] 500 554
Ne ORC powe ou pu / o al desalina ion
capaci y [kW/(1000 m3/day)] 125 118
Sola ield’s a ea (SM = 1)/ o al desalina ion
capaci y [m
2
/(1000 m
3
/day)] (low a
1
/high a
1
)
968 1175 1075 1303
S o age medium olume (1 h)/ o al
desalina ion capaci y [m3/(1000 m3/day)] 13.0 16.1
P ocesses 2022,10, 153 18 o 23
As can be seen, inc easing he condensing empe a u e o 70
◦
C o include he MED
uni equi es an ORC uni wi h less ne powe due o he u iliza ion o he hea ejec ed by
he cycle o eshwa e p oduc ion ou pu (118 kW ins ead o 125 kW pe 1000 m
3
/day
o capaci y, 6% less). Howe e , he nega i e e ec o he abo e is clea ly e lec ed in he
sola ield a ea and s o age olume equi emen s. In he i s case, 11% mo e a ea would be
equi ed o sola mul iple (SM) = 1 due o he highe he mal ene gy inpu o he ORC
uni , while in he second case, he inc ease would be 24%.
The con igu a ions o be compa ed when he hea ejec ed by he cycle is consumed in
he gene a o o an abso p ion cooling sys em a e shown in Figu e 11. To make he com-
pa ison consis en , a mechanical apo comp ession (VC) e ige a ion sys em is analyzed
when he condense ’s hea is no used o his pu pose. Fo he CSP
−
ORC + (RO + ARS)
con igu a ion, he auxilia y elec ical consump ion o he abso p ion e ige a ion uni is
conside ed negligible, in which case, bo h he cooling capaci y (
.
Qc,AR
) and he desalina ed
wa e low a e ( .
Vp,RO) a e easily compu ed as:
.
Qc,AR =COPAR·
.
Qin·(1−ηORC)(7)
.
Vp,RO =.
Qin·ηORC
SECRO
(8)
P ocesses 2022, 10, x FOR PEER REVIEW 18 o 23
𝑄󰇗, =𝐶𝑂𝑃
 ∙𝑄󰇗 ∙󰇛1−𝜂) (7)
𝑉󰇗, =𝑄
󰇗 ∙𝜂
𝑆𝐸𝐶 (8)
Fo he CSP − ORC + (RO + VC) con igu a ion, he 𝑉󰇗, can be ob ained om:
𝑉󰇗, =
 ∙󰇣𝜂 ∙𝑄󰇗 −󰇗,
󰇤 (9)
whe e 𝑄󰇗, is he cooling capaci y o he apo comp ession e ige a ion sys em.
Figu e 11. Con igu a ions conside ed o be compa ed: CSP − ORC + (RO + VC) (le ) and CSP − ORC
+ (RO + ARS) ( igh ).
Table 4. Compa ison o he CSP − ORC con igu a ion o desalina ion (same desalina ion capaci y).
SECRO = 3 kWh/m3 ; SECMED = 2 kWh/m3 ; PR = 10. Di ec TES wi h he mal oil The minol VP-1.
CSP − ORC +
RO
CSP − ORC +
(MED + RO)
ORC’s condensa ion empe a u e, Tcond 40 °C 70 °C
ORC’s e apo a ion empe a u e, Te ap 270 °C 270 °C
ORC’s he mal e iciency,
η
ORC [%] 24.98 21.37
HTF’s ou le empe a u e, THEX,o 226.5 234.5
A e age sola ield empe a u e [°C] 263 267
Sola ield’s e iciency,
η
SF (low a1/high a1) 0.608 0.501 0.606 0.500
S o age medium olume/s o ed ene gy [m3/MWh] 25.9 29.1
S o age medium olume/equi alen elec ic s o ed ene gy
[m3/MWhe] 103.7 136.1
ORC he mal inpu / o al desalina ion capaci y [kW/(1000
m3/day)] 500 554
Ne ORC powe ou pu / o al desalina ion capaci y
[kW/(1000 m3/day)] 125 118
Sola ield’s a ea (SM = 1)/ o al desalina ion capaci y
[m2/(1000 m3/day)] (low a1/high a1) 968 1175 1075 1303
G
M edium
e m p e a u e
ORC
Condense
Tu bine
Pump
Recupe a o
E apo a o -
supe hea e
P ehea e
1
2
3
y
x
4
Elec ici y
B ine
F eshw a e
C o o lin g Hea ing
Feedw a e
Abso p ion
Re ige a ion Sys em
RO
uni
G
Medium
em pe a u e
ORC
Condense
Tu bine
Pump
Recupe a o
E apo a o -
supe hea e
P ehea e
1
2
3
4
y
x
Elec ici y
B ine
F eshw a e
RO
uni
Feedw a e
Vapo
Comp ession
Re ige a ion
Cooling Hea ing
Figu e 11.
