Each megawatt of installed wind capacity requires about 250 liters of lubricants, Michael Mueller of the additive company Evonik RohMax, Darmstadt, Germany, told the STLE annual meeting in Atlanta in May. And the goal is for this lubricant to be serviced or changed once every three years. Thus, the coming boom in wind energy holds great promise for sales of the gear oils and greases for this equipment.

Unfortunately, gearboxes represent a weak point in wind turbines. They are subject to wear, require a lot of maintenance, and cause noise, says the WWEA, and their lubrication and maintenance issues are a sore spot with wind operators, who want better reliability and maintainability.

The top eight countries in wind energy consumption are the United States, China, Germany, Spain, India, Italy, France and United Kingdom. And by current sales, said Mueller, the worlds largest wind turbine manufacturers are Vestas, General Electric and Siemens, followed by Enercon, Gamesa, Goldwind and Dongfang; the last two are Chinese companies. China in fact installed more new wind energy capacity in 2010 than any other country and is rapidly becoming the worlds largest generator of electricity from wind.

According to the gearbox manufacturers Hansen and Winergy, the critical performance specifications for gear oils in this application are scuffing and micropitting for the gears, said Mueller. For the bearings, key parameters include micropitting, fatigue, and roller and cage wear.

Seal compatibility is increasing in importance, added Mueller, because the most prominent lubricant problem in the field is leakage. Lubricants must play nice with a large range of seal materials, so the oil or grease is held in place and the equipment not starved of lubrication.

The most widely used gear oil in wind turbines – 70 to 80 percent of the volume – is an ISO 320 grade, usually a semi-synthetic formulated with polyal-phaolefin 40 and 100, Mueller said. Evonik believes theres an opportunity here to use polyalkyl methacrylate base oils as well. PAMA is very viscous, and you can get good results if you combine it with an 8 centiStoke PAO or an 8 centiStoke API Group III base oil, he said. You can achieve very high viscosity index, and also attain a strong performance in seal compatibility, tests show. The PAMA blends also did well in FE8 wear tests, another key performance parameter.

In fact, the wind industry seems to have nothing but key performance parameters, the STLE meeting heard in another presentation, by Kevin Harrington of ExxonMobil Lubricants & Specialties, Fairfax, Va. Windmill lubricants are among the most demanding applications, with long lists of required tests to qualify for use, he declared.

The gearbox is the heart of the windmill, and the gear oil its lifeblood. Yeah, thats a bit dramatic, but its still true, Harrington said. This is not like a gearbox on a factory floor though, where a large foundation can support the unit and technicians have quick access for maintenance. Up in the air, every pound and stick has an impact on efficiency. So youre dealing with a compact design, with not much space to work around the unit, and remote locations that discourage service calls.

The lack of access means gear oils for windmills need a long drain interval, plus rust protection and resistance to wetness, especially offshore, Harrington said. Despite wide temperature ranges, the unit must be able to come up quickly from a cold start. That takes a good viscosity index, plus low-temperature fluidity. The lubricant must protect against micropitting, and retain its performance over the entire service interval. It needs to have wet and dry filterability, foam control and air release, rust and corrosion performance, plus compatibility with elastomers, paints, and cured and uncured sealants.

All this is not a priority list, Harrington said. We have to do it all, and do it well. Meanwhile, the demands keep getting higher. The OEMs keep raising the bar on compatibility for example, adding more materials and tests. If we formulate just for that, though, we could lose other important properties.

It may be that the industry is getting a little off-kilter on this one area, he mused. You cant say any one area is more important than the others. A lubricant without balanced performance is no more use than a chair with one leg thats longer than the others.

Lubricant manufacturers who want to qualify a product for windmill use face a daunting list of performance tests required for OEM approvals, he continued, some of which are proprietary tests and not widely available. For example, the test most requested for micropitting performance is not an industry standard test. Its critical to performance, but we see a wide variability in tests results from various independent laboratories. The FAG 4-stage bearing test and the Flender foam test are other examples. Suppliers are running compatibility tests too, but these are not standardized either.

Provided a new lubricant can pass all the costly tests, Harrington said, it maybe gets a chance to do a one year to two year field demo in one turbine.

Availability, he added, really is the key parameter for wind energy. You dont always know when the wind will blow, so the equipment always needs to be available. Its very difficult to formulate a windmill gear oil and bring it through to approval for this very risk-averse industry, he emphasized.

If the industry is risk-averse, that may be because theres so much invested, pointed out John Dunlop of the American Wind Energy Association. Speaking in early June to the NLGIs annual meeting in Palm Springs, Calif., the Washington, D.C.-based executive noted that the past two decades have seen enormous technical advances, and billions of dollars spent.

