Measuringirradiance,temperatureandangleofincidenceeffectsonphotovoltaicmodulesinAuburnHills,Michigan

by

MichaelS.Buday

Apracticumsubmittedinpartialfulfillmentoftherequirements

forthedegreeofMasterofScience/SustainableSystems(NaturalResourcesandEnvironment)

attheUniversityofMichiganAugust2011

Advisors

ProfessorGregoryKeoleian,ChairBillMarion,PrincipalScientistandProjectLeaderofPhotovoltaicResearchNREL

ii

ContentsAbstract...............................................................................................................................iiiAcknowledgments...............................................................................................................ivIntroduction........................................................................................................................1LiteratureReview................................................................................................................3Methods..............................................................................................................................8Results...............................................................................................................................15Conclusion.........................................................................................................................20Glossary.............................................................................................................................22References........................................................................................................................24Appendices........................................................................................................................27

ListofTables

Table1:IEC618531Matrixbasedtextconditions.........................................................................1Table2:Modulesundertest..........................................................................................................14Table3:TrendsinpowerbyPVmaterial.......................................................................................15Table4:Powerat1000W/m2byPVmaterial................................................................................16Table5:ObservationsfittingIEC618531parameters..................................................................19

ListofFigures

Figure1:ImpactsoftemperaturecoefficientsonPVpower...........................................................4Figure2:Solarspectrameasurementsandlinefitfromonedayfromexistingliterature.............5Figure3:CalculatedmismatchfactorforaSimodulefromexistingliterature..............................6Figure4:Systemdiagram.................................................................................................................8Figure5:SampleIVcurve................................................................................................................9Figure6:K12430IVCharacteristicspulsemode..........................................................................10Figure7:Fourwireconnection......................................................................................................10Figure8:KI#7053Switchcardconnectionfor5

iii

AbstractUnitedSolarOvonic(USO)installedaphotovoltaic(PV)moduletestingsystemin

AuburnHills,Michigan(Latitude42.6978,Longitude83.2419)inMarchof2010forthepurposeofevaluatingtheimpactsofirradiance,temperatureandangleofincidence(AOI)effectsonPVmoduleperformance.Weconsideredvarioustestbeddesignsandultimately,constructedasourcemeterbasedcurrentandvoltagemeasurementsystemcoupledwithadataacquisitionsystemrecordingreadingsfromweatherstationinstrumentsthattracksolarirradiance,temperatureandwindspeed.Current,voltageandpowerobservations,correlatedtoourweatherstationdevicereadings,werecollectedfromcommerciallyavailablePVmodulesmanufacturedfrommonocrystallinesilicon(cSi),amorphoussilicon(aSi)andcopperindiumgalliumselenide(CIGS).WeobservedthermalannealinginaSiandtheeffectsoftemperatureoncSiandCIGS.cSimoduletemperaturesabove25Cappeartodiminishpowerbyapproximately0.5%/C.Theresultswereconsistentwithourexpectationsbasedonexistingliterature.Fromthis,weinferthatthetestbediseffectiveatmeasuringmoduleperformance.

Ourresultssupport,butdonotconfirmthehypothesisthataSimodulesdelivermoreenergy(kWhrs)perpeakwatt(Wp)thanotherPVmaterials.ConfirmingthehypothesiswouldrequirebothtestingastatisticallysignificantnumberofPVmodulesandperformingaquantitativeanalysisoftheaccuracyofthetestbed.ThisisimportantbecausePVistypicallysoldona$/Wpbasis.TheWpratingisbasedonamodulesperformanceunderstandardtestconditions(STC)of1000W/m2,1.5airmass(AM)and25Cmoduletemperature.AnewPVratingsystemproposedbytheInternationalElectrotechnicalCommission(IEC)createsaseriesoftestingconditionsbasedonavarietyofweatherconditions.USOsPVmeasurementsystemiscapableofcollectingobservationsfittingmost,butnotallofthesetestconditions.Duetothearraysnorthernlocation,noneofourobservationsfittheIECshightemperatureconditions.

iv

AcknowledgmentsThisprojectwasmadepossiblethroughsponsorshipbyUnitedSolarOvonic

(USO)anditsChairmanSubhenduGuha.Iamverygratefulfortheopportunitythathegaveme.IworkedunderthedirectsupervisionofUSOSeniorResearchScientist,KevinBeernink.Hisguidance,instruction,supportandparticipationwerecriticaltothesuccessoftheproject.IalsoreceivedhelpfrommanyotherUSOemployees,mostnotably,ChrisWorrel,DavidWolf,GregDimaggio,LoganRowe,GingerPietka,KiasYounan,EricAhkashian,BrentBatchelder,MikeOndocsin,TonyTurkin,MikeWalters,andJacobWashington.

JosephdelCueto,BillMarionandDarylMyers,allattheNationalRenewableEnergyLaboratory(NREL)holdararelevelofexpertiseinthisfield.Theirpublications,guidanceandfeedbackwerevitaltothedevelopmentofthisreport.

TimothyDierauf,AdrienneKimberandJohnPrevitalisubmittedpresentationsattheAmericanSolarEnergySocietys2011nationalconferenceandprovidedmewithreferencematerials.Throughtheirwork,theyhavefurtherdemonstratedtheimportanceofthisfieldofresearchinindustry.

