Lack of water is a problem for many people. But remedying this deficiency, such as obtaining drinking water from seawater, is often a problem for nature. More than 1.4 billion people live in regions where water is scarce all year round, the UN children’s aid organization Unicef recently documented in a report. If these regions are by the sea, for example in North Africa and the Middle East, then they produce drinking water with the help of salt water systems. But these systems are energy-intensive – and they leave behind according to an article in the specialist magazine Science of the Total Environment more than 140 million tons of brine that is discharged into the sea or rivers. The consequence: the salts may carry poisonous substances with them and deprive the waters of oxygen.
Could the alternative be water from the air instead of water from the sea? Researchers at ETH Zurich have been pursuing this idea for some time. Now they have developed a pilot system that shows: water can be extracted cleanly and without operating energy from the atmosphere. According to the magazine, it could provide around three times the annual global water consumption in both wet and dry regions Science Advances, in which the ETH researchers present the process.
That’s the potential. However, the extraction of atmospheric water, like seawater desalination, is challenging for energetic reasons. How it is still possible to collect water from the air using natural energy alone is currently being demonstrated on the roof of the ETH building.
The “water collector” has a futuristic look, an equally shimmering, cone-shaped funnel stands on a shiny silver chamber. The heart of the system is located at the interface between these two objects – not visible from the outside. It is a multi-coated pane of glass with a diameter of less than ten centimeters on which moist air condenses.
The resulting heat is annoying. She is sent to heaven via a cone
The chamber is filled with ambient air, the relative humidity of which in the ETH experiments is between 65 and 95 percent. “The higher the humidity, the more efficient the condensation process on the coated glass plate,” says Iwan Hächler, lead author of the study and doctoral student in the group of thermodynamics professors Dimos Poulikakos at ETH. The chamber is coated with aluminized plastic film to reflect the sun’s rays. This means that the temperature of the chamber air and the ambient air always remains roughly the same.
The aim is to cool the coated glass pane as much as possible. The effect is then the same as one experiences in the car in winter, when the warm indoor air condenses on the cold windshield to form a film of water. Only: where the ETH researchers’ system is to be used, it is not cold, but hot. Cooling is therefore a major challenge. The different layers of the glass pane have different radiation properties: The top layer of plastic, a special polymer, and the glass pane emit long-wave heat rays in a special wave range into space, and the silver layer reflects visible sunlight. “Instead of a silver coating, the glass can also be painted white,” says Iwan Hächler.
When the moist air condenses on the pane, heat is also released that has to be dissipated. The researchers developed the aluminum-coated cone for this purpose. Experiments on the flow of heat have shown that this shape is suitable for carrying the heat away vertically towards the sky. This is the most effective way for thermal radiation to travel through the atmosphere into space. In addition, the cone protects against atmospheric heat radiation and sunlight, especially when the sun is low. In this way, the system can cool the condensation disk up to 15 degrees below the ambient temperature.
Condensation methods to collect water have been around for a long time. For example, water vapor is collected in desiccants such as silica gel, zeolite or salts in open chambers. When the substances are saturated, the chamber is closed and the absorbed water is evaporated by the ambient heat; it then condenses on the cooler chamber walls. However, this method only works during the day when exposed to sunlight.
Then there are systems with cooling foils, similar to the ETH method, on which the air humidity thaws. This process is optimized to collect water at night because, unlike the ETH version, the film is heated during the day. In Peru, people also use an old, simple method: in the higher regions of the country, fog arises in the morning and at night, which one tries to capture with large nets. The water then drips from the damp nets into open half-pipes and later into containers. From December to March, however, there is practically no fog there.
The system is cheap and low-tech, but it takes up a lot of space
The ETH system works all year round – but only at a humidity of more than 65 percent, the physics sets a limit here. However, the system reaches this physical limit without any energy supply. The development also has another special feature: the water vapor condenses on an extremely water-repellent layer on the glass pane, so that the droplets roll off the surface by themselves and can be caught in a vessel. In the previous systems, on the other hand, the condensate had to be regularly wiped off the surface of the capacitor disk, says Hächler.
The ETH system impresses with its simplicity. It is a low-tech method that is in demand in poorer, water-stricken countries. But the process also has a weakness; it is the yield. Under real conditions, the pilot plant produces just 4.6 milliliters of water per day. If you were to build a system on an area of 100 by 100 meters with elements from this prototype, Hächler reckons with 52,441 individual “water collectors” that would produce between 300 and 500 liters of drinking water per day. So the process takes up a lot of space.
But the costs are low. The production of the coated condensation disks is simple and inexpensive, says Hächler. Next, the ETH team wants to test the process for larger systems as well. Initially, it was primarily a matter of showing “that the process works without the addition of energy,” says Iwan Hächler. In any case, he does not see it as an option to meet the water needs of an entire country in the future. But he can imagine building the system where energy-intensive methods such as desalination plants or the construction of wells are out of the question. Or decentralized and without energy consumption in villages, where potable water will become even rarer in the future due to climate change and energy shortages.