On the role of critical properties in CO2-based binary mixture transcritical Rankine cycle performance

Abstract

The advantages of the CO2-based binary mixture transcritical Rankine cycle come from higher efficiency, lower operating pressure and higher condensing temperature. From the point of view of critical properties as: (1) the fundamental property of fluid mixtures; (2) the decisive parameters of cycle operating pressure and condensation conditions; and (3) the important effects of the screen criterion of working fluids, this work highlighted the composition of mixture and the critical point adjustment on the cycle performance. It is found that for the CO2 mixture, there is an optimal critical temperature increase range, which is 30~50 K higher than the critical temperature of CO2, for the best cycle performance and condensation temperature improvement. Because of the existence of the type I of critical phase behavior, the critical pressure of some CO2 mixtures will be higher than that of pure CO2 in certain composition, but this increase is marginal and will not worsen the cycle performance. When compared with pure CO2, it is the variation of the critical point that brings new characteristics to the CO2 mixture, namely lower saturation pressure and temperature glide, and beneficial for the advantages of mixture working fluid. The results of this work will provide guidance for the design, optimization, and thermophysical properties study of CO2-based mixture working fluids.