SondraAuerbach,JenniferTaylorandDianaWoodworthintheSchoolofNaturalResourcesandEnvironments(SNRE)OfficeofAcademicProgramswelcomedmetotheSNREcommunityandfacilitatedmyeducation.OutsideofSNRE,ProfessorDebraRowe,RobertPratt,DavidLankheetandProfessorIanHiskensweremykeyteachersinthisdiscipline.Thesepeopleallhelpedputmeonthistrack,makingthisreportpossible.

SNREProfessorandCoDirectoroftheCenterforSustainableSystemsGregoryKeoleianwasinstrumentalinapproachingUSOregardinganinternshipandmastersproject.Thetimehehasspentteachingandmeetingwithmeoverthepastthreeyearshasbeeninvaluabletoboththisprojectandmyeducation.Hedevotedasignificantamountoftimetooverseeingthisprojectandcontributingtothecontentandstructureofthisreport.HelaineHunscher,ProgramCoordinatorattheCenterforSustainableSystems,alsoprovidedvitalfeedback.

DonnaandJosephNapolitanogavememyfirstjobinthesolarindustry.TheyhavealwaysencouragedmetolearnandtoapplywhatIlearn.TheSNRE,ErbInstituteandGreatLakesRenewableEnergyAssociationcommunitiesalsoprovidedmewithcollaborativelearningenvironments.

Finally,Iwouldliketothankmyparents,EdnaandGene,fortheirunconditionalsupportandencouragementthroughalengthycareertransition.

IntroductionPhotovoltaic(PV)moduleandarrayperformanceisdifficulttopredictdueto

variationsinweather,airmass(AM),andnonlinearperformancecharacteristicsofvariousmoduletechnologies.Manufacturers,distributorsanddeveloperstypicallysellPVonacostperpeakwatt($/Wp)basis.AmodulesWprating,alsoknownasitsPmax,isbasedonitsperformanceunderStandardTestConditions(STC)consistingof1000W/m2,1.5AMand25Cmoduletemperature.However,theseconditionsrarelyoccursimultaneouslyinnatureandtheperformanceofPVmaterialsvariesovertimeandbygeographiclocationbasedprimarilyondifferencesintemperatureandAM(Marion,Kroposki,Emery,delCueto,Myers,&Osterwald,1999).TheInternationalElectrotechnicalCommission(IEC)hasproposedPVratingstandards(IEC61853)thatincludecharacterizingmoduleperformancebasedonamatrixofvariousweatherconditions,includinghightemperatureconditions(HTC),STC,nominaloperatingcelltemperature(NOCT),lowtemperatureconditions(LTC)andlowirradianceconditions(LIC).ThecriteriafortheseconditionsappearinTable1below.NotethatIEC618531doesnotincludeevaluatingperformanceunderphotovoltaicsforutilityscaleapplicationstestconditions(PVUSAorPTC)of1000W/m2,20Cambienttemperature,windspeedof1meter/secondand1.5AM.

Table1:IEC618531Matrixbasedtextconditions

Abbrev. Description Irradiance(W/m2)Module

Temp.(oC)AmbientTemp.(oC)

WindSpeed(m/sec)

AM

HTC Hightemperatureconditions 1000 75 1.5

STC Standardtestconditions 1000 25 1.5

NOCT Nominaloperatingcelltemperature 800 20 1 1.5

LTC Lowtemperatureconditions 500 15 1.5

LIC Lowirradianceconditions 200 25 1.5

Withitsproposedratingsstandards,theIECseekstoimprovethemethodbywhichPVmoduleperformanceisevaluatedbymeasuringPVpowerunderasetoftestingconditions,insteadofonlySTC.ThestandardsalsoestablishguidelinesforratingPVbasedonenergyyield(watthours)andperformanceratio(PR)(Poissant,Pelland,&Turcotte,2008)(delCueto,2007).

AdvancesininvertertechnologyhavereducedthecostofPVarrayperformancemonitoringsubstantially,tolessthan$1000orto1%10%ofoverallsystemcostfora1

2

20kWgridtiedsystem(EnphaseEnergy,2011)(FroniusUSALLC,2011).ThisiswithinreachofmanyhomeownersinstallingPVsystems.Thesemonitoringsystemsreportonanarraysenergyyield.However,thesesystemsdonotprovidethelevelofinformationacquiredfromperformingafullcurrentvoltage(IV)sweep.Theyalsodonotincludeweatherstations.TheequipmentrequiredtoeffectivelyanalyzetheelectricalcharacteristicsofPVmodulesandarraysaswellastheeffectsofirradianceandmoduletemperaturerangesbetween$10,000and$50,000(notincludingthemodulesthemselves).TobetterunderstandandpredictPVsystemperformance,NRELsPerformanceandEnergyRatingTestbed(PERT)andOutdoorTestingField(OTF)gobeyondperformingbasicenergyyieldandpowerratingmeasurements.ResearchscientistsatNRELdesignedsystemswiththeabilitytoexaminetheelectricalandopticalresponsecharacteristicsofvariousproductionmodulesandprototypesovertime.

MichiganbasedPVmodulemanufacturer,UnitedSolarOvonic(USO)soughttodevelopasystemwithPVmoduletestcapabilitiessimilartoNRELsstandalonetestingsysteminordertoconducttheirowncharacterizationofcompetitorsmodulesandUSOsnextgenerationofproductsonafractionofNRELsbudget.ThesystemwentliveinMarchof2010andcontinuestoperformandrecordregularmeasurements(IVsweeps)on20PVmoduleseverytenminutesduringdaylighthours.Italsorecordsbasicmeteorologicalconditionswitheachsweep,includingplaneofarray(POA)irradiance,ambientandmoduletemperature,andwindspeed.WecollectedandbundledfieldobservationsintothestandardscategoriesrepresentingvariousweatherconditionsinordertoverifythecapabilityofUSOsmeasurementsystemtotestIEC618531.

