Repurposing of supercritical coal plants as grid storage with adapted 620 °C nitrate salt technology

Energy storage is key to on demand generation of electricity from renewable energies such as wind and photovoltaics. In addition to the use of batteries, repurposing existing fossil-fired power plants with thermal energy storage may offer a very cost-effective option for implementing large amounts of grid-scale energy storage [1]. The maximum operation temperature of state-of-the-art molten salt storage systems is 565 °C [2], at higher temperatures the used nitrate salts start to decompose under ambient air [3]. However, in modern supercritical steam power plants, required steam temperatures exceed 600 °C in order to achieve maximum efficiencies. This gap can be bridged by using a closed gas system, so that gaseous decomposition products that would otherwise be extracted from the tanks are kept in the nitrate storage system. An ongoing decomposition of nitrate salts can thus be suppressed even at temperatures up to 620 °C [4, 5]. In this study, the optimal design of such a gas system is presented based on a echno-economic analysis. In addition, the increase in overall electrical efficiency of this new 620 °C storage system compared to the state-of-the-art solutions is determined for the first time. A cost saving potential of 13% is identified