Solar-driven system extracts drinkable water from ‘dry’ air

Technology News | October 20, 2020

By Rich Pell

The system builds on a proof-of-concept design initially developed three years ago. The latest design, say the researchers, brings the process closer to something that could become a practical water source for remote regions with limited access to water and electricity.

The system harnesses a temperature difference within the device to allow an adsorbent material – which collects liquid on its surface – to draw in moisture from the air at night and release it the next day. When the material is heated by sunlight, the difference in temperature between the heated top and the shaded underside makes the water release back out of the adsorbent material. The water then gets condensed on a collection plate.

The original device required the use of specialized materials – called metal organic frameworks (MOFs) – which are expensive and limited in supply, and the system’s water output was not sufficient for a practical system. Now, say the researchers, by incorporating a second stage of desorption and condensation, and by using a readily available adsorbent material, the device’s output has been significantly increased, and its scalability as a potentially widespread product is greatly improved.

Instead of using MOFs, the new design uses an adsorbent material called a zeolite, which in this case is composed of a microporous iron aluminophosphate. The material is widely available, stable, and has the right adsorbent properties to provide an efficient water production system based just on typical day-night temperature fluctuations and heating with sunlight.

The two-stage design, say the researchers, makes clever use of the heat that is generated whenever water changes phase: The sun’s heat is collected by a solar absorber plate at the top of the box-like system and warms the zeolite, releasing the moisture the material has captured overnight. That vapor condenses on a collector plate – a process that releases heat as well.

The collector plate is a copper sheet directly above and in contact with the second zeolite layer, where the heat of condensation is used to release the vapor from that subsequent layer. Droplets of water collected from each of the two layers can be funneled together into a collecting tank.

In the process, the overall productivity of the system, in terms of its potential liters per day per square meter of solar collecting area (LMD), is approximately doubled compared to the earlier version, though exact rates depend on local temperature variations, solar flux, and humidity levels. In the initial prototype of the new system, tested on a rooftop at MIT, the device produced “orders of magnitude” more total water than the earlier version, say the researchers.

While similar two-stage systems have been used for other applications such as desalination, say the researchers, they believe this approach has not been previously pursued for atmospheric water harvesting (AWH). Existing AWH approaches include fog harvesting and dew harvesting, but both have significant limitations. Fog harvesting only works with 100 percent relative humidity, and is currently used only in a few coastal deserts, while dew harvesting requires energy-intensive refrigeration to provide cold surfaces for moisture to condense on – and still requires humidity of at least 50 percent, depending on the ambient temperature.

By contrast, the new system can work at humidity levels as low as 20 percent and requires no energy input other than sunlight or any other available source of low-grade heat. Now that its effectiveness has been shown, say the researchers, even better adsorbent materials may be found that could further drive up the production rates.

The present production rate of about 0.8 liters of water per square meter per day may be adequate for some applications, but if this rate can be improved with some further fine-tuning and materials choices, this could become practical on a large scale. Already, say the researchers, materials are in development that have an adsorption about five times greater than this particular zeolite and could lead to a corresponding increase in water output.

The researchers are continuing to work on refining the materials and design of the device and adapting it to specific applications, such as a portable version for military field operations. The two-stage system could also be adapted to other kinds of water harvesting approaches that use multiple thermal cycles per day, fed by a different heat source rather than sunlight, and thus could produce higher daily outputs.

For more, see “Dual-Stage Atmospheric Water Harvesting Device for Scalable Solar-Driven Water Production.”