Effect of Gaseous Admixtures on Cycles With Supercritical Carbon Dioxide

Abstract

Supercritical carbon dioxide cycles are recently very perspective and they are researched all around the world. CO2 is an interesting medium for applications in many technologies, from nuclear energy through geothermal, solar and waste heat recovery systems. However, S-CO2 cycles have several issues which have to be researched, one of them being the presence of the so called pinch point in the heat exchangers design. Therefore, the Czech Technical University (CTU) conducts research on supercritical carbon dioxide cycles, which are focused on the effect of the gaseous admixtures in S-CO2 on different cycle components. The research is primarily focused on the pinch point shift within heat exchangers caused by gaseous admixtures. Previous work has shown that the pinch point can be removed with the addition of small amounts of another gases. However, it is also important to describe the effect on the performance of the cycles. This is the main topic of this paper. One of the reasons for this research is the positive effects on components are possible. The first part of the study is focused on the development of computational code for calculation of the basic S-CO2 cycles with mixtures. The second part of the study is focused on the calculation of basic cycles for binary mixtures. The calculation will be performed for pure CO2 and some binary mixture. He, CO, O2, N2, Ar will be used for the calculation as the most common admixtures, furthermore H2, CH4 and H2S will be used as well. The last part of the study will be focused on the optimization of individual cycles for different amount of admixtures in CO2. The result of this study will define the optimum ratio of admixtures and description of their effect on cycle efficiency.

Supercritical CO2 Mixtures for Advanced Brayton Power Cycles in Line-Focusing Solar Power Plants

“…The input parameters for the simulation are shown in Table 1. The results obtained were validated with the Thermoflex 27 software [27], UniSim Design R451 software [28] (UniSim uses Peng-Robinson equation of state for thermodynamic property estimations), and scientific articles about the effect of gaseous admixtures on cycles with s-CO2 [1,29,30]. [31,32] ηmc 0.89 -Re-compressor efficiency [31,32] ηrc 0.89 -Turbine efficiency [31,32] ηt 0.93 -Heat exchanger conductance for low temperature recuperator UALT 2500 // 5000 // 7500 kW/K Heat exchanger conductance for high temperature recuperator UAHT 2500 // 5000 // 7500 kW/K Split fraction γ Optimized - 1 CIT optimized for group A mixtures.…”

“…The input parameters for the simulation are shown in Table 1. The results obtained were validated with the Thermoflex 27 software [27], UniSim Design R451 software [28] (UniSim uses Peng-Robinson equation of state for thermodynamic property estimations), and scientific articles about the effect of gaseous admixtures on cycles with s-CO 2 [1,29,30].…”

New Knowledge on the Performance of Supercritical Brayton Cycle with CO2-Based Mixtures

“…On the other hand, in engineering application of Brayton cycle, S-CO 2 will be inevitably mixed with the gas impurities. Therefore, Vesely et al [24] investigated the effect of gaseous admixtures on the cycle efficiency at a fixed inlet temperature of main compressor. They found that all researched mixtures except CO 2 -H 2 S had negative effects on the cycle efficiency and net power output.…”

“…However, due to the fact that T c is different for various mixtures, this assumption will cause the performance comparison of mixtures under different compressor inlet temperatures. (2) For the work of Vesely et al [24,25] and Guo et al [23], the range of employed gas fraction is too less. Although Baik and Lee [22] considered the whole fraction range 0-1, most of the employed additives belong to organic fluids, which may decompose in the high temperature.…”

“…Table 1 lists the considered CO 2 -based mixtures in the existing studies. Thermodynamic properties of these mixtures are always obtained by REFPROP [19][20][21][22][23][24][25][26]. According to the shift direction of critical temperature, these mixtures can be classified into two groups: the ascending critical temperature group and the descending critical temperature group.…”