Pwr n Energy Storage

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POWER AND ENERGY STORAGE DEVICES FOR NEXT GENERATI ON HY BRID EL ECTRI CVEHICLE *Bimal K.BoseMin-Huei K im **

DepartmentofElectrical EngineeringTheUniversityof TennesseeKnoxville,TN 37996,USA

AbstractFuel conservation and environmental pollution control are theprincipal motivating factors that are urging at present widespread

research and development activities for eectric and hybrid vehiclesthroughout the world. The paper describes different possible energystorage devices, suchasWeq,flywhee and ultra capacitor, and powersources, suchas gasoline engine, diesel engine,gasturbine and fuel celfor next generation hybrid eectric vehicle. The technology trend andcorn-m energy storage and power devices indicate thatbatteryand gasoline en@e, respectively,will remain the most viable devicesfor hybrid vehicle at least in the near future.INTRODUCTION

Although the history of eectric vehicle (EV) goes back to nearlyhundred years, seriousR &D activity in EV started duringAraboilembargo of 1970's. The prime focus at that point was fuel saving sothat the dependence on importedoilcouldbereduced It is interestingto note that in USA oil constitutes approximately42%of total energyCansumption, and the majorpartof thisol isconsumed n automobiletransportation. Agan, more than 50% of our oil is imported fromoutside. In the 1980's, the environmentalpollution (parbcularly theu r hpollution) problemand the correspondinggobal warmng effectbecame a major concern inour society. In1990, the CaliforniaAirResource Board (CARB) established rules that mandate2% of al lvehicles sold in Californiain 1998 mustbezero emssion vehicles(ZEV). Thisquota increases to 5% by2001and to 10% by 2003. TheCalifomiadesnotonlyhadseriousmpact in otherstatesof USA, buthad wide reverberation in Europe and Japan. The govements,promnent auto industries and research laboratories around the worldare persuing electrichybrid vehicle research seriously, and publishedliterature nthisareaisswelling tremendously.In September 1993, the US Government along with the big threeautomakers (GhI, Ford and Chrysler) declared a historic partnershipprogramfornext generation vehicles (PNGV)[ ] .The salient PNGVgoas which are tobefully implemented before the year2003include:three times the present fuel efficiency (80 mledgallon) and theemssion level of 0.125/ 1.7/0.2 (HC/CQ/NOJ (gms/mle). Althoughnot mentionedasEV, it has to be hybrid eectric vehicle(HEV)withcoflsiderable regenerative braking energy capture capability n order tomeet the fuel efficiency and emssion goals.Ina federal urban drivingcycle(FUD), the ICEV gives a typical efficiency of 12.24% with theoverall fuel chain efficiency of 10%. On the other hand, an EV givesefficiency of 51.6%withthe otal fuel chain efficiency of 18%. The EVhas definite advantages over ICEV, but the disadvantages arethat therange of EV is imtedand thas limted acceleration capability, and canbattery sbulky, t ooexpawiveand it has limtedcycle ife. Thepractical

M.David KankamNASA Lewis]ResearchCenter

Cleveland. Ohio44135

not definitely meet the performance of a gasoline carA good solution is HEV where a power sourceassists the storagedevicetoenhancerange extension, fuel economy, versatility and makecompromse in emssion.Figure 1shows the general block diagram of dcdistribution systemof HEV with possible energy storage and power devicesinfuture.Basically, t is a series hybrid system where the power and storagedevices feed electrical power to a commondcbus that drives an acmotorthroughan inverter. The battery,utracapacitor and fuel cel canbe directly connected to the bus, but the flywheel and engme powerSOD require mache-converter interface. The control andoptimumenergy management of aHET1system is very complex, and requirespowerl l mcrocomputers.Although literature in EVMEV is very large, there is hardly anypaper that describes systematidly and compares the possible energystorage and power devicesfor future generations of EV/HEV. Thepaper aims to make that comparison.

