Thermodynamic processes in solid oxide and other fuel cells

Following a review of thermodynamic processes in a generalized fossil-fuelled power system, the theoretical efficiencies of systems based on the isothermal oxidation of methane are discussed. The fuel cell is introduced as a device that converts the change of Gibbs free energy (Delta G) in isothermal oxidation directly to electricity. Ideal efficiencies of fuel cell systems are presented. The Gibbs free energy change in isothermal oxidation of methane is 99 per cent of its lower heating value (LHV). However, direct electrochemical oxidation is not practical yet. Taking this and other factors into account the reversible electrical work from an internally reforming fuel cell could represent similar to 79 per cent of the lower heating value at 600 degrees C, falling to similar to 67 per cent at 1000 degrees C. With a bottoming heat engine the combined reversible work could be >80 per cent of LHV over this range. Adding such a device as a topping element to a conventional heat engine based system seems a logical next step in the evolution of the efficient fossil-fuelled power plant.