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Home Battery Storage Systems — A Look into the Market ......Nov 30, 2017 · This system combines...
Transcript of Home Battery Storage Systems — A Look into the Market ......Nov 30, 2017 · This system combines...
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HomeBatteryStorageSystems—
ALookintotheMarket,TheirEnergy
EfficiencyandPerformanceNovember30,2017
ExecutiveSummary
Thisreportlooksatemergingresidentialbatterysystemsthatcanprovidebackuppower,storeandreclaimexcesssolarenergy,andofferreservegridcapacitytoautilityorthirdparty.Whilethesesystemsareintheveryearlystagesofadoption,withonlyafewthousandunitsinstalledtodatenationally,thenumberofinstallationsisexpectedtoskyrocketgivendecreasingpurchasecosts,thegrowingnumberofhomeswithrooftopsolarpanels,andincreasinginterestinhavinground-the-clockaccesstoelectricityduring/afterextremeeventssuchashurricanes,forestfires,orearthquakes.Storagesystemscanalsohelpgridoperatorsintegratehigherfractionsofrenewableenergyintotheirsystemsandhelppolicymakersachievezero-netenergygoalsfornewhomes.Atypicalresidentialbatterysystemisaboutthevolumeofafilecabinetandiswall-orfloor-mountedinagarageorutilityspace.Asystemthatcostsabout$10,000–$15,000canstore10kWhofenergy(enoughtosupplyatypicalhomeforaday),withpeakpoweroutputof10kW.Currentcostsareintherangeof$1,000-$2,000perkWh;costreductionsto$250-500perkWh,assomeproject,wouldopenuplarge-scalemarketsforhomebatterysystems.Theresearchforthisreporttookahigh-levelviewofavailableproductsandtrends,withafocusonoverallefficiencyandenergylossesinstandbyandactivemodes.Thisreportalsoreviewsthestatusoftestproceduresandregulationsforbatterysystems.Themainfindingsinclude:
• ResidentialbatteriesformanexuswithsolarPVsystemsandelectricvehicles,withpotentialeconomicandperformancebenefitsflowingfromcombinedsystems.
• Thereiscurrentlyalackofofficialconsensusontestmethodsandstandardsforresidentialbatterysystems,althoughnationalstakeholdergroupsareawareoftheneedtodevelopthem.
• Long-termperformanceneedstobeconsideredinbatteryselection,sizing,andoperation,assystemcapacitydegradesovertime.
• Mostgrid-tiedresidentialbatteriesaresoldasbackuppowersystems,creatingthepotentialtoharnessunusedcapacityforgridservices,suchaspeakloadmanagement,voltagesupport,andspinningreserve.
• ThemostcommonstorageconfigurationisACcoupling(whereallDCdevicesconverttheirpowertoAC),butDCcouplingcouldimproveefficiency.Forexample,sendingDCpowerdirectlyfromsolarpanelstobatteries(withoutconvertingtoAC)reducesconversionlosses.
PreparedforNRDCby
DavidHoughtonPE
andChrisCalwell
EcosResearch
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• Residentialbatterysystemscan“consume”roughly300-500kWhperyear—aboutasmuchasatypicalhomerefrigeratorconsumesannually.Thisconsumptionorenergylossoccursintwoways:a)energylostduringconversionfromincomingDCpowerfromthesolarpaneltothebattery,andb)standbypowerlossesfromafullychargedbattery.
• Variationinpublishedround-trip(RT)efficienciesandstandbypowerlossesappeartobesignificant,butwithoutstandardizedtestingitisdifficulttotellthedifferencebetweenproducts,andharderstilltotellhowunitperformanceinthefieldwillcomparetoclaims.
• Opportunitiestosupportthedevelopmentofresidentialbatteriesinclude:o CoordinationofstandardsandtestmethodsbyworkingthroughIECTC120and
otherforumso Fieldtestingandmeasurementofreal-worldcostandperformanceo BuildingonEuropeanandAustralianexperience,wherethousandsofsystems
havebeenoperatingforayearormoreo Inclusionofenergystorageintobuilding,energy,andelectricalcodes.Measures
couldincludesafetyandsizingrequirements,efficiencyminimums,andelementssuchas“storage-ready”electricalsystemdesign.
