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Thin

    Thin

27卷第3

    ;20087

    ;.

    ;电工电能新技术

    ;AdvancedTechnolokwofElectricalEngineeringandEne

    ;Vo1.27.No.3

    ;July2008

    ;ThinFilmPh0tovoltaic/ThermalSolarPanels

    ;DavidJOHNSTON

    ;(PowerandControlResearchGroup,SchoolofComputing,EngineeringandIfiformationSciences,

    ;UniversityofNorthumbriaatNewcastle,NE18ST,UK)

    ;Abstract:Asolarpanelisdescribed,inwhichthinfilmsofsemiconductoraredepositedontoametal

    ;substrate.Thesemiconductor-metalcombinationformsathinfilmphotovohaiccel1,andalsoactsasa

    ;reflector-absorbertandem,whichactsasasolarselectivesurface,thusenhancingthesolarthermal

    ;performance0fthecollectorplate.1’heuseofthinfilmsreducesthedistanceheatisrequiredtoflowfromthe

    ;absorbingsurfacetothemetalplateandheatexchangeconduits.Computermodellingdemonstratedthat,by

    ;suitablechoiceofmaterials,photovohaicefficiencycanbemaintained,withthermalperformanceslightly

    ;reduced,comparedtothatforthermalonlypanels.Bygradingtheabsorberlayer-toreduc

    ethebandgapin

    ;thelowerregion.thethermalperformancecanbeimproved,approachingthatforathermal?onlysolarpane1.

    ;Keywords:solar;thinfilm;photovohaic;thermal;panel

    ;中图分类号:TK51文献标识码:A文章编号:1003.3076(2OHD8)030001.08

    ;1Introduction

    ;Photovohaicsolarpanelshavetypicalelectrical

    ;conversionefficienciesof15to20%(commercial

    ;devices),orupto25%(experimentaldevices)(NREL,

    ;2007).Theremainingenergyisconvertedtoheatwithin

    ;thecell(s).Incertainsituations,someofthisheatmay

    ;berecoveredforadditionalapplications.Thesewill

    ;generallybewherethepointofcollection(thesolar

    ;array)isclosetothepointofuse,wheretheshort

    ;distancesoverwhichheatistransfefredminimisestheheat ;losses.Examplesarebuildingintegratedpanels,where

    ;theheatrecoveredcanbeusedforspaceheatingorfor ;domestichotwater.Inaddition,someindustrial ;applicationsmaybeabletouseprocessheatattherange ;oftemperaturesavailablefromsolarpanels.Arangeof ;Photovoltatc/Thermal(PV/T)solarpanelshasbeen ;developed(TiwariandSodha,20O6).

    ;Flat?panelsolarthermalpanelstypicallyrecoverup ;to50%oftheincidentsolarenergy(Tiwari,A.

    ;2001.).(Evacuatedtubecollectorshavehigherrecovery ;efficiencies.butarenotsuitableforcombinedfunction.) ;Aphotovohaicpanelwithanelectricalconversion ;efficiencyof20%willconvert80%oftheincidentenergy ;toheat.Ifthethermalperformanceofsolarthermalpanels ;isreplicatedinPV/Tpanels,40%oftheincidentenergy ;wouldberecoveredasusableheat.1’hUS.60%ofthe

    ;incidentenergywouldberecoveredaseitherelectricityor ;heat.(Inpractice,designcompromisesmayreducethe ;efficienciesofoneorbothoftheseprocesses.) ;ThePV/Tpanelsproducedtodatearegenerally

    ;basedonsiliconcells.Anumberofsiliconwafersare ;bondedontothecollectorplateofasolarthermalpanel, ;theentireassemblybeingmountedinaninsulatingbox. ;ThinFilmPhotovohaic/Thermal(TFPV/T)solarpanels

    ;havetwopotentialadvantagesovertheconventional ;silicon-basedtype.Thefirstadvantageistherma1.Ina ;Sibasedpanel,thesolarenergyisabsorbedinthefront ;layerofthecell(typicallywithin100ptmofthefront ;surface).Asasiliconwaferisapprox.0.5mmthick, ;theheatmustCroSSthisdistancebeforereachingthemetal ;plate,withitshigherthermalconductivity.Incontrast,a ;thinfilmphotovohaiccellconsistsofthreeorfourlayers, ;eachofatmostafewttmthick.Thedistancethatheatis ;收稿日期:2008.0310

