Materials

$4 solar desalination system produces a family's daily drinking water

$4 solar desalination system p...
Two test devices of a new desalination system
Two test devices of a new desalination system
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Two test devices of a new desalination system
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Two test devices of a new desalination system
A diagram illustrating how the new desalination system works
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A diagram illustrating how the new desalination system works

Desalination is being explored as a way to solve the shortages of drinking water that plague much of the world, but it has a few issues to iron out. A new design for a solar-powered desalination device prevents the build-up of salt, making for an efficient and affordable system. Just four dollars' worth of materials should be enough for a device that can provide a family's daily drinking water.

Fouling is one of the main problems in desalination systems. As salt and other impurities are being filtered out of water, that material tends to build up on membranes or other surfaces in the device, requiring parts to be regularly cleaned, or worse, replaced. Wicking materials are among the most commonly fouled parts, so for the new project, scientists from MIT and Shanghai Jiao Tong University set out to design a wick-free solar desalination device.

Intended to float on the surface of a body of saltwater, the system is comprised of several layers. A material with 2.5-mm perforations draws water up from the reservoir below, forming a thin layer of water on top. With the help of a dark material that absorbs heat from sunlight, this thin layer of water is heated until it evaporates, so it can then be condensed onto a sloped surface for collection as pure water.

The salt stays behind in the remaining water, but this is where the team’s new idea kicks in. The holes in the perforated material are just the right size to allow for a natural convective circulation to occur. The warmer water above the material – which is now far more dense with salt – is drawn back down into the colder body of water below. A new layer of water is drawn up to the top of the material and the cycle begins again.

A diagram illustrating how the new desalination system works
A diagram illustrating how the new desalination system works

In test devices, the team says that this technique can achieve over 80 percent efficiency in converting solar energy to water vapor, even when the starting water had salt concentrations up to 20 percent by weight. No salt crystals were detected in the device after a week of operation.

Importantly, the system is made of everyday materials, so it should be scalable and inexpensive. The team calculates that a system with a collecting area of just 1 m2 (10.8 ft2) could provide enough drinking water for a family’s daily needs, and the materials to build it would only cost about US$4.

With more refinement, the team says that this desalination design could help bring drinking water to remote or developing areas, and be used as temporary setups to provide disaster relief after regular supplies are interrupted.

The research was published in the journal Nature Communications.

Source: MIT

11 comments
11 comments
guzmanchinky
This is fascinating, how many other easy solutions to huge problems are out there just waiting to be discovered?
Jennifer Rogers
Can we get the design? I’d love to try this out!
dan
wonderful, a big thanks to these people!
jerryd
I seriously doubt it'll make that much. While it can be done it needs a lot more area. as water takes a lot of heat to vaporize. 1sqm is only 1300wt at peak and downhill to and from there. Might make a pint, maybe 2. A lot of heat is wasted too.
While could keep a person alive, barely.
A person needs at least 1gal/day, the hotter it is, can triple that easily.
niio
@jerry Water heat of vaporization is about 0.6kwh/lit. With average insolation of 6kwh/m2/day and 80% efficiency each sqm of this device could produce about 3lit/day, not enough for the claimed family (unless it is one small person) but more than a pint or two.
Phaedrus
It looks promising!
christopher
Beautifully elegant solution (pardon the pun). "Desalination" is understating the output though? It's removing *all* the contaminants (not just the salt) that don't evaporate with the water.

@Jennifer - the design is fully explained: punch loads of 2.5mm holes in tin foil, arrange it to collect condensed water ( https://youtu.be/aYAZWcMaRwM?t=28 ), and "float" the result on something salty (e.g. your pool).
Expanded Viewpoint
J.R., if you click on the link to Nature Communications, you will see some color picture panels in the article. In Panel A, you will see most of what you need to know to build your own unit. Pick a size of box you want to make it, and on the side view, for simplicity, make it a trapezoid with one side being twice as tall as the other. Make a lid for the box out of clear, flat glass or plastic. You will need to glue a gutter inside the box, on the lower end of the lid to capture the water. Drill a small hole in the gutter for the water to flow out of into your collection cup via a hose to keep it from getting contaminated again. Keep the joints of the sides and top as tight as you can to keep the water vapor from getting out. The black plate with many holes in it can be clear plastic that is painted black. You will have to set the box on something that floats if you're doing a sea trial. Cheers!
jsopr
I wonder how hard it will be to get a 1m^2 sheet to be so flat and neutral buoyancy that a 1mm layer of water can be maintained over the whole thing.
Pedro TresPeros
I wonder if seawater could be desalinated on a huge scale by being used to cool tall buildings in Las Vegas? If it was pumped inland from off Los Angeles&sprayed down tubes in the middle of buildings.The heat of the building could boil off fresh condensate,that could be focused for collection for use by the building?The ,saline solution could slide down the tubes &be collected for sale. Turbines or solar installations were used to pump water up hill to vegas,,
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