An increasingly hotter and drier climate is set to put further strain on terrestrial water resources. But scientists are eyeing water in the atmosphere as an alternative source, XiaoZhi Lim reports
Water truly is everywhere – including in our atmosphere, points out Ruzhu Wang, a mechanical engineer at Shanghai Jiao Tong University, China. Depending on local humidity, a cubic litre of air could hold 10-20g of water, he points out. Altogether, Earth’s atmosphere holds 50,000km3 of fresh water that could be harvested, equivalent to 10% of all the water in lakes, or six times that in rivers. With water scarcity an issue facing every continent on earth, the real question, he says, is ‘how to capture this water?’
Obtaining water from the atmosphere is not a new practice (Joule, doi: 10.1016/j.joule.2018.07.015). Fog is already collected at small scales in coastal, yet arid, villages in South America, North Africa and parts of Asia. In ancient civilisations, piles of stones were used to direct droplets of dew water that condensed on them overnight to reservoirs.
Now, scientists are developing better materials for pulling moisture out of thin air. These materials are designed to work either passively or with energy from the sun. ‘Most of the areas where water has become an issue, energy at the same time is an issue,’ says Zongfu Yu, a materials scientist at the University of Wisconsin, Madison, US.
In April 2017, a landmark study demonstrated that metal-organic frameworks could potentially harvest 2.8L of water from dry, desert air with just 20% relative humidity in one day, using non-concentrated solar energy (Science, doi: 10.1126/science.aam8743). That study spurred intense interest in atmospheric water harvesting and since then, over 300 studies on the topic have appeared, says Wang.
Fog and dew potential
Some of the driest parts of the world receive fog instead of rain. Annual rainfall in the Namib Desert along the western coast of Namibia averages just 2-20mm, but thick fog appears for over 180 days/year. The driest place on Earth, the Atacama Desert that stretches across Chile and Peru, is also regularly shrouded in fog.
Fog occurs in these deserts when warm, humid air over the ocean moves inland and meets colder air. Such ‘fog deserts’ enjoy an advantage when it comes to harvesting atmospheric moisture as the airborne water is already in the liquid state. The downside, however, is that water collection must happen quickly. The day after a foggy day tends to be very hot and any uncollected water evaporates, says Kyoo-Chul Park, a mechanical engineer at Northwestern University, US. ‘If we do not move those water droplets to a reservoir, we will lose them.’
Airflow is key for catching these droplets, says Park, who has been investigating this problem for over seven years. Consider driving through the countryside – the faster the car goes, the more insects it collects on its windshield. Similarly, tiny water droplets in fog moving quickly through a net will naturally cling better to its threads compared with slow-moving fog, Park explains.
But most existing fog-catchers are stationary, comprising of a simple mesh net strung across two poles, and depend heavily on wind to blow fog through. Efficiencies of fog collection are thus low, hovering around a few percent.
Earth’s atmosphere holds 50,000km3 of fresh water that could be harvested, equivalent to 10% of all the water in lakes, or six times that in rivers.
Metal-organic frameworks could potentially harvest 2.8L of water from dry, desert air with just 20% relative humidity in one day.
Up to 90% of a hydrogel could be water, which means in its dehydrated form, a hydrogel could absorb and store a lot of water.
A second parameter that could help is the surface wettability of the threads or wires. In 2013, Park found that typical meshes benefit most from having a superhydrophobic surface (Langmuir, doi: 10.1021/la402409f). More recently, while investigating the fog-collecting behaviour of vertically placed single wires, he reported that a superhydrophilic surface performed better (Appl. Phys. Lett., doi: 10.1063/1.5087144). Park is currently working to find optimal combinations of fog-collector geometries and surface characteristics.
To access atmospheric moisture in other parts of the world that do not have fog, first, it needs to be condensed into liquid dew water. ‘The main difference between fog harvesting and dew harvesting is whether we use a cold surface or not,’ says Park.
Condensation occurs naturally overnight, when objects cool down by radiating heat to space. When the surface temperatures of grass blades, car windows, and other outdoor objects drop below dew point, dew forms on them.
Studying three types of life that thrive in dry deserts, Park in 2016 identified a combination of surface properties that promote condensation and enhance droplet transport – the miniscule bumps on the backs of Stenocara beetles, the asymmetrically sloped arrangements of cactus spines, and the slippery surfaces of pitcher plants (Nature, doi: 10.1038/nature16956). Park and his colleagues replicated all three features in a synthetic material. In one hour, a slippery surface with asymmetric bumps generated 35ml water/m2, while a flat surface of the same material produced under 5ml/m2.