AUXILIARY

POWER FLOWb-------4*IC ENGINE*GASTUR81NE I-'FUEL CELL . AITERY* FLYWHEELSUPERIULTRA CAPACITORSUPER CONDUCTING MAGNETENERGY STORAGE ISMESI$ 1

OPTIMAL ENERGYMANAGEMENT&CONTROL'IDISEL ENGINE]'ISTERLING ENGlNEl --.rUSER INTERFACEFig. 1General block diagram of HEV distribution system showingpossibleenerpj storage and power devices.ENERGY STORAGE DEVICK SBatteryInEV literature, the discussiononbattery has possibly them a x i "

* The project was supported by aresearch grant from NASA Lewis Research Center, Cleveland, USA.**Dr.Kimis currently a faculty memberinYeungnam Junior College,S.Korea1893 0-7803-3547- 3-7/164.0063 1996 lEEE


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coverage. The battery has possibly been the main barrier for consumeracceptance of EV in the market. Todays propulsion battery is tooheavy,tooexpensive,haslow cycle life and gives low practical vehiclerange. For this reason, the battery has been the prime focus of R&Dfor along h e.Unfortunatey, however, its technological evolution hasbeen very slow. In 1990, the US Advanced Battery Consorhum(USASC) was established to systematically sponsorR &DactivityinEVbatterytechnology.Unlike other applications, EV battery has to be specially designed tomeet some special performance and cost goals. These are specificenergy(Wh/kg), energy density(WWL), specific power (Wkg), powerdensity (WL), cycle life (years), and of course, finally the mostimportant parameter is cost($kWh).nergy storage capabihty isthekey pameter of-a battery that determnes the rangeof EV. Increasingthe battery weight can increase the energy storage which can givelonger range, butsoonthe vehicle becomes heavy and the cost becomesprohitive. Besides energy, the power ratmg of battery is equallyimportant from the viewpint ofacceleration/decelerationcapability andgradeability. For longer vehicle range, high accelerqtion capability athgh speed and steep grade driving, the assistance of a power source(hybrid vehicle) ismandatory. Note that inhi ghspeed decelerationofHEV at high brakjng torque the battery must absorb theful l power .An HEV battery, therefore, shouldbedesigned with high power-to-energy ratio (typically 10 or greater) compared to that for EVapplication(typically2).Besides the pnme performance factorsas discussed above, the otherconsiderations are maintainability, safety, reiability, materialrecyclability, abuse tolerance, charging deay, fuel gauging andcharge equalizationinthe series cells. Although off-peakutility powercharging of a batteryisan advantage, the charging deay is deffitely adisadvantage partlcdarly from a way-side station. Assumng that thefast charging problem of a battery is solved satisfactorily, typicaldomestic outlet can not handle the kW demand due to fast charging.Sirmar problem arises in utkty distribution systemiflarge number ofbatteries are charged simultaneously.The batterytypesthat are under consideration for EV are: Lead-Acid, Nickel-Cadmum, Sodi~~~~SulfUr,inc-&, Lithium-Polymer,Nickel Metal Hydride, Nickel Iron, Zinc Bromde, Sodium NickelChIoride, Nickel-Zinc, Nickel Hydrogen and LithiumIronDisulfide.Presently, lead-acid battery is possibly the best compromse in energydensity, power density, Me-cycle, cost and other criteria. It has beenwidely used in the past and present EV projects, andwill possiblyremainso mthe near future. It canbe shown that 40-50 mles range inurbandriving cycleiseasily achievable by lead-acid battery thatwillnothave more than30%of vehicle curb weight. Nickel-cadmum batteryis very expensive becauseof raw material cost. But, it has reativeyhgL ~nergy density andlongMecompared to lead-acid battery. Thesefactorsmake the vehicle range longer and the Me-cycle cost tends to beneutralized by long battery life. The battery is safe and sealable.C a d is not environmentally safe and recycling can be a problem.Sodium-sulfurbattery has high energy density but its power density issomewhat low. Its cost is comparable with Ni-Cd battery. *edrawbackisthatthebatteryoperatesa htgh temperature (about300C )to keep the sodium andsulfurmolten withm insulated capsules. Thebattery must be preheated to operating temperature and maintained atthat t e m m uring operation. Zinc-airhasgood energy capability,but low power and short life. L ithium-polymer battery may be acompromse of all the good properbes required for EV. The battery hasgood power and energycapdnhty,and the cost islower than that of Ni-Cdbattery. Nicke-metal-hydride battery seems to be more promsingfor high energy capability, and it has been under development forseveral years. It is more expensive than other types of batteries, and itis not easily available. Nickel-iron batteries have been underdevelopment for a number of years, and have demonstrated potentialfor long Me. The low cell voltage needs large number of cels in series

