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Title:
SODAR measurements in tilted position; Investigation in the use of SODAR in a tilted position as a possibility for use in offshore wind parks
 
Author(s):
 
Published by: Publication date:
ECN Wind Energy 1-4-2003
 
ECN report number: Document type:
ECN-C--03-043 ECN publication
 
Number of pages: Full text:
28 Download PDF  (501kB)

Abstract:
In this project the goal was to establish whether a normal, off-the-shelfSODAR could be used in an offshore wind farm in a tilted position at the base of a turbine. Measurements in tilted position have been carried out before but no valid data was received. Problems encountered in the first measurements were related to the setting of the tilt angle to avoid fixed echoes, and to the fact that the SODAR did not produce any data.

In this project a tilting mechanism has been constructed which allows the tilting of the SODAR between 40 and 70 degrees with a resolution better than 1 degree. This is accurate enough to adjust the instrument in such a way as to avoid fixed echoes from certain tilt angles if possible. The next step was to establish a working set of parameters with which the SODAR does produce data. After some tests this was achieved.

Two different measurements were carried out and compared to the data from the 213 metres high meteorological mast at Cabauw (NL). In general the SODAR produced data in the same order of magnitude as the anemometers fixed to the mast, and the data were also correlated. However, absolute and relative differences were very high.

We also found that fixed echoes in both measurements affected the data (in the direction of 340 degrees) this could not be corrected by adjusting the tilt angle because the reflection was caused by the guy-wires, and these go up to high tilt angles as well. In the direction of 250 degrees the fixed echoes could be avoided by changing the tilt angle but in this direction the measuring beam was at right angles to the wind direction and therefore no good data could be measured either. Because of the vicinity of the mast and trees in all other directions no other tests were possible at the Cabauw site. A more open site would probably give better results. We also saw that the SODAR software did not always detect the fixed echoes and that the fixed echoes also affected data below and above the altitude of the fixed echo.

From a design perspective we have drawn the conclusion that operating an off-the-shelf, commercially available SODAR is probably not going to work at all for a normal wind speed range. Because the SODAR has been designed for operation in a vertical position, measurements with a tilt angle will always be affected and give data with less quality and for a much smaller wind speed range. This will be unacceptable for wind energy purposes. A possibility to use the SODAR close to a meteorological mast has been described, but whether this will also work at the base of a turbine can only be answered after further research.


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