The nacelle holds all the turbine machinery. Because it must be able to rotate to follow the wind direction, it is connected to the tower via bearings.
Transport of a nacelle which includes the drive train, Photo by Fuhrländer
The drive train consists of the following components:
There are many ways to arrange the components, and the arrangements vary from one manufacturer to another.
Certification organizations have specifications for noise levels, oscillation response, and load profiles for these components. These specifications are very important because these components are subject to tremendous loads.
Drive Train: with Gearbox
Drive Train: Direct Drive System (German Only)
The rotor shaft connects the rotor to the gearbox. It is also called the slow drive shaft.
The gearbox converts the energy from the slowly rotating rotor at around 18-50 rpm to the approximately 1,500 rpm of the quickly rotating generator.
There are two types of gearboxes: helical and planetary gearboxes.
Josef Brenner, materials tester, Eickhoff Company, Bochum, Germany
© Jan-Peter Boening (www.unendlich-viel-energie.de)
Josef Brenner, materials tester, Eickhoff Company, Bochum, German
© Jan-Peter Boening (www.unendlich-viel-energie.de)
© BWE
No gearbox is needed if a specially developed multi-pole ring generator and sufficient diameter is used in the turbine. This alternative is especially interesting because gearboxes represent a weak point in wind turbines: they are subject to wear, require a lot of maintenance, and cause noise (see the report "Direct Drive System").
The coupling is located between the main shaft and the gearbox; due to the enormous torque, it is rigid.
There are generally two types of brakes: aerodynamic brake systems and mechanical systems.
The certification guidelines of Germanic Lloyd specify that two independent brake systems must be used: aerodynamic brakes (the tips of the blade can be adjusted or the entire rotor blade can be pitched) and another brake. The latter brake is generally a mechanical disc brake in most wind turbines. This type of brake is mainly used when the aerodynamic brake fails or the turbine is undergoing repairs.
The type of mechanical brake used depends on how power is controlled. In turbines with stall control, the mechanical brake has to take up all of the rotor's and the generator's motive energy in case of emergency; this brake therefore has to be very high-performant. In contrast, the mechanical brake used in turbines with pitch-controlled rotor blades can be smaller.
The generator in a wind turbine converts mechanical energy into electrical energy.
For high power wind turbines, doubly-fed asynchronous generators are most frequently used. Here, the operating rotation speed can be varied somewhat, unlike when using conventional asynchronous generators.
In general, a distinction is made between asynchronous and synchronous generators. Asynchronous generators are used most often; they allow for synchronization with the grid and are very robust and low-maintenance. However, synchronous generators are also used because they are more efficient. Synchronous generators can be directly connected to the grid, or an inverter can be used. But they require additional equipment for synchronization with the grid.
All generators have to be cooled. Usually, a ventilator is used for air cooling. Sometimes, water cooling is also used.
Slow-running multi-pole ring generators do without a gearbox as mentioned above.
Heier, Siegfried: Grid Integration of Wind Energy Conversion Systems, John Wiley & Sons, London, 2006
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470868996.html
Gasch, Robert / Twele, Jochen: Wind Power Plants: Fundamentals, Design, Construction and Operation, Berlin, 2002
http://www.fachbuch-erneuerbare-energien.de/gasch_engl.htm