Battery-operated Electric Vehicles (BEV)
From Thermal-FluidsPedia
Lead-acid batteries are the primary power source for many of today’s plug-in electric vehicles. More advanced nickel batteries (such as nickel-iron, nickel-cadmium, and nickel-metal hydride) offer better ranges, shorter recharge times, and longer lifetimes, but certain safety concerns and higher costs have precluded their widespread use. Cadmium is very toxic, while nickel-iron batteries tend to produce a buildup of hydrogen during charging. The battery with the greatest potential is the nickel-metal hydride (Ni-MH), which is nontoxic, has about four times the range of lead-acid batteries, and can be recharged in a few minutes. These batteries are currently used in the Honda EV Plus and Toyota RAV4-EV. A close competitor to the Ni-MH battery is the lithium-metal hydrite (Li-MH) battery. These batteries have a very high specific energy but must be operated at temperature of about 70o C. Li-MH batteries are currently being tested in the Nissan Altra.
Currently, the relatively low price of gasoline probably remains the most important impediment to wide acceptance of electric cars in the United States. As the price of gasoline rises, it is expected that electric cars will claim a larger fraction of the total vehicle market. Out of 230 million cars operating in the United States, about 40% are used as a second car, and 87% of automobile trips are less than 50 kilometers. Electric vehicles can satisfy some of these needs as around-town cars. Furthermore, EVs may have utility as fleet vehicles in inner city short route buses, delivery vehicles, airports, and mail delivery services.
Contents |
Advantages and Disadvantages of Electric Vehicles
The major advantages of electric over conventional vehicles are:
1. Better torque characteristics. Electric motors are simpler and have torque characteristics that match those demanded by vehicles more closely (high torques at low speeds and acceleration, relatively low torques during cruising). Internal combustion engines, on the other hand, deliver maximum torque at an optimum cruising speed (Figure 1). Because little or no torque is delivered during start ups and at lower speeds, internal combustion engines need starter motors. Electric motors produce the highest torque at zero speed when it is most needed. An electric motor can therefore be attached directly to the drive wheels and can accelerate the vehicle from rest to the desired speed without the need for a transmission or torque converter. EVs’ power trains are simpler and don’t usually need more than one or two gear ratios. Reverse gears are also absent because their function can be achieved simply by reversing the polarity of the electrical input.
2. Regenerative Braking. Electric motors can run as a generator by running in reverse. Electric vehicles take advantage of this feature by employing regenerative braking, where up to 50% of the kinetic energy of the vehicle can be reclaimed during urban stop-and-go traffic to recharge the battery. The experimental data shows that, depending on design and the driving cycle, regenerative braking extends driving range between 5 to 15 percent.
3. Lower noise and emission. Electric vehicles are much quieter during operation and do not consume any power or produce any emission when stopped. This is not true with gasoline cars, which continue to consume a substantial amount of fuel and produce pollution even when they idle. Some emissions, such as hydrocarbons, are actually higher during idling than when cruising at optimal speeds.
4. Superior efficiencies. Electric vehicles have efficiencies in the range of 40-45% compared to efficiencies of 18-25% common for most conventional vehicles (a). However, when losses associated with the generation of electricity and transport are considered, depending on how the electricity is generated, EV efficiency advantages are only 10-30%.
The major disadvantages of electric vehicles over conventional vehicles are:
1. Batteries have a relatively small capacity to store energy. Compared to gasoline, which has an energy density of 12,000 watt-hours per kilogram, lead-acid batteries have only 40 watt-hours per kilogram. Lower battery storage capability results in limitations on the distance electric vehicles can travel before they must be recharged. More batteries add to vehicle weight which indirectly limits performance. With current technology, EVs that are equipped with lead-acid batteries offer a range of 60-80 kilometers and take six to eight hours to charge. Ni-MH batteries offer a range of 400 kilometers and can be recharged faster. The EVs available in the market are, on average, 300 to 1000 kilograms heavier than similar conventional vehicles.
2. Infrastructure does not exist. There is no network of recharging stations. Charging must be accomplished in much faster times that are currently possible and in all types of weather conditions.
3. Electric vehicles cost more. Cost of the ownership (initial purchase price plus costs associated with maintenance and repair) are higher for electric vehicles.
References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) It should be noted that the efficiencies quoted here are first law efficiencies, the fraction of the input energy (electricity for EV and petrol for ICE) that is used to drive the vehicle.
Further Reading
Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004.
Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005.
Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001.
Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003.
Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002.
Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004.
Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science.
Transportation Research Part C: Emerging Technologies, Direct Science Elsevier Publishing Company; this journal focuses on scholarly research on development, application, and implications in the fields of transportation, control systems, and telecommunications, among others.
Fuel Cells Bulletin, Direct Science Elsevier Publishing Company, Fuel Cells Bulletin is the leading source of technical and business news for the fuel cells sector.
International Journal of Hydrogen Energy, Direct Science Elsevier Publishing Company, Quarterly journal covering various aspects of hydrogen energy, including production, storage, transmission, and utilization, as well as economical and environmental aspects.
External Links
US Department of Transportation (http://www.dot.gov).
US Department of Energy (http://www.doe.gov).
US Environmental Protection Agency (http://www.epa.gov).
National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (http://www.nrel.gov/vehiclesandfuels/hev).
FreedomCar (http://www.eere.energy.gov/vehiclesandfuels).