Supercritical CO₂ mixture Brayton cycle with floating critical points for concentrating solar power application: Concept and thermodynamic analysis

A novel concept of supercritical Brayton cycle with floating critical points is proposed to cope with the rapid deterioration of cycle performance at high heat sink temperatures. By coupling the power cycle with a distillation based composition tuning subsystem, the circulating mixture composition in the cycle can be altered dynamically to obtain the desired critical points at various ambient temperatures, thereby leading to a better match between the cold-end parameters of power cycle and the heat sink conditions. A thermodynamic model is established for the proposed novel system based on a recompression layout with single stage reheating. Four binary mixtures, including CO₂–H₂S, CO₂–Butane, CO₂–Isobutane, and CO₂–Butene are employed. Simulation results indicate that an increasingly larger improvement in cycle efficiency compared to the pure CO₂ cycle can be obtained as the ambient temperature increases from 20 to 50 °C. When the composition tuning process is not considered, the CO₂–H₂S cycle shows the best performance among the four mixture cycles, with an improvement between 1.3% and 7.4% being achieved over the entire temperature range. Several feasible schemes for composition tuning subsystem are proposed and compared in terms of energy consumption. Finally, the hourly efficiencies of the integrated system are evaluated based on typical daily weather conditions, with a piecewise control strategy for critical points. It is found that although the use of CO₂–H₂S causes relatively high energy consumption for distillation, a significant improvement in the daily average thermal efficiency of the integrated system can still be achieved, with efficiency being increased by 3.5%-5.1% in comparison with pure CO₂ cycle on hotter days.