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The high operating temperature of the SOFC puts numerous requirements (phase and conductivity stability, chemical compatibility, and thermal expansion) on material selection and development (5). Many of these problems could be alleviated with lower operating temperatures. However, a high temperature of approximately 1000 °C (1830 oF), i.e., the present operating temperature, is required in order to have sufficiently high ionic conductivities with the existing materials and configurations (5). 8.3.3 Utilization Both fuel and oxidant utilizations47 involve trade-offs with respect to the optimum utilization for a given system. High utilizations are considered to be desirable (particularly in smaller systems) because they minimize the required fuel and oxidant flow, for a minimum fuel cost and compressor/blower load and size. However, utilizations that are pushed too high result in significant voltage drops. One study (6) cites that low utilizations can be advantageous in large fuel cell power cycles with efficient bottoming cycles because the low utilization improves the performance of the fuel cell and makes more heat available to the bottoming cycle. Like almost all design parameters, the selection of optimum utilization requires an engineering trade-off that considers the specifics of each case. Fuel Utilization: High fuel utilization is desirable in small power systems, because in such systems the fuel cell is usually the sole power source. However, because the complete utilization of the fuel is not practical, except for pure H2 fuel, and other requirements for fuel exist, the selection of utilization represents a balance between other fuel/heat requirements and the impact of utilization on overall performance. Natural gas systems with endothermic steam reformers often make use of the residual fuel from the anode in a reformer burner. Alternatively, the residual fuel could be combusted prior to a gas expander to boost performance. In an MCFC system, the residual fuel often is combusted to maximize the supply of CO2 to the cathode while at the same time providing air preheating. In an SOFC system, the residual fuel often is combusted to provide high-temperature air preheating. The designer has the ability to increase the overall utilization of fuel (or the oxidant) by recycling a portion of the spent stream back to the inlet. This increases the overall utilization while maintaining a lower per pass utilization of reactants within the fuel cell to ensure good cell performance. The disadvantage of recycling is the increased auxiliary power and capital cost of the high temperature recycle fan or blower. One study by Minkov, et al. (6) suggests that low fuel and oxidant utilizations yield the lowest COE in large fuel cell power systems. By varying the fuel cell utilization, the electric power generation split between the fuel cell, steam turbine, and gas turbine are changed. The low fuel utilization decreases the percentage of power from the fuel cell while increasing the fuel cell performance. The increased power output from the gas turbine and steam turbine also results in their improved performance and economy of scale. The specific analysis results depend upon the assumed stack costs. The optimal power production split between the fuel cell and the gas and steam turbines is approximately 35 percent, 47 percent, and 17 percent for a 575 MW MCFC 47. Utilization - the amount of gases that are reacted within the fuel cell compared to that supplied. 8-49PDF Image | Fuel Cell Handbook (Seventh Edition)
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