home battery storage systems — a look into the market ......nov 30, 2017 · this system combines...

Applied Curiosity • Breakthrough Results San Luis Obispo, CA • Durango, CO • ecosresearch.com

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


• 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:


• 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.


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


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


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.


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


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


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.


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),


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.


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


• 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.


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(


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


$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.


• Monitorproductdevelopmentsandclaimsforveracity—dokWhcapacityandround-tripefficiencyclaimsholdupunderactualuse?

• Setminimumefficiencystandards,and/orprovideincentivesforhigher-efficiencysystems.

• EstablishcompatibilitycriteriaforDC-coupledbatteryproducts—namelystandardizedvoltages—thatwouldalloweasierretrofittingofbatterysystemstohomeswithexistingPVsystems

• Considerutilityincentivesthattargettheintersectionofgridmodernizationandcustomerbenefits.Furtherstudyisneededtodeterminethecaseforstorage.

home battery storage systems — a look into the market ......nov 30, 2017 · this system combines...

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