Located off the coast of Northern Ireland, the world's first commercial tidal current power plant is producing electricity for 1,500 households using energy generated by high and low tides.
Tidal flows represent a largely untapped source of clean energy. But with an energy density 800 times that of wind, water offers a highly-efficient and reliable source of power.
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The wind blows softly over the rich green hills that dot the countryside around the small coastal town of Strangford in County Down, Northern Ireland. Just a few steps away is the natural port of Strangford Lough — a deep-blue harbor that today fully lives up to its Celtic name Cuan, which means “the calm bay.” Nevertheless, large dark waves sometimes rip through the harbor. It’s therefore no coincidence that Strangford was called “the powerful fjord” by the Vikings who once settled there. The bay is 30 kilometers long and its total area of 150 square kilometers makes it the largest in the Irish Sea. It not only contains picturesque fishing boots but also a black and red steel tower that protrudes out of the water just off the coastline. This tower is part of SeaGen — the world’s first commercial tidal current power plant. The facility, which began operating in 2008, produces 1.2 megawatts (MW) of electricity solely from the power of the tides. That’s enough to supply a town of 1,500 households.
Tidal flows represent a largely untapped source of clean energy. This underutilization is due to the fact that the technology has remained in the development phase up until now, and installing it offshore is very expensive. Nevertheless, its potential is huge. Tidal current power plants can be built anywhere where the ebb and flood of the tides generate strong currents. The list of places offering ideal conditions includes Scotland, France, Canada, and East Asia.
Strangford Lough’s natural harbor is an attractive location for various reasons. First and foremost, it is relatively shallow. This has made it possible to anchor the power plant at a depth of around 30 meters, explains Kai Oliver Kölmel, who is responsible for Ocean Power at the Siemens Renewables Division. “Shallow water makes it easier to anchor a facility into the seabed,” he says. “In addition, the ebb and flow of the tides is stronger in shallow waters. For instance, the flow rate in the so-called Strangford Narrow gets as high as four meters of water per second; SeaGen needs a flow of at least one meter per second to generate electricity.”
Underwater Electricity Factory. The Strangford Lough plant is operated by Marine Current Turbines, a British company in which Siemens acquired a ten percent interest in 2010. The facility is similar to a wind turbine, the only difference being that it is driven by water instead of air. Each of its two drive trains weighs 27 tons and is equipped with a rotor measuring 16 meters in diameter.
The rotor blades can be turned through 180 degrees, which means they can produce electricity for up to 20 hours a day regardless of whether the tide is coming in or going out. The tower to which the two propeller turbines are attached via a cross-member has a diameter of three meters. Depending on the tide, the tower can protrude as much as 20 meters above the sea. The rotors can’t be seen above the water— and it’s even possible to take a small boat directly past the turbine because the rotors are located at least three meters below the surface. “Maintenance is easy,” says Kölmel, “because the facility can be easily accessed and the cross-members to which the turbines are attached can be raised out of the water using a hydraulic lifting system.”
Although extensive installation costs make an investment in tidal current power plants around twice as high as that for offshore wind power facilities, the resulting electricity offers several benefits. For example, the energy density of water is 800 times higher than that of wind, which makes generating electricity with water much more efficient. A 1.2 MW tidal plant like the one at Strangford Lough can produce as much electricity in a year as a 2.5 MW offshore wind turbine. The electricity yield from tidal facilities is also more precisely calculable, which enhances planning security. After all, tidal currents are determined by the moon and the Earth’s gravitation, so they’re not dependent on the weather and can be predicted years in advance.
The International Energy Agency estimates the global output potential of tidal power plants to be as high as 800 terawatt-hours per year — enough to supply 250 million households with electricity. Marine Current Turbines continues to invest in tidal technologies. Among other things, the company plans to start building a tidal turbine park near the Isle of Skye in northeastern Scotland in 2013. When it’s completed, the facility will supply up to 4,000 households with electricity from the sea.