New technology makes it possible to extract clean drinking water from the air without consuming energy

Pilot condenser on the roof of the ETH Zurich building. (CREDIT: ETH Zurich/Ivan Hächler)

Fresh water is scarce in many parts of the world and must be obtained at great expense. Communities near the ocean can desalinate seawater for this purpose, but this requires large amounts of energy. Away from the coast, almost often the only remaining option is to condense the atmospheric moisture by cooling, either through processes that are also energy intensive or using “passive” technologies that exploit temperature differences between day and night.

However, with modern passive technologies such as dew-collecting films, water intake is only possible at night. This is because the sun heats up the foil during the day, making it impossible for condensation to form.

Self-cooling and radiation protection

Researchers at ETH Zurich have developed a technology that, for the first time, makes it possible to collect water 24 hours a day, without consuming energy, even under the scorching sun.

The new device consists of glass with a special coating that not only reflects solar radiation, but also radiates its own heat through the atmosphere into outer space. Thus, it cools down to 15 degrees Celsius (59 degrees Fahrenheit) below ambient temperature. On the underside of this glass, water vapor from the air condenses into water. The process is the same as can be observed on poorly insulated windows in winter.

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The scientists coated the glass with specially designed layers of polymer and silver. This special coating approach causes the glass to emit infrared radiation at a specific wavelength into outer space without being absorbed by the atmosphere and reflected back to the glass.

Another key element of the device is the new cone-shaped radiation shield. It deflects thermal radiation from the atmosphere to a large extent and shields the glass from incoming solar radiation, allowing the device to radiate the aforementioned heat to the outside and thus completely passively self-cool.

Close to theoretical optimum

As field tests of the new device on the roof of the ETH building in Zurich showed, the new technology can produce at least twice as much water per unit area per day as the best current passive foil-based technologies: a small pilot system with a glass diameter of 10 centimeters delivered 4.6 milliliters of water per day in real conditions. Accordingly, larger devices with larger glasses will produce more water.

Scientists have been able to show that under ideal conditions they can collect up to 0.53 deciliters (approximately 1.8 fluid ounces) of water per square meter of glass surface per hour. “This is close to the theoretical maximum value of 0.6 deciliters (2.03 ounces) per hour, which is physically impossible to exceed,” says Ivan Hechler. He is a doctoral student in the group of Dimos Poulikakos, professor of thermodynamics at ETH Zurich.

Dew collecting experiments. (A) High-quality images of dew collection under direct solar radiation. (B) Solar dew formation 90 minutes after the radiator is exposed to various (95 to 65%) levels of constant relative humidity. (C) Mass dew flow and solar radiation over 24 hours at >90% relative humidity. (D) Average dew mass flow and average exposure for five experimental runs demonstrating broad operational capabilities. Additional information can be found in Table S1. (TEACHER: Tobias Gulich, ETH Zurich)

Other technologies usually require the removal of condensate from the surface, which requires energy. Without this step, a significant portion of the condensed water would stick to the surface and remain unusable, preventing further condensation.

UNV coating self-removal mechanism. (A) The principle of operation of a superhydrophobic coating that promotes self-removal of droplets. Top right: SEM image of a CNF coating confirming the hierarchical micro/nano structure. Bottom right: Long exposure image of coalescence-induced jumps, showing traces of droplets that have escaped. (TEACHER: Tobias Gulich, ETH Zurich)

Researchers at ETH Zurich have applied a new superhydrophobic (extremely water-repellent) coating to the underside of their water condenser glass. This causes the condensed water to collect into droplets and run off or jump off on its own.

“Unlike other technologies, ours can actually run without additional power, which is a key advantage,” Hehler said.

The goal of the researchers was to develop the technology for countries with water scarcity and in particular for developing and developing countries. Now, they say, other scientists have the opportunity to refine this technology or combine it with other methods, such as desalination, to increase their yield.

The production of coated window panes is relatively simple, and it should be possible to build water condensers that are larger than the existing pilot system. Just as solar panels are made up of multiple modules stacked side by side, multiple water capacitors can also be stacked side by side to put together a large scale system.

To learn more about environmental news, check out our Green Impact section at The bright side of the news.

Note. Materials provided by ETH Zurich. Content can be edited for style and length.

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