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The system provides electrical cooling



October 30, 2019

(Nanowerk NewsImagine a device that can sit outside in the sunlight on a clear day and use no force without consuming more than 23 degrees Fahrenheit (13 degrees Celsius). It sounds almost like magic, but a new system designed by researchers at MIT and Chile can do exactly that.

The device, which has no moving parts, operates by a process called radiation cooling. It prevents the sunlight from heating up and also efficiently emits infrared light that extends directly into the air and space and cools the device well below ambient air temperature – essentially heat.

The key to the operation of this simple and inexpensive system is a special type of insulation made of polyethylene foam called airgel. This light material, which looks and feels like a little marshmallow, blocks and reflects the visible rays of sunlight so that they cannot pass through it. However, it is very transparent to heat-bearing infrared rays and allows them to pass freely.Insulation material clogs and reflects visible lightIn the photo on the left, a disc of new insulation material blocks and reflects visible light by hiding the MIT logo beneath it. However, as seen in infrared light, the material on the right side is transparent and the logo is visible. (Courtesy of the researchers)

The new system is described in an article in the journal Scientific Developments ("High performance subambient radiation cooling provided by optically selective and thermally insulating polyethylene airgel insulation"), Mny graduate student Arny Leroy, mechanical engineering and department head Evelyn Wang and seven others at MIT and Chile. Pontifical Catholic University.

Such a system, for example, as a way of preventing spoilage of vegetables and fruits, could potentially double the freshness of the product in remote locations where there is no reliable power for cooling, he said.

Minimizing heat gain

Radiative cooling is the main process that most hot objects use to cool. Since they are highly transparent from air to infrared light, they emit moderate infrared radiation that transports heat energy directly from the object into space.

The new device is based on a concept that Wang and others showed a year ago, which also uses a physical barrier that uses radial cooling, but is a metal strip from direct sunlight to prevent the device from heating up. This device worked, but due to the high-efficiency insulation layer, it provided less than half the cooling power of the new system.

Yalıtım The biggest problem was isolation, L Leroy explains. The largest heat input that prevented the previous device from achieving deeper cooling was due to the heat of the surrounding air. Nasıl How do you keep the surface cold while still letting it radiate? Merak he wondered. The problem is that almost all insulation materials are very good at blocking infrared light and therefore interfere with the radiation cooling effect.

Gail E. Kendall Professor of Mechanical Engineering Wang, “Many research has been done on ways to minimize heat loss. However, this is a different issue that is less noticeable: how to minimize heat gain. “It's a very difficult problem, or he says.

The solution came from the development of a new type of airgel. Aerogels are lightweight materials that are mostly airborne and provide very good thermal insulation with a structure consisting of microscopic foam-like formations of some materials. The team's new insight was to make an airgel made of polyethylene, the material used in many plastic bags. The result is a soft, soft, white material that is light enough to weigh only 1/50 of the given volume of water.

The key to its success is that although it blocks more than 90 percent of the incoming sunlight and thus protects the surface beneath it from heating, it is very transparent to infrared light and allows about 80 percent of the heat rays to pass freely. Ik We were very excited when we saw this material, L says Leroy.

The result is that it can cool a plate made of a material, such as metal or ceramic, so as to be placed under the insulating layer called a transmitter. This plate can then cool a container connected to it to provide cooling for the product or air or water, or cool cold contacting through the coils.

Testing the device

To test their predictions of effectiveness, the team, together with Chilean collaborators, set up a proof of concept device in the Atacama desert of Chile, some of the driest in the world. They hardly get any rainfall, but because they are in the equator, they receive the burning sunlight that can put the device into a real test. The device provided cooling at 13 degrees Celsius under full sunlight at noon. Similar tests on MIT's campus in Cambridge, Massachusetts reached a cooling level below 10 degrees.

Researchers say there is enough cooling to make a significant difference in the protection of products in remote areas. In addition, it can be used to provide an initial cooling stage for electrical cooling, thus minimizing the load on these systems to enable them to operate more efficiently with less power.

Theoretically, such a device can achieve a temperature drop of up to 50 ° C, the researchers say, so they continue to work on ways to improve the system; The need for any power source. Radiation cooling is already integrated into existing air conditioning systems to increase their efficiency.

Nevertheless, in direct sunlight they have achieved greater cooling than other passive, irradiated systems, except for those using a vacuum system for insulation – which is very effective but also heavy, expensive and fragile.field test of radiation cooling deviceIn field tests, the performance of the radiation cooling device was measured under full sunlight, with both insulating material in place (left) and without it (right). (Courtesy of the researchers)

This approach may also be a low-cost add-on to another type of cooling system that provides additional cooling to support a more conventional system. “Whatever system you have, L says Leroy, takın install aerogels and you'll get better performance. L

Peter Bermel, an associate professor of electrical engineering and computer engineering at Purdue University, said, “The main potential benefit of the polyethylene airgel presented here may be its relative compactness and simplicity compared to a previous set. experiments. "

Olabilir It may be useful to compare and compare this method quantitatively in terms of performance (eg temperature variation), weight per unit area, and weight with some alternatives, such as polyethylene films and angle selective blocking. … By comparison, the practical benefit may be more important, and the cost / benefit tradeoff has significantly preferred these airogels. "


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