Storage of thermal solar energy

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Storage of thermal solar energy

Benoît Stutz, Nolwenn Le Pierrès, Frederic Kuznik, Kévyn Johannes, Elena Palomo del Barrio, Jean-Pierre Bedecarrats, Stéphane Gibout, Philippe

Marty, Laurent Zalewski, Jérôme Soto, et al.

To cite this version:

Benoît Stutz, Nolwenn Le Pierrès, Frederic Kuznik, Kévyn Johannes, Elena Palomo del Barrio, et al.. Storage of thermal solar energy. Comptes Rendus. Physique, Académie des sciences (Paris), 2017, Demain l’énergie – Séminaire Daniel-Dautreppe, Grenoble, France, 2016, 18 (7-8), p.401-414.

�10.1016/j.crhy.2017.09.008�. �hal-01655060�

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Contents lists available atScienceDirect

Comptes Rendus Physique

www.sciencedirect.com

Demain l’énergie – Séminaire Daniel-Dautreppe, Grenoble, France, 2016

Storage of thermal solar energy

Stockage thermique de l’énergie solaire

Benoît Stutz

^a

^,∗ , Nolwenn Le Pierres

^a

, Frédéric Kuznik

^b

^,

^c

, Kevyn Johannes

^b

^,

^c

, Elena Palomo Del Barrio

^d

, Jean-Pierre Bédécarrats

^e

, Stéphane Gibout

^e

, Philippe Marty

^f

, Laurent Zalewski

^g

, Jerome Soto

^h

, Nathalie Mazet

ⁱ

, Régis Olives

ⁱ

, Jean-Jacques Bezian

^j

, Doan Pham Minh

^j

aLOCIE,UniversitéSavoieMont-Blanc,CNRSUMR5271,73000Chambéry,France

bCETHIL,UniversitédeLyon,CNRS,INSALyon,CETHIL,UMR5008,69621Villeurbanne,France cUniversitéLyon1,69622Villeurbanne,France

dUniversitédeBordeaux,I2MUMR5295,33400Talence,France

eUniversitédePau&desPaysdel’Adour,Laboratoiredethermique,énergétiqueetprocédés–IPRA,EA1932,ENSGTI,avenueJules-Ferry, BP7511,64000Pau,France

fLEGI,Laboratoiredesécoulementsgéophysiquesetindustriels,BP53,38041Grenoble,France

gLGCGE,Universitéd’Artois,EA4515,Laboratoiredegénieciviletgéo-environnement(LGCgE),62400Béthune,France

hUniversitédeNantes,NantesAtlantiqueUniversités,CNRS,LaboratoiredethermocinétiquedeNantes,UMR6607,LaChantrerie,rue Christian-Pauc,BP50609,44306Nantescedex3,France

iCNRS-PROMES,UPR8521,Tecnosud,rambladelathermodynamique,66100Perpignan,France jUniversitédeToulouse,MinesAlbi,CNRS,CentreRAPSODEE,France

a r t i c l e i n f o a b s t r a c t

Keywords:

Sensibleheatstorage Latentheatstorage Thermochemicalheatstorage Mots-clés :

Stockagedechaleursensible Stockagedechaleurlatente

Stockagedechaleurenleverl’adjectiflatente thermochimique

Solarthermalenergystorageisusedinmanyapplications,frombuildingtoconcentrating solarpowerplantsandindustry.Thetemperaturelevelsencounteredrangefromambient temperaturetomorethan1000^◦C,andoperatingtimesrangefromafewhourstoseveral months. This paper reviews different types of solar thermal energy storage (sensible heat,latentheat,andthermochemicalstorage)forlow- (40–120^◦C)andmedium-to-high- temperature(120–1000^◦C)applications.

©²⁰¹⁷Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

r é s u m é

Le stockage thermiquede l’énergie solaire touchede très nombreuses applications, qui vont du bâtiment aux centrales solairesà concentration en passantpar l’industrie. Les niveauxdetempératurerencontrés vontde latempératureambianteàplusd’un millier dedegrés,etlesduréesd’utilisationdequelquesheuresàplusieursmois.Cetarticlepasse enrevuelesdifférentesfamillesdestockaged’énergiesolairethermique(stockagesensible,

*

Correspondingauthor.

E-mailaddresses:Benoit.Stutz@univ-savoie.fr,benoit.stutz@univ-smb.fr(B. Stutz).

https://doi.org/10.1016/j.crhy.2017.09.008

1631-0705/©²⁰¹⁷Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccessârticleûnder^the^CC^BY-NC-ND^license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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latentetthermochimique),pourdesapplicationsàbasses(40–120^◦C)etmoyennes–hautes températures(120–1000^◦C).

©²⁰¹⁷Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Solarenergyisavailablethroughouttheworldandissuﬃcienttosatisfyallhumanenergydemand.However,itisdiluted andintermittent.Therefore,energystoragesystemsmustbeassociatedwithsolarenergycapturingtocoverenergyneeds.

Among all the different energy storage systems (pumped storage hydropower, compressed air energy storage, ﬂywheel energystorage,electrochemicalenergystorage,capacitors,hydrogenstorage,powertogas,etc.), thermalenergystorageis one oftheleastexpensivesystemsthatcan beapplied toabroadspectrumofapplicationssuch aselectricityproduction (usingconcentratedthermodynamicsolarplants),industrialapplications(chemicalindustry,foodindustry,etc.)andbuilding applications (districtheating,domestic hotwater,thermalcomfort,etc.). Differenttechniquesareused forthermalenergy storage. The most widely used isthe sensible heat storage method.Other techniques such as latent energystorage and thermochemical energy storagehave appeared inthe last two decades, offering great heat storagecapacityand reduced heat loss during thestorageperiod. Newmaterial involvingphase changeandchemical reactions (peritectic compounds) appeartobepromisinggiventheirgreatheatstoragecapacity.

Sensibleheatstorageconsists inaccumulatingthermalenergyin asolid ora liquidmedium whosetemperaturethen rises. Fora giventemperatureelevation, amaterial’s greaterheat capacitywillresultin alarger amountofenergybeing stored(Eq.(1)).

Q

=

T₂

T1

V

ρ

cdTdV (1)

Water hasa particularlyhighheat capacity(about 4180 kJ

·

^m⁻³

·

^K⁻¹^), ^but^is ^limited^to applicationsunder 100^◦C un- less pressure is increased. The heat capacity of most materials used for sensible heat storage varies between 900 and 3000 kJ

·

^m⁻³

·

^K⁻¹^.^The ^thermalconductivityofsuch materials isalsolimited, ranging from0.5to4 W

·

^m⁻¹

·

^K⁻¹ ^[1].^Two majorapplicationsofsensibleheatstoragedeservetobediscussedfurther:storagefordistrictheatingnetworksandstorage forsolarpowerplants.

