By Kevin Wilcox
Researchers are testing state-of-the art coatings to dramatically improve the process of collecting water from fog in the arid regions of Chile.
The coastal mountains of Chile feature thick, abundant fog that can be harvested to supply potable water to remote residents in arid regions. Wikimedia Commons/McKay Savage
October 8, 2013—On a remote hillside 400 km north of Santiago, Chile, researchers from MIT and colleagues from the Pontifical Catholic University (PUC) of Chile are testing samples of mesh material that could dramatically improve "fog catching"—the process of producing potable water from fog—in the arid region.
Northern Chile includes some of the driest areas on earth. Some rain gauges in the Atacama Desert, for instance, have never recorded rain. Water resources are incredibly scarce and in high demand. Trucks deliver water to remote villagers and to miners extracting the region's plentiful copper, gold, and silver deposits.
Although it seldom rains, the region experiences heavy, abundant fog that rolls in off the Pacific Ocean, driven by strong winds. Engineers have been working for decades to develop effective systems to collect water from this fog.
The systems deploy large panels of mesh above a collection basin. As the wind blows fog through the mesh screens, water droplets are trapped in the fabric and run down into the basin. Although the systems have proven effective, they have historically captured only a small percentage of the water available in the fog.
To bolster this efficiency, Juan de Dios Rivera, Ph.D., a professor at PUC who researches fog harvesting systems in Chile, contacted two professors who research wettability at MIT: Robert E. Cohen, Ph.D., a professor of chemical engineering, and Gareth H. McKinley Ph.D., a professor of mechanical engineering, at MIT. The central question he asked was if it was possible to use cutting-edge wettability knowledge to develop more efficient screens for fog collectors, according to McKinley.
"If you've been sailing, you know a sail can trap a lot of water in the fabric, but it doesn't necessarily collect a lot," McKinley says. "The central idea behind our [research] was to try and start thinking about what happens if you systematically control and vary the surface wettability. What you want is for the fog droplets to impact the surface, stick on the mesh, and then drain down the surface to be collected before they get blown off the surface or the sun comes out and they evaporate away."
A fog-harvesting mesh developed by engineers at MIT is being tested in Chile. The mesh uses steel strands and coatings designed to optimize water collection. © G.H. Mckinley, MIT
Their research resulted in the paper "Optimal Design of Permeable Fiber Network Structures for Fog Harvesting," published recently in the journal Langmuir. The paper details a theoretical analysis and accompanying lab tests geared toward developing a more efficient mesh to collect water droplets from fog.
"What we show in that paper is basically that the current systems are operating at something like 2 percent efficiency, and we can increase that five-fold, up to about 10 percent efficiency, depending on wind speed and fog droplet size," McKinley says. "That's still not very high, but for a given wind speed and fog sizes, our experiments are very close to what we predict theoretically."
The meshes that the MIT team developed comprise thin steel strands coated with several different flexible polymer coatings that enable water drops netted by the mesh to quickly roll off into a collection vessel below. The team's analysis and experiments found that thinner fibers are more efficient at collecting and dispelling water droplets from fog.
"You have these very strong onshore winds in the morning. That brings very saturated air in from the Pacific and blows it up a mountain. As it blows up the mountain, it cools, the fog condenses, and you get essentially 100 percent saturated air, with very large fog droplets moving very fast. And under those conditions you could probably do even better than 10 percent," McKinley says.
McKinley explains that the efficiency of fog collection will always be limited because the mesh must be sufficiently open or porous to allow the wind-blown fog droplets to pass through. The 10 percent collection efficiency figure, however, does not capture the full benefit of a fog-collecting system; once they are installed the systems require no energy expenditure to collect the water. And because the systems are usually sited atop hills, gravity can provide the potential energy needed to bring water from the collection site to the users.
Traditionally, fog-harvesting mesh has been made from nylon. This installation in Majada Blanca, Chile, is thought to be the largest in the world. © G.H. Mckinley, MIT
"The really nice thing about these systems is they are completely passive," McKinley says. "Nature has already put in all the energy that you need. The sun has evaporated the water—what you would call solar desalination. When the water evaporates from the ocean, almost all of the impurities get left behind. Fog water is potable as is; you don't have to do anything. It's pure enough to drink. Condensation into fog droplets has also been taken care of, and then the wind does the rest of it for you."
The systems also require little operation and maintenance expense aside from periodic cleanings to remove insects and mold and to ensure the mesh netting remains intact. In laboratory conditions with constant, ideal humidity and continuous wind speeds of 2 m/s, 1 sq m panels made of the MIT mesh can capture as much as 12 liters of water per day. Field testing in Chile is under way.
"At the moment, we are comparing three surfaces: a control and two test surfaces with different coatings on the standard fog collectors [with 1 sq m panels]," McKinley says. "There is also a lot of research going on with much larger designs. Our colleagues in Chile are building some very, very large fog collectors to try to look at what happens when you change the scale of the structure."
The systems have possible applications far beyond Chile, in areas as diverse as Guatemala, Ethiopia, Nepal, and Morocco. The Canadian charity Fog Questhas been developing the systems in rural communities in developing countries for more than 25 years.