Cooling fluids for electric vehicle batteries are a potential avenue of exploration and investment for lube companies, but a key issue is what cooling systems EVs will employ in the thermal management of these critical components.
As internal combustion engines give way to electric motors, the need for crankcase lubricants decreases. The next radical change in fluid requirements will be in those used for cooling the various parts of an electric vehicle, including hybrids. The change is likely to be so dramatic that the term “thermal management” – or TM – has entered the lexicon in place of coolant.
While electric motors generate waste heat, that waste is much less than that created by an ICE. According to the United States Department of Energy’s Office of Energy Efficiency and Renewable Energy, “EVs convert about 59 to 62 percent of the electrical energy from the grid to power at the wheels. Conventional gasoline vehicles only convert about 17 to 21 percent of the energy stored in gasoline to power at the wheels.”
Electric motors and transmissions will still have to be cooled, but the issue for formulators is more one of materials compatibility, which may be easier to solve than the TM of batteries.
The dominant battery technology for the short-term will be lithium ion, due to lithium’s relative lightness, high availability (compared with rare earth metals) and more than 20 years of manufacturing and processing experience for use in the batteries of laptops and hand-held electronic devices.
Unfortunately, lithium-ion batteries have a narrow temperature range of operation, often referred to as their “Goldilocks” range. Too hot and the batteries rapidly lose capacity or, in extreme cases, thermal runaway can occur and cause battery fires or explosions. Too cold is also a concern, as the chemical reactions that produce the electricity are slower, reducing the available capacity of the battery. Prolonged low temperatures can induce lithium crystals to form in the battery, damaging the cells.
Charging causes the batteries to heat up, so the Goldilocks range is very important when the market requires faster charging for plug-in vehicles and for EVs to operate in hot or cold climates. Rapid charge-discharge cycles, where the battery is being recharged from regenerative braking, then discharged as it drives the wheels, can also cause issues.
Goldilocks can also play her part with permanent magnets in the motors, as they lose magnetization instantly if they are overheated.
The industry’s expectation is that the power density, or power per unit mass, of batteries and power electronics will increase in order to extend vehicle range. Original equipment manufacturers and fluid suppliers must also consider the chemical and electrical compatibility of the fluids with the materials used in the batteries, motors and power electronics. This is leading to a vast palette of possible cooling scenarios, incorporating many engineering and fluid options. Engineering solutions are beyond the scope of this article but include both direct (fluid contacts the vehicle component) and indirect (fluid contacts a part designed to transfer heat) options. Indirect options can also involve a second fluid.
At least five fluid options are under consideration:
Air cooling is used in the Nissan Leaf, and the simplicity of the system is attractive but is unlikely to be commonplace in the future as power densities rise and faster charging places greater thermal loads on the battery pack. The Nissan Leaf battery is heated by resistance heaters, which can lead to the incorrect notion that the battery powers resistance heaters in order to heat the battery.
Tesla and General Motors currently deploy conventional water-glycol mixes. The early Chevrolet Volt hybrid, for example, had four separate cooling systems or loops. Those for the battery charger/power inverter, the battery and the ICE used glycol coolants, whereas the electric drive was cooled by the transmission fluid. However, as charge rates for batteries become faster, the heat transfer characteristics of glycols are insufficient to keep lithium-ion batteries in their ideal range.
Option 3 is used in the BMW i3. Some companies have used the common air conditioning refrigerant R134a as a coolant, regardless of the vehicle power source. However, green activists and journalists have raised the issue that R134a is a highly potent greenhouse gas with 1,410 times the per-gram warming potential of carbon dioxide, so its use may be limited to avoid reputational damage.
Phase change materials are potentially the most exciting new fluids on the block. They consist of an emulsion of two immiscible liquids. One such example is of paraffin wax suspended in water. The paraffin is stabilized by surfactants to ensure consistent droplet size during the many melt-freeze cycles. As the temperature drops, the paraffin crystallizes, releasing heat that is quickly transferred to the vehicle component by the water. When the vehicle component is warm, the wax melts, absorbing heat. The advantage of such systems is that the specific heat capacity – the energy absorbed per unit temperature rise per unit mass – is many times that of conventional coolants in the temperature range required for battery thermal management systems. As the waxes can be tailored to the required temperature range, these materials are attracting significant interest.
The final option involves no fluids at all. Thermo-electric cooling relies on the Peltier effect, when a current flowing between two conductive materials can cause heat to flow. This can enhance radiative cooling or cause heat to flow from the cold material to the hot material. Recent advances in materials science means that this technology has transformed from heating and cooling very small volumes to being able to shift kilowatts of power.
There are many combinations of engineering and fluid solutions and little indication of which one will win out. OEMs will constantly push for the simplicity of a single fluid in a single TM loop, but the thermal demands for fast charging and chemical compatibility with new materials in the motors, for example, could dictate that different fluids are used.
The major fluid players are holding their cards close to the vest regarding their perceived direction of travel. Total launched four TM fluids specifically for batteries and power electronics in light- and medium-duty vehicles in late 2018. But it has no literature available on its website for the launch and did not respond when approached by Lubes’n’Greases, nor did Recochem, a leading supplier of coolant additives and packages.
BASF, a major supplier of glycols, declined to comment.
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