    ;作者简

    :DavidJohnston,ProfessoratUniversityofNorthumbfiaatatNewcastle,UK,i~seai’char

    ea-”newformsofenergy,

    ;specialisinginsolarenergyandelectricalengineering. ;

    ;2电工电能新技术第27

    ;requiredtotravelthroughthelowerconductivitymaterial ;isthusconsiderablylessthanforSi-basedpanels. ;Thesecondadvantageisoptica1.Inorderto

    ;maximisetheelectricalconversionefficiency,photovohaic ;materialsshouldhaveabandgapofapprox.1.4eV ;(Partain,1995).(Inpractice,bandgapsof1to1.8eV ;areused.)ThinfilmPVcellsaregenerallybasedon ;heterojunctions.Thelower(absorber)layerofthe ;junctionhastheoptimumbandgap,andhenceisstrongly ;absorbingatwavelengthsshorterthanapprox.880nm. ;Theoverlying(window)layerhasawidebandgap,to ;maximisethesolarradiationreachingthejunction.A ;metallaYerbelowtheabsorberlayerformstheelectrical ;backcontacttothecel1.Asolarthermalpanelrequires ;anabsorbing(black)surfacetoabsorbthemaximum ;fractionofthesolarspectrum.However,fromKirchhoff’s

    ;RadiationLaw.asurfacewhichabsorbsmostofthe ;incidentradiationwillal80emitradiationefficiently.As ;thepanelheatsup,re.radiationofthermalinfrared

    ;radiationbecomesasignificantheatlossmechanism.In ;ordertoovercomethisproblem,surfacesareused,which ;areabsorbentinthesolarspectrum(300to2000nm),

    ;andhavelowemissivityinthethermalinfra.red(10

    ;ttm)(AgnihotriandGupta,1971).Theseareonetypeof ;solarselective8u~ace,knownasreflection-absorption0r ;blackselectivesuCace(theothertypebeingreflection- ;transmission).Onetypeofreflection-absorptionsurfaceis ;theabsorber-reflectortandem.Theupperlayeris ;absorbentatshortwavelengths,andtransmissiveatlonger ;wavelengths.Thelowerlayerisreflectiveoverabroad ;wavelengthrange.Thecombinationisthereforeabsorbent ;atshortwavelengths,andreflectiveatlongwavelengths, ;asrequiredforareflection-absorptionsurface.The ;reflectivelayerisgenerallyameta1.Theselectively ;absorbentlayermaybeas,

    ;emiconductor.Inthisease,the

    ;absorber-reflectortandemisstructtirallyandoptically ;similartotheabsorberlayerandbackcontactofathin ;filmPVcel1.

    ;Thissimilarityallowsapaneltobeconstructed, ;whichcombinesbothfunctions.Athinfilmphotovoltaic ;cellisdepositedontothemetalcollectorplate,andalso ;actsasasolarselectivesurfacetotransferheattothis ;plate.Onthelearofthepanelsareattachedfins(for ;circulatingairasaheattransfermedium),orpipes(for ;circulatingliquid).Thisassemblyismountedinan ;insulatingbox(asforastandardsolarthermalpane1),

    ;andtheairductsorliquid-circulationpipesareconnected ;totherestoftheheattransfersystem.

    ;Althoughthethinfilmstructurecanbeusedbothfor ;electricitygenerationandheatrecovery,itmaynotbe ;possibletomeetthedesignrequirements(optimisation) ;foreachfunctionsimultaneously.Computermodellingis ;usedtooptimisethematerialparameters,etc,foreach ;function,andtodeterminethebestcompromisebetween ;bothfunctions.