Latentheatstorageinvolvestheheatabsorbedorreleasedwhenamaterialchangesfromonephysicalstatetoanother (whenitissubmittedtoaphasechange)andthesensibleheataccordingtoEq.(2):

Q

=

TF

T1

Vs

ρ

ScsdTdV

+

Vs

ρ

SLFdV

+

T₂

TF

Vl

ρ

lcldT (2)

Differentphase changescanbeencountered:solid

↔

solid,solid

↔

liquid,solid

↔

gas,andliquid

↔

gas.Latent heat storagemainlyinvolvessolid–liquidtransformationsofamaterialcalledaphase-changematerial(PCM).Overthelastfew years,variousreviewpapers[2–7]havepresentedtheavailablelatentthermalenergystoragetechnologiesandthecurrent research inthisfield, focusing mainly on theassessment ofthe thermalproperties ofthe various PCMs used.Moreover, the design ofa cost-effective latentheat-storage systemrequires thedevelopment of thermalperformance enhancement techniques. Forexample,thisconsistsin enhancing heattransfers using mostly high-conductivematerials, extendedheat transfer surface, intermediate heat transfer medium, or multiplePCMs. Other constraints,such asliquid–solid transition, whichsometimesexhibitsasupercoolingphenomenon,andthedevelopment ofreliablenumericalmodels,remainachal- lenge fromascientific pointof view.Thefirst one,ifitexists, cancausesystem-wideperformance downgrade andmust alsobeconsidered.Thesecondisnecessarytooptimizeandestimatethethermalenergyperformanceofsystems.

Thermochemicalthermalheat-storagesystems caninvolvevariousprocesses: asorptionprocess, whichconsistsinthe ﬁxation orthecaptureofareactive gasbyacondensedsubstance(eithera solid–theprocessiscalledadsorption–ora liquid–theprocessiscalledabsorption)orachemicalreaction,asdescribedbyEq.(3).

<

S

> +

^mG

⇔ <

SmG

> +

^m

Hr (3)

Thermochemicalthermalstoragesarepromisinggiventheirhigh-energydensitiesandthelowthermallossbetweenthe storageandrecoverysteps,becauseenergyisstoredaschemicalpotential.Intermsofthewholesystem,thermochemical storage requires the management of two materials in two different components: the sorbent (solid in a reactor or an adsorber/desorber; liquidinan absorber/desorber)andthe active gas(in an evaporator/condenser) renderingthe system morecomplex.Thechallengeistotakeadvantageofthiscomplexity:ﬁrst,itaddsdegreesoffreedomforthemanagement

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Nomenclature

Latin

C heatcapacity . . . J

·

^kg⁻¹

·

^K⁻¹ (G) reactivegas

L latentheat . . . J

·

^kg⁻¹ m stoichiometriccoeﬃcient

Q heat . . . J

<

S

>

reactivesolid

T temperature . . . ^◦C

V volume . . . m³ Greek symbols

ρ

density . . . kg

·

^m⁻³

Hr reactionenthalpy . . . J

·

^mol⁻¹ ^of^reactive^gas Subscript

l liquid

s solid

ofitsthermalpower(bycontrollingtheoperatingpressureandtemperatureofbothcomponents).Second,inadditiontothe heatstoragefunction,itallowscoldproductionorheatupgradingbyrelevantlymanagingtheendothermalandexothermal workings of all the components (reactor orabsorber/desorber andevapo/condenser) in both the storageand de-storage steps[8–10].Theabsorptionheatstoragetechnologyinvolvingliquidsisstillinitsearlystagesofdevelopment.Absorption storagetechnologyforlong-termsolarheat storagewassuggestedasearly as1981[11] –1982[12] forspaceheating –, butitsactualdevelopmentandprototypetestshaveonlyrecentlybeenundertakeninresearchlaboratories[13].Theenergy densityofthereactivesolutiontypicallyrangesfrom100to200 kWh

·

^m⁻³^.Îts^mainâdvantageîs^the^storageôf^the^sorbent andthesorbatinspecifictanks,andtheirtransporttothereactorusingpumps.

Thermochemicalprocessesbasedonsolid/gasreactionscanreachenergydensitiesfrom200to500 kWh

·

^m⁻³^of^porous reactivesolidandoperateinawiderangeoftemperatures(80–1000^◦Caccordingtothereactivepair).Suchthermochemical systemsarebeinginvestigatedforstoragepurposesinalargesetofapplicationsandtemperatures,fromspaceheating[14]

toconcentratedsolarplants[15,16].Agreatdealofresearchhasexaminedthecharacterizationandcontrolofheatandmass transferinporousreactivematerial[17–19].Pilotplantsuptoasigniﬁcantscalehavebeenexperimented[16,14].Regarding commercialproducts,someofthemhaveexistedforapproximately15yearsforthetransportationoftemperature-sensitive productsinthehealthandfooddomains.

Thispaperpresentsanoverviewoflow-,andmedium-to-high-temperatureheat-storagesystemsdevotedtosolarappli- cationsthat areunderdevelopmentto addressthechallengesofenergytransition. Consideringthemain techniquesused, medium- and high-temperaturesystems arepresentedseparately fromlow-temperaturesystems. Theperformance ofthe differentsystems aswell asthescientiﬁc challenge associatedwiththeir development are basedon theresultsobtained by ninedifferentlaboratoriesbelongingtotheFrench CNRSfederationonsolarenergy(FedEsolfederation) usingspeciﬁc prototypesandsetups.

2. Low-temperaturethermalheatstorage

2.1. Mainapplications

InEurope,26% ofthe ﬁnalenergyconsumption isrelatedtohousehold energysystems[20] and80% ofthisenergyis neededforheating purposes[21].Inthiscontext,thermalenergystorageisasolutiontorationalizetheuseofenergyby increasing thesolarfractioninbuildings[22,23].Spaceheatingandcoolingaswell asdomestic hotwaterproductionare key applications in thissector. In the generalcontext of energytransition, buildingswill have to consumelessand less energyforeconomicandenvironmentalreasonsandmustevolveintopositive-energybuildings: theywillbecome energy producers, but consumelow levels of energy. The solar energyreceived by buildings over one year covers their annual energyneeds.However,althoughthepeakofsolarirradiationoccursduringthesummerwhenthecoolingdemandishigh, theheatingdemandoccursduringthewinterwhensolarirradiationislowandsometimesnearlyinexistentforseveraldays.

