## 3 Results and Discussion
### Results of 'Group A' Mixtures (Substances for Reducing the Critical Temperature)  
For 'group A' mixtures, the maximum molar fractions for the s-CO\({}_{2}\)/He, s-CO\({}_{2}\)/Kr and s-CO\({}_{2}\)/CH\({}_{4}\) mixtures have been fixed at 90.0/10.0, 68.0/32.0 and 67.0/33.0 respectively because in these conditions their critical temperatures are close to 273.15 K. The molar fraction span for the s-CO\({}_{2}\)/C\({}_{2}\)H\({}_{6}\) was 100.0/0.0 to 0.0/100.0 because its critical temperature range is approximately 304.13 K to 290.21 K. This means that the CIT's range studied is approximately 304.13 K to 273.15 K, except for the s-CO\({}_{2}\)/C\({}_{2}\)H\({}_{6}\). The power cycle diagrams for pressure with respect to the enthalpy are shown in Figure 4, where it is possible to see the cycle has a different behavior when it uses sCO\({}_{2}\)-mixture as working fluid. For the study of the Brayton cycle performance, optimal values of CIT and CIP at the main compressor and recompressor inlet has been considered; it has been observed that the sCO\({}_{2}\)-mixtures produce a gain in cycle efficiency over pure s-CO\({}_{2}\) cycles of approximately 4%. Also, a linear trend is observed for s-CO\({}_{2}\)/Kr and s-CO\({}_{2}\)/CH\({}_{4}\) (cf. Figure 5). Figure 4: Real gas cycles with sCO\({}_{2}\)-mixtures as working fluid in the thermodynamic plane P-h. Group A mixtures with UA\({}_{\text{total}}\) = 15,000 kW/K. Table 2 shows the results summary (cycle thermal efficiency and working fluid properties using group A sCO\({}_{2}\)-mixtures) for the Brayton cycle working with UA\({}_{\text{total}}\) = 15,000 kW/K, CIT and CIP optimized. To summarize the results obtained, some working fluid properties which have a direct impact on the cycle performance and on the power cycle equipment detail design are shown in this table. The turbomachine dimensions are constrained to the working fluid density [33; 34]. If the density is increased, the compressor and turbine sizes will be minimized. The addition of krypton provides higher density values; hence, the turbomachines would have a minimized size. However, the isobaric heat capacity (Cp), kinetic viscosity and thermal conductivity must also be considered in the heat exchangers design. The heat exchanger's heat transfer coefficients are closely related to the viscosity (dynamic and kinetic), thermal conductivity, density and isobaric heat capacity [35]. 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 ethane mixture is the solution for maximizing the isobaric heat capacity. Moreover, the helium mixture and krypton mixture are the solution for minimizing the kinetic viscosity. Another important issue to be considered is the Ultimate Heat Sink (UHS) thermal storage system, this system design depends on the mixture critical temperature. Here, two possible technical solutions are going to be studied. The night cooling radiative panels could be proposed as the first technical solution, and this proposal has been discussed by [PERSON] [36]. The cooling fluid's lowest temperature for this proposal could be between 278.15 and 298.15 K; for this scenario, the ethane mixture, with a critical temperature around 290.82 K, is the optimal solution, as stated by [PERSON] [37] and [PERSON] [38].