Environmental Engineering Reference
In-Depth Information
in forested, steep terrain. Along with the mean wind speed and air density, the TI at
15 m/s enables a preliminary determination of the suitability of a turbine model for
the project site. The final determination is usually made by the manufacturer, who is
responsible for the warranty, and may take into account the frequency distribution of
turbulence as well as turbulence generated by upstream turbines. (See Chapter 16.)
10.1.5 Wind Power Density
WPD is defined as the flux of kinetic energy in the wind per unit cross-sectional area.
Combining the site's wind speed distribution with air density, it provides an indication
of the wind energy production potential of the site. It is calculated in the following
way:
N
i
=
1
ρ
i
v
i
1
2
N
w/m
2
WPD
=
(
)
(10.9)
where
N
=
the number of records in the period;
kg/m
3
ρ
i
=
the air density
(
)
;
=
v
i
the wind speed for record
i
(m/s).
The air density in this equation must be calculated from other information, as described
in the following section.
Note that the cubic equation must be evaluated for each record and then summed,
as shown, rather than being applied to the mean wind speed for all records. This is
because above-average wind speeds contribute much more to WPD than do below-
average speeds, thanks to the cubic exponent. Even then, the WPD estimate is not
exact since it ignores variations in speed within each recording interval. The true
WPD is generally a few percent greater than that calculated from this formula. This
is usually not important for wind resource assessment, since WPD is not used directly
in calculating energy production.
10.1.6 Air Density
The air density depends on temperature and pressure (and thus altitude) and can vary
by as much as 10-15% seasonally. If the site pressure is measured, the air density
can be calculated from the ideal gas law:
P
RT
(
kg/m
3
ρ
=
)
(10.10)
where
Pa or N/m
2
P
=
the site air pressure
(
)
;
R
=
the specific gas constant for dry air
(
287
.
04 J/kg
·
K
)
;
(
◦
C
T
=
the air temperature in degrees Kelvin
+
273
.
15
)
.
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