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Cooling load differences between radiant and air systems.pdf

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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/authorsrights  Author's personal copy EnergyandBuildings65(2013)310–321 ContentslistsavailableatSciVerseScienceDirect Energy   and   Buildings  journalhome   page:www.elsevier.com/locate/enbuild Cooling   load   differences   between   radiant   and   air   systems  Jingjuan   (Dove)   Feng ∗ ,   Stefano   Schiavon,   Fred   Bauman CenterfortheBuiltEnvironment,UniversityofCalifornia,Berkeley,390WursterHall,Berkeley,CA94720,USA a   r   t   i   c   le   i   n   f   o  Articlehistory: Received11April2013Accepted10June2013 Keywords: RadiantcoolingCoolingloadHeatgainAirsystemRadiantcoolingpanel(RCP)Embeddedsurfacecoolingsystems(ESCS)Thermallyactivatedbuildingsystems(TABS) a   b   s   t   r   a   c   t Unlike   the   case   of    air   systems   where   the   cooling   load   is   purely   convective,   the   cooling   load   for   radiantsystems   consists   of    both   convective   and   radiant   components.   The   objectives   of    this   simulation   studywere   to   investigate   whether   the   same   design   cooling   load   calculation   methods   can   be   used   for   radiantand   air   systems   by   studying   the   magnitude   of    the   cooling   load   differences   between   the   two   systems   overa   range   of    design   configurations.   Simulation   results   show   that   (1)zone   level   24-h   total   cooling   energyof    radiant   systems   can   be   5–15%   higher   than   air   systems   due   to   higher   conduction   load   through   theenvelope;   (2)peak   cooling   rate   at   the   radiant   cooled   surface   can   be   7–35%   higher   than   air   system   forzones   without   solar   load.   Thisdifference   can   increase   up   to   85%   for   floor   system   in   zones   with   solar   load;(3)   the   peak   cooling   rate   differences   srcinate   from:   (a)   radiant   cooling   surface(s)   reduce   radiant   heatgainaccumulation   in   the   building   mass;   (b)   onlypart   of    the   convective   heat   gain   becomes   instantaneouscooling   load.   As   aresult,   toolsusing   response   factor   methods   such   as   radiant   time   series   method   forcooling   loadcalculations   are   not   appropriate   for   radiant   system   design.©2013   Elsevier   B.V.   All   rights   reserved. 1.Introduction Water-basedradiantcoolingsystemsaregainingpopularityasanenergyefficientapproachforconditioningbuildings[1–3].Thedesignofradiantsystemsiscomplicatedbecauseofthecou-plingbetweenthermalload,buildingstructureandthehydronicsystemandbecauseoftheimportantimpactofbothradiationandconvectiononthermalcomfort.Dedicatedradiantsystemdesignandtestingstandardshavebeendevelopedtoaddressissueslikesystemsizing,installation,operationandcontrol[4–9].How-ever,radiantcoolingsystemsarestillconsideredasaninnovativeapproach,andtheirapplicationinNorthAmericaisstilllimited[10,11].Inthisstudy,weinvestigatedtheimpactsofthepresenceofactivatedcooledsurfaceonzonecoolingloads.CoolingloadcalculationsareacrucialstepindesigninganyHVACsystem.Comparedtoairsystems,thepresenceofanactivelycooledsurfacechangestheheattransferdynamicsintheroom,andtwopotentialimpactsonzonecoolingloadsstudiedhereare(1)cooledsurfacesmay   