An entropy analysis model and irreversibility assessment of solid oxide fuel cells based on the Gibbs relation of charged particles

Abstract

Energy conversion is the main purpose of various fuel cells, but the irreversibility of thermodynamic processes – including electrochemical reactions and heat and mass transfer in fuel cells – lead to a decrease in energy efficiency. In this paper, we develop a systemic theoretical model to evaluate the irreversibility of solid oxide fuel cells (SOFCs). By combining the conservation equations for energy, mass, and momentum with the Gibbs relation for charged particles, we establish the energy-balance equation for SOFCs and derive the entropy generation rates (EGRs) associated with electrochemical reactions and charged particle transfer processes. We solve the coupling multiphysics model in a three-dimensional computational unit of a SOFC, and calculate the EGRs and entropy flux rate based on the proposed thermodynamic model and the numerical results. We analyse the conversion and transfer mechanisms of energy based on the distributions of different local EGRs and the entropy flux rate, and the influence of various thermodynamic processes, such as mass transfer, heat transfer, and flow, on the overall operation of SOFCs. The results show that the EGR due to electrochemical reactions in the SOFC is the largest and is mainly due to activation and concentration polarisation. Complex heat and mass transfer processes occur in the corner region formed at the intersection of the channel, the current junction, and the electrode, leading to a higher localised EGR. We also evaluate the effects of variations in the anode gas mass flow rate, the oxygen fraction, and the channel length on thermodynamic efficiency. This study contributes to the understanding of the complex mechanisms of irreversible losses in SOFCs and provides guidance for further improving the thermodynamic efficiency of SOFCs.