热泵

Air-sourceheatpumps Workingprinciple: Airsourceheatpumpsarerelativelyeasy(andinexpensive)toinstallandhavethereforehistoricall beenthemostwidelyusedheatpumptype.However,theysufferlimitationsduetotheiruseofthe outsideairasaheatsourceorsink.Thehighertemperaturedifferentialduringperiodsofextreme coldorheatleadstodecliningefficiency,asexplainedabove.Inmildweather,COPmaybearound 4.0,whileattemperaturesbelowaround−8°C(17°F)anair-sourceheatpumpcanachieveaCOPof2.5orbetter,whichisconsiderablymorethantheCOPthatmaybeachievedbyconventionalheatingsystems.TheaverageCOPoverseasonalvariationistypically2.5-2.8,[9]withexceptionalmodelsabletoexceed6.0(2.8kW). Theworkingfluid,initsgaseousstate,ispressurizedandcirculatedthroughthesystembya compressor.Onthedischargesideofthecompressor,thenowhotandhighlypressurizedgasis cooledinaheatexchanger,calledacondenser,untilitcondensesintoahighpressure,moderate temperatureliquid.Thecondensedrefrigerantthenpassesthroughapressure-loweringdevicelikeanexpansionvalve,capillarytube,orpossiblyawork-extractingdevicesuchasaturbine.Thisdevicethenpassesthelowpressure,(almost)liquidrefrigeranttoanotherheatexchanger,theevaporatorwheretherefrigerant evaporatesintoagasviaheatabsorption.Therefrigerantthenreturnstothecompressorand thecycleisrepeated. Insuchasystemitisessentialthattherefrigerantreachasufficientlyhightemperaturewhen compressed,sincethesecondlawofthermodynamicspreventsheatfromflowingfromacold fluidtoahotheatsink.Practically,thismeanstherefrigerantmustreachatemperaturegreater thantheambientaroundthehigh temperatureheatexchanger.Similarly,thefluidmustreach asufficientlylowtemperaturewhenallowedtoexpand,orheatcannotflowfromthecoldregion intothefluid,i.e.thefluidmustbecolderthantheambientaroundthecold temperatureheat exchanger.Inparticular,thepressuredifferencemustbegreatenoughforthefluidtocondens atthehotsideandstillevaporateinthelowerpressureregionatthecoldside.Thegreaterthe temperaturedifference,thegreatertherequiredpressuredifference,andconsequentlythe moreenergyneededtocompressthefluid.Thusaswithallheatpumps,theenergyefficiency (amountofheatmovedperunitofinputworkrequired)decreaseswithincreasingtemperature difference.