Yuyu Zhou and Steven Smith from the Pacific Northwest National Laboratory looked at the spatial and temporal patterns of global onshore wind-speed distribution and found that there are large variations between regions.

"While average wind speed is an important parameter for calculating wind-power potential, it does not give the whole picture," Zhou told environmentalresearchweb. "We found that different areas of the world have different wind-speed frequency distributions, and ignoring these differences means that inaccuracies will be introduced into wind-power potential calculations."

Zhou and Smith used Weibull distributions to describe the wind-speed distributions of different regions around the world. For example, two regions could have the same average wind speed but different Weibull k parameters. A Weibull k parameter of 1 indicates a region with a flatter distribution and a larger range of wind speeds, while a Weibull k value of 4 indicates a tall distribution with relatively steady wind speeds.

"When calculating wind power potential, the default Weibull k parameter used is 2," explained Smith. "But we have found that, while there are not many regions of the world with k=1 or k=4, there are a few, and knowing which k value to use makes a big difference when calculating wind power potential."

The researchers found, for example, that in some areas of Africa the wind-power potential could be overestimated by as much as 35% if the default k value was used. "We were surprised by the large spatial variations in frequency distributions that we found," said Zhou. "For example, the wind-speed frequency distribution for most areas of Australia is a totally different shape (taller) than for Asia (flatter)."

These variations become more important at higher mean wind speeds, with the error in calculations increasing as the mean wind speed increases. In areas of mean wind speeds of around 6 m/s, the difference between generated power (MW) calculations that use k=1 and k=5 is negligible. But in areas where the mean wind speed is 9 m/s or more, estimations for generated power can vary by as much as 100%, depending on what k value is used.

"Our work shows that wherever you are in the world it is important to know what k value to use in your calculations of wind-power potential, and not to simply assume that the wind-speed distribution is a typical Rayleigh distribution (k=2)," said Smith.

Zhou and Smith reported their findings in Environmental Research Letters (ERL).

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