PEM Fuel Cell Progress Reported at International Energy Conversion Engineering Conference

Henry Oman
Consulting Engineer
Seattle, WA

In the past the efficiency with which fuel energy can be converted to electric power has been limited by two important factors. The first is the Carnot cycle which limits the efficiency of fuel-burning heat engines to the ratio of the temperature difference between the heat source and sink temperatures, to the absolute temperature of the heat source. The second has been the high cost of platinum, currently around $700 per troy ounce. The cost of fuel-cell power generation must include the costly rare-earth platinum catalyst in its fuel-to-hydrogen converter. Now two new discoveries suggest that fuel cells can become the major converter of fuel-energy to electric energy in the future as the world’s petroleum production diminishes.

On June 27, 2003 chemical and biological engineers at the University of Wisconsin reported that they have found a cost-effective nickel-tin catalyst that can replace the expensive platinum metal in a new process for making hydrogen from corn syrup, sugar beets, and biomass waste such as paper-mill sludge, cheese whey, and wood waste (1). In the 15 August issue of Science, a team in the Department of Chemical and Biological Engineering at Tufts University reported results from work on ceria-based water-gas shift catalysts. (2)

News reports about producing hydrogen fuel with catalysts of nickel-tin, instead of high-cost platinum, produced great excitement in the fuel-cell sessions of the International Energy Conversion Engineering Conference (IECEC) that was held in Portsmouth, Virginia, August 18 to 21, 2003. This conference, sponsored by the American Institute of Aeronautics and Astronautics, dealt with energy conversion for generating electric power in Earth-orbit satellites, space stations, Mars-surface exploration, propulsion of deep-space probes, earth-surface vehicles and other applications. The latest developments in energy and power fields were described in the 206 technical papers presented. In seven panel sessions government and industry leaders described future energy- and power-related developments.

Fuel Cells That Power Spacecraft

Proton-membrane (PEM) fuel cells, rated 1.0kW, powered the Gemini spacecraft and produced drinking water for its crew on eight flights from 1965 to 1966. Alkaline fuel cells, rated 1.5kW and weighing 250 pounds, powered 18 flights of the Apollo space vehicles. These fuel cells operated for nearly 11,000 hours total between 1966 and 1978. The Shuttles, that have flown 113 flights since 1981, were powered by 12kW alkaline fuel cells that have now accumulated more than 90,000 hours of operation. The reliability of fuel cell power made possible the success of these missions. With new developments the cost and weight of fuel cells for coming missions can be reduced, but reliability and safety must be assured.

M. A. Hoberecht in his IECEC presentation reviewed the difficulties encountered during operation of the Shuttle’s three 12kW fuel cells, each of which powered a 28V DC bus (3). Each fuel cell consisted of a power section, where the chemical reaction occurs, and an accessory section that monitors the power section’s performance. The power section, where hydrogen and oxygen are transformed into electric power, water, and heat, consisted of 96 cells that were contained in three substacks. Manifolds run the length of these substacks to distribute hydrogen, oxygen, and coolant to each cell. Each cell contains an electrolyte consisting of potassium hydroxide and water, an oxygen electrode (cathode) and a hydrogen electrode (anode). Each fuel cell power plant is 14 inches high, 15 inches wide, and 40 inches long, and weighs 260 pounds.

Reactant consumption in a fuel cell is directly proportional to the current produced. If no load is connected to the cell, no reactants are consumed. Leaks can be detected from the fuel being consumed by an unloaded cell. The water produced by the power-generating reaction must be removed or the cells will become saturated with water and their efficiency will drop. With a 7kW load it takes only a few minutes to flood the fuel cell with product water, halting power generation. Therefore, hydrogen is pumped through the stack. It picks up the water vapor, which is next condensed and separated from the hydrogen. This water is available for life support and environmental control. A relief valve set at 45psia discharges water overboard if it is not used. Each fuel cell is serviced between flights, and is usually replaced after 1400 to 1500 hours of operation due to accessory component problems.

The hydrogen and oxygen are stored as cryogenic liquids in double-walled, thermally insulated spherical tanks with a vacuum annulus between the inner pressure vessel and outer shell of the tank. Having cryogenic liquid hydrogen and oxygen on board complicates the orbiter processing after it lands. Only essential personnel are allowed onboard until the fuel cells are shut down, a detank is completed, and systems are inerted. This takes seven eight-hour shifts.