Hydrogen Fuel Cell Technologies

A fuel cell is an energy conversion technology that allows the energy stored in hydrogen to be converted back into electrical energy for end use. Although fuel cells can use a variety of fuels including gasoline, hydrogen is usually preferred because of the ease with which it can be converted to electricity and its ability to combine with oxygen to emit only water and heat. Fuel cells look and function very similar to batteries. A fuel cell continues to convert chemical energy to electricity as long as fresh hydrogen fuel is fed into it.

Aside from being pollution-free at their point of use, fuel cells are quiet because they are non-mechanical. In addition, through concerted R&D efforts, fuel cell efficiencies continue to grow. Automotive fuel cells manufactured today have achieved a conversion efficiency of more than 50 percent of the energy in hydrogen to electricity, depending on the type of fuel cell. For stationary fuel cells, the conversion efficiency is approximately 40 percent; but when combined with the use of byproduct heat, the overall efficiency can approach 90 percent. Size, flexibility, and their corresponding electrical output make fuel cells ideal for a wide variety of applications, from a few kilowatts to power a laptop computer to several megawatts at a central power generation facility. For automotive applications, 70- to 120-kilowatt systems are typically required. Fuel cells are classified by their electrolyte and operational characteristics:

• The Polymer Electrolyte Membrane (PEM) fuel cell is lightweight and has a low operating temperature. PEM fuel cells operate on hydrogen and oxygen from air. Other fuels can be used, but must they must be reformed onsite, which can reduce fueling cost but also drives up the purchase price and maintenance costs and results in CO2 emissions. PEM systems are typically designed to serve in 70- to 120-kilowatt transportation applications and may be useable as uninterruptible power supplies (UPS) in special commercial applications. Current PEM stack life is typically around 1,350 hours, as used in automotive applications.

• Alkaline fuel cells (AFCs) are one of the most mature fuel cell technologies. AFCs have a combined electricity and heat efficiency of 60 percent efficient and have been used for the production of electrical power and heated water on the Gemini and Apollo spacecrafts. However, their short operating time renders them less than cost effective in commercial applications. Their susceptibility to poisoning by even a small amount of CO2 in the air also requires purification of the hydrogen feed.

• A newer cell technology is the Direct Methanol Fuel Cell (DMFC). The DMFC uses pure methanol mixed with steam. Liquid methanol has a higher energy density than hydrogen, and the existing infrastructure for transport and supply can be utilized. Research and development of DMFCs are about 3 to 4 years behind other fuel cell technologies.

• For stationary power applications, Phosphoric Acid Fuel Cells (PAFCs) are commercially available today. Over 200 PAFCs have been placed into operation. PAFCs are less efficient than other fuel cell designs, and they tend to be large, heavy and expensive. Nevertheless, they have been used in emergency power and remote power applications.

• Molten Carbonate Fuel Cells (MCFCs) and Solid Oxide Fuel Cells (SOFC) are high temperature designs that promise higher operating efficiencies. The newest fuel cell technology is the Unitized Regenerative Fuel Cell (URFC) that can produce electricity from hydrogen and oxygen while generating heat and water. The URFC is lighter than a separate electrolyzer and generator, making it desirable for weight-sensitive applications.