However, in the same breath as those bullish projections, the study points out an important weakness in wind energy. By 2030, global generating capacity of wind turbines will reach 800 gigawatts, surpassing nuclear facilities. [B]ut wind utilization rates will be far lower, the study said, estimating that wind turbines will generate less than 300 gigawatts.

Improving utilization rates is one of the biggest challenges facing the wind energy industry. Industry insiders say that one of the keys to accomplishing this is to help components such as gearboxes last longer and avoid breakdowns. Lubrication is a focal point for those working to improve the situation, along with turbine design and maintenance, but there appears to be little or no consensus about what will provide the solution.

Bigger Towers, More Power

The drivetrain of a wind turbine consists of rotor blades that catch the wind, a hub round which they turn, a gearbox to transmit power between the hub and a generator, and the generator itself. A yaw drive turns the head of the windmill so that it faces into the wind.

Wind turbines have grown in terms of generating capacity in recent years, and the trend looks likely to continue. Capacities of 2.5 to 3.5 megawatts are typical for units being installed these days, and several original equipment manufacturers are developing turbines that are even larger. These advances have been accomplished largely by increasing the length of the rotor blades which allows them to capture more wind and hence to produce more energy. The downside is that this also results in more torque and pressure being exerted on the main shaft and the gearbox. These are the key elements of a wind turbine, essential in transmitting the motion of the rotating blades into usable power.

Bearings in the gearboxes and main shafts of many turbines have been wearing out and breaking with frequency that operators find intolerable. Some experts believe that, although OEMs give warranties of up to 20 years for new turbines, owners must be prepared to replace the gearbox and their bearings three to five times during that span. This could significantly increase the maintenance and downtime costs.

Engineering consultant Erwin W. Zaretsky is not confident that modern wind turbine designs are capable of 20-years lifetimes.

In terms of designing bearings for a certain wear, there is no formula for it, Zaretsky told the Society of Tribologists and Lubrication Engineers at its annual meeting in Atlanta, Georgia, U.S., in May. The problem could be mitigated with lubrication and modification of bearing design, and instead of getting three to five years, an operator could get not more than eight years for a gearbox and main shaft bearing.

Unplanned wind turbine downtime can lead to high financial losses, Kirsten Tschauder of BP Europa SE remarked at the OilDoc conference in Rosenheim, Germany, in February. Roughly one third of such losses can be minimized or avoided by considering special ambient and application conditions in which a turbine operates. BP Europa SE is a subsidiary of British oil giant BP.

Turbines Rising

Many nations such as Germany, Great Britain and the United States are enthusiastically building clean power generation capacities. The U.S. government pledged to satisfy 20 percent of the countrys power demand using only wind turbines by 2030. Germany is even more enthusiastic and, in the eyes of many observers, too optimistic. The European Unions biggest member wants renewable energy for its large economy. Wind and solar facilities now supply 17 percent of the countrys electricity, and in future years will likely receive even more emphasis because of nuclear energys fall from favor. After the Fukushima, Japan, nuclear disaster in March, Germany decided to phase out half of its nuclear capacity.

Observers say such dependence on wind energy may be premature, given the current level of unreliability, and that the public may be in for a rude awakening.

Bearing in mind the unreliability of the wind turbines, and the fact that thousands of them have been built in this past decade, 20 percent of peoples electricity bills soon might cost more than they cost today, said Zaretsky who is based in Cleveland, Ohio, U.S.

While many agree that problems exist, there is a lack of agreement about what causes them. Danish OEM Vestas has in recent years experienced a number of gearbox failures, which it says is influenced by the gear oil itself. These problems were not discovered in the qualification testing executed in the labs on the affected oil, Ole Jensen, senior specialist at Vestas research and development center in Aarhus, Denmark, told the OilDoc conference.

Vestas said it has observed a number of problems in wind turbine gear oils, including extreme foaming and air entrainment, drops in viscosity and chemical attack on zinc in bearing cages. The company further reported forming of deposits, corrosion and micropitting of gears.

Foremost, lubricants are influencing an occurrence of flaking of bearings, Jensen said, adding that his company is working to develop bench tests that would check for such problems. Prior to field testing, these tests could screen lubes for their performance with respect to the problems experienced with todays gear oils.

BPs Tschauder added that bearing manufacturers have recently adopted new requirements for lubes approved for use with their components.

