Solar System Sizing - Open Electrical

5/22/2015 SolarSystemSizingOpenElectrical

http://www.openelectrical.org/wiki/index.php?title=Solar_System_Sizing 1/9

SolarSystemSizingFromOpenElectrical

Contents1Introduction

1.1Whydothecalculation?1.2Whentodothecalculation?

2CalculationMethodology2.1Step1:EstimateSolarIrradiationattheSite

2.1.1BaselineSolarIrradiationData2.1.2SolarIrradiationonanInclinedPlane2.1.3SolarTrackers2.1.4NonStandardApplications

2.2Step2:CollecttheSolarPowerSystemLoads2.3Step3:ConstructaLoadProfile2.4Step4:BatteryCapacitySizing2.5Step5:EstimateaSinglePVModule'sOutput

2.5.1EffectivePVCellTemperature2.5.2StandardRegulator2.5.3MPPTRegulator

2.6Step6:SizethePVArray2.6.1StandardRegulator2.6.2MPPTController

3WorkedExample3.1Step1:EstimateSolarIrradiationattheSite3.2Step2and3:CollectLoadsandConstructaLoadProfile3.3Step4:BatteryCapacitySizing3.4Step5:EstimateaSinglePVModule'sOutput3.5Step6:SizethePVArray

4ComputerSoftware5WhatNext?

Introduction

Thiscalculationoutlinesthesizingofastandalonesolarphotovoltaic(PV)powersystem.StandalonePVsystemsarecommonlyusedtosupplypowertosmall,remoteinstallations(e.g.telecoms)whereitisn'tpracticalorcostefficienttorunatransmissionlineorhavealternativegenerationsuchasdieselgensets.

AlthoughthiscalculationisbiasedtowardsstandalonesolarPVsystems,itcanalsobeusedforhybridsystemsthatdrawpowerfrommixedsources(e.g.commercialPV,hybridwindPVsystems,etc).LoadsmustbeadjustedaccordingtothedesiredamountthatthesolarPVsystemwillsupply.

ThiscalculationisbasedoncrystallinesiliconPVtechnology.TheresultsmaynotholdforothertypesofsolarPVtechnologiesandthemanufacturer'srecommendationswillneedtobeconsulted.

Whydothecalculation?



Figure1.SolarPVarray

Figure2.Worldsolarirradiationmap

ThiscalculationshouldbedonewheneverasolarPVpowersystemisrequiredsothatthesystemisabletoadequatelycaterforthenecessaryloads.Theresultscanbeusedtodeterminetheratingsofthesystemcomponents(e.g.PVarray,batteries,etc).

Whentodothecalculation?

Thefollowingprerequisiteinformationisrequiredbeforeperformingthecalculation:

LoadsrequiredtobesupportedbythesolarPVsystemAutonomytimeorminimumtolerabledowntime(i.e.ifthereisnosun,howlongcanthesystembeoutofservice?)GPScoordinatesofthesite(ormeasurementsofthesolarinsolationatthesite)Outputvoltage(ACorDC)

CalculationMethodology

ThecalculationislooselybasedonAS/NZS4509.2(2002)(http://infostore.saiglobal.com/store/Details.aspx?ProductID=315646)"StandalonepowersystemsSystemdesignguidelines".Themethodologyhasthefollowingsixsteps:

Step1:Estimatethesolarirradiationavailableatthesite(basedonGPScoordinatesormeasurement)Step2:CollecttheloadsthatwillbesupportedbythesystemStep3:ConstructaloadprofileandcalculatedesignloadanddesignenergyStep4:CalculatetherequiredbatterycapacitybasedonthedesignloadsStep5:EstimatetheoutputofasinglePVmoduleattheproposedsitelocationStep6:CalculatesizeofthePVarray

Step1:EstimateSolarIrradiationattheSite

Thefirststepistodeterminethesolarresourceavailabilityatthesite.Solarresourcesaretypicallydiscussedintermsofsolarradiation,whichismoreorlessthecatchalltermforsunlightshiningonasurface.Solarradiationconsistsofthreemaincomponents:

DirectorbeamradiationismadeupofbeamsofunscatteredandunreflectedlightreachingthesurfaceinastraightlinedirectlyfromthesunDiffuseradiationisscatteredlightreachingthesurfacefromthewholesky(butnotdirectlyfromthesun)



Albedoradiationisislightreflectedontothesurfacefromtheground

Solarradiationcanbequantitativelymeasuredbyirradianceandirradiation.Notethatthetermsaredistinct"irradiance"referstothedensityofthepowerthatfallsonasurface(W/m2)and"irradiation"isthedensityoftheenergythatfallsonasurfaceoversomeperiodoftimesuchasanhouroraday(e.g.Wh/m2perhour/day).

