Clean Energy Inspired by Oil Rigs

Scientists at the University of Michigan are beginning the first large-scale test of a new technology that takes a common problem for oil platforms and turns it into a method for reliably generating clean electricity from ocean and river currents. They are working with the U.S. Navy to build a prototype in the Detroit River this year with the capacity to power a 20,000-square-foot building.

When word first surfaced of the VIVACE Converter (short for Vortex-Induced Vibration for Aquatic Clean Energy Converter), it sparked a flurry of pop-sci articles struggling to explain the fluid dynamics with anything remotely accessible to the public.

The concept—absorbing energy from a phenomenon called vortex-induced vibration (VIV)—has been likened to Leonardo Da Vinci’s research into “Aeolian Tones,” the infamous Tacoma Narrows Bridge disaster, and the device’s own sexy aquatic biomimetics: It imitates fish. Like fish, whose muscular power alone could not propel them at the speeds they travel, the invention harnesses forces created by a disrupted current.

Previous methods for collecting energy from currents, like turbines and water mills, required an average flow of five or six knots, while most of the earth's currents are slower than three knots.

VIVACE promises to generate power from these much slower flows.

The initial research has even suggested improvements for offshore oil rig construction—making it one of the few areas in alternative energy research to offer simultaneous long-term and short-term gains for petroleum company investors.

“When I first came up with this idea, people were thinking that VIVACE was kind of exotic,” says inventor Michael Bernitsas, a professor at the University of Michigan and CEO of Vortex Hydro Energy.

With the pressing need for practical and reliable forms of alternative energy, ‘exotic’ was the last descriptor investors wanted to hear. It was also inaccurate.

Vortex-induced vibration is actually one of the better understood and most predictable phenomena in the world of fluid dynamics and structural engineering. The installation of everything from telephone poles to nuclear fuel rods to undersea pipelines has required an understanding of VIV sufficient enough to suppress its effects: namely, damage to a structure via the oscillating forces induced by the periodic creation and shedding of vortices.

Whether it’s wind singing around Aeolian Chimes or coolant water flowing past a nuclear reactor, certain ratios of a fluid’s velocity and density relative to the characteristics of a given obstruction will create a repeating pattern of paisley-shaped swirls of turbulent fluid in the “empty space” behind the structure. These vortices exert a force on the structure, pushing themselves downstream as they do. At certain ratios, these forces develop a piston-like regularity that can create energy. VIV is so predictable that high-end velocimeters use its forces to measure fluid flow.

“We know a lot about VIV, and we haven’t used it properly,” says Bernitsas, who spent much of his academic life working with oil companies to suppress VIV on offshore oil rigs and pipeline mechanics.

One of the problems with solar, wind and wave power is reliability. In comparison, Bernitsas says:

The advantage that currents have is that—as long as we have the motions of the earth, the moon and the sun—the currents are going to be predictable.

This is due in part to what Bernitsas alternately calls the “lock-in phenomena,” synchronization and non-linear resonance. Over a range of current velocities—which always experimentally seems to be the distance between a velocity and its double (e.g. 2 to 4 knots, 3 to 6 knots, etc.)—VIV occurs at a periodic frequency allowing for the regular generation of power.

Bernitsas estimates that VIVACE energy could sell at 5.5 cents per kilowatt-hour, a scant 1.5¢/kWh cents from matching pulverized coal (see graph). If 0.1 percent of the ocean’s energy were harnessed, it could support the energy needs of roughly 15 billion people, he says.