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Vertical-Axis Wind Turbines, VAWTs, are wind turbines that rotate about a vertically-oriented axis that is perpendicular to the wind direction (sometimes termed a “cross-flow” turbine). The significant distinction between a Horizontal-Axis Wind Turbine, HAWT, and a VAWT is the orientation of their rotational axis relative to the wind direction. The HAWT’s propeller type rotor is aligned with its axis of rotation essentially parallel to the direction of the wind and the VAWT’s rotor is aligned with its axis of rotation essentially perpendicular to the direction of the wind.Vertical-axis wind turbines. (VAWTs)

In typical, modern designs, the center axis is a vertical shaft (tower) that is connected to a speed-increasing transmission (gearbox). The transmission’s output shaft, in turn, drives a motor/generator that converts the mechanical torque of the rotor to electrical power. Typical designs, see the figure, include (a) the full-Darrieus (or eggbeater) [also see the first photo], (b)the “H”, (c) the “V” (or “Y” or “sunflower”), (d) the tower-less "V", (e) the “Diamond” and (f) the “Gyromill” configurations. Many additional configurations have been proposed, even some that turn the turbine on its side (a “squirrel cage” configuration) and ones that have no center shaft.

Configurations
VAWTs tend to come in two main configurations: the Savonius and the Darrieus turbines. In the case of the former, power is generated using momentum transfer (a drag device) and, in the latter, using aerodynamic forces (the lift force on an airfoil). The Savonius in characterized by its high torque, low speed and low efficiency (less than half the Betz limit). The Darrieus rotor is characterized by its high speed and high efficiency (approaching the Betz limit). [Betz's law (or limit), developed in 1919 by the German physicist Albert Betz, states that no turbine can capture more than 59.3 percent of the kinetic energy in wind.]

Savonius Turbine


Savonius turbines use rotors that typically have a “bucket” design. These rotors, studied by numerous investigators since the 1920s, have been used extensively in high-torque low-speed applications, i.e., water pumping and ventilation.3 These rotors cannot compete with other configurations on an aerodynamics performance basis, but their ease of fabrication has yielded many applications, particularly in developing countries and do-it-yourself projects. A 2-bucket design reaches maximum efficiencies in the mid-twenties percentage range and a 3-bucket design reaches efficiencies in the high-teens percentage range. These efficiencies are obtained at rotational velocities (at the outside edge of the rotor) that are significantly less than the inflow wind speed. Practically, the efficiency is, at the very best, thirty percent and is only obtained at very low rotation rates. At higher rotational rates, the efficiency of the rotor decreases dramatically.

Darrieus Turbine


The Darrieus configuration was patented by the French inventor Georges Jean Marie Darrieus in France in 1925 and in the US in 1931. This configuration was reinvented in the late 1960s in Canada by Rangi, South and Templin.

The full-Darrieus-rotor VAWT is a high efficiency design whose aerodynamic efficiency approaches the Betz limit, as does its HAWT counterparts. All of its heavy equipment (i.e., the gearbox and the generator) are stationary and located at or near ground level, where they are easy to mount and maintain. This is in contrast to HAWTs where this equipment is mounted at the top of a support tower and moves with the nacelle when the blades are aligned (yawed) with the wind.

By the very nature of the design, the blades of the VAWT move essentially perpendicular to the direction of the wind. Thus, this design works equally well with winds from any direction, enabling it to easily accommodate horizontal and directional wind shear. This is in contrast to HAWTs, which must be yawed into or out of the prevailing wind using aerodynamic forces or a mechanical system and which cannot readily accommodate strong wind shears.

Blade Design:
One of the main perceived disadvantage of the Darrieus-rotor VAWT is that its blades are essentially twice as long as those of a HAWT counterpart of comparable swept area. However, in the traditional full-Darrieus design, the blades are connected to the center tower at both ends; thus, the VAWT blades are loaded mainly in tension and can be made lighter than their cantilevered HAWT counterparts. Also, the blades can be manufactured with constant chord and no twist with only a small effect on the aerodynamic performance of the rotor. When the blades are shaped into a troposkien, their flatwise (radial) bending stresses during operation are reduced to essentially zero and the blades are loaded only in tension, a very favorable loading scenario for composite materials.[''The troposkien shape is the curve formed by spinning a rope with a constant angular velocity and with its ends anchored. Structurally, the rope is under only tensile forces along its length (a rope cannot support bending loads). The troposkien shape varies depending upon the orientation of the axis (vertical or horizontal), due to the action of gravity.'']