OncewedesignedandinstalledourPVmoduletestingsystemandbegancollectingdata,wethenhadtheinformationneededtoevaluatemoduleenergyproductionaswellastheelectricalandbasicopticalresponsecharacteristicsofthevariousPVmodulesundertest.Weappliedourtestmethodandfoundresultsconsistentwithourexpectationsbasedonexistingliterature,whichishighlightedinthenextsection.Inthispaper,wereportontherelationshipswefoundbetweenpowerandirradianceaswellasbetweenpowerandmoduletemperaturebasedonobservationstakenfromselectamorphoussilicon(aSi),crystallinesilicon(cSi),andcopperindiumgalliumselenide(CIGS)modulesfromJulythroughDecemberof2010.(ObservationsfromDecember14ththrough20thwereignoredbecausemanyofthemoduleswerecoveredinsnow.)Wewereunabletoobtaincadmiumtelluride(CdTe)modulesduetothemanufacturerstightcontrolofitsdistributionchannel.

Giventhatpoweristheproductofvoltage(V)andcurrent(I),theproposedstandardcallsforevaluatingopencircuitvoltage(Voc)andshortcircuitcurrent(ISC),and

3

fillfactor(FF)againstbothirradianceandmoduletemperature(delCueto,2007).FFindicatesamodulesrelativeefficiency.WedesignedandbuiltasystemcapableofmeasuringalloftheseparametersandothersincludingVmp.(ManyoftheserelationshipsappearinAppendices5through7.)However,themeasurementsystemisnot100%accurate.Thereisalevelofuncertaintyinthedataitgenerates.

LiteratureReviewThirtyyearsago,researchersinthePVfieldacknowledgedtheneedtogo

beyondSTC,suggestingthatmoduleperformancebecharacterizedbycategoriesofweatherconditions(hotsunny,coldsunny,hotcloudy,coldcloudy,andnice)(Marion,Kroposki,Emery,delCueto,Myers,&Osterwald,1999)(Gay,Rumberg,&Wilson,1982).TheIECscurrentproposedstandardseekstoaddressthisneed(delCueto,2007).

ThemainpurposeoftheliteraturereviewwastoexamineexistingresearchinordertoidentifythekeyvariablesaffectingPVpowergenerationandtodeterminehowtomeasurethosevariableseffectively.Existingliteraturewashelpfulinestablishingoursystemarchitecture,indefiningperformancemetricsandinidentifyingareasofopportunityforfurtherlearning.WesoughttodevelopmeasurementandtestingcapabilitiesapproachingthoseofawellfundedgovernmentlaboratoryandsofocusedourresearchonpublicationsfromNRELandSandiaLabs.WealsoreviewedarticlesrelatingtoperformanceanalysisofinstalledPVarraysranginginsizefrom2500kWp.

IrradiancehasthegreatestimpactonPVpower.Beyondirradiance,moduletemperature,angleofincidence(AOI)andAMalsoaffectamodulesoranarrayspowerandproduction(delCueto,2007)(Myers,2009)(King,Kratochvil,&Boyson,1997).Moduletemperatureisinturn,influencedbyambienttemperature,cloudpatternsandwindspeed.ResearchershaveusedsophisticatedtestingandmeasurementdevicesinPVperformancetestingforover30years,yetpredictingmoduleperformanceremainscomplexandforecastersmustacceptarelativelyhighlevelofuncertaintyinpredictingenergyproductionfromaPVarray.Additionally,underrapidlychangingandextremeweatherconditions,inverterramptimesandclippingbothdiminishACpowergeneration(vanCleef,Lippens,&Call,2001).

Researchersusepyranometerstomeasureirradiance,buttherearedifferentclassesofpyranometers.SecondarystandardandfirstclassthermopiletypepyranometersmeasureirradiancethroughouttherangeoffrequenciestowhichPVresponds(3002800nm).Silicondiodepyranometersonlyrespondtoanarrowerrangeoffrequencies(4001100nm),butarestillusedinPVweatherstationsbecausetheyarelessexpensivethanthermopilesandbecausetheyareactuallymoresensitivethan

4

thermopilesintherangeoffrequenciestowhichtheydorespond.Furthermore,siliconbasedPVisalsoprimarilyresponsivewithinthisrange.

MeasuringmoduletemperatureisimportantbecausecSiandotherPVmaterialsproducelesspowerathightemperatures.Figure1(below)indicatestheimpactsofmoduletemperatureonvariousPVmaterials.Estimatingmoduletemperatureisachievedwiththeuseofathermocoupleattachedtothebackofthemodule.However,undermostconditions,themoduletemperatureislikelytobewarmerthanthebackofthemodule.Furthermore,thetemperatureofthemoduleisnotlikelytobeconsistentthroughoutitsentiresurface.Forthesereasons,researchersapplyastandardadjustmenttothebackofthemoduletemperaturereadingtocompensateforthetemperaturedifferencebetweenthebackofthemoduleandsurfaceofthemoduleandtheymayplacemorethanonethermocoupleinseveralspecificpositionsonthebackofamoduleinordertocalculateanaveragetemperature.

