Lithium-ion batteries are quite sensitive to temperature variations. Their optimum temperature range is between 20 and 35°C. But already during a short summer ride the battery can heat up beyond this limit, potentially causing significant damage. If such a battery is operated at 45°C instead of 35°C, the service life will be reduced by 50%. For this reason, traction batteries require a temperature management. Today’s batteries are either not cooled at all – with the abovementioned consequences. Or they are cooled by air, which however is not regarded as a satisfying solution. Air has a low thermal capacity and is a poor thermal conductor. In addition, the battery cells need a relatively large distance between each other to enable the designers to implement air conducts.
Researchers from the Fraunhofer Institute for environmental, safety and energy technology (UMSICHT) in Oberhausen (Germany) have developed a cooling agent with excellent properties to cool batteries. CryoSolplus is a dispersion, consisting of water, paraffin, stabilizing surfactants and small quantities of the antifreeze agent glycol. Since its thermal capacity is three times higher than water, the storage tank can be much smaller than the tank of a water-cooled system with comparable performance. In addition, CrySolplus features good thermal conduction; the heat can be transported away from the battery calls quickly. The additional costs are moderate; according to UMSICHT they amount to some 50 to 100 euros per vehicle.
The cooling agent functions as follows: As CryoSolplus absorbs heat, the paraffin globules melt – they become drops and store the heat. As soon as the solution cools off, the globules solidify again. Thus, the effect of CryoSolplus is based on a phase change. "The challenge was to get the dispersion stable enough", said UMSICHT scientist Tobias Kappels. The globules must not clump and, since they are lighter than water, they must not concentrate at the surface. This is why the researchers added surfactants: They accumulate at the globules and thus form a protecting cover.
The next research step will be to equip a test vehicle with the new cooling agent.