Modern Wind Turbines: A Lubrication Challenge

Wind turbines have been used in one form or another for the last 7,000 years. Versions of wind turbine-generated power helped early Egyptians propel cargo vessels along the Nile River. Wind turbines were used in Persia, modern-day Iran, to crush grain. These early vertical shaft designs were the forerunners of the designs eventually adopted for use in Europe and America during the second millennium.

Travel and trade brought the concept to Europe, and by the 11th century, the Dutch refined and adapted the wind turbine mostly for draining lakes and marshes helping to reclaim Holland from the sea. In the early 19th century, wind turbines were further used throughout Europe for water pumping.

Although used extensively in the United States for electricity supply mainly to farms, by the 1950s, the central power grid was extended to almost every American home, which effectively put wind turbine developments on hold.

In the early 1900s, wind turbines were a major U.S. export. However, with the rapid development of coal and oil-based energy alternatives, interest in wind-generated energy options waned.

It wasn’t until the Organization of Petroleum-Exporting Countries (OPEC) oil embargo of the 1970s, and a rise in the price of oil that the world became seriously interested in this natural energy source again. Market growth for wind energy is now occurring in North America, Europe and Asia, following a 30 percent growth in installations through the 1980s and 1990s.

Currently, Germany is the world’s leading market and the country with the largest wind power base, with Spain and Denmark together equaling approximately two-thirds of Germany’s output, with the United States having about half. The majority of wind turbine activity in the United States is centered in California, with an installed base of more than 17,000 machines rated between 30 and 350 kilowatts each.

Rating of Wind Turbines

Wind turbines have a power rating often called a nameplate power. For example, 750 kW means that the wind turbine will produce 750 kilowatts (kW) of energy per hour of operation, when running at its maximum performance (see Table 1 for conversions).

Wind turbines generate between 0.75 MW and 2.50 MW according to their design limits. The Flender Corporation, a major international manufacturer of drive systems and components, is seeking to develop an advanced wind turbine in the four-to-five MW capacity range.

Wind turbines will usually run about 75 percent of the year, but run at their maximum rated power only during a limited number of hours of the year. In order to find out how much energy the wind turbines produce, one must know the distribution of wind speeds for each turbine.

In Spain’s case, the average wind turbines will return 2,300 hours of full-load operation per year. To get total energy production, multiply the 3,337 MW of installed base capacity by the hours of operation (3,337 x 2300 = 7,675,100 MWh) to arrive at the total output, which is 7.7 Terawatts (TWh) of energy.

To put Spain’s power into perspective, the world’s total installed wind generation capacity is about 25,000 MW, which is equivalent to roughly 10,000 large wind turbines. One hundred and fifty of these large machines could match the output of a nuclear power plant.

In 2001, Europe produced 17,000 MW of generation capacity (TW = installed base x average hours operating per unit at full load). This is enough energy to support 10 million average homes. Sixteen million tons of coal would be required to generate equivalent power from coal-fired turbines. Burning this amount of coal would also produce 24 million tons of CO2 emissions.

Main Components

The key mechanical and power-generating elements in a wind turbine are a gearbox and the generator to which it is attached. Various designs of wind turbines include the original Dutch windmills of old to strange hoop-shaped Darrieus “eggbeater” turbines. For the purpose of this explanation we’ll examine a typical propeller-type wind turbine (Figure 1).

Figure 1. Propeller Type Wind Turbine

Stated simply, the wind turbine propeller captures the wind’s energy, which spins a shaft, which drives a generator and produces electricity.

The following structural components make up most modern wind turbines alongside the systems that help to run them most efficiently:

The tower raises the turbine’s assembly above the turbulent air currents close to the ground. Innovative tower designs allow towers to be built at reduced costs with increases in height to more than 300 feet.

The blades, which spin in the wind to drive the turbine generator, along with the hub are called the rotor. A turbine with a 600 kW electrical generator will typically have a rotor diameter of 44 meters (144 feet) but newer designs have blades spanning 75 meters.

The rotor attaches to the nacelle, which sits atop the tower and includes the gearbox, generator, controller and brake. A cover protects the components inside the nacelle. The entire nacelle pivots to maintain point contact with the shifting wind.

The yaw drive, with the help of computer controls, keeps the nacelle pointed into the wind. Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds that are too high or too low to produce electricity.

A disc brake, which can be applied mechanically, electrically or hydraulically, is used to stop the rotor in emergencies or when the temperature is too high. Modern turbines have protection systems that prevent damage in excessively high winds.

The generator is usually an off-the-shelf induction generator that produces 50 or 60-cycle AC electricity. The electricity is transferred to storage, exported to the grid or hard-wired directly to an application. There are variable-speed generators in use that can take advantage of inconsistent wind conditions.

The wind turbine controller evaluates wind conditions and adjusts the turbine operation to maximize the amount of power generated, while protecting it from wear and tear. These smart controllers start the machines when wind speeds reach between 8 and 16 miles per hour (mph) and shut off the machine when wind speeds reach about 65 mph.

Some turbines are configured to transmit operational and mechanical condition to a control center for observation and analysis.

Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 40 to 60 rotations per minute (rpm) to about 1,500 to 1,800 rpm, the rotational speed required by most generators to produce electricity. This requires massive gears and shafts. For example, in a 3.2 MW wind turbine, the ingoing torque is 2.5 million N-m.

Reliability

Considering the extreme environmental and mechanical pressures wind turbines must endure, their reliability is impressive. It is well above that of most conventional generating technologies, and extensive studies show that the best turbine manufacturers consistently achieve availability - a commonly used operational measure of reliability - of more than 98 percent.

Structural and mechanical failures (which can result in a tower collapse) are primarily due to control system errors and lack of effective maintenance.

