Demand for wind turbine lubricants is expected to grow at a compound annual rate of 7.5 percent until 2020, thanks to a healthy increase in wind turbine installations. In 2016, 54.85 gigawatts of wind power were installed globally, growth of 11.8 percent over 2015 figures. Much of this expansion was in China and Latin America, according to the World Wind Energy Association.

According to Grease Technology Solutions President Chuck Coe, speaking at the ICIS North American Industrial Lubricants conference held in Chicago in September, wind turbine lubricant demand was between 31.75 million kilograms and 36.28 million kg in 2015. Gear oil makes up 70 percent of that demand, followed by hydraulic fluid at 25 percent, with grease making up the remaining demand.

Greases are only 5 percent but, frankly, its the most challenging application. Its 5 percent of volume but Id be willing to bet its probably 15 percent to 20 percent of the lubricant suppliers margin, said Coe.

Gear oil is used for the gearboxes that increase rotational speed and hydraulic fluid is used in the hydraulic systems that control blade angle in the wind, known as pitch. Though it makes up the smallest amount of the demand, grease is used in the most locations in turbine equipment, such as the main rotor shaft bearing, yaw bearing, pitch or blade bearings, pitch drive gears and generator bearings.

The main shaft rotates through the main rotor shaft bearing, which transfers motion to the gearbox. The yaw bearing enables the nacelle to rotate into the wind, and the pitch drive gears drive the pitch bearings. The two generator bearings support the shaft that goes through the generator to allow the shaft to rotate at the speed of the generator, Coe told LubesnGreases. The gearbox and hydraulic systems use considerably more volume of lubricants compared to the greased bearings.

Typically, National Lubricating Grease Institute (NLGI) 1.5 to 2 grade lithium, lithium complex and calcium greases are used in the pitch, yaw and main bearings, and NLGI 2 grade lithium, calcium and lithium complex greases are used in generator bearings.

Extreme pressure additives are needed for greases used on pitch, yaw and main bearings because high load and slow rotation along with bidirectional oscillation of the pitch and yaw bearings results in boundary lubrication. Greases used on the yaw and pitch bearings also need to stand up to fretting corrosion because the bearings are subject to vibrations.

Demand is a function of many things, including operating capacity and relubrication frequency. In terms of operating capacity, a larger turbine doesnt tend to use more lubricants, so you get more power with less lubricant, Coe said. This means that there may not be a direct correlation between lubricant demand growth and wind energy capacity.

Most wind turbines are a minimum of 80 meters high and in remote locations, meaning servicing can be difficult leading to a push to decrease the frequency of relubrication. Theres a huge drive from both the wind turbine operators and the wind turbine original equipment manufacturers to up lubricant life and reduce relubrication frequency, said Coe. This dampens lubricant growth rate.

Because of the drive to reduce relubrication intervals and because weather conditions around wind turbines can be extreme, penetration of synthetics among wind turbine greases is roughly 75 percent. The higher viscosity index of synthetics offers better low-temperature mobility and high-temperature thermal stability, increasing lubricant life. You want that long life to reduce maintenance, said Coe.

The condition of wind turbine greases must be closely monitored because failed bearings cost between U.S. $300,000 and $700,000 to replace, not including downtime, said Coe. Grease condition monitoring is critical to reduce unplanned maintenance, extend equipment life and prevent catastrophic failures, he noted.

Consistency, along with presence of water, wear metals and contamination should be regularly checked. The remaining antioxidant needs to be examined.

The harsh conditions of wind turbines lead to demanding parameters for greases. They must be able to withstand a wide temperature range, oscillation, moderate to high speeds, fretting, moisture and shock loads while maintaining low relubrication needs. To ensure greases are able to meet these requirements, the product is tested for mechanical stability, rust and corrosion prevention, fretting, load carrying and wear prevention, and oil separation.

Mechanical stability, which assesses grease consistency after shearing, is tested in the Roll Stability Test. Grease is put in a cylinder containing a 5 kg roller and rotated at 165 rotations per minute room at temperature at for two hours.

An SKF Emcor Bearing Corrosion Test is used to determine corrosion protection in roller bearings. Roller bearings are rotated at 80 rpm without a load in distilled, acidic or synthetic saltwater for 110 hours, and rust is rated on a scale.

To measure the greases ability to reduce fretting wear an RE-IME False Brinnelling Test is used, which simulates the load situation required for false brinnelling in roller bearings. The four-point contact bearing is exposed to a sinusoidal alternating load of 70 kilonewtons while a 1 percent sodium chloride solution passes through the bearing.

The longest test, clocking in at 500 hours, is the FAG FE8 test, which evaluates the effect of grease on the wear behavior of rolling element bearings.

Finally, greases are put through a Static Oil Bleeding Test to see if the oil will separate under pressure, elevated temperature or shear. A weighted grease sample is placed on a mesh screen and tested at varying temperatures and times. The results are given as the weight percent of oil separation.