Major manufacturers are promoting ultra-fast charging technology, advertising that the battery can be charged from 0-80% SOC in just 15 or 20 minutes, which means for some high-capacity batteries, the charging rate can exceed 4C. The battery generates a significant amount of heat, making traditional liquid cooling plates difficult to dissipate the heat, and the battery temperature can easily soar above 60℃. Due to the inconsistency of battery manufacturing processes and lithium precipitation due to aging, fires occur frequently. These two demands have led to the development of new battery thermal management cooling methods, with immersion cooling being one of the current research directions.
Which OEMs or parts manufacturers are researching immersion cooling?
Due to the limitations of liquid cooling and the need to suppress thermal runaway, companies like Geely, Great Wall, and BYD have taken the lead in researching immersion cooling. Although BYD is currently promoting direct refrigerant cooling across all its models, the company's vision is long-term, as it has already begun studying the feasibility of applying immersion cooling to the next generation of vehicles.
Major component suppliers such as AVL, 3M, Ricardo, Castrol, Valeo, Mahle, and others are currently conducting research, validating with some vehicles in their raw form.
Domestic state-owned OEMs are relatively conservative, still adhering to the liquid cooling method, which is destined to have a slower technological iteration compared to private enterprises like Geely and BYD.
What is Immersion Cooling?
The proposed solution involves dielectric immersion cooling, where the battery is directly in contact with the electrically insulating working fluid. This method offers the advantage of achieving a high heat transfer rate through direct contact between the battery cells and the immersion fluid, especially when using a two-phase fluid system. Here, the latent heat of vaporization associated with the liquid-gas transition enhances convective heat transfer, and effects such as nucleate boiling increase the amount of turbulent mixing. Furthermore, many immersion liquids can act as fire retardants, reducing the risk and impact of thermal runaway. However, to date, challenges such as fluid costs, uncertainties in system lifetime benefits, and integrated weight losses have prevented the large-scale industrial implementation of immersion cooling systems.
The electric vehicle industry is already seeing applications, including Kreisel using Shell's thermal fluids, Xing Mobility utilizing 3M's Novec fluids, and Rimac Automobili employing Solvay's Galden fluids.
This immersion cooling medium is available in both single-phase and two-phase configurations.
(3) A Brief Discussion on the Coupling Relationship Between Immersion Cooling, Heat Generation, and Fluid Flow
Due to immersion cooling's single-phase and two-phase immersion fluids, the following illustration showcases key parameters such as thermal conductivity, specific heat capacity, and viscosity, along with their coupled properties with battery performance, as depicted in Figure 1. Understanding this diagram aids in comprehending why immersion cooling holds such promising applications.
(4) Why Immersion Cooling Can Cool Down Batteries
Consider this: The liquid-cooled plate is one side that comes into contact with the battery cell. The cell is composed of multiple layers (positive electrode, negative electrode, separator, and electrolyte), with resistances within the layers and contact resistances between them. This assembly forms the internal resistance of the battery, with inconsistencies between layers leading to uneven current flow and State of Charge (SOC) across each layer. This inconsistency contributes to cell aging and reduced lifespan, which is a significant factor. Liquid cooling has limited heat exchange and contact area, whereas immersion cooling directly contacts the battery cells, ensuring a higher degree of temperature uniformity.
(5) Immersion Cooling Solution
As one of the emerging cooling technologies, direct liquid cooling, also known as immersion cooling, involves submerging the battery in an electrically non-conductive dielectric fluid, thereby achieving direct contact with the battery. Candidate dielectric fluids include hydrocarbons, silicone oils, and fluorinated hydrocarbons. This unique cooling method offers several advantages. Firstly, immersion cooling has the potential to provide superior battery and battery temperature uniformity among all cooling methods. This is because all battery surfaces are immersed in the fluid, providing a uniform and high thermal capacity heat transfer path for dissipation. This direct contact with the battery surface further reduces the thermal contact resistance in indirect cooling systems. Immersion cooling simplifies system design and reduces complexity. Additionally, thermal runaway is often suppressed in immersion cooling systems, as some dielectric fluids are also flame retardants, thereby enhancing the safety of the battery pack. Depending on the immersion level, flow type, and fluid operating state, there are different implementation methods for immersion cooling:
(6) Challenges and Drawbacks of Immersion Cooling
Additional complexities/costs associated with condensing evaporative steam, higher potential pumping losses in high-viscosity fluids, increased fluid costs, material compatibility issues, and increased fluid weight.