Thermalstoragesystemsarethereforeneededtomatchsupplywithdemand.Themostwidelyusedtechniquesforthermal energystorageinvolvehot watertanksandthethermalinertiaofbuildings.Thesetechniquesmakeitpossibletousethe heatproduced duringsunnydaysto coverthe heatingneeds duringthenightandpeakdemands relatedto domestichot wateruse.Suchsystemsareeﬃcientduringspringandautumn,butarenotabletocovertheenergyneedsduringwinter at the individual scale. Other heat-storage systems must be included to increase the storing capacities of single-family houses.Large-scaleheatstorageisanothergreatproblem.Foragiventemperaturecondition,thereductionoftheexternal surface/volumeratiowiththeincreaseofheat-storagesystems aswell asthereductionofthestoring capacitieswiththe increaseinthenumberofusersisfavorabletothedevelopmentoflarge-scaleheat-storagesystemsassociatedwithdistrict heating.Low-temperatureheat-storagesystemsarenotverywidespreadinindustry.Somesolarcoolingsystems,involving watertankPCMsorthermochemicalcoldstoragehaveappearedfortelecommunications,food,andmedicalapplications.

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Fig. 1.Diagram of composite solar wall.

2.2. Sensibleheatstorage

Low-temperature sensibleheatstoragemainlyconcerns solarwaterheatersfordomestichot waterapplicationsatthe individualscale,anddistrictheatingatthelargescale.Solarthermalsystemsarerelativelycomplex,involvingmajordraw- backssuchascost,storagetanklocationrequirementsandtechnicalmaintenance.Thedevelopmentofpassivesolarsystems isclearlyaveryinterestingstrategy.Inthiscontext,thenewconceptofintegralcollectorstoragewithheatpipes(thermal diode) andafullyinsulatedstoragecavityisa verypromisingresearchﬁeld.Oneoftheobjectivesistoimprovearchitec- turalintegrationbyreducing thesystem’sweightandstoragethickness.Both waterstorage[24]andPCMs[25]havebeen considered.Similarenergyperformancecanbeobtainedwithsimplersystems.

Twomillionapartmentsandhousesarecurrentlyconnectedtoadistrictheating networkinFrance.Thecorresponding energyisashighas20TWhperyear.Storingheatlocally,quiteclosetoeachbuilding,wouldallowasmallertimeresponse fromthe heatmanagement controllingsystemaswell asincreasedcomfortfortheconsumer.A sensiblestoragestrategy alreadyexists,involvinghugewatertowers(1000 m³ inBrestandupto200,000 m³ inVojens,Denmark,inahugewater pit,heatedbysolarenergyduringthesummerseason).Thelatterexamplescanbeconsideredasinterseasonalheatstorage.

Geothermal heat-storage systems(GHSSs) havegood prospects forthe massivestorage oflow-temperaturesolar thermal energy[26].Dependingontheundergroundconditions(nativerock,clay,gravel)andthedepthofthewatertable,theGHSS canconsistofaclusterofboreholes(afewtensofmeterstoapproximately100minheight),oraninsulatedvolumeafew metersinheight.AGHSSmayormaynotincludeheatpumpsdependingonthestoragetemperature[27].

2.3. Latentheatstorage

PCMs inbuilding construction havebeen developedfor about10 yearsto increase the thermalinertia of wallsor to increase the compactness ofsolardomestic hot waterstorage systems.This isparticularlyadvantageous when, forspace reasons,traditionalsolutionsemployingsensibleheatcannotbeused,suchasincompositesolarwallsequippedwithheat storage (Fig. 1). Solar wallsallow buildings to be provided withfree andrenewable energy. Theyensure a minimum of comfort, maintain suﬃcient ambient temperatures, and thus avoid condensation problems, which is a source of molds, compromising thehealthoftheoccupantsandthedurabilityofthebuilding. Standardstoragewallsaremadeofconcrete blocks;thestoredenergyisinthiscaseonlysensible[28].

PCMs can beintegratedinto thewall tostore moreenergyinthe samestoragevolume: a2.5-cm-hydrated saltPCMs canstoreasmuchenergyasa15-cmconcretewall[29].However,certainconstraintsareencounteredusingsaltPCMs,such assegregationphenomena.PCMsmustbeprotectedfromexcessivelyhightemperaturestoavoidtheirthermaldegradation.

Hence, safety roller shutters must be installed to prevent overheating. Energy management can take advantage of this componentusingclosedshuttersduringthenighttolimitthermallossestotheoutsideorstopheatcontributionsduringthe summerperiods[30].Analternativesolutionproposedistheuseofacompositematerialcomposedofamicroencapsulated PCMintegratedintoabuildingmaterial(cementmortar)ratherthanusingapolyoleﬁncontainerﬁlledwithaPCM.

Thephasechangephenomenaarecomplexandmakemodelingdynamicthermaltransfersdiﬃcult.Tocarryoutnumer- ical simulationsthat arefaithful tothethermalbehavior ofthecompositematerial (mortarcement

+

microencapsulated PCM),itisessentialtocharacterizethethermophysicalpropertiesofthecompositematerialandtodevelopanadaptednu- mericalmodel.Thenumericalmodelimplementedisbasedonthederivativeoftheintrinsicenthalpycurveofthecomposite material.Thethermophysicalpropertiesofthecompositematerialareestimatedfromexperimentaldataobtainedfroman originaldevice[31,32]coupledwithidentiﬁcationmethods[33],[34].Modelingshowsthatinthethermaloperatingranges ofthesolarwall,mortarcontaining20%PCMstores41%moreenergythanaconventionalmortar[32–34].

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Fig. 2.Functioning principle of the interseasonal absorption heat storage process, with water as the sorbate.

2.4. Absorptionheatstorage

Absorption technologyis an interesting possibilityto store space heating in solarbuildings, because it allows storing excessheat available during thesummeruntil thefollowingwinter.Its operatingconditions arecompatiblewiththeuse ofconventionalsolarheatcollectors.Theabsorptionheat-storagesystemincludesthesamecomponentsasthewell-known absorptionchiller,towhichtwoorthreestoragetanksmustbeadded(Fig. 2)[35].

Duringthechargingphase(desorption),thepoorsolution(withalow massfractionofsorbent)inthesolutionstorage ispumped tothe generator/desorber,where itis heatedbya solarflat platecollector.The absorbateinthe solutioncan be endothermicallydesorbedandtransferred tothe condenser,whereits latentheat isreleasedtothe surroundings.The condensed absorbateflows toits tank, whereit can be stored. The rich solution(with a highmass fractionof sorbent), whichisproducedinthegenerator,flowsbacktothesolutiontank.Duringtheinertstorageperiod,theliquidsolutionand absorbatearekeptintheir respectivetanks.Thedifferentconnectionvalvesareclosed,sonomasstransferoccursduring thisperiod.Thechemicalpotentialofthesystemisthuspreservedduringthisphase.Duringthedischargingphase(absorption), theabsorbateis circulatedfromtheabsorbate storagetothe evaporator,whereit isvaporized bylow-temperature heatfromthesurroundings.Theabsorbateisthenabsorbedbytherichsolution,whichiscirculatedintotheabsorber.Heat isreleasedandthisproductionmeetstheneedsforbuildingheating.Thepoorsolutionflowsbacktothesolutiontank.