Introduction
Althoughbatterysystemshavebeenavailableforhomeuseformanyyears,theyareprimarilyolderlead-aciddesignsrequiringregularmaintenanceandintendedsolelyforoff-griduse.Withtheadventofcompact,maintenance-freelithiumiontechnologiesandsteadydeclinesincost,grid-connectedhomeenergystoragesystemsarenowemergingintheU.S.marketplace.Thistechnologypromisesmultiplebenefitsforelectricityconsumersandproviders,butitisnotyetclearhowbesttousethesesystems,andhowwelltheywillperform.RecentadvancesinLi-ionbatteryperformanceandcostshavedriventhreemaingrid-connectedproductareas:utility-scalesystems(front-of-meter),commercial-scalesystems(behind-the-meter),andresidential-scalesystems(behind-the-meter).Ofthesethree,residentialsystemshavereceivedthemostattentioninthepopularpress(thankslargelytotheTeslaPowerwall),yettodatehavereceivedrelativelylittleattentionfromutilitiesandregulators.Residentialbatterystoragesystemsareintheveryearlystagesofadoption.Toputthingsintoperspective,welloveramillionresidentialrooftopPVsystemsareinplacetodayacrosstheUS,butonlyafewthousandbatterysystems—lessthanonepercentofsolar-equippedhomes.Inthispaper,wereviewtoday’sproductofferings,performanceclaims,keymetrics,andrecommendtechnicalandpolicystepstosupportthisemergingtechnology.Whywouldahomeownerwantabatterysystem?Themainreasons:
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• Backuppower.Vendorsmaintainthatthisiscurrentlythemostcompellingbenefitthatleadstosales.Brochuresoftenwillshowablacked-outresidentialstreet,withonehouselitup.
• Environmentalbenefits.Manysolarcustomersareincreasinglyuncomfortablewithusingcoal-dominatedbaseloadpowerfromthegridwhenthesunisnotshining,leadingtointerestinstoringtheirsolarenergyproductionforuseatnight.
• Utilitybillsavings.Savingscancomefromarbitrage(buycheappowerlateatnight,sellexpensivepowerduringtheafternoon/evening)orimprovedsolarself-consumptioninareaswherenetmeteredisdiscouraged,disallowed,orhobbledbyunfavorablerates.
• Sellinggridservices.Groupsofhomebatteriescanprovidevoltagesupport,frequencyregulation,renewablesfirming,spinningreserve,andotherservicesthatgeneraterevenueoravoidcostsfortheutilityorforthird-partyaggregators.
• Off-gridoperation.Lead-acidbatterieshaveofferedgrid-freelivingfordecades;cheaperandbetterbatteriesmayencourageexistingutilitycustomerstodetachfromthegridandbecometheirownelectricalislands.Mostobserverscontendthatmassive“griddefection”isunlikely,however.
Batterysystemtaxonomy&schematics
Batteriesandtheirsurroundingelectricalcomponentscanbedeployedinseveralconfigurationsandusedinawidevarietyofways.Thebatterycellsinhomesystemshavenovalueforconsumersuntiltheyareintegratedintoaproductandinstalledasasystem.
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Energystoragevendorsareessentiallysystemsintegratorsthatcombinebatterycellsormoduleswithpowerconversion,softwareandcontrolstocreateproductsforspecificmarkets.Mostbatterieswillbeinstalledinhomesthatalreadyhavesolarpower,orinstalledtogetherwithnewsolarpowersystems.Nearlyeveryhomebatterybrochureincludesaspaghetti-likediagramshowingelectricalcomponentsandconnections,sometimesinoverwhelmingdetail.Chemicalbatteriesaredirectcurrent(DC)devices,soconnectingtothealternatingcurrent(AC)grid,requiresinverterstoconvertfromDCtoACpower.Similarly,goingfromACtoDCrequiresarectifier.Eachoftheseconversionsconsumessomeenergyexactsan“energypenalty”,sothefewerthebetter.SomebatterysystemsareabletochargewithDCpowerdirectlyfromsolarPVpanels(alsoDCdevices),whichcanpotentiallyimproveoverallefficiencybyeliminatingtwopowerconversions.SomemayalsooffertheabilitytooutputDCpowerdirectlyfromthebatterytoloadsthatcanutilizethepowerinthatform,suchaselectricvehiclecharging,avoidingadditionalinverterandrectifierlosses.Thepotentialsystemarrangementsdependonthecombinationofstorageelements,PVpanels,electricvehicle(EV)chargers,andbackupcircuitry.Ingeneral,theoptionscanbesimplifiedintothefollowingtypes.
ACCoupledBattery+PV
Inthisarrangement,thebatteryandPVsystemseachhavetheirowninverters,andthewiringcombinesonacommonACbusat120Vor240V.Thisisthemostcommonsystemtype,asitavoidsthecomplicationsofcoordinatingDCvoltageandcurrentmanagement.ThebatterysystemvendorcanselltheproductregardlessofthetypeofPVsystem,anditcanberetrofittedtooneofthemorethanonemillionhouseholdsthatalreadyhaverooftopPV.DCCoupledBattery+PV
ThissystemcombinesthePVandbatteryinvertersintoasingleunit.Theadvantagesaretwofold:costreduction(oneinverterinsteadoftwo),anddirectDC-DCbatterychargingfromthePVsystemforhigherefficiency.However,thePVandbatterysystemsgenerallymustbecompatible,andeitherinstalledorplannedatthesametime.
