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.
The Prototype |
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 |
Key Words: electric cars, fuel cell, Helios, hydrogen, Partnership for a New Generation of Vehicles, zero-emission vehicles.
References
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 (fm@llnl.gov).