Engineers at the University of Maryland‘s School of Engineering have investigated ways of using energy from the sun and a block of wood smaller than an adult’s hand to heat water to produce steam which can be condensed to provide purified water in an efficient, easily accessible, environmentally friendly, biodegradable way and at an extremely low cost.
Inspired by the process by which water is carried through trees from roots to small pores on the underside of leaves, the research team tested several ways in which water can be transported through wood, purifying it for safe use.
Basically, a small block of wood about five centimetres square is floated on a bowl of unpurified water sitting in a sunny spot. The side of the wooden block facing up is darkened. As the sun heats the wood, the water below is drawn up through the wood’s natural channels. The hot dark surface evaporates the water, which can be condensed and distilled off. The salt or other contaminants are too heavy to evaporate, so they stay below.
The team tried this with the natural wood’s channels oriented up-and-down, just as they would be inside the tree.
One design used carbon nanotubes – tiny, naturally dark structures grown in a lab – to coat one side of the wood and heat the water inside. Another used metal nanoparticles to achieve the same results. In the third design, the top layer of wood was carbonized (essentially burned) to create a dark surface.
The team measured how efficient the solar steam generation devices are. The most efficient device was the burned-top wood, with 87% efficiency. It was also the least expensive to produce, coming in at just $1 per square meter.
The other techniques also had advantages. The carbon nanotube-topped version was flexible, because the lignin, which is the component that makes wood stiff, was removed. It could be rolled into a tube.
The device coated with metal nanoparticles showed a self-cleaning aspect when it was placed in salt water. During the day, the salt was too heavy to evaporate and was left behind. During the simulated night (12 hours without sunlight) the salt dissolved off the wet surface.