Evaluating the thermodynamic performance of an SOFC and PEMFC combined system

This study investigates the integration of solid oxide fuel cells (SOFC) with proton exchange membrane fuel cells (PEMFC) to improve the system efficiency and minimise exergy losses from unused hydrogen. The paper offers new insights into the efficiency-power density trade-off of SOFC+PEMFC combined systems by simultaneously evaluating the systems’ efficiency trends and their overall volume and mass. The SOFC+PEMFC is thermodynamically analysed and compared for the first time against an SOFC stand-alone system with anode off-gas recirculation (AOGR), another approach to increase efficiency by maximising the direct conversion of fuel into power. Simulations are run to reveal the impact of varying stack operating parameters, fuel utilisation, cell voltage, and operating temperature on the system efficiency, shape of the system’s operational envelope, and overall volume and mass. An exergy analysis identifies major loss sources in the system and proposes pathways for improvement. The results demonstrate that integrating an SOFC with a PEMFC increases system efficiency to 55%, comparable to AOGR, while reducing the total system volume and mass by 20% and 23%, respectively. This study provides new insights into the potential use of SOFCs in volume and mass-limited applications such as long-distance transportation to reduce pollutant emissions. • A high-fuel utilisation SOFC+PEMFC combined system is thermodynamically analysed. • The combined system is benchmarked against SOFC-only systems with and without AOGR. • The system’s efficiency is superior at lower cell voltage and fuel utilisation levels. • The system’s volume and mass are reduced by up to 20% while efficiency is maintained. • An exergy analysis identified pathways to further enhance the system’s efficiency.