On the intrinsic transient capability and limitations of solid oxide fuel cell systems

The intrinsic transient performance capability and limitation of integrated solid oxide fuel cell (SOFC) systems is evaluated based on the system balance-of-plant response and fuel cell operating requirements (i.e., allowable deviation from nominal operation). Specifically, non-dimensional relations are derived from conservation principles that quantify the maximum instantaneous current increase that a solid oxide fuel cell system can safely manage based on (1) the desired fuel cell operating point, (2) the maximum allowable fuel utilization, (3) the maximum average fuel cell temperature deviation, (4) the response delay and (5) the operating requirements of the system balance-of-plant components. New non-dimensional numbers representing the ratio of species or thermal convection to volumetric capacitance in the fuel cell during balance-of-plant delay have been developed.The analyses indicate: (1) SOFC intrinsic transient performance is largely limited by fuel processor flow delays that can cause fuel depletion in the anode compartment and (2) that with proper system actuators and control design transient operation of SOFC systems should be possible while maintaining SOFC average temperature within a degree. The SOFC system fuel processor lag appears to be the cause of SOFC load following limitation, while lag in air handling appears to be manageable. To demonstrate methods to avoid fuel depletion limitation a fuel flow lead compensator is developed. Integrated system simulations with proper control demonstrate significant SOFC transient performance within fuel cell operating requirements.