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A Turn for the Better

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Wind turbines have been used for thousands of years, beginning in ancient Persia, to mill grain, pump water and perform other work. Those creaky structures are a far cry from todays aerodynamic versions: slender steel towers capped by nacelles sprouting 50-meter-long blades that turn the winds energy into great swells of electricity. The key point where this happens is the gearbox, which is one of the largest components in todays installations and contributes about 10 percent of a wind-mills total weight.

In 2008, approximately 120 gigawatts of installed turbine capacity was at work worldwide. That figure could grow as much as 30 percent a year over the next five years, according to Raj Patel, European marketing manager for synthetic fluids at Cognis UK Ltd. in Liverpool. The worlds increasing demand for electricity is bringing a flood of investment – and putting pressure on wind turbine manufacturers and operators to step up the reliability of their equipment.

Even the United States is feeling the gusts. Although its still a sliver of the overall electric power supply, the U.S. wind industry installed some 10,000 megawatts (10 gigawatts) of capacity in 2009, said the American Wind Energy Association, pushing the domestic total to over 35,000 megawatts. The trade group listed another three dozen installations under construction in this years first quarter, which will add another 1,400 units to the current U.S. population of 33,000 wind turbines.

The most common size turbine being installed in the United States produces about 1.5 MW of power, and its becoming common now to find 2.5 MW to 3.6 MW turbines operating, AWEA said. To feed this growth, 38 wind turbine, gearbox and component manufacturing facilities were brought on line last year, despite the recession.

The outlook in Europe is no less bright, Patel indicated. The European Union is committed to generating 20 percent of its energy and 35 percent of its electricity from renewable resources by 2020. Doing this will require rapid construction of wind power, and leading OEMs are responding with even bigger turbines. A number of 5 MW designs have entered commercial operation, and turbine manufacturers such as General Electric, Vestas, Siemens and Suzlon are said to have even larger models on their drawing boards.

These large-scale turbines make wind energy more economical, Patel noted, but more difficult to service. They also place greater loads on the gearboxes that transfer the blades rotational motion to the generator set inside the nacelle. Designers ideally try to match the rotational speed of the turbine blades to levels high on the generator shaft, and this increases the load. Designers also are adopting gearboxes with one or more planetary stages, followed by several helical bevel stages, he noted.

Despite this cutting-edge technology, gearbox failures remain a leading cause of windmill failure, Patel told Januarys International Colloquium Tribology at the Technische Akademie Esslingen in Ostfildern, Germany. In a presentation co-authored by Markus Scherer, Dirk Rettemeyer and Rudolf Iking of Cognis GmbH in Germany, he outlined the most common failure modes (micropitting, scuffing, gear tooth breakage and bearing failure), and offered a concept for how lubricant formulators may meet the markets needs.

Protect and Prevent

Current lubricants for wind-mill gearboxes are mostly based on polyalphaolefins (PAO), followed by polyalkylene glycol (PAG). More and more, these synthetics are displacing the mineral oil base stocks used in smaller turbines, Patel said.

The four key areas where the lubricant has to prevent gear box failure are protection against micropitting, preventing bearing failure, avoiding viscosity loss or change, and inhibiting foam. The lubricant also cannot form sludge, and must provide oxidative and thermal stability for long life.

Gearbox manufacturers have set some high performance hurdles for lubricants to assure their needs are met. There are probably more than 150 tests to satisfy to make sure you meet the performance requirements for wind turbines, Patel pointed out. Many of these are outlined in a new ISO Standard 51517, Part 3, which sets out basic lubricant criteria such as viscosity index, corrosion control, wear protection, foaming resistance and seal compatibility.

Other gear oil tests include the ASTM D2893 test for oxidative stability, showing limited viscosity change over 312 hours at 100 degrees C. This assures the oil is suitable for extended longevity and can endure prolonged drain intervals, Patel noted. Shear stability is measured by DIN 51350-6, run at 60 degrees C for 300 hours, and one of the most critical factors, micropitting resistance, is measured under arduous conditions on an FZG test rig.

Some OEMs pile on additional requirements. SKF wants approved gear oils to show resistance to the impact of water/salt water, for example, even though the gearbox is a sealed system. Even though this aggressive test may not reflect conditions in the field, passing it can be a problem, and many commercial gear lubes fail to meet the SKF salt water corrosion requirements, Patel commented.

Formulation Challenges

The most common gear oil viscosity grade specified by wind turbine manufacturers is a quite heavy. ISO 320. Viscosity retention is one of the most critical issues, Patel pointed out, because if the oil loses viscosity (thins), the oil film on the gears metal surfaces could be insufficient to protect the meshing parts. Likewise, the oil must have excellent oxidative stability, so that viscosity does not climb and increase the load.

Polyalkylene glycol (PAG) base oils are one way to meet the viscosity requirement, because they can be synthesized with specific molecular weight distributions. Wind turbine gearbox lubes made with PAG typically contain 95 to 98 percent base fluid, Patel said, with additives making up the balance.

Another strategy has been to create gear oils with less-expensive polyalphaolefins (PAO). Usually, these contain a 4 centiStoke or 6 cSt PAO, plus a large component of a heavyweight PAO such as 40 cSt or 100 cSt, and just 2 percent to 5 percent additives. One drawback here is the cost of the high-vis PAO. PAO 40 and 100 vis base oils are expensive, Patel continued, and they also have seen periodic tightness in the market. Few manufacturers make heavy-vis PAO, and if one of them is knocked out of action – as ExxonMobil Chemical was for a period in late 2008, when Hurricane Ike swamped its plant in Beaumont, Texas – shortages can abruptly hit the marketplace. Heavyweight PAO is also in strong demand for applications such as transportation lubes, said Patel, so gear oil blenders must compete with others for the same molecules.

This led Cognis to see if it could formulate a PAO based lubricant without using any heavy PAO. Our concept is to use PAO 4 or PAO 6, at 30 to 40 percent of the blend, plus a polymer that is 60 to 70 percent, and an additive package thats 2 to 5 percent, he said. We set a goal of a top-tier product with better performance than the top-tier PAO based product can offer. It should protect the gearbox from micropitting, be shear stable, have good foaming resistance and air release, and long drain intervals. Shear stability of the thickening polymer was very important to maintain the required viscosity, while including PAO meant the formulation would be compatible with other mineral oil and PAO based gear lubricants.

The researchers also wanted a polymer that could be made at multiple locations around the world, and from sustainable, renewable raw materials with a wide supplier base. The answer was an exclusive polymer system developed by Cognis. This could enable environmentally friendly, biodegradable and low-tox lubes for wind-mill gear boxes, Patel declared. The polymer is ester-based, he indicated, but declined to go into more detail.

Failure of a wind turbine is very expensive, especially offshore, he stressed, so the wind industrys stakeholders are all motivated to improve the uptime of windmills and their gearboxes. Gearbox OEMs want to reduce their warranty claims, as do wind turbine manufacturers. And power generators want to increase their time up on the power grid.

The Cognis researchers have high hopes for their formulation concept, not least because it avoids the use of heavyweight PAOs while meeting the requirements for viscosity retention, shear stability, oxidation stability, micropitting, and more. We also use an additive pack thats already in commercial use with a proven record, Patel added. With further development, the Cognis scheme could give a boost to turbine designers and operators, and help the wind energy market continue to grow.

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