Exergy efficiency improvement of the solid oxide fuel cell cogeneration system considering operation safety of the stack

Solid oxide fuel cells can simultaneously generate electricity and provide heat due to their high operating temperatures, thereby enhancing energy utilization efficiency. Although numerous studies have employed exergy analysis to better understand the energy conversion within cogeneration systems, they have typically treated stack current as a decision variable. This approach often forces the stack's operating point off-design, failing to reduce the system's exergy destruction during the heat recovery process. To address these issues, this study improves the exergy efficiency of a 2 kW system while considering operating safety of the stack. Based on detailed analysis of exergy destruction and the off-gas exergy-temperature relation, the primary source and cause of exergy destruction are identified. Then the exergy destruction is reduced via the system layout design and heat exchange parameter exploration. Results indicate that the afterburner is the dominant destruction source, accounting for over 60% of total system losses. Driven by the non-linear exergy-temperature relation, raising the afterburner off-gas temperature improves the system exergy efficiency by 10%. However, at low fuel utilization, the off-gas reaches approximately 1150 °C, causing thermal shock risks. Placing the afterburner downstream of the hydrogen preheater helps to control the off-gas temperature below the safety limit of 1000 °C while reducing afterburner exergy destruction by 0.69 kW without increasing air preheater losses. Consequently, the improved system achieves 71.3% exergy efficiency at 77% fuel utilization.