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Chapter 1 Solar Radiation - Eastern Mediterranean University

Chapter 1 Solar RadiationTHE SUN 109m Itis,ontheaverage, 1011mawayfromtheearth. Thesunrotatesonitsaxisaboutonceevery4wee ks, Itdoesnotrotateasasolidbody Thesunhasaneffectiveblackbodytemperature of5777K Thetemperatureinthecentralinteriorregion sisvariouslyestimatedat8 106to40 106K SUN Sun sdensityisabout100timesthatofwater. Severalfusionreactionshavebeensuggestedt osupplytheenergyradiatedbythesun. Theoneconsideredthemostimportantisaproce ssinwhichhydrogencombinestoformhelium Theenergyproducedintheinteriorofthesolar sphereis firsttransferredouttothesurface andthenradiatedintospaceTHE Solar CONSTANT sorbit Nearlyfixedintensityofsolarradiationreac hestotheoutsideoftheearth satmosphere. ThesolarconstantGsc=1367W/m2 DEFINITIONS AirMassm:Theratioofthemassofatmosphereth roughwhichbeamradiationpassestothemassit wouldpassthroughifthesunwereatthezenith( ,directlyoverhead) BeamRadiation:Solarradiationreceivedfrom thesunwithouthavingbeenscatteredbytheatm osphere.

horizontal surface, often referred to as global radiation on the surface.) • Irradiance, W/m2: The rate at which radiant energy is incident on a surface per unit area of surface. The symbol G is used for solar irradiance, with appropriate subscripts for beam, diffuse, or spectral radiation. • Irradiation or Radiant Exposure, J/m2: The incident

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Transcription of Chapter 1 Solar Radiation - Eastern Mediterranean University

1 Chapter 1 Solar RadiationTHE SUN 109m Itis,ontheaverage, 1011mawayfromtheearth. Thesunrotatesonitsaxisaboutonceevery4wee ks, Itdoesnotrotateasasolidbody Thesunhasaneffectiveblackbodytemperature of5777K Thetemperatureinthecentralinteriorregion sisvariouslyestimatedat8 106to40 106K SUN Sun sdensityisabout100timesthatofwater. Severalfusionreactionshavebeensuggestedt osupplytheenergyradiatedbythesun. Theoneconsideredthemostimportantisaproce ssinwhichhydrogencombinestoformhelium Theenergyproducedintheinteriorofthesolar sphereis firsttransferredouttothesurface andthenradiatedintospaceTHE Solar CONSTANT sorbit Nearlyfixedintensityofsolarradiationreac hestotheoutsideoftheearth satmosphere. ThesolarconstantGsc=1367W/m2 DEFINITIONS AirMassm:Theratioofthemassofatmosphereth roughwhichbeamradiationpassestothemassit wouldpassthroughifthesunwereatthezenith( ,directlyoverhead) BeamRadiation:Solarradiationreceivedfrom thesunwithouthavingbeenscatteredbytheatm osphere.

2 (Oftenreferredtoasdirectsolarradiation) DiffuseRadiation:Solarradiationreceivedf romthesunafteritsdirectionhasbeenchanged byscatteringbytheatmosphere.(Referredtoi nsomemeteorologicalliteratureasskyradiat ionorsolarskyradiation)SOME DEFINITIONS TotalSolarRadiation:Thesumofthebeamandth ediffusesolarradiationonasurface.(Themos tcommonmeasurementsofsolarradiationareto talradiationonahorizontalsurface,oftenre ferredtoasglobalradiationonthesurface.) irradiance ,W/m2 ,withappropriatesubscriptsforbeam,diffus e,orspectralradiation. IrradiationorRadiantExposure,J/m2:Theinc identenergyperunitareaonasurface,foundby integrationofirradianceoveraspecifiedtim e, DEFINITIONS Insolation:isatermapplyingspecificallyto solarenergyirradiation. Hisusedforinsolationforaday. Iisusedforinsolationforanhour(orotherper iodifspecified). HandIcanrepresentbeam,diffuse,ortotaland canbeonsurfacesofanyorientation.

