©2019, elsevier. this manuscript version is made available
TRANSCRIPT
NomenclatureC
geometricconcentrationratioofthelens-walledCPC
CV(RMSE)
coefficientofvariationoftherootmeansquarederror
CPV/D
Daylightingcharacteristicsandexperimentalvalidationofanovelconcentratingphotovoltaic/daylightingsystem
QingdongXuana
GuiqiangLib,⁎
YashunLua
BinZhaoa
XudongZhaob
GangPeia,⁎
aDepartmentofThermalScienceandEnergyEngineering,UniversityofScienceandTechnologyofChina,96JinzhaiRoad,HefeiCity230026,China
bSchoolofEngineering,UniversityofHull,HullHU67RX,UK
⁎Correspondingauthors.
Abstract
Daylightplaysanimportantroleontheenvironmentalcomfortlevelforbuildings.Asfortheenergyconsumptioninthebuilding,lightingisoneofthemaincontributors.However,traditionalbuildingintegratedsolar
utilization systems suchas flatphotovoltaic or concentratingphotovoltaic systemscanonly supply theheat or theelectricity forbuildings.Thus, anovel concentratingphotovoltaic/daylightingwindow isproposedasa
strategytoeffectivelygeneratetherenewableelectricityforthedomesticusewhileprovidingabetterdaylightperformance.Theindoorexperimentandraytracingsimulationarebothconductedtoidentifytheeffectofthe
“daylightingwindow”on theopticalperformanceof theconcentrator.Theannualdaylightperformanceofa typicalofficebuilding installedwith theconcentratingphotovoltaic/daylightingwindowatvarious installation
angles,window-to-ceilingratiosandunderdifferentclimateconditionsisinvestigatedthroughRADIANCE.Theaccuracyandconfidenceofthesimulationmodelisvalidatedthroughtheoutdoorexperiment,andthedeviation
betweentheexperimentalandsimulationresultsisaslowas8.7%,whichisindicatedbythecoefficientofvariationoftherootmeansquarederror.Thesimulationresultsshowthattheconcentratingphotovoltaic/daylighting
windowprovidesagooddaylightperformanceontheworkingplaneoftheofficeroom:thepercentageoftheworkinghoursunderdaylightthatliesintheusefulrange(100–2000 lx)canbeupto92.00%.Italsoachievesa
homogenousdistributionofdaylightwithintheinternalworkingspaceandeffectivelyreducesthepossibilityofglare.Throughthesimulationresultsunderdifferentclimateconditions,besidesofthesolarirradiance,the
latitudealsohasanobviouseffectontheannualdaylightperformance.Sofortheapplicationindifferentlatitudes,it’shighlyrecommendedtobeinstalledwiththeinclinationangelnearthelocallatitudeforahigherannual
electricityoutputandbetterannualdaylightperformance.
Keywords:Concentratingphotovoltaic/daylightingwindow;Opticalefficiency;RADIANCE;Daylightperformance
©2019, Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
ConcentratingPhotovoltaic/Daylighting
DA
DaylightingAutonomy
DF
DaylightingFactor
UDI
UsefulDaylightIlluminance
(A)
shortcircuitofthenon-concentratingPV
(A)
ShortcircuitcurrentoftheconcentratingPV
LWCPC
Lens-walledcompoundparabolicconcentrator
Greeksymbols
(°)
incidenceangle
daylightingefficiency
opticalefficiency
actualopticalefficiencyoftheconcentrator
1IntroductionIthasbeenreportedthatthebuildingsectorismorethan40%responsiblefortheworldtotalenergydemand(Omer,2008),whichisstillintheincreasingtrendwiththerapiddevelopmentofthesociety.Inthisregards,itwas
notingthatthelightingisoneofthemaincontributortothebuildingenergyconsumption.ItwasestimatedbytheEnergyInformationAdministration(EIA)thattheconsumptionoftheelectricityforlightingfortheresidentialand
commercialsectors in2016 is279billionkWh,whichwasaround10%of the totalelectricityconsumptionby thesesectorsandaround7%of the totalelectricityconsumption in theUnitedStates (EIA,2017).Thus, it’s of great
importancetointroducetherenewableenergytechnologyintobuildings(Lvetal.,2017).What’smore,combiningthenaturaldaylightingwiththeexistrenewableenergytechnologiessuchasPV,PV/T(Photovoltaic/Thermal)orCPV,
CPV/T (ConcentratingPhotovoltaic/Thermal)wouldbeamoreefficientway touse thesolarenergy forbuildings. Inaddition, ItwashighlightedbyWong thatalthoughartificial lightinghasbeingusedassupplementary lighting
resourceintheinteriorsofbuildingsforalongterm,manyreportssuggestedthenegativeeffectsofartificiallightingonhumanhealth(Wong,2017).Onthecontrary,itoffersalotofbenefitstothehumanhealthanditcanevencure
someofthemedicalailmentsbyusingnaturallight(Hraska,2015)anditcanalsoreducepsychologicalsadnessrelatedtotheSeasonalAffectiveDisorder(Sapia,2013).
