Wind-generated electricity is becoming an increasingly attractive source of power, as it is recognized as providing an environmentally compatible source of electricity. Wind energy is the world's fastest growing energy source on a percentage basis (with an annual increase approaching 32%) and its growth is forecast to continue at a double-digit pace into the next decade. The process of the wind turbine is a fairly simple one. The wind is used to generate mechanical power or electricity. A range of different wind speeds passing over the blades is converted into mechanical power. Wind turns the blades and the power from the rotation of the wind turbine rotor is transferred to the generator through the nacelle that houses the gearbox, low and high speed shafts, generator, controller, and brake. To get the most reliable energy production possible, machines have to be kept in excellent condition. Lubrication is another important aspect that contributes to turbine reliability and long service intervals.
Key Wind Turbine Lubrication Points
The promise of clean, unlimited wind energy presents many technical challenges for the components in the windmill nacelle. Often located in extremely remote locations and tens of meters in the air, reliability is paramount and failure not an option. Components need to be compact and lightweight yet provide exceptional service life in the most demanding of operating environments. For manufacturers and operators of wind energy generating turbines, reliability, remote monitoring and ease of component maintenance are critical factors in their successful and viable operation. High efficiency lubrication should be an integral part of wind turbine design and maintenance.
Wind turbines are complex mechanical installations which require three types of special lubricant: for the hydraulic circuits and brakes, greases for the slewing rings and bearings. Among the most critical components of the nacelle are the blade bearings which have to operate under extreme stresses and operating conditions. The higher power of the latest wind energy technology is a consequence of design changes and component development including much longer blades, taller towers and improved mechanical and electrical components like gearboxes and generators. Also, improved lubricants and adapted maintenance intervals together with automatic lubricant dispensing systems have contributed to this growth.
The points to lubricate in an indirect drive wind turbine are:
1. Blade bearings with pitching gears, Azimuth bearings
2. Main shaft bearing
3. Releasable shaft connections i.e. shrink disc connections
4. Gear boxes
5. Generator bearings
6. Disc brakes
7. Most of the external and internal threaded connections
Many bearings, including the blade bearings, are lubricated with an automatic greasing system. Greases for blade bearings and azimuth bearings have to show the following properties:
- High load resistance
- Resistance to vibrations as a result of the light weight design and the unsteady wind
- Medium to low speeds
- High temperature resistance and broad service temperature range
- Resistant to corrosive environments etc.
- Good compatibility to the sealing materials and varnishes involved
- Good pumpability in centralized lubricating systems
High loads are occurring due to the wind gusts and inner vibrations in the turbine. The intermediate bodies of the bearings like balls or rollers are pressed against the outer and inner rings under vibrations so that the grease is squeezed out of the highly loaded contact zones. Wear marks in form of the contact area of the intermediate bodies against the rings are generated. This 'riffeling' effect is damaging the bearings severely. The effect can be reproduced in screening tests, and at the same time the corrosion of the bearing is checked.
Specialty greases i.e. Lithium or Calcium soap greases with Mineral, PAO or Ester base oils are in use (the later when compatibility to the plastic materials involved is assured). In addition, solid lubricants and anti-corrosion additives can ensure high wear reduction and less corrosion leading to the long service intervals needed.
With a trend in wind farms moving offshore so as to capture maximized wind speeds and therefore run more efficiently, maintenance will increasingly become more difficult. This emphasizes the importance of proper lubrication at the design stage. To date, very poor maintenance of wind turbine lubrication has been seen in the field. But Molykote hopes its expertise will be called upon to improve this.
Smart Lubrication Tip
Problem: You are faced with short life of your bearings which operate under high load in a wet, corrosive environment
Solution: You can extend the equipment life by applying Molykote G-0102 heavy duty bearing grease, which is well suited in the presence of water
This information has been sourced, reviewed and adapted from materials provided by Dow Corning.
For more information on this source, please visit Dow Corning.