createdifferentinsidesurfacetempera-turesofthenon-activeexteriorbuildingwalls,causingdifferentheatgainthroughthebuildingenvelope,andinturndifferentzoneleveltotalenergy,and(2)changestheeffectofthermalmassoncoolingloads,andthereforecreatingdifferentpeakcoolingload. ∗ Correspondingauthorat:CenterfortheBuiltEnvironment,Universityof California,Berkeley,373CWursterHall,Berkeley,CA94720,USA.Tel.:+15103663139. E-mailaddresses:  jjfeng@berkeley.edu,dovefeng@gmail.com(J.Feng). Tworesearchstudieswereidentifiedthatlookedatheatingloadcalculationsintermsoftheimpactoftheradiantsystemonwallsurfacetemperaturesandtheresultantroomload[12,13].However,bothstudiesfocusedonheatingloadcalculationundersteady-stateconditions.Inanotherstudy,Chen[14]suggestedthatthetotalheatingloadofaceilingradiantheatingsystemwas17%higherthanthatoftheairheatingsystembecauseoftheroleofther-mal   massandhigherheatlossthroughthebuildingenvelopeduetoslightlyhigherinsidesurfacetemperatures.Forcoolingapplica-tions,nostudieswerefoundonthistopic,andincurrentradiantsystemdesignguidelines[4,8],suchimpactsarenotconsideredorevaluated.Secondly,theinteractionofbuildingmasswithheatsourceisinfluencedbythepresenceofactivatedradiantcoolingsurface(s).Onephenomenonmentionedintheliteraturewasradiantsur-face(s)aspartofthebuildingmass,insteadofstoringthemalenergyasinthecaseofairsystems,removesradiantheatgain(e.g.solar,radiativeinternalloadandradiativeenvelopeload)thatisdirectlyimpingingonit.Thisphonomenonfundamentallychangesthecool-ingloaddynamicsinaroom.Niu[15]pointedoutthatthisdirectradiationmay   createhighpeakcoolingloads.Hemodifiedthether-mal   analysisprogramACCURACY [16]toaccountforthedirectradiantheatgainasinstantaneouscoolingloadforradiantsystems.However,noinformationcanbefoundonhowheimplementedthemodificationandthesoftwareisnotaccessibleforthepublic.Inanefforttodevelopanewcoolingloadcalculationapproachforradi-antsystems,Corgnati[17]alsotackledthedirectradiantheatgaineffectusingasimilarstrategytoNiu.BasedonCorgnati’swork,Causoneetal.[18]focusedonthecaseswiththepresenceofdirect 0378-7788/$–seefrontmatter©2013ElsevierB.V.Allrightsreserved.http://dx.doi.org/10.1016/j.enbuild.2013.06.009  Author's personal copy  J.Fengetal./EnergyandBuildings65(2013)310–321 311 Nomenclature RCPradiantcoolingpanelsESCSembeddedsurfacecoolingsystems(lightweight)TABSthermallyactivatedbuildingsystemsG1–G6simulationgroupindex q  heatflux(W/m 2 ) q  surf   heatfluxattheexposedfaceofthecoolingsurface(s)(W/m 2 ) q  surf,cond  conductionheattransferattheexposedfaceofthecoolingsurface(s)(W/m 2 ) q  surf,con v  convectionheattransferattheexposedfaceofthecoolingsurface(s)(W/m 2 ) q  surf,rad  radiationheattransferattheexposedfaceofthecoolingsurface(s)(W/m 2 ) q  lwsurf   netlongwaveradiationfluxtoradiantactivesurfacefromothersurfaces(W/m 2 ) q  lw int  longwaveradiantexchangefluxfrominternalload(W/m 2 ) q  swsol  transmittedsolarradiationfluxabsorbedatsurface(W/m 2 ) q  sw int  netshortwaveradiationfluxtosurfacefrominternalload(lights)(W/m 2 ) q  surf,pk  specificpeakradiantsystemsurfacecoolingload(W/m 2 ) q  hyd,pk  specificpeakradiantsystemhydroniccoolingload(W/m 2 ) q  air,pk  specificpeaksensiblecoolingloadforairsystem(W/m 2 )˙ q surf,tot   specific24-htotalsurfacecoolingenergy(kJ/m 2 )˙ q hyd,tot   specific24-htotalhydroniccoolingenergy(kJ/m 