Immersion cooling of lithium-ion batteries with dielectric fluids

This PhD proposal will investigate the operational mechanisms around immersion cooling of lithium-ion batteries (LIBs)
using dielectric fluids.
LIBs are a key enabling technology for consumer electronics, electric vehicles (EVs) and for the integration of zeroemission renewable technologies such as wind and solar. Whilst their commercial adoption continues at pace, their cost,
performance and lifetime still needs to be improved to be widely economically viable. Increases in the cost when translating
from a single cell to a battery pack are due to the additional integration of components such as battery management
systems and thermal management systems (TMS), with the thermal management system current contributing to ca. 20% of
the total pack cost. With the battery performance and lifetime being highly thermally coupled, the effective design of a TMS
is critical for achieving long term performance targets. In terms of power density, the 2035 target is 12 kW/kg which is still a
long way away from the current performance of 3 kW/kg with a lifetime of ca. 15 years compared to current values of ca. 8
years. To compound this, future TMSs must also be able to operate over a temperature range of -40-80 degC which means
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Research Organisation
Supervisor
* = Main Supervisor
alternative cooling media to water must be explored.
Currently, there are a range of TMSs being commercially exploited. The simplest approach is the use of natural convection
to reject heat generated from the operation of LIBs. This is an approach taken by the Nissan Leaf, and whilst this is lowcost, the limited specific heat capacity (~1 kJ/kg.K) and convective heat transfer coefficient (~1-10 W/m2K) of the natural
convection of air means that batteries can experience accelerated degradation due to higher temperature operation and
also limited power capabilities due to thermal limits being reached. Tesla battery packs on the otherhand use a copper tube
which is arranged in a serpentine pattern around the individual cylindrical cells of its pack to pass water to remove the heat.
The advantage here is that water has a much higher specific heat capacity (~1,000 kJ/kgK). The disadvantage of this
approach is the additional weight of the heat pipes, the suboptimum thermal contact between the cells and the cooling
media as well as uneven cooling between cells which can lead to accelerated degradation.
Research efforts have investigated various alternative cooling methods such as the use of phase change materials (PCM)
like Paraffin. Whilst these PCMs do have very good specific heat capacitances (~2,000 kJ/kgK) due to the additional latent
heat energy (~200 kJ/kgK) associated with the phase change from solid-to-liquid, their performance once in the melted
state is not ideal since PCMs do not dissipate heat well in part due to the stagnant nature of the material
when solid preventing circulation of the cooling media.
Motivated by these challenges, this work will develop a fundamental insightinto the feasibility of immersive cooling of LIBs
using dielectric fluids including commercially available single and dualphase fluids but extending out to the investigation of
more proprietary solutions currently being developed by Shell. This will be achieved by first developing a 3D thermally
coupled electrochemical model of a LIB in COMSOL multi-physics to predict the heat generation
characteristics as well as the performance and lifetime implications. In large form factor and multi-cell battery packs,
commonly found in automotive battery packs, the heat generation can be high heterogeneous and it is critical to
understand this.
The insights from this project will help Shell develop improved e-fluids for the growing EV industry and add to academic
knowledge around the use of single/multi-phase dielectric fluids for immerson cooling of LIBs.