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Atmospheric Water Harvesting

July 11, 2022

Tikalon's home in Northern New Jersey is blessed with abundant rainfall, which leads to lush vegetation (and pollen allergies!). Indeed, New Jersey is known as the Garden State. However, other parts of the United States and much of the world are facing water scarcity. I wrote about water scarcity in an earlier article (Future Water Scarcity, March 28, 2016).

The United Nations has compiled a list of water statistics, as follows.[1]

• 72% of all water withdrawals are used by agriculture, 16% by municipalities for services and households, and 12% by industry.

• Five out of 11 regions withdraw 25% more water than is replenished by renewable freshwater resources, such as rainfall.

• 2.3 billion people live in water-stressed countries, and 733 million of these live in highly water-stressed countries.

• 1.2 billion people, about a sixth of the world's population , live in severely water-constrained agricultural areas.

• Today, 1.42 billion people, including 450 million children, live in areas of high or extremely high water vulnerability.

• About 4 billion people, nearly two-thirds of the world's population, experience severe water scarcity during at least one month of the year. This could increase to up to 5.7 billion in 2050.

• 700 million people worldwide could be displaced by intense water scarcity by 2030.

World water stress map for 2019. Sources: World Resources Institute, National Water Stress Rankings, and World Resources Institute, Projected Water Stress Country Rankings. (Wikimedia Commons image by Genetics4good. Click for larger image.)

Adult humans are more than 50% water by weight, while nearly 75% of an infant is water. A human living in a desert climate needs to consume several gallons of water each day to survive. Increased industrial activity has led to significant pollution of water sources in developing countries.

The Earth has a lot of water, but most of it is seawater.

Desalination of saline water, typically by reverse osmosis, helps to mitigate water shortage along sea coasts.

In reverse osmosis, pressure on one side of a semipermeable membrane induces water permeation through the membrane while rejecting salts.

(Rendered using Inkscape.)

Most mornings, I'm reminded that there's a lot of water in the air. Cool summer mornings find my lawn covered in a thick blanket of dew. My automobile is likewise covered with a thick layer of water, condensed from the atmosphere. The same is true on winter mornings when the dew appears as a thick layer of ice. In an earlier article (Fog Water Harvesting, December 2, 2010), I did a back-of-the-envelope calculation of how effective my automobile is at collecting dew.

I estimate that after a heavy dew, the surface of my car is covered by about a liter of water. Approximating the geometry of the car as two rectangular parallelepipeds , one atop the other, having about 220 square feet exposed to the air (about 20 square meters) when the undercarriage is ignored. This results in a collection efficiency of about 50 milliliters per square meter. Trees, which have leaves or needles with a large collecting area hoisted high into the air, are natural fog water harvesters. This illustrates the idea that potable water can be harvested from the air.

Retreating from the idea that you need a cool night to harvest water from the air, you can use electrical refrigeration to condense water vapor through cooling. This is an energy-intensive process, but it could be enabled by storing solar energy during the day. Plastic netting is a more effective fog water harvester than automobile metal and glass, since air can flow though it. Fog water harvesting has been demonstrated using large plastic nets in Lima, Peru .[2] During the cooler months of May through November, 32 square meter nets have captured as much as 590 liters in a single day.[2] A double-net system produced an average of 300 liters per day throughout the year, with 2650 liters being produced in a single day.[2]

Scientists have been researching better atmospheric water harvesting materials. Recently, a team of scientists and mechanical engineers from the University of Texas at Austin (Austin, Texas) have developed an inexpensive gel film, created from abundant materials, that's capable of extracting water from air in even the driest climates.[3-4] Their results are published as an open access article in Nature Communications.[3]

As chemists know, some salts, such as lithium chloride (LiCl), calcium chloride (CaCl₂), and magnesium chloride (MgCl₂) are hygroscopic, and they have high water uptake at even low relative humidity. My parents had a primitive dehumidifier in the cellar of their house that used calcium chloride as the hygroscopic agent. However, the aggregation of salt crystals during hydration creates a passivation layer, so the water cycling performance is reduced.[3]

Spider graph comparison of atmospheric water harvesting technologies.

Radar chart comparison of atmospheric water harvesting technologies. (Fig. 1d of ref. 3, licensed under the Creative Commons Attribution 4.0 International License.[3] Click for larger image.)

The material used in the present study is made from cellulose, konjac glucomannan, a water-soluble polysaccharide that's a food additive used as an emulsifier and thickener, and LiCl.[3-4] The open-pore structure of the glucomannan increases the rate of the moisture-capturing process, while the cellulose becomes hydrophobic when heated to assist in the water removal.[4] The material is a super hygroscopic polymer film with an hierarchical porous structure with sorbent-air interfaces and rapid water vapor transport pathways.[3]

The research builds on previous work by the same research team on a super moisture-absorbent gel composed of hygroscopic polypyrrole chloride penetrating an hydrophilic-switchable polymer network of poly N-isopropylacrylamide.[5-6] This super sponge can collect large amounts of water from the atmosphere, and removing the water merely involves heating by exposure to sunlight for a few minutes.[6] The reaction to produce the cellulose-glucomannan material is so easy it can be done at home.[4] The resultant gel film, freeze-dried after casting in a mold, is flexible, it can be created in a desired shape, and it's created in just two minutes.[4]

The starting materials for creation of the cellulose-glucomannan material cost just $2 per kilogram. A single kilogram can produce more than 6 liters of water per day in areas with less than 15% relative humidity and 13 liters per day at 30% relative humidity.[4] Six liters seems like a small amount, but thicker films and arrays of films will enhance the yield.[4] This material can be used at 14-24 cycles per day in arid environments to produce a water yield of 5.8-13.3 L/kg.[3] The 14 daily cycles are realized at 15% relative humidity, and the 24 daily cycles at 30% relative humidity.[3] The 5.8 L/kg yield is obtained at 15% relative humidity, and 13.3 L/kg yield is obtained at 30% relative humidity.[3] This research was funded by the Defense Advanced Research Projects Agency (DARPA).[4]

Atmospheric water harvesting material and a comparison of atmospheric water harvesting materials.

Left, the atmospheric water harvesting material can be molded into different shapes. Right, a comparison of atmospheric water harvesting materials at ∼30% relative humidity. (Left, a University of Texas image. Right, figure 3f of ref. 3, licensed under the Creative Commons Attribution 4.0 International License. Click for larger image.)

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