Heat can damage the batteries of electric vehicles – even just driving fast
on the freeway in summer temperatures can overheat the battery. An innovative
new coolant conducts heat away from the battery three times more effectively
than water, keeping the battery temperature within an acceptable range even in
extreme driving situations.
Batteries provide the “fuel” that drives electric cars – in effect, the
vehicles’ lifeblood. If batteries are to have a long service life, overheating
must be avoided. A battery’s “comfort zone” lies between 20°C and 35°C. 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: operating a battery at a temperature of
45°C instead of 35°C halves its service life. And batteries are expensive – a
new one can cost as much as half the price of the entire vehicle. That is why it
is so important to keep them cool. Thus far, conventional cooling systems have
not reached their full potential: either the batteries are not cooled at all –
which is the case with ones that are simply exchanged for a fully charged
battery at the “service station” – or they are air cooled. But air can absorb
only very little heat and is also a poor conductor of it. What’s more, air
cooling requires big spaces between the battery’s 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 away heat, 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.
More space under the hood
In future, another option will be available for keeping batteries cool – a
coolant by the name of CryoSolplus. It is a dispersion that mixes water and
paraffin along with stabilizing tensides and a dash of the anti-freeze agent
glycol. The advantage is that CryoSolplus can absorb three times as much heat as
water, and functions better as a buffer in extreme situations such as trips on
the freeway at the height of summer. This means that the holding tank for the
coolant can be much smaller than those of watercooling systems – saving both
weight and space under the hood. In addition, CryoSolplus is good at conducting
away heat, moving it very quickly from the battery cells into the coolant. With
additional costs of just 50 to 100 euros, the new cooling system is only
marginally more expensive than water cooling. The coolant was developed by
researchers at the Fraunhofer Institute for Environmental, Safety and Energy
Technology UMSICHT in Oberhausen.
As CryoSolplus 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 Dipl.-Ing. Tobias Kappels, a scientist at UMSICHT.
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 stabilize the dispersion, depositing themselves on the paraffin
droplets and forming a type of protective coating. “To find out which tensides
are best suited to this 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 such as being pumped through pipes? And
how stable is it when exposed to thermal stresses, for instance when the
paraffin particles freeze and then thaw again?” says Kappels. Other properties
of the dispersion that the researchers are optimizing 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.