Skip to Content
Find More Like This
Return to Search

Wind Turbine Blade Testing System Using Base Excitation

Base Excitation Test System (B.E.T.S.)

National Renewable Energy Laboratory

Contact NREL About This Technology



Technology Marketing Summary

Recently, there has been a rapidly growing demand for renewable energy, including wind energy. To meet this demand, wind turbine designers are working to provide blade designs that allow a turbine connected to the wind turbine blades or to the rotor to effectively convert wind into electricity. The blades must also be designed properly to withstand inertial forces, aerodynamic forces, and structural forces so as to provide a relatively long service life and safe operation. Like all rotating machines, wind turbines are generators of fatigue, and every revolution of its components including the turbine blades produces a load or fatigue cycle, with each of these cycles causing a small, finite amount of damage that eventually may lead to fatigue cracks or other failures.

Modeling may be used in some cases to determine service life of a turbine blade during normal operations. However, modeling has its limitations including variations in the as-built/manufacture blade design and the difficulty in accurately modeling operational conditions with varying and sometimes random loading. As a result, wind turbine blades are typically laboratory tested to determine that their fatigue strength or characteristics are adequate for a desired service life. Wind turbine or rotor blade testing is used to verify that laminations in the blade are safe, e.g., the layers used to fabricate a blade do not separate/delaminate and to verify that the blade will not break under repeated stress.

Presently, wind turbine blades are fatigue tested in the flapwise direction (i.e., out of the rotor plane or in the direction transverse to a plane extending through the blade) and in the edgewise direction (i.e., in the plane of rotation or in a direction parallel to a plane extending through the blade). For large blades (greater than forty-meter blade lengths), these two fatigue tests are typically run sequentially, and, to simulate a typical service life of a blade, each test may involve placing a blade through one million to ten million or more load or fatigue cycles, which may take three to twelve months or more to complete for each tested direction. There is a trend for wind generator systems to become increasingly larger. Unfortunately, however, the larger blades associated with larger wind generator systems are subjected to greater static and dynamic loads and the facilities for testing these larger blades are also very large as newer generation turbine generators are being designed with blades 40 meters or more in length. It is very desirable, and often necessary, to advance test a proposed blade design to ensure that it will be capable of withstanding the expected loads with structural failure and to evaluate the fatigue resistance or the blade design, and these advanced tests may significantly delay implementation of a new blade design. The test equipment can also be relatively expensive to purchase and operate, which can drive up the costs of blades and wind energy. Hence, there is a need for blade testing techniques that are less expensive to use and take less time to complete while still providing accurate fatigue testing results.

Description

Engineers at the National Renewable Energy Laboratory (NREL) have developed a system which uses a motor to resonate wind turbine blades by oscillating the system at the root. One advantage of this system is that it removes the need to have specialized hydraulic equipment such as pumps, hoses and actuators. This is done using a motor to create oscillatory motion and resonate a blade in either the flapwise or edgewise direction. The motor and flywheel system rotate a link, which is attached to a frame with a prismatic joint. The frame’s vertical deflection causes the system frame to rotate about a revolute joint mounted to the ground. The blade is mounted to the frame and oscillates with the frame. In contrast to traditional test stands, the frame is mobile because it is self-supporting and requires little anchoring to the ground. A variation of the concept shown below would be to cantilever a weight from the blade stand to reduce the loads at the follower carriage.

The supporting structure includes a tunable spring element for applied excitation forces, the tunable spring enabling dual axis testing by modification to the spring constant in different directions.

Benefits
  •   Less expensive testing technique
  •   Faster than conventional techniques
  •   Requires less specialized equipment
  •   Mobile frame
Applications and Industries
  •  Wind turbine blade testing
  •  Wind energy
Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 8,601,878
Patent
8,601,878
Base excitation testing system using spring elements to pivotally mount wind turbine blades
A system (1100) for fatigue testing wind turbine blades (1102) through forced or resonant excitation of the base (1104) of a blade (1102). The system (1100) includes a test stand (1112) and a restoring spring assembly (1120) mounted on the test stand (1112). The restoring spring assembly (1120) includes a primary spring element (1124) that extends outward from the test stand (1112) to a blade mounting plate (1130) configured to receive a base (1104) of blade (1102). During fatigue testing, a supported base (1104) of a blade (1102) may be pivotally mounted to the test stand (1112) via the restoring spring assembly (1120). The system (1100) may include an excitation input assembly (1140) that is interconnected with the blade mounting plate (1130) to selectively apply flapwise, edgewise, and/or pitch excitation forces. The restoring spring assembly (1120) may include at least one tuning spring member (1127) positioned adjacent to the primary spring element (1124) used to tune the spring constant or stiffness of the primary spring element (1124) in one of the excitation directions.
National Renewable Energy Laboratory 12/10/2013
Issued
Patent 8,677,827
Patent
8,677,827
Wind turbine blade testing system using base excitation
An apparatus (500) for fatigue testing elongate test articles (404) including wind turbine blades through forced or resonant excitation of the base (406) of the test articles (404). The apparatus (500) includes a testing platform or foundation (402). A blade support (410) is provided for retaining or supporting a base (406) of an elongate test article (404), and the blade support (410) is pivotally mounted on the testing platform (402) with at least two degrees of freedom of motion relative to the testing platform (402). An excitation input assembly (540) is interconnected with the blade support (410) and includes first and second actuators (444, 446, 541) that act to concurrently apply forces or loads to the blade support (410). The actuator forces are cyclically applied in first and second transverse directions. The test article (404) responds to shaking of its base (406) by oscillating in two, transverse directions (505, 507).
National Renewable Energy Laboratory 03/25/2014
Issued
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROIs 07-21, 08-46DevelopmentAvailable03/17/201603/18/2016

Contact NREL About This Technology

To: Erin Beaumont<erin.beaumont@nrel.gov>