## 2 System Description  
The layout of the solar tower integrated with the S-CO\({}_{2}\) recompression Brayton cycle is depicted in Figure 1. The system consists of the heliostats field, solar receiver, thermal energy storage and the S-CO\({}_{2}\) recompression cycle. The solar radiation in the heliostat field (HF) is reflected and concentrated on the solar receiver (R), which is located at the top of the tower. This concentrated radiation heats the HTF, which flows through the solar receiver. The HTF is used to provide thermal power to the S-CO\({}_{2}\) power cycle. The thermal storage system is used to take advantage of times when there is a surplus of solar radiation. In conditions of elevated solar radiation, an excess of thermal energy is stored. Conversely, in conditions of reduced solar radiation, the thermal energy that has been stored is discharged. Both operation modes are oriented to maintain the desired electricity production of the cycle. The thermal storage system can consist of two tanks or a single thermocline tank. In this work, the two-tank system is adopted because this method is more reliable and, therefore, more used in commercial projects despite the fact that the method based on the thermocline tank presents lower costs [14]. The intermediate heat exchanger connects the HTF circuit with the S-CO\({}_{2}\) cycle. This cycle consists of the following thermodynamic processes:  
* Process 1-2 and 13-14: Heat transfer in intermediate heat exchanger (IHE) between the molten salts (states 13-14) to the S-CO\({}_{2}\) circulating through the power block (states 1-2).
* Process 2-3: Non-isentropic expansion process in the turbine (T).
* Process 3-4 and 1-10: Heat exchange in high-temperature recuperator (HTR).
* Process 5-6 and 7-8: Heat exchange in low-temperature recuperator (LTR).
* Process 6-7: Non-isentropic compression process in main compressor (MC).
* Process 5-9: Non-isentropic compression process in recompression compressor (RC).
* Process 9, 8 and 10: Adiabiatic mixed flow.
* Process 5-6 and 11-12: Heat exchange process in precooler (PC) between the S-CO\({}_{2}\) (states 5-6) and cooling water (states 11-12).  
This paper examines two types of molten salts: solar salts and MgCl\({}_{2}\)-KCl. Table 1 summarizes the thermophysical properties of both molten salts. The thermophysical properties of the solar salts and MgCl\({}_{2}\)-KCl were collected by Equation Engineering Solver (EES) [15] from the reference materials [16; 17], respectively. The minimum operating temperatures are set with a safety margin of 70 \({}^{\circ}\)C above the melting temperature [6]. The specific heat for MgCl\({}_{2}\)-KCl is 1.56 kJ/kg\(\cdot\)K and is minimally influenced by temperature throughout the operating range. Generally, the specific heat of all chloride salts exhibits little dependence on temperature [5]. Although the MgCl\({}_{2}\)-KCl system presents higher operating temperatures, it has limitations due to its lower density and specific heat capacity, which should be taken into account.  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Parameter** & **Solar Salt** & **MgCl\({}_{2}\)–KCl** \\ \hline  & 60\% NaNO\({}_{3}\) & 62\% KCl \\ Mass Composition & 40\% KNO\({}_{3}\) & 38\% MgCl\({}_{2}\) \\ Maximum temperature [\({}^{\circ}\)C] & 565 & 800 \\ Minimum temperature [\({}^{\circ}\)C] & 290 & 495 \\ Melting temperature [\({}^{\circ}\)C] & 220 & 424.4 \\ Density at minimum temperature [kg/m\({}^{3}\)] & 1905 & 1652 \\ Density at maximum temperature [kg/m\({}^{3}\)] & 1725 & 1510 \\ Specific heat at average temperature [k/kg\(\cdot\)K] & 1.52 & 1.56 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Thermophysical properties of the Solar salts and MgCl\({}_{2}\)–KCl [16; 17].  
Figure 1: Layout of S-CO\({}_{2}\) recompression Brayton cycle. The red line indicates the S-CO\({}_{2}\), the orange line represents the molten salts and the blue line is the cooling water.