Dual-temperature molten-salt thermal storage for bidirectional flexibility in coal-fired power units

The increasing penetration of variable renewable energy requires coal-fired power units to provide greater peak-shaving capability and faster load-following support. This study proposes a dual-temperature molten-salt hybrid integrated thermal storage configuration (HITSC) for a 600 MW supercritical coal-fired unit and evaluates its steady-state and dynamic performance. A two-tank Solar Salt storage system operating between 250 and 560°C is coupled to the steam cycle through charging and discharging pathways. In charging mode, main-steam heat extraction transfers high-grade heat to the molten salt and creates downward regulation capacity. In discharging mode, stored heat is released to produce auxiliary intermediate-pressure steam, thereby increasing unit output. The model results show that HITSC provides 30.445 MW of downward regulation at 50% turbine heat acceptance and up to 45.999 MW of upward regulation at 75% turbine heat acceptance. The condenser-outlet extraction route performs better than the post-high-pressure-heater route, reducing coal consumption and CO2 emissions under part-load discharging conditions. The dynamic analysis further confirms stable charging behavior and rapid discharging response, while the exergy assessment identifies finite-temperature-difference heat exchange as the dominant source of irreversibility. The proposed configuration therefore offers a practical retrofit option for improving the bidirectional flexibility of existing coal-fired units in renewable-rich power systems.