Scientists tap motion in the ocean for energy

Design based on dolphin flukes
For a separate project, Fish’s lab is studying dolphin flukes as inspiration for generating better thrust for boats or submarines. The key is the flexibility of a dolphin’s fluke, or its tail fin.

Fish believes mimicking the fluke’s unusual mechanics could lead to more efficient and flexible propellers. “What we have with dolphin flukes is a wing,” he said. “Instead of that wing simply staying static against a flow, it moves up and down. It just doesn’t wag like the tail of a dog — there’s a joint at the base of the fluke, and it varies the angle of attack.”

If the joint is key to maintaining the best angle of attack and the thrust though the dolphin’s entire stroke cycle, modifications at the tip of the fluke influence the water flow so the animal receives as much lift as possible. Fluke tips, in fact, seem to act like the winglets that extend along the tail end of airplane wings during takeoffs and landings. Winglets modify airflow at the tips of wings to help prevent energy loss, which can sometimes be seen as a large contrail following in the wake of some jets.

The upshot of winglets? Effectively longer wings, and more lift for less energy.

The flexible fluke of a dolphin creates a similar increase in lift, and when applied to a variable pitch propeller, the design could increase efficiency to 90 percent (compared to the 70 percent typically associated with standard rotational propellers). Plus, Fish said, the propeller could maintain that efficiency over a much larger range of speeds.

Sizing up an energetic ‘snake’
British researchers have borrowed the moniker of another aquatic creature for their Anaconda Wave Energy Converter, envisioned as a rubber tube running the length of two football fields and undulating with passing waves while tethered to the ocean floor.

The Anaconda, still fairly early in its development, is meant to generate electrical power through the action of successive bulge waves traveling along its length. Each bulge’s movement through the nearly 8-yard diameter tube, at least as depicted in models, is more than a little reminiscent of how you might imagine an unlucky victim makes its way through the digestive tract of a real anaconda, only faster.

Both ends of the water-filled tube are sealed off, with one end facing oncoming waves. When a wave hits and begins squeezing the tube, a bulge wave forms and becomes progressively bigger as it moves forward by the continued squeezing of the sea wave racing along the tube’s outside. At the other end, the bulge wave’s flow turns a turbine and the resulting power zips ashore and feeds the power grid through a cable. If testing continues to pan out, the United Kingdom’s coast could see its first full-scale Anaconda invasion in about five years, with offshore farms of 20 or so tubes capable of powering up to 40,000 homes.

John Chaplin, a professor of applied fluid mechanics at the University of Southampton in the United Kingdom, said Anaconda inventors Francis Farley and Rod Rainey first approached him with their idea a few years ago. Chaplin, who studies the fundamental mechanics of wave forces on structures such as offshore oil and gas installations, was a natural choice and began testing a small-scale version of the tube in 2006.

“There have been ideas around for about 100 years for using waves to generate electricity,” he said. “But in comparison with wind and tidal streams, it has obviously developed much more slowly.”

Engineering challenges
One of the biggest engineering challenges facing the field, he said, is designing something that can survive extremely hostile conditions in the open ocean.

“The variation between the everyday waves that you see and the extreme conditions that exist during a storm that’s raging is very much greater there than the range that wind turbines experience or a tidal stream turbine experiences.”

Of the Anaconda, he said, “we intuitively expect these things to be able to withstand waves much better than steel and concrete and things with hard joints.” The system’s commercial developer, Checkmate SeaEnergy, is likewise betting on it.

But does it work?

In his initial lab tests, Chaplin said, “I could see the bulges traveling pretty much in the way they had envisioned.”

After another two years of models and successful testing of tubes one-twentyfifth the envisioned size, a new grant from the UK’s Engineering and Physical Sciences Research Council will allow Chaplin and Southampton colleague Grant Hearn to direct tests on a model about one-tenth the eventual size —  or about the range of the real anacondas slithering around in South American swamps.

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