Enhancing thermal management in a reversible solid oxide cell system utilizing thermal energy storage

Reversible solid oxide cell (ReSOC) systems hold promise for providing cost-effective and efficient solutions for both long-duration and seasonal-energy storage applications. Endothermic processes in electrolysis mode using steam-hydrogen chemistry require electrical heaters to supply high-grade thermal energy to sustain stack operating temperature, leading to decreased system efficiency. Both high- and low-grade thermal energy are needed in electrolytic operating modes. Thermal energy storage (TES) can be incorporated with stand-alone ReSOC systems to capture exothermic fuel cell waste heat and discharge it for endothermic electrolysis mode when "free" waste heat is not available. This study examines the economic and technological feasibility of integrating various TES approaches with a reversible system, such as two-tank energy storage, steam accumulator, phase-change heat exchangers, and packed-bed regenerator. The thermal energy from the exhaust gas is utilized to charge a TES medium in fuel cell mode, which is later discharged in electrolysis mode to improve the system round-trip efficiency (RTE) and reduce utility heating requirements. A physics-based ReSOC cell-stack model combined with balance-of-plant component models is utilized to assess overall system performance and levelized cost. A techno-economic analysis is conducted to evaluate the levelized cost of storage (LCOS) of the system. This study shows that given free waste heat, the system RTE can be improved by 14.4 % points from its stand-alone case value of 50.4 %, leading to an LCOS of 12.1 cent/kWh by eliminating external heaters. In the absence of free waste heat, the results show that systems integrated with steam accumulators that enable lowgrade TES achieve the highest RTE (57 %) and the lowest LCOS. The increase in RTE corresponds to a 6.6 % points improvement over systems not employing any TES, while the LCOS decreases slightly to 13.3 cent/kWh. The study further indicates that integrating hardware for both high- and low-grade TES via packed beds and steam accumulators, respectively, increases the LCOS by 7 % points compared to systems without any TES.