A coolant exhibiting a high thermal conductivity is desired for it allows for a more effective heat removal from the cells, which can help reduce the cooling load and energy consumption. It indicates how much heat can be transferred from the surface of the cells into the fluid, before considering the fluid will itself be moved away due to natural or forced convection. A thermal conductivity above 0.14 W/m/K at 40°C is recommended for acceptable performance, and can be assessed using the standard ASTM D7882. This property can vary substantially between oils, with typical mineral oils having a thermal conductivity of about 0.13 W/m/K at 40°C, while ester oils reach 0.16 W/m/K and specific transformer oil could exhibit thermal conductivities above 0.35 W/m/K.

Volumetric heat capacity is an important thermal property of dielectric fluids when used in immersion cooling. This thermal property refers to the amount of heat that a specific volume of a material can absorb, and it is calculated as the heat capacity of a sample of the substance divided by the volume of the sample. The standards ASTM7896 and ASTMD4052 offer guidance on the measuring method.

Common dielectric fluids used for immersion cooling have a volumetric heat capacity more than 1,000 times higher than that of air. One liter of dielectric fluid will therefore carry more heat than 1 m³ of air. At any given time, the power supplied to the dielectric fluid can be viewed as the product between the mean temperature, the fluid’s volumetric heat capacity and its flow rate. For a given flow rate, the average battery temperature depends on the volumetric heat capacity of the dielectric fluid, making it one of the key characteristics for efficient dielectric fluids for immersion cooling.

To achieve high thermal efficiency, dielectric fluids should have volumetric heat capacity values greater than 1.5 kJ/L/K at 40°C (preferably above 1.9 kJ/L/K).

The dynamic viscosity is a measure of a fluid’s internal friction, or, in other terms, a measure of a fluid’s resistance to flow. In immersion cooling, dynamic viscosity is an important property because it affects the convection coefficient and the pumping requirement. A coolant with high viscosity will flow more slowly over a hot surface, resulting in a lower convection coefficient and a slower rate of heat transfer.

It is important to note that the relationship between viscosity and convection coefficient is not a simple one-to-one correlation. Other factors, such as the velocity and turbulence of the fluid, also affect the convection coefficient. In general however, a coolant with a low viscosity will have a higher convection coefficient and will be able to transfer heat more effectively than a coolant with a high viscosity.

Kinematic viscosity is defined as the ratio of a fluid’s dynamic viscosity to its density.

In immersion cooling, the fluid needs to be able to flow around the cells, busbars and electric components in order to effectively remove heat away from them. In some configurations, the electronic control circuits supporting the Battery Management System (BMS) can also be immersed. If the kinematic viscosity of the fluid is too high, it will not flow easily and may not be able to effectively remove heat from the components.

In addition to its impact on the efficiency of heat transfer, the kinematic viscosity of immersion cooling fluids has also a significant effect on the performance and lifespan of the pumps used in the cooling system.

Indeed, for a given flow rate the pumping losses are given by the ratio of the dynamic viscosity to the square product of the density and the specific heat. Therefore, it is important to select an immersion cooling fluid with a kinematic viscosity that is suitable for the specific cooling application, as the pumping losses depend on it. W ˙ p u m p = c s t . μ ρ . C p 2

Finally, the viscosity also impacts how easily the pump will be able to operate in cold start situations. Based on typical viscosity-temperature relationships, the following specifications can be considered relevant for this application, and can be evaluated using the standards ISO3104, ASTM D7042 or D445/446: ν 25 ° C < 5   c S t ν 40 ° C < 3.5   c S t

Exoes has found that using the Mouromtseff number (Mouromtseff, 1942) is a valuable tool for ranking the performance of dielectric fluids in immersion cooling. With reference to a flow inside a fixed geometry at a given velocity, the highest heat transfer rate is achieved by the liquid coolant with the highest Mouromtseff number. This indicator encapsulates in one number most of the properties discussed previously.

For single-phase forced convection, Mouromtseff found this figure of merit, Mo, to follow the form: M o = ρ a   .   k b   .   C p d μ e where ρ, k, Cp, and µ represent the density, thermal conductivity, specific heat (at constant pressure) and dynamic viscosity of the fluid. The exponents a, b, d, and e take on values appropriate to the heat transfer mode of interest and the corresponding heat transfer correlation.

Exoes has developed a definition of the Mouromtseff number based on a Nusselt correlation built from experimental results conducted at their facilities on full immersion modules.

The standard shape correlation for the Nusselt number. N u = C ⋅ R e α ⋅ Pr 0.33 , with Re the Reynold’s number and Pr the Prandtl number

Was used in a 50 nodes analytical GTsuite model of the module. The model was fitted to experimental results with one fluid on the following criteria:

- Module pressure drop,

C and α parameters were chosen to minimize the average surface temperature error between the tests and the model. Five steady state tests at different flowrates and C-rates were taken into account.

