An overview of energy sources for electric vehicles

An overview of energy sources for electric vehicles

Energy Conversion & Management 40 (1999) 1021±1039 An overview of energy sources for electric vehicles K.T. Chau*, Y.S. Wong, C.C. Chan Department of...

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Energy Conversion & Management 40 (1999) 1021±1039

An overview of energy sources for electric vehicles K.T. Chau*, Y.S. Wong, C.C. Chan Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Received 17 April 1998; accepted 21 December 1998

Abstract With ever increasing concerns on energy eciency, energy diversi®cation and environmental protection, electric vehicles (EVs) have launched a revenge for road transportation. Among those multidisciplinary EV technologies, energy sources are the key technology for possible commercialization and popularization of EVs. This paper not only reviews the current status of EV energy sources, but also assesses their suitability and potentiality. Moreover, the concept of multiple energy sources for EVs is identi®ed, hence the corresponding near term and long term measures are discussed. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Energy sources; Electric vehicles

1. Introduction What is an electric vehicle (EV)? The simplest answer is that the vehicle motion is propelled by an electric motor, rather than by a gasoline/Diesel internal combustion engine [1]. As shown in Fig. 1, a basic EV system consists of an energy source, a power converter, an electric motor and a mechanical transmission, in which the energy ¯ow can be forward and backward during motoring and braking, respectively [2]. How old is an EV? Truly, it is about 125 years old. The ®rst practical EV was built in Britain by Robert Davidson in 1873, whereas the ®rst gasoline powered vehicle did not appear until 1885, nearly 12 years later. The historical golden period of EVs was only about 10 years, between 1895 and 1905. With the drastic improvement in the internal combustion engine, gasoline-powered vehicles showed much better performance than EVs and received much * Corresponding author. Tel.: +852 2859 2704; Fax: +852 2559 8738. E-mail address: [email protected] (K.T. Chau) 0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 0 2 1 - 7

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Fig. 1. Basic EV system.

attraction. Thus, the use and development of EVs were almost absent from the 1930 s to the 1950 s. Why does an EV revive? The rekindling of interests in EVs started at the outbreak of the energy crisis and oil shortage in the 1970 s. Over a single year between 1973 and 1974, the price of a barrel of oil increased fourfold. The actual revival of EVs should be due to the ever increasing concerns on energy conservation and environmental protection throughout the world [3]. The advantage on energy conservation is due to the following reasons: . EVs o€er high energy eciency. In general, the overall energy conversion eciencies from crude oil to vehicle motion for EVs and gasoline powered vehicles are 12.5 and 9.3%, respectively. Moreover, EVs can perform ecient braking by converting the kinetic energy back to electricity. This regenerative braking can lengthen the driving range of EVs up to 15%. . EVs allow energy diversi®cation. Electricity can be generated not only from thermal power using oil and coal, but also from hydro power, wind power, geothermal power, wave/tidal power, solar power and nuclear power. Recently, the generation of electricity by on board fuel cells in EVs has been becoming practical and attractive. . EVs enable load equalization of the power system. By recharging EVs at night, these nonstockable energy at non-peak hours and, hence, the power generation facilities can be e€ectively utilized, contributing to energy saving and stabilization of power cost. On the other hand, the bene®t on environmental protection is due to the gain in air quality and the reduction in noise level: . EVs show zero local exhaust emissions. Even globally, the emissions due to the generation of

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electricity for EVs are only 2% in carbon monoxide, 76% in carbon dioxide, 56% in nitrogen oxides and 9% in hydrocarbons exhausted by gasoline powered vehicles. Increasingly, the emissions generated by power plants can be further minimized by dedicated ®ltering and even recycled, such as the use of carbon dioxide recycling for electricity generation. . EVs operate quietly and almost vibration-free, whereas gasoline powered vehicles are inherently noisy and with sensible vibration. Thus, EVs are welcomed by drivers and appreciated by local residents. Consequently, some governments have set aside emission-free zones and have enforced stricter emissions regulations, encouraging the promotion of EVs. In October 1990, the California Air Resources Board established rules that 2% of all vehicles sold in the state between 1998 and 2002 be emission-free, and 10% of vehicles put on the market must have zero emissions by 2003. In 1996, the Board decided to scrap the 2% mandate, since there is no way for the automobile industry to meet that timetable [4]. Which is the bottleneck technology of an EV? The bottleneck of the development of EV technologies is the energy sources, including both energy storage and energy generation systems. Among those EV energy sources, all of them have a common problemÐeither high speci®c energy or high speci®c power, but not both. The feature of high speci®c energy is favorable for long driving range, while the high speci®c power is desirable for high acceleration rate and hill climbing capability. It is the purpose of this paper to review the current status of EV energy sources and, hence, to assess their suitability and potentiality for EVs. Moreover, the concept of multiple energy sources, the so-called hybridization of energy sources, for EVs will be described. The possible near term and long term energy source hybrids will be discussed.

