Most people are aware of the necessity to cool a standard ICE vehicle: the ignition of fuel in the cylinders and the friction between moving components generate heat, which is managed by lubrication, coolants along with heating, ventilation, and air conditioning systems. But as a matter of fact, ICE engines prefer it hot, hence these systems only cool the engine to roughly 95 degrees Celsius during typical operation.
EVs Need Cooling Too
What’s less generally known is that electric vehicles’ batteries and electric motors create a lot of heat and unlike ICE vehicles, EVs don’t love the heat. Tesla discovered this at the cost of a series of car fires. In an EV, adequate cooling, particularly of the battery pack, is critical. The high working temperatures for the ICE and the low temperatures sought for batteries, electric motors, and inverters make thermal management for hybrids even more difficult.
The Battery & The Motor
The Battery’s efficiency is dependent on maintaining the ideal temperatures as the working temperature ranges influence efficiency, lifespan, range, and charging times. As we know, a chemical process produces electricity in lithium-ion EV batteries, and the colder the reactants, the slower the reaction and the less energy generated. Cold batteries take longer to charge and have a shorter range and acceleration. Self-discharge occurs when hot batteries deteriorate quicker and leak lithium from the anode.
Because of the small window between charging and discharging, a battery management system must constantly monitor heating and cooling to keep cells between 20 and 40 degrees Celsius. Most EVs utilize 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 they use a refrigerant (Nissan Leaf, VW e-Golf) (BMW i3).
Cooling electric motors are also important for optimum performance. In an electric motor, effective thermal management of the space between the stator and rotor is crucial, and the bearings can achieve temperatures of 150°C and 170°C for brief durations.
But how does the Thermal management system in electric vehicles work? Let us take a closer look.
Thermal Management In EVs
EV manufacturers have tried at least six different cooling methods for electric motors and inverters, 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.
Although the system may have phase change materials and fins the liquid coolants are the ones worth discussing because the other two have limited application and scope in the sector.
Choosing a Coolant
The thermal conductivity, heat storage capacity (known as specific heat capacity – the amount of heat energy required to raise the temperature of a substance per unit of mass), and viscosity of a fluid determine its cooling efficiency, and there are various fluid alternatives now available for the purpose.
Specific heat is an important factor to consider when choosing a cooling fluid. Plain water rises to the top, but it can’t be utilized by itself, so it’s combined with a glycol (an alcohol-like compound) to keep it from freezing or boiling. Even yet, these mixes maintain heat better than petroleum oil.
Another important component is viscosity, which is a measurement of how readily a liquid flows. Low-viscosity fluids are desired, but they must be balanced against two factors: volatility (the proclivity for the oil to evaporate at high heat, avoiding possible fires) and density (which affects how much fluid you need, affecting the vehicle’s total weight). These aspects must be taken into account, but the question of whether oil-based solutions can compete with glycol-water solutions remains unanswered.
Now let us check out some of the potential coolants.
#1. Glycol
A glycol-water mix is a low-cost, well-known cooling fluid that generally contains roughly 50% glycol, 45% water, and 5% additives such as antioxidants, antifreeze, corrosion inhibitors, solvents, and dyes. Glycol has high heat transfer qualities, with thermal conductivity in the 0.35 watts per meter kelvin(W/m K), compared to 0.13 W/m K for a traditional base stock, such as a polyalphaolefin, or 0.06 W/m K for a fluorocarbon refrigerant. It also outperforms other fluids, such as a PAO, in terms of specific heat capacity.
#2. PAOs and Mineral Oils
Due to the fact, that oil is lighter than glycol water, contamination and corrosion concerns would be considerably minimized. This implies the fluid will last longer and won’t lose its effectiveness over time. Furthermore, keep in mind that an oil-based solution may be fill-for-life, reducing maintenance and improving the driving experience. It also has the potential to provide more efficiency than the glycol-water solution.
Afton tested the cooling and heat transfer capacity of different base oils, and while propylene and ethylene glycols fared the best, they had conductivity issues. Polyalphaolefins may be a viable alternative, with the added benefit of being a dielectric fluid. The additive treat rate does not affect thermal performance, according to Afton.
In this application, Afton discovered that improving the quality of a typical base oil helps, but it still falls short of glycol-based alternatives. Esters, also known as Group V oils, have been demonstrated to be more effective, but they are also more costly.
So, are there any additional ingredients that can help? Battery cooling might be a profitable outlet for the oil industry’s goods, but only if it can produce a better and more cost-effective overall solution and an area ripe for innovation.
Now moving on to the cooling systems the EVs employ.
#1. Indirect Cooling
Indirect cooling is provided by water-glycol systems or simply air cooling. Glycols are poured around the battery through pipes, or air flows around it. Air cooling systems are the most basic and inexpensive option, but they are insufficient for addressing the demands of big EV batteries and therefore incapable of giving maximum range and lifespan. This is especially true given the wide range of ambient temperatures encountered by cars and the amount of surface area that must be cooled. Air cooling requires up to three times more energy than liquid cooling systems to maintain the same average temperature, according to tests conducted by the National Renewable Energy Laboratory in the United States and the National Active Distribution Network Technology Research Center in China.
#2. Direct Cooling
Direct cooling is much simpler in theory because the components are directly in touch with the coolant(immersed in the coolant) whereas, in indirect cooling, there is a mediator between the component and coolant. Because they are directly in touch with the components, direct liquid cooling systems have varying coolant needs. The coolant must be a low to no conductivity fluid in systems where the battery will be immediately exposed to the coolant, such as in Fuel Cell Vehicles or direct liquid cooling.
Direct cooling in EVs will be much different and complex from traditional ICE coolants with high conductivity. In EV direct cooling, ‘low or no’ conductivity is required for safety reasons: electrons move throughout the battery, and if they come into contact with a high conductivity fluid, the battery will fail and explode. Employing deionized water as a fluid medium or using a non-salt-based fluid media are two examples of approaches to keep coolant conductivity low.
From Research to Implementation
Petronas Lubricants International is one of the companies creating EV fluids specifically for electric vehicle cooling and lubrication. The company believes that using specifically formulated lubricants and coolants for electric vehicles, may enhance efficiency while also expanding range. As we have discussed earlier the indirect cooling of electrical and electronic components is being phased out in favour of direct cooling.
The fluid indirect cooling comes into direct touch with electrical components such as circuit boards, seals, copper, and plastic components, and it must be dielectric to avoid a major short circuit (incapable of conducting electricity). As EV drivetrains become integrated rather than separate, the scenario becomes more convoluted. The fluid must then lubricate the gears as well as cool the motor and its electronics directly.
If the cooling of the battery and charging equipment can be enhanced, ultra-rapid charging may be made even quicker. The rate of charge of cars capable of 350kW charging (such as the Porsche Taycan and Hyundai Ioniq 5) peaks early and then progressively declines as the battery management systems ‘throttle’ the current to avoid damage. According to Petronas, the Taycan takes 41 minutes to charge from empty, but that time might be cut in half if it could charge at 350kW till full. It’s not saying it’s a given, but it does suggest that there’s a lot of room to improve charging speeds by focusing on cooling and the fluids that do it.
Conclusion
Longer battery life and better power output are in high demand now that electric cars are so widely adopted. To do that, battery thermal management systems must be able to move heat away from the battery pack while it is charged and discharged at increasing rates. The heat created while the battery is utilized might endanger the passengers’ safety. The suitable coolant and additive package is even more important because of the tremendous stress and heat created by the batteries.
While firms like Tesla, BMW, and LG Chem may employ standard liquid coolants in their indirect cooling systems, further research and development on battery packs and coolants are needed to improve electric car safety.