Harnessing tidal energy takes new turn

With the idea of underwater windmills in mind, Peter 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. 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. 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 tide's 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.