The batteries represent a significant part of the cost of a electric or hybrid vehicle and, therefore, the effort to improve management in this area is not surprising. In turn, the thermal control of the battery packs gains prominence, since the temperature influences the performance, safety and duration of the battery pack itself.
Any change in temperature can cause a decreased performance and shortened battery life. The ideal would be that the temperature variations were minimal between the battery cells; too low a temperature can reduce energy output, while excessive heat can decrease life expectancy. This is why interest in how to improve cooling through liquid immersion to manage the heat generated by the batteries themselves.
Liquid Immersion Cooling
For the 3M company, liquid immersion cooling has proven to be one more alternative economic that air and a way fast, effective and safe when removing heat without causing damage. In this regard, there have been various technical advances in recent years.
This does not mean that other forms of refrigeration –using air or thermal interface materials– are unable to perform their role in automotive electronics. Although there are many examples where the latter are ideal for heat management, in electric vehicles there is a consensus on the adequacy of liquid immersion cooling in future battery designs.
Let’s take a look at the liquid cooling options available, with their respective pros and cons. The subcategories are a mixture of glycated water and oil-based fluids and other advanced chemicals.
Regardless of the type, there are certain attributes to analyze:
Thermophysical properties– Heat transfer fluids remove heat through convection, so factors such as coefficient of expansion, high density, low viscosity, and low surface tension are beneficial.
Direct contact: A high dielectric strength makes fluids good insulators and ideal for direct contact with power electronics systems.
Temperature range: Depends on the application, but a low freezing point is usually required. The boiling point, meanwhile, is chosen based on the upper operating temperature.
Long duration: chemical and thermal stability throughout the life of the component or device to be refrigerated.
Material compatibility: Non-corrosive behavior with electronic components, as well as compatibility with a wide range of plastics.
Cost of ownership: throughout the life of the component or device. For example, complexity has an impact on cost, as some systems require fewer components, such as a separate pump.
Sustainability: Is the refrigerant subject to any environmental and safety legislation or restrictions?
Low or no flammability: an obvious requirement in the design of an electric vehicle.
Security: Is the refrigerant safe or toxic?
Weight: Hydrofluoroether (HFE) products are heavy, but tend to use less fluid.
Alternatives to traditional refrigeration
Traditionally, liquid immersion cooling in the automotive industry has been based on a mixture of glyced water or just water which by nature is highly conductive and corrosive. In order to be able to enter the water, the components to be cooled must be sealed to avoid any risk of damage and this limits the efficiency of the refrigeration, since there is no direct contact. It also imposes other design limitations on engineers.
Immersion cooling with hydrocarbon based fluids It is another alternative that works well in many applications, but the inflammability This method calls into question its use in the automotive market. Furthermore, the combination of the low coefficient of thermal expansion and the high viscosity of lubricants makes the convective properties less favorable. And it also requires a bomb to remove heat from components. Lastly, the nature of oils complicates maintenance processes, not to mention the danger of exposure to flammable liquids.
As a consequence, many industry organizations are searching for the next generation of liquid immersion cooling, based on especially fluorinated chemical agents. These elements have the benefit of allowing direct contact and, therefore, thermal management is faster and more efficient. And last but not least, they allow full immersion, since they do not cause damage or leave residues.
These fluids are neither corrosive nor do they usually need auxiliary equipment, so their impact on battery size is minimal.
Another aspect to consider with any battery refrigerant is the environmental legislation, which may affect its use in the future. Although dielectric chemicals have their advantages, many of them are viewed with a magnifying glass by regulations. For example, HFCChemicals normally used in heat transfer applications are in the phase of phase-out under the F-gas regulation. Its production and use are expected to be reduced by 79 percent by 2030 (compared to now).
However, there is good news: the use of some fluids, such as ‘3M Novec Engineered Fluids’, is not affected by the F-gas regulation, contrary to what happens with HFE-based chemicals, and offers very low values of the global warming potential index (GWP) and, consequently, low permanence in the atmosphere and zero ozone depletion potential.
The ‘3M Novec’ Engineered Fluids range not only meets current and future heat dissipation needs in a vehicle battery, but is also used successfully in many other applications.
Furthermore, these fluids are neither flammable nor combustible, guaranteeing maximum safety in the vehicle. They also do not need an additional pump.