A review of solid oxide steam-electrolysis cell systems: Thermodynamics and thermal integration

Abstract

As the world is advancing towards a hydrogen society, solid oxide electrolysis cell systems are gaining increasing attention owing to their overwhelming thermodynamic advantages. Solid oxide electrolysis cell systems operate at high temperatures; therefore, they can achieve a substantially high energy conversion efficiency, which is critical in determining the techno-economic feasibility of green hydrogen production. However, one key concern to be resolved prior to market deployment is maintaining high operating temperatures. In this study, the thermal aspects of currently verified solid oxide electrolysis cell systems are reviewed based on standardized criteria. First, the basic concept of the thermal integration of solid oxide electrolysis cell systems is introduced. Second, standardized thermodynamic indices are introduced to enable consistent performance evaluation and capture thermal characteristics from a system-level perspective. Particularly, this review presents complete information on the system efficiency, specific energy consumption, specific thermal energy consumption, and system thermoneutral voltages. Third, based on the aforementioned discussion, the recently verified solid oxide electrolysis cell systems are reviewed, focusing on their thermal aspects. The analysis on ten verified SOEC systems from different institutions suggest that system thermal integration and operating conditions must be designed considering external thermal energy consumption. Furthermore, several future milestones in solid oxide electrolysis cell system verification are discussed. Currently, five institutions are developing kilowatts to megawatt scale SOEC systems and most of them are considering ironworks and nuclear power plants as external heat sources. This review is expected to shed light on the hitherto overlooked thermal aspect of solid oxide electrolysis cell systems and suggest a future direction for system design and demonstration.