## 3 Results and Discussion
### Results of Group B Mixtures (Substances for Increasing the Critical Temperature)
The first one is the mixture density; as already explained in this work, the density has a direct impact on the turbomachine's final dimension. It has been seen benzene mixtures provide the maximum density at the compressor inlet (Table 4). Other important considerations to be assessed are related to the isobaric heat capacity and kinetic viscosity. As mentioned before in this study, the highest Cp and density values and the lowest kinetic viscosity value minimize the recuperator (low-temperature recuperator (LTR) and high-temperature recuperator (HTR)) dimensions. The s-CO\({}_{2}\)/H\({}_{2}\)S and s-CO\({}_{2}\)/C\({}_{4}\)H\({}_{10}\) mixtures have higher Cp values, and the s-CO\({}_{2}\)/C\({}_{6}\)H\({}_{6}\) mixture has a higher density value. The s-CO\({}_{2}\)/C\({}_{4}\)H\({}_{8}\) and s-CO\({}_{2}\)/C\({}_{4}\)H\({}_{10}\) mixtures have lower kinetic viscosity value. Another important factor impacting in the power cycle performance is the mixture maximum operating temperature. At an operating temperature of around 823.15 K at the turbine inlet, hydrocarbons pyrolysis or chemical decomposition could occur. Also, air infiltration in the power cycle working fluid due to leakages could provoke added substance auto-ignition. The mixture substances which are more stable in these scenarios are the cyclic hydrocarbons, short-string hydrocarbons, and hydrocarbons with double links. Based on the REFPROP fluid property library information [26], the maximum operating temperature for benzene and hydrogen sulfide is 725 K and 760 K, respectively. The advantage of benzene is that it has a lower mole fraction than hydrogen sulfide. Figure 14: LF’s solar field estimated cost vs concentration of s-CO\({}_{2}\). Group B mixtures with a CIT = 318.15 K and CIP = just above the critical pressure of the working fluid. Figure 13: PTC’s solar field estimated cost vs concentration of s-CO\({}_{2}\). Group B mixtures with a CIT = 318.15 K and CIP = just above the critical pressure of the working fluid. This study paves the way for future works which should achieve a detailed equipment design and cost assessments. The final recommendation extracted from this analysis is to compare the cycle performance and detailed equipment design with benzene and hydrogen sulfide mixtures. Subsequent analyses should be focused on comparing the results with the other mixtures in Table 4.  
Another complementary power cycle design work will be focused on adding two or more substances to the sCO\({}_{2}\)-mixtures, considering the benefits provided by each individual substance, and balancing the required critical temperature according to the CSP-sCO\({}_{2}\) location ambient conditions. For the case where CIT equal to 323.15, 328.15 and 333.15 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 are shown in Table 5. Bearing in mind that in these conditions the critical temperatures are below to CIT's range studied, i.e., if the additives molar fraction exceeds these values, the mixture critical temperature will be greater than CIT's range studied. A power cycle efficiency improvement from 3% to 4% has been obtained for group B sCO\({}_{2}\)-mixtures. Thermodynamic results show savings in PTC's installations could reach up to 11 million USD for CIT = 323.15 K, 9 million USD for CIT = 328.15 K, and 10 million USD for CIT = 333.15 K. Alternatively, in the case LF's installations, the savings could reach 8 million USD for CIT = 323.15 K, 7 million USD for CIT = 328.15 K, and 7 million USD for CIT = 333.15 K.  
As discussed previously for Table 4, there are several factors when selecting the optimum s-CO\({}_{2}\) mixture, namely power cycle efficiency impact, critical temperature, the plant location's ambient temperature, equipment, performance, and cost, etc. When increasing the CIT (cf.