Repurposing of supercritical coal plants into highly flexible grid storage with adapted 620 °C nitrate salt technology

Abstract

Energy storage is essential for on-demand electricity generation from renewable sources like wind and photovoltaics. Repurposing fossil-fired power plants with thermal energy storage (TES) offers a cost-effective solution for large-scale grid energy storage. This paper explores converting supercritical coal plants into flexible grid storage systems using adapted nitrate salt technology. State of the art TES systems are limited by their maximum operating temperatures at up to 560 °C, but higher temperatures cause nitrate salts to decompose. Supercritical steam power plants require steam temperatures above 600 °C for optimal efficiency. To address this, a closed gas handling system can keep gaseous decomposition products within the nitrate storage system, stabilizing the salt at temperatures up to 620 °C. This study presents the optimal design of such a gas system based on a techno-economic analysis and determines the overall electrical efficiency improvement of the supercritical power plant equipped with the adapted 620 °C storage compared to a subcritical power plant with 560 °C. The costs of repurposing power plants with two-tank and single-tank (thermocline) storage systems are evaluated, identifying potential cost savings of up to 18 % with the 620 °C single-tank system. The gas handling system costs are minimal. Compared to grid-scale lithium-ion batteries with a 10-h discharge duration, the levelized cost of storage (LCOS) for the proposed system is lower for low charging electricity costs. The 620 °C nitrate salt technology could further reduce LCOS in most cases worldwide. This paper demonstrates the economic feasibility of a 620 °C molten salt system, highlighting cost savings over conventional options like batteries. The research provides valuable insights into repurposing existing fossil fuel infrastructure for a sustainable and efficient renewable energy transition.