Wind turbines are designed to produce electrical energy as cheaply as possible. Wind turbines are therefore generally designed so that they yield maximum output at wind speeds around 15 m/s. Its does not pay to design turbines that maximise their output at stronger winds, because such strong winds are rare.
In case of stronger winds it is necessary to waste part of the excess energy of the wind in order to avoid damaging the wind turbine. All wind turbines are therefore designed with some sort of power control.
There are a number of options for controlling power aerodynamically. The selection of which of these is used will influence the overall design in a variety of ways. The following presents a brief summary of the options.
Stall control takes advantage of reduced aerodynamic lift at high angles of attack to reduce torque at high wind speeds. Blades in stall-controlled machine are fastened rigidly to the rest of the hub, resulting in a simple connection.
Variable pitch machines have blades which can be rotated about their long axis, changing the blades' pitch angle. Variable pitch provides more control options than does stall control. On the other hand the hub is more complicated, because pitch bearings need to be incorporated.
Some wind turbines utilize aerodynamic surfaces on the blades to control or modify power. These surfaces can take a variety of forms, but in any case the blades must be designed to hold them and means must be provided to operate them. An example is the usage of ailerons. Another option for controlling power is yaw control.
Pitch controlled wind turbines
On a pitch controlled wind turbine the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again. The rotor blades thus have to be able to turn around their longitudinal axis (to pitch) as shown in the picture. Note, that the picture is exaggerated: During normal operation the blades will pitch a fraction of a degree at a time - and the rotor will be turning at the same time.
Designing a pitch controlled wind turbine requires some clever engineering to make sure that the rotor blades pitch exactly the amount required. On a pitch controlled wind turbine, the computer will generally pitch the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximise output for all wind speeds. The pitch mechanism is usually operated using hydraulics.
Stall controlled wind turbines
(Passive) stall controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle. The geometry of the rotor blade profile, however has been aerodynamically designed to ensure that the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade which is not facing the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor. In other words, as the actual wind speed in the area increases, the angle of attack of the rotor blade will increase, until at some point it starts to stall.
If you look closely at a rotor blade for a stall controlled wind turbine you will notice that the blade is twisted slightly as you move along its longitudinal axis. This is partly done in order to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value (other reasons for twisting the blade are mentioned in the previous section on aerodynamics).
The basic advantage of stall control is that one avoids moving parts in the rotor itself, and a complex control system. On the other hand, stall control represents a very complex aerodynamic design problem, and related design challenges in the structural dynamics of the whole wind turbine, e.g. to avoid stall-induced vibrations. Around two thirds of the wind turbines currently being installed in the world are stall controlled machines.
Active stall control wind turbines
An increasing number of larger wind turbines (1 MW and up) are being developed with an active stall power control mechanism. Technically the active stall machines resemble pitch controlled machines, since they have pitchable blades. In order to get a reasonably large torque at low wind speeds, the machines will usually be programmed to pitch their blades much like a pitch controlled machine at low wind speeds (often they use only a few fixed steps depending upon the wind speed).
When the machine reaches its rated power, however, there is an important difference from the pitch controlled machines: if the generator is about to be overloaded, the machine will pitch its blades in the opposite direction from what a pitch controlled machine does. In other words, it will increase the angle of attack of the rotor blades in order to make the blades go into a deeper stall, thus wasting the excess energy in the wind.
One of the advantages of active stall is that one can control the power output more accurately than with passive stall, so as to avoid overshooting the rated power of the machine at the beginning of a gust of wind. Another advantage is that the machine can be run almost exactly at rated power at all high wind speeds. A normal passive stall controlled wind turbine will usually have a drop in the electrical power output for higher wind speeds, as the rotor blades go into deeper stall.
The pitch mechanism is usually operated using hydraulics or electric stepper motors. As with pitch control it is largely an economic question whether it is worthwhile to pay for the added complexity of the machine, when the blade pitch mechanism is added.
Other power control methods
Some older wind turbines use ailerons (flaps) to control the power of the rotor, just like aircraft use flaps to alter the geometry of the wings to provide extra lift at takeoff. Another theoretical possibility is to yaw the rotor partly out of the wind to decrease power. This technique of yaw control is in practice used only for tiny wind turbines of 1 kW or less, as it subjects the rotor to cyclically varying stress which may ultimately damage the entire structure.
- Control system: general for aim and objectives
- Main control concepts (A): variable-constant speed, stall-pitch
- Main control concepts (B): companies choices for control
- Main control concepts (C): more on pitch, stall, active stall
- Pitch control above Vrated (A): explanation with airfoil graph
- Pitch control above Vrated (B): more
- Stall control above Vrated (A): explanation with airfoil graph
- Stall control above Vrated (B): more
- Stall control requires constant RPM: synchronous-induction generators
- Control at grid failure: for pitch and stall control
- Comparison of concepts: table with +/- of concept