Figure: AC-Coupled Battery+PV System
Figure: DC-Coupled Battery+PV System
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Stand-aloneBattery(noPV)
MoststoragesystemsarecombinedwithPVgeneration.Stand-alonesystemsareasimplifiedversionoftheACcoupledsystem,butwithoutPV.Battery/PV/EV
Today’selectricvehicles(EVs)aregenerallychargeableonlywithACpower,usuallyviaaLevel2(240V)charger.Inthissystemtype,thechargerisjustanotherconnectionontheACbus,andtheAC-DCconversiontothecar’sbatteryishandledwithonboardelectronics.
IfthecarcanbechargedwithDCpower,anothersimplificationispossible—asingleDCpowerconverterthathandlesPVinput,batteryinputandoutput,andEVoutput.Thisoffersthefewestconversionlossesandhighestefficiencyofallpossiblearrangements.BackupPowerSubpanel
Atthispointinthedevelopmentofthehomebatterymarket,backuppowerisakeypartofthesalespitch.Toactuallydothis,however,requireseitheracritical-loadssubpanel(veryunusualinmosthomewiring)orasystembigenoughtohandletheentirehouseforameaningfullengthoftime(uneconomicalifthehousehaslargeloadssuchasairconditioning).AnyofthesystemtypesdescribedabovecanbemodifiedtodothisbyconnectingtheACsideofthebatteryinvertertoa“criticalloads”subpanel—onesucharrangementisshownhere.
Figure: Stand-alone Battery System
Figure: AC-Coupled Battery+PV+EV System
Figure: DC-Coupled Battery+PV+EV System
Figure: Battery-Critical Backup System
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Severalyearsago,California’sTitle24buildingcodebeganincludingprovisionstomakenewconstruction“solarready”byallocatingphysicalspaceandelectricalinfrastructureforPVsystems.Similarprovisionscouldbeemployedinfuturecodestoencourageorrequirebuildingtobe“storageready.”Thesemeasurescouldincludewiringforacritical-loadssubpanel,physicalspaceforbatteries,androbustelectricalconnectionstoallowforhigh-amperage240Vsourcesandloads—PV,EVs,andbatteries.Keymetrics
Batteryspecificationsheetslistseveralattributes.Themostcommonare:
• Energycapacity(inkWh)—mayormaynotbede-ratedtoprovideaperformancebuffer
• Powercapacity(inkW)
• Powercapacity(inkVA)—mayincludeshortburstsofpowerabovenormalcapacity
• Round-tripACefficiency
• Round-tripDCefficiency(ifapplicable)
• Maximumchargerate(inampsorwatts,oftenexpressedasarateoftotalcapacityperhour,i.e.1Cequalscompletedischargeinonehour,2Cequalsdoublethatrate,andsoon)
• Maximumdischargerate(sameunitsaschargerate,i.e.1C,2C,etc.)Additionalmetricsthatareimportantforresidentialsystems—butarelesscommonlyprovidedinproductinformation—include:
• Dailydischarge(lossesfromsimplyholdingacharge,in%perday,orstandbypower,inwatts)
• Degradationovertimeandcharge/dischargecycles,bothinenergycapacityandinround-tripefficiency
• Degradationinperformanceunderextremeoperatingtemperaturesorotherenvironmentalstresses.Thisisparticularlyimportantinsolar-friendlyclimatessuchasArizonaandNevada,wherebatteriesmaybelocatedoutsideandsubjecttoveryhightemperatures.
Metricsthataremorerelevanttolargerutility-dispatchedsystemsincluderesponsetime,ramprate,internalresistanceorimpedance,andreactivepowermeasurements.Someofthesequalitiesmaybeapplicableforfleetsofresidentialsystems.Testmethods
Forthisstudy,welookedforevidenceofanyuniformtestmethodsforresidentialbatterysystemperformance.Thissearchincludedphonecallsanddirectdiscussionswithresidentialbatterystakeholders(includingEPRI,CSA,NEMA,PNNL,CEC,EPA,andPG&E)andmanufacturers/vendors(includingEnphase,Sonnen,Simpliphi,Sunverge,Eguana,IdealPower,Adara,andPika).Allpartiesagreedthattherearecurrentlynosuchstandardizedtestmethods.Mostvendorsseemedwillingtosupportstandardizedtesting,afewwereneutral,andatleastonewaswary;thoseinthelattertwocategoriesexpressingtheopinionthattheydidnotthinklackofuniformtestingwasamajorproblem.
