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Scientists devise simple passive cooling

Two versions of the device, using a strip of metal to block direct sunlight. Photo by Bikram Bhatia

USA: Researchers at MIT claim to have devised a new way of providing cooling on a hot sunny day, using inexpensive materials and requiring no fossil fuel-generated power.

The passive system, which they say could be used to supplement traditional air conditioning and refrigeration systems, is described as a high-tech version of a parasol.

The system is said to allow the 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. Unlike other radiative cooling systems, the MIT researchers claim their idea is both simple and inexpensive.

Other groups have attempted to design passive cooling systems that radiate heat in the form of mid-infrared wavelengths of light, but these systems have been based on complex engineered photonic devices that can be expensive to make and not readily available for widespread use, the researchers say. They claim that the devices are complex because they are designed to reflect all wavelengths of sunlight almost perfectly, and only to emit radiation in the mid-infrared range, for the most part. That combination of selective reflectivity and emissivity requires a multilayer material where the thicknesses of the layers are controlled to nanometer precision.

The MIT researchers claim that similar selectivity can be achieved by simply blocking the direct sunlight with a narrow strip placed at just the right angle to cover the sun’s path across the sky, requiring no active tracking by the device. Then, a simple device built from a combination of inexpensive plastic film, polished aluminium, 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.

They stress that simple radiative cooling systems have been used since ancient times to achieve nighttime cooling. However, such systems didn’t work in the daytime because the heating effect of the sunlight was at least 10 times stronger than the maximum achievable cooling effect.

But, as the sun’s heating rays travel in straight lines, shading the device by essentially putting an umbrella over it, and supplementing that with insulation around the device to protect it from the ambient air temperature, is said to make passive cooling more viable.

It’s claimed that the system could provide cooling of as much as 20ºC below ambient in a location like Boston. So far, in their initial proof-of-concept testing, they have achieved a cooling of 6ºC. For applications that require even more cooling, the remainder could be achieved through conventional refrigeration systems or thermoelectric cooling.

A limiting factor is humidity in the atmosphere, which can block some of the infrared emission through the air. In drier environments, such as the southwestern US or many desert or arid environments around the world, the maximum achievable cooling could actually be much greater, the researchers claim, potentially as much as 40ºC.

“This would be useful for refrigeration applications, such as food storage or vaccines,” said professor of mechanical engineering and department head Evelyn Wang. Even if the system wasn’t sufficient to bring down the temperature all the way to needed levels, “it could at least reduce the loads” on the electrical refrigeration systems, Wang added.

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