Thermal cloak passively keeps electric vehicles cool in the summer and
warm in the winter

Date:
July 11, 2023
Source:
Cell Press
Summary:
When an electric vehicle is parked outside, its temperature can
swing wildly from day to night and season to season, which can
lead to deterioration of the battery. To dampen these fluctuations
and extend the battery's lifespan, researchers have designed an
all-season thermal cloak that can cool an electric vehicle by
8DEGC on a hot day and warm it by 6.8DEGC at night. The cloak,
made predominantly of silica and aluminum, can do so passively
without outside energy input and operates without any modification
between hot or cold weather.


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FULL STORY ==========================================================================
When an electric vehicle is parked outside, its temperature can
swing wildly from day to night and season to season, which can lead to deterioration of the battery. To dampen these fluctuations and extend the battery's lifespan, researchers have designed an all-season thermal cloak
that can cool an electric vehicle by 8DEGC on a hot day and warm it by
6.8DEGC at night. The cloak, made predominantly of silica and aluminum,
can do so passively without outside energy input and operates without
any modification between hot or cold weather.

This prototype is described July 11 in the newly launched Device, an application-oriented sister journal to Matter,Joule, and Cell.

"The thermal cloak is like clothes for vehicles, buildings, spacecrafts,
or even extraterrestrial habitats to keep cool in summer and warm in
winter," says senior author Kehang Cui, a materials scientist at Shanghai
Jiao Tong University.

To dampen natural temperature fluctuations, the cloak isolates the car --
or any other object beneath it -- from the surrounding environment. The
cloak has two components: an outer layer which efficiently reflects
sunlight and an inner layer that traps heat inside. Whatever heat the
outer layer does absorb is emitted in such a way that it can be readily dissipated to outer space. This design earns it the name of Janus thermal cloak, inspired by the two-faced Roman god Janus.

"The cloak works basically the same way the earth cools down, through
radiative cooling" says Cui. "The earth is covered by the atmosphere,
and the atmosphere is transparent to a certain range of electromagnetic
energy we radiate." While this process is desirable in the summer, it
would make the car colder during winter months. "You have to develop
something that can turn on and off by itself without external energy
input, and that's extremely difficult," says Cui.

Cui and his team designed the cloak to automatically counteract this
effect in the winter. The cloak employs an effect called "photon
recycling" - - essentially, any energy that is trapped under the cloak
will bounce back and forth between the car and cloak rather than escape
to the surroundings outside.

To assess the performance of the thermal cloak, the researchers conducted
tests on electric vehicles parked outside under typical ambient conditions
in Shanghai. While the cabin temperature of an uncovered car reached
50.5DEGC at mid-day, the cabin of the cloak-covered car reached 22.8DEGC
-- 27.7DEGC lower than the uncovered car and 7.8DEGC lower than the
temperature outside. At midnight, the covered car stayed 6.8DEGC higher
than the temperature outside, never dropping below 0DEGC.

"This is the first time that we could achieve warming above the ambient temperature by almost 7DEGC 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." The outer component of the cloak is made of
thin fibers of silica that were then coated in flakes of hexagonal boron nitride, a ceramic material similar to graphite that enhances the fibers'
solar reflectivity. These fibers are then braided and woven together into
a fabric and adhered to the inner layer, which is made of aluminum alloy.

The team purposefully designed the cloak to make scaling up production
easier in the future. For example, using thinner silica fibers would have increased solar reflectivity, but they would be weaker and couldn't be
made with high- volume, industrial-level production techniques already available. In addition, the materials used, including the aluminum,
silica, and boron nitride, are all low-cost and make the cloak
lightweight, durable, and fire-retardant.

* RELATED_TOPICS
o Matter_&_Energy
# Vehicles # Physics # Automotive_and_Transportation #
Quantum_Physics
o Earth_&_Climate
# Energy_and_the_Environment # Renewable_Energy #
Environmental_Issues # Environmental_Science
* RELATED_TERMS
o Battery_electric_vehicle o Season o Temperature o Fuel_cell o
2005_Atlantic_hurricane_season o Electric_power o Hybrid_vehicle
o Hurricane_Camille

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========================================================================== Journal Reference:
1. Huaxu Qiao, Zhequn Huang, Jianming Wu, Jie Shen, Heng Zhang, Qixiang
Wang, Wen Shang, Wei Tang, Tao Deng, Hongxing Xu, Kehang
Cui. Scalable and durable Janus thermal cloak for all-season
passive thermal regulation. Device, 2023; 100008 DOI:
10.1016/j.device.2023.100008 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230711133111.htm

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