## 2 Slag Based TES Design  
For a better impression, Figure 1 shows a schematic flow sheet of a CSP plant with slag as TES inventory material. The TES design consideration, the comparative assessment and the lead design selection are described in detail by [PERSON] et al. [22]. The CSP plant target specifications and lead concept specifications are already introduced in [24]. Therefore, subsequently only a short overview of the lead design selection process is given, and the TES properties required for the investigations presented here are listed.  
The entire design process includes the development of various possible TES designs, their calculation and evaluation using established management tools. The considered designs differ in the arrangement of the container, which is either axial standing or horizontal lying. Another distinguishing feature is the flow direction, which is divided into axial, radial or meander-shaped. Further information, especially the pros and cons of each arrangement is given in [22]. [PERSON] et al. [22] set up simplified continuous solid phase models, based on the \(\Lambda\)-II method to obtain an initial estimation of the thermal design and heat storage dimensions. The basics of the method are described in [25, 26]. CSP plant specifications used as boundary conditions for the TES design consideration and the slag material properties are given in Table 1.  
[PERSON] et al. [22] applied management tools normally used in production development in order to identify a lead concept. This includes an aptitude and risk analysis based on the quality function deployment (QFD) method. For the aptitude analysis, criteria for technical and economical demands were collected. Each concept was evaluated according the criteria, which resulted in individual aptitude values. Various possible risks were identified for the risk analysis and assessed in terms of potential damage. A risk value was calculated for each TES design after considering a probability of occurrence of the respective damaging event. Both values led in an aptitude to risk ratio. The best ratio was shown by the vertical storage design with axial flow direction. This TES design is depicted in Figure 2. It was taken as the lead concept and is investigated in detail in further research. The step-by-step implementation of the method as well as detailed results for the individual TES designs are given in [22].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**a**} & \multicolumn{2}{c}{**b**} \\ \hline
**Parameter** & **Value** & **Slag Properties** & **Value** \\ \hline TES capacity & 6.5 h (2.21 GWh) & Thermal conductivity & 1.43 W (m K)\({}^{-1}\) \\ TES inlet temperature (charging) & 700 \({}^{\circ}\)C & Specific heat capacity & 0.933 kJ (kg K)\({}^{-1}\) \\ TES inlet temperature (discharging) & 120 \({}^{\circ}\)C & - & - \\ Max. temperature drop at TES & 60 \({}^{\circ}\)C & - & - \\ outlet while discharging & & & \\ Max. TES pressure loss & 100 mbar & - & - \\ Discharging mass flow & 780 kg s\({}^{-1}\) & - & - \\ Max. TES charging mass flow & 1080 kg s\({}^{-1}\) & - & - \\ Mean TES charging mass flow & 706 kg s\({}^{-1}\) & - & - \\ Charging duration & 8 h & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 1: Operation specifications for a 150 MW\({}_{\text{el}}\) CSP plant (a) and used slag parameter (b) [27].  
Figure 1: Flow sheet of a concentrated solar power (CSP) plant with a thermal energy storage based on slag inventory [22].