United States. Can you imagine a future in which every home has a device that draws all the water the home needs from 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 collector that uses only sunlight to draw liters 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 combine, reported in the journal Science, was built at the Massachusetts Institute of Technology using a special material – a metal-organic frame, produced at the University of California, Berkeley.
"This is a major breakthrough in the long-standing challenge of collecting water from the air at low humidity," said Omar Yaghi, one of the paper's two lead authors, who holds the James and Neeltje Tretter Chair in Chemistry at UC Berkeley and is a faculty scientist at Lawrence Berkeley National Laboratory. "There is no other way to do it 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 extract 2.8 liters (3 quarts) of water from the air over a 12-hour period, using one kilogram (2.2 pounds) of MOF. MIT rooftop tests confirmed that the device works in real conditions.
"One vision for the future is to have the water off the grid, where you have a device in the home running in a solar environment to deliver water that meets the needs of a home," said Yaghi, who is the founding director of the Berkeley Global Science Institute, a co-director of the Kavli NanoSciences Energy Institute and the California Research Alliance for BASF (chemical company). "For me, that will be possible thanks to this experiment. I call it custom water."
In 2014, Yaghi and his uc Berkeley team synthesized a MOF — a combination of zirconium metal and adipic acid — that binds to water vapor, and suggested to Evelyn Wang, a mechanical engineer at MIT, that they join forces to turn the MOF into a water harvesting system.
The system Wang and his students designed consisted of more than two pounds of powder-sized MOF crystals compressed between a solar absorber and a condenser plate, placed inside an open-air chamber. As the ambient air diffuses through the porous MOF, the water molecules preferentially attach to the inner surfaces. X-ray diffraction studies have shown that water vapor molecules often gather in groups of eight to form cubes.
Sunlight entering through a window heats the MOF and conducts the bound water into the condenser, which is at the temperature of the outside air. The steam condenses as liquid water and drips into a collector.
"This work offers a new way to collect water from the air that does not require high relative humidity conditions and is much more energy efficient than other existing technologies," Wang said.
This sample of the concept combine leaves plenty of room for improvement, Yaghi said. The current MOF can absorb only 20 percent of its weight in water, but other MOF materials could absorb 40 percent or more. The material can also be adjusted to be more effective at higher or lower humidity levels.
Source: University of California, Berkeley.
