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A new way of cooling without power

Two versions of the device designed by MIT researchers, with a metal strip to block direct sunlight, were built and…

Two versions of the device designed by MIT researchers, with a metal strip to block direct sunlight, were built and tested on the roof of an MIT building to confirm that they could cool well below ambient temperature. Credit: Bikram Bhatia

MIT scientist has designed a new way of cooling on a hot sunny day, with cheap materials and requires no fossil fuel-produced power. The passive system, which could be used to supplement other cooling systems to conserve hot and hot food and medicine, is essentially a high-tech version of a parasol.

The system allows heat emissions at the midfield infrared light area that can pass straight through the atmosphere and radiate into the outer cold and punch through the gases that act as a greenhouse. To prevent heating in direct sunlight, a small metal strip above the unit blocks a sunbeam.

The new system is described this week in the journal Nature Communications in a paper by scientist Bikram Bhatia, graduate student Arny Leroy, Professor of Mechanical Engineering and Department Manager Evelyn Wang, Professor of Physics Marin Soljacic and six others at MIT.

Theoretically, the system allows the design to cool as much as 20 degrees Celsius (36 degrees Fahrenheit) below ambient temperature in a location like Boston, researchers say. So far, in their original test-of-concept test, they have achieved a cooling of 6 C (about 1

1 F). For applications that require even more cooling, the residue can be achieved by conventional cooling systems or thermoelectric cooling.

Other groups have attempted to design passive cooling systems radiating heat in the form of mid-infrared wavelengths, but these systems are based on complexly constructed photonic devices that can be expensive to do and not available for widespread use, researchers say. The devices are complex because they are designed to reflect all wavelengths of sunlight almost perfect and only to emit radiation in the middle infrared area, for the most part. The combination of selective reflectivity and emissivity requires a multi-layer material where the thickness of the layers is controlled to nanometer precision.

However, it appears that similar selectivity can be achieved by simply blocking direct sunlight with a narrow strip located at just the right angle to cover the sun’s way across the sky, which requires no active tracking of the device. Then a simple device made of a combination of cheap plastic film, polished aluminum, white paint and insulation can enable necessary heat emissions through mid-infrared radiation, which is how most natural objects cool while protecting the device from being heated by direct sunlight. In fact, simple radiative cooling systems have been used since antiquity to achieve night cooling; The problem was that such systems did not work during the daytime because the heat output of the sunlight was at least 10 times stronger than the maximum achievable cooling effect.

But the sun’s radiant rays move in straight lines and are easily blocked – as we experience, for example by entering the shade of a tree on a hot day. By shading the device by essentially putting an umbrella over it and completing it with insulation around the device to protect it from ambient air temperature, the researchers made passive cooling more viable.

“We built the layout and experimented outdoors with an MIT ceiling,” said Bhatia. “It was done with very simple materials” and clearly demonstrated the system’s efficiency.

“It’s kind of deceptively easy,” said Wang. a separate shade and an emitter to the atmosphere – two separate components that can be relatively inexpensive – the system requires no special ability to deliver and absorb selectively. We use angle selectivity to enable blocking of direct sun as we continue to emit the heat-carrying wavelengths to the sky. “

This project” inspired us to rethink the use of “nuance,” said Yichen Shen, a research-related and co-author of the newspaper. “Previously, people only thought about using it to reduce warming. But now we know if the shadow is used smart along with a little supportive light filtering, it can actually be used to cool the object down,” he says.

A limiting factor for the system is humidity in the atmosphere, says Leroy, which can block some of the infrared emissions through the air. In a place like Boston, near the ocean and relatively humid, this limits the total cold that can be achieved, limiting it to about 20 degrees Celsius. drier environments like the southwest of the United States or many desert or dry environments around the world, the maximum achievable cooling can actually be much bigger, he points out, potentially as much as 40C (72F).

While most research around radiant cooling has focused on larger systems that can be used to cool entire rooms or buildings. This method is more located, Wang says: “This would be useful for cold applications, such as cooking or vaccines. “In fact, the protection of vaccines and other drugs from destruction in hot, tropical conditions has been a major ongoing challenge that this technology may be well positioned to deal with.

Although the system was not enough to lower the temperature all the way to needing levels, “it could at least reduce the load” on the electric cooling systems, to give only the last bit of cooling, “said Wang.

The system may also be useful for certain types of concentrated solar cells, where mirrors are used to focus sunlight on a solar cell to increase its efficiency. However, such systems can easily overheat and generally require active thermal handling with liquids and pumps. Instead, the back of such concentration systems may be equipped with the mid-infrared emissive surfaces used in the passive cooling system and can control the heating without any active intervention.

As they continue to work on improving the system, the biggest challenge is finding ways to improve the insulation of the device, to prevent it from warming too much from the surrounding air while not blocking its ability to radiate heat . “The biggest challenge is finding insulating materials that would be infrared transparent,” says Leroy.

The team has applied for a patent for the invention and hope that it can begin to find real applications fairly quickly.


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More information :
Bikram Bhatia et al., Passive directed sub-surrounding day radiative cooling, Nature Communications (2018). DOI: 10,1038 / s41467-018-07293-9


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Massachusetts Institute of Technology

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