## 3 Results and Discussion
### Results of Group B Mixtures (Substances for Increasing the Critical Temperature)  
For group B mixtures, the cycle works with a CIP just above critical pressure and a CIT's range from 318.15 K to 333.15 K. This CIT's range has been studied in this work because most of CSPs are located in sites where the ambient temperature is above the s-CO\({}_{2}\) critical temperature, i.e., CIT = 318.15, 323.15, 328.15, and 333.15 K [43; 44]. In the case of CIT equal to 318.15 K and UA\({}_{\text{total}}\) = 15,000 kW/K, the maximum molar fractions for the s-CO\({}_{2}\)/H\({}_{2}\)S, s-CO\({}_{2}\)/C\({}_{3}\)H\({}_{8}\), s-CO\({}_{2}\)/C\({}_{4}\)H\({}_{10}\), s-CO\({}_{2}\)/C\({}_{5}\)H\({}_{10}\), s-CO\({}_{2}\)/C\({}_{5}\)H\({}_{12}\), s-CO\({}_{2}\)/C\({}_{4}\)H\({}_{8}\) and s-CO\({}_{2}\)/C\({}_{6}\)H\({}_{6}\) mixtures have been fixed at 66.0/34.0, 72.5/27.5, 90.0/10.0, 97.5/2.5, 97.5/2.5, 92.5/7.5, and 92.5/7.5 respectively, because the critical temperatures in these conditions are just below to 318.15 K. The power cycle diagrams for pressure with respect to the enthalpy are shown in Figure 10, where it is possible to see the cycle has a different behavior when it uses sCO\({}_{2}\)-mixture as working fluid. Comparing the recompression Brayton cycle using pure s-CO\({}_{2}\) and the same cycle using 'group B' sCO\({}_{2}\)-mixtures, it can be observed that the cycle efficiency with group B mixtures decreases when it works with CIT and CIP just above the critical point. In contrast, a better efficiency has been obtained if the cycle works with sCO\({}_{2}\)-mixtures and higher compressor inlet temperatures, i.e., 318.15, 323.15, 328.15 and 333.15 K. Most series of cycle efficiency versus molar fraction have a linear trend, with a minority of lines following an exponential trend, such as the mixtures s-CO\({}_{2}\)/H\({}_{2}\)S and s-CO\({}_{2}\)/C\({}_{3}\)H\({}_{8}\)  
Figure 10: Real gas cycles with mixtures as working fluid in the thermodynamic plane P-h. Group B mixtures with a CIT = 318.15 K and UA\({}_{\text{total}}\) = 15,000 kW/K. Figure 9: LF Solar Field cost vs. a concentration of s-CO\({}_{2}\) for ‘group A’ mixtures. The cycle efficiency improves by about 3 to 4%, when the cycle works with group B sCO\({}_{2}\)-mixtures, CIP just above critical pressure and CIT = 318.15 K, as shown in Figure 11.  
The same analysis of 'group A' mixtures (substances added for decreasing the critical temperature) has been developed with 'group B' mixtures (substances added for increasing the critical temperature). As a result of this analysis, it has been observed that the cycle efficiency increases when \(\delta_{total}\) increases. Therefore, the influence of the parameters \(w_{mc}/w_{t}\), \(w_{rc}/w_{t}\) and \(\Delta\)h\({}_{a}\)/\(w_{t}\) on the cycle efficiency that works with UAtotal = 15,000 kW/K and CIT = 318.15 K can be seen in Figure 12.  
Due to the mixtures used in the recompression Brayton cycle, savings in PTC installations could reach up to 7 million USD for CIT = 318.15 K (cf. Figure 13). The maximum saving was obtained with the s-CO\({}_{2}\)/H\({}_{2}\)S mixture (66.0/34.0), and the minimum saving was obtained with s-CO\({}_{2}\)/C\({}_{5}\)H\({}_{12}\) mixture (97.5/2.5). Alternatively, in the case of linear Fresnel installations, savings could reach 6 million USD for CIT = 318.15 K (cf. Figure 14). In the same way as for the PTC installations, the maximum saving for LF installations was obtained with the s-CO\({}_{2}\)/H\({}_{2}\)S mixture (66.0/34.0). Figure 11: Cycle efficiency vs. a concentration of s-CO\({}_{2}\). Group B mixtures with a CIT = 318.15 K.  
Figure 12: Variation of the parameters \(w_{mc}/w_{t}\), \(w_{rc}/w_{t}\) and \(\Delta\)h\({}_{a}\)/\(w_{t}\) of the s-CO\({}_{2}\) mixtures with respect to the pure s-CO\({}_{2}\). Recompression Brayton cycle with UAtotal = 15,000 kW/K, CIP = just above the critical pressure of the working fluid and CIT = 318.15 K for group B mixtures. There are different issues to be considered when selecting the optimum sCO\({}_{2}\)-mixture.