In a patent filed in 2013, Dagher describes the tower as a tube fabricated from two thin layers of FRP composite material around a foam core (although the patent allows for use of other suitable material, including concrete or steel).
“When you have a floating turbine, the topside weight is very important, just like on a boat. You don’t want a steel mast on a sailboat, as it’s going to be very top-heavy,” Dagher explains. “For the same reason you don’t want to see a 50-foot-tall steel tower on a floating foundation to hold up to the turbine.”
Reducing the tower weight also reduces the hull size, because it no longer has to support the weight of a steel tower. “For this reason, composite towers are more likely to be financially attractive for floating versus fixed-bottom turbines,” Dagher predicts.
There’s another factor that makes composite towers appealing. Because of the cost of maintaining offshore turbines, they must be corrosion resistant. This both extends the life of the turbines and reduces the need for costly ongoing maintenance. It’s a demand that gives composites a decided advantage over steel.
The next VolturnUS iteration is expected to support a 350-foot-tall tower, about 20 feet in diameter at the bottom. “It’s not a very standard composite part, so how we manufacture this tower is key to driving the cost down,” Dagher says. As with many of the players in this fast-evolving market segment, Dagher is close-lipped on details about the manufacturing process.
Stronger, Stiffer Blades
Equally important to the floating platform and tower are the turbine blades, which, together with the hub that connects the blades to the tower, form the rotor. The 6 MW tower deployed in 2013 featured a 450-foot-diameter rotor design but, as is the trend, the next version is planned to be much larger – 525 feet in rotor diameter, or nearly the length of two football fields.
Achieving this massive size and the strength necessary to withstand the significant offshore wind loads demands the use of composites. In fact, Dagher notes that the turbine blades represent the largest composite use on the Maine Aqua Ventus project.
“You’re concerned about the bending strength of the blade under the wind loads, but you’re also concerned about stiffness because if the blade flexes too much it will touch the tower,” Dagher points out. “So, you want a stiff design and a strong design. That puts a lot more demand on the materials you use.”