Fog harps could wrest water from the clouds

Fog harp
© Bob Embleton

Inspired by coastal redwoods, scientists have created a new kind of fog harvesting design that appears to increase the capacity of clean water collection by threefold.

Some of us live in climates where water pours from the sky and kindly fills our reservoirs. Others, not so much; and given our distinct reliance on water, those people have had to get inventive in their collecting of it. Like, pulling it out of the air. While fog harvesting may seem whimsical and more like the work of elves and fairies, fog nets have actually proven to quite productive for people in semi-arid and arid climates around the world.

In use since the 1980s, the nets work anywhere that has frequent, moving fog. The method involves giant screens strung across hillsides; as the fog moves through, its microscopic water droplets get caught in the mesh, gather, and drip to troughs below. Although it may sound like a laborious process, the larger fog harvesting schemes collect an impressive 6,000 liters of water each day.

One problem with the nets, however, is that they have long posed a Goldilocks dilemma. If the holes are too large, the water passes through them; too small and the water clogs the mesh and doesn’t drip down. The just-right size allows the water to collect, but doesn’t yield as much as water as the system could.

But now, now an interdisciplinary research team from Virginia Tech has worked on the traditional design with a promising result: An increased collection capacity by threefold. The solution? A harp, of sorts, that retains the vertical wires while eliminating the horizontal ones.

"From a design point of view, I've always found it somewhat magical that you can essentially use something that looks like screen door mesh to translate fog into drinking water," says Brook Kennedy, one of the study's co-authors. "But these parallel wire arrays are really the fog harp's special ingredient."

As it turns out, Kennedy specializes in biomimetic design, and he went to one of nature’s crowning accomplishments for inspiration; California’s colossal coastal redwoods. (Unabashed superfan here, see 11 facts about coastal redwoods, the tallest trees in the world.)

"On average, coastal redwoods rely on fog drip for about one-third of their water intake," says Kennedy. "These sequoia trees that live along the California coast have evolved over long periods of time to take advantage of that foggy climate. Their needles, like those of a traditional pine tree, are organized in a type of linear array. You don't see cross meshes."

The team built a few scale models of the poetically dubbed fog harp with different sizes of wires, before testing the small prototypes in the lab and developing a theoretical model of the experiment.

"We found that the smaller the wires, the more efficient the water collection was," says co-author Jonathan Boreyko. "These vertical arrays kept catching more and more fog, but the clogging never happened."

Fog harp© Virginia Tech

The team has now constructed a larger prototype of the harp (above, with study co-author Josh Tulkoff) that they plan to test in the wild at a nearby farm. It certainly seems like they are on the right track, learning low-tech lessons from the trees and putting them to good use … with a nice assist from the fog.

See more at Virginia Tech.

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