New tech purifies water using sunlight

Scientists have developed a new technology that uses the sunlight to purify water with near-perfect efficiency.

The idea of using energy from the sun to evaporate and purify water was reportedly described by Greek philosopher Aristotle over 2,000 years ago.

By draping black, carbon-dipped paper in a triangular shape and using it to both absorb and vaporise water, researchers have developed a method for using sunlight to generate clean water with near-perfect efficiency.

"Our technique is able to produce drinking water at a faster pace than is theoretically calculated under natural sunlight," said Qiaoqiang Gan, an associate professor at the University at Buffalo in the US.

"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 per cent efficient. Our system has a way of drawing heat in from the surrounding environment, allowing us to achieve near-perfect efficiency," said Gan.

The low-cost technology, described in the journal Advanced Science, could provide drinking water in regions where resources are scarce, or where natural disasters have struck.

Researchers have launched a startup, Sunny Clean Water, to bring the invention to people who need it. The company is integrating the new evaporation system into a prototype of a solar still, a sun-powered water purifier.

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 vapour cools and returns to a liquid state, at which point it's collected in a clean container.

The technique has many advantages. It is simple, and the power source - the sun - is available just about everywhere. However, even the latest solar still models are somewhat inefficient at vaporizing water.

Researchers addressed this challenge by increasing 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. Since 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 vaporisation process.

Using this set-up, researchers evaporated the equivalent of 2.2 litres of water per hour for every square metre of area illuminated by the regular sun, higher than the theoretical upper limit of 1.68 litres, 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.

"Most groups working on solar evaporation technologies are trying to develop advanced materials, such as metallic plasmonic and carbon-based nanomaterials," Gan said.

"We focused on using extremely low-cost materials and were still able to realise record-breaking performance," he said.