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Harnessing Wind Power
Power transmission plays critical role in the efficiency and reliability of wind turbines
By Al Presher, Contributing Editor -- Design News, April 1, 2009
Capturing the power of the wind is pushing industry to develop power transmission technology that can deal with highly dynamic conditions harnessing wind power creates. Efficiency of the turbines is critical because it plays a key role in energy generation, but reliability and effective maintenance are also essential for success.
"Wind energy is exciting technology that can be cost-competitive, but the challenge is improving the reliability of the wind turbines," says Douglas Lucas, manager of application engineering, wind energy at The Timken Co. "The dynamics of the wind, deflections in the turbine system and inertias in the drive train components all cause dynamic complexities in the gearbox specifically and the whole drive train system in general."
Bearings in industrial applications are typically designed to last from 20 to 30 years. But the variability of wind, the flexibility of the towers and torque spikes that occur from all of those variations can result in impact loading in the turbine. The impact loadings, along with the variable torque throughout the entire drive train from the gear box to the generator, can cause significant power transmission challenges.
Lucas says these challenges are providing the focus for research and development efforts. The National Renewable Energy Lab. has a project looking at the system dynamics of wind turbines and gearboxes. There is also a focus on condition monitoring and understanding how these systems are operating dynamically.
Power transmission components for wind applications are being designed and calculated to achieve a 20- to 30-year life. But based on his experience in the industry, Lucas says the turbines and gearboxes are not achieving that level of reliability.
"In other applications for a power generation piece of equipment, we can get units to last 20 to 40 years but that's a gearbox that is being mounted on a solid pad, seeing constant speed and predictable load conditions. The unit is inspected and proper maintenance is performed on a regular basis," Lucas says. "There is no variable loading. Add in the dynamic movements of the tower, which can be moving back-and-forth several meters, and the fact that the wind is never constant and is always changing. Plus the position of the blades is also changing, so the loading throughout the entire system is very dynamic."
Bad weather creates an additional set of problems. The industry does consider the extreme load events and calculations to analyze power transmission components from the standpoint of both normal operating and extreme loads. But there are factors which may not be able to be accounted for.
In response to these engineering challenges, Timken and other power transmission suppliers are optimizing their fundamental technologies and offering specific products aimed at increasing the efficiency and reliability of the power train for wind power applications.
A key goal is friction management. Designs and bearing configurations that are more optimal can increase the efficiency of the turbines and gearboxes by reducing the amount of friction in the bearings, and heat generation that affects the overall cost per kilowatt hour.
One solution is low torque or fuel-efficient bearings where the geometry of the raceways and surface finishes are optimized to reduce the rolling friction. In other industrial market segments, this approach reduces bearing power consumption, the result of the amount of friction between the components, by more than 30 percent and has helped fuel efficiency up to 2 percent.
Concentration on a group of fundamental technical issues is also important to improving system performance. Key technical breakthroughs include work on engineered surfaces where coatings and finishes can enhance bearing fatigue life, corrosion resistance, speed capabilities and friction reduction.
Another area is debris solutions. Bearing raceways are frequently required to operate in lubricants that are contaminated with large-particle debris emitted by gears and other sources. These large particles dent bearing raceways and typically reduce the fatigue life of standard bearings. Specialized bearings help withstand these effects to maintain gear box reliability.
Lubrication options and condition monitoring systems are also important, especially for effective system maintenance. Single-point lubricators can help reduce service intervals and ensure that an adequate supply of fresh grease is delivered to the bearings at all times.
"Part of our product focus is on use of cleaner steels," says Lucas. "We try to keep the size and number of inclusions down to a low value to increase the life of the product. When a bearing starts to get damaged, subsurface rolling contact fatigue can be a problem. With fewer inclusions in the steel, there is less chance for rolling contact fatigue to occur and for the stress profile underneath the raceway to create high maximum shear stress. If you can minimize those stress concentrations under the surface, you can improve the reliability."
The company's DuraSpexx™ bearings are manufactured using highly refined steel, providing twice the fatigue life of normal bearings and making it easier for wind turbine design engineers to improve the power density of their projects.
Promoting power-dense designs is important for these applications. Power density is achieved by using smaller bearings and or putting more power into the same volume of steel. In some wind power gearboxes, some of the bearing components have been integrated into the gear or shafts to reduce weight and system inertia which can be affected by some of the system dynamics.
One example of advanced gear drive technology developed for wind applications is Timken's Flex Drive System with Integrated Flexpin Bearing Technology, which uses an integrated flexible bearing in the planetary portion of the drive train to allow the bearing and planet gear to deflect into the system while still maintaining accurate and reliable gear tooth contact.
"This is an innovative design that can be used to optimize and reduce the size of the gearbox," Lucas says. "If the customer wants to put additional power into that same gearbox, Flex Drive System can be added to allow more power regardless of the deflections in the rest of the system."
A final critical issue for these systems is maintenance. Wind turbines are often quite inaccessible for maintenance purposes because they are located on mountains, in deserts or on the coastlines of the North Sea. Condition monitoring systems help monitor performance, and predictive maintenance increases performance and reliability of the drive train systems. But generally the goal is to achieve a one-year maintenance cycle on the turbines, so timing of maintenance is typically longer than the six month maintenance schedule for turbines mounted on land.
Until the economic changes at the end of 2008, Timken's Lucas says he expected the wind energy market to grow at an exponential rate. But now the wind energy market is expected to grow at a slower rate, even though funding from the stimulus plan should have a positive effect.
One concern is the connection of wind turbines to the mainland grid system. Typically where there is good wind, there aren't many people living there and so industry is dependent on the grid to transport power. Particularly in North America, grid efficiency is absolutely one of the key issues for industry to meet the goals for renewable and wind energy.
"We are pleased that governments in our growth areas, such as Europe, Asia and North America, understand this and intend to take aggressive actions to address the issue," says Hans Landin, director of the front end unit for wind energy at Timken. "We are very optimistic for continued growth in China, where they understand this issue and are making infrastructure investments to facilitate expansion of wind turbines and wind energy."
The Timken Co.: http://designnews.hotims.com/23101-606
The National Renewable Energy Lab.: http://designnews.hotims.com/23101-607
For more fluid power/power transmission news, visit http://designnews.hotims.com/23101-599.
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Harnessing Wind Power
