可再生能源电力存储白皮书

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ELECTRICITYSTORAGEWHITEPAPER

ForRenewableTechnologiesWorkingGroupMeeting,September8,2008

DRAFT

SteveIsser,JD,PhD

VicePresidentandGeneralCounsel

GoodCompanyAssociates

816CongressAvenue,Suite1400

Austin,Texas78701

512-279-0766phone

\o"/"

www.GoodCompanyA

I. STORAGETECHNOLOGIES

Energystoragetechnologiesconvertelectricitytootherenergyforms,withacharacteristicturnaroundefficiencydrivenbythecomplexityofconversionandreconversionbetweenelectricityandthestoredenergyform:

90-95percentefficienttoconvertelectricitytokineticenergyandbackagainbyspeedinguporslowingdownaspinningflywheel.

~70-80%efficiencyforbatteries(electrochemicalenergystoragedevices)ifchargedanddischargedatmoderaterates.

~75%efficiencyforcompressedairstorage,asrapidcompressionheatsupagas,increasingitspressureandthusmakingfurthercompressiondifficult.

~30-50%efficiencyforhydrogenstorageofelectricityfromthecombinationofelectrolyserefficiencyandre-conversion

Therearefourkeycharacteristicsofenergystoragedevices:

EnergyDensity:Theamountofenergythatcanbesuppliedfromastoragetechnologyperunitweight(measuredinWatt-hoursperkg,Wh/kg).

EnergyRating:(expressedinkWhorMWh)isimportantindetermininghowlongadevicecansupplyenergy.

PowerCapability:(ExpressedinkWorMW)determineshowmuchenergycanbereleasedinasettime.A100kWhdeviceratedat20kWcansupply20kWofoutputfor5hours(20x5=100kWh).

DischargeTime:Theperiodoftimeoverwhichanenergystoragetechnologyreleasesitsstoredenergy.

CostsofStorageTechnologies

Dr.RobertB.Schainker,EPRI,EmergingTechnologiestoIncreasethePenetrationandAvailabilityofRenewables:EnergyStorage–ExecutiveSummary,July31,2008,p.8

.EnergyStorageCouncil-

Costsofenergystoragedevicesareusuallyquotedintermsofcost/kWhorcosts/kW.Theseareusuallyrelatedtotheapplicationthedevicewasdesignedtosatisfy.SomedeviceswillhaveahighcostperkWhbutrelativelylowercost/kWwhileotherswillbethereverse.Theeconomicsofastoragetechnologywilldependbothuponcostanditsoperatingcharacteristics,andthustheeligiblemarketsinwhichitcouldexpecttoparticipate.Theeconomicswillalsodependuponthecustomerandpurpose,forexample,marketarbitrageorancillaryservicesforanindependentgenerator,ortransmission/distributioninvestmentdeferralforatransmissionanddistributionserviceutility.

A. Battery,FlywheelandCapacitorTechnologies

1. Batteries

Thereareawiderangeofbatterytechnologies,somewhichhavebeenemployedforalmostacentury,suchaslead-acidbatteries,andsomeofwhicharestillindevelopandhaveyettobecommercialized.

a. LithiumIon

Lithiumionbatterytechnologyhasprogressedfromdevelopmentalandspecial-purposestatustoaglobalmass-marketproductinlessthan20years.Lithiumionbatteriesofferhigh-powerdensities,typically110–160Wh/kgandgenerallyacceptablecyclelife.Nano-compositeelectrodesystemsmayofferevenhigherenergydensities.Charge/dischargeefficienciesof90%(i.e.roundtripefficiencyfrominitialchargetocompletedischarge)arereportedforLithiumbatteries.Duringcharging,lithiumionsmoveout(de-intercalate)fromthelithiummetaloxidecathodeandintercalateintothegraphite-basedanode,withthereversehappeningduringthedischargereaction.Theconductingelectrolytetakesnopartinthereactionexceptforconductingthelithiumionsduringthechargeanddischargecycles.Lithiumionsystemsmustbemaintainedwithinwell-definedoperatinglimitstoavoidpermanentcelldamageorfailure.Thetechnologyalsolackstheabilitytoequalizetheamountofchargeinitscomponentcells.

