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.