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01/07/2010

Great Britain hopes to soon to take the global lead in tapping the power of tides to produce clean energy. A single power plant is expected to cover 5 percent of the country's electricity needs by providing 8.6 gigawatts of CO2-free electricity, the equivalent of about eight nuclear power plants -- but without the pesky waste.

The tidal range at the mouth of the Severn, Britain's largest river, is 15 meters. The country now wants to put this enormous force to use with its Severn Barrage project. The barrage will hold back water at high tide like a dam, and 216 giant turbines -- each about 9 meters in diameter -- will be powered up as the water ebbs.

Critics of the project fear it will massively disrupt the river's ecosystem and note that its financing remains an open question. Nevertheless, the moon energy that drives the ebb and flow of tides is inexhaustible -- and that's why so many are viewing the project with great interest. Despite some protest, experts around the world are working to tap the eternal power.

Tidal power could also be put to good use in Germany, argues Kai-Uwe Graw, a professor for hydraulic engineering at the University of Leipzig. Graw says sufficient flow velocities -- around three meters per second is optimal -- can be found on parts of the island of Sylt near the German-Danish borner.

Graw began raising awareness about tidal power's potential back in the 1990s. In his opinion, tidal power plants can easily be compatible with coastal conservation efforts. One benefit is that the facilities extract some of the energy from tides, which could help to reduce the kind of beach and coastal erosion that has been a major problem for Germany on North Sea islands like Sylt in recent years.

Is Tide Power Ready for Prime Time?

Until recently, though, the technology wasn't ready for prime time. One major obstacle was the fact that the turbines, located underwater, are put under extreme pressure as the water forces tug at the rotor blades -- a pressure that can be variable on different parts of the equipment and thus required a highly resistant design. Rotors must also be able to face the direction of the changing tidal flow in order to optimize energy production. In the past, there was also no simple and inexpensive solution for providing maintenance or wiring the plants up to the electrical grid.

But Graw argues that the technology is now more mature and that its time has come. The fact that the technology now works, he says, is proven by the SeaGen tidal power plant along Strangford Lough on the coast of Northern Ireland that has been in operation since summer 2008. There, the equipment works like an underwater windmill. The rotor blades are propelled by the force of the tides, which have a velocity of 2.4 meters per second. Two rotors with a diameter of 16 meters each are capable of generating up to 1.2 megawatts at that velocity -- enough to power about 1,000 homes. Currently, SeaGen is the most powerful tidal power plant in the world. By comparison, a large wind turbine is capable of producing 5 megawatts of electricity.

Alternative Technology

In Strangford, the blades are attached to a crossbar and a towering beam that enables them to be lifted out of the water for easy maintenance work. The beam is 40.7 meters tall, and more than half, about 24 meters, is underwater. It's a design that won't work for the German coast. But the firm Voith Hydro in the German city of Heidenheim wants to present a prototype of a tidal power plant at the end of the year -- together with German utility giant RWE's Innogy subsidiary -- that does not rely on a beam structure and would instead reside underwater, attached to the ocean floor. The blades on the prototype look more like the propeller of a motorboat than a windmill; and they are designed symmetrically so that they don't have to be repositioned during the ebb and flow of tides.

The prototype is part of a planned power station project off the South Korean coast. On long term, enough turbines are expected to be added there to generate a total of 600 megawatts of electricity -- as much as a small coal-fired power plant -- and provide electricity for around 400,000 Korean households. Pilot projects are also planned for Europe.

Is Tidal Power Compatible with the Environment?

The primary sticking point with tidal power right now is environmental protection. Experts are divided over the environmental impact of tidal power stations. But today's generation of ocean current turbines are more environmentally friendly than the tidal power plants they succeeded.

When the large tidal power plant in Saint-Malo, France, at the mouth of the Rance River went online in 1966, it cause considerable disruption to plant and animal life. The old barrage system works like a dam at the mouth of the river when operating, bringing once moving water to a standstill and also altering living conditions for flora and fauna.

With the latest generation of tidal-power systems, however, fish and other species can easily avoid the units by swimming around them. In addition, they don't impact the coastline because the turbines are attached to the ground.

In terms of cost, though, tidal power doesn't present much competition. The use of tidal power is still very expensive -- even compared to other renewable energies. According to British government calculations, electricity derived from the Severn Barrage project will cost around £317 (€352/$503) per megawatt hour. By comparison, offshore windpower costs about £85 pounds per megawatt hour.

Tidal power wouldn't come inexpensively in Germany, either. But it would still make sense for the country to pursue the technology, argues expert Graw. After all, German companies are already among the global leaders in water power. Building a prototype at home, he says, is a must.