Theapplicationofthetechnologytolarger-scalesystemsisrelativelylimitedtodate,althoughvariousdevelopmentsareinhandinrelationtotheautomotive,powerutility,submersibleandmarinesectors.ThemainhurdleassociatedwithmassenergystoragesystemsusingLibatteriesisthehighcost(above$600/kWh)duetospecialpackagingandinternaloverchargeprotectioncircuits.SeveralcompaniesareworkingtoreducethemanufacturingcostofLi-ionbatteriestocapturelargeenergymarkets,especiallytheautomotivemarket.

InAugust2007AESCorp.andAltairNanotechnologiesannouncedajointdevelopmentandequipmentpurchaseagreement.Thecompaniesfirstprojectwasamodularunitwhichcontainedtwo1MW,250kWhbatterystorageunits,consistingofalithiumionbatterystack,anAC-to-DCpowerconversionsystem,HVACunit,andacontrolsystem,mountedinaportabletractortrailersizecontainer.Thebatterystackswerecomposedofaseriesarrangementoflithiumioncellpackagesmountedinrackswithinatrailer.Powerconversionwasperformedbycommercially-availableinverterswithcontrolcoordinatedbyaprogrammablelogiccontroller(PLC).ThetwobatterystoragesystemprototypeswereinstalledanddemonstratedatasubstationownedandoperatedbyIndianapolisPower&Light(IPL).TheIPLtestsitewasselectedtobecapableofdispatching1-MWtoapowergridinresponsetoaregulationcommand.Eachofthestoragedeviceswasabletooperatecontinuouslybetween1-MWchargeto1-MWdischargewithpowerdispatchresponseoccurringwithinonesecond.Additionaltestingincludedsimulatedfrequencyregulation,whichinvolvedswitchingtheunitsfromchargetodischargeatupto1MWeveryfoursecondsforseveralhours.Batterystackefficiencymeasuredusingcycliccharge/dischargetests(at50%stateofcharge)variedfrom97%at250kWdispatchto91%at1MWdispatch.Efficiencydropsoffwiththepowerdispatchlevelduetointernallossesthatareproportionaltothecurrentsquared.FactoringintheDC-to-ACpowerconversionsystem,theaverageconversionefficiencymeasuredvariedbetween93%at250kWdispatchto86%at1MWdispatch.ThisdoesnotincludeHVACortrailerauxiliaryload.SummaryofKEMAValidationReport,TwoMegawattAdvancedLithium-ionBESSSuccessfullyDemonstratesPotentialforUtilityApplications,June27,2008.

AEShasinstalleda2MWAdvancedLi-ionbasedsystematitsAESHuntingtonBeachPowerPlantinCalifornia,whichwentonlineinNovember,2008.Theramprateis999MW/sec

withroundtripefficiencyof90%.Theunitcancompletelychargeordischargein15minutes.DaurenKalish,EnergyStorageRoleinSmartGrid,CPUCSmartGridRulemaking,June2009.

TheAESsystemhasbeenacceptedforregulationservicesinthePJMmarket.

b. SodiumSulfur(NaS)battery

NaSbatterytechnologyinvolveshighoperatingtemperatures,from290°to360°C.Thecellconstructionusesliquidsulfurasthepositiveelectrodeandliquidsodiumasthenegativeelectrode,separatedbyasolidelectrolyteofbeta-alumina.Theelectrolyteallowsonlythepositivesodiumionstopassthroughitandcombinewiththesulfurtoformsodiumpolysulfides.Itsoperatingtemperaturemustbemaintained,byroutineoperationorbyexternalheating.

NaSbatterieshavearelativelyhighenergydensity,withintherange150–240Wh/kg.NaSisdesignedforlongdischargecycles(8hours),buthasthecapacitytodischargeveryrapidlyandatmultiplesofratedpower.Thesebatterieshaveanestimatedlifetimeof15yearswithacyclelifeof2500andcharge/dischargeefficienciesupto90%.

