Harvesting the clouds: how to make water out of fog
Even dry areas have fog. So why don't we use it?
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Generally, when scientists look to nature for inspiration, they look to environments that mimic what they’re trying to build. In the case of collecting water from air, that meant desert plants and animals — seeing what special structures they might have to harvest fog water. So it was surprising when a fog-collecting discovery came from a far wetter source: the beaks of shorebirds. And it could inspire a pretty radical redesign of fog collection devices.
The new beak-inspired fog collector consists of two plates that come together and then move apart, collecting and transporting deposited water droplets. Video courtesy of Xin Heng
Right now, most fog collection is done using mesh fabric. The Standard Fog Collector, used all over the world since Robert Schemenauer's first paper on it in 1994, consists of a fabric mesh 1 square meter in size and faces the wind like little perforated sail. As the wind blows fog through the device, water droplets accumulate on the mesh and drop down into a trough, from which they flow down a tube into a container.
"Fog has a really big impact on the world"
Daniel Fernandez, a fog researcher at California State University, Monterey Bay, has set up 20 standard fog collectors along the California coast. While most such fog collectors gather just a few liters a day, where they’re placed can make all the difference; Fernandez says he has collected almost 40 liters, or about 10 gallons, per square meter in a day from some parts of Big Sur.
"Fog has a really big impact on the world we live in, particularly in the coastal areas in California," Fernandez says. On a summer day along the winding Northern California coast, the cool waters of the Pacific can cause the moisture in the air above to condense, forming low-lying clouds. As the breeze brings these clouds onto the shore and close to the ground, they form fog. It’s the same kind of fog that spreads into San Francisco, giving the city its iconic fog cover — and keeps temperatures along the California coast cool.
Fog is made of tiny suspended water droplets. When these droplets hit California’s coastal redwoods, they are intercepted by the trees’ leaves and drip down to make the surrounding areas wet. That’s crucial in the coastal ecosystem, since fog can contribute water when other sources such as rain are scarce. "What’s nice about fog is it tends to happen when we don’t have rain, typically," Fernandez says. Even now, during one of California’s most severe droughts, there’s been plenty of coastal fog to go around. For instance, 2013 was California’s driest calendar year since the state started measuring rainfall in 1849, with just 7 inches of aggregated rain, and the period from October 2013 to September 2014 has been the third-driest on record. But the fog rolled in as usual this summer, and contributed its water both to the redwoods and to Fernandez’s fog collectors.
Fog collectors are even more useful in areas with more extreme conditions than California, particularly in parts of South America such as Chile. High up in the Andes, in desert areas that receive almost no rainfall, dense fog can serve as a significant source of water. "You get more water from fog in many parts of Chile than you get from rain," Fernandez says.
Fog collection is a relatively cheap and environmentally friendly source of water in many remote arid regions. The collected water is often of good quality, so it can provide drinking water where it’s otherwise unavailable. Fog collecting projects have been implemented all over the world, in countries such as Chile, Peru, Guatemala, Namibia, Eritrea, Oman, and Nepal, many of these implemented by a Canadian non-profit called FogQuest.
fog collection can serve as a relatively cheap source of water in many arid regions
Right now, fog collection is still useful in a limited range of places — ones with large amounts of fog and few other sources of water — but that may not always be the case. Several research groups are working on devising better fog collectors made of new materials and designs, which could collect water more efficiently and expand the regions where fog collection could be useful. As we try to reduce our water consumption and look for more sources of fresh water, we may need to rely on more such innovative approaches.
Daniel Fernandez stands in front of three Standard Fog Collectors with different types of meshes. Photo courtesy of Jeremiah Tysporin.
This technique is for the birds
Xin Heng, a doctoral student with professor Cheng Luo at the University of Texas at Arlington, was working on replicating a cactus’s fog-collecting apparatus when he chanced upon an article about the feeding mechanism of shorebirds. Many shorebirds dip their beaks into the water while swimming, capturing their tiny prey in water droplets held between their beaks. They then open and close their beaks repeatedly to transport the prey-containing droplets into their mouths.
Heng saw stills from high-speed videos showing the birds' open-and-close technique, and wondered if he could harness it to catch water from air, rather than prey from water. He used two hinged plates that come together and then move apart to mimic the movement of the shorebirds’ beaks. Droplets of fog that collect on the plates are transported en masse to the hinge each time the plates clamp down and then separate. At the moment, Heng moves the plates by hand, but he expects to motorize future models.
Heng saw stills from high-speed videos showing the birds' technique and wondered if he could harness it.
Mesh-based fog collectors rely on gravity to passively drive water droplets down into a trough. "In our case, we actively transport the water droplets," Heng says. He expects active transport to be much better at transporting small droplets that otherwise would remain on the mesh. "Because we use an active transport process, we expect that all the drops can be transported," he says.
Fog on Tanbark trail in Julia Pfeiffer Burns State Park (Christian Holland).
Heng and his fellow researchers "are developing an interesting technique," says Otto Klemm, a professor of climatology at the University of Münster who has been interested in fog for the past 30 years. Klemm says even a small increase in the amount of fog water collected "would be helpful, definitely."
Heng has tested both a tiny fog collector that’s a little smaller than a matchbook, and a larger model that’s a little smaller than a license plate, and published his results in September 2014. The larger model collected about a tablespoon’s worth, or about 15 mL of water, in 36 minutes in lab tests. That makes it several hundred times more efficient at collecting water than individual desert beetles, grasses, or metal wires, Heng says. The larger version can collect both fog and dew over short periods of time and on a small scale — so it could also be developed into a portable model. Heng is currently testing a much larger version made of plastic acrylic plates 70 square centimeters in size. He hopes to have an even larger collector ready in a year. "I need to do more experiments to try to optimize our setup, but after that we can try to commercialize our device," Heng says.
the unorthodox design could prompt researchers to look more broadly for inspiration
Once he’s finished lab tests of his largest device he plans to take his fog collectors out to a nearby lake or river to try them out in the morning fog and dew. He also plans to compare the device to existing mesh-based fog collectors, and to his original cactus-based designs.
