Why do Darrieus vertical-axis wind turbines need an external motor to start?

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Rows of white wind turbines with spinning blades stand on a dry, grassy landscape at a wind farm near Tehachapi, California. — turbine Wind turbines near Tehachapi, California.

The Darrieus turbine is not self-starting because of its aerodynamic design. Unlike other wind turbine configurations, the curved, airfoil-shaped blades of a Darrieus rotor do not generate sufficient torque at a standstill to overcome the inertia of the system and begin rotation when exposed to wind.1

Because the turbine cannot initiate movement on its own, it requires an external motor to accelerate the rotor to a speed where the blades can generate enough lift to sustain rotation and produce power.1