Picture a bowl of unpurified water sitting in a sunny spot. On top of it floats a small block of wood about five centimetres by five centimetres. The side of the block facing up is darkened, to catch the sun’s rays. 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 too heavy to evaporate stay below.
Inspired by the process by which water is carried through trees from roots to small pores on the underside of leaves, the research team at the University of Maryland’s James Clark School of Engineering has created a suite of new ways in which water can be transported through wood, purifying it for safe use. Energy from the sun and a block of wood smaller than an adult’s hand are the only components needed to heat water to its steaming point in these devices.
With safe water difficult to secure for one billion of the world’s people, and cost and manufacturing key challenges in using solar-steam technology for seawater desalination, wood-based structures can potentially provide solutions.
These suite of solar steam generation devices are at once efficient, easily accessible, environmentally friendly, biodegradable, and extremely low cost. The team is working to scale up the devices for commercial use, which includes designing ways to easily manufacture them and bring down their cost in poorer countries.
Twists
In order to ensure the best products, the team is trying out a few twists on the basic idea of using a darkened surface on the wood to heat the water, then pulling it through the wood’s natural porous structures. One design uses carbon nanotubes—tiny, naturally dark structures grown in a lab—to coat one side of the wood and heat the water inside. Another uses metal nanoparticles to achieve the same results. Both of these designs are very efficient, but higher cost to produce.
Another innovative design involves carbonising—essentially, burning—the top layer of wood to create a dark surface. The team tried this with the natural wood’s channels oriented up-and-down, just as they would be inside the tree.
By the same measure used to test solar cells’ efficiency, the team measured how efficient the solar steam generation devices are. The most efficient device was the burned-top wood, with 87% efficiency at ten suns of light. It was also the least expensive to produce, coming in at only $1 per square metre.
But though they may not be the most efficient or cheap, the other devices have their advantages. The carbon nanotube-topped version is flexible, because the component that makes wood stiff, lignin, was removed – it can 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, but during 12 hours without sunlight the salt dissolved off the wet surface.
Source: phys.org
