In areas where clean water isn't easily accessible,
solar stills can help purify available water that might be dirty
or salty. These devices absorb heat from sunlight and use it to
evaporate water, leaving behind contaminants and reforming as a
liquid in a separate container, and although they work, they can
be relatively expensive and inefficient. Researchers have now
developed a new type of solar still using carbon-coated paper
that they say is cheaper and more than twice as efficient as
existing devices.
Solar stills can be live-saving devices for people
in developing countries or disaster-affected areas, but there's
room for improvement according to the team made up of members
from the University at Buffalo (UB), China's Fudan University
and the University of Wisconsin-Madison.
"People lacking adequate drinking water have
employed solar stills for years, however, these devices are
inefficient," says Haomin Song, a co-author of the study. "For
example, many devices lose valuable heat energy due to heating
the bulk liquid during the evaporation process. Meanwhile,
systems that require optical concentrators, such as mirrors and
lenses, to concentrate the sunlight are costly."
To overcome these issues, the researchers built
their own version of the device. The "solar vapor generator," as
they call it, is about the size of a mini-fridge and is made of
expanded polystyrene foam and a porous, hydrophilic paper coated
in carbon black. By floating the device on a body of water, the
carbon black absorbs sunlight, to heat and evaporate the water
absorbed by the paper.
Rather than heating the bulk of a body of water, the
new device focuses its energy on just the surface water, which
evaporated at 44° C (111° F). That allows the still to reach a
reported efficiency of 88 percent, which the team believes is a
record for thermal efficiency. As a result, the device could
produce between 3 and 10 liters (0.8 and 2.6 US gal) of purified
water per day, compared to the 1 to 5 liters (0.3 to 1.3 US gal)
per day possible with most commercial stills of comparable size
currently available.
"Using extremely low-cost materials, we have been
able to create a system that makes near maximum use of the solar
energy during evaporation," says Qiaoqiang Gan, PhD, associate
professor of electrical engineering in the University at Buffalo
School of Engineering and Applied Sciences and lead researcher
on the study. "At the same time, we are minimizing the amount of
heat loss during this process."
Where systems that use lenses and mirrors to
concentrate the sunlight can cost upwards of US$200 per square
meter, the team claims that its new device could be put together
for around $1.60 per square meter – a much more manageable price
tag for people in need.
"The solar still we are developing would be ideal
for small communities, allowing people to generate their own
drinking water much like they generate their own power via solar
panels on their house roof," says Zhejun Liu, co-author of the
study.
The research was published in the journal
Global Challenges.