θ
ηdaylighting
ηopt
ηopt,ac
SpeakingofcombingthenaturaldaylightingwiththePVsystem,comparedwiththetraditionalflatPVmodule,theconcentratingPVtechnologyoffersmoredesignimaginations,fortheconcentratorsareusuallymadeofthe
transparentmaterialsuchaspolymethylmethacrylate(PMMA).ThetraditionalplatPVmodulewillblockthesunraysfromenteringtheroomwhichmakeitinefficienttoaddthefunctionofthedaylighting.Onthecontrary,theCPV
modulemadebythetransparentmaterialwhichwouldallowaportionof“escape”sunraysintotheroomfordaylighting.Bymeansof“escape”,itrepresentsthesunraysthatcan’treachthePVcellfortheelectricitygeneration.It’s
worthnotingthatthekeycomponentoftheCPVmoduleistheconcentrator,adeviceusuallymakesuseofgeometricalopticsinthedesignofreflectiveand/orrefractivetypesofconcentratingdevicestofocusthesolarfluxontoa
receivermodulewherethePVcellisattached(Abu-Bakaretal.,2015;Xuetal.,2016).Duetothefactthatthehighgeometricconcentrationratiosneedsadditionaltrackingsystemsandhighcostopticalmaterials,itwouldnotbe
suitableforthebuildingapplication(Fengetal.,2015).Incontrasttothat,thecompoundparabolicconcentrator(CPC)ismoresuitableforbuildingintegrationbecauseitcanworkasastaticconcentrator,whichhasalsobeenpaida
lotofattentionsinceitwasfirstinventedintheU.Sin1970s(Rabletal.,1980;Winston,1974).
In theresearchareaof the lowconcentrationconcentrator forbuildingapplication, theworkdonebyprofessorProfessorT.K.Mallickshouldbehighlighted.Since thebeginningof the21stcentury,T.K.Mallicketal.have
designed several low concentrators for building application which are suitable for the installation on the building rooftop and façade. These concentrators included the traditional compound parabolic concentrator besides its
optimizationstructure,new-styleconcentratorandsoon.T.K.Mallicketal.designedanovelasymmetricconcentratorandoutdoorexperimentalresultsindicatedthattheconcentratorcanincreasethemaximumpowerbyafactorof
62% as compared with the non-concentrating solar cell (Mallick et al., 2004). Furthermore, based on the same outer contours, they further proposed the second generation asymmetric concentrator i.e. dielectric asymmetric
concentratorthatadoptsthetotalinternalreflectiontocollectsunrays,whichwasprovedtobeamoreefficientenergycollectionway.Theexperimentalresultsshowedthatthesecond-generationconcentratordeliveredapowerratio
of2.01whencomparedtoasimilarnon-concentratingsystem(i.e.increasethemaximumpowerbyafactorof103%)(MallickandEames,2007).Forthissameasymmetricstructure,Sharmaetal.usedthephasechangematerialto
enhancetheelectricalperformanceofit,andanincreaseintherelativeelectricalefficiencyby1.15%at500 W m−2,4.20%at750 W m−2and6.80%at1200 W m−2wasobserved(Sharmaetal.,2016).Muhammad-Sukkietal.proposeda
mirrorsymmetricaldielectrictotallyinternallyreflectingconcentrator(MSDTIRC)(Muhammad-Sukkietal.,2014),thestructureofwhichwasfirstcalculatedinthesoftwareMatlab®andthentransferredinto3-Dmodellingsoftware
toplotthefinalstructure.AndtheirexperimentalresultsshowedthattheMSDTIRC-PVstructurewascapableofprovidingamaximumpowerconcentrationratioof4.2×whencomparedtoasimilarcellwithouttheconcentrator
(Muhammad-Sukkietal.,2013).Baigetal.presentedalowconcentratorphotovoltaicsystemwiththermal(LCPV/T)extraction,andanincreaseinpowerof141%(powerratio2.41)wasfoundthroughtheexperimentaltestcompared
totheanalogousnon-concentratingcounterpart(Baigetal.,2018).Reddyetal.designedanEllipticalHyperbolicCollector(EHC)andcarriedouttheexperimentalinvestigationoftrapezoidal/concavecavitysurfacereceiver(TSR)for
it.Basedontheexperimentalresults,fortheflowrateof0.03 kg/minand0.5 kg/min,thefluidoutlettemperatureisestimatedtobe87 °Cat768 W/m2and49 °Cat908 W/m2respectively.Thecorrespondinginstantaneousefficiency
wascalculated tobe9%and40%respectively (Reddyetal.,2018).Baigetal.designedabuilding integratedconcentratingphotovoltaic (BICPV)system.ThesystemunderstudyareessentiallycomposedofSymmetricElliptical
Hyperboloid(SEH)concentratingelementswiththegeometricconcentrationratioof6X(Baigetal.,2015).Lamnatouetal.conductedanlife-cycleassessmentforthedielectric-based3Dbuilding-integratedconcentratingphotovoltaic
modulesanditwasfoundthatenergypaybacktimesrangesfrom2.30to4.10 years(Lamnatouetal.,2017).
