Wind turbines attached to floating buoys can harness stronger, more sustained winds in the open ocean. But the floats now used for such deep-water installations may prove prohibitively expensive because the buoys needed to keep them above water are enormous. Now a project in France is turning the turbine design on its head for what developers hope will be a low-cost alternative.
French oil and gas engineering company Technip and wind-power startup Nenuphar recently announced Vertiwind, a two-megawatt wind turbine that they plan to float in Mediterranean waters by the end of 2013. The project employs a turbine with a main rotor shaft that is set vertically, like a spinning top, rather than horizontally, as in a conventional wind turbine.
The benefit of the vertical-axis design is that it lowers the turbine's center of gravity. Vertiwind's design stands 100 meters tall, but places the generator, which weighs 50 tons, inside a sealed tube beneath the turbine's rotating blades, 20 meters above the sea. This makes the turbine less top-heavy, allowing for a significantly smaller flotation system, which would extend only nine meters below the surface of the ocean.
In contrast, a horizontal-axis turbine with the same power output and blades also reaching 100 meters high would need its generator to be 60 meters above the sea. A buoy built by Technip for a 2.3 megawatt horizontal-axis floating turbine prototype, owned by the Norwegian energy company Statoil, extends 100 meters below the surface.
"You save a lot of material" with a vertical axis, says Stephane His, vice president of biofuels and renewable energy at Technip. "But more than that, you ease the process of installing the machine itself."
Technip and Nenuphar plan to build two vertical-axis turbines with a power output of two megawatts each, one onshore and one offshore, at a cost of $28 million. The figure is still significantly more than shallow-water turbines fixed to the seafloor (which cost around $5 million per megawatt) but much less than the approximately $70 million spent on construction of the prototype owned by Statoil for construction, deployment, and ongoing research.
By pursuing a vertical-axis design, Vertiwind is using technology that was all but abandoned for onshore wind power more than a decade ago. Vertical-axis designs, which are inherently low to the ground, usually cannot compete with taller horizontal wind turbines that catch stronger winds at higher altitudes.
This should be less of a disadvantage offshore, since wind speed increases less with height over open water than it does over land, says Walter Musial, who leads offshore wind energy research activities for the U.S. Department of Energy's National Renewable Energy Laboratory in Golden, Colorado.
However, Musial has doubts about the design that Vertiwind is pursuing. Few large-scale vertical-axis turbines have been built, and all have had a curved-blade design that connects to the turbine's main shaft at the top and bottom of the blade, thereby evenly distributing the load placed on the structure, Musial says. Vertiwind, however, will use a straight-blade design that is only connected to the central shaft by two supports or struts near the bottom of the blade.
"That blade is going to bend as it is rotating due to centrifugal force, and the connections of those struts are carrying all the load," Musial says. "Those joints are getting hammered—that is the most challenging engineering aspect of this design."
Nenuphar CEO Charles Smadja says he is confident that the design will handle the strain, based on tests of a 35-kilowatt prototype that spins at higher speeds. Smadja concedes, however, that ramping up to a two-megawatt turbine will present new challenges. "What you can do properly at a small scale can be difficult to do at a very big scale," he says.
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