Investigation on the effect of mixtures physical properties on cycle efficiency in the CO2-based binary mixtures Brayton cycle

Abstract S–CO2 recompression Brayton cycle has attracted much attention with the development of compact heat exchangers. However, the critical point of CO2 is a constraint on the minimum cycle condition. Mixing additional gas into pure CO2 working fluids could reduce the critical point and bring improvement potential of the cycle efficiency. However it is not determined what kind of mixtures can improve cycle efficiency; and there is no criterion for choosing proper binary mixtures components. In this study, the feasibility of using mixtures physical properties as criterion to judge if CO2-based binary mixtures can improve cycle efficiency is discussed. The detail parametric study is performed to reveal the effects of mixtures physical properties on the performance of cycle efficiency. According to the analysis results achieved, three factors have been identified to have significant contributions to the improvement of the cycle efficiency: the entropy difference of a certain temperature range of CO2-based binary mixtures should be larger than that of S–CO2, the specific heat capacity within the temperature range of the compressor and cycle minimum pressure should be higher, and the maximum enthalpy of CO2-based binary mixtures should be lower than that of S–CO2.