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USAIDGRID-SCALEENERGYSTORAGETECHNOLOGIESPRIMER

USAIDGRID-SCALEENERGYSTORAGETECHNOLOGIESPRIMER

Authors

ThomasBowen,IlyaChernyakhovskiy,KaifengXu,SikaGadzanku,KamyriaConey

NationalRenewableEnergyLaboratory

July2021

Acompanionreporttothe

USAIDEnergyStorageDecisionGuideforPolicymakers

Preparedby

NOTICE

Thisworkwasauthored,inpart,bytheNationalRenewableEnergyLaboratory(NREL),operatedbyAllianceforSustainableEnergy,LLC,fortheU.S.DepartmentofEnergy(DOE)underContractNo.DE-AC36-08GO28308.

FundingprovidedbytheUnitedStatesAgencyforInternationalDevelopment(USAID)underContractNo.IAG-17-2050.TheviewsexpressedinthisreportdonotnecessarilyrepresenttheviewsoftheDOEortheU.S.Government,oranyagencythereof,includingUSAID.

ThisreportisavailableatnocostfromtheNationalRenewableEnergyLaboratory(NREL)at

/publications.

U.S.DepartmentofEnergy(DOE)reportsproducedafter1991andagrowingnumberofpre-1991documentsareavailablefreevia

www.OSTI.gov.

Frontcover:photofromiStock506609532;Backcover:photofromiStock506611252

NRELprintsonpaperthatcontainsrecycledcontent.

PAGE\*roman

viii

ThisreportisavailableatnocostfromtheNationalRenewableEnergyLaboratory(NREL)at

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Acknowledgments

Theauthorsaregreatlyindebtedtoseveralindividualsfortheirsupportandguidance.WewishtothankDominiqueBain,MarcusBianchi,NateBlair,AnthonyBurrell,PaulDenholm,GregStark,andKeithWipkeattheNationalRenewableEnergyLaboratory(NREL),andOliverSchmidtatImperialCollegeLondonfortheirreviews.AndwewishtothankIsabelMcCan,ChristopherSchwing,andLizBreazealeforcommunications,design,andeditingsupport.Anyerrorsoromissionsaresolelytheresponsibilityoftheauthors.

ThisworkwasfundedbyUSAID.

