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The method of exergy analysis is presented for a solid oxide fuel cell (SOFC) power plant involving external steam reforming and fed by methane and ethanol. The parameters for optimal operation of the integrated SOFC plant are specified after minimizing the existing energy and exergy losses. A comparison of methane and ethanol as appropriate fuels for a SOFC-based power plant is provided in terms of efficiency of exergy assuming the minimum allowable (for carbon-free operation) reforming factors for both cases. Then, a parametric analysis provides guidelines for practical design. The analysis employs parameters such as the extension of the steam reforming reaction and the hydrogen utilization in the SOFC and proves that the appropriate adjustment of the plant performance can be achieved by simply interrelating them in order to control the energy losses to environment and the participation of combustion processes in the power cycle. Energy losses from the SOFC stack are found also of negative impact to plant efficiency and are minimized through an appropriate thermal management between the mixtures incoming and outgoing the stack, attaining the adiabatic regime of SOFC operation. It is concluded that the exergy calculations pinpoint the losses accurately and that the exergy analysis gives a better insight of the system’s process.

A simplified model has been formulated in order to easily and adequately determines the efficiency of a solid oxide fuel cell (SOFC) stack system, independently on the fuel choice. Simple analytical formulas for the calculation of electromotive force (emf) generated in a SOFC system as well as for the estimation of its efficiency are proposed. It was found that both emf and efficiency are functions of the amount of carbon atoms of the fuel (oxygenated or not) as well as of the operational temperature. The steam to fuel molar ratio, defined as reforming factor, m, for each fuel and temperature has been adjusted to be low enough so as to ensure optimal conditions in the SOFC operation but high enough to avoid carbon formation within the cell. It has been also demonstrated that the efficiency values predicted by the model are of low relative difference compared with the numerical ones and, therefore, the approach presented here can be considered as sufficient enough for practical use.

An energy–exergy analysis was undertaken in order to optimize the operational conditions of a SOFC-based power plant fueled by ethanol. A certain plant configuration was contemplated, equipped with an external steam reformer, an afterburner, a mixer and two heat exchangers (preheaters). The most significant operational parameters are enunciated and their influence on the energy and exergy balances of the plant is discussed and optimized. An optimization strategy is presented and optimally controlled unit operations are specified through minimization and allocation of exergy costs.