Figure1:ImpactsoftemperaturecoefficientsonPVpower

AhighAOIcansignificantlyreducePVpowergeneration.RedaandAndreasprovidedthesolarpositionalgorithm(Reda&Andreas,2008)weusedtofindzenith,declination,andazimuthanglesrequiredtocalculateAOIinordertoplotitagainst%Pmaxandothervariables.

PVperformanceistypicallyreportedtoAM1.5.(ThisvaluewaschosenbecauseitrepresentstheaverageAMatsolarnoonforoptimallytiltedPVarraysatlatitudesinthecontinentalUS.)AM1.5reference(orstandard)referstotherelativepathlengthofthedirectsunlightthroughtheatmosphere.Withthesundirectlyoverhead(zenith)AMis1.0(uncorrected).Withlongerpathlengths,thereismorescatteringandabsorptionofsolarradiationbyatmosphericconstituentssuchaswatervaporandaerosols.Similarto

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7

sensitivetowatervaporandturbiditythancSi,CIGSandCdTe.AsstatedpreviouslyoursystemdoesnotincludeaCdTemoduleundertestbecauseofthemanufacturerstightlycontrolleddistributionchannel.Instead,wereliedonexistingliteraturetoprovideabasiccomparisontotheresultswefoundforcSi,aSiandCIGSmodules(delCueto,2007).

WealsorecordedwindspeedandambienttemperaturewitheachmodulesIVsweep,eventhoughMyersshowednostrongcorrelationbetweenpowerandeitherwindspeedorambienttemperature(Myers,2009).

Theliteraturealsoprovidedabasisforunderstandingthelevelofuncertaintywecouldexpectfromouranalysis.Thereareseveralsourcesofuncertaintyincludingalackofprecisioninthemeasurementdevicesandrapidlychangingconditions(e.g.,irradiance)duringtestperiods.TheprecisionratingsofourmeasurementdevicesareavailableinAppendix1.Duringoneexperimentconductedin1998,MarionatNRELfoundthatBecauseoferrorsinmeasurementsandenergyratingmethodology,differencesof8%orlessintheenergyratingsoftwoPVmodulesarenotsignificant.IfoneofthemodulesisaSi,differencesof13%orlessintheenergyratingsoftwoPVmodulesarenotsignificant.(Marion,2000)

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9

Figure5:SampleIVcurvewithYaxisA(left)andPowercurvewithYaxisW(right)

NotethattheillustrationaboveshowsastandardfirstquadrantIVcurve,butthatthesysteminfactcarriesoutfourthquadrantsweeps.InadditiontoVocandIsc,thefilealsorecordsPmax(thepointatwhichtheproductofVxIreachesitsmaximumpowervalue),FF[=(VmpxImp)/(VocxIsc)]andatimestamp.ThesysteminitiatesanIVsweepeverytenminuteswhenirradianceisgreaterthan20W/m2(2%offullsun).Thisisnotasfinearesolutionassomeothermonitoringsystems(e.g.,NRELsOTF)whichtakemeasurementseveryminuteorevenmorefrequently.

Alternatedwithitssweeps,thesoftwarecallsforreadingsfromdevicesconnectedtothedataacquisitionsystem,namelyaMaximummodel#41threecupanemometer,typeKthermocouplesattachedtothebackofeachmoduleandtwoambientpoints(shadedandnotshaded),andseveralKippandZonenSPLite2photodiodedetectorpyranometers.WealsoaddedasecondarystandardKippandZonenCMP21thermopileinMarch2011.Aspreviouslystated,alloftheotherfindingsinthisreportarebasedonobservationstakenbetweenJulyandDecember2010.However,thefindingsbasedonCMP21measurementsweretakenbetweenMarchandJuly2011.(ThemeasuredcorrelationbetweenaSPLite2andtheCMP21appearsinAppendix2.)However,theCMP21wascalibratedbythemanufacturerandnotputthroughNRELsmorerigorouscalibrationprocess(Emery,etal.,2005)(DevicePerformance,2006).

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Wecalculatedenergyyieldsas:

Equation3

,withatimeincrementbetweenmeasurementssetto10minutesduringdaylighthours.

Wethenindexedpowerandenergybyarea(m2)andweestimatedmoduletemperatureasmeasuredbackofmoduletemperatureplus3Cper1000W/m2,basedontheindustrystandard.

InfluencedbydelCuetosrecentstudy,wereportedourresultsinaseriesofgraphsplotting%Pmax,Isc,VocandFFagainstPOAirradiance,aswellas%PmaxagainstAOI,%PmaxandFFagainstmoduletemperature,andFFand%Pmaxovertimeintervals(delCueto,2007).Wealsocalculatedlinearandpolynomiallinefitsfor%Pmaxversusirradiance.Finally,usingtheequationbelow,wecalculatedforpowercorrectionbasedontemperaturecoefficientinordertohelpassesstheimpactofmoduletemperatureandtoestimatetheeffectsofAMandotherfactors:

Equation4

% % withtemperaturecoefficient,

delCuetofilteredoutobservationstakenduringhazyskyconditionsconsistingofprimarilydiffuseradiationinordertoanalyzePVperformanceunderclearskyconditionsordirectradiation(delCueto,2007).Ourworkdidnotincludesignificantfilteringortheapplicationofdatacorrectionfactors,buttheseareareasofpotentialfurtherresearch.Instead,wefocusedonreportingpowergenerationandelectricalcharacteristicsundertherealworldconditionsthatthetestmodulesexperienced.PVmodulesundertestappearinTable2below.