Thebatterymoduleconsistsofcellsconnectedinseries/parallelorseriesarrayswithinathermallyinsulatedenclosure.Modulesarethenconfiguredinseriesand/orparalleltosupportmulti-megawattloads.Highlycorrosivematerialrequireprotectivemeasuressuchasasafetytubeincorporatedinthecelldesign,hermeticallysealedcells,double-layerstainlesssteel,vacuuminsulatedenclosurewithsandfillerpackingbetweencells.Atradeoffexistsbetweenpoweroutputandbatterylife.Operatingathigherpowerlevelsresultsinasignificantriseinoperatingtemperature,whichacceleratescellcorrosionandincreasecellresistancewhichshortensbatterylife.Expectedlifeis15yearsor2500fullcharge/dischargecycles,butcanbeasshortas500cyclesinlongduration(15minute)mode.BenjaminL.Norris,JeffNewmiller,GeorgiannePeek,NAS®BatteryDemonstrationatAmericanElectricPower,SandiaReport,SAND2006-6740,March2007.

ResearchanddevelopmentintoNaSbatterieshasbeenpioneeredinJapansince1983bytheTokyoElectricPowerCorporation(TEPCO)andNGKInsulators.NGKbroughttheNASbatterytomarketin2002,andinitiatedcommercialscaleNASmanufacturinginApril,2003.Todate,theinstalledcapacitybaseisover300MW,acrosssome200sites,principallyinJapan.An8MW,58MWhsysteminstalledataHitachiautomotiveplantinJapaniscurrentlytheworld’slargestbatteryintermsofstoragecapacity.

AEP’sfirstcommercialenergystoragesystemwasaNGKNASBatteryatCharleston,WV.Ithasacapacityof1.2MW,7.2MWh(6hourduration)andhasbeenoperationalsinceJune26,2006,ninemonthsaftercontractsweresigned.Theprimaryapplicationispeakshaving.Thebatteryhelpsshavetransformerpeakloads,andreducestransformertemperaturesbyseveraldegrees.Peakshavingimprovedthefeeder’sloadfactorfrom0.75to0.80,onaverage,andprovidedaPJMmarketenergyvalueof$5,500permonth.AliNourai,InstallationoftheFirstDistributedEnergyStorageSystem(DESS)atAmericanElectricPower(AEP),SandiaReport,SAND2007-3580,June2007.

Whilecostswillvarywithlocalsiteconditions,itistheunderstandingatAEPthatthenextNASbasedenergystorageprojectwillcostapproximately$2,500/kW,installed.AEPhasinstalledthree2MWbatteriesatsitesinOhio,WestVirginiaandIndiana,allwithdynamicislanding.AEPisalsoinstallinga4MWbatteryatPresido,Texas,attheendofatransmissionline,todeferatransmissionupgrade.AliNourai,UtilityDeploymentofEnergyStorage,Presentation,October2008.

c. FlowCellBatteries

Electrochemicalflowcellsystems,alsoknownasredoxflowcells,convertelectricalenergyintochemicalpotentialenergybymeansofareversibleelectrochemicalreactionbetweentwoliquidelectrolytesolutions.Thenameredoxflowbatteryisbasedontheredoxreactionbetweenthetwoelectrolytesinthesystem.Inaflowcellthetwoelectrolytesareseparatedbyasemi-permeablemembrane.Thismembranepermitsionflow,butpreventsmixingoftheliquids.Astheionsflowacrossthemembrane,anelectricalcurrentisinducedintheconductors.Flowcellsstoreenergyintheelectrolytesolutions,andthepowerandenergyratingsofredoxflowcellsareindependentvariables.Theirpowerratingisdeterminedbytheactiveareaofthecellstackassemblyandtheirstoragecapacitybytheelectrolytequantity.Overthepast20years,developmentanddemonstrationactivitieshavecenteredaroundfourprincipalelectro-chemistriesforflowbatteries:vanadium/vanadium(VanadiumRedoxBatteries,VRB),zincbromine(ZBB),polysulfidebromideandzinccerium.Installationstodatehaveprincipallyusedthevanadiumredoxandzincbromine.Severaldozenareinplace,mainlyinJapanandNorthAmerica.Amajoradvantageofthetechnologyistheabilityofthetechnologytoperformdischargecyclesindefinitelysotherearenosignificantwasteproductsassociatedwithoperation.Thesesystemshavequotedefficienciesvaryingfrom70%(ceriumzinc)to85%(VRB).Oneproblemwithflowbatteriesisthatmultiplepumpingcircuitsindicatethatregularmaintenanceactivitywillberequired.

