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increase in sulfur partial pressure. The current practice for internal reforming systems up to 3 MW is atmospheric operation. Pressurization of an SOFC yields a smaller gain in fuel cell performance than either the MCFC or PAFC. For example, based on the pressure relationships presented earlier, changing the pressure from one to ten atmospheres would change the cell voltage by ~150, ~80, and ~60 mV for the PAFC, MCFC, and SOFC, respectively. In addition to the cell performance improvement, pressurization of SOFC systems allows the thermal energy leaving the SOFC to be recovered in a gas turbine, or gas turbine combined cycle, instead of just a steam bottoming cycle. Siemens Westinghouse is investigating the possibilities associated with pressurizing the SOFC for cycles as small as 1 to 5 MW. Large plants benefit the most from pressurization, because of the economy of scale on equipment such as compressors, turbines, and pressure vessels. Pressurizing small systems is not practical, as the cost of the associated equipment outweighs the performance gains. Pressurization in operating PAFC systems demonstrates the economy of scale at work. The IFC 200 kWe and the Fuji Electric 500 kWe PAFC offerings have been designed for atmospheric operation, while larger units operate at pressure. The 11 MWe plant at the Goi Thermal Power Station operated at a pressure of 8.2 atmospheres (2), while a 5 MWe PAFC unit (NEDO/ PAFCTRA) operates at slightly less than 6 atmospheres (3). NEDO has three 1 MWe plants, two of which are pressurized while one is atmospheric (3). Although it is impossible to generalize at what size a plant would benefit by pressurization, when plants increase in size to approximately 1 MW and larger, the question of pressurization should be evaluated. 8.3.2 Temperature Although the open circuit voltage decreases with increasing temperature, the performance at operating current densities increases with increasing temperature due to reduced mass transfer polarizations and ohmic losses. The increased temperature also yields higher quality rejected heat. An additional benefit to an increased temperature in the PAFC is an increased tolerance to CO levels, a catalyst poison. The temperatures at which the various fuel cells can operate are, however, limited by material constraints. The PAFC and MCFC are both limited by life shortening corrosion at higher temperatures. The SOFC has material property limitations. Again, the fuel cell and system designers should evaluate what compromise will work best to meet their particular requirements. The PAFC is limited to temperatures in the neighborhood of 200 oC (390 oF) before corrosion and lifetime loss become significant. The MCFC is limited to a cell average temperature of approximately 650 oC (1200 oF) for similar reasons. Corrosion becomes significant in an MCFC when local temperatures exceed 700 oC (1290 oF). With a cell temperature rise on the order of 100 oC (210 oF), an average MCFC temperature of 650 oC (1200 oF) will provide the longest life, highest performance compromise. In fact, one reference (4) cites "the future target of the operating temperature must be 650 °C +30 °C (1290 °F +55 °F)." 8-48PDF Image | Fuel Cell Handbook (Seventh Edition)
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