Many field-operating failures are a consequence of gearbox bearing failure. This type of failure is believed to be directly related to poor lubrication and lack of routine maintenance.

The bearings in the wind turbine gearbox must take extremely high loads, and throughout the gearbox, the bearing performance criteria will be different. In some operating conditions, the requirement is to carry medium-sized loads at low speeds, while elsewhere the bearings need to carry much lower loads but at far higher speeds.

The high-load/low-speed conditions that arise when winds are light can possibly lead to the breakdown of the lubricating film that is normally required for a long bearing life. This has been identified by developers and will be corrected in bearings tests for future wind turbine gear oil specifications.

Routine Maintenance and Relubrication

The gearbox is situated just where the winds are the strongest - as high as 300 feet. In addition, offshore installations encounter rough seas. The engineer will have to gain access up the tower via an internal ladder (or elevator in some cases), which is demanding and specialized work.

Many bearings are lubricated with an automatic greasing system. A special gearbox oil filter, separate from the normal oil cooling system, ensures high oil cleanliness. This is a key factor in desert or arid conditions where airborne dust can get into gearboxes, act as an abrasive, and eventually lead to (three-body) contact fatigue failures.

Nonetheless, oil drain intervals have rested between 8 and 12 months, with one major manufacturer just extending its interval to 16 months following a six-year field evaluation of lubricants. The expectations for new generation oils for offshore applications could be a drain interval of up to three years.

Trends for Wind Turbine Lubricants

Most of the wind turbine gearbox manufacturers have compiled or are in the process of compiling new lubrication specifications. These specifications are more stringent than those for industrial gear applications, and more accurately reflect true operating conditions, including low-temperature conditions.

Performance expectations for lubricants used in offshore wind turbines are higher due to demand for extended life. Some new trends and measures include:

1. The FAG FE 8 bearing test, which is part of the standard DIN 51517 Part III specification, is required by all gearbox manufacturers. Other bearing tests are under evaluation for inclusion in the new oil specifications as well.
2. There is a move toward synthetic formulations (PAO/ester, ester and PAG-based). The synthetic oils offer longer life and so reduce the need for oil changes.
   • PAOs (Poly Alpha Olefin) provide excellent viscosity index and low pour point. These properties make them a fluid of choice for applications characterized by wide ranges of operating temperatures.
   • PAO/ester Blend. There have been hydrolysis issues (breakdown in the presence of water), making selection of hydrolytically stable products a critical issue.
   • PAGs (Polyalkalene Glycol) offer improved resistance to micropitting but have compatibility problems with coatings and seal material.
3. New oil compatibility specifications for paints, lacquers, sealants and bearing materials.
4. New extended lifecycle seal tests for both static and dynamic seals.
5. The SKF Emcor rust test is being evaluated to include testing with salt water.

Dr. Helen Ryan Head of Global Industrial Development at Ethyl Petroleum Additives said, “The tests specified by the gearbox manufacturers are known entities and technology is already in place to meet these requirements. It is the inclusion of the new bearing tests, which are evaluating not only wear on the bearing and bearing cages but also corrosive pitting and staining of the bearings, that will cause a paradigm shift in how industrial gear lubricants are formulated.

Preventing this type of bearing damage will require a move away from very active and aggressive EP additives. The ultimate gearbox oil for wind turbine application should have the thermal stability of a top-tier hydraulic oil combined with the EP properties of current gear oils. In addition, the components added to prevent micropitting need to be carefully selected to ensure that surface activity is balanced.”

Implications for the Future

With the latest trend of offshore wind turbine parks, accessibility is even more difficult than on land, so proactive prediction of the useful life of lubricants becomes the new maintenance strategy rather than the reactive strategy based on measuring acid number and viscosity.

Turbine operators, analysis labs and component manufacturers are collaborating in the development of methods to characterize the conditions of in-service lubricants to meet the new challenges. For instance, bearing manufacturer SKF solicited the expertise of Fluitec to develop a test procedure for in-service bearing lubricants and greases, which could be recommended to the (SKF) customers, in order to predict the remaining lubricant life.

Another collaborative effort has produced a monitoring system for in-service wind turbine lubricants that detect and trend the remaining concentration of antioxidants. As a quick check for fluid condition, at a minimum the turbine operators should be measuring: cleanliness (contamination by ISO class), oxidation, water and viscosity.

By trending these four major parameters, 90 percent of the lubricant and component information is accessible onsite in a short time. Also of key importance is quality control of incoming oil batches. When refilling with new greases and lubricants, it is important to control the quality and make sure the right oil is added to the reservoir to avoid mixing and creating deposits into the gearbox.

“To date, we have seen very poor maintenance procedures in the field. These will have to change dramatically, especially for the larger sizes of wind turbines and gearboxes where exposure to higher oxidative and wear stresses will occur quickly,” said Jo Ameye, global sales and marketing manager of Fluitec International.

Winds of Change

What conclusions can be drawn about general machinery lubrication from the task of maintaining effective lubrication conditions on a modern wind turbine? Wind turbine lubrication exists at the very extremes of industrial gear applications in terms of temperature, load weights, bearing wear, maintenance, accessibility and basic lubricant performance.

Increasingly for offshore applications, synthetic and biodegradable fluids are being developed. Additionally, turbine gear oil specifications are beginning to reflect demand for higher lubricant performance through testing for enhanced oxidation and corrosion resistance, and improved bearing and long-term operational performance.

Wind power is a high-growth, rapidly evolving industry. The lubrication developments for this mechanical application are moving at a similar pace. The players developing lubricants and maintenance strategies for this ‘extreme’ application are paving the way for a new standard in gear and bearing lubrication.

Photos Courtesy of Nordex GmbH.