Thermoelectric Generator in Recovering Waste Heat of Engine Exhaust

Thermoelectric Generator in Recovering Waste Heat of Engine Exhaust

THERMOELECTRIC GENERATOR IN RECOVERING WASTE HEAT OF ENGINE EXHAUST

ABSTRACT

The main objective of this paper is to recover the waste heat from the automotive engine exhaust into useful electrical energy by using “thermoelectric power generator”.Automotiveenginesreject a considerableamountofenergytotheambiencethroughthe exhaustgas.Significantreductionofenginefuelconsumptioncouldbeattainedbyrecoveringof exhaustheat byusingthermoelectricgenerators. Changingtheheat energyoftheexhaustgasesintoelectricpowerwouldbringmeasurable advantages.Moderncarsequippedwithcombustionenginestendtohavelargenumbersof electronicallycontrolledcomponents.Contemporarycarenginesexchangeapto30-40%ofheatgeneratedintheprocessof fuel combustionintousefulmechanicalwork and losing roughly 15 terawatts of power in the form of heat to the environment.Thermoelectric devices could convert some of this waste heat into useful electricity.Therefore,evenpartialuseofthe wastedheat wouldallowasignificantincreaseoftheoverallcombustionengineperformance. Theobservedtendencyisto replacemechanicalcomponentswiththeelectronicones.This increasesthedemandforelectricpowerreceivedthroughthepowersupplysystemsofthe vehicle.Thistendencywillundoubtedlyremainatleast duetothelegal regulationsconnected withtheon-boarddiagnosticsystems,whichforceamorecomprehensivecontrolofoperationof thevehiclecomponentsintherespectofsafetyimprovementandemissioncontrol.

KEYWORDS:

Automotive Thermoelectric Generators, Thermoelectric effect, TE modules.

INTRODUCTION:

Automotive Thermoelectric Generators (ATEG) recovers heat that escapes from a vehicle powered by an internal combustion engine, and generate electricity with the heat. Thisleadstothesignificantincreaseofdemandforelectricpowerinthevehiclewhichhastobe generatedbythealternator.It ispredictedthat ifonly6%oftheheatcontainedintheexhaust gaseswas changedintoelectricpower,itwouldallowtolowerfuel consumptionby10%dueto thedecreasedwasteresultingfromtheresistanceofthealternatordrive.Power generation systemusingthethermoelectricgeneratorshouldgenerallyconsistofthefollowingcomponents:heat exchanger,thermoelectricmodule,coolingsystemandDC/DCvoltageconverter.

Oneof themostimportantdesignissuesrelatedtotheconstructionofthethermoelectricgeneratorTEG istodevelopan efficient heat exchanger,whichshouldprovideoptimalrecoveryofheat from exhaustgases.Through theapplication ofthermoelectricgenerators(TEG),inthefutureaproportion of the energycarried bythe exhaustgasesthatwould previouslyhavebeenlostwillberecovered aselectrical energy. TheGerman AerospaceCenter (DLR)hasdevelopedTEG foruse in vehiclesfordoingthisand resultingfromcollaboration withtheBMWGroup, 200 Wofelectricalpower inatestvehiclewas achieved in ademonstration. Given theuseoffuturematerials, performancesofup to 600 Wcan be expected, yieldingausagepotentialof

5percentage.

PRINCIPLE OF TEG:

When heat is applied to one of the two conductors or semiconductors, heated electrons flow toward the cooler one. If the pair is connected through an electrical circuit, direct current (DC) flows through that circuit. This effect is called as seebeck effect. The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances.

THERMOELECTRIC EFFECT:

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. The thermoelectric effect refers to phenomena in which a temperature difference creates an electric potential or electric potential creates a temperature difference.

THERMOELECTRIC MATERIALS:

A commonly used thermoelectric material in such applications is Bismuth Telluride (Bi2Te3).Thermoelectric materials have a nonzero thermoelectric effect; in most materials it is too small to be useful. However, low cost materials that have a sufficiently strong thermoelectric effect could be used for applications.