and makes reduction of non-active components weight important forhigher system voltage. Zinc-bromde battery systems are kingdeveopedworld-wide. This technology hasapotential for low cost, asthe systems components lend themselves to mass production and aremadebf readily available and ine-nsive materials. Nickel hydTogenbattery offers excellent energy density, high abuse tolerance andhighest level of reiability of any battery system. The battery issomewhat expensive, it hasalong Mecycle which can be guaranteedfor the operating life of a vehicle.FlywheelFlywhee@W) s an energy storage element has been investigated fora long period of time for different applications. It has been consideredas propulsion source for commercial transit bus, and anumber ofexperimental vehicles have been built using flywheels for automobileapplications. A running flywheel stores mechanical energy which isconverted to electrical form through a coupled machine-convertersystem. The machine actsasa generator when the mechanical energyof the aywheel isextracted for vehicle acceleration, whereas it actsasa motor in brakmg when the vehicle kinetic energy charges theflywhee. Since thestoredenergy variesasthe square of the speed,75%of the stored energy can be utked by varying the speed in 2:l range.A flywheel normally uses light-weight and high strength compositematerial to economze the cost, but the storage energy is increased byi ncg the speed that sometime exceeds 100,000 rpm The bearingfriction and aero-dyiatuic lossof the FW-machme system shouldbecritically reduced. The bearing fiction can be elimmated by activebearing,whereas the aero-dynamclosscan be highly attenuated byplacingthe system in a vacuum chamber. Maintamingagood vacuumby avacuumpumpinan automobile environment may be difficult. Thegyroscopic efffect of the flywheel can be a safety concern. Of course,gymbol suspension can be considered to deviate thisproblem.Allthese eements add cost to the system. The machine-converter systemhas to handle the high frequency because of high machme speed. Aspccdrcduction gear canbeused at theexpenseof additional cost andlosses. Either induction or synchronous machine can be used throughvoltagefdor current-fed converter. A flywheel energy storage has theadvantages of higherpowercapability, quick charging, long service lifeand higher round trip efficiency (total energy in vs. Energy out) overthe battery.

Flywhee storage hasbeenconsidered both forEV andHEV. It hasbeen considered for load levelling in a battery-fed EV where itsupplies/absorbs thepeak power during acceleratioddeceleration andheps to prolong the battery life. Prototype flywheel systems have beendemonshratedwith a specific energy of 5Wh/kg and specific power of375 Wkg for transit bus applications. Additional research anddeveopment efforts n flywhee systems should focus on increasing thespecific energy and reducing the cost .Ultra Capacitor

Recent advances in superhltra capacitor (UC) technology hasrenewed interest for its potential applications in EV/HEV and otherareas. The energy storage density of UC istwoorder of magnitudeshigher than the traditional electrolyhc capacitors. However, comparedto battery, the energy density is very low.

A UC is basically a chemcal double layer capacitor (CDLC) thatutilizes large surface area electrodes and liquid electrolyte to formacharge storage ayer. The thickness of this layer isontheorder of a fewangstroms.Materials suchas carbon blacks and Raney metalswithsurfaceareas of severa hundred square meters per gram had been usedin the early CDLCs. For new CDLCs, the energy density baspractically approached to one-tenth that of lead-acid battery. Ultracapacitorsarepresently manufactured by Panasonic,As& Glass,NEC,Pinnacle Research, etc.


