Fresh water is scarce in many parts of the world and must be obtained at great expense. Communities near the ocean can desalinate sea water for this purpose, but doing so requires a large amount of energy. Further away from the coast, practically often the only remaining option is to condense atmospheric humidity through cooling, either through processes that similarly require high energy input or by using “passive” technologies that exploit the temperature swing between day and night. However, with current passive technologies, such as dew-collecting foils, water can be extracted only at night. This is because the sun heats the foils during the day, which makes 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 harvest water 24 hours around the clock, with no energy input, even under the blazing sun. The new device essentially consists of a specially coated glass pane, which both reflects solar radiation and also radiates away its own heat through the atmosphere to the outer space. It thus cools itself down to as much as 15 degrees Celsius below the ambient temperature. On the underside of this pane, water vapour 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 specifically designed polymer and silver layers. This special coating approach causes the pane to emit infrared radiation at a specific wavelength window to the outer space, with no absorption by the atmosphere nor reflection back onto the pane. Another key element of the device is a novel cone-shaped radiation shield. It largely deflects heat radiation from the atmosphere and shields the pane from incoming solar radiation, while allowing the device to radiate the aforementioned heat outward and thus to self-cool, fully passively.
