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%.