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Schools of fish. Barnacles. Seaweed. Dolphins. Whales. The highly corrosive nature of salt water. Fishing nets and lines. Submarines. Ships and boats. All things, and more, for designers and engineers of ocean energy devices to worry themselves with. Still, with the oceans covering two-thirds of the planet, capturing their energy for clean power is a challenge worth pursuing.

Ocean currents, such as the Atlantic Ocean’s Gulf Stream, could feasibly be harnessed for significant energy. Ocean waves have been tapped already in a few locations, even though winds to build powerful waves can be fickle. The ocean’s thermal qualities – warm at the surface, cold at great depths – are another energy delivering possibility. Capturing energy from based on the difference in density between really salty water and not so salty is also being tried, with some success. However, the Earth’s Moon-driven tides and currents created by the twice daily event, seem the best bet for now as a guaranteed 24/7 source of energy.

Of the tidal technologies being developed, in-stream devices, which use the kinetic energy of flowing water to generate electricity, seem preferred over other possibilities such as barrages or tidal lagoons. Both of those use the difference in height between high and low tides behind hydropower dam-like structures to generate electricity. Cost and limited site locations, as well as environmental concerns, have dampened interest in these tidal energy schemes.

Aside from clock-precision reliability, tidal currents have another nice feature: Some of the best tidal resources are near population centers where power is needed. For reasons of commerce, travel, recreation and resources, over the centuries people have tended to settle near the world’s coastlines with hundreds of large population centers now on bays, inlets and estuaries; the mouths of rivers that meet the oceans.

The best of the best tidal energy resource is Canada’s Bay of Fundy where the tide rises and falls as much as 55 feet. That’s a lot of water moving at good clip.

There, it is estimated that up to 2500 megawatts of power could be captured, more than province of Nova Scotia, which borders one side of the Bay, can use. To attract tidal energy development, as well as meet a renewable energy goal, the Nova Scotia government has signed into law two feed-in tariffs (FIT) for tidal generated electricity: One fit is for large scale tidal generation projects, the other for small. The FITs will give tidal energy developers guaranteed rates for electricity sold that is higher than the market rate for conventional electricity. The FIT incentives could also help the overall economy as tidal operators bring new industry to the province.

Nova Scotia would like to be a world leader in tidal energy. "The Minas Basin is like the Mount Everest of tidal technology," says Sandra Farwell, director of Sustainable and Renewable Energy with the Department of Energy. "If you can deploy a device here, you can deploy one anywhere in the world."

In 2007, the province established North America's only tidal testing center, the Fundy Ocean Research Centre for Energy (FORCE), in the Minas Passage, in the Northeast corner of the Bay. It also set up two independent, nonprofit associations, Offshore Energy Environmental Research (OEER) and Offshore Energy Technical Research (OETR), to research the environmental and technological aspects of tidal deployment.

"What the province has done to create the two FITs is the first in the world," says Dana Morin, president of Fundy Tidal. "This is the only place in the world to make money on tidal power. We are now internationally renowned because of FORCE. With the addition of the feed-in tariffs, we have really set the bar internationally for tidal power development."

As an affirmation of potential of tidal power, Siemens, of Germany, has acquired the majority stake in Marine Current Turbines (MCT), of the UK, which develops and builds in-stream tidal power systems it calls SeaGen. MCT already has a commercial scale 1.2 megawatt SeaGen tidal current generator planted in the seabed off the coast of Northern Ireland.

The MCT’s SeaGen technology is much like a wind turbine, but under water. With the energy density of water more than 800 times greater than wind, turbine blade lengths are much shorter than wind turbines for the same rated capacity. The SeaGen device has twin rotors that can be adjusted to optimized the power output from tidal flows. One version of the SeaGen is attached to a pillar-like structure with its foundation built into the seabed. With this version the rotors and generator can be raised above the surface for cleaning and maintenance. Another variation, for deeper waters, keeps the rotors and genset well below the waves.

With its acquisition of MCT, Siemens now has a presence in Nova Scotia through FORCE. There, with partner Minas Basin Pulp and Power, of Nova Scotia, a SeaGen device will be deployed at an unannounced time. Siemens is joined at FORCE with partnerships of ALSTOM/Clean Current (which are developing an in-stream, ducted turbine); Atlantis/ Lockheed Martin/ Irving Shipbuilding (with an in-stream, seabed-planted, propeller-like turbine); and OpenHydro/ NS Power (another in-stream seabed planted turbine with a twist: a centerless or hubless rotor. Power is generated through a ring attached to the blade tips that rotates within an outer ring.)

Siemens says the worldwide potential for power generated by tidal power plants is estimated at 800 terawatt-hours (TWh) annually. That’s approximately 25 percent more than the total power demand of Germany, for instance, and is equivalent to 3-4 percent of global power consumption. Coastal regions with strong tidal currents like those in the UK, Canada, France and East Asia offer major potential for the utilization of tidal current technology.

Article posted by: (21 February 2012)
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  • Bruce Mulliken, Green Energy News (February 20, 2012 – Vol.16 No.49)  
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