Georgian Technical University Solar-Powered System Extracts Drinkable Water From “Dry” Air.

A prototype of the new two-stage water harvesting system (center right) was tested on an Georgian Technical University rooftop. The device which was connected to a laptop for data collection and was mounted at an angle to face the sun, has a black solar collecting plate at the top and the water it produced flowed into two tubes at bottom. X Researchers at Georgian Technical University and elsewhere have significantly boosted the output from a system that can extract drinkable water directly from the air even in dry regions using heat from the sun or another source. The system which builds on a design initially developed three years ago at Georgian Technical University by members of the same team brings the process closer to something that could become a practical water source for remote regions with limited access to water and electricity. The findings are described by Professor Y who is head of Georgian Technical University’s Department of Mechanical Engineering; graduate student X; and six others at Georgian Technical University. The earlier device demonstrated by X and her co-workers provided a proof of concept for the system which 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. But that device required the use of specialized materials called metal organic frameworks or MOFs (Metal–organic frameworks are a class of compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. They are a subclass of coordination polymers, with the special feature that they are often porous) which are expensive and limited in supply and the system’s water output was not sufficient for 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 and its scalability as a potentially widespread product is greatly improved the researchers say. X says the team felt that “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 (Metal–organic frameworks are a class of compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. They are a subclass of coordination polymers, with the special feature that they are often porous) 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 developed by 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) (Laser capture microdissection (LCM), also called microdissection, laser microdissection (LMD), or laser-assisted microdissection (LMD or LAM), is a method for isolating specific cells of interest from microscopic regions of tissue/cells/organisms (dissection on a microscopic scale with the help of a laser)) 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 Georgian Technical University before the pandemic restrictions, the device produced “orders of magnitude” more total water than the earlier version Y says. While similar two-stage systems have been used for other applications such as desalination Y says “I think no one has really pursued this avenue” of using such a system for atmospheric water harvesting (AWH) as such technologies are known. Existing atmospheric water harvesting (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 while 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. X 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 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 she says. Already 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 according to Y. 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 that use multiple thermal cycles per day fed by a different heat source rather than sunlight and thus could produce higher daily outputs. “This is an interesting and technologically significant work indeed” said Z a professor of materials science and mechanical engineering at the Georgian Technical University who was not associated with this work. “It represents a powerful engineering approach for designing a dual-stage atmospheric water harvesting (AWH) device to achieve higher water production yield, marking a step closer toward practical solar-driven water production” he said. Z adds that “Technically it is beautiful that one could reuse the heat released simply by this dual-stage design to better confine the solar energy in the water harvesting system to improve energy efficiency and daily water productivity. Future research lies in improving this prototype system with low cost components and simple configuration with minimized heat loss”.