[May 14, 2022: Fabio Bergamin, ETH Zurich]
Pilot condenser atop the ETH Zurich building. (credit: ETH Zurich/Evan Hechler)
Fresh water is scarce in many parts of the world and must be obtained at great cost. Communities near the ocean can desalinate ocean water for this purpose, but doing so requires a large amount of energy. Further off the coast, practically often the only remaining option is to condense atmospheric humidity through cooling, either through processes that require high energy inputs or by using “passive” techniques that can last for days. And take advantage of temperature swings in the middle of the night. However, with current passive technologies, such as dew-collecting foils, water can only be removed at night. This is because the sun heats the foil during the day, making condensation impossible.
Self-cooling and protection from radiation
Researchers at ETH Zurich have now developed a technology that for the first time allows them to collect water round the clock with no energy input, even under the blazing sun. The new instrument essentially consists of a specially coated glass pane, which both reflects solar radiation and also radiates its own heat through the atmosphere into outer space. Thus it cools itself to 15 °C (59 °F) above ambient temperature. Below this face, water vapor from the air condenses into water. The process is the same as can be observed on poorly insulated windows in winter.
The scientists coated the glass with layers of specially designed polymer and silver. This special coating approach causes the vane to emit infrared radiation into outer space over a specific wavelength window, with neither absorption by the atmosphere nor reflection back on the vane. Another key element of the device is a novel cone-shaped radiation shield. This substantially deflects heat radiation from the atmosphere and shields the vane from incoming solar radiation, while allowing the device to radiate the above heat outwards and thus completely passively itself. cools.
close to the theoretical optimum
As revealed by testing of the new equipment in real-world conditions on the roof of an ETH building in Zurich, the new technology can produce at least twice as much water per day, which is the best current passive technologies based on foils. They are: Small pilot system with a vane diameter of 10 cm delivers 4.6 ml of water per day under real-world conditions. Larger appliances with larger pans will accordingly produce more water.
The scientists were able to show that, under ideal conditions, they could harvest 0.53 deciliters (about 1.8 fluid ounces) of water per hour per square meter of plank surface. “This is close to the theoretical maximum value of 0.6 deciliters (2.03 oz) per hour, which is physically impossible to exceed,” says Evan Hechler. He is a doctoral student in the group of Dimos Poulikkos, Professor of Thermodynamics at ETH Zurich.
Schematic of the condenser. (Credit: Hechler I et al. Science Advances)
Other techniques typically require wiping condensed water off the surface, which requires energy. Without this step, a significant portion of the condensed water would stick to the surface and remain unusable, hindering further condensation.
Researchers at ETH Zurich applied a novel superhydrophobic (extremely water-repellent) coating to the bottom of the vane in their water condenser. This causes the condensed water to bead and run or jump on its own. “Unlike other technologies, ours can actually function without any additional energy, which is a significant advantage,” Hechler said.
The goal of the researchers was to develop a technology for countries with water scarcity, and in particular for developing and emerging countries. Now, he says, other scientists have the opportunity to further develop this technology or to combine it with other methods, such as water desalination, to increase their yield.
Production of coated pans is relatively simple and it should be possible to manufacture large water condensers from existing pilot systems. Just as solar cells consist of multiple modules installed next to each other, multiple water condensers can also be stacked together into one large-scale system.
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Note: Materials provided above by ETH Zurich. Content can be edited for style and length.
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