Wind turbines used to generate electrical power are well known in the art and can be divided into two main categories relative to their axis of rotation. Thus, vertical axis wind turbines (VAWT's) have a plurality of blades or airfoils rotating about a vertically extending axis and horizontal axis wind turbines (HAWT's) having the blades or “propellers” thereof rotating about a horizontally extending axis. There exist advantages and disadvantages with each type. Current art HAWT's are generally more efficient at extracting electrical power from a given wind current, where for example, large wind generators in the megawatt range are exclusively of the horizontal type. However, in order to achieve efficient operation a HAWT is desirably mounted on a high tower to lift it and the blades thereof above air obstructions that create turbulent or obstructed wind flow that hinder its performance. A HAWT is also generally limited to two or three propellers which travel at high tip speed, well above the incident wind speed. The rotating shaft typically couples a generator to a gear box, both mounted to the tower and hence difficult to install and service. The propeller blades often use variable pitch to more efficiently harvest a major part of all the energy in the wind over the entire disk aperture swept by the blades As a result thereof, HAWT's can require powered assistance to initiate rotation once a minimal wind speed is sensed.
Conversely, VAWT's can have many individual blades allowing for operation in low wind speeds and do not have to be “facing” into the wind as initiation of rotation can occur from any wind direction. This ability to work immediately from any wind direction and in low wind speeds makes VAWT's more ideally suited for smaller low ground installations. A VAWT then obviates the cost of a high tower and reduces the difficulty of access to various turbine components for replacement or repair. VAWT's therefore hold out the possibility of providing a lower cost wind power alternative and for areas where an HAWT would not be suitable due to lower than required average wind speeds.
However, in VAWT's of the Savonius type the blades only produce power during half of their rotation and fight against the wind during the return half thereof. Thus, such VAWT's can not rotate at a speed greater than the wind speed thereby severely limiting their ability to harvest power from the available wind.
A Darrieus or “eggbeater” type VAWT's are known that produce power in both directions, but suffer from the fact that they often need assistance to get their rotation started.
A third type of VAWT, sometimes referred to as a Gyromill, uses airfoils rotating around a vertical axis that also provide power through each full rotation. Frequently the vanes are designed to provide adequate torque to provide for self start-up at zero rotational velocity, but due to their inherent large amount of drag are limited in the peak attainable velocity. In some designs airfoil orientation can be maximized for start-up and then adjusted for high speed running. It is also known to cycle or vary the angle of attack as the vane moves from the “upwind” to the “downwind” part of the rotational path to improve wind energy harvest performance. However, a major problem therewith concerns the cost, complexity, increased maintenance and reduction of generation efficiency that result from the mechanisms that exist to provide for such airfoil adjustment.
Accordingly, it would be desirable to have a VAWT that can maximize the ability to harvest wind power through airfoil adjustment and do so with a mechanism that is simple, reliable and low in cost.