Besidesofsomeexistingnoveldesignsofthelowconcentrationconcentrators,mostoptimizationstructuresofthelowconcentrationconcentratorarederivedfromthetraditionalCPC,andthelens-walledcompoundparabolic
concentrator(LWCPC)isoneofthetypicalrepresentatives.Suetal.proposedthefirstgenerationofLWCPCbyrotatingtheoutercontourofthetraditionalCPCwithacertainangle(usually3-–5°)toformthelens-walledstructure(Su
etal.,2012a,2012b).ThemainpurposeoftheoriginalLWCPCwastoreducethemanufacturematerialthustoreducetheoverallcostandweightoftheconcentratingPVsystemsandusetherefractiontoenlargetheacceptancerange
oftheconcentratortomakeitmoresuitableforbuildingapplication(Lietal.,2013).InordertofurtherenhancetheopticalperformanceoftheLWCPC,Lietal.proposedtoadoptthetotalinternalreflectionbysettinganairgap
betweenthelens-walledstructureandthemirrorconcentratorwiththesamestructure,whichwecalledasthesecondgenerationLWCPC(Guiqiangetal.,2014).Inthisway,theopticalefficienciesatvariousincidenceanglescanbe
increasedbymorethan10%withamoreuniformfluxdistributiononthereceiver(Guiqiangetal.,2013)andithasbeenconcludedthattheuniformfluxdistributionisgoodforthePVoutput(Lietal.,2018b).AconcentratingPV/T
systemwiththeuseofthesecondgenerationLWCPCforbuildingapplicationwasalsobuilt(Lietal.,2014).Outdoorexperiment(Lietal.,2015a)andnumericalsimulation(Lietal.,2015b)resultsindicatedagoodconcentratingPV/T
performancewhichprovedagoodsolutionforBICPVorBICPV/T.Inspiredbythisandconsideringthefactthattheconcentratorinsymmetricgeometryismoresuitablefortheintegrationwiththebuildingroof,whileduetodifferent
irradiationcondition,theasymmetricstructuremightbeabetterchoiceforthebuildingfaçade.Xuanetal.designedanasymmetricairgaplens-walledCPCfortheintegrationwiththebuildingsouthwall(Xuanetal.,2017b),andthe
opticalperformanceof itweredetailed studied through theexperimentand ray-tracingsimulation (Lietal.,2018a;Xuanetal.,2017a).Considering that the sun raysnear thebasearea for theLWCPCwouldescapeoutof the
concentrator,it’sfeasibletoaddthefunctionofthedaylighting.BasedonthefirstgenerationLWCPC,theoutersurfacenearthebaseareaisn’tcoatedtosetupthe“daylightingWindow”,whichwewouldliketocallitasthethird
generationoftheLWCPC,toachievethemulti-functionoftheelectricitygenerationanddaylighting.It’sworthnotingthatsettingupthe“daylightingwindow”won’tdecreasetheopticalefficiencyoftheconcentratorbutachievea
transmittanceofaround10%(Lietal.,2018c).Inthisway,amultifunctionoftheconcentratingphotovoltaic/daylighting(CPV/D)windowcanbeformed.Comparedwiththetraditionalwindow,theCPV/Dwindowofferstheoptimization
potentialfortheenergyconsumptionwithinthebuildings.Forinstance,duringthesummerwhenthesolarradiationisuncomfortablyhighwillmostlybeabsorbedbythePVcellsattachedwiththeabsorberoftheconcentratorinthe
generationoftherenewableelectricityandonlyallowsasmallportionofthesolarradiationintothebuildingforlighting.Howeverinconventionaldesigns,thehighsolarradiationiscontrolledbytheshadingdeviceandtherefore
dissipateasheat;Inthewinter,lightandheatpreferentiallypassthroughthesystemhelpingtooffsetheatingandlightingenergydemands(Lietal.,2018c).
Thequantity,qualityanddistributionofdaylightthatpassesthroughawindowsystemandilluminatesaspace,playsanimportantroleinenergyefficiencyandachievingacomfortableindoorenvironment.Thus,inthispaper,
theactualdaylightperformancewhenusingtheCPV/Dwindowaredetailedpresentedwiththeuseofthedynamicdaylightperformancemetrics.ThesoftwareRADIANCEisusedforindicatingthedaylightingperformanceofaCPV/D
windowinstalledonatypicalofficeroom.Inthesimulation,acellularofficeroominstalledwithdifferentCPV/DwindowsismodelledwiththeactualweatherdatafromEnergyPlus,andtheilluminancedistributionacrosstheworking
planeiscalculatedfor1 htime-stepthroughthecourseofayear.Thepredictedluminousenvironmentduringworkinghourswereanalyzedusingadvancedmetrics(e.g.usefuldaylightilluminance(UDI)).Theeffectoftheinstallation
anglesaswellas thewindow-to-ceilingratiohasbeendetailedstudiedtomatchwith thedifferentbuildingdesigns.Theperformanceof thechosenCPV/Dwindowhasalsobeen investigatedunderdifferentclimateconditions to
provideanindicationofhowsite-specificvariablesinfluenceperformance.
2ThedescriptionoftheCPV/DwindowAsillustratedinFig.1isthekeycomponentoftheConcentratingPhotovoltaic/Daylighting(CPV/D)window,i.e.thelens-walledcompoundparabolicconcentrator(LWCPC),whichhasbeenwidelystudiedinthepreviousstudies
(Lietal.,2015a,2014).Thedetaileddescriptionoftheformationprocessofthelens-walledcompoundparabolicconcentratorhasbeendetailedintroducedin(Li,2018).Thelens-walledcompoundparabolicconcentratoriscoatedon
theoutersurfacetoconcentratethesolarradiationthroughthespecularreflection.Inordertoachievethefunctionofthedaylight,thelowerpart(nearthebasearea)isnotcoatedtosetthe“daylightingwindow”,whichallowsa
portionofsunraystoreachtheroom(Fig.1(c)).
Fig.1(a)3Dview;(b)sectionviewofthelens-walledCPCpaneland(c)CPV/Dwindow.
OnemajorconcernabouttheCPV/Dwindowisthattheopticalefficiencymightbedecreasedduetothe“daylightingwindow”.Inordertoaddressthisissue,theopticalperformanceofthelens-walledCPCwithandwithout
settingthe“daylightingwindow”arestudiedthroughtheraytracingsimulatingandtheindoorexperiment.Theopticalefficiencyanddaylightingefficiencyaredefinedas:
Andtheactualopticalefficiencyoftheconcentratorcanbeinvestigatedby:
Wherewhere istheshortcircuitcurrentoftheconcentratingPV; istheshortcircuitofthenon-concentratingPV.Cisthegeometricconcentrationratio.