2 )˙ q air,tot   specific24-htotalsensiblecoolingenergy(kJ/m 2 ) P  surf,pk  percentagedifferenceofsurfacepeakcoolingratebetweenradiantandairsystem(%) P  hyd,pk  percentagedifferenceofhydronicpeakcoolingratebetweenradiantandairsystem(%) P  surf,tot   percentagedifferenceofsurfacelevel24-htotalcoolingenergybetweenradiantandairsystem(%) P  hyd,tot   percentagedifferenceofhydroniclevel24-htotalcoolingbetweenradiantandairsystem(%) Subscript surf  variablemeasuredatradiantsurfacelevel hyd variablemeasuredatradiantcoolingwaterloop  pk peakcoolingload tot  24htotalcoolingenergysolargain.However,themethodsproposedintheseresearchstud-iesonlylookedattheeffectofdirectradiantheatgainoncoolingload,andtherestoftheradiantheatgainandtheconvectiveheatgainarestillconsideredtointeractwithbuildingmassasiftheradi-antsystemdoesnotexist.Inaddition,noresearchcanbefoundthatfundamentallystudiesthedifferencesoftheheattransferprocessinzonesconditionedbyanairandaradiantsystem,andhowthesedifferencesaregoingtoimpactthecoolingloadcalculationandwhatcouldbethemagnitudeofthedifferences.Althoughresearchhasdemonstratedthatcoolingloadsforradi-antsystemsneedtobeconsidereddifferentlythanforairsystems,currentradiantdesignstandardsdonotexplicitlyacknowledgethesedifferences.Severalstandardsandhandbookswerereviewed,including:Chapter6of   ASHRAEEquipmentandHVACsystems [19],radiantheatingandcoolinghandbook(2002)[9],Chapter18of   ASHRAEFundamental (2012),ISO11855(2012)[4],andEuropeanstandardEN15377(2008)[8].Thefirstthreedonotofferanyguid-anceontheselectionofthecalculationmethodswhenradiantsystemsareinvolved.Inchapter18of   ASHRAEFundamental (2012)handbook,thedescriptionofthecoolingloadcalculationprocessisbasedontheimplicitassumptionthatanairsystemisusedforconditioningthespace.Somesimplifiedcoolingloadcalculationmethods,suchastransferfunctionmethod(TF)[20]andradianttimeseriesmethod(RTS)[21],havealsobeendevelopedforairsystem.Thesealgorithmsarewidelyimplementedinbuildingther-mal   simulationorloadcalculationtools,includingHAP(TF),TRANETRACE(RTS),BLAST,andDOE-2(TF)basedtoolssuchaseQuest,Energy-pro,GreenBuildingStudioandVisualDOE.Thesetoolsareoftenusedforcoolingloadestimatesduringinitialdesignstageandfordetailedenergyandcomfortanalysisevenwhenradiantsys-temsareinvolved[22].TheEuropeanstandardsreviewedindireclyreferenceEN15255[23]forcoolingloadcalculationprocedure.EN15255classifiedallcoolingloadcalculationmethodsintodif-ferentcatogoriesaccordingtotheircapabilitytomodeldifferenttypesofcoolingsystemandcontrolmethod.MethodsthatareabletosimulateradiantsystemscontrolledbyoperativetemperatureareinClass4b.Thisimpliesthatcoolingloadcalculationmethodforradiantsystemsshouldbeproperlydistinguishedfromairsys-tems.However,thisstandarddoesnotexplicitlyprovidecoolingloadcalculationmehtodsforradiantsystem.Arecentsurveyconductedbytheauthorsofradiantcool-ingdesignpractitionersrevealedthatthedifferencesincoolingloadbetweenradiantandairsystemsarenotfullyunderstood.Someofthemostexperiencedprofessionalsacknowledgethecomplicationsandlackofguidanceinthestandardsanddevel-opedrule-of-thumbmethodsforinitialsystemdesigncalculation.Amongthosemethods,eitherheatgainisdirectlyusedascool-ingloadforsystemsizing[24,25],oraportionoftheheatgainisconsideredasdirectheatremovalbytheactiveradiantsurface.Thepercentagesofthedirectremovaldependonloadtype(light-ing/people/equipment),andareobtainedbasedonexperience[26].Indesignpractice,itisnotoftenthatdynamicsimulationtoolsthatcanproperlymodelradiationheattransferareusedatthecoolingloadestimationstage.Radiantsystemmanufacturershavedevel-opedsometoolsforsystemsizing[27],buttheyaremainlyusedforheatingapplications,wheresteady-stateheattransferisadequatetocapturethethermalbehavior.Theobjectivesofthissimulationstudyareto(1)assessthecool-ingloaddifferencesbetweenthetwo   