Marine Current Turbines Ltd., England, develops turbines that work much like submerged windmills. They can sit in the sea where tidal currents are strong. The submerged turbines can each put out 750 to 1,500 kW depending on the local flow pattern and peak velocity. A company goal is to deploy them in arrays or farms.
MCT turbines consist of twin axial-flow rotors of 15 to 20-m diameter, each driving a generator. The twin power units mount on winglike extensions either side of a 3-m tubular tower set into the seabed. Turbines and power units can be hoisted up above sea level for maintenance. This eliminates underwater work by divers or remotely operated vehicles.
The design's environmental impact is considered negligible. The rotors turn slowly, 10 to 20 rpm, and stay in one place, so marine creatures can easily avoid them.
A 10-MW tidal farm under consideration will supply energy to about 5,500 homes. Commercially available units are scheduled for next year.
In another development, inventor Douglas Selsam has introduced a two-rotor, 7-ft-diameter wind generator that produces up to 1 kW. Selsam says the second rotor on his Superturbine ST 1.2 will exceed the output of other 7-ft-diameter turbines, even when his sits on a shorter tower, a distinct advantage in locales with height restrictions. "It's good practice to get a turbine as high as possible, but a tall tower may not be an option," says Selsam. "The extra rotor is more effective than doubling tower height in most locations and uses less material."
The design puts both rotors on the same shaft and then tilts it a bit so they both catch fresh wind. The result is power output equaling that of a 14-ft-diameter turbine, says Selsam. Unidirectional carbon-fiber blades operate quietly and efficiently at all speeds, he adds. Electrical power comes from a three-phase alternator using rare-earth supermagnets to produce 100 A at 12, 24, or 48 V. Side furling and a shock absorber lets the blades turn out of high wind for overspeed protection over 30 mph.