热能储存技术

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CHAPTER12

THERMALENERGYSTORAGETECHNOLOGIES

CliffordK.HoandAndreaAmbrosini,SandiaNationalLaboratories

Abstract

Thermalstoragetechnologieshavethepotentialtoprovidelargecapacity,long-durationstoragetoenablehighpenetrationsofintermittentrenewableenergy,flexibleenergygenerationforconventionalbaseloadsources,andseasonalenergyneeds.Thermalstorageoptionsincludesensible,latent,andthermochemicaltechnologies.Sensiblethermalstorageincludesstoringheatinliquidssuchasmoltensaltsandinsolidssuchasconcreteblocks,rocks,orsand-likeparticles.Latentheatstorageinvolvesstoringheatinaphase-changematerialthatutilizesthelargelatentheatofphasechangeduringmeltingofasolidtoaliquid.Thermochemicalstorageconvertsheatintochemicalbonds,whichisreversibleandbeneficialforlong-termstorageapplications.Currentresearchineachofthethermalstoragetechnologiesisdescribed,alongwithremainingchallengesandfutureopportunities.

KeyTerms

Thermalstorage,sensiblestorage,latentstorage,thermochemicalstorage,long-durationstorage

Introduction

Increasingpenetrationsofintermittentrenewableenergysources(e.g.,photovoltaics[PV]andwindenergy)haveincreasedtheneedforenergystoragetechnologiestoaccommodatedailyperiodsofovergenerationandpeakloads.Thesediurnalenergy-storagerequirementsarecategorizedinthischapterasshort-durationandspanperiodsfromsecondstohourswithcapacitiesrangingfromkilowattstogigawatts.Previousstudieshavesuggestedthatthedecreasingcostsofbatteriesandassociatedtechnologiesmayenablebatterysystemstomeettheshort-durationneedsofthegridwithhighpenetrationsofintermittentrenewableenergysystems[1,2].However,recentstudieshaveshownthatlong-durationenergystorage(daystomonths)willbeneededtoaccommodate100%renewable(orcarbon-free)energygeneration[3].Inaddition,long-durationenergystoragewillbeneededtoincreasethesecurityandresilienceoftheelectricalgridinthefaceofincreasingnaturaldisastersandintentionalthreats.

ThermalStorageApplications

Figure1

showsachartofcurrentenergystoragetechnologiesasafunctionofdischargetimesandpowercapacityforshort-durationenergystorage[4].Withintherangeofshort-durationenergystoragecapacities,applicationsincludereserveandresponseservices(1–100kW),transmissionanddistributionsupportgrid(100kW–10MW),andbulkpowermanagement(10MW–1GW).Althoughthermalstoragetechnologyisincludedinthechartascryogenicenergystorage,hotthermalstorageusingsensible,latent,orthermochemicalmethods[5,6]isnotshown.Commercialconcentratingsolarpower(CSP)usingsensibleheatstoragehasdemonstratedtheabilitytoprovideontheorderof100MWofpowercapacityover10hours(~1GWh)forbothgridsupportandbulkpowermanagement.

Thermalstoragetechnologiesarealsobeingconsideredfornuclearpowerplantstoincreasetheflexibilityofthesetraditionallybaseloadsystems[6].Attimesoflowornegativeelectricityprices,

Chapter12ThermalEnergyStorage

heat(orelectricity)generatedbythenuclearreactorwouldbesenttothermalstorage.Attimesofhighelectricityprices,theheatfromthereactorandthermalstoragewouldbeusedtoproducemaximumelectricityoutput(

Figure2

).NewGenerationIVnuclearreactorsdeliverhighertemperaturestothepowercyclerelativetowater-cooledreactors,whichisbeneficialforthermalstoragebecauseathighertemperatures,lessstoragematerialisrequiredtodeliveradesiredamountofthermalpower.Inaddition,thehighertemperaturesenablemoreefficientthermal-to-electricpowerconversion.Addingthermalenergystoragetogeothermalpowerplantstoincreaseflexibilityanddispatchabilityhasalsobeenconsidered[7].

Figure1.Dischargetimeandcapacityofvariousenergystoragetechnologies[4].Hotthermalstoragetechnologiesarenotshownbutcanprovidehundredsofmegawattsformanyhours

Chapter12ThermalEnergyStorage

Figure2.Diagramillustratinghowthermalstoragecanincreasetheflexibilityoftraditionalbaseloadpowerplantsthatrelyonthermalenergy[6].

