Since the start of this century, the world has witnessed a revolution in the development of renewable energy. Solar panels and rechargeable batteries that power homes and industries are mushrooming, with automotive company Tesla a trendsetter. But no other renewable energy source has grown at such breakneck speed as the one that allows production of clean energy from gusts of wind.
A number of countries are leading the way in wind energy. In Europe, Germany and the Scandinavian countries have erected thousands of turbines in wind farms on land and offshore in the last few years. Germany for example, following the Fukushima nuclear plant disaster in Japan in 2011, decided to phase out its nuclear power by 2022 and cover the gap with other sources, including wind.
The trend is growing in North America and Asia, in countries such as the United States, China, India and South Korea. The United States government has set a target to generate at least 20 percent of its electricity requirement from wind by 2030, versus 4.5 percent today.
The Global Wind Energy Council found that between 2005 and 2014, the cumulative global market growth rate for wind energy was nearly 23 percent. Last year was a record year for the wind power industry – new installations worldwide surpassed the 50 gigawatt mark for the first time, and the annual market for new capacity grew by 44 percent.
Oil additives supplier Evonik has also noticed the trend. From 2004 to 2009 we have seen tremendous growth in wind power capacity additions, Anatoly Smirnov, the companys technical support manager, told GBCs Base Oils, Lubricants and Fuels conference held in Moscow in late spring. During the preceding two years, we observed less dramatic growth compared to the second half of this past decade.
Most wind towers are equipped with main gearboxes, and these typically are lubricated with formulations made of API Group IV base oils (polyalphaolefins) or Group V base oils such as esters and polyalkylene glycol. These specially tailored products are sold with premium prices, and some players such as Evonik are eyeing ways to penetrate the market by offering competitive analogues with similar or identical properties as these expensive oils. Ideally, Smirnov told the GBC event, this would be a cost and performance balanced product based on Group III base oils.
The gearbox is an essential part of a wind turbine. Its job is to convert the low-speed, high-torque spin of the turbine blades – which varies widely with wind speed – into a consistent, low-torque high-speed rotation of the generator shaft. It can be designed with a one-, two- or three-step transmission arrayed in a combination of planetary and spur gears. It is lubricated with high viscosity synthetic or semi-synthetic ISO VG 320 gear oil, and depending on the size, it can be filled with 100 liters to up to 1,000 liters of fluid, Smirnov said. The oil change interval is every three to five years – while in offshore turbine gearboxes, it can be longer.
Seventy-five percent of all wind turbines contain a main gearbox, but in the years to come, sales of gearless or direct-drive systems that feature a rotor and generator positioned on the same shaft is expected to grow, the company found. We anticipate the use of gearboxes to decline to 70 percent [market share] in 2020, while a significant number of new installations will come on stream with an increasing share of large, offshore units, Smirnov said, citing research by from ZF Wind Power. Gearless systems are more expensive though, and gearbox manufacturers are countering them with greater power density and durability.
Smirnov added that the larger the wind turbines are, the more oils and greases are needed to lubricate them. Globally, the average installed capacity for a single wind tower is between 3.5 and 6 megawatts, and he estimates that around 200 liters of lubricant are needed for each megawatt of generating capacity.
In all, Evonik expects global wind turbine gear oil consumption to soar from 16,000 metric tons in 2012 to 41,000 metric tons in 2020.
Gear oils for wind turbines need to have high viscosity, and so they are mostly formulated with non-polar polyalphaolefin base stocks. Base oil can also be polar, and the addition of ester should provide [additive] package compatibility. If biodegradability is needed, then there are ester-only formulations. There are also polyglycol based formulations, Smirnov said.
PAO, esters and polyglycols are relatively expensive however, so Evonik has sought an alternative formulation for wind turbine gear oils that does not rely on them. This formulation, called Nuflex 8175, features a methacrylate polymer chemistry in combination with hydroprocessed and wax isomerized Group III base oils, which overall is less costly than PAO or PAO-ester blends. It is a cost and performance advantaged formula. Unlike Group I, Group III base oils have excellent shear stability, very low aromatics and sulfur, high saturation and oxidation stability, and good low-temperature performance, Smirnov pointed out.
The backbone of the formulation is an alkyl methacrylate oligoester which has great thickening power (Evoniks trademarked Viscobase 11-522), in an 8 cSt Group III base oil from Finlands Neste which is more than 50 percent of the finished fluid. With the correct additives, this combination can deliver the same advantages as a PAO-ester formulated wind turbine transmission oil, Smirnov stated. They exhibit excellent solvency for all kinds of lubricant additives. Also, [the polymers] can be combined with low-viscosity PAOs.
What makes this important for wind turbine operators is that gear lubes made using Group III can be up to 50 percent more cost effective than PAO and ester formulations. For example, Viscobase has a lower treat rate than a PAO 100 product, it costs 50 percent less, and it has [additive] package compatibility, Smirnov said. In addition, we used the same additive system as for PAO and ester. The tested formulation had a higher viscosity index and better high-temperature viscosity properties, versus PAO-ester formulations of the equivalent viscosity grade. Cold-temperature performance was quite similar, too, with pour point at -39 degrees C.
This alternative gear oil formulation meets the rigorous DIN 51517 (Part 3) industrial gear oil standard, and Evonik says factory and field tests have shown it to have good oxidation stability, rust and corrosion protection, micropitting and seal and coating compatibility.
It also gives micropitting protection for gears and bearings, plus balanced foam and filtration behavior, Smirnov said. Moventas tested the wind turbine formulation of the product in one of its gearboxes. Comparable to the commercial PAO gear oil, it passed the factory tests and was then permitted in field testing, he continued, adding that Moventas found that gear contact patterns did not show any abnormal behavior, and no hard-end contacts or particle marks were detected in these tests.
Another consideration for blenders, marketers and users is that the new formulation has been evaluated according to the OEM specifications of Siemens, Winergy, Hansen, Eickhoff and other wind turbine gearbox manufacturers, and has received approval letters for Winergy, Moventas and Flender gear units as well as bearing manufacturer FAG Schaeffler, Evonik said.
What is more important, this product is based on a very economic base oil concept that can easily compete with the pricey PAO formulations, Smirnov concluded.