Investigating the partial load of reversible solid oxide cell systems: A focus on balance of plant and thermal integration

Solid oxide cells are promising electrochemical devices capable of operating in both electrolysis and fuel cell modes with high electrical efficiency. This work investigates the design and partial-load operation of a reversible solid oxide cell (rSOC) system for steam electrolysis and hydrogen-based power generation, when adopting a unified balance of plant for both modes and molten salt thermal energy storage for thermal integration. Different configurations are compared with the aim of widening the part-load window, taking into account the electrochemical behavior as well as the changes in heat exchange properties. The definition of system efficiency losses with respect to the stack efficiency is proposed, helping in identifying the main causes of efficiency degradation throughout the part-load window. Results show that pre- or post-stack heaters are required when switching from exothermic to endothermic conditions. Moreover, they prove essential in keeping the rSOC in thermal balance also when the reaction is slightly exothermic. The use of electric heaters and hydrogen combustors is compared, and electric heaters appear to have the least impact on system efficiency at lower loads. For all configurations, the highest efficiency is obtained close to the thermoneutral point, which optimizes the trade-off between stack efficiency and system efficiency losses. Heat recovery in fuel cell mode is prominent at nominal load and could be beneficial in facilitating thermal integration between the two operational modes. However, the magnitude of its reduction at partial load is greater than the corresponding reduction in heat demand in electrolysis mode, leading to increased thermal imbalances between fuel cell and electrolysis modes.