2. EV energy sources The viable energy sources being proposed for EVs include batteries, fuel cells, capacitors and ¯ywheels. Among them, the batteries, capacitors and ¯ywheels are energy storage systems in which electrical energy is stored during charging, whereas the fuel cells are energy generation systems in which electricity is generated by chemical reaction. At present and in the near future, the batteries have been identi®ed to be the major EV energy source because of their technological maturity and reasonable cost [5,6]. Recently, both the fuel cells and ultracapacitors have received substantial attention and have shown promising application to EVs in the mid term. In the long term, the use of ultrahigh-speed ¯ywheels as the energy source for EVs may be a possible solution. 2.1. Batteries Even excluding those primary batteries, there are numerous secondary (rechargeable) batteries [7]. Those viable secondary batteries for EVs, loosely called as EV batteries, consist of valve-regulated lead±acid (VRLA), nickel±iron (Ni±Fe), nickel±zinc (Ni±Zn), nickel±cadmium

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(Ni±Cd), nickel±metal hydride (Ni±MH), zinc/chlorine (Zn/Cl2), zinc/bromine (Zn/Br2), iron/ air (Fe/Air), aluminum/air (Al/Air), zinc/air (Zn/Air), sodium/sulfur (Na/S), sodium/nickel chloride (Na/NiCl2), lithium±aluminum/iron monosul®de (Li±Al/FeS), lithium±aluminum/iron disul®de (Li±Al/FeS2), lithium±polymer (Li±Po) and lithium±ion (Li±Ion) types. As shown in Fig. 2, these batteries are classi®ed into lead±acid, nickel-based, zinc/halogen, metal/air, sodium±beta, high-temperature lithium and ambient-temperature lithium categories. The United States Advanced Battery Consortium (USABC), jointly funded by the Department of Energy, Electric Power Research Institute, Detroit's Big Three and other companies engaged in battery research, has spent about US$200 million over the past several years to improve the EV battery technology [8]. Without setting a timetable, the USABC has set long term performance goals that a battery for EVs must meet in order to compete with gasoline powered vehicles in performance and costs [9]. In Table 1, some typical characteristics of those viable EV batteries, including speci®c energy, energy density, speci®c power, cycle life and projected cost, are given to compare with the USABC's goals. The lead-acid battery has been a successful commercial product for over a century. It uses metallic lead for the negative electrode, lead dioxide for the positive electrode and sulfuric acid

Fig. 2. Classi®cation of EV batteries.

Table 1 Typical characteristics of EV batteries, NA = Not applicable

a

Energy densitya (Wh/L)

Speci®c powerb (W/kg)

Cycle life (Cycles)

Projected cost (US$/kWh)

30±50 30±55 60±65 40±50 50±70 65 65±75 75 190 230 100 86 130 180 155 120±140 200

60±100 60±110 120±130 80±100 100±140 90 60±70 100 190 269 150 149 220 350 220 240±280 300

200±400 25±110 150±300 150±350 150±300 60 90±110 60 16 105 120 150 240 400 315 200±300 400

400±600 1200±4000 100±300 800±2000 800±2000 200 300 300±600 NAc NAc 800 1000 1000 1000 600 1200 1000

120±150 NA NA 300±350 150±200 NA 150 NA NA 100 250±500 160±300 NA NA 125 150±180 100

At 80% depth-of-discharge. At 3-h discharge rate. c Mechanical recharge.

b

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VRLA Ni±Fe Ni±Zn Ni±Cd Ni±MH Zn/Cl2 Zn/Br2 Fe/Air Al/Air Zn/Air Na/S Na/NiCl2 Li±Al/FeS Li±Al/FeS2 Li±Po Li±Ion USABC

Speci®c energya (Wh/kg)

