## 4 Proof of Concept: First Experimental Validation  
For proofing of the storage concept and for validation, a test rig was erected focusing preliminary experimental investigations on the charging process and statements regarding the quality of the porous simulation model. Central components within the test rig include the honeycomb with integrated heating wires and the thermal insulation. Through its high flexibilities in terms of electrical voltage (10-230 V), electrical current (maximum 16 A) and \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Honeycomb (S)** & **Thermal Insulation (Ins)** & **Heating Wire (P)** \\ \hline \(m\) **[kg]** & 7.8 & 4.2 & 0.4 \\ \hline \(V\) **[l]** & 3.4 & 18.8 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Central specifications of the selected design option. Figure 9: Temporal characteristic of a favoured design option: heating wire and (spatial-averaged) honeycomb temperature as well as the normalized electrical heating power and thermal charging state. exchangeable storage media, wide experimental investigations of different honeycomb-heating-wire-configurations are enabled. The test rig includes all essential components and requirements for the storage concept, allows a validation of central characteristics of the porous simulation model and the identification of critical design aspects for the futural target scale. For the first experimental setup-up, a ceramic honeycomb (alumina porcelain) with a mass of 6.8 kg, a thermal insulation (calcium silicate) with an insulation thickness of 100 mm and a ferritic FeCrAl based heating wire were [33] selected. The honeycomb has a square cross-section area of 150 \(\times\) 150 mm, a length of 300 mm and a specific surface of 273.4 m\({}^{2}\)/m\({}^{3}\) with a void fraction of 62.2%, the heating wire a diameter of 1 mm and a total length of 27.3 m. The heating wire was uniformly integrated in the honeycomb structure, resulting in a heating wire assignment of 43.2%. Within the honeycomb, two thermocouple sets were installed each triangularly at a distance of 100 mm from the frontal areas and placed close to the honeycomb wall. Central aspects of the experimental setup-up can be seen in Figure 10.  
First experimental tests on the shown setup-up were performed with an electrical heating power of 0.7 kW at a voltage of 180 V. The transient charging process with the associated experimental and calculated temperatures based on the model explained in Sections 2.1 and 2.2 are shown in Figure 11.  
The experimental results show a homogeneous heating despite slightly lower temperatures of the thermocouples (T4-6) located closer at the edge region compared to the thermocouples located inside the core region (T1-3) of the honeycomb. This is caused due to the thick-walled thermal insulation leading to low heat losses and thus to low  
Figure 11: Validation: experimental (Exp.) vs. simulation (Sim.) results. Figure 10: Test rig for the experimental investigation of the charging process: honeycomb and thermal insulation (**left**); thermocouple sets (T1-3 green colored, T4-6 magenta colored) and heating wires (**right**). local temperature differences within the honeycomb. Analogous to the experiment, the local temperatures resulting from the porous 2D simulation model showed only slight differences of less than 30 \({}^{\circ}\)C. In comparison between the experimental and the averaged simulative temperatures (Ts, avg), a good agreement in the temporal heating process over the whole charging period is visible. For the first experimental setup-up with a honeycomb structure close to the specifications of the favored design option as described in Section 3.2.3, maximum temperature-related deviations of 11.1% and mean deviations of 4.1% are determined. The first experimental investigations on the charging process confirm a good agreement between experimental and simulation results with regard to the temperature characteristics of the honeycomb and a homogeneous heating of the whole storage medium.