Numericalinvestigationonthermalperformanceofgroundheatexchangersusingphasechangematerials

asgroutforgroundsource

heatpumpsystem匯報人：張恒1.Introduction2.Methods3.Resultsanddiscussion4.Conclusions1.IntroductionThereisnodoubtthattheenergyshortagehasbecomeahugeproblem,bothlocallyandglobally.Groundsourceheatpumpsystemisoneofeffectiveandefficientwaytotackletheproblem.Theverticalboreholegroundheatexchanger(GHE)iswidelyutilizedinChina.GSHPsystemsneedalargeamountoflandarea,however,Chinesepracticalsituationsisalargepopulationwithrelativelylittleland.Inordertoreducethelandutilization,phasechangematerialswereproposedtouseasbackfillmaterialinsteadofcommonmaterials.ItcandecreaseboreholenumberforthefixedenergydemandUsingPCMisaneffectivemeasuretoreducethesuddenheatingorcoolingwaveuponthegroundandsmooththethermalwavegeneratedfromtheoperationofaGSHP.2.Methods2.1.Modeldomains2.MethodsAsshowninFig.2,itwasclearlyseenthatwhenthegridnumbercontinuestoincreaseabove750,000,thedeviationofresultsbetweenthelastthreemodelswerelessthan1%.ThemodelparametersarelistedinTable1indetails.2.Methods2.2.MathematicalmodelsTheliquidfraction

β,canbedefinedasleft.Liquidfractionisanotherparametertodescribephasechangerate.2.Methods2.3.ModelvalidationAsshowninFig.3,thetemperaturevariationtrendofr1,r2,r3withtimewasinagoodagreementwiththemeasuredvalues.Therefore,itcouldbedemonstratedthatthepresentsimulationmodelwassufficientlyaccuratetoanalyzethermalperformanceofgroundheatexchangersusingphasechangematerialsasgrout2.Methods2.4.Evaluationmethodologies2.4.1.Effectiveness2.4.2.Thermaleffectsradius3.ResultsanddiscussionThosePCMbackfillmaterialwereusedindiscontinuousairconditioningsystem,whichwasrunfor10handstopfor14h.TheGSHPoperationhoursareselectedtorepresenttypicalworkingconditionsattheofficebuilding:8am–18pmfromMondaytoFriday.

Thefollowingworkisappliedtothecoolingmode,whichthefluidiswarmerthanthesoilunderthesummercondition.BothofthemarefilledwiththesameflowrateintheU-tube.3.Resultsanddiscussion3.1.EffectsofbackfillmaterialsonthermalperformanceofGSHEFourdifferentmaterialswerechosenasthebackfillmaterialforGHEs.Soilisthecommonbackfillmaterial,andanotherthreematerialsarePCMs,suchasParaffinRT27,Acid

andEnhancedacid.3.ResultsanddiscussionTheheattransferratedropswiththeincreaseoftime.Itisunderstandablebecausethermalfluidinthetubereleasesheattothesoil.Thenthetemperatureofsoilincreasedwiththetime,whilethetemperaturedifferencebetweenthermalfluidandgroutdecreased

withtime.Theeffectivenessdroppedmonotonouslywithtimeinallcasedconsidered.ComparedtheeffectivenessofacidtothatofRT27,thehigherphasechangetemperaturebroughtthehighereffectiveness.3.ResultsanddiscussionLiquidfractionisanotherparametertodescribephasechangerate.OnecanseefromFig.4(c)thattheliquidfractiongrewmonotonouslywithtimeinallcasesconsideredIfcomparedtheliquidfractionofacidwiththatofRT27,thehigher

phasechangetemperaturebroughtthelower

liquidfraction.Moreover,comparedtheliquidfractionofacidwiththatofenhancedacid,thehigherthermalconductiveresultedinthehigherliquidfraction3.Resultsanddiscussion3.ResultsanddiscussionThethermaleffectsradiusofgroundsourceexchangersat1.5mdeepafterrunning10hwaspresentedinTable3.Inspiteoflowthermalconductivities,therearemanyadvantagesofPCM.Obviously,thePCMtemperaturekeepsconsistentinsolidificationandmeltingprocess,whichisbeneficialforstableoperationoftheGSHPsystem.Moreover,temperaturechangeofsurroundingsoilcanbereducedwithPCMsasbackfill.Thus,thespacingofboreholecanbelessened,andlanddemandcanbereducedeffectivelyaswell.Therefore,PCM,tosomeextent,wassuitableforuseasbackfillmaterialintheGSHPsystem.However,heattransferenhancementsmustbeadopted.(λ:0.2-0.5)3.Resultsanddiscussion3.2.EffectsofinitialgroundtemperatureonthermalperformanceofGSHEFig.6illustratestheeffectivenessofPCMsystemsandheattransferrateunderdifferentinitialgroundtemperatureconditions.Moreover,theeffectivenessofPCMsystemsincreasedasinitialgroundtemperaturedecreased,whichisconsistentwiththeheattransferrateinthisprocess.3.ResultsanddiscussionItmeansthattheproportionofliquidincreasedastheinitialtemperatureclosedtothephasechangetemperature.Specifically,theLiquidfractionincrementinthethreeinitialgroundtemperatureincreased

