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ThermoelectricCooling-TheBasics1IntroductionAlthoughthermoelectric(TE)phenomenawerediscoveredmorethan150yearsago,thermoelectricdevices(TEcoolers)haveonlybeenappliedcommerciallyduringrecentdecades.Forsometime,commercialTECshavebeendevelopinginparallelwithtwomainstreamdirectionsoftechnicalprogress–electronicsandphotonics,particularlyoptoelectronicsandlasertechniques.Lately,adramaticincreaseintheapplicationofTEsolutionsinoptoelectronicdeviceshasbeenobserved,suchasdiodelasers,superluminescentdiodes(SLD),variousphotodetectors,diodepumpedsolidstatelasers(DPSS),charge-coupleddevices(CCDs),focalplanearrays(FPA)andothers.TheprogressinapplicationsisprovidedbyadvantagesofTEcoolers–theyaresolidstate,havenomovingpartsandareminiature,highlyreliableandflexibleindesigntomeetparticularrequirements.2WhatisThermoelectricCooling?ThermoelectriccoolingusesthePeltiereffecttocreateaheatfluxbetweenthejunctionsoftwodifferenttypesofmaterials.APeltiercooler,heater,orthermoelectricheatpumpisasolid-stateactiveheatpumpwhichtransfersheatfromonesideofthedevicetotheothersideagainstthetemperaturegradient(fromcoldtohot),withconsumptionofelectricalenergy.SuchaninstrumentisalsocalledaPeltierdevice,Peltierheatpump,solidstaterefrigerator,orthermoelectriccooler(TEC).ThePeltierdeviceisaheatpump:whendirectcurrentrunsthroughit,heatismovedfromonesidetotheother.Thereforeitcanbeusedeitherforheatingorforcooling(refrigeration),althoughinpracticethemainapplicationiscooling.Itcanalsobeusedasatemperaturecontrollerthateitherheatsorcools.Thistechnologyisfarlesscommonlyappliedtorefrigerationthanvapor-compressionrefrigerationis.ThemainadvantagesofaPeltiercooler(comparedtoavapor-compressionrefrigerator)areitslackofmovingpartsorcirculatingliquid,anditssmallsizeandflexibleshape(formfactor).Itsmaindisadvantageisthatitcannotsimultaneouslyhavelowcostandhighpowerefficiency.ManyresearchersandcompaniesaretryingtodevelopPeltiercoolersthatarebothcheapandefficient.APeltiercooleristheoppositeofathermoelectricgenerator.InaPeltiercooler,electricpowerisusedtogenerateatemperaturedifferencebetweenthetwosidesofthedevice,whileinathermoelectricgenerator,atemperaturedifferencebetweenthetwosidesisusedtogenerateelectricpower.Theoperationofbothiscloselyrelated(botharemanifestationsofthethermoelectriceffect),andthereforethedevicesaregenerallyconstructedfromsimilarmaterialsusingsimilardesigns.3WhatisThermoelectricCooler?Thermoelectriccooler(TEC),orPeltierCoolerisasolid-stateheatpumpthatusesthePeltiereffecttomoveheat.ThemoderncommercialTECconsistsofanumberofp-andn-typesemiconductorcouplesconnectedelectricallyinseriesandthermallyinparallel.Thesecouplesaresandwichedbetweentwothermallyconductiveandelectricallyinsulatedsubstrates.TheheatpumpingdirectioncanbealteredbyalteringthepolarityofthechargingDCcurrent.TECschematicisillustratedinFigure1.ThetypicalmaterialsusedforconstructingTECare:1.Substrate:aluminumoxide(Al2O3),aluminumnitride(AlN),orbariumoxide(BaO)2.Conductor:Copper3.Thermoelectricsemiconductori.n-type:bismuth-telluride-selenium(BiTeSe)compoundii.p-type:bismuth-telluride-antimony(BiTeSb)compound4.Assembledandjoinedbysolder.Figure1.ThermoelectricCoolerSchematicTheTECcanbemadeindifferentshapesandsizes,butmostcommonshapeisasquareorarectangularsubstratedevice.ThepracticalsizeofasinglestageTECrangesfrom3mmx3mmupto60mmx60mm.Thesizelimitationof60mmx60mmisduetothethermalstress.ThisstresscomesfromthermalexpansiondeformationsbetweenthecoldandthehotjunctionsoftheTEC.Toobtainalargertemperaturedifference,amultistageTECcanbebuild.ThemultistageTECisusuallyincascadeshapeand6stagesarethemaximumpracticallimit.Figure2depictsvarioussizesofTEC.Figure2.VariousSizesofTEC4WhentoconsiderTEC?TECcanbeusedindifferentapplicationwherecoolingortemperaturecontrolofanobjectisrequired.Ingeneral,TECismostoftenusedwhenanobject:1.Needstobecooledbelowtheambienttemperature,or2.Requirestobemaintainedataconsisttemperatureunderafluctuatingambienttemperature.TECisperfectforcoolingasmall,lowheatloadobject.DuetothelowCOP(CoefficientofPerformance)comparedwithcompressorcooling,TECloosesitsadvantageifthecoolingloadishigherthan200watts.But,becauseTECshavenomovingparts,theyarelightweightandreliable;theycreatenoelectricalnoise,andcanbeoperatedatanyorientationorenvironment,insomeinstancesTECsareusedtocoolkilowattsofheat.TECisexceptionallysuitableforaprecisiontemperaturecontrolofanobjectsuchasalaserdiode,CCDorothersmallobjects.PairedwithaDCpowersupplyandanelectronicsproportional/integral(PI)controllerpackagedinasinglechipdevice,TECisabletocontrolanobjectto+/-0.1oCaccuracy.Today,noothercoolingmethodyetcanprovidesuchprecise,simpleandconvenienttemperaturecontrol.5ApplicationofTECTECismainlyusedforcoolingofelectroniccomponents,especiallyofopticaldevices,soastoimprovetheperformanceofelectroniccomponents,suchascomputerchips,infrareddetectors,lasersandsoon.Thesedevicestypicallyrequirelesscoolingpower.U.S.MarlowCompanyhassetupalasercoolingsegment,andaboutonethirdofthepersonnelareengagedinthisstudyarea.TEC’sapplicationsonthebiologyandmedical.PCR