An average-sized living room in a temperate climate contains about a kilogram of water vapor. Even in deserts, where groundwater and surface water are scarce, there’s usually plenty of potentially useful water in the air—if only it could be converted into liquid form. Nature obligingly condenses water vapor into dew, and systems for collecting dew have been around for decades. But the collectors require the temperature to drop below the dew point; at low humidity, that may require active cooling, which expends a lot of energy.
Now Omar Yaghi from the University of California, Berkeley, Evelyn Wang of MIT, and their colleagues have collaborated to create a water-harvesting device that works at relative humidity as low as 20% and requires no external energy source. Their concept is based on a metal–organic framework (MOF), a highly porous material whose properties can be tuned by varying the metal and organic components and how they’re put together. Yaghi and his group identified advantageous water-uptake properties in a zirconium-based MOF (shown in the figure, with the Zr atoms in blue, the organic components in red and black, and the pores highlighted in yellow, orange, and green). At 25 °C, even at low humidity, water molecules readily cling to the MOF’s large surface area; at 65 °C, the water is released.
Even on a 25 °C day, a dark-colored surface under direct sunlight can reach 65 °C with no additional heat input. Wang and her group designed a passive thermal system that incorporated Yaghi’s MOF, a graphite solar absorber, and an ambient-temperature heat sink. At night, the MOF was cool, so it took in water from the air; during the day, the graphite heated the MOF, and the water was released. The hot, moist air surrounding the heated MOF then came into contact with the heat sink, where most of its water condensed into liquid form.
For their proof-of-principle demonstrations—in a humidity-controlled lab and on an MIT rooftop—Wang and colleagues used less than 2 g of MOF, and they collected less than a milliliter of water in each temperature cycle. If the device can be scaled up and made to cycle 12 times a day instead of one, the researchers estimate that they can harvest almost three liters of water per kilogram of MOF per day. (H. Kim et al., Science, 2017, doi:10.1126/science.aam8743.)