The currentdevelopment ofthe absorptiontechnologyforlong-termlow-temperatureheat storagepurposesis stillat theresearchstage.Themainadvantagesofthistechnologyarethehighheatstoragecapacityandthepossibilityofstoring heat overlong periods [36]. Atthe research anddevelopment level,theoretical, modelingand experimentalresearch has been conducted by different teams. Three different prototypes using LiBr/H2O andCaCl2/H2O asthe absorption couples have been designed andtested at the LOCIE. The latter uses the LiBr/H₂Oabsorption coupleand ﬂat plate exchangers.

The desorption heat is provided by solar collectors (at about 90^◦C). Water vapor absorption during the winter allows low-temperature household heating at up to 35^◦C. The theoretical energydensity ofthe solution is330 kWh

·

^m⁻³^.^The usefulmaterial energydensity is120 kWh

·

^m⁻³ ⁱⁿinterseasonal operating mode. Thereleased speciﬁc energydensityis 66 kWh

·

^m⁻³ ôf^the^small^small^size ^prototype^(notôptimized)ⁱⁿinterseasonal operatingmode.The resultsobtainedare encouraging, butthe technologymust still solve the significant scientific, technical,andfinancial challenges, atboth the micro- andmacro-scalelevels,beforeitcanbemarketed.

Amajorissueinthedevelopmentofthistechnologyisthedefinitionandchoiceoftheidealabsorptioncouple.Research continuesin an attemptto findunhazardous andlow-cost sorbentmaterials withlong-termstability under thesystem’s operating conditions,while presentinga sufficiently highheat storagecapacity tomake the systemprofitable. Moreover, heatandmassexchangerdesignproblemspersist,forexamplewettinganddistributionissues,whichmustbeovercometo reach acceptablesystemefficiency. Inmostcases,componentsmust bedesignedto operateinvacuumconditions,which changes theusual dimensioningrules usedforengineeringpurposes.Theserules remainto bedevelopedatthe research levelandthenintegratedbytheengineeringcommunity.Variousprototypeshavebeendevelopedtostudytheperformance ofabsorption systemsatthemacro-scale level.Mostofthemachievedtemperaturessuitable forspaceheating andsome evenreachedtemperatureshighenoughfordomestichotwaterproduction.

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Fig. 3.Experimental adsorption setup using the zeolite 13X as storage media.

2.5. Adsorptionandsolid/gasreactionheatstorages

Solid/gas physisorption andchemisorption provide valuable perspectivesfor long-termlow-temperature buildingheat storage. One of their advantages compared to absorption processes is their ability to be used in an open heat-storage system. Thethermalenergystoragesystemisthenintegratedintothebuilding’sventilationsystem.Duringdesorption,the air comingfromthebuildingorfromtheoutside isheatedusingaheat sourcesuch assolaraircollectorpanels[37] or electrical heaters, provided that thedehydrationtemperatureis reached. Duringsorption,moist airis extractedfromthe buildingandpassesthroughthereactor.Thehotairﬂowleavingthereactorisusedtoheatcoolair.

A real-scaleopenheat-storagesystembasedonzeolite13Xmadeoftwo separatereactors thatcan beconnectedina serialorparallelconfigurationtotesttheadvantagesandlimitationsofeachconfiguration,hasbeendesignedandtestedat CETHIL.Eachreactorisa72-cm-diametercylinder,witha20-cmbedthickness.Moistairflowsverticallyinsidethereactor.

A sketch of the prototype and a picture of the experimental setup are presented in Fig. 3. The reactors are ﬁlled with Faujasitezeolite86Na

−

^X^(Alfa^Aesar,^13X,^beads^1.6–2.5^mm).

Theplanofexperimentshasbeendevelopedtoaddress,withaminimumoftests,theinﬂuenceofthedesorptiontem- perature(120^◦Cvs.180^◦C),theairﬂowrate(180m³

·

^h⁻¹ ^vs.^{90 m}³

·

^h⁻¹ ^vs.^{60 m}³

·

^h⁻¹^), ^relative^humidityⁱⁿdischarging mode(50%vs.70%),bedthickness(20cmvs.10 cm)andtheserial/parallelconﬁgurations.Themainresultsarethefollow- ing:the releasedheating poweris27.5 W

·

^kg⁻¹ ^or^{4.5 kW}

·

^m⁻³ ^for^the ^prototype.^The^released^specific ênergy^densityîs 0.2 kWh

·

^kg⁻³ ^or³³^kWh

·

^m⁻³ ^for^the^prototype.Îtîs^worth^mentioning^that^the^materialênergy^densityîs^{180 kWh}

·

^m⁻³^. MoreinformationonhowthetemperatureevolvesduringthetestscanbefoundinJohannesetal.[38].

AsecondtypeofseasonalthermochemicalstorageprocesshasbeenbuiltandexperimentedatPROMES[14]:itisbased on a solid–gas reaction involving an environmentally friendly water-based reactive pair, i.e. SrBr₂/H₂O. For the sake of compactnessandcost,thissystemhasbeendesignedasanopensystem,i.e.itoperateswithmoistair,asdescribedforthe previouszeolitestoragesystem.Thehydrationreactionenthalpyallowsheatingtheairﬂowupto35^◦C.Thethermalpower during the de-storage step is highly dependent on the gapbetween the operating and the thermodynamic equilibrium temperaturesofthereactivepair.Thisinterseasonalapplicationinvolveslargeamountsofstoredenergy,butitonlyrequires verylowspeciﬁcpower(W/kg)becausethereactiontimeisbasicallyverylong(uptoawholeseason).Theprototypewas designedforapproximatelyone-tenthoftheheatingdemandofatypicalhouse,andareactiontimeof12days,with400 kg ofreactivesalt.Theenergydensityofthereactivesaltbedis390 kWh

·

^m⁻³^,^and^{190 kWh}

·

^m⁻³ ^when^related^to^the^whole volume ofthereactor (non-optimized).Thissystemcouldbe considered asa partofa modularfullinterseasonal storage or astoragesystemdedicatedto peakdemand periods.Thisexperimentdemonstratedthe feasibilityandperformance of suchstorageandoutlinedtheneedfordeepoptimizationofthereactor,includingheatandmassdiffusernetworks[39],to enhancetheoverallenergydensity.