Table2:PVModulesundertest

Tech Module Pmax(W)ISC(A)

IMP(A)

VOC(V)

VMP(V)

FillFactor

TempCoeffPwr(%/oC)

Area(m2) Efficiency

cSi STP160S24/Ab1 160 5.0 4.65 43.2 34.4 0.74 0.48 1.2766 12.5%

aSi KanekaGSA060 60 1.19 0.9 92 67 0.55 NA 0.9504 6.3%

CIGS GSEPN33030O 30 2.2 1.7 25 17.5 0.54 0.5 0.3937 7.6%

CIGS SolyndraSL001165 165 2.74 2.37 93.9 69.6 0.64 0.24 1.9656 8.4%

aSi USOPVL68 68 5.1 4.13 23.1 16.5 0.58 0.0021 1.1225 6.1%

R

cVte

PPirir

T

inasttera

ResultsThere

Si,andCIGSVocandFFagemperature,

TheIEmaxwithrespmaxtoirradiarradiance.Tarradiance.

able3:Tren

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esultsofourSmodulesreainstPOAir,andFFand

ECproposedpecttotempanceandtheable3(below

ndsinpower

ModuMateri

aSi

CIGS

cSi

earrelationsAppendices5tion,especiarelationshipandairmasmaxreadings

rnumericalaespectively.radiance,%P%Pmaxover

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shipbetween57a).Apolyallybelow10pbetweenposs)alsoaffecatfullsun(

15

analysesappEachappendPmaxagainsttimeinterva

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npowergenynomialequ000W/m2.Aowerandirrctpowergen1000W/m2)

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rlinearinteaswellasav1xln(POAthatwefou

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endices5throfgraphsplandFFagain

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0.98

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rough7foraotting%Pmanstmodule

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16

Table4:Powerat1000W/m2byPVmaterial

ModuleMaterial

%PmaxatFullSun(+/0.5%)

Mean Low High

aSi 101.1% 90.2% 106.1%

CIGS 78.9% 76.6% 86.0%

cSi 86.2% 79.8% 94.1%

Inaccuratemeasurementdevicesandrapidlychangingconditionsduringtestperiodsalsoimpactresults.

AsindicatedinFigure14below,theaSimoduleclearlydemonstratesasuperiorpowerindextoirradianceperformanceratio.ThiscorrespondstotheequationspresentedinTable3above.

Figure14:PowervsPOAIrradianceforaSi,cSiandCIGSmodules

aSimodulesdeliversuperiorperformanceindexresultsathigherlevelsofirradianceduetoafavorabletemperaturecoefficient.Themanufacturersstate

0%

10%

20%

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cylindricalCIGS

17

temperaturecoefficientsof0.38%/C,0.48%/C,and0.0021%/CfortheirCIGS,cSiandaSimodulesrespectively.

ForaSiandcSi,moduletemperaturestypicallyaverage5060Catfullsun.ForcSi,onecanexpecta5060CmoduletemperaturetoreducePmax12.517.5%[(6025)x0.48%=17.5%]toaPmaxbetween82.587.5%ofSTC.Thisisconsistentwithourresults.

CorrectingfortemperatureonthecSimodule,alinearfitofPmaxcorr=0.00106xPOAIrr0.0582(R2=0.976)yields100.2%Pmaxat1000W/m2.ApolynomialfitofPmaxcorr=0.00000012POAIrr2+0.00118POAIrr0.711yields98.9%Pmaxat1000w/m2.TemperaturecorrectedobservationsandtheircorrespondinglinearandpolynomialfitsappearinFigure15.Thisimpliesthatathighirradianceconditions,whenAMtypicallyrangesbetween1and2,AMdoesnotsignificantlyimpactpowergeneration.However,AMcanexceed10neardawnandduskandhasamuchgreaterinfluenceoverpowerunderthose,butnotalllowirradianceconditions.

Figure15:TemperaturecorrectedPmaxcSi

BecausetheUSOtestbedlackstheabilitytomeasureAMandbecausetheremaybeinaccuraciesinitsmeasurementsofmoduletemperature,ourabilitytomeasureandisolatetemperaturedependenceislimited.However,havingcapturedbothnearbyambientandbackofmoduletemperaturesalongwithpowergeneration,POAirradianceandwindspeed,wewereableobserveperformanceunderavarietyofrealworld

y=0.00106x 0.05823R=0.97634

y=0.00000012x2 +0.00117777x 0.07105246R=0.97727378

20%

0%

20%

40%

60%

80%

100%

120%

140%

0 200 400 600 800 1000 1200 1400

%P m

ax

POAIrradiance(W/m2)

Linear(TempCorr) Poly.(TempCorr)

18

conditions.SimilartoMyersfindings,observationsfromourstudy,presentedinAppendix8indicatethatwindspeeddoesnotstronglyimpactpower(Myers,2009).

AlsoasexpectedforaSi,weseeanoscillationinfillfactors(i.e.,efficiency)throughouttheseasonsfromapproximately0.60inmidJulyto0.53inlateDecember(Appendix5c).Intermsofmoduletemperature,FFrangesfrom0.53near0Cto0.60between30and50C(Appendix5h).ThedownwardtrendfromsummertowinteristheresultofanincreasedStaeblerWronskieffectunderlowtemperatureconditionsandthermalannealingduringwarmperiods(Gregg,Blieden,Chang,&Ng,2005).cSiandCIGSmoduleFFs,ontheotherhand,remainsteadierduringthetestperiodat0.70and0.64,respectively(Appendices6and7c).ThemanufacturerslistFFsof0.74,0.64,and0.55forcSi,CIGSandaSi,respectively.

