Wind energy

At least some of our energy future is blowing in the wind. The industry is growing at a quick 25% annually, and net sales have doubled in the last three years for most manufacturers of turbines and components such as gearboxes and bearings.

December 10, 2010
by Steve Gahbauer, Contributing Editor

In addition to land-based wind turbines, Emerging Energy Research forecasts more than 6,000 megawatts offshore wind energy will be developed in North America over the next 10 years.
Photo: iStockphoto

At least some of our energy future is blowing in the wind. The industry is growing at a quick 25% annually, and net sales have doubled in the last three years for most manufacturers of turbines and components such as gearboxes and bearings.

Today, Denmark derives more than one-third of its energy from wind turbines. China’s wind energy market is also booming. Currently, it ranks fifth in the world with 6,000 megawatts of installed capacity but the country’s aim is to have 30,000 megawatts by 2020. Canada trails far behind with only 2,550 megawatts of wind power but by 2015 there should be 12,000 megawatts.

There are more than 830 offshore wind turbines operating in nine European countries, and thousands more are being built. Although land-based wind farms are popping up in Alberta and Ontario, there’s not a single offshore wind turbine in North America. However, Vestas, a wind turbine manufacturer based in Denmark, has plans to build some for Ontario’s Great Lakes.

Data analyzed by FC Business Intelligence Ltd., England in its Wind Energy Operations and Maintenance Report, states that the average operation and maintenance (O&M) costs are $0.027 per kilowatt hour, and the average lifespan of wind turbines is 20 years. O&M costs do, of course, increase over that lifespan and are associated with generation, drive trains and blades, but mostly with gearboxes, which are the weakest link in the mechanism. There is an ongoing R&D effort related to gearbox reliability, because many of those designed for a 20-year lifespan fail after only six to eight years and they can cost up to $500,000 to fix.

Gearboxes must be lightweight and capable of handling large stresses, variable-speed loads, torsion from uneven wind loads and, at least in Canada, extreme temperature fluctuations. Complicating these requirements is the fact that their design is approaching practical limits of available materials.

The gearbox connects a low-speed shaft turned by the rotor blade with a high-speed shaft that drives the turbine. The low-speed shaft is typically supported by two large bearings. Lubricants in the gearbox and bearings play a vital role in ensuring that a wind turbine operates effectively. Gearboxes are relatively small, but very complex with their low- and high-speed shaft geometry. Their position in the wind turbine mechanism makes them hard to reach. Replacement parts are very pricey as are long-reach cranes needed to move the gearboxes, which can add between $75,000 and $100,000 to the bill.

Repair and refurbishment involves taking the machine apart and shutting down the turbine’s operation. Adding to the costs is a loss-of-revenue factor of $1,000 per day. As with every other type of equipment, over the long haul proactive maintenance is cheaper than reactive maintenance. Proactive and preventive strategies should include:

• early detection through diligent PM and condition monitoring by remote sensors;
• using up-to-date inspection technologies;
• managing mechanical loads; and
• improving gear lubrication.

Since gearboxes are the weakest link and the most expensive component to repair or replace, failures are central to the lifecycle of wind turbines. Diligent care, proper lubrication and adequate condition monitoring are imperative.

Equally important is the careful choice of a reputable gearbox manufacturer. There are many to choose from. One of the best is Stiebel Getriebebau GmbH, a privately owned German company in Waldbröl near Cologne. Stiebel designs and builds gearboxes and drives that have earned a reputation for robustness and reliability over three generations.

Increasing power density
There are also consulting companies that can help with gearbox selection. One of them is Ortech Consulting Inc. in Mississauga, Ont. Under the leadership of its president, Uwe Roeper, Ortech is Canada’s foremost atmospheric science consulting firm, providing technology-based consulting services and expertise in environmental science and engineering. The company works in partnership with Deutsches Wind Energie Institut GmbH in Germany.

Key to the development of multi-megawatt units is the need to increase the power density, and that is a challenge for bearing manufacturers. Bigger turbines have longer blades. That means a slower rotational speed. To get the same power rating out of a slower speed requires much greater torque, and that torque increases exponentially. New gear units needed to produce that kind of torque must be lighter and, consequently, the large gear-supporting bearings must be a match.

SKF came up with a high-capacity cylindrical roller bearing design that achieves a higher load-carrying capacity in the limited space. Combining this with a planet wheel design and an integrated outer ring creates a very high power density and increases reliability. The design has the outer ring raceway integrated into the bore of the planet wheel, and the bearing comes with a mounting sleeve that makes installation easy. Simply, more rollers create more contact area and extend bearing life.

Extended bearing life, reliability, and long-term operation with minimal maintenance are the chief characteristics that large wind turbine bearings must deliver. To assure these qualities, SKF has built a special test rig in Schweinfurt, Germany, in proximity to its factory for wind turbine bearings. The newer 3-megawatt turbines are 90 metres high and large bearing systems are required at the heart of the top-mounted mechanisms. The test rig supports the validation of new designs and advanced calculation tools for these types of bearings.

The trend toward greater installed power brings with it the need for larger, yet compact and lightweight bearings. The most commonly produced main rotor shaft bearing from SKF is a special double row tapered roller bearing with an outside diameter of 2.40 metres. These bearings are the largest main rotor bearings for wind turbines currently manufactured in series production. And now they can be fully tested at the Schweinfurt test rig before they’re installed, without all the implications of accessibility, tall cranes and easement fees. The bearings test at speeds of up to 60 rotations per minute, more than three times the speed of real applications. The test stand also enables close monitoring of various lubricants, lubricant distribution and lubrication conditions, while the bearing loads can be increased up to two times the load-carrying capacity of the bearing.

Wind turbines are not only getting larger, they are also getting smarter. Solutions for clever condition monitoring abound. Wind turbines are now equipped with new sensors that direct rotor blade position changes to better accommodate unexpected or suddenly changing wind conditions. The sensors respond to two types of wind conditions: quasi-static wind loading, which represents the average wind speed or background wind; and dynamic wind loading, which includes unexpected wind events, such as turbulent gusts and shear.

Researchers will place sensors on the gearbox to gauge the impact of vibration on bearings and overall performance.

The function of sensors on the rotor blade is to read changes in the orientation and deformation of the rotor blade caused by the wind loading and to feed that information to the data acquisition system in the turbine. In turn, the information is provided to a control system in the closed loop, which will instruct changes to be made in the rotor blade.

The key sensors used on the rotor blades are called accelerometers. They measure the magnitude of wind acceleration by using the piezoelectric effect. A mass excited by the acceleration generates a force that impinges on piezoelectric material. The result is the generation of a proportional electric charge that can be measured.

An effective lubrication survey for wind turbines ensures that gearbox and bearing lubrication is always up to snuff. Start with a criticality assessment and analysis of the key operating factors. You also need to include high maintenance costs due to component location.

A lubrication best practice incorporates details about machine design, installation and operating mode, such as speed, load, duration and environment. Some of the specific things to keep in mind are the requirements for lubricant type, quantity and frequency of change; contamination control; oil analysis; and the planning, then scheduling of maintenance.

Wind turbines place special demands on extremely high-torque gearboxes and large double row tapered roller bearings. As with all equipment, these giant energy generators need diligent maintenance.

Steve Gahbauer is an engineer, a Toronto-based freelance writer, and the former engineering editor of PLANT. E-mail