## 4 Conclusions  
This study was mainly focused on quantifying the impact of CSP-sCO\({}_{2}\) thermal efficiency due to the mixing of the substances (He, Kr, CH\({}_{4}\) C\({}_{2}\)H\({}_{6}\), C\({}_{3}\)H\({}_{8}\), C\({}_{4}\)H\({}_{8}\), C\({}_{4}\)H\({}_{10}\), C\({}_{5}\)H\({}_{10}\), C\({}_{5}\)H\({}_{12}\), and C\({}_{6}\)H\({}_{6}\)) with pure s-CO\({}_{2}\). The obtained results confirmed that the variations in working fluid properties directly impact in the cycle thermodynamic efficiency. On one hand, it is confirmed that He, Kr, CH\({}_{4}\) and C\({}_{2}\)H\({}_{6}\) addition reduces the critical temperature, and hence it increases the power cycle thermal efficiency. Detailed results are summarized in Table 2. The working fluids mixtures s-CO\({}_{2}\)/He (90.0/10.0) and s-CO\({}_{2}\)/Kr (68.0/32.0) increase the power cycle efficiency from 49.0% to 53.0%. Further, krypton and helium are inert gases and the addition of them to the pure s-CO\({}_{2}\) is very beneficial for avoiding the equipment materials corrosion. The group sCO\({}_{2}\)-mixtures, for reducing the sCO\({}_{2}\) critical temperature, are very suitable for the CSP-sCO\({}_{2}\) night sky cooling technology. On the other hand, another CSP ultimate heat sink technical solution is the dry-cooling system with air-cooled heat exchangers. Most CSPs are located in sites where the ambient temperature is above the s-CO\({}_{2}\) critical temperature (CIT = 318.15, 323.15, 328.15, and 333.15 K). In this scenario, the target is setting the CIT near the working fluid's critical temperature, hence, it is required to increase the working fluid critical temperature by adding H\({}_{2}\)S, C\({}_{3}\)H\({}_{8}\), C\({}_{4}\)H\({}_{8}\), C\({}_{4}\)H\({}_{10}\), C\({}_{5}\)H\({}_{10}\), C\({}_{5}\)H\({}_{12}\) or C\({}_{6}\)H\({}_{6}\) to the cycle's working fluid. In Tables 4 and 5 the results obtained for the CSP with dry-cooling scenario are summarized. As a result of this work, it is concluded that the solar power plant design-point efficiency is improved by about 3-4% when adding the mentioned substances in comparison with the reference pure s-CO\({}_{2}\). In works developed by [PERSON] [22] and [PERSON] [45], it was observed that an increase in the turbine inlet temperature (TIT) implies an improvement in the performance of the recompression Brayton cycle. As a result of this study, it is concluded that this effect of increasing the TIT is equivalent to the use of mixtures of supercritical fluids in the Brayton cycles. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Mole Fraction** & **CIT (K)** & **Plant Efficiency** & **PTC Cost** & **LF Cost** \\  & **(\%)** & **CIT (K)** & **Efficiency** & **(Million USD)** & **(Million USD)** \\ \hline s-CO\({}_{2}\) pure & 100.0 & 323.15 & 0.42 & 97.23 & 76.65 \\ s-CO\({}_{2}\)/H\({}_{2}\)S & 60.0/40.0 & 323.15 & 0.45 & 86.66 & 68.37 \\ s-CO\({}_{2}\)/C\({}_{3}\)H\({}_{8}\) & 65.0/35.0 & 323.15 &  
Future works should align with the working fluid chemical composition customization according to the ambient conditions where the solar power plant will be located. The environmental impact should also play an important role, and the substances added should be environmentally friendly. For this reason, inert gases in combination with other substances are advisable in future CSP-sCO\({}_{2}\) conceptual designs. Additional simulations are required to analyze more complex Brayton cycle configurations: the recompression with partial cooling cycle (RCPC) and recompression with main compressor intercooling cycle (RCMCI), both with reheating. In addition, it will be necessary to analyze the variation of increasing the CIP for compensating the CIT increment in hot locations. Also, detailed experimental analysis is required to verify that the possible reactions and chemical decomposition of mixtures do not affect the plant's performance. [PERSON] and [PERSON] developed the computer program SCSP (Supercritical Concentrated Solar Power Plant); All authors wrote the paper.