Modeling and analysis of a reversible solid oxide cell system

Abstract

Energy storage systems are an inevitable part of future energy systems with the world switching to renewable energy sources and smart grid technologies. A reversible solid oxide cell (ReSOC) system is an electrochemical power to gas to power system poised to serve as an intermediary between energy demand and supply. The power generation is the fuel cell (SOFC), and the power storage is the electrolysis (SOEC) mode. In this study, a small scale ReSOC system comprising of the ReSOC stack and balance of plant (BOP) components (such as compressor, heat exchangers, tanks, etc.) is modeled using the electrochemical and thermodynamic relations. Engineering Equation Solver (EES), a powerful tool for thermodynamic analysis by FChart is used for the modeling and analysis of the ReSOC system. The performance of both the cell and the stack were validated with literature data. The energy and exergy analysis of the stack and system was carried out using performance metrics such as power, energy and exergy efficiency, exergy destruction, roundtrip efficiency, and exergetic performance coefficient. The system was further analyzed at base case conditions using the Levelized cost of storage (LCOS) and storage cost method. The result of the analysis carried out in this thesis can be summarized as follows. The stack overall performance is better than the system overall performance primarily because of the extra power consumption by the BOP components. Furthermore, the performance of the system is not only dependent on the system operating condition but also on the method of operating the stack and the composition of the reactant gas in the system. The SOEC mode (83% and 78% exergy and energy efficiency, respectively) performs better than the SOFC mode (68% and 65% exergy and energy efficiency, respectively) both exergetically and energetically and the system had a roundtrip efficiency of 0.51 at the base case. The economic analysis results showed that for both storage cost and LCOS, the system considered in this study is competitive with conventional battery storage technologies and flow batteries. With a storage cost of 13 cents/kWh and LCOS of 32 cents/kWh, the system is expected to be competitive with large scale compressed air energy storage systems after performance improvements. Exergoeconomic analysis showed that the major drivers of the exergetic cost rates are the storage tanks and ReSOC stack capital costs. The SOFC mode of operation had a better exergoeconomic performance than the SOEC mode of operation despite the SOEC having a better exergy performance.