TechnologyOverview

Theremainderofthischapterprovidesasummaryofthermalstoragetechnologies,whichcanincludesensible,latent,andthermochemicalsystems.Sensiblestoragereliesonatemperaturedifferencewithinthestoragemediumtoenableusefulworktobeperformed,suchasusinghotmoltensalttoheatwaterandgeneratesteamtospinaturbineforelectricityproduction.Latentstorageinvolvesstoringheatinaphase-changematerialthatutilizesthelargelatentheatofphasechange,forexample,duringisothermalmeltingofasolidtoaliquid,whichrequiresheat,andsubsequentfreezingoftheliquidtoasolid,whichreleasesheat,isothermally.Thermochemicalenergystorage(TCES)reversiblyconvertsheatintochemicalbondsusingareactivestoragemedium.Whentheenergyisneeded,areversereactioncombinesthereactants,releasingenergy.

Table1

summarizesthedifferentthermalstoragetechnologiesandkeyattributes.

Table1.Summaryofthermalstoragetechnologies

SensibleHeatStorage

[5,8-12]

LatentHeatStorage

[5,9,10,12,13]

ThermochemicalStorage

[9,11,13]

Storagemechanism

Energystoredastemperaturedifferenceinsolid(e.g.,concrete,rock,sand)orliquidmedia

(moltensalt)

Energystoredusingphasechangematerials(e.g.,salts,metals,organics)

Energystoredinchemicalbonds

EnergyDensity

~200–500kJ/kg(for

~200–400°C

temperaturedifferential)

~100–200kJ/kgfornitratesalts;~200–500kJ/kgformetals;~1000

kJ/kgforfluoridesalts

~300–6,000kJ/kg

Chapter12ThermalEnergyStorage

SensibleHeatStorage

[5,8-12]

LatentHeatStorage

[5,9,10,12,13]

ThermochemicalStorage

[9,11,13]

Advantages

Demonstratedlargeenergycapacity(~GWh)

Inexpensivemedia

Solidmediadoesnotfreezeandcanachieve

>1000°C

GoodforisothermalorlowTapplications

Canprovidelargeenergydensitywithcombinedsensibleandlatentheatstorage

Largeenergydensities

Smallheatlosses

Potentialforlong-termstorage

Compactstoragesystem

OxideTCESStableathightemperatures(>

1000°C)

Challenges

Requiresinsulationtomitigateheatlosses

Lowerenergydensityrequireslargervolumes

Moltensaltsfreezeat

~200°C.

Potentialforcorrosion

ForlargerT,mayneedcascadedsystems(addscostsandcomplexity)

Lowmaturity

Highercomplexity

Lowmaturity

Highercapitalcosts

Mayrequirestorageofgaseousproducts

Maturity

High

Low

Low

Cost

~$1/kgformoltensaltsandceramicparticles

~$0.1/kgforrockandsands

~$1/MJ–$10/MJ(systemcapitalcost)

~$4/kg–$300/kg

~$10/MJ–$100/MJ(systemcapitalcost)

~$10/MJ–$100/MJ(systemcapitalcost)

StateofCurrentTechnology

Sensibleheatstorage

Sensibleheatstorageconsistsofheatingamaterialtoincreaseitsinternalenergy.Theresultingtemperaturedifference,togetherwiththermophysicalproperties(density,specificheat)andvolumeofstoragematerial,determineitsenergycapacity(JorkWh):

Esensible

=ρV

THc

∫

TC p

(T)dT

(1)

Desirablefeaturesofsensiblestoragematerialsincludelargedensities,ρ(kg/m3),largespecificheats,cp(J/kg-K),andlargetemperaturedifferencesbetweenthehotandcoldstates,TH–TC(K).Keyadvantagesincludealowcostofsensiblestoragematerials,highmaturitylevel,andlargeenergycapacities.

Table2

providesasummaryofthermophysicalpropertiesofvarioussensiblesolidandliquidstoragemedia.

Chapter12ThermalEnergyStorage

Table2.Thermophysicalpropertiesofsensiblestoragemedia(adaptedfrom[5]).Calculationofvolumetricandgravimetricstoragedensitiesassumeatemperaturedifferentialof350°C.

