## 5 Conclusions  
A key element for improving efficiency and flexibility are thermal energy storage systems. They store efficiently heat available or generated in processes at times of low thermal energy demand (charging period) and provide it at times of increased thermal energy demand (discharging period). Such solutions, extended by power-to-heat options, are already commercially available in large-scale applications or are in focus in a wide range of R&D activities, but new in the transport sector. By transferring existing ideas as new thermal management concepts for battery electric vehicles an alternative heat supply during cold seasons can be enabled reducing the battery-powered heating demand and thus increase the effective range.  
In particular, through vehicle-specific weight restrictions solutions are needed that achieve high systemic storage and power densities while at the same time meeting thermal requirements in terms of performance and temperature level. For this purpose and for the first time, a novel thermal storage system based on an electrically heated solid medium has been developed, constructively realized, systematically measured and successfully validated. High-temperature heat is generated electrically in heating wires during the charging period and transferred efficiently via thermal radiation to ceramic honeycombs. During discharging period, the stored thermal energy is used to heat the interior at defined temperature levels with high thermal powers by a bypass control.  
Despite test rig-related limitations, the experimental results confirm the feasibility of meeting all vehicle-specific requirements and simultaneously achieving high systemic storage and power densities. Under the existing laboratory infrastructure, the thermal energy storage system reaches constant heating powers of 6.8 kW up to average inventory temperatures of approx. 843 \({}^{\circ}\)C. During discharging period constant thermal powers of up to 6.4 kW at adjustable temperature levels are reached leading to systemic storage densities of about 155 kWh/kg. In addition, the developed component also allows high flexibility through a bifunctional operational use: on the one hand as storage system with successive charging/discharging periods and on the other hand as convective heating system.  
Future activities will focus on the implementation of improvement measures based on the identified optimization potentials regarding inventory materials, thermal insulations, electric heating wires and double-walled storage containment. With a successful implementation, systemic storage densities of up to 200 Wh/kg and maximum heating powers of up to 19.2 kW will be expected. In addition to a prototypical realization with reduced amount of measurement devices and thus significantly high volume-specific improvements, conceptual investigations on system extensions for generation and storage of cold are performed. Here, basic idea is to increase the seasonal utilization of the novel  
Figure 15: Heating mode—transient outlet temperature (T\({}_{out}\)) as a function of the air mass flow rate (\(\dot{\mathrm{m}}\)).  
component by using it in hot seasons as source for cold supply and thus to achieve further efficiency improvements for battery electric vehicles.  
Conceptualization, L.H.; investigation, V.D.; methodology, V.D.; validation, V.D. and L.H.; writing--original draft, V.D. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.