Year-to-year Correlation, Record Length, and Overconfidence in Wind Resource Assessment

Interannual variability (IAV) of wind speeds presents a fundamental source of uncertainty in preconstruction wind energy resource assessment. To quantify the role of IAV, we analyze a set of long (62-year) wind-speed records from 60 stations in Canada. Deviations from mean resource levels persist over many decades and make actual uncertainty 2-3 times larger than would be expected using typical wind energy industry resource assessment approaches. For example, performance levels averaged over the last 20 years of each site’s data diverge more widely than expected from the P50 levels estimated from the first 42 years of data: half the sites have either fewer than 5 or more than 15 years exceeding the P50 estimate. In contrast to this 10-year interquartile range, a 4-year range (2.5 times narrower) was found for “control” records where statistical independence was enforced by randomly permuting each station’s historical values. Similarly, for sites with capacity factor of 0.35 and interannual variability of 6%, one would expect 9 years in 10 to fall in the range 0.32–0.38; we find the actual 90% range to be 0.27–0.43, or three times wider. The presence of serial correlations favors a “persistence-forecasting” approach: for this dataset, no improvement in P50 error is gained by using records longer than 4-5 years, and use of records longer than 20 years actually degrades accuracy.