StorageMedium

SpecificHeat(kJ/kg-K)

Density(kg/m3)

TemperatureRange(°C)Cold Hot

GravimetricStorageDensity

(kJ/kg)

VolumetricStorageDensity(MJ/m3)

Solids

Concrete

0.9

2200

200

400

315

693

Sinteredbauxiteparticles

1.1

2000

400

1000

385

770

NaCI

0.9

2160

200

500

315

680

Castiron

0.6

7200

200

400

210

1512

Caststeel

0.6

7800

200

700

210

1638

Silicafirebricks

1

1820

200

700

350

637

Magnesiafirebricks

1.2

3000

200

1200

420

1260

Graphite

1.9

1700

500

850

665

1131

Aluminumoxide

1.3

4000

200

700

455

1820

Slag

0.84

2700

200

700

294

794

Liquids

Nitratesalts

(ex.KNO3-0.46NaNO3)

1.6

1815

300

600

560

1016

TherminolVP-1®

2.5

750

300

400

875

656

Siliconeoil

2.1

900

300

400

735

662

Carbonatesalts

1.8

2100

450

850

630

1323

CaloriaHT-43®

2.8

690

150

316

980

676

Sodiumliquidmetal

1.3

960

316

700

455

437

Na-0.79Kmetaleutectic

1.1

900

300

700

385

347

Hydroxidesalts(ex.NaOH)

2.1

1700

350

1100

735

1250

Silicon

0.71

2300

1900

2400

250

575

CommercialCSPplantsthatemploysensiblethermalstoragewithover1GWhofstoragehavebeendeployedworldwide.Forcomparison,Figure3showsthetotalnumberoflarge-scalebatterydemonstrationfacilitiesintheUnitedStatesattheendof2017alongwithtwoCSPplants.EachCSPplantprovidesmoreenergystoragecapacitythanall~100PVdemonstrationfacilitiescombined.

Chapter12ThermalEnergyStorage

1800

1600

EnergyStorageCapacity(MWh)

1400

1200

1000

800

600

400

200

0

Large-ScaleBatteryStorage(~100plantsinU.S.atendof2017)

CrescentDunesCSPPlant(molten-saltstorage)

1680

1100

742

SolanaCSPPlant(molten-saltstorage)

Figure3.ComparisonofenergystoragecapacityforbatteryandCSPplants.Batterydatafrom

U.S.EnergyInformationAdministration[14].

CurrentImplementation

Currentimplementationofhigh-temperaturesensibleheatstorageforelectricityproductionusesliquids(e.g.,moltensalts)andsolids(concrete,rocks).

Liquid

Moltennitratesalt(60%NaNO3,40%KNO3)isbeingusedincommercialCSPplantsaroundtheworldtoprovidegigawatt-hoursofthermalenergystorage.Ithasalowvaporpressure,soitisnotpressurizedattypicalstoragetemperaturesupto~600°C,anditcanbepumpedfromonelocationtoanother.

Figure4showsaphotographandschematicofthe110MWCrescentDunesCSPplantwith1.1GWhofthermalstorageusingmoltennitratesalt.Moltensaltisheatedinareceiverontopofatowerbyconcentratedsunlightfromafieldofheliostats.Thehotmoltensalt(~565°C)flowstoahotstoragetank(righttankinFigure4).Whenneeded,moltensaltispumpedfromthehotstoragetanktoaheatexchangerwhereitheatswaterandgeneratessteamtospinaturbine/generatorforelectricity.Thecooledmoltensalt(~300°C)ispumpedtoacoldstoragetank(lefttankinFigure4)andbacktothereceivertobeheatedwhenthesunisshining.CSPplantscanoperatewithlargecapacityfactors(70–80%)andprovidedispatchableenergy.

Chapter12ThermalEnergyStorage

SolartoThermalConversion ThermalStorage ElectricityGeneration

SolarThermal

Receiver

HotStorage

HeatExchanger

PowerBlock

HeliostatField ColdStorage

Figure4.Top:110MWCrescentDunesCSPplantwith1.1GWhofthermalstorageusingmoltennitratesalt[15].Bottom:Schematicofsensibletwo-tankthermalstoragesysteminaCSPplant.

Solid

Solidthermalstoragehasbeenusedinseveralcommercialanddemonstrationfacilities.In2011,GraphiteEnergydevelopeda3MWeCSPplantinLakeCargelligoinNewSouthWales,Australia,thatusedgraphiteblocksinthereceiversontopofmultipletowers.Thegraphiteblocksinthereceiver,irradiatedbyconcentratedsunlight,servedasboththestoragesystemandboilertogeneratesteamforpowerproduction.

