Posted 02 June 2011, by Paul Marks, New Scientist, newscientist.com
THE thought of wind power brings visions of giant turbines, high-altitude kites and graceful sailboats to mind. But the breeze has a more sinister side, full of turbulence that can wreak havoc with bridges and other structures.
Now Hyung-Jo Jung and Seung-Woo Lee at the Korea Advanced Institute of Science and Technology in Daejeon, South Korea, plan to harness these destructive forces to generate energy. They have built a prototype that produces energy using a specific type of unstable airflow called wake galloping.
Wake galloping is a form of vigorous vibration that affects cylindrical parts of structures, such as the cables on suspension bridges, exposed to seemingly harmless airflow. When the wind passes a horizontal cylinder, eddy currents called wake vortices are created on the lee side. These induce a lifting force on a cylinder in the path of these eddies – but only if the two have the same diameter and the second cylinder is three to six diameters away from the first.
The leeward cylinder’s weight counteracts the lift by pulling it back down again, resulting in the cylinder repeatedly moving up and down as the wind continues to blow. It is this movement that Jung and Lee hope to harness as energy.
To do this, they built a device containing two 85-centimetre long, 5-cm-diameter perspex rods spaced appropriately from each other. The rod on the leeward side was attached to a magnet, which was free to move within a copper coil (see diagram). As the cylinder moved, so did the magnet, generating a current in the coil.
The team found that even at wind speeds between 2.5 to 4.5 metres per second, when traditional wind turbines are inefficient, the system generated nearly half a watt of electrical power. They think this could be improved if the magnets and coils were optimised. The team report their findings in Smart Materials and Structures (DOI: 10.1088/0964-1726/20/5/055022).
“One of the most promising applications for this is monitoring a structure’s health using wireless sensors,” Jung says. “The device could supply the monitors with power in a bridge or a high-rise building.” And if enough of the generators were grouped together, he adds, they could power a bridge’s street lighting.
The pair are now attempting to find what Jung calls the “most efficient size of the device” before they attempt to commercialise it.
Jung and Lee are not alone in harnessing unusual aspects of the wind. Humdinger Wind Energy of Honolulu, Hawaii, led by founder Shawn Frayne, is interested in a similar phenomenon called aeroelastic flutter, in which aerodynamic forces reinforce a structure’s natural resonance, causing it to vibrate. The most famous example of this occurred in 1940, when the newly opened Tacoma Narrows bridge in Washington state began undulating in a light breeze, earning it the nickname Galloping Gertie. Within four months of completion, the bridge had shaken itself to shreds in a storm.
Frayne has created the Windbelt, which uses aeroelastic flutter to vibrate a plastic ribbon in light to moderate breezes. The vibration moves a magnet through a coil, which generates current in a similar fashion to Jung and Lee’s device.
Both will likely find a market, says Matthew Wright of the Institute of Sound and Vibration Research at the University of Southampton in the UK. “There are situations where you need a small amount of power, but in a location that’s too remote to connect to the grid, or to deliver fuel or batteries to. One of these devices might well find a niche there.”