公用事业规模能源 贮存

UnitedStatesGovernmentAccountabilityOfficeReporttoCongressionalAddressees

March2023

GAO-23-105583

TECHNOLOGYASSESSMENT

Utility-ScaleEnergyStorage

TechnologiesandChallengesforanEvolvingGrid

Thecoverimagedisplaysimagesofagas-poweredturbineforelectricitygeneration,andpumpedhydroelectric,flywheel,andbatteryenergystoragetechnologies.

Coversources:GAO(illustration);contributor_aerial/Regan/malp/filins/(photoslefttoright).|GAO-23-105583

Highlightsof

GAO-23-105583,

areporttocongressionaladdressees

March2023

WhyGAOdidthisstudy

TheU.S.electricitygridconnectsmorethan11,000powerplantswitharound158millionresidential,commercial,andotherconsumers.Energystoragetechnologieshavethepotentialtoenableseveralimprovementstothegrid,suchasreducingcostsandimprovingreliability.Theycouldalsoenablethegrowthofsolarandwindenergygeneration.

GAOconductedatechnologyassessmenton(1)technologiesthatcouldbeusedtocaptureenergyforlaterusewithintheelectricitygrid,

(2)challengesthatcouldimpactenergystoragetechnologiesandtheiruseonthegrid,and(3)policyoptionsthatcouldhelpaddressenergystoragechallenges.

Toaddresstheseobjectives,GAOreviewedagencydocumentsandotherliterature;interviewedgovernment,industry,academic,andpowercompanyrepresentatives;conductedsitevisits;andconvenedavirtualmeetingofexpertsincollaborationwiththeNationalAcademiesofSciences,Engineering,andMedicine.GAOisidentifyingpolicyoptionsinthisreport(seep.2).

View

GAO-23-105583.

Formoreinformation,contactBrianBothwellat(202)512-6888,

bothwellb@.

TECHNOLOGYASSESSMENT

Utility-ScaleEnergyStorage

TechnologiesandChallengesforanEvolvingGrid

WhatGAOfound

Technologiestostoreenergyattheutility-scalecouldhelpimprovegridreliability,reducecosts,andpromotetheincreasedadoptionofvariablerenewableenergysourcessuchassolarandwind.Energystoragetechnologyusehasincreasedalongwithsolarandwindenergy.SeveralstoragetechnologiesareinuseontheU.S.grid,includingpumpedhydroelectricstorage,batteries,compressedair,andflywheels(seefigure).Pumpedhydroelectricandcompressedairenergystoragecanbeusedtostoreexcessenergyforapplicationsrequiring10ormorehoursofstorage.

Lithium-ionbatteriesandflywheelsareusedforshorter-durationapplicationssuchaskeepingthegridstablebyquicklyabsorbingordischargingelectricitytomatchdemand.Flowbatteriesrepresentasmallfractionoftotalenergystoragecapacityandcouldbeusedforapplicationsrequiring10ormorehoursofstorage.Metal-airbatteriesarebeingevaluatedforapplicationsrequiring10ormorehoursofstorage.

PumpedHydroelectric(left)andLithium-IonBattery(right)EnergyStorageTechnologies

Energystoragetechnologiesfacemultiplechallenges,including:

Planning.Planningisneededtointegratestoragetechnologieswiththeexistinggrid.However,accurateprojectionsofeachtechnology’scostsandbenefitscouldbedifficulttoquantify.Further,refinementofcosts,benefits,andotherdataareneededtoinformtheplanningprocess.

Regulation.Rulesandregulationsvaryacrossregionsandstates,whichforcesenergystorageprojectdeveloperstonavigateapatchworkofpotentialmarkets.Developersthatwanttodeploystorageacrossmultiplemarketsmayneedtoconductseparateanalysestodetermineeachregion’sregulatoryoutlookandprofitpotential.

