Wind Turbine Aerodynamics Part A: Basic Principles - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 342

Figure 5-42. Typical effect of rotor solidity on the power coefficient of a VAWT. Rotor
shape is that of the Sandia 17-m rotor, but blade chord is a variable.
A second (and smaller) effect is caused by changes in Reynolds number . At a constant
rotor speed, increasing the blade chord directly increases local Reynolds numbers. When the
Reynolds number is less than one million, increases in performance with increases in Reyn-
olds number are most noticeable.
Vortex Generators
Vortex generators have been employed on VAWTs as well as HAWTs to improve per-
formance. The test power curves shown in Figure 5-43 are from three tests run on the DAF
Indal 50-kW VAWT [Quinlan 1986]. The blades on this rotor were approximately 9 m long.
The first test, without vortex generators, was made to establish the basic power curve of the
turbine. A second test was made with vortex generators installed over 3 m of the equatorial
section of each blade, on both surfaces. Finally, a third test was run with vortex generators
placed over the entire blade length, again on both surfaces.
As shown in Figure 5-43, the basic power curve ( i.e. , without vortex generators) exhibits
the same flattening at high wind speeds as was observed during the slowest test on the Sandia
17-m VAWT discussed earlier (Fig. 5-38). As a result of placing vortex generators only on
the equatorial sections of the blades, there was a reduction in power output over the entire
range of wind speeds. At the highest wind speed in the test (22 m/s) the reduction reached
over 20 percent. Utilization of vortex generators over the entire blade resulted in a small
decrease in power output under low wind conditions, but a substantial increase in power
output at high winds. The tip-speed ratio at the transition from power loss to power gain was
about 5.
The conclusion drawn from these tests is that the vortex generators on the equator sec-
tions of a VAWT rotor will reduce power output, while vortex generators on the upper and