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

The maximum electrical efficiency of fuel cell system, eta(max)(e), is important for the understanding and development of the fuel cell technology. Attempt is made to build a theory for eta(max)(e) 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 eta(max)(e) are deduced. Unlike the Carnot efficiency, eta(max)(e) is found to be fuel specific. Except for hydrogen fuel, chemical equilibrium calculations are necessary to compute eta(max)(e). 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 H-2 on eta(max)(e) for systems with various degrees of waste heat recovery. Contrary to the common perception concerning methane and propane fuels, eta(max)(e) decreases substantially with the increase of T. Moreover, eta(max)(e) 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 degrees C <= T <= 900 degrees C.