Down the Line: Underwater Power Plant
The world’s first tidal stream power plant is producing electricity in Kvalsundet, Norway. The unique underwater facility will produce 32 GWh per year.
Power is generated by a series of windmill-like devices located 50 meters under the sea at the bottom of a remote strait. Tidal currents moving at 2.5 meters per second turn three 10-meter fiberglass blades attached to a row of 20 water mills. The blades rotate a hub that is connected to a gearbox and a shaft that drive a generator contained within a nacelle. The generator converts the rotational shaft energy into electricity, which is transmitted to shore via a cable on the seabed.
“Marine current turbines are, in principle, much like submerged windmills,” says Martin Wright, managing director of Marine Current Turbines Ltd. (Chineham, Hampshire, UK). They are installed in the sea at places with high tidal current velocities, to take out energy from the huge volumes of flowing water.
“These flows have the major advantage of being an energy resource as predictable as the tides that cause them, unlike wind or wave energy which respond to the more random quirks of the weather system,” claims Wright. His company is currently developing twin axial flow rotors that will measure 15 to 20 meters in diameter, each driving a generator. The power units of each system will be mounted on wing-like extensions on both sides of a 3-meter diameter tubular steel monopile. Each piling will be set into a hole drilled into the seabed from a jack-up barge.
“The submerged turbines, which will generally be rated at from 500 to 1000 kilowatts each, depending on the local flow pattern and peak velocity, will be grouped in arrays or ‘farms’ under the sea, at places with high currents, in much the same way that wind turbines in a wind farm are set out in rows to catch the wind,” explains Wright. “The main difference is that marine current turbines of a given power rating are smaller, because water is 800 times denser than air and they can be packed closer together, because tidal streams are normally bidirectional whereas wind tends to be multi-directional.” Wright claims the technology has a “low profile” and involves negligible environmental impact.
“The technology does not offer any serious threat to fish or marine mammals,” adds Wright. “The rotors turn slowly—10 to 20 rpm. A ship propeller, by comparison, typically runs 10 times as fast.
“Another advantage of this technology is that it is modular,” Wright points out. “Small batches of machines can be installed with only a small period between investment in the technology and the time when revenue starts to flow. This is in contrast to large hydroelectric schemes, nuclear power stations or other projects involving major civil engineering, where the lead time between investment and gaining a return can be many years.”