## 3 Results
### Thermal Storage System
#### 3.2.3 Selected Design Option: Temporal Characteristics and Dimensions  
Within the wide solution spectrum, a favored design option was selected considering systemic storage densities, maximum heating wire surface loads and thermal discharging requirements. The selected solution is based on a honeycomb structure with a specific surface of 350 m\({}^{2}\)/m\({}^{3}\), a void fraction of 42.5% and a heating wire assignment of 38.4%. Central transient characteristics are shown in Figure 9.  
As can be seen, a constant electrical heating power is achieved over a period of about 20 min, whereby a linear temperature elevation of the honeycomb structure from \(-10\)\({}^{\circ}\)C to 800 \({}^{\circ}\)C or a thermal charging state of 80% is reached. From this point on, the implemented control algorithm leads to a reduction of the heating power to limit the heating wire temperature at 1000 \({}^{\circ}\)C. The heating wire itself shows significantly higher temperature gradients at the beginning of the loading process due to lower thermal masses compared to the honeycomb. However, from a heating wire temperature of about 800 \({}^{\circ}\)C the dominating radiation heat transport leads to substantial increasing honeycomb temperatures and thus to reduced heating wire temperature gradients.  
Figure 8: Maximum heating wire surface load depending on specific surface (\(a_{V}\)) and void fraction (\(\varepsilon\)).  
For the exemplary favored design option presented here, systemic storage densities of 201 Wh/kg and 113 kWh/m\({}^{3}\) with a maximum heating power of 6.4 kW are reached. During charging maximum heating wire surface loads of 4.6 W/cm\({}^{2}\) occur at heating wire length and diameters of 30 m and 1.5 mm, respectively. Despite the high storage temperatures, only maximum heat losses of about 76 W occur resulting from the restrictive specification regarding the maximum permitted surface temperature of 60 \({}^{\circ}\)C (Table 4) and the small storage dimensions. Central specifications of the selected design option are summarized in Table 5.  
The systematically prepared results regarding systemic storage densities and heating wire surface loads confirm the feasibility and efficiency of such storage systems for the heat supply in BEV. Comparable results are also reached for different electrical power supplies and for charging durations of less than 30 min.  
The contexts presented here elucidate the need of high specific surfaces of solid media thermal energy storages in addition with adequate selected void fractions. Comparable geometric conditions are also required during thermal discharging, in order to allow efficient convective heat transport to the heat transfer medium (air). The associated thermodynamic synergies in the cyclic storage process, the achievable systemic storage densities and the commercial availability of such ceramic honeycomb structures confirm the potential of the technology as alternative thermal management concept of battery electric vehicles.  
For proof of concept a test rig was erected, enabling due its high flexibility in terms of electrical voltage, heating power and exchangeable storage media wide experimental investigations. First results of an exemplarily honeycomb-heating-wire-configuration were performed and used for validation of the described porous simulation model.