Titel: The sodar as a screening instrument
Författare/Author: Anders Nilsson
Utgivare/Publisher: Department of Earth Sciences, Program for Air, Water and Landscape Science. Uppsala University
Årtal/Year: 2010
Sökord: Sodar, long-term correction, wind measurement, wind power
Rapport via www.w-program.nu/
Sammanfattning/Summary: A site assessment including wind measurements is of major importance when planning a wind farm. Small differences in wind speed can determine if a project will be economically viable since the power in the wind is proportional to the cube of the wind speed. It is also important to have knowledge about the long-term wind climate since the technical lifetime of a wind turbine is about 20 – 25 years.
Traditionally, masts mounted with cup anemometers and wind wanes have been used for wind measurements during site assessments. However, an instrument called sodar which measures the wind based on sound waves has entered the market some years ago and offers some advantages in terms of e.g. mobility compared to the mast equipped with cup anemometers and wind vanes. Since the sodar is a relatively new instrument in the wind energy business, it is important to gain knowledge on how the instrument operates and its performance. To do that, sodar measurements have been compared to wind measurements performed by a cup anemometer and a wind vane.
In order to estimate the long-term wind climate at a site, a long-term calculation is performed based on short-term measurements from the site and a long-term reference wind data. At least one year of wind measurements at the site are needed in order to obtain a valid result from the calculation. However, for an early evaluation of a site it is
desirable to measure the wind for a shorter period of time. Therefore, it has been studied in this thesis how the result of the long-term correction depends on the length of the site measurement period. The seasonal impact on the results has also been studied to distinguish what season is the most suitable for short measurements.
According to the results, the measurements performed by the sodar used in this study are as valid as those performed by the cup anemometer and the wind vane. Considering the advantages with the sodar compared to the mast equipped with cup anemometers and wind vanes, the sodar is the preferable choice.
The results from the long-term correction calculations improve most rapidly during the first four months of measurements. It is therefore important to measure the wind for at least four months. Best results are obtained when the measurements are performed during the spring and the autumn compared to the summer and the winter.