While it’s still too early to know how the new plate-based fog collector will fare, it could prompt researchers to look more broadly for inspiration. Heng's device bears a resemblance to dew collectors, which often consist of a plastic or metal sheet on which water condenses — and as a consequence, Heng expects his design will also harvest dew. He also plans to incorporate additional tweaks, such as including microstructures on the plates to try to enhance their efficiency.
It’s not until the new design has been used in the field that it can be properly evaluated, Klemm says. "That’s a very, very large step, maybe larger than the engineering itself," he says. "Out there it’s a different world, and many things don’t behave as nicely in the field as they do in the laboratory," Klemm says.
Fog in Chile's Atacama desert (Danielle Periera)
It's different outside the lab
One of the advantages of the mesh-based fog collectors is that they’ve been extensively tested in the real world. The Chilean-made mesh material is hardy and inexpensive, and has been used in successful projects all over the world. "Existing meshes are well-tested and work well," says Klemm. "We’re still looking for other materials, but the Chilean version is a very good one, and it’s cheap."
The mesh design is also better able to withstand the strong winds present in many fog-heavy areas than a solid plate would be, says Klemm. In addition, while high-tech designs could be more efficient, they could also result in fog collectors that are more expensive and harder to maintain in remote arid regions, Klemm says. "The more engineering there is, the less is the chance that these will be applicable for poor people in semi-desert regions," he says.
To harvest large amounts of water, researchers use a bigger version of the Standard Fog Collector called a Large Fog Collector, with a mesh that’s about 40–50 square meters in size. Both versions of the fog collector use a double-layered polypropylene mesh manufactured in Chile, with a pattern of vertically stretched triangles.
Researchers have been trying to optimize the mesh-based fog collectors, and Cal State’s Fernandez says he’s currently testing three different types of meshes. "We’re comparing the standard one made in Chile, we’re comparing one made in Germany, which is supposedly a bit more effective, and we’re comparing one that’s made by researchers made at MIT, which is supposedly optimal," he says. Fernandez says he’s collecting data from all three types of meshes, and should have the results sometime in 2015.
The MIT mesh was developed by mechanical engineering professor Gareth McKinley and chemical engineering professor Robert Cohen. The new mesh is much finer, made of stainless steel filaments 3-4 times the thickness of a human hair, and has smaller holes and a special coating that allows water to flow down more easily. Laboratory experiments showed that these changes could make it five times as efficient as traditional meshes, and the group published their results in July 2013.
The MIT group notes that the increased efficiency of fog collectors made of their optimized mesh could expand the geographic range where collectors would be useful. Fog collection is currently most useful only in certain areas — but in those areas, it can be invaluable.
Location, location, location
Fog collectors could make a significant difference to the water supply of many arid regions. A United Nations report notes that "fog collection technology appears to be an extremely promising and low-cost water harvesting system for drinking water, crop irrigation, livestock beverage, and forest restoration in dryland mountains."
"Fog collection makes sense in very specific regions of the world," Klemm says. "These are areas where there’s a lot of fog but no rain, and a lot of wind, and they can collect huge amounts of fog water there," he says. It’s not just the abundance of fog that makes these areas suitable for fog collectors, but also their sparse populations and lack of other freshwater sources, he says.
Klemm cites Guatemala as a successful example. In the high mountain regions, some villages don’t have fresh water close at hand. Villagers must go down to the valley to get water and then carry it back up the mountain. With fog collection, though, each villager gets the few liters of water they need daily without the strenuous hike.
"I think in some areas one can really develop nice networks of fog collectors, that really supply several hundreds or thousands of people," Klemm says. "But you need the right environment for it."
Given the current extreme drought in California, and how much fog the state gets both at the coast and as Tule fog in the central valley, it’s tempting to think that fog water could help alleviate California’s water deficit. The problem is that unlike the inhabitants of remote desert regions in Chile or Eritrea, an average US citizen uses about 100 gallons of water a day. That’s without taking into account how many people live in California, or the even vaster amounts of water needed for the state's agriculture. Fog collection would struggle to meet the water needs there, especially since California could get water more efficiently and cost-effectively from its snowpack and rivers and dams, Münster’s Klemm says.
"If somebody was surviving on a few gallons a day, that might make sense, but given the vast amounts of water we consume and waste, I would question the value of trying to hook up fog into a municipal water system," says Fernandez. We’re better off first learning to use less water and to minimize wastage, he says. "Just the amount of water wasted would far exceed the amount that one could physically bring in by setting up fog collectors."
There are situations where the collectors could be useful, though. There are areas in Spain and in the Canary Islands where fog water is collected to help fight forest fires, Klemm says, and one could envision similar initiatives in California. Fog collectors could be placed in remote areas to collect drinking water for wild animals, to collect extra water during a dry summer, or to supplement rainwater capture in a building that already has a cistern, Fernandez says.
It’s premature to think that any new technology, whether for fog collection or for something else, could solve all our water problems, Fernandez says. But fog collection could help in small ways, and a number of different methods like this could all play a part. "That’s where I think the excitement is, in the innovations that people can come up with to apply things like this," he says. "Fog is such an amazing phenomenon, beautiful and mysterious."