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Is It Always Windy Somewhere? Occurrence of Low-Wind-Power Events over Large Areas

Mark A. Handschy, Stephen Rose, and Jay Apt

As wind power grows from its present 4% market share in the US, knowing how often the wind fails and power must be supplied by other generators becomes important. The statistics of these low prob­ability events have “thin tails”; the wind fails less frequently than would be predicted by a Gaussian distribution. In order to investigate a future in which wind plants are geographically numerous, we examine the occur­rence frequency of low wind-power levels for arrays of wind generators simulated from anemometer data at nine tall-tower sites spread across the contiguous United States. We find that the number of low-power hours per year declines exponentially with the number N of sites comprising the array. Power levels below 5% of total capacity, for example, drop by a factor of about 60, from 2140 h/y for the median single site to 36 h/y for the generation aggregated from all nine sites. The systematic dependence of the low-power duration on both N and on power threshold is in accord with an explanation based on the theory of Large Deviations. Combining this theory for tail behavior with the normal distribution for behavior near the mean allows us to estimate the entire generation duration curve as a function of the number of sites in the array.

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