Recent estimates for a 2.0 MW turbine intended for an 80-m tower provide an example of the uncertainty in power predictions based on data from short (60m) met towers. The plots below were calculated from several wind shear models (in the box). Wind shear is a way of estimating higher level winds from lower-level measurements.
The Mean wind speeds (second column) in the table were collected during a site assessment at a particular location.
“The standard deviation in the middle column represents the uncertainty in wind shear,” says WindPole president Steve Kropper.
“The uncertainty in the mean wind speed (Col 3) over the sample set and the resulting uncertainty over the measured power-production time frame are for multiple sensor heights at one location,” he adds. Results indicate that without a sensor at the 80-m height, there would be an additional uncertainty in energy production of roughly 14.1% versus having a sensor installed at the turbine’s hub height. To find the 90% confidence interval (i.e. P90) for the cumulative annual-energy production, wind speed data from the 80m sensor is rescaled several times by randomly sampling errors from a normal distribution of annual mean wind speeds (i.e. the mean wind speed of 6.65 m/s provides a standard error of 5.5%) and applying each newly rescaled time series to the power curve of a sample Gamesa G87 2.0 MW turbine at 80m whose MWh output is summed for the entire year for each sample created.
With regard to the P-90 column, the error is the percent difference in P90 when the sensor at 80m is missing. One must perform vertical extrapolation from 40-m height.
P90 represents the 90% confidence interval in cumulative annual energy production. Only the uncertainty in wind shear is considered for this calculation (others may be present).
“We believe other sites may experience significantly more uncertainty when wind speeds are extrapolated. This research supports earlier findings that actual hub height tall tower data (versus extrapolation from short towers) reduces forecast risk. What’s more, the improved P-90 outlook lets developers negotiate more attractive financing terms,” says Kropper.
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