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solution for the electrolyte. The corresponding electrochemical reaction is Pb + PbO2 + 2H2SO4 t 2PbSO4 + 2H2O. Recently, the valve-regulated type, the so-called VRLA, has been widely accepted for EVs because it o€ers the advantages of mature technology, high speci®c power (over 200 W/kg), low initial cost (presently US$200±400/kWh), rapid recharge capability and maintenance-free operation. However, it su€ers from very low speci®c energy (about 40 Wh/kg) and short cycle life (about 500 cycles). Up to now, the VRLA battery is still the most popular energy source for EV application, especially those commercially available EVs. Besides the lead±acid, the nickel-based batteries, including Ni±Fe, Ni±Zn, Ni±Cd and Ni± MH, are becoming mature. Among them, the Ni±Fe and Ni±Zn batteries are relatively unattractive because the Ni±Fe su€ers from very low speci®c power (25±110 W/kg) while the Ni±Zn has very short cycle life (100±300 cycles). On the other hand, both the Ni±Cd and Ni± MH batteries have been accepted by EVs. The Ni±Cd battery is the most mature technology among all nickel-based batteries. Its active materials are metallic cadmium for the negative electrode, nickel oxyhydroxide for the positive electrode and potassium hydroxide solution for the electrolyte. The corresponding electrochemical reaction is described as Cd + 2NiOOH + 2H2) t Cd(OH)2 + 2Ni(OH)2. It shows the advantages of high speci®c power (150±350 W/kg), very long cycle life (800±2000 cycles), rapid recharge capability and maintenance-free operation. Apart from the disadvantages of low speci®c energy (about 45 Wh/kg) and high initial cost (presently US$600± 800/kWh), the Ni±Cd su€ers from a severe drawback, namely the carcinogenicity and environmental hazard of cadmium. Before the advent of the Ni±MH battery, the Ni±Cd used to be widely accepted to power EVs. The Ni±MH battery is being considered to be the near term battery of choice for EVs. Its active materials are metal hydride for the negative electrode, nickel oxyhydroxide for the positive electrode and potassium hydroxide solution for the electrolyte. The metal hydride is generally of a rare-earth alloy based on lanthanum nickel, known as an AB5 alloy, or a vanadium±titanium±zirconium±nickel based alloy, known as an AB2 alloy. The corresponding electrochemical reaction is described as MH + NiOOH t M + Ni(OH)2. It is so attractive because it o€ers the highest speci®c energy among all nickel-based batteries (about 60 Wh/kg), high speci®c power (150±300 W/kg), very long cycle life (800±2000 cycles), environmental friendliness, rapid recharge capability and maintenance-free operation. At present, the key drawback is its very high initial cost (about twice that of Ni±Cd). Due to its attractive characteristics and environmental friendliness, the Ni±MH is going to supersede the Ni±Cd for EV application. Since this battery can potentially reduce to about US$175/kWh on mass production, it is rapidly accepted by new EVs. The zinc/halogen batteries, consisting of Zn/Cl2 and Zn/Br2, have ever been identi®ed as viable EV energy sources. The Zn/Cl2 battery technology was the subject of active development on both EV and stationary energy storage applications during the 1970 s. However, the requirements of considerable plumbing to operate the battery and frequent maintenance of the electrolyte have diverted its recent development focusing on power utility batteries. The Zn/Br2 battery has ever been attractive for EV application because of its high speci®c energy (about 70 Wh/kg), low material cost and rapid recharge capability. With the drawbacks of very low speci®c power (about 100 W/kg), high reactivity of bromine as well as bulky auxiliary systems

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for electrolyte circulation and temperature control, the recent development of the Zn/Br2 battery for EVs has been signi®cantly slowed. Because of the inexhaustible air, the development of the metal/air batteries, including Fe/Air, Al/Air and Zn/Air, have received a signi®cant amount of e€ort. Among them, the Zn/Air battery is preferred to both the Fe/Air and Al/Air and is being commercialized. Although the Al/Air battery can o€er very high speci®c energy (about 190 Wh/kg), its exceptionally low speci®c power (about 16 W/kg) has greatly obstructed its direct application to EVs. Compared with the Zn/Air, the Fe/Air battery o€ers much lower speci®c energy and speci®c power (only about 75 Wh/kg and 60 W/kg), and is no longer being actively developed. The Zn/Air battery is receiving great attraction for EV application. It uses metallic zinc for the negative electrode, air for the positive electrode and potassium hydroxide solution for the electrolyte. The corresponding electrochemical reaction can simply be described as 2Zn + O2 t 2ZnO. It can be designed to be either electrically rechargeable or mechanically rechargeable (automatic refueling of zinc). The mechanically rechargeable one is particularly attractive for EVs because it o€ers very high speci®c energy (about 230 Wh/kg), fast refueling (comparable to gasoline powered vehicles) and low material cost. On the other hand, it has the disadvantages of relatively low speci®c power (about 105 W/kg) and incapability of accepting regenerative braking energy. This Zn/Air battery and the associated refueling system are being actively evaluated by a ¯eet of EVs for the German Postal Service. Because of the common features that molten sodium as one reactant and solid beta0± alumina ceramic as the electrolyte, the Na/S and Na/NiCl2 batteries are classi®ed as the sodium±beta technology. Although the Na/S has been developed for over 25 years, it still su€ers from some problems, such as the highly corrosive products of reaction 2Na + xS t Na2Sx (x = 5 . . . 2.7) and the requirement of thermal management to maintain the operating temperature at 300±3508C, which have confronted its application to EVs. In contrast to the Na/S, the Na/NiCl2 can potentially o€er easier solutions to these confrontations. In the Na/NiCl2 battery, the active materials are molten sodium for the negative electrode, solid nickel chloride for the positive electrode, solid beta0±alumina ceramic for the primary electrolyte and molten sodium aluminum chloride for the secondary electrolyte. The secondary electrolyte functions to conduct sodium ions from the primary electrolyte to the positive electrode. At the operating temperature of around 3008C, the electrochemical reaction is 2Na + NiCl2 t Ni + 2NaCl. This battery takes advantages over the Na/S on wider operating temperatures (250±3508C), safer products of reaction (less corrosive than molten Na2Sx) and more reliable failure mode (any violent distortion will ®rst break the primary electrolyte so that any molten sodium can rapidly react with the secondary electrolyte to form common salt and aluminum). Nevertheless, the Na/NiCl2 battery still su€ers from relatively low speci®c power (about 150 W/kg) and the inevitable need of thermal management. At present, it is the most attractive high temperature battery ready for EV application. Besides the sodium±beta batteries, another high temperature battery technology being developed for EVs is the high temperature lithium batteries. By adopting a lithium±aluminum alloy to control the lithium activity and iron sul®de to relieve the corrosiveness associated with the use of sulfur, the Li±Al/FeS and Li±Al/FeS2 batteries operating at the temperature range of 375±5008C have been most interesting. Based on the lithium±aluminum negative electrode, iron sul®de positive electrode and molten lithium chloride±potassium chloride electrolyte, their

