Image shows a schematic arrangement of a horizontal axis wind machine. Although the common wind turbine with a horizontal axis is simple in principle yet the design of a complete system, especially a large one that would produce electric power economically, is complex. It is of paramount importance that the components like rotor, transmission, generator and tower should not only be as efficient as possible but they must also function effectively in combination.
Horizontal axis wind turbine (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.
Downwind machines have been built, despite the problem of turbulence (mast wake), because they don’t need an additional mechanism for keeping them in line with the wind, and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclic (that is repetitive) turbulence may lead to fatigue failures most HAWTs are upwind machines.
- Variable blade pitch which gives the turbine blades the optimum angle of attack.
- The tall tower base allows access to stronger wind in sites with wind shear.
- High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation.
- The face of a horizontal axis blade is struck by the wind at a consistent angle regardless of the position in its rotation.
- The tall towers and blades up to 90 meters long are difficult to transport.
- Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
- Massive tower construction is required to support the heavy blades, gearbox, and generator.
- Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
- Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.
- Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower’s wind shadow.
- HAWTs require an additional yaw control mechanism to turn the blades toward the wind.