Thermal High Performance Storages for electric bus heating - overview on the current state of development

Abstract

Introduction: Battery electric buses are facing huge challenges when interior heating is required on winter term conditions, since the heating demand can exceed the power that is required for traction. As state-of-the-art solutions for providing interior heating, diesel fueled heaters, electric resistance heaters or electric heat pumps are used. However, when using diesel fueled heaters, local emissions are emitted. When using electric resistance heaters, electric energy from the battery is consumed cutting the range of the vehicle up to more than 50 %. When using heat pumps, overall energy consumption for interior heating is reduced as well as range reduction. However, it is still very significant on cold ambient conditions. Additionally, a second heater as to be used, since the thermal output of available heat pumps is not sufficient on temperatures below -10 °C. To overcome this challenge, the use of a Thermal High Performance Storage (THS) as stand-alone heating solution or additional heater can be used, as already proposed in past publications. The THS can be electrically charged simultaneously to the traction battery. The stored energy afterwards is used for heating the interior. The benefit of using a THS for interior heating is that no local emissions occur and no energy from the battery has to be used for heating. Compared to currently used batteries, a THS can be up to 90 % cheaper, 50 % lighter and 40 % smaller. Materials and Methods: Within the targeted oral presentation, on overview on the state of development of THS for the application as bus heating system will be given. For this, the THS will be split in its single components storage material, housing, charging system and discharging system. Also, a look on design considerations for the overall assembly is given. Results: Regarding storage materials, the most suitable metallic Phase Change Materials (mPCM) will be shown and the current state of investigation of their thermophysical properties. Regarding housing, the problematic of potential reaction with the mPCM is addressed. Potential solutions of how a suitable housing container has to be designed will be shown. For charging, electric resistance heating or inductive heating are potential solutions. The potential of solutions regarding power density is shown as well as advantages and disadvantages of both solutions for the described application. For discharging, several concepts using liquid fluid, gaseous fluid or the phase change of a working fluid are considered. For this, different solutions and their potentials will be shown as well. Last thing to be presented are consideration concerning the overall assembly of the storage system. Especially the relative position of the discharging system to the charging system is of main interest, due to the formation of materials defects and its high impact on the power density. Conclusion: Thermal High Performance Storages could be a highly relevant solution to solve the issues regarding interior heating of battery electric buses. The current state of development underlines the potential of this technology, however also shows the challenges on the further development.