The sun burns out and dries up everything – but even in desert regions there are always masses of water – in the form of humidity. Now Swiss researchers are presenting a surprisingly uncomplicated concept through which this treasure can be tapped around the clock for the first time: They have developed a particularly efficient condenser system that can use self-cooling to extract water from the humidity during the day without consuming any energy. Since the production costs are low, the system could make a useful contribution to the water supply in arid regions, say the developers.
So far, fresh water can only be obtained with a great deal of effort: In coastal areas, seawater is desalinated for this purpose, which requires a lot of energy. In the interior of the country, on the other hand, water can often only be obtained through the condensation of humidity. Energy-intensive cooling technology is also used for this purpose – so-called passive technologies have also already been developed, such as dew-collecting foils or absorber systems. With the previous methods, however, water can only be extracted from the humidity at night. In the heat of the day, however, the previous passive technologies cannot enable condensation. Because the principle is like breathing on a pane of glass: the tiny water droplets are only formed through the clear temperature difference between the air you breathe and the pane of glass.
Refined passive cooling
But as the researchers led by Iwan Hächler from the Swiss Federal Institute of Technology in Zurich report, they have now developed a technology with which they can for the first time obtain water around the clock without the need for energy – with realistic humidity conditions and even in blazing sunshine. The system essentially consists of a specially coated pane of glass that both reflects solar radiation and can also effectively radiate its own heat. As a result, it cools down astonishingly: The disk reaches up to 15 degrees Celsius below the ambient temperature, the scientists report. Due to this strong temperature difference, humidity condenses on the underside – tiny water droplets form. The process is basically the same as that which can be observed on poorly insulating window panes in winter.
As the developers report, the secret of the “cool” disc is a sophisticated coating made of polymers and silver. This material causes the pane to emit infrared radiation of a certain wavelength range. This frequency is not absorbed by the atmosphere and is not reflected back onto the pane by the molecular components of the air, the scientists explain. Another essential component of the system is a newly developed conical radiation protection shield. This keeps heat radiation from the air and solar radiation from the pane to a large extent, but at the same time allows it to radiate heat and thus cool it.
Promising dribble
The researchers report that the underside of the condensation disk also has a sophisticated coating. As they explain, with other technologies the condensed water usually has to be stripped from the surfaces, which is associated with effort. Otherwise some of the condensed water would stick to the surface, could not be used and would hinder further condensation. In the new system, the water condenser has an extremely water-repellent – superhydrophobic – coating. This causes the condensed water to roll off it intensely. “In contrast to other technologies, we really get by without any additional energy, which is a decisive advantage, says Hächler.
Tests of a small test facility under real conditions have shown that with the new technology, at least twice as much water can be obtained per area and day as with the best previous film passive technologies. The researchers’ calculations show that under ideal conditions, larger systems could extract up to 53 milliliters of water per square meter of pane area and hour. Similar to solar cells, several water capacitors can be set up next to each other in order to provide a more abundant water yield. The important thing is that the production of the coated panes is relatively easy, say the researchers. The system could make an interesting contribution to supply, especially in developing and emerging countries with water scarcity. Hächler and his colleagues are now calling on other research teams to further develop the technology or to combine it with other methods in order to further increase yields.
Source: ETH Zurich, specialist article: Science Advances, doi: 10.1126