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Although characterized by low energy density, UC's have highspecific power and high charge/discharge energy efficiency. Thefmerpennitsveq high charge/dischargerate. TherefOre, combininghghenergydensitybattery withUC capable of handling high transientpower requirement (load evelling) may provide an attractive solutionfor highpowerhergy ratioi~EVwithout impairing the battery life.The UC's haveasobeenmsihed to operateinparalle with fuel celswhichwillbe described later. Ultra capacitors that are commerciallyavailable today have a specific energy of about 1-2 W g . Recentlaboratory prototypes have been demonstrated with specific energy ofabout 2 to 5 W g nd power density of 2 to 4 kWkg. The keytechtllcal challenges are to improve theUC'slow specific energy andreducecosts. Research and deveopment offers the potential to improveUCmahiah, suchascarbodmetal fibe~omposites, monolith foamedcarbq foamed carbon with a binder, doped polymer layersoncarbonpaper, and mxed metal oxides (ceramc) on metal foil.Ultracapaci tors are definitey very expensive forEV application andtheir present availability is poor. Another problem is that they areavailable in very low voltage rating.A large number of them can beconnected n series and connected directly across the battery for loadleveling.Or ese, low voltage capacitorbank canbeinterfaced to thedcbusthrougha dc-dc converter. This, O course, adds additional cost,powerloss and sacrifke of reiability.Comparison of Energy Storage DevicesTable 1 gives a general comparison of energy storage devices. TheComparison,parbcularly the numerical figures, can be considered veryapproximate because adequate numerical nformation s not availableand some of these technologes are in early stage of evolution. Thelead-acid battery is taken as the basis for comparison. Agai n, asdiscussed above, a hybrid system, suchasbatteym or batteryAJCcanbeused combining the advantages of both although at a highercost. Fromthe table, itcanbeconcluded that batteryremains the mostviable storage devicein future.

Table.1: Comparision of Energy Storage Devices. .... I_;;__i_i=_=EL_i__=====~~~=~==~.~=~.==~ == =_ ~~~~-~~.-...;; .~.~

Battery Fl ywheel Ultra capacitor-----___-

Spsciiic Energy (pu) I 0.125 0.1Specifi c P ower(po) 1 3 20Cost(pu) I 8 ' 20*A\ ailability Good Medium I'oorSafety Good notgood GoodMaintenance Good Medium Very GoodCycle Life L imited Large Very LargeChargdDischarge Good Moderate Very GoodEfficiencyComments Avail able Avail able NeedsR& Dtechnology technology

* Nunmerical fi gure maynotbe accurate

POWER DEVICE SThe power device in an HE V helps increasing the range,acceleration capability athi gh speed, gradeability and maintaining amnimumstate of chargeinthe storage device. The important criteriafor selection of power source are fuel economy, emssioncharacteristics, specific power and cost. 'Two possible power sources,

i.e., heat engine andfuel cel canbeused although the formertypehasbeen traditionally preferred. The engine typescan be classifiedasgasoline engine, diesel engine ,gas turbine and Stirling engine.Different types of power deviceswillbe described below and then befollowed by a comparison.Gasoline Engine

Spark-ignited, homogeneous charge reciprocating type internalcombustion engine (commonly knownasgasoline engine) haslongbeenthe traditiona power source for a passenger automobile. Since tsinvention n late nineteenth century and the subsequent invention ofeectricself-starter inthe earlypartof thiscentury, the enginehas gonethrough slow evolutionary mprovement over a long period of time.Modemturbo-chargedengine has smaller size and improved efficiencywhichisimproved M e rby mcroprocessor-controlled fuel injection.Itispossibly the best power source forHEV,andwill remainso intheearly part of the next century181.Figure2shows thetypicalcharslcteristics[7]of a four-stroke gasoline+e where he constantefficiencycontoursare given in he planeofnormalizedmeaneffectivep m e reated o engine output torque) vs.SMpeed Thesecontoursare also specified ntermsofbrake specificfuel consumption (BSFC) with units ofgramsof fuel per kwh of