Con igu a ions conside ed o be compa ed: CSP
−
ORC + (RO + VC) (
le
) and CSP
−
ORC + (RO + ARS) ( igh ).
Fo he CSP −ORC + (RO + VC) con igu a ion, he .
Vp,RO can be ob ained om:
.
Vp,RO =1
SECRO
·"ηORC·
.
Qin −
.
Qc,VC
COPVC #(9)
whe e .
Qc,VC is he cooling capaci y o he apo comp ession e ige a ion sys em.
The nume ical esul s o he compa ison o he same cooling and desalina ion capaci y
a e shown in Table 5, whe e a condensing empe a u e o 90
◦
C is assumed o he ope a ion
o he abso p ion e ige a ion uni .
P ocesses 2022,10, 153 19 o 23
Table 5.
Compa ison o he CSP
−
ORC con igu a ion o desalina ion and e ige a ion (same
desalina ion and cooling capaci y). SEC
RO
= 3 kWh/m
3
; di ec TES wi h he mal oil The minol VP-1.
CSP −ORC +
(RO + VC)
CSP −ORC +
(RO + AR)
ORC’s condensa ion empe a u e, Tcond 40 ◦C 90 ◦C
ORC’s e apo a ion empe a u e, Te ap 270 ◦C 270 ◦C
ORC’s he mal e iciency, ηORC 24.98% 19.12%
HTF’s ou le empe a u e, THEX,o 226.5 ◦C 239.6 ◦C
COP 4 0.7
A e age sola ield empe a u e [◦C] 263 270
Sola ield’s e iciency, ηSF 0.608 0.501 0.604 0.493
Sola ield’s a ea (SM = 1), ASF [m2]4553 5526 3440 4215
S o age medium olume/s o ed ene gy
[m3/MWh] 25.9 31.5
S o age medium olume/equi alen elec ic
s o ed ene gy [m3/MWhe]103.7 164.9
ORC he mal inpu / o al desalina ion capaci y
[kW/(1000 m3/day)] 871 654
Ne ORC powe ou pu / o al desalina ion
capaci y [kW/(1000 m3/day)] 218 125
Sola ield’s a ea (SM = 1)/ o al desalina ion
capaci y [m2/(1000 m3/day)] 1685 2045 1273 1560
S o age medium olume (1 h)/ o al
desalina ion capaci y [m3/(1000 m3/day)] 22.6 20.6
Unlike he use o MED desalina ion, he use o he condense ’s hea o cooling
combined wi h RO desalina ion has ad an ages o e a con igu a ion in which he cooling
p oduc ion is pe o med by a comp ession sys em o a oid inc easing he condensa ion
empe a u e o he cycle. Mo eo e , his si ua ion is obse ed o a ela i ely high COP
alue o he comp ession chille . E en in ha case, he be e pe o mance o he ORC does
no lead o a smalle size o he ORC uni , which inally implies a la ge s o age olume
and a la ge equi ed sola ield a ea.
5. Conclusions and Recommenda ions
The ollowing ends ha e been de ec ed a e he upda e o he design p oposals
ecen ly ound in he speci ic li e a u e on sola desalina ion sys ems based on ORCs. In
he medium- empe a u e ange, i is common o ind p oposals in which he hea ejec ed
by he cycle is used, in gene al, o d i e he mal desalina ion p ocesses (mainly MED) o
abso p ion cooling p ocesses. In he low- empe a u e ange, he e a e nume ous p oposals
o he use o sola ponds as sola collec ion and ene gy s o age sys ems. Rega ding
he coupling o desalina ion sys ems wi h sola supe c i ical CO
2
powe cycles, he mos
common op ion is he use o he hea ejec ed by he cycle by means o MED uni s.