    ;2DesignConsiderations:OpticalandOther

    ;PropertiesofSemiconductorLayers

    ;Thetical,electricalcharacteristicsofphotovoltaic ;materials,andtheoptical/thermalcharacteristicsofsolar ;selectivesurfacesaremodelled,todemonstratethe ;optimalparametersforeachfunction,andtoidentifythe ;designcompromisesneededtomeettherequirementsof ;bothfunctionssimultaneously.

    ;TheCroSSsectionofaTF-PV,Tpanelisshownin ;Fig.1.Thisisbroadlysimilartoastandardsolarthermal ;pane1.exceptthatmulti-layerthin-filmPVcellsreplace ;thesimpleabsorber-reflectortandemover,

    ;someorallof

    ;thearea.Specifically,theabsorberlayermustproducean ;electricaloutput,aswellasformingpartofasolar ;selectivesurfaces,andtheTCOlayerwillactasaheat ;mirror,inadditiontoanysuchfilmsdepositedonthe ;coverglasses.Eachofthesemodificationsaffectsthe ;opticalandthermalperformance.

    ;Fig.1CrosssectionofarF.PV/Tsolarpane1.showingthe ;layersofathinfilmsolarcellsuccessivelydepositedontoa ;metalsubstrate.Alsoshownistheinsulatinglalyer,which ;isolatesthecellelectricallyfromthesubstrate.Inthisexample, ;finsaleshownbondedtothelearofthepane~,nsusedinan ;air?heatingpane1.Foraliquid-heatingpanel,pipeso1”similar

    ;conduitswouldbeused.

    ;

    ;

    ;

    ;4电工电能新技术第27

    ;WindowLayer

    ;Fig.5Thinfilmphotovoltaiccellwithgraded ;absorberlayer.Inthedepletionregion,thebandgap ;isoptimisedformaximumphotovohaicefficiency. ;Inthelowerpartoftheabsorberlayer,thebandgap

;isoptimisedforthermalperformance

    ;1.41.5eVistypica1.Inthelowerpartofthelayer ;(belowthedepletionregion),thisgradestoamaterial ;withalowerbandgap.

    ?一?2)materials,being ;Thechalcopyrite(I

    ;ternarycompounds,canbeproducedingraded ;composition,whichgivestherequiredbandgapgrading ;(Bube,1998).AmaterialbeingdevelopedforPVcells ;isCuInS2,withabandgapof1.45eV.Materialssuchas ;Cu(Ga,In)Se2andCu(A1,In)Se2canbeproducedwith ;bandgapssuitableforPVcells(Zoppieta1.).These ;materialswouldbesuitableforthedepletionregion.Of ;theehalcopyritematerials,whichhavebeendevelopedfor ;PVcells,CuInSe2hasthelowestbandgap1.05eV

    ;givingacutoffwavelengthof1.18um.Thisgivesan ;absorptivity,forthesolarspectrum,of0.81,while ;maintainingphotovohaicefficiency.Furtherimprovements ;inthermalperformancecouldbeachievedbyusinga ;materialoflowerbandgap.

    ;Multi-junctioncellsuseabsorberlayersofdifferent ;bandgapstoimproveelectricalperformance.Asthelower ;absorberlayerhasabandgapof(1.2eV),thiswould

    ;alsomsuhinalongercutoffwavelength,andhence

    ;higherabsorptivityandthermalperformance,comparedto ;asinglejunctioncel1.Thatpartofthelowestabsorber ;layerbelowthedepletionregioncouldbegradedtoa ;lowerbandgap,inordertofurtherimprovethermal ;performance.

    ;Thesecondmodification.comparedtoathermal

    ;panelistheeffectofthewindowlayerofthePVcel1. ;Thishasacutoffwavelengthduetoitsbandgap,below ;whichitisabsorbent.Tomaximiseperformance,ahigh ;bandgapmaterialisused,givingashortcut?off ;wavelength.Thematerialsusedincludemetaloxides, ;whichalsohaveacut?-offwavelength-betweenbeing

    ;transmissiveandbeingreflectiveatalongerwavelength,

    ;duetoreflectionfromfreeelectronsinthemateria1.is ;wavelengthdependsonthefreechargecarrierdensity, ;butistypicallyafewum.Thisopticalbehaviouris ;characteristicofreflectiontransmissionsurfaces,also

    ;knownasheatmi~on(AgnihotriandGupta,1971). ;Theabsorbing,transmittingandreflectingregionsofa ;typicalmetaloxidelayerareshowninFig.6. ;O2345

;Wavelength/nm

    ;一一

    ;Absorption——Tsmii??,.Reflection

    ;Fig.6Absorption,transmissionandreflectionofameml ;oxidefilm.Thebandgapis3.4eV,givingacut-off ;wavelengthof370am.Thechargecarrierdensityis ;3×10.cm.