Standardization.Codesandstandardsmayneedrevisingandmustkeeppacewithmaturingtechnologiestominimizepublicsafetyandwelfarerisks.However,thetechnology’sevolutionanddeploymentisoutpacingcodesandstandardsdevelopment.Asaresult,entitiesseekingtodeploynewtechnologiesmayfacechallengesapplyingexistingcodesandstandardstonewtechnologies.

Valuation.Realizingthepotentialofenergystoragetechnologiesmaydependontheabilitytovalueinvestments.Forexample,profitpotentialcanvarybecauseregionsandstatesvaluestoragedifferently,reflectinglocalmarketrulesandregulations.

UnitedStatesGovernmentAccountabilityOffice

UnitedStatesGovernmentAccountabilityOffice

GAOdevelopedsixhigh-levelpolicyoptionsinresponsetothesechallenges.Thesepolicyoptionsareprovidedtoinformpolicymakersofpotentialactionstoaddressthepolicychallengesidentifiedinthistechnologyassessment.Theyidentifypossibleactionsbypolicymakers,whichincludeCongress,federalagencies,stateandlocalgovernments,academicandresearchinstitutions,andindustry.Thestatusquooptionillustratesascenarioinwhichpolicymakersdonotintervenewithongoingefforts.

PolicyOptionstoAddressChallengestoUtility-ScaleEnergyStorage

Policyoptionsandimplementationapproaches

Opportunities

Considerations

Statusquo(reportp.

48

)

Policymakerscouldmaintainthestatusquothrough:

Taxcreditsandfunding

Researchanddevelopment

Previousplansandprogramsbystateswouldcontinue,includingactionsforenergystorage.

Thefederalgovernmenthasvariousnationalcapabilitiestosupportenergystoragetechnologyincentivesanddemonstration.

DOEsupportforstorageresearchanddevelopmentwouldcontinue.

Somepolicymakersmaylacksufficientinformationtomakedecisionsonevolvingstoragecapabilities.

Storagedevelopment,deployment,andusecouldbeleftdependentonforcesoutsidepolicymakers’control.

Integration(reportp.

50

)

Policymakerscouldincludecleargoalsandnextstepsinplanstohelpintegratestorage,by:

Establishingroadmaps,basedonstoragecostsandbenefits

Assessingstorageinplans

Storageplanningcouldhelppolicymakersidentifyandremovebarrierstoenergystoragedeployment.

Planscouldincreaseinvestors’confidenceandhelpthemdeterminestorageinvestments.

Plansthatseektoalterconventionalgridplanningcouldbedifficulttoexecute.

Stakeholdershavesetdifferentgoalsforlow-carbonelectricgeneration.

Planningdependsonfactorssuchaslocationsuitability;noteverytechnologyissuitedforeverylocation.

Regulation(reportp.

52

)

Policymakerscouldreviseandenactrulesandrequirementsforhowstorageisdefined,used,orownedby:

Identifyingmarketbarriers

Establishingtargetsormandates

Modernizingownershipmodels

Couldpromoteenergystoragetechnologiesbyimprovinggridefficiencywhilereducingcostsforallcustomers.

Couldhelplowercostsandreducethetimelineforinterconnection.

Couldacceleratepermitapprovaltimelines.

Regulationsdifferacrossstates,whichcouldmakefindingtherightregulatorymodeltoachieveenergygoalsachallenge.

Integratingnewtechnologieswithconventionalgridplanningcanbechallenging.

Changestorulesandregulationscouldexcludecertaintechnologies.

Standardization(reportp.

54

)Policymakerscouldupdateorcreatenewcodesandstandardsandprovideeducationonstoragesafetyrisks.

Couldhelpstakeholdersoperatestoragesystemsmoresafely.

Standardsplacedintoregulations

couldhelpaddressstorageperformancerequirements.

Codesandstandardstaketimetodevelopandcouldbeoutdatedifnotadoptedinatimelymanner.

Standardsmaybeambiguous,whichcouldmakeitdifficulttodesignstoragesystems.

Supportmanufacturingandadoption(reportp.

