## 4 Results
### Discharging Period
Together with low requirements of the exiting constant air temperature compared to the storage inventory (see Figure 10), the described exergy losses effects only slightly the overall efficiency and thus the discharging duration. However, in the last period of discharging significant temperature changes are visible in both flow paths. This effect is due to the specification of a constant mixing outlet  
Figure 11: Transient temperature characteristics at the outlet of the storage inventory (T\({}_{I-out}\)), the bypass path (T\({}_{B-out}\)) and the storage system (T\({}_{out}\)). Figure 12: Transient characteristics of the mass flow rates inside the bypass path (\(\dot{\mathrm{m}}_{B}\)) and storage inventory (\(\dot{\mathrm{m}}_{I}\)). temperature--thus constant thermal discharge power--and related to the mass flow rates as shown in Figure 12.  
Due to the specification of a constant mixing temperature, the mass flow rate in the bypass and inventory path shows an opposite behavior compared to the local outlet temperatures in time. Particularly in the last discharge period, high gradients are visible analogous to Figure 11. The reason for this is the interaction of increasing temperature drop of the air leaving the storage inventory with increasing mass flow rate. This positive feedback leads to growing gradients in the temporal mass flow rates and outlet temperatures and can be seen in good agreement in both the measurement and the simulation results. Comparable to the characteristics explained here, good agreements between the experimental and simulative results with regard to constant mixing temperature, total mass flow rate and average inventory temperature at the end of charging period were found in all the investigations within this measurement campaign. For a comprehensive analysis and for evaluating the performance of the storage system, the thermal discharging power (\(\bar{m}\)) and the effective discharge duration (\(\tau\)), which reflects the period of a constant mixing temperature (T\({}_{out}\)), are suitable criteria. Here, the effective discharge duration is determined uniformly according to Equation (2) on the base of a maximum permitted deviation of the time dependent temperature difference (T\({}_{out}(\tau)-\mathrm{T}_{in}\)) to the specified temperature difference (T\({}_{out}-\mathrm{T}_{in}\)). \[\left|1-\frac{\mathrm{T}_{out}(\tau)-\mathrm{T}_{in}}{\mathrm{T}_{out}- \mathrm{T}_{in}}\right|<0.1 \tag{2}\]  
The results for this as a function of the average inventory temperature at the end of charging period (\(\overline{\mathrm{T}}_{I}\)) are summarized in Figure 13.  
The investigations conducted show a direct correlation between these central quantities in both the simulative and the experimental results: increasing average inventory temperatures--thus thermal energy stored--as well as lower thermal discharge powers enable longer effective discharge durations. It is also obvious that the specified target values--constant thermal discharging with 5 kW over 30 min--were reached, whereby average inventory temperatures of approx. 700 \({}^{\circ}\)C are required for this. These temperatures can be achieved thereby with the currently available heating power of 6.8 kW in less than 30 min during charging period as illustrated in Figure 9.  
Based on the findings obtained here, central results for evaluation of the systemic performance and optimization potentials of the storage system are presented in the following. Figure 10:0 Effective discharge durations (\(\tau\)) as a function of constant thermal discharge powers (\(\dot{\mathrm{Q}}\)) and average inventory temperatures at the end of charging period (\(\overline{\mathrm{T}}_{I}\) ).