## 3 Modelling and Simulation
### Basic Model  
An APROS 6 model of a thermal storage tank was developed in this work and used as the basis for the modelling. The schematic structure of the basic thermal storage model can be seen in Figure 4. The model consists of two tanks (CST and HST), two valves (V\({}_{1}\) and V\({}_{2}\)), two pumps (P\({}_{1}\) and P\({}_{2}\)), and two heat exchangers (Q\({}_{1}\) and Q\({}_{2}\)). The individual components are connected by pipes.  
In contrast to Figure 1, the charging and discharging of the salt storage in the APROS model is realised with two different pipe systems. The reason for this is that APROS can only calculate flows in a certain direction. Thus, if the fluid in the same component is allowed to flow in the opposite direction, the simulation is aborted. However, the use of two different pipe systems instead of one should not affect subsequent results.  
The salt storage tanks are modelled with the Heat_Tank component from the APROS library. The dimensions are taken as described above [25]. At an initial temperature of 282 \({}^{\circ}\)C, a temperature difference of 28 \({}^{\circ}\)C was determined over a period of six weeks with an outside temperature between 2 and 6 \({}^{\circ}\)C. This corresponds to a daily loss of 0.66 \({}^{\circ}\)C, which is negligible. The pumps are modelled with the BASIC_PUMP component. This defines the pump by the following five parameters: nominal flow rate, nominal delivery head, maximum delivery head, length, and flow cross-section.  
The pump is controlled via the rotational speed, as a percentage, depending on the maximum speed. The power calculated from this assumes that the pump works isentypically [26].  
The valves are modelled with the component CONTROL_VALVE. To fully define a valve, the program requires the following five parameters: valve length, flow cross-section, nominal mass flow, loss of nominal pressure, and nominal density.  
The valves are controlled by their percentage of opening, where 100% represents a fully open valve.  
The heat exchangers are modelled with the components HEAT_PIPE and BOUNDARY_CONDITION. With the component HEAT_PIPE, only a pipe with thermal insulation is initially implemented. The BOUNDARY_CONDITION is used to supply the fluid in the pipe with a heat flow in W\({}_{\text{th}}\) in one or more nodes. In this case, the heat is transferred through three nodes. This promises a more accurate result than feeding through a single node. Alternatively, it would also be possible to model a countercurrent heat exchanger  
Figure 4: Schematic structure of the APROS basic model.  
with an HTF mass flow on one side and the molten salt mass flow on the other. However, it would not be possible to precisely record the supplied thermal energy, which is essential for subsequent validation of the model. Unknown characteristic values are left at the standard value (from APROS) or modified according to the cycle process so that APROS does not display any error messages. After the basic model was implemented in APROS 6, the model was further developed in the next step. The following concrete objectives were set for this: implementation of molten salt as a storage medium; implementation of the constant pressure of 1 bar in the storage tanks during a complete charging cycle; implementation of heat supply and removal that is as realistic as possible; limitation of the working temperature in each working step to the working range of the liquid salt that is possible in reality; and expansion of the control range of the valves (in the basic model, these constantly operated in an opening range of less than 1%). In addition, it should be determined whether the pumps must run continuously at maximum output (as in the basic model) or whether power reduction is possible during certain periods of the work cycle.