An example of a drag-based wind turbine.

The basic classification of wind turbines is based on the principle used to extract kinetic energy from the moving air.

Drag Turbines

Some low-output wind turbines use mainly the drag principle. A surface exposed to the wind experiences a drag force, which can be used to produce rotation of a shaft. Drag-based wind turbines typically achieve power coefficients of around 10—20%, depending on their design.

An example of a lift-based wind turbine.

Lift Turbines

Larger, modern propeller-type turbines are based mainly on the lift principle. Their rotor blades have an aerodynamic profile, and the airflow around them produces a pressure distribution that generates lift. The magnitude and direction of this force depend strongly on the angle of attack, defined as the angle between the blade chord and the relative airflow. The lift force acts approximately perpendicular to the relative airflow, and its tangential component produces the torque that turns the rotor. Modern lift-based wind turbines can achieve power coefficients of around 40—50%, depending on operating conditions and rotor design.

An example of a vertical-axis wind turbine.

Horizontal or Vertical Rotor Axis

Another way of classifying wind turbines is by the orientation of their main rotor axis, which can be horizontal or vertical. A horizontal axis is typical of modern three-bladed lift-based turbines. The main drivetrain components, including the generator, are usually housed in a nacelle at the top of a tall tower, making maintenance more demanding. A yaw system turns the nacelle so that the rotor faces the wind. In some vertical-axis wind turbines, the generator and other drivetrain components can be located near ground level. These turbines do not need a yaw system to face the wind, but some designs, such as certain Darrieus turbines, may require assistance to start rotating.

The tip speed ratio λ (lambda) is the ratio of the circumferential speed of the blade tips to the wind speed. Different turbine designs operate most efficiently at different tip speed ratios. As a general guide, turbines with a tip speed ratio below about 6 can be considered low-speed, while modern high-speed turbines typically operate at values of 6 or more. In general, rotors with more blades tend to operate at lower tip speed ratios, rotate more slowly and produce higher torque.