Harnessing Tidal Energy Takes New Turn

Could the application of the windmill principle produce a sea change?

3 min read

For well nigh two decades, Peter Fraenkel had worked on renewable energy technologies, seeking new ways of producing clean, reliable power. But it was only in the late 1990s that he hit upon the idea of what he now calls ”underwater windmills,” he told IEEE Spectrum.

This windmill-like tidal turbine is off the coast of Devon, in southwest England. The rotor and box, normally 20-30 meters deep, can be raised for servicing, as shown.

With the idea of underwater windmills in mind, Fraenkel, a mechanical engineer, assembled investors in 1999 to launch the Seaflow project and formed the company Marine Current Turbines Ltd. (MCT, Basingstoke, England). The mission of Seaflow was to build a commercial tidal energy plant based on the windmill concept. Backing came from the British government, the European Commission, and eight private British and German companies. Now, four years after the project began, MCT and its partners have built a working tidal mill in the English Channel off the coast of Devon, in southwest England.

The 130-metric-ton unit is cemented to the seabed about 1.1 km from the coast, rising a few meters above sea level [see photograph, right]. Tidal currents turn the 11-meter-long rotor, but as they reverse direction, the rotor's blades can be pitched to accept flow from the opposite direction.

Though the rotor turns slowly in water, which is 800 times as dense as air, at 17 rpm the speed is sufficient, with appropriate gearing, to harness the tides' immense energy and drive a turbine. Rotor speed varies, as with variable-speed wind turbines on land or sea, and a power-conditioning system--involving ac-dc-ac conversion--is used to obtain a current output at the grid frequency of 50 Hz. The result is an average of 100 kW and a peak of 300 kW of power, enough for 200 average British homes when hooked into the grid, according to MCT.

How to site and service?

For the tidal turbine to generate adequate power, finding the right location is crucial, Fraenkel emphasizes. Most important is that the rotor can be placed about 20 to 30 meters deep and that currents have a maximum speed of at least 2.25 to 2.5 meters per second.

There are perhaps 100 suitable locations in England and northern France. Especially in the English Channel, however, the seas are notoriously stormy, making the repair and servicing of a tidal turbine challenging. ”To get onto the rig, we use a rubber inflatable boat, which can be a white-knuckle ride,” says Fraenkel.

A key innovative aspect of the Seaflow design is that its gearbox and turbine can be raised above sea level for servicing. Ultimately, the goal is to achieve such a level of reliability that servicing will be required no more than once a year.

As much as possible, operation of the turbine is to be fully automated. Right now, signals can be sent and received via a radio telephone network to the on-board control PC, which monitors and controls the tidal mill and feeds back performance data.

How much tidal mill potential?

As Seaflow's commissioning approaches, MCT also has plans to install a twin-rotor system on a second rig, probably nearby. The rotors will be somewhat larger, boosting power output to as much as 500-700 kW per rotor. A third phase, scheduled for 2004-2005, would see the construction of a tidal farm with five units and a total generating capacity of 4-5 MW.

The first, and still by far the largest, tidal generating facility was built in northern France in 1966, at the mouth of the La Rance River, near St. Malo on the Brittany coast. The plant, with an impressive capacity of 240 MW, relies on the ”barrage” design. Basically, water is trapped in an artificial lagoon behind a dam at high tide; the water escapes as the tide recedes and drives a turbine.

Similar but much smaller barrage plants--a type increasingly seen as too costly--have been built in Nova Scotia, Canada (20 MW) and in Kislogubsk, near Murmansk, Russia, on the Barents Sea (40 kW).

Norway's Hammerfest tidal turbine, built in a town that bills itself as the world's northernmost, will resemble Seaflow, but it does not provide for raising of the gearbox. As a result, divers must be sent down into extremely cold and turbulent waters to make repairs. Securing the turbine structure to the sea floor was itself no mean feat.

That's why a new concept being developed at Robert Gordon University (Aberdeen, Scotland) is of some interest. Dubbed Snail, it attaches a rotor to a squat, heavy base. The unit is dropped into the sea, falls to the sea floor, and counts on water pressure to anchor it there. A unit based on this idea is due to be tested in the Orkney Islands in Scotland. It can take the turbine to any depth, says Alan Owen, a member of the university's faculty of design and technology.

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