Page - 27 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 1217 Table11.Comparisonofdifferent fuel cell configurations.Data from[2]. PAFC AFC MCFC SOFC SPFC DMFC Workingtemp. (◦C) 150–210 60–100 600–700 900–1000 50–100 50–100 Powerdensity (W/cm2) 0.2–0.25 0.2–0.3 0.1–0.2 0.24–0.3 0.35–0.6 0.04–0.25 Estimated life (kh) 40 10 40 40 40 10 Estimatedcost (USD/kW) 1000 200 1000 1500 200 200 PAFC:Phosphoricacid fuelcell;AFC:Alkaline fuelcell; SOFC:Solidoxide fuelcell; SPFC:Solidpolymer fuelcell, alsoknownasprotonexchangemembranefuelcell. Fuel cells havemany advantages for EVuse like efficient production of electricity from fuel, noiseless operation, fast refueling, noor lowemissions, durability and the ability toprovidehigh densitycurrentoutput [24,60].Amaindrawbackof this technology is thehighprice.Hydrogenalso havelowerenergydensitycomparedtopetroleumderivedfuel, therefore largerfuel tanksarerequired forFCEVs, these tanksalsohave tocapableenoughtocontain thehydrogenproperlyandtominimize riskof anyexplosion in caseof anaccident. FC’s efficiencydependson thepower it is supplying; efficiencygenerallydecreases ifmorepower isdrawn.Voltagedropin internal resistancescausemost of the losses. Response timeofFCs iscomparativelyhigher toUCsorbatteries [35]. Becauseof these reasons, storage likebatteriesorUCs isusedalongsideFCs. TheToyotaMiraiusesbatteries topower itsmotorandtheFCisusedtocharge thebatteries. Thebatteries receive thepowerreproducedby regenerativebrakingaswell. Thiscombinationprovidesmoreflexibilityas thebatteriesdonotneed tobecharged,only the fuel for theFChas tobereplenishedandit takes far less timethanrecharging thebatteries. 4.4. Flywheel Flywheels are used as energy storage byusing the energy to spin the flywheelwhich keeps on spinningbecause of inertia. Theflywheel acts as amotorduring the storage stage. When the energy isneededtoberecovered, theflywheel’skineticenergycanbeusedtorotateagenerator to producepower.Advancedflywheelscanhavetheir rotorsmadeoutofsophisticatedmaterials like carboncompositesandareplacedinavacuumchambersuspendedbymagneticbearings. Figure33 showsaflywheelused in theFormulaOne(F1) racingkineticenergyrecoverysystem(KERS).The majorcomponentsofaflywheelaredemonstrated inFigure34. Flywheelsoffera lotofadvantages overotherstorageformsforEVuseas theyare lighter, fasterandmoreefficientatabsorbingpower fromregenerativebraking, fasterat supplyingahugeamountofpower inashort timewhenrapid acceleration isneededandcangothrougha lotof charge-dischargecyclesover their lifetime. Theyare especiallyfavoredforhybridracecarswhichgothroughalotofabruptbrakingandacceleration,which arealsoatmuchhigherg-force thannormal commuter cars. Storage systems likebatteriesorUCs cannotcapturetheenergygeneratedbyregenerativebrakinginsituations likethisproperly. Flywheels, on the other hand, because of their fast response, have a better efficiency in similar scenarios, by makinguseofregenerativebrakingmoreeffectively; it reducespressureonthebrakepadsaswell. The Porsche911GT3Rhybridmadeuseof this technology. Flywheelscanbemadewithdifferentmaterials, eachwiththeirownmeritsanddemerits.CharacteristicsofsomethesematerialsareshowninTable12; amongtheonesdisplayedinthe table, carbonT1000offers thehighestamountofenergydensity,but it ismuchcostlier thantheothers. Therefore, thereremainsa trade-offbetweencostandperformance. 27