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electrochemical reactions can be described by 2Li±AlFeS t 2Al + Fe + Li2S and 2Li± Al + FeS2 t 2Al + Li2FeS2. Although they have shown promising advantages on both speci®c energy and speci®c power (about 130±180 Wh/kg and 240±400 W/kg), these high-temperature batteries still su€er from the need of thermal management and consumption of stored energy to maintain the operating temperature. At present, they are still being investigated in laboratories and have not yet been applied to EVs. Instead of operating at high temperatures, some lithium batteries can be operated at or near ambient temperature. These ambient temperature lithium batteries consist of two main types, the Li±Po and Li±Ion, which are being actively developed for EVs. The key di€erence between them is that the Li±Po uses metallic lithium as one reactant, whereas the Li±Ion contains no lithium metal in the cell. The Li±Po battery is getting a big boost from the USABC's award of US$60 million to bring it from concept to prototype. It uses metallic lithium for the negative electrode, vanadium oxide (V2O5 or V6O13) for the positive electrode and solid polymer (polyethylene oxide with a lithium salt) for the electrolyte. The operating temperature is normally maintained at 60±808C so as to achieve fast ion migration in the polymer electrolyte. The corresponding electrochemical reaction can be described as xLi + VyOz t LixVyOz. The Li±Po o€ers advantages on both speci®c energy and speci®c power (about 155 Wh/kg and 315 W/kg) but su€ers from short cycle life (about 600 cycles) and poor low temperature performance. It is on the verge of being applied to EVs for evaluation. The Li±Ion battery has been viewed to be the battery that might hit the USABC's long-term goals for EVs. It uses lithiated carbon for the negative electrode, lithiated transition metal oxide (Li1ÿxCoO2, Li1ÿxNiO2 or Li1ÿxMn2O4) for the positive electrode and liquid organic solution for the electrolyte. Lithium ions are deintercalated (extracted) from the negative electrode and intercalated (inserted) to the positive electrode on discharge and vice versa on charge. Equivalently, these ions are swinging through the electrolyte between the negative and positive electrodes, and no metallic lithium will be deposited. The corresponding electrochemical reaction can be described as LixC + Li1ÿxMyOz t C + LiMyOz. It exhibits both high speci®c energy and speci®c power (about 130 Wh/kg and 250 W/kg) as well as long cycle life (about 1200 cycles). At present, the key drawback is its extremely high initial cost (up to 7 times as high as Ni±Cd), though the projected value (about US$165/kWh) is reasonable. Anyway, it has recently been applied to a new EV for evaluation. Table 2 Possible EV battery suppliers and recent applications to EVs

VRLA Ni±MH Zn/Air Na/NiCl2 Li±Ion

Possible suppliers

Recent applications

GS, Horizon, Panasonic, Sonnenschein, YUASA GP, GS, Ovonic, Panasonic, SAFT, Varta YUASA Electric Fuel Zebra GS, SAFT, Sony, Varta

Chrysler Voyager, Daihatsu Hijet, Ford Ranger, GM EV1, Mazda Bongo Friendee, Suzuki Alto Honda EV Plus, Mazda Demio, Peugeot 106, Solectria Force, Toyota RAV4L GM-Opel Corsa Combo, Mercedes-Benz MB410 BMW AG, Mercedes-Benz Vito Nissan Prairie Joy

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Having reviewed those viable EV batteries, it can be found that none of them can fully satisfy the USABC's long term goals which aim to enable EVs competing with gasoline powered vehicles. Nevertheless, in order to meet the mandate of 10% zero-emission vehicles by 2003, the development of EV batteries is being focused on several types, namely the VRLA, Ni±MH, Zn/Air, Na/NiCl2 and Li±Ion. Table 2 summarizes some suppliers of these batteries and their recent application to EVs. 2.2. Fuel cells In 1839, William Grove discovered that electrolysis can work in reverseÐcombining hydrogen and oxygen can produce both water and electricity. This discovery was nearly forgotten until the 1960 s when fuel cells were developed to provide the power source for Gemini and Apollo Missions. In contrast to batteries, the fuel cells generate electrical energy as long as the fuel supply can be maintained, thus virtually having no cycle-life limitation. In terms of fuel eciency, the fuel cells can generally o€er 40±50%, almost twice that in today's gasoline engines. At the present status of fuel cell technology, they are generally classi®ed into six types, namely the alkaline fuel cell (AFC), the phosphoric acid fuel cell (PAFC), the molten carbonate fuel cell (MCFC), the solid oxide fuel cell (SOFC), the solid polymer fuel cell (SPFC) and the direct methanol fuel cell (DMFC) [10]. Except the DMFC in which liquid methanol is directly used as the fuel, the other ®ve fuel cells are generally fed by hydrogen. Typical characteristics of these fuel cells are summarized in Table 3. Accordingly, both the MCFC and SOFC su€er from very high temperature operation, respectively over 600 and 9008C, making them practically dicult to be applied to EVs. Among the others, the PAFC and AFC are relatively unattractive, because their theoretical power densities (the product of cell potential and current density) are much lower than that of the SPFC. Therefore, recent research and development on fuel cells for EVs have been focused on the SPFC technology. The SPFC, recently named as the proton exchange membrane (PEM) fuel cell by some developers, uses a solid polymer membrane as the electrolyte. As shown in Fig. 3, the membrane is sandwiched between two platinum-catalyzed porous electrodes. The overall electrochemical reaction is simply 2H2 + O2 4 2H2O in which the generation of electricity can be described by both H2 4 2H+ + 2e ÿ and O2 + 4H+ + 4e ÿ 4 2H2O that occur in the anode and cathode, respectively. The SPFC has the advantages of the highest power density among Table 3 Typical characteristics of fuel cells