systemsbycomparingthezonelevelpeakzonecoolingloadand24-htotalcoolingenergyforaradiantcoolingsystem(withactivatedchilledsurface)vs.anairsystem;and(2)suggestpotentialimprovementsincurrentdesignguidelinesforradiantcoolingsystem. 2.Backgroundandtheory  Inthissection,we   giveabriefintroductiontothethreetypesofradiantcoolingsystemsinvestigatedinthispaperandexplainhowtheirthermalcharacteristicsaffectthedesignapproach.Sinceradiantandairsystemsaredifferentinmanyways,thesimulationstudyhadtobedesignedcarefullytoprovideafaircomparison.  2.1.Radiantcoolingsystems TheREHVAguidebookonradiantsystems[7]hasroughlycat-egorizedthesesystemsintothreetypes:radiantcoolingpanels(RCP),water-basedembeddedsurfacecoolingsystems(ESCS),andthermallyactivatedbuildingsystems(TABS).AsshowninFig.1,RCParemetalpanelswithintegratedpipesusuallysuspendedunderthe  Author's personal copy 312  J.Fengetal./EnergyandBuildings65(2013)310–321 Fig.1. Schematicofthethreetypesofradiantsurfaceceilingsystems(nottoscale). ceilingwithheatcarriertemperaturerelativelyclosetoroomtem-perature.ESCShavepipesembeddedinplasterorgypsumboardorcementscreed,andtheyarethermallydecoupledfromthemainbuildingstructure(floor,wallandceiling)bytheuseofthermalinsulation.Theyareusedinalltypesofbuildingsandworkwithheatcarriersatrelativelyhightemperaturesforcooling.Finally,“systemswithpipesembeddedinthebuildingstructure(slab,walls),TABS,whichareoperatedatheatcarriertemperaturesveryclosetoroomtemperatureandtakeadvantageofthethermalstor-agecapacityofthebuildingstructure.”Thesesystemsusuallyhavedifferentapplicationsduetotheirthermalandcontrolcharacteris-tics,andtherefore,thedesignanddimensioningstrategiesforthesesystemsvary.  2.2.Radiantvs.airsystems Acomparisonbetweenradiantandairsystemsischallenging.Inthissection,wediscussthedifferencesbetweenthetwosystemsthatdictatethemodelingapproachusedinthisstudy.Besidesthosementionedintheliterature[28],themaindifficultiesinclude: ã  Typesofload(sensible/latent)andtheexpectedamountofloadtobehandledbythetwosystemsaredifferent.Airsystemsareusuallydesignedtobetheonlysystemtohandlebothlatentandsensibleloads,whileradiantsystemsmustoperateinhybridmodewithareduced-sizedairsystem(forventilationandlatentloads).Radiantcoolingsystemsarealwayssizedtohandleaportion(asmuchaspossible)ofthesensible-onlycoolingload.Toaddressthisissue,neitherthelatentloadnorventilationsystemwassimulated.Thiswastosimplifyouranalysis. ã  Thedesigncoolingloadconceptisdifferentforthetwo   systems.AccordingtoASHRAEHandbook[29],thesensiblecoolingloadforanairsystemiscalculatedintermsofmaintainingaconstantzoneairtemperature,whileradiantsystems,particularlyTABS,arenotcapableofmaintainingaconstantzoneairtemperatureduetolargethermalinertiaoftheactivesurfaces.Forthisreason,inthiscomparisonstudy,wesizedandcontrolledthesimulatedradiantsystemstomaintainanacceptablethermalcomfortrangeduringthesimulationperiod.Operativetemperaturewasusedasthecontroltemperatureforbothsystems[28,30].Toensureequivalentcomfortconditionsbetweenthetwosystemsforfaircomparison,allsimulationsoftheairsystemweresubsequentlycontrolledtocloselytrackthehourlyoperativetemperaturepro-filederivedfromtheradiantsystemsimulationfortheidenticalinputconditions. ã  