More recently, new passive radiative cooling materials that can stay cooler than their surroundings even under hot sun have been reported. These materials are optimised to reflect the sun’s rays almost perfectly, while continuing to radiate heat away as they do at night. Most research groups developing these materials are targeting cooling applications. Yu, with Qiaoqiang Gan from the University of Buffalo, hopes to boost atmospheric water harvesting with them instead (Proc. SPIE 11121, doi: 10.1117/12.2525125). ‘The major limitation [in water production] is the condensation rate,’ Gan says.
On a 1mm-thick aluminium substrate, the researchers first deposited a 150nm-thick layer of silver, then covered that with polydimethylsiloxane (PDMS) around 100µm-thick. They placed this material on the roof of a campus building at the University of Wisconsin-Madison with a control and pumped humidified air over them. Compared with a white low-density polyethylene foil containing titanium dioxide nanoparticles, a standard material recommended for condensation by the International Organization for Dew Utilization, the PDMS-coated aluminium material produced double the water.
Hot and dry
The challenge of absorbing water vapour is greater in a dry environment with low relative humidity. Not only is there less moisture available, but lower temperatures are also required for condensation. Still, dry, arid regions need water most, and that’s where atmospheric water harvesting has the greatest potential, Wang says.
One strategy to collect water vapour from dry air is to first concentrate it within an absorbent, then use the water-saturated material to raise humidity within a chamber. That allows the vapour to be more easily condensed into a liquid and collected, Wang explains. For this, a material that can absorb and release water vapour is needed.
Metal-organic frameworks (MOFs) have long been touted as useful materials that can capture small molecules like hydrogen, natural gas, carbon dioxide or water in their pores, owing to their large surface area (see Digital DNA page 22). Omar Yaghi at the University of California, Berkeley, US, pioneered several MOF materials, and then realised MOF-801 – composed of zirconium ions linked with fumarate molecules – has a very high affinity for water.
Yaghi reached out to Evelyn Wang at the Massachusetts Institute of Technology to develop a practical device prototype, reported in their widely cited 2017 study (Science, doi: 10.1126/science.aam8743). The device is a simple acrylic enclosure with a condenser at the bottom. The researchers first allowed a thin layer of copper foam loaded with MOF-801 to accrue water outdoors overnight, then placed it inside the enclosure the next morning. During the day, the sun heats the MOF-801 layer up to almost 70°C, releasing hot water vapour that condensed into a collectable liquid.
Yaghi’s team tested a large version of the device in Scottsdale, Arizona (Sci. Adv., doi: 10.1126/sciadv.aat3198). In one seven-hour cycle, with 825g of MOF-801, 55g of liquid water was collected. Later, Yaghi developed a second, cheaper MOF material based on aluminium, called MOF-303. This material, tested in the Mojave Desert, produced 700g of liquid water/kg of MOF-303/day (ACS Cent. Sci., doi: 10.1021/acscentsci.9b00745).
Other inorganic materials such as anhydrous metal salts can also easily capture water. Many metal salts naturally contain water of crystallisation in their lattices. Compared with specialised materials like MOFs, such anhydrous salts are inexpensive and readily available at a large scale.
A research group at King Abdullah University of Science and Technology (KAUST) led by Peng Wang tested 14 anhydrous salts for their capability to absorb water (Environ. Sci. Technol., doi: 10.1021/acs.est.7b06373). Of the 14 salts, the researchers found three that could hold and release at least 20% of their own weight in water. When integrated into a water collection device, copper (II) sulfate and magnesium sulfate have slightly higher water capacities between 0.3-0.35g/g of salt, compared with copper (II) chloride, which produced just 0.21g of water/g of salt. However, copper (II) chloride can begin absorbing water at the lowest relative humidity, just 10%.
Combining the two types of materials, Shanghai Jiao Tong’s Wang and his team encapsulated an anhydrous metal salt, lithium chloride, inside the pores of a cobalt-based MOF called MIL-101 (Angew. Chem. Intl. Ed., doi: 10.1002/anie.201915170). This improved the water harvesting ability of both materials. At 30% relative humidity, 10g of material generated 4.5g of water in one 90-minute cycle.
Air plants that live on rocks or glass bowls do not have functional roots, drawing water instead from the atmosphere through their leaves. A group of researchers from the Ningbo Institute of Material Technology and Engineering, China, developed an organogel that mimics the structure of those leaves (Angew. Chem. Intl. Ed., doi: 10.1002/anie.202007885).
