## 3 Experimental Set-Up  
Central elements of the thermal storage system test rig include the ceramic, channel shaped inventory (honeycomb) with integrated heating wires, the shell sided thermal insulation and the double-wall containment with integrated bypass. The test rig design was based on simulation studies on the storage system [24; 26] reaching a constant thermal output of 5 kW over 30 min during the discharging period and is heated in less than 30 min during the charging period. Based on these simulative results, commercially available inventories, heating wire options and microporous thermal insulations, technical specifications for the overall design were developed and realized in exchange with suppliers. Central parameters for favorite design options and for the finally erected test rig with an electrical power supply of 400 V and a maximum current of 16 A are summarized in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Subsystem** & & **Simulation Results** & **Test Rig** \\ \hline \multirow{4}{*}{**Storage Inventory**} & Material & Al\({}_{2}\)O\({}_{3}\) & 91\% Al\({}_{2}\)O\({}_{3}\) \\  & Mass [kg] & 8.2 & 2 \(\times\) 5.9 \\  & Specific Surface [m\({}^{2}\)/m\({}^{3}\)] & \(>\)200 & 620 \\  & Void Fraction [\%] & 40–70 & 66 \\  & Length-to-Diameter Ratio & 2–4 & 3.65 \\ \hline \multirow{4}{*}{**Heating Wire**} & Material & FeCrAl Alloy & FeCrAl Alloy \\  & Maximum Operation Temperature [\({}^{\circ}\)C] & 1000 & 1300 \\ \cline{1-1}  & Length [m] & 30 & \(3\times 30\) \\ \cline{1-1}  & Diameter [mm] & 1.5 & 1.5 \\ \hline \multirow{2}{*}{**Thermal Insulation**} & Typ & microporous & microporous \\ \cline{1-1}  & Thickness [mm] & 20–40 & 25 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Central specification for the favorite design solution: simulation results vs. test rig. The storage inventory selected here (honeycomb) with an Al\({}_{2}\)O\({}_{3}\) content of 91% shows high specific surface areas of 620 m\({}^{2}\)/m\({}^{3}\) at void fractions of 66% and exceeds significantly the required geometric specifications in terms of heat transferring surface compared to the simulation results from Table 1. Due to manufacturing constraints, two honeycombs were installed here behind each other, with a total mass of 11.8 kg, a length (L) of 566 mm with a diameter of 155 mm. Inside the honeycombs three heating wires were integrated allowing operating temperatures of 1300 \({}^{\circ}\)C, whereby each has a diameter of 1.5 mm, a length of 30 m and were selected for maximum permitted currents of 16 A at a voltage of 400 V. In total, this results in a mass of 1.2 kg and a maximum heating power of 19.2 kW, which is significantly higher than the target values to be achieved. Unfortunately, the delta circuit required therefor using a 400 V three-phase alternating current could not be implemented due to laboratory restrictions, since the resulting maximum current in the outer conductor would exceed the permitted electric fuse of 16 A. Hence, the three heating wires were connected in a star circuit, which limited the maximum heating power in total to 6.8 kW. However, by adapting the laboratory infrastructure on a 32 A system and a delta circuit, heating powers of 19.2 kW can be achieved in the future. Central insights into the honeycombs with integrated heating wires are shown in Figure 2.  
The double-walled containment with integrated bypass including the air-side piping connections was made of aluminum with a wall thickness of 0.6 mm and reaches a total mass of 5.9 kg. The heating wires were integrated into the storage system through thermally decoupled electrode line bushings and additionally thermally insulated within the flow distribution chamber. Microporous thermal insulations with a layer thickness of 25 mm were installed inside the containment on the shell side and at the conical outlet of the storage inventory. For other zones still internally uninsulated (inlet and outlet areas), high-temperature fiber blankets were used, resulting in a mass of 2.5 kg for all thermal insulations.