Cooling without power

Print edition : January 04, 2019

Two versions of the device designed by MIT researchers, built and tested on the roof of an MIT building as proof of concept.

RESEARCHERS at the Massachusetts Institute of Technology (MIT) have devised a new way of providing cooling using inexpensive materials and no fossil fuel-based power. The passive system is essentially a high-tech version of an umbrella and can be used to supplement other cooling systems to preserve food and medications in hot, off-grid locations. The system allows emission of heat at mid-infrared range of light that passes through the atmosphere and radiates into the cold of outer space, unhindered by gases that act like a greenhouse.

The new system is described in a recent issue of“Nature Communications” by a team led by Bikram Bhatia. In theory, it could provide cooling of as much as 20°C below the ambient temperature in a location such as Boston (-23 °C). So far, in the initial proof-of-concept testing, a cooling of 6°C was achieved. Other groups that have tried to design passive cooling systems that radiate heat in the form of mid-infrared wavelengths of lightthat are based on expensive photonic devices. These devices are designed to reflect all wavelengths of sunlight, and only to emit radiation in the mid-infrared range. That combination of selective reflectivity and emissivity requires a multilayer material where thicknesses are controlled to nanometer precision.

Similar selectivity can be achieved by blocking direct sunlight with a narrow strip placed at the right angle to cover the sun’s path across the sky, requiring no active tracking by the device. This is like putting an umbrella over it, and by supplementing that with insulation around the device to protect it from the ambient air temperature, the researchers have made passive cooling more viable.

A simple device built from a combination of inexpensive plastic film, polished aluminium, white paint and insulation allows for the emission in the mid-infrared, which is how most natural objects cool off, while preventing the device from being heated by direct sunlight, which can be up to 10 times the cooling effect during daytime. “We built the setup and did outdoors experiments on an MIT rooftop,” said Bhatia. “It’s kind of deceptively simple,” said Evelyn Wang of the group. “By having a separate shade and an emitter to the atmosphere the system doesn’t require a special ability to emit and absorb selectively.”

One limiting factor, according to Arny Leroy of the team, is humidity in the atmosphere, which can block some of the infrared emission through the air. That is why in a place like Boston, the cooling gets limited to about 20° C. But in drier environments, the maximum achievable cooling could be as much as 40°C.

While most of the research on radiative cooling has focussed on large systems that are applicable to cooling entire rooms or buildings, this approach is localised, said Wang. “This would be useful for refrigeration applications, such as food storage or vaccines,” Wang added.

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