A solar-powered device, working under conditions of 20-30 percent humidity, was able to pull 2.8 liters, or 3 quarts, of water from the air over a 12-hour period, using one kilogram, or 2.2 pounds, of certain metal-organic framework, or MOF, material.
The prototype, known as a water harvester, was constructed at the Massachusetts Institute of Technology (MIT) using the 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, one of two senior authors of a paper published in the journal Science, who is a professor in chemistry at UC Berkeley and a faculty scientist at Lawrence Berkeley National Laboratory. "There is no other way to do that right now, except by using extra energy."
Yaghi invented metal-organic frameworks more than 20 years ago, combining metals like magnesium or aluminum with organic molecules in a tinker-toy arrangement to create rigid, porous structures ideal for storing gases and liquids. Since then, more than 20,000 different MOFs have been created by researchers worldwide, some hold chemicals such as hydrogen or methane, others are able to capture carbon dioxide from flue gases, catalyze the reaction of adsorbed chemicals or separate petrochemicals in processing plants.
In 2014, Yaghi and his UC Berkeley team synthesized a MOF, a combination of zirconium metal and adipic acid, that binds water vapor, and he suggested to Evelyn Wang, a mechanical engineer at MIT, that they join forces to turn the MOF into a water-collecting system.
"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. "To me, that will be made possible because of this experiment. I call it personalized water."
The system Wang and her students designed consisted of dust-sized MOF crystals compressed between a solar absorber and a condenser plate, placed inside a chamber open to the air. As ambient air diffuses through the porous MOF, water molecules preferentially attach to the interior surfaces. 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 vapor 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," Wang was quoted as saying in a news release from UC Berkeley.
The 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 can also be tweaked to be more effective at higher or lower humidity levels.
Yaghi and his team are at work improving their MOFs, while Wang continues to improve the harvesting system to produce more water.
"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," Yaghi 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."
