Chances are you have seen an air conditioner dripping water in summer. So, what’s the big deal about harvesting water from air?
Well, can your gizmo squeeze a canful of the good stuff out of pale blue Karoo air inside an hour? Running on ambient sunlight? Is it, in other words, a compelling solution to providing much of the water a household needs in an arid country such as ours, in a single appliance?
Metal-organic frameworks or MOFs are such a solution. And as such, they’re destined to play a significant part in relieving near-future water scarcity around the world.
A MOF can harvest water from low-humidity (20%) air, at pace, and without throwing extra energy at the problem, its creators explained in the journal Science. “An electric dehumidifier essentially produces very expensive water.”
What is it?
So, what is a MOF? Synthesised by University of California (Berkeley) scientist Omar Yaghi, MOF compounds combine a metal (such as magnesium) with organic molecules (such as carbon, hydrogen or nitrogen) in various permutations.
Yaghi’s special compound combines zirconium with adipic acid to form a crystalline ‘sponge’ that binds water vapour.
Thus, he hit upon the idea to turn the material into a water-collecting system.
How does it work?
Yaghi’s prototype harvester compresses MOF crystals between a solar absorber and a condenser plate, placed inside a chamber open to the air. As ambient air diffuses through the porous MOF, water molecules preferentially attach to the interior surfaces.
Sunlight entering through a window heats up the MOF and drives the bound water towards the condenser, which is at the temperature of the outside air. The vapour condenses as liquid water and drips into a collector.
Using one kilogram of the material, the harvester (assembled by MIT) can pull 2.8 litres of water out of thin air over a 12-hour period. It works as well in real-world conditions as it does in labs.
But while the prototype doesn’t require high relative humidity and is much more energy-efficient than available technologies, there’s room for improvement. The current model can absorb only 20% of its weight in water. Other MOF materials could possibly absorb 40% or more.
The material can also be tweaked to be more effective at higher or lower humidity levels, Yaghi surmises. But considering what it does, these are no deadly sins. It’s really just a matter of further engineering.
And did we mention you can take it out into the desert with you?