A Review of Carnot Battery Technologies for Retrofitting Fossil Fuel Power Plants

This paper reviews the development trends of Thermal Energy Storage (TES), focusing on Carnot batteries as a key solution for addressing both grid variability and carbon reduction. A Carnot battery stores surplus electricity as heat using electric heaters or high-temperature heat pumps, and later reconverts the stored heat back into electricity through a steam turbine when needed. From a retrofit perspective, replacing a coal boiler with TES and integrating it with existing steam turbines allows for the reuse of transmission and balance-of-plant assets, providing a capital-efficient pathway to long-duration, dispatchable capacity. As a result, Carnot batteries are gaining attention as a viable option for large-scale, long-duration storage and for repurposing stranded coal assets. Government-led demonstrations and policy support are advancing in Germany, the United States, and Korea, with multiple pilot projects reported in Europe and the U.S. Building on these developments, this paper examines the market prospects and policy needs for Carnot battery TES, synthesizes current retrofit design approaches, and outlines future directions necessary for practical deployment at fossil fuel plants.