Performance Assessment of Electric Energy Storage (EES) Systems Based on Reversible Solid Oxide Cell

This paper focuses on the performance assessment of a novel and efficient EES (electric energy storage) system based on ReSOC (reversible solid oxide cell) technology. The ReSOC is an electrochemical energy conversion device working at high temperature (600-1000°C) that can operate reversibly either as a fuel cell (SOFC) or as an electrolyzer (SOEC). In this study, a ReSOC unit fed by mixtures of CH4, CO, H2O and H2 is proposed and analyzed. In particular, in the SOFC mode, where electricity is generated, the reactant gas, mainly formed by CH4 and H2, is converted into a mixture of H2O and CO2. The exhausts from the SOFC are used as the reactant gas for the SOEC operation. During the electrolysis process, CH4 can be also produced thanks to the methanation reaction that, under proper operating conditions, occurs at the cathode of the solid oxide cell. The ReSOC unit behavior is investigated by developing a thermo-electrochemical model, able to predict its performance (i.e. roundtrip efficiency, polarization curve, thermally self-sustaining conditions) under different operating conditions. The ReSOC model, built with a modular architecture, is performed through thermodynamic, thermochemical and electrochemical sub-models taking into account mass and energy balances, chemical reactions (reforming, shifting reactions and methanation) and electro-chemical relationships. Available literature data have been used for the model validation and a calibration procedure has been performed in order to evaluate the best fitting values for the model parameters. Furthermore, in order to estimate the thermoneutral conditions in SOEC operating mode, the ReSOC thermal behavior has been analyzed under different operating temperatures. Results pointed out that, by feeding the cell with a syngas mixture, the reforming reaction (in the SOFC mode) and the methanation reaction (in the SOEC mode) allow to simplify the cell thermal management. Moreover, the best performance in terms of stack roundtrip efficiency (about 70%), can be reached operating the ReSOC at low temperature (700°C).