Design and simulation of reversible solid oxide cell systems for distributed scale energy storage

Abstract

Reversible solid oxide cells (ReSOCs) are an energy conversion technology that can be used to either produce power from fuel when electricity is needed (fuel cell mode), or produce fuel from electricity when excess energy is available (electrolysis mode). By leveraging C-O-H reaction chemistry and operating at intermediate temperatures, these cells can be mildly exothermic in both operating modes, eliminating the need for external heat input or high over-potential operation during electrolysis. Tanked storage of fuel (hydrogen, carbon monoxide, methane) and exhaust (water, carbon dioxide) allows ReSOC systems to provide stand-alone electrical energy storage services. However, tanks are inherently dynamic, and balance-of-plant hardware such as heat exchangers and compressors must be selected to operate efficiently in both modes. This work assesses the effects of tank dynamics and hardware off-design behavior on system design, performance, and cost for a 100 kW/800 kWh ReSOC system operating with a 16-hour round-trip cycle, suitable for distributed energy applications. A novel floating piston storage tank that allows near-constant storage pressure and reduced overall tank volume is proposed and modeled, and off-design performance of balance-of-plant components is evaluated through the use of performance maps and flow correlations. Optimization is employed to minimize the levelized cost of storage (LCOS). These efforts determined that the floating piston tank concept significantly reduces the impact of tank dynamics on system design and performance, allowing efficient and reversible operation. Additionally, separate storage of water as a liquid yields lower costs and improved hardware compatibility relative to a system configuration that stores water as vapor with other exhaust gases. ReSOC system optimization indicates that a pressurized stack system with pre-storage water removal accomplishes a levelized storage cost of 16.4 cents/kWh-cycle, an energy density of 100 kWh per cubic meter, and round trip efficiency of 68.8%. These results are shown to compare well with re-dox flow and conventional battery technologies.