The need to cool a conventional ICE vehicle is generally understood by most people – the ignition of a fuel in the cylinders and friction between moving parts create heat, which is controlled by lubrication, coolants and the heating, ventilation and air conditioning systems. But ICE engines like it hot, so those systems only cool the engine to around 95 degrees Celsius during normal operation.
What is less well known is that batteries and e-motors in an EV also generate lots of heat, but they prefer it cool. Adequate cooling, especially of the battery pack, is essential, as Tesla found to its cost after a series of fires in its vehicles. Lubrizol suggests that thermal management is also complicated in hybrids by the high operating temperatures for the ICE and the low temperatures desired for batteries, e-motors and inverters.
Looking first at the battery, its effectiveness depends on maintenance of the optimum temperature. Extremes of hot or cold impact efficiency, longevity, range and charge times. Lithium-ion EV batteries produce power by a chemical reaction and the colder the reactants, the slower the reaction is and the less energy is produced.3 Cold batteries take longer to charge and have reduced acceleration and range. Hot batteries degrade faster and bleed lithium from the anode resulting in self-discharge.4
This narrow window necessitates a battery management system continuously monitoring heating and cooling and keeping cells between 20 C and 40 C. There are a few cooling methods including, in most BEVs, a liquid coolant, such as a glycol-water mix (Tesla Model 3, Jaguar I-Pace, Audi E-tron), or they are air cooled (Nissan Leaf, VW e-Golf) or use a refrigerant (BMW i3).
Cooling e-motors is also key to ensure peak operational efficiency. Effective thermal management of the area between the stator and rotor in an e-motor is critical, and the bearings can reach 150 C and can hit 170 C for short periods.
To date, EV manufacturers have tried at least six approaches for cooling e-motors and inverters, which turn DC battery power into AC for the e-motor, including forced air cooling, hollow shaft rotor cooling, outer jacket liquid cooling, and a combination of the two, oil spraying or immersion in dielectric oils.
The last two could use a mineral oil. However, there is consensus that more work is needed both to optimize coolant performance in today’s EVs and to meet the needs of tomorrow’s vehicles. Ford contends that there is very limited data in the literature on the heat-reducing properties of existing lubricating base oils. The company says that more research needs to be done on existing base stocks and that a solution may ultimately require discovery of new fluids.
The cooling performance of any fluid depends on its thermal conductivity, its capacity to store heat (known as specific heat capacity – the amount of heat energy required to raise the temperature of a substance per unit of mass) and its viscosity, and there are several fluids options currently available.
Specific heat is a key consideration for the selection of any cooling fluid. Plain water comes out on top, but water cannot be used on its own and so it is mixed with a glycol (a substance in the alcohol family) to prevent it from freezing or boiling. These mixtures are still better than a petroleum oil at retaining heat.
Viscosity, a measure of how easily a liquid will flow, is another key factor. A low-viscosity fluid preferred, but this must be balanced against two factors: volatility – the propensity for the oil to evaporate in high heat, so as to avoid potential fires – and density, which determines how much fluid you need, as this will impact the overall weight of the vehicle. These factors must be considered, but the question remains if oil-based solutions can compete with glycol-water solutions.
A glycol-water mix is an inexpensive, well-established cooling fluid, typically comprising about 50% glycol, 45% water and 5% additives, including an antioxidant, antifreeze, corrosion inhibitor, solvent and dye. Glycol has good heat transfer properties, with thermal conductivity in the region of 0.35 watts per meter kelvin5 versus 0.13 W/m K for a conventional base stock, such as a polyalphaolefin, or a fluorocarbon refrigerant at 0.06 W/m K. It also has superior specific heat capacity than other fluid options, such as a PAO.
The disadvantages of a glycol-based coolant are that it loses potency over time, is susceptible to contaminants and corrosion and is a major maintenance item for a BEV. Other disadvantages are its incompatibility with current additive chemistry, as well as potential electrical conductivity concerns, according to research by Afton Chemical.
Mineral Oils and PAOs
Oil is lighter than glycol-water and issues of contaminants and corrosion would be greatly reduced. This means the fluid would last longer and would not lose potency over time. In addition, one must remember than an oil-based solution could be fill-for-life and reduce maintenance, as well as enhance the driving experience. It could also bring efficiencies and more direct cooling that glycol-water cannot.
Afton performed tests on other base oils’ cooling and heat transfer capacities, and while it found propylene and ethylene glycols performed the best, they have conductivity concerns. Polyalphaolefins could be an alternative and have the advantage of being a dielectric fluid. Afton also found that the additive treat rate did not affect thermal performance.
Afton found that increasing the quality of a traditional base oil in this application helps but still falls short of glycol-based solutions. Esters, or Group V oils, have shown some better results but are also expensive.
So, are there other additives that can boost properties? This will be a key challenge for the oil industry, as battery cooling could become a lucrative outlet for their products but only if it can drive a better and cost-effective overall solution and an area ripe for innovation.
Direct or Indirect Cooling
Water-glycol systems and air cooling are indirect cooling. Glycols are pumped through pipes surrounding the battery or air circulates around it. Air cooling systems are the simplest, cheapest solution but are insufficient at handling the requirements of large EV batteries and therefore delivering maximum range and lifecycle. This is especially true when considering the range of ambient temperatures vehicles experience and the surface area that must be cooled. Tests by the U.S. National Renewable Energy Lab and the National Active Distribution Network Technology Research Center in China found that air cooling needs up to three times more energy than liquid cooling methods to maintain the same average temperature.6
There are efforts to introduce direct cooling systems that flood the battery with a dielectric mineral oil pumped through a heat exchanger to maintain optimum temperature. A common system could serve both the battery and motor. But the balance is a complex one between keeping the e-motor cool and the battery at an optimum – and different – temperature. Having components operate at peak efficiencies also impacts the size of the e-motor and the battery pack. The smaller the motor, the less overall weight leads to further energy efficiency. And as battery performance increase, you need less battery or range can be extended. Less e-motor and battery to cool means less coolant is required.
The joint U.S.-China study found that indirect liquid cooling is more practical than direct liquid cooling but has slightly lower cooling performance.7
Whatever the liquid solution, it needs to also have little to no conductivity to prevent current from the battery transferring to other parts of the vehicle in the liquid, as mentioned above. So far, direct cooling has only been achieved in the lab, but in the future, it will hopefully be implemented in the real world.
A downside of liquid cooling is that any kind of leak would be a major problem for the car. As mentioned, batteries do not operate well in the cold, and the start-up and ambient temperatures also play a role in efficiency. Batteries also heat up during charging, and designers must consider maintaining temperatures during normal operation and during charging, further complicating matters. Overnight 120- or 240-Volt charging versus fast charge must also be factored in.
Cooling is an area where the lubricant industry needs to proactively collaborate with automakers to best understand needs and then bring new solutions to the table. Until then, glycol-water will likely remain the cooling fluid of choice, and, of course, this presents opportunities for better glycol-based fluids that can resist corrosion and perform for longer intervals.