Electric vehicles parked outside are subject to drastic temperature changes from day to night and across different seasons, leading to potential battery degradation. In order to mitigate these temperature swings and enhance battery longevity, a team of researchers has created an all-season thermal cloak.
Image Credit: Huaxu Qiao (CC BY-SA)
The cloak can lower the temperature of an electric vehicle by 8°C during hot days and increase it by 6.8°C during cool nights. Made primarily of silica and aluminium, the thermal cloak performs these functions passively, without requiring external energy, and adjusts seamlessly between warm and cold weather. The prototype is detailed in the inaugural issue of Device, a practical application-focused sister publication to Matter, Joule, and Cell.
The senior author of the study, Kehang Cui, a materials scientist at Shanghai Jiao Tong University, likens the thermal cloak to clothing for vehicles, buildings, spacecraft, and potentially even off-world habitats. It can maintain cooler temperatures during the summer and warmth during the winter.
To achieve this, the cloak separates the car—or any other object beneath it—from the surrounding environment, thus reducing natural temperature fluctuations. The cloak comprises two parts: an external layer that effectively reflects sunlight and an internal layer that retains heat. Any heat absorbed by the external layer is emitted in a way that allows it to dissipate easily into outer space, earning the cloak its name of Janus thermal cloak, after the two-faced Roman deity Janus.
Senious author Kehang Cui explains that the cloak employs a process called "radiative cooling," similar to the natural cooling of the Earth. While this is beneficial during summer, it could make the car too cold in winter. Hence, Cui and his team devised a mechanism known as "photon recycling" to automatically counterbalance this effect during the colder months. This means that any energy caught beneath the cloak is reflected between the car and the cloak instead of escaping to the external environment.
Photograph of the phononic metafabric - Image Credit: Huaxu Qiao (CC BY-SA)
The researchers validated the effectiveness of the thermal cloak through testing on electric vehicles parked outdoors under typical environmental conditions in Shanghai. They found that while an unshielded car's internal temperature reached 50.5°C at noon, a car covered with the cloak only registered 22.8°C—27.7°C lower than the unshielded vehicle and 7.8°C lower than the outdoor temperature. At midnight, the covered car's temperature was 6.8°C higher than the outdoor temperature and never fell below freezing.
Cui expressed his surprise at achieving warming above ambient temperature by nearly 7°C during winter nights without the need for energy input or sunshine.
The thermal cloak's outer component comprises thin silica fibers coated with flakes of hexagonal boron nitride, a ceramic material akin to graphite that enhances the fibers' solar reflectivity. The fibers are then interlaced into a fabric and bonded to the inner aluminium alloy layer.
The researchers deliberately designed the cloak to facilitate potential mass production. For instance, while using thinner silica fibers would have enhanced solar reflectivity, these fibers would have been more fragile and not suitable for large-scale industrial manufacturing techniques. Furthermore, the materials used, such as aluminium, silica, and boron nitride, are all affordable, making the cloak lightweight, robust, and fire-resistant.
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