New Energy Storage Parameter: Cell Cooling Coefficient (CCC)
Aug 09, 2019 Dr Richard Collins
One of the most rapidly evolving areas of energy storage in electric vehicles is the thermal management of lithium-ion cells. Although most of the focus is on cell chemistry advancements, improvements to the battery pack can have a significant advantage in safety, performance, and longevity.
Preventing thermal runaway while at the same time allowing for effective longevity and performance of the cells is complicated. Many different OEM strategies are in play, from passive air cooling to active liquid or refrigerant processes. Active cooling brings more weight, complexity, and material requirements (such as thermal interface materials and coolant) but will certainly benefit performance and will likely be essential for future high-power fast charging. This is all in the backdrop of a shifting regulatory field, variable cell module designs, new players, and market acquisitions.
New academic research has come out from Imperial College London that defines a new parameter called the Cell Cooling Coefficient (CCC). This is with the aim to universally benchmark the thermal performance of all cell designs. If taken up as standard it could prove to be as relevant as energy or power density. The abstract is shown below and full text available on the open access journal.
This same academic group has received prominence through their research into cell tab cooling; this provides an interesting alternative to surface cooling that could lead to reduced temperature gradients across the cell and enhance their lifetime. However, re-designs would likely be required to have sufficient size tabs to draw off enough heat - more of this approach can be seen in this video.
Lithium-ion battery development is conventionally driven by energy and power density targets, yet the performance of a lithium-ion battery pack is often restricted by its heat rejection capabilities. It is therefore common to observe elevated cell temperatures and large internal thermal gradients which, given that impedance is a function of temperature, induce large current inhomogeneities and accelerate cell-level degradation. Battery thermal performance must be better quantified to resolve this limitation, but anisotropic thermal conductivity and uneven internal heat generation rates render conventional heat rejection measures, such as the Biot number, unsuitable. The Cell Cooling Coefficient (CCC) is introduced as a new metric which quantifies the rate of heat rejection. The CCC (units W.K−1) is constant for a given cell and thermal management method and is therefore ideal for comparing the thermal performance of different cell designs and form factors. By enhancing knowledge of pack-wide heat rejection, uptake of the CCC will also reduce the risk of thermal runaway. The CCC is presented as an essential tool to inform the cell down-selection process in the initial design phases, based solely on their thermal bottlenecks. This simple methodology has the potential to revolutionise the lithium-ion battery industry.
IDTechEx has released a new market report focussing on Lithium-ion Batteries for Electric Vehicles 2020-2030. This gives a granular overview of this industry with a section dedicated to thermal management for implementation in electric vehicles.
IDTechEx will be running business insight forums for those looking for a detailed technical and commercial understanding of the electric vehicle industry. These will be run in Tokyo and Seoul in September and Stuttgart in December. This will again include a forum looking at the strategies and disruption from in the thermal management market.
Top image: Pixabay