Thermal Management Mode of Pure Electric Vehicles:
Battery Immersion Cooling
In new applications of vehicle electrification, it is important to cool and heat electrical components to maintain them at optimal operating temperatures, as this ensures their longevity and performance. Therefore, a suitable thermal management system is essential. In other words, it is necessary to design a suitable thermal management system specifically for the electrification components used.
If the battery operating temperature is too high, it may result in a loss of battery capacity and, in extreme cases, thermal runaway. If the battery operating temperature is too low, it may lead to reduced battery efficiency, increased resistance, reduced battery capacity, and the formation of lithium dendrites (lithium plating layer). The lithium plating layer will cause accelerated aging and failure of the battery core.
The goal of thermal management is to ensure that the system is at optimal operating and safe temperatures. To further complicate matters, the optimal temperature of a battery system may change depending on the operating mode. The optimal temperature when fast charging may be different from the optimal temperature when driving or parking (parking).
Currently used battery thermal management systems mainly include air cooling, indirect liquid cooling, direct liquid cooling (also called immersion cooling) and phase change materials.
Description of various cooling systems
Air-cooled systems are the most widely used because they are simple to design, cost-effective, and have no leakage issues. Air cooling is divided into active type using forced convection and passive type using natural convection. Air has a small heat capacity and low thermal conductivity compared to media such as liquids, so air cooling is unlikely to be the technology of choice for the next generation of electric vehicles with larger battery packs and faster charging.
Liquid cooling can be divided into two methods: indirect and direct. Coolant has a greater heat capacity and higher thermal conductivity than air. Indirect liquid cooling is currently one of the most common solutions for battery thermal management due to balanced temperature control. The most commonly used coolant is a mixture of water and ethylene glycol. The principle of indirect cooling is to allow coolant to flow through channels at the bottom or side of the cell/battery module to transfer heat away from the system.
Cooling can be improved by using specific thermal interface materials (TIMs). The disadvantage of indirect liquid cooling compared to air cooling is the complexity of the system. More parts and channels/tubing can lead to more failures, extra weight, and leakage issues.
Another emerging cooling technology is direct liquid cooling, also called immersion cooling, which completely submerses the battery in a dielectric liquid. This is a non-conductive liquid with high resistance to electrical breakdown. The introduction of this technology means that the complexity of battery process and component design can be greatly reduced, which also helps to reduce the weight and volume of the system, and significantly improves the stability and balance of battery temperature control. Immersion cooling can heat or cool the battery as needed without the use of a heat exchanger, resulting in significant improvements in efficiency.
The characteristics of the coolant play an important role in thermal management and should meet the following requirements:
Good electrical insulation
High specific heat capacity and high thermal conductivity
Non-flammable and/or high flash point
Easy to produce and available in large quantities
Has a suitable operating temperature range
Liquids have a long shelf life
In addition to the above requirements, material compatibility, low density, low viscosity and environmental friendliness must also be considered when selecting a suitable immersion coolant