Dunlop has been with AWEA since 1993. We had nine people on our staff then, and 90 now. In those years, hes watched the size of U.S. windmills grow at a similar pace.

In the 1980s, Dunlop said, the typical windmill had a 17-meter rotor, and averaged 56 to 100 kilowatts in power output. By 2000, with 150,000 towers built, average output had reached 200 KW. And post-2000, the size has bulked up even more. Rotors sprouted to 77-to-104 meters long, and generators of 3.6 megawatts were not uncommon. (The average installed in 2010 was 1.77 MW, Dunlop told the NLGI meeting.) With scale, the operating cost has also come down; electricity from wind now costs 4 cents to 9 cents per kilowatt-hour to create.

Last year, U.S. operators added 5,000 megawatts of wind capacity, 15 percent more than 2009s new build. That brought the U.S. total to 40,181 MW of nameplate generating capacity, or about 35 gigawatts of delivered power – 2 percent of the countrys electricity needs. AWEA forecasts supply will reach 305 GW by 2030, satisfying 20 percent of demand. This will require a lot of lubricants, Dunlop estimated: The annual lubrication need in 2030 will be greater than $1 billion.

Now comes the hard part: the equipment itself. When it comes to windmills, the higher the better, Dunlop pointed out. Air flows slower when youre closer to the ground. So if you want to have a larger turbine it means having a taller tower, to reach where the wind is smoother and stronger and assures more energy per unit of blade area, he explained. So now we see engineers putting turbines on larger towers.

That sounds good, until you realize that an 80-meter tower is roughly the height of a 25-story building, he continued. This is one of the first technical hurdles for maintenance personnel. Technicians must climb 25 stories every day, and do it two or three times per day to service several towers. New tower owners are looking seriously at installing service lifts, Dunlop said, so technicians could spend more time in skilled work, and less time simply hauling themselves, their tools and needed parts up the towers and back down again. Reducing the climbing stress will help prolong the technicians careers, too, Dunlop pointed out.

At full power, turning at top efficiency, the blades will go completely around their hub about once every four or five seconds, for a relatively slow speed of 12 to 14 rpm. That must be upgraded by the gearbox to higher speed to serve the generator.

Besides the rotor hub, the gearbox components include a main pivot bearing that must hold up the nacelle and its contents (about 100 tons of weight in all), the yaw system and the pitch drive. The yaw system alters the orientation of the rotor in the wind, while the pitch drive adjusts the angle of the blades. And heres where the lubrication issues start, Dunlop said.

Windmills tend to be in areas where the wind is strong, so we dont like to live there, and that makes them remote and inaccessible, he said. The temperature extremes in such places are very wide, from -30 degrees C to 100 C. In addition to the temperature extremes, the gear oils and greases will have to endure high loads, exposure to saltwater, fretting, humidity and condensation. Vibrations, variable speeds and oscillation are also part of the package, because the wind is not intermittent, Dunlop stressed. It doesnt stop like your lights going on and off – it slows and varies, but its not just off.

Saltwater corrosion could become a bigger issue for U.S. operators if wind installations move offshore. Offshore wind turbines are a very large part of the market in some countries, such as Denmark, but there are none in the United States yet, Dunlop noted. The challenges offshore become even higher, with high run times and 90 to 92 percent uptime expected. How can regular maintenance be done if the unit is only down 8 percent of the time? Thats another puzzle lubricants will have to help solve.

The pitch, yaw and main bearings all need extreme-pressure additives, and the pitch and yaw need to be protected against fretting wear from the vibration. Automatic lubrication systems are good, because we dont like to visit the towers too frequently, Dunlop said. The operating temperatures make synthetic greases mandatory, but the lubricants also must be compatible with all the windmills materials, like paints, plastics and seals. The holy grail would be to have a single grease that could serve all three of the systems: pitch, yaw and main bearings.

Aside from the lubricants alone, there are operational and maintenance hurdles, too. The warranty period on a new windmill is typically two to five years, although some OEMs offer extended maintenance contracts, Dunlop said.

Owners need to consider future lubrication needs when purchasing the equipment, he said. And they should ask about maintenance before the service contract ends. Will there be an automatic grease system to dispense grease as needed? Should there be a service lift? Are there condition monitors to watch the lubricant condition, and send an alert if attention is needed? Owners may not even know basic lubrication practices, such as not to mix grease types to avoid compatibility issues that could harden the grease, or cause it to soften and ooze out.

Owners should also ask if warranty specifications are shared, and insist on having full end-of-warranty information for future service. When the warranty ends, the equipment owner will make the decisions about lubrication – what to buy, how and when to apply it, Dunlop said.