地源热泵外文翻译英.docx

地源热泵外文翻译英.docx

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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