Inthissection,wewillestimatethesolarradiationavailableatthesitebasedondatacollectedinthepast.However,itneedstobestressedthatsolarradiationisstatisticallyrandominnatureandthereisinherentuncertaintyinusingpastdatatopredictfutureirradiation.Therefore,wewillneedtobuildindesignmarginssothatthesystemisrobusttopredictionerror.

BaselineSolarIrradiationData

Theeasiestoptionistoestimatethesolarirradiation(orsolarinsolation)byinputtingtheGPScoordinatesofthesiteintotheNASASurfaceMeteorologyandSolarResource(http://eosweb.larc.nasa.gov/cgibin/sse/sse.cgi)website.

ForanygivensetofGPScoordinates,thewebsiteprovidesfirstpassestimatesofthemonthlyminimum,averageandmaximumsolarirradiation(inkWh/m2/day)atgroundlevelandatvarioustiltangles.Collectthisdata,chooseanappropriatetiltangleandidentifythebestandworstmonthsoftheyearintermsofsolarirradiation.Alternatively,forUSlocationsdatafromtheNationalSolarRadiationDatabase(http://www.nrel.gov/rredc/solar_data.html)canbeused.

Theminimum,averageandmaximumdaytimetemperaturesatthesitecanalsobedeterminedfromthepublicdatabaseslistedabove.ThesetemperatureswillbeusedlaterwhencalculatingtheeffectivePVcelltemperature.

Actualsolarirradiationmeasurementscanalsobemadeatthesite.Providedthatthemeasurementsaretakenoveralongenoughperiod(orcrossreferenced/combinedwithpublicdata),thenthemeasurementswouldprovideamoreaccurateestimateofthesolarirradiationatthesiteastheywouldcapturesitespecificcharacteristics,e.g.anyobstructionstosolarradiationsuchaslargebuildings,trees,mountains,etc.

SolarIrradiationonanInclinedPlane

MostPVarraysareinstalledsuchthattheyfacetheequatorataninclinetothehorizontal(formaximumsolarcollection).Theamountofsolarirradiationcollectedoninclinedsurfacesisdifferenttotheamountcollectedonahorizontalsurface.Itistheoreticallypossibletoaccuratelyestimatethesolarirradiationonanyinclinedsurfacegiventhesolarirradiationonanhorizontalplaneandthetiltangle(therearenumerousresearchpapersonthistopic,forexampletheworkdonebyLiuandJordanin1960).

However,forthepracticalpurposeofdesigningasolarPVsystem,we'llonlylookatestimatingthesolarirradiationattheoptimaltiltangle,whichistheinclinethatcollectsthemostsolarirradiation.Theoptimaltiltanglelargelydependsonthelatitudeofthesite.Atgreaterlatitudes,theoptimaltiltangleishigherasitfavourssummertimeradiationcollectionoverwintertimecollection.TheHandbookofPhotovoltaicScienceandEngineering(http://www.amazon.com/HandbookPhotovoltaicScienceEngineeringAntonio/dp/0471491969)suggestsalinearapproximationtocalculatingtheoptimaltiltangle:

Where istheoptimaltiltangle(deg)



isthelatitudeofthesite(deg)

Thehandbookalsosuggestsapolynomialapproximationforthesolarirradiationattheoptimaltiltangle:

Where isthesolarirradiationonasurfaceattheoptimaltiltangle(Wh/m2)

isthesolarirradiationonthehorizontalplane(Wh/m2)istheoptimaltiltangle(deg)

Alternatively,theestimatedirradiationdataontiltedplanescanbesourceddirectlyfromthevariouspublicdatabaseslistedabove.

SolarTrackers

SolartrackersaremechanicaldevicesthatcantrackthepositionofthesunthroughoutthedayandorientthePVarrayaccordingly.Theuseoftrackerscansignificantlyincreasethesolarirradiationcollectedbyasurface.Solartrackerstypicallyincreaseirradiationby1.2to1.4times(for1axistrackers)and1.3to1.5times(for2axistrackers)comparedtoafixedsurfaceattheoptimaltiltangle.

NonStandardApplications

Asolarirradiationlossfactorshouldbeusedforapplicationswheretherearehightiltangles(e.g.verticalPVarraysaspartofabuildingfacade)orverylowtiltangles(e.g.NorthSouthhorizontaltrackers).Thisisbecausethethesolarirradiationissignificantlyaffected(detrimentally)whentheangleofincidenceishighorthesolarradiationismainlydiffuse(i.e.noalbedoeffectsfromgroundreflections).Formoredetailsonthislossfactor,consultthestandardASHRAE93,"Methodsoftestingtodeterminethethermalperformanceofsolarcollectors"(http://www.techstreet.com/standards/ASHRAE/93_2010?product_id=1703551).

