Thermodynamic and techno-economic analysis of a cascade thermal energy storage system integrating molten salt and hot-water tanks with a 350 MW CHP unit

To enhance the peak-shaving flexibility of combined heat and power (CHP) units under high renewable penetration while ensuring district-heating security, this study investigates a 350 MW supercritical extraction CHP unit and proposes a cascade thermal energy storage (TES) scheme comprising a unit-side high-temperature molten-salt TES and a district-heating-side hot-water tank. A thermodynamic model is developed in EBSILON and validated under multiple operating conditions. Four charging paths and three discharging configurations are developed around the charge–discharge coupling path to determine the optimal cascade TES configuration. Under heating conditions, the molten-salt system is coordinated with the hot-water tank to realize “energy time shifting + cascade utilization,” and the economic performance is quantified using an intra-day dispatch model under typical-day constraints. Results indicate that the B2 charging path (reheat-steam extraction with condensate return to the deaerator) provides the best overall performance. Based on B2, the B2(a) discharging configuration (raising deaerator outlet feedwater to boiler feedwater temperature) achieves the best thermodynamic performance, with a full charge–discharge cycle thermal efficiency of 41.338%. The maximum heating capacity increases from 312.2 MW (baseline) to 412.4 MW with molten-salt coupling and further to 562.3 MW with additional hot-water storage, significantly expanding the heat–power feasible operating region. Under the specified electricity–heat constraints, the typical-day net profit increases from 5.0362 to 5.1146 million CNY/day, and the annual net profit increases from 1.7293 to 1.7556 billion CNY; the hot-water tank contributes an additional annual benefit of about 1.334 million CNY with a simple payback period of about 0.742 years.