A conventional horizontal-axis wind turbine.
Two related concepts would make the nacelle of a horizontal-axis wind turbine considerably lighter and smaller, by removing the gear transmission and generator to a location on the main support and near ground level. This will result in greater reliability because the step-up gear transmission is the main cause of turbine downtime. The generator will also be considerably easier to service, and the generation efficiency will increase because of the elimination of gear losses.
A geared transmission is required in conventional horizontal-axis wind turbines because the optimum blade-tip speed for a two- or three-bladed turbine is about five times the wind speed. Wind turbines generally operate at a maximum wind speed of about 20 m/s (44 mile/h). The blade speed at the tip is around 100 m/s regardless of size. A large turbine rotates slowly and a small turbine has a higher rotating speed even if the blade-tip speed is the same.
The diameter of the rotor of a nacelle-mounted generator is a small fraction of that of the blade tips, perhaps one-tenth or one-twentieth of the diameter. If the generator is directly coupled to the turbine rotor, the generator rotor will have a maximum peripheral speed of 5 – 10 m/s. This low speed requires a large and heavy generator for the specified power output. The optimum peripheral speed of generator rotors is around 100 m/s. To allow a near-optimum generator to be used, a “step-up” transmission of around 20:1 is required. (Some wind-turbine gear transmissions have step-up ratios of over 100.)
Circular shroud around blade tips
One concept that avoids the need for the gear transmission is to mount a circular shroud around the turbine-blade tips. Such shrouds have been proposed previously, for instance in “Energies: A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology” by Yuji Ohya and Takashi Karasudani. In this case, the purpose of the shroud was to produce an increased wind lift force on the blade tips by shaping the shroud to accelerate the flow.
The present concept would not preclude that use, but the principal purpose would be to mount magnets, conductors or high-permeability stacks of iron all the way around the rotating shroud in such a way that they would pass through appropriate stationary devices (stators) to generate current, as occurs in traditional electrical generators.
Single permanent magnet
The second concept has a single permanent magnet or high-permeability-iron stack at the end of each blade of a wind turbine. A “stator” would extend in an arc on two sides of the main turbine column. If the turbine had three blades, the arcs would extend 60 degrees on each side of the column. The arcs would house the stator components of the generator. There would be a continuous torque on the turbine rotor, exerted on each blade in turn. It may be desirable to have a tensioning wire connecting the three tips to avoid vibratory responses.
Advantages and improvements
Both the concepts will result in wind turbines that are more reliable, more efficient (because of the elimination of the gear losses), lower installation and maintenance costs, and lower purchase cost. Both concepts require that the stator components of the generator be mounted on a sleeve surrounding the main turbine pillar so that it is oriented correctly to the plane of the blades. The sleeve could be airfoil shaped and thus would have a lower drag with lower buffeting forces and lower noise.