Metal That Can Automatically Cool Off Without Consuming Energy

2021-07-12 Academic

Professor Sunkyung Kim at the Department of Applied Physics and his team developed the world's first nanostructure that lowers the surface temperature of metal without consuming energy

In midsummer, the cabin temperature of a vehicle parked outside can rise over 90℃. Metal reflects sunlight well but also absorbs some, which is then converted into heat energy, and metal with its low emissivity can easily heat up after absorbing even a little sunlight. Metal structures, such as automobiles and buildings, require cooling to maintain an appropriate temperature under intense sunlight. Thick heat sinks or wind generators have been frequently used to address this issue.

Professor Kim and his research team adopted the gap plasmon principle from nano-optics. When applied to the surface of metal, gap plasmon resonance should help increase heat emission while maintaining sunlight reflectivity. This hypothesis was proven by their outdoor solar heating experiments. The findings are to be utilized further for designing diverse metal structures, ranging from small electronic devices to large buildings. The research result was published under the title, “Colling Metals via Gap Plasmon Resonance” in the April online issue of Nano Letters, an international academic journal in nanoscience.

Efficient cooling by introducing a sandwich-like array of gap surface plasmon.
The team applied the nano-optical principle of gap plasmon to metal for improved radiated emission. Square metal tiles are placed on a thin dielectric layer that coats the surface of metal plate. The dielectric layer that is sandwiched in-between traps sunlight intensely, and this multilayer sustains gap plasmon cavity modes.

Professor Kim explained: “This structure basically consists of metal with high reflectivity. If designed well, it can also function like a blackbody that exhibits improved radiation. Combining high reflectivity and radiation helps metal remain cool on its own when exposed to sunlight without energy-consuming forced cooling methods.

When outdoor structures are exposed to sunlight, more than half of the thermal radiation is within the 8-13㎛ infrared wavelength range. The team fabricated the gap plasmon cavities by coating a copper plate with zinc sulfide to a thickness of 500 nanometers and then applying square copper tiles on top. Professor Kim said, “In this structure, tiles of the same size emit the same wavelength. Excellent cooling hinges on designing a resonator that emits all the wavelengths in the infrared radiation spectrum of 8 to 13 micrometers.”

Outdoor solar heating experiments predict the cooling effect of at least 10 degrees C in summer.
In the array of gap plasmon cavities, the copper tile size determines the radiation wavelength. Therefore, it seems necessary to introduce as many copper tiles of different sizes as possible. However, many tiles in a confined area cut down distances between adjacent tiles, which reduces overall radiation. Professor Kim noted, "It may be another area that needs distancing. We have confirmed that five sizes of copper tiles arranged on a copper plate produce the best result.”

The team members conducted experiments outdoors on the days with the country’s average winter temperature of 0℃ and witnessed the new structure’s temperature was lower by at least four degrees than that of conventional copper plates. This gap plasmon resonator is much more effective in hot environments. The team’s thermal simulation predicts that the array will lower the metal temperature by at least 10 degrees when the air temperature is about 25℃, a typical summer temperature in Korea.

Professor Kim’s heat dissipation via radiation technology does not consume additional energy for cooling, and the Cu/ZnS/Cu multilayer is sufficiently thin and flexible that it can attached to heating elements in various shapes. Moreover, it does not require maintenance costs. Combined, this new design will become a standard for eco-friendly, low-carbon cooling.

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