The optical simulation is conducted using the software Lighttools®. The geometricmodel of the CPV/Dmodule andCPVmodule are first built in SolidWorks®and then transferred into Lighttools® for the ray tracing
simulation.Detailedsimulationparameterscanbefoundin(Xuanetal.,2017a).InordertofindouttheactualopticalperformanceoftheCPV/DandCPVmodule,theprototypeoftheCPV/DandCPVmodulearemanufacturedand
assembledasshowninFig.2.ThematerialisselectedastransparentPolymethylMethacrylate(PMMA)withtherefractiveindexof1.49.PolymethylMethacrylateiswidelyusedfortheconstructionofopticalconcentratorsduetoits
hightransmittance(92%per10 mm)andgoodresistancetophotodegradation.Theevaporatedaluminumcoatingtechnologywasemployedtoprocessthemirrorreflectors(withthespecularreflectivityofaround85%).Asillustrated
inFig.2,istheindoorexperimenttestrigusedtoevaluatetheelectricalcharacteristicsoftheCPV/DandCPVmodule.Detaileddescriptionoftheexperimentaltestrigandhowthetestsareconductedcanalsobefoundin(Xuanetal.,
2017a).Theexperimentsareconductedunderthestandardtestcondition(STC,thesolarsimulatorgeneratesarayintensityof1000 W m−2withtheroomtemperatureof25 °C).
ThesimulationandexperimentalopticalefficiencyoftheCPV/DandCPVmodulearepresentedinFig.3.Frombothsimulationandexperimentalresults,itcanbeseenclearlythattheopticalefficienciesoftheCPV/Dmodule
atvariousincidenceanglesarebasicallyconsistentwiththatoftheCPVmodule.Sotheconcerntowardstheeffectof“daylightingefficiency”canbeexpelledbecausetheCPV/Dmoduleonlyusestheportionofsunraysthatcan’tbe
used for theelectricitygeneration fordaylighting.Combinedwith thedaylightingefficiencyresultsatvarious incidenceangles (Fig.4), itcanbeconcluded thatsetting“daylightingwindow”hasnonegativeeffecton theoptical
performanceofthelens-walledcompoundparabolicconcentratorbutaddthefunctionofthedaylightingtoachieveahigherusageratioofthesolarenergywhichcanalsobettersuitwiththebuildingenergydemands.Thedeviationof
around10%isobservedfromthesimulationandexperimentalresults,whichcanbecausedbyallkindsoferrors(Lietal.,2018a).
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Fig.2TheprototypeoftheCPV/D,CPVmoduleandtheindoorexperimenttestrig.
3Daylightingsimulationandthevalidationofthemodel3.1Simulationmethodanddaylightperformanceassessmentmetrics
Inthisstudy,acomprehensivedaylightingmodelwasbuiltbasedonRADIANCE,whichwasdevelopedbyLawrenceBerkeleyNationalLaboratory(LBNL),USA.RAIDANCEisasoftwaretoolbasedonabackwardray-tracing
algorithm,whichmeansthattheraysareemittedfromthepointofinterestandtracedbackwardsuntiltheyeitherhitalightsourceoranotherobject(Sunetal.,2017).
Theannualsimulationofaspacethatadoptedthecomplexfenestrationsystem,suchasconcentratingphotovoltaic/daylightingwindow,wouldrunintoalotofdifficulties,becauseofthecomplicatedinternalmultipleinter-
reflectionsthatoccurwithintheconcentratingphotovoltaic/daylightingwindow.Thusinthispaper,ratherthansimulateaspecificdaylightcondition,the“three-phasemethod”wasusedtoconducttheannualdaylightperformance
predictionoftheCPV/Dwindowinanoffice.Withtheuseofthe“three-phasemethod”,fluxtransferisbrokenintothefollowingthreephasesforindependentsimulation(McNeil,2010):
1. Skytoexterioroffenestration;
2. Transmissionthroughfenestration;
3. Interioroffenestrationintothesimulatedspace.
Amatrixisusedtocharacterizeeachphaseoflighttransport.Theinputcondition,skyluminance,isavector.Theresult,illuminancevaluesorarendering,isalsovector.Theresultisachievedbymultiplyingthesunvectorby
eachmatrixrepresentingeachphaseoffluxtransfer.Thisprocessisdescribedbythefollowingequation(McNeil,2010):
where:i is point in time illuminanceor luminance result; I ismatrix containing time series of illuminanceor luminance result;T is transmissionmatrix, relating incidentwindowdirections to exitingdirections;V is viewmatrix
andD isdaylightmatrix,whichcanbeobtainedthroughanembeddedcommand inRADIANCEconsideringthemodel’sorientation,surroundingenvironment,geometryandsurfacepropertiesof the indoorspace;s is skyvector,
assigning luminance values to patches representing skydirections;S is skymatrix, a collection of sky vectors. Skymatriceswere obtained fromCSTWweather data for different citieswithdifferent latitudes and climates. The
transmissionmatrixfortheCPV/DwindowsystemswasexpressedusingBidirectionalScatteringDistributionFunctions(BSDFs).DetaileprocessesabouthowtheBSDFfilesfortheCPV/Dwindowaregotcanbefoundinreferences
(McNeil,2010;Sunetal.,2017).
Fig.3ExperimentalandsimulationopticalefficiencycomparisonoftheCPV/DmoduleandCPVmodule.
Fig.4DaylightingefficiencyoftheCPV/Dmoduleatvariousincidenceangles.
