Vertical-axis wind turbine

The world's tallest vertical-axis wind turbine, in Cap-Chat, Quebec.

Vertical-axis wind turbines (VAWT) are a type of wind turbine where the main rotor shaft runs vertically. Among the advantages of this arrangement are that generators and gearboxes can be placed close to the ground, and that VAWT do not need to be pointed into the wind. Major drawbacks for the early designs (Savonius, Darrieus, giromill and cycloturbine) included the pulsatory torque that can be produced during each revolution and the huge bending moments on the blades. Later designs solved the torque issue by using the helical twist of the blades almost similar to Gorlov's water turbines.

A VAWT tipped sideways, with the axis perpendicular to the wind streamlines, functions similarly. A more general term that includes this option as well is "transverse axis wind turbine." For example, the original Darrieus patent , includes both options.

Drag-type VAWT, such as the Savonius rotor, typically operate at lower tipspeed ratios than lift-based VAWT such as Darrieus rotors and cycloturbines.

General aerodynamics

The forces and the velocities acting in a Darrieus turbine are depicted in figure 1. The resultant velocity vector, ${\vec {W}}$, is the vectorial sum of the undisturbed upstream air velocity, ${\vec {U}}$, and the velocity vector of the advancing blade, $-{\vec {\omega }}\times {\vec {R}}$.

${\vec {W}}={\vec {U}}+\left(-{\vec {\omega }}\times {\vec {R}}\right)$

Fig1: Forces and velocities acting in a Darrieus turbine for various azimuthal positions

File:Tassa 5KW 2 ElectronSolarEnergy2.jpg

5 Kilowatt Vertical Axis Wind Turbine

Thus, the oncoming fluid velocity varies, the maximum is found for $\theta =0{}^{\circ }$ and the minimum is found for $\theta =180{}^{\circ }$, where $\theta$ is the azimuthal or orbital blade position. The angle of attack, $\alpha$, is the angle between the oncoming air speed,W, and the blade's chord. The resultant airflow creates a varying, positive angle of attack to the blade in the upstream zone of the machine, switching sign in the downstream zone of the machine. From geometrical considerations, the resultant airspeed flow and the angle of attack are calculated as follows:

$W=U{\sqrt {1+2\lambda \cos \theta +\lambda ^{2}}}$

$\alpha =\tan ^{-1}\left({\frac {\sin \theta }{\cos \theta +\lambda }}\right)$

where $\lambda ={\frac {\omega R}{U}}$ is the tip speed ratio parameter.

The resultant aerodynamic force is decomposed either in lift (F_L) - drag (D) components or normal (N) - tangential (T) components. The forces are considered acting at 1/4 chord from the leading edge (by convention), the pitching moment is determined to resolve the aerodynamic forces. The aeronautical terms lift and drag are, strictly speaking, forces across and along the approaching net relative airflow respectively. The tangential force is acting along the blade's velocity and, thus, pulling the blade around, and the normal force is acting radially, and, thus, is acting against the bearings. The lift and the drag force are useful when dealing with the aerodynamic behaviour around each blade, i.e. dynamic stall, boundary layer, etc; while when dealing with global performance, fatigue loads, etc., it is more convenient to have a normal-tangential frame. The lift and the drag coefficients are usually normalised by the dynamic pressure of the relative airflow, while the normal and the tangential coefficients are usually normalised by the dynamic pressure of undisturbed upstream fluid velocity.

$C_{L}={\frac {F_{L}}{{1}/{2}\;\rho AW^{2}}}{\text{ }};{\text{ }}C_{D}={\frac {D}{{1}/{2}\;\rho AW^{2}}}{\text{ }};{\text{ }}C_{T}={\frac {T}{{1}/{2}\;\rho AU^{2}}}{\text{ }};{\text{ }}C_{N}={\frac {N}{{1}/{2}\;\rho AU^{2}}}$

A = Surface Area

Advantages compared with horizontal wind turbines

Vertical wind turbines can be packed closer together in wind farms allowing more in a given space than with horizontal wind turbines. This is not due to them being smaller size but rather due to the slowing effect on the air that horizontal wind turbines have, forcing a designer to have to place them ten times their width apart

See also

• List of wind turbine manufacturers

External Links

• Cellar Image of the Day Shows a VAWT traverse to wind, yet with axis horizontal relative to horizon, but such does not admit the machine to be HAWT.