Alarming Slowing in Winds Could Lessen Efficiency of Wind Turbines!!
October 18, 2010 1 Comment
Increasing amounts of vegetation could be causing up to 60% of a slowing in wind speed across the Northern Hemisphere, according to researchers analyzing three decades of wind-speed data in Nature Geoscience today.The decline is a potential concern for wind-turbine efficiency. But researchers can’t tell whether the effect, an average 10% slowdown, will make much difference to wind turbines; the slowing winds measured are are at 10 metres above the ground, whereas turbines operate at 50–100 metres up, where there is little global data. Several previous regional studies looking at the United States, Australia, China and parts of Europe have shown decreasing wind speeds just above the planet’s surface. Climate change, afforestation and urban development had been mooted as possible causes.
People always said the data were rubbish. There was no quality-controlled global archive of data. Researchers collected data from about 10,000 weather stations, although they removed all but 822 stations from their list because of incomplete records. That left records stretching back to 1979, taking in Europe, central Asia, eastern Asia and North America. Researchers expected a study spanning such a large area to show speeds increasing in some areas and decreasing in others. Annual wind speeds had declined at 73% of the stations, dropping by 5–15% over almost all of the land areas examined. The most pronounced effect was seen across Eurasia. The researchers also found that stronger winds have been affected more than weak ones.
Hot air and greenery
Changing patterns of atmospheric circulation at high altitudes have previously been blamed for slowing wind speeds- as air temperatures and pressures in different parts of the atmosphere shift with a gradually heating Earth. But Vautard and his team say that only 10–50% of the change they observed could be explained by current understanding of how changes in high-altitude circulation affect surface wind speeds. Another factor that has been suggested is thriving vegetation. More abundant and taller plants increase the ‘surface roughness’ of the ground, absorbing some of the wind’s energy and slowing it down. Surface roughness is a factor in climate models used by weather predictors such as the Met Office, but it had not been thought to effect wind speed over such a wide area. Increased vegetation-resulting from ex-agricultural land becoming overgrown, afforestation and changing landscape-management practices-could account for 25–60% of the observed stilling in wind speed, says Vautard. The researchers used satellite images to estimate increases in the volume and height of vegetation, and computer climate models to see how this would affect wind speed. Vautard said:
We need further investigation of whether afforestation affects wind speeds, but it’s almost certainly having an effect.
Vegetation changes did not wholly explain wind-speed changes seen in eastern Europe and China. In central Asia the height of vegetation would have to have tripled to account for the stilling. Vautard admits this is “fairly unrealistic”. He suspects that changes in general atmospheric circulation may be more important in these parts of the world than in others. Golding says that changes in vegetation could explain the drop, but adds that increases in urban density, which would also absorb wind energy and increase surface roughness, could be having a similar effect. Vautard admits that urbanization could be an important factor, but says he and his team were unable to investigate its effects, because estimates to plug into computer models of how buildings affect surface roughness are not available.
[Source: Nature and Vautard, R., Cattiaux, J., Yiou, P., Thépaut, J.-N. & Ciais, P.Nature Geosci. advance online publication doi:10.1038/NGEO979(2010).]