©2019, Elsevier. This manuscript version is made available

NomenclatureC

geometricconcentrationratioofthelens-walledCPC

CV(RMSE)

coefficientofvariationoftherootmeansquarederror

CPV/D

Daylightingcharacteristicsandexperimentalvalidationofanovelconcentratingphotovoltaic/daylightingsystem

QingdongXuana

GuiqiangLib,⁎

[email protected]

YashunLua

BinZhaoa

XudongZhaob

GangPeia,⁎

[email protected]

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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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.

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