Study on the retrofit of coupled Carnot battery in retired coal-fired power units for grid energy storage transformation

This study investigates the retrofitting of decommissioned coal-fired power plants into long-duration energy storage systems by integrating a Carnot battery. The proposed system couples a high-temperature heat pump, two-tank molten salt thermal energy storage, and the original steam turbine to form a closed-loop electricity–heat–electricity pathway, enabling full reuse of existing infrastructure. A detailed thermodynamic model is developed to evaluate the system's performance under varying working fluids and operating conditions. Results show that argon achieves the highest round-trip efficiency (59.67 %) at a moderate compression ratio, while CO₂ offers superior volumetric power density. Exergy and sensitivity analyses reveal that major irreversibilities occur in the molten salt evaporator and heat pump subsystems. To enhance adaptability under variable conditions, a MATLAB–EBSILON-based closed-loop control strategy is proposed, which reduces exergy losses by up to 16.2 % and improves part-load efficiency. Techno-economic evaluation indicates an annual arbitrage profit of USD 77.01 million, a payback period of 6.39 years, and a levelized cost of storage of 149.09 USD/MWh, significantly lower than mainstream battery technologies. These findings confirm the strong technical and economic viability of the proposed system, offering a scalable pathway for repurposing coal-fired plants and advancing the low-carbon energy transition.