How to keep cool without costing the Earth
A film worth watching
ABOUT 6% of the electricity generated in America is used to
power air-conditioning systems that cool homes and offices. As countries such
as Brazil, China and India grow richer, they will surely do likewise. Not only
is that expensive for customers, it also raises emissions of greenhouse gases
in the form both of carbon dioxide from burning power-station fuel and of the
hydrofluorocarbons air conditioners use as refrigerants.
As they describe in a paper in this week’s Science, Ronggui
Yang and Xiaobo Yin of the University of Colorado, in Boulder, have a possible
alternative to all this. They have invented a film that can cool buildings
without the use of refrigerants and, remarkably, without drawing any power to
do so. Better yet, this film can be made using standard roll-to-roll
manufacturing methods at a cost of around 50 cents a square metre.
The new film works by a process called radiative cooling.
This takes advantage of that fact that Earth’s atmosphere allows certain
wavelengths of heat-carrying infrared radiation to escape into space unimpeded.
Convert unwanted heat into infrared of the correct wavelength, then, and you
can dump it into the cosmos with no come back.
Dr Yang and Dr Yin are not the first to try to cool
buildings in this way. Shanhui Fan and his colleagues at Stanford University,
in California, demonstrated a device that used the principle in 2014. Their
material, though, consisted of seven alternating layers of hafnium dioxide and
silicon dioxide of varying thicknesses, laid onto a wafer made of silicon. This
would be difficult and expensive to manufacture in bulk.
Dr Yang’s and Dr Yin’s film, by contrast, was made of
polymethylpentene, a commercially available, transparent plastic sold under the
brand name TPX. Into this they mixed tiny glass beads. They then drew the
result out into sheets about 50 millionths of a metre (microns) thick, and
silvered those sheets on one side. When laid out on a roof, the silver side is
underneath. Incident sunlight is thus reflected back through the plastic, which
stops it heating the building below.
Preventing something warming up is not, though, the same as
cooling it. The key to doing this is the glass beads. Temperature maintenance
is not a static process. All objects both absorb and emit heat all the time,
and the emissions are generally in the form of infrared radiation. In the case
of the beads, the wavelength of this radiation is determined by their diameter.
Handily, those with a diameter of about eight microns emit predominantly at
wavelengths which pass straight through the infrared “window” in the
atmosphere. Since the source of the heat that turns into this infrared is, in
part, the building below, the effect is to cool the building.
That cooling effect, 93 watts per square metre in direct
sunlight, and more at night, is potent. The team estimates that 20 square
metres of their film, placed atop an average American house, would be enough to
keep the internal temperature at 20°C on a day when it was 37°C outside.
To regulate the amount of cooling, any practical system
involving the film would probably need water pipes to carry heat to it from the
building’s interior. Manipulating the flow rate through these pipes as the
outside temperature varied would keep the building’s temperature steady. Unlike
the cooling system itself, these pumps would need power to operate. But not
much of it. Other than that, all the work is done by the huge temperature
difference, about 290°C, between the surface of the Earth and that of outer
space.
This article appeared in the Science and technology section
of the print edition under the headline "A film worth watching"
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