储能技术方案

PDHonlineCourseE320(4PDH)

EnergyStorageTechnology

Instructor:LeeLayton,PE

2020

PDHOnline|PDHCenter

5272MeadowEstatesDriveFairfax,VA22030-6658

Phone:703-988-0088

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www.PDH

PDHonlineCourseE320

www.PDH

©LeeLayton.

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EnergyStorageTechnology

TableofContents

Section Page

Introduction:BottlingElectricity 3

Chapter1–EnergyStorageApplications 9

Chapter2–BatteryEnergyStorage 20

Chapter3–Non-ElectrochemicalStorage 36

Chapter4–Plug-inHybridElectricVehicles 43

Summary 45

Introduction:“BottlingElectricity”

Electricityisoneofthemajorcommoditiesinoureconomyanditisoneofthefewcommoditiesthatneverhadaneconomicalorpracticalmethodtostoretheproduct.

Electricpowerisproducedanddeliveredatvirtuallytheinstantitisdemanded.

Generationandtransmissionsystemsmustbedesignedtomeetthepeakinstantaneous

demandthatmayoccuronthesystem.Consideringalittleextracapacityforreliability,thishasresultedinamodelwherethecapacityfactoroftheentiresystemislessthan50%.Althoughitisdifficulttostoreelectricitydirectly,electricenergycanbestoredinotherforms,suchaspotential,chemical,orkineticenergy.Advancedenergystoragetechnologiesbasedontheseprinciplesareemergingasapotentialresourceinsupportinganefficientelectricitymarket.Thetermenergystoragerefersspecificallytothecapabilityofstoringenergythathasalreadybeengeneratedaselectricityandcontrollablyreleasingitforuseatanothertime.

Onlyabout2.5%ofthetotalelectricpowerdeliveredintheUnitedStatespassesthroughenergystorage,almostallofwhichispumpedhydroelectricstorage.Therestructuringoftheelectricityindustry,alongwithincreasedrequirementsforpowerreliabilityandquality,hasmadeutility-scaleenergystorageasubjectofcurrentinterest.

Althoughthepresent-dayelectricgridoperateseffectivelywithoutstorage,cost-effectivewaysofstoringelectricalenergycanhelpmakethegridmoreefficientandreliable.Electricenergystorage(EES)canbeusedtoaccumulateexcesselectricitygeneratedatoff-peakhoursanddischargeitatpeakhours.Thisapplicationcouldyieldsignificantbenefitsincludingareducedneedforpeakgenerationandreducedstrainontransmissionanddistributionnetworks.energystoragecanalsoprovidecriticallyimportantancillaryservicessuchasgridfrequencyregulation,voltagesupport,andoperatingreserves,therebyenhancinggridstabilityandreliability.

Technicalapplicationsofenergystorageincludegridstabilization,gridoperationalsupport,powerqualityandreliability,loadshifting,andcompensatingforthevariabilityofrenewableenergysources.Restructuredelectricitymarketsprovideopportunitiesforenergystoragetoparticipateinenergyarbitrageandancillaryservices.

Thefirstapplicationoflarge-scaleenergystorageintheUnitedStatesoccurredin1929,whenthefirstpumpedhydroelectricpowerplantwasplacedintoservice.Pumpingwaterfromalowerelevationtoahigherelevationwasthemostpracticalwaytostorelargeamountsofenergythatcouldthenbereleasedduringperiodsofhigh,orpeak,demand.Thesepowerplantsarestillused

tohelpmanagegridfrequencyandprovidecleanreservegeneration,knownasancillaryservices.Duringa30-yearperiodfromthelate1950stothelate1980s,approximately19,500MWofpumpedhydroelectricstoragefacilitieswerebroughtintoserviceintheUnitedStates.By2000,about3%ofthetotalpowerdeliveredbythenation’sgridwassuppliedthroughtheseenergystoragefacilities.Becauseoftheneedforsignificantelevationchangesinpumpedhydroelectricplantdesigns,thenumberofenvironmentallyacceptablesitesforfuturepumpedhydroelectricfacilitiesislimited.Thesitingofnewplantswillfacethesameobjectionsthatthesitingofnewtransmissionlinesfacestoday.

