Electrical efficiencies of methane fired, high- and low- temperature fuel cell power plants

The important system difference between power plants based on low temperature and high temperature fuel cells is that gas reforming and shift conversion is thermally decoupled from the cell in low temperature cell power plants whereas the gas process steps are performed at close to the elevated fuel cell temperatures in high temperature fuel cell power plants. This article elucidates the consequences: assuming equal electrical efficiencies for the respective cells (50%) it is shown that thermal decoupling leads to energy and exergy losses and sizably lower electrical system efficiencies because heat for the generation of the process steam necessitates the combustion of methane. Also hydrogen losses in the step for preferential oxidation of carbon monoxide (Selox process) and several heat transfer steps add to the lower efficiency of low temperature systems. Low temperature fuel cell power plants need 15–17% more fuel than high temperature fuel cell power plants for the same amount of electric energy. The theoretical comparison of an adiabatic LT and HT fuel cell process reveals that, with postulated electrical cell efficiencies of 50%, the theoretical electrical efficiency of the LT process is 6–7% points lower than that for the HT-process (35 vs. 41%). For exergy efficiencies also taking into account rejected heats, the numbers read 43 and 58%.