Doubling the wind speed delivers 8 times the power

Posted Thursday 22nd of May 2014 at 13:39

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We often get asked which is the best wind turbine for a site with low wind speeds. Sadly such a turbine doesn't exist because a gentle wind simply doesn't contain much in the way of usable energy. The cut-in speed for horizontal axis wind turbines is 3m/s (6.7mph) -- this is the point at which it starts to generate power. Winds below this speed can't be harnessed.

The good news is that as wind speed increases then the power that can be extracted increases exponentially. Doubling the wind speed delivers eight times as much power. We can see how important a small increase in wind speed makes to overall power generation if we look at two sites one with an average annual wind speed of 4m/s and another with an average annual wind speed of 5m/s.

At Site A where the wind speed averages out at 4m/s 31.2% of the time the wind is below the 3m/s start-up speed. At Site B with a wind speed of 5m/s this drops down to 21.3%.

weibull wind speed 4mPerSec
wind speed distribution 5mPerSec

A 2kW wind turbine such as the LE-2000 will produce 2478.52kWh a year on Site B but only 1263.02kWh on Site A where the average annual wind speed is 1m/s less. The result is that Site B will produce twice as much energy as Site A.

Another consideration is the height at which a wind turbine is mounted. Wind close to the ground it loses some of its speed due to friction but increases steadily with height up to about 10 meters so it's advisable to mount the wind turbine as high as possible for a small wind turbine this ideally is at a height of 10m.