Performance analysis and optimization of combined-cycle solar tower power generation systems

Concentrated solar power (CSP) has emerged as an effective low-carbon pathway for large-scale renewable electricity generation in global energy. Among them, solar tower power generation (STPG) is characterized by the high optical concentration ratios, large thermal energy storage capacity, and delivering dispatchable power at scale. Particularly, supercritical carbon dioxide (SCO₂) recompression Brayton cycle–based STPG (SCRBC-STPG) can be expected for the favorable thermophysical properties of SCO₂, compact turbomachinery, and the potential for high thermal efficiency among various power-block configurations. However, existing SCRBC-STPG configurations have suffered from a high levelized cost of electricity (LCOE) and low generation efficiency, thus hindering large-scale commercialization and practical deployment. In this study, an integrated configuration was proposed to couple a steam Rankine cycle (SRC) with an SCRBC-STPG, termed as SCRBC+SRC-STPG. The SRC was designed to recover residual thermal energy from both the SCO₂ Brayton cycle and the molten-salt thermal energy storage subsystem. Additional low-grade heat was extracted to enhance the effective thermal storage capacity. The SRC then lowered the required operating temperature of the low-temperature molten-salt tank. A series of simulations was conducted on a baseline SCRBC-STPG and the SCRBC+SRC-STPG model using the EBSILON Professional platform. Key subsystems—including the heliostat field, receiver heat transfer, molten-salt storage, and power conversion units: Were validated for the model reliability. Following model validation, a parametric analysis was then performed to evaluate the thermodynamic and economic performance of two configurations over a wide range of operating conditions. The results demonstrate that the SCRBC+SRC-STPG system consistently achieved higher net power generation efficiency than the standalone SCRBC-STPG ones. Specifically, the efficiency was improved by 4.97% at a turbine inlet temperature of 550 °C and a high-pressure turbine inlet pressure of 30 MPa, compared with the baseline. From an economic perspective, once the power output of the steam Rankine cycle (PSRC) was limited to 16 MW, the optimal performance of the SCRBC+SRC-STPG system achieved an LCOE of 0.814 CNY/kWh, which was 0.091 CNY/kW·h lower than that of the conventional ones. Multi-objective optimization was conducted to enhance efficiency with low COE. The optimal operation was then determined to balance the system performance and economy. The optimal variables included the cycle flow split ratio, high-pressure turbine inlet pressure, and turbine inlet temperature. The Pareto frontier can represent the trade-offs between thermodynamic efficiency and economic competitiveness under the impacts of key parameters. In conclusion, an SRC with an SCO₂ recompression Brayton cycle and molten-salt thermal storage can significantly enhance the technical and economic performance of solar tower CSP plants. The SCRBC+SRC-STPG concept can represent a viable pathway to improve conversion efficiency with low LCOE. The findings can offer valuable insights to optimize the high-performance, cost-effective STPG system.