Arthur J. Villasanta – Fourth Estate Contributor
Berkeley, CA, United States (4E) – A larger, next-generation harvester has been successfully tested in the Arizona desert and proved that the water harvester developed by a team from the University of California, Berkeley can extract drinkable water every day/night cycle at very low humidity and at low cost. This makes this new water harvester ideal for people living in arid, water-starved areas of the world.
Each kilogram of the material called a metal-oxide framework (MOF) produces 200 mL of water per day/night cycle from dry air, using only solar energy, said the team. MOFs consist of linked molecules that create a super porous material with lots of surface area. MOFs can be very “hydrophilic,” meaning they attract water.
The MOF used by the UC Berkeley team can extract water from the air and store it in its pores, which it does during the night. During daytime, sunlight is used to release the water and a condenser harvests it. The system is completely passive and the research team showed it works in the arid Arizona climate.
“There is nothing like this,” said Omar Yaghi, who invented the technology underlying the harvester. “It operates at ambient temperature with ambient sunlight, and with no additional energy input you can collect water in the desert. This laboratory-to-desert journey allowed us to really turn water harvesting from an interesting phenomenon into a science.”
Yaghiand his team will report the results of the first field test of a water-collecting harvester in the June 8 issue of the journal Science Advances.Yaghi is the James and NeeltjeTretter chair in chemistry at UC Berkeley and a faculty scientist at Lawrence Berkeley National Laboratory.
The trial in Scottsdale, where the relative humidity drops from a high of 40 percent at night to as low as eight percent during the day, demonstrated that the harvester should be easy to scale up by simply adding more of the MOFs. Researchers anticipate that with the current MOF (MOF-801), made from the expensive metal zirconium, they will ultimately be able to harvest about 200 mL (about seven ounces) of water per kilogram (2.2 pounds) of MOF, or three ounces of water per pound.
Yaghi also reports that he’s developed a new MOF based on aluminum (called MOF-303) that is at least 150 times cheaper and captures twice as much water in lab tests. This will enable a new generation of harvesters producing more than 400 mL (3 cups) of water per day from a kilogram of MOF, the equivalent of half a 12-ounce soda can per pound per day.
“There has been tremendous interest in commercializing this, and there are several startups already engaged in developing a commercial water-harvesting device,” said Yaghi. “The aluminum MOF is making this practical for water production, because it is cheap.”
For the new paper, the UC Berkeley team (graduate student Eugene Kapustin and postdoctoral fellows MarkusKalmutzki and FarhadFathieh) collected and measured the water and tested the latest generation harvester under varying conditions of humidity, temperature and solar intensity.
The harvester is a box within a box. The inner box holds a two-square-foot bed of MOF grains open to the air to absorb moisture. This is encased in a two-foot plastic cube with transparent top and sides. The top was left open at night to let air flow in and contact the MOF, but was replaced during the day so the box could heat up like a greenhouse to drive water back out of the MOF. The released water condensed on the inside of the outer box and fell to the bottom, where the researchers collected it with a pipette.
Extensive field tests lay out a blueprint allowing engineers to configure the harvester for the differing conditions in Arizona, the Mediterranean or anywhere else, given a specific MOF.
“The key development here is that it operates at low humidity, because that is what it is in arid regions of the world,” said Yaghi. In these conditions, the harvester collects water even at sub-zero dew points.
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