Design and development of small wind turbines - Wind Power Generation and Wind Turbine Design - page 263

Figure 9: Bergey 10kW with tilt-up tower and furling tail (credit: AWEA, Don
Marble).
can be put into three categories: rotor aerodynamics, rotor overspeed control, and
rotor manufacturing considerations.
1.2.1 Rotor aerodynamics
In the early days of grid-connected wind turbines, rotors were usually “stall-con-
trolled”, i.e. maximum power was limited via aerodynamic stall. As wind turbines
grew in size, pitch control has become the universal method to limit power output
during high winds. Stall control is still commonly used on small wind turbines.
Figure 10 shows two power curves illustrating the two types of power limitation.
The reason for the use of stall control is simplicity, and therefore low cost. The
blades can be fi xed to the hub without the need for pitch bearings and a pitch
mechanism. Few small wind turbines feature pitch control.
Using blade pitch to effectively “dump” wind when turbine rated power is
reached is a natural and obvious application of blade pitch. Stall control is simple
and elegant, although somewhat less effi cient. It is typically used on a constant
speed turbine, e.g. one with an asynchronous generator. For example, suppose the
blade tip speed is a constant 100 mph. In light winds the blade angle of attack
would be very shallow, i.e. the wind is coming directly at the leading edge (the
blade pitch angle is only a few degrees). In high winds the blade tip would see
wind coming at it from a much steeper angle, and stall would occur, limiting power
output with no moving parts. There are certain airfoils that exhibit a particularly
useful stalling characteristic (e.g. the NACA44 series) which are commonly used
on stall-controlled small wind turbines.