Wind Projects - Site Selection - Rules of Thumb

Estimating annual wind energy output

The forecasting of annual energy generated by a single wind turbine or a wind park at a specific site is a very complex task requiring very much work in advance for wind measurement and the following site evaluation. However, some basic relationships like the increase of wind power with wind speed, the increase in wind speed with height, the dependency of wind power towards air density and general assumptions about the wind speed distribution can be used to create a rough estimation of the annual wind energy output.

The relationship of wind speed to wind power

The power $P$ of a wind stream with the speed $v$ is given by 

$P=1/2\rho AV^{3}$

where $\rho$ is the density of air and A is the swept area by the rotor of the wind turbine. Using air density at sea level $\rho$=1,225 kg/m³ the relationship can be expressed as $P=0,6125AV^{3}$. The power in the wind stream is influenced by the wind speed with a cubic exponent. This means even a small increase in wind speed substantially raises the power in the wind and stresses the need for a good estimation of wind speeds choosing a new site for a turbine. The increase in power caused by a certain increase in wind speed can be easily estimated: Leaving the influence of air density and rotor area apart assume a site with a wind speed $v_{1}$ of 10 m/s and another site with an average wind speed $v_{2}$ of 12 m/s. The difference in wind speed is only 20 % or $v_{2}/v_{1}=12/10=1.2$. Nevertheless this relatively small increase in wind speed results in bigger increase of wind power:

$P_{2}/P_{1}=(V_{2}/V_{1})^{3}$

$P_{2}=1,2^{3}P_{1}=1,73P_{1}$

The wind power at the site with an average wind speed of 12 m/s is almost 70 % higher than the wind power at the other site.