Vertical axis wind power
Unlike horizontal axis wind turbines, vertical axis wind turbines are provided with a rotor whose axis of rotation is generally perpendicular to the flow of the fluid, and therefore very often vertical. The technology has existed since the beginning of the 20th century and brings together different technologies: Darrieus, Savonius, Cycloturbine ...
Darrieus wind turbines are the most efficient and have advantages that make them the technology of the future.
The essential principle of these so-called "vertical axis" systems is to convert the mechanical power of the flow by a movement of a solid, the rotor, which rotates on an axis generally orthogonal to the velocity vector of the air flow. This feature makes the system "isotropic along the Z axis", meaning insensitive to a change in wind direction.
The vertical wind turbines are all made from a structure, a rotor and a conversion system.
Savonius wind turbines, the simplest to manufacture, have the drawback of low efficiency, slow rotation speed, and considerable wind resistance in stormy weather.
Whereas Darrieus wind turbines are fitted with profiled blades, identical to airplane wings, and their efficiency is close to that of modern horizontal axis wind turbines.
Several forms of Darrieus wind turbine D with vertical axis exists: troposkin (the original according to the design of M. Darrieus), in H (straight blades), helical and hyperboloid.
The performance of wind turbines is linked to their aerodynamics.
To be efficient, the blade must move faster than the wind and take advantage of the lift of the profile. In wind power, we speak of TSR (tip speed ratio), equal to the speed of the blade divided by the wind speed. In horizontal axis wind turbines, the speed of the blade depends on the point of the blade considered: the speed is low near the hub and fast at the end. By convention, the TSR is calculated with the speed at the end of the blade, and the best yields are obtained with TSRs of 7. In vertical axis wind turbines, the speed of the blade is identical from the beginning to the end of the blade, the optimum yields are obtained with TSRs of 2.5.
The wind turbine’s noise
The noise is linked to the speed of the blades:
- For horizontal axis wind power (conventional), the tip of the blades goes 7 times faster than the wind. This is the origin of the marginal vortex, which explains the origin of the noise of wind turbines.
- For vertical axis wind turbines (Darrieus), the entire blade progresses at 3 times the wind speed. The technology is silent.
The aerodynamics of Darrieus wind turbines
Wind turbines designed and manufactured by EOLIE are quieter, more aesthetic, and more efficient in changing winds than horizontal axis wind turbines.
The aerodynamics of the blades have been worked on and optimized. The blades are patented, and allow unrivaled levels of performance to be achieved.
During a rotation, the blade will see the relative wind coming from the right (crossing the air flow), then from the left (returning to its position). During this journey, the angle of incidence of the relative wind will vary, and reach incidences which exceed the angle of aerodynamic setback. The phenomenon which is feared in aeronautics and deliberately sought after in vertical axis wind power. This short and transient moment makes it possible to increase the lift of the blade and the torque of the propeller.
EOLIE is one of the only companies to master this phenomenon.