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NREL's Wind Technology Patents Boost Efficiency and Lower Costs

March 22, 2013

Wind energy research conducted at the National Wind Technology Center (NWTC) at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) during the last decade has earned the lab two patents, one for adaptive pitch control and one for a resonance blade test system that will ultimately help its industry partners increase the efficiency of wind technologies and reduce the cost of wind energy. The most recent patent for adaptive pitch control for variable-speed wind turbines was granted in May 2012.

Variable-speed wind turbines use rotor blade pitch control to regulate rotor speed at the high wind speed limit. Although manufacturers and operators have been interested in developing a nominal pitch to improve energy capture in the lower wind speed regions, the methods of selecting the nominal pitch have not been entirely effective because the models used to predict wind turbine behavior are inaccurate. Wind turbine models are based on predictive design parameters that differ from operating parameters because operating parameters change with variances in components and operating conditions. These differences between design and operating parameters in addition to model inaccuracy often result in nominal pitch settings that are either too high or too low for optimal energy capture.

NREL's adaptive pitch control system, which includes an adaptive torque gain control, works while the wind turbine is operating. The system modifies the pitch of one or more blades in continuous increments based on power measurements taken over a period of time until optimal pitch and power production are achieved. This adaptive control system enables wind turbines to produce more power at lower wind speeds. Energy production increases of 0.5% to 5% or more are anticipated for a typical variable-speed wind turbine.

Photo showing part of a turbine blade locked into NREL's resonance blade test system.

NREL's resonance blade test system at the NWTC performs a blade flap fatigue test on a 50-m wind turbine blade.
Photo by David Snowberg, NREL 20449.

The second patent, granted to NREL in May 2011, is for a resonance blade test system that significantly reduces the cost and time required for testing the wind turbine blades used on today's multimegawatt machines. Multimegawatt wind turbines use larger, longer blades that are subjected to greater static and dynamic loads. To avoid the risks of failure in the field, and hence, costly replacements, manufacturers must have prototypes of their designs tested before they are commercially deployed. But as wind turbine blades become longer and stronger, they become more difficult to test for endurance. Traditional test methods developed for smaller blades are not suitable for larger blades because they cannot be cost effectively scaled up to accommodate the much larger loads and resulting deflections experienced by the blades under dynamic test conditions.

NREL's hydraulic resonance blade test system can be scaled up to test blades 90 meters or longer in length. It uses a series of modules that consist of a servo-controlled hydraulic actuator and oscillating mass. The displacement of the mass is precisely controlled to oscillate up and down, which excites the blade at the system's natural flap or edge frequency. The number of modules, amount of oscillating mass, and placement on the blade depends on the size of the blade and is tailored to match the bending moment target levels. NREL's system uses one-third the energy that some of the conventional methods use and oscillates a blade at more than twice the rate. Instead of taking as long as 4 months to apply 3 million cycles to fatigue test a blade, researchers can now do it in less than 2 months. Test durations of 1 to 5 million cycles, at accelerated load amplitudes, are typically used to represent the 20-year lifespan that blades are expected to perform in the field.

NREL's resonance blade test system is used to test blades for manufacturers in the United States at the NWTC near Boulder, Colorado, and at the Wind Technology Testing Center in Boston, Massachusetts. In addition, whereas many patents are not implemented beyond their point of origin, NREL's patented blade test technology has been adopted by wind turbine manufacturers in the United Kingdom, Spain, and China. NREL has worked with industrial test system supplier, MTS Systems, to provide a commercial version of the test system.