Regenerative fuel cells for energy storage: Efficiency and weight trade-offs

In optimizing regenerative fuel cell systems for a given application, the operating efficiency of both the fuel cell and electrolyzer may be traded against the stack mass. Both efficiency and mass are important characteristics for energy storage, particularly for aerospace applications. This paper reports the results of a trade-off study conducted to optimize the design of regenerative fuel cell systems used in conjunction with a PV array. The results suggest that high efficiency does not necessarily provide the highest specific energy, and that the highest specific energy of the energy storage subsystem does not result in the lowest mass of the entire power system. Optimum cell voltage (in both fuel cell and electrolyzer modes) strongly depends on the duty cycle of the system, i.e., duration of charging/discharging and capacity factor in charging and discharging, as well as the fuel cell / electrolyzer polarization curves and hydrogen/oxygen storage characteristics.