Wind Turbine Aerodynamics Part A: Basic Principles - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 343

Figure 5-43. Effect of vortex generators on the power output of the DAF Indal 50-kW
VAWT. [Quinlan 1986]
lower portions of the blades (which are the regions of highest angles of attack) can increase
power output by counteracting the “flattening” of the power curve at high wind speeds.
Starting and Stopping
Darrieus rotors require mechanical power input for starting, although they have been
known to start by themselves (several VAWTs have been lost this way). The usual method
of starting a VAWT is to run the generator as a motor. Motoring the rotor is required only to
bring the VAWT up to a fraction of its rated speed, after which aerodynamic forces are suffi-
cient to bring it up to operating speed. Auxiliary rotors, such as Savonius turbines, were tried
on early VAWTs for starting, but this practice was discarded in favor of motor starting.
The process of stopping a VAWT in the absence of aerodynamic control surfaces is pres-
ently accomplished almost entirely by means of mechanical brakes. Aerodynamic brakes
were attempted on the Canadian Eolé VAWT, but these were not successful, primarily be-
cause of their complexity and the fact that they had to operate reliably in the high acceleration
environment of the rotor equator. Aerodynamic control surfaces such as flaps and air brakes
have not been employed on U.S. VAWTs to date.