Prospects for converting decommissioned coal power plants into large-scale thermal energy storage systems are being advanced by researchers from the Technical University of Denmark and the German Aerospace Center. Retrofitting fossil fuel-fired power plants with an electric heater and thermal storage capacity to serve as solar-powered Carnot batteries offers an economically and environmentally sound solution for the decarbonization of power production.

These battery systems store electricity in the form of heat through storage media and transform the heat back to electricity when needed. To demonstrate the feasibility of this technology, a techno-economic analysis focused on the retrofitting of an existing Chilean 300 MW coal power plant with an electric heater and a steam generator that uses air as the heat transfer fluid. The packed rock bed thermal energy storage system facility offers a storage capacity of 1.37 GWh and a storage temperature of 730° C.

Heaters, valves, and inlet and outlet pipes are placed on top of the system to avoid additional excavation and allow the rock bed to be installed partially below ground level. Photovoltaic plants power the storage system's electric heater connected to the storage tank, which in turn provides heat to the steam generator connected to the coal power plant's steam turbine and condenser.

The analysis outlined in the Journal of Energy Storage indicates that the retrofitted plant may achieve an annual net power production of around 443 GWh for 4 hours of storage capacity, 797 GWh for 8 hours of storage capacity, and 1,150 GWh for 12 hours of storage. The system's highest annual round-trip efficiency is achieved for a storage capacity of 8 hours and the lowest levelized cost of energy is reached for the largest storage capacity at $95.97/MWh. The researchers maintain that this value is competitive with both state-of-the-art conventional power plants and alternatives such as those powered with thermal storages based on molten salt.