Europe gets 11% of its total energy from wind. But just a 10% shift in wind speed can change the amount of energy produced by up to 30%. The fluctuating nature of wind power is reflected in the variability of its price (between 0.03 and 0.16 US$ per kWh) compared to fossil fuel's more predictable prices (between 0.04 and 0.14 US$ per kWh). Differences in local wind conditions are thought to account for much of this variability, with hilly and forest regions often being the greatest loss makers.

Many wind turbines are deliberately placed near escarpments and ridges because these features are associated with increased wind speeds. Energy production at these locations, however, is often severely overestimated because the wind flow around escarpments is more complex than anticipated.

In order to investigate the impact that small changes in terrain can have on wind power output, Jakob Mann from the Technical University of Denmark and his colleagues made small changes to a 1/25 scale model of the well-studied "Bolund peninsula" in Denmark. They measured the resulting changes in wind flow in the Wind Engineering Energy and Environment Dome (WindEEE), a three-dimensional wind-testing chamber at the Western University in Canada. Using modelling clay, the team altered the shape of the edge of the model escarpment from rounded to sharp.

The researchers found that the mean wind, wind shear and turbulence level were all extremely sensitive to the sharpness of the edge of the escarpment. For a wind turbine positioned close to the edge of the escarpment they showed that annual energy production near a sharp-edged escarpment is only 20 to 51% of the round-edge case, and turbulence was increased by a factor of five.

"Steep terrain can give more energy, but also significantly less," said Mann, whose findings are published in Environmental Research Letters (ERL) . "It depends on whether the turbine is within the turbulent 'recirculation zone', where the wind can turn 180 degrees, which typically lies downwind of very steep terrain features."

It isn't just steep escarpments that wind farm developers need to be aware of: any obstacle that disturbs the wind flow, including buildings and trees, will create turbulence and impact wind power generation.

Now Mann and his colleagues are scaling their experiments up, mapping the wind flow out in the field near the Perdigão ridge in eastern Portugal. In the meantime, the scientists advise that wind farm developers take care when planning developments near escarpments and preferably carry out on-site field measurements before installing their turbines.

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