56

)Policymakerscouldsupportactionstohelpenergystoragemanufacturingandadoptionchallengesby:

Enactingbatteryreuseandrecyclingpolicies

Conductingoutreach

Targetingactivitiestosupportstoragedevelopmentanddeployment

Reuseandrecyclingpoliciescouldincreasetherecoveryofproductsandmaterials.

Stakeholderoutreachandinformationalprogramscouldhelpovercomeawarenessandfamiliaritychallenges.

Federalandstatefinancialsupportforlonger-durationenergystoragedevelopmentanddemonstrationcouldbeimportantinafutureelectricitysystempoweredbywindandsolargeneration.

Incentivesandmotivationtoinvestinnewrecyclingapplicationsislimited.

Fundingmayfluctuateyeartoyearorfavorshort-termprojects.

Developmentofnewsystemscouldbedifficultbecauseofengineeringandeconomicuncertainty,particularlyforlonger-durationstorage.

Low-cost,flexiblenaturalgasgenerationcouldmakeitmoredifficultfornewpumpedhydroelectricfacilitiestocompete.

Provideincentives(reportp.

58

)Policymakerscouldcreatemechanismstoincentivizestoragedeployment,by:

Providingincentives,suchasloanguaranteesortaxcredits

Consideringpoliciestoencouragethecaptureofmultiplerevenuestreams

Financialincentivescouldhelpdevelopersandcompaniesdevelopstoragetechnologies.

Technologieswithlongerdurationsmaybenefitfrompoliciesthathelpindustrytocapturetheirfullvalue.

Incentivescouldleadtounintendedoutcomesforgovernmentsordevelopers,andsomestakeholdersmaynotbelievetheyarenecessary.

Technologyvaluevariesbyregion,whichmayaffectstorageincentives,valuation,andrevenuestreams.

Environmentalandsocialcostsandbenefitscouldbedifficulttoquantify.

Source:GAO.|GAO-23-105583

ThisisaworkoftheU.S.governmentandisnotsubjecttocopyrightprotectionintheUnitedStates.ThepublishedproductmaybereproducedanddistributedinitsentiretywithoutfurtherpermissionfromGAO.However,becausethisworkmaycontaincopyrightedimagesorothermaterial,permissionfromthecopyrightholdermaybenecessaryifyouwishtoreproducethismaterialseparately.