2.6. Comparison

Tocompare thedifferentheatstoragetechnologies, commonkeyperformance indicators(efficiency, compactness,eco- nomiccriteria,life-cycleassessment,etc.)aswellasclassesofoperatingconditionsmustbedefined.Theproblemiscomplex andthereiscurrentlynoconsensusonthissubject.Nevertheless,arough comparisonbasedon theresultsobtainedwith fivedifferentheatstorageprototypes(sensible(SEN),PCMs,absorption(ABS),physicaladsorption(PAD)andchemicalreac- tion(CR) heat-storagesystems)isproposed.Thesystemsareusedforinterseasonalheatstorageapplicationsforbuildings.

The following hypotheses are considered: the energy needs of the low-consumption single-family home covered by the heat-storagesystemare2000kWh.ThethermalloadingofthesystemsoccursfromMaytomid-September.Then,solaren- ergyisusedfordomestichotwaterproduction.Theheat-storagesystemprovidesheatfrommid-Octobertomid-Marchto thefamilyhomefollowingasinuslaw.Thetemperatureneededbytheheatingsystemis30^◦C.ThesensibleandPCMtanks are insulatedwitha 30-cm-thickpolystyrenewall.The tanksusedinthe sorptionheat-storagesystemsare not insulated (the sensiblepartoftheenergyislost).Fig. 4comparestheenergydensityoftheactivematerial(sensiblematerial,PCM, absorptionsolution,solidadsorbents,solidreactant)andtheusefulenergydensityoftheheat-storagesystems(ratioofthe energyusedtoheatthesingle-familyhomeoverthevolumeofthesystem).

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Fig. 4.Energydensityofthesystemscomparedwithenergydensityofthereactants. :SEN(T=⁶⁰^◦^C); ^:^PCM⁽T=²⁰^◦^C); ^:^PAD; ^:^CR; ^: ABS.

ThevolumesofthesensiblehotwaterandthePCMtankarecomparable(50 m³).Heatlossesgeneratedbythesensible heat-storagesystemare veryhighandcorrespondtoabout43%ofthetotalenergystored.Theheatlossgeneratedbythe PCM tank aretwice aslow asthat obtainedwithsensibleheatandcorresponds to aboutone-third oftheenergy stored duringthesummer.Thefirstresultsobtainedwiththephysicaladsorptionsystemleadstoasystemvolumeofabout60 m³, whichistoolargeforasingle-familyhome.Systemvolumesforabsorptionandchemicalreactionareequalto16and10 m³, respectively,whichbeginstobefeasibleforasingle-familyhome.Asthefigureshows, theenergydensityincludedinthe active material is about twice as high asthe useful energy density of the heat-storage systems. This difference can be reduced by optimizing thedesign of the system andlimiting heat loss,which also affects the sorption systems.Finally, thenecessaryone-thirdgain incompactnessshouldbeobtainedbyworkingontheseaspects.Whenprototypeshavebeen developedatanearly commercialscale,additionaleconomicconsiderations relatedtothesystem’slifetimeandoperation aswellasmaintenancecostsshouldbeincluded,withtheaimofdefiningandevaluatingabusinesscase.

3. Medium- andhigh-temperaturethermalheatstorage

3.1. Mainapplication

Athightemperatures,theapplicationsforthermalenergystoragefromsolarenergymainlyinvolveelectricitygeneration bythermodynamiccyclesconcentratingsolarpower(CSP).Theobjectivesare:

•

^for^short-term^storage⁽³⁰^min),^to^maintainelectricityproductionforpartlycloudyperiods,sothatthestorageplaysthe roleofregulatingtheincomingenergytotheturbinebyprovidingathermalbufferandthenpreventingnonstationary solarproductionduetoweatherconditions;

•

^formedium-termstorage(afewhours),toextendtheproductiontothenighttimewhenthehome electricitydemand peaks,ortomaximizetheﬁnancialincometoproduceonlyduringpeakhours.

Dependingonthesolarcollectortechnology,differentoperatingtemperaturescanbeobtained:from200to500^◦Cwith solarFresnelandforparaboliccollectorsandabove600^◦Cwithheliostatﬁeldcollectortechnology[40].Energyconversionis thenoperatedusingasteamturbineortheOrganicRankineCycle(ORC)system.Industrialapplicationsforhigh-temperature thermalstorageaccountforlessthan1%oftheapplicationfromsolarenergy[41].Sensible,latent,orchemicalsystemsare thethreetypesofheatstorageindecreasingorderoftechnologicalreadinessandincreasingenergydensity.Eightypercent ofthermalstorageinCSPisdesignedwithtwotanksabletocontaincoldorhotmoltensalt.Researchhasbeenconducted ononethermoclinetank aswell astheuseofPCMstostabilizetheoutﬂowtemperature(in packedbedthermalstorage) [42].

3.2. Sensibleheatstorage

TwotypesofThermalEnergyStorage(TES)shouldbeconsidered:activeandpassivesystemsPyandOlives[43].Inthe activesystem,thestoragesystemisoftencomposedoftwotanks,oneforthecoldestfractionsofthestoragemediaandthe otherforthewarmestfraction.Thestoragemediaisafluidwithintherangeoftemperatureconsidered,andisalsousedas aheattransferfluid(HTF).Itcanflowthroughheatexchangerstotransferheatduringthecharginganddischargingsteps.

Fig. 5presentstheprincipleofthetwo-tankstoragesystem.

The essential currentindustrial technology is based on the two-tank molten-salt systems used inthe chemistry and metallurgyindustriesandinconcentratedsolarpowerplants(CSPs)suchasThemisinthe1980sandAndasolandGemasolar

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Fig. 5.Sensible heat-storage system principle: a) two-tank storage system; b) single-tank thermocline-based TES integrated into a CSP plant.

today[44,45].Forexample,intheAndasolCSP,theTESabsorbsapartoftheheatproducedduringthedaytorestoreitat night orduring cloudyperiods. TheTESis composedoftwo circulartanks36 mindiameterand14m inheight, which contain 28,000 tonsof moltennitrate saltsbetween293and 393^◦C.The quantity ofstorage mediacorresponds to 1010 MWh, allowing the equivalent electric power of 50 MW over 7.5 h dueto energy conversion. In this case, the energy densityisabout35 kWh

·

^m⁻³^for^the^two-tank^volume^or^{36 Wh}

·

^kg⁻¹ ^of^storage^material.

The passive system corresponds to a regenerator. In this case, a HTF circulates through a storage media. Practically speaking,thestoragemediacanbesolidssuchasrocks,sand,concrete,ceramics,orrecycledmaterialsfromwastes[1,46].