ListofAcronyms

A-CAES adiabaticcompressedairenergystorage

CAES compressedairenergystorage

CHP combinedheatandpower

CSP concentratedsolarpower

D-CAES diabaticcompressedairenergystorage

FESS flywheelenergystoragesystems

GES gravityenergystorage

GMP GreenMountainPower

LAES liquidairenergystorage

LADWP LosAngelesDepartmentofWaterandPower

PCM phasechangematerial

PSH pumpedstoragehydropower

R&D researchanddevelopment

RFB redoxflowbattery

SMES superconductingmagneticenergystorage

TES thermalenergystorage

VRE variablerenewableenergy

TableofContents

TOC\o"1-2"\h\z\u

Introduction 1

ElectrochemicalEnergyStorageTechnologies 6

Lithium-ionBatteryEnergyStorage 8

FlowBatteryEnergyStorage 12

Lead-AcidBatteryEnergyStorage 14

Sodium-SulfurBattery 16

MechanicalEnergyStorageTechnologies 18

PumpedStorageHydropower(PSH) 19

FlywheelEnergyStorage 21

CompressedAirEnergyStorage 23

GravityEnergyStorage 26

AdditionalEnergyStorageTechnologies 28

HydrogenEnergyStorageSystems 29

ThermalEnergyStorage(TES) 34

Supercapacitors 36

SuperconductingMagneticEnergyStorage(SMES) 37

Glossary 39

References 40

ListofFigures

Figure1.Ecosystemofenergystoragetechnologiesandservices 2

Figure2.U.S.annualnewinstallationsofelectrochemicalenergystoragebychemistry 8

Figure3:Lithium-ionbatterychemistrymarketshareforecast,2015–2030 10

Figure4.Pathwaysinthehydrogeneconomyfromfeedstocktoendapplication 32

ListofTables

Table1.QualitativeComparisonofEnergyStorageTechnologies 3

Table2.ComparisonofElectrochemicalStorageTechnologies 6

Table3.AdvantagesandDisadvantagesofSelectElectrochemicalBatteryChemistries 7

Table4.OperatingCharacteristicsofSelectLithium-IonChemistries 9

Table5.ComparisonofMechanicalStorageTechnologies 18

Table6.TypicalCharacteristicsofSelectFlywheelTechnologies 21

Table7.MethodsforProducingHydrogen 31

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Introduction

Powersystemsworldwideareexperiencinghigherlevelsofvariablerenewableenergy(VRE)aswindandsolarpowerplantsconnecttothegrid.ThistrendisexpectedtocontinueascostsforVREresourcesdeclineandjurisdictionspursuemoreambitiouspowersectortransformationstrategieswithincreasedVREpenetrations.

1

HigherpenetrationsofVREcandriveadditionalneedforpowersystemflexibilityinbothshort-termessentialgridservicesandlonger-termenergyshiftingandpeakingcapacityservices(Chernyakhovskiyetal.2019).Energystorageisoneofseveralsourcesofpowersystemflexibilitythathasgainedtheattentionofpowerutilities,regulators,policymakers,andthemedia.

2

Fallingcostsofstoragetechnologies,particularlylithium-ionbatteryenergystorage,andimprovedperformanceandsafetycharacteristicshavemadeenergystorageacompellingandincreasinglycost-effectivealternativetoconventionalflexibilityoptionssuchasretrofittingthermalpowerplantsortransmissionnetworkupgrades.

Thisprimerisintendedtoprovideregulatorsandpolicymakerswithanoverviewofcurrentandemergingenergystoragetechnologiesforgrid-scaleelectricitysectorapplications.Transportationsectorandotherenergystorageapplications(e.g.,mini-andmicro-grids,electricvehicles,distributionnetworkapplications)arenotcoveredinthisprimer;however,theauthorsdorecognizethatthesesectorsstronglyinteractwithoneanother,influencingthecostsofenergystorageasmanufacturingcapacityscalesupaswellasimpactingelectricitydemand.Thestoragetechnologiescoveredinthisprimerrangefromwell-establishedandcommercializedtechnologiessuchaspumpedstoragehydropower(PSH)andlithium-ionbatteryenergystoragetomorenoveltechnologiesunderresearchanddevelopment(R&D).Thesetechnologiesvaryconsiderablyintheiroperationalcharacteristicsandtechnologymaturity,whichwillhaveanimportantimpactontherolestheyplayinthegrid.Figure1providesanoverviewofenergystoragetechnologiesandtheservicestheycanprovidetothepowersystem.

SeveralkeyoperationalcharacteristicsandadditionaltermsforunderstandingenergystoragetechnologiesandtheirroleonthepowersystemaredefinedintheGlossary.

Table1

providesseveralhigh-levelcomparisonsbetweenthesetechnologies.ManyofthesecharacteristicsareexpectedtochangeasR&Dforthetechnologiesprogresses.Sometechnologycategories,suchaslithium-ionorlead-acidbatteries,comprisemultiplesubtypesthateachfeatureuniqueoperationalcharacteristics;comparisonsofsubtypeswithintechnologiesareconsideredintheirrespectivesections.

Thisreportservesasacompanionpiecetothe

USAIDEnergyStorageDecisionGuideforPolicymakers,

whichoutlinesimportantconsiderationsforpolicymakersandelectricsectorregulatorswhencomparingenergystorageagainstothermeansforpowersystemobjectives.

1Bypowersectortransformation,theauthorsreferto“aprocessofcreatingpolicy,marketandregulatoryenvironments,andestablishingoperationalandplanningpracticesthataccelerateinvestment,innovationandtheuseofsmart,efficient,resilientandenvironmentallysoundtechnologyoptions”(IEA2019).Formoreinformationonsuchpowersectortransformations,seeCoxetal.(2020).

2Powersystemflexibilityisdefinedhereas“theabilityofapowersystemtoreliablyandcost-effectivelymanagethevariabilityanduncertaintyofdemandandsupplyacrossallrelevanttimescales,fromensuringinstantaneousstabilityofthepowersystemtosupportinglong-termsecurityofsupply”(IEA2018).Forinformationonandsourcesofpowersystemflexibility,seeIEA(2018)andIEA(2019).

