Cost-effective solar stills made from bubble wrap developed by researchers

Solar stills have been used for thousands of years to make tainted water or seawater fit to drink. But modern solar stills typically use expensive lenses or other equipment to produce more than a trickle. Now, researchers have created a more cost-effective solar still made from bubble wrap and other simple materials.

The most basic versions of solar stills are water-filled vessels with black bottoms that absorb the sun’s rays, increasing evaporation of the water inside. Glass or other clear material on top captures the vapor, and the condensate drips into a collection vessel.

Modern versions use lenses or mirrors to collect 100 times more sunlight and speed up the process. But the high cost of these solar concentrators, typically on the order of US$200 per sq m, makes them unaffordable for many people.

Researchers led by Gang Chen, a mechanical engineer at the Massachusetts Institute of Technology in Cambridge, unveiled an efficient solar absorber made from a layer of graphite on floating carbon foam two years ago.

The two layers were perforated, allowing the water below to wick up to the graphite, where it was warmed by the sun. The device worked, but they still needed additional devices to concentrate 10 times the ambient sunlight to overcome the infrared losses and be able to boil water.

Chen and his colleagues kept their idea of a spongy insulator floating on water. For their current experiment, the researchers replaced the graphite solar absorber with a thin layer of a bluish metal and ceramic composite material used in commercial solar water heaters. This material selectively absorbs visible and ultraviolet rays from the sun, but it doesn’t radiate heat in the infrared. Between this layer and the foam, they placed a thin sheet of copper, an excellent heat conductor. The researchers then punched holes through the sandwich-like layers as before.

But much of the energy absorbed by the composite was still being swept away by convection, heat lost to the air moving above the still’s top surface.

Luckily, Chen’s 16-year-old daughter was designing a cheap greenhouse for a science fair experiment. She found that a top layer of bubble wrap acted as an excellent insulator. So Chen and his student George Ni covered their solar still in bubble wrap that allowed them to boil and distill water with no extra solar concentrator. Down the road, Chen estimates that this will allow them to make large-area solar stills for about one-twentieth the cost of conventional technology.

Chen believes the low-cost apparatus could help purify wastewater near fracking sites, for example. Typically, companies work to evaporate water from wastewater ponds to concentrate and remove the contaminants. A cheap solar sponge could speed the cleanup.

He said that for the device to be useful for desalination or other drinking water applications, it will need another plastic or glass layer on top to collect the water vapor, which could increase the system’s efficiency by trapping more heat and boosting evaporation.

Creating a purification system would be no small task. Chen estimates it would require 20 to 40 sq m of the solar still material to provide 50 liters of water per day, the minimum that United Nations says a person needs for daily life.