Utility-ScaleEnergyStorageGAO-23-105583

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TableofContents

Introduction 1

Background 3

Howdoesthegridwork? 3

Whatisenergystorage? 9

Whyenergystorage? 10

Historyofenergystoragetechnologies 12

Factorsaffectingeconomicviability 13

Legalandregulatoryconsiderations 15

Utility-ScaleEnergyStorageTechnologies 18

Multiplestoragetechnologiesareavailable 18

Differentenergystoragedurationshavedifferentusesonthegrid 35

SeveralChallengesMayHinderEnergyStorageTechnologyDevelopmentandUse 39

Planningforstoragetechnologies 39

Challengingregulatoryenvironment 42

Existingcodesandstandardsdonotfullyaddressenergystoragetechnologies 43

Crosscuttingchallenges 44

Valuingenergystorage 46

PolicyOptionstoAddressEnergyStorageTechnologyChallenges 48

Statusquo 48

Integratingstoragetechnologies 50

Revisingandenactingrulesandrequirements 52

Updatingorcreatingcodesandstandards 54

Addressingcrosscuttingchallenges 56

Incentivizingenergystorage 58

AgencyandExpertComments 61

AppendixI:Objectives,Scope,andMethodology 63

AppendixII:ExpertParticipation 68

AppendixIII:GAOContactsandStaffAcknowledgments 69

Figures

Figure1:Theelectricitygrid 4

Figure2:Exampledepictingelectricitystystemload

5

Figure3:U.S.electricpowermarketsandinterconnections 7

Figure4:Independentsystemopertorsandregionaltransmissionorganizations

8

Figure5:Selectedenergystoragetechnologyperformancecharacteristics 9

Figure6:Examplesofenergystorageapplicationsontheelectricitygrid

11

Figure7:Hypotheticalexampleofcurtailedwindenergyonagrid

usingsimulateddata 11

Figure8:Totalinstalledcosts(energycapacity)oflarge-scalebatterystorage

systemsfrom2015-2019

13

Figure9:Percentofutility-scaleenergystorageinoperationbytechnologytype 19

Figure10:Simplifiedinterconnectionstudyprocess

41

Figure11:Examplesofstateenergystorageefforts 51

Abbreviations

DOE

DepartmentofEnergy

EIA

EnergyInformationAdministration

FERC

FederalEnergyRegulatoryCommission

ISO

IndependentSystemOperator

MW

Megawatt

NationalAcademies

NationalAcademiesofSciences,Engineering,andMedicine

NERC

NorthAmericanElectricReliabilityCorporation

RTO

RegionalTransmissionOrganization

Utility-ScaleEnergyStorageGAO-18-3071

441GSt.N.W.

Washington,DC20548

March30,2023CongressionalAddressees

Energystoragetechnologies—suchasbatteries,flywheels,compressedair,andpumpedhydroelectricpower—haveseveralpotentialbenefits.

1

Forexample,theabilitytostoreenergy—especiallyforseveralhoursorlonger—couldreducecosts,increasetheelectricitygrid’sreliability,andimproveitsabilitytorecoverfromdisruptions.Storagetechnologiescouldalsopromoteincreasedadoptionofrenewableenergysourcessuchassolarandwindbycapturingtheirexcesspowerandreturningittothegridwhenthesesourcesarelessavailable.However,energystorage,alongwithrenewableenergygeneration,mayrequirechangesinthewaythepowersystemisorganizedandoperated.

2

Thefederalgovernmenthastakenseveralstepstoexploreorpromoteenergystoragetechnologies.Forexample,in2021theInfrastructureInvestmentandJobsActappropriated

$505milliontotheDepartmentofEnergy(DOE)forenergystoragedemonstrationprojectsforfiscalyears2022to2025.

3

TheactalsorequiredDOEtostudycodesandstandardsforenergystoragesystemsandestablishagrantprogramtoenhanceU.S.batterymanufacturing.Further,theInflationReductionActof2022createdandexpandedtaxcreditsforinvestmentinenergystoragetechnology.

4

Withintheexecutivebranch,theFederalEnergyRegulatoryCommission(FERC)issuedordersin2018and2019toremovebarrierstomarketparticipationforenergystoragetechnologies.

WepreparedthisreportundertheauthorityoftheComptrollerGeneraltoassistCongresswithitsoversightresponsibilities,inlightofbroadcongressionalinterestinutility-scaleenergystoragetechnologies.

5

Weexamined(1)technologiesthatcouldbeusedtocaptureenergyforlaterusewithintheelectricitygrid,(2)challengesthatcouldimpactenergystoragetechnologies

1

Forthepurposesofthisreport,wediscusspumpedhydroelectricstorage;lithiumion,andotherbatterytechnologies;compressedairenergystorage;andflywheelsasexamplesofenergystoragetechnologies.Wedonotdiscussconcentratedsolarthermalenergyforthisreport,becauseitcannottakeenergyfromthegrid,orhydrogen,becauseitwasnotsufficientlywellestablishedduringourreview.

2

Energystoragetechnologiesaresystemsthatarecapableofreceivingelectricenergyfromthegridandstoringitforlaterinjectionofelectricenergybacktothegrid.

3

Pub.L.No.117-58,135Stat.429(2021).

4

Pub.L.No.117-169,§13102,136Stat.1818,1913-21.

5

Forthepurposesofthisreport,wearedefiningutility-scaleassystemsthathaveatleast1megawatt(MW)ofoutput,arelocatedinacentralizedlocation,andareontheutility’ssideofthemeter.

Utility-ScaleEnergyStorageGAO-23-105583

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andtheiruseonthegrid,and(3)policyoptionsthatcouldhelpaddressenergystoragechallenges.

