## 2 Technical Solution
### Carnot Battery Integration  
#### 2.3.1. Carnot Battery with Baseline: Scenario II  
To leverage the excess electricity described earlier, a Carnot battery has been selected due to its natural compatibility with the solar thermal power plants previously detailed. Carnot batteries enable the conversion of electrical energy into thermal energy during the charging phase using electric heaters. Subsequently, when the stored energy is required, the reverse conversion from thermal to electrical energy is performed through the power block turbine. This process relies on creating a temperature gradient between two heat reservoirs, effectively utilizing surplus electricity [47]. In this case, a system has been selected in which surplus electricity from the grid is used to generate a hot reservoir via the Joule effect. This can be achieved using heat pumps or electric resistances, for example [47; 48]. For this study, an electric resistance has been chosen due to several factors:  
* High efficiency: Electric resistances with molten salts can achieve efficiencies of up to 100% [49]. * Preference for large-scale projects: Electric resistances are commonly used in large-capacity demonstration or pre-commercial projects [48; 50]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Working Fluid/Solid** & **Temperature [\({}^{\circ}\)C]** & **Pressure [bar]** \\ \hline Hot particles & 650 & 1 \\ Cold particles & 150 & 1 \\ Superheated steam & 600 & 285 \\ Reheated steam & 620 & 65 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Operating conditions of the working fluids and solids [13]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Area** & **Typical Efficiencies (\%)** \\ \hline Receiver & 77–82 [43] \\ Steam generator & 93–99 [44] \\ Power block & 45.4 [15] \\ Storage & 99 [45; 46] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Typical efficiencies in Scenario III. \begin{table}
\begin{tabular}{c c} \hline \hline
**Property** & **Value** \\ \hline Sauter mean diameter [\(\upmu\)m] & 63.9 \\ Density [kg/m\({}^{3}\)] & 3210 \\ Specific heat capacity [J/kg\(\cdot\)K] @ 500 \({}^{\circ}\)C & 1150 \\ Thermal conductivity [W/m\(\cdot\)K] @ 500 \({}^{\circ}\)C & 109 \\ Cost [USD/kg] & 1.54 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Key properties of silicon carbide particles [13; 43]. * Simplicity and cost-effectiveness: Electric resistances require less investment compared to alternatives like heat pumps [49], which also cannot reach temperatures beyond 130-150 \({}^{\circ}\)C [50,51]. For this work, the electric resistance will heat the molten salts up to 568 \({}^{\circ}\)C [2] and will be installed in parallel with the solar field and the storage system, as suggested in other studies [52, 53, 54]. This configuration allows the electric resistance to operate alongside the heliostat field, enabling, for instance, the transfer of molten salts from the cold tank to the hot tank during nighttime without relying on the tower. Additionally, this bypass installation provides extra temperature control in the cold tank and prevents issues related to salt solidification [54]. Furthermore, the parallel configuration allows for managing the amount of surplus electricity the plant can accept, prioritizing energy from the solar field. For example, during periods of solar irradiation when storage is at capacity, the plant can reject surplus electricity, optimizing the energy flows. The charging system for this setup follows the approach developed in [2]. It consists of a shell-and-tube heat exchanger, where the tubes themselves act as the electric resistance. This design ensures that, whenever the plant can absorb surplus electricity, the electric resistance is activated to heat the molten salts passing through the exchanger. These heated salts are then stored in the thermal reservoirs of the previously described solar thermal plant.