Dynamic Flexibility and Screening Assessment of a 550 MW Combined-Cycle Gas-Turbine Plant With Two-Tank Molten-Salt Thermal Storage

Abstract

The increasing penetration of variable renewable generation requires combined-cycle gas-turbine plants to follow load more frequently, while repeated cycling reduces part-load efficiency and increases thermal stress. The unresolved problem is whether a utility-scale plant retrofitted with two-tank molten-salt storage can follow a daily dispatch while respecting inventory, thermal, control, degradation, and economic constraints. This study develops a component-based MATLAB/Simulink model of a 550 MW plant and a 488.6 MWhth storage system, including the gas-turbine island, heat-recovery steam generator, steam cycle, molten-salt heater and steam generator, tanks, pumps, heat tracing, supervisory control, direct emissions, a repairable-component availability framework, a normalized thermal-cycle severity indicator, and a screening-level economic model. The design point is calibrated with deviations below 2%. Dynamic credibility is assessed through conservation checks, state-bound and limiting-case tests, and comparison with an internal transient benchmark trajectory; independent plant transient validation remains unavailable. Under a 24 h renewable-rich dispatch, net output ranges from 313.542 to 557.990 MW and daily generation is 10 296.98 MWh, 58.34 MWh above the storage-free trajectory. The storage charges 230.50 MWhth and delivers 399.20 MWhth using both initial inventory and newly stored heat; a closed tank-side balance gives a final state of charge of 9.06%. Direct carbon intensity varies from 0.302 to 0.364 tCO₂/MWh. The value 1.70 × 10⁻⁴ is retained only as a normalized cycle-severity indicator, not a life fraction. With 31 USD/kWhth installed cost, an 8% discount rate, a 25-year life, and 300 equivalent cycles per year, the levelized cost of stored heat is 15.39 USD/MWhth and ranges from 11.54 to 23.08 USD/MWhth for 400 to 200 cycles. The results demonstrate that storage can widen the dispatch range and shift recovered exhaust heat to high-value periods; however, design and investment decisions require plant transient data, disclosed component maps and controller settings, material-specific degradation laws, vendor reliability data, and annual market-based optimization.