Figure1.Ecosystemofenergystoragetechnologiesandservices

Table1.QualitativeComparisonofEnergyStorageTechnologies

Source:(Chenetal.2009;Mongirdetal.2019a;Mongirdetal.2020)

Category

Technology

DevelopmentStageforUtility-ScaleGridApplications

CostRange

TypicalDurationofDischargeatMaxPowerCapacity

ReactionTime

Round-TripEfficiency

3

Lifetime

Electro-ChemicalBatteries

Lithium-ion

Widelycommercialized

1,408-1,947

($/kW)

352-487($/kWh)?

Minutestoafewhours

Subsecondtoseconds

86-88%

10years

Flow

Initialcommercialization

1,995-2,438

($/kW)

499-609($/kWh)?

Severalhours

Subsecondtoseconds

65%–70%

15years

Lead-acid

Widelycommercialized

1,520-1,792

($/kW)

380-448($/kWh)?

Minutestoafewhours

Seconds

79-85%

12years

Sodium-sulfur

Initialcommercialization

2,394–5,170

($/kW)

599–1,293

($/kWh)??

Severalhours

Subsecond

77%–83%

15years

Mechanical

PSH

Widelycommercialized

1,504-2,422

($/kW)

150-242

($/kWh)???

Severalhourstodays

SeveralSecondstoMinutes(dependsontechnologychoice)

80+%*

40years

Compressedairenergystorage(CAES)

Initialcommercialization

973-1,259($/kW)

97-126($/kWh)???

Severalhourstodays

SeveralMinutes

52%**

30years

Flywheels

Widelycommercialized

1,080-2,880

($/kW)

4,320-11,520

($/kWh)??

Secondstoafewminutes

Subsecond

86%–96%

20years

Gravity

R&Dstage

Insufficientdata

Severalhours

SeveralMinutes

Insufficientdata

Insufficientdata

Chemical

Hydrogenproductionandfuelcells

Pilotstage

2,793-3,488

($/kW)279-349

($/kWh)????

Severalhourstomonths

Subsecond

35%

30years

Thermal

Thermalenergystorage

Initialcommercialization

1,700-1,800

($/kW)

20-60($/kWh)

Severalhours

SeveralMinutes

90+%

30years

3Assomeenergystoragetechnologiesrelyonconvertingenergyfromelectricityintoanothermedium,suchasheatinthermalenergystoragesystemsorchemicalenergyinhydrogen,weuseefficiencyheretorefertotheround-tripefficiencyofstoringandreleasingelectricity(electrons-to-electrons),asopposedtotheefficiencyofusingelectricitytoproduceheatforheatingneedsorhydrogenfortransportationfuelneeds.

Electrical

Super-capacitors

R&DStage

930($/kW)

74,480($/kWh)??

Secondstoafewminutes

Subsecond

92%

10–15

years

Superconductingmagneticenergystorage(SMES)

Initialcommercialization

200–300($/kW)

1,000–10,000

($/kWh)

Seconds

Subsecond

~97%

20years

*:ThisreferstonewerPSHinstallationsandolderPSHsystemsmayhaveefficienciesclosertothe60-75%range.

**:AsCAESreliesonbothelectricitytocompressairandafuel(typicallynaturalgas)toexpandtheair,itsefficiencycannotbereadilycomparedtootherstoragetechnologies.Thevalueusedinthisreportrepresentstheratiooftheoutputofelectricalenergytothecombinedinputofelectricalenergyforthecompressorandthenaturalgasinputforexpansion,usingtheheatingvalueofnaturalgastoconvertitsenergytohowmuchelectricityitcouldhaveproduced(Mongirdetal.2019).

?Thisrangereferstoa10MW4-hourbatteryin2020costs.Forlithium-ion,thisreferstotheNMCchemistry(seeSection

2.1

foradditionalinformationonlithium-ionchemistries).SeeMongirdet.al.(2020)foradditionalenergystoragesizesanddurationsandestimatesforfutureyears.

??:Thisrangerefersto2018costs.SeeMongirdet.al.(2019)forfutureyears.

