Wind Power

Power generation from the wind

Theoretically available power in the wind can be expressed with the formula

P = 1/2 ρ A v³ (1)

where

P = power (W)

ρ = density of air (kg/m³)

A = area wind passing through perpendicular to the wind (m²)

v = wind velocity (m/s)

Be aware that the density of air decreases with temperature and altitude. Note that the major factor is the wind velocity. A 20% increase in wind velocity increases the power generation with 73%.

Theoretical and rated wind power generation for typical windmills are indicated in the "wind speed-power curve" below. Cut-in wind speed, rated wind speed, shut-down wind speed and rated power for windmills with 20% and 40% efficiency are indicated. 

Actual available Power

can be expressed as

P_a = 1/2 ρ μ A v³ (2)

where

μ = efficiency of the windmill (in general less than 0.4, 40%)

Online Wind Power Calculator

The calculator below can be used to calculate available power from windmills:

Density (kg/m³)

Efficiency

Wind velocity (m/s)

Windmill area (m²)

Generated Wind Energy

The energy generated by a windmill depends on the power generation as indicated above - and how often, or how many hours the wind blows - or more scientifically - the "wind speed frequency distribution" at the actual location.

The total energy generated over a year can be calculated by summarizing the power generation for all velocities (ranging from the actual windmill cut-in speed to the shut-down speed) multiplied with the no. of hours the wind blows at the actual speeds.

A typical wind speed frequency distribution diagram with energy produced at the different velocities are indicated below.

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