Anotherenergystoragetechnologyiscompressedairenergystorage(CAES).ACAESdemonstrationpowerplantwasplacedinserviceintheearly1990sandhasproventobeeffective.Undergroundformations,suchassaltdomesanddepletedgasfields,canbeadaptedforusewithCAEStechnology.Thesesystemsappeartobepracticalinapowerrangefromabove100MWuptoseveralthousandMW.

Themostcommonformofenergystorageinusetodayisbasedonbatteries.TheisalargeinstalledbaseofleadacidbatteriesinUPSsystem.TherapidgrowthoftheinformationagehasspawnedtheconstructionofdatacenterstosupporttheInternetandcommunicationscenters.

Thesefacilitiesaresensitivetopowersupplydisruptions,solargebattery-poweredprotectionsystemshavebeenandwillcontinuetobedeployedtoachieveahighlevelofprotection.

Poweringthesetypesofloadscurrentlyaccountsforover1.5%ofthetotalutilitypowerconsumptionintheUnitedStates.

Thereareseveralotherelectrochemicaltechnologiesinuseforelectricbackuppowerapplications.Thesebatterytechnologiesarealsobeinginvestigatedordeployedforutility-scaleapplications.Batterytechnologiesincludelithiumion,sodiumsulfur,zinc-bromine,vanadiumredox,andpolysulfide-bromideredoxflowbatteries,amongothers.

Thetwomainclassesofbatteriesinthisdistributedenergystoragecategoryareflowbatteriesandhigh-temperaturebatteriessuchassodiumsulfurandsodiumnickelchloridebatteries.

Industryexpertshavefoundthat,unlikelead-acidbatteries,thesedevicescancycledailyandhaveusefuloperatinglivesintherangeof10to20years.Thesesystemscanbedesignedforcharge/dischargedurationsuptoeighthoursperday.Allthesedevicesarescaledchemistrieswithnoemissionsandquietoperation.

Flowbatterytechnologyutilizesanactiveelementinaliquidelectrolytethatispumpedthroughamembranelikeafuelcelltoproduceanelectricalcurrent.Thesystem’spowerratingisdeterminedbythesizeandnumberofmembranes,andtheruntime(hours)isbasedonthegallonsofelectrolytepumpedthroughthemembranes.Pumpinginonedirectionproducespowerfromthebattery,andreversingtheflowchargesthesystem.

High-temperaturebatteriesoperateabove250Candutilizemoltenmaterialstoserveasthepositiveandnegativeelementsofthebattery.Thesechemistriesproducebatterysystemswithveryhigh-powerdensitiesthatservewellforstoringlargeamountsofenergy.TheSodium-Sulfurbattery,suchastheunitshownontheright,iscurrentlybeingdeployedintheUnitedStatesbyseverallargeutilitiesindemonstrationprojects.

Otherenergystoragedevicessuchasflywheelsandsupercapacitorsarebeingappliedforpowerqualityapplicationsandfrequencyregulationforutilitiesandotherload-balancingusestoreduceemissionsfromdieselgenerator-powereddevicessuchasportcranes.Forthesesystems,energystorageismeasuredinminutes.

OneenergystoragetechnologythatmaybethefutureofutilityenergystorageisPlug-inHybridElectricVehicles(PHEVs).Theacceptanceofthesevehiclesandtheensuingrateofadoptionbythepublicwilldeterminethetimingoftheirimpactontheoverallpowerdemandoftheutilitygrid.AssumingmostchargingofPHEVsoccursatnight,therelativeimpactonthegridovertimeshouldbepositiveinconjunctionwiththeanticipatedsignificantgrowthofwindenergy.

