Don’t be fooled by Ol’ Man River. Below the surface of even the laziest stream surges an eternally youthful source of energy—water coursing its way from the highlands back to the ocean in a driven, ineradicable flow.
The energy is there but tapping it is difficult. Water mills did their small part to ease chores like grinding wheat into flour. And on a larger scale, hydroelectric dams provide energy for entire towns. But they interrupt essential dynamics of the river, stanching the flow of essential nutrients to portions of the river downstream, and disrupting the reproductive habits of fish that time their spawning to yearly floods.
University of Michigan Professor Dr. Michael Bernitsas in front of Vivace. Photo/Scott Galvin, U-M Photo Services
Michael Bernitsas, a marine engineer at the University of Michigan has added his own invention to this timeline. He predicts that his system will convert river currents into large amounts of power without altering natural structures or even being visible from the surface.
Bernitsas’ Vortex Induced Vibration for Aquatic Clean Energy converter, or VIVACE, is driven by a rig of cylinders running parallel to the bed of a river. Each cylinder is mounted on a spring-loaded runner, giving it a vertical motion. In a current, the cylinders plunge up and down like a robot marching in place. “It’s like a reciprocating engine with four cylinders. But they are driven by the fluid dynamics” of the river, says Bernitsas.
As water flows around a cylinder, it sweeps off the back and forms two vortices that swirl in opposite directions. The same thing happens as fish swim through the water, and indeed when any blunt object encounters a current. Fish, however, use their tails to flick away these tiny whirlpools, allowing them to steer strait through the rushing water. But as a vortex sheds from the back of the VIVACE cylinder, it propels the cylinder either up or down until the spring action sends it back into place, The second vortex then swoops it up, shooting it in the opposite direction.
“When the vortex sheds, there is a pressure change. That pressure change moves the body,” explains Bernitsas, who started a company called Vortex Hydro Energy to commercialize the technology. “That’s about it. It’s a very simple machine.”
In a steady current, the cylinders fall into a regular pattern of oscillation that Bernitsas can control by changing the width and length of the cylinders and by giving them an asymmetrical shape. A magnet is attached the cylinder and as it moves up and down a coil, it creates a electrical current.
See VIVACE at work:
With these controls, a system like VIVACE could harness energy from a large range of water supplies, even slow moving rivers. And here, especially, is where VIVACE could compete with other water-powered renewable energy devices.
“The vast majority of the currents we have around the world are slower than 3 knots,” says Bernitsas. “Turbines require an average of 5-7 knots to be financially viable.”
In a current moving at only 3 knots (about 3.5 miles per hour) VIVACE produces between 51 and 239 watts for every cubic meter of river bed covered. And unlike turbines, they can operate in shallow water, and are built with off-the-shelf materials.
The system has been optimized for currents running 2-3 knots, but Bernitsas has successfully tested it at a low limit of 0.8 knots. If proven to be efficient in such slow moving water, VIVACE systems could be deployed in remote locations in the ocean and used as recharging stations for sensors and unmanned undersea vehicles, a use the U.S. Navy is investigating.
“There are applications at all levels,” says Bernitsas.
But whether VIVACE succeeds as part of the solution to our broader energy demands will ultimately depend on economics.
“It’s a very exciting project. It’s a very interesting project,” says Jason Dahl, a professor in the Department of Ocean Engineering at the University of Rhode Island. “What it comes down to is how much does the energy cost.” At this point Bernitsas predicts that it would cost $2,700 per killowatt to install the VIVACE system, with energy being supplied at $0.08 to $0.12 per killowatt hour, which is more expensive than coal but slightly cheaper than wind, depending on how you calculate it.
Bernitsas has plans to try out an updated version of VIVACE, with electronics instead of springs, in Michigan’s St. Clair River early next year, at which point he expects a more detailed cost analysis to emerge.