An international team of researchers has significantly boosted the output from a renewable energy powered system that can extract potable water directly from the air, even in dry regions.
The system uses heat from sources such as the sun and builds on a design initially developed three years ago at the Massachusetts Institute of Technology (MIT) by members of the same team.
It brings the process closer to one that could become a practical water source for remote regions with limited access to water and electricity. The findings are described in a paper appeared recently in the journal Joule by Evelyn Wang, the head of MIT’s Department of Mechanical Engineering, graduate student Alina LaPotin, and six others at MIT and in Korea and Utah.
The earlier device demonstrated by Wang and her co-workers provided a proof of concept for the system. It 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 over the course of the next day.
When the material is heated by sunlight, the difference in temperature between the heated top surface and the shaded underside releases the water back out of the adsorbent material. The water then condenses and collects on a collection plate.
But that device required the use of specialised materials called metal organic frameworks, or MOFs, which are expensive and limited in supply. The system’s water output was also not sufficient for use as a practical system.
Now, 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. Its scalability as a potentially widespread product has also been greatly improved, the researchers say.
Wang says the team posed the question, “It’s great to have a small prototype, but how can we get it into a more scalable form?” The new advances in design and materials have now led to progress in that direction.
Instead of the 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 and 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 developed by LaPotin 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 vapour condenses on a collector plate, in 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 vapour from that subsequent layer. Droplets of water collected from each of the two layers can be funnelled together into a collecting tank.
In the process, the overall productivity of the system, in terms of its potential litres per day per square metre of solar collecting area (LMD), is approximately doubled compared to the earlier version. Exact rates, however, depend on local temperature variations, solar flux, and humidity levels.
In the initial prototype of the new system, tested on a rooftop at MIT before the restrictions put in place for the novel coronavirus (COVID-19) pandemic, the device produced water at a rate “orders of magnitude” greater than the earlier version, Wang says.
Similar two-stage systems have previously been used for other applications such as desalination. But Wang says, “I think no one has really pursued this avenue” of using such a system 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% relative humidity, and is currently used only in a few coastal deserts. Dew harvesting requires energy-intensive refrigeration to provide cold surfaces for moisture to condense on, and still requires humidity of at least 50%, depending on the ambient temperature.
By contrast, the new system can work at humidity levels as low as 20%, and requires no energy input other than sunlight or any other available source of low-grade heat.
LaPotin says that the key is this two-stage architecture. Now that its effectiveness has been shown, people can search for even better adsorbent materials that could further drive up the production rates.
The present production rate of about 0.8 litres of water per square metre per day may be adequate for some applications. If this rate can be improved with some further fine-tuning and materials choices, however, this could become practical on a large scale, she says.
Materials are already being developed that have an adsorption about five times greater than this particular zeolite, and could lead to a corresponding increase in water output, according to Wang.
The team continues 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. These would use multiple thermal cycles per day, fed by a different heat source rather than sunlight, with the potential to produce higher daily outputs.
