With the direction of the wind energy market moving toward bigger wind plants and bigger turbines, the role and sophistication of motion control is growing. With offshore installations expected to grow to five megawatts in the next few years, wind turbine OEMs are moving to individual blade control, advanced sensing and control techniques plus electromechanical systems to assure reliability, durability and precision.
"Five years ago, the average size of a turbine was somewhere between 750 kilowatts and one megawatt. In 2008, the average size was approaching 1.6 to 1.8 megawatts," says Dheeraj Choudhary, business unit manager for Global Renewable Energy at Parker Hannifin Corp . "Over the next five years there will be a lot of turbines installed in the 2, 2.50, 3 and 5 megawatt range."
Choudhary says that with wind turbines of this size, the diameter or span of the blades is more than 100m. Each blade is close to 50m and as long as 60 to 65m for the high end of the turbine size. No matter how lightweight the blades become, they need to have the structural integrity to last 20 years or more. And because each blade weighs anywhere from 10 to 20 tons, the engineering goal is effectively moving and controlling these large, awkward "wings" in high winds.
"These blades almost never rotate in a smooth circular fashion. While spinning, they move around in response to wind gusts, side loading and torsional loading," Choudhary says. "So the challenges that we see in pitch control systems are not just reliability, durability and precision, but also controlling these blades so that the overall life of the turbine is preserved."
Dan Foster, engineering manager for Moog Industrial Group , says that "as the systems continue to grow in diameter, the loads on the blades, rotors and hubs have more variation. Increased loads on the blades, the rotor shaft and whole turbine translate into component wear, premature breakdown, imbalances of loading and torque on the towers and even the tower structure itself."
Foster says that this problem statement is what the industry needs to address. One of the biggest trends moving forward is the potential benefits of individual blade control and how it might help this situation. Today, most turbines are using independent pitch control with each blade independently controlled by a servo actuator device. However, all of the blades respond to the same command as they go through their cycle.
"What individual pitch blade control provides is real-time feedback from blades or monitoring devices. An approach we are exploring is to embed sensors into the blades for real-time load feedback," says Foster. "The system closes the loop at the turbine level using that feedback to really significantly reduce the load variation from blade to blade."
The result is an ability to handle peak gusts better and more quickly. And also, as blades rotate 360 degrees, the system is able to provide dynamic control based on system parameters at any angle. At the blade's highest point, it has a significantly different load versus the wind flow as it's sweeping past