## 3 Modelling and Simulation
### Design of the Process for Other Storage Fluids  
After modelling and model validation are completed, three new models with different storage media or other temperature ranges were created based on the developed model in order to analyse differences and optimisation options. First, the TSS system is operated by the fluid Solar Salt, but the temperature range is extended to the maximum possible range. Because the melting temperature of Solar Salt is 220 \({}^{\circ}\)C and the decomposition temperature is 580 \({}^{\circ}\)C, the temperature range was set to 240 to 560 \({}^{\circ}\)C. The following components must be adapted to the new process requirements: regulation of the mass flows, pump parameters, and valve parameters. The rest of the modelling could be performed. Second, the Hitec fluid from the APROS database is used. This has a melting temperature of 142 \({}^{\circ}\)C and a decomposition temperature of 535 \({}^{\circ}\)C. Accordingly, the temperature range is set to 162 to 515 \({}^{\circ}\)C. Here, the same components must be adapted as when the working area of the Solar Salt was expanded. Third, the carbonate salts are used, which have a working temperature higher than nitrate salts. The melting temperature for the carbonate salt specified as optimal in [20] is 400 \({}^{\circ}\)C. The decomposition temperature is correspondingly higher (830 \({}^{\circ}\)C), which not only promises a greater temperature range than nitrate salt, but also a higher degree of efficiency can be achieved. The specific heat capacity is discussed in the next section in more detail later when implementing carbonate salt in APROS. Implementation of Carbonate Salt in APROS 6  
As the last storage medium, carbonate salt is included in the process so that it can also be compared with the other storage media. Carbonate salt is not one of the predefined fluids that APROS provides, which is why a new fluid must be created in APROS. As described above, the following values are required to define the carbonate salt in APROS: the density, the dynamic viscosity, the specific heat capacity, the thermal conductivity, and the adiabatic compressibility--all depending on the temperature. The selection of the carbonate salt follows the study [20], with the 36 carbonate salts in terms of their usability  
Figure 11: A validation model for a thermal storage system during a working cycle. as a storage medium in sensible heat storage as part of a solar power plant. This identified two suitable carbonate salt compositions: 3% K\({}_{2}\)CO\({}_{3}\), 10% Li\({}_{2}\)CO\({}_{3}\), 60% Na\({}_{2}\)CO\({}_{3}\) and 20% K\({}_{2}\)CO\({}_{3}\), 10% Li\({}_{2}\)CO\({}_{3}\), 70% Na\({}_{2}\)CO\({}_{3}\). The first composition was used in this work because it has a higher decomposition temperature (850 \({}^{\circ}\)C). The working temperature range is set to 446 to 830 \({}^{\circ}\)C because a distance of 20 degrees from the melting and decomposition point must be maintained. Melting and decomposition points were also taken from [20]. Based on [27], the density of the salt is modelled as a linear function. However, the literature values, which are given in Kelvin, must be converted to degrees Celsius, as APROS only accepts temperatures in Celsius:  
\[\rho\left[\text{kg}\cdot\text{m}^{-3}\right]=2323-0.4613\times(T) \tag{4}\]  
In the following, the correlation of the dynamic viscosity is required. This can be found in [28]:  
\[\eta[\text{mPa}\cdot\text{s}]=0.1012\times e^{33400/R\cdot T} \tag{5}\]  
With  
\[R\left[\text{J}\cdot\text{K}^{-3}\cdot\text{mol}^{-3}\right]=8.3143 \tag{6}\]  
However, only correlations in the form of polynomials can be inserted into APROS, which is why the formula given in Equation (5) is approximated with a second-degree polynomial for the range from 446 to 830 \({}^{\circ}\)C.  
A polynomial of the 2 nd degree can be formed with these values. The temperature is converted back to Celsius when setting up the polynomial.