The thermal management of an Electric Vehicle (EV) is a completely different challenge to an Internal Combustion (IC) car. Unlike conventional IC engines, which simply require a cooling circuit to lower engine temperature, EV batteries need to be regulated within a specific temperature window, as this is where the cells can achieve maximum efficiency. The higher the efficiency, the more energy a battery can store and preserve throughout its use; increasing the vehicle’s driving range.
The optimum temperature of EV roadcar batteries is typically around 21 degC (70 degF). However, efficiency can significantly drop if the battery gets too hot or too cold. A study conducted by Geotab [1] analysed 5.2 million trips taken by 4,200 EVs and found that at temperatures of 0 degC (32 degF) and 40 degC (104 degF), an EV can drop to around 80% of its rated driving range. While at more extreme temperatures such as -20 degC (-4 degF), an EV can only achieve 49% of its rated range. This means that on a particularly cold day in countries like Canada, an electric vehicle that is rated to do 250 miles (402 km) would only be capable of 122.5 miles (197 km) on average. Consequently, EV cooling circuits require both heating and cooling elements to warm the battery up when it is too cold and cool it down when it is too hot.
The complexities of Electric Vehicle (EV) Thermal Management Systems
It is this delicate balance between battery temperature and performance that requires an innovative approach to the design of thermal management systems. Typically, an EV thermal management system features three circuits: a standard vapour compression cycle for the cabin; a radiator coolant loop for the inverter and electric motor; and a battery loop connected to the AC refrigeration circuit via a chiller [2].
However, currently the most revolutionary thermal management system in production features four circuits and was developed by Tesla for the Model Y. This design uses a multi-functional heat pump which links both the cabin and coolant circuits, allowing heat to be transferred between the two. Controlling the flow of heat is an octovalve which is a rotation valve with eight ports that allows coolant to flow around the four coolant loops across 15 different operating modes [2].
Air to water cooling for EVs
The air to water heat exchangers found in most EV roadcars consist of a tube and fin arrangement and are manufactured using traditional methods. However, modern additive manufacturing techniques can now achieve heat exchangers with complex 3D geometries that were previously impossible to manufacture. This allows engineers to maximise the cooling capacity within the available space; achieving efficient heat exchange in small areas.
‘We can vary our geometry to achieve the optimum surface area ratio to suit the fluid as it changes thermal-physical properties,’ explains Glenn Rees, Head of Engineering at Conflux Technology. ‘Air has a lower specific heat capacity than water, so it requires a larger surface area to achieve the same cooling capacity. In conventional heat exchangers it is difficult to vary the surface area ratio without increasing weight and potentially pressure drop. Whereas our technology achieves the same heat exchange within the same volume without increasing weight or pressure drop. That’s our advantage – it’s not the fact we package heat exchangers into smaller volumes, it’s the fact we can achieve greater performance than conventional coolers for less volume, weight and cost.’
Conflux Technology’s Water Charge Air Cooler (WCAC) has over 60,000 fins and achieves an 82% reduction in water side pressure drop, a 24% reduction in air side pressure drop and is around 40% lighter than other leading microtube WCAC’s with constant heat exchange.
Electric Vehicle Battery Cooling
The most common approach to cooling an EV battery is either through air cooling or indirect cooling using coldplates with a water-glycol coolant. However, recent improvements in battery technology have developed EV’s that are now capable of fast charging. This is where DC current is supplied directly to the battery, instead of AC current that then needs to be converted on-board. Consequently, more power can be delivered to the battery in a shorter time, which in some cases can fully recharge an EV within 15 minutes [3].
Unfortunately, these faster charging rates significantly increase battery temperature, which then needs to be managed to avoid thermal runaway and battery degradation. To help solve this problem, direct cooling can be used which is where the battery enclosure is flooded with a dielectric fluid. These fluids are electrically non-conductive liquids that have a higher thermal conductivity and specific heat capacity than air [4]. This, together with the fact that the fluid is in direct contact with a larger surface area, makes direct cooling more efficient at extracting heat from the battery than air or indirect cooling.
‘Dielectric fluids are necessary in many sub-systems of EVs due to their electrical insulating properties. There are some compromises in the thermal conductivity of the fluids, however, Conflux can compensate for these with novel 3D printed heat exchanger geometries tailored for the specific thermal physical properties of the dielectric fluids,’ says Rees. ‘This means we can use dielectric fluids to cool other elements of the powertrain such as the motors, inverters and power electronics.’
Electric Vehicle Motor Cooling
Another cooling application that can exploit the capabilities of additive manufacturing is motors. There are two main sources of heat within a motor: 1) the internal resistance of the copper windings 2) hysteresis losses due to the changing direction of the magnetic field. If either of these sources are not managed effectively, the rotor magnets could demagnetise and the insulation of the windings could deteriorate.
‘In a similar way to battery cooling, instead of using coldplates, additive manufacturing could be used to 3D print an external cooling jacket that surrounds the motor, providing an even distribution of cooling,’ highlights Rees. ‘You could even cool the motor windings directly by printing the windings and an integrated cooling solution within one assembly. This would require a level of complexity that would be impossible to cast or machine.’
Electric Vehicle Transmission Cooling
The transmission system is typically cooled via a cartridge style heat exchanger. These consist of hundreds of straight thin-walled microtubes within a compact package. This layout can result in areas of ‘dead space’ within the heat exchanger that get blocked off. However, additive manufacturing can fill the entire volume with heat transfer geometry, resulting in a complex 3D printed core that can be easily integrated into an existing casing.
‘Cartridge heat exchangers are particularly suited for additive manufacturing because of the potential for highly complex geometry in small packaging sizes,’ says Rees. ‘These coolers can be very small, for example 40mm3 with no compromise on the very high surface area density. We can also print small parts quickly and fill large build plates so suddenly, we can make hundreds of parts within one build, which reduces costs.’
See how our 3D printed cartridge heat exchangers performed against incumbent
Conflux Technology recently developed a cartridge heat exchanger for cooling transmission oil. After a comprehensive design and development process, the finished concept achieved a heat transfer rate of 5.7kW on a test rig and weighed only 43g dry.
‘Managing the thermal requirements of an electric vehicle effectively is a real challenge,’ concludes Rees. ‘The complexity of cooling these vehicles will only grow as manufacturers continue to hunt for efficiency. However, additive manufacturing is the key to unlocking cooling performance whilst reducing weight, volume and pressure drop. That’s why at Conflux Technology we continue to challenge our expertise to innovate thermal solutions that can bring performance benefits to our customers.’
References
[1] Argue, C. 2020. To what degree does temperature impact EV range? [Online]. Available from: https://www.geotab.com/blog/ev-range/
[2] Wray, A., Ebrahimi, K., 2022. Octovalve Thermal Management Control for Electric Vehicle. Energies, 15, 6118
[3] EVBOX. DC charging is driving electric mobility forward. [Online]. Available from: https://evbox.com/uk-en/ev-chargers/fast-charger
[4] CRODA. Battery cooling and thermal management. [Online]. Available from: https://www.crodaenergytechnol...
