Environmental Engineering Reference
In-Depth Information
preliminary attempts to simulate it with a mass-consistent numerical flow model have been
reported by Phillips [1979], and Wendell [1984], The subject of wake modeling is treated
in more detail in Chapter 6.
Veenhuizen and Lin [1987a] have proposed a set of computer models that estimate
wind flow over complex terrain and account for wake interference on the productivity of
individual turbines. For verification, measured energy output from 50 turbines in a 275-
turbine wind power station was compared with predicted output based on an earlier micro-
siting study in which kite anemometers were used to survey the wind field over the station.
Monthly energy production was used to calculate an effective monthly mean wind speed
for each turbine site, and these were found to be about 10 percent less than the wind speeds
determined by the micrositing study. Errors in power output were found to be about twice
the size of errors in estimating wind speed, on a percentage basis.
Characterizing Wind Flow Over a Large Area
The so-called field measurement method of determining the spatial variation of the wind
resource across a wind power station is currently the procedure generally followed by
private wind power developers. This method consists of subjectively interpolating between
on-site measurements of the wind speed at several locations to estimate wind speeds at
potential turbine sites. Two alternative approaches are wind tunnel physical models and
computerized numerical models , but these have not yet been used extensively by private
industry.
The Field Measurement Method
A very complete description of the field measurement method for micrositing of wind
turbines is given by Wade and Walker [1988]. A basic assumption for determining spatial
variations in the wind resource is that power-producing winds are associated with a persis-
tent set of characteristic meteorological conditions such that when these conditions occur,
the wind field is spatially well-correlated. It is operationally assumed that the wind flow
pattern is fixed and that the temporal variation in speed can be scaled locally with speed,
once the wind direction at a given reference point is known from the prevailing wind direc-
tion. Ratios of wind speed measured simultaneously at separate locations on the terrain are
constants under this assumption.
The general procedure of the field measurement method is to establish the wind speed
ratios between the short-term (4 to 12 months) data from on-site anemometers and an
existing long-term data base (several years) from a nearby reference station. Typically, two
to three short-term anemometer stations are located per square kilometer, although the trend
recently has been toward twice that density. Correlation of wind speeds between short- and
long-term data records is obtained by linear regression analysis. For large-scale turbines,
it is economically justified to utilize one anemometer per turbine site for at least one year.
Kite anemometer measurements can be made with commercially available equipment
to provide additional definition of the spatial variation of the wind resource to supplement
the fixed anemometer data. Kite anemometers typically add approximately 60 to 80 esti-
mates of wind speed and direction to the data already available from fixed anemometers.
About one kite measurement per 20,000 to 60,000 m 2 is recommended, depending on the
steepness of the terrain. Several kite anemometers are normally flown simultaneously in
relatively close proximity, and one is always located near a fixed anemometer which acts
as a reference for calibration. Data records are about 20 min long, and three or more flights
are generally made per kite location on different days. Kite elevation is usually the same
as that of the center of the swept area of the prospective turbines.
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