## 3 Modelling and Simulation
### Basic Model
#### 3.1.2 Regulation and Control  
Then, the working temperature limits are dealt with at each work step of the real working range of the molten salt. As in the comparison process, this is achieved by regulating the mass flow. The associated APROS automation can be seen in Figure 9 and is explained and described in more detail below:  
The temperature at the heat exchanger outlet is measured from the APROS process (Figure 9 top left) and converted into a digital signal with the measurement component TI01. This is compared in the PI controller with the setpoint SP09, which sets a target temperature of 386 \({}^{\circ}\)C. A PI controller is used because, according to [19], this is the type of controller commonly used in the industry. The controller then forwards a signal between 0.1 and 1, which is converted into a valve position between 10% and 100% with the aid of the ACT05 actuator component. A valve position of over 10% is used here to ensure a constant mass flow of >0 kg/s. If the controller has to regulate the mass flow to 0 kg/s in the meantime, nothing will change in the temperature after the heat exchanger, because the heated fluid will not reach the measuring point. The binary_switch ASW01 between the actuator and the PI controller has the function of completely closing the valve if necessary because the PI controller cannot do this due to its working range of 0.1-1 described above. This is needed in two cases. The first case describes the end of the charging process, that is, when no more heat flows into the system. If the measured heat flow (middle left) falls below the value of 0.5, the  
Figure 8: Control circuit for heat flow during discharging by APROS. output signal of the limit-value checker LVC03 changes from true to false. The following OR component then also changes its output signal to false (because one of its input values is false), which leads to the binary_switch jumping to the setpoint SP02 = 0, which results in a valve position of 0%. The second case in which the valve needs to be fully closed occurs when the system is fully charged; this means when the CST falls below 0.65 m level. As shown in Figure 9, this is also modelled using a measured signal and a limit-value checker. The control of the discharge line is equivalent to that of the charging line, except that the control parameters are adapted to the discharge process (e.g., target temperature = 292 \({}^{\circ}\)C). The next step is to try to optimise the pump performance. In the basic model, these pumps ran at 100% performance regardless of the fill level of the tanks and the required mass flow. The reason for this is the associated simplification of the regulation, because the valve regulates continuously at the same pump output and can therefore easily keep the mass flow constant compared to when the pump output changes. However, full power is not always required. For example, when the HST is low, the power required is less than when the level is high. However, it must be ensured here that the pump performance is not directly dependent on the mass flow because this can lead to an interference with the control of mass flow by the valve. As a solution, the pump is controlled based on the charging capacity and the geodetic height difference. The associated APROS automation can be seen in Figure 10 and is described below:  
Figure 9: Mass flow controller during charge mode. Automation begins with the design principle of the pump to overcome a height difference of 13 m with a capacity of 100% while providing the required mass flow at full discharge capacity. On this basis, the automation shown in Figure 10 calculates the required power and passes it on to the pump via the ACT02 actuator. As described above, two variables are used to determine the power: the geodetic height difference and the charging power. The discharge power is flowed in the upper line, divided by the maximum discharge power (109.83 \(MW_{th}\)) and then multiplied again by a factor of 100.