The “inverted solar still”, a new technology to extract drinking water from the air 24 hours a day


Engineers from ETH Zurich have designed a new device capable of extracting drinking water from the air. The system operates day and night and requires no additional power supply, making it suitable for use in remote or developing areas.

It is a fact that water is literally all around us in the air, but much of it is inaccessible. There are different collection methods: many use materials or microstructures with large surface areas that trap water molecules and release them when heated by sunlight, but many require external energy to heat up, while than those that do not often depend on temperature fluctuations during a day and night cycle, so they cannot work 24 hours a day.

The new device from the ETH team aims to solve both problems. In a way, it’s the opposite of a solar still: it’s basically a big box designed to cool drastically from the outside environment, so that water vapor condenses more easily. .

This process begins with the most important feature of the device: a large cone at the top. This acts as a radiation shield that deflects ambient heat into the air, while keeping sunlight on the top surface of the box.

This surface has a few tricks of its own to help keep the cold out. It is made of glass coated with layers of a specially designed polymer and silver, which allows it to reflect sunlight in the first place to prevent it from heating up. The heat it picks up is emitted at a specific infrared wavelength so that it passes through the atmosphere and is “ventilated” directly into the cold of space.

The end result is a box that the team says can be up to 15°C cooler than the outside temperature. This creates ideal conditions for condensation, which accumulates on a special surface on the ceiling inside the box. This surface is coated with a superhydrophobic material that repels water, so that it forms into drops and falls into a collection container below.

In their prototype, the researchers were able to collect 4.6 ml of water per day, using a 10 cm wide surface. According to the team, if you increase it, it equates to about 1.3 liters per square meter per day.

It’s not the highest performance ever seen from a water collector like this: that honor still belongs to a Johns Hopkins University design that can squeeze 8.66 liters per day per pound of material.

But the performance of the new device is the same as other recent designs, except that these require power, while this new one operates entirely passively. This meant it could be left outside wherever it needed to work, producing water around the clock with little effort.

Unlike other technologies, ours can actually operate without the need for additional power, which is a key advantage.

Iwan Hächler, author of the study.

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