Fuel eciency (%) Power density (kW/m2) Working temperature (8C) Projected cost (US$/kW) Lifetime (kh)

PAFC

AFC

MCFC

SOFC

SPFC

DMFC

40±45 2±2.5 180±210 1000 >40

40±50 2±3 60±80 >200 >10

45±50 1±2 600±700 1000 >40

45±50 2.4±3 900±1000 1500 >40

45±50 3.5±6 50±100 >200 >40

30±40 1.5±3.2 <100 >200 >10

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Fig. 3. Schematic of SPFC.

all fuel cells, solid electrolyte with no corrosive liquid in the cell and insensitivity to carbon dioxide in the oxidant. At present, the major challenge is how to reduce signi®cantly the material cost of the solid polymer membrane and platinum-catalyzed electrodes. The corresponding cost of about US$2700±5400 per kilowatt is extremely expensive compared to the internal combustion engine cost of about US$11±27 per kilowatt [11]. Of course, the best way to reduce considerably this cost is to go into mass production. In order to introduce the SPFC into EVs, the key issue is on the supply or storage of hydrogen. Since hydrogen is not a primary fuel, it is generally derived from other primary fuels such as oil, coal and natural gas by means of fuel reformers. The viable methods of storing hydrogen gas are generally based on the following three ways: . It is stored as a compressed gas, so-called compressed hydrogen gas (CHG). Similar to compressed natural gas, the CHG can be stored at 20±34.5 MPa (3000±5000 psig) in ®breglass-reinforced aluminum containers. . It is chilled below its boiling point (ÿ2538C) to form liquid hydrogen, which is then stored in cryogenic containers. . It is brought to react with some metals such as magnesium and vanadium to form metal hydrides. The reaction is reversible, depending on the temperature of dissolution (up to about 3008C).

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Table 4 shows the theoretical energy content of some prominent fuels, including hydrogen stored in various forms, liquid methanol and liquid gasoline [12]. The CHG storage can o€er the advantages of light weight, low cost, mature technology and good fast refueling capability but su€er from bulky size and safety concerns. The liquid hydrogen o€ers both high speci®c energy and good refueling capability but has the drawbacks of expensive production and distribution costs as well as high volatility. Although the metal hydrides can provide the merits of compact size and inherent safety, they su€er either too high temperature of dissociation, such as magnesium hydride (2878C), or relatively low speci®c energy, such as vanadium hydride (700 Wh/kg), leading to be impractical for EV application. Starting from 1993, a Canadian ®rm, namely Ballard Power Systems, has pioneered the development of CHG-fueled SPFC technology for EV application. Instead of using hydrogen as the fuel, the DMFC possesses a de®nite advantage that the methanol is the simplest organic liquid fuel which can economically and eciently be produced on a large scale from the relatively abundant fossil fuels, such as coal and natural gas. Increasingly, liquid methanol can readily be distributed and marketed via the existing infrastructure used for gasoline. Similar to the SPFC, the DMFC uses a solid polymer membrane as the electrolyte which is sandwiched between two platinum-catalyzed electrodes [13]. The principle of operation can be described as Ch3OH + H2 4 CO2 + 6H+ + 6e ÿ at the at the cathode, resulting in anode and O2 + 4H+ + 4e ÿ 4 2H2O 2CH3OH + 3O2 4 2CO2 + 4H2O. Di€erent from the SPFC, the DMFC produces carbon dioxide gas, thus not exactly zero emissions. Although this DMFC has been developed for over 30 years, it still needs to overcome a number of challenges before practically applying it to EVs, especially its low power density and high cost of catalysts. By retaining the de®nite advantage of liquid fuel while avoiding those shortcomings of the DMFC, the concept of methanol-fueled SPFC EVs is becoming more and more attractive [14,15]. As shown in Fig. 4, methanol and water are mixed, vaporized and then converted into hydrogen and carbon dioxide gases via an on-board reformer. The resulting hydrogen gas is fed to the SPFC to generate the desired electricity and reusable pure water. The puri®er functions to prevent any undesirable reformer byproducts, such as carbon monoxide gas, from poisoning the precious catalysts of the SPFC. Although this technology seems to be Table 4 Theoretical energy contents of prominent fuels

Compressed hydrogen gasa Liquid hydrogenb Magnesium hydride Vanadium hydride Methanol Gasoline a b

At ambient temperature and 20 MPa. At cryogenic temperature and 0.1 MPa.