Wefocusedthistechnologyassessmentonutility-scaleenergystoragesystems,selectingpumpedhydroelectricstorage,batteries,compressedairenergystorage,andflywheelsasexampletechnologies.Wedonotdiscussconcentratedsolarthermalenergyinthisreportbecauseitcannottakeenergyfromthegrid,andhydrogenbecauseitwasnotsufficientlyestablishedduringourreview.Wereviewedagencydocumentsandotherliterature;interviewedagencyofficials,expertsandstakeholdersfromindustry,andpowercompanies,amongothers;conductedsitevisits;andheldavirtualmeetingofexperts.Themeetingincludedanon-generalizablesampleof15expertsselectedbasedontheirtechnical,economic,regulatory,operational,orpolicyexpertise.SeeappendixIforadetaileddescriptionofourobjectives,scope,andmethodology.

WeconductedourworkfromDecember2021toMarch2023inaccordancewithallsectionsofGAO’sQualityAssuranceFrameworkthatarerelevanttotechnologyassessments.Theframeworkrequiresthatweplanandperformtheengagementtoobtainsufficientandappropriateevidencetomeetourstatedobjectivesandtodiscussanylimitationstoourwork.Webelievethattheinformationanddataobtained,andtheanalysisconducted,provideareasonablebasisforanyfindingsandconclusionsinthisproduct.

Background

Howdoesthegridwork?

Theelectricitygridisamassivefeatofengineering,whichoneauthorcalled“themostcomplexmachineevermade.”

6

IntheU.S.,itconnectsmorethan11,000powerplantswithover158millionresidential,commercial,andothercustomersviamillionsofpowerlines.Ithasfourdistinctfunctions:generation,electricitytransmission,distribution,andgridoperations.

7

Seefigure1

6

Schewe,PhillipF.,Thegrid:ajourneythroughtheheartofourelectrifiedworld(Washington,DC:JosephHenryPress,2007).

forarepresentationofthegrid.Powerplantsgenerateelectricitybyconvertingotherformsofenergy,suchaschemicalenergyfromfuel,mechanicalenergyfromwindorwater,andnuclearenergy.Oncegenerated,electricityisauniformresourcethatisinterchangeablewithelectricityfromanyothersource.Thegridcarriesthiselectricityfirstthroughhigh-voltage,high-capacitytransmissionlines.Theelectricityisthentransformedtoalowervoltageandsentthroughthelocaldistributionlinestohomesandbusinesses.

7

Generationfacilitiesproduceelectricity.Transmissionlinesmoveelectricitybetweenpowerplantsandpointswhereitisdeliveredtocustomersorotherelectricsystems.Distributiondeliversenergytoretailcustomers.

Gridoperatorsmustensurethatelectricitysupplyconstantlymatchespowerdemand.Thisbalancingactrequiresthemtoforecastelectricitydemandandscheduleandoperatepowerplantstomeetdemand,whichvariesbytimeofdayandyear,sinceitisdifficulttoeconomicallystorelargequantitiesofelectricity.Assuch,electricitymustbeproducedtheinstantitisneededandused.Todothis,gridoperatorssendminute-by-minutesignalstopowerplantstoadjust

output.Onekeypatterntheymustfollowistheriseinconsumerelectricitydemandthroughouttheday,inmanyareas,reachingpeakdemandinthelateafternoonorearlyevening.Typically,gridoperatorsuseasteadyflowofelectricityfrombaseloadpowerplants,whichruncontinuouslyandaretheleastexpensivetooperate.Asdemandincreasestoitspeak,operatorsprogressivelyincreasetheelectricitysuppliedbypeakerplants—electricitygeneratorsreservedfor

operationduringthehoursofhighestdaily,weekly,orseasonalelectricityloads—andothergeneratorsthataremoreexpensivetooperatebutcanbequicklybroughtonline(seefig.2).

aPeakinggenerationiselectricityreservedforoperatingduringthehoursofhighestdaily,weekly,orseasonalelectricityloads.

bIntermediateloadgenerationisnormallyoperatedonadailycycletoserveon-peakloadsduringtheday,butnotoff-peakloadsduringnightsandweekends.

cBaseloadgenerationservestheminimumlevelofelectricpowerdemandofaregion,orcustomerrequiredoveragivenperiodoftimeatasteadyrate.

dRenewablesgenerationrepresentsvariablegenerationprimarilyfromwindorsolarsources,whosepeakgenerationdoesnotnecessarilycoincidewithelectricitysystemperiodsofpeakdemand.

