Dielectric Immersion Cooling

Dielectric immersion cooling for a battery pack is perhaps the ultimate method of controlling cell temperatures.

Dielectric Fluid: an electrically non-conductive liquid that has a very high resistance to electrical breakdown, even at high voltages.

The cells are immersed in a dielectric and the dielectric then flows through a heat exchanger to extract the heat. The dielectric is in direct contact with the cells and busbars and hence thermal barriers are minimised.

This isn’t though a simple solution as the dielectric is just a heat transfer medium and there are lots of parameters to consider.

The heat transfer from the cell surface directly into the coolant medium is a significant advantage based on the area. However, the dielectric fluids have a lower heat capacity and conductivity than water-glycol. This means that the fluid flow path and rate are important design parameters to avoid local static boundary conditions on the surface of the cell.

Change in space between the cells due to cell swelling causes an imbalance in the flow. If we increase the space between the cells to eliminate the sensitivity we end up increasing the overall fluid volume and therefore the mass of the system.

Single and Two-Phase Dielectric Fluids: two-phase fluids increase the heat transfer by using latent heat of evaporation of the liquid-to-gas phase transition, this does increase the complexity of the design.

The selection of a dielectric fluid is complex and you will need to consider a number of factors for your application. AVL did the high level comparison in a 2020 presentation looking at the realities of dielectric cooling in a series production battery pack [7]:

The key parameters for dielectric fluids have been proposed in different formats and hence important to look at those to ensure that you don’t miss a factor in any given application.

Opportunities

• more cooling capability allows the battery to be run hotter
  • lower internal resistance (DCIR)
  • more power
• reduce busbar cross-sectional area as heat is dissipated to dielectric fluid
• extend lifetime with lower temperature gradients
• sensing properties of dielectric fluid to pre-detect electrical breakdown due to particles or water ingress
• contained system without a header tank to remove service requirements

Examples

Kreisel

Kreisel designed a cylindrical cell cooling system design where the main body of the cell is cooled with dielectric fluid.

“Lowest temperature spreads of less than 1°C on pack level avoid early single cell aging”

Note that the coolant does not flow over either end of the cylindrical cell and hence does not cool the busbars.

Lion Smart

LION Smart developed a light-weight battery pack with integrated immersive cooling technology using 3M Novec fluids, that can be used in automotive or aviation productions.

The project is EU funded and a part of LIBAT Clean Sky 2 aiming towards climate neutral aviation by 2050.

McLaren

McLaren P1 GTR original battery and upgraded battery are both dielectrically cooled.

McLaren SpeedTail has a very small, energy dense and powerful (5.2 kW/kg) dielectrically cooled battery pack.

Mercedes

Mercedes C63 AMG uses dielectric fluid to cool the central section of the cell body.

Rimac

Koenigsegg Regera – appears to use the patented dielectric cooling where it cools the busbars at each end of the cells before then flowing the fluid over the main cell body

Patent: WO2022069910A1 [3]

The cooling patent from RIMAC Automobili appears to show the dielectric cooling system for the Koenigsegg Regera. The dielectric enters the module and flows over both ends of the cells before returning over the centre section of the cell and exiting the module.

XING Mobility

Working with a number of companies who produce dielectric fluids.

• test results shows 10°C delta between immersion cooling and indirect liquid cooling method on cell surface temperature
• 20°C delta on busbar temperature
• No fire propagation was detected when during nail penetration to a single cell, causing it to undergo thermal runaway.
• Immersion cooled battery modules tested 10% longer life cycle compared to conventional indirect liquid cooled module at -4C/+2C charge/discharge rates.

Other Application Areas

HV Transformers – dielectric cooling has been used for HV power transformers for a very long time and hence this area is a good source of information.

IT datacentres – moving towards dielectric cooling to increase density, reduce hardware failures, minimize water usage and to reduce costs [4].

References:

1. Charlotte Roe, Xuning Feng, Gavin White, Ruihe Li, Huaibin Wang, Xinyu Rui, Cheng Li, Feng Zhang, Volker Null, Michael Parkes, Yatish Patel, Yan Wang, Hewu Wang, Minggao Ouyang, Gregory Offer, Billy Wu, Immersion cooling for lithium-ion batteries – A review, Journal of Power Sources, Volume 525, 2022
2. MIVOLT – Liquid Immersion Cooling
3. Choosing the Right Fluid for Electronics Cooling: Seven Dimensions of a Decision: An Aid for Thermal Engineers, DSi
4. submer – What’s immersion cooling?
5. Kreisel Electric – dielectric cooling design for cylindrical cells
6. Xing Mobility – Technology
7. How to develop battery technologies for large series applications – on the example of immersion cooling, AVL

Battery Cooling Options

There are many different options for battery cooling (and heating). These range in capability and complexity from Passive through to Fully Immersed Dielectric.