Thermodynamic analysis and comparison for different direct-heated supercritical CO2 Brayton cycles integrated into a solar thermal power tower system

In this paper, a complete mathematical model is developed to carry out the thermodynamic analysis and comparison for different direct-heated S-CO2 Brayton cycles (simple, pre-compression, recompression, partial-cooling, and intercooling) integrated into a solar power tower (SPT) system. Based on the model, the effect of turbine inlet temperature (TIT) on the thermodynamic performances of the receiver, the thermal energy storage unit, the S-CO2 power cycle blocks and the integrated SPT systems is investigated respectively for these cycles. Additionally, a comparison of cycle efficiencies and overall integrated SPT system efficiencies is performed for five S-CO2 cycles at a series of total recuperator conductance (UAtotal) values. The results reveal that the TIT exhibits a parabolic effect on the overall efficiencies for each S-CO2 cycle, and the intercooling S-CO2 cycle achieves the highest overall efficiencies followed by the recompression, the partial-cooling, the pre-compression, and the simple cycles at different TIT values. Furthermore, the partial-cooling cycle possesses the highest overall specific work at each TIT and offers higher overall efficiencies than the recompression cycle at a constant TIT (650 °C) as the UAtotal is rather low, having the potential to reduce the costs of integrated SPT systems with limited UAtotal values.