AZBBdemonstrationprojectforPG&Eusedatransportable2MW/2MWHZBBbatteryenergystoragesystematasubstationtodemonstrateandassessvalueofT&Dupgradedeferral.PremiumPower,anewmanufacturer,claimsa30yearlifeforitsZinc-Flowtechnologyandtheabilitytowithstandanunlimitednumberofcycles,whetherfull-orpartial-dischargeevents.ItsTransFlow2000providesupto500kWofpowerand2.8MWhofenergystoragecapacityinasingleenclosurethatfitsontoa53'trailer.Thebatteryhasyettobedeployed,thoughCPSEnergyhaschosenitforapilotprojectandthecompanyisrumoredtohavearelationshipwithDukeEnergy.Untilactualoperatingdatabecomesavailable,thecompany’sclaimsshouldbetreatedwithhealthyskepticism,sincetheyfarexceedoperatingexperiencewithotherflowbatteries.

TheleadingproducerofvanadiumredoxflowbatteriesywasVRBPowerSystem,whichbecameinsolventandwasacquiredbytheChinesefirmPrudentEnergy,whothenformedaCanadiansubsidiarytomanagetheassets.Thevanadiumredoxsystemhasanadvantageoverthehybridsystemasthedischargetimeatfullpowercanbevaried.VRBscanbefullydischargedwithoutreducinglifeexpectancy.AVRBinSapporo,Japanhasundergonearound14,000dischargecycles.TheVRBsystemiscurrentlybeingdeployedatanumberofsitesaroundtheworld,includinga250kW,2MWhbatterybyPacifiCorpinUtahanda4MWunitinJapan.

d. Lead-Acid

Lead-Acidbatteriesareelectrochemicalcells,baseduponchemicalreactionsinvolvingleadandsulfuricacid.Lead-Acidisoneoftheoldestandmostdevelopedbatterytechnologies,usedinelectricalpowersystemsformorethanacentury.Theyprovideacost-competitiveandprovensolutiontoarangeofstoragerequirements.Leadacidbatteriesarelowcostcomparedtootherbatterytechnologies.Buttheyhavesomedisadvantagesincludingrelativelylimitedcyclelife,low-energydensityandalargefootprint.Thetypicalenergydensitiesarelowerthanotherbatteriesat25–45Wh/kg.Charge/dischargeefficienciesforlead-acidbatteriesare60–95%withself-dischargeratesof2to5%permonth.Thechemicalreactionwithinalead-acidrecombinationcellfavorsseveralhoursoflow-ratedischarge,ratherthanafewsecondsofhigh-rateduty.Dependinguponthedesignthatisusedandthequalityofthebattery,theusercanexpectbatterylifetorangefrom3yearstoaslongas9yearsat>80%capacity.Maximizingbatteryliferequireskeepingthebatteryroomtemperatureat20°C,asforevery10°above20°Cthedesignlifeofabatterywillbehalved.

e. Nickel

ThereareanumberofNickelbasedbatteriescurrentlyavailableorunderdevelopment,includingNickel-Cadmium(NiCd),Nickel-MetalHydride(Ni-MH),Nickel-Zinc(NiZn)andSodium-NickelChloride(NaNiCl2).NiCdandNi-MHarethemostdevelopedoftheNibatteries.ThesevariousNibatterytypescovertheenergydensityrange20–120Wh/kg.TheNiCdandNiMHbatteriescanreachuptoaround1500deepcycles.Ni-ZnandNa-NiCl2haveashorterlifetime.

