Thermodynamic analysis of CO2–SF6 mixture working fluid supercritical Brayton cycle used for solar power plants

To overcome the poor cycle thermal efficiency of supercritical CO2 Brayton cycle caused by the high rejection temperature used for solar power plants, mixture of CO2–SF6 as working fluid for the Brayton cycle system is proposed and evaluated. Detailed thermodynamic analysis is conducted to the CO2–SF6 mixture Brayton cycle compared with the referenced s-CO2 cycle. Influence of MC inlet temperature and mole fraction of SF6 on the thermodynamic performances is discussed. Moreover, exergy efficiencies and exergy loss distribution of the components in the CO2–SF6 mixture Brayton cycle are analyzed. The results suggest that the system cycle thermal efficiencies of CO2–SF6 mixture cycle and s–CO2 cycle both decrease with the increase of the MC inlet temperature, however, the thermal efficiency of CO2–SF6 mixture cycle is significantly higher than that of s-CO2 cycle. An optimal mole fraction of SF6 exists in the CO2–SF6 mixture for the CO2–SF6 mixture cycle. The optimal value is 0.356 in this context. The heliostat field and solar receiver has the lowest exergy efficiency among all the components of the CO2–SF6 mixture cycle, which is as low as 31.59%.