The Wind Energy Technologies Office (WETO) has invested in blade and drivetrain testing facilities since the 1990s, providing crucial knowledge and expertise to the ongoing expansion of commercial wind power—both domestically and globally.
Wind Energy Technologies Office
May 16, 2024
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Blade Testing History at the National Renewable Energy Laboratory and Beyond
In the 1990s, the wind turbine industry was still young. Blade designers had settled on fiberglass as the best material to use (versus wood, metal, or other alternatives), but manufacturing methods and the structure of blades was still immature. Many design ideas were borrowed from the aerospace industry in a trial-and-error fashion, and there was no clear methodology a designer could use to improve reliability. As international industry standards came into place, the U.S. Department of Energy’s (DOE) Wind Energy Technologies Office (WETO) supported the National Renewable Energy Laboratory (NREL) to develop the facilities, equipment, methods, and procedures for validating a wind turbine blade design and certifying its compliance with standards. In 1990, NREL commissioned its high bay testing facility at the National Wind Technology Center (NWTC). This facility was sufficiently large to accommodate blades of that era (less than 30 meters long) and included the ability to apply loads for blade-strength tests. NREL and industry engineers devised and demonstrated many testing innovations at this facility. In 1996, with a $2 million investment from WETO, NREL built the Structural Testing Laboratory (STL) at NWTC. The STL added the capability of testing blades up to 30 meters long.
In the early 2000s, test facilities focused on enabling and accommodating rapid growth in rotor sizes. Longer blades can capture more power from the wind leading to more cost-efficient turbines. Before STL was even 10 years old, the industry had eclipsed 30-m capacity, so—in 2005—NREL installed a 50 meter blade test stand in the STL.
From 2001-2004, NREL developed a resonance testing method that induced fatigue loads that applied the equivalent amount of damage on a blade that occurs over a lifetime of field operation. The method works by oscillating the blade at its natural frequency. By this method we were able to apply a lifetime of damage to the blade over a 3-6 month period to validate that the blade was designed and manufactured properly.
However, the challenge for the test facilities was not only to certify longer blades, but also to validate new technologies. To reach longer blade lengths while maintaining reliability, engineers could not simply stretch older designs; instead, they needed to develop new blade designs. These designs impacted the structural makeup and manufacturing process of the blades.
By 2010, wind turbine rotor sizes had eclipsed the capabilities of NREL’s facilities. A new test center was needed near a shipping port to allow for easy transportation of very large blades. For long-term viability, this new test center would also need active participation by industry. In 2011, DOE invested more than $27 million to build the Wind Technology Testing Center (WTTC) facility in Boston, MA. Operated by the Massachusetts Clean Energy Center, WTTC can test blades up to 90 meters in length. NREL worked directly with the facility design team and kept staff onsite for two years after WTTC’s commissioning to ensure methods, procedures, and experience were transferred effectively.
In 2019, the WTTC received an $7 million from WETO to enable structural testing of 85- to 120-meter-long blades. While the WTTC has advanced blade testing, trends point to blade sizes that will likely exceed the capabilities of the WTTC; in fact, some recently announced offshore turbine models already reach this limit. Therefore, ongoing research at NREL and the WTTC includes methods to test blades broken down as subcomponents but findings comparable to those that would be obtained from full-scale testing. Subcomponent testing will help to ensure that facilities requiring considerable investments and time to establish are not rapidly eclipsed by blade growth.
(a) Forced hydraulic loading system
(b) Linear hydraulic resonance blade fatigue test system
NREL Drivetrain Testing History
Experiences from the early years of the wind industry showed that wind turbine drivetrains (the collection of shafts, bearings, and gearing that connect the rotor to the generator) suffered high rates of failure and were expensive to maintain and replace. In fact, in the 1990s, the wind industry was facing a major crisis due to gearbox failures. Some wind original equipment manufacturers (OEMs) and gearbox companies were going out of business due to the warranty cost of these failures. Other wind turbine OEMs simply bought gearbox manufacturers to safeguard the supply chain of their products. Significant improvements in drivetrains and gearboxes were needed to improve overall wind turbine reliability and maintain industry viability.
The standard drivetrain test method in the 1990s was the “four-square” approach, where two gearbox systems are connected back-to-back and the applied torque is increased above normal expected conditions in order to perform a highly accelerated life test. The “four-square” type of testing does not replicate all the real-life wind conditions and loads. These tests did not include the complete drivetrain and were only accessible to the gearbox manufacturer, not the wind turbine OEM. Complete drivetrain reliability testing could be performed only with fully installed turbines.
DOE and NREL responded to this need in 1996 by beginning to plan for a heavily instrumented full drivetrain and generator test facility. At a cost of approximately $4 million, NREL commissioned its 2.5 megawatt (MW) dynamometer research facility in 1999—the first of its kind in the world. Industry and laboratory researchers used the facility extensively. Furthermore, once NREL demonstrated the benefits of the facility through industry experience and numerous published reports, at least six other multi-MW drivetrain testing facilities were built in the United States in the 2000s, some with NREL assistance. As the size and capacity of wind turbines grew, so did the drivetrains. To align with this growth, NREL commissioned a 5-MW dynamometer facility in 2013.
