Cell Voltage Analyisis of a 6 kW polymeric electrolyte fuel cell stack designed for hybrid power systems

A polymeric electrolyte Fuel Cell System (FCS) was experimentally studied for application as power generator in hybrid power systems. The electrochemical device was based on 96 individual cells and produced a maximum power of 6.2 kW. A dedicated DC -DC converter was inserted between the FCS, a Pb battery pack used as storage system, and a resistive variable electric load. The whole system was preliminarily characterized to evaluate FCS and DC-DC converter efficiency in the whole power range. The FCS operated at low pressure, without external humidification and in anode recirculation mode. As irregularities of cell voltages could be a sign of draining conditions for catalyzed electrodes and electrolytes, the cell voltage uniformity analysis was effected in both steady state and dynamic conditions. The coefficient of variation (Cv), was used as a measure of dispersion of cell voltage around the mean value. The experimental data evidence a satisfactory behavior of all cells in steady-state conditions in the entire power range (1-6 kW), with FCS+DC-DC converter efficiencies close to 50% and very low Cv values (<1.6%). Stack and cell performance remained high and reliable during most of transient phases of a dynamic test characterized by a high frequency of load variation steps. Power variation rates higher than 300 W/s determined Cv values close to 3-3.5%. These load ramp rates could be considered as a threshold range for the management of a low pressure self-humidified FCS in hybrid power system configurations.