The key to the water harvester is a new class of materials called metal-organic frameworks (MOFs). These MOFs are solid but porous materials with enormous surface areas—an MOF the size of sugar cube can have the internal surface area as big as many football fields. This means they can absorb gases and liquids, and then release them quickly when heat is added.
“Certain MOFs as we showed here have an extraordinary ability to suck in water vapor from the atmosphere, but then at the same time do not hold on to the water molecules inside their pores too tightly so that it is easy to get the water out,” says Omar Yaghi, a professor of chemistry at Berkeley, who led the research.
The harvester itself is a box inside a box. The inner box contains a bed of MOFs. The outer box is a two-foot transparent plastic cube. At night, the researchers left the top off the outer box to let air flow past the MOFs. In the day, they put the top back on so the box would be heated by the sun. The heat would pull the water out of the MOFs, where it would condense on the inner walls of the plastic cube before dripping to the bottom, where it could be collected.
“The most important aspect of this technology is that it is completely energy-passive,” says Eugene Kapustin, a Berkeley graduate student who worked on the research.
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