http://news.mit.edu/2020/passive-solar-powered-water-desalination-0207
[Unclear to me how this would work in an environment where freezing of
water is a regular ambient feature. However, not needing to remove
brine or sediment as a regular process seems like an advance to me.]
Simple, solar-powered water desalination
System achieves new level of efficiency in harnessing sunlight to make
fresh potable water from seawater.
David L. Chandler | MIT News Office
February 6, 2020
A completely passive solar-powered desalination system developed by
researchers at MIT and in China could provide more than 1.5 gallons of
fresh drinking water per hour for every square meter of solar collecting
area. Such systems could potentially serve off-grid arid coastal areas
to provide an efficient, low-cost water source.
The system uses multiple layers of flat solar evaporators and
condensers, lined up in a vertical array and topped with transparent
aerogel insulation. It is described in a paper appearing today in the
journal Energy and Environmental Science, authored by MIT doctoral
students Lenan Zhang and Lin Zhao, postdoc Zhenyuan Xu, professor of
mechanical engineering and department head Evelyn Wang, and eight others
at MIT and at Shanghai Jiao Tong University in China.
The key to the system’s efficiency lies in the way it uses each of the
multiple stages to desalinate the water. At each stage, heat released by
the previous stage is harnessed instead of wasted. In this way, the
team’s demonstration device can achieve an overall efficiency of 385
percent in converting the energy of sunlight into the energy of water
evaporation.
The device is essentially a multilayer solar still, with a set of
evaporating and condensing components like those used to distill liquor.
It uses flat panels to absorb heat and then transfer that heat to a
layer of water so that it begins to evaporate. The vapor then condenses
on the next panel. That water gets collected, while the heat from the
vapor condensation gets passed to the next layer.
Whenever vapor condenses on a surface, it releases heat; in typical
condenser systems, that heat is simply lost to the environment. But in
this multilayer evaporator the released heat flows to the next
evaporating layer, recycling the solar heat and boosting the overall
efficiency.
“When you condense water, you release energy as heat,” Wang says. “If
you have more than one stage, you can take advantage of that heat.”
Adding more layers increases the conversion efficiency for producing
potable water, but each layer also adds cost and bulk to the system. The
team settled on a 10-stage system for their proof-of-concept device,
which was tested on an MIT building rooftop. The system delivered pure
water that exceeded city drinking water standards, at a rate of 5.78
liters per square meter (about 1.52 gallons per 11 square feet) of solar
collecting area. This is more than two times as much as the record
amount previously produced by any such passive solar-powered
desalination system, Wang says.
Theoretically, with more desalination stages and further optimization,
such systems could reach overall efficiency levels as high as 700 or 800
percent, Zhang says.
Unlike some desalination systems, there is no accumulation of salt or
concentrated brines to be disposed of. In a free-floating configuration,
any salt that accumulates during the day would simply be carried back
out at night through the wicking material and back into the seawater,
according to the researchers.
Their demonstration unit was built mostly from inexpensive, readily
available materials such as a commercial black solar absorber and paper
towels for a capillary wick to carry the water into contact with the
solar absorber. In most other attempts to make passive solar
desalination systems, the solar absorber material and the wicking
material have been a single component, which requires specialized and
expensive materials, Wang says. “We’ve been able to decouple these two.”
The most expensive component of the prototype is a layer of transparent
aerogel used as an insulator at the top of the stack, but the team
suggests other less expensive insulators could be used as an
alternative. (The aerogel itself is made from dirt-cheap silica but
requires specialized drying equipment for its manufacture.)
Wang emphasizes that the team’s key contribution is a framework for
understanding how to optimize such multistage passive systems, which
they call thermally localized multistage desalination. The formulas they
developed could likely be applied to a variety of materials and device
architectures, allowing for further optimization of systems based on
different scales of operation or local conditions and materials.
One possible configuration would be floating panels on a body of
saltwater such as an impoundment pond. These could constantly and
passively deliver fresh water through pipes to the shore, as long as the
sun shines each day. Other systems could be designed to serve a single
household, perhaps using a flat panel on a large shallow tank of
seawater that is pumped or carried in. The team estimates that a system
with a roughly 1-square-meter solar collecting area could meet the daily
drinking water needs of one person. In production, they think a system
built to serve the needs of a family might be built for around $100.
The researchers plan further experiments to continue to optimize the
choice of materials and configurations, and to test the durability of
the system under realistic conditions. They also will work on
translating the design of their lab-scale device into a something that
would be suitable for use by consumers. The hope is that it could
ultimately play a role in alleviating water scarcity in parts of the
developing world where reliable electricity is scarce but seawater and
sunlight are abundant.
“This new approach is very significant,” says Ravi Prasher, an associate
lab director at Lawrence Berkeley National Laboratory and adjunct
professor of mechanical engineering at the University of California at
Berkeley, who was not involved in this work. “One of the challenges in
solar still-based desalination has been low efficiency due to the loss
of significant energy in condensation. By efficiently harvesting the
condensation energy, the overall solar to vapor efficiency is
dramatically improved. … This increased efficiency will have an overall
impact on reducing the cost of produced water.”
The research team included Bangjun Li, Chenxi Wang and Ruzhu Wang at the
Shanghai Jiao Tong University, and Bikram Bhatia, Kyle Wilke, Youngsup
Song, Omar Labban, and John Lienhard, who is the Abdul Latif Jameel
Professor of Water at MIT. The research was supported by the National
Natural Science Foundation of China, the Singapore-MIT Alliance for
Research and Technology, and the MIT Tata Center for Technology and Design.
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