# Dynamic Investigation of a Solar-Driven Brayton Cycle with Supercritical CO2
## Material and Methods
### Simulation Strategy  
In the present study, the solar-driven Brayton cycle system is modeled using the Dymola software. Each component of the total system is modeled based on the Modelica programming language. The present model of the system, which is depicted in Figure 1, models the simulation of the STP for a year. Each model is created based on certain parameters, which are different for each one. The parameters needed to define each component are presented in the following tables. Then, the interfaces between the models are created to establish cooperation between the models. Interfaces are called connectors in the Dymola software. These connectors are specific model types for the Modelica programming language. These cannot be simulated, but they contain variables that, two components connected, need to transfer from one to the other. For each component, the needed type of connector is selected. For example, the solar tower model interacts with the heliostat field model to receive the value of the total available solar irradiance (Q\({}_{\text{sol}}\)). For the heliostat model, the heliostat \"Sanlucar 120\" is chosen [42]. The distance between two consecutive heliostats is chosen to be 1 m, while the distance between the first heliostat and the solar tower is set to be 30 m. The input in this model is the DNI, where the output is the available solar irradiance of the field (Q\({}_{\text{sol}}\)). Table 1 shows the heliostat field parameters. For the solar tower model, both parameters for the central receiver and the molten salt need to be provided. This model requires three inputs. Firstly, Q\({}_{\text{sol}}\) from the heliostat model is needed to calculate the total absorbed heat (Q\({}_{\text{abs}}\)) by the central receiver. In addition, the ambient temperature has to be known in order to calculate the thermal losses. Finally, the DNI is used as a control parameter to avoid negative values for the useful heat (Q\({}_{\text{u}}\)). These negative values may appear since, although there is no heat input in the central receiver during the night, there are still thermal losses because of the high temperature of the central receiver's surface [14]. The material of the central receiver is chosen to be aluminum [43], while the overall coefficient of heat transfer for the central receiver's inner surface is set to be 22 W/m\({}^{2}\)K [22]. The emissivity is doubled in order to approach the real radiation losses for high temperatures. Table 2 shows the parameters of the solar tower. The individual efficiencies of the solar field are also shown in Table 2.  
The molten salt chosen for this simulation is \"NaNO\({}_{3}\) (18%)-KNO\({}_{3}\) (53%)-LiNO\({}_{3}\) (29%)\". The composition is presented in percentage by weight (wt%). This molten salt has a lower melting point and higher specific heat capacity in comparison to the most used salt, \"KNO\({}_{3}\) (40%)-NaNO\({}_{3}\) (60%)\", also known as \"solar salt\". The mass flow rate of the molten salt in the solar thermal system is chosen so that the maximum temperature of the molten salt does not exist as the maximum limit of stability [17]. Moreover, when the temperature of the stored molten salt is less than a certain temperature level, the mass flow of the molten salt is \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Values/Description** \\ \hline Heliostat & Sanlucar 120 \\ \hline Count & 700 \\ \hline Length & 12.84 m \\ \hline Width & 9.45 m \\ \hline Reflectivity (\(\eta_{\text{mir}}\)) & 0.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Heliostat field parameters [42]. salt in the heat exchanger of the STP is reduced to 0.1 kg/s to allow the thermal storage to charge. Table 3 shows the parameters of the molten salt. Finally, the thermophysical properties of the air are needed to calculate the heat transfer coefficient between the central receiver and the ambient. The wind speed is set to be constant at 10 m/s, where the parameters that affect the Prandtl number of the air are functions of the ambient temperature [31].