Smarter wind turbines

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2009

New sensors direct rotor blade position changes to better accommodate unexpected wind conditions. 

KEY CONCEPTS
• Sensors and an active control system are being incorporated into wind turbines to maximize their performance and reliability.
• The sensors need to respond to two types of wind conditions. Quasi-static wind loading represents the average wind speed or background wind and dynamic wind loading includes unexpected wind events including turbulent gusts and wind shear.
• One of the main sources of wind turbine failure is the gearbox. The researchers hope to place sensors on the gearbox in the future to determine the impact of vibrational measurements on performance.

As the rising cost of petroleum has spurred research into viable alternatives, wind power has emerged as a serious option. The two leading suppliers of wind power in the world are the U.S. and Germany with a combined 50,000 megawatts (MW) of capacity. Growth has been particularly rapid in the U.S. with 8,358 MW of new capacity installed in 2008. This accounted for 42% of all new energy capacity installed in the U.S.

The U.S. Department of Energy estimates that wind power can cover 20% of the nation’s energy needs by 2030. The power of the wind is captured as it rotates a rotor blade that spins a shaft which turns a turbine to generate electricity.

A gearbox in the wind turbine connects a low-speed shaft turned by the rotor blade with a high-speed shaft that drives the turbine. The low-speed shaft is typically supported by two large bearings in most turbines. Lubricants in the gearbox and bearings play an essential role in ensuring that the wind turbine operates effectively.

Wind turbines are being built in hard-to-reach locations such as offshore, which makes maintenance very difficult. Lubricant reliability is essential to ensure that the turbines can operate effectively over a long time frame.

One of the main causes of wind turbine failure is gearbox failures due to lubrication problems. This is in part due to the extremely high torque placed on the gearbox. Wind turbines can have blades ranging in diameter from 22 meters to 90 meters that mainly rotate at slow speeds. The torque on the gearbox can reach up to 4 million newton-meters.

Minimization of wind turbine failures is needed to reduce downtime and maintenance and maximize power production. Any measures to improve the efficiency of the wind turbines would hopefully reduce the torque on the gearbox and improve operating life. Such an approach is in the process of being developed.

SENSORS
Researchers at Purdue University, in conjunction with Sandia National Laboratories, are working to incorporate sensors and an active control system into wind turbines to maximize their performance and reliability, particularly when encountering unexpected windy conditions. Jonathan White, doctoral candidate in the School of Mechanical Engineering at Purdue University, says, “We are working on a visionary project that has the objective of placing sensors on the rotor blade. The sensor’s function is to read changes in the orientation and deformation of the rotor blade caused by the wind loading and feed the information to a data-acquisition system in the turbine. In turn, the information is provided to a control system in a closed loop, which will instruct changes to be made in the rotor blade.”

The objective is to measure the magnitude and direction of the wind force exerted on the rotor blade. Then instructions are sent to bend and twist the rotor blade in the wind to maximize performance and minimize torque. These steps need to happen instantaneously in response to wind conditions, according to White. In addition, the change in the torque placed on the wind turbine needs to be minimized by reducing torque during high wind conditions and increasing torque during low wind periods to maximize power production.

Initial work is focused on determining the load placed on the rotor blade and monitor data accumulation. Concurrent work is being done to incorporate actuators such as flaps on the rotor blade. This means that the wind turbine could have a similar appearance to an airplane wing in the future. A rotor blade equipped with sensors and actuators is shown in Figure 2. Such a blade is a key component in a smart wind turbine.


Figure 2. A smart wind turbine has been developed which contains sensors used to detect changes in wind conditions and to transmit information that can be used to maximize performance and minimize torque. (Courtesy of Purdue University)

The key sensors utilized on the rotor blades are known as accelerometers. White explains, “They measure the magnitude of wind acceleration by utilizing the piezoelectric effect. A mass excited by the acceleration generates a force that impinges on a piezoelectric material. The result is the generation of a proportional electric charge that can be measured.” 

One of the biggest challenges faced by the researchers is placement of the sensors on the rotor blade. White says, “The sensors need to last for at least 20 years or be inexpensive and easy to replace. Positioning the sensors is not easy, and they do not necessarily go where we would expect. An optimization method has been developed to determine where sensors should be placed and how many should be used on a specific rotor blade.”

The sensors also are faced with handling two types of wind conditions. Quasi-static wind loading represents the average wind speed or background wind seen in a specific location. Dynamic wind loading include unexpected wind events that can occur. White adds, “Included in this category are turbulent wind gusts, wind shear and unexpected bursts of wind.” 

A trial using sensors placed on a composite wind turbine in Bushland, Texas, was just completed after a sixmonth period. White says, “We oversensored the rotor blade intentionally to determine what locations can best assess the wind load.” The operational testing is complete, and the data is being analyzed to determine the accuracy of the methods. The sensored rotor blade has been removed from the turbine and will be shipped to the National Renewable Energy Laboratory’s National Wind Technology Center for static and fatigue testing.

White also noted that the sensors are hermetically sealed in protective housing so that they cannot be adversely affected by moisture and other harsh environmental conditions.

In future work, White would like to place sensors on the gearbox to determine the impact of vibrational measurements on performance. Data taken at the rotor blade also would be used to correlate with torque detected on the gearbox. The hope is that this work can improve the efficiency and reliability of wind turbines by reducing sudden increases in torque that can cause gearboxes to fail.

Further information on this work can be obtained from a presentation made at the Windpower 2009 Conference & Exhibition (1).

REFERENCE
1. White, R., Adams, D., Rumsey, M. and Zayas, J. (2009), “Measurement of Operational Loading and Deflection with a Smart Wind Turbine Rotor Blade,” Windpower 2009 Conference & Exhibition.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.