By Bob Moser, US correspondent

Innovation has radically changed wind energy. Turbines are subject to Galileo's square cube law, and as they get larger and heavier they should face the common economic struggle of costs growing faster than performance.

But that hasn't been the case in wind turbines. As turbines scale up, designers have become smarter, introducing changes like more aggressive control of loads, adding strategic flexibility in the system, and changing the solidity of rotors to lower weight.

As to what's behind gearbox failure, the answer is not straightforward. “There's no one thing causing all the problems,” said Sandy Butterfield, a 34-year veteran of the wind power industry, former chief engineer of the NREL's Wind Program and lead of the Gearbox ReliabilityCooperative, and current CEO of Boulder Wind Power.

“If you go out and look at cases, you find each one is suffering from slightly different failures. We've learned that flexibility in the gearbox is important, lubrication is very important, cleanliness and quantity of lubrication supply, and type of lube supplied to different stages of the gearbox is important. We don't have answers yet because the (Collaborative study) is still ongoing, but we do have better insight.”

Turbulence ahead

NREL researchers, along with a host of turbine manufacturers, parts suppliers and owners in the GRC, completed their first dataset in July 2009 from field and dynamometer tests on two 750-kilowatt gearboxes. The multi-year GRC is already providing a deeper understanding of factors that impact gearbox performance, and operating lifespan.

The root of the problem lies with turbulence, according to Butterfield. A steady, consistent environment isn't realistic for wind energy. Since the 1990s, operators have gained a better understanding on how to control the loads on gearboxes, evidenced by the emergence of international standards for gearbox design.

But as turbines get bigger, rotor speed decreases, producing more power but a lot more torque on gearboxes. Gears are becoming wider, but when operating at a low speed lubricant has difficulty achieving a consistent layer between metal surfaces.

As turbines grow and vary in size and complexity, different types of lubricant may be needed based on the turbine's speed and torque. Bearings in gearboxes may also be changed from rollers to journal bearings, but there are drawbacks with the more advanced journals as well.

Much of the fault lies with turbine owner-operators, said Terry Callahan, vice president of operations for Suzlon Wind Energy. “Consistent, persistent maintenance, and following that religiously,” should be rule no. 1 for the industry. He says this is the most foolproof way to catch issues like proper lubrification for bearings and expansion valve reliability for heat release, before they escalate.

“When you see a gearbox having issues, look at the whole fleet. Was the failure in the metal in the box itself? Was it bearing failure? You have to look at the entire population and dig into the root cause of failure before deciding there are a lot (of failures),” Callahan said. “If there is a common cause, is it a bad batch of metal tied back to parts suppliers? Companies are standing behind their gearboxes too and repairing them.”

Designing out the problem

What you see in terms of failure now has lined the design process for many years, Butterfield said. “But the fleet of machines coming out today are much more diligently designed than those five years ago.”

Gearbox designs have continued to improve as industry experience has grown. One area in particular, is bearing applications. While not suitable for all bearing locations within the WTG gearbox, the Tapered Roller Bearing pair has become popular because of its ability to resolve radial and axial loads, while allowing more flexibility than other designs, said Michael Tyson, vice president of engineering for Suzlon Wind Energy.

He adds that increased focus on refining material processing methods, including more routine quality checks during manufacturing, has improved gearbox reliability.

“I personally don't think we have a 20-year gearbox operating yet, but of course that's the requirement,” Butterfield said. “When we design the machines, we designed for a 20-year lifespan. Today, I think all owners are thinking they'll need to replace the gearbox once in a (turbine's) life. What we're trying to avoid is replacing it three to four times.

“The Holy Grail of wind turbine design is to eliminate the gearbox altogether and go with direct drive generators,” Butterfield added. “Those are expensive and heavy, so it hasn't been worth it. But for offshore where maintenance is costly, some people are judging it's worth going after this new generator for higher reliability and lower maintenance cost.”

To respond to this article, please write to:

Bob Moser: bobmoser333@gmail.com

Or write to the editor:

Rikki Stancich: rstancich@gmail.com