Experimental analysis of thermal performance of fl

ExperimentalanalysisofthermalperformanceofflatplateandevacuatedtubesolarcollectorsinstationarystandardanddailyconditionsE.Zambolin,D.DelCol*DipartimentodiFisicaTecnica,Universita`degliStudidiPadova,ViaVenezia1,35131Padova,ItalyReceived3June2009;receivedinrevisedform20April2010;accepted26April2010Availableonline11June2010Communicatedby:AssociateEditorBrianNortonAbstractNewcomparativetestsontwodifferenttypesofsolarcollectorsarepresentedinthispaper.Astandardglazedflatplatecollectorandanevacuatedtubecollectorareinstalledinparallelandtestedatthesameworkingconditions;theevacuatedcollectorisadirectflowthroughtypewithexternalcompoundparabolicconcentrator(CPC)reflectors.Efficiencyinsteady-stateandquasi-dynamicconditionsismeasuredfollowingthestandardEN12975-2anditiscomparedwiththeinput/outputcurvesmeasuredforthewholeday.Thefirstpurposeofthepresentworkisthecomparisonofresultsinsteady-stateandquasi-dynamictestmethodsbothforflatplateandevacuatedtubecollectors.Besidethis,theobjectiveistocharacterizeandtocomparethedailyenergyperformanceofthesetwotypesofcollectors.Aneffectivemeanfordescribingandanalyzingthedailyperformanceisthesocalledinput/outputdiagram,inwhichthecollectedsolarenergyisplottedagainstthedailyincidentsolarradiation.Testrunshavebeenperformedinseveralconditionstorepro-ducedifferentconventionaluses(hotwater,spaceheating,solarcooling).Resultsarealsopresentedintermsofdailyefficiencyversusdailyaveragereducedtemperaturedifference:thisallowstorepresentthecomparativecharacteristicsofthetwocollectorswhenoperatingundervariableconditions,especiallywithwiderangeofincidenceangles.2010ElsevierLtd.Allrightsreserved.Keywords:Flatplatecollector;Evacuatedtubecollector;Efficiency;Quasi-dynamicmethod;Dailyefficiency1.IntroductionMainlytwotypesofliquidsolarcollectorsfordomesticheatingandhotwaterproductionareusedpresently:flatplatecollectorsandevacuatedtubecollectors.Theyarecharacterizedbydifferentcostandperformance,soitisveryimportanttochoosetherightcollectorforeachappli-cationinordertooptimizethebehaviourofthewholesys-tem,theenergysavingsandthefinancepayback.Glazedflatplatecollectorsusuallypresentametalabsorberinaflatrectangularhousing.Theglasscoverontheuppersurfaceandtheinsulationontheothersidelimitthethermallosses.Thesolarenergyabsorbedbytheplateistransferredtotheliquidflowingwithinthecollectortubes.Thetubesareingoodthermalcontactwiththeabsorbersurface.Airispresentinthespacebetweentheplateabsorberandthetransparentcover.Incomparison,evacuatedtubecollectorsallowtoreducetheconvectionandtheconductionthermallosses.Thiscollectorconsistsofglassvacuum-sealedtubes;theabsorbersurfaceislocatedintotheinnerglasstubeanditcanhaveseveralshapes.0038-092X/$-seefrontmatter2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.solener.2010.04.020*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](D.DelCol).(2010)1382–1396Theevacuatedtubecollectorsmaybesubdividedintwotypes.Inthedirectflowthroughcollectortheheattransferliquidispumpedinthetubes.Thesecondtypeconsistsofheatpipesinsidevacuumsealedglasstubes.Areflectorcanbepresenttooptimizetheabsorptionofthesolarradiation.Thechoiceoftheoptimalcollectordependsonthetem-peraturelevelrequiredbythespecificapplicationandontheclimaticconditionsofthesiteofinstallation.Therefore,intermsofefficiency,eachcollectordisplaysfeatureswhichmakeitmostsuitabletoacertainapplication.Inconventionalusessolarcollectorscanprovideenergyfordomestichotwaterorspaceheatingincombinationwithlowwatertemperaturesystems(approximately35–50C),whereasthisheathastobeprovidedaboveamin-imumtemperatureof75–80Cinabsorptioncoolingmachines(Schmidetal.,1984).Inareaswithhighsunshine,solarcollectorscouldalsobeusedincookingprocess(Husseinetal.,2008)orinstillplants(Badranetal.,2005).Theknowledgeofthethermalperformanceofasolarcollectorisessentialtomaketherightchoice.WiththepublicationsoftheEuropeanStandardEN12975-2auniquestandardexiststhroughoutEuropeforsolarther-malcollectortesting.Thisstandardspecifiesareproducibleprocedureandguaranteesthuscomparableresults.Itincludestwoalternativetestmethodsforthethermalper-formancecharacterizationofsolarcollectors:steady-stateandquasi-dynamictests.SomestudiesontheperformanceofflatplatecollectorsfollowingEN12975(Kratzenbergetal.,2005;Fisheretal.,2004)andthecomparisonofuncertaintycalcula-tionmethodsofthisperformance(Kratzenbergetal.,2006)canbefoundintheliterature.ComparativestudiesbetweendifferentnormalizedtestmethodsforflatplatecollectorsarealsoreportedinRojasetal.(2008)andCucumoetal.(2008).Instead,limitedresultsareavailableontransienttestmethodsappliedtoevacuatedtubecol-lectors.In(Ro¨nnelidetal.,1997)datafromoutdoortest-inghasbeenusedforcharacterizingthebehaviourofaCPCcollectorwithincidenceangle;inPerers(1997)theextendedMLRprocedureisappliedtoquasi-dynamicmethodforcharacterizationofevacuatedtubecollectorsNomenclatureAaapertureareaofcollector(m2)aheatlosscoefficientforlinearregression,Eq.(5)(W/(m2K))a1heatlosscoefficient,Eq.(4)(W/(m2