Thermodynamic analysis and parameter optimization of a hybrid system based on SOFC and graphene-collector thermionic energy converter

The exhaust heat released from SOFCs (solid oxide fuel cells) possesses tremendous energy that can be recovered for energy cascade utilization. Nevertheless, low conversion efficiency or power density limits the SOFC's high-grade waste heat recovery capabilities for the cogeneration of electric power. To address this challenge, a novel hybrid system coupling a SOFC with a GTEC (graphene-collector thermionic energy converter) is proposed, where the GTEC harvests the high-grade exhaust heat produced by the SOFC and generates extra electricity. It is found that the maximum power density of the hybrid system can reach 0.774 W/cm2 at 1073 K, which is 1.20 times higher than that of the sole SOFC, indicating that the hybrid system offers a considerable improvement in output performance. Additionally, the optimal operating conditions and major parameter designs of the hybrid system are determined from the perspective of finite-time thermodynamics. Choosing the optimal area ratio, increasing the SOFC operating temperature, enhancing the heat transfer coefficient, decreasing the thermal emissivity, and fabricating the perfect optical reflector can further improve the optimal performance of the hybrid system. Compared with other SOFC-based hybrid systems, the SOFC-GTEC provides better output performance, proving that the GTEC can more efficiently utilize the exhaust heat produced by SOFC than other energy harvesting devices. This work provides crucial theoretical guidance on the optimal designs and parametric analysis of SOFC-GTEC hybrid systems, thus paving the way towards developing high-performance SOFC cogeneration systems.