Simultaneously, in November 2009, DOE provided Clemson University's Restoration Institute a $45-million financial-assistance award funded through the Recovery Act to design, build, and operate a facility to test next-generation wind turbine drivetrain technologies. The university matched the grant with $53 million in public and private funds and developed the Energy Innovation Center, which can test wind turbine drivetrains on two dynamometer test rigs: 7.5 MW and 15 MW in capacity rating.
NREL Blade Testing Impact
DOE investment in testing from 1995 to 2006 improved blade reliability for all turbines by developing the certification methodology used today; NREL engineers devised the testing methods and equipment that are now commonplace across the wind industry. This includes the inertial resonant test systems that simulate fatigue loads more accurately and comprehensively than previous approaches. With the ability to simulate fatigue, testing also enabled blade designers to develop pathways to improved reliability by significantly reducing failures resulting from fatigue. NREL tests discovered fatal flaws in a design that—left undiscovered—would have led to turbine failure in the field and could even have caused nascent wind companies to fail. With better understanding of ways to ensure reliability, wind turbine designers could pursue development of longer blades. In fact, the period of peak activity in the DOE/NREL blade testing program, 1995–2006, coincided with the most significant annual percentage growth in rotor diameter. Essentially, sped up the improvements in turbine blade technology that would otherwise have taken longer if all testing was conducted in the field. The improved testing process allowed designers and developers to identify potential blade failures before they were deployed on wind turbines.
DOE’s investments in testing also helped the U.S. blade design and manufacturing industry, including major U.S. wind firms such as TPI Composites and GE Wind. For instance, NREL tested multiple iterations of GE’s 1.5-MW turbine blades and offered design improvements before they were finalized. The improved design became widely used in GE’s commercial wind turbines. NREL test facilities also played a key role in proving out the blade technology innovations of bend-twist coupling, flatback airfoils, and carbon-fiber blade spars. This gave industry confidence to incorporate these innovations into their blade designs. these innovations into their blade designs.
NREL also collaborated with Clemson University to develop these technologies. DOE invested in a research facility at Clemson, which has 7.5-megawatt and 15-megawatt dynamometers that can conduct hardware-in-the-loop testing—meaning simultaneous mechanical, electrical, and controller validation of offshore-scale wind turbine systems.
The legacy of the DOE investment in blade testing continues to influence the industry today. DOE-funded engineers transferred their experience into the international standards that define the design and testing methodology for all blades. Modern blade test facilities, such as the WTCC, and also those at ORE-Catapult (UK) and CENER (Spain), are replications of NREL’s STL facility and testing methodology. Furthermore, without DOE’s investment, blade testing expertise would almost certainly have been centered in Europe and the U.S. industry would not have benefited from the close partnership with NREL test engineers.
NREL Drivetrain Testing Impact
The tests and studies NREL carried out in its 2.5-MW Dynamometer facility helped bolster gearbox reliability by determining the failure mechanisms and publicizing a number of critical findings. Much of this work was carried out in conjunction with the Gearbox Reliability Collaborative (GRC), which DOE and NREL formed in the early 2000s along with industry, to investigate gearbox issues. First, dynamometer tests showed that gear teeth design and manufacturing tolerances were deficient. The tests also indicated lubricant and its chemical additives beyond the simplest oil were critical for gearbox health and reliability. Finally, the tests showed that the use of a high-speed brake imposed enormous loads on the gearbox. If these loads were properly accounted for as design loads, they would lead to an extremely heavy and expensive drivetrain. NREL recommended that the high-speed brake be removed from the design.
The design changes recommended by the NREL studies were quickly adopted by industry. As a result, the frequency of gearbox failures declined substantially, and most remaining failures were traced back to the bearings and not the gear teeth. Moreover, the early drivetrain workshops and meetings hosted by NREL spawned the GRC, which continues to meet and innovate to this day. The conclusions that NREL and the GRC participants reached in their studies required a carefully controlled laboratory environment and could not have been ascertained from tests of fully installed turbines alone.
DOE’s investment in a dedicated drivetrain test facility—NREL’s 2.5-MW and 5-MW dynamometer—contributed to improvements in drivetrain reliability that have been critically important for commercial success. Similar to the blade test program, the NREL dynamometers also had a mandate to focus first on the U.S. wind industry. With substantially improved gearbox reliability, wind turbine manufacturers could safely increase the machine power ratings of their products to make them even more cost effective. Additionally, DOE also funded the Controllable Grid Interface (CGI) at NREL during this same period. The CGI allowed for the validation of novel technologies, refinement of simulation models, verification of compliance with standards. The period of time where the NREL drivetrain team was making significant contributions to the industry coincided with parallel growth in turbine ratings.
Overall, DOE investments in testing facilities for turbine blades and drivetrain components—along with the research enabled by these facilities—have provided knowledge and experience critical to the ongoing successful expansion of commercial wind power globally, and especially to the U.S. wind industry.