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Alternative technology
<< Continued from page 37
In ambient sunlight, the researchers found that, while the
black graphite structure absorbed sunlight well, it also
tended to radiate heat back out into the environment. To
minimise the amount of heat loss, the team looked for
materials that would better trap solar energy.
The secret’s in the bubbles
In their new design, the researchers settled on a spectrally
selective absorber: a thin, blue, metallic-like film that is
commonly used in solar water heaters and possesses
unique absorptive properties. The material absorbs
radiation in the visible range of the electromagnetic
spectrum, but it does not radiate in the infrared range,
meaning that it both absorbs sunlight and traps heat,
thereby minimising heat loss.
The researchers used a thin sheet of copper, chosen for its
heat-conducting abilities, and coated it with the spectrally
selective absorber. They then mounted the structure on a
thermally insulating piece of floating foam. However, they
found that even though the structure did not radiate much
heat back out into the environment, heat was still escaping
through convection, in which moving air molecules —
such as wind — would naturally cool the surface.
A solution to this problem came from an unlikely source:
Chen’s 16-year-old daughter, who at the time was working
on a science fair project in which she constructed a
makeshift greenhouse from simple materials, including
bubble wrap.
“She was able to heat it to 160°F [71.11°C], in winter!”
Chen says. “It was very effective.”
Chen proposed the packaging material to Ni as a costeffective way to prevent heat loss by convection. This
approach would let sunlight in through the material’s
transparent wrapping, while trapping air in its insulating
bubbles.
Science Daily
MIT graduate student,
George Ni, holding the device.
“I was very sceptical of the idea at first,” Ni recalls. “I
thought it was not
a high-performance
material. But we
tried the clearer
bubble wrap with
bigger bubbles for
more air trapping
effect and, as it
turns out, it worked.
Now because of this
bubble wrap, we
don’t need mirrors to
concentrate the sun.”
November 2016 Volume 22 I Number 9
The bubble wrap, combined with the selective absorber,
prevented heat from escaping the surface of the sponge.
Once the heat was trapped, the copper layer conducted
the heat towards a single hole (or channel) that the
researchers had drilled through the structure. When they
placed the sponge in water, they found that water crept
up the channel, where it was heated to 100°C and then
turned to steam.
Tao Deng, professor of material sciences and engineering
at Shanghai Jiao Tong University in Ch ina, says the
researchers’ use of low-cost materials will make the
device more affordable for a wide range of applications.
“This device offers a totally new design paradigm for solar
steam generation,” says Deng, who was not involved in
the study. “It eliminates the need for the expensive optical
concentrator, which is a key advantage in bringing down
the cost of the solar steam generation system. Certainly
the clever use of bubble wrap and commercially available
selective absorber also helps suppress the convection
and radiation heat loss, both of which not only improve
the solar harvesting efficiency, but also further lower the
system cost.”
Chen and Ni say that solar absorbers based on this general
design could be used as large sheets to desalinate small
bodies of water or to treat wastewater. Ni says other
solar-based technologies that rely on optical-concentrating
technologies typically are designed to last 10–20 years,
though they require expensive parts and maintenance. This
new, low technology design, he says, could operate for one
to two years before needing to be replaced.
“Even so, the cost is pretty competitive,” Ni says. “It’s kind
of a different approach, where before, people were doing
high technology and long term [solar absorbers]. We’re
doing low technology and short term.”
“What fascinates us is the innovative idea behind this
inexpensive device, where we have creatively designed
this device based on a basic understanding of capillarity
and solar thermal radiation,” says Zhang. “Meanwhile, we
are excited to continue probing the complicated physics of
solar vapour generation and to discover new knowledge for
the scientific community.”
This research was funded, in part, by a co-operative
agreement between the Masdar Institute of Science
and Technology and MIT, as well as by the Solid-State
Solar Thermal Energy Conversion Centre, which is an
Energy Frontier Research Centre funded by the US
Department of Energy. PA
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