Stall-control basics

Following on from the airfoil aerodynamics post, here is an overview of the important aerodynamic concepts for stall-controlled wind turbines. Stall control was the first really practical control system for large wind turbines, and was a logical development of the disc-area-regulation concept used on small wind turbines that regulated the power through varying the frontal disc area (typically pitching or yawing the rotor out of the wind after rated speed). A typical stall-control turbine is shown here, which is an NM52-900 that I took a picture of in Spain a few years ago (Pena Amada in Gallicia). It has a 52m rotor diameter, with the blades directly mounted to the hub with a fixed pitch.

The concept of stall control is that the power is regulated through stalling the blades after rated speed is achieved. As the rotational speed of the rotor is effectively constant, the AoA of the blades increase with increasing wind speed. As the wind speed increases the blades begin to stall, the lift drops, and the drag increases to a more inefficient L/D relationship at a higher AoA and thereby reducing the driving torque. There is little control of the blade aerodynamics, which means that the blade is typically designed such that stall occurs at rated wind speed with the most optimal AoA setting (the L/D sweet spot) occurring much earlier (around average wind speed if you are lucky!). You can really see this in the power curve when you have a good look at it (that will be a future post!).

As the blade pitch is fixed, the turbine rotor is quite simple as there is no blade pitch system required (no additional hydraulics, electrics, or pitch bearings!) making the turbine cheap and simple - these are very important characteristics for a turbine in my opinion, as the cost per kW.hr should really be the market driver.

Some of the drawbacks I think with this type are:

• Turbines have reduced efficiency (Cp) towards rated speeds due to the higher AoA (reduced L/D)
  
• Higher thrust coefficient (Ct) due to high AoA means:
• Higher loads, particularly on blades and tower
• Larger wake deficits resulting in greater array losses for the farm
• Increased wake turbulence
  
• Noisier due to higher AoA during operation
  
• Can induce severe vibrations due to transition period around rated speed where blades are coming in and out of stall during each rotation
• Sensitive to the blade's initial pitch setting (must be correctly set to account for local density)
• Sensitive to dirty blades (rated power can be significantly reduced)

A derivative of this control system is the active-stall system, where the blades have a small amount of pitch control to allow power regulation after rated power is achieved; effectively the control system utilises the negative lift-curve slope characteristic of the airfoil in the post-stall condition, increasing the stall will reduce the power. This is a very popular system, and was first introduced on the NEG-Micon 54-950 and has been further optimised for use on the popular NM82-1650. Here's a picture of some NM82-1650 turbines I checked in Australia (Wattle point, if you are really interested!).