Past experiences in the field have shown that some gear oils were demonstrating quite excellent gear protection, she said, but due to aggressive [interaction] with bearings, damage could be observed.

Seeking Answers

Some experts stated that there are solutions to these problems. The key to defeating the micro-pitting is reducing the shear stresses, Gary Doll, chief technologist at U.S. bearing manufacturer Timken Co., told the STLE conference. Surface treatment such as coatings on rollers, and [choice] of lubricant are the keys for mitigating these problems. Statistically, the number of shaft bearings that failed is not big – only 10 percent of all installed equipment, according to Doll. Some failures coincide with the environmental conditions, some with maintenance practices, but I believe they are not caused by design flaws in the turbines.

Shawn Sheng, senior research engineer at the U.S. National Renewable Energy Laboratory, in Golden, Colorado, contends that operators need more operation and maintenance education. In OEM manuals it is recommended to not allow gear oil temperatures to reach 90 degrees C, he said. To generate more power, and more profits, operators allow gear-boxes to work [with oil temperatures] up to 110 degrees. Sheng is located at labs National Wind Technology center in Boulder, Colo.

Often what we see is that operators first experience some vibration, which can be an early indication of some bearing issues, he continued. However, replacing the gearbox is a complicated task, so the operators decide to do a minimal overhaul, replacing a couple of bearings, and put back in operation the same gearbox. The wind farm operators are essentially running that gearbox to failure.

Vestas Jensen said the

results of that companys lab tests placed the blame on the oil formulators. Specifications for wind turbine gear oils include requirements for anti-foaming and corrosion prevention, he said, but those are not stringent enough.

Current requirements do not at all address the shear stability, compatibility with zinc and early flaking of bearings. He said the OEMs and lube manufacturers have to cooperate to streamline the lubricant standards.

Gearbox life can also be impacted by effectiveness of oil filtration, the type of oil used and the frequency with which it is changed. All lubrication points in a wind turbine are grease lubricated except the gearbox, Doll said. The gearbox itself has an oil circulation system with fine filtration intended to keep it clean. The vast majority of turbines in megawatt class are running on synthetic gear oils. However, some OEMs still choose mineral gear oils for factory fill.

Cost of Saving Money

Drain intervals of 18 months are recommended for mineral oils, while three years are prescribed for synthetics. However, we have case studies of some operators that are changing the oil after five years, Doll said. Operators wanting to follow such intervals need excellent condition monitoring programs with regular oil analysis, he said. They also need to consider the interests of all stakeholders. Their gearbox manufacturer may strongly recommend that they dont risk further extending the drain interval. The insurance companies also would not be happy with pushing the drain intervals on the gearboxes.

In addition to severe loads, wind turbine gear oils face challenges due to the fact that many are built offshore and therefore are likely to be contaminated by saltwater. Given such difficult conditions, operators should think hard about trying to stretch drain intervals, observers said.

Of course, long drain intervals are keeping the maintenance costs down, Jesus Teradillos of Tekniker Foundation, a research institute in Eibar, Spain, said at the Rosenheim conference. But we are faced with a complex situation here: longer lubricant life equals prolonged wear and longer need for temperature protection.

Commercially viable land-based wind turbines have come under pressure from environmental groups calling them eyesores and sources of noise pollution. For operators, this increases the attractiveness of offshore sites. The drawback is that offshore turbines are more costly to install. They are also more difficult for maintenance personnel to access.

Gearboxes may be the biggest tribological problem in wind turbines, but they are not the only one. OEMs and lubricant companies were surprised to see how the performance life of bearings in yaw and pitch systems depleted depending on weather patterns. If the wind direction is relatively constant, the position of the nacelle remains unchanged and bearings never rotate. This causes some of these bearings to be constantly exposed high loads, while others seldom are. The result is uneven wear on pitch and yaw bearings.

Thunderstorms cause another type of problem. Off-shore windmills are as high as 40-story buildings and, with nothing else nearby to rival their height, become frequent targets for lightning. When a bolt strikes, it discharges across the entire system, sometimes causing significant damage to lubricants and components.

Enthusiasm for wind power was a boon for manufacturers of wind turbines. Likewise it creates an opportunity – albeit smaller – for companies supplying lubricants used in turbines. Those opportunities will continue to grow if turbines show that they can be a viable part of energy supply. So far, a lack of durability has prevented them from meeting expectations. Certainly a great deal of attention will be given to lubrication and other aspects of the problems. If not solved, the opportunities could vanish like the wind.