sin αst

CONCEPTUALISATION
The combination of complex geometry and airfoils make the Darrieus turbine moderately
difficult to manufacture. In addition to the previously described pulsating loads, many
Darrieus designs have resonant modes at particular occurring rotational speeds, which
make the blades prone to structural fatigue problems [9, p. 4]. Another key disadvantage of
the classic Darrieus design is that the rotor is unable to start itself due to a low starting
torque and therefore requires external excitation. Both active and passive solutions to the
problem have been developed, but none of them have been proven beyond the research
stage [24].
Both the self-starting and the fatigue issues are improved when using a helical version of
the Darrieus turbine, see figure 4.12.
Figure 4.12: Helical Darrieus wind turbine [25]
Using a helical twist of 60 degrees has shown to spread the torque evenly over the entire
revolution, thus preventing destructive pulsations and furthermore enabling self-start of
the rotor [26, p. 94]. The helical version is commonly used in urban settings due to its ap-
pealing aesthetics and proclaimed low noise [59].
� Giromill
The patent that Darrieus filed in 1927 also covered other possible arrangements using
vertical airfoils. One of these is the Giromill or H-rotor that can be seen on figure 4.13.
25