Page - 84 - in Hybrid Electric Vehicles

vehicle (HEV),electricvehicle (EV),andfuelcellvehicle (FCV)areeffective technicalapproaches for energy conservation andemission reduction [1, 2]. Currently, EVandHEVcannot compre- hensively fulfill the requirements of transit buses due to their poor durability and high cost. However,HEVisa feasiblesolutionwithhighreliabilityandrelatively lowcost. There are twokindsof power source for anHEV: one is internal combustion engine, and the other iselectricenergystoragesystem(ESS).AnESScandischargeelectricpower topropel the vehicleorabsorbelectricpowerduring the regenerativebrakingprocess.Generally, thearchi- tecture of anHEVpowertrain can be classified as series hybrid, parallel hybrid, andpower- splithybrid [3].Taking intoaccount theheavy-dutyhybridpowertrain for transitbuses, series hybridandparallelhybridarewidelyadoptedpresently.For instance,OrionVIIandManLion use series hybrid powertrain for their transit buses. Volvo andEaton designed two types of parallelhybridpowertrain forheavy-dutyapplications. With the progress of technology, the battery used for the ESS of anHEV has been gradually shifted from lead-acid,NiCad, andNi-MH to lithium-ion battery. Lithium-ion batterieswill be usedwidelyas theESSforvariousvehiclesbecauseof theirhigh-energydensity,goodsafety,and long durability [4]. Currently, the materials of lithium-ion batteries are mainly lithium iron phosphate and nickel-cobalt-manganese ternary composite [5]. AnHEV transit bus undergoes frequent acceleration anddecelerationduring itsworking time and requires largeworking cur- rentsof theESSfor theseprocesses.Becausethedischargeandchargeratesof lithium-ionbattery arelimited, if theESSconsistsofonlylithium-ionbatteries,alargecapacityof lithium-ionbatteries isrequired,whichwill increasethecostandweightofanHEVgreatly.Toovercomethisproblem, hybrid energy storage system (HESS) composed of lithium-ion batteries and supercapacitors is employed. Incontrast toa lithium-ionbattery,asupercapacitorcanchargeordischargewithvery large instantaneous currents. This characteristic canprovide sufficient electric powerduring the acceleration process and store electric energy during the regenerative braking process. Because supercapacitorsuse aporous carbon-basedelectrodematerial, a veryhigh effective surface area can be obtained by this porous structure compared to a conventional plate structure. Supercapacitorsalsohaveaminimaldistancebetweentheelectrodes,whichresult inaveryhigh capacitance compared to a conventional electrolytic capacitor [6, 7].Apart from the fast charge/ dischargeratesandthehigh-powerdensity,supercapacitorshavemuchlonger lifetimes(>100,000 cycles)comparedtolithium-ionbatteries[8–10].However,supercapacitorsnormallyhaveamuch smallerenergycapacitycomparedwith lithium-ionbatteries.Therefore,usinganHESScan fully utilizetheadvantagesofthesetwokindsofenergystoragedevicesandavoidtheirdisadvantages. Figure1showsfourprimary topologiesofanHESS,whichencompasspassivehybrid topology, supercapacitor semi-active hybrid topology, battery semi-active hybrid topology, and parallel activehybrid topology [11, 12]. Thepassivehybrid topology is the simplest to combinebattery andsupercapacitortogether.Theadvantageofthistopologyisthatnopowerelectronicconverters areneeded.BecausethevoltageoftheDCbusisstabilizedbythebattery,thestoredenergyofthe supercapacitor cannot be utilized sufficiently. In the supercapacitor semi-active topology, the battery isconnectedtotheDCbusdirectly,while thesupercapacitorusesabidirectionalDC/DC convertertointerfacetheDCbus.Asaresult,thevoltageoftheDCbusequalstheoutputvoltageof the battery so that it cannot be varied toomuch. But the voltage of the supercapacitor can be changedinawiderange.ThedisadvantageofthistopologyisthatalargesizeofDC/DCconverter Hybrid Electric Vehicles84