## 3 Results
### Thermal Storage System
#### 3.2.2 Optimized Systems with Maximum Gravimetric Storage Density  
As explained in Section 3.2.1, maximum possible storage densities are reached based on appropriate configurations regarding the specific surface and the heating wire assignment, whereby the void fraction (\(\varepsilon\)) was kept constant so far. For a holistic investigation of the storage system, additional variation studies are performed on this. Hereby, the presented results refer to the maximum possible gravimetric storage densities determined through iterative optimization simulations to the heating wire assignment depending on the specific surface. Central results regarding the iteratively optimized systemic storage densities are illustrated in Figure 7.  
The results show increasing maximum possible systemic storage densities with higher specific surfaces. This is caused by the already mentioned effect regarding the number of honeycomb channels, the corresponding heating wire lengths and thus only a moderate temperature elevation. For those solutions, the iteratively determined results force a maximum heating wire assignment of 100% to reach their maximum possible systemic storage densities. In contrast, at high specific surfaces of 500 m2/m3-thus a maximum temperature elevation of the honeycomb structure-only heating wire assignments of less than 20% are needed.  
Additionally, the results point out that for maximum possible storage densities a specific surface of above 200 m2/m3 is required, whereby its magnitude only depends on the void fraction. But with higher void fractions increasing honeycomb and proportionally significantly increasing thermal insulation volumes are associated leading to reductions in systemic storage densities, especially for the volumetric. Furthermore, the results show an absence of solutions at high specific surfaces and small void fractions. This is caused by associated insufficient low channel diameter of the honeycombs compared to the heating wire diameter.  
The presented results and contexts point out the significance of high specific surfaces as central prerequisite for maximum systemic storage densities. However, for a holistic evaluation of such systems, additional statements regarding the heating wire surface load  
Figure 7: Maximum possible gravimetric and associated volumetric systemic storage densities depending on specific surface (\(a_{V}\)) and void fraction (\(\varepsilon\)).  
(\(\hat{q}\)) and thus the lifetime are necessary. Therefore, the maximum heating wire surface loads occurring in the transient process are shown in Figure 8 based on the iteratively optimized variation studies.  
The results show decreasing maximum heating wire surface loads with higher specific surfaces. This is due to the fact of increasing heating wire diameters and lengths with slight void fraction-related dependencies. However, it is visible that well-suited heating wire surface loads of less than 5 W/cm\({}^{2}\)[35] are reached with sufficient high specific surfaces of more than 250 m\({}^{2}\)/m\({}^{3}\) minimizing lifetime-critical conditions in addition to the already moderate heating wire temperature of 1000 \({}^{\circ}\)C.  
Based on these results for optimized systemic storage densities and corresponding maximum heating wire surface loads, a favored design option is selected meeting also geometrical requirements in terms of specific surface and void fraction for an efficient thermal discharging operation with low pressure drops as describe in [23]. For this, transient characteristics as well as central storage dimensions are presented in the following.