## 4 Results
### Charging Period: Electrical Heating  
The charging of the storage system is based on heating wires, which are homogeneously distributed within the honeycombs heating the storage inventory via thermal radiation. For investigation of the transient heating process as well as the (optional) active cooling in the bypass path, variation studies on the heating power (1-6.8 kW) at a given charging duration (30 min) were conducted in this measurement campaign. The maximum permitted heating wire and bypass temperatures, which cause the activating of the active cooling when exceeded, were set to values of 1050 \({}^{\circ}\)C and 60 \({}^{\circ}\)C, respectively. For an exemplary measurement at a heating power of 6.8 kW, the resulting transient characteristics in the bypass path and the inventory temperatures are shown in Figures 7 and 8. The results during charging period of the storage system show increasing temperatures in the bypass path due to heat conduction and radiation coming from the heated honeycombs until the permitted maximum temperature of 60 \({}^{\circ}\)C is exceeded. From this point on, the active cooling is activated, whereby its magnitude is determined according to Equation (1). In comparison between the experimental and simulated results, a good agreement is evident both in characteristics of the bypass temperatures and in the time  
Figure 8: Transient temperatures in the honeycombs (T\({}_{I}\)) and electric heating power (P). Figure 7: Transient characteristics in the bypass path: outlet temperature (T\({}_{B-out}\)) and normalized active cooling mass flow rate (\(\dot{\mathrm{m}}_{B}/\dot{\mathrm{m}}_{B,c}\) ). for starting the active cooling. Due to the specified charging duration of only 30 min, the steady state condition of the bypass outlet temperature is not reached here. Even in the temporal characteristics of the inventory temperatures, a good agreement between measurement and simulation is reached. After the beginning of electrical heating, the storage inventory temperatures increase until average values of 790 \({}^{\circ}\)C are reached in the measurement and 819 \({}^{\circ}\)C in the simulation at the end of charging period. The significantly higher spread in experimental results is due to the fact of only three radially distributed measurement locations (see Figure 4), whereby the outermost measuring zone was also integrated close to the shell-side thermal insulation. As a result, heat loss effects in combination with a limited number of thermocouples are disproportionately visible leading to elevated temperature spread inside the experimental results. Comparable good agreements between the measurements and simulations--both in temporal characteristics of the temperatures and in times of activating the active cooling--were found in all investigations inside this measurement campaign. Central results on maximum inventory temperatures at the end of charging period as a function of heating power and on times of activating the active cooling are summarized in Figure 9 and in Table 3.  
The results show--in good agreement between experiment and simulation--linear increasing inventory temperatures with raising constant heating powers. Relating to time of activating the active cooling summarized in Table 3 slightly lower values in the simulations compared to the measurements are visible. This is due to neglection of the double-walled containment mass in the model leading to a lower thermal capacity of the whole storage system. At low heating powers (1 kW) with the associated inventory temperatures of less than 200 \({}^{\circ}\)C, the permitted maximum bypass temperature of 60 \({}^{\circ}\)C was not exceeded, resulting in a non-activating of the active cooling in both the experimental und simulation results. Based on the contexts explained here during charging period, investigations into the thermal discharging process of the storage system were conducted in the subsequent measurement campaign.