TURBINE OPERATION/ENERGY OUTPUT

Power density function

In the page Energy of wind it was shown that the energy potential per second, the power, varies in proportion to the third power of the wind speed and in proportion to the density of the air, its weight per unit of volume.

Combining things from previous pages it comes that if the power of each wind speed will be multiplied with the probability of each wind speed from the Weibull graph, then the distribution of wind energy at different wind speeds may be calculated. This distribution is called the

Notice, that the previous Weibull curve changes shape, because the high wind speeds have most of the power of the wind.

The graph above was drawn using the Wind Turbine Power Calculator of this web site. The following different areas may be distinguished:

- The area under the grey curve (all the way to the axis at the bottom) gives the amount of wind power per square metre wind flow we may expect at this particular site. In this case there is a mean wind speed of 7 m/s and a Weibull k=2, so it is 402 W/m

- The area under the blue curve tells how much of the wind power can be theoretically converted to mechanical power. According to the Betz' law, this is 16/27 of the total power in the wind.

- The total area under the red curve tells us how much electrical power a certain wind turbine will produce at this site. This will be shown in the page Power curve.

The most important thing to notice is that the bulk of wind energy will be found at wind speeds above the mean (average) wind speed at the site. This is not as surprising as it sounds, because as already explained, high wind speeds have much higher energy content than low wind speeds.

Usually, wind turbines are designed to start running at wind speeds somewhere around 3 to 5 m/s. This is called the

The wind turbine will be programmed to stop at high wind speeds above, say 25 m/s, in order to avoid damaging the turbine or its surroundings. The stop wind speed is called the