Optimal Design and Steady‐State Operation

Abstract

The focus of this chapter is on describing procedures for the optimal design and operation of microfabricated power generation processes. In contrast to the problem of selection of alternatives, as described earlier in Chapter [Selection of Alternatives and Process Design], a fixed process structure is considered throughout this chapter. In many applications, the power demand remains essentially constant during the operation, with rapid changeovers, and, therefore, steady-state operation must be considered thoroughly. For a given power demand, or a power demand varying in a specified range, the design and steady-state operation problem is to determine values of the design variables (e.g., sizes of the individual components such as fuel processing reactor and fuel cell) as well as operational variables (e.g., fuel flow rates and operating temperature) so as to maximize its (steady-state) performance, in light of safety, reliability, as well as other considerations. In other applications, the power demand may change rapidly and the devices may be operated periodically, with frequent start-ups and shut-downs. In this case, special attention must be paid to the dynamics of these processes, in addition to their steady-state operation. Ideally, one would like to optimize the design and operation over an entire operation cycle, including start-up, steady-state operation, and shut-down. However, because the duration and power demand profile of a mission is rarely known in advance, such an optimization is hardly tractable. Instead, the start-up phase can be optimized separately from the subsequent steady-state operation. For start-up purpose, the devices will most likely be coupled with a small battery or ultra-capacitor, whose role will be to ensure that the power demand is met when the fuel cell is unavailable or can only satisfy part of the demand, and to provide the