Basis for Teetering

One of the principal problems involved in designing horizontal axis wind turbines is wind shear, which is the variation of wind velocity with height above ground level. Wind velocities tend to increase with altitude due to aerodynamic surface drag and the viscosity of air. As a result, turbine blades at the top of a rotary path experience higher wind velocities than blades at the bottom of the rotary path. If this vertical wind velocity gradient is not addressed in the design of the wind turbine, then it will subject the components of the wind turbine to damaging stresses during rotary operation. In addition to wind shear due to natural differences in wind velocity with altitude, teetering can also be induced by improper alignment of the main shaft axis, i.e., not placing the axis at the optimal angle with respect to wind direction. Most often, improper alignment results from changes in wind direction. Other sources of teetering include wind turbulence, shadowing from a turbine’s own tower as well as shadowing from neighboring turbines (e.g., for wind turbines located within a wind farm). Additionally, for turbines deployed in aqueous environments, there are significant differences in the flow rate of the water. Typically water at the top of a flowing stream runs faster than water at the bottom of the stream. The lift generated by turbine blades during rotation is applied both in the direction of rotation and in a backward direction. Forces applied in the direction of rotation are designated as in-plane forces and forces applied in a backward direction are designated as out-of-plane forces. Because of this, wind shear will cause more backward force to be applied to blades experiencing the greater effective wind speed. With a rigid hub, the unbalance in backward forces creates a cyclical stress on the blades and bearings that can cause excessive wear and maintenance problems, and can shorten the useful life of the wind turbine generator.

Two Bladed Teetering

One approach for addressing the problems associated with wind shear involves use of a hub incorporating a “teeter pin” that enables a turbine rotor to pivot back-and-forth like a playground seesaw. This back-and-forth rotation results in balancing of the torque on the blades around a teeter axis because blades experiencing higher wind velocity move with the wind and blades experiencing lower wind velocity move into the wind. Such teeter pins are useful as applied to two-bladed wind turbines, as they allow the upper blade to tilt backward while the lower blade tilts forward. Thus, the teetering motion of a two-bladed wind turbine tends to equalize the effective wind speeds for both blades, thereby maintaining a more constant tip speed ratio. The pivotal movement enabled by teeter pins, however, is inadequate to compensate for wind shear in turbines having three or more blades. This is because teetering is limited to one blade moving forward and the other moving backward in an equal and opposite manner across a single rotating teeter axis.

Three Bladed Teetering

[0006] One approach for addressing problems associated with wind shear involves use of a ball-and-socket hub that enables teetering of a turbine rotor with three or more blades, such as described in U.S. Patent Nos. 8,708,654 and 9,194,366.  Finally, a new patent entitled "HUB ASSEMBLY FOR HORIZONTAL AXIS, FLUID-DRIVEN TURBINE ENABLING TEETERING "was filed in December, 2016. The new hub does not rely upon a ball-and-socket design.