???Thisrangerefersto1000MW10-hoursystems.SeeMongirdet.al.(2020)foradditionalenergystoragesizesanddurationsandestimatesforfutureyears.

????Thisrangerefersto100MW10-hoursystems.SeeMongirdet.al.(2020)foradditionalenergystoragesizesanddurationsandestimatesforfutureyears.

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USAIDGRID-SCALEENERGYSTORAGE

TECHNOLOGIESPRIMER

ElectrochemicalEnergyStorageTechnologies

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ElectrochemicalEnergyStorageTechnologies

Electrochemicalstoragesystemsuseaseriesofreversiblechemicalreactionstostoreelectricityintheformofchemicalenergy.Batteriesarethemostcommonformofelectrochemicalstorageandhavebeendeployedinpowersystemsinbothfront-of-the-meterandbehind-the-meterapplications,aswellasinelectronicsandtransportationapplications.Broadlyspeaking,batteriestendtohavedurationslastinguptoseveralhoursandcanchangeoutputinthesubsecondtoseveralminutesrange.

Table2.ComparisonofElectrochemicalStorageTechnologies

Source:(Fanetal.2020;DNVGL2016;Kintner-Meyeretal.2010;DiazdelaRubiaetal.2015;Mongirdetal.

2020)

Technology

ReactionTime

Round-TripEfficiency

EnergyDensity(Wh/kg)

PowerDensity(W/kg)

OperatingTemperature(°C)

CycleLife(Cycles)**

Lithium-Ion

Subsecondtoseconds

86-88%

210–325*

4,000–

6,500*

-20–65

1,000–2,000*

Flow

Subsecond

65%–70%

10–50

0.5–2

5–45

12,000–14,000

Lead-Acid

Seconds

79-85%%

30–50

30-50

18–45

500–1,000

Sodium-Sulfur

Subsecond

77%–83%

150–240

120–160

300–350

~4,500

*Valuesmayvaryacrossdifferentcelldesigns,chemistries,andpowerelectronicsconfigurations.Foroperationalcharacteristicsbrokendownintocommonlithium-ionchemistries,see

Table5.

**Itshouldbenotedthatcyclelifeisintrinsicallyrelatedtothebehaviorandenvironmentofthestoragesystem(e.g.,someusecasescanleadtolowercyclelifeasitstressesthestoragesystem,andmanyelectrochemicalstoragetechnologiesperformworseorsuffershortercyclelifeoutsidetheirnormaloperatingtemperaturerange).

Table3.AdvantagesandDisadvantagesofSelectElectrochemicalBatteryChemistries

Adaptedfrom(Fanetal.2020)

StorageType

Advantages

Disadvantages

Lithium-Ion

Relativelyhighenergyandpowerdensity

Lowermaintenancecosts

Rapidchargecapability

Manychemistriesofferdesignflexibility

Establishedtechnologywithstrongpotentialforprojectbankability.

Highupfrontcost($/kWh)relativetolead-acid(potentiallyoffsetbylongerlifetimes)

Poorhigh-temperatureperformance

Safetyconsiderations,whichcanincreasecoststomitigate

Currentlycomplextorecycle

Relianceonscarcematerials.

Flow(Vanadium-Redox)

Longcyclelife

Highintrinsicsafety

Capableofdeepdischarges.

Relativelylowenergyandpowerdensity.

Lead-Acid

Lowcost

Manydifferentavailablesizesanddesigns

Highrecyclability.

Limitedenergydensity

Relativelyshortcyclelife

Cannotbekeptinadischargedstateforlongwithoutpermanentimpactonperformance

Deepcyclingcanimpactcyclelife

Poorperformanceinhightemperatureenvironments.

Toxicityofcomponents

Sodium-Sulfur

Relativelyhighenergydensity

Relativelylongcyclelife

Lowself-discharge.

Highoperatingtemperaturenecessary

Highcosts.

