Maximum Thermodynamic Electrical Efficiency of Fuel Cell System and Results for Hydrogen, Methane, and Propane Fuels

The maximum electrical efficiency of fuel cell system, η_e^max, is important for the understanding and development of the fuel cell technology. Attempt is made to build a theory for η_e^max by considering the energy requirement of heating the fuel and air streams to the fuel cell operating temperature T. A general thermodynamic analysis is performed and the energy balances for the overall operating processes of a fuel cell system are established. Explicit expressions for the determination of η_e^max are deduced. Unlike the Carnot efficiency, η_e^max is found to be fuel specific. Except for hydrogen fuel, chemical equilibrium calculations are necessary to compute η_e^max. Analytical solutions for the chemical equilibrium of alkane fuels are presented. The theoretical model is used to analyze the effects of T and the steam contents of CH₄, C₃H₈, and H₂ on η_e^max for systems with various degrees of waste heat recovery. Contrary to the common perception concerning methane and propane fuels, η_e^max decreases substantially with the increase of T. Moreover, η_e^max of hydrogen fuel can be higher than that of methane and propane fuels for a system with a medium level of waste heat recovery and operated at 700 ℃≤T≤900 ℃.