Researchers of the University at Buffalo School of Engineering and Applied Sciences have developed a method for using sunlight to generate clean water with near-perfect efficiency. The idea of using energy from the sun to evaporate and purify water is ancient. The Greek philosopher Aristotle reportedly described such a process more than 2,000 years ago. Now, researchers are bringing this technology into the modern age.

This technique is able to produce drinking water at a faster pace than is theoretically calculated under natural sunlight. Usually, when solar energy is used to evaporate water, some of the energy is wasted as heat is lost to the surrounding environment. This makes the process less than 100 percent efficient. This system has a way of drawing heat in from the surrounding environment, allowing us to achieve near-perfect efficiency.
Solar stills have been around for a long time. These devices use the sun’s heat to evaporate water, leaving salt, bacteria and dirt behind. Then, the water vapor cools and returns to a liquid state, at which point it’s collected in a clean container.
The technique has many advantages. It’s simple, and the power source — the sun — is available just about everywhere. But unfortunately, even the latest solar still models are somewhat inefficient at vaporizing water.

Lead researcher and associate professor of electrical engineering in the University Qiaoqiang Gan’s team, addressed this challenge through a neat, counterintuitive trick: They increased the efficiency’ of their evaporation system by cooling it down.
A central component of their technology is a sheet of carbon-dipped paper that is folded into an upside-down “V” shape, like the roof of a birdhouse. The bottom edges of the paper hang in a pool of water, soaking up the fluid like a napkin. At the same time, the carbon coating absorbs solar energy and transforms it into heat for evaporation.
the paper’s sloped geometry keeps it cool by weakening the intensity of the sunlight illuminating it. (A flat surface would be hit directly by the sun’s rays.) Because most of the carbon-coated paper stays under room temperature, it can draw in heat from the surrounding area, compensating for the regular loss of solar energy that occurs during the vaporization process.
The low-cost technology could provide drinking water in regions where resources are scarce, or where natural disasters have struck. The advancements are described in a study published on May 3 in the journal Advanced Science.
Gan, Song and other colleagues have launched a startup, Sunny Clean Water, to bring the invention to people who need it.