Step2:CollecttheSolarPowerSystemLoads

Thenextstepistodeterminethetypeandquantityofloadsthatthesolarpowersystemneedstosupport.Forremoteindustrialapplications,suchasmeteringstations,theloadsarenormallyforcontrolsystemsandinstrumentationequipment.Forcommercialapplications,suchastelecommunications,theloadsarethetelecomshardwareandpossiblysomesmallarealightingformaintenance.Forruralelectrificationandresidentialapplications,theloadsaretypicallydomesticlightingandlowpoweredapppliances,e.g.computers,radios,smalltv's,etc.

Step3:ConstructaLoadProfile

RefertotheLoadProfileCalculationfordetailsonhowtoconstructaloadprofileandcalculatethedesignload( )anddesignenergy( ).Typically,the"24HourProfile"methodforconstructingaloadprofileisusedforSolarPowerSystems.

Step4:BatteryCapacitySizing

InasolarPVpowersystem,thebatteryisusedtoprovidebackupenergystorageandalsotomaintain



outputvoltagestability.RefertotheBatterySizingCalculationfordetailsonhowtosizethebatteryforthesolarpowersystem.

Step5:EstimateaSinglePVModule'sOutput

ItisassumedthataspecificPVmoduletype(e.gSuntechSTP070S12Bb)hasbeenselectedandthefollowingparameterscollected:

Peakmodulepower, (Wp)Nominalvoltage (Vdc)Opencircuitvoltage (Vdc)Optimumoperatingvoltage (Vdc)Shortcircuitcurrent (A)Optimumoperatingcurrent (A)Peakpowertemperaturecoefficient (%perdegC)Manufacturer'spoweroutputtolerance (%)

ManufacturersusuallyquotethesePVmoduleparametersbasedonStandardTestConditions(STC):anirradianceof1,000W/m2,thestandardreferencespectralirradiancewithAirMass1.5(seetheNRELsite(http://rredc.nrel.gov/solar/spectra/am1.5/)formoredetails)andacelltemperatureof25degC.StandardtestconditionsrarelyprevailonsiteandwhenthePVmoduleareinstalledinthefield,theoutputmustbederatedaccordingly.

EffectivePVCellTemperature

Firstly,theaverageeffectivePVcelltemperatureattheinstallationsiteneedstobecalculated(asitwillbeusedinthesubsequentcalculations).ItcanbeestimatedforeachmonthusingAS\NZS4509.2equation3.4.3.7:

Where istheaverageeffectivePVcelltemperature(degC)

istheaveragedaytimeambienttemperatureatthesite(degC)

StandardRegulator

Forasolarpowersystemusingastandardswitchedchargeregulator/controller,thederatedpoweroutputofthePVmodulecanbecalculatedusingAS\NZS4509.2equation3.4.3.9(1):

Where isthederatedpoweroutputofthePVmoduleusingastandardswitchedchargecontroller(W)

isthedailyaverageoperatingvoltage(Vdc)isthemoduleoutputcurrentbasedonthedailyaverageoperatingvoltage,atthe

effectiveaveragecelltemperatureandsolarirradianceatthesitemoreonthisbelow(A)isthemanufacturer'spoweroutputtolerance(pu)isthederatingfactorfordirt/soiling(Clean:1.0,Low:0.98,Med:0.97,High:0.92)



Toestimate ,youwillneedtheIVcharacteristiccurveofthePVmoduleattheeffectivecelltemperaturecalculatedabove.Foraswitchedregulator,theaveragePVmoduleoperatingvoltageisgenerallyequaltotheaveragebatteryvoltagelessvoltagedropsacrossthecablesandregulator.

MPPTRegulator

ForasolarpowersystemusingaMaximumPowerPointTracking(MPPT)chargeregulator/controller,thederatedpoweroutputofthePVmodulecanbecalculatedusingAS\NZS4509.2equation3.4.3.9(2):

Where isthederatedpoweroutputofthePVmoduleusinganMPPTchargecontroller(W)

isthenominalmodulepowerunderstandardtestconditions(W)isthemanufacturer'spoweroutputtolerance(pu)isthederatingfactorfordirt/soiling(Clean:1.0,Low:0.98,Med:0.97,High:0.92)isthetemperaturederatingfactorseebelow(pu)

ThetemperaturederatingfactorisdeterminedfromAS\NZS4509.2equation3.4.3.9(1):

Where istemperaturederatingfactor(pu)

isthePowerTemperatureCoefficient(%perdegC)istheaverageeffectivePVcelltemperature(degC)

isthetemperatureunderstandardtestconditions(typically25degC)

Step6:SizethePVArray

ThesizingofthePVarraydescribedbelowisbasedonthemethodoutlinedinAS/NZS4509.2.Therearealternativesizingmethodologies,forexamplethemethodbasedonreliabilityintermsoflossofloadprobability(LLP),butthesemethodswillnotbefurtherelaboratedinthisarticle.Thefactthatthereisnocommonlyacceptedsizingmethodologyreflectsthedifficultyofperformingwhatisaninherentlyuncertaintask(i.e.apredictionexercisewithmanyrandomfactorsinvolved).