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(5)
There are severalmetrics such as Daylighting Factor (DF), Daylight Autonomy (DA), Useful Daylight Illuminance (UDI), and IlluminanceUniformity Ratio (UR), etc. that are used for the dynamic daylight performance
assessment(Reinhartetal.,2006).Sunetal.summarizedthecharacteristicsofthesemetrics(Sunetal.,2017).DaylightingFactorwasatraditionalmetric thataremainlybasedontheruleof thumbwiththesimplifiedcalculation
algorithm,whichalsoseemstobeincreasinglydeemedinadequateforthemoreandmorecomplexdaylightequipment(Sunetal.,2017).Nowadays,DaylightingAutonomyandUsefulDaylightIlluminance(UDI)aretwocommonlyused
metrics.Thesemoresophisticatedmetricsarecurrentlyavailablefromseveralfreedaylightingsimulationsoftware,suchasRADIANCE(ReinhartandAndersen,2006)andDAYSIM(Chengetal.,2018).ThemetricofDaylightAutonomy
wasfirstlyproposedbytheAssociationSuissedesElectriciensin1989,whichindicatesthepercentageofworkinghoursinayearataspecificsensorpointinwhichaminimumilluminancethresholdcouldbeachievedbydaylight
alone(Reinhartetal.,2006).However, thedrawbackof thismetric isobvious, thismetriconlyconsider theminimum illuminance levelbut fail toconsiderstrongglareeffectunder theexcessivedaylighting. In thiscase,Daylight
AutonomywasmodifiedwiththemetricofUDI(NabilandMardaljevic,2006;Nabil,2016),whichisdeterminedbyclassifyingthesimulatedhourlyilluminanceatthesensorpointsinto3bins:
(1) anundersuppliedbin(illuminancevalue < 100 lx);
(2) ausefulbin(100 lx < illuminancevalue < 2000 lx);
(3) anoversuppliedbin(illuminancevalue > 2000 lx).
ThusUDIisanallarounddynamicmetricthatcouldavoidthepotentialofglareortoodark(Pengetal.,2015).
3.2ModelvalidationTheaccuracyoftheRADIANCEalgorithmdaylightcoefficientmethodandPerezskymodelhavebeenanalyzedformorethan10,000skyconditionsincludingovercastskies,clearskiesandpartlycloudyskiesbyReinhartetal.
(Reinhart and Andersen, 2006; Reinhart andWalkenhorst, 2001). The results of theirs proved that theRADIANCE is able to efficiently and accurately predict the annual indoor illuminance distribution of the buildings adopting the
complicateddaylightingelementsbasedonthebuildinggeometry,opticalpropertiesofthesurfacesanddirectanddiffusesolarirradiance.Inthisresearch,inordertodisplaytheannualpredictionresultsfortheactualofficeroom
withtheCPV/Dwindowmoreconfidentlyandaccurately,aCPV/Dmoduleintegratedwiththeintegratingbox(Fig.5)ismanufacturedandinstalledontheEngineeringBuilding2,UniversityofScienceandTechnologyofChina,Hefei,
China(31.86°N,117.27°E).Thephotometricintegratingboxisacubicboxwithitsinternalsurfacepaintedmattwhitesothatlightcanbediffuselyreflectedtotheinternalsensor,whichwasproposedbytheUKBuildingResearch
Establishment.Theilluminancesforthetestrigweremeasuredandcomparedwithilluminancesfromsimulationunderthesameconditions.
Themeasurementsweretakenon10thAugust2018withtheclearskycondition.TheilluminanceonthesouthwalloftheboxwasmeasuredbytheilluminometerTES-1339R(withanaccuracyof±3%).Thecomparisonofthe
simulationandexperimentalresultswasmadeasshowninFig.6.Thepictureoftheexperimentalboxwhichwastakenduringtheexperiment,andasimulatedrenderimagefromthesimulationareshowninFig.7.Fromtheresults,it
canbeseenclearlythatthesimulationresultsarebasicallyconsistentwiththesimulationsdespitesmalldeviations.ThedeviationscanbequantifiedbyCv(RMSE)(coefficientofvariationoftherootmeansquarederror)indicatesthe
overalluncertaintyinthepredictionofsimulation(YunandKim,2013).Alowervaluemeansthebettermodelaccuracy:
Fig.5Theoutdoorexperimenttestrig.
RMSEiscalculatedby:
whereNisthenumberofthetimeintervals,Isim,iandIexp,iaresimulatedandmeasureddatarespectively.
Themeanofthemeasureddataiscalculatedby:
ThenCV(RMSE)iscalculatedby:
AfterthecalculationofthesimulationandexperimentresultspresentedinFig.6,thevalueofCV(RMSE)is8.7%,whichindicatethehighlevelreliabilityintheuseoftheRADIANCEtopredictthedaylightingperformanceof
theofficeroomusingtheCPV/Dwindow.
3.3ThedescriptionoftheactualofficemodelingAfterthevalidationoftheCPV/DwindowintheRADIANCE,themodeloftheCPV/Dwindowcanbeusedtopredicttheannualdaylightingperformanceoftheactualofficeroom(withthesizeof4.0 m × 3.5 m × 3.3 m,Fig.8(a))
integratedwiththeCPV/Dwindow.A12 × 18analysisgridcomprising216pointsareusedtoestimatetheannualilluminancedistributionontheworkingplane(usually0.75 mabovethegroundlevel)asdepictedinFig.8(b).ACPV/D
Fig.6Validationofthemodel.
Fig.7(a)Photooftheboxand(b)Renderedimage.
