## 4 Experimental Investigation
### Uniaxial Compression Tests  
During thermal charging and discharging of the TES system, the inventory and insulation undergo thermal expansion and shrinkage. This effect has two major consequences. Firstly, the risk of individual particles being damaged increases due to the increasing contact forces between the pebbles. Secondly, the pressure exerted on the surrounding container insulation walls increases, which must be taken into account during container and insulation design. Thermomechanical experiments are carried out to determine possible damage in cyclic TES operation. They are based on so-called uniaxial compression tests (UCT's) [29]. The used test rig and its specifications are illustrated in Figure 6. The test rig is custom-built and essentially consists of two parts. The basic structure is a hydraulic press which supplies the samples with a specific, axial load. Loads up to 160 t can be supplied. The second element is a cylindrical, insulated and heatable stainless-steel container located under the piston and measures 0.5 m in diameter as well as 0.5 m in height. The packed bed to be investigated is placed within the container. The slag pebbles can also be heated up to 600 \({}^{\circ}\)C. The piston load and axial change in bed height are measured with the internal measuring system of the press [29]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Date of the Year** & **Mode/-** & **Temperature/\({}^{\circ}\)C** & **Mass Flow/kg\(\cdot\)h\({}^{-1}\)** & **Time/h** \\ \hline
21 March (design point) & Charging & 700 & 612 & 5.20 \\  & Discharging & 120 & 677 & 4.20 \\
21 December (winter) & Charging & 700 & 612 & 4.17 \\  & Discharging & 120 & 677 & 3.38 \\
21 June (summer) & Charging & 700 & 612 & 6.19 \\  & Discharging & 120 & 677 & 4.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Test parameter [27]. Figure 5: Schematic drawing of the thermal energy storage (**a**) with different levels of thermocouple placement (**b**) [27]. The experimental methods for the investigation of the inventory and the isolation options are almost identical. The only difference is the filling. For the inventory tests, slag is filled into the container up to the top edge, while for the insulation tests the insulation bricks are placed on the bottom of the container and a slag layer of approx. 10 cm is put on top. This is done to obtain an accurate representation of the TES slag to insulation penetration. On top of the filled container, a cover is placed to distribute the piston load over the entire packed bed. Cyclic compression tests with a load starting from 20 kN increasing to 140 kN are carried out. Between these two load points, 500 cycles are performed, which represents about two years of TES operation in a CSP plant. The reason for the short test time compared to the real operating time is, on the one hand, that the highest particle damages occur after a few cycles. This is due to pre-damage, production-related damage and micro-cracks in the particles. On the other hand, the investigation time is limited. Figure 7 illustrates the load force acting on the slag packed bed over (here only for 100 cycles). The starting load of 20 kN represents the static, non-thermal load at the bottom of the 12 m high full-scale heat storage. This was calculated using the Janssen equation for silo stress calculation (Equation (9)) and the boundary conditions of the full-scale TES system given in Figure 2.  
\[A\,\sigma_{\mathrm{v}}+g\,\rho_{\mathrm{b}}\,A\,\mathrm{dz}=A(\sigma_{\mathrm{ v}}+\mathrm{d}\sigma_{\mathrm{v}})+\tau_{\mathrm{w}}\,U\,\mathrm{dz} \tag{9}\]  
\(\rho_{\mathrm{b}}\) is the effective packed bed density and corresponds to the product of the packing factor and the slag density. The parameters \(A\) and \(U_{\mathrm{c}}\) are the circular area and the circumference of the backed bed. \(\sigma_{\mathrm{v}}\) represents the vertical pressure. The highest load of 140 kN represents the \"worst-case\"  
Figure 6: Uniaxial compression test rig (**a**) and its specifications (**b**).