heat pumps

Air-sourceheatpumps Workingprinciple: Airsourceheatpumpsarerelativelyeasy(andinexpensive)toinstallandhavethereforehistorically 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,ispressurizedandcirculatedthroughthesystemby acompressor.Onthedischargesideofthecompressor,thenowhotandhighlypressurizedgasis cooledinaheatexchanger,calledacondenser,untilitcondensesintoahighpressure,moderate temperatureliquid.Thecondensedrefrigerantthenpassesthroughapressure loweringdevice likeanexpansionvalve,capillarytube,orpossiblyawork-extractingdevicesuchasaturbine.Thisdevicethenpassesthelowpressure,(almost)liquid refrigeranttoanotherheatexchanger,theevaporatorwheretherefrigerantevaporatesintoa gasviaheatabsorption.Therefrigerantthenreturnstothecompressorandthecycleisrepeated Insuchasystemitisessentialthattherefrigerantreachasufficientlyhightemperaturewhen compressed,sincethesecondlawofthermodynamicspreventsheatfromflowingfromacoldfluid toahotheatsink.Practically,thismeanstherefrigerantmustreachatemperaturegreaterthan theambientaroundthehigh-temperatureheatexchanger.Similarly,thefluidmustreachasufficientlylowtemperaturewhen allowedtoexpand,orheatcannotflowfromthecoldregionintothefluid,i.e.thefluidmustbe colderthantheambientaroundthecoldtemperatureheatexchanger.Inparticular,thepressure differencemustbegreatenoughforthefluidtocondenseatthehotsideandstillevaporatein thelowerpressureregionatthecoldside.Thegreaterthetemperaturedifference,thegreater therequiredpressuredifference,andconsequentlythemoreenergyneededtocompressthe fluid.Thusaswithallheatpumps,theenergyefficiency(amountofheatmovedperunitofinput workrequired)decreaseswithincreasingtemperaturedifference.