In addi ion, he c i ical analysis o he e iewed con igu a ions leads o he ollowing
conclusions and design ecommenda ions:
•
In emo e loca ions, sola desalina ion sys ems based on sola -pond-d i en ORC/RO
should be s udied in compa ison o sola PV/RO plan s o supply only he eshwa-
e demand.
•
Conce ning he selec ion o he mal desalina ion e sus e e se osmosis, design p o-
posals should conside he ex a cos s a ibu able o cooling low, namely, capi al
expenses o seawa e in ake and pumping sys ems, along wi h pumping elec ici y
consump ion (see Figu e 5).
P ocesses 2022,10, 153 20 o 23
•
A compa a i e analysis o sola ORC/RO combined wi h cooling echnologies has
been pe o med. The use o he hea ejec ed by he cycle o d i e an abso p ion
e ige a ion sys em is supe io since esul s show ha a bigge sola ield and s o age
sys em would be needed o supply he same cooling capaci y wi h a mechanical apo
comp ession e ige a ion sys em (see Table 5).
•
A compa a i e analysis has also been ca ied ou o compa e sola ORC/MED and
sola ORC/RO desalina ion. Auxilia y elec ici y consump ion o 0.75 kWh/m
3
is
assumed in seawa e RO desalina ion, a ibu able o he con ol sys em and eed
pumping, whe eas 2 kWh/m
3
has been conside ed in MED plan s due o he addi ional
equi emen s o a acuum sys em and cooling low pumping (see Figu e 5). Resul s
(see Table 4) show ha he mal desalina ion is no ecommended o exploi he hea
ejec ion o he powe cycle since he condensing empe a u e mus be inc eased o
couple he MED uni .
•
On he con a y, hea ejec ion o sCO
2
cycles in CSP plan s may be economically
exploi ed by inno a i e MED desalina ion plan s wi h he so-called cascade design
depending on bo h local egula ions o discha ge empe a u e and he co esponding
auxilia y pumping o cooling low. Wa e p oduc ion is limi ed o he was e hea
a ailable, so in he case o highe wa e demands, RO desalina ion is he only op ion
ecommended. Ei he seawa e eed o concen a e lows o he RO plan can be used as
he cooling low o he powe cycle, up o he maximum limi o empe a u e discha ge.
Based on he esul s o he p esen li e a u e upda e and assessmen , esea ch on he
imp o emen o sola ORC con igu a ions o poly-gene a ion, wi h eshwa e p oduc ion
h ough desalina ion, is conside ed a u u e wo k o in e es , in addi ion o sola sCO
2
coupled o no el MED plan s wi h enhanced ene gy e iciency.
Au ho Con ibu ions:
Concep ualiza ion, A.M.D.-T. and L.G.-R.; w i ing—o iginal d a p epa a-
ion, A.M.D.-T. and L.G.-R.; w i ing— e iew and edi ing, A.M.D.-T. and L.G.-R.; unding acquisi ion,
L.G.-R. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding:
This esea ch was unded by he Eu opean Regional De elopmen Fund, In e eg A lan ic
A ea, and he EERES4WATER P ojec (Second Call, P io i y 2, EAPA_1058/2018). The Uni e si y o
Se ille is also g a e ully acknowledged o suppo ing his esea ch h ough i s In e nal Resea ch
P og amme (Plan P opio de In es igación) unde con ac no. 2019/00000359.
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Acknowledgmen s:
L. Ga cía-Rod íguez wishes o hank he Eu opean Regional De elopmen Fund,
In e eg A lan ic A ea, o i s inancial assis ance wi hin he amewo k o he EERES4WATER P ojec
(Second Call, P io i y 2, EAPA_1058/2018).
Con lic s o In e es :
The au ho s decla e no con lic o in e es . The unde s had no ole in he design
o he s udy; in he collec ion, analyses, o in e p e a ion o da a; in he w i ing o he manusc ip , o
in he decision o publish he esul s.
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