|October 15, 2018|
Can stripping the air of its moisture quench the world's thirst?
|We live in a thirsty world. Each person on Earth needs about 50 L of water each day to meet basic needs, including water for drinking, food preparation, sanitation, and personal hygiene, according to the World Health Organization. Despite the planet's rich water resources, scientists estimate that 4 billion people---more than half the world's population---don't have enough water for at least one month each year, and 500 million of them don't have enough throughout the entire year. Factor in climate change and a global population expected to reach 10 billion by 2050, and it becomes clear the world is only going to get thirstier.|
To slake that thirst, scientists and engineers have been searching for solutions to the growing problem of water scarcity. One source of water they've been trying to tap is moisture in the air. For one thing, it's everywhere---even in the desert---so it has the potential to help water-deprived people no matter where they're located.
The atmosphere contains as much as 1.29 x 1016 L of water, in the form of clouds, fog, and water vapor. In global terms, that number is small, only 0.001% of the world's total water. But in relative terms, it's a deluge---six times as much water as what's in the world's rivers, a major source of drinking water.
Scientists and a growing number of companies that make air-from-water devices are working on harvesting water from the atmosphere in ways that are practical, efficient, and inexpensive. But mining moisture from air isn't trivial, and scientists agree there won't be a single technology that's best for every locale. In foggy areas, the answer may be putting up systems that can coax water droplets suspended in the air to coalesce. In humid climates, where the air is thick with water vapor but the vapor hasn't nucleated into droplets, devices that can condense and collect it might be best. And in arid regions, specialized machines that can sop what little moisture that's around will need to be developed.
Air holds two completely different kinds of water, "and we have different tricks for harvesting them," says Jonathan Boreyko, who leads the Nature-Inspired Fluids & Interfaces Lab at Virginia Tech.
With water vapor, it's necessary to force gaseous water in the air to form liquid dew drops on a collecting surface. "It's a pretty multifaceted process, especially if you want to collect condensation in a low-humidity region," Boreyko says. With fog, he adds, the process is a lot more like instant gratification.
"Fog is microscopic droplets that are already in the liquid phase, and you're simply catching them using some kind of net or collecting surface," Boreyko continues. Coastal areas, for example, have winds that are laden with millions and millions of tiny water drops. "You simply catch them," he explains, and eventually the water droplets grow on whatever has caught them so they fall via gravity into a water collector.
Existing fog-harvesting systems look like volleyball nets, Boreyko says, except they're made with a fine mesh that's similar to what's in a window screen. "There's actually a huge problem with this design," he notes. "If the mesh wires are too small and close together, the water you catch can clog all the holes. The water gets stuck by surface tension and has a hard time draining to a collector. If you clog the holes, now the wind goes around the mesh, not through it, and you stop collecting water." But if the holes are too big, they won't catch the water.
Scientists have been trying to address this problem with chemistry by adding coatings to the mesh materials, but Boreyko says the coatings aren't durable. They tend to wear away from the mesh when exposed to the elements. Boreyko's lab, in collaboration with Virginia Tech industrial design professor Brook S. Kennedy, has taken a different tack by redesigning fog-collecting devices, ones that don't require coatings. Like many in the field of fog harvesting, Boreyko and Kennedy found inspiration in plants, which can't move to collect the water they need and have therefore developed clever ways to hydrate themselves.
Specifically, the researchers noted how giant sequoia trees collect water from fog on their needles. These needles are parallel to one another, so water collects on the needles, rolls downward, and then drips onto the ground to nourish the tree's roots. Boreyko and Kennedy reasoned they could achieve the same effect by creating a fog-harvesting apparatus made from vertical parallel wires---a fog harp
"If you only have the vertical wires, there are no horizontal wires in the way of the droplets," Boreyko explains, and no clogging can occur. Small, lab-scale harps can harvest three times as much water as similar-sized meshes. Looking to scale up the advance, the researchers have made a meter-scale harp out of aluminum and are looking into methods to make fog-harp farms.
Durability is a major challenge in developing practical fog-harvesting surfaces, says Jas Pal Badyal, who studies surface morphology and surface chemistry at Durham University. The other major challenge, he says, is cost. "That's been our main motivation," Badyal explains. "To come up with ways that water can be collected on a reasonably large scale but at low cost."