    ;givingaplasmafrequency

    ;of1.5×10Hz.andthusacutoffwavelengthof2,um

    ;Metaloxidesarecommonlyusedasheatmirrorsin ;conventionalsolarthermalpanels.Theytransmitsolar ;radiationtotheabsorbingpanel,andreflectreemitted

    ;radiationbacktotheplate.InaPV,Tpanel,thewindow ;layerispartofthepanelfrontsurface,andwouldreflect ;radiationatthispoint.Thiswouldaltertheheatflow, ;andequilibrium,fromthatofaconventionalsolarthermal ;panel,wheretheheatmirrorsarecoatedontothefront ;coverglasses.somedistancefromthecollectorplate.(In ;aF|panel.heatmirrorsmaybeappliedtothe

    ;coverglasses,inadditiontotheTCOlayerwithinthePV ;cel1.)

    ;InaPVpanel,thecutoffwavelength(between

    ;transmissiveandreflective)shouldbelongerthanthecut- ;offwavelengthoftheabsorberlayer(9OOnm)to

    ;ensurethatal1usablephotonsaretransmittedtothe ;O8642O

    ;1OOOO

    ;oA2rB.一蕾0

    ;

    ;3DavidJOHNSTON:ThinFilmPhotovohaic/ThermalSolarPanels5 ;junction.Thisplacesanupperlimitonthecharge ;concentration,andhenceconductivity,ofthewindow ;layer.Duetoitshighbandgap,theintrinsic

    ;concentrationofthewindowlayerislow;thisisincreased ;bydoping.Ideally,thedopinglevelshouldbeashighas ;possible,tomaximiseconductivity,i.e.,itshouldgivea ;plasmafrequency,whichresultsinacut-offwavelength ;slightlylongerthanthatfortheabsorberlayer.In ;practice,themaximumconductivityproduciblebydoping ;islimited.Forinstance,zincoxide(ZnO)isdopedwith ;aluminiumoxide(A12Os)(Songeta1.).At

    ;concentrationsabove3%,thisformsgrainsofA12O3,

    ;ratherthandopingtheZnOlattice.Thislimitsfurther ;increaseinelectronconcentration,andreduceselectron

    ;mobility,thusreducingconductivity.Theoptical ;transmissionisalsoreduced,thusfurtherreducing ;photovohaicperformance.Themaximumusefuldoping ;density(3%)isequivalenttoachargeconcentrationof

    .Theresultingplasmafrequencyis(ul0. ;3×10

    ;givingacut-offwavelengthofapprox.2/zm.Thisis ;similartothatrequiredforoptimisationofthermal ;performance,andthereforenodesigncompromiseis ;requiredforthisparameter.

    ;Tosummarise;foroptimumphotovohaic

    ;performance,thebandgapoftheabsorberlayershouldbe ;approx.1.4eV(cut-offwavelength:890nm),andthe ;cut-offwavelengthofthewindowlayershouldbeslightly ;longerthanthis.Foroptimumthermalperformance,these ;cut-offwavelengthsshouldbeapprox.2/zm.Itmaybe ;possibletoimprovetheperformanceoftheabsorber-for ;combinedfunction-bygradingthecomposition.A ;windowlayer,withacut-offwavelengthoptimisedfor ;thermalperformance,isalsowithintheregionofoptimal ;photovohaicperformance.