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Wealsoobservedarangeofmanufacturingclaimsregardingefficiencyandstandbypoweruse.Forexample,somepublishedefficiencyvaluesdon’tstatewhethertheyareAC-ACorDC-DC,asignificantdifference,andstandbyvaluesareseldomgiven.Furtherstandardizationoftestmethodsandpresentationofperformanceclaimswillcreateamorelevelplayingfieldformanufacturersandhelpshieldconsumersfromexaggeratedorunrealisticclaims.Althoughhomebatteryteststandardsdonotyetexist,thelastyearhasseensignificantprogressincharacterizingenergystoragesystemperformance.TheEPRIESICTestManualV1.0,publishedinDecember2016,laysoutfoundationalprinciplesanddetailedproceduresfortestinggrid-connectedbatterysystems.Thismanualwascreatedforlargersystems,butitsguidanceisrelevanttohome-scalesystems.Underwriter’sLaboratories(UL),SandiaNationalLaboratory,andPacificNorthwestNationalLab(PNNL)arealsoinvolvedinadvancingtestprocedures.HerearekeyelementsofthetestingproceduresdefinedintheESICTestManual:
• Definitions.Morethantwentyterms—suchasAvailableDischargeEnergyCapacity,StateofCharge,andRatedContinuousPower—aredefinedtobuildaframeworkforcalculations.
• Testsetup.TheManualspecifiesatestchambermaintainedat73F±4F,butalsohasallowancestotestatothertemperaturesifthesystemwillbeoperatedoutdoors,orifaclimate-controlledtestchamberisnotavailable.
• DutyCycle.Forround-tripefficiencytesting,aseven-daytestisspecified,withafull-powerdischarge/chargecycleoneachofthefirstthreedays,andreduced-power(75%/50%/25%)discharge/chargecyclesonthelastfourdays.DifferentdutycyclesarespecifiedtomeasureRatedContinuousPower,ResponseTime,andSettlingTime.
• Auxiliaryloads.Loadsforheating,cooling,controls,andanythingelserequiredtooperatethebatterysystemmustbemeasured(andifpossible,measuredseparately)tofactorintopowerandefficiencycalculations.
• DataLogging.One-minutedatamustbeloggedforcurrent,voltage,reactivecurrent,andothervariables.
• OtherTests.Theproceduresfortestingefficiencyandpoweravailabilityarefairlywelldefinedinthecurrentversion(1.0),whileothertestmethods—todetermineRemaining
Figure: ESIC Test Manual Duty Cycle for roundtrip efficiency
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UsefulLife,Self-DischargeRate,andBlackStartcapability,amongothers—arestilltobedraftedbytheESICgroup.
InApril2016,PNNLupdatedtheProtocolforUniformlyMeasuringandExpressingthePerformanceofEnergyStorageSystems,adocumentthatwascoordinatedwiththeESIC.TheProtocolappliestoalltypesandsizesofelectricenergystorage,andwasfirstpublishedin2012.Itscontentisorganizedaroundmeasurementsforspecificgridapplications,suchasPVsmoothingorvoltagesupport.Thisissignificant,astheperformanceandefficiencyofresidentialenergystoragesystemswillvaryiftheyareusedmerelyforpeak-shavingvsdailyarbitragebetweenpeakandoff-peakrateperiodsvsgridbackuponly.Thediscussionaboveappliestoelectricalperformancemetricsforbatterysystems.Safetystandardsforbatteriesandinvertersarealreadyinplace,including:
• UL1741(forinverters)
• UL1973(forbatteries)
• UL9540(forsystemsthatcombinebatteriesandinverters)
• IEEE1547
• FCCClassBTestmethodgapsanddifficulties
Theobvioustestmethodgapisthelackofastandardmetricforroundtripefficiency,whichdependsonrateofdischarge/charge,depthofdischarge,temperature,andotherfactors.StandardizedmeanstomeasureroundtripefficiencyisimportantbecausethereisnotyetagreementonwhetheritshouldbemeasuredonaDC/DCbasis(whichwillproduceahigherefficiencyrating)orAC/ACbasis.Somebatterychemistriesdoverywellwhileinactive(i.e.lead-acid)whileotherswithhighauxiliaryloadsfarepoorlyiftheyarenotdeeplyandregularlycycled(i.e.liquidelectrolyte“flow”batteries).Residentialsystemsnearlyalluselithium-ionbatterycells,sothereissomeuniformityinthiscategory,andtheyperformrelativelywellinbothstandbyandheavycyclingmodes.Anotherfactoristhemodularityofhomebatterysystems.Forexample,Sonnenofferssystemsrangingfrom4to16kWhofnominalstorage,withdifferentratiosofinvertercapacitytocellcapacity.Theefficiencyofthesevariedsystemcombinationsisunlikelytobeexactlythesame—aproblemsimilartotheissueofsizingsolarpanelstotakemaximumadvantageofinvertercapacity.Thisgivesrise
Figure: Sonnen Battery in modular cabinet
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toseveralquestions:
• Ifseveralbatterypackagescanberununderasingleinverter/controller,dotheyneedtobetestedineachconfigurationtoearnanefficiencyrating?
