Thermal management mode of pure electric vehicles: battery immersion cooling

In a previous article, we discussed the role and related technologies of thermal interface materials (TIMs) in battery systems, as well as the development of thermal conductivity properties of these materials and how they support the transfer of battery heat in thermal management systems to the cooling circuit.

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. [1]

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. Compared with media such as liquid, air has a small heat capacity (Cp = 1.006 kJ/kgK at standard temperature) and low thermal conductivity, so air cooling is unlikely to be the solution for the next generation of electric vehicles with larger battery packs and faster charging. Preferred technology. [1]

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 (TIM). The topic of thermal interface materials, including their overview, properties and importance in indirect liquid cooling has been discussed in a previous article1. 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.

还有一种新兴的冷却技术是直接液冷，也叫做浸液式冷却，它将电池完全浸没在介电液体中。这是一种不导电的液体，具有很高的抗电击穿能力。这项技术的引入意味着电池工艺和部件设计的复杂性可以大大降低，也有助于减轻系统的重量和体积，显著提高电池温度控制的稳定性和均衡性。浸液式冷却可以按照需要加热或冷却电池，而无需使用热交换器，这在效率方面带来显著提升。电动汽车电池的浸液式冷却目前仍处于初级阶段，但已经有一些使用案例出现，例如法拉第未来公司的拥有专利的全浸式电芯系统 [2]、达喀尔拉力赛汽车奥迪RS Q e-tron的浸液式冷却技术 [3] 或者松下和特斯拉前员工创立的行兢科技的IMMERSIO系统  [4]。

Figure 1 The immersion cooling system in the Audi RS Q e-tron used in the Dakar Rally

The dielectric fluids commonly used in immersion cooling are flame retardant and can inhibit thermal runaway events. Currently, there are several groups of cooling media available on the market - hydrofluoroethers, hydrocarbon oils, silicone oils and fluorinated hydrocarbons. There is growing interest in biodegradable dielectric fluids.

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

- Long shelf life of liquids

When selecting a suitable immersion coolant, in addition to the above requirements, material compatibility, low density, low viscosity and environmental friendliness must also be considered.

Description of various coolants

氢氟醚 - 来自电力电子应用领域的氢氟醚在纯电动汽车的浸液式冷却领域得到了极大的关注。文献表明，与风冷系统相比，其冷却效率显著提高。由于氢氟醚的不易燃性，并且几乎不存在闪点，系统安全性也得到了改善。这种氢氟醚浸液式冷却系统在全生命周期内的性能表现仍有待研究。此外，氢氟醚的密度大约比水-乙二醇系统高40%，这对纯电动汽车的重量和续航里程都有不利影响。此外，材料成本和环保性也是阻碍氢氟醚在电动车热管理系统中大量使用的重要因素。

Hydrocarbons - Includes mineral oils, polyalphaolefins (PAO) and synthetic hydrocarbon oils. Hydrocarbons are distillation products of petroleum, which makes them low-cost, low-toxicity, and suitable for use in immersion cooling within the appropriate operating temperature range. The disadvantage of such liquids is that they can be flammable and have a flash point.

Esters - They have found widespread use in various industries due to their low cost, high flash point, good dielectric properties and biodegradability. Esters are divided into two types: synthetic and natural. Synthetic esters are the product of a chemical reaction between polyols and carboxylic acids, while natural esters are the product of vegetable oils. The two sources also correspond to different properties. Synthetic esters have good oxidation stability, which has a positive impact on extending vehicle maintenance intervals; however, they generally have a lower flash point than natural esters. The disadvantage of ester-based systems is that as the material ages, its viscosity increases, which in turn reduces its cooling ability.

Silicone Oil - The main advantage of silicone oil is its good temperature resistance and dielectric properties under both high and low temperature conditions.

Water/Glycol - A mixture of water and glycol. Water/glycol has considerably higher electrical and thermal conductivities and offers cost advantages compared to other systems. However, the conductivity of the water-based glycol mixture limits its use to indirect liquid cooling, and during use, the sealing measures of the indirect liquid cooling system are very important, as this can prevent the mixture from leaking into the battery or wiring. on, thereby preventing short circuits and eventual thermal runaway. [5]

In short, there are many solutions for thermal management; at Datwyler, we believe that thermal management systems for pure electric vehicles can be improved through the judicious use of the right materials and composites. As the mobility industry moves steadily towards electrification, the focus is now on supporting coolant manufacturers, suppliers and OEMs in upgrading thermal management systems; in this regard, Datwyler's materials expertise is supported High level application. Chemical compatibility is key with direct immersion cooling, as different coolants may result in different sealing solutions.

Experimentation is necessary when selecting the best sealing solution for every immersion cooling system. As far as the electric vehicle industry is concerned, perfluoropolyethers (PFPE) appear to be at the forefront of coolant applications due to their non-flammable and low viscosity characteristics. However, no matter which coolant is used in an electric vehicle, it must be ensured to be compatible with the sealing solution to avoid seal corrosion/degradation over time in this harsh environment and prevent the resulting The problem. [1]

Security is paramount

The most important safety issue for battery systems is to prevent fire and thermal runaway. Since an immersion-cooled battery system is directly immersed in the coolant, the entire area must be sealed with a specialized sealing elastomer component, which must be chosen correctly and ideally throughout the vehicle's lifetime. A certain level of chemical and weather resistance.

At Datwyler, we are conducting experiments and precise analysis of various types of coolant in contact with different materials; through these tests we can determine which types of polymers or elastomers are best suited for sealing these liquids.

An important parameter that needs to be controlled even during the simulation analysis phase is thermal aging. Materials are tested at high temperatures (up to 100°C) for long periods of time (up to 1000 hours). Judging from the first set of tests on PFPE, silicone oil and seed oil, Datwyler's formulations based on IC-DAT10 and IC-DAT30 perform well in most coolants and should generally be the sealing material of choice. Each IC-DAT code represents a different elastomer family, so the study compared the performance of different polymer families in selected fluids.

Following this rigorous testing process, the physical properties of the materials before and after being immersed in these liquids were compared. The volume change shown in the figure below is used as an indicator of chemical stability over time after immersion in coolant. In addition, thermal stability, changes in tensile properties, compression set and leakage were examined and understood.

Figure 2 Chemical stability results of standard rubber formulations after immersion in different types of coolants used in immersion cooling systems (seed oil, silicone oil, and various PFPEs)

In the second set of tests shown in Figure 2, we focused on halogen-free, non-toxic bio-based and biodegradable coolants. All tested coolants were classified into two categories with global warming potential (GWP) of 0 and <1. In addition, this study also includes so-called mixed liquids, which provide not only a cooling effect but also a lubricating effect for the propulsion train.

Volume changes of standard rubber formulations after immersion in these liquids showed significant changes in most of the rubber formulations tested. It can be seen that among all the tested materials, only the formula with code IC-DAT41 is a suitable candidate formula.