Speci®c energy (Wh/kg)

Energy density (Wh/L)

33 600 33 600 2400 700 5700 12 400

600 2400 2100 4500 4500 9100

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Fig. 4. Block diagram of methanol-fueled SPFC system.

contradictory to the pursuit of zero-emission vehicles, it is still environmentally friendly as it does not generate carbon monoxide, nitrogen oxides or hydrocarbons. The ®rst methanol fueled SPFC EV has just been presented, namely the Daimler-Benz Necar 3, which can travel over 400 km on 38 L of methanol. Toyota has also announced that its fuel cell RAV4 EV can achieve 500 km per tank of methanol. Even so, these methanol fueled SPFC EVs are far from economically viable within the next 5 years. Further extending the concept of liquid fueled fuel cell EVs, research on extracting hydrogen from gasoline using an on-board reformer has been launched [16]. The argument of this research is simpleÐhundreds of billion dollars have been invested in the way gasoline is distributed, and it is impossible to change this infrastructure just because there are fuel cell EVs that run on hydrogen and methanol. No matter whether this argument is agreeable or not, the success of this concept can de®nitely move fuel cell EVs closer to reality. Although Chrysler has decided to realize this concept by demonstrating a gasoline fueled fuel cell EV within the next two years, there is still a long way to go. In the foreseeable future, it is hardly possible to expect those battery powered EVs attaining the per-charge driving range similar to that of gasoline powered vehicles. As indicated in Table 4, the fuel cell EVs can potentially provide such an opportunity. At the present status of technology, the CHG fueled SPFC system is relatively mature. Including the container and auxiliaries, it can achieve the system speci®c energy of about 500 Wh/kg which is much higher than the maximum value o€ered by any batteries. However, the corresponding system speci®c power is only about 60 W/kg, which limits its application to those EVs desiring high acceleration rate and hill climbing capability. It should also be noted that the fuel cell itself can not allow for accepting electrical energy regenerated by EVs during braking or downhill. 2.3. Ultracapacitors Since capacitors have inherently high speci®c power and long cycle life for rapid and deep discharges, the development of ultracapacitors has recently shown promising application to EVs. Starting from 1991, the US Department of Energy has established a program to develop ultracapacitors. Some long term goals for the development of ultracapacitors have been set,

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namely speci®c energy of 15 Wh/kg, energy density of 37 Wh/L, speci®c power of 1600 W/kg, cycle life of 100 000 cycles and cost of US$1/Wh [17]. The double layer capacitor technology was ®rst described by Hermann von Helmholtz, and has a high potential to attain the ultracapacitor goals. As shown in Fig. 5, when a voltage is applied across the electrodes, the charge carriers of the electrolyte solution adsorb at the entire surface of the electrodes and form the dielectric of the double-layer capacitor [18]. This polarization is used to store energy E = 1/2CV 2 in the capacitor with capacitance C = eA/d, where e is the e€ective dielectric constant, d the separation distance, A the electrode surface area and V the applied voltage. By minimizing d to a few angstroms and maximizing A to 2000 m2/g, the speci®c capacitance can be brought up to 500 F/g. In order to provide this large A with a small geometrical surface, the electrode material is generally based on activated carbon, structured polymers or noble-metal oxides. Although the electrolyte solution can be either aqueous or organic, the organic one is usually preferable for EV application because it can o€er both higher speci®c energy and cell voltage. At present, there are a number of ®rms actively engaged in the development of ultracapacitors, such as Maxwell Energy Products, Panasonic, Powercell, SAFT and Federal Fabrics-Fibers. The Maxwell ultracapacitor o€ers 2300 F at 2.3 V with 0.63 kg and 0.462 L, achieving speci®c energy and speci®c power of about 5 and 1000 W/kg, respectively. The Panasonic ultracapacitor can also achieve 1800 F at 2.3 V. On the other hand, by internally stacking many ultracapacitors, Powercell has o€ered a single unit of 135 V, 47 Wh, 4 kW (peak), 8 kg and 6.5 L.

Fig. 5. Schematic of double-layer ultracapacitor.

