Phosphoric Acid. This is the most commercially developed type of fuel cell. It is being used in such diverse applications as hospitals, nursing homes, hotels, office buildings, schools, utility power plants, and an airport terminal. Phosphoric acid fuel cells generate electricity at more than 40% efficiency -- and nearly 85% if steam this fuel cell produces is used for cogeneration -- compared to 30% for the most efficient internal combustion engine. Operating temperatures are in the range of 400 degrees F. These fuel cells also can be used in larger vehicles, such as buses and locomotives.

Proton Exchange Membrane. These cells operate at relatively low temperatures (about 200 degrees F), have high power density, can vary their output quickly to meet shifts in power demand, and are suited for applications, -- such as in automobiles -- where quick startup is required. According to the U.S. Department of Energy, "they are the primary candidates for light-duty vehicles, for buildings, and potentially for much smaller applications such as replacements for rechargeable batteries in video cameras."

Molten Carbonate. Molten carbonate fuel cells promise high fuel-to-electricity efficiencies and the ability to consume coal-based fuels. This cell operates at about 1,200 degrees F.

Solid Oxide. Another highly promising fuel cell, the solid oxide fuel cell could be used in big, high -power applications including industrial and large-scale central electricity generating stations. Some developers also see solid oxide use in motor vehicles. A 100-kilowatt test is being readied in Europe. Two small, 25-kilowatt units are already on line in Japan. A solid oxide system usually uses a hard ceramic material instead of a liquid electrolyte, allowing operating temperatures to reach 1,800 degrees F. Power generating efficiencies could reach 60%. One type of solid oxide fuel cell uses an array of meter-long tubes. Other variations include a compressed disc that resembles the top of a soup can.

Alkaline. Long used by NASA on space missions, these cells can achieve power generating efficiencies of up to 70 percent. They use alkaline potassium hydroxide as the electrolyte. Until recently they were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility.

Other Fuel Cells. Direct methanol fuel cells (DMFC) are a relatively new member of the fuel cell family. These cells are similar to the PEM cells in that they both use a polymer membrane as the electrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120-190 degrees F. Higher efficiencies are achieved at higher temperatures. Regenerative fuel cells, also a very young member of the fuel cell family, would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyser. The hydrogen and oxygen are fed into the fuel cell which generates electricity, heat and water. The water is then recirculated back to the solar-powered electrolyser and the process begins again. These types of fuel cells are currently being researched by NASA and others worldwide.