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NFCRC Tutorial: Fuel Cell

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The following provides some background information on fuel cells in general followed by a descriptions of the characteristics of fuel cells by major type. An excellent in-depth treatment of the subject may be found in the U. S. Department of Energy publication titled "Fuel Cells - A Handbook" by Hirschenhofer et. al., January 1994.

Fuel cells as energy conversion devices have a number of advantages; some of these advantages being: high energy conversion efficiency which is relatively independent of size good part load characteristics modular design and flexibility of size low environmental impact rejection of heat at high temperature in some of the fuel cell types, which is suitable for cogeneration citing ability due to the favorable environmental signature quick response to load changes.

Fuel cells, however do have certain disadvantages which are listed below: sensitivity to certain contaminants that may be present in the fuel such as sulfur and chlorides current capital costs on a $/KW basis are high lack of the field data on endurance/reliability.

One of the main advantages of fuel cells is the high conversion efficiency which may range from 40 to 60% based on lower heating value (LHV) of the fuel. The fuel conversion efficiency is higher than that of most energy conversion systems, the efficiency advantage becoming more significant at the smaller scales since the efficiency of fuel cells is nearly constant with size. Furthermore, the heat rejected from some of the fuel cell types such as the Molten Carbonate and the Solid Oxide fuel cells is at high temperature and thus, is available for cogeneration applications. Thus, fuel cell plants can be constructed in a wide range of electrical output, from less than a KW to sizes in excess of a MW. Fuel cells produce virtually no gaseous, solid or noise emissions except unless turbomachinery is included in the plant.

Fuel cells are sensitive to certain fuel contaminants such as sulfur and chlorides. Furthermore, the fuel entering the cell has to be gaseous and in some cases has to be hydrogen which leads to the requirement of a fuel pre-processor such as a reformer. In some fuel cell designs, however, the reformer has been incorporated within the fuel cell itself.

Thus, fuel cells can be classified by whether the fuel is processed outside (externally reformed) or inside (internally reformed), by the type of electrolyte or by the temperature of operation. A brief description of the major types of fuel cells characterized by the type of electrolyte employed is presented in the following.

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