Characterization of Wind Power Resource in the United States and its Intermittency
by Gunturu, U.B. and C. A. Schlosser (December 2011)
Joint Program Report Series, 65 pages, 2011
(Superseded by Reprint 2012-23)
Wind resource in the continental and offshore United States has been reconstructed and characterized using metrics that describe, apart from abundance, its availability, persistence and intermittency. The Modern Era Retrospective-Analysis for Research and Applications (MERRA) boundary layer flux data has been used to construct wind profile at 50m, 80m, 100m and 120m turbine hub heights. The wind power density estimates at 50m are qualitatively similar to those in the US wind atlas developed by the National Renewable Energy Laboratory (NREL), but quantitatively a class less in some regions, but are within the limits of uncertainty. The wind speeds at 80m were quantitatively and qualitatively close to the NREL wind map. The possible reasons for overestimation by NREL have been discussed. For long tailed distributions like those of the wind power density, the mean is an overestimation and median is suggested for summary representation of the wind resource. The impact of raising the wind turbine hub height on metrics of abundance, persistence, variability and intermittency is analyzed. There is a general increase in availability and abundance of wind resource but the there is an increase in intermittency in terms of level crossing rate in low resource regions. The key aspect of geographical diversification of wind farms to mitigate intermittency - that the wind power generators are statistically independent - is also tested. This condition is found in low resource regions like the east and west coasts. However, in the central US region which has rich resource the condition fails as widespread coherent intermittence in wind power density is found. Thus large regions are synchronized in having wind power or lack thereof. Thus, geographical diversification in this region needs to be planned strategically. The annual distribution of hourly wind power density shows considerable variability and suggests wind floods and droughts that roughly correspond with La-Nina and El-Nino years respectively. The collective behavior of wind farms in seven Independent System Operator (ISO) areas has also been studied. The generation duration curves for each ISO show that there is no aggregated power for some fraction of the time. Aggregation of wind turbines mitigates intermittency to some extent, but each ISO has considerable fraction of time with less than 5% capacity. The hourly wind power time series show benefit of aggregation but the high and low wind events are lumped in time, thus corroborating the result that the intermittency is synchronized. The time series show that there are instances when there is no wind power in most ISOs because of large-scale high pressure systems. An analytical consideration of the collective behavior of aggregated wind turbines shows that the benefit of aggregation saturates beyond ten units. Also, the benefit of aggregation falls rapidly with temporal correlation between the generating units.
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