Key Parameters for Dielectric Fluids

What are the key parameters for dielectric fluids as used in a battery pack? We take a look at a few reference papers to build an initial list.

There are many options for cooling batteries. However, dielectric immersion cooling is seen as perhaps the ultimate high performance cooling method for battery cells. In this particular case we are focusing on the single phase dielectric cooling fluids.

Daccord [1] lists the key parameters with an explanation and gives a range of values/limits. Here we have pulled out the main headings:

• Thermal conductivity
• Volumetric heat capacity
• Viscosity
• Flash point
• Volume electric resistivity
• Dissipation factor
• Electric permittivity of the fluid
• Breakdown voltage

DSi [2] take a higher level and more encompassing look at the fluid parameters and list the eight key characteristics:

1. Heat Transfer Effectiveness
2. Electrical Safety
3. Flammability and Fire Safety
4. Oxidation Stability
5. Material Compatibility
6. Worker Health and Safety
7. Biodegradation and Environmental Fate
8. Cost

AVL [4] built a pugh matrix based on the following list:

• Compatibility with other materials
• Dielectric properties
• Water tolerance
• Heat transfer and viscosity
• Fire safety
• Low temperature
• Environmental
• Toxicity of decomposition products
• Recyclable
• Sustainable
• Stability and ageing
• Coolant cost

All of these lists are very similar and have a level of overlap.

One parameter missing from these lists and an important consideration when engineering battery packs to use a dielectric fluid for cooling is something within the definition of what a dielectric is [3]: “A dielectric material is a poor conductor of electricity but an efficient supporter of electrostatic fields.” This support of electrostatic fields can cause problems when flowing the fluid through pumps, around cells, through heat exchangers and over sensitive electrical equipment. Hence we should add a key characteristic:

• Electrostatic build up

This post has been built based on the support and sponsorship from: About:Energy, AVANT Future Mobility, Quarto Technical Services, TAE Power Solutions and The Limiting Factor. 

Key Parameters

Finally we can pull these together into one comprehensive list:

• Compatibility with other materials
  • Cell materials – should be checked against all cell materials in the event of a cell seal failure.
  • Pack materials – include all components and glues, check over temperature range of normal operation.
• Dielectric properties
  • Volume electric resistivity
  • Dissipation factor
  • Electric permittivity of the fluid
  • Breakdown voltage
  • Electrostatic build up
• Water tolerance
  • Open to atmosphere limits
  • Detection options
  • Hydroscopic materials in system – some materials absorb water and this might then be released to the dielectric fluid over time.
• Heat transfer and viscosity
  • Thermal conductivity
  • Volumetric heat capacity
  • Viscosity
• Flammability and Fire safety
  • Flash point
  • Oxidation Stability – this is not just will it catch fire, but what are the compounds if it does when it is subjected to the very high temperatures of thermal runaway.
• Low temperature
  • Thermal properties
  • Viscosity
  • Pourability
• Environmental
  • Water – impact on water in case of spill or crash.
• Toxicity of decomposition products – this could be from a fire or from ageing and decomposition.
  • Dielectric fluid
  • In combination with materials inside battery pack and cell itself
• Recyclable
  • Process to enable reuse
• Sustainable
  • Life Cycle Analysis
• Stability and ageing
  • Dielectric properties
  • Thermal properties
• Coolant cost
  • Dielectric fluid
  • System
  • Processing

References

1. Rémi Daccord, Thiébaut Kientz, Alexandre Bouillot, Aging of a dielectric fluid used for direct contact immersion cooling of batteries, Front. Mech. Eng., 03 July 2023, Sec. Engine and Automotive Engineering, Volume 9 – 2023
2. Dielectric Fluids vs Mineral Oils in Electrical Cooling Applications, Part 2: Electrical and Fire Safety DSI Ventures, Inc.
3. What is a dielectric material, WhatIs.com
4. How to develop battery technologies for large series applications – on the example of immersion cooling, AVL