Fuel Cell Types

Fuel cells can use a variety of fuels (natural gas, petroleum, methanol, hydrogen, etc.) to produce electricity through a noncombustion electrochemical reaction. The fuel used directly is hydrogen, usually reformed from hydrocarbon fuels. A catalyst splits the hydrogen molecules into electrons and protons. The protons pass through an electrolyte membrane, while the electrons create an electrical current. The electrons and protons are then reunited and combined with oxygen to create water. The process also creates heat.

Several types of fuel cell technology are being developed for commercial use.

Phosphoric acid fuel cell (PAFC) systems are commercially available and currently installed at utility power plants, hospitals, hotels, schools, office buildings, and an airport terminal. Operating at about 200 Celsius PAFCs offer 40-45 percent electrical efficiency, with the potential for greater than 80 percent efficiency when used in a cogeneration arrangement. The system's high operating temperature requires a high-priced support system and costly maintenance, which makes PAFC more suitable for large-scale stationary and mobile applications.

Molten carbonate fuel cell (MCFC) systems are in the demonstration phase, with systems ranging from 250 kilowatts to 2.5 megawatts. Operating at about 600 to  700 Celsius, this technology, too, is best suited for large-scale centralized power applications. Developers project that MCFCs will provide electricity in the 50-60 percent efficiency range, with cogeneration efficiency approaching 85 percent. The high operating temperature of an MCFC necessitates the use of expensive materials and presents electrolyte vaporization, leakage, and corrosion challenges.

Proton exchange membrane fuel cell (PEMFC) systems are in the precommercial beta testing stage. Operating at around 80 Celsius, PEMFCs provide electrical efficiency at less than 40 percent, with no potential for cogeneration applications. However, the system's high power density and fast output shifting make it suitable for automobiles, small stationary power plants, and applications as small as battery replacements for portable devices.

Solid oxide fuel cell (SOFC) systems are currently being demonstrated. With an operating temperature of about 600 to 1000 Celsius, SOFCs demonstrate 50-60 percent electrical efficiency, with cogeneration efficiency in a range of 70-85 percent. According to Frost & Sullivan's 1999 report on the North American stationary fuel cell market, SOFCs "are considered the only fuel cell technology with a wide span of possible market applications ranging from 2-kilowatt residential systems to wholesale distributed generation systems of 10-25 megawatts."

Other types of technology are under development as well.

Alkaline fuel cells, long used by the National Aeronautics and Space Administration on space missions, 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.

Direct methanol fuel cells (DMFCS) are a relatively new member of the fuel cell family. These cells are similar to the PEMFCS 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 percent are expected with DMFCS, with would operate between 50 to 80 Celsius .

Regenerative fuel cells, 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. Both molecules are fed into the cell, which then generates electricity, heat, and water--the water is then recirculated to the electrolyser.

Hydrogen peroxide fuel cells are under development at Purdue University. Based on reactions between hydrogen and aluminum, they have the capacity to generate more than 20 times the power of traditional car batteries per pound.