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Out of thin airTech could someday let people even in dry climates
get clean water straight from the atmosphere 13 Mar 2023
On a desert planet in a galaxy far, far away, the land is hot, dry and devoid of any large bodies of surface water. It is a parched world, desolate in that way only a planet illuminated by a pair of binary stars can be. Fantastical, yes; a pop-culture icon, also yes.
But there are two reasons to start with a description of Tatooine, the desert planet that appears in the Star Wars franchise: The technology seen here has become a reality, and we can test it in the real-world places that inspired the fictional landscape.
We’re talking moisture farming.
The moisture vaporators, also known as vapor spires in the Star Wars lingo, are devices used on Tatooine moisture farms to capture water from the atmosphere. Tall and slender, they were stationed at ground level and used refrigerated condensers to pull water from the air around them. Captured water was then pumped or gravity-directed into a storage cistern. These devices could collect 1.5 liters of water every day, even when the relative humidity of the air was only 1.5 percent. An amazing idea, and now becoming real as new technologies and materials emerge to harvest previously untappable water from the atmosphere.
The basic concept is simple. If you take an ice-cold glass of water outside on a hot day, you’ll quickly notice water droplets forming on the outside of the glass. If you cool warm, humid air, it loses its capacity to maintain its water content and you can produce and collect condensation, whether it’s on the outside of your glass or in a moisture vaporator straight out of science-fiction.
Rather than waiting for the rain, bring the rain to you
When clean drinking water comes out of the tap at home, it’s easy to think that it will always be plentiful, but fresh water is actually incredibly rare. Only 3 percent of the world’s water is potable, and two-thirds of that is stored away, frozen in glaciers, or otherwise unavailable for our use.
Almost everyone has bought something and found a packet of silica gel beads placed inside to absorb moisture while items are waiting to make their way to the customer. Read more›››
Silica is commercially available, inexpensive and a highly effective desiccant.
Silica can also be also used in water production via the conventional condensation approach.
Silica gel is one of the most commonly used materials in moisture harvesting, and Lisa Klein, professor of materials science and engineering at Rutgers University, has investigated using patterns on silica gel to facilitate water-droplet formation.
She conducted a series of experiments to condense water vapor on the hydrophilic pattern of silica gels. Although the pattern was hydrophilic, the gel itself was hydrophobic so the water droplets slide down the surface and collect in a container rather than absorb into the gel. This represents another potential area of investigation for harvesting water from the atmosphere. ‹‹‹ Read less
As a result, more than 1 billion people worldwide lack access to clean water year-round. Global warming may be melting those glaciers, but as humans continue to pump carbon dioxide into the atmosphere, weather and water patterns will change, combining to make less water available for people around the world. By 2025, predicts the World Wildlife Fund, two-thirds of the world’s population may be facing water shortages.
Technologies such as filtration, desalination and solar purification have been developed to use seawater or wastewater. However, because they depend on terrestrial water sources, these technologies are feasible only in coastal areas. Atmospheric water, however, is present everywhere, and the global water cycle enables a sustainable supply of water to the air, providing a resource equivalent to about 10 percent of all the fresh waterin lakes on Earth.
At 100 percent humidity, the air at 40 degrees Celsius contains about 51 milliliters of water per cubic meter of air. For the same humidity at 10 degrees Celsius, the air contains only 9.3 milliliters. Bring that 40-degree air down to 10 degrees and you should be able to extract that water difference. Scale that up and you could produce an awful lot of water on one of those sticky, hot Arabian Peninsula days.
Technologies already exist to catch fog or collect dew that condenses overnight, but pulling water directly from the air, without consuming lots of electricity, is still under development. Still, Ruzhu Wang, professor at Shanghai Jiao Tong University, says atmospheric water harvesting is accessible everywhere and can be easily co-operated with a renewable energy source for local needs.
The problem, Wang writes in Joule, is that there are few commercial water-harvesting systems available.
But when those systems do become available?
“In general, any viable atmospheric water-harvesting technology must satisfy five primary criteria: It should be efficient, cheap, scalable, wide-band and stable enough to operate for a whole year or at least a monsoon season,” Wang writes.
None of the existing commercial atmospheric-water generators meets all five criteria. Wang says this is mainly due to the energy inefficiency of the process.
So, the ideal moisture harvester has a high water uptake, low-energy demand for water release, fast water capture/release cycling, high cycling stability and a low cost — a tall order but one that could be achieved with advances in material science.
Living in a material world
Atmospheric water harvesting based on moisture harvesters captures vapor from the air via adsorption, where water molecules adhere to the surface of a material through chemical or physical interactions.
For chemical adsorption, the surface needs to adsorb water through strong chemical bonding; releasing the water requires a large energy supply.
Physical adsorption needs pores in the surface, where water molecules can pool and collect. Energy is still required to release the water, but at a significantly lower rate than chemical adsorption.
Porous materials capture the water from the atmosphere, but said pores need to be perfect; you can’t just place a sponge in the desert and expect water to collect.
Enter the metal-organic framework (MOF): a network of metal and organic materials that can easily trap water vapor, which is then released using energy captured from the sun.
Water load of options
But MOFs aren’t the only material vying for a slice of the water-harvesting pie: hydrogels and zeolites have also entered the ring.MOFs work great in areas with lower humidity, but they have a finite number of pores. Fill those, and your harvesting device stalls until they can be emptied.
The United Arab Emirates has all the necessary ingredients for fog as dry desert conditions exist next to the warm seas of the Arabian Gulf, with moist air carried inland by the afternoon sea breeze cooled by the night desert surface. Read more›››
Tendrils of fog snake their way through the dunes in the early morning and could be captured by the fog-farming technologies already available. At the same time, the humidity that plagues the region during the hot months makes atmospheric-water generation viable.
