Increased daily use of fully electrical vehicles (EV) makes high demands on the lithium-ion battery, and used fast charging processes. Consequently, efficient charging requires advanced cooling systems which are capable of handling strong temporal peaks. In Figure 1, one such example of a cooling system for fast charging of lithium-ion batteries is shown.
Using water as coolant is problematic, since it requires a minimum cooling pipe cross-section of 6 to 10 mm for necessary efficiency meaning that water cooling of thin-walled sections and smaller areas occurring in lithium-ion batteries is ineffective, time consuming or even not possible.
To address this, it is possible to exploit direct refrigerant cooling which is effective for small passages, and works typically, as shown in Figure 1. The refrigerant is fed into the hot region in a liquid state from the cooling unit (temperature control unit) via a pipe connection. At the position to be cooled the refrigerant expands in an evaporation space and can be thought of as releasing “cold”. The gaseous refrigerant extracts and absorbs thermal energy from the surrounding area. This process is referred to as the saturation of the refrigerant, and the saturated refrigerant gas is then fed back into the cooling unit, where it is compressed to a high pressure and again absorbs thermal energy. The hot refrigerant gas subsequently passes through a heat exchanger, which brings this gas back into the liquid state. In this way, the refrigeration cycle runs in a closed system without any losses. A numerical optimization of this process is desirable to optimize energy efficiency.
From a thermal management point of view, it is a challenge to compute and optimize the cooling process of direct refrigerant cooling since it includes transient, two-phase flows in one- and three-dimensions. At VEAS, a similar CFD procedure has been developed for the computations of transient one-phase flows (cooling air/water) on encapsulated engines and in engine bays. This procedure uses merged one- and three-dimensional computational codes for one-phase transient flows and has been shown to operate successfully for optimizing the energy efficiency on power trains of both light and heavy vehicles. Figure 2 flow chart of the developed procedure.
Results from these studies will enable development of faster charging processes for lithiumion batteries and thereby open possibilities for an increased and more flexible use of fully electrical vehicles, hence, a more sustainable society. The complexity of the problem requires, in the start-up phase, activities at the Professor and Technical Specialist level since this will form a solid foundation for up-coming national and international research applications: FFI, Swedish Energy Agency and European sources like the Horizon 2020.
Forskare vid Chalmers, Mechanics and Maritime Sciences, Vehicle Engineering and Autonomous Systems
Professor vid Chalmers, Electrical Engineering, Electric Power Engineering, Electrical Machines and Power Electronics
Funding Chalmers participation during 2018
Areas of Advance