Tidal turbines: wave of the future?

Tidal power plants are an attractive energy option because they don't shut down at night (like solar plants) and aren't dependent on annual rainfall (like freshwater hydropower plants). However, large tidal installations like the massive 240-megawatt tidal dam across the mouth of La Rance estuary in France can modify currents, silt flows, and shoreline habitats. A new generation of technology may offer a less environmentally intrusive way to harness limitless, renewable power from the sea.

In September 2003, a subsea turbine manufactured by research and development firm Hammerfest Strom was anchored in Norway's Kvalsund Channel and began generating electricity for the small nearby city of Hammerfest.

The submerged turbine looks like a cross between an airplane propeller and a win&hill, with three 10-meter blades that turn with the tides to produce up to 300 kilowatts (kW) of electricity, enough to light and heat about 35 Norwegian homes. If the pilot project is successful, Hammerfest Strom plans to install up to 20 additional turbines in the area that would produce a minimum of 700 kW each.

The Hammerfest Strom turbine rests on 20-meter tripods anchored to the sea floor. "If one could extrapolate from the experience of offshore wind power, for which we have a bit more experience, the installation period is disruptive but nor damaging," says Rick Sellers, head of the Renewable Energy Unit of the International Energy Agency in Paris. Fish concentrations may even increase around such installations because they serve as artificial reefs, providing habitat for algae and other organisms that fish eat.

According to project leader Bjorn Bekken, the blades of the Hammerfest Strom turbine do not change the waterflow, so they don't impede silt or nutrient flows. He adds that the blades turn so slowly (about 7 revolutions per minute) that marine mammals and fish should be able to avoid them, and the blade's leading edges are rounded to minimize injury if a collision does occur.

Not all tidal power plants have large exposed blades or are rigidly attached to the sea floor. AquaEnergy Group of Mercer Island, Washington, develops offshore power plants using floating buoys that convert the bobbing of the waves into a high-pressure flow that powers an enclosed generator. The 85-foot buoys, which have been tested in the North Sea, produce about 80-250 kW each. "They're no different than navigational buoys in their impact on the environment," says AquaEnergy CEO and president Alia Weinstein.

In Norway, current environmental policy does not allow the construction of any more major hydropower plants, the country's only current source of electricity. Norwegians will thus have to look elsewhere to increase or replace generating capacity in the future. "I think it's very important to start looking at alternative sources of energy like tidal power," says Bekken. "Once we pass peak production of fossil fuels, the difference between supply and demand decreases day by day, and energy prices will go up."