Effect of Multicomponent Mixtures on Cycles With Supercritical Carbon Dioxide

With the increasing interest in solar and geothermal power plants as well as waste heat recovery systems from many technologies, the whole world is more focused on gas power cycles. Especially, the supercritical carbon dioxide (S-CO₂) cycles are very interesting for these applications. This is due to many advantages of the S-CO₂ cycles over the other cycles such as a steam-water cycle or helium cycle. On the other hand, S-CO₂ cycles have also disadvantages. One of the disadvantages is presence of impurities in the cycles. The big question is the effect of these impurities in the CO₂, which can occur as impurities or can be suitably added to the pure CO₂. From the previous research, it is obvious that binary mixtures affect the cycle as they influence cycle component design and thus the overall efficiency of the power cycle. The biggest effect of mixtures is on the heat exchangers and compressor, which operate close to the critical point. The positive effect of the binary mixtures is observed in the recuperative heat exchanger. On the other hand, negative effects occurs in the cooler. Therefore, the Czech Technical University in Prague (CTU) conducted research on supercritical carbon dioxide cycles, which is focused on the effect of the gaseous admixtures in S-CO₂ on the different cycle components. The main goal of this paper is to describe the effect of gaseous admixtures on the efficiency of the cycles and their effect on each component. The first part of the study is focused on the calculation of the basic cycles for binary mixtures and description of the effect on the compressor and the cycle efficiency. The second part of the study is focused on the calculation of the basic cycles for multicomponent mixtures. In this part, the effect of the mixtures for different compositions and amounts of the individual mixture components will be presented. The calculations are performed for pure CO₂ and then for selected multicomponent mixtures. A basic multicomponent mixture includes mixtures from technology of carbon capture and storage. Other multicomponent mixtures are combinations of previously investigated gaseous admixtures such as He, CO, O₂, N₂, H₂, CH₄ and H₂S. The last part of the study is focused on the optimization of individual basic cycles for different amount of admixtures in CO₂. The result of this study defines the optimum composition of multicomponent mixtures and describes their effect on the cycle efficiency for the particular utilization of S-CO₂ cycle.