Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 235

Today, Darrieus wind turbines are again commercially available, especially for use in
urban areas or even on top of buildings. Most designs have peak powers less than 20 kW and
emphasize characteristics of good performance in wind conditions that are variable in direc-
tion and low in speed. Figure 4-9 shows an example of a commercially available Darrieus
wind turbine with helical blades.
Figure 4-9. Quietrevolution QR5 6-kW Darrieus wind turbine.
Design Improvements in Darrieus VAWTs
The most promising design improvements for Darrieus machines appear to be in the ar-
eas of increasing aerodynamic performance and reducing manufacturing costs. VAWT ana-
lytical codes are available now that yield satisfactory agreement between design calculations
and test data for mean (average) aerodynamic and structural characteristics. One approach
to improving aerodynamic performance is the use of a combination airfoil that has tapered
(or stepped) sections to optimize the local airfoil shape from the upper and lower hubs to
the equator. This change alone may increase energy capture by 0 to 20 percent. Another
approach to increasing energy output which is receiving consideration in the industry is the
use of either a multiple constant-speed or a continuously-variable speed power train. Either
method permits lower cut-in wind speeds (higher cut-in speeds are a problem with Darrieus
VAWTs) and higher energy capture over the wind speed range at a particular site.