EnergyNest,basedinNorway,developedaconcrete-basedthermalenergystoragesystemthatconsistsofanarrayofmodularpipesfilledwithconcreteandsteeltubes.Thetubescarryheat-transferfluidthatcanheattheconcretewhenchargingandextractheatfromtheconcretewhendischargingtopoweraturbine/generatororprovideprocessheating.Thesystemcancharge/dischargein~30minutesandthestoredenergycanlastforseveraldayswithlessthan2%heatlossper24hoursforlarge-scalesystems.

Chapter12ThermalEnergyStorage

SiemensGamesainGermanyhasdevelopeda130MWhtElectricThermalEnergyStorage(ETES)systemcomprisesrocksstoredinabuilding.Airisresistivelyheatedusingelectricity(whenpriceislow)andpasseddirectlythroughthebedofrocks.Therocksareheatedto~600°C,and,whenneeded,airispassedthroughthehotrockstoheatsteamforaRankinepowercycle.The130MWhtdemonstrationplantbecameoperationalin2019,andthecompanyisplanningadesignfora30MWcommercialpilotplant.

Challenges

Therelativelylowenergydensityofsensible-heatstoragematerialsrequireslargevolumesofmaterialforlarge-capacityenergystorage,whichincreasestheoverallstoragecost.Inaddition,somepowercyclesthatemployrecuperationtoincreasethethermal-to-electricefficiencyrequirerelativelylowtemperaturedifferentialsbetweenthehotandcoldstatesofthestoragematerial.Forexample,thesupercriticalCO2recompressionBraytoncyclerequiresatemperatureincreaseofonly~200°Cintheprimaryheatexchanger[16].Asaresult,therequiredmassinventoryofstoragematerialmustincreasetodeliverthesameamountofenergyforalowertemperaturedifferential,whichincreasescosts.ThetargetcapitalcostfortheU.S.DepartmentofEnergy(DOE)CSPprogramis$15/kWhfortheentirethermalstoragesystem.

Moltensaltsfreezeat>200°C,whichrequiresexpensivetraceheatingtomaintainallcomponentsattemperatureswellabovethefreezingpoint.Ifthesaltfreezes,flowcanbeblocked,andthawingmustoccurbeforeoperationcanbegin.StresswithinthelargestoragetankshasalsocausedissuesatCSPplants.Thermalgradientsatthebaseofthetankcancreatethermomechanicalstressesthatdamagethetankstructure.Appropriateconsiderationofthermomechanicalstressesiscriticaltothedesignoflarge-scalethermalstoragetanks.

Opportunities

Anumberofinstitutionshavebeenpursuingsmall,sand-likeparticle-basedthermalstorageforCSPplantsandstand-alonethermalenergystoragesystems.Unliketheprevioussolid-basedthermalstoragesystems,ratherthanpassingairoraheat-transferfluidthroughthestoragemedia,theparticlesareheateddirectlyandconveyedthroughaheatexchangertoheattheworkingfluid[8].Theparticlesareliftedtothetopofthereceiverwheretheyareirradiatedandheatedbyconcentratedsunlight.Thehotparticlesflowintoaninsulatedstoragetankwheretheycanbeheldforhoursordays.Whenneeded,theparticlesarereleasedthroughaparticleheatexchangertoheataworkingfluidthatspinsaturbine/generatorforelectricityproduction(

Figure5

).

Chapter12ThermalEnergyStorage

Particleelevator

Particlehot

storagetank

Particle-to-working-fluidheat

exchanger

Particlecold

storagetank

Particlecurtain

Particlecurtain

Apertur

Aperture

Fallingparticlereceiver

Figure5.Illustrationofahigh-temperaturefallingparticlereceiverwithtower-integratedstorageandheat-exchangerfordispatchableelectricityproduction[17]

Liketheothersolid-basedthermalstoragetechnologies,inexpensiveparticlestoragecanaccommodateincreasingpenetrationsofrenewablesbyallowingheattobestoredwhenelectricitydemandislow,andthenusingthatstoredheattoproduceelectricitywhendemandandpricesarehigher.Thistime-shiftingofenergyproductionandusecanincreasetheflexibilityoftraditionalbaseloadpowerplants,includingnuclearandgeothermal.

