Thermodynamic Cycle of a Solid Oxide Fuel Cell with Internal Methane Convertion under the One-Way Conversion Approach

Isobaric-isothermal processes in a fuel cell with internal methane convertion are considered in terms of increments of thermodynamic potentials, enthalpy, and Gibbs energy in a thermodynamic cycle using the one-way conversion approach. This approach appears to be the simplest for determining the thermodynamic, or ideal thermal, efficiency of an electrochemical reaction and readily explains the paradoxical cases of efficiency exceeding unity found in the literature. It is shown that a solid oxide fuel cell operating on methane is an example of a system with a complex process that requires identification of the electrochemical step in order to determine the thermodynamic efficiency. In this case, the ideal electrical efficiency can additionally be defined as a criterion for the thermodynamic effectiveness of the direct conversion of methane chemical energy into electricity. This criterion allows for a reasonably objective evaluation of the thermodynamic efficiency of a fuel cell with methane conversion without analyzing the entire power system, including regenerative heating of fuel and oxidant and a heat recovery unit. Using the example of the dependence of the ideal electrical efficiency of a methane–air fuel cell on the oxidant excess ratio, it is demonstrated that the stage of fuel and oxidant mixing must be separated from the electrochemical stage itself for a correct thermodynamic analysis of the multistep process in a fuel cell.