SCIENTISTS are searching for cleaner ways to power vehicles and to make better use of domestic energy resources. The fuel cell, an electrochemical device that converts the chemical energy of a fuel directly to usable energy without combustion, is one of the most promising of these new technologies. Running on hydrogen fuel and oxygen from the air, a 50-kilowatt fuel cell can power a lightweight car without creating any undesirable tailpipe emissions.|
If the fuel cell is designed to operate also in reverse as an electrolyzer, then electricity can be used to convert the water back into hydrogen and oxygen. (See Figure 1.) This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). Lighter than a separate electrolyzer and generator, a URFC is an excellent energy source in situations where weight is a concern.
Weight was a critical issue in 1991 when scientists at Lawrence Livermore National Laboratory and AeroVironment of Monrovia, California, began looking at energy storage options for an unmanned, solar-powered aircraft to be used for high-altitude surveillance, communications, and atmospheric sensing as part of the Strategic Defense Initiative. Called Pathfinder, the aircraft set an altitude record for solar-powered flight in 1995, flying to 15,400 meters (50,500 feet) and remaining aloft for about 11 hours. Pathfinder's successor, Helios, will remain aloft for many days and nights. For that aircraft, storage devices were studied that would provide the most energy at the lowest weight, i.e., the highest energy density. The team looked at flywheels, supercapacitors, various chemical batteries, and hydrogen- oxygen regenerative fuel cells. The regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density--about 450 watt-hours per kilogram--ten times that of lead-acid batteries and more than twice that forecast for any chemical batteries.
Fuel cells have been used since the 1960s when they supplied on-board power for the Gemini and Apollo spacecraft. Today, fuel cells are being used for Space Shuttle on-board power, power plants, and a variety of experimental vehicles. However, none of these applications uses the URFC because early experience did not uncover the usefulness of the reversible technology, and little research had been funded. Recent results of Livermore research indicate otherwise, based on more thorough systems engineering and improved membrane technology.
Challenged by a lack of information on the technology, Livermore physicist Fred Mitlitsky was determined to uncover just how to make the combination of technologies work. Mitlitsky continued in 1994 with a little funding from NASA for development of Helios and from the Department of Energy for leveling peak and intermittent power usage with sources such as solar cells or wind turbines. (See Figure 2.)
The 50-watt prototype that Mitlitsky's team developed is a single proton-exchange membrane cell (a polymer that passes protons) modified to operate reversibly as a URFC. It uses bifunctional electrodes (oxidation and reduction electrodes that reverse roles when switching from charge to discharge, as with a rechargeable battery) and cathode-feed electrolysis (water is fed from the hydrogen side of the cell). By November 1996, the prototype had operated for 1,700 ten-minute charge-discharge cycles, and degradation was less than a few percent at the highest current densities.1|
Testing will continue in a variety of forms. Larger, more powerful prototypes will be created by increasing the size of the membrane and by stacking multiple fuel cells. For use on Helios, a prototype will likely provide 2 to 5 kilowatts running on a 24-hour charge-discharge cycle. As funding becomes available, prototypes may also be tested for other uses. A lunar rover, for example, would require cycles of about 29 days.
URFC-Powered Electrical Vehicles
In a 1994 study for automotive applications, Livermore and the Hamilton Standard Division of United Technologies studied URFCs. They found that compared with battery-powered systems, the URFC is lighter and provides a driving range comparable to gasoline-powered vehicles. Over the life of a vehicle, they found the URFC would be more cost effective because it does not require replacement.2
In the electrolysis (charging) mode, electrical power from a residential or commercial charging station supplies energy to produce hydrogen by electrolyzing water. The URFC-powered car can also recoup hydrogen and oxygen when the driver brakes or descends a hill. This regenerative braking feature increases the vehicle's range by about 10% and could replenish a low-pressure (1.4-megapascal or 200-psi) oxygen tank about the size of a football.
In the fuel-cell (discharge) mode, stored hydrogen is combined with air to generate electrical power. The URFC can also be supercharged by operating from an oxygen tank instead of atmospheric oxygen to accommodate peak power demands such as entering a freeway. Supercharging allows the driver to accelerate the vehicle at a rate comparable to that of a vehicle powered by an internal-combustion engine.
The URFC in an automobile must produce ten times the power of the Helios prototype, or about 50 kilowatts. A car idling requires just a few kilowatts, highway cruising about 10 kilowatts, and hill climbing about 40 kilowatts. But acceleration onto a highway or passing another vehicle demands short bursts of 60 to 100 kilowatts. For this, the URFC's supercharging feature supplies the additional power. A URFC-powered car must be able to store hydrogen fuel on board, but existing tank systems are relatively heavy, reducing the car's efficiency or range. Under the Partnership for a New Generation of Vehicles, a government-industry consortium dedicated to developing high-mileage cars, the Ford Corporation provided funding to LLNL, EDO Corporation, and Aero Tec Laboratories for development of a lightweight hydrogen storage tank (a pressure vessel). The team combined a carbon fiber tank with a laminated, metalized, polymeric bladder (much like the ones that hold beverages sold in boxes) to produce a hydrogen pressure vessel that is lighter and less expensive than conventional hydrogen tanks. Equally important, its performance factor--a function of burst pressure, internal volume, and tank weight--is about 30% higher than that of comparable carbon-fiber hydrogen storage tanks. In tests where cars with pressurized carbon-fiber storage tanks were dropped from heights or crashed at high speeds, the cars generally were demolished while the tanks still held all of their pressure--an effective indicator of tank safety. Unlike other hydrogen-fueled vehicles whose refueling needs depend entirely on commercial suppliers, the URFC-powered vehicle carries most of its hydrogen infrastructure on board.3 But even a highly efficient URFC-powered vehicle needs periodic refueling. Until a network of commercial hydrogen suppliers is developed, an overnight recharge of a small car at home would generate enough energy for about a 240-kilometer (150-mile) driving range, exceeding the range of recently released electrical vehicles. With the infrastructure in place, a 5-minute fill up of a 35-megapascal (5,000-psi) hydrogen tank would give a 580-kilometer (360-mile) range. Commercial development of unitized regenerative fuel cells for use in automobiles is perhaps 5 to 10 years away. With their long life, low maintenance requirements, and good performance, URFCs hold the promise of someday supplying clean, quiet, efficient energy for many uses.
Key Words: electric cars, fuel cell, Helios, hydrogen, Partnership for a New Generation of Vehicles, zero-emission vehicles.
1. F. Mitlitsky, B. Myers, and A. H. Weisberg, Lightweight Pressure Vessels and Unitized Regenerative Fuel Cells, LLNL, Livermore, California, UCRL-JC-125220 (November 1966). Presented at the 1996 Fuel Cell Seminar, San Diego, California, November 17-20, 1996.
2. F. Mitlitsky, N. J. Colella, and B. Myers, Unitized Regenerative Fuel Cells for Solar Rechargeable Aircraft and Zero Emission Vehicles, LLNL, Livermore, California, UCRL-JC-117130 (September 1994). Presented at the 1994 Fuel Cell Seminar, Orlando, Florida, November 28-December 1, 1994.
3. "Getting along without Gasoline--The Move to Hydrogen Fuel," Science & Technology Review, UCRL-52000-96-3 (March 1996), pp. 28-31.
For further information contact Fred Mitlitsky (510) 423-4852 (email@example.com).
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