## 4 Results
### Overall Evaluation  
The overall evaluation of the thermal storage system in terms of performance and optimization potentials is based on three key criteria: systemic storage density, effective heating power and parasitic losses. **Systemic storage density**  
The primary objective in the development of the novel thermal energy storage system for an alternative heat supply in battery electric vehicles is to achieve comparable or higher systemic storage densities in relation to today's battery-powered PTC heating elements. For this purpose, the results obtained in the previous section regarding effective discharge duration and constant thermal discharge power are considered in relation to the total mass of the storage system. The resulting systemic storage densities from this--thus the effectively usable heat at a constant temperature or thermal power level--are illustrated in Figure 14.  
The results show a constant level of systemic storage densities within the considered thermal discharge power range of up to 6.4 kW, whereas their magnitudes depend on the average inventory temperatures at the end of charging period. Minor decreases in systemic storage density with increasing constant thermal discharge power are visible, associated with the higher requirements for heat transport of the air passing the honeycombs. For the maximum average inventory temperatures currently achieved at the end of charging period of approx. 843 \({}^{\circ}\)C, systemic storage densities of approx. 155 Wh/kg are reached in the experimental and in the simulative results. Compared to today's PTC heaters with battery-specific storage densities of up to 160 Wh/kg [25], the performance of the first thermal storage system test rig is already visible. Further increases of the systemic storage density up to 200 Wh/kg can be achieved by optimizations, regarding the inventory material and its void fraction, by layered thermal insulation concepts and reduced heating wire lengths as well as design improvements on the double-walled containment. **Effective heating power**  
In addition to the high performance achieved by the thermal storage system during discharging period, the results presented in Section 4.2 also show high effective heating powers during charging. Thus, with maximum permitted heating wire temperatures of 1050 \({}^{\circ}\)C, a constant heating power of 6.8 kW could be achieved up to average inventory temperatures of approx. 843 \({}^{\circ}\)C. From this level, the implemented control leads to a reduction of the heating power, whereas currently a further increase of the average inventory temperatures up to 880 \({}^{\circ}\)C was reached. Additional improvements in performance during charging period can be obtained by a more homogeneous distribution of the heating wires  
Figure 14: Systemic storage density (Q/m\({}_{tot}\)) as a function of constant thermal discharge powers (\(\dot{\text{Q}}\)) and average inventory temperatures at the end of charging period (\(\overline{\text{T}}_{I}\)). within the honeycombs and by higher permitted heating wire temperatures up to the operational limit of 1300 \({}^{\circ}\)C. Apart from that, the electrical connection of the three heating wires limits the maximum heating power to approx. 6.8 kW due to the current laboratory restrictions. Despite these limitations, the heat of 2.5 kWh required for thermal discharge could be generated and stored in less than 30 min during the charging period fulfilling successfully the target values specified for the investigated test rig. By futural adapting the laboratory infrastructure on a 32 A system and a delta circuit of the heating wires, heating powers of up to 19.2 kW can be reached allowing significant improvements in charging durations on less than 11 min. **Parasitic losses**  
The central issue for optimization includes the balance between permitted heat losses and the amount of thermal energy used for preheating the interior via the double-walled bypass path during charging period, respectively.