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Keeping electric vehicle batteries cool

2012-10-1 0:01:37 Category:Professional News

Batteries provide the ‘fuel’ that drives electric cars – in effect,it's the vehicles’ lifeblood.

If batteries are to have a long service life, overheating must be avoided. A battery’s ‘comfort zone’ lies between 20 ºC and 35 ºC. But even a Sunday drive in the midday heat of summer can push a battery’s temperature well beyond that range. The damage caused can be serious, as operating a battery at a temperature of 45 ºC instead of 35 ºC halves its service life.
Batteries are expensive – a new battery can cost as much as half the price of the entire vehicle. That is why it is important to keep them cool. Thus far, conventional cooling systems have not reached their full potential. They are either not cooled at all – which is the case with those that are simply exchanged for a fully charged battery at the ‘service station’ – or are air-cooled. But air can absorb only very little heat and is also a poor heat conductor. What’s more, air cooling requires big spaces between the battery cells to allow sufficient fresh air to circulate between them. Water-cooling systems are still in their infancy. Though their thermal capacity exceeds that of air-cooling systems and they are better at conducting heat away, their downside is the limited supply of water in the system, compared with the essentially limitless amount of air that can flow through a battery.
In future, another option will be available for keeping batteries cool – a coolant called CryoSol-plus – a dispersion that mixes water and paraffin, along with stabilising tensides and a sash of the antifreeze agent glykol. The advantage is that CryoSol-plus can absorb three times as much heat as water and functions better as a buffer in extreme situations like trips on the freeway at the height of summer. This means that the holding tank of the coolant can be much smaller than that of water – saving both weight and space under the hood.
In addition, CryoSol is good at conducting heat away, moving it quickly from the battery cells into the coolant. With additional costs of just €50 to €100, the new cooling system is only marginally more expensive than water cooling systems. The coolant was developed by German researchers at the Fraunhofer Institute for Environmental Safety and Energy Tech- nology, in Oberhausen.
As CryoSol-plus absorbs heat, the solid paraffin droplets within it melt, storing the heat in the process. When the solution cools, the droplets revert to their solid form. Scientists call such substances phase-change materials, or PCMs.
“The main problem we had to overcome during development was to make the dispersion stable,” explains Tobias Kannels, a scientist at the institute.
The individual solid droplets of paraffin had to be prevented from agglomerating or – as they are lighter than water – collecting on the surface of the dispersion. They need to be evenly distributed throughout the water. Tensides serve to stabilise the dispersion, depositing themselves on the paraffin droplets and forming a type of protective coating.
“To find out which tensides are best suited to the purpose, we examined the dispersion in three different stress situations. How long can it be stored without deteriorating? How well does it withstand mechanical stresses? And how stable is it when exposed to thermal stresses – for instance, when the paraffin particles freeze and then thaw again?” asks Kappets.
Other properties of the dispersion that the researchers are optimising include its heat capacity, its ability to transfer heat and its flow capability.
The scientists’ next task will be to carry out field tests, trying out the coolant in an experimental vehicle.