Generally small horizontal-axis wind turbines are kept facing into the wind by a tail. The control mechanism to reduce power in high winds is that the rotor axis is offset from the pivot point, axis of connection to the tower (Figure 6.24). Therefore, there is more force on one side of the rotor than the other, which tries to move the rotor parallel to the wind; however, the wind force on the tail keeps the rotor perpendicular to the wind. For high winds the unequal force on the rotor is greater than the force of the tail; therefore, the rotor moves parallel to the wind. For very small rotors the tail may be fixed, and during medium to high winds with rapid change in direction, sometimes these wind turbines will turn completely around the yaw axis, a 360° revolution. Most of the wind turbines have a hinge for the tail, and for high winds, the rotor moves to a position closer to parallel to the tail, called furling [47-49]. Then when the winds decrease, the tail returns to a position perpendicular to the rotor by a force due to springs or gravity. Dampers, like shock absorbers, can keep this movement from happening too rapidly, both for furling and for restoration to normal operation. The farm windmill uses springs, the length of which is adjustable for the restoring force. One mechanism to use gravity is to have the tail hinge at a slight inclined angle to the vertical plane.
Performance was measured for a 2 kW wind turbine for water pumping for changes in the parameters of the offset of the rotor axis to the yaw axis, length of tail boom, area of tail, and pitch angle . Four different tails and two different yaw axis offsets were tested because the furling behavior was critical to the performance . Overall, nine different configurations were tested, which included two sets of blades with different pitch angles to try to improve the performance at low wind speeds.
The pivot point does not have to be around the vertical axis (yaw); it can also be about a horizontal axis, which would produce vertical furling. The rotor and generator on the North Wind high-reliability turbine had a horizontal pivot for the rotor and generator, a coil spring damper, and the restoring mechanism was gravity. Another horizontal pivot was unique in that the rotor
was downwind and the tail with flat plate and fins hung down (moderate winds to 13 m/s). In high winds the force of the wind on the rotor and also on the flat part of the tail moved the tail and rotor, alternator to the horizontal position, and vertical furling (Figure 6.25). There is no hinge on the tail and the restoring force is gravity.
Small wind turbines are mostly mounted on pole and lattice towers. Very small wind turbines are mounted on almost anything, even on buildings, and of course on sailboats they are mounted on a short pole. A short pole is sometimes referred to as a stub mast.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.