Vertical axis wind turbine
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. This arrangement allows the generator and gearbox to be located close to the ground, facilitating service and repair. VAWTs do not need to be pointed into the wind, which removes the need for wind-sensing and orientation mechanisms. Major drawbacks for the early designs (Savonius, Darrieus and giromill) included the significant torque variation during each revolution, and the huge bending moments on the blades. Later designs solved the torque issue by providing helical twist in the blades.
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, US Patent 1835018, 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.
The forces and the velocities acting in a Darrieus turbine are depicted in figure 1. The resultant velocity vector, , is the vectorial sum of the undisturbed upstream air velocity, , and the velocity vector of the advancing blade, .
Thus the oncoming fluid velocity varies during each cycle. Maximum velocity is found for and the minimum is found for , where is the azimuthal or orbital blade position. The angle of attack, , 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:
where is the tip speed ratio parameter.
The resultant aerodynamic force is resolved either into lift (F_L) - drag (D) components or normal (N) - tangential (T) components. The forces are considered acting at the quarter-chord point, and the pitching moment is determined to resolve the aerodynamic forces. The aeronautical terms "lift" and "drag" refer to the forces across (lift) and along (drag) the approaching net relative airflow. The tangential force acts along the blade's velocity, pulling the blade around, and the normal force acts radially, pushing against the shaft bearings. The lift and the drag force are useful when dealing with the aerodynamic forces around the blade such as 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 tangential coefficients are usually normalised by the dynamic pressure of undisturbed upstream fluid velocity.
A = Surface Area R = Radius of turbine
The amount of power, P, that can be absorbed by a wind turbine:
Where is the power coefficient, is air density, is the swept area of the turbine, and 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. They are quiet, omni-directional, and they produce lower forces on the support structure. They do not require as much wind to generate power, thus allowing them to be closer to the ground where windspeed is lower. By being closer to the ground they are easily maintained and can be installed on chimneys and similar tall structures.
Research at Caltech 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.
VAWTs are much quieter than HAWTs, with dB levels at ground level ten meters from the tower measured at around 95 dB for a HAWT – the sound of passing traffic on a busy highway measured from the road shoulder – versus 38dB for a VAWT – the sound of whispered conversation. This is due to several factors, starting with the much lower tip speed of VAWTs.
Disadvantages of vertical axis wind turbines
VAWTs have a tendency to stall under gusty winds. They produce very low starting torque, and exhibit 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 due to the wide variation in applied forces during each rotation. The vertically oriented blades used in early models twisted and bent during each turn, causing them to 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) sought to overcome the inefficiencies associated with VAWTs by optimising 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".
The Windspire, a small VAWT intended for individual (home or office) use was developed in the early 2000s by US company Mariah Power. The company reported that several units had been installed across the US by June 2008.
Arborwind, an Ann-Arbor (Michigan, US) based company, produces a patented small VAWT which has been installed at several US locations as of 2013.
In 2011, Sandia National Laboratories wind-energy researchers began a five-year study of applying VAWT design technology to offshore wind farms. The researchers stated: "The economics of offshore windpower are different from land-based turbines, due to installation and operational challenges. VAWTs offer three big advantages that could reduce the cost of wind energy: a lower turbine center of gravity; reduced machine complexity; and better scalability to very large sizes. A lower center of gravity means improved stability afloat and lower gravitational fatigue loads. Additionally, the drivetrain on a VAWT is at or near the surface, potentially making maintenance easier and less time-consuming. Fewer parts, lower fatigue loads and simpler maintenance all lead to reduced maintenance costs."
A 24-unit VAWT demonstration plot was installed in southern California in the early 2010s by Caltech aeronautical professor John Dabiri. His design was incorporated in a 10-unit generating farm installed in 2013 in the Alaskan village of Igiugig.
Dulas, Anglesey received permission in March 2014 to install a prototype VAWT on the breakwater at Port Talbot waterside. The turbine is a new design, supplied by Wales-based C-FEC (Swansea), and will be operated for a two-year trial. This VAWT incorporates a wind shield which blocks the wind from the advancing blades, and thus requires a wind-direction sensor and a positioning mechanism, as opposed to the "egg-beater" types of VAWTs discussed above.
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-  C-NET, "Vertical-axis wind turbine spins into business" (2 June 2008)
-  Arborwind (website), "History"
-  Renewable Energy World, August 2012, "Offshore Use of Vertical-Axis Wind Turbines Gets A Closer Look"
-  Technology Review, "Will Vertical Turbines Make More of the Wind?"
-  C-FEC (website)
-  Renewable Energy Focus (website), 05 March 2014
-  C-FEC (website), "Turbine"
|Wikimedia Commons has media related to Vertical-axis wind turbines.|
- Cellar Image of the Day Shows a VAWT transverse to the wind, yet with the axis horizontal, but such does not allow the machine to be called a HAWT.