Electric Vehicles

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Electric Vehicles

Batteries

Advantages

Hybrids

Challenges still exist

Resources

Electric vehicles (EVs, also called battery electric vehicles or BEVs) are vehicles whose wheels are turned by electric motors rather than by a gasoline-powered drivetrain. EVs have been long touted as saviors of the environment due to their apparently zero emissions of pollutants, but have been also been criticized as limited in range and of less environmental benefit than claimed.

The electric vehicle is at least as old as the internalcombustion vehicle. Thomas Davenport is credited with building the first practical EV in 1834, which was quickly followed by a two-passenger electric car in 1847, and then an electric car in 1851 that could go 20 mph (32 km/h). The Edison Cell, a nickel-iron battery, was developed in 1900 and was a key factor in the development of early twentieth century electric vehicles. By 1900, electric vehicles had a healthy share of the pleasure car market. Of the 4, 200 automobiles sold in the United States at the turn of the century, 38% were powered by electricity, 22% by gasoline, and 40% by steam. But by the 1920s, both electricity and steam had lost out to gasoline.

Automakers began working on electric vehicle batteries again during the 1960s as an offshoot of the U.S. space programs. (In 19711972, several battery-powered cars called Lunar Roving Vehicles were driven by astronauts on the moon.) Research continued during the oil embargo of the 1970s and beyond. The results include a handful of commercially available electric automobiles capable of driving between 70300 mi (113160 km) before recharging. A number of auto manufacturers are researching to produce commercially viable EVs. As of 2006, however, most major manufacturers were shifting their attention to hybrid electric and fuel-cell electric vehicles that produce power onboard by running an efficien gasoline engine or fuel cell. Hybrid vehicles produce about a tenth of the pollution made by conventional gasoline vehiclesperhaps even less than EVs, when the whole pollution cost of generating electricity at power plants so that it can be transmitted to EVs for charging is taken into accountwhile having much greater range than EVs, refueling at the pump in less than a hundredth the time it takes an EV to recharge, and using the same fuel-distribution infrastructure (refineries, gas stations, and so forth) that conventional vehicles use. They are also lighter and cheaper than allelectric vehicles, mostly because they do not need such a large battery load. Using fewer batteries, they impact the environment more lightly by not requiring as much toxic metal. However, some EV models remained in production as of 2006 and several new models were scheduled.

The key components of an electric vehicle include energy storage cells, a power controller, and motors. Transmission of energy in electrical form eliminates the need for a mechanical drivetrain. A special braking design, called regenerative braking, uses the motor as a generator. This system feeds energy back to the storage system each time the brakes are used.

Batteries

The three main batteries employed today are leadacid; nickel-metal hydride; and lithium-based batteries. Of these, experts predict nickel-metal hydride and lithium-based batteries have the greatest potential. Lithium batteries are used in most EVs and hybrids today.

The lead-acid battery uses lead oxide and spongy lead electrodes with sulfuric acid as an electrolyte. Generally, they consist of several cells put in series to form a battery, such as an automobile battery. The group of cells is generally in a polypropylene container. The advantages of the lead-acid battery are commercial availability, recyclability and low cost. The disadvantages are that they are heavy and the amount of energy stored per kilogram is less than other types of batteries.

Nickel-metal hydride operates by moving hydrogen ions between a nickel-metal hydride cathode and a nickel hydroxide anode. During discharge, hydrogen moves from cathode to anode. During charging, ions move in the opposite direction.

There are two types of lithium batteries, the lithium ion and the lithium polymer. A lithium ion type works by dissolving lithium ions, and transporting them between the anode and cathode. The battery has an anode made of lithium cobalt dioxide and a cathode from a non-graphitizing carbon. During operation, lithium ions move through a liquid electrolyte that contains a thin, microporous membrane. The lithium polymer uses lithium as an electrochemically active material and the electrolyte is a polymer or polymer-like material that conducts lithium ions.

Advantages

Electric vehicles are more efficient than internal combustion engines for several reasons. First, the electric motor is directly connected to the wheels, so it consumes no energy while the car is at rest or coasting. Secondly, a regenerative braking system can return as much as half an electric vehicles kinetic energy to the storage cells. Third, the motor converts more than 90% of the energy in its storage cells to motive force, whereas internalcombustion drives use less than 25% of the energy in a gallon (3.75 L) of gasoline. A full comparison of efficiency must, however, take into account conversion losses from the fuels being used, usually, to generate the electricity that charged the electric vehicle to begin with. The consensus among experts seems to be that even taking into account all fuel-to-wheel losses in both conventional and electric cars, electric vehicles are significantly more efficient though still far less than 90% efficient.

The world land speed record for an electric car is just under 200 mph (320 km/h) as of 1996. Many operators of heavy vehicles, such as subway trains, locomotives and mining equipment, prefer electric motors because of the amount of instantaneous torque they offer; gasoline engines have to build power before they reach the peak rpm range that allows them to shift gears. Additionally, the average daily use of private vehicles in major U.S. cities is 40 mi (64 km); todays

Key Terms

Battery A battery is a container, or group of containers, holding electrodes and an electrolyte for producing electric current by chemical reaction and storing energy. The individual containers are called cells. Batteries produce direct current (DC).

Cell Basic unit used to store energy in a battery. A cell consists of an anode, cathode and the electrolyte.

