The Janus Thermal Cloak Can Protect EV Batteries by Cooling in Summer and Warming in Winter

Researchers at the Shanghai Jiao Tong University, Shanghai Academy of Spaceflight Technology, and Wuhan University have developed clothes for cars, but they're not supposed to be fashionable: instead, they can help regulate the temperature of electric vehicles in hot and cold conditions, to help prolong the life of the battery.

"The thermal cloak is like clothes for vehicles, buildings, spacecrafts, or even extraterrestrial habitats to keep cool in summer and warm in winter," explains senior author Kehang Cui, a materials scientist at Shanghai Jiao Tong University, of the device the team has built. "The cloak works basically the same way the earth cools down, through radiative cooling. The earth is covered by the atmosphere, and the atmosphere is transparent to a certain range of electromagnetic energy we radiate."

Dubbed the Janus Thermal Cloak, for the Roman two-faced god of duality, though, being able to bleed heat off into space in order to reduce temperatures is only half the device's capabilities: in winter, it can do the opposite and warm whatever it covers above ambient temperature through "photon recycling" — bouncing energy trapped between the covered object and the cloak to prevent heat loss. Better still, it does so entirely passively — cooling when it needs to cool and heating when it needs to heat.

The cloak itself is built using a layer of silica fibers coated in hexagonal boron nitride, designed to increase the cloak's albedo — the amount of solar energy it can reflect. The fibers are braided and woven together to form a soft fabric, which is then coated with an inner layer of aluminum alloy — a production process the team claims is wholly scalable for mass production using existing fabrication techniques.

In tests on vehicles, the cloak was able to reduce the cabin temperature to 22.8°C (around 73°F), nearly eight degrees below the ambient temperature outside the vehicle and a whopping 27.7°C cooler than the cabin of an uncovered vehicle in the same conditions. At night, the reverse was observed: the covered vehicle stayed 6.8°C higher than ambient, never dropping below freezing.

"This is the first time that we could achieve warming above the ambient temperature by almost 7°C during winter nights," Cui notes of the testing. "This is also kind of surprising to us — there's no energy input or sunshine and we can still get warming."

The team's work has been published under open-access terms in the journal Device.