Performance implications of rapid load changes in carbonate fuel cell systems

There are several key performance objectives of molten carbonate fuel cell systems operating under load perturbations. Among these include: regulation of stack temperature, regulation of differential pressure between anode and cathode, maintaining acceptable fuel utilization and steam-carbon ratio, and electrical load tracking. Utilities are interested in rapid load cycling of carbonate fuel cell systems while the military is interested in the response of carbonate fuel cell systems under sudden application and removal of electrical load. An integrated fuel cell system may endure these types of disturbances while satisfying performance goals, depending largely on the control system. In this paper, the authors examine two important performance variables: fuel utilization and steam-carbon ratio, under both ramping operation and sudden increase in load. Setpoint control laws are proposed for determining the proper steam and natural gas flows corresponding to steady state or transient conditions. In the case of a sudden load increase, they illustrate a trade-off between good load tracking and good utilization/steam-carbon ratio when considering rate constraints on valves.