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Can I increase wind speeds at the site?

Yes and no. Obviously wind is a naturally occurring phenomenon beyond humankind’s manipulation, but there is one thing that can be done to increase wind speeds experienced by a wind turbine at a given location; go upwards by increasing the tower height.

As wind gets closer to the ground it loses some of its speed due to friction as it rubs against any ‘surface roughness’. Surface roughness can be low in areas of wide open ploughed fields or short grassland, or higher over forested areas and more complex terrain with buildings, trees and hedgerows interspersed. A typical UK rural location with a mixture of open areas, some trees and hedgerows and some small buildings would have a ‘roughness class’ of 2. Compare this to open sea that has a roughness class of 0, and cities or dense forest with a roughness class of 4.

The impact of surface roughness on annual average wind speed reduces disproportionately with height, as shown in the figure below. The figure shows how the wind speed increases with height above the ground (based on a site with an annual mean wind speed of 6 m/s at 20 meters above the ground). Wind turbines on shorter towers benefit from tower height increase more than wind turbines on higher towers, though all turbines with hub heights less than 150 metres (which is all currently available wind turbines) will see a significant increase in annual mean wind speed.

Very roughly speaking, for each 1 m increase in the hub-height of a farm wind turbine the annual energy production increases by 0.5%. Therefore it is always financially better to opt for the highest tower available, provided you can get planning consent for the higher tower.

Annual mean windspeed increasing with height - increase wind speed

Annual mean windspeed increasing with height

To use an example, a site that measured an annual mean wind speed of 6 m/s using a 20 metre high met mast would actually have an annual mean wind speed of 7.2 m/s at 59 metres and 7.45 m/s at 72 metres (these are the shorter and taller tower options for an Enercon E53 wind turbine). This would mean an increase in annual energy production of 6% by simply increasing the tower height by 13 m – well worth the modest increase in wind turbine cost!

In summary, an annual mean wind speed at 10 or 20 metres above the ground may not be great for a wind turbine, but when this is extrapolated to the actual operating hub height of commercially available wind turbines it may be much better, and in any case the additional cost of higher towers is always recouped in higher annual energy capture, and therefore income.


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