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地源热泵外文翻译英
EnergyEfficiencyTechnologies–AirSourceHeatPumpvs.GroundSourceHeatPump
Abstract.Heatpumpdeservesthenameofeco-innovation.Ituses‘freeenergy’-warmthcollectedinthesoilorintheairtoprovideheatingandcooling.Therearetwomaintypesofheatpumps–airsourceheatpumpandgroundsourceheatpump.Howdotheywork?
Whatarethebenefitsofeachsystem?
Howdotheycompare?
Firstofall,questionslike“whatisheatpump,howdoesitwork,whatareASHPsandGSHPs”willbeanswered.ThenadetailedcomparisonbetweenASHPsandGSHPswillbecarriedout,fromtechnologicalparameterstosocial,practical,economicalparameters.Finally,theconclusionsaredrawnfromtheseparametersastodecidewhichkindofheatpumpisbetteroffunderdifferentconditions.
Keywords:
Heatpump,Efficiency,AirSourceHeatPump,GroundSourceHeatPump
Nomenclature
Qe:
condensationheatrate,kJs-1
Qc:
evaporationheatrate,kJs-1
W:
compressorinputenergy,kW
COP:
coefficientofperformance
˙m:
massflowrate,kgs-1
h:
enthalpy,kJkg-1
˙Q:
therateofheattransfer,kJs-1
Uc/eAc/e:
overallheattransfercoefficientofcondenser/evaporator,kWoC-1
K:
constant,equalsUeAe/UcAc.
Tc:
temperatureincoldregion(heatsource),oC
Th:
temperatureofwarmregion(heatsink),oC
Tre:
temperatureofrefrigerantinevaporator,oC
Trc:
temperatureofrefrigerantincondenser,oC
Teout:
temperatureofoutletfluidinevaporator,oC
Tcout:
temperatureofoutletfluidincondenser,oC
1.Introduction
Thephysicallawtellsusthatheatnormallyflowsfromawarmermediumtoacolderone.Butcanwemoveheatfromourcoolerhouseanddumpittoahigheroutsideenvironmentinsummer?
Andcanweextractheatfromalowertemperatureoutside,toourwarmerroomsinwinter?
Theanswerisyesifweuseaheatpump.Theheatpumpdoessobyessentially“pumping”heatupthetemperaturescale,transferringitfromacoldmaterialtoawarmeronebyaddingenergy,usuallyintheformofelectricity.Themostcommontypeofheatpumpistheair-sourceheatpump,which
transfersheatbetweenindoorandtheoutsideair.GroundSourceheatpumps(GSHPs)havebeeninusesincelate1940s,theyusetheconstanttemperatureoftheearthastheexchangemediuminsteadoftheoutsideairtemperature.
2.Background
2.1HeatPumps
Heatpumps(vaporcompressionheatpumps)transferheatbycirculatingaphasechangingsubstancecalledarefrigerantthroughacycleofevaporationandcondensation(Figure1).
Acompressorpumpstherefrigerantbetweentwoheatexchangercoils.Inonecoil,therefrigerantisevaporatedatlowpressureandabsorbsheatfromheatsource,therefrigerantisthencompressedenroutetotheothercoil,whereitcondensesathighpressure,atthispoint,itreleasestheheatitabsorbedearlierinthecycletotheheatsink(NRCOEE,2004,p.4).
Figure1.BasicVaporCompressionCycle
Source:
NRCOEE(NaturalResources’CanadaOfficeofEnergyEfficiency),2004,p.4
2.2EnergyBalanceandEfficiencyofHeatPumps
Fromthelawsofthermodynamic,theEnergyBalanceofaheatpumpsystemis:
Qc=Qe+W①
WhereQeistheheatabsorptionratebytheevaporatorandQcistheheatgivenoffatthecondenser(Figure1).
Theoperatingtemperaturesofthevapor-compressionrefrigerationcycleareestablishedbythetemperatureTctobemaintainedinthecoldregionandthetemperatureThofthewarmregiontowhichheatisdischarged;therefrigeranttemperatureintheevaporatormustbelessthanTcandtherefrigeranttemperatureinthecondensermustbegreaterthanThtoallowheattransferring(Figure2).
Figure2a.Vapor-compressionrefrigerationcycle:
T-Sdiagram
Source:
Refrig.LabofQueen’sUniversity,2007,p.9
Figure2b.Vapor-compressionrefrigerationcycle:
ln(p)-hdiagram
Source:
Rademacher,2005,p.70
Asfluidspassthroughevaporatorandcondenser,whicharetwoheatexchangersthatexchangeheatbetweenrefrigerant
andsurroundingfluids.Therateofheattransferfromrefrigeranttothesurroundingfluidincondenserandtherateof
heattransferfromsurroundingfluidtotherefrigerantinevaporatorcanbeexpressedby˙Q:
Q=m*Δh=UAΘLMT②
Here˙misthemassflowrateofrefrigerant,Δhistheenthalpychangeofrefrigerant,Uistermedtheoverallheattransfercoefficient,AistheareaofthesurfaceseparatingthefluidsthroughwhichtheheattransferoccursandΘLMTisthemeandifferencesbetweenthetemperaturesofthetwofluids.Theclassicparameterthathasbeenusedtodescribetheperformanceofaheatpumpisthecoefficientofperformance(COP),whichistheratioofthequantityofheattransferredtotheheatsink(usefulenergyoutput)tothequantityofworkdrivingthecompressor(totalenergyinput)(Reynolds1977,pp.287-289).