Solidstoragemediahastheadvantageofbeinginert,inexpensive,non-corrosive,andeasytohandle.Inaddition,manysolidmaterialsexhibitamuchwideroperatingtemperaturerangethanmoltensalts.Rock,sand,andsinteredbauxitehaveallbeenutilizedinthermalstoragesystemsandcanoperateinsub-freezingto>1000°Ctemperatures.Largevolumesofbulksolidmaterialcanalsoprovideself-insulationfromthecoolerambientenvironment.Asthevolumeofthebulkstoragetankincreases,theratioofitssurfaceareatovolumedecreases,whichreducesheatloss.So,largestoragetanksorcontainmentsystemsyieldbothperformancebenefitsandeconomiesofscale.

Pumpedthermalenergystorageuseselectricityinaheatpumptotransfersheatfromacoldreservoirtoahotreservoirsimilartoarefrigerator.Whenelectricityisneeded,theheatpumpisreversedtoallowtheheatfromthehotreservoirtodriveaheatengineandspinaturbine/generator.Thelargepotentialtemperaturedifferencesbetweenthehotandcoldreservoirscanenablehighlyefficientpowercycles.Malta,aspinofffromGoogleX,isdesigningapumped-thermalenergystoragesystem(Figure6).

Chapter12ThermalEnergyStorage

Figure6.Malta’spumpedthermalenergystorageconcept[Malta,2020#13799]

MITisinvestigatinganotherstoragetechnologythatwouldusecheaporexcesselectricitytosensiblyheatmoltensilicontoultra-hightemperaturesinlarge,insulatedgraphitetanks.Themoltensiliconwouldbeheldat“cold”temperaturesof~1900°C(aboveitsmeltingpointof1414°C)andheatedwithelectricalheatingelementstonearly2400°C,whereitisstoredinasecond“hot”tank.Whenelectricityisneeded,themoltensiliconispumpedfromthehottankthroughtubesthatemitthermalradiationtomultijunctionphotovoltaiccellsthatgenerateelectricity.Thecooledmoltensiliconisthencollectedinthecoldstoragetank.

Latentheatstorage

Latentheatstoragesystemsusethelatentheatofphasechangetostoreenergy.Latentheatoffusionistheenergyrequiredtochangethestateofsubstancefromasolidtoaliquid,andlatentheatofevaporationistheenergyrequiredtochangethestateofsubstancefromaliquidtoagas.Saltsandmetalscanbemelted,andthecombinedsensibleandlatentheatcanbeusedtostoretheaddedthermalenergy.

Table3

summarizesthethermophysicalpropertyvaluesofdifferentlatent-heatstoragematerials.Thelatentheatofreaction(kJ/kg)showninthesecondcolumnwouldbeaddedtothesensibleheatcapacityinEq.(1)todeterminethetotalheatcapacityoflatentheatstoragematerialsbeingheatedfromonestatetoanother.Inmostcases,thematerialsaresolid/liquidphasechangematerialsthatarestoredasliquidsthatcansubsequentlyreleaseenergywhenconvertedbacktoasolidstate.Someliquid/gassubstances(nitrogenandoxygen)arealsoshownbecausecryogenic“liquidair”storagehasalsobeendemonstratedforgridenergystorageapplications.

Chapter12ThermalEnergyStorage

Table3.Thermophysicalpropertiesofphase-changestoragematerialsatstandardconditions,unlessotherwisenoted(adaptedfrom[5])

StorageMedium

SpecificHeat(kJ/kg-K)

LatentorReactionHeat

(kJ/kg)

Density(kg/m3)

MeltingPoint(°C)

BoilingPoint(°C)

GravimetricStorageDensity(kJ/kg)

VolumetricStorageDensity

(MJ/m3)

Liquid/SolidPhaseChangeMaterials

Aluminum

1.2

397

2380

660

-

397

945

Aluminumalloys(ex.Al-0.13Si)

1.5

515

2250

579

-

515

1159

Copperalloys(ex.Cu-0.29Si)

-

196

7090

803

-

196

1390

Carbonatesalts(ex.Li2CO3)

-

607

2200

726

-

607

1335

Nitratesalts

(ex.KNO3-0.46NaNO3)

1.5

100

1950

222

-

100

195

Bromidesalts(ex.KBr)

0.53

215

2400

730

-

215

516

Chloridesalts(ex.NaCI)

1.1

481

2170

801

-

481

1044

Fluoridesalts(ex.LiF)