WORKING OF TEG:

When two different conductors are placed in contact, electrons flow from one to the other if the energy levels of the electrons are different in the two materials. The higher energy electrons cross the junction until the energy levels are the same on both sides. The thermoelectric module is made from two conductors whose energy levels change at different rates when the temperature changes. If the junctions are not at the same temperature, there are unequal differences in energy levels across the junctions. Thus, unequal numbers of electrons have to cross the junctions and unequal voltages are established. Since there is a net voltagearound the loop, a current will flow.

IMPARTANCE:

Through theapplication ofthermoelectricgenerators(TEG),inthefutureaproportion of the energycarried bythe exhaustgasesthatwould previouslyhavebeenlostwillberecovered aselectrical energy. TheGerman AerospaceCenter (DLR)hasdevelopedTEG foruse in vehiclesfordoingthis, and, resultingfromcollaboration withtheBMWGroup, 200 Wofelectricalpower inatestvehiclewas achieved in ademonstration. Given theuseoffuturematerials, performancesofup to 600 Wcan be expected, yieldingausagepotentialof5%.The currentresearchfocusisaimed at improvingthe manufacturingtechniquewithaview tofuturestandard production applications.

LOCATIONOFTEG IN AUTOMOTIVE ENGINES:

The location of the thermoelectric generator is an important factor, decisive of its operability.The TEG generatorcan be installed on the exhaust pipe immediatelybetween the collectorandthecatalyticconverterorbehindthecatalyticconverter.TheheatisabsorbedfromtheexhaustpipeandlaterconvertedintoelectricitybyusingTEGgenerator.

Thetestingofthetemperaturedistributionwasperformedatdifferentenginespeeds:2300 and 3300rpm.Thetemperaturesreceivedatlowerenginespeedare higher,eventhough thedifferenceoftemperatureoftheexhaustgases measuredforthebothenginespeedsinfront ofandbehindtheheat exchangerisateveryengineloadpointhigherforthe3300rpmby50°C onaverage.However,inthefirstcasethecoolant flowof21l/hwasused.At3300rpmtheflow was10timeshigher,whichleadtosmallerdifferencesin coolanttemperaturein frontof andbehindthecoolers,andat thesametimetothe greaterefficiencyofheat absorptionfromexhaust gases.ThissituationisillustratedbyFig.,whichshowsthat themostefficientoperationofthe systemwas at3300rpm.

CONCLUSION:

Theperformanceoftheheat exchangersystemformsthebasisforcontinuingtheprocess ofdesignoptimization.Thedesignedmodelofheat exchangerallowed fortheutilizationof0.6to5.0kWofexhaustgasenergydependingontheoperatingparametersoftheengine. However,theanalysisof temperaturedistributionpointsoutthat,uponintroductionofspecificchangesintothedesign,it ispossibleto recovereven25kWofheat energy.Assuming the5%efficiencyofthethermoelectricmodulesitcouldallowtoobtainthemaximumelectric powerof app.750W.Thispoweris comparabletothepoweroftypical alternatorsusedincars with1.3dm3enginecapacity.It shouldbeexpectedthat muchgreatergeneratorperformancecanbeobtainedby buildingit intheexhaustsystemofspark-ignitionenginetypes,duetothehigher temperaturesofexhaustgases.

REFERENCES:

  • Fuel Economy in furnaces and Waste heat recovery-PCRA
  • Heat Recovery Systems by D.A.Reay, E & F.N.Span, London, 1979.
  • J. LaGrandeur et al, “Vehicle Fuel Economy Improvement

Through Thermoelectric Waste Heat Recovery”, 2005 Diesel

Engine Emissions Reduction (DEER) Conference Presentations, Chicago, Illinois, August 21-25, 2005

  • thermoelectric-generator-harnesses-exhaust-heat-to-power-cars
Top View