Retrofitting coal-fired power plants for grid energy storage by coupling with thermal energy storage

Grid energy storage is key to the development of renewable energies for addressing the global warming challenge. Although coal-fired power plant has been coupled with thermal energy storage to enhance their operational flexibility, studies on retrofitting coal-fired power plants for grid energy storage is lacking. In this work, molten salt thermal energy storage is integrated with supercritical coal-fired power plant by replacing the boiler. Electric resistive heating is applied for the charging process using curtailed electricity or during periods with low grid demand. During discharging, the heat stored in the molten salt is exchanged to steam for electricity generation. A techno-economical analysis is performed on the coupled system and compared with other grid energy storage technologies. Results show that the integrated plant has higher thermal efficiency than the original coal-fired power plant, especially at low load for peak shaving purpose, which is due to the less exergy loss by avoiding the exhausted flue gas and the large temperature difference of heat transfer in boiler of the original plant. At full load, the round-trip efficiency of the integrated plant is ∼ 41.8 %. Although this efficiency is lower than other energy storage technologies, the levelized cost of electricity of the integrated plant is generally lower due to the reduction in initial investment utilizing existing infrastructures of coal-fired power plants. With longer discharging duration (≥10 h), the integrated system shows a similar levelized cost of electricity as that of compressed air energy storage. Replacing the boiler with thermal energy storage would also greatly reduce the CO2 emission and various pollutants from coal combustion.