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windowwiththedimensionsof4.00 m(length)and3.50 m(width)forthe100%windowtoceilinginstallationratioislocatedontherooftopoftheofficeroomintheEast-Westorientation.ThedimensionsoftheCPV/Dwindowsforthe
75%and50%windowtoceilinginstallationratioare3.46 m(length)/3.03 m(width)and2.83 m(length)/2.47 m(width)respectively.TheCPV/Dwindowistestedtohavealighttransmittanceofaround10%.Thecoordinatesystem
presented inFig.8 is used to locate theanalysisgrid,whichwill beused in theFigures that illustrate the simulation results. TheX-axis andY-axis represent southwall andwestwall respectively. In the simulation, illuminance
distributionontheanalysisgridiscalculatedfor1 htime-stepsoverthecourseofayear.Thepredictedluminousenvironmentduringworkinghourswereanalyzedusingtheadvancedmetric,i.e.UDI.Theresultsaregivenasthe
proportionoftheworkinghoursinanentireyearatthelocationpoints(Fig.8(b))generatesadesireilluminancelevel(100to–2000 lx).Forexample,thevalueof0.85ataspecificpointmeansthat85%ofayear’stimeatthispointcan
maintainthedesireilluminancelevelof100to–2000 lx.
3.4LocationsandweatherdataInordertostudytheperformanceoftheCPV/Dwindowunderdifferentgeographicalandweatherconditions,fivedifferentcitiesarechosen:Haikou(20.02°N,110.20°E),Lhasa(29.39°N,91.08°E),Hefei(31.86°N,117.27°E),
Beijing(39.90°N,116.40°E),Harbin(45.44°N,126.36°E).TheweatherdatafilesofthesecitiesaregottenfromEnergyPlus.Thesimulationsarerunat1-htimestepfortheentireyearwiththeCSTWweatherfile.
4ResultsanddiscussionsFortheconcentratingdevices,duototherestrictionoftheacceptancerangeoftheconcentratorandsunmotion,thelocal latitudeandinstallationanglehaveagreat impactontheannualperformanceofthem(Lietal.,
2018c).Thus,inthissection,thesimulationsareconductedfortheCPV/Dwindowwithdifferentinstallationangles(i.e.0°,10°,20°,30°,40°),andtheeffectoftheCPV/Dwindowtoceilingratioisalsodetailedanalyzed.Forthe
Fig.8(a)Simulationmodel;(b)Selectedpointsforevaluatingilluminancedistributionontheworkingplaneoftheoffice.
analysisofeffectofthesetwofactors,theofficeisassumedtobelocatedinHefei.Atlast,fivedifferentcitiesinChinawiththelatitudesfromlowtohighareselectedtoidentifytheperformanceoftheCPV/Dwindowunderdifferent
geographicalandweatherconditions.
4.1TheeffectoftheinclinationangleAsdescribedbyLietal.(2018c)thattheCPV/Dwindowisdesignedtobeinstalledonthebuildingrooftopasanalternativetotheskylight,thusitmustsuitwiththebuildingrooftopinclinationanglebymeansofchangingthe
installationangle(inclinationangle).ThefigurespresentedinFig.9areUDIbinsfortheofficeroomwiththedifferentCPV/Dinclinationanglesandthecolorfulcontourrepresentsthepercentageofworkinghoursatthetestpoints
generateadesireilluminancelevelof100to2000 lx.Astheinclinationangleoftheconcentratorchanges,theactualincidenceangles(definedastheanglebetweenthesunrayandthenormalofthebaseoftheconcentrator)will
changeaswell,whichfinallyleadstochangeofthedaylightingefficiencyandopticalefficiency.Thefivecaseswith0°,10°,20°,30°and40°inclinationangleswillbecalledasCase0,Case10,Case20,Case30,Case40inthefollowing
interpretation.
Fromtheresults,theeffectoftheinclinationangleoftheCPV/DwindowontheUDIbinisobvious.Fortheoverallcomparisonbymeansofthecomparisonoftheaveragevaluefrom216sensorpoints,theUDI100–2000values
fromCase40arehighestamongfivecases,whiletheresultsfromCase10,Case20,Case30arealmostatthesamelevel,whichare6%-10%lowerthanthatfromCase40.It’snowonderthatthesolarenergyutilizationsystemsincluding
theconcentratingdeviceisbettertobeinstalledattheinclinationanglearoundthelocallatitude,whichcancapturemoresolarenergyannually.TheaverageUDI100–2000valuesonthe216pointsforfivecasesare80.52%,76.98%,
76.00%,77.99%and85.57%andthemediansofthesevaluesare81.00%,77.00%,76.00%,78.00%and86.00%respectively.Toinvestigatethenon-uniformityfactoroftheUDI100–2000resultsforeachcase,thefollowingequationis
Fig.9UDI100–2000binsfortheofficeroomwiththedifferentCPV/Dinclinationangles:(a)0°;(b)10°;(c)20°;(d)30°;(e)40°.
used(Xieetal.,2016):
where and are the maximum andminimum values displayed in Fig. 9. Thus the non-uniformity factors for five cases are 8.46%, 11.26%, 10.53%, 15.23% and 8.24%. The non-uniformity factors of the
UDI100–2000resultsareallnotveryhighacrosstheanalysisgridforfivecases,whichindicatearelativelyuniformillumianceilluminanceleveldistributionontheworkingplane.Therefore,drawfromtheyearlypredictionresults,itcanbe
concludedthataround80%ofworkinghoursontheworkingplaneoftheofficeroomwithdifferentCPV/Dwindowinstallationanglescanattainthedesireilluminancelevel,whichprovesthattheCPV/Dwindowcansuitwithdifferent
rooftopslopeanglesthusprovidesawiderapplicationscope.
