Thermodynamic Analysis of Brayton Cycles Using Supercritical Carbon Dioxide and its Mixture as Working Fluid

In this paper, the first and the second laws of thermodynamics are applied to analyze the thermodynamic performance of the split flow, pre-compression and modified recompression Brayton cycles. Firstly, the influences of the compressor and turbine inlet conditions (temperature, pressure) on the cycles performance are discussed. Furthermore, the Brayton cycles using mixture working fluid based on carbon dioxide (CO₂) are proposed, and the typical modified recompression cycle is taken as an example to show the performance. The effects of different adding gases and the proportion of a gas in CO₂ are analyzed. The results indicate that higher turbine inlet conditions can improve the efficiency of different cycles; the influence of the comperssoer inlet pressure on the cycle efficiency is greater than that of the compressor inlet temperature; adding krypton gas and xenon gas can improve the cycle thermal efficiency; the largest improvements of the modified recompression cycel thermal efficiency are 1.44% and 3.04% when adding 50% krypton gas and 26% xenon gas, respectively; however, adding nitrogen gas decreases the cycle efficiency.