Horizontal Axis Wind Turbines (HAWT)

The Horizontal Axis Wind Turbine (HAWT) is the most efficient design for turning wind into electricity. The basic design allows two or more rotor blades to face into the wind. Since they are all being simultaneously moved, they form the least possible resistance to wind forces.

A Horizontal Axis Wind Turbine (HAWT)

The rotor blades of a Horizontal Axis Wind Turbine usually have an aerodynamic design. On a wing or rotor blade, the top side of the blade has a longer surface area than the bottom. When the air moves over the top of the blade, the air must move faster than the air going under the bottom of the blade. This higher speed creates lift because the denser underside air pushes against the blade. The blades are hooked to a shaft so the lift on the blade forces the shaft to spin.

HAWT designs vary in size from the very small to the commercial grade generators. Most are installed on towers to help them reach high above the ground where airflow is strongest and most constant. Even so, the larger the rotors the more likely there is to be varying forces on different parts of the turbine. This means that HAWTs must be rugged and well supported right down to the foundation. Also, at very high wind-speeds, generators and rotors can be damaged, so in most windmills safety features are built in that allow the rotors to not take so much wind and even to disconnect the rotors from the generator. Braking systems are also used to slow turbines in very high wind-speeds1. Another method of avoiding damage in high winds: some wind generators are designed to tilt back at higher wind-speeds so that the rotors become less efficient, thereby creating less stress.

Smaller turbines generally spin at faster rpm speeds than the large commercial models. The larger models are slowed to avoid killing birds and also because higher speeds create greater wear and tear on larger devices.

Most of the wind generating devices available for both home and commercial use are Horizontal Axis Wind Turbines. However, Vertical Axis Wind Turbines are also coming to their own.

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