Thermal Self-Sufficiency of a Proton-Conducting rSOC System: Is It Feasible?

Reversible solid oxide cell (rSOC) systems suffer from a thermal mismatch between fuel cell (FC) and electrolysis cell (EC) operation, which limits overall system efficiency. Although thermal energy storage (TES) has been widely proposed to mitigate this imbalance, the feasibility of achieving thermal self-sufficiency remains unclear. In this study, thermal self-sufficiency is defined as a condition in which external electrical heating demand is negligible (<5% of EC stack power). A comprehensive thermodynamic model is developed from a system-level perspective to quantify energy flows throughout the rSOC system and evaluate the effects of key operating parameters. The results show that thermal self-sufficiency is not an inherent characteristic of the rSOC system but is strongly governed by operating conditions. Thermally self-sufficient operation is achieved only above a critical current density of 0.42 A cm–2, at operating temperatures of 550 °C and below, and at steam utilization (SU) values above 0.4. Higher current density and lower operating temperature promote thermal self-sufficiency by enhancing polarization heat generation, whereas higher SU reduces the steam-generation duty. However, the first two approaches inevitably reduce electrical round-trip efficiency (RTE). Reducing the area-specific resistance (ASR) effectively alleviates this trade-off. When the ASR is reduced to half of its baseline value, the electrical RTE increases from 34.3% to 61.5% while thermal self-sufficiency is maintained. These findings provide quantitative criteria for thermally self-sufficient operation and highlight ASR reduction as a key pathway toward the commercialization of competitive low-temperature rSOC systems.