A fog-harvesting system that is up to five times more efficient than previous systems at turning airborne water into drinking water has been developed by researchers at MIT in collaboration with colleagues in Chile.
Fog harvesting is not a new technique -- it's already used to pull drinking water out of the air in at least 17 different countries. Systems generally consist of some sort of vertical mesh, a little like a large tennis net. The technique is inspired by specialised plants and insects that survive in some of the world's driest regions by drawing water from the air in this way.
The research team have managed to optimise the nets by fine-truning the size of the filaments in the nets, the size of the holes between the filaments and the coating applied to the filaments.
Most existing systems use a woven polyolefin (a plastic) mesh which have holes that are far too large and as a consequence only extract around two percent of the water available in mild fog. The researchers showed that a finer mesh could extract 10 percent or more of the water from the fog, while multiple nets deployed one behind another could extract even more. The right coating will cause the droplets that form on the screen to slide down to be collected at the bottom and be funneled into tanks.
The best performance of any of the test meshes came from one made from stainless-steel filaments around four times the thickness of a human hair with a spacing double that size between fibres.
This mesh is then dip-coated using a solution that decreases a characteristic called "contact-angle hysteresis", which causes droplets to stick onto windows and other objects. The coating ensures that the droplets instead slide down more easily into a gutter.
Test screens were installed in collaboration with Chilean researchers at the Pontifical Catholic University in Santiago Chile. Screens made out of different materials were placed on hilltops in a semi-arid region north of Santiago, where there is scarcely any rainfall but a strong coastal fog known as
camanchaca, which comes inland from the Pacific.
The mesh systems currently used in the coastal mountains in the Atacama Desert tend to produce a few litres of drinking water per day for every square metre of mesh. Theoretically, the new systems could yield up to 12 litres per day or more in strong winds with dense fogs, the researchers have calculated.
The team believes that large structures with hundreds of square metres of mesh could be set up very cheaply and would cost very little to operate, requiring no energy and just an occasional brushing to remove insects and dirt. If just four percent of the water in camanchaca could be captured it would meet the water needs of the nation's four northernmost regions, which occupy a space the size of the Atacama Desert.
The findings have been published online by the journal Langmuir, a publication of the American Chemical Society, in a paper by MIT postdoc Kyoo-Chul Park and colleagues including Shreerang Chhatre, SIddarth Srinivasan, chemical engineering professor Robert Cohen, and mechanical engineering professor Gareth McKinley.
McKinley says: "Nature has already done the hard work of evaporating the water, desalinating it and condensing the droplets.
We just have to collect it."
This article was originally published by WIRED UK