UncontrolleddaytimeorearlyeveningchargingbyPHEVs,bycontrast,couldposechallengestosystemeconomicsandcapacity,astheextrademandcouldincreasecongestionorpeakuse.

Therearemanybenefitstodeployingenergystoragetechnologiesintothenation’sgrid.Energystoragecanprovide:

Ameanstoimprovegridoptimizationforbulkpowerproduction.

Awaytofacilitatepowersystembalancinginsystemsthathavevariableordiurnalrenewableenergysources.

Facilitationofintegrationofplug-inhybridelectricvehicle(PHEV)powerdemandswiththegrid.

Awaytodeferinvestmentsintransmissionanddistribution(T&D)infrastructuretomeetpeakloadsforatime.

Aresourceprovidingancillaryservicesdirectlytogrid/marketoperators.

Dependingupontheprincipalapplicationoftheenergystoragetechnology,energystoragemaybeviewedasageneration,transmission,distribution,orend-userresource.

PumpedhydroelectricandCAEStechnologiesareconsideredbulkpowerenergystoragesystems.Incontrast,newclassesofbatterieshavebeendevelopedthatareconsideredsuitableforsmallerapplicationsandarereferredtoas“distributed”utilitystoragesystems.(Inthiscontext,theterm“distributed”isusedasadifferentiationfrom“largecentralized”energystoragetechnologies,analogoustolarge-centralizedpowerplants.)Thetermdistributedenergystoragemeansdeploymentofthesedevicesclosetoloadcenters,transmissionsystempointsofreinforcement,orrenewablegenerationsources,typicallyinornearutilitysubstations.Inothercontexts,theterm“distributed”denoteslocationondistributionfeedercircuitsoratconsumerpremisesbehindthemeter.

Islanding:Continuingtopoweraportionofagridindependentlyfromtheutilitysource.

Fullintegrationofnewsourcesofenergydemandcoupledwiththeoverallincreaseinelectricityuseisamajorchallengefacingthedesignersoftheelectricgridofthefuture.Energystoragetechnologiesneedtobeexaminedcloselytounderstandwherestoragecanaddvaluetotheoverallelectricityinfrastructure.Examplesofthevalueofenergy

storagetechnologiescouldincludecapitaldeferral,energymaintenanceduringislanding,andbetterutilizationofgenerationincoordinationwiththevariableoutputnatureofrenewableenergygeneration.

Theratioofstorageenergycapacitytocharge/dischargepowerrating,orthedurationoftheenergystoragethatisrequired,variesdependingupontheapplicationandfavorsdifferenttechnologiesaccordingly.Energydensity,cost,efficiencies,andenvironmentalconcernsareadditionalfactorsthataffecttheapplicabilityofdifferenttechnologiestodifferentpurposes.TheelectricvehicleapplicationdrivesmostR&Dforadvancedmaterialstoday,butitshouldbenotedthatitisalsothemostdemandingapplicationandthustheonethatjustifieshighercosts.Inthelongterm,thebestenergystoragetechnologiesforutility-scaleapplicationsmaybedifferentfromthoseusedforelectric-drivevehicles.

Determiningtheamountandoverallvalueofenergystoragethatshouldbeaddedtothegridbeginswithanexaminationofthemarginalcostofgeneratingelectricity.Theelectricpowerindustryrunsatlowcapacityfactors.Thislevelofcapacityhasbeenacceptabletotheindustrybecausegenerationresourceshavetraditionallybeenmorecost-effectivesourcesofcapacitythanenergystorageresources.Thegrowthofrenewableenergywilllikelyleadtoevenlowercapacityfactorsfortraditionalgenerationsources.

ManyofthedriversforaSmartgridarebasedonadesiretoimprovecapacityfactorsbyshiftingthedemandcurvethrougheitherincentivesorcontrols.Beyondsomepointthatremainstobedetermined,thereislikelytobesomepublicresistancetothedegreeofloadshiftingentailedinthedeploymentofdemandresponseprograms.Energystoragetechnologyoffersanotherpathtohelpbalancethesystemtoadaptproductiontodemandwhileimprovingcapacityfactors.

