Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 50

Wind atlases are normally produced with Wind Atlas Analysis and Application
Program (WAsP, a micro-scale model, see Section 4.3) or combined models which
involve the use of both meso- and micro-scale models, and are presented as a col-
lection of wind statistics. The usefulness of these wind statistics depends very
much on the distance between the target site and the stations, the input data they
are based on, the site as well as on the complexity of the area, both regarding
roughness and orography. Typically the main source of information for wind
atlases is meteorological stations with measurements performed at a height of
10 m. Meso-scale models additionally use re-analysis data (see Section 3.1.1).
Care has to be taken since the main purpose of these data is to deliver a basis for
general weather models, which have a much smaller need for high precision wind
measurements than wind energy. Thus the quality of wind atlases is not suffi cient
to replace on-site measurements [1].
Nature itself frequently gives reasonable indications of wind resources. Particu-
larly fl agged trees and bushes can indicate a promising wind climate and can give
valuable information on the prevailing wind direction.
A very good source of information for a fi rst estimate of the wind regime is
production data from nearby wind farms, if available.
No other step in the process of wind farm development has such signifi cance to
the fi nancial success as the correct assessment of the wind regime at the future
turbine location. Because of the cubic relationship between wind speed and energy
content in the wind, the prediction of energy output is extremely sensitive to the
wind speed and requires every possible attention.
2 Wind speed measurements
2.1 Introduction
The measured wind climate is the main input for the fl ow models, by which you
extrapolate the spot measurement vertically and horizontally to evaluate the energy
distribution across the site. Such a resource map is the basis for an optimised lay-
out. The number and height of the measurement masts should be adjusted to the
complexity of the terrain as with increasing complexity, the capability of fl ow
models to correctly predict the spatial variation of the wind decreases. The more
complex the site, the more and the higher masts have to be installed to ensure a
reasonable prediction of the wind resource.
Unfortunately wind measurements are frequently neglected. Very often the
measurement height is insuffi cient for the complexity of the site, the number
of masts is insuffi cient for the size of the site, the measurement period is too
short, the instruments are not calibrated, the mounting is sub-standard or the
mast is not maintained. It cannot be stressed enough that the most expensive
part when measuring wind is the loss of data. Any wind resource assessment
requires a minimum measurement period of one complete year in order to
avoid seasonal biases. If instrumentation fails due to lightning strike, icing,
vandalism or other reasons and the failure is not spotted rapidly, the lost data