## 4 Results
### System Analysis of the Andasol 2 Model  
First, the process values of the Andasol 2 system were analysed and it was determined whether the objectives set in the modelling were achieved. For this purpose, as described above, 24 h were simulated. This was implemented with the help of the full day of the work queue, with which process values and input variables were changed at certain time intervals. The simulation begins at a virtual time of 8:00 a.m. with the loading of the initial conditions (initial condition = 8:00 a.m.) of the process and is thus reset to the initial state, as shown in Figure 11. In this, the mass flow of the two molten salt lines is equal to 0 kg/s, there is no heat flow on the system, the CST is completely filled with a fill level of 13.6 m, and the HST is completely emptied with a fill level of 0.6 m. At 9:30 a.m. the heat flow of the charging line is set to 109.82 \(MW_{th}\)--charging begins. After 10 h the maximum charging capacity of HST is reached. At 19.30 p.m. the charging process is completed and the heat flow is set to 0 \(MW_{th}\) due to the discontinuation of electrical energy from renewable energy sources. Then, the discharging process begins for 7 h and 30 min. For this, the value of the heat flow in the discharge line is set to 136.67 \(MW_{th}\), which is slowed by the filter after approximately 100 s. At 3:00 a.m. the end of the discharge time is reached and the heat flow of the discharge line is set to 0 \(MW_{th}\) again. From 3:00 a.m. to 8:00 a.m., the system maintains its state without charge or discharge processes, so that at 8:00 a.m., with ideal modelling, the initial conditions should be given again. In the first step, it is simulated whether the temperature of the molten salt remains within the real working range during the entire process. For this purpose, Figure 12 shows the temperature of the molten salt after the heat exchangers (referred to as heat exchanger in the illustration) in the two charging lines over a 24 h period. As can be seen, the working range of 220 to 580 \({}^{\circ}\)C is not exceeded. Furthermore, the set temperature is missed by a maximum of 14 \({}^{\circ}\)C (start of discharge) due to the transient behaviour of the control, which corresponds to a maximum control deviation of 4.79%. This means that the goal of delimiting the work area can be classified as achieved. It should be noted that the modelling of sunrise and sunset lengthens the transient process, which can be clearly seen when comparing the two temperature curves. This is an important finding for further simulations and a confirmation of the relevance of modelling the sunrise and sunset. Another relevant finding from the simulation is that the valve after the heat exchanger must be designed for the entire temperature range. As can be seen in the simulation, the valve in the discharge line is only set with a temperature of 292 \({}^{\circ}\)C during the discharge period, but it is exposed to the full 386 \({}^{\circ}\)C in the rest phase. One explanation for this is that the mass flow of the discharging line is only set to 0 kg/s after the discharge process has ended, so that small amounts of hot fluids can still reach the valve. If an attempt is made to end the discharge process and the mass flow at the same time, a temperature peak occurs shortly before the discharge power and the mass flow are stopped. Figure 12: Simulated temperature after the heat exchangers during charging. In the second step, it is simulated in which area the valves work in the further developed form of the model. The current valve position is plotted as a percentage of the maximum degree of opening. As can be seen in Figure 13, the valves operate in a range of 0-75%, which means that the goal of expanding the control range of the valves has been achieved. This is important for the subsequent construction of the system because with a low degree of opening there are constant flow losses, which further confirms the success of the simulation.