## 2 Assumptions and Methods  
This work proposes a comparison between the performance of the cycles operating with pure s-CO\({}_{2}\) and the cycles of admixtures of s-CO\({}_{2}\) with He, Kr, H\({}_{2}\)S, 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\({}_{6}\). These mixtures have been used because their physical properties stimulate the better behavior of the plant cycle. The computer program SCSP (Supercritical Concentrated Solar Power Plant) is used for the simulations of the performance of the recompression Brayton cycle using pure s-CO\({}_{2}\) and sCO\({}_{2}\)-mixtures. This program has been developed by members of the Energy Engineering Department at the Technical University of Madrid [21] and it is based on the software core developed by [PERSON] in his doctoral thesis [22]. SCSP sets a constant heat recuperator total conductance for the performance calculation methodology and uses optimization algorithms (SUBPLEX [23], NEWUOA [24] and BOBYQA [25]) to calculate the optimal cycle performance. Figure 2 illustrates the iteration process for the cycle models integrated into the SCSP software. Figure 1: Recompression Brayton cycle layout. MC: main compressor; RC: recomressor; G: generator; T: turbine; PC: precooler; FS: fluid split; FM: fluid mixture; LTR: low-temperature recuperator; HTR: high-temperature recuperator; PHX: primary heat exchanger; SF: solar field. The fluid properties were obtained from the REFPROP (Reference fluid Properties) database developed by NIST (National Institute of Standards and Technology) in the USA [26]. Figure 3a,b show, respectively, the critical pressure and temperature of the mixture when the mole fraction of the additive fluid changes. Figure 3a specifies that the tendency of the pressure lines is very different among the different binary mixtures based on s-CO\({}_{2}\), as there a non-linearity between the critical pressure and the mole fraction. Figure 3b shows that the critical temperature distributions of mixtures follow a monotonous, nearly linear trend. The critical temperatures of s-CO\({}_{2}\)/He, s-CO\({}_{2}\)/Kr, s-CO\({}_{2}\)/CH\({}_{4}\), and s-CO\({}_{2}\)/C\({}_{2}\)H\({}_{6}\) have a downward slope, while the critical temperatures of 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\({}_{6}\) have an upward slope. Therefore, based on the critical temperature distribution trends, the s-CO\({}_{2}\) binary mixtures can  
Figure 2: Iteration process for the cycle models integrated into the SCSP software. be divided into two groups: 'group A' mixtures (substances for reducing the critical temperature) and 'group B' mixtures (substances for increasing the critical temperature). The performances of the pure s-CO\({}_{2}\) and sCO\({}_{2}\)-mixtures' recompression Brayton cycle have been analyzed. 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].