A thermodynamic analysis of an air-cooled proton exchange membrane fuel cell operated in different climate regions

A fundamental thermodynamic analysis of an air-cooled fuel cell, where the reactant air stream is also the coolant stream, is presented. The adiabatic cell temperature of such a fuel cell is calculated in a similar way as the adiabatic flame temperature in a combustion process. Diagrams that show the dependency of the cathode outlet temperature, the stoichiometric flow ratio and the operating cell voltage are developed. These diagrams can help fuel cell manufacturers to identify a suitable blower and a suitable operating regime for their fuel cell stacks. It is found that for standard conditions, reasonable cell temperatures are obtained for cathode stoichiometric flow ratios of ξ = 50 and higher, which is in very good agreement with manufacturer's recommendations. Under very cold ambient conditions, the suggested stoichiometric flow ratio is only in the range of ξ = 20 in order to obtain a useful fuel cell operating temperature. The outside relative humidity only plays a role at ambient temperatures above 40 ◦C, and the predicted stoichiometric flow ratios should be above ξ = 70 in this region. From a thermodynamic perspective, it is suggested that the adiabatic outlet temperature is a suitable definition of the fuel cell operating temperature.