Passive device relies on a
layer of material that blocks incoming sunlight but lets heat radiate away.
David Chandler
| MIT News Office
October 30, 2019 -- Imagine a device that can sit outside
under blazing sunlight on a clear day, and without using any power cool things
down by more than 23 degrees Fahrenheit (13 degrees Celsius). It almost sounds
like magic, but a new system designed by researchers at MIT and in Chile can do
exactly that.
The device, which has no moving parts,
works by a process called radiative cooling. It blocks incoming sunlight to
keep from heating it up, and at the same time efficiently radiates infrared
light — which is essentially heat — that passes straight out into the sky and
into space, cooling the device significantly below the ambient air temperature.
The key to the functioning of this
simple, inexpensive system is a special kind of insulation, made of a
polyethylene foam called an aerogel. This lightweight material, which looks and
feels a bit like marshmallow, blocks and reflects the visible rays of sunlight
so that they don’t penetrate through it. But it’s highly transparent to the
infrared rays that carry heat, allowing them to pass freely outward.
The new system is described today in a
paper in the journal Science Advances, by MIT graduate student Arny
Leroy, professor of mechanical engineering and department head Evelyn Wang, and
seven others at MIT and at the Pontifical Catholic University of Chile.
Such a system could be used, for
example, as a way to keep vegetables and fruit from spoiling, potentially
doubling the time the produce could remain fresh, in remote places where
reliable power for refrigeration is not available, Leroy explains.
Minimizing heat gain
Radiative cooling is simply the main
process that most hot objects use to cool down. They emit midrange infrared
radiation, which carries the heat energy from the object straight off into
space because air is highly transparent to infrared light.
The new device is based on a concept
that Wang and others demonstrated a year ago, which also used radiative cooling
but employed a physical barrier, a narrow strip of metal, to shade the device
from direct sunlight to prevent it from heating up. That device worked, but it
provided less than half the amount of cooling power that the new system
achieves because of its highly efficient insulating layer.
“The big problem was insulation,” Leroy
explains. The biggest input of heat preventing the earlier device from
achieving deeper cooling was from the heat of the surrounding air. “How do you
keep the surface cold while still allowing it to radiate?” he wondered. The
problem is that almost all insulating materials are also very good at blocking
infrared light and so would interfere with the radiative cooling effect.
There has been a lot of research on ways
to minimize heat loss, says Wang, who is the Gail E. Kendall Professor of
Mechanical Engineering. But this is a different issue that has received much
less attention: how to minimize heat gain. “It’s a very difficult problem,” she
says.
The solution came through the
development of a new kind of aerogel. Aerogels are lightweight materials that
consist mostly of air and provide very good thermal insulation, with a
structure made up of microscopic foam-like formations of some material. The
team’s new insight was to make an aerogel out of polyethylene, the material
used in many plastic bags. The result is a soft, squishy, white material that’s
so lightweight that a given volume weighs just 1/50 as much as water.
The key to its success is that while it
blocks more than 90 percent of incoming sunlight, thus protecting the surface
below from heating, it is very transparent to infrared light, allowing about 80
percent of the heat rays to pass freely outward. “We were very excited when we
saw this material,” Leroy says.
The result is that it can dramatically
cool down a plate, made of a material such as metal or ceramic, placed below
the insulating layer, which is referred to as an emitter. That plate could then
cool a container connected to it, or cool liquid passing through coils in
contact with it, to provide cooling for produce or air or water.
Putting the device to the test
To test their predictions of its
effectiveness, the team along with their Chilean collaborators set up a
proof-of-concept device in Chile’s Atacama desert, parts of which are the
driest land on Earth. They receive virtually no rainfall, yet, being right on
the equator, they receive blazing sunlight that could put the device to a real
test. The device achieved a cooling of 13 degrees Celsius under full sunlight
at solar noon. Similar tests on MIT’s campus in Cambridge, Massachusetts,
achieved just under 10 degrees cooling.
That’s enough cooling to make a
significant difference in preserving produce in remote locations, the
researchers say. In addition, it could be used to provide an initial cooling
stage for electric refrigeration, thus minimizing the load on those systems to
allow them to operate more efficiently with less power.
Theoretically, such a device could
achieve a temperature reduction of as much as 50 C, the researchers say, so
they are continuing to work on ways of further optimizing the system so that it
could be expanded to other cooling applications such as building air
conditioning without the need for any source of power. Radiative cooling has
already been integrated with some existing air conditioning systems to improve
their efficiency.
Already, though, they have achieved a
greater amount of cooling under direct sunlight than any other passive,
radiative system other than those that use a vacuum system for insulation —
which is very effective but also heavy, expensive, and fragile.
This approach could also be a low-cost
add-on to any other kind of cooling system, providing additional cooling to
supplement a more conventional system. “Whatever system you have,” Leroy says,
“put the aerogel on it, and you’ll get much better performance.”
Peter Bermel, an associate professor of
electrical and computer engineering at Purdue University, who was not involved
in this work, says, “The main potential benefit of the polyethylene aerogel
presented here may be its relative compactness and simplicity, compared to a
number of prior experiments.”
He adds, “It might be helpful to
quantitatively compare and contrast this method with some alternatives, such as
polyethylene films and angle-selective blocking in terms of performance (e.g.,
temperature change), cost, and weight per unit area. … The practical benefit
could be significant if the comparison were performed and the cost/benefit
tradeoff significantly favored these aerogels.”
The work was partly supported by an MIT
International Science and Technology Initiatives (MISTI) Chile Global Seed Fund
grant, and by the U.S. Department of Energy through the Solid State Solar
Thermal Energy Conversion Center (S3TEC).
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