101 I I 5 \ I I I I

Fig.2: Typical Characteristicsof Four-stroke Gasoline Enginedeiveredenergy. Using the heat content of gasoline(33.2kWh/g), thepercentage efficiency vauesar ecalculated and noted on these contours.The heavy line contours indicale the effect of five-speed transmssion(donot usually apply for HETI).Also shown are the constant powerhyperbolas with normalized power levels of 1 to 36. An optimalefficiency locus spanning the range of engine power is shown by thedashed contour FGCDE. The highest efficiency(30%) is obtained atthe pointDwhere BSFC=274gkwh. Ina normal vehicle operation,the enginespeedvarieswidely and bwause of the wastage of brakingpower the total efficiency hade exceeds 15% In paralle hybridsystem the enginealsorunsat variable speed but the braking energyisrecovered in the battery. Hwwever, in a series hybrid systemasconsidered here, the engine speed canbeindependently controlled foranypowerdemand, and therefore, optimal efficiency canbeachieved.Commercia automobile engines are available n various power and

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speedratings dependingonthe size and performance needs of the car.A typical V4 engine of 93 kW and 6200 rpm has shipping weight of130 kg which correspondingly gives 0.73 weight-specific power(kW/kg) and 52 volume-specific power (kW/L). These parametersdecreaseasthe engine power increases. The use of unleaded gasolineand catalybc converter in the tail pipe have significantly reducedemssion characteristics of recent automobiles. The typlcal emssion(gmsimle) of a md-size car can be given as 0.20/0.63/3.43/0.35(SO,/NOJCO/HC)whereas the PNGV goals are 0/0.20/1.70/0.125.Various alternative fuels, such as natural gas, LPG, ethanol andmethanol canasobe usedinthe engine. The gaseous fuels give lowervolumehc efficiency and consequently decreased power. The on-boardstorage of gaseous fue ipvolves bulky and heavytanks,oraltematively,reduced range on a tank of fuel. Ethanol, normally available fromagricultural products,will never be available in sufficient quantity.Methanob principally made from natural gas and possibly some day insignificant quantity from coal, isstirring nterest for long range use inautomobile engine.

The fuel economy and the correspondmg emssion charachisticsofanIC engjnecanbe improved by increasing compression ratio, air-fue ratio, and charge stratifkation. ncreased compression ratio causescombustion knock. The octane numh imt to 92 has reinforced thisbarrier. The lean-burn characteristics are shown in Fig. 3 [SI. Thetheontical efficiency increases with air-fue ratio, but the shasp breakoccurs at the stoichiometric ratio. The experimental curves belowfollow the trend for theory. However, with increasing dilution, theflame speed falls which increases the time loss to offset the theonticalgans.A faster burn rate increases optimum dilution and its associatedefficiency.

5 5 , I I I I I

Uwi

0 351, I3 0 0o 15 20 25 30 35 4 0

AIR-FUEL RATIO

Fig.3: Effect of Air-Fuel RatioonThermal EfficiencyDiesel Engine

The diesel reciprocating engine which uses compression ignitionprinciple is also a good candidate for HEV drive. The principaladvantage of diesel engineisits superior fuel economy,asindicated nFig.4,comparedtothat of agasolinecar. The&fisioncombustionofitsfuel spray avoids the knock problem pmniting higher compressionratio, and therefore, improved efficiency. Again, diffusion combustionalows the load tobe controlled by varying the overall air-fuel ratio, thusavoiding the part-load pumping loss of atrahtional gasoline engineduring the intake stroke.Allpassenger-car diesels in the market todayareof induect injectiontype.Research s in progress to develop directinjectiontypewhich promses 10%-15% higher efficiency.The diesel is more popular in Europe than in the USA, its marketpenel" in USA peaked at 6% in 1981, and now it is less than 1%.There are a variety of reasons behind the disaffectionof the USconsumer for the diesel car. Among them are the decline of gasolineprices (which once had been projected to be t wo to three times their

c m t eve), imposition of extra federaltaxon diesel fuel, higher firstcost of diesel car, higher noise level and exhaust odor and fumesassociated with today's diesel engine. Despite such shortcomngs, the