InAppendices5through7b,theupperbandofobservationsupto105AOIrepresentsclearskyconditionswhereasthelowermassofobservationsreflectmeasurementstakenunderovercastconditions.Outliersabovethebandmostlikelyindicatemostlysunnyconditionswithscatteredcloudsenhancingpowerthroughdiffuseirradiancethatenhancesoverallirradiancewithoutobstructingdirectsunlight.Intheseextremecases,totalPOAirradianceexceedsfullsun(POAIRR>1000W/m2).Thisworkdidnotincludeseparatingclearskyobservationsfromcloudyskies,butdelCuetomeasuredaspecificPVmodulesperformanceunderclearskyconditionswithPOAIrradiance=A+Bxcos(AOI)andfound:

A B 1standarddeviation

87.47.8W/m2 1142.520.9W/m2 93.711.2W/m2

HedidthisbyfittingitsphotoresponseasafunctionofAOIintosegments(

19

Table5:UnitedSolarOvonicsTestbedPVPower(W)ObservationsfittingIEC618531parameters

aSI cSi CIGS

HTC*

%Pmax 0.999 0.821 0.709

Avg 60 131.3 117

Min 55.8 124.8 102.1

Max 63.9 139.2 130.8

Obs 17 47 30

STC

%Pmax 0.968 0.921 0.643

Avg 58.1 147.4 106.0

Min 53.1 138.7 86.3

Max 63.2 160.9 127.2

Obs 5 4 91

NOCT

%Pmax 0.766 0.72 0.632

Avg 46.0 115.1 104.3

Min 39.3 104.8 97.1

Max 52.6 132.6 125.7

Obs 95 82 63

LTC

%Pmax 0.408 0.507 0.455

Avg 24.5 71.6 75.0

Min 20.5 63.7 62.4

Max 30.4 81.2 105.7

Obs 24 20 122

LIC

%Pmax 0.185 0.157 0.195

Avg 11.1 25.2 32.2

Min 5.3 14.3 20.7

Max 15.8 35.2 50.2

Obs 191 195 360

*Thecool,humidconditionattheAuburnHills,MichigantestsitedidnotyieldHighTemperatureConditions,sotheseHTCobservationsincluderesultsformoduletemperaturesaslowas60C,ratherthantheIECproposedstandardof75C.

20

ConclusionTheoutdoorPVmoduletestingsystemdevelopedbyUnitedSolarOvonic

dramaticallyincreasedtheorganizationscapabilitiestotesttheperformanceofitscompetitorsanditsownPVmodulesincludingprototypes.USOalsoreliesonSpiresimulators,acceleratedtestingandothermeansoftestingPVinordertobetterunderstandandultimatelyfacilitatetheadvancementofPVtechnology.

TheUSOtestbediseffectiveattestingPVmoduleswithIsc

21

ResearchersanalyzetheconstituentparametersofPVpower,butdevelopersandconsumersunderstandablyonlycareaboutpowergeneration.Themodulesinthisstudyproducedbetween0.76and0.95WperkWhofPOAirradianceperSTCWPmax.TheseresultsappearinAppendix9.

Themodulesundertestproducedbetween51.5and105.1dcWperkWhofPOAirradianceperm2.GiventhatPVmodulesaresoldona$/Wpbasis,efficiencybecomesasecondaryfactorwhenselectingamodule.However,efficiencyquicklycomesbackintoplayassystemdevelopersandbuyersconsiderspaceconstraints(i.e.,roofrent)andbalanceofsystemcosts.1000squaremetersofarraywillrequireapproximatelythesameamountofracking,wire,overcurrentprotection,laborcosts,etc.regardlessofthetechnologyandefficiencyofthemodules.Inthiscase,asystemwithahigherefficiencymodulewillgeneratemoreenergyinthesameamountofspaceasalessefficientpanelandthoughthemoduleswouldhavecostmorebasedontheirSTCratings,modulecostrepresentsonlyafractionoftheoverallsystemcost.ThebalanceofsystemsarelikelytocostapproximatelythesameamountregardlessofthePVmodulematerial.ForthesereasonsPVmoduleefficiencyremainsanimportantfactorforconsideration.

22

Glossary Efficiencywithrespecttoreferenceconditions

Wavelength

A Testmodulearea

aSi AmorphousSilicon

AM AirMass

AOI AngleofIncidence

cSi CrystallineSilicon

CIGS CopperIndiumGaliumSelenide

Energyyield Whrs/Wp

Eref() Referencespectralirradiance

Es() Measuredspectralirradianceofthelightsource

Et Totalirradiance

FF Fillfactor

Imp CurrentatPmax

Isc Testmoduleshortcircuitcurrent

IEC InternationalElectrotechnicalCommission

Inverterramptimes

ACpowerlossesthatoccurduringsuddenfluctuationsinirradiance

Inverterclipping ACpowerlossesthatoccurwhenarraypowerexceedsinvertercapability

IRR Calibratedcurrentofthereferencecellunderthereferenceconditions

ITM Measuredtestcellcurrent

ITR Calibratedcurrentofthetestcellunderthereferenceconditions

IV Currentversusvoltage

K Spectralcorrectionfactor,inverseofM

M Spectralmismatchparameter

Pmax Testmodulemaximumpowerunderreferenceconditions

POA PlaneofArray

St() Measuredspectralresponsivityofthetestmodule

23

Sr() Measuredspectralresponsivityofthereferencemodule

STC StandardTestConditions(1000W/m2,25Cmodtemp,1.5AM)