significantlyasinitialgroundtemperaturegrew,andthisincrementgrewovertime.3.ResultsanddiscussionTable4presentsthatthethermaleffectsradiusofgroundsourceexchangersat1.5

mdeepafterrunning10hunderthreeinitialgroundtemperatureconditions.Admittedly,lowerinitialgroundtemperaturecontributedtolargerheattransferrateandhighereffectiveness.Incontrast,theliquidfractionofPCMandthethermaleffectsradiussawanoppositetrend.Thatistosay,theheattransferrateandthethermaleffectsradiusvarytotheoppositedirectionwiththeinitialgroundtemperature.(undetermined)3.Resultsanddiscussion3.3.EffectsofpipespacingonthermalperformanceofGSHEAsFig.8(a)and(b)show,theheattransferrateandtheeffectivenesshavethesimilartrendsduringthese10h.Fig.9(a)and(b)presentthattheheattransferrateandtheeffectivenessincreasedaspipespacingrose.Especially,heattransferrateofthe60mmpipespacingcasedroppedquickerthanthatoftheothertwocases,whichmeansthattheheattransferconditiondeterioratedseriouslyunderthe60mmpipespacingcondition.3.ResultsanddiscussionFromFig.8(c),liquidfractionofPCMwith80mmpipespacingandthatwith60mmpipespacinghavethesimilartrendduringthe10h.However,liquidfractionofPCMwith100mmpipespacingwaslowerthanthosetwo.Thethermaleffectsradiusofgroundsourceexchangersat1.5mdeepafterrunning10hwerepresentedinTable5.Table5showsthatthethermaleffectsradiuswith100mmpipespacingislargestamongthethreecases.3.ResultsanddiscussionFromFig.9(a),thetemperatureofr0isconstant

inthefirstthreehours,andexperiencedadramaticgrowthinthenextsixhours.After10h,itdecreasedduetothestopofthesystem.Itisunderstandablethatthelatentheatplayedadominaterolewhentheacidwasmeltinginthefirstthreehours,andthenthesensibleheatplayedadominaterolewhenthematerialhadmelted.ThisphenomenoncanbeprovedbyFig.9(d)with60mmpipespacingU-tube.Therewasasharpincreaseintemperatureofr=0with80mmpipespacingU-tubeafter8h,asFig,9(b)shows.Itdelayed5hthanthatwith60mmpipespacingUtube,whichisbecausethemeltingtimeofthematerialbetweentwopipesislonger(Fig.9(d)).Incontrast,thetemperatureofr=0with100mmpipespacingU-tubekeptconstant

afterreachingthemelttemperature(seeFig.9(c)).Thematerialbetweenthetwopipesdidnotfullymelt,andtherewassomesolidacidbetweenthem,asshowninFig.9(d)100mm.Consequently,larger

pipespacingledtolargerheattransferrateandhighereffectiveness.Meanwhile,liquidfractionofPCMwith100

mmpipespacingU-tubewaslowerthantheothertwocases.In

contrast,thethermaleffectsradiusislargestamongthethreecases.Thatistosay,theheattransferrateandthethermaleffectsradiuschangeintheoppositedirection3.Resultsanddiscussion3.4.Combinedeffectsofpipespacingandinitialgroundtemperature4.ConclusionsInthepresentstudy,theprocessofmeltingofaPCMgroutinGSHPhasbeenexplorednumerically.TransientnumericalsimulationswereperformedusingANSYSFLUENT15.0software.Tobeginwith,thethermalperformancesoffourbackfillmaterialswereinvestigated.Furthermore,theinitialgroundtemperaturefactorandpipespacingfactorwerestudied,respectively.Finally,thecombinedeffectsofpipespacingfactorandinitialgroundtemperaturefactoronthethermalperformanceofGSHPwereanalyzed.4.ConclusionsThemainconclusionsaresummarizedasfollows:(1)Duetoitssmallthermaleffec