Control of fuel cell breathing

Abstract

In this article we analyzed and designed air flow controllers that protect the fuel cell (FC) stack from oxygen starvation during step changes of current demand. The steady-state regulation of the oxygen excess ratio in the FCS cathode achieved by assigning an integrator to the compressor flow. Linear observability techniques were employed to demonstrate improvements in transient oxygen regulation when the FCS voltage is included as a measurement for the feedback controller. The FCS voltage signal contains high frequency information about the FC oxygen utilization, and thus, is a natural and valuable output for feedback. We used linear optimal control design to identify the frequencies at which there is a severe tradeoff between the transient system net power performance and the stack starvation control. The limitation arises when the FCS system architecture dictates that all auxiliary equipment is powered directly from the FC with no secondary power sources. This plant configuration is preferred due to its simplicity, compactness, and low cost. The FCS impedance given the closed-loop air flow and perfect humidification and temperature regulation captures the FC current-voltage dynamic relationship. It is expected that the closed-loop FCS impedance will provide the basis for the systematic design of FC stack electronic components.