New solar-powered device can pull water straight from the desert air
This new solar-powered device can pull water straight
from the desert air
By Robert ServiceApr. 13, 2017, 2:00 PM
You can’t squeeze blood from a stone, but wringing water
from the desert sky is now possible, thanks to a new spongelike device that
uses sunlight to suck water vapor from air, even in low humidity. The device
can produce nearly 3 liters of water per day, and researchers say future
versions will be even better. That means homes in the driest parts of the world
could soon have a solar-powered appliance capable of delivering all the water
they need, offering relief to billions of people.
The new water harvester is made of metal organic
framework crystals pressed into a thin sheet of copper metal and placed between
a solar absorber (above) and a condenser plate (below). Wang Laboratory at MIT
There are an estimated 13 trillion liters of water
floating in the atmosphere at any one time, equivalent to 10% of all of the
freshwater in our planet’s lakes and rivers. Over the years, researchers have
developed ways to grab a few trickles, such as using fine nets to wick water
from fog banks, or power-hungry dehumidifiers to condense it out of the air.
But both approaches require either very humid air or far too much electricity
to be broadly useful.
To find an all-purpose solution, researchers led by Omar
Yaghi, a chemist at the University of California, Berkeley, turned to a family
of crystalline powders called metal organic frameworks, or MOFs. Yaghi
developed the first MOFs—porous crystals that form continuous 3D networks—more
than 20 years ago. The networks assemble in a Tinkertoy-like fashion from metal
atoms that act as the hubs and sticklike organic compounds that link the hubs
together. By choosing different metals and organics, chemists can dial in the
properties of each MOF, controlling what gases bind to them, and how strongly
they hold on.
Over the past 2 decades chemists have synthesized more
than 20,000 MOFs, each with unique molecule-grabbing properties. For example,
Yaghi and others recently designed MOFs that absorb—and later release—methane,
making them a type of high-capacity gas tank for natural gas–powered vehicles.
In 2014, Yaghi and his colleagues synthesized a MOF that
excelled at absorbing water, even under low-humidity conditions. That led him
to reach out to Evelyn Wang, a mechanical engineer at the Massachusetts
Institute of Technology (MIT) in Cambridge, with whom he had previously worked
on a project to use MOFs in automobile air conditioning. After synthesizing the
new zirconium-based MOF, dubbed MOF-801, Yaghi met Wang at MIT and said,
“Evelyn we have to come up with a water harvesting device.” She agreed to give
it a shot.
At night setup soaks up water vapor from air, and uses
heat from the sun to release it as liquid water during the day.
The system Wang and her students designed consists of a
kilogram of dust-sized MOF crystals pressed into a thin sheet of porous copper
metal. That sheet is placed between a solar absorber and a condenser plate and
positioned inside a chamber. At night the chamber is opened, allowing ambient
air to diffuse through the porous MOF and water molecules to stick to its
interior surfaces, gathering in groups of eight to form tiny cubic droplets. In
the morning, the chamber is closed, and sunlight entering through a window on
top of the device then heats up the MOF, which liberates the water droplets and
drives them—as vapor—toward the cooler condenser. The temperature difference,
as well as the high humidity inside the chamber, causes the vapor to condense
as liquid water, which drips into a collector. The setup works so well that it
pulls 2.8 liters of water out of the air per day when run continuously, the
Berkeley and MIT team reports today in Science.
“It has been a longstanding dream” to harvest water from
desert air, says Mercouri Kanatzidis, a chemist at Northwestern University in
Evanston, Illinois, who wasn’t involved with the work. “This demonstration … is
a significant proof of concept.” It’s also one that Yaghi says has plenty of
room for improvement. For starters, zirconium costs $150 a kilogram, making
water harvesting devices too expensive to be broadly useful. However, Yaghi
says his group has already had early success in designing water-grabbing MOFs
that replace zirconium with aluminum, a metal that is 100 times cheaper. That could
make future water harvesters cheap enough not only to slake the thirst of
people in arid regions, but perhaps even supply water to farmers in the desert.
Posted in: Technology DOI: 10.1126/science.aal1051
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