NiCdbatterysystemsrankalongsidelead-acidbatteriesintermsoftheirmaturity.NiCdbatterieshavebeenproducedsincetheearly20thcenturyandformedthemajorityoftherechargeablebatterymarketinconsumerelectronicsbythe1990s.NiCdisarobustandprovenalternativetolead-acidbatteries,withhigherenergydensity,alongercyclelifeandlow-maintenancerequirements.Despitebeingusedwidelyinelectricvehicles,therearefewexamplesoftheirapplicationtoelectricitymarkets.GoldenValleyElectricAssociation(GVEA)inFairbanks,Alaskahasinstalledwhatisclaimedtobetheworld’smostpowerfulbattery.Thelarge-scaleNiCdBatteryEnergyStorageSystem(BESS)canprovide27MWofelectricityforaminimumof15minutestostabilizethelocalpowergridintheeventoflossofgeneration,andhasdelivered46MWfor5minutes.In2006theBESSrespondedto82events.TheBESSwasdesignedandbuiltbySaft,aninternationalbatterymanufacturer,andcomprises13,760SaftSBH920NiCdcellsarrangedinfourparallelstrings.TheNiCdbatteriesthemselvesareexpectedtocomplete100completeand500partialdischargesinthesystem’s20yeardesignlife.StevenEckroad,EPRI,GoldenValleyCooperativeProjectinAlaska-40MWNickel-CadmiumBattery,CaliforniaEnergyCommissionStaffWorkshop,February24,2005.

Concernsaboutcadmiumtoxicityandassociatedrecyclingissuesareabarriertogainingconsentforfuturelarge-scalestoragesystemsbaseduponNiCdtechnology.

TheNaNiCl2battery,otherwiseknownastheZEBRAbattery,isahigh-temperaturebatterysystem,developedandproveninvarioustractionandpropulsionapplications.Itscellconstructioncomprisessodiumandnickelchlorideelectrodes,separatedbyabeta-aluminaelectrolyte,whichisabletoconductsodiumionsbutnotelectrons.Itoffersanumberofadvantagesrelativetosodium–sulfursystems,includingbettersafetycharacteristics,highercellvoltageandtheabilitytowithstandlimitedoverchargeanddischarge.NiCdandNi-MHofferthelowestefficiency,dischargingaround70%oftheenergyusedduringcharging.Incomparison,NiZnbatteriesofferefficienciesof~80%andNaNiCl2batterieshaveanefficiencyofaround90%.BothNiCdandNi-MHbatteriesareexpensivetomanufacturerelativetootherbatterytechnologies.

2. Flywheels

Theflywheelactsasamechanicalbatteryandcomprisesashaftmountedmassrotatingin(orcarrying)amotor-generatorwinding–convertingelectricalenergyintokineticenergyasitaccelerates(chargeswhenspeedingup)andthen,whenadischargeofenergyisrequired,reversestheflowofenergyandslowsdownasitgivesupitsstoredenergyintheformofelectricalpower.

Ingeneral,flywheelscanbeclassifiedaslowspeedorhighspeed.Theformeroperateatrevolutionsperminute(rpm)measuredinthousands,whilethelatteroperateatrpmmeasuredinthetensofthousands.Increasingrpmsignificantlyincreasestheenergydensityofaflywheel,butahighermassflywheelcanstoremoreenergyperrpm.Operatingathigherrpmnecessitatesfundamentaldifferencesindesignapproach.Whilelow-speedflywheelsareusuallymadefromsteel,high-speedflywheelsaretypicallymadefromGFRE(graphitefiberreinforcedepoxy)andfiberglasscompositematerialsthatwillwithstandthehigherstresses.High-speedflywheelsuniversallyemploymagneticbearings(allowingtheflywheeltolevitate)andvacuumenclosurestoreduceoreliminatefrictionlossesfrombearingsandairdrag.Whilesomelow-speedflywheelsuseonlyconventionalmechanicalbearings,mostflywheelsuseacombinationofthetwobearingtypes.Vacuumsarealsoemployedinsomelow-speedflywheels.ThebenefitsofincreasedperformanceofferedbyGFREcompositesmustbebalancedagainstthefarlowerrawmaterialcostofhighqualitysteels.