Foranairsystemthezonecoolingloadisequaltotheheatextractionratebythemechanicalsystemwhentheroomairtem-peratureandhumidityareconstant.Butthisisnotalwaysthecaseinaradiantsystem.Otherthanpanelsystems,radiantcool-ingsystems(ESCSandTABS)areintegratedwiththebuildingstructurewithhydronicpipesembeddedinthemass.Asaresult,heatremovedfromthezoneatthechilledsurfacecanbequitedifferentfromtheheatremovedbythehydronicloop.Sizingof theradiantsystemcoolingequipmentishighlydependentonspecificationsofthecoolingsurface(slabmaterial/thickness,tubespacing,andsurfacefinishing).Thisindicatedthatweneededtoinvestigateheattransferoftheradiantsystematboththesurfaceandhydroniclevels,whichisdiscussedindetailbelow.  2.3.Heattransferatradiantsurfaceandhydroniclevel Radiantsystemsremovethesensibleheatinaroomatthecool-ingsurface.We   definethiscoolingrateassurfacecoolingrate.Definethecontrolvolumeastheinsidefaceofthecoolingslab,withpositivesignmeansheatbeingtransferredintothecontrolvolumeandnegativeindicatesheatleavingthecontrolvolume,theheatbalanceforthecoolingsurfacecanbewrittenasfollows(1)[31]: q  surf   = q  surf,con v   + q  surf,rad  =− q  surf,cond  (1)Surfacecoolingrateservesasonekeydesignparameterfordeterminingrequiredradiantsystemareaandselectionofsystemtype.Hydroniccoolingrateistheheatextractionratebasedonanenergybalanceonthehydroniccircuit.Thehydroniccoolingrateisimportantforsizingofwatersideequipment,suchaspumps,chillersandcoolingtower.HydroniccoolingratecanbecalculatedbyEq.(2)[31]: q  hyd  = ( ˙ mc   p ) water  ( T  wi  − T  wo )(2)BothRCPandmostESCSoperateduringoccupiedhourstomaintainarelativelyconstantcomfortconditioninthespace,sothediffer-encebetweenthesurfaceandhydronicrateisonlyafunctionof thermalpropertiesofthepanel/slab.ForRCPsystems,ifinsulationisinstalledonthebacksideofthepanel,hydroniccoolingratecanbeassumedtobethesameassurfacecoolingoutputduetohighcon-ductivityofthesurfacematerial[6],whichisusuallydesired.TABSareusuallydesignedandoperatedtotakeadvantageofthethermalstorageeffectoftheslab,sothedifferencebetweenthesurfaceandhydronicrateisalsoafunctionoftheoperationalstrategies,whichwillbediscussedlater. 3.Methodologyandmodelingapproach Toinvestigatetheimpactsofthepresenceofactivatedcooledsurfaceonzonecoolingload,weadoptedthefollowingmethodol-ogy: ã  Two   singlezonemodels,oneconditionedbyanairsystemandonebyradiantsystemweredevelopedinEnergyPlusv7.1forcom-parison.Allthreeradiantsystems(RCP/ESCS/TABS)werestudied.Becausetheconstructionofeachradiantsystemtypeisdifferentandishighlyinfluentialonoverallbuildingresponse,thecom-parisonairmodelswereconfiguredtomatchtheconstructionof theradiantsystems. ã  Themodelswereparameterizedforstudyingtheinfluencesof envelopethermalinsulation,thermalmass,typeofinternalgain,solarheatgainwithdifferentshadingoptions,andradiantsurfaceorientation(ceiling,floor).EnergyPlusv7.1wasusedforthesimulationstudybecauseitperformsafundamentalheatbalanceonallsurfacesinthezone.Theheatbalancemodelensuresthatallenergyflowsineachzonearebalancedandinvolvethesolutionofasetofenergybalance
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