The rotor is the heart of a wind turbine. It consists of multiple rotor blades attached to a hub. The rotor converts the wind energy into a rotation. In the following, the structure and operation of rotors are discussed and concepts of power control presented.
Rotor blades are a crucial and elementary part of a wind turbine. Various demands are placed on them, and they must withstand very great loads.
Rotor blades take the energy out of the wind. They "capture" the wind and convert its motive energy into the rotation of the hub. The profile is similar to that of airplane wings. Rotor blades utilize the same "lift" principle: below the wing, the stream of air produces overpressure; above the wing, a vacuum. These forces make the rotor rotate.
Rotor blade at Bonn International Renewable Energies
Conference venue 2004, Photo by WWEA e.V.
Today, most rotors have three blades, a horizontal axis, and a diameter of between 40 and 90 meters.
Three bladed wind turbine at wind farm
Sintfeld, Germany, Photo by WWEA
In addition to the currently popular three-blade rotor, two-blade rotors also used to be common in addition to rotors with many blades, such as the traditional windmills with 20 to 30 metal blades that pump water in the United States.
Over time, it was found that the three-blade rotor is the most efficient for power generation by large wind turbines. In addition, the use of three rotor blades allows for a better distribution of mass, which makes rotation smoother and also provides for a "calmer" appearance.
The rotor blades mainly consist of synthetics reinforced with fiberglass and carbon fibers. The layers are usually glued together with epoxy resin. Wood, wood epoxy, and wood-fiber-epoxy compounds are less widely used. One of the main benefits of wooden rotor blades is that they can be recycled.
Aluminum and steel alloys are heavier and suffer from material fatigue. These materials are therefore generally only used for very small wind turbines.
Each manufacturer has its own rotor blade concepts and conducts research on innovative designs; there are many variations that are quite different. In general, though, all rotor blades are constructed similar to airplane wings.
The hub is the center of the rotor to which the rotor blades are attached. Cast iron or cast steel is used.
The hub directs the energy from the rotor blades on to the generator. If the wind turbines have a gearbox, the hub is connected to the slowly rotating gearbox shaft, converting the energy from the wind into rotation energy. If the turbine has a direct drive, the hub passes the energy directly on to the ring generator.
Rotor Hub Enercon E82 at Hannover Energy International Fair, Photo by WWEA
The rotor blade can be attached to the hub in various ways: either in a fixed position, with articulation, or as a pendulum. The latter is a special version of the two-blade rotor, which swings as a pendulum anchored to the hub.
Most manufacturers currently use a fixed hub. It has proved to be sturdy, reduces the number of movable components that can fail, and is relatively easy to construct.
The power that a wind turbine absorbs has to be controlled. If the wind is too strong, power is reduced to prevent damage to the system. There are basically two concepts of power regulation:
Stall control (regulation by flow separation)
Rotor blades with stall control are attached to the hub at a fixed angle. The profile of the rotor blade is designed to cause turbulence behind the rotor blade at a particular wind velocity. At the same time, when the wind is too strong the asynchronous generator also limits power generation automatically. It restricts the speed of the system to the frequency of the power grid, so that the rotor cannot turn faster when the wind blows stronger. In this concept, the rotor blades are designed to cause flow separation at a certain wind velocity (stall), the power input is reduced even though the blades are not themselves pitched.
In active stall control, the pitch of the rotor blades can also be changed. This control system is used mainly in large wind turbines (> 1 MW). When the wind is too strong, the rotor blades are turned into the wind, increasing turbulence.
Active stall regulation allows for power to be regulated more accurately than passive stall regulation does.
Pitch control
This control concept was developed from 1990 up to 2000. Here, each individual rotor blade can be infinitely turned into or out of the wind. The drive for pitch adjustment is either mechanical (for systems with an output below 100 kW), hydraulic (starting at 300 kW), or electric (the most common one, especially for large turbines > 500 kW).
A controller constantly monitors the turbine's power output. If the wind is too strong, the rotor blades are turned out of the wind along their axis, generally only by a fraction of the degree. This reduces the lift, so that the rotor continues to generate power at rated capacity even at high wind speeds.
Heier, Siegfried: Grid Integration of Wind Energy Conversion Systems, John Wiley & Sons, 2006, London
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
BWE Bundesverband WindEnergie, German Wind Energy Association
http://www.wind-energie.de/de/die-technik/