Development and Characterisation of an Innovative Battery Thermal Management System for Electric Vehicles with Loop Heat Pipes and Graphite Sheets

Abstract

A new era for passenger transportation is beginning, where Electric Vehicles have emerged as an immediate solution to reduce emissions from passenger cars, when charged from renewable electric energy sources. However, their distribution is currently limited to only the 1% of the global fleet, due to key issues such as long charging time, limited all-electric range and elevated cost. A properly designed Thermal Management System (TMS) can improve on these points by reducing both the maximum temperature of the battery pack during fast charge and parasitic power consumed. The present thesis utilises a combination of numerical and experimental investigations to propose an innovative TMS concept using Loop Heat Pipes (LHPs) and graphite sheets. In the proposed concept the LHP acts as the main thermal vector, transferring heat from the bottom of a battery module to a remote heat exchanger, connected to the HVAC system already present in the vehicle. LHPs are reliable, low-maintenance, safe and not expensive systems, already used in electronics, space and aeronautical applications. Since LHPs do not need electrical power to operate, they reduce the parasitic power consumption compared to forced air or liquid basedTMS. Graphite sheets have the twofold function of promoting heat transfer along the cell plane direction, whilsthindering it in the transverse direction. The feasibility of the presented innovative design was first verified thanks to a Lumped Parameter Model, which was validated against in-house experimental data, using a copper flat plate LHP applied to a module of dummy prismatic cells and two different working fluids, ethanol and water. Results showed that this design complies with the requirements of maximum cell temperature and temperature difference across the cell and, when compared against an equivalent liquid cold plate design, its achieved maximum temperature after fast charge was lower by 3.6°C. Furthermore, following automotive industry demands for environmentally friendly systems, Novec™ 649 is, for the first time,experimentally investigated as the LHP working fluid, due to its exceptional environmental and safety properties. The results showed that with this fluid the proposed design provides excellent cooling at fast charge conditions. Moreover, compared to ethanol, maximum temperatures were only 0.7°C higher, proving that the adoption of Novec™ 649 improves on safety and environmental impact with no detriment to the thermal performance. Additionally, aiming at practical implementations of the proposed TMS concept, the developed numerical model was used to study the effect of active heating zone and thickness of the evaporator and the number of LHPs applied to a 12-cell module. Moreover,an extensive parametric analysis on various combinations ofLHP materials and working fluids, with particular attention to their safety and environmental properties, was performed. Finally, after a comparison against the use of passive free convection and an active liquid cold plate, the LHP-based TMS resulted in the lowest temperature after fast charging.

Date of Award	Jan 2022
Original language	English
Awarding Institution	University of Brighton
Supervisor	Marco Marengo (Supervisor), Nicolas Miche (Supervisor), Anastasios Georgoulas (Supervisor) & Cedric Rouaud (Supervisor)