Comparative Study on the Performance and Economics of Different Heat-Release Pathways in a Coal-Fired Power Unit Coupled with Molten Salt Thermal Storage

To improve the flexibility of coal-fired power units and support renewable energy integration, molten salt thermal storage has been widely considered a promising retrofit option. However, under unified operating conditions, the comparative effects of different heat-release pathways and steam extraction ratios on flexibility, full-cycle thermodynamic performance, and economic performance have not been sufficiently clarified. In this study, a thermodynamic model of a 600 MW subcritical coal-fired power unit coupled with a two-tank molten salt thermal storage system was established in Ebsilon and validated against the design heat-balance data under typical load conditions, with maximum relative deviations of 0.06% for power output, 0.95% for main steam flow rate, and 1.24% for heat consumption rate. Three representative heat-release pathways were comparatively investigated under identical heat-storage conditions, with steam extraction ratios ranging from 2% to 18%. The results show that increasing the extraction ratio raises the thermal storage capacity from 9.762 to 84.636 MWh and enhances the downward peak-shaving capability, but weakens the full-cycle thermodynamic performance. Among the three schemes, Scheme 2 exhibits the strongest upward peak-shaving performance, with upward peak-shaving energy increasing from 2.893 to 24.395 MWh, and also yields the highest annual net profit (0.546–4.342 million CNY). Scheme 3 exhibits the best full-cycle thermal and exergy efficiencies, with full-cycle thermal efficiency of 42.76–41.56% and full-cycle exergy efficiency of 38.34–37.27%. In addition, Schemes 1 and 2 show significantly higher round-trip efficiencies than Scheme 3, with Scheme 2 becoming more advantageous at higher extraction ratios. Scheme 1 exhibits the shortest static payback period (7.12–7.63 years) and the highest internal rate of return (12.77–11.65%). These results indicate that the three schemes have distinct advantages in peak-shaving performance, full-cycle thermodynamic performance, and economic performance, and provide a comparative basis for engineering selection and parameter optimization of molten-salt-based flexibility retrofits in coal-fired power units.