Vmp TestmodulevoltageatPmax

Voc Testmoduleopencircuitvoltage

24

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27

AppendicesAppendix1:MeasurementDeviceDataSheets...............................................................28Appendix2:Comparisonofpyranometerreadings.........................................................32Appendix3:SystemDesign...............................................................................................33Appendix4:ModuleDataSheets.....................................................................................36Appendix5:aSimoduleResults......................................................................................42Appendix6:cSimoduleResults.......................................................................................47

GraphAxis

x ya POAIrrW/m2 %Pmaxb AOI %Pmaxc Date FillFactord POAIrrW/m2 FillFactore POAIrrW/m3 ISCf POAIrrW/m4 VOCg ModuleTempoC %Pmaxh ModuleTempoC FillFactori Date %Pmax

Appendix7:CIGSmoduleResults.....................................................................................52

GraphAxis

x ya POAIrrW/m2 %Pmaxb AOI %Pmaxc Date FillFactord POAIrrW/m2 FillFactore POAIrrW/m3 ISCf POAIrrW/m4 VOCg AmbientTempoC %Pmaxh AmbientTempoC FillFactori Date %Pmax

Appendix8:PowerversuswindspeedcSimodule.........................................................57Appendix9:Conclusions...................................................................................................58

Appendix

x1:Measur

K

rementDevi

ippandZon

2

iceDataShe

nenSPLite2P

28

eets

PhotodiodePyranometter

Kipp

SpecificISOClasResponsZerooff(a)therm(b)tempNonstaNonlineDirectiobeam)TemperTilterroSensitivImpedaLevelacOperatiSpectraTypicalsMaximuExpecteRecomm

andZonen

cationsssificationsetime(95%fsetsmalradiatioperaturechability(changearity(0to1onalerror(up

raturedepenor(at1000Witynceccuracyngtemperatlrange(50%signaloutpuumirradianceddailyuncemendedapp

CMP21Sec

%)

n(200W/m2ange(5K/hrge/year)1000W/m2)pto80owit

ndenceofseW/m2)

ture%points)utforatmospceertaintylications

29

condaryStan

2)r)

)th1000W/m

ensitivity

phericapplic

ndardTherm

CMSe

KeithleeyInstrume

3

ntsModel#

30

#2700DataAAcquisitionSSystem

31

KI2430SourceMeterSpecifications:

VoltsRanges 0.2,2,20,100VBasicVSourceAccuracy 0.02%BasicVMeasureAccuracy 0.015%

1,10,100A1,10,100mA1,3,10A

BasicISourceAccuracy 0.045%BasicIMeasureAccuracy 0.035%

2,20,2002,20,200k2,20,200M

BasicOhmsMeasureAccuracy 0.06%110WDC

1000WPulse

IRanges

OhmsRanges

MaximumPower

32

Appendix2:Comparisonofpyranometerreadings

0.7

0.8

0.9

1.0

1.1

1.2

1.3

0 200 400 600 800 1,000 1,200 1,400

CMP2

1/SPLite2Irrad

iance

CMP21Irradiance(W/m)

CMP21POAIrr=1.0346xSPLite2POAIrr

0

200

400

600

800

1000

1200

1400

1600

0 200 400 600 800 1000 1200 1400 1600Irrad

iancefrom

CMP2

1Pyrano

meterat

25deg

tilt(W

/m)

IrradiancefromSPLite2Pyranometerat25degTilt(W/m)

33

Appendix3:SystemDesign

SystemComponentsKeithleyInstruments(KI)SourceMeter#2430KISwitchingMainframe#7002FourKISwitchingModules#70534Achannels(x2=8A)100mH/LchannelballastresistorsKIDMMDataAcquisitionSystem#2700/7700Kipp&ZonenCMP21thermopilepyranometerThreeKipp&ZonenPyranometersSPLite2(flat,15,and27)MaximumAnemometer#41Type(K)Thermocouples(eachmoduleandtwoambient)PCwithGPIBinterfaceVBA.netFixedloadresistors(MinimumPowerRating=Vmp/ImpX1.25)Blockballastednonpenetratingracks

3

System

34

Diagram

Ro

35

ooftopSchematic

Appendixx4:ModuleeDataShee

3

ts

36

37

338

39

440

41

42

Appendix5:aSimoduleResults

y=0.001x 0.0074R=0.9859

0%

20%

40%

60%

80%

100%

120%

140%

0 200 400 600 800 1000 1200 1400

P max

POAIrradianceW/m2

0%

20%

40%

60%

80%

100%

120%

140%

0 10 20 30 40 50 60 70 80 90 100 110

P max

AngleofIncidence

5(a)

5(b)

43

aSimodulefillfactorovertimeandPOAIrradiance

Notethedropinefficiency(fillfactor)duringcoldermonths(above).

