August 15, 2007 4:00 AM PDT
FAQ: Energy on the high seas
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The skate ramp: With the Wave Dragon, wave reflectors more than 100 meters long guide waves up a ramp, where the water dumps into a reservoir. The added pressure forces a turbine at the bottom to turn. The device's developer, a company that is also called Wave Dragon, is building a 7MW prototype off of Wales in 2008 and wants to do a 77MW project in the Celtic Sea by 2010. The Wave Dragon is slack moored, so that it can flow with the power of the ocean. A 20-kilowatt prototype running off Denmark's shore since 2003 has helped iron out the technical kinks, wrote Wave Dragon founder Erik Friis-Madsen in an e-mail. "The device can be up scaled to whatever size is needed, and the efficiency of the device grows in line with the size," he wrote. "The biggest difficulty seems to be to secure the funding for the first years of commercial development."Tidal power, wave power: What's the difference?
Tidal advocates want to put turbines, similar to wind tunnels, where tides snake in the ocean. The tides would turn a turbine and generate power. Marine Current Turbines has had a prototype for four years in southern England generating 300 kilowatts of power. It will start installing a 1.2MW system off Northern Ireland this fall.
Why are none of these past the prototype stage?
Cost and unknowns. The 1.2MW system from Marine Current, for instance, will run 7.5 million pounds, or nearly $15 million. It has been delayed several times because of corporate management changes, a lack of funds, and other projects getting priority on construction equipment. "It is like wind was in 1980," said Marine Current Turbines' Fraenkel. At the moment, investors aren't gushing. Few venture capital firms have paid much attention to sea power. In April, Ocean Power Technologies, which makes a buoy system, held a U.S. initial public offering, selling its shares for $20. At the end of trading on Tuesday, it was down to $12.39.
What are some of the environmental risks?
Animals, plants and birds are the main concerns. Ocean-power advocates, however, point to the relatively minimal impact wind turbines have had on birds over the past 20 years. Other dangers include debris and escaping oils, but these can be minimized.
Overbuilding, conceivably, could also present problems by attenuating the force of tides or waves. A large, dense wave facility could reduce wave power by 10 percent to 15 percent in its vicinity, NREL projected, although the impact will be minimal a few kilometers away. The impact on the biological community is unknown. Fraenkel argues that high capital costs will help minimize that problem.
Can you leverage the sea in other ways?
European utilities have already erected offshore wind farms in the United Kingdom and Denmark. These farms--which sport turbines with blades that can measure more than 100 meters long--sit in about 30 meters of water. Offshore winds can be steadier, thus generating more power. Putting the turbines offshore also eliminates some of the "not in my backyard" problems.
RePower Systems of Germany has created a 5MW offshore turbine. Right now, it is creating a 300MW field in water 82 feet deep and 18 miles off the coast of Belgium. Combined, the six turbines in Belgium will be only 54 megawatts smaller than the largest solar thermal plant in the world, located in California.
A significant portion of the U.S. offshore wind potential is located in deep-water areas, but that would require building more robust turbines that can withstand harsher winds, waves and tides. Instead of being anchored directly into bedrock, deep-water turbines might have to be anchored to floating platforms, which in turn are anchored into earth. Corrosion and maintenance are issues, as are environmental concerns. The turbines also need to be connected to the grid via electrical transmission cables. Luckily, a large portion of the U.S. population lives near the sea. The same would go for solar power farms at sea, which could convert sunlight or heat into electricity. (Such facilities do not exist, however.)
Hydrogen on the high seas?
Put this in the "Great idea, but I might be on Social Security then" category. Under this scenario, offshore platforms would harvest power from deep-sea waves and then exploit that power to split water molecules into hydrogen and oxygen. The hydrogen could then be pumped into a carrier and then shipped to shore. (Transporting hydrogen through pipelines or liquefying it presents whopping technical and economic problems).
But right now, demand for hydrogen is low and converting wave power to hydrogen and then to electricity is not cost-effective. Some have proposed delivering power generated at something like the deep sea platform described above over a submarine cable. This would eliminate the conversion inefficiencies, but then there's the question of how far offshore you can go.
What is ocean thermal technology?
Another far-off experimental technology. Electricity can be generated from differentials in temperature. Some companies are already trying to employ temperature differentials to power small sensors. Because the surface of the ocean is warmer than water deeper down, it could be possible to harvest the difference for electricity, but the colder water would have to be made to come in contact with the warmer water.
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