According to the US. Department of Energy, heating and cooling
accounts for 48 percent of residential energy usage, making it the largest
expense in most homes. Air conditioning accounts for 6 percent of the total electricity
generated in the U.S., contributing roughly 117 million metric tons of carbon
dioxide to the atmosphere.
Cool air is truly essential in many cases,
such as food refrigeration,
healthy environments for children and the elderly or in datacenters.
Radiative sky cooling is a method of
transferring heat with little to no power cost. The earth’s surface constantly radiates
heat into space, simply due to the second law of thermodynamics. However, this
radiant heat is blocked by clouds and particulates, and during the day the heat
input by the sun far exceeds the heat released to space. On a cloudless night,
the cooling effect of radiant heat is greatest, because these obstacles are out
of the way.
Radiative sky cooling panels, with running water through tubes underneath. (Image courtesy of Aaswath Raman)
However, if radiant sky cooling is to be
useful, it needs to be effective during the day, too.
Since 2013, a professor of electrical
engineering at Stanford University, Shanhui Fan, and his team have been working on a
mirror-like optical coating that lets radiant heat out, but not in.
In 2014, the team published research proving that their multilayer silicon dioxide and hafnium oxide material
can cool itself. Now, the team has proven that a system of panels can cool
flowing water to a temperature lower than the surrounding air. "It's very
intriguing to think about the universe as such an immense resource for cooling and
all the many interesting, creative ideas that one could come up with to take
advantage of this," Fan said.
The researchers built a system where panels
covered in the specialized optical surfaces sat atop pipes of running water, and
ran tests on the roof of the David Packard Electrical Engineering Building at
Stanford’s California campus in September 2015. These panels were just over two
feet in length on each side, and the team ran as many as four panels at a time.
With the water moving at a relatively fast rate, they found the panels were
able to consistently reduce the temperature of the water three to five degrees
Celsius below the ambient air temperature over a period of three days.
"This research builds on our previous
work with radiative sky cooling, but takes it to the next level. It provides
for the first time a high-fidelity technology demonstration of how you can use
radiative sky cooling to passively cool a fluid and, in doing so, connect it
with cooling systems to save electricity," said Aaswath Raman, one of
Fan’s associates working on the project. Passively cooling water with little to
no input electricity could have big implications for the U.S. power grid.
In a computer simulation of this system operating
on the roof of a two-story office building in Las Vegas, where the hot, dry
conditions are ideal for this technology, the panel-cooled system saved 14.3
megawatt-hours of electricity, a 21 percent reduction in the electricity used
to cool the building. In the simulation, the special optical panels covered the
roof, with the chilled water cooling a vapor-compression system. Over the
entire period, the daily electricity savings fluctuated from 18 percent to 50
percent.
Fan and his associates, Eli Goldstein and
Aaswath Raman, have founded the company SkyCool
Systems, under which they are further testing and commercializing their technology.
Next steps for SkyCool include making their
panels integrate easily with standard air conditioning and refrigeration
systems. Fan and his team are
particularly excited at the prospect of applying their technology to the
serious task of cooling data centers.
Improving the
efficiency of datacenters is a major
goal for companies like Facebook, Google and HP. Hit the links to learn
more about power-saving engineering at these companies.