Environmental Engineering Reference
In-Depth Information
in some regions, their impact is much greater than the “high” estimate cited here.
Curtailment may also be imposed at certain times for environmental reasons, such as
to avoid interfering with bird and bat movements, satisfy nighttime noise restrictions,
and reduce shadow flicker (caused by sunlight reflecting from blades).
Combining Losses.
The individual loss factors are combined not by direct addi-
tion but by multiplying the efficiencies (defined as one minus the loss). The total loss
is given by,
L
total
=
100%
−
(
100%
−
L
1
)(
100%
−
L
2
)(
100%
−
L
3
)...
(16.4)
where the values
L
n
are the individual losses in percent (only the first three being
shown in this equation).
16.7 SPECIAL TOPICS
Although one can run wind plant design software without understanding the details of
the underlying calculations, it is useful to know something about how the programs
work, as this can help the user make sensible choices and diagnose problems. The
following sections discuss methods of extrapolating the wind resource from one or
more masts to the turbine locations, the gross energy calculation, and wake modeling.
16.7.1 Extrapolating the Wind Resource from Mast to Turbine
Although the details differ, all the leading programs make use of a concept called the
speedup ratio
, which is the ratio of the speed predicted at a point to the speed at a
mast for a particular direction. This ratio is calculated using the information contained
in the WRG and point WRG files. The speedup ratio for each direction is multiplied
by the observed speed at each mast to estimate the corresponding speed at the point.
v
pi
=
R
pmi
v
mi
(16.5)
v
pi
is the predicted speed at point
p
for direction
i
,
v
mi
is the corresponding speed at
the mast
m
, and
R
pmi
, is the speedup ratio from the mast to the point for the same
direction.
The speedup ratio for a given direction is generally assumed to be constant with
speed. Thus, the observed speed frequency distribution at the mast is merely scaled up
or down by the same factor but is not otherwise changed. In fact, of course, the speed
frequency distribution
can
vary from point to point. Some programs use the predicted
variation in the Weibull
k
parameter in the WRG file in an attempt to estimate changes
in the observed frequency distribution. However, there is little hard data to confirm
whether this option improves accuracy or not.
The method of directional speedups also generally assumes that the directional
frequencies do not vary across the project area. Within the software, this assump-
tion is realized by setting the directional frequencies at all points equal to the mast
frequencies. Although this approach works reasonably well in many cases, it can
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