Anotherpositiveaspectoftheimplementationofenergystoragetechnologiesisthepotentialtocaptureandstoreelectricityfromwindenergywhenthereisalackoftransmissioninfrastructure.Forexample,windcurtailmenthasalreadybecomecommoninTexasbecauseofalackoftransmissioncapacitytomovethatpowerfromwesternTexastoloadcentersinotherpartsofthestate.Inmanyregions,includingTexas,transmissionprojectsaremovingforwardtobetterconnectwindpowerplantswithloadcenters,althoughenergystoragetechnologiesmayhavepotentialvalueintheinterim.Inaddition,aswindpowerdeploymentincreases,windoutputmaybegintoexceedelectricitydemandduringcertaintimesoftheyear,whichwouldnecessitatecurtailment.Thisproblemmayalsobeaggravatedbyinflexiblenuclearandcoalpowerplantsthathavelimitedabilitytodecreasetheiroutput,giventhedifficultyofpoweringuporpoweringdowntheselargebaseloadfacilities.

Windisagrowingcontributorofenergy,butonlyasmall,insignificantcontributortoelectricalgeneratingcapacity.Windpower’sintermittency—whichresultsingenerationthatisnotdispatchable—iswelldocumented.Theoutputofawindfarmcanvaryfromzerotothefullratedoutputofthefacility.Thisisanissueevenwithlargewindfarms,whichhavesomeself-compensatingabilitybecausetheyaregeographicallydispersed.Formodernwindturbinefarms,theyearlyaveragecapacityfactor—theportionoftimetheyproducefulloutput—isaround40%.Asthepercentagecontributionofwindgrows,sodoesitseffectonthegrid,creatingproblemsoffrequencystabilizationandsystemreliability.Energystorageoptionscouldbeemployedtosupplementorcompensateforthevariabilityofthewindpower’soutput.

Muchlikewindenergy,photovoltaicenergyisalsoanintermittentsourceofelectricity.Theoutputfromasolararraywillvarywiththelocation,weatherconditions,andtimeofday.Italsovariesthroughouttheday,increasingfrommorningtomiddayanddroppingoffintheafternoon.Inmanycases,photovoltaicenergyproductiondoesnotcoincidentwithlateafternoonsummerpeakdemandsthatmostutilitiesexperience.Thereisalsotheintermittencycausedbypassingcloudcover,whichcanmomentarilyreduceaphotovoltaicarray’soutputtovirtuallyzero.

Energystoragecansmooththeoutputofphotovoltaicsbyfillingtheshoulderperiod—theafternoondrop-offofpowerfromthesun.Itcanalsobuffertheeffectofmomentarypowerlossduetopassingcloudcover.BecausetheoutputfromasolararrayisDC,itdoesnotrequiretheACtoDCconversionthatwindenergyneeds.ThisallowsdirectconnectionofthebatterytothesolarDCbusthroughelectronics,butwithoutAC/DCconversion.Thecapitalcostshouldthereforebelessandtheefficiencyhigherthanthoseofwindpowerconversionequipment.

Inanalyzingenergystoragealternatives,Figure1showsthecurrentcostestimatesforvarioustypesofenergystoragetechnologiesavailabletoday.ExceptforCAES,allotherformsofenergystoragehavenoemissionsassociatedwiththeenergydischargecycle.CAESsystemsburnamixtureofcompressedairandnaturalgastogeneratepower.CAEStechnologyrequiresfuelcostsfordischarging,whicharenotcapturedinFigure1.Ifthesystemoperatedoncompressedairalone,thecostsperkilowatt(kW)wouldbeapproximatelythreetimesgreater.

