## 4 Results
### Discharging Period  
During thermal discharging, cold entering air is passing the honeycombs and the bypass path via a mass flow control in order to achieve constant mixing temperatures at the outlet of the storage system. The fluid flow temperature is elevated convectively either from the previously electrical heated honeycombs or from the heat losses inside the bypath path. For investigation of the reachable discharging durations, variations studies were conducted regarding the constant mixing temperature (\(\overline{T}_{out}\)), the total mass flow rate (m) and the average inventory temperature (\(\overline{T}_{I}\)) at the end of charging period as summarized in Table 4.  
Here during charging period, the electrical heating's of the honeycombs were performed with a (constant) heating power of 6.8 kW up to the specified average inventory temperature for each measuring point. As in the previous section, the maximum permitted heating wire and bypass temperatures, which cause the activation of active cooling when exceeded, were set to values of 1050 \({}^{\circ}\)C and 60 \({}^{\circ}\)C, respectively. Central results regarding transient characteristics during discharging for an exemplary selected measurement at \(\overline{T}_{I}\) = 843 \({}^{\circ}\)C and T\({}_{out}\) = 120 \({}^{\circ}\)C each with m = 183 kg/h are shown in Figures 10-12. With regard to the temporal and local temperatures inside the honeycomb, the results in Figure 10 illustrate the typical undulatory character of solid media thermal energy storage systems (regenerator): the entering cold air is convectively heated from the honeycomb over the flow path length leading locally to a cooling of the solid medium over time. Here, the efficiency of heat transport depends only on the heat transfer coefficient, the thermal resistance inside the solid medium and the heat transferring surface and is reflected in local and temporal temperature gradients. Within the scope of the honeycombs investigated here with their large specific surface areas of 620 m\({}^{2}\)/m\({}^{3}\), high heat transfer rates are achieved, which are visible both in the experimental and comparably in the simulat \begin{table}
\begin{tabular}{c c c} \hline \(\overline{T}_{I}\) [\({}^{\circ}\)C] & \(\hat{m}\) [kg/h] & \(T_{out}\) [\({}^{\circ}\)C] \\ \hline
340 & 64 & 60 \\
517 & 125 & 120 \\
701 & 183 & \\
843 & 230 & \\ \hline \end{tabular}
\end{table} 
Table 4: Central variation parameters: average inventory temperature (\(\overline{T}_{I}\) ) at the end of charging period, total mass flow rate (\(\hat{m}\) ), constant mixing temperature (T\({}_{out}\) ). Figure 10: Transient temperature profiles in the honeycombs at z/L = 0.05 (T\({}_{I-A-1}\)), 0.25 (T\({}_{I-A-2}\)), 0.45 (T\({}_{I-A-3}\)). In addition to the local characteristics within the honeycombs, the outlet temperatures of the storage inventory and the bypass path as well as the resulting constant mixing temperature are of central importance for the efficiency of the whole storage system. The results on this are shown in Figure 11.  
As can be seen during discharging period, constant mixing temperatures of 120 \({}^{\circ}\)C are reached with a duration of 33 min in the simulation results and of approx. 29 min in the experimental results. Thereby over a long period of time, a relative constant level of temperatures at the outlet of the storage inventory and the bypass path are reached. At the solid medium side significant higher deviations between simulation and measurement are visible with average temperatures of approx. 870 \({}^{\circ}\)C and 650 \({}^{\circ}\)C, respectively. The main reasons for this are neglected heat losses at the front surfaces of the honeycombs inside the model. These lead locally to increased exergy losses of the exiting air, but only to slight energetic losses related to the stored thermal energy.