Conversion of combined heat and power coal-fired plants to Carnot batteries - Prospective sites for early grid-scale applications

Abstract

Repurposing fossil fuel-fired plants to electricity storage systems known as Carnot batteries (CB) has been proposed before. This technology provides a prospect of high-power, high-capacity systems for medium and long duration storage. Significant benefits on capital cost from second use of the existing infrastructure and low cost per added MWh are expected to outweigh low round-trip efficiency. Retrofitting large coal plants is still demanding in terms of absolute capital, lack of previous experience and uncertain business. This work proposes smaller coal-fired combined heat and power plants with 50 MWe output as a suitable prospective site for conversion to CB instead of large power plants. Higher overall efficiency with heat providing an additional stream of revenue, plant flexibility, or smaller system size make such application more beneficial, especially for first-of-a-kind installations. Therefore, this paper illustratively investigates the techno-economic prospects of the conversion of a cogeneration coal plant to a CB. The proposed system is described. It is based on direct electrical heating for the power to heat process, thermal energy storage based on either molten salts or gravel packed bed and finally, heat exchangers integrated to operate as steam generators. The sizing of major components is followed by costing. In case of a backpressure turbine, packed bed storage and discharge duration of 5 h the specific cost of electrical storage system is around 200 €/kWhe, while in the case with no heat production and condensing turbine (full condensation regime), it is around 100 €/kWhe. The operation regimes and expected revenue streams from both electricity and heat are evaluated. A comparison with existing technologies is performed and the levelised cost of storage in presented cases varies from 35 to 179 €/MWhe for the fully condensing regime and 35 to 291 €/MWhe for the backpressure cogeneration regime.