Figure1

CostEstimatesforselectESSTechnologies

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CAES

Li-Ion

Flywheel

PSH

FlowBattery

NaS

Energystoragetechnologytypescanbedividedintotwocategoriesbasedontheireconomicallypracticalduration:thosewithhoursofruntime,andthosewithminutesofruntime.Currently,flywheelsandbatteriesratedforsmalleramountsofenergyareappearinginthegridtodayforancillaryserviceusesuchasfrequencyregulation.Allotherenergystoragetechnologiescanprovidehoursofenergyruntimeinadditiontouseinancillaryservicessuchasfrequencyregulation.

Chapter1

EnergyStorageApplications

Inthischapterwewilllookattheapplicationsofelectricenergystoragesystemstotheutilitygrid.Wewilllookatthebenefitstothethreesectorsontheutilityindustrywhichare,

Generation

Transmission&Distribution

End-Users

Electricityhastraditionallybeenusedatthetimeatwhichitisgenerated.Itisnotoftenstored,eventhoughenergystoragewouldallowfortheoptimizationofpowergeneration.Currently,theUnitedStateshasadaptedgenerationtomatchpeakload,resultinginlowcapacityfactorsfortheelectricpowerindustry,asmuchofthecapacityisusedinfrequentlytomeetpeakdemand.Theshiftingenerationresourcesfromfossilfuelstorenewableenergyresourcesasasourceofelectricpowerwillaggravatethislowcapacityfactorbecausewindpowerisoftenstrongestattimeswhenelectricdemandislow.Whenusedtolevelizetheproduction/demandmismatchovervarioustimedomains,energystoragetechnologieshaveseveralgenerationapplications.Inaddition,storagealsohastransmissionapplicationsthatimprovetransmissioncapacityandreliability.

Energystorageapplicationsmayofferpotentialbenefitstothetransmissionanddistribution(T&D)systembecauseoftheabilityofmodernpowerelectronics,andsomeelectro-chemistries,tochangefromfulldischargetofullcharge,orviceversa,extremelyrapidly.Thesecharacteristicsenableenergystoragetobeconsideredasameansofimprovingtransmissiongridreliabilityorincreasingeffectivetransmissioncapacity.Atthedistributionlevel,energystoragecanbeusedinsubstationapplicationstoimprovesystempowerfactorsandeconomicsandcanalsobeusedasareliabilityenhancementtoolandawaytodefercapitalexpansionbyaccommodatingpeakloadconditions.

Energystoragecanalsobeusedtoalleviatediurnalorothercongestionpatternsand,ineffect,storeenergyuntilthetransmissionsystemabletodelivertheenergytothelocationwhereitisneeded.

Oneareainwhichenergystoragetechnologiescouldprovidegreatbenefitsisinconjunctionwithrenewableenergyresources.Bystoringenergyfromvariableresourcessuchaswindandsolarpower,energystoragecouldprovidefirmgenerationfromtheseunits,allowtheenergyproducedtobeusedmoreefficiently,andprovideancillarytransmissionbenefits.

Attheend-uselevel,energystoragetechnologiescanbeusedtocapturedistributedrenewablegeneration—photovoltaicsolarorwindpower—andstoreituntilitisneeded,bothforoff-gridandgrid-connectedapplications.Assuch,end-userenergystoragetechnologyapplicationsalsohavethepotentialbenefitofimprovinggridutilization,especiallyifend-userenergystoragecanbecoordinatedwithutilityoperations.Oneexampleofsuchcoordinationistheuseofenergystorageinlargecommercialbuildingstoallowpeakshavinganddemandresponsetooccurwithoutreducingactualbuildingservicesandheating,ventilation,andairconditioning(HVAC).

Apotentialbenefitofanend-userenergystoragetechnologyisvehicle-to-grid(V2G)technology,wherebyplug-inhybridelectricvehicles(PHEVs),withtheaddedcapabilityofdischargingbacktothegrid,areusedtoimprovegridutilization,levelizedemand,andimprovereliability.BecauseexpectationsforPHEVdeploymentaresohigh,thereisgreatinterestintheelectricpowerutilityindustryaboutthepotentialforV2Gtoprovidemanyofthebenefitsofenergystorageatthedistributionandend-userlevel.

