Optimal Design and Steady-State Operation

The focus of this chapter is on describing procedures for the optimal design and operation of man-portable power generation processes. For a given power demand, or a power demand varying in a specified range, the design and 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 performance, in light of safety, reliability, as well as other considerations. At the micro scale, a different paradigm to the unit-operations paradigm of macro-scale process design and operation is necessary. The reason for this is that the different units constituting a microfabricated system are tightly spatially integrated and can no longer be considered to operate independently from each other. Accordingly, operational decisions must be taken at an early stage of development, together with design decisions. For example, increasing the operating temperature of a microfabricated reactor increases the heat losses per unit surface area, but because the reaction rates are also enhanced, the volume needed to achieve a given conversion is reduced. In the case where the latter effect dominates, one then obtains the counter-intuitive result that increasing the operating temperature lowers the overall heat losses for the system. In other words, it is of paramount importance to determine the operation policy simultaneously with the design and sizing of the units. Because the underlying physicochemical phenomena are complicated and intrinsically coupled, one cannot rely on engineering intuition only to find out the optimal design and operation. The use of mathematical models along with systematic optimization methods based on mathematical programming is clearly warranted. Because optimization algorithms may require hundreds,