Environmental Engineering Reference
In-Depth Information
The data should normally be entered for the hub height of the turbine to be modeled.
While most software allows the data to be projected from a different height to the hub
height, this means that any dependence of the wind shear on time of day and direction
and other factors is lost. For reasons discussed in Chapter 11, this can produce errors in
the frequency distribution of wind speeds at hub height and therefore in the estimated
energy production.
It is very important to enter the mast coordinates as precisely as possible and to
verify the position against the topographic data and landmarks within the software.
Along with the modeled WRG (described in the next section), the model uses the
mast position to extrapolate the observed wind resource to the turbines. In complex
terrain, even a small error in the position can produce large errors in the estimated
energy production.
16.3.2 Modeled Wind Resource Grids
The second key piece of resource information required by wind plant design software
is a WRG. This is almost always generated by a numerical wind flow model of some
kind. The conventional WRG format (made nearly universal by the WAsP software) is
a text file containing predicted frequencies and Weibull parameters ( A and k ) for each
of the 12 or 16 direction sectors for every point within the project area. An example is
shown in Figure 16-4. The points are typically spaced anywhere from 10 to 50 m apart
in a regular grid, providing good definition of the spatial variation of the wind resource.
A related file that is usually required is called a point WRG . This is the modeled
wind resource at the precise location of each mast, in a format very similar to a WRG
file but with a single line of data. Since masts are not generally found at exactly the
center of a grid point, the point WRG values usually differ from those of the nearest
grid point in the WRG. The information in the point WRG is used by the plant
design software to calculate the speedup ratios for the extrapolation of the observed
resource to other points in the project area. (The speedup ratio method is described
in Section 16.7.1.)
Although the conventional WRG format is widely used, it has significant limita-
tions, including limited data precision and an inability to incorporate wind resource
data other than the Weibull factors (which, as discussed in Chapter 10, do not always
represent actual wind speed frequency distributions with good accuracy). Improve-
ments in this format are consequently desirable. One new file format that emerged
recently is the plant data grid (PDG), which is supported by the openWind software
and some wind flow models. This flexible, binary format has full floating precision and
can support multiple data layers including turbulence, inflow angle, mean shear, and
others.
16.4 SELECTING A TURBINE
Once the project is set up and the wind resource information has been loaded, the
analyst is ready to design the turbine layout. This process starts by selecting a turbine
model. Sometimes the choice is obvious; perhaps, the developer has a relationship
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