+GAS TURBINEz i I r' ./STIRLING

%STE AMw-1 T I JO' 19175 1980 1985Y E A RFig.4: Fuel Economy Index for Various Engine Types

diesel remains the most fue-efficient engine for passenger carpropulsion.Faced with the long rangegrimprospects for petroliusn, tis difficult to ignorethis engine until amore efficient alternative isidenlitidHowever, the diese continues strong in the heavy-duty truckmarket. It has also found a niche market in lightand medium dutytrucks and vans some of which are used in personal transportation.However, promulgatedUSparticulatestandadspose serious threht tothe future of the diesel.GasTurbine

A gas turbineis an external combustion rotory heat engme wherethe processofcombustion sconimuous unlike intermttent combustionof IC enwe. Nearlyal automotive gas turbines demonstrated to datehave been of free or dual-shaft arrangement. The t wo shaftmangement permts independent speed for the compressor and powerproducing turbine. A generator is coupled to the power turbinenormallythrougha reduction gear.Althoughgas turbines have been traditionally usedinpower plants,asmallerscae mede isa desirable candidate forHEVbecause it is freeofnoiseandvibrationassociated with IC engines, it can be made smallandhght, and it normally has excellent torque-curve for vehicle use. Inlaboratoly setting, thasdemonstrated ow emssion level (0.41 3.4/0.4)(HC/CO/NO,) at low mleage.A parhcular advantageof gas turbineis that variety of fuels, such as diesel oil, natural gas, gasoline ormethanol can be used. The single pint n he1 economy index of Fig.4 from a turbine-powered car built by Chrysler compares vequnfavourably withother typesof engines. The thermal efficiencyofgasturbine increases with inletar temperature, regenerator effectivenessand other component efficiencies. It has been established that toimprove efficiency to the desirable figure automotive gas turbine hastoincorporate structural ceramcs n hot parts, particularly in thehighlystressed turbine rotor. This is expected to pennit raising dettemperature from todayk level of around 1050' C with hi ghtemperature metal alloystoasmuch as1350.C. The present researchisnow headinginthsdirection. Itisinteresting to note that recently ABVolvo of Sweden has announced its Environmental Concept Car(ECC) [9] which is an HEV usinggas turbmeaspower source thatdirectly drives the generator. The20 kW, 100,000rpmturbine usesnickel aloy in the rotor and is fuelled by diesel oil.Thediscussionon heat engmeswiUremain incomplete without a brieftouch on Stirling engine. The Stirling engine [14] receives heat ateevated tempxature roman external source and converts a fraction ofit to work reecting the remainder at lower temperature. Stirling enginehas severa advantageous characteristm. These include silent operation,

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low exhartst emssion levels, superior fue economyat steady stateoperation, the ability to operate onany liquid fuel (gasoline, dieseloil,kerosine, propane or methane), low cyclic torque variation, and a flatpart-load characteristics. It is conceivable that they couldbeused inautomobile.In fact, annmberof research projcctsinlastdecadehasdenionsWthe Stirling engine propulsion for cars, trucks and buses.Despite the favourable factors, they are very expensive, have largewarm-up time and have poor performanceinurban driving cycle.Excessive manufacturing costwill remainamain barrier for consumeracceptance.Fuel Cell

Fuel cel is a potentially viable power source for HEV and has beenconsidered for trrmsportatonapplications over a numberofyears. Theyare attractive because thay convert chemcal energy of a fuel directlyinto eectricity without combustionprocess, an achieve high efficiency(2 to3 times better than gasoline engine), virtually givesnoemssion,inherently modular, static, and noise-free. The operating principle offuel cel is indicated in Fig. 5 . Basically, t is a reverse process of

HYDROGEN FUEL (Hz)

2H' ELECTROLYTE

I1/2 0% 2H'+ 2e -+HzO

connected n series inafuel cel stack. Inaddition to thestack, theauxiliary system components include controls, cooling fans,recirculation pumps andai r compressor.

