Researchers at MIT and elsewhere have dramatically increased the performance of a system that can extract drinking water directly from the air, even in dry regions, using heat from the sun or another source.
The system, which is based on a design initially developed three years ago at MIT by members of the same team, brings the process closer to something that could become a practical source of water for remote regions with limited access to water and electricity. The system is described in an article published in Joule magazine, by Professor Evelyn Wang, head of the Department of Mechanical Engineering at MIT; graduate student Alina LaPotin; and six other people from MIT and from Korea and Utah.
The earlier device demonstrated by Wang and his collaborators provided a proof of concept for the system, which takes advantage of a temperature difference within the device to allow a material that collects liquid on its surface to attract moisture from the air at night and release it into the air. next day. When the material is heated by sunlight, the difference in temperature between the heated upper part and the shaded lower part causes the water to be released again from the absorbent material. The water is then condensed on a collection plate.
But that device required the use of specialized materials called MOFs (metal organic frameworks), which are expensive and of limited supply, and the water output of the system was not enough for a practical system. Now, by incorporating a second stage of desorption and condensation, and by using a readily available absorbent material, the output of the device has been significantly increased, and its scalability as a potentially extended product has been greatly improved, the researchers say.
Wang says the team felt “it’s great to have a little prototype, but how can we bring it to a more scalable form?” New advances in design and materials have now led to progress in that direction.
Instead of MOFs, the new design uses an absorbent material called zeolite [What is Zeolite?]. The material is widely available, stable, and has the right absorbent properties to provide an efficient water production system based only on typical day-to-night temperature fluctuations and heating with sunlight.
The two-stage design developed by LaPotin makes smart use of the heat that is generated when water changes phase. The heat from the sun is collected by a solar absorbing plate on top of the box-shaped system and heats the zeolite, releasing the moisture that the material has captured overnight. That vapor condenses on a collector plate, a process that also releases heat. The collector plate is a copper foil directly above and in contact with the second layer of zeolite, where the heat of condensation is used to release the vapor from that subsequent layer. The water droplets collected from each of the two layers can be channeled 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), roughly doubles compared to the previous version, although the exact rates depend on local temperature variations, solar flux and humidity levels. In the initial prototype of the new system, tested on an MIT rooftop prior to pandemic restrictions, the device produced water at a rate “orders of magnitude” higher than the previous version, Wang says.
The new system can operate in humidity levels as low as 20 percent and does not require any power input except sunlight or any other available source of low-grade heat.
LaPotin says the key is this two-stage architecture; Now that it has been proven effective, even better absorbent materials can be sought that could further increase production rates. The current production rate of around 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 adjustments and material choices, this could be practical on a large scale, he says. Materials are already being developed that have an absorption about five times greater than this particular zeolite and that could lead to a corresponding increase in water production, according to Wang.
The team continues to work on refining the device’s materials and design 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 types of water harvesting methods that use multiple thermal cycles per day, powered by a different heat source instead of sunlight, and thus could produce higher daily yields.