Horizontal Axis Wind Turbines (HAWT) Horizontal axis wind turbines, also shortened to HAWT, are the common style that most of us think of when we think of a wind turbine. A HAWT has a similar design to a wind mill, it has blades that look like a propeller that spin on the horizontal axis.
Horizontal axis wind turbines have the main rotor shaft and electrical generator at the top of a tower,
and they must be pointed into the wind. Small turbines are pointed by a simple wind vane placed
square with the rotor (blades), while large turbines generally use a wind sensor coupled with a servo
motor. Most large wind turbines have a gearbox, which turns the slow rotation of the rotor into a faster
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. Wind
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, 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 turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines.
For more info:
Power of the wind turbine (efficiency calculation) - download PDF
Types of wind turbines - download PDF