Fuel cels are classifiedonthebasisof the electrolyte and includephospharic acid he1cel (PAFC),molten carbonate fuel cel (MCFC),solidoxydefuel cel (SOFC), direct methanol fuel cel (DWLFC) andpolymer or proton exchange membrane fuel cel (PEMFC). Table2summarizes [lo] the charactenstics of some of these fuel cells.ThePAFC is reativey more mature technology and is nearest tocommercialization. However,PEMFC is the strongest candidate forvehicle application due to its use of solid eectrolyte, cold startcapablity,reativey high power density, and efficiency characteristics.The solid polymer electrolyte (referred to asamembrane) consists ofthin sheets. The cel electrodes have a thn filmplatinum catalystsupported on carbon and are bonded to the faces of the eectrolyte.

Fue celshave recentlybeenused as low to medium capacity powerplants in utility system, power generation in space vehlcles andoccasionally to power urban transit buses. However, their excessivecost ($/kW) and bulkiness makes them impractical for automobileapplications. Besides, their sl;art-uptime and transient response ared& The PEMFC basicmatedcost aloneisestimatedtobe over$2OOO/kW( .e.,$60,000fora30kW unit inanHEV), nd includingthe auxiliary system thespecificpower (kWkg)may be 0.04to0.06.The fuel cel, as indicated besore, has higher efficiency than gasolineengine, and ithasbeen ehnahl hat the payback period for fuel savingis 34 years foranautomobile, eight years for cross-country track andfour years for a commercial bus.

At present, research and development activitiesin fuel cels arevery heavy by government and private agencies. Hopefully, thelimtationswillbe solved in future and he1 cel willcomeout as aviable power sourcenot only for automobiles but also for otherapplications.Comparisonof Power Devices

Table3 gives the approximate comparison for different powerdevices excluchng the Stirling engine. Consideringall the factors, t canbe concluded that gasoline enginewill remain the most viable powersource at least in the earlypial of the next century.

AI R (Q z AND NS CONCLUSIONFig.5: Fuel Cell wration Principle

eectrolysisofwater where water breaksdownto hydrogen and oxygengases with the help of electncity. The construction of fuel cel issomewhat sdar to a battery, except that it does not undergo anymateria change, and consequently, t operatesas longas the fuel supplyexists. The hydrogen and oxygen gases in he1 cel are suppliedexternally and react through eectrolyte. First, the electrons areseparatedliomthe hydrogen molecules by a catalist creating hydrogenions and electrons, as indicated. The ions then pass through theeectrolyte to the oxygen side. The electrons are forcedtopass throughthe externa electncal circuit to the oxygen side. When electrons reachthe oxygen side, they combine with the hydrogen ions and oxygencreating water. Heat is generatedas a resultof thisreaction that raisesthe cel temperature. Hydrogen canbe stored n atank,or a hydrogen-Contairungfuelsuchasmethane or methanol can be stored which canberefmedtohydrogen for feeding the fuel cell. The oxygen can bedirectly supplied from air through 3 compressor. The theonticalefficiency of a hydrogen-oxygen fuel cel is 83%.Efficiencies ofpractica fuelcelsusing pure hydrogen and oxygen range&om50% to65% based on lower heating value. The theontical voltage of such acellat25 C is 1.23 volts. Under eectrical load, the cel voltage fallsto0.6 - 0.8 due to polarization effect. Therefore, multiple cels are

