Thermal cloak keeps electric vehicles cool in the summer, warm in winter

When electric vehicles are parked outside, their temperature can swing widely from day to night and season to season, which can lead to deterioration of the car’s battery. To mitigate such fluctuations and extend the battery’s lifespan, a team of researchers led by the Shanghai Jiao Tong University (SJTU) in China has recently designed an all-season thermal cloak that can cool electric vehicles by 8°C on a hot day and warm it by 6.8°C on cold, winter nights. 

This cloak, build mainly of silica and aluminum, can achieve this passively, without any outside energy inputs, while efficiently operating in both hot and cold weather.

“The thermal cloak is like clothes for vehicles, buildings, spacecrafts, or even extraterrestrial habitats to keep cool in summer and warm in winter,” said senior author Kehang Cui, a materials scientist at SJTU.

Janus thermal cloak

In order to dampen natural variations in temperature, this so-called “Janus thermal cloak” (a name inspired by the two-faced Roman god Janus) can efficiently isolate cars or other objects from the surrounding environment due to its innovative two-component design, consisting of an outer layer that reflects sunlight and an inner one that traps heat inside. The heat the outer layer absorbs can be readily dissipated to outer space.

“The cloak works basically the same way the earth cools down, through radiative cooling,” Cui explained. “The earth is covered by the atmosphere, and the atmosphere is transparent to a certain range of electromagnetic energy we radiate.”   

However, while this process is desirable in the summer, it would make the vehicles colder during winter months. As Cui puts it, “you have to develop something that can turn on and off by itself without external energy input, and that’s extremely difficult.” 

Photon recycling 

To automatically counteract the winter cooling effect, the experts made use of a phenomenon called “photon recycling,” in which any energy that is trapped under the cloak will bounce back between the car and cloak instead of escaping to the surroundings outside.

To assess the cloak’s performance, the researchers conducted a series of tests on electric vehicles parked outside in Shanghai. The experiments revealed that, while the cabin temperature of an uncovered car reached up to 50.5°C at mid-day during summer, the cabin of the cloak-covered car reached 22.8°C—27.7°C lower than the uncovered car and 7.8°C lower than the temperature outside. Moreover, during cold nights the covered car stayed 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 night. This is also kind of surprising to us – there’s no energy input or sunshine and we can still get warming,” Cui said.

Low-cost materials 

The scientists purposefully designed the cloak to make scaling up production easier. Its outer component is made of thin fibers of silica, which are coated in flakes of hexagonal boron nitride, a ceramic material similar to graphite which increases the fibers’ solar reflectivity. These fibers are then braided and woven together into a fabric and adhered to the aluminum alloy-based inner layer. 

All of these materials are low-cost and make the cloak durable, lightweight, and fire-retardant. Although using even thinner silica fibers would have increased solar reflectivity, they would be weaker and could not be made with the high-volume, industrial-level production methods already available.

An in-depth description of this revolutionary cloak can be found in the journal Device.     

More about electric cars

Electric cars, also known as electric vehicles or EVs, run at least partially on electricity. Unlike conventional vehicles that use a gasoline or diesel-powered engine, electric cars and trucks use an electric motor powered by electricity from batteries or a fuel cell. Here are a few points about them:

Types

There are three main types of electric vehicles (EVs), classified by the amount or extent of electricity they use as their energy source.

BEVs, or Battery Electric Vehicles, run entirely on electricity. Most have all-electric ranges of 60 to over 300 miles. When the battery is depleted, it can be recharged from an external source. Examples include the Tesla Model 3 and Nissan Leaf.

PHEVs, or Plug-in Hybrid Electric Vehicles, can recharge the battery through both regenerative braking and “plugging in” to an external source of electrical power. While they can run exclusively on electricity, they also have a gasoline engine for when the battery is depleted. Examples include the Chevrolet Volt and Mitsubishi Outlander PHEV.

HEVs, or Hybrid Electric Vehicles, are powered by both gasoline and electricity. The electric energy is generated by the car’s own braking system to recharge the battery. This is called ‘regenerative braking’, a process where the electric motor helps to slow the vehicle and uses some of the energy normally converted to heat by the brakes. An example is the Toyota Prius.

Charging

Electric vehicles can be charged at home using a standard outlet or a home electric vehicle supply equipment (EVSE), at a public charging station, or at a workplace charging station. The time it takes to charge depends on the size of the battery and the speed of the charging point.

Performance

Electric vehicles are generally quieter than conventional vehicles. They also have faster acceleration but slower top speed compared to conventional vehicles. However, high-end electric cars like the Tesla Roadster have shown top speeds comparable to high-performance gasoline vehicles.

Environmental impact

Electric vehicles produce less pollution than conventional vehicles. They can be zero-emission vehicles if the electric power they use comes from renewable sources (like wind or solar), but even when the power comes from fossil fuels, they are usually more efficient than conventional vehicles.

Economic impact

While the upfront cost of electric vehicles can be higher than traditional cars, they can be cheaper to maintain and fuel. Many countries offer incentives for electric car purchases to offset the initial cost.

Battery

The battery in an electric vehicle is a significant part of the cost. Batteries for these vehicles are large, expensive, and may need to be replaced over the life of the vehicle. However, the technology and manufacturing processes are improving rapidly, resulting in lower costs and better performance.

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By Andrei Ionescu, Earth.com Staff Writer

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