According to the United Nations, about one-fifth of the world’s population lives in areas where water is scarce. Therefore, technologies are urgently needed to obtain clean water from unsuitable sources of drinking – such as sea, river or lake water, as well as polluted water. Scientists have developed a wood-based steam generator that uses solar materials to purify water using nanomaterials produced by bacteria. The study publishes the journal Nano Letters.
A solar steam generator is a device that uses an abundance of solar energy to separate clean water from pollutants by evaporation. Previously, many different versions of these devices with different efficiencies were invented. To develop better solar steam generators, researchers must find ways to improve light absorption, heat management, water transfer, and evaporation.
Professor Shu-Hong Yu and his colleagues from the University of Science and Technology of China wanted to combine all four improvements in one device. They chose wood as the basis for their generator because of its stability and porous structure, which provides fast water transport.
Scientists made their device using bacteria that produced long cellulose nanofibres. They also connected the layers of the device together. A team of researchers added bacteria to the surface of a wooden block and allowed them to roam.
Scientists then sprayed an aerosol of glass bubbles – tiny hollow spheres that provide excellent thermal insulation – to the surface. Spheres began to penetrate into cellulose nanofibers produced by bacteria, forming a hydrogel. Finally, the researchers added carbon nanotubes, which messed up with cellulosic nanofibers, forming a vaporizing top layer that absorbs light.
The device works by transporting water up through the wood to a light-absorbing layer that is heated by the sun. Water evaporates, and steam is collected and condensed to produce clean water. The insulating layer of the glass spheres prevents heat transfer down through the device and its loss. At this time, nanoscale structures reduce the energy needed to evaporate water. As a result, the new device has a higher evaporation rate and efficiency than most existing solar steam generators.