• VendorsoftenswitchsuppliersfortheirLi-ioncells,asthebatteriesthemselvesapproachcommoditystatus—doesachangeincellsupplierrequireretesting?
• Cancombinationsofcomponentsearnratingsbasedoncomputersimulations,aswasdonewithNFRCwindowcalculationsinthe1990’s?
• Shouldbatteriesberatedonthebasisoftheirnormalstoragecapacitywhennew,ortheirtypicalaveragelifetimestoragecapacity?
Batterytestingproceduresdependonanimportantbutdifficult-to-measurequantity:StateofCharge(SOC).Thisisnotassimpleasmeasuringbatteryvoltageandapplyinganalgorithm,whichishowmobilephoneswork.Onutility-scalebatterysystems,thebatterymanagementsystem(BMS)doesitsowninternalcalculationsbasedoncellandmodulevoltages(whichcannumberinthehundredsorthousandsofmeasurements),aswellasintegrationsofcurrentvalues.Thismethod,knownas“coulombcounting,”isnon-linear,andnon-trivial.Therelianceoftestmethodsonself-reportingSOCcalculationsisapotentialweakspotinanyattempttostandardizetesting,asitishiddenfromviewandcouldbetiltedtoproducefavorableresults.Anotherchallengeisquantifyingbatteryperformanceovertime.Asweknowfromourlithium-battery-equippedcellphones,capacitydegradeswithfrequentuse.Howmuchcapacityislostovertimedependsoncellchemistryanddesign(whicharecontinuallyevolving),depthandrateofdischarge,numberofdischargecycles,operatingtemperature,theextenttowhichadvertisedcapacityisunder-orover-stated,andotherfactors.Developmentoftestingstandardsshouldincludesomeprovisionforevaluatingthisaspectofbatterysystems.Thiscouldinclude,forexample,ratingbatterycapacityonthebasisoflifetimeaverageperformanceratherthannew(similartothecomparisonsinlightingbetweeninitialandmeanlumens),orstatingthatcapacitymayonlybeclaimedinmarketingmaterialstotheextentitiscoveredbywarrantytoacertainnumberoffulldischargecycles,yearsofoperation,ortotalcapacitycycled.Asonerecentcomparisonamongtheleadingresidentialenergystoragesystemsfound,Teslaprovideda10yearwarrantyforits13.5kWhPowerWall2.0system,butnospecificsonhowperformanceandremainingstoragecapacitymightdegradeoverthatperiod.ItswarrantyfortheoriginalPowerWallsystemwasmorespecificbutlessencouraging.Itonlyguaranteed85%ofinitiallyclaimedcapacityinthefirst2years,and60%inthefirst10years,providedthatcapacityismeasuredunderoptimaltemperatureanddischargeconditionsandthedevicehasnotalreadyexceededpre-specifiedaggregatedischargeamounts.1Sonnenwarrantsits4kWhproductsfor10yearsto70%remainingcapacity.AquionAspenwarrantedits2.2kWhmodularproductsfor10yearsto70%remaining
1JohnPeterson,“WhyTesla’sSemiWillAlmostCertainlyBeShelved,”SeekingAlpha,11/20/17,https://seekingalpha.com/article/4126621-teslas-semi-will-almost-certainly-shelved.
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capacity(beforethecompanyrecentlydeclaredbankruptcy).LGChemwarrantsits3.3,6.5and10kWhproductsfor10yearsto60%remainingcapacity.2EnergyConsumption
Allbatteriesconsumesomeamountofenergyduringthepowerconversionprocessandwhilestoringpower,sometimesreferredtoas“leakage”.Whiletheinclusionofbatterystoragesystemsinahomemayprovideanarrayofbenefits,theywillincreasethehome’stotalnetenergyconsumption.Therearegenerallytwotypesoflosses:theenergylostthroughadischarge-chargecycle(“roundtrip”(RT)losses),andtheenergyassociatedwithsimplymaintainingafullcharge(“parasitic,”“standby,”or“self-discharge”losses).Theformeriscapturedbytheroundtripefficiencymetric,andisusuallyquotedinproductspecificationsheets,althoughinconsistently.Publishedefficiencydescriptorsinclude“AC,”“DC,”“AC-AC,”“DC-DC,”“grid-battery,”orsometimessimply“roundtrip”(withoutanyqualifier).Standbylossesareseldommentionedincurrentliterature;ourreviewofthirteenproductsfoundvaluesof3W(Eguana),
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annualenergylossesassociatedwithround-tripenergyconversionandhastenlossofbatterycapacitywithage.Table: Effect of Roundtrip Loss (Single System)
Item Qty Unit Notes
Systemsize(power) 5 kW Typicalvalue
Systemsize(energy) 10 kWh Typicalvalue
Cyclesperyear 365 d/y Onecycleperday
Cycledepth 80% % PercentofnominalkWhcapacity
Efficiency 90% % Midpointoftypicalvalues
Energydischarged 2920 kWh/y Pervaluesabove
Energytocharge 3244 kWh/y Dischargeenergy/effcy
Energyloss 324 kWh/y Charge-dischargeenergy
Energyrate $0.121 $/kWh USaverageresidentialrate,2013(EIA)
Energycost $39.26 $/y Ataverageelectriccost
Whatistheeffectofhighervslowerroundtripefficiency?Thetablebelowshowsthatthisworksouttoabout$39peryearforthetypicalresidentialcustomer.(Theenergycostfromthiscomparisonissimilartotheenergycostoftheaverageroundtriplossescalculatedabove—bothareabout10percentofthroughput.)
