Device
developed at MIT could provide refrigeration for off-grid locations.
By David L. Chandler | MIT News Office
By David L. Chandler | MIT News Office
November 28, 2018 -- IT
researchers 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 could be used to supplement other cooling systems to
preserve food and medications in hot, off-grid locations, is essentially a
high-tech version of a parasol.
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 new system is described this
week in the journal Nature Communications in a paper by research
scientist Bikram Bhatia, graduate student Arny Leroy, professor of mechanical
engineering and department head Evelyn Wang, professor of physics Marin
Soljačić, and six others at MIT.
In theory, the system they designed
could provide cooling of as much as 20 degrees Celsius (36 degrees Fahrenheit)
below the ambient temperature in a location like Boston
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. 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.
But it turns out 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 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. In fact, simple
radiative cooling systems have been used since ancient times to achieve
nighttime cooling; the problem was that 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 the sun’s heating rays travel
in straight lines and are easily blocked — as we experience, for example, by
stepping into the shadow of a tree on a hot day. By 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, the
researchers made passive cooling more viable.
“We built the setup and did
outdoors experiments on an MIT rooftop,” Bhatia says. “It was done using very
simple materials” and clearly showed the effectiveness of the system.
“It’s kind of deceptively simple,”
Wang says. “By having a separate shade and an emitter to the atmosphere — two
separate components that can be relatively low-cost — the system doesn’t
require a special ability to emit and absorb selectively. We’re using angular
selectivity to allow blocking the direct sun, as we continue to emit the
heat-carrying wavelengths to the sky.”
This project “inspired us to
rethink about the usage of ‘shade,’” says Yichen Shen, a research affiliate and
co-author of the paper. “In the past, people have only been thinking about
using it to reduce heating. But now, we know if the shade is used smartly
together with some supportive light filtering, it can actually be used to cool
the object down,” he says.
One limiting factor for the system
is humidity in the atmosphere, Leroy says, which can block some of the infrared
emission through the air. In a place like Boston
While most research on radiative
cooling has focused on larger systems that might be applied to cooling entire
rooms or buildings, this approach is more localized, Wang says: “This would be
useful for refrigeration applications, such as food storage or vaccines.”
Indeed, protecting vaccines and other medicines from spoilage in hot, tropical
conditions has been a major ongoing challenge that this technology could be
well-positioned to address.
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, to
provide just the final bit of cooling, Wang says.
The system might also be useful for
some kinds of concentrated photovoltaic systems, where mirrors are used to
focus sunlight on a solar cell to increase its efficiency. But such systems can
easily overheat and generally require active thermal management with fluids and
pumps. Instead, the backside of such concentrating systems could be fitted with
the mid-infrared emissive surfaces used in the passive cooling system, and
could 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 heating up too much from the
surrounding air, while not blocking its ability to radiate heat. “The main
challenge is finding insulating material that would be infrared-transparent,”
Leroy says.
The team has applied for patents on
the invention and hope that it can begin to find real-world applications quite
rapidly.
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