# Dynamic Investigation of a Solar-Driven Brayton Cycle with Supercritical CO2
## Material and Methods
### Simulation Strategy
For simplicity, the thermophysical properties of the air are set to be constant, considering the properties at 30 \({}^{\circ}\)C. Table 4 shows the thermophysical properties of air. For the thermal storage system, two tanks are needed for the STP [14]. Using the Dymola software allows for the creation of only one model for thermal storage. By duplicating the model and setting different parameters for each copy of the model, two different tanks \begin{table}
\begin{tabular}{l l} \hline \hline
**Properties** & **Values** \\ \hline k\({}_{\text{air}}\) & 0.026 W/mK \\ \hline v\({}_{\text{air}}\) & \(13\times 10^{-6}\) m\({}^{2}\)/s \\ \hline \(\mu_{\text{air}}\) & \(17\times 10^{-6}\) kg/ms \\ \hline C\({}_{\text{p,air}}\) & 1100 J/kgK \\ \hline \end{tabular}
\end{table} 
Table 4: Thermophysical properties of air at 30 \({}^{\circ}\)C [31]. \begin{table}
\begin{tabular}{l l} \hline \hline  & **Solar Tower’s Geometry [14]** \\ \hline Tower’s height & 190 m \\ \hline Receiver’s shape & Cylindrical \\ \hline Receiver’s height & 18.2 m \\ \hline Receiver’s diameter & 15.2 m \\ \hline \multicolumn{3}{c}{**Central Receiver’s Material**[22, 43]} \\ \hline Material & Aluminum 1100-H12 \\ \hline Emissivity & 0.1 \\ \hline \multicolumn{3}{c}{**Internal Heat Exchanger**[22]} \\ \hline U\({}_{\text{tex}}\) & 22 W/m\({}^{2}\)K \\ \hline A\({}_{\text{tex}}\) & 40,000 m\({}^{2}\) \\ \hline \multicolumn{3}{c}{**Solar Field Efficiencies**[42]} \\ \hline \(\tau_{\text{latm}}\) & 0.95 \\ \hline \(\tau_{\text{spill}}\) & 0.99 \\ \hline \(\tau_{\text{cos}}\) & 0.83 \\ \hline \(\tau_{\text{IS\&B}}\) & 0.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Design parameters for the receiver. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Values/Description** \\ \hline Molten salt & NaNO\({}_{3}\) (18\%)-KNO\({}_{3}\) (53\%)-LiNO\({}_{3}\) (29\%) \\ \hline Maximum temperature & 540 \({}^{\circ}\)C \\ \hline Melting point & 120 \({}^{\circ}\)C \\ \hline Specific heat capacity & 1640 J/kgK \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermophysical properties of molten salt [17]. can be modeled based on the same thermal storage model. For this model, the ambient temperature needs to be an input in order to calculate the total thermal losses from the tank to the ambient. The storage surface consists of three layers [44]. The first material is chosen to be carbon steel [45], the second layer is an insulation layer of firebricks [46], and finally, the last layer is stainless steel [47]. Table 5 shows the materials chosen for the storage's surface, leading to an overall heat transfer coefficient of 1.5 W/m\({}^{2}\)K.  
The initial conditions of the simulation are the total molten salt's stored mass and temperature in the two thermal storages. The stored volume of the molten salt (V\({}_{\text{s}}\)) must be less than the total volume of the storage. This value depends on the mass flow rate at the inlet and the outlet of the storage but also on the temperature of the stored molten salt, since this variable affects its density. Table 6 shows the density parameters of the chosen molten salt [35], as well as the initial conditions for the simulation. Table 6 shows the geometry parameters of the TES\({}_{\text{s}}\). The heat exchanger is modeled based on the NTU method [37]. Its type is chosen to be counter-current. The parameters of the heat exchanger are chosen to achieve high effectiveness. Additionally, the recuperator's model, as well as the gas cooler's parameters, are chosen to achieve high effectiveness. Table 7 presents the parameters for all the heat exchangers of the total system model. The turbomachinery modeling consists of three separate models: the turbine model, the compressor model and the generator model. All three machines are connected to the same shaft. The mass flow rate of the CO\({}_{2}\) is constant. The pressure ratio value must be between two and three to achieve the highest efficiency [28].