Posted in | Water | Sustainability

Fog Harvesting Device Boosts Collection Capacity for Clean Water

Study co-author Josh Tulkoff constructs a large prototype of the fog harp, which consists of a vertical array of 700 wires and is based on initial experimental results. Tulkoff was part of an interdisciplinary research team at Virginia Tech that discovered parallel wire arrays could increase the water collection capacity of fog nets by threefold. (Image credit: Virginia Tech)

Fog harvesting may look like quirky work. After all, fixing giant nets along mountaintops and hillsides to catch water out of thin air sounds more like idiocy than science. However, the practice has become a significant way to clean water for many people who live in dry and semi-arid climates across the globe.

A passive, robust, and effective technique of water collection, fog harvesting comprises capturing the tiny droplets of water suspended in the wind that constitute fog. Fog harvesting is possible - and has gained attention over the past decades - in areas of Africa, Asia, the Middle East, South America, and even California. As exemplified by recent headlines of South Africa's countdown to "Day Zero," or the day the water taps are anticipated to run dry, water scarcity remains a growing issue worldwide. Leading experts estimate that two-thirds of the world's population is already undergoing conditions of severe water scarcity at least for a month every year.

Fog harvesting could help ease that scarcity, and presently an interdisciplinary research team at Virginia Tech has enhanced the traditional design of fog nets to boost their collection capacity by threefold.

Reported in ACS Applied Materials & Interfaces and partly sponsored by the Virginia Tech Institute for Creativity, Arts, and Technology, the team's research shows how a vertical array of parallel wires may alter the forecast for fog harvesters. In the researcher’s design nicknamed as the "fog harp," the vertical wires shed microscopic water droplets faster and more efficiently compared to the traditional mesh netting used in fog nets.

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. But these parallel wire arrays are really the fog harp's special ingredient.

Brook Kennedy, Co-Author & Associate Professor of Industrial Design - College of Architecture and Urban Studies

Since the 1980s fog nets have been in use and can yield clean water in any area that undergoes recurrent, moving fog. As wind pushes the fog's microscopic water droplets through the nets, some get caught on the suspended wires of the net. These droplets collect and unite until they have sufficient weight to roll down the nets and settle into collection troughs placed at the bottom. In a few of the largest fog harvesting projects, these nets gather an average of 6,000 liters of water per day.

However, the traditional mesh design of fog nets has for a long time posed a twin constraint issue for engineers and scientists. If the holes in the mesh are very big, water droplets simply pass through without getting trapped on the net's wires. If the mesh is too fine, the nets catch more water, but the water droplets tend to congest the mesh without rolling down into the trough and wind then is stopped from moving through the nets.

Therefore, fog nets target a middle ground, a Goldilocks zone of fog harvesting: mesh that is neither too big nor too small. This compromise results in the nets avoiding clogging, but not being able to catch as much water as they could be.

"It's an efficiency problem and the motivation for our research," said Jonathan Boreyko, assistant professor in the Department of Biomedical Engineering and Mechanics in the College of Engineering. As the study’s co-author, Boreyko consulted on the theory and physical characteristics of the fog harp's design.

"That hidden regime of making the wires smaller but not clogging is what we were trying to accomplish. It would be the best of both worlds," he said.

Since the water droplets trapped in a fog net roll downward with gravity, Boreyko hypothesized that eliminating the horizontal wires of the net would lessen some of the clogging. In the meantime, Kennedy, who concentrates on biomimetic design, got his inspiration for the fog harp from nature.

On average, coastal redwoods rely on fog drip for about one-third of their water intake. 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.

Brook Kennedy, Co-Author & Associate Professor of Industrial Design - College of Architecture and Urban Studies

Mark Anderson, a study co-author and then-undergraduate student in the Department of Mechanical Engineering, constructed several scale models of the fog harp with different sizes of wires. Weiwei Shi, a doctoral student in the engineering mechanics doctoral program as well as the study's lead author, analyzed the small prototypes in an environmental chamber and created a theoretical model of the experiment.

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

The researchers have already built a larger prototype of the fog harp - a vertical array of 700 wires that measures 3 feet by 3 feet - in an effort headed by Josh Tulkoff, study co-author and a then-undergraduate student in the industrial design program. They plan to try out the prototype on neighboring Kentland Farm.

Through its exclusive combination of science and design, the team anticipates the fog harp will one day make a big impact where it is needed most - in the bottom of the water trough.

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