Severalfactorshavemadethetaskofmatchingelectricitysupplyanddemandevenmorecomplex.Variableelectricitysourcessuchaswindandsolarpoweraresupplyinganincreasingshareofelectricity,buttheiroutputvarieswiththeweatheranddoesnotalwaysmatchdemand.Further,theincreasinguseofvariableenergyresources,interactionofsuchenergysourceswithtraditionalgenerationsources,andchangingroleofelectricitycustomershaveincreasedthe

complexityofmatchingelectricitysupplywithdemandatalltimes.

Gridoperatorsconductplanningactivitiestodeterminegridinfrastructureadequacy,identifycapacityneeds,andevaluatethecostandeffectivenessofpotentialsolutionstoaddresstheseneeds.Utilitiesdealwithuncertaintypartlybyproducingarangeofforecastsbasedondemographicandeconomicfactors,andbymaintainingexcessgenerationcapacity,knownasreserves.

Additionally,utilitiesusemodelstohelpchoosetheleast-costcombinationofelectricitygeneratingresourcestomeetdemandinordertoreducecosts.Stateregulatorsapproveofutilityinvestmentsbeforefacilitiesarebuiltorwhenutilitiesseektorecovercostsintheratesconsumersarecharged.Further,somestatesuseintegratedresourceplanningprocessestodeterminewhichfacilitiesshouldbebuilt.Thisprocessisintendedtomeetfuturepowerdemandbyidentifyingtheneedforgeneratingcapacityanddeterminingthebestresourcemixtomeetsystemneedsatthelowestcosts.

Theelectricitygridinthelower48statesismadeupofthreemainparts,knownasinterconnections,whichoperatelargelyindependentofeachother,withlimited

powertransfersbetweenthem.

8

Seefigure3formapsofinterconnectionsandU.S.electricpowermarkets.Further,howpowerisboughtandsoldvariesbyregionandthereisamixofregulatorymarketenvironments.Someutilitiesmayoperateunderamixofmarketenvironments.Further,someutilitiesmaybeinvestor-ownedandregulatedbypublicpolicy,whileothersmaybepubliclyownedandregulatedthroughtheirownership,inadditiontomanystateandfederallaws.U.S.utilitiesoperateintraditionallyregulatedandderegulatedmarkets.

Traditionallyregulatedmarkets.Intraditionallyregulatedmarkets,utilitiesaretypicallysolelyresponsiblefor

8

TheWestern,Eastern,andElectricReliabilityCouncilofTexas(ERCOT)interconnectionsconsistofbalancingauthoritieswhichcanbeindependentsystemoperators,regionaltransmissionorganizations,orindividualpowercompanies.

Balancingauthoritieshavebalancingresponsibilitiesforaspecificportionofthepowersystemandensurethatpowersystemsupplyanddemandarebalanced,whichisrequiredtomaintainsafeandreliableoperationofthepowersystem.

generating,transmitting,anddistributingelectricitytotheircustomers.

Deregulatedmarkets.Inderegulatedmarkets,utilitiesthatserveretailcustomerscannotownpowerplants;theyareonlyresponsiblefordeliveringelectricitytocustomers,andforcustomerbilling.

9

Insuchmarkets,electricitygeneratingentitiestypicallyselltheelectricitytheygeneratethroughcompetitivepowermarkets.Independentsystemoperators(ISO)andregionaltransmissionorganizations(RTO),formedinresponsetoFERCorders,aregroupsthatcoordinate,control,andmonitortheelectricgridintheseareas.Seefigure4foramapofISOsandRTOs.