Table: Effect of Higher vs Lower Roundtrip Efficiency (Single System)
Item Qty Unit Notes
EnergyDischarged 2920 kWh/y Percalculationsabove
LowerRTefficiency 86% % PublishedRTACeffcyforSonnen
HigherRTefficiency 95% % PublishedRTACeffcyforEguana
Lowereffcyenergy
tocharge 3395 kWh/y Dischargeenergy/effcy
Highereffcyenergy
tocharge 3074 kWh/y Dischargeenergy/effcy
Lowereffcyloss 475 kWh/y Charge-dischargeenergy
Highereffcyloss 154 kWh/y Charge-dischargeenergy
Energysavings 322 kWh/y Deltaoflosses
Energyrate $0.121 $/kWh USaverageresidentialrate,2013(EIA)
Energycostsavings $38.92 $/y Ataverageelectriccost
Extendingthiscalculationtotheprojected2022USresidentialbatteryfleet(769MW—about150,000systems,assumingaveragesizingof5kW/10kWh)showsthattheefficiencyspreadmakesadifferenceofnearly$6millionperyearacrosstheprojectedUSfleet.
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Table: Effect of Higher vs Lower Roundtrip Efficiency (US Residential Fleet—2022)
Item Qty Unit Notes
Totalbatteryfleet 2562 MW GTMQ12017Forecastfor2022
Res’lbatteryshare 30% % GTMForecast—approximate
Res’lbatteryfleet 768.6 MW Totalfleet*Residentialshare%
Averagesystemsize 5 kW Typicalvalue
Qtyofres’lsystems 153720 # Fleetcapacity/systemsize
Fleetenergysavings 49446 MWh/y Singlesystemsavings*Qtyofsystems
Averageelectric
cost $0.121 $/kWh USaverageresidentialrate,2013(EIA)
Energycostsavings $5,983,000 $/y Ataverageelectriccost
Atthemoment,theseimpactsarelowenoughthatthecallforpublicactionmaybechallengingtomake.However,policiesfocusedonnetzeroenergyhomesoronmakingsolarhomesstorage-readycoulddemonstratesignificantimpactswithinthoseparticularhomes.Butalargerpointalsoemerges.Ifaresidentialenergystoragesystemhasthepotentialtoincreaseahome’sannualenergyconsumptionbyanamountsimilartoaddinganotherrefrigerator,shouldtheirpurchaseandinstallationbewidelypromotedunlessspecificconditionsaremet?Willtheirsocietalbenefitsoutweighthoseextracosts?
Activitiesbyotherstakeholders
CaliforniaEnergyCommission
WerecentlycorrespondedwithastafferattheCaliforniaEnergyCommission(CEC)togettheCommission’sinformalopiniononitscoverageofhomebatterysystemsintheirregulations.Homebatterystoragesystemsarecurrentlynotcoveredbythestate’sapplianceorbuildingenergycoderegulations.Californiarecognizesthepotentialbenefitsofresidentialstoragesystems,butareconcernedaboutpotentialunintendedconsequencesofthetechnology.Theywillbelookingintogatheringadditionaldataontheirperformancetoinformpotentialfutureregulationsofthisequipment.
InternationalElectrotechnicalCommission
TheIECisaddressingenergystoragethroughitsTechnicalCommittee120:ElectricalEnergyStorage(EES)Systems.ThestatedscopeofTC120is:“StandardizationinthefieldofgridintegratedEESSystems,”and:
• TC120focusesonsystemaspectsonEESSystemsratherthanenergystoragedevices.
• TC120investigatessystemaspectsandtheneedfornewstandardsforEESSystems.
• TC120alsofocusesontheinteractionbetweenEESSystemsandElectricPowerSystems(EPS).