DCflywheelenergystoragesystemsaregenerallymorereliablethanbatteries,soapplicabilityismostlyanissueofcost-effectiveness.Batterieswillusuallyhavealowerfirstcostthanflywheels,butsufferfromasignificantlyshorterequipmentlifeandhigherannualoperationandmaintenanceexpenses.Thus,flywheelswilllookespeciallyattractiveinoperatingenvironmentsthataredetrimentaltobatterylife,suchasfrequentcycling.

BeaconPowerCorphasdevelopeda100kWmodulebasedonhigherrotationalspeedsratherthanmasstoincreasetheenergystored.Apatented,co-mingledrimtechnology(PCRT)hasbeendevelopedtopreventcracksdevelopingduetocentrifugalforces,leadingtosafetyimprovements.BeaconPowerquotesalifetimeof20yearsforitsflywheels.Thetechnologyhastheabilitytodischargeoverperiodsupto30minutes.BeaconPowerCorpenvisagesarraysofthe100kWmodulesinsystemsofaround20MWcapacity,providingupanddownregulationequalto40MWofswing.

3. UltraCapacitors

Themostdirectwayofstoringelectricalenergyiswithacapacitor.Acapacitorconsistsoftwometalplatesseparatedbyanonconductinglayercalledadielectric.Whenoneplateischargedwithelectricityfromadirect-currentsource,theotherplatewillhaveinducedinitachargeoftheoppositesign.Tobuildstandardcapacitorsthatcanholdasignificantamountofenergyrequiresaverylargedielectric,makingtheuseoflargecapacitorsuneconomical.Theultracapacitors(alsoknownassupercapacitorsordouble-layercapacitors)solvesthisproblemthroughtheuseofahighsurfaceareamaterialsuchasactivatedcarbonastheconductorwithanaqueousornon-aqueouselectrolyte.Ultracapacitorscontainasignificantlyenlargedelectrodesurfaceareacomparedtoconventionalcapacitors,aswellasaliquidelectrolyteandapolymermembrane.Theenergystoragecapabilitiesofultracapacitorsaresubstantiallygreaterthanthatofconventionalcapacitors,byapproximatelytwoordersofmagnitude.

Ultracapacitorsarecapableofchargingsubstantiallyfasterthanconventionalbatteries,

beingrechargedalmostindefinitelycomparedtobatteriesthatonlyhavearelativelysmallnumberofrechargesbeforeneedingreplacement;andcanoperatedowntotemperaturesof-25°C.Energydensitiesof20-30Wh/kghasbeenreportedforultracapacitors,whilerecentresearchatMITsuggeststhatenergydensitiesofgreaterthan60Wh/kgandalifetimelongerthan300,000cyclesisachievable.Typicalefficienciesforultracapacitorsarehigh(85–98%),makingthemanattractivestoragetechnologyformanyapplications.Ultracapacitorshavebeenmarketedsincethe1980s,withthefirstapplicationinmilitaryprojects,startingtheenginesofheavyequipmentsuchasbattletanksandsubmarinesorreplacingbatteriesinmissiles.In2005,theultracapacitormarketwasbetweenUS$272millionand$400million,andisgrowing,especiallyintheautomotivesector.ChrisNaish,IanMcCubbin,OliverEdbergandMichaelHarfoot,OutlookofEnergyStorageTechnologies,forEuropeanParliament'scommitteeonIndustry,ResearchandEnergy(ITRE),February2008,p.11.

.

B. CompressedAirEnergyStorage(CAES)

ThefirstcommercialscaleCAESplantintheworldwasthe290MWplantinHuntorf,Germany,operatedbyNordwestDeutscheKraftwerke(NDK)since1978.TheHuntorfplant,withtwosaltcaverns,hasacapacityof290MWandrunsonadailycycleinwhichitchargestheairstoragefor8hoursandprovidesgenerationforupto4hours.Itisprimarilyusedtoprovideancillaryservices.TheAlabamaElectricCo-operativeinMcIntosh,Alabama,USAbuiltthesecondcommercialscaleCAESplant,withacapacityofabout110MWofpowergeneration.Theplantisconstructedinconnectionwitha100MWcoalplantandactsasaregulatingcapacitybetweenthecoalplant’scapacityandtheelectricitydemand.TheAlabamaplantwasbuiltbasedonacompetitivelyawardedfixedprice,turnkeycontract,costingabout$460perkW(1991dollars).Theplantwasbuiltinabout2.5years.TheonemajordesigndifferencebetweentheGermanandAlabamaCAESplantsistheAlabamaplanthadanexhaustgasheatexchangerinittoheattheairafteritcamefromstorage,whichreducedtheplant’sfueluseby25percent.

