High-Performance Solid Medium Thermal Energy Storage System for Heat Supply in Battery Electric Vehicles: Proof of Concept and Experimental Testing

Abstract

The reduction of global CO2 emissions requires cross-sectoral measures to reduce fossil energy consumptions and to strengthen the expansion of renewable energy sources. One element for this purpose are thermal energy storage systems. They enable, due to their time-decoupled operation, increases in systemic efficiency and flexibility in various industrial and power plant processes. In the electricity and heat sector such solutions are already commercially available for large-scale applications or are focused in diverse R&D projects, but are largely new in the transport sector. By transferring existing concepts specifically to the requirements for the heat supply of battery electric vehicles, efficiency improvements can also be achieved in the transport sector. The idea is to provide the required heat for the interior during cold seasons via a previously electrical heated thermal energy storage system. Thus, battery capacities can be saved, and the effective range of the vehicle can be increased. Basic prerequisites for this concept are high systemic storage densities and high performances, which must be justified to commercial battery powered PTC-elements. Compared to large-scale applications, this results in new challenges and design solutions needing finally a proof of concept and experimental tests under vehicle typical specifications. For the first time, a novel thermal energy storage system based on ceramic honeycombs with integrated heating wires and a double-walled, thermally insulated storage containment was developed and constructively realized. This storage system meets all the requirements for the heat supply, reaches high systemic storage and power densities and allows due to its high flexibility a bifunctional operation use: a cyclic storage and a conventional heating mode. In the focused storage operation, high-temperature heat is generated electrically through heating wires during the charging period and transferred efficiently via thermal radiation to the ceramic honeycombs. During the discharging period (driving) the stored thermal energy is used for heating the interior by a bypass control system at defined temperatures with high thermal output. The systematic measurement campaigns and successful model validations confirm high electrical heating powers of 6.8 kW during the charging period and a heat supply with a thermal output of 5 kW over more than 30 min during the discharging period. Despite current infrastructure and test rig restrictions, high systemic storage densities of 155 Wh/kg with constant discharging outlet temperatures are reached. Compared to battery powered heating systems, the experimental results for the developed thermal energy storage system confirm an excellent level of competitiveness due to its high performance, operational flexibility and low-cost materials.