2.4

1044

2200

842

-

1044

2297

Lithiumhydride

8.04

2582

790

683

-

2582

2040

Hydroxidesalts(ex.NaOH)

1.47

160

2070

320

-

160

331

Silicon

0.71

1800

2300

1414

-

1800

4140

Liquid/GasPhaseChangeMaterials

Nitrogen

1.04

199

809

(liquid)

-

-196

199

161

Oxygen

0.92

213

1140

(liquid)

-

-183

213

243

CurrentImplementation

Phasechangematerials(PCMs)havebeenencapsulatedinspherestoformpackedbedsofencapsulatedPCMs[9].Heat-transferfluidcanbepassedthroughthepacked-bedofspherestochargeordischargeenergyto/fromtheencapsulatedPCMs.Thephasechangeoccursatnearlyisothermalconditions,sothismethodisusefulforapplicationswheretheheatadditionneedstooccurataspecifictemperature.Atlargertemperatureranges,cascadedPCMsystemscanbedesigned,butwithadditionalcomplexityandcost.Todate,encapsulatedPCMsystemshavebeentestedanddemonstratedatsmallscales.Commercialsystemshavenotbeendemonstrated.

Moltensiliconsystemshavebeendevelopedtoexploitthelargeheatofphasechangewhenmelting/solidifyingsilicon(~1800kJ/kg).TheAustraliancompany,1414Degrees,hasdesignedthermalenergystoragesystemsrangingfrom10–200MWh,andtheybeganoperatingaprototypefacilityin2019.Thesystemsmeltsiliconat~1400°Candrecoupthelatentenergyduringsolidificationtopowercombinedcycles.

Ontheoppositeendofthetemperaturescale,HighviewPowerhasdemonstratedcryogenicenergystorageusing“liquidair”atdemonstrationfacilitieswith2.5kWh(300kWpeakpower)and15MWh(5MWpeakpower)ofenergystorage.Thesystemoperatesbyusingelectricitytocoolairfromambienttemperaturesto-195°CusingtheClaudeCycle.Theliquifiedairisstoredatatmosphericpressureinlargevacuum-insulatedtanks.Thevolumeoccupiedbytheliquidairis

~1,000timeslessthanthatofairatambientconditions.Whenelectricityisneeded,theliquidairispumpedathighpressuresthroughaheatexchangerthatexposestheliquidairtoambienttemperatures(orwasteheatfromanindustrialheatsource).Theliquidairvaporizes,causingsuddenexpansion,whichspinsandturbine/generatorforelectricityproduction.Theheat

Chapter12ThermalEnergyStorage

exchangercanconsistofagravelbedthatservesasacoldstoreoflow-temperaturematerialaftergivingupitsenergytovaporizetheliquidair.Thelow-temperaturematerialcanthenbeusedtohelpcooltheairduringthenextrefrigerationcycle.

Challenges

ChallengeswithPCMsincluderelativelyhighcostsandnarrowoperatingtemperatureranges.UsingPCMstoprovideenergytoaheatenginewilltypicallyrequireacascadedsystemwithmultiplePCMswithdifferentmeltingpoints.Theuseofmoltensiliconathightemperaturesprovideschallengeswithmaterialscontainmentandheatloss.Phase-changesystemsmuststillbewellinsulatedtopreventheatlossandsubsequentphasechange.

Opportunities

1414Degreesappearstohavesuccessfullydevelopedaprototypemolten-siliconsystemthatexploitsveryhighlatentheatsoffusion.Othersystemsandmaterialsthatcanexploithighlatentheatsoffusionatlowcostsmayprovidealternativethermalstoragecapabilities.

Thermochemicalstorage

Thermochemicalenergystorage(TCES)isapromisingstoragetechnology,especiallyathightemperatures(>700°C),asitallowsforthestorageofheatthroughchemicalreactions,forexample,thebreaking/reformingofbonds.AconceptualillustrationofTCESisshownin

Figure

7

.

Figure7.SchematicofstepsinvolvedinTCES:charging,storage,anddischarging[18]

Thethermochemicalstoragereaction,initsmostbasicform,canbewrittenas

AB+ΔHrxn↔A+B (2)

Inthisequation,ReactantABisdissociatedintoProductsA+Bviatheapplicationofheat(heatofreactionshownin

Table3

)inanendothermicreaction.Theindividualproductscanbestoredseparatelyforanindefiniteamountoftime.Intimesofthermaldemand,A+Brecombineinanexothermicreaction,releasingheat(thereactionproceedstotheleft).

