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Aimed at providing water to people in dry and off-grid areas worldwide, the material was created by Omar Yaghi and colleagues at the University of California, Berkeley. Used in a new water-harvesting device, each kilogram of the material reportedly produced 0.7 litre of water – about one-and-a-quarter pints – per day from the arid air of California’s Mojave Desert. Even on the driest test day, with extremely low relative humidity of 7% and temperatures over 26.6ºC, each kilogram produced 0.2 litre of water.
“It is well known that in order to condense water from air at a low humidity – less than 40% relative humidity – you need to cool down the air to below freezing, to 0ºC, which is impractical. With our harvester, we are doing this at very low humidity without such cooling. There is no other material that can do that,” said chemistry professor Yaghi.
“Some people say that 0.7 litre is not a lot of water – but it’s a lot of water if you don't have water.”
The material used is a very porous substance known as a metal organic framework (MOF) – so porous that 1g has a surface area equivalent to a football field. First developed by Yaghi in the mid-1990s, MOFs can absorb a range of chemicals.
Water molecules in ambient air stick to the internal surface – a process called adsorption – and increase the humidity inside the MOF to a point where the water condenses even at room temperature, just as water condenses on cooler surfaces when the humidity is high. When the MOF is heated slightly, the water comes back out and can be condensed and collected.
The latest project uses aluminium-based MOF-303. It can hold 30% more water than a previous zirconium-based MOF and can adsorb and release water in 20 minutes under ideal conditions.
The microwave-sized device used in testing used solar panels to power fans blowing ambient air over MOF contained within a cartridge. The cartridge, about 65cm² and 12.7cm thick, is intersected by two sets of channels: one for adsorbing water, the other for expelling it to the condenser, allowing continuous cycling. The solar panels, attached to batteries so the harvester can run at night, also power small heaters that drive the water out of the MOF.
A start-up from Yaghi, Water Harvesting Inc., is testing and hoping to sell microwave-sized devices capable of supplying 7-10 litres of water per day – enough drinking and cooking water for two to three adults. The researcher predicts a refrigerator-sized unit will provide 200-250 litres of water per day – enough for a household to drink, cook and shower – and a village-scale harvester could eventually produce 20,000 litres per day.
Jordanian-born researcher Yaghi said he receives nearly daily enquiries from people in dry areas around the world. “The atmosphere has almost as much water at any one time as all the rivers and lakes,” he said. “Harvesting this water could help turn dry deserts into oases.”
The research, which was funded by Saudi Arabia’s King Abdulaziz City for Science and Technology, was published in ACS Central Science.
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