Vertical axis wind turbine

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

Vertical-axis wind turbines (VAWTs) are a type of wind turbine where the main rotor shaft is set vertically and the main components are located at the base of the turbine. Among the advantages of this arrangement are that generators and gearboxes can be placed close to the ground, which makes these components easier to service and repair, and that VAWTs do not need to be pointed into the wind. Major drawbacks for the early designs (Savonius, Darrieus and giromill) 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 is "transverse axis wind turbine". For example, the original Darrieus patent, includes both options.

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

A unique, mixed Darrieus - Savonius VAWT type has recently been developed and patented. The main benefits obtained are improved performance at lower wind speeds and a lower r.p.m. regime at higher wind speeds resulting in a silent turbine suitable for residential environments.

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

Five-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

The amount of power, P, that can be absorbed by a wind turbine.

$P=\frac{1}{2}C_{p}\rho A\nu^{3}$

Where $C_{p}$ is the power coefficient, $\rho$ is the density of the air, $A$ is the swept area of the turbine, and $\nu$ is the wind speed.

Advantages of vertical axis wind turbines

VAWTs offer a number of advantages over traditional horizontal-axis wind turbines (HAWTs). They can be packed closer together in wind farms, allowing more in a given space. This is not because they are smaller, but rather due to the slowing effect on the air that HAWTs have, forcing designers to separate them by ten times their width.

VAWTs are rugged, quiet, omni-directional, and they do not create as much stress on the support structure. They do not require as much wind to generate power, thus allowing them to be closer to the ground. By being closer to the ground they are easily maintained and can be installed on chimneys and similar tall structures.

Research at Cal Tech has also shown that carefully designing wind farms using VAWTs can result in power output ten times as great as a HAWT wind farm the same size.

However most of the above claims are considered debatable by people with experience of wind engineering. There is very little history of successful VAWT operation, despite their very long history. During this history they have repeatedly been portrayed as innovative, more efficient, quieter, more bird-friendly, more cost-effective etc whereas there is no factual evidence for any of these claims.

Disadvantages of vertical axis wind turbines

Some disadvantages that the VAWTs possess are that they have a tendency to stall under gusty winds. VAWTs have very low starting torque, as well as dynamic stability problems. The VAWTs are sensitive to off-design conditions and have a low installation height limiting operation to lower wind speed environments.

The blades of a VAWT are prone to fatigue as the blade spins around the central axis. The vertically oriented blades used in early models twisted and bent as they rotated in the wind. This caused the blades to flex and crack. Over time the blades broke apart, sometimes leading to catastrophic failure. Because of these problems, VAWTs have proven less reliable than HAWTs.

Research programmes (in 2011) have sought to overcome the inefficiencies associated with VAWTs by reconfiguration of turbine placement within wind farms. It is thought that, despite the lower wind-speed environment at low elevations, "the scaling of the physical forces involved predicts that [VAWT] wind farms can be built using less expensive materials, manufacturing processes, and maintenance than is possible with current wind turbines".

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.