When theHTFthermalcapacityissigniﬁcant andconsequentlycontributestothe wholestoragecapacity,thesystemisa dual-medium system. This technologyisbased onthe thermoclineconcept. The storagematerial remains inplacein the storagesystemandaHTFisusedtochargeanddischarge it.Forexample,atthePROMES-CNRSlaboratory,a thermocline that contains325kg ofquartzitecanstore 8.3kWhbetween160and210^◦C.The TESisa singlecirculartank measuring 0.4 min diameterand1.8m high. The energydensityis about36 kWh

·

^m⁻³ ^of single-tankvolume or25.5 Wh

·

^kg⁻¹ ^of storagematerial.Nevertheless,duetoits smallsize, thisTEShasastoragecapacityalsoensuredby HTFanda steelwall.

Hoffmann et al. [47] showed that the thermocline tank presents an optimum mass flow corresponding to a maximum dischargeefficiencynear75%.ThisexperimentalsetupcantestdifferenttypesofmaterialfillerandHTFcouple.

Inthesetechnologies,differentcriteriacanbeconsidered:energyperformance,availabilityofstoragematerials,compati- bilitybetweenstoragematerialsandHTF,embodiedenergy,energypay-backtime,capitalcost,lifecyclecost,levelizedcost ofthestoredenergyandenvironmentalimpacts suchasgreenhousegasemissions[7].Low-costandeco-friendlysystems are proposedtooffersustainable optionsintheindustry.Tosatisfy theobjectivesofIEAaboutthedevelopmentofCSP,a hugequantityofstoragematerialsisnecessary.Inthisway,theuseofrecycledwastesisaninterestingalternativetomolten salts,whichmaybeunavailableatacceptablecost[48].ThecompatibilitybetweenstoragematerialandHTFisakeypoint andcanbeanalyzedonacase-by-casebasis[49,50].

3.3. Latentheatstorage

Tothebest ofourknowledge,thereisnocommercialthermalenergystoragemoduleusingPCMstorageathightem- peratures,andonlyresearchanddevelopmentmodulesexist,stemmingforthemostpartfromaneconomicbottleneck.To extendtheuseoflatentheat-storagesystems,manyscientiﬁcandtechnologicalobstaclesneedtobeovercome.Foragiven application, the choiceofa PCMresultsfroma compromisebetweenits energystorageproperties,heatexchange performance,compatibilitywiththecontainer(i.e.thestructure)andeconomicconstraints(mainlycost).Fornominaloperationof latentheat-storagesystems,manycriteriamustbe consideredandsometechnological barrierspersist:PCMatacostsuit- ablefortheapplication,agingofthematerial(performancedegradation),andreturnofpower.Thesetechnologicalbarriers correspondtoscientiﬁcobstacles:thermophysicalcharacterizationandimprovementofPCMs.Themethodologytoselecta PCMoftencombinesexperimentationandmodeling.ThepropertiesofagoodPCMcomprise:adaptedmeltingtemperature (high latentheat),highdensity,lowcost,low dangerandtoxicity,stability overtime,reliabilityofcontainmentmaterials, andlowsupercooling(delayattheliquid–solidtransition).

Many shortcomings remain in the development of reliable and practical storage systems using PCM, such as incon- gruentmelting,supercooling,lowthermalconductivity,andinsuﬃcientlong-termstability.Theseshortcomingslimittheir applications. However, research and development of newmaterials is overcoming manyof theshortcomings. Toprevent incongruent melting (phaseseparation where the liquidphase does not haveexactly the same chemical composition as the solid phase), it ispossible to add suspension media orthickening agents. Nevertheless,thickening agents displacea part of thePCM in the system. As a result, the volumetric heat storagecapacity is lower than the capacityofthe pure

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substance.AnotherpossibilityconsistsinmixingthePCMwhenitisintheliquidstate.Whilethismethoddoesnotreduce thevolumetricstoragecapacity,itdoesrequireameansofmixingthePCM,withaddedenergyconsumption.

Allsubstancesdonotcrystallizeatthemeltingtemperature(liquid–solidequilibrium),butatalowertemperature.This delay,calledsupercooling,canbe reducedbyaddingnucleationcatalysts(whose crystalstructureissimilar tothatofthe PCM)orusinganucleatingdevicethatismaintainedatacoolertemperaturethanthemaximumsupercoolingtemperature orusingasurfaceroughnesstocreatenucleationsites.

PCMsare knowntohavelow thermalconductivity,usually between0.2and0.7 W/m

·

^K.^This^low^thermalconductivity reducesthetransfer ofenergyinandout ofthePCM.Duringcrystallization,the phasechangestarts attheheat transfer surface,causingthePCMsolid–liquidinterfacetomoveawayfromtheheattransfersurface.ThesolidlayerofPCMbetween theexchange surface andthe interfaceactsasan insulatorreducing theheat transfer,thus increasingthermalresistance.

Therefore,evenifthesameamountofenergyisexchanged,thedynamic(i.e.theheatflux)maybesmall.Generally,dueto theconvectioneffect,lowconductivityislessdisadvantageousduringthemeltingprocess.Toresolvetheproblemofalow heattransferrate,severalheattransferenhancementtechniquesareavailable.Heattransfersurfaceareascanbeincreased, using,forexample,metalfinswithhighthermalconductivity.ThethermalcharacteristicsofthePCMcanbeimprovedusing ahighconductivitymatrixorgraphiteparticles.Scrapedheatexchangersordirectcontactlatentheat-storagesystemscan alsobeused.Allthesolutionscaneffectivelyimproveheattransfer,buttheyincreasethefinalcost.

AppropriatePCMs mustundergomanymeltingandfreezingcycleswithoutdegrading theirproperties.The stabilityof PCMsandsometimesthecorrosionbetweenPCMsandcontainersmustbeexperimentallytested.

Finally,althoughseveralproblemspersistintheuseofPCMs,solutionsexistandPCMstoragetechnologyisonethemost promisingtechnologies.Nevertheless,continuous researchanddevelopment effortsareneededtoreduce costsfurtherand scaling-upthistechnology.

3.4. Thermochemicalheatstorage

High-temperaturethermochemicalstorageforCSPisaddressedbyseveraltypesofreactivepairsandreactors.Themain onesare:(1)redoxcyclesusingairflow[51,52]and(2)oxide/hydroxidecycles[53,54]infixed-bedreactors[15]orfluidized reactors[55].

Forsuch systems to be fullydeveloped,the challenge is todefine reactors andprocesses that are simple andsecure, with highenergy density, acceptable cost, anda design upscalable ata pilot plant level.The CaO/Ca(OH)2 cycle imple- mentedinfixed-bedreactorsandaclosedprocess(operatingwithpurevaporwithinthe300–600^◦Crange)isapromising configuration.

