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.

"It seems pretty viable," says Peter Lucon, a mechanical engineer with Resodyn Corporation, who has also been harnessing resonance-induced turbulence—not for energy generation, but for über-efficient industrial mixing. However, Lucon expressed some skepticism regarding the technology's energy efficiency.

As presently described, a VIVACE system about the size of a running track would be capable of powering about 100,000 houses. Bernitsas has shown repeatedly in the lab that the system can generate 6.375 Watts per cubic meter of water in a flow of 1.5 knots, 51 W/m3 of water in a flow of 3 knots, and 408 W/m3 of water in a flow of 6 knots.

Lucon worries that—like early attempts to pack wind turbines close together—these projections might fall victim to another concept familiar to students of fluid dynamics: pressure drop.

When wind turbines were packed close to one another, pressure would build up in front of the system and a significant low-pressure zone would develop behind it. "Once you create that kind of pressure difference, it's going to try and go around it," Lucon says. Air circulated around, instead of through, the wind turbines without generating energy.

This is just one of the many issues Bernitsas hopes to resolve by studying fish:

I'm sure you've seen in aquaria, fish swimming in compact formation. Each fish glides between the vortices generated by the fish ahead of him. That's why they go so fast and why they travel in compact formation.

At present, the VIVACE team tries to keep its units far apart in order to avoid interference. "If I was as smart as fish," Bernitsas says, "I'd probably have sensors that would allow the cylinders to be much closer together."

Each individual VIVACE unit consists of one metal cylinder placed parallel to the river bed and perpendicular to the flow. This metal cylinder is attached to springs whose movements collect energy as the cylinder is subjected to the vortex-induced motion. In Nuclear engineering, studies of VIV created by the placement of cylindrical fuel rods in the cooling flow have shown a strong correlation between the frequency of the vibrations and the closeness of the fuel rods to one another, Bernitsas says. It's his hope that the increased energy of these increased vibrations means that a tightly packed VIVACE system will actually be synergistically more efficient than a loosely packed system.

By strategically making portions of the cylinders roughly textured (a concept inspired by the rough texture of fish skin), the team has been able to better control the creation of vortices. Recent test results suggest this method of "passive turbulence stimulation" increases the range of velocities in which power generation occurs.

Bernitsas is eager to continue optimizing the many parameters (length, stiffness, thickness) that arise merely by adding passive fish-like tails to the individual cylinders. "Our experiments have shown that, depending on the length and the stiffness, you get totally different answers," he says. "In some cases you enhance VIV, and in some other cases you convert VIV to galloping. Galloping is a much slower phenomenon but larger amplitude."

Because galloping is a much more gentle process (think large undulating motions like a manatee's or a whale's), the team hopes to deploy this form of VIVACE in small rivers where a more vigorous system would disrupt the marine life.

The care Bernitsas and his team have for VIVACE's ecological effects is just one of the many refreshing aspects of their research. This month, they will be publishing the results of experiments designed to mitigate VIV disturbance of ocean floors and river beds, areas with either "contaminated soft sediments or marine life habitats." It will appear in the Journal of Offshore Mechanics and Arctic Engineering.

Bernitsas is fond of humbly pointing out the distance VIVACE still needs to go by joking, "We're not as smart as fish, at this point."

That being said, I don't see any fish generating electricity by mimicking us.

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