The following fitted correlation is then proposed: N u = 0.175 ⋅ R e 0.65 ⋅ Pr 1 / 3

The following correlation is used by Exoes to define the Mouromtseff number: M o = ρ 0.65 .   k 0.67 .   C p 0.33 μ 0.32

However, in addition to a fluid’s heat transfer capability, the fluid’s ability to store and move heat away from a heat source and the fluid’s hydraulic performance should also be considered for optimum system performance. It is desirable to maximize the caloric and heat transfer capability while at the same time minimizing the hydraulic behavior, which is characterized by system pressure drop and required pumping power, as introduced in Section 2.1.1.2.

A figure of merit (FoM) representing the relationship among key thermophysical properties for comparative purposes has been suggested by Yeh and Chu (L-T Yeh, 2002) for single phase forced convection cooling that captures this desire: F o M = C p . h W ˙ p u m p , with h the heat transfer coefficient

The safety of electric vehicles is a critical concern for manufacturers, particularly with regards to the possibility of battery thermal runaway. To minimize this risk, non-flammable fluids or high flash points fluids are used for safe operation. The flash point of a fluid is the lowest temperature at which its vapors will ignite when given an ignition source. While high flash point fluids could increase the safety of EVs, currently, there is no consensus among EV manufacturers on the requirements and limits for flash point, leaving a gap in both academic and industrial understanding.

Moreover, common dielectric fluids with viscosities in the ranges described in Section 2.1.1.2 have extremely low volatility and high flash points. More research is needed to understand their behavior under thermal runaway conditions. It is then worth noting that the flash point does not necessarily reflect a material’s flammability behavior in the context of immersion cooling. To address this, companies such as Lanxess use a hot plate droplet ignition test to assess the risk of ignition.

The volume electric resistivity, measured in Gohm. m, can be defined as the resistance of a cube with a unit length between two opposite faces on which metallic electrodes are applied. It indicates the ability of the fluid to conduct current. The electrical resistivity of insulating mineral oils, which can exceed 10 3 Gohm. m, generally decreases during use due to chemical alteration. Indeed, the oxidation products of the oil, which are favored by the presence of metals, exposure to air, operation at high temperature, and the presence of polar pollutants such as water or solid particles, strongly increase conductivity. Water can indirectly contribute to the increase in oil conductivity by promoting the miscibility of contaminants in the oil.

The ISO-6469 regulation requires that for an 800 V battery in electric vehicles, there must be a 500Ω/V insulation resistance to the chassis, which results in a required electric insulation resistance greater of 400 kΩ for a complete powertrain, including a 800 V battery.

Assuming that:

- the typical distance between live parts and the chassis is 1 mm,

The electrical dissipation factor of an insulating material is defined as the tangent of the loss angle (delta). The loss angle delta is the complementary angle of the phase shift between the applied voltage and the current. For dielectric materials, delta is often small and therefore can be approximated by its tangent. In the case of a perfect dielectric material, the phase shift between current and voltage is 90°. The electrical dissipation factor thus reflects energy losses due to Joule heating. The dissipation factor is an important consideration for transformer oils, but in the case of DC batteries, it is used as a sensitive indicator to assess the quality of the fluid with inexpensive and readily available testing methods.

The electric permittivity is defined as the quotient of the electric induction by the electric field, and is expressed in farads per meter (F/m). It represents the ability of the fluid to form a capacitor under an electric field and tells how much the molecules oppose an external electric field. For mineral oils, the permittivity is typically between 2 and 2.5 F/m, but it strongly depends on the nature of the oil. For instance, the permittivity of aliphatic hydrocarbons is around 2 F/m, while that of aromatic hydrocarbons is around 2.3 F/m. The permittivity of oils increases with the presence of polar compounds such as impurities. This effect is even more pronounced in alternating current, where dielectric losses occur due to a phase shift between current and voltage. In insulating transformer oils, the permittivity is often considered complex. With the real part being related to the stored energy within the material. The imaginary part relates to the loss of energy in the material.

The breakdown voltage, which measures the minimum voltage at which an arc appears between two electrodes, typically separated by 2.5 mm (0.1 inch) according to the standards, is crucial for safety considerations in EV batteries. A minimum breakdown voltage of 2.5 kV or 1,000 V/mm is typically required for 800 V batteries. Impurities and moisture usually result in a decrease of the breakdown voltage in transformer oils (M.S.M. Abeyrathna, 2021).

These classic dielectric properties play a vital role in ensuring the optimal performance and safety of the immersion cooling fluids used in EV battery applications.