Therearealsohigh-valuebenefitstonicheenergystorageapplicationsassociatedwithspecificend-usesectors.Specificindustrialapplicationswillbedevelopedasmegawatt-scaleenergystoragetechnologybecomesprovenandeconomicandthatwillprovideaddedbenefitsofenergystoragetechnologies.

GenerationApplications

Thefollowingisasummaryofafewofthekeyareaswhereelectricenergystoragesystemsmaybenefittheelectricutilitygridfromthegeneratortotheend-user.

Energystoragecanhelpwithgridstabilizationbyassistingwiththegrid’sreturntoitsnormaloperationafteradisturbance.Energystoragecanbeusedtoremedythreeformsofinstability:rotorangleinstability;voltageinstability;andfrequencyexcursions.

Inadditiontostabilizingthegridafterdisturbances,energystoragecanalsobeusedtosupportnormaloperationsofthegrid.Fourtypesofsupportoperationscanbeperformedwithenergystorage,

FrequencyRegulationServices:Energystoragecanbeusedtoinjectandabsorbpowertomaintaingridfrequencyinthefaceoffluctuationsingenerationandload.

ContingencyReserves:Atthetransmissionlevel,contingencyreserveincludesspinning(orsynchronous)andsupplemental(non-synchronous)reserveunits,thatprovidepowerforuptotwohoursinresponsetoasuddenlossofgenerationoratransmissionoutage.

VoltageSupport:Voltagesupportinvolvestheinjectionorabsorptionofreactivepower(VARs)intothegridtomaintainsystemvoltagewithintheoptimalrange.Energystoragesystemsusepower-conditioningelectronicstoconvertthepoweroutputofthestoragetechnologytotheappropriatevoltageandfrequencyforthegrid.

BlackStart:Blackstartunitsprovidetheabilitytostartupfromashutdownconditionwithoutsupportfromthegrid,andthenenergizethegridtoallowotherunitstostartup.Aproperlysizedenergystoragesystemcanprovideblackstartcapabilities,provideditiscloseenoughtoagenerator.

Energystoragecanalsohelptoimprovepowerqualityandreliability.Mostgrid-relatedpowerqualityeventsarevoltagesagsandinterruptionswithdurationsoflessthantwoseconds,phenomenathatlendthemselvestoenergystorage-basedsolutions.

Loadshiftingisanotherareawhereenergystorageisutilizedduringperiodsoflowdemandandreleasingthestoredenergyduringperiodsofhighdemand.Loadsiftingcomesinseveraldifferentforms;themostcommonispeakshaving.Peakshavingdescribestheuseofenergystoragetoreducepeakdemandinanarea.Itisusuallyproposedwhenthepeakdemandforasystemismuchhigherthantheaverageload,andwhenthepeakdemandoccursrelativelyrarely.Peakshavingallowsautilitytodefertheinvestmentrequiredtoupgradethecapacityofthenetwork.Theeconomicviabilityofenergystorageforpeakshavingdependsonseveralfactors,particularlytherateofloadgrowth.

Thissectionfurtherdiscussesthepotentialbenefitsofenergystorageacrossdifferentinfrastructureandtimedomainsandgivessomeindicationsoftheperformancecharacteristicsrequiredbyeachapplicationandtheestimatedeconomicgains.Table1summarizesgenerationapplicationsandtheirbenefits.

Manyofthegenerationservicesthatarepotentialenergystorageapplicationsareexistingenergymarket-definedproducts(e.g.,ancillaryservicesandbalancingenergy),andassuch,marketcostsfortheseservicesarereadilyavailable.Wheremarketsarenotderegulated,theamountofenergystoragecapacitythatcouldbeusedisroughlylinkedtosystemorgeneratorsizes.Inmostcases,theoveralleconomicbenefitscanbeusedtofinanceenergystoragetechnologyprojectsvianormalmarketmechanisms.

