Thermal Battery for Electric Vehicles: High-Temperature Heating System for Solid Media Based Thermal Energy Storages
Authors: Dreißigacker Volker 
Published: November 2021

Thermal energy storage systems open up high potentials for improvements in efficiency and flexibility for power plant and industrial applications. Transferring such technologies as basis for thermal management concepts in battery-electric vehicles allow alternative ways for heating the interior and avoid range limitations during cold seasons. The idea of such concepts is to generate heat electrically (power-to-heat) parallel of charging the battery, store it efficiently and discharge heat at a defined temperature level. The successful application of such concepts requires two central prerequisites: higher systemic storage densities compared to today’s battery-powered PTC heaters as well as high charging and discharging powers. A promising approach for both requirements is based on solids as thermal energy storage. These allow during discharging an efficient heat transfer to the gaseous heat transfer medium (air) due to a wide range of geometric configurations with high specific surfaces and during charging high storage densities due to use of ceramic materials suitable for high operating temperatures. However, for such concepts suitable heating systems with small dimensions are needed, allowing an efficient and homogeneous heat transfer to the solid with high charging powers and high heating temperatures. An appropriate technology for this purpose is based on resistance heating wires integrated inside the channel shaped solids. These promise high storage densities due to operating wire temperature of up to 1300 °C and an efficient heat transport via radiation. Such electrically heated storage systems have been known for a long time for stationary applications, e.g., domestic storage heaters, but are new for mobile applications. For evaluation such concepts with regard to systemic storage and power density as well as to identify preferred configurations extensive investigations are necessary. For this purpose, transient models for the relevant heat transport mechanisms and the whole storage system were created. In order to allow time-efficient simulations studies for such an electrical heated storage system, a novel correlation for the effective radiation coefficient based on the Fourier Number was derived. This coefficient includes radiation effects and thermal conduction resistances and enables through its dimensionless parameterization the investigation of the charging process for a wide range of geometrical configurations. Based on application-typical specifications and the derived Fourier based correlation, extensive variation studies regarding the storage system were performed and evaluated with respect to systemic storage densities, heating wire surface loads and dimensions. For a favored design option selected here, maximum systemic storage densities of 201 Wh/kg at maximum heating wire surface loads of 4.6 W/cm2 are achieved showing significant benefits compared to today’s battery powered PTC heaters. Additionally, for proofing and confirming the storage concept, a test rig was erected focusing experimental investigations on the charging process. For a first experimental setup-up including all relevant components, mean temperature-related deviations between the simulative and the experimental results of 4.1% were detected and storage temperatures of up to 870 °C were reached. The systematically performed results confirm the feasibility, high efficiency, thermodynamic synergies with geometric requirements during thermal discharging and the potential of the technology to reach higher systemic storage densities compared to current solutions.