May 15, 2008

A new wave: Ocean power

By Sonal Patel

Tidal power

The most significant difference between wave and tidal energy is that waves occur only in water closest to the surface, whereas the entire water body moves from surface to seabed in a tide. In tides, moreover, energy is due to a net movement of water—unlike waves, where the water acts as a carrier for energy. Unlike wave energy, therefore, tidal energy is location-specific.

Although only a few regions in the world harbor ideal conditions for tidal power, a range of diverse devices have been designed and are ready for testing and deployment. The UK, notably, has surged to the forefront of the tidal power race, propelled by a recent Sustainable Development Commission (SDE) report estimate that, considering how geographically well-suited the islands are to the technology, tidal energy has the potential to generate about 10% of the UK’s power.

Barrage systems. The concept of building dams composed of gated sluices and low-head hydro turbines across channels to harness water-level differentials has been proven productive by long-standing installations like the Annapolis station in the Bay of Fundy and La Rance, in France. Costs and environmental issues aside, researchers at Utrecht University in the Netherlands have deemed the concept feasible and have proposed that a dam constructed across the 20-mile Bab al Manab strait of the Red Sea could generate as much energy as 50 GW (see "Dam the Red Sea?" in POWER, January 2008, Global Monitor).

And now, more than 80 years after the first feasibility study, the UK government has renewed its interest in constructing a 10-mile-long barrage across the Severn tidal estuary running between the English and Welsh coasts, prompted by an SDE finding that a location-specific hydroelectric barrier on the estuary could generate 8.6 GW—meeting 5% of Britain’s power needs. Britain has allocated £14 million ($28 million) for the feasibility study, which is expected to culminate in early 2010.

Meanwhile, to reap energy from the Bay of Fundy, Canadian company Blue Energy International is advancing technologies used in tidal dam power with a “tidal fence” concept—a horizontal array of stand-alone vertical-axis turbines. This configuration, which can capture energy from both directions of a tide, has so far seen six prototypes and is currently being assessed by the National Research Council of Canada.

Tidal stream turbines. Making a marked departure from the traditional barrage system, tidal stream turbines are massive stand-alone turbines that work much like wind turbines—but with a much higher energy density, because saltwater is 850 times denser than air.

7. First commercial-scale tidal energy turbine. SeaGen’s 52-foot-diameter twin rotors will operate for up to 18 to 20 hours per day off the coast of Northern Ireland, producing up to 1.2 MW. Courtesy: Marine Current Turbines Ltd.

In April 2008 the first commercial-scale tidal turbine from Bristol-based company Marine Current Turbines, a 122-foot long inverted windmill dubbed the SeaGen, was installed in Strangford Laugh, a shallow inlet in Northern Ireland where tides gush in at speeds of up to 9 mph (Figure 7). Energy produced by the 1.2-MW device will be purchased by ESB Independent Energy, a retail subsidiary of Ireland’s national electricity company—and one of the first utilities to provide tidal energy to its customers.

Deep-sea tidal farms. British tidal company Lunar Energy plans to construct two deep-sea tidal farms. One would consist of eight underwater turbines on the sea bottom in Pembrokeshire, South Wales; the other would be a colossal 300-turbine field in the Wando Hoenggan Water Way off the South Korean coast. The latter project, a collaboration with Korean Midland Power Co., is a $1 billion scheme that extracts power from fast-moving deep-sea tidal streams. Lunar Energy will utilize 60-foot-tall Rotech Tidal Turbines, each with a 2,500-ton frame containing a pump, generator, motor, and electronics (Figure 8). The project entails a full resource research and feasibility study, and if successful, it will be operational by 2015.

8. Harnessing tidal power. Each 60-foot tall Rotech Tidal Turbine consists of a 2,500-ton frame containing a pump, generator, motor, and electronics. Courtesy: Lunar Energy