In field tests, device harvests water from desert air

By running a test device on a rooftop at Arizona State University in Tempe, Wang says, the team “was field-testing in a place that’s representative of these arid areas, and showed that we can actually harvest the water, even in subzero dewpoints.”

The test device was powered solely by sunlight, and although it was a small proof-of-concept device, if scaled up its output would be equivalent to more than a quarter-liter of water per day per kilogram of MOF, the researchers say. With an optimal material choice, output can be as high as three times that of the current version, says Kim. Unlike any of the existing methods for extracting water from air at very low humidities, “with this approach, you actually can do it, even under these extreme conditions,” Wang says.

Not only does this system work at lower humidities than dew harvesting does, says Rao, but those systems require pumps and compressors that can wear out, whereas “this has no moving parts. It can be operated in a completely passive manner, in places with low humidity but large amounts of sunlight.”

Whereas the team had previously described the possibility of running the system passively, Rao says, “now we have demonstrated that this is indeed possible.” The current version can only operate over a single night-and-day cycle with sunlight, Kim says, but “continuous operation is also possible by utilizing abundant low-grade heat sources such as biomass and waste heat.”

The next step, Wang says, is to work on scaling up the system and boosting its efficiency. “We hope to have a system that’s able to produce liters of water.” These small, initial test systems were only designed to produce a few milliliters, to prove the concept worked in real-world conditions, but she says “we want to see water pouring out!” The idea would be to produce units sufficient to supply water for individual households.

The team tested the water produced by the system and found no traces of impurities. Mass-spectrometer testing showed “there’s nothing from the MOF that leaches into the water,” Wang says. “It shows the material is indeed very stable, and we can get high-quality water.”

“This technology is fantastic, because of the practical demonstration of an air-cooled water harvesting system based on MOFs operating in a real desert climate,” says Yang Yang, a professor of materials science and engineering at the University of California at Los Angeles, who was not involved in this work.

“This provides a new approach to solving the problem of water scarcity in arid climates,” Yang says. “This technology, if one can further increase its production capacity, can have a real impact in areas where water is scarce, such as southern California.”

The team also included graduate student Eugene Kapustin at the University of California at Berkeley; graduate student Lin Zhao and postdoc Sungwoo Yang at MIT; and professor of chemistry Omar Yaghi at Berkeley and at King Abdulaziz City for Science and Technology, in Saudi Arabia.

— Read more in Hyunho Kim et al., “Adsorption-based atmospheric water harvesting device for arid climates,” Nature Communications 9, Article number: 1191 (22 March 2018) (doi:10.1038/s41467-018-03162-7); and Hyunho Kim et al., “Response to Comment on “Water harvesting from air with metal-organic frameworks powered by natural sunlight,” Science 358, no. 6367, eaao3139 (1 December 2017) (DOI: 10.1126/science.aao3139)

Reprinted with permission of MIT News