On turbines with stall regulation, the blades are rigidly fixed to the hub and are designed so that the airflow over them causes a loss of lift and an increase in drag in high winds; this limits the power output. The disadvantage of stall regulation is that the maximum power output may depend on the air density and on the surface roughness of the blade. On pitch regulated turbines, the blades are mounted on the rotor hub and can twist around their longitudinal axis. In high winds the pitch setting of the blades is continuously adjusted by reducing the angle of attack, thus reducing the blade lift to yield the maximum power specified. This requires a sensitive active regulation system, as small changes in blade angle can produce large changes in power.
With the Bonus CombiStall, bearings are inserted between the blades and the rotor hub, as with any other pitch regulated system, thereby allowing adjustment of the pitch setting during operation. Actual power limitation in high winds, however, is achieved by pushing the blades further into stall -- increasing the angle of attack instead of reducing it. As blade torque is less sensitive to changes in angle of attack beyond the stall, power control is easier.
Power control by pushing blades into stall is not new. Howden of Scotland used the concept (with partial span pitch control) over ten years ago. The fear has always been, however, that instabilities might arise due to the turbulent nature of the flow beyond the stall. In practice, though, it is being proved that turbine blades operate at a range of angles of attack along their span and this tends to smooth out any fluctuations. Bonus is sending a clear message that it can be done.