Multi-scale performance evaluation of direct and indirect-heated supercritical CO₂ Brayton cycles for solar power tower plants with alternative high-temperature chloride salt

The adoption of high-temperature chloride salt as alternative heat transfer and storage medium has been demonstrated to enable indirect-heated solar power tower (SPT) plants to achieve operational temperature parity with direct-heated SPT plants. In this paper, two classical supercritical CO₂ (S-CO₂) Brayton cycles are integrated into direct and indirect-heated SPT plants. A multi-scale evaluation framework, combining design-point optimization with off-design operation analysis, is employed to determine optimal configurations for next-generation concentrated solar power deployment. The results indicate that employing a direct-heated layout in a SPT plant can enhance both specific work and overall thermal efficiency compared to those of an indirect-heated layout under identical conditions, regardless of the cycle configuration. Exergy analysis reveals that superior performance is achieved by the direct-heated layout through the reduction of exergy destruction at the receiver and heat exchanger. The optimization results confirm that the recompression cycle is more suitable for the direct-heated layout in SPT plants, demonstrating the highest specific work of 0.314 MJ/kg and a greater thermal efficiency of 38.06 %. However, under actual operating conditions, the indirect-heated SPT plant using a recompression cycle demonstrates the most stable performance, with an efficiency variation of only 18.09 %. Therefore, it is recommended that the indirect-heated layout with a recompression cycle be considered the optimal solution for next-generation SPT plant deployment.