The paper discusses diflknmttypesof energy storage and powerdevicesthat areappropnzr te for next generatron electnchybnd vehicles.Approximatecomparison isaso given inthe respective class of devicesto indicate ther viability of application. The study indicates that batteryand gasoline enginewill r emai n the most viable energy storage andpower device, respectively ai least in the near-term hybrid vehicle.Although power electronics and drives areahas attained a reasonabledegree of maety, t appears that considerable amount of R &D isneeded in storage and power device technologiesnorder to make thehybrid vehicle economcally acceptable to the consumer. Thetechnology evolution inthisarea isV ~ I Yslow inspiteof large numberof publicationsintherecentliterature. Thebattery is the "weakestlink"inelectrichybrid vehicle, andwillremain so in the near future.REFERENCES[1 Partnership for Next Generation of Vehicles (PNGV) ProgramPlan, July 1994.[2] B. K. Bose and M. H. Kim, "Advanced propulsion powerd stributi onsystem fornext generationelectrichybridvehicle,Phase 1prelimmry system studies", Fina ReportsubmttedtoNASA LewisResearch Center, June 1995.[3]X.XuandV.A. Sankaran, "Power electronics n eectric vehicles:chalenges and opportunities", EEE/IASAnnu.Meet. Cod. Rec.,pp.463468,1993.

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Table2: Characteristicsof VariousFuel Cell TypesCell Type Major EstimatedApplication Fuel CommercialPotential

PhosphoricAcid(PAFC)

MoltonCarbonate(MCFC)

Solid Oxide(SOFC)

Polymer(PEMFC)

DispersedElectricOn-Site

DispersedElectricOn-SiteCogen.Central PowerOn-Site Nat. 2002Cogen. GasCentral Power Coal 2002+Vehicle Methanol 2000+

Nat.GasNat.GasNat.GasNat.GasCoal

1996

1992-5

1996-8

1996-8

2000+

41(80)

36-40(70)

48-55

45(70)

>50 12(70) 1,000

I5

(a) Fuel higher heating value to net ac electricity; cogeneration in parenthesis(b) So, based on 4 percent sul fer coal

[4]D.Coates, Advancedbattery systems for electric vehicleapplications,American Chem. Society Magazine, pp. 2.273-2.278.1993.[j] C. W. Setz, Industrial battery technologies and markets, EEEAES Syst. Mag., pp. 11-15, May 1994.[6] J . S. Lai et al., Highenergy density double layer capacitors forenergy storage applications,BEE AESMagazine, pp. 14-19, April1992.[7] . Meisel, A hybrid electric powertranfor meeting the super carmandate, Environmental Vehicle 95Cod. Rec.,pp. 133-147, an.1995.[8] C. A.A, How shal we power tomorrows automobile,Automotive Engine Alternative,pp. 1-35,(Ed.) R. L. Evans,Plenum Press,NY.[9] M. Valenti, Hybrid car promses high performance and lowemissions,Mechanical Engg., pp. 47-49, J uly 1994.[101 J. H. Hirschenhofer, Latest progressin fuel cel technology,JEEEAESSyst.Magazine,pp.18-27,Nov.1992.[l11 . Douglas, Solid utures nfuel cells,EPRI Joumds, pp.6-13,March 1994[121D.H. Swan and A. J . Appleby, Fuel cels and other longrangetechnology options for eectric vehicles knowledge gaps anddevelopment priorities, OECD Documents (The Urban ElectricVehicle), pp. 457-468, May 1992.[13] J.R.H aetal., Fuel cel intransportation, BatterySymposium,Long Beach, CA., 1987.[141G. Waker et al. Automotive applicationsof Stirhg engine,Automotive Engine Alternative, pp. 105-124, @d.)R. L. Evans.

MainCellMateriaisCarbons

Steels

Ceramics

Polymers

EmissionsdG JNO, So

1.1 0

I . I C oc

4.1 0

4.1 0

5.7 11.42.6 0

14.2 34.0

KdMWhCO*

450

5 10-460

385-335

410

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