TC120hasproduceddocumentsincluding:
• UnitParametersandTestingMethods-Generalspecification
• PlanningandInstallation-Generalspecifications
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• GuidanceOnEnvironmentalIssues-Generalspecifications
• Safetyconsiderationsrelatedtogridintegratedelectricalenergystorage(EES)
systems
• UnitParametersandTestingMethods-Generalspecification
TheTC120documentsareavailableonlytomembersoftheCommittee.USEnvironmentalProtectionAgency
TheEPAhasproducedscopingreportstoconsiderincludingsmallandlargeelectricenergystoragesystemsintheENERGYSTARlabelingprogram.ElectricPowerResearchInstitute
EPRIhoststheEnergyStorageIntegrationCouncil,asmentionedabove.TheirworkisdividedintothreeWorkingGroups:
• WG1—GridServicesandAnalysis
• WG2—TestingandCharacterization
• WG3—GridIntegrationTheESICTestMethoddescribedaboveisaproductofWG2,chairedbyNaumPinksy(SCE).JayHendersonofPG&EistheTestingSubgroupco-leader.NationalElectricalManufacturersAssociation
NEMAisawareoftheemergenceofenergystoragedevicesandunderstandstheneedfortestingstandards.Theirparticipationwouldbeorientedaroundtheinterestsoftheirmembercompanies.CSAGroup
TheoriginalCanadianStandardsAssociationhasexpandedintoCSAGroup,whichprovidestestingandcertificationglobally.Theyalsoareawareoftheneedforstandardizedtestingproceduresforenergystoragesystems.
Additionaltopicstiedtohomeenergystorage
Thisreportprovidesasurvey-levelviewofarapidlydevelopingtechnology,withalimitedscopeofinquiry.Severalaspectsofenergystoragearecandidatesforfollow-upstudy.ThePV-EV-Battery“Triangle”
BatteriessharemanycommonalitieswithsolarPVsystemsandelectricvehicle(EVs)—allarelargeDCenergyloadsand/orsourcesthathavejustemergedoverthelastdecade.Ascarmanufacturersshifttoelectricvehicleproduction,theyarealsodevelopingterrestrial(non-mobile)storagesystems—examplesincludeNissan,Mercedes,andTesla.ResidentialstorageinstallationsoverlapheavilywithsolarPVsystemssincetheybothuseinverters,andarelikelytooverlapwithEVownershipasthatmarketdevelops.Astheabovediagramsindicate,thesesystemsofferopportunitiesforperformanceandefficiencybenefitswhenconsideredtogether.Theconnectionsbetweenthesemarketsshouldbemonitoredforleverageonallthreetechnologies.
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Oneexampleofthecrossoverbetweencarsandhomeenergystorageistheareaofvehicle-to-grid(V2G)technology,whereacarbatterycanstoreelectricityforpotentialuseinthehomeduringcertaintimesoftheday,orforhomebackuppower.ManyEVsnowofferonboardenergystorageof40to70kWh—significantlylargerthantypicaldailyhouseholdconsumption,whichaveragesabout27kWhperdaynationally.ThepotentialbenefitsofV2Ghavegenerallybeendiscouragedbycarmanufacturerstoprotecttheirwarrantedbatteries,asfrequentand/ordeepcyclingcandegradethebatteryovertime.However,usingavehicleforoccasionalhousebackup(i.e.3or4timesperyear)couldprovidesubstantialbenefitwithouttaxingthebatteryhealth.EVmanufacturerscouldalsochargeafeeforV2Gservicesthatmightprovemorecost-effectivethaninstallingastationarysysteminthehome.Eventually,itislikelythatsomeformoftwo-wayenergyflowwillbepossible,orevencommon,fromelectriccars.Nissan’s2018Leafnowoffersthiscapability.(NotethatthereisnoreasontoattempttochargeEVsfromhomestorage;thelatterhaslowercapacity,andthecontrollabilityofEVchargingtimeeliminatesanysuchimperative.)Marketingapproachesandmarketcharacterization
Sellinghomebatterysystemsforanythingotherthanbackupprotectioniscompletelyreliantonutilityratesandtheregulatoryenvironmentinwhichtheyareset.FormostoftheUS,homebatteriescannotbejustifiedonthisbasis,butseveralstatesareevolvingratestructuresandincentivesthatarechangingthesituation.Thefigurebelowshowsthatatleasttwelvestatesarepromotingenergystorage,themostprominentexamplesbeingHawaii,California,Nevada,Massachusetts,andArizona.California’sSelf-GenerationIncentiveProject(SGIP)isoffering$57millioninfundingforresidential(
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Applied Curiosity • Breakthrough Results San Luis Obispo, CA • Durango, CO • ecosresearch.com
BenefitBundlingor“Stacking”
Residentialenergystoragesystemscanpromiseseveralseparatevaluepropositions.Thesewilldifferregionally,butthefollowingexamplefromGreenMountainPower’spilotresidentialenergystorageprograminVermontisillustrative,inwhichfiveseparatebenefitsadduptojustifysystemcost.(Novalueisgiventobackuppowercapability—thecurrentleaderinahomeowner’sperceivedvalueofbatterysystems!)Severalotherstudieshavereachedsimilarconclusions:thatbatterysystemsmustservemultiplefunctionstoearntheirkeep.Thisisatoddswiththecurrentresidentialstoragemarketwherebackuppoweristhemainsalespitch,butrepresentsthepotentialthatwilldrivefuturesystemmarketing.
