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Solar Stills

Paul Kando

In the early 1940s, World War II battles in the Pacific would often leave US soldiers stranded at sea for days, and lacking drinking water posed a serious threat to their survival. That is when my late solar research colleague, Maria Telkes — then a research associate at MIT — adapted her solar still design to a lightweight solar water distillation device made for the U.S. military out of clear plastic film. The device could be included in soldiers’ emergency medical kits and could produce, from sea water, enough fresh drinking water for one person.

Maria Telkes and her Solar Still
photo credit: therobertabondarfoundation.org

The Telkes solar stills consist of a shallow black-bottomed vessel filled with water and topped with a slanted clear glass or plastic pane. The black bottom absorbs sunlight, heating the water so that it evaporates and leaves salt and other contaminants behind. The water vapor then condenses on the clear covering and trickles into a collection trough. Larger versions of the solar still have been used to provide drinking water on the US Virgin Islands.

Such solar devices are easy to construct, but their output is limited by the fact that the sun’s rays must heat the entire volume of water in the shallow pan before evaporation can begin. Commercially available stills today produce about 0.3 liters of water per hour per square meter (L/h/m2) of the covered water’s surface area. An average person requires about 3 liters of water a day for drinking, so providing enough drinking water for a small family requires a still around 5 square meters in size, increasing the still’s otherwise low cost. Operating at their theoretical best, such devices produce only 1.6 L/h/m2 of potable water. So, even large, expensive stills can produce only enough water for a very small family.

Yet, according to UNICEF, 783 million — nearly one in 10 — people around the world lack clean water access. They spend a collective 200 million hours a day fetching water from distant sources, and the water they fetch is often of questionable quality. Technologies do exist for purifying contaminated water and desalinating seawater. However, these typically require expensive infrastructure and lots of energy, which puts them beyond the reach of many water-hungry communities.

Recently, researchers have been working to upgrade solar stills as a cheap, low-tech alternative, by improving their rate of evaporation. Guihua Yu, a materials scientist at the University of Texas, Austin, and colleagues have developed hydrogels — polymer mixtures that form a 3-dimensional porous, water-absorbent sponge-like layer — which they placed atop the water’s surface in a solar still. The two polymers are polyvinyl alcohol (PVA), a water-binding polymer; and polypyrrole (PPy), a light absorber.

Inside the transparent, light-absorbing gel-layer, water molecules chemically bond to the PVA by means of hydrogen bonds. With so much of their bonding ability tied up with the PVA, these water molecules bind only loosely to the mass of water beneath the gel. This allows the gel-tied water to evaporate more readily and, as it evaporates, to be immediately replaced by water molecules from outside the gel. Using this technology, Yu’s experimental solar still produced 3.2 L/h/m2 of water, twice the theoretical limit, as reported last year in the journal Nature Nanotechnology.

Since then Yu and his colleagues have created an even better hydrogel by adding a third polymer — Chitosan — another strongly hydrophilic material that enables the gel to hold and bond more water. The result, as reported in Science Advances, is the highest solar distillation rate ever reported: 12 times the distillate produced by today’s commercially available solar stills.

At this higher water production rate, a solar still 1 square meter in size could produce about 30 liters of clean drinking water per day, enough for a small family. And all three polymers in the hydrogel are commercially available and inexpensive. This means that if the stills can be scaled-up and made rugged enough at a reasonable cost, they could provide access to clean drinking water for millions of impoverished people worldwide.

There are important lessons in this story about how science works at its best. First, the best scientific discoveries are often the simplest—the Telkes solar still is testament to this. Second, the best science builds on earlier discoveries. This is why science works best when it is not “owned” and exploited for money. Knowledge is the common wealth of the human race and sharing it, building on it, is a capacity unique to the human species.