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Engineers on Earth are also increasingly looking to the atmosphere for water. They have good reason to do so. Even in the depths of Chile’s Atacama Desert, often called the driest place on Earth, estimates show that fog and dew can generate about 200 milliliters of water per square meter. Elsewhere the atmosphere is more liberal. It is estimated to contain 12,900 cubic kilometers of water worldwide, which is approximately the size of Lake Superior. Furthermore, models indicate that evaporation induced by global warming will increase these levels by 27% during the next 50 years.
Tapping this invisible resource is a priority. As the Earth’s temperature rises and its population grows, more and more people are likely to face water shortages. More than 2.3 billion people currently live in water-scarce countries and analysts predict that nearly a third of these people will be forced from their homes by 2030 due to further drought.
Collecting water from the air is nothing new. The Incas, who are widely believed to have invented this technique, placed buckets under trees to collect condensation from dense fog drifting in from the ocean. Laurel, juniper and pines on the Canary Islands are known as “fountain trees” due to their association with fog harvesting. People living in the dry mountains of Oman have long built pools under trees for this reason.
Modern atmospheric water harvesting follows many of the same principles. However, rather than using leaves as condensation traps, which trap dripping over an impractically large area, modern traps consist of sheets of very fine polymer mesh. As the fog flows through the sheets, tiny water droplets stick to the polymer fibers. These droplets grow until gravity pulls them into a denser trough and from there into a reservoir. While collectors vary in size, a 40-metre-square collector in a reasonable fog area produces about 200 liters per day, enough to supply drinking water to 60 people.
Further improvement is possible. A team led by Urszula Stachewicz at the AGH University of Kraków in Poland found that the sheet could be made even more productive by changing the way its polymer threads were manufactured. Dr Stachewicz theorized that careful manufacturing through a process called electrospinning could give the sheet a slight electrical charge that would prove attractive to the water droplets in the fog. In experiments conducted in 2021, he and a team of colleagues found that such sheets lost 50% more water.
Last August, she and Gregory Parisi, a PhD student at Rensselaer Polytechnic Institute in New York, reported further improvements by adding titanium dioxide (TiO2) to the trap. Previous work had shown that titanium dioxide could be made superhydrophilic (extremely attractive to water) when exposed to ultraviolet light – a hindrance in extremely hazy conditions, as water would stick to the mesh rather than drip into the tank. However, when the fog was light, Dr. Stachewicz and Mr. Parisi found that the TiO2-enhanced trap became 30% more effective. Its fog collectors are now being used at sites on three continents.
Further inland, where fog is less frequent, other solutions are needed. One effective approach involves harnessing the water already present in the air. When the temperature falls, the water carrying capacity of the air also decreases with it. This causes excess water to condense on surfaces, a process often seen as dew. This is common in water-saturated places such as Britain, but anywhere with little wind and an average relative humidity of 70% or more, water can leach out of the air.
A major way to do this is through radiative cooling, a phenomenon that occurs at night when some materials (such as aluminum) emit enough heat to cool below the temperature of their surroundings. After sunset, water condenses on these materials, forming drops and dripping. Chambers made of these radiation materials sometimes include adsorbent internal surfaces to which water in the air readily adheres. When humid air flows into such chambers, it loses its water when exposed to cold conditions before escaping. A major advantage is that such technologies work best in places like deserts, where there are clear skies, high temperatures during the day and cool nights.
A significant limitation of radiative cooling has long been its relative ineffectiveness on day-to-day basis. That changed in 2021 when Dimos Polyakos and his then-doctor-student Ivan Hechler at ETH Zurich designed a piece of glass with a layer of silver on the bottom and a layer of silicon polymer between layers of chromium on top. , The silver layer reflected incoming sunlight while the sandwich polymer allowed the device to release heat in the form of infrared radiation. This cooled the glass by 15°C below ambient temperature, allowing condensation to occur even during the heat of the day. Along with a heat shield, a condensation chamber made of this glass helped produce 1.2 liters of water per square meter per day.
Another challenge posed by radiation cooling systems is that water must be wiped off the surface of the collection chambers. This requires electricity, usually from nearby turbines or solar panels, which can be expensive. To cut costs, Dr. Polyakos and Dr. Heckler applied a superhydrophobic coating to the surface of the chamber, which forced water droplets away from the surface and made it possible for the device to operate without electricity.
Such technology is actually inexpensive, with prototypes costing less than $50. But in many areas where water is desperately needed, humidity levels are too low for dew accumulation. In such places, the most promising options are those that use superabsorbent materials.
Many salts, chemical cousins of the familiar sodium chloride, will readily strip water from the air. With this in mind, an engineering team led by Peng Wang at King Abdullah University of Science and Technology in Saudi Arabia studied the effectiveness of hollow nanocarbon capsules filled with lithium chloride. In 2020 researchers reported that these capsules could capture more than double their weight in water vapor from ambient air when relative humidity was below 60%. Similar techniques using other salts have proven capable of collecting water at humidity levels as low as 10%.
The findings are promising, but the technology has yet to progress beyond the prototype stage. The problem is incompetence; Even Dr Wang’s world-leading capsules can produce only 1.6 liters of water per kilogram of lithium chloride over the course of ten hours, even in extremely dry conditions. Better than nothing, but insufficient to sustain a community.
However, between the two, these technologies suggest that a brighter future is possible. Those areas have become so dry that there has been no rainfall since modern records began, and there may one day be enough water to sustain settlement. And not just on an imaginary planet.
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© 2024, The Economist Newspaper Limited. All rights reserved. From The Economist, published under license. Original content can be found at www.economist.com
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