A passive, renewable and more efficient way to extract water from the atmosphere

Freshwater scarcity affects more than two billion people worldwide, mostly in arid and remote regions, as well as islands and coastal areas without freshwater sources. Climate change and population growth are only making the problem worse, and existing methods require energy input, usually electricity.

Renewable energy can solve this problem and is essential in these regions for drinking water and irrigation, using water drawn from the atmosphere. (The atmosphere is estimated to hold about 13 trillion tons of water, or six times more than the world’s freshwater rivers; global warming is causing the air to hold more water vapor, theoretically by 7% for every degree Celsius of warming.)

Now, engineers and scientists from Saudi Arabia and China have created a system that uses solar energy to extract up to 3 liters (0.8 gallons) of water per square meter per day from the air, completely passively, requiring no maintenance or operators. The study was published in the journal Nature communication.

The system was tested by using collected water to grow cabbage for two seasons in Thuwal, Saudi Arabia.

“Our goal is to implement this technology to extract water from the air to compensate for the water demand for sustainable agriculture, which is essential for safe food production in the Middle East,” said Yu Han, co-author from South China University of Technology.

Existing solar-powered atmospheric water extraction (SAWE) systems typically rely on the absorption of water vapor from the air. Once the absorbent material reaches saturation, the system is sealed and exposed to sunlight, which begins to release the captured water. They are an improvement over passive atmospheric water technologies such as fog and dew collection, and are more widely available in other geographic areas and climate-constrained locations.

However, such SAWE systems only allow for one absorption and release cycle per day, capturing moisture at night and desorbing it during the day, with the slow absorption phase limiting the amount of water that can be extracted.

Their widespread acceptance is also limited by the expensive absorbent nanomaterials involved, making larger-scale prototyping difficult, and switching cycles require either an active system prone to failure or a labor-intensive operation with moving parts, making the systems complex and energy-intensive.

To design a passive, efficient, easily scalable, and work-minimizing system, the group used a structure of many vertical microchannels, called mass transfer bridges. The tubes, placed in a container, are filled with a liquid salt solution that acts as a liquid absorber; they used lithium chloride.

Depending on the temperature distribution, the ambient temperature area exposed to the environment continuously captures atmospheric water and stores it in the container. When the system receives sunlight, the absorber converts the light into heat and generates concentrated water vapor in the high temperature area.

The water vapor condenses on the chamber wall, producing fresh water. More of the captured water from the absorber container moves continuously to the high temperature area.

At the same time, concentrated liquid in the high-temperature region is transported back to the ambient-temperature region by diffusion — the movement of molecules from an area of ​​higher concentration to an area of ​​lower concentration — and by convection — the movement of warmer, lower-density solution through cooler, denser regions — allowing for the continuous capture of water vapor as long as sunlight is available.

To implement the system, the team created a solar absorber from partially oxidized carbon nanotubes on a glass fiber membrane. The black carbon nanotubes and light-blocking microstructures absorbed about 96% of sunlight when wet. They found that the optimal heights for the vapor-generating zone and the atmospheric water-capturing zone were 3 and 5 cm, respectively.

To test this setup over an eight-day production period, they used eight hours of sunlight followed by 16 hours of darkness. They found that as relative humidity increased from 60% to 90%, the water production rate increased from about 0.04 to about 0.65 kilograms per square meter per hour.

In a real-world test in Saudi Arabia, the evaporation area was increased to 13.5 cm by 24 cm, or 36 times larger than the prototype. This configuration produced 2.9 liters per square meter per day, depending on the solar energy received and relative humidity.

This amount is four times greater than the amount foreseen in the 2021 Atmospheric Water Project and 27 times greater than the amount foreseen in the 2017 SAWE Project.

In a test in Papua New Guinea, that increased to 4.6 liters per square meter per day. “Remarkably, the collected water was successfully used for off-grid irrigation of Brassica rapa (Chinese cabbage),” said co-author Qiaoqiang Gan of King Abdullah University of Science and Technology in Saudi Arabia, “showing the potential for unattended gardening in areas without access to liquid water sources.”

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