    ;3InitialModelling:ElectricalandThermal ;PerformanceofaSolatCollectorPlate

    ;Intheinitialmodelling,onlyradiativeexchangesare ;considered.Theseincludeabsorptionofsolarradiation, ;photovoltaicgenerationandreradiationfromthesolar

    ;selectivesurface.Heatlossesareneglectedatthisstage, ;asaretheopticaleffectsofthefrontcoverass. ;Thecut-offwavelengthsofthesemiconductorslayers ;are-

    ;Windowlayer:Bandgap

    ;Plasmafrequency

    ;Absorberlayer:Bandgap

    ;350nm(typicalvalue)

    ;wP

    ;2000nm

    ;AB

    ;5001100nm

    ;(dependingonbandgap)

    ;Thesecut-offwavelengthsdefineanumberofregions ;ofthespectrum.Theopticalpropertiesofthecellineach ;regionareshownbelow.

    ;<wBAbsorptioninwindowlayer

    ;wB<<ABAbsorptioninabsorptionlayer ;AB<<wPReflectionbybackmetalcontact

;w『】<Reflectionbywindowlayer

    ;Thus,thereflection-absorptiontransitionOCCUrsat ;thecut-offwavelengthoftheabsorberlayer,whichis ;determinedbyitsbandgap.

    ;Thefractionofthesolarspectrumabsorbedisa ;functionofthebandgapoftheabsorberlayer,asgiven ;byEq.(2).Energyoutputsarethe(photovohaic) ;electricaloutput,extractedheatandre.radiationfromthe ;pane1.ThelatterisgivenbyEq.(3)andisafunctionof ;bandgapandtemperature.

    ;Themodelling,atthisstage,consistsofthe ;followingsteps.

    ;1.Foreachvalueofthebandgap,theabsorbed ;poweriscalculated-basedonAM1atnormalincidence. ;2.Thephotovoltaicoutputpoweriscalculated, ;basedonthedatashowninFig.2,andmaking

    ;reasonableestimatesforotherlosses(seriesresistance, ;etc).

    ;3.There-radiatedpoweriscalculatedoverarange ;ofusabletemperatures.(Itisassumed,atthisstage, ;thatthetemperatureattheoutletmanifoldisequaltothe ;surfacetemperatureofthecollectorplate.) ;4.Bycomparingthevaluesfromsteps1,2and4. ;theresidualpoweriscalcu1.

    ;ated,whichrepresentsan

    ;upperlimitontheextractableheat.

    ;TheresultsofthemodellingareshowninFig.7. ;Fromthegraph,thefollowingpointsCanbe

    ;identified.

    ;1.Fortherangeoftemperaturesused,there

    ;radiatedspectrumisatamuchlongerwavelengththanthe ;cutoffwavelength,duetothebandgapoftheabsorber ;layer.Therefore,thereradiatedpowerisdetermined

    ;

    ;6电工电能新技术第27

    ;BandgspleV

    ;Fig.7Photovoltaic,thermalandtotalefficiency ;asafunctionofbandgap

    ;largelybytheemissivityofthebackmetalcontact.Asthe ;reflectivityofmetalsisclosetounityinthiswavelength ;range.theemissivityislow,andthere-radiatedpoweris ;afewpercentoftheincidentpower.(Thisisgenerally ;lessthantheotherlossmechanisms,whichwouldbe ;presentinrealpanels.)

;2.Thephotovohaicoutputisatamaximumfora

    ;bandgapofapprox.1.5eV.asforaPV-onlypane1. ;3.Thenetabsorbedpowerisamaximumforthe

    ;lowestbandgap.Asthere-radiatedpowerisnegligible, ;thenetpowerisdeterminedlargelybythefractionofthe ;solarspectrumabsorbed.Thisfavoursalongercut-off ;wavelength,andhencealowerbandgap.Thispower ;wouldbethe(theoretica1)maximumextractableheatofa ;thermal-onlypane1.

    ;4.111eextractableheat(afterdeductionof ;photovoltaicpower)isamaximumforthelowestband ;g?

    ;Fromtheseobservations,designchoicescanbe ;made.Ifthebandgapisselectedtophotovohaic ;maximiseefficiency(15%at1.5eV),theresulting ;thermalefficiencyis42%,andthetotalefficiency?

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