Cooling Without Electricity
A thin film to radiate infrared heat, couples radiative properties with reflective ones, to throw back nearly all the heat in sunlight.
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Transcript
Today,
it's estimated that worldwide electricity costs total roughly $2.5 trillion a
year and that cooling systems consume 15% of that electricity. With experts
forecasting demand for cooling to grow ten-fold by 2050, that means electricity
expenditure for cooling alone could rise to nearly $4 trillion per year.
Therefore, improving the efficiency of cooling systems is a critical part of
the twenty-first-century energy challenge.
Fortunately,
there appears to an extremely clever and cost-effective way of doing just that.
Here's how it works.
All
objects give off heat in the form of thermal radiation. But the air around
them, mainly because of water molecules, absorbs and radiates back most of that
heat. However, a sliver of those emissions in the mid-infrared range, can slip
past these compounds, enabling surfaces that emit radiation at those wavelengths to become cooler than the
surrounding air.
A team of Stanford researchers developed a thin film tuned to
radiate infrared heat in exactly this band. Then, in an even bigger advance,
they coupled those radiative properties with reflective ones, enabling the
materials to throw back nearly all the heat in sunlight. That's crucial because
without this reflective capability, the sun would more than offset the
radiative cooling effect during the daytime.
Recently,
the team demonstrated that retrofitting radiative panels to an office building
could cut its cooling electricity needs by 21 percent in summer. Extrapolated
to the expected global electricity demand for cooling in 2050, that amounts to
roughly $800 billion a year.
To
commercialize this technology, team members Shanhui Fan, Aaswath Raman and Eli Goldstein, founded a
company called SkyCool Systems.
The
underlying scientific phenomenon called "radiative sky cooling" is a natural
process that everyone and everything does, when their molecules release heat.
You can witness it for yourself in the heat that comes off a road as it cools
after sunset. This phenomenon is particularly noticeable on a cloudless night
because, without clouds, the heat we and everything around us radiates can more
easily make it through Earth's atmosphere, all the way to the vast, cold
reaches of space.
If
you have something that is very cold, like outer space, and you can dissipate
heat into it, then you can do cooling without any electricity or work. The heat
just flows! For this reason, the amount of heat continuously flowing off the
Earth into the universe is enormous.
But
on a hot, sunny day, radiative sky cooling doesn't work that well for the human
body or for most other objects. This is because sunlight will warm them more
than radiative sky cooling will cool them. To overcome this problem, the
SkyCool team created a surface using a multilayer optical film that reflects
about 97 percent of the sunlight while simultaneously being able to emit the
surface's thermal energy through the atmosphere.
Without
absorbing heat from the sunlight, the radiative sky cooling effect can enable
cooling below the air temperature even on a sunny day.
That
means we're no longer limited by what the air temperature is, we're limited by
something much colder: the temperature of outer space.
The
first experiments published in 2014 were performed using small wafers of a
multilayer optical surface, about 8 inches in diameter, and only showed how the
surface itself cooled.
Naturally, the next step was to scale up the technology
and see how it works as part of a larger cooling system.
In
their late 2017 paper in Nature Energy, the
researchers described a system where panels covered in the specialized optical
surfaces sat atop pipes of running water and tested it on the roof of the
Packard Building at Stanford
They
also applied data from this experiment to a simulation where their panels
covered the roof of a two-story commercial office building in Las Vegas - a
hot, dry location where their panels would work best. They calculated how much
electricity they could save if, in place of a conventional air-cooled chiller,
they used a vapor-compression system with a condenser cooled by their panels.
They found that, in the summer months, the panel-cooled system would save 14.3
megawatt-hours of electricity, a 21 percent reduction in the electricity used
to cool the building. Over the entire period, the daily electricity savings
fluctuated from 18 percent to 50 percent.
Right
now, SkyCool Systems is measuring the energy saved when panels are integrated
with traditional air conditioning and refrigeration systems at a test facility,
and Fan, Goldstein and Raman are optimistic that this technology will find
broad applicability in the years to come.
But,
according to Nick Fernandez, an energy analyst at the Pacific Northwest
National Laboratory, far larger energy savings may be possible for developers
who opt to incorporate radiative cooling systems directly into new buildings during the design phase.
According
to a simulation analysis published
in 2015, on which Fernandez
was the lead author, if the system were coupled with a hydronic radiant cooling
system, a rare but highly-efficient way of cooling buildings that works by
circulating water instead of blowing air, the energy savings for heating,
cooling, and ventilation could reach nearly 70 percent in ideal climate
conditions.
Translated
into dollars and cents, that could mean global electricity savings in 2050
totaling around $2.5 trillion a year.
Given
this trend, we offer the following forecasts for your consideration.
First, the adoption
rate for radiative cooling systems will vary greatly depending on climate and
type of construction.
The
Pacific Northwest Lab study estimated that if a retrofit rooftop radiator of
the type SkyCool is developing could be produced and installed for less than 58
cents per square foot, the energy savings would cover those costs in about five
years based on typical savings. Buildings with a large roof area in hot, dry
climates are ideal. The southwest United States and the Middle East are obvious
targets. The Pacific Northwest and the UK are less appealing near-term targets.
Second, SkyCool will
not be the only firm developing products designed to address the enormous
opportunity in radiative sky cooling systems.
In
February 2018, a team of engineers at the University of Colorado, Boulder,
published a paper in Science describing
a glass-polymer hybrid material that achieved "noon-time radiative cooling power
of 93 watts per square meter under direct sunshine."
According to a university publication, the CU
Boulder researchers stressed that they've already figured out how to affordably
manufacture rolls of the film-like material, "making it a potentially viable
large-scale technology for both residential and commercial applications." Like
the Stanford team, the Boulder researchers raised money from ARPA-E, applied
for a patent, and formed a company, which is called Radi-Cool.
According to Ronggui Yang, a professor
of mechanical engineering, who is a coauthor of the paper and acting CEO of the
startup, the CU Boulder scientists are now in talks with potential investors
and manufacturers. And,
Third, with trillions
in potential savings, this is precisely the kind of "green technology" that
will capture the imagination of policy-makers, consumers and investors.
Unlike
many so-called, "environmentally-friendly solutions," radiative sky cooling
technology reduces costs, conserves finite resources, cleans up the environment
and improves the lives of consumers. It's the kind of win-win innovation that
benefits everyone.
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