Controller Device managing electricity flow from batteries to motor(s), from on-off function to vehicle throttle control.

Direct current (DC) Electrical current that always flows in the same direction.

Electrolyte The medium of ion transfer between anode and cathode within the cell. Usually liquid or paste that is either acidic or basic.

Flywheels Rapidly spinning wheel-like rotors or disks that store kinetic energy.

Hybrid vehicle Vehicles having two or more sources of energy. There are two types of hybrid electric vehicles (HEVs), series and parallel. In a series hybrid, all of the vehicle power is provided from one source. For example, with an IC/electric series hybrid, the electric motor drives the vehicle from the battery pack and the internal combustion engine powers a generator that charges the battery. In a parallel hybrid, power is delivered through both paths. In an IC/electric parallel hybrid, both the electric motor and the internal combustion engine power the vehicle.

Motor Electromechanical device that provides power (expressed as horsepower and torque) to driveline and wheels of vehicle.

Regenerative braking A means of recharging batteries using energy created by braking the vehicle. With normal friction brakes, a certain amount of energy is lost in the form of heat created by friction from braking. With regenerative braking, the motors act as generators. They reduce the energy lost by feeding it back into the batteries resulting in improved range.

Ultracapacitors These are higher specific energy and power versions of electrolytic capacitorsdevices that store energy as an electrostatic charge. They are electrochemical systems that store energy in a polarized liquid layer at the interface between an ionically conducting electrolyte and a conducting electrode.

Watt The basic unit of electrical power equal to 1 joule per second.

EVs can handle these trips with ease. An EV averages 40100 mi (34160 km) per charge.

Recognizing the need for alternative fuel vehicles (AFVs), U.S. President Bill Clinton issued Executive Order 13148, Greening the Government Through Federal Fleet and Transportation Efficiency on the twenty-fifth anniversary of the first Earth Day, April 21, 2000. The executive order sought to ensure that the federal government exercises leadership in the reduction of petroleum consumption through improvements in fleet fuel efficiency and the use of AFVs and alternative fuels. This includes procurement of innovative vehicles capable of large improvements in fuel economy such as hybrid electric vehicles. In 2003, President George W. Bush called for a federal institute to advance fuel cell development and use.

Hybrids

While pure electric vehicles are some time in the future, the world is ready today for the hybrid electric vehicle (HEV, or simply hybrid)a vehicle that combines a small internal-combustion engine with electric motors and batteries. These reduce emissions even more than battery-powered electric vehicles recharged from the present-day grida hybrid produces about a tenth as much air pollution as an equivalent-size conventional gasoline vehicleas well as offering the extended range and rapid refueling that consumers get from conventional vehicles. Hybrid power systems were conceived as a way to compensate for the shortfall in battery technology. Because batteries could supply only enough energy for short trips, an onboard generator, powered by an internal combustion engine, could be installed and used for longer trips. Hybrids carry a much smaller battery load than electric vehicles and are therefore lighter. They do not require plug-in recharging, but refuel exactly like a conventional vehicle.

The HEV is able to operate approximately twice as efficiently as conventional vehicles. Hondas Insight, the first hybrid car to be sold in the United States, goes 700 mi (1, 127 km) on a single tank of gasabout 70 miles per gallon (mpg). The Toyota Prius goes over 500 mi (724 km) on a tank of gas, comparable to a conventional carabout 5060 mpg. The most efficient small conventional cars get mileage that is almost as good, but produce approximately 10 times as much pollution per mile. For the driver, hybrids offer similar or better performance than conventional vehicles, with rapid acceleration due to the electric motors. Hybrids are an off-the shelf alternative approach. The Insight and Prius, first introduced into the US market in 1999 and 2001 respectively, have become rapidly-growing major sellers; in 2004, for example, the Prius sold about 50, 000 units, and in 2005 it sold over 100, 000. Toyota has had difficulty keeping up with demand worldwide for the Prius. Because of the success of the Insight and Prius, American car manufacturers have begun offering hybrid cars, including hybrid SUVs (sport utility vehicles). Due to their larger size, these hybrid models do not have the mileage or pollution characteristics of the Insight or Prius, but are still more efficient than their conventional counterparts.

Challenges still exist

Battery engineers still struggle with energy density. Average energy density in todays EV batteries is about 80135 W-hr/kg (one W-hr/kg is roughly one mile of range in a four-passenger sedan), as compared to about 25 W-hr/kg for a lead-acid battery (typical car battery). In order to increase driving ranges, battery makers must find new alloys for cathodes and anodes. Merely loading more batteries into each vehicle is not sufficient; a typical EV design already carries as much as it can without becoming too heavy or running out of room for other purposes.

Resources

BOOKS

Ehsani, Mehrdad, et al. Modern Electric, Hybrid Electric, and Fuel Cell Vehicles. New York, CRC, 2005.

Erjavec, Jack and Jeff Arias. Hybrid, Electric, and Fuel-Cell Vehicles. Clifton Park, NY: Thomson Delmar Learning, 2006.

Mom, Gijs. The Electric Vehicle: Technology and Expectations in the Automobile Age. Baltimore, MD: Johns Hopkins University Press, 2004.

PERIODICALS

Saranow, Jennifer. The electric car gets some muscle: Confluence of factors gives alternative cars power to roll. The Wall Street Journal. September 4, 2006: Autotimes, p. A4.

Laurie Toupin