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In the foreseeable future, the ultracapacitor can not be used as the sole energy source for EVs, simply because its speci®c energy (about 5 Wh/kg) is too low for such application. Nevertheless, it can o€er de®nite edges in speci®c power (about 1000 W/kg) and cycle life (over 300 000 cycles) as well as instantaneous and deep charge/discharge capabilities. Provided that the average energy necessary for EV application can be supplied by other energy sources, the use of ultracapacitors is very attractive to give sudden energy for acceleration and hill climbing as well as to accept instantaneous regenerative energy during braking and downhill. Very recently, an ultracapacitor unit, consisting of 40  2 ultracapacitors (Panasonic, 2.3 V, 1800 F), has been installed in the Mazda Bongo Friendee for experimentation. 2.4. Ultrahigh-speed ¯ywheels The use of ¯ywheels for storing energy in mechanical form is not a new concept. The traditional ¯ywheel is a massive steel rotor with hundreds of kilograms that spins at hundreds of radians per second. On the contrary, the advanced ¯ywheel is a light weight composite rotor with tens of kilograms and rotates at thousands of radians per second, the so-called ultrahighspeed ¯ywheel. Since the mechanical energy E = 1/2Jo2 (where J is the moment of inertia and o is the rotational speed) depends on the square of speed but only linearly with the inertia, there is a signi®cant gain in energy storage even when the increase in speed is o€set by the decrease in inertia with the same ratio. For example, a ¯ywheel of 50 kg running at 5000 rad/s can store 10 times that of 500 kg at 500 rad/s. Although the higher the rotational speed the more the energy that can be stored, there is a limit in which the tensile strength of the material constituting the ¯ywheel can not withstand the stress resulting from the centrifugal force. As the maximum stress acting on the ¯ywheel depends on its geometry, speci®c density and rotational speed, the maximum bene®t can be obtained by adopting the ¯ywheel material having the maximum ratio of tensile strength to speci®c density [19]. Table 5 summarizes these characteristics of some composite materials for ultrahigh-speed ¯ywheels. Moreover, a constant-stress principle can be adopted to maximize the energy storage further. Based on this principle, every element in the ¯ywheel should be equally stressed to its maximum limit, resulting in a shape of gradually decreasing thickness that theoretically approaches zero as the radius approaches in®nity. To charge and discharge the ¯ywheel, which is directly coupled to the machine rotor, the permanent magnet (PM) brushless machine has been accepted to be the most appropriate type. Apart from possessing high power density and high eciency, this machine has a unique Table 5 Composite materials for ultrahigh-speed ¯ywheels

E-glass Graphite epoxy S-glass Kevlar epoxy

Tensile strength s (MPa)

Speci®c density r (kg/m3)

Ratio s/r (Wh/kg)

1379 1586 2069 1930

1900 1500 1900 1400

202 294 303 383

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feature that no heat is generated inside the PM rotor. It should be noted that this feature is particularly advantageous for the rotor to work in a vacuum to minimize the associated windage losses. Apart from encasing the rotor in a vacuum shell, magnetic bearings are employed to further minimize the rotational friction losses. These magnetic bearings have the added attraction of extending the ¯ywheel life because there are no mechanical contacts and, hence, no wear-and-tear problems. Fig. 6 shows the con®guration of a typical ultrahigh-speed ¯ywheel in which composite material, constant stress principle, PM brushless machine and magnetic bearings have been employed. On the basis of present technologies, the whole ultrahigh-speed ¯ywheel system should be able to achieve the speci®c energy of about 14 Wh/kg and speci®c power of about 800 W/kg in the near term and about 30 Wh/kg and 900 W/kg in the mid term. Recently, a prototype has been built which can achieve 35000 rpm, 800 Wh and 50 kW [20]. The current activities of these ¯ywheel systems are focused on the overall optimization of weight, size and cost, as well as experimental demonstrations. Based on the development progress of this technology, it has a long way to go for practical application to EVs. 3. Hybridization Based on the aforementioned facts, none of the available energy sources, including batteries, fuel cells, ultracapacitors and ultrahigh-speed ¯ywheels, can ful®l all the demands of EVs to enable them to compete with gasoline powered vehicles. In essence, these energy sources can not provide high speci®c energy and high speci®c power simultaneously. Instead of limiting use to a sole energy source, EVs can adopt the concept of multiple energy sources, the so-called hybridization of energy sources. Since both the control and packaging complexities of this concept increase with the number of energy sources involved, only the

Fig. 6. Schematic of ultrahigh-speed ¯ywheel.

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hybridization of two sources (one high speci®c energy, the other high speci®c power) is considered to be viable. The advantages resulting from the use of this hybrid energy system for EVs are summarized below: . Since the EV requirements on energy and power can be decoupled, it a€ords an opportunity to design EV sources, such as batteries and fuel cells for high speci®c energy and to optimize other sources, such as ultracapacitors and ultrahigh-speed ¯ywheels, for high speci®c power. . Since there is no need to carry out trade-o€s between the pursuits of speci®c energy and speci®c power, the cycle life and production cost of these sources can readily be lengthened and minimized, respectively. . The unique advantages of various EV energy sources can be fully utilized, such as the technical maturity and reasonable cost of batteries, the outstanding speci®c energy and fuel

Fig. 7. Operation of hybrid energy system.

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eciency of fuel cells, the enormous speci®c power and instantaneous charge/discharge capability of ultracapacitors, as well as the outstanding speci®c power and practically unlimited cycle life of ultrahigh-speed ¯ywheels. The principle of operation of this hybrid energy system, consisting of both high speci®c energy and high speci®c power sources, is illustrated in Fig. 7. Firstly, during the normal driving condition of EVs, the high speci®c energy source supplies the necessary energy to the electric motor via the power converter. To enable the system to be ready for sudden power demand, this source can pre-charge the high speci®c power source in the light load period. Secondly, during the acceleration or hill climbing condition, both the sources need simultaneously to supply the desired energy to the electric motor. Thirdly, during the braking or downhill condition, the electric motor operates as a generator so that the regenerative energy ¯ows back to recharge the high speci®c power source via the power converter. If this source can not fully accept the regenerative energy, the surplus can be diverted to recharge the high speci®c energy source, provided that it is energy receptive.