4.2TheeffectofthewindowtoceilingratioThecontourdataofUDI100–2000fortheofficeroomontheworkingplaneof0.75 mwithdifferentwindowtoceilingratio(WCR)ispresentedinFig.10.Thewindowtoceilingratiosof50%,75%and100%areselectedtobe
studiedinthissection.Asitcanbeseenfromtheresultsthatwiththeincreaseofthewindowtoceilingratiosfrom50%to100%,theUDI100–2000valuesacrosstheanalysisgridexperienceanobviousincrease.Theaveragevalue
increasesfrom69.00%to80.00%.Themaximumvaluesforthreecasesareclose,whichallexistsinthemiddleareaoftheroom.However,theminimumvaluesvariesalot,whichare74.00%,61.00%and48.00%forWCR-100%,WCR-
75%andWCR-50%respectivelyandthesevaluesareallclosetothewallarea.Themainreasonforthisisthat,theCPV/Dwindowareinstalledatthecenterareaoftherooftop,whichshowslittleeffectonthecenterareaoftheoffice
room,butwillinfluencethedaylightinglevelnearthewallsignificantly.However,fortheactualhumandaylilylife,theuserateoftheareanearthewallismuchlowerthanthatofcenterareainthesameofficeroom,nottomention
thattheareanearthewallisalsoclosetothewindowonthewall.Inthiscase,itcanbeconcludedthatalthoughthedecreaseofthewindowtoceilingratiodecreasetheUDI100–2000ontheanalysisgrid,theeffectontheactualhuman
activityisrathersmall.ItshouldbenotedthattheanalysisoftheeffectofthewindowtoceilingratioalsoemphasizethefeasibilityofinstallingtheCPV/Dwindowonrooftopswithdifferentconditionsjustliketheanalysisoftheeffect
oftheinclinationangle,whichalsoprovidesthebasicdesignreferencefortheactualengineering.
(9)
4.3TheapplicationoftheCPV/DwindowindifferentclimatesInthissection,theperformanceoftheCPV/DwindowunderdifferentclimateconditionsinChina,i.e.Haikou,Lhasa,Hefei,BeijingandHarbin,whichspanalatituderangefrom20.02°to45.54°thatcoversmostlatitudesof
Chinesearea.ThediurnalaveragedirectanddiffusesolarradiationfromtheCSWDweatherfilesforthesefivecitiesareshowninFig.11.Thecharacteristicsoftheweatherconditionforthemcanbedescribedas:Haikouhasa
tropicalmonsoonclimate,andtheannualsolarradiationisverystronghere.Sometimesthediffusesolarradiationisalittlelargerthanthedirectsolarradiation;Lhasahasaplateaumountainclimate,andithasverystrongannual
directsolarradiationwhichismuchlargerthanthediffusesolarradiation;Hefeihasasubtropicalmonsoonclimateandtheannualdirectsolarradiationisapproximatelyequaltothediffusesolarradiation;Beijinghasatemperate
monsoonclimateand ithasverystrongdirectsolar radiation inwinter,skiesdiffuse insummer;Harbinalsohasa temperatemonsoonclimatebutunlikeBeijing, ithasstrongerdirectsolar radiation thandiffusesolarradiation
throughouttheyear.TheCPV/DwindowisinstalledintheEast-Westorientationandforamorecomprehensiveandreasonablecomparison,theCPV/Dwindowforfivecitiesareallinstalledwithnoinclinationangle.Thedistribution
dataoftheUDIbinsselectedtoquantifytheperformanceforfivecitiesacrosstheanalysisgridarepresentedinFigs.12and13.TheanalysisfarinthisresearchmainlyfocusedonthemetricofUDI100–2000bins,howeveritisdefinedby
ChinaStandardfordaylightingdesignofbuildings(MOHURD,2013)thattheminimumacceptableilluminancelevelfortheofficebuildingis450 lx.Thus,inthissection,thedesiredUDIrangeof100–2000 lxisfurtherdividedintotwo
Fig.10UDI100–2000binsacrosstheanalysisgridfordifferentWCR:(a)50%;(b)75%;(c)100%.
UDIbins:asub-desiredrangeof100–450 lx(UDI100–450)andadesiredrangeof450–2000 lx.
Fig.11ThediurnalaveragedirectanddiffusesolarradiationfromtheCSWDweatherfilesfor:(a)Haikou;(b)Lhasa;(c)Hefei;(d)Beijing;(e)Harbin.
Fig.12UDI100–2000binsacrosstheanalysisgridforfivecities:(a)Haikou;(b)Lhasa;(c)Hefei;(d)Beijing;(e)Harbin.
Fig.11showstheUDI100–2000valuesacrosstheanalysisgridforHaikou,Lhasa,Hefei,BeijingandHarbin.Fromtheannualpredictionresults,itcanbeseenclearlythatdespitethedifferenceoftheannualsolarradiationforfive
cities,withtheincreaseofthelocalaltitudes,theoveralldaylightingperformanceoftheCPV/Dwindowalsoincreases.Themainreasonforthismightbe:ontheonehand,thedistributionpercentageofthediffusesolarradiationhasa
greatinfluenceonthedaylightingperformanceoftheCPV/Dwindow,forthatthetransmittanceoftheCPV/Dwindowwithintheacceptancerangeoftheconcentratorforthedirectsolarradiationisaround10%,butthisvalueforthe
diffusesolarradiationcanbehigher.ThisiswhythelatitudesofHefeiandLhasaarealmostatthesamelevel,buttheannualdaylightingperformanceinHefeiisbetterthanLhasa,becausealthoughtheannualdirectsolarradiationof
LhasaislargerthanthatofHefei,thediffusesolarradiationinHefeiislarger.Ontheotherhand,withtheincreaseofthelatitudeangle,theequivalentincidenceanglefortheconcentratorincreasesaswellandcombinedwiththe
resultsshowninFig.4,it’sobviousthatthelargerincidenceanglesarecorrespondingwiththehigherdaylightingefficiencyespeciallywhentheincidenceanglesareoutofthescopeoftheacceptancerangeoftheconcentrator.
