“Water, water, everywhere, nor any drop to drink” is the mournful cry of Coleridge’s stranded ancient mariner. But the same sentiment could be applied to a number of drought-stricken regions that experience abundant fog. Many of these occur in remote areas—in places like the west coasts of the Americas and Africa as well as in Yemen, Dubai, and the Canary Islands, which also lack electricity. How, then, can precious water be effectively and passively harvested from fog, making life healthier and easier for people living in these areas?
Researchers at Virginia Tech have developed a "fog harp" capable of collecting three times more water than a conventional fog net. (Image source: Virginia Tech)
Indeed, efforts to harvest fog have been around since at least the 1990s. Most of them consist of fine mesh nets that, when hung in foggy areas, can collect fog, which drips down into some kind of collection vessel. A number of efforts to enhance collection have been based on the behavior of the Namibian Desert Beetle, an amazing little creature. It lives in one of the driest places on Earth by pulling water from the air, which is funneled from its back and into its mouth. A number of researchers came to believe that the combination of hydrophilic and hydrophobic coatings on the beetle’s shell gave it this ability, leading one company that makes such coatings to take its name from the beetle.
It’s the Geometry of the Beetle
More recent studies, however, have shown that the beetles’ collection ability is more a function of the geometric arrangement of tiny bumps on the beetle’s back rather than their surface chemistry. Furthermore, according to Jonathan Boreyko at the Department of Biomedical Engineering and Mechanics at Virginia Tech, while these patterns of differential coatings may work well in the lab, “you get hydrophobic particulates from hydrocarbons in the air that will deposit on the hydrophilic coatings over time”—eventually diminishing the wettability contrast on which the water collection was based.
That led Boreyko to look elsewhere for inspiration on the subject. It turns out that his colleague at the School of Architecture and Design, Brook Kennedy, had also been looking at natural processes for fog harvesting. Rather than looking down, however, Kennedy looked up into the branches of giant Sequoias. These trees, native to California’s Sierra Nevada region, consume as much as 800 gallons of water per day during the growing season, yet are considered drought-resilient. As much as a third of their water intake comes from fog. The sequoias utilize their needles to capture the fog that is often present at the towering heights that the trees rise to, funneling it to aerial roots as well as to roots on the ground.
What was unique about the Sequoia needles was their parallel structure, which is different from the meshes typically used in fog nets due to the absence of horizontal cross-members that could serve to obstruct the downward flow of the collected droplets. Meanwhile, Boreyko’s previous work on the movement of droplets across super-hydrophobic surfaces had found that the need for droplets to separate from obstructions in the path will often constitute the majority of the friction restricting their mobility.
Capturing Water out of Fog
So the two teamed up, along with several others, to come up with a new fog capture device based on those observations, from which emerged the fog harp. As the name suggests, the structure contains only wires running in the same direction as they do in a harp. This provides an unobstructed parallel channel for the droplets to flow down. It was Kennedy and his design students that came up with the mechanism to wind the wires and maintain them under tension. The absence of cross-members does indeed result in better harvesting performance, generating as much as three times as much water as the mesh-type devices currently being used.
While fog harvesting meshes are constrained as the wires get smaller, due to clogging, this is not the case with a fog harp. In fact, the harp performs better as wires get smaller. The apparent limit is the need to maintain low enough aerodynamic impedance so that the fog goes through, rather than around, the harp.
Next steps include finding ways to mass-produce the harps, which the researchers feel could likely be woven on some sort of modified loom. They also plan to test a number of other parameters including overall harp size, wire tension, inlet geometries, and even the possibility of passive wind tracking. With a little luck in making the right connections, these fog harps could be out collecting water in the not-too-distant future.
RP Siegel, PE, has a master's degree in mechanical engineering and worked for 20 years in R&D at Xerox Corp. An inventor with 50 patents, and now a full-time writer, RP finds his primary interest at the intersection of technology and society. His work has appeared in multiple consumer and industry outlets, and he also co-authored the eco-thriller Vapor Trails.
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