Our planet doesn’t suffer from a lack of water, but there is a geographical mismatch between freshwater demand and its availability. This means that two-thirds of the global population lives under conditions of severe water scarcity for at least one month of the year. One billion of those live in India and 0.9 billion live in China. Other areas facing longer periods of severe water scarcity include: Bangladesh (130 million people), the western and southern states of the USA (130 million), Pakistan (120 million), Nigeria (110 million) and Mexico (90 million).
‘If you have freshwater available through flowing river systems, or you are in a region with high rainfall, freshwater security is generally good,’ says Bahaa Khalil, who researches water security at McGill University in Montreal. ‘But there will come times when those sources aren’t available.’
Communities use a range of techniques to overcome water scarcity. In dry summers, those near the coast can desalinate sea water, although the process requires a large amount of energy and expenditure. Some remote inland communities have devised alternative methods that suit their particular conditions: ‘Many high-altitude Chilean communities in the Andes have specialised nets that harvest water from the fog,’ says Khalil.
Elsewhere, there’s another option available. It is estimated that the water locked away as humidity in the atmosphere could provide an additional 15 per cent of freshwater to the existing sources. Harvesting this vapour involves sufficiently cooling a surface so that water vapour condenses on it. The downside is that this usually requires large amounts of energy or relies on environmentally damaging refrigerants. ‘Passive’ options that don’t require energy exist, but as they mostly exploit the temperature swing between day and night, water can only be extracted during the hours of darkness.
To overcome this, researchers at ETH Zurich have developed a new device that passively harvests water from the air around the clock. The device is made up of a specially coated glass pane that reflects solar radiation while also radiating away its own heat. In doing so, it cools down by as much as 15 C below ambient temperatures. This difference in temperature between the air and the pane causes water vapour to condense on its underside, which can then be easily collected.
Tests so far have been conducted on a university building in Zurich and suggest that the system can produce twice as much water per day as other passive water-collection technologies. Under ideal conditions, researchers harvested 0.53 decilitres of water per square metre of the surface per hour. ‘That’s close to the theoretical maximum value of 0.6 decilitres per hour, which is physically impossible to exceed,’ says Iwan Hächler, a doctoral student who is working on the technology.