Thisdiagramtreatsallofthevaluestreamssimilarlyandsumsthem,butofcoursesomeofthemonetizedbenefitsflowtothehomeowner/billpayer,someofthemflowtotheutility,andsomeflowtosocietyasawhole.Ifthehomeownerisbeingaskedtomaketheinvestmentintheenergystoragesystem,thedirectfinancialbenefitstothemmustbelargeenoughtojustifytheexpense,ortheutilityorgovernmentwouldlikelyneedtoofferincentivesreflectingthevaluetotheutilityandsocietyoftheotherbenefitsofthesystem.Finally,theutilitywouldneedtobeabletohavesomecontroloverthetimingofcharginganddischargingofthebatterytorealizethosebenefits.
Costtrends
Dataontherealinstalledcostsofbatterysystemsishardtocomeby,butcurrentlypublishedcostsrangefrom$1000to$2000perkWhofcapacity.Ifcostscanbereducedto$250-$500perkWh,manymorepossibilitieswillopenup.Assuminga10kWhsystemandthemidpointofthesecostranges,thisisthedifferencebetweenspending$15,000todayor
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Applied Curiosity • Breakthrough Results San Luis Obispo, CA • Durango, CO • ecosresearch.com
$3,750inthefuture.Batteriesareunlikelytogetcheaperasfastassolarpanelshaverecently,however.
Microgriddevelopments
Everybackup-capablebatterysystemiseffectivelyamicrogrid.Researchanddevelopmentofmicrogrids,particularlyintheareaofcontrolsandcommunications,willspurhomebatterycapabilities.Micro-inverters
Mostofthisreportisorganizedaround“stringinverters”withrelativelyhighelectricalcapacity(1-5kW)inonedesktop-sizedbox.Micro-inverters,likethosefromEnphase,aresandwich-sizedunitsthatconnectsinglesolarpanels(200-300W)withACwiring,whichsimplifiessomeaspectsofsystemdesign.Themodularityofmicro-invertershasthepotentialtoharmonizetheACandDCloadsandsourcesinahome.Recommendations
Thisstudyraisestechnicalandpolicyissuesaroundresidentialbatterytechnology,andisintendedtospurdiscussionofpossibleactionstosupporttheemergenceofproductsthataddresstheveryrealchallengesfacingthefutureofourelectricalsystem—renewableenergypenetration,gridstability,utilityreliability,andconsumerprotection.Somestartingpointsforthatdiscussionandfutureaction:
• Developuniformtestingmethodsforhomebatteries.Thebuildingblockstocreatetestmethodsalreadyexist—thankstoeffortsorientedtowardlargersystems—butstandardizedproceduresforresidentialsystemsarenotyetinplace.
• Designandimplementfieldtestingofresidentialbatteriestodeterminehowthepromiseofthesesystemssquaresupwithreal-worldoperation,bothwhenfirstinstalled,overtime,andwithaggressivecycling.
• Accessmanufacturerdatashowinghowtheirproductsareactuallybeingused,includingdataoncyclefrequencyanddepth.
• Establish“storageready”criteriafornewconstructionandmajorretrofits,similartothe“solarready”requirementsinTitle24.
• Encourageuniformlabelingandminimumwarrantycoverageforresidentialbatterysystems.
• FollowactivitiesinEuropeandAustralia,wherethousandsofresidentialbatterieshavealreadybeeninstalled.
• ParticipateinIECTC120(requiresmembershiponthecommittee),wheredevelopmentoftestmethodsandstandardsarediscussed.
• Establishanetworktofacilitateinformationsharingandtocoordinateactivitiesbetweenstakeholdersincludingthoselistedabove.
• Educatethepubliconthemultiplebenefitsofbatterysystems.Currently,mostbatteriesarebeingmarketedasbackupsystemsorincombinationwithsolarPVinstallations.Fleetsofbatteriesthatsitidleuntilapoweroutageareamissedopportunity,andrepresentnewresidentialload.
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Applied Curiosity • Breakthrough Results San Luis Obispo, CA • Durango, CO • ecosresearch.com
• Monitorproductdevelopmentsandclaimsforveracity—dokWhcapacityandround-tripefficiencyclaimsholdupunderactualuse?
• Setminimumefficiencystandards,and/orprovideincentivesforhigher-efficiencysystems.
• EstablishcompatibilitycriteriaforDC-coupledbatteryproducts—namelystandardizedvoltages—thatwouldalloweasierretrofittingofbatterysystemstohomeswithexistingPVsystems
• Considerutilityincentivesthattargettheintersectionofgridmodernizationandcustomerbenefits.Furtherstudyisneededtodeterminethecaseforstorage.