In the 1970’s, 80’s and 90’s, these blade characteristics yielded VAWT designs that utilized aluminum blades that were shaped into the necessary airfoil cross section using an extrusion process to minimize manufacturing costs. Unfortunately, in this time frame, the fatigue loads on wind turbines (both VAWTs and HAWTs) were not well understood, and the properties for extrudable aluminum (i.e., 6063 T5) yielded blades with poor fatigue properties. The resulting premature failures of the VAWT aluminum blades, especially at joint structures where large stress concentrations were present, led to the perception that VAWTs were inherently prone to fatigue. In reality, VAWTs are no more prone to fatigue failure than are HAWTs, With the current understanding of fatigue loads, VAWT blades that reliably withstand the fatigue loads imposed upon them can and have been designed.

Struts:
The lighter structural designs of the VAWT blades do lead to large flexures (both static and dynamic) of the blade. In many cases, the blades must be reinforced using struts. While struts provide the necessary stability at minimal capital costs, they may cause a significant reduction in rotor performance by introducing aerodynamic drag at the strut-to-blade joint(s). They should be placed as close to the blade-to-tower joints as possible to minimize this reduction. Aerodynamic fairings around these joints have had mixed results in reducing this joint drag and restoring rotor efficiency.

Self-Starting:
Although most HAWTs are self-starting, the Darrieus-rotor VAWT may or may not self-start, depending upon the wind conditions. Thus, to ensure that a VAWT is started when desired, the turbine must be equipped with a starting system. Typically, this system uses the generator as a motor to rotate the rotor until it has reached sufficient speed to start producing power. Although this is a relatively simple solution to the need for a starting system, it imposes the requirement that the gearbox be bi-directional, thus increasing the capitol costs of the turbine.

Torque Ripple:
A VAWT blade produces positive torque when it crosses the wind and produces little or negative torque when it moves parallel to the wind. Thus, each VAWT blade produces two “pulses” of torque on each revolution. In even-number bladed VAWTs and, in particular, 2-bladed VAWTs, these pulses align, producing a highly variable output torque that approaches a sinusoid with a positive mean. As the gearbox and the generator do not operate well with a highly varying torque, the VAWT power train can be problematic. However, even with the earliest designs, this problem was handled effectively simply by adding compliance (in torque) to the drive train. The Test Bed design included such a coupling.

Rotator Stabilization:
Many VAWT designs use guy-cables as a cost effective technique to stabilize the top of the rotor (obviously not the “H,” “V” and “Giromill” configurations shown in Figure 2). However, this design does present some problems. When guy cables are used, the main support bearing at the bottom of the rotor must be designed to not only support the rotor weight, but also the downward force due to tension in the cables. A thrust bearing is also needed on the top of the rotor to allow for the rotation of the rotor beneath the cables. The required increase in capacity of these bearings due to the cable can contribute significantly to the capital cost of the turbine. Also, the guy cables and their anchors give the turbine a large footprint. For typical land-based installations, this footprint is usually not a problem, but for turbines in farming country, this large footprint can be detrimental. “Spindeless designs,” as shown in Figure 2d, have been proposed to eliminate the large foortprint of a guyed rotor.

Aerodynamic Controls:
Finally, active aerodynamic controls (i.e. variable-pitch blades and aerodynamic brakes) are relatively difficult to implement in VAWT designs. When fitted with conventional, airplane type airfoils, the output power of a fixed-speed VAWT increases monotonically with increasing wind speed. Without proper controls, this output can overdrive the system, leading to a “run-away” turbine that self-destructs. However, modern airfoil designs (for both VAWTs and HAWTs) have yielded airfoils that shed loads at high inflow wind speeds (through a controlled stall of the blade) and alleviate this problem; these have resulted in stall-controlled turbines. Variable speed operation can also be used to reduce excessive output power by lowering the rotation rate of the rotor to prevent it from reaching a “run-away” condition.