Lithium-ionBatteryEnergyStorage

TechnologySummaryforPolicymakers

Lithium-ionisamatureenergystoragetechnologywithestablishedglobalmanufacturingcapacitydriveninpartbyitsuseinelectricvehicleapplications.Theoverlapbetweenthetransportationandpowersystemsectorshaveenabledsteeppricedeclinesintechnologycostsforlithium-ionbatteries,drivinghigherdeployments.Inutility-scalepowersectorapplications,lithium-ionhasbeenusedpredominantlyforshort-duration,high-cyclingservicessuchasfrequencyregulation,althoughitisincreasinglyusedtoprovidepeakingcapacityandenergyarbitrageservicesincertainjurisdictions.Lithium-ionhasatypicaldurationinthe2-to4-hourrange,withpricecompetitivenessdecreasingatlongerdurations.Onemajortechnicalissuewithlithium-ionisfiresafety,asthechemistrycansufferthermalrunawayleadingtofireconcerns.Recentbatterypacktechnologyandsoftwareinnovationsareaddressingsafetyconcernsrelatedtothermalrunaway.

Lithium-ionbatterystoragecurrentlydominatesthelandscapefornew,utility-scaleinstallationsforelectrochemicalstationarystorageapplicationsandisonlysurpassedbypumpedhydrostorageforcumulativecapacity.Since2010intheUnitedStates,over90%ofannualadditionsofutility-scalestationarybatterystorageinthepowersectorhasbeenlithium-ion(

Figure2

).Thistrendisdrivenbyseveralfactors,includingrobustmanufacturingcapabilities,well-developedsupplychains,increasingdemandinthetransportationsector,andaprecipitousdropinlithium-ionbatterypackpricesoverthepastseveralyears:lithium-ionbatterypackpricesdeclined89%from2010to2020(Frith2020).

4

Figure2.U.S.annualnewinstallationsofelectrochemicalenergystoragebychemistry

Source:(EIA2019)

Aswithallbatteryenergystoragetechnologies,lithium-ionbatteriesconvertchemicalenergycontainedinitsactivematerialsdirectlyintoelectricalenergythroughanelectrochemicaloxidation-reductionreaction(Warner2015).Lithium-ionbatteries,however,havesignificantlyhigherenergydensitiesrelativetootherelectrochemicalstoragetechnologiessuchaslead-acidandflowbatteries,whichallows

4Notethatthispricedeclinerefersonlytobatterypackprices,whichreflectlithium-ionbatterypackhardwarecostsanddonotincludeadditionalhardwarecomponentsorsoftcoststhatwouldaccumulatewhenconstructingaproject.

thesameenergyneedstobemetwithsmallerandlighterbatteries.Lithium-ionbatteriesarealsoabletochargeanddischargethousandsoftimesbeforereachingtheendofthebatterypacklife.

Theprimarysafetyconcernsurroundinglithium-ionbatteriesisfire-riskscausedby“thermalrunaway.”Thermalrunawayreferstoapointatwhichthetemperatureinsidethebatterycellsbecomeshotenoughtocauseself-sustainingheatgeneration,whichcanquicklyleadtobatteryfailureorevenfires(Warner2015).Eventhoughthermalrunawayisnotuniquetolithium-ion,lithiumtendstohavealowerrunawaytemperature,whichmeansthermalmanagementandfiresuppressionareimportantfactorstoconsiderwhenoperatinglithium-ionbatteries,eventhoughtheymayincreaseoverallprojectcosts.

5

Lithium-ionbatteriescanconsistofvariouschemistryconfigurationsandeachchemistryexhibitsslightlydifferentoperatingparameters.

Table4

comparesthekeyoperatingmetricsforafewofthecommonlithium-ionchemistries(Warner2015).AlthoughLithiumNickelManganeseCobalt(NMC)iscurrentlythedominatechemistry,competingchemistriesLithiumNickelCobaltAluminum(NCA)andLithiumIronPhosphate(LFP)areexpectedtogrowinpopularityoverthenextseveraldecades(

Figure3

).

