A significant uncertainty in the estimate of energy from a wind plant is the interannual and interseasonal variation in energy output. These estimates are typically based on wind data that have been collected on-site for one or more years. Rarely have the wind data records exceeded three years. Variations in the mean annual wind speeds have been assumed to be ±10% of the long-term mean wind speed with a 90% confidence interval. This assumption, however, is based on previous work using long-term airport wind data and it has not been verified at high wind locations. In addition, most of the strong wind sites that have been developed for wind energy display distinct seasonal variations in the wind speed. In the Altamont and San Gorgonio Passes about 80% of the energy is produced during the spring and summer months. If the winds are abnormally weak during any of these months (particularly May-August), the annual energy production can be significantly reduced. Similarly, stronger than normal winds during this time period can greatly enhance the annual production. Over 10 years of hourly wind data have been collected at several moderate-to-high wind sites in the Pacific Northwest for the Bonneville Power Administration. This data base is used to determine the year-to-year and the interseasonal variations in the mean speeds and in wind tirbine energy production. Wind turbine power curves are used with the speed frequency distributions to produce seasonal and annual energy estimates. © 1990.