FirstgenerationCAESusedasimpledesignwiththecompressorandgeneratoronthesameshaft.Thesingle-shaftturbomachinerytrainwithmultiplecomponentshassomeoperationalandmaintenancecomplications.Toinitiatecompressionoperation,theturbinetypicallybringsthemachinerytraintospeed.Aftersynchronization,theturbineisdecoupledandshutoffandthecompressorsareleftoperating.Thismeansthattheturbinesarecalledupontoinitiatebothcompressionandgeneration.Newdesignswouldusemultiplecompressorsonaseparateshaft,providingmoreoperationalflexibility.Thenewdesignseliminateoperationswitchovertimebydecouplingthecompressionandturbo-expandertrains,permittingdirectswitchingbetweencompressionandexpansionoperation.Thischangemeanscompressorsizecanbeoptimizedindependentlyoftheturbo-expanderdesignandpermitsstandardproductioncompressorstobeusedinthesystemconfiguration.ItalsomeansthataCAESwouldbeanidealancillaryservicemachine,sinceitwouldprovideacontrollableloadincompressormodeandafaststartgenerator,bothabletooperateindependently.ElectricPowerResearchInstitute,CompressedAirEnergyStorageScopingStudyforCalifornia,fortheCaliforniaEnergyCommission,PIERFinalReport,November2008,p.5.

TherampratesforaCAESsystemarebetterthanforanequivalentgasturbineplant.

TheMcIntoshplantcanrampatapproximately18MWperminute,whichisabout60%greaterthanfortypicalgasturbines.Proposedplantshavebeendesignedtoreachfullpowerin14minutes(or7minutesforanemergencystart)—whichtranslatesto9.6to19MWperminuteper135MWmodule.SamirSuccarandRobertH.Williams,CompressedAirEnergyStorage:Theory,Resources,AndApplicationsForWindPower,PrincetonEnvironmentalInstitute,April8,2008,p.23.

CAESstorageisdependentontheavailabilityofsuitablesaltdomeformationsorrockcaverns.TexasdomalsaltintheEastTexasandGulfCoastbasinsisrelativelypureandhomogeneous,butthelateralextentofdomesislimited,however,andrestrictingtheareausefulforcaverndevelopment.BeddedsaltofthePermianBasinismuchlesspurethanTexasdomesalt.Thedistributionoflow-solubilityimpuritiesisoneofthelimitationsofengineeringsolution-minedcaverns.However,thesesaltbedsaretypicallycontinuousoverlargeareas,manyclosetoareasslatedforextensivewinddevelopment.

Abovegroundairstoragevesselsorairstoragepipelinesystemscanalsobeusedtostorecompressedair.SuchsystemsareattractivebecausetheyallowCAESplantstobesitedalmostanywhere,sincenoundergroundgeologicformationisneeded.Thismethodusesbanksofhighstrengthsteelpipinginverticalconfigurationstoachievethedesiredresult.Thestorageunitswouldbeplacedinanexcavation,resultinginafourtosixstorybuilding.Thisprovidesflexiblestoragequantitiesataconstantpressureandtemperaturewithouttheconventionalgeologicalconstraints.However,amini-CAESfacilityhasonlyafewhoursofenergystorage.Suchsystemsareestimatedtobemoreexpensivethanundergroundsalt-basedairstoragecaverns,atabout$2,500/kW.NYSERDA,Mini-CompressedAirEnergyStorageforTransmissionCongestionReliefandWindShapingApplications,July2008.

C. OtherTechnologies

1. Hydrogen

Theessentialelementsofahydrogen-basedenergystoragesy