Efficiency analysis and operating condition optimization of solid oxide electrolysis system coupled with different external heat sources

•Three scenarios of coupling between SOEC system and external heat sources are studied.•Efficiency is analyzed from specific system loss and specific energy consumption.•The system performance maps and stack operating windows are obtained.•The selection criteria of operating parameters in the three scenarios are established. Coupling external heat sources effectively reduces energy consumption and improves the efficiency of the solid oxide electrolysis (SOEC) system. While an in-depth analysis of the SOEC systems in different coupling scenarios is still needed. According to the internal heat demand and the temperature of external heat sources, three coupling scenarios are defined in this study, i.e., no heat source coupling (Scenario 1), coupled low-temperature heat source (Scenario 2), and coupled high-temperature heat source (Scenario 3). Firstly, the influence of critical parameters on system efficiency is analyzed from two perspectives, including specific system loss and specific energy consumption. The results present that efficiency η1 mainly increases with the rise of steam utilization. Efficiency η2 is not significantly affected by the parameters and is primarily positively correlated with the fuel electrode steam concentration. Efficiency η3 is negatively related to steam utilization, fuel electrode steam concentration, and fuel electrode flow rate, while positively to stack operating temperature. Meanwhile, for Scenarios 1 and 2, the exothermic state of the stack module should be avoided. Furthermore, the system performance maps and stack operating windows in the three scenarios are obtained by changing multiple operating parameters simultaneously and setting constraints. For Scenarios 1 and 2, high system efficiency, high hydrogen production, and long lifetime cannot be achieved simultaneously, so operating parameters need to be selected at a compromise. For scenario 3, high fuel electrode flow rate and low steam utilization can basically meet the above requirements. Both high stack operating temperature and high fuel electrode steam concentration can improve hydrogen production, while the former is mainly beneficial to efficiency η3, and the latter can improve the three efficiencies. The results obtained can be used not only to choose the optimal working conditions of various external heat sources but also to guide the online control and load regulation during system operation.