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New Thermal Cloak Helps Keep Your Car Battery Cool in the Summer

An uncovered car (right) and car covered in the Janus cloak (left) are parked next to each other on a sunny Shanghai street.
Credit: Huaxu Qiao / Shanghai Jiao Tong University.
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A team at Shanghai Jiao Tong University has developed a prototype “thermal cloak” that can dampen the extreme temperature fluctuations felt in cars left out in the sun. The research was published today in the journal Device.

Making a passive, dual-action thermal cloak

The sweltering hot interior of a car left in the summer sun is not a very comfortable place to be – both for humans and electric vehicle (EV) batteries. Large temperature variations, such as those seen from day to night or season to season, can be very detrimental to the lifespan of an EV battery.

In a bid to dampen these fluctuations and preserve EV battery performance, researchers have developed a prototype all-season thermal cloak that can cool a vehicle’s interior on a hot day.

“The cloak works basically the same way the earth cools down, through radiative cooling,” said senior author Kehang Cui, a materials scientist at Shanghai Jiao Tong University. “The earth is covered by the atmosphere, and the atmosphere is transparent to a certain range of electromagnetic energy we radiate.” 

But to stop the car from becoming even colder during the winter months, the cloak also has to be able to counteract this behavior at times. “You have to develop something that can turn on and off by itself without external energy input, and that's extremely difficult,” said Cui.

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The team’s solution to this problem is a prototype two-layer fabric cloak, which they have named the Janus thermal cloak after the two-faced Roman god.

The cloak’s outer layer is made of thin fibers of silica coated in flakes of hexagonal boron nitride – a ceramic material similar to graphite – which helps to enhance the fibers’ solar reflectivity. Once braided and woven into fabric, the silica outer layer is adhered to an aluminum alloy inner layer that reflects energy back inside the car and stops it from escaping into the surrounding environment.

Cloak can keep cars up to 27°C cooler

To demonstrate the cloak’s operation, the research team conducted an experiment with electric vehicles parked outside under typical ambient conditions in Shanghai.

Firstly, they measured the internal cabin temperatures of cars left uncovered or covered with the cloak at midday. They found that the cabin of the uncovered car reached a peak temperature of 50.5°C at midday. The car covered with the Janus thermal cloak was 22.8°C inside – 27.7°C cooler than the uncovered car and 7.8°C lower than the ambient temperature outside.

When this experiment was repeated at midnight, the temperature recorded inside the cloaked car remained 6.8°C higher than the temperature outside and never dropped below 0°C.

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

As the researchers explain, the Janus thermal cloak was not made to be optimized for heat retention or solar reflection. Instead, the prototype was deliberately designed in a way to maximize its scalability and potential for commercial production in the future. For example, they say that thinner silica fibers would increase solar reflectivity, but this would make the material weaker and more difficult to deal with in high-volume production spaces. Additionally, the aluminum, silica and boron nitride materials used were all selected intentionally as they are low-cost while helping to make the cloak lightweight, durable and fire-retardant.

Additionally, while the Janus cloak appears to help protect EV batteries, the researchers believe that this material also could have more broad applications.

“The thermal cloak is like clothes for vehicles, buildings, spacecrafts or even extraterrestrial habitats to keep cool in summer and warm in winter,” said Cui.

Reference: Qiao H, Huang Z, Wu J et al. Scalable and durable Janus thermal cloak for all-season passive thermal regulation. Device. 2023. doi: 10.1016/j.device.2023.100008

This article is a rework of a press release issued by Cell Press. Material has been edited for length and content.