COP=Qc/W=Qc/(Qc–Qe)=1/(1-Qe/Qc)③
3.AirSourceHeatPumps
AirSourceHeatPumpsarethemostwidelyusedheatpumpnowadays,ambientairisfreeandwidelyavailable,anditisthemostcommonheatsourceforheatpumps.
Inanair-to-airheatpumpsystem,heatisremovedfromindoorairandrejectedtotheoutsideofabuildingduringthecoolingcycle,whilethereversehappensduringtheheatingcycle(Figure3).
Figure3.ASHP-Splitsystem
Source:
EERE(EnergyEfficiencyandRenewableEnergy),2008a
Forapplication,ASHPistypicallyrooftopunitseithercompletelypackagedorsplitpackagedsystems.Splitpackageheatpumpsaredesignedwithanairhandlingunitlocatedinsidetheconditionedspacewhilethecondenserandcompressorarepackagedforoutdoorinstallationontheroof.Packagedsystemsusuallyhavebothcoilsandthefanoutdoors.Heatedorcooledairisdeliveredtotheinteriorfromductworkthatprotrudesthroughawallorroof(EERE,2008).
Inreality,thecapacityandperformanceofair-sourceheatpumpsdecreaserapidlywithdecreasingambienttemperatureduringheatingseason,andwithincreasingambienttemperatureduringcoolingseason.Especiallyoverextendedperiodsofsub-freezingtemperatures,astheairsourcetemperaturedropsbelow4oCthereispotentialfortheevaporatorstosufferperformancedegradationduetoiceformation(LuAye,2003).
Nonetheless,comparedwithconventionalheatingmethods,anASHPhasfollowingbenefits:
-Typicallydrawsapproximately1/3to1/4oftheelectricityofastandardelectricalheatingforthesameamountofheating,reducingutilitybillsandgreenhousegasemissionsaccordingly(HyperPhysics2005).
-TypicalCOPofanASHPisabout200%-400%comparesto100%foraresistanceheaterand70-95%forafuel-poweredboiler(HyperPhysics2005).
-Asanelectricsystem,noflammableorpotentiallyasphyxiatingfuelisusedatthepointofheating,reducingthepotentialdangertousers,andremovingtheneedtoobtaingasorfuelsuppliesexceptforelectricity(HyperPhysics2005).
4.GroundSourceHeatPumps
Likeanyheatpumps,groundsourceheatpumpsareabletoheat,coolthehouse.Althoughpublicawarenessofthisbeneficialtechnologyislow,GSHPsinfacthavebeenincommercialuseforover50years.ThefirstsuccessfuldemonstrationofGHPsoccurredbackin1946,attheCommonwealthBuildinginPortland,Oregon(Stuebi2000).
4.1ComponentsofGSHPSystems
-TheEarthConnection:
Theearthconnectiontransfersheatintooroutofthegroundorwaterbody.Itoftentakestheformofanoutdoorheatexchanger.Thisisacoilorpipecarryingwater,anantifreezemixture,oranotherheattransferfluid.Itmaybeburiedintheground,inwhichcaseitiscalledaground-coupledsystem,orsubmergedinalakeorpond,inwhichcaseitiscalledasurfacewatersystem(NRC(NaturalResourcesCanada),2008).
-AHeatPump:
ThisistheheartofaGSHPSystems,theyoperatesaccordingtothesameprincipleasconventionalheatpumps.Allthecomponentsoftheheatpumparetypicallyhousedinasingleenclosurewhichincludestheearthconnection-to-refrigerantheatexchanger,thecompressor,controls,thefan,anairfilter,anairhandler,andrefrigerant-to-airheatexchanger(NRC,2008).
-TheInteriorHeatingorCoolingDistributionSystem:
Thedistributionsystemisneededfordistributingheatingandcoolinginsidethebuilding.GSHPsystemstypicallyuseconventionalductworktodistributehotorcoldairandtoprovidehumiditycontrol(NRC,2008).
4.2TypesofGroundSourceHeatPumpSystems
TherearedifferentwaystocategorizeGSHPs.DependingontheWhenitusesearthasheatsource/sink,theearthconnectioncouldbeeitherhorizontalorvertical,andtheyarebothclosed-loop.Whenituseswaterasheatsource/sink,itcouldbeclosed-looporopen-loop(Fig4).
-Horizontal(Closed-Loop)
Thistypeofinstallationisgenerallymostcost-effectiveforresidentialinstallations,particularlyfornewconstructionwheresufficientlandisavailable(EERE,2008b).Itrequirestrenchesatleastfourfeetdeep(EERE,2008b).
-Vertical(Closed-Loop)
Verticalsystemsareusedwhenlandarearequiredforhorizontalloopswouldbeprohibitiveorthesoilistoorockyfortrenching,theyarewidelyusedforlargebuildings(NRC,2008).Althoughitrequireslesssurfacearea,itrequiresmuchdeepertrenchesthanhorizontalloops(NRC,2008).
-Pond/Lake(Closed-Loop)
Aclosedpondloopisnotascommon,ituseswaterasheatsource/sink(NRC,2008).Ifthesitehasanadequatewaterbody,thismaybethelowestcostoption(EERE,2008b).
-Open-LoopSystem(Open-Loop)
Anopenloopsystempullswaterdirectlyfromawell,lake,orpond.Waterispumpedfromoneofthesesourcesintotheheatpump,whereheatiseitherextractedoraddedandthewateristhenpumpedbackintothegroundorsourcebodyofwater(SEW(ShankyEngineeri