|CryoSolplus is a dispersion that can absorb three times as much heat as water, and can prevent batteries from overheating. Click to enlarge.|
Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen have developed a new phase-change material (PCM) coolant that conducts heat away from a traction battery for electric vehicles three times more effectively than water, keeping the battery temperature within an acceptable range even in extreme driving situations.
Avoiding overheating EV battery packs is important to maintaining long service life; operating a battery at a temperature of 45°C instead of 35°C halves its service life. Conventional cooling systems have not reached their full potential, Fraunhofer suggests:
Air cooling systems are limited, FraunhoferÚ says, because 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.
Fraunhofer researchers have developed a coolant they call CryoSolplus—a dispersion that mixes water and paraffin along with stabilizing tensides (also known as surfactants: chemical species that act as wetting agents to lower the surface tension of a liquid)—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 €50–100, the new cooling system is only marginally more expensive than water cooling.
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 (i.e., phase change materials).
The main problem to be overcome during development of CryoSolplus was making the dispersion stable, said 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?—Tobias 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.