Currents can be powerful enough to tip canoes, damage docks and even topple bridges. The force of all that moving water can also provide a clean, affordable and unobtrusive source of renewable energy, says engineer Michael Bernitsas, of the University of Michigan in Ann Arbor.
Bernitsas has invented a device, named VIVACE, that converts river and ocean currents into electricity. Like fish, the device takes advantage of powerful phenomena called vortex-induced vibrations.
These vibrations occur when water flows past a round or cylindrical object. Swirls of water form downstream from the object and in an alternating pattern on either side of it. That causes the object to oscillate, or vibrate, up and down.
Vortex-induced vibrations also occur in air. And they have been responsible for some high-profile disasters, including the 1940 collapse of the Tacoma Narrows Bridge in Washington.
"We always try to suppress vortex-induced vibrations because they are very destructive," said Bernitsas, who has designed offshore mooring systems for oil companies. "It dawned on me that instead of suppressing those vibrations, maybe we could enhance them, control them, and harness the energy from currents."
Spinning electricity out of water is not a new idea. Hydroelectric plants harness the power of falling water. Turbines tap into currents.
But these techniques are limited to systems that contain lots of fast-moving water flowing at speeds of at least five knots. (One knot is equivalent to just under one mile per hour.) However, the vast majority of ocean currents around the world flow slower than three knots. Most river currents are slower than two knots.
The advantage of VIVACE, Bernitsas said, is that vortex-induced vibrations form even at slow rates of flow, and the device can adjust to capture energy at a wide range of flow speeds.
The essence of the device is a spring-supported cylinder that is submerged in water. As water flows past the cylinder, vortex-induced vibrations move it up and down. A generator then converts that motion into electricity.
In Bernitsas' lab, 8,000 gallons of water circulate through a tank equipped with a VIVACE cylinder. The test section measures 10 feet long, 3 feet wide, and 3 feet deep. At a flow speed of three knots, experiments show, the device produces 51 watts of electricity, which illuminates six light bulbs. Bernitsas published a paper about the work recently in the Journal of Offshore Mechanics and Arctic Engineering.
Small versions of the device — with just a few cylinders — could eventually be used to charge batteries on anchored ships, Bernitsas said. Larger units could power entire communities.
With further refining, a VIVACE device could produce electricity for about 5.5 cents per kilowatt-hour (kWh). By comparison, coal-derived power costs about 4 cents per kWH. Wind energy costs about 7 cents. And solar power costs between 16 and 48 cents.
Fish manage to swim through powerful currents by collecting and shedding vortex-induced vibrations on the sides of their bodies as they wiggle. As Bernitsas continues to tinker with the technology, he is adding more fishlike features, including a tail and scale-like textures.
"Both have a profound impact on how much energy we can harness," he told Discovery News. "We are getting amazing results."
Bernitsas' invention has potential, said Roger Bedard, Ocean Energy Leader at the Electric Power Research Institute in Palo Alto, Calif.
"This is a very emerging technology," Bedard said. "I would recommend his device get built and tested to see if his claims are true or not."
Answers should be available soon. Bernitsas is currently working on a prototype for the Detroit River.