Driven by the global energy transition and the “dual-carbon” goals, enhancing the operational flexibility of coal-fired units has become pivotal for improving the grid’s renewable energy accommodation capacity. To enhance the frequency-regulation flexibility of coal-fired units and address the pinch-point bottleneck in molten-salt/steam heat exchange, this study proposes a three-tank cascaded thermal energy storage (TES) system integrated with a subcritical unit.

A steady-state model of a 330 MW unit was developed in EBSILON. It incorporated high-, intermediate-, and low-temperature molten-salt loops, and achieved partial boiler-turbine decoupling via main-steam and feedwater bypass circuits.

The simulation results indicate that the cycle electric efficiency reached 63.51% at 100% THA; the heat rate increase of the integrated system was about 1% in hot standby working state of each working condition, and 13.2% at 40% THA with a 5%Pe regulation depth; the coal consumption penalty grew exponentially with regulation depth, providing a basis for defining an economic dead band.

The cascaded TES structure preserves steam exergy while significantly improving load-response rate and frequency-regulation capability, offering both theoretical insights and a practical design paradigm for flexibility retrofits of coal-fired units in power systems with high renewable penetration.