## 2 Simulations  
The hybrid power plant evaluated in this article (Figure 1) coupled a concentrated solar plant with a conventional coal power plant. The simulation of the solar plant is based on the solar power tower plant GEMASOLAR, located in Seville, Spain [34]. The coal power plant follows the structure of the power plant Litoral (Spain) [35]. The proposed hybrid power plant is assumed to be located in Almeria. The simulation and analysis of the hybrid plant are thus based on data from this region.  
Sunlight is reflected on the heliostats of the plant toward the receiver of the solar tower, heating up, in this way, the molten salts that flow through the tower. The salts, composed of 40% KNO\({}_{3}\) and 60% NaNO\({}_{3}\)[32], are used to heat up steam, and water streams from the Rankine cycle of the plant. In order to ensure continuous operation when solar energy is not adequate to generate the required electricity, a thermal storage system with a capacity of 15 hours is included in the solar plant.  
Preheated water (34) is heated up using thermal energy from the molten salts and is then led to the boiler of the plant to be heated up further to the inlet temperature of the steam turbine (560 \({}^{\circ}\)C). The superheated steam (36) then expands to 47 bar in the high-pressure turbine, and it is then partly reheated in the boiler (38) to reach the maximum temperature of the main stream. Reheated steam (15) at 560 \({}^{\circ}\)C and 41 bar is led to the intermediate-pressure turbine and gradually expands to the operating pressure of the condenser of the plant (0.61 bar) [36]. Once the water exits the condenser, it flows through several water preheaters. The water preheaters are heat exchangers that use successive steam extractions from the steam turbine to progressively increase the water temperature. The water exiting the water preheaters flows through the molten-salt heat exchanger, completing the cycle.  
Water preheaters can have open, closed, or mixed structures. In this specific application, we find that preheaters of the closed type are the thermodynamically best choice to implement. The role of the water preheaters is to increase the efficiency of the plant by reducing the difference between the temperature of the water that returns to the boiler and the temperature of the steam exiting the turbine. Based on investment cost and efficiency calculations, the optimized number of extractions is found to be seven. [37]. An iterative process to calculate the conditions of each extraction was followed to achieve operating conditions that led to the required electricity generation (582 MW). The process used to calculate the pressure at each extraction was that indicated by General Electric [37]:  
\[\mathrm{P_{x}=}\frac{\dot{m}_{x+1}\cdot\mathrm{f}}{10^{6}} \tag{1}\]  
The pressure of extraction x (psia) was calculated using the mass flow rate (lb/h) through the extraction and the f factor shown in Table 1.  
The proposed hybrid power plant was simulated using the software EpsilonProfessional [38]. The base simulation was further modified to account for several weather conditions, using data during daytime (12:00, 17:00) and night time (03:00) for spring, summer, and winter [39; 40]. During daytime,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**N\({}^{\circ}\)****EXTRACTION (x)** & **1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline f FACTOR & 210.3 & 110.1 & 59.3 & 23.87 & 8.9 2 & 4.9 9 & 2.5 9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Factors for the steam turbine.  
Figure 1: The proposed hybrid power plant that couples a coal plant with a solar power tower.  
the radiation conditions allow the plant to operate using both coal and solar sources, while during night time the plant operates using coal or stored thermal energy.  
The thermodynamic data at the stream level of the plant for the different scenarios are presented in the Appendix A of the manuscript.