9

Transmissionsystemsarelinesandequipmentthatmoveelectricityfromwhereitissuppliedtowhereitisdeliveredtocustomersorothersystems.

aPJMinterconnectionandSouthwestPowerPoolareRegionalTransmissionOrganizations.

Responsibilityforpowerindustryregulationisdividedamongstatesandthefederalgovernment.Forexample,theFederalPowerActgivesFERCtheresponsibilitytoregulatethetransmissionandwholesalesaleofelectricityininterstatecommerce,andtoensurethattheratesforsuchtransmissionandwholesalesalesarejustandreasonable.

10

Stateentities,suchaspublicutilitycommissions,regulateutilitymanagement,operations,andelectricityratestructures.Insomeregions,ISO’sandRTOsmanageelectricitytransmissionandwholesaleelectricitymarkets.AccordingtotheNational

10

16U.S.C.§§824,824d.

AcademiesofSciences,Engineering,andMedicine(NationalAcademies),thisdividedresponsibilitycontributestomakingitdifficulttomakegeneralizationsaboutmanyaspectsoftheU.S.electricitysystem.

AccordingtoaNationalAcademiesconsensusstudy,itcanbechallengingtodeterminewhoisinchargeofplanning,developing,andensuringfuturepowersystemintegrity.

11

IntheU.S.,nosingleplannerordesignerisresponsiblefortheelectricitysystem.Thegridhasbeendevelopedinanincrementalandpiecemealprocessdrivenbythesometimes

11

NationalAcademiesofSciences,Engineering,andMedicine,TheFutureofElectricPowerintheUnitedStates(Washington,D.C.:NationalAcademiesPress.2021).

divergentinterestsoffederal,state,regional,andlocalauthoritiesoperatingdifferentlyintheirrespectiveareas.Thisincrementalprocesshasshapedhowthegridhasevolved,andaccordingtothisNationalAcademiesstudy,howitwillcontinuetoevolve.

Whatisenergystorage?

Typesofenergystoragetechnologiesincludepumpedhydroelectricstorage,lithium-ionandotherbatterytechnologies,compressedairenergystorage,andflywheels.

12

Thesetechnologieshavedifferentperformancecharacteristicsthatmaymakethemmoresuitableforsomegridservicesthanothers.Forexample,theyhavedifferentroundtripefficiencies,ameasureoftheamountofenergylostwhentheenergystoragesystemchargesanddischarges.Theyalsohavemanydifferentdurationtimes—theamountoftimethatastoragetechnologycanproduceelectricity.Thesedurationsrangefromsecondstohours.Theyalsohavedifferentcapacities,ormaximumamountsofpowerthattheycandischargeontothegrid.

Capacitycanreach1,000megawatts(MW)forpumpedhydroelectricandcompressedairenergystoragesystems.

13

Technologieslikebatteriesandflywheelshavesmallercapacitiesandshorterdischargetimes.Seefigure5forinformationonselectedtechnologypower,themaximumamountofelectricitythatthestoragecanprovide,andduration.

12

Wedonotdiscussconcentratedsolarthermalenergyforthisreportbecauseitcannottakeenergyfromthegrid,andhydrogenbecauseitwasnotsufficientlyestablishedduringourreview.

13

Amegawatt(MW)isaunitofelectricpower.Onegigawattis1,000megawatts.Abatterywith1MWcapacityanda

Note:Becausetechnologycharacteristicsarequicklyevolvingthisfiguremaynotberepresentativeofthefullrangeoftechnologycapabilities.

Thegridwasnotdesignedwithadvancedenergystorageinmind.Energystoragemaybechallengingtointegratewiththeexistinginfrastructurebecauseitmaynotfitintotheexistingpolicyandregulatoryframework.Forexample,itmayactastransmission,electricitydemand,andinfrastructure,alongwithitsabilitytoshift