Plants have also inspired Badyal's group. "In developing countries, we've been looking at how local plants collect moisture or water, and we've been trying to replicate those systems by combining surface architecture and surface chemistry." For example, the tiny hairs on the Salsola crassa plant, a small shrub that grows in arid climates, collect fog and turn it into water droplets that drip along the plant's leaves and onto the ground to provide hydration. Scientists on Badyal's team found they could mimic this behavior by applying a hydrophilic coating to inexpensive nonwoven fabrics, which have a fibrous structure and are used to make dust masks.
To gather fog from rooftops, Badyal and coworkers plan to combine this material with a different bioinspired approach they developed to collect water during a heavy rainfall. They create tiles that resemble the branchlets of the coniferous Thuja plicata, which grows in the Pacific Northwest. By getting the pore size, tile angle, and layering of the structure just right, they can direct lots of water into a single stream for collection. They've made plastic prototypes, but Badyal says they could also create similar tiles by punching holes in nonwoven materials that would then be able to collect and channel water from fog.
Advances in chemistry and materials can improve all water-harvesting systems, says Tak-Sing Wong, an engineering professor at Pennsylvania State University. Wong's lab, in collaboration with Xianming (Simon) Dai's group at the University of Texas, Dallas, has also developed a bioinspired surface for collecting and directing droplets of water.
Wong and Dai's inspiration: Rice plants' leaves possess parallel grooves, which direct water to their roots, and carnivorous pitcher plants have a slippery lubricant on their surface to move water and insect prey down into a trap. The scientists combined these two aspects by making a surface etched with parallel microgrooves to which they applied a thin coating of the hydrophilic lubricant hydroxy-terminated poly(dimethylsiloxane). Nanoscale texture within the grooves keeps the lubricant in place. The surface quickly gathers droplets of water in simulated fog and then channels them for collection, outperforming smooth surfaces with similar slippery coatings.
Even though the researchers have built the surface on the square-meter scale, they say the major challenge in commercializing it is the durability of the lubricant. It lasts about two weeks, which isn't long enough for practical applications. "We are trying to select a better lubricant that will last longer or find a way to replenish the lubricant," Dai says.
Gathering water by fog harvesting is attractive, researchers say, because it doesn't require an energy source. But it also has its limits. Amounts of water collected can be small, and the technology works best in regions of high humidity, where fog is plentiful. But water scarcity is a bigger threat in arid parts of the world.
"In the U.S., water availability is not too big of a problem, and water comes at a very low cost," Wong says. "But in many other regions of the world, water is not as accessible, particularly for those that are far from the ocean" and therefore don't have easy access to desalinated water. Water-from-air technologies, he says, don't rely on gathering and cleaning water at a centralized location and then distributing it, so they have the potential to deliver potable water almost anywhere on Earth.
Although there's no clear market leader, several companies currently sell water-from-air machines. Drinkable Air, for example, has been in the business for eight years. The company's products range from small units designed for in-home use to a trailer-sized setup that could provide enough water for a large hotel, about 150,000 L per day. Like most commercial water-from-air devices, it collects water vapor in air using a cooling system.
"It's a dehumidifier on steroids," explains Michael Bourgon, Drinkable Air's international business development director. The machine pulls in air through an electrostatic filter, which removes dust and other particles. Then the air passes over the cold surface of a condenser, which is coated with a lubricant that's safe to consume. Vapor collects on the condenser's surface as liquid water, which drops into a collection tank. Drinkable Air's patented ozone purification system cleans the water further. Before it's ready for drinking, the water travels through a carbon filter and a mineral cartridge, which adds calcium, magnesium, and sulfates to make the water more alkaline and improve its taste.
Currently, Bourgon says, the technology's cost comes out to about 6 cents per liter of water. That's about six times the cost of a liter of desalinated water, but it's far less than bottled water, which ranges from 21 cents per liter in Turkey to $1.25 per liter in Denmark.
Some cities facing water scarcity appear to rely on importing bottled water to make up for shortfalls in water supply, says Roland V. Wahlgren, a principal with Atmoswater Research and president of Canadian Dew Technologies, which develops water-from-air systems. "But that's probably not sustainable or affordable in the long run," he says.
Ultimately, it comes down to cost, Wahlgren says. "Even if you had a dirty, polluted source of liquid water, it would be cheaper to clean that up with conventional water treatment methods than to use a water-from-air machine to provide the same volume of water per day" when you factor in equipment and energy costs, he explains.
