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John O | December 2018

Researchers design passive cooling system that could provide refrigeration for off-grid locations


By Josh Perry, Editor
[email protected]

 

Researchers at the Massachusetts Institute of Technology (MIT) have devised a new passive cooling solution that uses inexpensive materials, requires no fossil fuel-generated power, and that could provide refrigeration for food or medicine in locations with no access to the power grid.

 


Two versions of the device designed by MIT researchers, using a strip of metal to block direct sunlight, were built and tested on the roof of an MIT building. (Bikram Bhatia/MIT)

 

According to a report from the university, “The system allows emission of heat at mid-infrared range of light that can pass straight out through the atmosphere and radiate into the cold of outer space, punching right through the gases that act like a greenhouse. To prevent heating in the direct sunlight, a small strip of metal suspended above the device blocks the sun’s direct rays.”

 

The system can lower temperatures as much as 20°C below ambient, although proof-of-concept prototypes have achieved only 6°C of cooling so far.

 

This is considered a breakthrough for passive cooling systems because previous attempts have required expensive photonic devices, but the MIT researchers achieved the same level of performance by blocking direct sunlight with a strip placed at a specific angle.

 

“Then, a simple device built from a combination of inexpensive plastic film, polished aluminum, white paint, and insulation can allow for the necessary emission of heat through mid-infrared radiation, which is how most natural objects cool off, while preventing the device from being heated by the direct sunlight,” the article noted.

 

Essentially, researchers covered the device with an umbrella and used insulation to protect the device from ambient temperatures. Humidity in the atmosphere is a limiting factor for the device’s performance, but researchers believe the device could reach as much as 40°C cooling in desert environments.

 

The research was recently published in Nature Communications. The abstract stated:

 

“Demonstrations of passive daytime radiative cooling have primarily relied on complex and costly spectrally selective nanophotonic structures with high emissivity in the transparent atmospheric spectral window and high reflectivity in the solar spectrum.

 

“Here, we show a directional approach to passive radiative cooling that exploits the angular confinement of solar irradiation in the sky to achieve sub-ambient cooling during the day regardless of the emitter properties in the solar spectrum. We experimentally demonstrate this approach using a setup comprising a polished aluminum disk that reflects direct solar irradiation and a white infrared-transparent polyethylene convection cover that minimizes diffuse solar irradiation.

 

“Measurements performed around solar noon show a minimum temperature of 6 °C below ambient temperature and maximum cooling power of 45 W m–2.

 

“Our passive cooling approach, realized using commonly available low-cost materials, could improve the performance of existing cooling systems and enable next-generation thermal management and refrigeration solutions.”

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