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Vortex-induced vibrations: hydro energy of the future?

Attempts to mine the rich energy sources from the ocean through wave or tidal devices are limited to areas that have fast flowing water. But E&T discovers there are plans to use water vibrations to generate electricity from even the slowest running source.

Many modern inventions have been attributed to the genius of Leonardo da Vinci, but until now he had not been credited with extracting energy from the ocean.

The 15th century artist and inventor designed flying machines that were the inspiration of the helicopter and hang glider; he drew sketches for hydraulic pumps, steam cannons and huge single span bridges. But it is one of his lesser known discoveries, Aeolian Tones, which is driving a remarkable technology to extract energy from even the lowest flow of water.

Aeolian Tones, as da Vinci discovered, are the audible sounds created by wake-eddy - vortex-induced air pressure fluctuations produced as air flows around obstacles, such as wires and twigs. Their pitch is controlled by the rate at which eddies are formed and detached in the wake region on the downwind side of an obstacle.

A good example would be the whistling sounds created by power lines in a windstorm. These are pure Aeolian Tones. Some musical instruments - such as wind harps - create Aeolian Tones directly. However, other more common wind instruments utilise the Aeolian effect to create sound that can be controlled in pitch and volume. Most notable among these are the various types of block flute that are common in both aboriginal and modern cultures around the world.

Vortex-induced vibrations

In engineering, these tones have been the bane of engineers throughout the ages. Named vortex-induced vibrations, they can have a devastating effect on all sorts of man-made structures.

But how is this all relevant to extracting energy from the oceans? Existing technologies that use water power, relying on the action of waves, tides or faster currents created by dams, are far more limited in locations where they can be used; they also cause greater obstructions when they are built in rivers or the sea. Turbines and water mills need an average current of six knots to be financially viable, while most of the Earth's currents are slower than three knots.

This water flows continuously, but in most places too slowly for modern technology to harness its power. That could change soon thanks to an invention by University of Michigan researchers. Their machine is called Vivace (Vortex Induced Vibrations for Aquatic Clean Energy), and it can tap into the energy locked in slow-moving ocean and river currents.

As Professor Michael M Bernitsas sees it, the cylinder-based device he invented is a short step away from a commercially viable version that might be the key to a cheap, inexhaustible supply of clean energy to power the entire world, even regions far removed from sources of water.

The vibrations produced by vortices can cause damage to structures built in water, like docks and oil rigs. But, because Vivace is designed to harness the hydrokinetic energy, the technology is not destructive. Oscillations are slow, imitating the swim strategy of a school of fish, and thus less likely to be harmful to aquatic life than dams or water turbines. And, because the installations can be positioned far below the surface of the sea, there would be less interference with shipping, recreational boat users, fishing and tourism.

"The vast majority of currents around the world are slower than three knots," Professor Bernitsas explains. "Vivace greatly expands our ability to harness energy from slow currents, even as slow as one knot. There's no other technology that can go into such a slow flow and harness the energy."

How Vivace works

The Vivace system is a new concept in hydro kinetic energy. A cylinder is placed into the water across the direction of the current. The cylinder causes the water flowing around it to form into vertical whirl pools that alternate above and below the cylinder.

The whirlpools push or pull on the cylinder causing it to bounce up and down in vortex-induced vibrations. These hitherto toublesome vibrations are what the machine captures.

The most dramatic evidence of the negative effect of vortex-induced vibrations was the collapse of the 1,800m Tacoma Narrows suspension bridge in November 1940.

"The Tacoma Narrows Bridge got into torsional vibrations and in a matter of hours the bridge collapsed," Professor Bernitsas explains. "So in all disciplines of engineering - navel, marine, offshore, nuclear, aerospace engineering, civil engineering - we try to suppress vortex-induced vibrations.

"At some point it dawned on me that maybe what we could do instead of suppressing this phenomenon is to enhance them in a totally different domain and then try to harness the energy. But as we started studying this phenomenon and moving from the general idea of man-made structures into water we started realising that fish use a lot of the same forces.

"They basically curve their bodies, they collect a large vortex of the curved side, they straighten their bodies to shed that vortex, and collect one on the other side. They basically glide between these vortices, so, when you go to an aquarium and you see schools of fish swimming in formation. Every fish is gliding between vortexes generated by a fish ahead of them. It's a very efficient way of propulsion.

"Marine renewable energy has its advantages and disadvantages. The big advantage is it's abundant - it's everywhere. The vast majority of the energy the Earth collects from the Sun is absorbed by water. So we need technology that's new and environmentally friendly to harness this energy. That's where Vivace comes in.

"It looks exotic to humans because we live in air and all we see around us is lifting surfaces like airplane wings, birds wings, sail boats, windmills. But if we put ourselves in water then the picture is totally different. Anything that moves in dense fluid from tiny sperm to huge whales is basically a bluff body with a tail, so to fish and marine life this is the natural thing."

The technology is being commercialised through Professor Bernitsas's company Vortex Hydro Energy, and a pilot project in the Detroit River is in its early stages. A prototype of Bernitsas' invention could be set up in about a year for an underwater field test in the Detroit River. The major hurdle now is neither creative nor technical; it's a matter of getting clearances from some two dozen federal and state agencies.

The six-month-long field test - to be conducted in water 13.5m deep at a site near Detroit's Renaissance Centre - will have to demonstrate that the system is safe, has no technical issues and poses no threat to nesting fish or other aquatic life.

"We're where cars were 100 years ago," Bernitsas, says. "Hopefully it won't take us 100 years to get where we need to be."

Detroit river authorities

Actually, he says, securing permits for the Vivace test should be relatively easy, since it is considered a scientific instrument.

"I do not see the regulatory agencies as a problem," adds Bernitsas. "They are helpful in identifying environmental issues that we have to look into. In my opinion we are living with the consequences of decisions we made 100 years ago to go with fossil fuels and other decisions we did not make since then, and now that we see these consequences, we are more careful."

The prototype for the Detroit River field test will operate seven or eight cylinders, each of them about 25cm in diameter and 2m long. The device itself is expected to cost about $25,000, but Bernitsas estimates the entire development process of building, testing and launching to come in at above $1m.

Besides ensuring that marine life can live with Vivace, the test will aim to confirm the energy output results he produced in the University of Michigan lab, in addition to gauging the device's durability, resistance to fouling, robustness in dealing with variable current speeds, and the functioning of adoptive controls.

A second field test is expected to be run in an undetermined ocean environment.

The technology can generate electricity in water flowing at a rate of less than one knot - a knot is a bit more than one mile an hour - meaning it could operate on most waterways and sea beds around the globe. Electricity can be sent to other locations through power grids, just as it is now.

The potential is huge. If we could harness 0.1 per cent of the energy in the ocean, we could support the energy needs of 15 billion people. "In the English Channel, for example, there is a very strong current, so you produce a lot of power."

Further information

Tacoma Bridge (Youtube) [new window]

Vortex Hydro Energy [new window]

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