Solar-powered system extracts drinkable water from 'dry' air
Engineers have made their initial design more practical, efficient, and scalable

Date:
October 14, 2020
Source:
Massachusetts Institute of Technology
Summary:
Researchers 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.



FULL STORY ========================================================================== 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 regions, using heat from the sun or another source.


==========================================================================
The system, which builds 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 water source for remote regions
with limited access to water and electricity. The findings are described
today in the journal Joule, in 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 earlier device demonstrated by Wang 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, 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.

Wang 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, 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 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 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 before the pandemic restrictions, the device produced water at
a rate "orders of magnitude" greater that the earlier version, Wang says.

While similar two-stage systems have been used for other applications
such as desalination, Wang 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 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.

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 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 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 that use multiple thermal
cycles per day, fed by a different heat source rather than sunlight,
and thus could produce higher daily outputs.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by David
L. Chandler. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Alina LaPotin, Yang Zhong, Lenan Zhang, Lin Zhao, Arny Leroy,
Hyunho Kim,
Sameer R. Rao, Evelyn N. Wang. Dual-Stage Atmospheric Water
Harvesting Device for Scalable Solar-Driven Water Production. Joule,
2020; DOI: 10.1016/j.joule.2020.09.008 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201014114648.htm

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