The researchers built a porous scaffold from two polymers and filled it with a hygroscopic liquid, glycerin, to create the organogel. A polymer carrying hydrophilic side chains attracts water molecules to the gel’s surface, traps them, and hands them off to the glycerin liquid. As with the cells of air plant leaves, glycerin then rapidly transports the water molecules to the gel’s interior. A second polymer, which converts light to thermal energy, heats the gel up and dries it out in the sun.
Hot and humid
Another class of materials useful for atmospheric water capture are hydrogels, which are able to hold multiple times their own weight in water, explains Wang, who is also developing them for atmospheric water harvesting. Up to 90% of a hydrogel could be water, which means in its dehydrated form, a hydrogel could absorb and store a lot of water.
Hydrogels could also address one limitation of strongly water-absorptive materials like MOFs – releasing the water that they have captured. ‘I think that is one of the key challenges in the field,’ says Guihua Yu at the University of Texas, Austin, US. ‘When you want to release the water, you have to pay a penalty.’ He notes that in the case of MOF materials, typically, a temperature over 65°C is needed for water release.
Hydrogels can absorb water from the atmosphere, and release it once above a critical temperature.
Yu and his team designed a hydrogel with a combination of two polymers serving two different functions (Adv. Mater., doi: 10.1002/adma.201806446). Clusters of hygroscopic polypyrrole chloride, which absorbs water, are embedded in the fibres of a poly-N-isopropylacrylamide (NIPAM) network, which helps with water release.
NIPAM is well known for its thermal switching ability, Yu explains. Normally, the NIPAM polymer is straight and linear, forming a neat 3D network. But above a critical temperature, around 37-40°C, NIPAM crumples up and the network collapses in on itself, physically squeezing the stored water out. Over 28 cycles each lasting one hour, a gram of gel produced 55g of water in an environment with 75% relative humidity.
A slippery surface with asymmetric bumps generated 35ml water/m2 in one hour, while a flat surface of the same material produced under 5ml/m2.
For now, the gels require high humidity to perform well. But that is not necessarily a bad feature, Yu thinks, as the materials could serve densely populated and thirsty areas that are hot and humid such as South and Southeast Asia.
With 2.2bn people around the world lacking access to safe drinking water in 2019, according to the WHO, and 4bn people faced with water shortages for at least one month every year, solutions to tackle the burgeoning problem of water scarcity can’t come soon enough. If not, it’s been suggested that intensifying water scarcity could force the displacement of 700m people worldwide by 2030.
The white plumes billowing from power plants are often mistaken for pollution. But really, they are clouds of water vapour and fog, produced when power plants cool down by evaporating water.
‘Everyone agrees that water is precious, but no one is willing to pay for it,’ says Kripa Varanasi, a chemical engineer at the Massachusetts Institute of Technology, US. ‘So, I wanted to look at where people pay for water.’
Instead of harvesting water from the atmosphere, Varanasi identified the power sector as a better source – and market – for fog. Globally, energy-producing companies already spend $50bn/year just to secure the water supplies they need. In the US, 39% of freshwater is earmarked for cooling power plants, Varanasi says.
Whether natural gas or nuclear, power plants need a cold sink to generate electricity, typically supplied by cooling towers. Water sourced from a nearby lake, river, or the ocean cycles through these towers, warming up and evaporating off. A single 500MW power plant could send 2000 gallons of water every minute to the atmosphere.
Part of this lost water could be easily recovered. By Varanasi’s estimate, 20-30% of the hot water vapour condenses into tiny liquid droplets and turns into fog when it leaves the cooling tower and meets colder outdoor air.
To recover this fog, Varanasi’s team came up with the idea of introducing an electrical charge to the airborne water droplets (Sci. Adv., doi: 10.1126/sciadv.aao5323). Traditional mesh or net fog collectors are inefficient because aerodynamic forces drag the airborne droplets around a collector’s threads so they don’t even contact each other, Varanasi explains. Using an electrode to ionise the water droplets just before they leave a tower allows the charged water droplets to be directed by an electric field to a collector.
Some of them even U-turn around,’ Varanasi says. In a small laboratory experiment, a coffee sieve captured almost 100% of the fog that received an electrical charge.
Through start-up company Infinite Cooling, the team is now working to build a pilot at the 20MW plant on campus. Beyond power plants, Varanasi hopes this idea could be extended to water-based cooling towers elsewhere, such as data centres, hospitals or commercial office buildings.
Image credit: Guihua Yu, University of Texas, Austin
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