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

7/1/2010 8/1/2010 9/1/2010 10/2/2010 11/2/2010 12/3/2010

FillFactor

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0 200 400 600 800 1,000 1,200 1,400

FillFactor

POAIrradianceW/m2

5(c)

5(d)

44

aSimoduleelectricalcharacteristicscurrent(I)andvoltage(V)versusPOAIrradiance

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 200 400 600 800 1000 1200 1400

Isc

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

Voc

POAIrradianceW/m2

5(e)

5(f)

45

aSimodulePowerandEfficiencyversusModuleTemperature

0%

20%

40%

60%

80%

100%

120%

140%

10 0 10 20 30 40 50 60 70

P max

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

20 10 0 10 20 30 40 50 60 70

FillFactor

ModuleTemperatureOC

5(h)

5(g)

46

aSimoduleEnergyyieldsforthreeconcurrentdaysinJuly,SeptandDec2010

0%

20%

40%

60%

80%

100%

120%

7/3/2010 7/4/2010 7/5/2010 7/6/2010

Pmax

0%

20%

40%

60%

80%

100%

9/21/2010 9/22/2010 9/23/2010 9/24/2010

Pmax

0%

20%

40%

60%

12/24/2010 12/25/2010 12/26/2010 12/27/2010

Pmax

5(i)

47

Appendix6:cSimoduleResults

y=0.000898x 0.013802

0%

20%

40%

60%

80%

100%

120%

140%

0 200 400 600 800 1000 1200 1400

P max

POAIrradianceW/m2

0%

20%

40%

60%

80%

100%

120%

0 10 20 30 40 50 60 70 80 90 100 110

P max

AngleofIncidence

6(a)

6(b)

48

cSImodulefillfactor

0.30

0.40

0.50

0.60

0.70

0.80

0.90

7/1/2010 8/1/2010 9/1/2010 10/2/2010 11/2/2010 12/3/2010

FillFactor

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 200 400 600 800 1,000 1,200 1,400

FillFactor

POAIrradianceW/m2

6(c)

6(d)

49

cSimoduleelectricalcharacteristicscurrent(I)andvoltage(V)versusPOAIrradiance

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 200 400 600 800 1000 1200 1400

Isc

10

20

30

40

50

0 200 400 600 800 1000 1200 1400

Voc

POAIrradianceW/m2

6(e)

6(f)

50

cSimodulePowerandEfficiencyversusModuleTemperature

0%

20%

40%

60%

80%

100%

120%

140%

10 0 10 20 30 40 50 60 70

P max

0.3

0.4

0.5

0.6

0.7

0.8

0.9

10 0 10 20 30 40 50 60 70

FillFactor

ModuleTemperatureOC

6(g)

6(h)

51

cSimoduleEnergyyieldsfromthreeconcurrentdaysinJuly,SeptandDec2010

0%

20%

40%

60%

80%

100%

120%

7/3/2010 7/4/2010 7/5/2010 7/6/2010

P max

0%

20%

40%

60%

80%

100%

9/21/2010 9/22/2010 9/23/2010 9/24/2010

P max

0%

20%

40%

60%

12/24/2010 12/25/2010 12/26/2010 12/27/2010

P max

6(i)

52

Appendix7:CIGSmoduleResults

y=4E07x2 +0.0012x 0.0187R=0.9744

0%

20%

40%

60%

80%

100%

120%

0 200 400 600 800 1000 1200 1400

P max

POAIrradianceW/m2

0%

20%

40%

60%

80%

100%

120%

0 10 20 30 40 50 60 70 80 90 100

P max

AngleofIncidence

7(a)

7(b)

53

CIGSmodulefillfactorovertimeandintensityoflight(POAIrr)

0.30

0.40

0.50

0.60

0.70

0.80

7/1/2010 8/1/2010 9/1/2010 10/2/2010 11/2/2010 12/3/2010

FillFactor

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0 200 400 600 800 1000 1200 1400

FillFactor

POAIrradianceW/m2

7(c)

7(d)

54

CIGSmoduleelectricalcharacteristicscurrent(I)andvoltage(V)versusPOAIrradiance

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 200 400 600 800 1000 1200 1400

Isc

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

Voc

POAIrradianceW/m2

7(e)

7(f)

55

CIGSmodulePowerandEfficiencyversusAmbientTemperature

0%

20%

40%

60%

80%

100%

120%

10 5 0 5 10 15 20 25 30 35 40

P max

0.20

0.30

0.40

0.50

0.60

0.70

0.80

10 5 0 5 10 15 20 25 30 35 40

FillFactor

AmbientTemperatureOC

7(g)7(g)

7(h)

56

CIGSmoduleEnergyyieldsfromthreeconcurrentdaysinJuly,SeptandDec2010

0%

20%

40%

60%

80%

100%

7/3/2010 7/4/2010 7/5/2010 7/6/2010

P max

0%

20%

40%

60%

80%

100%

9/21/2010 9/22/2010 9/23/2010 9/24/2010

P max

0%

20%

40%

60%

12/24/2010 12/25/2010 12/26/2010 12/27/2010

P max

7(i)

57

Appendix8:PowerversuswindspeedcSimodule

0%

20%

40%

60%

80%

100%

120%

0 5 10 15

P max

Windspeed(m/sec)

AppendixPe

rcen

tageSTC

Pmax

DCPow

erGen

erated

(W

/m2 )

0.0

0.2

0.4

0.6

0.8

1.0

m2 /kW

Whr

x9:Conclus

0%

20%

40%

60%

80%

100%

120%

0

0

20

40

60

80

100

120

140

0

(W/m

)

0

2

4

6

0.8390.879

r/(IrrkWh

sions

200

aSi

200

90.757

0.951

hr/m2)/ST

5

400

mcSi

400 6

POAIrrad

TCPmax

cSi

CIGS@P

CIGS@T

aSi(1x)

58

600 80

cylindric

600 800

diance(W/

POAIrr

TiltIrr

)

2

00 1000

calCIGS

0 1000

/m2)

0

20

40

60

80

100

120 105

m2

Whr/(Ir

0 1200

1200

.1

73.863.6

rrkWhr/m

60.1

m2)