Researchers at MIT and elsewhere have significantly boosted the output from a system that can extract drinkable water directly from the air even in dry areas, using heat from the sun or another source.
The machine, which builds on a design initially developed three years ago at MIT by members of the identical team, brings the process closer to something that could develop into a practical drinkable water source for remote regions with limited access to water and electricity. The findings have been described in the journal Joule, at a paper by Professor Evelyn Wang, who is head of MIT’s Department of Mechanical Engineering; graduate student Alina LaPotin; and six others at MIT and in Korea and Utah.
The previous apparatus exhibited by Wang and her co-workers supplied a proof of concept for the system, which exerts a temperature difference within the apparatus to permit an adsorbent material — which collects liquid on its surface — to draw moisture in the atmosphere during the night and release it the next day. When the substance is heated by sunlight, the difference in temperatures between the heated top and the shaded underside leaves the drinkable water release back out of this adsorbent material. The water then gets condensed onto a collection plate.
That apparatus required the use of technical materials called metal organic frameworks, or MOFs, that can be costly and limited in distribution, along with the system’s water output was not enough for a practical system. Now, by integrating a second period of desorption and moisture, and by using a readily accessible adsorbent material, the device’s output continues to be significantly improved, and its scalability as a possibly widespread product is significantly improved, the investigators state.
Wang says that the team believed that “It’s great to have a small prototype, but how can we get it into a more scalable form?” The newest advances in design and materials have led to progress in this direction.
Instead of this MOFs, the new layout utilizes an adsorbent material called zeolite, which in this case is composed of a microporous iron aluminophosphate. The substance is widely accessible, secure, and has the right adsorbent properties to provide an efficient water manufacturing system based just on average day-night temperature fluctuations and heating with sunlight.
The two-stage design developed by LaPotin makes smart use of this heat that is created whenever water changes phase. The sun’s warmth 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. This vapor condenses on a collector plate — a process which releases heat too. The collector plate includes a copper sheet directly over and connected with the next zeolite layer, in which the heat of condensation is used to release the vapor from that succeeding layer. Droplets of water accumulated from each of these two layers could be funneled together into a tank.
In the process, the overall productivity of this machine, in relation to its possible liters per day per square meter of solar collecting area (LMD), is approximately doubled compared to the earlier version, although accurate rates depend on local temperature variations, solar flux, and humidity levels. In the initial prototype of the new system, analyzed on a rooftop in MIT before the pandemic restrictions, the device produced water at a rate “orders of magnitude” greater that the earlier version, Wang says.
While comparable two-stage systems have been used for additional applications Such as desalination, Wang says, “I think no one has really pursued this avenue” of using such a system for atmospheric drinkable water harvesting (AWH), as these technologies are known.
But both have significant constraints. Fog harvesting only functions with 100 percent relative humidity, and is presently used only in a few coastal deserts, while dew harvesting demands energy-intensive pipes to provide cold surfaces for moisture to condense on — and requires humidity of 50 percent, based on the ambient temperatures.
By comparison, the new system may work at humidity levels as low as 20 percent and needs no energy input other than sunlight or any other available source of low-grade heat.
LaPotin says that the secret is that this two-stage structure; today that its effectiveness was shown, people can search for much better adsorbent materials that may further push the production rates. The present production rate of approximately 0.8 gallons of water per square meter every day may be adequate for some applications, but when this speed could be made better 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 specific zeolite and may lead to a corresponding growth in water output, according to Wang.
The team continues focus on optimizing the materials and design of the device and adapting it to particular programs, like military programs. The two-stage system may also Be adapted to other kinds of drinkable water harvesting methods that use multiple thermal cycles daily, fed with a different heat source rather than sun, and therefore can generate higher daily outputs.
Source: https://mit.edu/