By using solid paraffin droplets, the team at the Fraunhofer Institute created a phase change material (PCM) that should last the life of the battery without the need to be changed.
‘The battery is the most expensive part of an electric vehicle, and so you do not want to be changing the battery so often. But to have a long life, it has be cooled effectively,’ said project lead Tobias Kappels of the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen.
Indeed, for a typical lithium-ion battery, the optimum range lies between 20°C and 35°C — which can be exceeded in summer heat even with modest driving.
Current EV batteries are generally air-cooled; however, air is a poor conductor of heat and also requires large spaces between the battery’s cells to allow sufficient circulation.
Water-cooling systems are still in their infancy. Although their thermal capacity exceeds that of air-cooling systems, the downside is the limited supply of water in the system compared with the essentially limitless amount of air that can flow through a battery.
Kappels’s team began looking at alternatives based on a dispersion that mixes water and paraffin along with stabilising tensides and a dash of the anti-freeze agent glycol.
The CryoSolplus formula acts as a PCM, such that when it absorbs heat, the solid paraffin droplets within it melt, storing the heat in the process; conversely when the solution cools, the droplets revert to their solid form.
‘The main work of the research so far was to get a stable dispersion — that’s very critical and it wasn’t easy, because when it’s not at the right concentration, the paraffin tends to agglomerate and plug the pipes,’ said Kappels.
‘If you have microencapsulated particles, it’s possible to damage the particles, then the paraffin gets out into the water. But because we have surfactants, they always get back to the paraffin particle, and it’s refreshed, it’s stable and there will be no wear.’
According to the team, the CryoSolplus absorbed up to three times as much heat as water in test scenarios. The upshot of this is that less coolant is needed — only 20 litres, compared with around 60 litres required for water — meaning that the holding tank for the coolant can be much smaller, saving weight and space.
Still, Kappels acknowledges that 20 litres of coolant would add around €50 to €100 (£39 to £78) on the price of the battery. The next task will be to carry out field tests, trying out the coolant in an experimental vehicle.
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