Energy Dense Storage Using Intermediate Temperature Reversible Solid Oxide Cells
Fig. 1 shows a simplified schematic of the envisioned process. The system operates in either fuel cell (SOFC) or electrolysis (SOEC) modes with intermediate tanked storage of reactants and products. The energy storage system charges by operating the ReSOC stack as an electrolyser, in which exhaust species – primarily H2O and CO2– are discharged from a storage tank, heated, delivered to the stack, and co-electrolyzed to produce a fuel mixture with an input of electricity. The generated fuel is cooled and compressed to a separate storage tank for later use. The system discharges by operating in SOFC mode, in which the tanked fuel mixture is preheated, delivered to the stack and electrochemically oxidized, producing electricity and exhaust species to re-fill the exhaust tank.
An oxidant flow is required in the SOFC mode to provide oxygen for the electrochemical reactions and regulate stack temperature. In SOEC mode, the airflow acts as a sweep gas to increase electrical efficiency by diluting generated oxygen in the oxygen channel and serves as a heat sink for exothermic operation. Some unique challenges arise in designing ReSOC systems, including: (i) overcoming the thermal disparity between fuel cell (typically exothermic) and electrolysis (typically endothermic or near thermoneutral) operation using a unitized cell-stack and common hardware, (ii) selecting configurations and operating conditions (T, p, utilization, composition) that promote high efficiency in both operating modes, and (iii) thermal integration between high temperature stack operation and lower temperature, pressurized storage. Furthermore, because reaction products are tanked for use in the opposite mode of operation, they must be processed to enable compression to storage pressure with minimal energetic cost.
ReSOC systems simplify system thermal management by combining co-electrolysis with in-situ fuel synthesis (i.e., methanation) and electrochemical oxidation with internal fuel reforming such that the stack is slightly exothermic in both SOFC and SOEC modes. By using this strategy, the cell can operate exothermically at electrolysis voltages that would otherwise be endothermic, enabling increased electrical efficiency without utilizing an external heat source. This approach enables high efficiency and thermally self-sustaining operation, but requires operating the ReSOC stack under conditions that promote methane formation in electrolysis mode. Methanation is catalysed on nickel present in the ReSOC fuel electrode and is promoted by low temperature and high pressure stack operation. Our previous analyses have demonstrated the potential for roundtrip efficiencies of nearly 74% with 20-40 kWh/m3 tanked energy density depending on the type of water management employed in the system. Separately storing condensed water increases energy density of storage to 38 kWh/m3, but limits efficiency to 68% based on the energetic cost of evaporating reactant water during electrolysis operation. Further increases in energy density (to 90 kWh/m3) require higher storage pressures (e.g., 50-bar nominal) which lower roundtrip efficiency to about 65%.
To be competitive with advanced batteries such as sodium sulphur and vanadium redox flow batteries, ReSOC technology would benefit from higher energy storage density. In the present work, we explore system design strategies which integrate methanation reactors to achieve >300% increase in volumetric energy density while roundtrip efficiency exceeds 68%. The ReSOC technology explored within the present study is based on intermediate temperature (600-650°C) LSGM cells on SLT supports and nickel nano-particle infiltrated fuel electrodes for enhanced triple-phase boundary areas which lead to high power density. System design and operational modes are discussed, along with significant balance-of-plant thermal integration and performance analysis. The resulting energy density of the system for modest tank storage pressures of 25 bar exceeds 130 kWh/m3.