"Water-from-air machines come into play if there's a real scarcity of liquid water supplies," Wahlgren says, such as places that are far from the ocean where all the groundwater is spoken for. He points out that many cities, such as Mexico City; Beijing; and Bangalore, India, face looming water shortages in the coming decades. "Water from air may be the only solution for providing enough drinking water for everybody," he says.
Even so, Wahlgren thinks the move to water harvesting might be too slow for these cities. "Perhaps some water-from-air companies will fail because they can't stay in business long enough until the demand makes those businesses viable," he says.
DRYING AIR WITH DESICCANTS
Jonas Wamstad, CEO of the water-harvesting company Drupps, is hoping his company, which spun off from the humidity-control company Airwatergreen in 2017, will stand out from the competition, thanks to a different kind of water-collecting technology. In the Middle East, he points out, water-harvesting technologies based on cooling tend not to work well because the relative humidity is low during the day, so there's less water to gather. Humidity is higher after dusk, but low nighttime temperatures make cooling systems energy inefficient because you have to cool the condensers to lower temperatures.
Drupps's technology doesn't use cooling but instead relies on a liquid desiccant to pull moisture from the air, Wamstad explains. Sitting in one module of the device that's roughly the size of a shipping container, this proprietary slurry soaks up moisture from the air. The water-laden desiccant then moves into a second module, where water is boiled off, cooled, and collected. Dried of its water, the liquid desiccant returns to the first module to repeat the process.
"Depending on the climate and the number of modules, we can have a system producing up to 700,000 L of water a day," Wamstad says. The technology, he says, works most places on Earth, except for the driest desert, and costs about 2 cents per liter of water. Drupps's technology, like condensers, is still an energy-intensive system. But it can be powered with thermal energy---burning garbage, for example---rather than electricity, which means it can be used in places that don't have access to the electrical grid. "Our main aim is to make atmospheric water cheap enough for everyone to use it," Wamstad says.
Evelyn Wang, an engineering professor at Massachusetts Institute of Technology, also recently developed a water-harvesting system that uses a desiccant to pull water from the air. The desiccant, developed by Omar Yaghi's group at the University of California, Berkeley, consists of dust-sized crystals of a metal-organic framework known as MOF-801. The crystals are embedded in a porous layer where the top side is coated black and serves as the solar absorber. These crystals are inside a chamber that's open to the air. As air flows through the chamber, water attaches to the MOF. Sunlight then heats the MOF, driving the water onto a condenser, where it cools and drips into a collector.
The device is designed so that the relative humidity in the chamber is almost 100% "so you can condense the water near ambient conditions," Wang explains. That means the device doesn't require any active cooling to get the water to condense, making it significantly more energy efficient.
The device can harvest water even when the relative humidity of the air is as low as 20%---a level that's lower than what you'd find in the Sahara desert. Earlier this year, Wang's lab field-tested the device in Tempe, Ariz., where the relative humidity ranges from 10 to 40%. The prototype was able to harvest 0.25 L of water each day for every kilogram of MOF-801 it contained.
The biggest challenge in scaling up this prototype is the availability of MOF-801. "It is difficult to make the material at a large enough scale, which also makes it expensive," Wang says.
Looking to bring down the cost of water-from-air systems, the Water Abundance XPrize is offering a $1.75 million purse to anyone who can come up with an atmospheric water-harvesting device that can deliver 2,000 L of water per day at a cost of 2 cents per liter using only renewable energy.
Zenia Tata, XPrize's chief impact officer, acknowledges that there are already water-from-air devices on the market. But, she says, "they use high amounts of energy and usually are large and expensive to operate."
Registration for the competition opened in October 2016, and by May of 2017, 98 teams from 25 countries had entered. In March, XPrize announced five finalists, whose technologies are being evaluated in a final round of testing. The grand-prize winner will be announced in a few weeks, Tata says.
So, will water-from-air technologies help quench the world's thirst? They're likely to be part of a multifaceted approach to addressing water scarcity, experts say, but none will be a stand-alone solution. Other technologies, as well as improved water conservation and distribution systems, will be needed too. Even so, devices that pull water from the air show promise and with continued innovation could make the thirstiest parts of the world a little less parched.﻿