Thistypeofsystemhasbeeninvestigatedfromtwoperspectives:

•

^heatând^mass^transferⁱⁿ^the^porous^bed:^storage^systemsâim^to^reach^highênergy^density^(kWh

·

^m⁻³^),^but^they^also mustprovidethethermalpowerrequiredbythepowercycle.Thesetwocriteriaareantagonistic:enhancingthedensity ofthestoragebedrequiresprimarilyreducingtheporosityofthereactivebed,whichcanresultinalimitationofvapor transferandconsequentlyadecreaseofthethermalpowerreleased[56].Ontheotherhand,aninertconductivebinder is usually added to improvebed heat transfer(expanded graphitecalled ENG[57]). Since both transfers impact the thermal power, they must be carefullyinvestigated to determine the best compromise betweenenergy density and thermalpowerforeachapplication;

•

performance of CaO/Ca(OH)₂ fixed-bedreactors: they havebeen measured on approximately1-kg reactive beds and forvarious configurationparameters(energy density,conductive binder) andoperating conditions.Fig. 6 displaysthe releasedpowermeasuredasafunctionofthestoredenergyofthesebeds(bothrelatedtothevolumeofthereactive bed,i.e.kW/m³ vskWh/m³)andhighlightsthesignificantincidenceoftheoperatingtemperature.

Therefore,withinthe rangeinvestigated,thespeciﬁc powerrangesfrom10to70 kW/m³ ofreactivebed;thereaction timeforthisde-storagesteprangesfrom2to12h.Suchstoragesystemsarethereforesuitablefortheoperationofasolar powerplantforvariousstorage/de-storagescenarios.

Future investigations should focus on developing knowledge and tools to optimize the reactive solids in fixed beds, experimentsatasignificantpilotscaleandmoredetailedanalysisofefficientintegrationsofsuchthermochemicalprocesses inaCSPplant.

3.5. Peritecticheatstorage

Theterm“peritectic”refers toreactions inwhichaliquidphase(L)reactswithatleastone solidphase (

α

) toforma newsolid phase (

β

).The reaction isreversibleandtakes placeatconstant temperature. Thephase (

β

) formed isa solid solutionof one ofthecomponents,an allotropic phase ofone ofthe componentsora newstoichiometric compound.In thesematerials,thermalenergyisstoredby twoconsecutiveprocesses:amelting/solidiﬁcation processandaliquid–solid chemicalreaction.Underthermodynamicequilibriumconditions,thepro-peritecticphase

α

(s)startstonucleateoncooling (dischargeprocess)oncetheliquidphasecrossestheliquidustemperature;thenitgrowsuntiltheperitectictemperatureis

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Fig. 6.Speciﬁcpowersforthede-storagestepforvariousreactivebeds.Energydensitiesandamountofexpandedgraphitebinder(ENG)inthebeds: : bed1(131 kWh/m³,37 kgENG/m³); :bed2(196 kWh/m³,24.3 kgENG/m³); :bed3(228 kWh/m³,0 kgENG/m³); :bed4(132 kWh/m³,37kg ENG/m³); bed5(162 kWh/m³,46kgENG/m³).Temperature:thegapbetweentheoperatingandequilibriumtemperatures(Teq)islabeledneareach point.Blacklines:pointsoftheiso-reactiontimeforthede-storagestep.

reached.Atthispoint,theliquidphasereactswith

α

(s)toform

β

(s).Onheating(chargingprocess),thesolid

β

decomposes attheperitectictemperatureintoaliquidphaseandthesolid

α

;thenthesolid

α

melts.

Theuseofperitecticcompounds(PCs)forTESathightemperatures(300–600^◦C)hasrecentlybeenproposedbyAchchaq etal.[58].PCsinsalt-basedandmetallicbinarysystemshavebeenextensivelysearchedusingFactSage6.4^®software[59].

A signiﬁcant number of PCs withhigh theoretical potential for compact TEShas been found. Undernormal equilibrium cooling,thevolumetricenergydensityprovidedbytheperitecticreactionrangesfrom200 kWh/m³to400 kWh/m³,which is comparabletotheeffectiveenergydensityprovidedbythe bestgas–solidreactionsinvestigatedso far.Consideringthe latentheatdeliveredduringthepro-peritecticformationaswell,volumetricenergydensitiesfrom400to650 kWh/m³ can beachieved.

Li2K(OH)3 was chosenforpreliminaryexperimentalanalysis.ThisstoichiometricPCappearsintheLiOH/KOHsystemat 58%wtofKOH/(KOH

+

^LiOH).Îtîs^formedât^314.8^◦^C^by^the^liquid

+

^LiOH

→

^Li2K(OH)3 reaction,whosetheoreticalenthalpy is 534J/g (threetimeshigherthan thelatentheat ofNaNO₃).It hasbeensynthesized bythe melting/solidificationroute and submitted to thermal analysis. The main experimental results obtained are summarized in Fig. 7. As shown in the figureinsert,theenthalpychangemeasuredduringsuccessivecyclesofheatingandcoolingisveryhigh(480J/g),although it is lessthan the theoretical value (534 J/g) dueto the incompleteannihilation ofthe pro-peritectic phase.Indeed, the SEMimagesofthefinalmaterial(Fig. 7,top)showacore-segregatedstructureinwhichtheremainingpro-peritecticphase (LiOH)isencapsulatedbyathinlayerofLi₂K(OH)₃ andembedded inabackgroundformedbydirectprecipitationofLiOH and Li2K(OH)3 fromthemelt. It is well knownthat a PC formsby means oftwo mechanisms: a peritectic reactionand peritectic thickening.Theperitecticreaction,whereall threephases(L,

α

,

β

) areincontactwitheach other,proceedsby short-range diffusionthroughthe liquidandisusually veryfast.Incontrast,thethickening oftheperitecticlayerformed around the pro-peritectic phase takes place by long-range atom diffusion through the peritectic solid and is a sluggish process. Unlessthe coolingrateisunrealistically slow, anout-of-equilibriumstructure willbe obtainedattheendofthe discharge process.Thevaluesofenthalpymeasuredoncoolingaredisplayedintheenthalpy-temperaturegraph(points)in Fig. 7andcomparedtothosecalculatedassumingbothnormalequilibriumcooling(blueline)andmetastableScheil–Gulliver cooling (green line).An intermediatebehavior betweenthesetwocooling situationsis observed,againindicatingthat an out-of-equilibriummicrostructurehasbeenobtained.