3.1. Near-term hybrids Based on the available technology of various energy sources, there are several viable hybridization schemes for EVs in the near term: . battery and battery hybrids; . battery and ultracapacitor hybrids; . fuel cell and battery hybrids. In the battery and battery hybrids, one battery provides high speci®c energy while another battery o€ers high speci®c power. Taking into account the maturity and cost, the Zn/Air and VRLA hybrid seems to be a natural choice. It combines the merits of the 230 Wh/kg of Zn/Air for long driving range and the 300 W/kg of VRLA for acceleration and hill climbing. This choice also overcomes the incapability of the mechanically rechargeable Zn/Air which can not accept the precious regenerative energy during braking or downhill. Other possible choices can be the Zn/Air and Ni±MH hybrid or the Zn/Air and Li±Ion hybrid. Since the ultracapacitor can o€er the speci®c energy far below the minimum requirement for EV application, it has to work together with other EV energy sources. Typically, it collaborates with various batteries to form the battery and ultracapacitor hybrids. During the hybridization, an additional two quadrant d.c.±d.c. converter is usually placed between the battery source and the ultracapacitor source because the working voltage of the ultracapacitor source is quite low (generally less than 100 V) even when many ultracapacitors have already been internally stacked. Recently, the VRLA and ultracapacitor hybrid has already received attention such that the average energy consumption is supplied by the VRLA, while the peak power demand is by the ultracapacitor source. The ultracapacitor source is recharged during regenerative braking or from the VRLA at periods of low power demand. Other viable combinations are the Ni±MH and ultracapacitor hybrid and the Li±Ion and ultracapacitor hybrid.

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Although the fuel cell can o€er outstanding speci®c energy, it su€ers from low speci®c power and incapability of accepting regenerative energy. Thus, the fuel cell and battery hybrid is a good collaboration in which the battery can be purposely selected to compensate those shortcomings of the fuel cell. The SPFC and VRLA, SPFC and Ni±MH and SPFC and Li±Ion hybrids are typical choices, since these three batteries are of high speci®c power and rapid recharge capability. 3.2. Long-term hybrids In the long term, the ultracapacitor should be improved to such a level that its speci®c energy is suciently high to provide all necessary instantaneous energy for acceleration and hill climbing, as well as to accept all regenerative energy during braking and downhill. Similarly, the ultrahigh-speed ¯ywheel should also be able to attain such speci®c energy in the long term. Consequently, they can possibly replace batteries to hybridize with the fuel cell to form the following hybrids: . fuel cell and ultracapacitor hybrids; . fuel cell and ultrahigh-speed ¯ywheel hybrids.

4. Conclusions In this paper, all prominent energy sources for EVs have been reviewed. Their suitability and potentiality for EV application have also been assessed. Since none of them can ful®ll all demands for EVs to compete with gasoline powered vehicles, the concept of multiple energy sources is proposed. Both near term and long term hybridization schemes are also discussed. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Chan CC. IEEE Proceedings 1993;81:1201. Chan CC, Chau KT. IEEE Transactions on Industrial Electronics 1997;44:3. Electrical Progress Enables Environment-Friendly Life. Kansai Electric Power, 1996. Toyoda S. In: Proceedings of the 13th International Electric Vehicle Symposium. 1996. Hoolboom GJ, Szabados B. IEEE Transactions on Vehicular Technology 1994;43:1136. Batteries for electric vehicles. Electric Power Research Institute, March/April 1996. Linden D. Handbook of batteries, 2nd ed. New York: McGraw-Hill, 1995. Brown SF. Fortune. October 1997. Proceedings of Annual Automotive Technology Development Contractors' Coordination Meeting. Society of Automotive Engineers, November 1992. Blomen LJMJ, Mugerwa MN. Fuel Cell Systems. New York: Plenum Press, 1993. Ebner J. UIP Electric and Hybrid Vehicle Technology '97. 1997. p. 42. McAuli€e CA. Hydrogen and energy. New York: Macmillan Press, 1980. Halpert G. UIP Electric and Hybrid Vehicle Technology '97. 1997. p. 58.

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[14] Krauss R, zur Megede D, Panik F. In: Proceedings of the 14th International Electric Vehicle Symposium. December 1997. [15] Nonobe Y, Kimura Y, Ogino S. In: Proceedings of 14th International Electric Vehicle Symposium. December 1997. [16] Robertson B. UIP Electric and Hybrid Vehicle Technology '97. 1997. p. 30. [17] Burke AF. In: Proceedings of the 12th International Electric Vehicle Symposium. December 1994. [18] Schmid M. In: Proceedings of the 13th International Electric Vehicle Symposium. October 1996. [19] Anerdi G, Brusaglino G, Ancarani A, Bianchi R, Quaglia G, Barberis U, Ravera C, Mellor PH, Howe D, Zegers P. In: Proceedings of the 12th International Electric Vehicle Symposium. December 1994. [20] Grudkowski TW, Polley EC. UIP Electric and Hybrid Vehicle Technology '95. 1995. p. 138.