The world’s river and ocean currents carry an enormous amount of kinetic energy, but most of this water flows slower than four miles per hour. Existing turbine and water-mill technologies can’t generate enough electricity at such speeds to make their deployment economically viable.
Researchers at the University of Michigan say that they have overcome this limitation by taking advantage of energy-packed vortices that are formed when water flows past a cylindrical object, even at low speeds. Salmon and trout are known to leverage the force created by these naturally occurring water swirls so that they can swim upstream. A new mechanical device designed to economically harvest that energy and convert it into electricity could turn waterpower into a much larger part of the world’s renewable-energy mix.
“Anywhere we have currents, we can use it,” says Michael Bernitsas, a professor in the department of marine engineering at the University of Michigan. He says that the first test of the device will be in the Detroit River, likely in 2010. “If we make it work, and I believe it will, it’s going to be a major development,” he says.
The device works on the well-known principle of vortex-induced vibrations, which in an ocean setting are known to play havoc with the cylindrical steel risers and mooring lines that anchor offshore oil platforms. As current flows past a cylinder, a thin layer of water gets entrained along each side of the rounded surface until, at some point at the back of the object, the layer of water separates from the surface and swirls into a vortex.
Part of the phenomenon, however, is that the separations on the left and right sides don’t take place at the same time: one side lags. The result is an alternating pattern of vortices that can impose tremendous force on underwater structures. When a cylinder-shaped object can move more freely in its environment, like a fishing lure being pulled by a river’s current, the alternating vortices will vibrate the object from left to right.
Bernitsas says that the alternating vortices “lock on” to the oscillating frequency of the object. “The bottom line is we get synchronization between the shedding of the vortices and the motion of the cylinder,” he explains.
As part of his research for the oil industry, Bernitsas has spent much of his career trying to figure out ways to suppress these destructive natural vibrations. Four years ago, it occurred to him that if he enhanced and tapped into these vortex forces, he could design a device that generates emission-free electricity. This led to the development of the VIVACE (vortex-induced vibration for aquatic clean energy) converter, a modular system that in the lab generates 51 watts per cubic meter of water flowing at three knots, or about 3.5 miles per hour.
In its most primitive form, VIVACE is a horizontal cylinder on springs that moves up and down between two upright tracks as water flows past it, creating mechanical energy that is converted into electricity. Bernitsas envisions the system as stackable and deployable in different configurations and generation capacities, from kilowatts to multimegawatts. And it wouldn’t occupy much space: one megawatt, he estimates, would take up about 90 cubic feet.
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