StandardRegulator

ThenumberofPVmodulesrequiredforthePVarraycanbefoundbyusingAS\NZS4509.2equation3.4.3.11(1):

Where isthenumberofPVmodulesrequired

isthederatedpoweroutputofthePVmodule(W)isthetotaldesigndailyenergy(VAh)istheoversupplycoefficient(pu)isthesolarirradiationafterallfactors(e.g.tiltangle,tracking,etc)havebeencaptured

(kWh/m2/day)



isthecoulombicefficiencyofthebattery(pu)

Theoversupplycoefficient isadesigncontingencyfactortocapturetheuncertaintyindesigningsolarpowersystemswherefuturesolarirradiationisnotdeterministic.AS/NZS4509.2Table1recommendsoversupplycoefficientsofbetween1.3and2.0.

Abatterycoulombicefficiencyofapproximately95%wouldbetypicallyused.

MPPTController

ThenumberofPVmodulesrequiredforthePVarraycanbefoundbyusingAS\NZS4509.2equation3.4.3.11(2):

Where isthenumberofPVmodulesrequired

isthederatedpoweroutputofthePVmodule(W)isthetotaldesigndailyenergy(VAh)istheoversupplycoefficient(pu)isthesolarirradiationafterallfactors(e.g.tiltangle,tracking,etc)havebeencaptured

(kWh/m2/day)istheefficiencyofthePVsubsystem(pu)

Theoversupplycoefficient isadesigncontingencyfactortocapturetheuncertaintyindesigningsolarpowersystemswherefuturesolarirradiationisnotdeterministic.AS/NZS4509.2Table1recommendsoversupplycoefficientsofbetween1.3and2.0.

TheefficiencyofthePVsubsystem isthecombinedefficienciesofthechargeregulator/controller,batteryandtransmissionthroughthecablebetweenthePVarrayandthebattery.Thiswilldependonspecificcircumstances(forexample,thePVarrayalargedistancefromthebattery),thoughanefficiencyofaround90%wouldbetypicallyused.

WorkedExample

Asmallstandalonesolarpowersystemwillbedesignedforatelecommunicationsoutpostlocatedinthedesert.

Step1:EstimateSolarIrradiationattheSite

Fromsitemeasurements,thesolarirradiationatthesiteduringtheworstmonthattheoptimaltitleangleis4.05kWh/m2/day.

Step2and3:CollectLoadsandConstructaLoadProfile

Forthisexample,weshallusethesameloadsandloadprofiledetailedintheEnergyLoadProfileCalculationexample.Theloadprofileisshowninthefigurerightandthefollowingquantitieswerecalculated:

DesignloadSd=768VA



Figure3.Loadprofileforthisexample

DesignenergydemandEd=3,216VAh

Step4:BatteryCapacitySizing

Forthisexample,weshallusethesamebatterysizescalculatedintheBatterySizingCalculation]workedexample.Theselectednumberofcellsinseriesis62cellsandtheminimumbatterycapacityis44.4Ah.

Step5:EstimateaSinglePVModule'sOutput

APVmodulewiththefollowingcharacteristicsischosen:

Peakmodulepower,Wp

Nominalvoltage VdcPeakpowertemperaturecoefficient %perdegCManufacturer'spoweroutputtolerance %

Supposetheaveragedaytimeambienttemperatureis40C.TheeffectivePVcelltemperatureis:

degC

AnMPPTcontrollerwillbeused.Thetemperaturederatingfactoristherefore:

Givenamediumdirtderatingfactorof0.97,thederatedpoweroutputofthePVmoduleis:

W

Step6:SizethePVArray

Givenanoversupplycoefficientof1.1andaPVsubsystemefficiencyof85%,thenumberofPVmodulesrequiredforthePVarrayforaMPPTregulatoris:

10.9588modules



ForthisPVarray,12modulesareselected.

ComputerSoftware

ItisrecommendedthatthesolarPVsystemsizedinthiscalculationissimulatedwithcomputersoftware.Forexample,HOMER(http://www.homerenergy.com/)isapopularsoftwarepackageforsimulatingandoptimisingadistributedgeneration(DG)systemoriginallydevelopedbytheNationalRenewableEnergyLaboratory(NREL).

ScreenshotsfromHOMERsoftware

PVOutput BatteryOutput

WhatNext?

Withthesizingcalculationcompleted,thesolarPVequipment(PVarray,batteries,chargecontrollers,etc)canbespecifiedandacostestimateorbudgetenquiry/requisitionpackageissued.Theapproximatedimensionsoftheequipment(especiallythePVarrayandbatteries)canalsobeestimatedandadesignlayoutcanbeproduced.

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