Whenbenefitsaredescribedasalleviatingconventionalgenerationcapacitytoprovideenergy,itisbecausetheprovisionofanancillaryservicerequiresthatthegeneratoroperateatlessthanfullcapacity.Thus,theownerofthatgeneratorincursanopportunitycostinthatthemarginsonproductionaredecreased;thiscostisalargepartofthepricingdemandedforancillaryprovision,especiallyatpeakload.Insomecases,generatingunitsthatarenot“inthemarket”andwouldbeuneconomicalareusedtoprovideancillaryservices,generallyathigherprices.

Replacingtheseunitswithenergystoragetechnologieswouldreducethesecostsandtheassociatedemissionsfromtheseunits,potentiallyenablingtheretirementofolderpowerplants.

Someoftheapplicationsarealreadyunderearlycommercialdevelopment;severalmerchantenergystoragedevelopersarepilotingfastenergystoragetechnologiesforuseinsystemregulation.Inaddition,somewinddevelopersthatexperiencecurtailmentduetoinsufficienttransmissioncapacitiesareinvestigatingenergystoragesolutions.

TransmissionandDistributionApplications

Transmissioncapacitytobringremotegenerationtoloadcentersiscurrentlylimited,althoughnewtransmissioninfrastructureisbeingplannedandbuiltinmanyareas.Increasingly,newgenerationmustbesitedfarfrompopulationcenters,whichcanplaceadditionalstrainonthegrid.Windpowergenerationisoftenlocatedinremoteorrurallocations,whichrequirestheinstallationofnewtransmission.Becausewindresourcestypicallyhavecapacityfactorsofaround40%,itisoftenthecasethatassociatednewtransmissionratedatthefullpowercapacityoftherenewableresourceisnoteconomical.

Forsomewindpowerprojects,itmaybecost-effectivetoeither,buildtransmissioncapacityforslightlylessthanthefullnameplatecapacityoftheprojectandsimplycurtailoutputduringthesmallnumberofhoursperyearwhenoutputexceedstheavailabletransmissioncapacity,ortoaddenergystoragetoenablethedispatchoftheenergyatadifferenttime.

Energystoragetechnologiesmayprovideawaytocapturepowerproductionthatwouldotherwisebecurtailedandreserveitforatimewhenthetransmissiongridisnotloadedtocapacity.Energystoragealsoaffordsthetransmissionowner/gridoperatorachancetodefer

transmissionexpansionforaperiod;transmissioncapacityisgenerallynotincrementallyincreased.Thisabilitytodefertransmissionexpansionisanexampleofenergystorageprovidingmutualbenefitstogenerationandtransmission.However,thecostsofenergystorageoptionsneedtobecomparedtootheroptions,includingtheconstructionofnewtransmissioninfrastructure,thatbenefitallgeneratorsaswellasconsumersviaenhancedreliabilityandloweroverallcosts.

Transmissioncongestionisalreadyanissueinmanypartsofthecountry.Congestionchargesaretypicallyconsideredaspartoffuelcostadjustmentsbymostregulatedload-servingentitiesandcanbetenstohundredsofmillionsofdollarseachmonth.Theimpactofcongestionistoforcetheuseofexpensivegenerationresourcesclosertotheloadcenterinsteadoflessexpensivecoalandhydroelectricresources,whichcanbeusedinremotelocations.Therefore,large-scaleenergystorageisanotherwaytomitigatetransmissioncongestioniftheeconomicsareviable.

Aspecialcaseofcongestionreliefoccurswhenthelimitingtransfercapacitiesarenotthephysicalcapacitiesofthetransmissionpathsinquestion,butratherarereliabilitylimitsarisingfrompost-contingencyloadingorstabilityconditions.InthewesternUnitedStates,systemdynamicandtransientstabilitylimitsimposerestrictionsonthenort