Estate Living Magazine The Slow Movement - Issue 39 March 2019 | Page 48

L I v E
use radiative sky cooling to passively cool a fluid and, in doing so,
connect it with cooling systems to save electricity,’ said Raman, who
is co-lead author of the paper detailing this research, published in
Nature Energy on 4 September 2017.
Together, Fan, Goldstein and Raman have founded the company
SkyCool Systems, which is working on further testing and
commercialising this technology.
Sending our heat to space
Radiative sky cooling is a natural process that everyone and
everything does, resulting from the movements of molecules
releasing 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 space – and you can
dissipate heat into it, then you can do cooling without any electricity
or work. The heat just flows,’ explained Fan, who is senior author of
the paper. ‘For this reason, the amount of heat flow off the Earth that
goes to the universe is enormous.’
Although our own bodies release heat through radiative cooling to
both the sky and our surroundings, we all know that on a hot, sunny
day, radiative sky cooling isn’t going to live up to its name. This is
because the sunlight will warm you more than radiative sky cooling
will cool you. To overcome this problem, the team’s surface uses a
multilayer optical film that reflects about 97% of the sunlight while
simultaneously being able to emit the surface’s thermal energy
through the atmosphere. Without heat from sunlight, the radiative
sky cooling effect can enable cooling below the air temperature
even on a sunny day.
‘With this technology, we’re no longer limited by what the air
temperature is; we’re limited by something much colder: the sky
and space,’ said Goldstein, co-lead author of the paper.
The experiments published in 2014 were performed using small
wafers of a multilayer optical surface, about eight 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.
Putting radiative sky cooling to work
For their latest paper, the researchers created a system in which
panels covered in the specialised optical surfaces sat atop pipes
of running water, and tested it on the roof of the Packard Building
S M A R T
in September 2015. These panels were slightly more than two feet
in length on each side, and the researchers ran as many as four 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 3 to 5 degrees Celsius below ambient air temperature over a
period of three days.
The researchers also applied data from this experiment to a
simulation where their panels covered the roof of a two-storey
commercial office building in Las Vegas – a hot, dry location where
their panels would work best – and contributed to its cooling system.
They calculated how much electricity they could save if, in place of
a conventional air-cooled chiller, they used a vapour-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% reduction in the electricity
used to cool the building. Over the entire period, the daily electricity
savings fluctuated from 18% to 50%.
The future is now
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. The researchers are focused on making their panels
integrate easily with standard air-conditioning and refrigeration
systems, and they are particularly excited at the prospect of applying
their technology to the serious task of cooling data centres.
Fan has also carried out research on various other aspects of
radiative cooling technology. He and Raman have applied the
concept of radiative sky cooling to the creation of an efficiency-
boosting coating for solar cells. With Yi Cui, a professor of materials
science and engineering at Stanford, and of photon science at SLAC
National Accelerator Laboratory, Fan developed a cooling fabric.
‘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,’ he said.
Fan is also director of the Edward L. Ginzton Laboratory,
a professor, by courtesy, of applied physics, and an affiliate
of the Stanford Precourt Institute for Energy. This work was
funded by the Advanced Research Projects Agency – Energy
(ARPA-E) of the Department of Energy.
Taylor Kubota/Stanford University News Service