Theaveragesvaluesof theUDI100–2000bins for fivecitiesare66.00%,75.00%,81.00%,84.00%and84.00%respectively,with thenon-uniformity factorsof9.09%,17.48%,8.64%,9.20%and11.25%.The largervaluesof
UDI100–2000binsareallbasicallyfallsinthemiddleareaoftheworkingplaneforfivecities.
TheUDI450–2000valuesforHaikou,Lhasa,Hefei,BeijingandHarbinacrosstheanalysisgridareshowninFig.13.DifferentfromresultsofUDI100–2000,thevaluesofUDI450–2000fromLhasaandHaikouarehighestamongfivecities
(theaveragevaluesare46.00%and43.00%)whilethosefromHarbinarelowest,andUDI450–2000valuesfromHefeiandBeijingarealmostatthesamelevel.Thismeansthat,althoughtheresultsofUDI100–2000fromHaikouandLhasa
are lowest, the illuminance levelof them is relativelyhigher than thatofothercities.Harbinhas thehighestUDI100–2000 valueson theworkingplane,but in themost timeof theyear, the illuminance level iswithin therangeof
100–450 lx.FromtheUDI450–2000binsresultsforfivecities,itcanbeseenclearlythatthelargervaluesacrosstheanalysisgridtendtobeclosetothecenterareamoreobviously,sotheCPV/Dwindowhasanadvantageofredirecting
Fig.13UDI450–2000binsacrosstheanalysisgridforfivecities:(a)Haikou;(b)Lhasa;(c)Hefei;(d)Beijing;(e)Harbin.
thesunraystothecenterareaoftheofficeroom,whichmakethenaturaldaylightingmoreefficient.
5ConclusionsThis research displays a novel concentrating photovoltaic/daylighting window with the optimization lens-walled CPC. The annual daylight performance of a cellular office room installed with different concentrating
photovoltaic/daylightingwindowsismodelledwiththeactualweatherdatafromEnergyPlusforarangeofdifferentsitesandwindowinstallationanglesusingRADIANCE.Thefollowingconclusionscanbedrawn:
(1) Theindoorexperimentandraytracingsimulationarebothconductedtoidentifytheeffectofthe“daylightingwindow”ontheopticalperformanceoftheconcentrator.Fromsimulationandexperimentalresults,itcanbeseenclearlythatthe
opticalefficienciesoftheCPV/DmoduleatvariousincidenceanglesarebasicallyconsistentwiththatoftheCPVmodule,whichcanbeconcludedthatsetting“daylightingwindow”hasnonegativeeffectontheopticalperformanceof the
concentratorbutaddthefunctionofthedaylightingtoachieveahigherusageratioofthesolarenergywhichcanalsobettersuitwiththebuildingenergydemands.
(2) Thedaylightingsimulationmodelfortheconcentratingphotovoltaic/daylightingwindowintegratedwiththetypicalofficeroomisvalidatedthroughtheoutdoorexperiment,andthedeviationbetweentheexperimentalandsimulationresultsis
aslowas8.7%,whichisindicatedwiththecoefficientofvariationoftherootmeansquarederror.Thustheaccuracyandconfidenceofthesimulationmodelisproved.
(3) Theconcentratingphotovoltaic/daylightingwindowoffersagoodannualdaylightperformanceintermofUsefulDaylightIlluminance(UDI).ThepercentageofworkinghourswhentheUDIliesintherangeof100–2000 lxisbetween60.00%
and89.00%fordifferentgeographicalsitesandweatherconditions.Thisvaluecanbeupto92.00%withthechangeofinstallationangles.
(4) Theinstallationangleshastheeffectontheannualdaylightperformanceoftheconcentratingphotovoltaic/daylightingwindow,butthiseffectisnotsoobvious,whichindicatesthattheconcentratingphotovoltaic/daylightingwindowcansuits
withdifferentbuildingdesigns.
(5) Theapplicationoftheconcentratingphotovoltaic/daylightingwindowsatdifferentlatitudessuggestthatwiththeincreaseofthelatitude,thepercentageofworkinghourswhentheUDIliesintherangeof100–2000 lxacrosstheworking
planedecreasesdespitethedifferenceoftheyearlysolarradiation.Andit’sfoundthatthediffusesolarradiationhasanmoreobviouseffectonthedaylightperformanceoftheconcentratingphotovoltaic/daylightingwindow:thelatitudesof
HefeiandLhasaarecloseandtheannualsolarradiationinLhasaisstrongerthanthatinHefei,buttheconcentratingphotovoltaic/daylightingwindowshowsabetterdaylightperformanceinHefeiratherthaninLhasa.Themainreasonforthis
isthatthediffusesolarradiationinHefeiislargerthanthatinLhasaandtransmittanceofthediffusesolarradiationismuchlargerthanthatofthedirectsolarradiation.
AcknowledgementThe studywas sponsoredby theProjectofEUMarieCurie International IncomingFellowshipsProgram (745614), theNationalNatural ScienceFoundation ofChina (NSFC51476159,51776193, and5171101721),
International TechnologyCooperationProgramof theAnhui Province ofChina(BJ2090130038),andFundamental ResearchFunds for theCentralUniversities (WK6030000099). The authorswould like to thankProf. Zheng
Hongfei(SchoolofMechanicalEngineering,BeijingInstituteofTechnology,China)forhisassistanceinthesoftwaresimulation.
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Highlights
• ThedesignofanovelCPV/Dwindowbasedonthelens-walledCPC.
• ThecomparisonoftheopticalperformancewasmadefortheCPV/DandCPVmoduleexperimentally.
• ThedaylightingsimulationmodelinRADIANCEwasexperimentallyvalidated.
• ThedaylightperformanceoftheCPV/Dwindowwasinvestigated.