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, using it to sanitize water at what they report to be record-breaking rates. This could provide drinking water in regions where resources are scarce, or where natural disasters have struck.

Credit: Huaxiu Chen

By draping black, carbon-dipped paper in a triangular shape and using it to both absorb and vaporize water, they have developed a method for using sunlight to generate clean water. Gan explains, “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. Our system has a way of drawing heat in from the surrounding environment.”

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. As Gan explains, 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. Using this set-up, researchers evaporated the equivalent of 2.2 liters of water per hour for every square meter of area illuminated by the regular sun, higher than the theoretical upper limit of 1.68 liters, according to the new study. The team conducted its tests in the lab, using a solar simulator to generate light at the intensity of one regular sun.

Source (University at Buffalo. “Engineers upgrade ancient, sun-powered tech to purify water with near-perfect efficiency: Low-cost device — shaped like a birdhouse — could help provide drinking water to people affected by natural disasters.” ScienceDaily. ScienceDaily, 3 May 2018.)

Original paper: Song, H., Liu, Y., Liu, Z., Singer, M.H., Li, C., Cheney, A.R., Ji, D., Zhou, L., Zhang, N., Zeng, X. and Bei, Z., 2018. Cold vapor generation beyond the input solar energy limit. Advanced Science, 5(8), p.1800222.