18th April 2017

Device pulls water from dry air, powered only by Sun

The University of California, Berkeley, has created a device that pulls water from dry air, powered only by the Sun. Even under conditions of relatively low (20-30%) humidity, it can produce 2.8 litres of water over a 12-hour period.



Credit: University of California, Berkeley

Imagine a future in which every home has an appliance that pulls all the water the household needs out of the air, even in dry or desert climates, using only the power of the Sun. That future may be just around the corner, with the demonstration of a water harvester that uses only ambient sunlight to pull litres of water out of the air each day in conditions as low as 20 percent humidity, a level common in arid areas.

The solar-powered harvester, reported in the journal Science, was constructed at the Massachusetts Institute of Technology using a special material called a metal-organic framework – or MOF – produced at the University of California, Berkeley.

"This is a major breakthrough in the long-standing challenge of harvesting water from the air at low humidity," said Omar Yaghi from UC Berkeley, one of two senior authors of the paper. "There is no other way to do that right now, except by using extra energy. Your electric dehumidifier at home 'produces' very expensive water."

The prototype, under conditions of 20-30 percent humidity, was able to pull 2.8 litres (3 quarts) of water from the air over a 12-hour period, using one kilogram (2.2 pounds) of MOF. Rooftop tests at MIT confirmed that the device works in real-world conditions.



Schematic of a metal-organic framework (MOF). Credit: UC Berkeley, Berkeley Lab image.

"One vision for the future is to have water off-grid, where you have a device at home running on ambient solar for delivering water that satisfies the needs of a household," said Yaghi, who is the founding director of the Berkeley Global Science Institute, a co-director of the Kavli Energy NanoSciences Institute and the California Research Alliance by BASF. "To me, that will be made possible because of this experiment. I call it personalised water."

Yaghi worked with Evelyn Wang, a mechanical engineer at MIT, alongside students at the university. The system they designed consists of approximately two pounds of dust-sized MOF crystals compressed between a solar absorber and a condenser plate, inside a chamber open to the air. As ambient air diffuses through the porous MOF, water molecules preferentially attach to the interior surfaces. X-ray diffraction studies have shown that the water vapour molecules often gather in groups of eight to form cubes.

Sunlight entering through a window heats up the MOF and drives the bound water toward the condenser, which is at the temperature of the outside air. The vapour condenses as liquid water and drips into a collector.

"This work offers a new way to harvest water from air that does not require high relative humidity conditions and is much more energy efficient than other existing technologies," said Wang.

This proof of concept harvester leaves much room for improvement, Yaghi said. The current MOF can absorb only 20 percent of its weight in water, but other MOF materials could possibly absorb 40 percent or more. The material could also be tweaked to be more effective at higher or lower humidity.

"It's not just that we made a passive device that sits there collecting water; we have now laid both the experimental and theoretical foundations so that we can screen other MOFs, thousands of which could be made, to find even better materials," he said. "There is a lot of potential for scaling up the amount of water that is being harvested. It is just a matter of further engineering now."

Yaghi and his team are working to improve their MOFs, while Wang continues to improve the harvesting system to produce more water.

"To have water running all the time, you could design a system that absorbs the humidity during the night and evolves it during the day," he said. "Or design the solar collector to allow for this at a much faster rate, where more air is pushed in. We wanted to demonstrate that if you are cut off somewhere in the desert, you could survive because of this device. A person needs about a Coke can of water per day. That is something one could collect in less than an hour with this system."

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