PERFORMANCE EVALUATION OF A DIRECT AIR-COOLED SUPERCRITICAL RECOMPRESSION BRAYTON CYCLE USING CO₂/N-BUTANE AS THE WORKING FLUID OPERATING UNDER OFF-DESIGN CONDITIONS

In order to improve the air-cooled supercritical carbon dioxide (CO₂) Brayton cycle for solar power systems, the use of mixed working fluid is a potential direction. The thermal stability of the CO₂-based mixture at the operating temperatures, however, has not received as much attention as the thermodynamic performance. In this study, the degradation of CO₂/n-Butane mixture is investigated using ReaxFF simulation method. Based on design results and off-design model of a direct air-cooled supercritical CO₂/n-Butane Brayton cycle, the influence of thermal decomposition of the working fluid is evaluated on the cycle thermodynamic behavior. The results show that the fluid degradation greatly reduces the ability of the power cycle to absorb heat input from the solar field. When the totally decomposition of CO₂/n-Butane mixture occurs, the net electricity is decreased by 32% at the design ambient temperature, while the cycle thermal efficiency is near the design-point efficiency. In addition, the impact of fluid degradation is much greater than above-design ambient temperature on the off-design performance of the supercritical Brayton cycle. This study indicates that the design of novel CO₂-based mixture Brayton cycles should simultaneously take into account the cycle performance and thermal stability of the potential CO₂ mixture.