The world's first commercial tidal-power system has been connected to the National Grid in Northern Ireland. Built by the British tidal-energy company Marine Current Technologies (MCT), the 1.2-megawatt system consists of two submerged turbines that are harvesting energy from Strangford Lough's tidal currents. The company expects that once the system, called SeaGen, is fully operational, it will be able to provide electricity to approximately one thousand homes.

The system is currently being tested and has briefly generated 150 kilowatts of power into the grid. But it has also damaged one of its rotors due to a failure in the control system when the rotor began turning too fast. Although the problem was a minor setback, the unit is not expected to start running continuously and at full capacity until November, says Peter Fraenkel, the technical director at MCT.

The technology works like a wind turbine, but instead of wind, the turbines are driven by the flow of tidal currents. It offers a significant advantage over wind because currents are predictable, says James Taylor, the general manager of environmental planning and monitoring at Nova Scotia Power, a company that also has plans for a one-megawatt tidal-power project. "Wind is intermittent and, because of that, is much more difficult and expensive to integrate in a power system," he says.

Generating power from currents in the form of "watermills" was first demonstrated by MCT in 1994 with a 15-kilowatt system in Loch Linnhe, off the west coast of Scotland. In 2003, MCT installed a 300-kilowatt system off the coast of Lynmouth, England. At the same time, a Norway-based energy company, Hammerfest Strom, installed a like-sized system in the Kvalsund strait. In the spring of 2007, Verdant Power submerged six 35-kilowatt turbines in New York City's East River. SeaGen, however, is much larger than any of these systems and is not an experimental device, says Fraenkel.

SeaGen uses two rotors that are 16 meters in diameter and can each produce 600 kilowatts of power. Fraenkel says that using two rotors is a "cost-effective solution" because the depth of the seas limits the size of the rotors. "We have to grow sideways," he says.

The researchers also have complete control over the rotors. "They are pitched like the propeller on an old aircraft, so by changing the angle--which dictates how much force is produced--of the blades, it allows us to optimize the rotor," says Fraenkel. The researchers can start and stop the rotor, and make it go faster or slower. And to prevent any damage to the ecosystem, it is important that the researchers keep the rotors at about 14 revolutions per minute, a speed that is too slow for marine life to run into the blades or to alter tides. The rotors are connected to a crossbar on a large steel beam that is held in place by four legs cemented into the seabed. The crossbar can be raised above or lowered below the surface of the water for easy assess to the turbines.