Design and analysis of reversible solid oxide cell systems for electrical energy storage

Abstract

Electrical energy storage is projected to be a critical component of the future world energy system, performing load-leveling operations to enable increased penetration of renewable and distributed generation. Reversible solid oxide cell (ReSOC) technology has the potential to play a major role in stationary electrical energy storage markets. ReSOCs operate in two distinct modes: fuel producing (electrolysis mode) and power producing (fuel cell mode). A stand-alone energy storage system is realized from this technology by coupling the two operating modes with intermediate storage of reactant and product species. In this dissertation, ReSOC energy storage systems are designed and analyzed with computational modeling to establish suitable system configurations and operating conditions that achieve high roundtrip efficiency. A critical feature of the ReSOC system that enables high roundtrip efficiency is that the ReSOC is operated at conditions where methane is generated in electrolysis mode to offset the typically endothermic conversion process. Methanation is promoted by low temperature and high pressure conditions, meaning that intermediate-temperature ReSOCs (<700°C) are important to achieving high system performance. Doped lanthanum gallate (LSGM)-electrolyte ReSOC characteristics are leveraged in this study. The results include thermodynamic analysis of ReSOC systems, physically-based calibrated modeling of intermediate temperature ReSOCs, steady-state system simulation at distributed (100 kW) and bulk (>10 MW) scales, and bottom-up system costing. System modeling results suggest that dc roundtrip energy storage efficiency of 65-74% are achieved for a 100 kW system. Maximum efficiency is achieved when the tanked species are maintained in the vapor phase to mitigate the energetic requirement of steam generation; although the energy density suffers within this configuration. The bulk scale system achieves 74% roundtrip efficiency at optimal stack operating conditions of 680°C, 20 bar, and 70% fuel utilization. Economic calculations estimate bulk-scale (250 MW / 500 GWh) storage cost of 1.7 ¢/kWh based on the system capital cost. This storage cost is lower than compressed air and battery technologies and comparable to pumped hydro, but improvements in cell technology and additional system simulation and hardware selection must be addressed before commercialization.