Basics: Terminology

Available Wind Power

power = ½ x air density x swept area x wind velocity³

• power in watts
• air density = 1.23 kilograms per cubic meter at sea level
• swept area, A=πr²
• wind velocity in meters per second, 1mph = 0.447 meters per second

Betz Limit

The maximum power that can be extracted from the wind is 59.26% of the power available in the wind.

Coefficient of Power cP

The ratio of the amount of power that a wind turbine can extract from the wind to the amount of power that is available in the wind.  It can never be more than the Betz Limit.

Tip Speed Ratio, TSR

The ratio of how fast the blade tips are moving in their axis of rotation compared to the speed of the oncoming wind.   The best TSR range to maximize blade efficiency is 5 to 6.

Start-up Speed

The wind speed at which a wind turbine rotor starts to rotate.

Cut-in Speed

The wind speed at which a wind turbine alternator starts producing electricity, generally a few mph higher than the start-up speed.