Table4.OperatingCharacteristicsofSelectLithium-IonChemistries

Source:(Warner2019;DNVGL2016;Mongirdetal.2020)

Technology

EnergyDensity(Wh/L)

PowerDensity(W/L)

OperatingTemperature(°C)

CycleLife

Self-Discharge(%/month)

LithiumIronPhosphate

220–250

4,500

-20to+60

~2,000

<1%

LithiumNickelCobaltAluminum

210–600

4,000–

5,000

-20to+60

>1,000

2%–10%

LithiumNickelManganeseCobalt

325

6,500

-20to+55

~1,200

1%

5Batterycelldegradationthatcanleadtothermalrunawaycanbeginattemperaturesaslowas80°C.At80°C,lithiumionsbegintoreactwithchemicalsintheelectrolyte,decomposinglayersaroundtheanodeinaheat-generatingreaction(exothermic)(Warner2019).

Figure3:Lithium-ionbatterychemistrymarketshareforecast,2015–2030

Source:(WoodMackenzie2020)

CurrentApplications

Inadditiontowidespreadelectricmobilityapplicationsandconsumerelectronics,lithium-ionbatterystorageisincreasinglyusedforstationaryenergystorageapplications,bothinutility-scaleandbehind-the-meterapplications.Lithium-ion’squickresponsetime,longcyclelife,andlimiteddurationlenditselfwelltoshorter-termapplicationsthatmayrequirefrequentanddeepcycling.

6

Currently,lithium-ionisusedinfrequencyresponseandotheressentialgridreliabilityservicesthathelpsystemoperatorsmaintainbalancebetweenloadanddemandatshorttimescales(uptoafewhours)(Bowenetal.2019).Lithium-ionbatterieshavealsoseendeploymentforprovidingpeakingcapacity,chargingduringtimesofenergysurplus,anddischargingduringtimesofhigherdemandtohelputilitiesmeetpeakdemand.Duetoitslimitedduration,lithium-ion’scontributiontosystempeakdemandstronglydependsontheshapeofthedemandcurve(DenholmandMargolis2018).Similarly,lithium-ioncanalsobeusedtoreducegridcongestionanddefertransmissionanddistributionsystemupgradesbystoringenergyduringtimesofexcessgenerationandmeetingloadlocallyduringtimesofhighdemand.

EmergingApplicationsandR&DEfforts

Futureimprovementsinlithium-ionbatteriesareprimarilyfocusedonincreasingenergydensity,increasingthepoweroutputoflithium-ioncells,makingthebatteriessafertooperate,reducingoverallcosts,andreducingrelianceonscarceminerals.Twonovelconfigurationscurrentlybeingexploredare

6“Deep”and“shallow”cyclingareusedtoqualitativelyrefertothedepthofdischargeanenergystoragesystemexperiencesduringoperation.Thedepthofdischargereferstotheshareofthestoragesystem’scapacitythathasbeendischargedandisinverselyrelatedtoitsstateofcharge.Althoughthereisnosetdefinition,deepcyclingmayrefertooperationswhenthestoragesystemdischargesthemajorityofitsstoredenergy(suchaswhileprovidingprolongedpeakingcapacity)whereasshallowcyclingreferstooperationswhenthestoragesystemalternatesbetweencharginganddischargingsuchthatitsstateofchargeremainsrelativelyhigh(suchasprovidingfrequencyregulation).Thedepthofdischargecanhavesignificanteffectsonthelifetimeofthestoragesystem,andtechnologiesvaryintheirsensitivitytothedepthofdischargetheyexperience.

solid-statelithium-ionbatteries,whichusesolidelectrolytesandhaveimprovedenergydensitiesandlowersafetyriskscomparedtoliquid-electrolytelithium-ionbatteries,andlithium-airbatteries,whichhaveimprovedenergydensitiesandhavethepotentialtobeverylowcostandcouldreducerelianceonscarceminerals(Warner2019).

ExampleDeployment

Lithium-ionhasseenextensiveglobaldeploymentintheenergysector.OneprominentexistingprojectistheHornsdalePowerReserve,a100-MW/129-MWhlithium-ionbatteryinSouthAustraliacompletedin2017forfrequencyregulationandtransmissioncongestionrelief.TheSouthAustraliapowersystemisrelativelyisolatedandcandisconnectfromthelargerAustralianpowersystemifthepointofinterconnectionisoverloaded.Oneofthebattery’sadditionalfunctionsistoprovideinjectionsofpowertopreventtheinterconnectionfromdisconnecting.Onatleasttwooccasions,duringeventswhenlargecoalplantstrippedoffline,theHornsdalePowerReserverespondedwithinmillisecondstoimmediatelyinjectlargeamountsofpowerintothegridoverafewminutestosupportthegridfrequencyuntilotherpowerplantscouldincreasetheiroutput,arrestingthefallinfrequencyandpotentiallyavoidingpowerreliabilityissuesanddisconnectionfromthelargergrid(AEMO2018).

