Fog is the latest innovation in the hunt for clean and accessible water. In order to catch clean water from fog, researchers install giant nets along hillsides and mountaintops to actually catch the fog water in arid and semi-arid climates. I know it sounds like a method straight from a weird, futuristic novel, but it actually works!
In fog harvesting, the tiny water droplets that make up fog are caught by a “fog net.” Fog harvesting has been proven possible for decades in Africa, South America, Asia, the Middle East and California. But researchers are starting to focus on using fog harvesting to help the two-thirds of the population on Earth who experience severe water shortages for at least one month of the year. Researchers believe that fog harvesting could be the key to helping these water-scarce areas, but further development of the water nets needs to happen first. That is where a Virginia Tech research team comes in.
The VT team has focused on improving the net design so the nets are able to increase their water capacity by threefold. Their research has proven that a vertical array of parallel wires could change the forecast for fog harvesters. This new design has been named the “fog harp.” The vertical wires grab tiny water droplets faster and more efficiently than the previous design.
"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," said Brook Kennedy, associate professor of industrial design in the College of Architecture and Urban Studies and one of the study's co-authors. "But these parallel wire arrays are really the fog harp's special ingredient."
The fog nets from the 1980s have been able to gather clean water in any consistently foggy area. As the fog moves through the air due to the wind, the water droplets from the fog move through the nets and some of these water droplets get caught in the net. The water droplets gather in the wires until they are heavy enough to travel down the nets and then settle in the collection areas. Some of the largest fog-harvesting projects have gathered almost 6,000 liters per day.
This old method has had its own sets of problems, including a dual constraint problem. If the mesh holes are too big, the water passes through without getting caught, but if the mesh is too fine, the nets catch more water but the water clogs up the mesh without running into the collection area. Fog nets need to be in the middle.
"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, "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."
Borekyo believes that if the horizontal wires in a net are removed, the chances of clogging will be reduced. Kennedy found inspiration for the new design in nature.
"On average, coastal redwoods rely on fog drip for about one-third of their water intake," said 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 prototype of the new net and then tested it in a laboratory setting. The new results from these tests proved that the new design works!
"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."
A larger prototype of the nest has been developed. The team plans on testing the new net on a farm close to the laboratory. Once the initial testing is finished, they hope to launch this net to help water-scarce areas.
The paper on this research was published in Applied Materials and Interfaces.