The efficiencyofPCsshould beincreasedbyboosting thekineticsoftheperitectic formation.Onesolutionconsistsin providingappropriatesitesfortheheterogeneousnucleationofthepro-peritecticphasesothatthelatterhasafinerdistri- bution.Thisrequiresanincreasedspecificsurfaceareabetweenthereactants,namelytheliquidphaseandthepro-peritectic phase, andthiscan be achievedeither byinfiltratingthe storagematerialwithin appropriate porous/fibrousstructuresor bydispersingappropriateparticleswithinthestoragematerial.Differenttypesofcarbonstructuresprovidingawidevariety of structural andtextural characteristics can be produced and investigated asthe PC’s host medium (within the Pc2TES project).Twotypesofcarbon–saltcompositeshavealreadybeenpreparedtotesttheconcept.Thefirstoneisobtainedby dispersionofsmallparticles(micrometricsize,20%wt)ofvitreouscarbonintotheLiOH/KOHmelt.Thesecondoneconsists in a graphitic carbon foam (82% porosity,0.24 g/cc apparent density) filled withLi₂K(OH)₃. Both composites have been submitted to100successivecyclesofheating/coolingintheDSC.Itwasobservedthat theonsettemperatureofthecom- positesremainsunchangedwhiletheenthalpychangeissignificantlyimprovedandbecomesevenhigherthatthepredicted theoreticalvalue.

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Fig. 7.SummaryoftheexperimentalresultsobtainedforLi2K(OH)3.Bottom:enthalpy-temperaturecurve(symbols:experimentalpoints;continuouslines:

theoreticalcalculations);insert:enthalpychangemeasuredduringsuccessivecyclesofheatingandcooling.Top:SEMimageoftheﬁnalmaterial.

Table 1

Mainfeaturesofstoragematerialandsystemsinvestigatedinthesetoflaboratories.

Material Material

availability

–Developmentofsystems –Keypointsoftheprocess

Estimated cost€^/kWh Project & references Sensible heat storage

Molten salt (nitrate) 0.8MT/an –Commercialsystem 20–50 Gemasolar

–15hthermalstorageforasolar plantoperating22houtof24

(storage system) 2-tanks tower solar plant

Molten salt (nitrate) 0.8MT/an –Commercialsystem 20–50 Andasol 2-tanks parabolic trough

–7hthermalstorage (storage system)

Alumina–oil 90MT/an –Demonstration(6.3tofsolid) 20€/kWh (material) Micro sol plant –Commonsolidmaterialwithuseful

thermalpropertiesathighT

Quartzite-vegetable oil Abundant –Labexperiment(325kgsolid) 1€/kWh (material) Dual media thermocline –Lowenvironmentalimpact

Coﬁlin–air Abundant

(wastes recycling)

–Labexperiment(7kg)

–Usefulthermalproperties,recycled ceramicsfromasbestos-containing waste

Low cost (waste) Air–cobalt, Pease case

Latent heat storage

Conﬁdential Good –Labexperiment NA STEEP

–Simplesystem(commontubesand shellheatexchanger),TRL5

ANR-13-SEED-0007 Thermochemical heat storage

CaO/Ca(OH)2 Veryabundant –Labexperiment(∼¹^kg) 0.04 to 0.4 (material) Instore –Managementofthermalpowerand

energydensity

ANR-12-SEED-0008 Peritectic heat storage

Li2K(OH)3 NA –Experimentalmaterial

characterization

NA Pc2TES

–Compactness,technology,simplicity ANR-16-CE06-0012

Thetheoretical analysisaswellasthepreliminary experimentalresultsachievedindicates thatusing PCsmightbe an appealingsolution forTES athightemperatures.Compared tothe state ofthe art, themain expectedadvantages of PCs are:(i)compactness,withvolumetricenergydensitycomparabletothatofthebestgas–solidreactionsinvestigatedsofar;

(ii) simple storage technology, withcharge/discharge processes atatmospheric pressure andreactants that separate and recombinebythemselves,and(iii)acceptablecostduetoenhancedcompactnessandtechnologysimplicity.

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Fig. 8.EnergydensityofTESsystemsexperimentedinFedelelaboratories.Upperlinesrefertothematerialvolume.Lowerlinesrefertothetankorsystem volumes andareindicativevaluesmeasuredon non-optimizedlaboratorydevices orprototypes.Onlyblackandgraylinescorrespondtowell-known commercialCSPplants.

3.6. Comparison

Research on high-temperature thermal storage carried out in FEDESOL laboratories is summarized in Table 1. They exploreallthethermalstorageroutes,fromalreadycommercialsystems(sensibleheat)toveryinnovativesystems(peritectics),coveringawiderangeoftechnologyreadinesslevels(TRLfrom8to3).Thecorrespondingoperatingtemperaturesand energydensitiesaregatheredinFig. 8.Nevertheless,thesestudiesmainlyfocusedonlab-scaleexperiments.Moredetailed investigations ontheimplementationofstoragematerials,optimizationoftransfersandhigher-scaleexperimentsmustbe conductedbeforereliableconclusions andperformance canbeobtained. Manycriteriacanbe consideredwhendeﬁninga storage system:energydensity,thermalpower, butalsocost,availability, technical-complexity,etc. (Table 1). Theyshould leadtodifferentcompromisesdependingontheapplications.

4. Conclusions

Among thedifferent energystoragesystems, thermalenergy storageis one ofthe cheapest onesthat can be applied to abroadspectrumofapplications.Themostwidelyusedisthesensibleheatstoragemethod.Other techniquessuchas latentenergystorageandthermochemicalenergystoragehaveappearedinthelasttwodecadesandoffergreatheatstorage capacityandreducedheatlossduringthestorageperiod.

Thispaperpresentedanoverviewoflow-,andmedium-temperatureheat-storagesystemsdevotedtosolarapplications thatarecurrentlyunderdevelopmenttoaddressthechallengesofenergytransition.Thepaperreviewstheresultsobtained byninedifferentlaboratoriesbelongingtotheFrenchCNRSfederationonsolarenergy:theFedelefederation.Theselabora- toriesexploreallthethermalstorageroutes,fromcommercialsystems(sensibleheat)toveryinnovativeones(peritectics), coveringawiderangeoftechnologyreadiness(TRLfrom8to3).

Acknowledgements

We thank the French “Agence nationale de la recherche” for its ﬁnancial support, through the following projects:

ANR-12-SEED-0008 In-STORES“IntegrationofathermochemicalstorageprocessinaRankinecycledrivenbyconcentrated solar energy”, ANR-11-SEED-0011-01 PROSSIS2 “Procédépour le stockage solaireintersaisonnier – 2” andANR-10-STKE- 0009STAID“Stockageinteraisonnierdel’énergiethermiquedanslesbâtiments”,ANR-10-STKE-0003MICMCP“Utilisationde méthodesd’identiﬁcationpourlacaractérisationdematériauxàchangementdephase(MCP)”,PREBATINPASOL-09“Integra- tionofcompositesolarwallsintheenvelopeofahighenvironmentalperformancebuilding”,ANR-16-CE06-0012“Peritectic compoundsforcompactthermalenergystorageathightemperature”.

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