By Fort Felker, Director, National Wind Technology Center, NREL
Wind turbines appear so simple—tall white sentinels cranking gracefully on the horizon. But up close, a wind turbine is an industrial workhorse. Inside the nacelle hundreds of feet off the ground, hot metal gears strain as shifting winds pull and twist the long flexible blades. Sometimes, something has to give under such strain and the turbine gearbox has been an occasional culprit.
The National Renewable Energy Laboratory’s (NREL) National Wind Technology Center (NWTC) has embarked on the task of determining why some gearboxes tend to fail too soon—sometimes within a few years of installation. That’s a problem because wind turbines are expected to operate for 20 years. Early equipment failure, especially in turbine gearboxes, reduces power production and drives up wind power costs just as the industry captures a greater share of U.S. generating capacity. The NWTC is working to improve gearbox reliability by bringing together the parties involved in the gearbox-design process to achieve the common goal of improving the lifetime of gearboxes. This is being accomplished through the Gearbox Reliability Collaborative (GRC).
Since gearboxes are one of the most expensive components of the wind turbine system, the higher-than-expected failure rates are adding to the cost of wind energy. The future uncertainty of gearbox life expectancy is contributing to wind turbine price escalation, since increased warranty reserves are required to cover the risk of premature gearbox failure. Further, owners and operators build contingencies into the project’s revenue and income projections to account for problems that may show up after the warranty expires. To help reduce the cost of energy, an increase in long-term gearbox reliability needs to be demonstrated.
The collaborative extensively tests instrumented gearboxes to identify weaknesses in current design approaches and aims to point out ways to improve initial designs and retrofit packages. The project identifies equipment failures that are common throughout the industry and targets deficiencies in the design process that are contributing to these problems. NREL has overcome intellectual property concerns by running tests on representative wind turbine configurations and parts, but not replicating any specific manufacturer’s wind turbine model.
Focusing on design is especially important because the expanding wind industry is trying to rapidly meet market demand for new and larger-capacity turbines. Designers and manufacturers are all following very stringent design-quality tolerances.
Yet the problems persist. This implies the design standards are missing some critical loads or conditions that are unique to wind turbines, or that key assumptions in the design and analysis process are flawed. Without identifying the fundamental issues behind previous failures, the same problems could find their way into new generations of turbines.
Here are some observations on the basic problem:
- Most of the problems with the current fleet of gearboxes are widespread in nature, meaning that the problems are not specific to a single manufacturer or turbine model. Over the years, most wind turbine gearbox designs have converged to a similar architecture with only a few exceptions.
- The preponderance of gearbox failures suggests that poor adherence to accepted gear industry practices, or poor workmanship, are not the primary cause of failures.
- Most gearbox failures do not begin as gear failures or gear-tooth design deficiencies. The observed failures appear to initiate at several specific bearing locations, which may later advance into the gear teeth as bearing debris and excess clearances cause surface wear and misalignments.
The problems that manifested themselves in the earlier 500 kW to 1,000 kW sizes five to 10 years ago still exist in many of the larger 1 to 2 MW gearboxes being built today with the same architecture. As such, it is likely that lessons learned in solving problems at today’s scale can be applied directly to future wind turbines at a larger scale, but with less cost.