Wind Speed Variability

Short term variability of the wind: 
Turbulence

Seeing that the wind speed is always changing, it is easily to understand that the energy content of the wind is also always varying. But many factors influence these variations, like the weather, the obstacles or the local surface conditions.

As experienced during hailstorms or thunderstorms when wind speed and direction both change very frequently because of gusts, turbulence can also be created in areas with a very uneven terrain surface or behind obstacles such as buildings or trees. Wind flows are very irregular, often in whirls in the neighbourhood.

Turbulence decreases the possibility of using the energy in the wind effectively for a wind turbine. It also imposes more tear and wear on the wind turbines which is why towers for wind turbines are usually tall enough to avoid turbulence from the wind close to ground level.

Diurnal (night and day) variations of the wind

Generally, wind speeds are higher during the day than during the night. This phenomenon is mainly due to the temperature differences between the sea and the land surfaces generated by the solar activity. The wind is also more turbulent and tends to change direction more frequently during the day than at night. But electricity consumption being higher during the day, the fact that most of the wind energy is produced during daytime represents a great advantage.

Seasonal Variations of the Wind

As we can see on the graph, wind speeds vary during the year as the weather changes from summer to winter.

As example, in temperate zones, winter winds are usually stronger than summer winds. But electricity consumption is also higher in winter than in summer. The use of wind turbine seems therefore particularly well adapted, seeing that wind electricity generally fits well into the electricity consumption pattern.

Annual Variation in Wind Energy

Wind patters may vary from year to year. But these variations are generally quite small. In the case of Denmark, the output from wind turbines typically varies of from 10 percent.

This graph represents the variations of wind speeds with height in the roughness class 2, assuming that the wind speed is 10m/s at a height of 100 meters. The fact that the wind speed decreases as we move closer to ground level is called wind shear. This phenomenon is crucial in the design of a wind turbine. Indeed, considering a wind turbine with a hub height of 40 meters and a rotor diameter of 40 meters, the previous graph shows that the wind blows at 9.3 m/s when the tip of the blade is in its uppermost position, whereas it blows at only 7.7 m/s when the tip is in the bottom position. Therefore, the forces acting on the rotor blade when it is in its top position are far larger than when it is in its bottom position.

The wind speed at a certain height z above the ground is given by:

where v is the reference speed, i.e. a wind speed we already know at height z.

z is the roughness length in the current wind direction ( height above ground level where the wind speed is theoretically zero).

z is the reference height, i.e. the height where we know the exact wind speed v ref.

To give an example, assuming a roughness length of 0.1m a wind speed of 7.7 m/s at 20 m height, the wind speed at 60 m height will be:

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