## 4 Results
### Charging Period: Active Cooling  
The design of the storage system is based on a shell sided integration of the bypass path around the thermal insulation with the idea to utilize heat loss during discharging period and to enable preheating of the interior during charging period by a slight active cooling. To investigate these effects during charging period, a moderate thin thermal insulation with a thickness of 25 mm was chosen to achieve significant temperature changes in the bypass path. For this purpose, steady-state series of measurements were performed that included variations of the bypass mass flow rate (\(\dot{m}_{B}\)) and the average inventory temperature (\(\overline{\Gamma}_{I}\)). In this case, the tTe storage system was heated at the beginning with an electrical heating power of 5 kW and kept stationary--after reaching the defined average inventory temperature--by the implemented power control. Central variation parameters are summarized in Table 2.  
For two exemplary selected measurements at \(\overline{\Gamma}_{I}\) = 835 \({}^{\circ}\)C and \(\mathrm{m}_{B}\) = 14 kg/h as well as at \(\overline{\Gamma}_{I}\) = 835 \({}^{\circ}\)C and \(\mathrm{m}_{B}\) = 113 kg/h, the resulting steady-state temperatures at the outlet of the bypass path (\(\Gamma_{B-out}\)) are shown in Figure 5. \begin{table}
\begin{tabular}{c c} \hline \hline \(\overline{T}_{I}\) [\({}^{\circ}\)C] & \(\dot{m}_{B}\) [kg/h] \\ \hline
365 & 14 \\
555 & 40 \\
755 & 65 \\
835 & 89 \\
113 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Central variation parameters: average inventory temperature (\(\overline{\Gamma}_{I}\)) and bypass mass flow rate (\(\dot{m}_{B}\) ). Figure 5: Transient characteristics of the bypass outlet temperature (\(\Gamma_{B-out}\)). The results under steady state conditions show the well-known relationship of increasing mass flow rates with simultaneously decreasing outlet temperatures in both the measurement and simulation characteristics. Related to the experimental data, slightly higher steady-state active cooling powers of 570 W result here at a mass flow of 113 kg/h compared to 420 W at a mass flow of 14 kg/h. Similar orders of magnitude with 540 W and 440 W are also reached in the simulative results. These effects are caused by increasingly convective heat transfer coefficients associated with higher mass flow rates. In addition to comparable steady-state results, the transient characteristics between the experimental and simulative temperatures also show a good agreement. Larger deviations can be seen especially at low mass flow rates, which can be explained by the neglection of the thermal mass of the bypath channel inside the model. Analogous to the exemplarily presented contexts, all results within the conducted measurement campaign for active cooling show comparably characteristics with good agreements between experiment and simulation. The resulting central steady-state results based on the investigations according to Table 2 are summarized in Figure 6.  
As can be seen, increasing steady-state temperature differences between the outlet and inlet bypass are associated with higher average inventory temperatures and decreasing active cooling mass flow rates. Furthermore, the simulation results also show comparable magnitudes and characteristics and thus confirm the quality of the implemented heat transport mechanisms within the model. For further measurement campaigns presented in the following, the active cooling mass flow rate was activated after a maximum permitted temperature in the bypass path has been exceeded. The magnitude of the active cooling mass flow rate (\(\dot{m}_{B,c}\)) was selected in such a way, that a maximum bypass outlet temperature of 60 \({}^{\circ}\)C results under steady state condition. For this purpose, the results according to Figure 6 were used at a temperature difference of 40 \({}^{\circ}\)C, since the inlet temperatures within the measurement campaigns showed constant values of approx.