3 SubscriptsonG,H,andIareasfollows: o referstoradiationabovetheearth satmosphere,referredtoasextraterrestrial Radiation ; b and d refertobeamanddiffuseradiation; T and n refertoradiationonatiltedplaneandonaplan enormaltothedirectionofpropagation. Ifneither T nor n appears, DEFINITIONS RadiosityorRadiantExitance,W/m2:Theratea twhichradiantenergyleavesasurfaceperunit areabycombinedemission,reflection,andtra nsmission. EmissivePowerorRadiantSelf-Exitance,W/m2 :Therateatwhichradiantenergyleavesasurfa ceperunitareabyemissiononly. Anyoftheseradiationterms,exceptinsolatio n,canapplytoanyspecifiedwave-lengthrange (suchasthesolarenergyspectrum)ortomonoch romaticradiation. DEFINITIONS Anyoftheseradiationterms,exceptinsolatio n,canapplytoanyspecifiedwave-lengthrange (suchasthesolarenergyspectrum)ortomonoch romaticradiation. DEFINITIONS SolarTime:Timebasedontheapparentangularm otionofthesunacrossthesky Solarnoon:Thetimethesuncrossesthemeridia noftheobserver.

4 Solartimeisthetimeusedinallofthesun-angl erelationships; DEFINITIONS Itisnecessarytoconvertstandardtimetosola rtimebyapplyingtwocorrections 1stthereisaconstantcorrectionforthediffe renceinlongitudebetweentheobserver smeridianandthemeridianonwhichthelocalst andardtimeisbased . Thesuntakes4mintotransverse1 oflongitude. 2ndcorrectionisfromtheequationoftime,whi chtakesintoaccounttheperturbationsinthee arth srateofrotationwhichaffectthetimethesunc rossestheobserver smeridian. Tofindthelocalstandardmeridian, DEFINITIONSDIRECTION OF BEAM Radiation Thegeometricrelationshipsbetweenaplaneof anyparticularorientationrelativetotheear thatanytimeandthepositionofthesunrelativ etothatplane,canbedescribedintermsofseve ralangles orL:Latitude, :Declination, :Slope, :Surfaceazimuthangle orh:Hourangle, :Angleofincidence, z:Zenithangle, s:Solaraltitudeangle, s:SolarazimuthangleANGLES FOR TRACKING SURFACES Somesolarcollectors track thesunbymovinginprescribedwaystominimize theangleofincidenceofbeamradiationonthei rsurfacesandthusmaximizetheincidentbeamr adiation.

5 Theanglesofincidence( )andthesurfaceazimuthangles( )areneededforthesecollectors. Trackingsystemsareclassifiedbytheirmotio ns: Rotationcanbeaboutasingleaxis(whichcould haveanyorientation)butwhichinpracticeisu sually horizontaleast-west, horizontalnorth-south, vertical, orparalleltotheearth saxis orrotationcanbeabouttwoaxesANGLES FOR TRACKING SURFACES Figureshowsextraterrestrialradiationonaf ixedsurfacewithslopeequaltothelatitudean dalsoonsurfacesthattrackthesunaboutahori zontalnorth-southoreast-westaxisatalatit udeof45 atthesummerandwintersolstices Summersolstices: = Wintersolstices: = FOR TRACKING SURFACES Itisclearthattrackingcansignificantlycha ngethetimedistributionofincidentbeamradi ation. Trackingdoesnotalwaysresultinincreasedbe amradiation; FOR TRACKING SURFACES Foraplanerotatedaboutahorizontaleast-wes taxiswithasingledailyadjustmentsothatthe beamradiationisnormaltothesurfaceatnoone achday, Theslopeofthissurfacewillbefixedforeachd ayandwillbe Thesurfaceazimuthangleforadaywillbe0 or180 dependingonthelatitudeanddeclination:ANG LES FOR TRACKING SURFACES Foraplanerotatedaboutahorizontaleast-wes taxiswithcontinuousadjustmenttominimizet heangleofincidence, Theslopeofthissurfaceisgivenby Thesurfaceazimuthangleforthismodeoforien tationwillchangebetween0 and180 ifthesolarazimuthanglepassesthrough 90.

6 Foreitherhemisphere,ANGLES FOR TRACKING SURFACES Foraplanerotatedaboutahorizontalnorth-so uthaxiswithcontinuousadjustmenttominimiz etheangleofincidence, Theslopeisgivenby Thesurfaceazimuthangle willbe90 or 90 dependingonthesignofthesolarazimuthangle :ANGLES FOR TRACKING SURFACES Foraplanewithafixedsloperotatedaboutaver ticalaxis,theangleofincidence( ) ,theangleofincidenceis Theslopeisfixed,so ThesurfaceazimuthangleisANGLES FOR TRACKING SURFACES Foraplanerotatedaboutanorth-southaxispar alleltotheearth saxiswithcontinuousadjustmenttominimize , Theslopevariescontinuouslyandis Thesurfaceazimuthangleis WhereANGLES FOR TRACKING SURFACES Foraplanethatiscontinuouslytrackingabout twoaxestominimizetheangleofincidence,RAT IO OF BEAM Radiation ON TILTED SURFACE TO THATON horizontal SURFACE Forsolarprocessdesign,itisnecessarytocal culatethehourlyradiationonatiltedsurface ofacollectorfrommeasurementsorestimateso fsolarradiationonahorizontalsurface.

