Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 51

will falsify the results and as a consequence the measurement period has to
start all over again. Otherwise the increased uncertainty might jeopardise the
feasibility of the whole project.
2.2 Instruments
2.2.1 General
Wind speed measurements put a very high demand on the instrumentation because
the energy density is proportional to the cube of the mean wind speed. Further-
more, the instruments used must be robust and reliably accumulate data over
extended periods of unattended operation. The power consumption should be low
so that they can operate off the grid.
Most on-site wind measurements are carried out using the traditional cup ane-
mometer. The behaviour of these instruments is fairly well understood and the
sources of error are well known. In general, the sources of error in anemometry
include the effects of the tower, boom and other mounting arrangements, the ane-
mometer design and its response to turbulent and non-horizontal fl ow characteris-
tics, and the calibration procedure. Evidently, proper maintenance of the
anemometer is also important. In some cases, problems arise due to icing of the
sensor, or corrosion of the anemometer at sites close to the sea. The current version
of the internationally used standard for power curve measurements, the IEC stan-
dard 61400-12-1 [2], only permits the use of cup anemometry for power curve
measurements. The same requirements for accuracy are valid for wind resource
measurements. Therefore it is advisable to use also these instruments for wind
resource assessment.
Solid state wind sensors (e.g. sonics) have until recently not been used exten-
sively for wind energy purposes, mainly because of their high cost and a higher
power consumption. These have a number of advantages over mechanical ane-
mometers and can further provide measurements of turbulence, air temperature,
and atmospheric stability. However, they also introduce new sources of error which
are less known, and the overall accuracy of sonic anemometry is lower than for
high-quality cup anemometry [3].
Recently, remote sensing devices based either on sound (Sodar) or on laser
(Lidar) have made an entry into the market. Their clear merit is that they
replace a mast which can have practical advantages. However, they often
require more substantial power supplies which bring other reliability and
deployment issues. Also more intensive maintenance is required since the
mean time between failures does not allow unattended measurements for peri-
ods required for wind resource assessment. While the precision of a Lidar
seems to be superior to the Sodar, and often comparable to cup anemometry
[4], both instruments suffer at the moment from short-comings in complex ter-
rain due to the fact that the wind speed sampling takes place over a volume,
and not at a point.
Remote sensing technologies are currently evolving very rapidly and it is
expected they will have a signifi cant role to play in the future.