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Wind profile assessment for wind power purposes

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Title: Wind profile assessment for wind power purposes
Author(s): Sointu, Iida
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Discipline: Meteorology
Language: English
Acceptance year: 2014
Preliminary estimation of wind speed at the wind turbine hub height is critically important when planning new wind farms. Wind turbine power output is proportional to the cube of wind speed which means that even small uncertainties in wind speed estimation will greatly affect the estimated energy yield. Wind resource estimation is usually based on wind measurements using meteorological masts and SODAR (SOnic Detection And Ranging) instruments. Modern wind turbine hub height typically ranges from 100 to 140 meters. Wind speeds measured with a mast must often be extrapolated to turbine hub height, while SODAR measures a continuous wind profile up to approximately 200 meters height. The goal of this study is to assess the uncertainty of SODAR measurements and to analyse the vertical wind profile variability in Finnish conditions from a wind power perspective. Mast and SODAR data collected at 17 sites across Finland covering a total of 381 months of measurements was available for this study. Both the amount and type of equipment at the sites as well as temporal data coverage varied greatly among the sites. Both coastal and inland sites were represented. Mast and SODAR data were quality controlled using manual inspection. The main reason for this was to identify periods with anemometer freezing, which results in erroneous wind data. Quality controlled data was used to calculate various parameters utilized in the wind resource assessment such as annual wind speed, wind shear represented by the power law exponent (alpha), atmospheric stability category and turbulence intensity. SODAR uncertainty was studied using the difference of wind speed measured by co-located mast and SODAR in relation to wind speed, humidity, alpha and turbulence intensity. Wind profile variability was studied with emphasis on hub height wind speed extrapolation, particularly in terms of profile shape and wind speed magnitude. Based on the results, the most common stability categories are neutral and slightly stable, which are related to relatively strong wind shear. Wind profiles were found to vary significantly with season. Therefore wind speed extrapolation should not be performed based on seasonal data when aiming for statistical representativeness of the wind resource. As a result of this study a statistical method to account for the seasonality is presented. SODAR was observed to overestimate horizontal wind speed in rainfall as well as in high wind speed and weakly turbulent conditions. Due to these tendencies, conducting short SODAR measurement campaigns in which the afore-mentioned conditions prevail is not recommended. As winters have generally higher wind speeds in Finland, short wintertime SODAR campaigns are discouraged. A verification measurement period for mast-SODAR intercomparison ought to be carried out before using SODAR as a standalone instrument. This should be conducted close to a meteorological mast and the period should include as much meteorological variability as possible. This would ensure some level of certainty in SODAR measurements, especially in the absence of an acknowledged calibration standard.

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