Combining both approaches could reduce dependency on desalination and provide clean water for the many farms found far out in the desert.‹‹‹ Read less
Hydrogels, on the other hand, can expand to hold more water. The soft, pliable and thin material that makes up more than 90 percent of contact lenses prescribed in the United States is a hydrogel: a water-swollen polymeric material that maintains a 3D structure.
The 3D network of hydrophilic polymers can swell in water while maintaining its structure and is tunable, dynamic, biodegradable and, most importantly, capable of encapsulating huge amounts of water.
Let’s just use hydrogels, then. Well, they’re not the best in low-humidity areas — they like it muggy outside.
Although they may not be suited to the deserts of the Middle East, there are plenty of places with high humidity that are also water-stressed. Lima, Peru, is one such place.
Just south of Lima is the village of Bujama. Despite being in an area where air humidity reaches 98 percent, Bujama is almost a desert, and its people live in tough conditions with little access to clean water.
Researchers from the University of Engineering and Technology in Lima installed panels in advertising billboards that trap the humidity and transform it into drinking water for the people on the ground. These panels comprise filters and condensers and produced 96 liters of water a day in 2013.
People here may already have one solution to water scarcity, but that doesn’t mean hydrogels couldn’t also work in Bujama.
Zeolites are often considered “molecular sieves” as they can selectively sort molecules based primarily on a size-exclusive process. They are easy to manufacture and have a large internal surface area full of pores to adsorb the tiny quantities of water held in desert air — another contender for the low-humidity application.
Water is running out and we know that desalination is not the solution. It’s not just drinking water, it’s all the water used in industry, in agriculture.
– Michael Rutman, co-CEO of Watergen
The zeolite can collect water vapor overnight, and heat from the sun can then be used to extract the water for use. However, compared to MOFs and hydrogels, the water capacity of zeolites is relatively low, and releasing the water requires a high energy consumption that, even when supplied by solar power, make zeolites a less efficient option.
In areas where water scarcity is a problem — and climate change is putting more areas at risk — it’s important to consider different technologies and approaches.
Condensing the problem
The billboard in Bujama is just one example of the condenser approach. Michael Rutman is co-CEO of Watergen, a company creating drinking water from air. Based in Israel, “which has a very similar climate to the UAE,” Watergen uses a system involving food-grade polymer condensers and filters to draw water out of the air around us.
“Adsorption can only generate so much water,” Rutman explains. “It also requires a much larger resource footprint than condensation, and much more energy. Metal-organic frameworks that don’t need quite so much energy input to draw the water out are under development, but the metal part of a MOF should also be a concern.”
Rutman points out that an air conditioning unit does much the same thing as a Watergen condensing system: pull warm air out of the environment and cool it, producing water as a by-product. However, the heat exchanger material in an AC unit is usually made of metal, and that metal leeches into the resulting water.
“That’s why you don’t drink from your AC,” Rutman says, laughing. “An AC unit produces tons of water, but that water is contaminated with heavy metals. The Watergen systems use food-grade polymers in the heat-exchanger technology, so the water produced is immediately potable, but we also add minerals to further improve the quality.”
Watergen didn’t set out to save the world from its water problem; the company started by trying to make dehumidifiers more efficient and less power-hungry.
It was Michael Mirilashvili, an Israeli-Georgian businessman, who declared they were wasting this technology. Now president of Watergen, Mirilashvili realized these highly efficient polymers could be used to solve the world’s biggest problem and spent the past five years pivoting the company to producing water from the air for everyone.
Their system works, too. It works in areas of high humidity, such as Colombia and South Africa, but it also works in the driest of places, like Arizona in the U.S., where the average relative humidity is 38.5 percent.
Rutman says he believes mass use of atmospheric water generation is the future.
“Water is running out and we know that desalination is not the solution. It’s not just drinking water, it’s all the water used in industry, in agriculture. It can take 160 liters of water to make a pair of jeans, and 60 liters for a loaf of bread. All this water can be replaced by water produced from the air. I believe we’re less than ten years away from this point. Our pilot technology works, and it’ll work everywhere.”
Speaking of everywhere, we should also start thinking about portability.
Conventional water supply starts with a large centralized plant that distributes water to the population, but if a device were small enough to incorporate into a home, gaps in water supply could be plugged.
Make them even smaller and they could travel to all sorts of now-uninhabitable regions: the middle of the desert, the polar extremes, Mars?
Back down to Earth
Understandably, research institutes in the Middle East are particularly invested in this new type of technology. Many of the projects showing promise in the U.S. were funded by Saudi Arabia’s King Abdulaziz City for Science and Technology, including projects designed by Omar Yaghi, pioneer in the MOF space, and his teams.
Similar technology is behind an industry-funded project at Masdar City, a hub for sustainability research and innovation in the MENA, with whom Khalifa University does research.
“As freshwater scarcity is becoming a global challenge, a promising route to overcoming water shortage is to extract water from air with innovative atmospheric water production (AWG) technologies,” says Samuel Mao, director of Masdar Institute at Khalifa University. “KU’s research team at Masdar Institute is performing comprehensive assessment of different AWG approaches, and developing advanced technologies to enable water extraction from air with better energy efficiency and lower cost.”
Almost half of all people on Earth live in water-threatened conditions, with demand growing drastically, while supply remains constant, according to the World Health Organization.
The United Nations recognizes that access to clean water and sanitation is at the core of sustainable development, and ensuring access requires innovation. Atmospheric water generation could be the solution, and it’s already here.
Residents of the world’s driest regions might someday owe a debt to the Namib desert beetle, which is giving up secrets to harvesting water from the air›››
'Blue diesel'-powered ships, COVID-19's effect on the oceans
and microbots to the rescue›››
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