7 Themostcommonlyavailabledataaretotalradi ationforhoursordaysonthehorizontalsurfac e, OF BEAM Radiation ON TILTED SURFACE TO THATON horizontal SURFACE ThegeometricfactorRb,theratioofbeamradia tiononthetiltedsurfacetothatonahorizonta lsurfaceatanytime, Figureindicatestheangleofincidenceofbeam radiationonthehorizontalandtiltedsurface s. TheratioGb,T/Gbisgivenby Thesymbol G isusedinthisbooktodenoterates,while I isusedforenergyquantitiesintegratedovera nhour. andcos andcos ( ).RATIO OF BEAM Radiation ON TILTED SURFACE TO THATON horizontal SURFACE Theoptimumazimuthangleforflat-platecolle ctorsisusually0 inthenorthernhemisphere(or180 inthesouthernhemisphere). Thusitisacommonsituationthat =0 (or180 ). Inthiscase, zandcos ,respectively,leadinginthenorthernhemisp here,for =0 ,to Inthesouthernhemisphere, =180 andtheequationisRATIO OF BEAM Radiation ON TILTED SURFACE TO THATON horizontal SURFACE AspecialcaseofinterestisRb,noon,theratio forsouth-facingsurfacesatsolarnoon.

8 ,forthenorthernhemisphere, Forthesouthernhemisphere, ,SHADING Threetypesofshadingproblemsoccursofreque ntlythatmethodsareneededtocopewiththem. Thefirstisshadingofacollector,window,oro therreceiverbynearbytrees,buildings,orot herobstructions. Thesecondtypeincludesshadingofcollectors inotherthanthefirstrowofmultirowarraysby thecollectorsontheadjoiningrow. Atanypointintimeandataparticularlatitude , , ,and arefixed. Fromtheequationsgivenabove,thezenithangl e zorsolaraltitudeangle sandthesolarazimuthangle Asolarpositionplotof zand sversus sforlatitudesof 45 isshowninFigure. Theangularpositionofbuildings,wingwalls, overhangs,orotherobstructionscanbeentere donthesameplot. Forexample ifabuildingorotherobstructionofknowndime nsionsandorientationislocatedaknowndista ncefromthepointofinterest( ,thereceiver,collector,orwindow), theangularcoordinatescorrespondingtoalti tudeandazimuthanglesofpointsontheobstruc tion(theobjectazimuthangle oandobjectaltitudeangle o)canbecalculatedfromtrigonometricconsid erations Alternatively, Thesolarpositionatapointintimecanberepre sentedforapointlocation.

9 Collectorsandreceivershavefinitesize,and whatonepointonalargereceivingsurface sees maynotbethesameaswhatanotherpointsees. Theproblemisoftentodeterminetheamountofb eamradiationonareceiver. Ifshadingobstructionsarefarfromthereceiv errelativetoitssize,sothatshadowstendtom oveoverthereceiverrapidlyandthereceiveri seithershadedornotshaded, Ifareceiverispartiallyshaded,itcanbecons ideredtoconsistofanumberofsmallerareas,e achofwhichisshadedornotshaded. Orintegrationoverthereceiverareamaybeper formedtodetermineshadingeffects. Overhangsandwingwallsarearchitecturalfea turesthatareappliedtobuildingstoshadewin dowsfrombeamradiation. Thesolarpositionchartscanbeusedtodetermi newhenpointsonthereceiverareshaded. TheprojectionPisthehorizontaldistancefro mtheplaneofthewindowtotheouteredgeoftheo verhang. ThegapGistheverticaldistancefromthetopof thewindowtothehorizontalplanethatinclude stheouteredgeoftheoverhang.

10 Theangleofincidenceofbeamradiationonasha dingplanecanbecalculatedfromitssurfaceaz imuthangle anditsslope =90+ Theangle ofshadingplane1is Andthatforshadingplane2is Notethatiftheprofileangle pislessthan90 ,theoutersurfaceoftheshadingplanewill see thesunandbeamradiationwillreachthereceiv erSHADING Shadingcalculationsareneededwhenflat-pla tecollectorsarearrangedinrows. Normally,thefirstrowisunobstructed,butth esecondrowmaybepartiallyshadedbythefirst ,thethirdbythesecond,andsoon. Aslongastheprofileangleisgreaterthanthea ngleCAB,nopointonrowNwillbeshadedbyrowM. IftheprofileangleatapointintimeisCA B andislessthanCAB,theportionofrowNbelowpo intA willbeshadedfrombeamradiation.