Chapter12ThermalEnergyStorage

TheTCESprocesscomparedtootherthermalstoragetechnologiesissummarizedin

Table1

.ThepotentialbenefitsofTCESinclude(1)enablingmoreefficienthigh-temperaturepowercycles(sCO2orairBrayton)thatareinaccessibleusingcurrentmoltensalttechnologies,(2)potentialhigher-densityandlong-termstorage,and(3)higherexergy.Inaddition,certainTCESprocesses(e.g.,redox-activeoxides)arealsoamenabletogeneratinghydrogenviawater-splitting.Thehydrogencanthenbeusedon-sitetorunafuelcellforback-upgeneration.ForTCEStobeapracticalstoragetechnology,thematerialsmusthavealargereactionenthalpyandfastreactionkinetics,highthermalconductivity,goodcyclicstabilitywithouttheformationofunwantedphasesorsidereactions.Theyshouldalsoconsistofabundantandeconomicallyinexpensiveelements[19-22].

Implementation

AvarietyofpotentialTCESprocessesexist,thoughnoTCESmaterialhasbeenimplementedonanindustrialscale.TCEScanbeapplicableoverawiderangeoftemperaturesandconditions.Heatsource,thetypeofpowercycle,operatingtemperature,andreceiverconfigurationallinfluencetheselectionofacandidateTCESmaterial.

Table4

liststhemostpromisingTCESreactionsbytype,reactiontemperatures,enthalpies,andgravimetricstorageenergies.Theoperatingtemperaturesandstoragedensitiesarerepresentativevalues,butcandifferdependingonoperatingconditions,suchaspressure,aswellasthemorphologyofthesolidspecies.Thesolidspeciescanbeparticles,monoliths,orsupportedoninertorreactivescaffoldstoavoidsinteringordeactivationofthematerial[23].

Table4.Candidatematerialssystemsforthermochemicalenergystorage

StorageMedium

ReactionEnthalpy(kJ/mol)

TemperatureRange(°C)

GravimetricStorageDensity(kJ/kg)

VolumetricStorageDensity(MJ/m3)

Carbonates

CaCO3(s)+ΔH↔CO2(g)+CaO(s)+CO2(g)

178

850-1273

1764

2491

SrCO3(s)+ΔH↔SrO(s)+CO2(g)

234

900-1200

300-1000

1200-1500

BaCO3(s)+ΔH↔BaO(s)+CO2(g)

273

~1290

Hydroxides

Ca(OH)2(s)+ΔH↔CaO(s)+H2O(g)

104

400600

1406

1640

Mg(OH)2(s)+ΔH↔MgO(s)+H2O(g)

81

350-

1340

1396

Hydrides

MgH2(s)+ΔH↔Mg(s)+H2(g)

75

300-480

2880

2088

Mg2FeH6(s)+ΔH↔2Mg(s)+Fe(s)+H2(g)

74

300-500

2106(theo.),

1921(expt)

5768(theo)2344(expt)

Mg2NiH4(s)+ΔH↔Mg2Ni(s)+2H2(g)

77

300-500

1160

3142

NaMg2H3(s)+ΔH↔NaH(s)+Mg(s)+H2(g)

87

430-585

1721

~1721

NaMgH2F(s)+ΔH↔NaF(s)+Mg(s)+H2(g)

97

510-605

1416

1968

CaH2(s)+ΔH↔Ca(s)+H2(g)

186

1000-1400

3587

7374

Chapter12ThermalEnergyStorage

StorageMedium

ReactionEnthalpy(kJ/mol)

TemperatureRange(°C)

GravimetricStorageDensity(kJ/kg)

VolumetricStorageDensity(MJ/m3)

Ammonia

NH3(g)↔½N2(g)+3/2H2(g)

67

400-700

3924

2682

RedoxActiveOxides*

-

2Co3O4(s)+ΔH↔6CoO(s)+O2(g)

205

900

844

-

2BaO2(s)+ΔH↔6BaO(s)+O2(g)

79

693-780

474

-

6Mn2O3(s)+ΔH↔4Mn3O4(s)+O2(g)

32

1000

204

-

4CuO(s)+ΔH↔2Cu2O(s)+O2(g)

64

1030

-

-

Ca0.95Sr0.05MnO3(s)+ΔH↔Ca0.95Sr0.05MnO2.7(s)+

0.15O2(g)

-