In2018,theelectriccooperative,UnitedPower,completedtheinstallationofa4-MW/16-MWh(4-hourduration)lithium-ionbatteryinFirestone,Colorado.Thecooperativeaimstostoreexcessenergyovernightwhendemandislowanduseittomeetpeakdemandduringtheday,reducingoperatingcostsfortheutility.Thelocalutilityexpectstobeabletosave$1millionperyearinavoidedwholesalecapacitycharges(UnitedPower2018).

FlowBatteryEnergyStorage

TechnologySummaryforPolicymakers

Flowbatteriesareintheinitialstagesofcommercialization.Thetechnologyismarkedbylongdurations,theabilitytodeeplydischargeitsstoredenergywithoutdamagingthestoragesystem,andexceedinglylonglifecycles.Flowbatteriesmaybeuniquelysituatedforlongerdurationservicessuchasloadfollowingorpeakingcapacity.Whileflowbatterieshavehigherupfrontcoststhanlithium-ion,theirlongerlifecyclecanleadtosignificantlylowerlifetimecosts.Flowbatteriesarealsotypicallysaferandarelessreliantonrarematerials,dependingonthespecificchemistry.Givenflowbatteries’lowenergyandpowerdensity,thesesystemstendtobelargerthanotherequivalentstoragetechnologies.

Flowbatteryenergystorageisaformofelectrochemicalenergystoragethatconvertsthechemicalenergyinelectro-activematerials,typicallystoredinliquid-basedelectrolytesolutions,directlyintoelectricalenergy(NguyenandSavinell2010).Therearevariousformsofestablishedflowbatteryenergystoragetechnologies,includingredoxflowbatteries(RFBs)andhybridflowbatteries.RFBs,whichincludevanadiumredoxflowandpolysulphidebromideflowbatteries,havetheelectro-activematerialdissolvedinaliquidelectrolytethatisstoredexternaltothebattery.Thebatterychargesanddischargesbasedonredoxreactions,whicharechemicalreactionsbetweentwoelectrolytesolutionsatdifferentoxidationstates.Theelectrolytesaretypicallyliquid-based,separatedbyamembrane,andstoredinlargetanks.

Hybridflowbatteries,whichincludezinc-bromineandzinc-ceriumflowbatteries,haveoneoftheirelectro-activecomponentsdepositedonasolidsurface,asopposedtobeingdissolvedinaliquidelectrolyte(Alotto,Guarnieri,andMoro2014;NguyenandSavinell2010).

TheglobalflowbatterymarketisdominatedbyvanadiumRFBs,whichisthemoststudiedandcommercializedflowbatterytype(MinkeandTurek2018;Weberetal.2018).Zinc-bromine(Zn-Br)andpolysulphidebromideflowbatterieshavealsobeenwidelystudiedwithsomeinitialcommercializationbutfacetechnicalandeconomicbarriersthathavestalledtheircommercialization.Zn-Brbatteriesarerelativelylowcostandexhibithighenergydensity,highdesignflexibility,rapidcharge,andhighdepthofdischargecapabilities,butsufferfromlowcycle-life,lowenergyefficiency,anddendriteformation,whichimpactsperformance.

7

Polysulphidebromideshaverapidresponsesbutsufferfromexpensivematerialrequirements,limitedenergydensity,relativelylowefficiencies(~60%–75%),andcross-contaminationconcernsduringlong-termbatteryoperation.ThesechallengescurrentlymakeZinc-bromineandpolysulphidebromidemoreexpensiveandinefficientthanthemoreestablishedvanadiumRFBs(Fanetal.2020).

Inprinciple,flowbatterieshaveseveraladvantagesoverotherelectrochemicalstoragetechnologies.Astheactiveelectrolyticmaterialisseparatedfromthe