Environmental Engineering Reference
In-Depth Information
rotation with frictional and pressure forces tends to produce a rotation of the wind
vector with increasing height above ground. This effect, however, is quite small, so
a constant direction is usually a safe assumption. Therefore, the directions recorded
at the top anemometer vane (with substitutions as needed) are generally projected to
hub height with no alteration.
There are sites where, because of the influence of terrain or strong temperature
gradients, substantial directional changes with height (veer) are frequently observed.
Such shifts can reduce turbine power production since the wind vector may not be
perpendicular to the rotor throughout the rotor plane. Remote sensing using sodar or
lidar can detect these situations.
11.3.2 Air Density
Two factors affect how the air density varies with height: the pressure (or elevation)
and air temperature. The air temperature is usually extrapolated from the thermometer
height to the hub height using the temperature lapse rate of the standard atmosphere
of 6
5
◦
C (K) per 1000 m. For a thermometer height of 3 m and a hub height of
80 m, this represents a drop in temperature of about 0
.
5
◦
C. Applying the change in
temperature and height to the density equation (Eq. 10.12), the effect is to decrease
the air density by 0.6-0.8% (independent of elevation).
.
11.3.3 Turbulence Intensity
The TI at hub height is normally estimated by assuming that the 10-min speeds
increase with height above the top anemometers, while their standard deviations remain
constant. Thus, the TI for each record decreases with height, and the speed-averaged
TI follows suit. The mean TI for the 15 m/s bin—a standard reference parameter for
determining if a particular turbine model is suitable for a site—may change slightly,
if at all.
11.4 QUESTIONS FOR DISCUSSION
1. What are the advantages and disadvantages of using hub-height monitoring
masts or remote sensing systems to measure the hub-height wind resource?
How might the answer differ in simple and complex terrain?
2. Define what the convergence height is and how its presence can be applied for
the shear analysis of a site. When does the convergence height corollary not
hold true?
3. Name three methods for scaling a time series of wind speed data or a wind
speed frequency distribution to hub height. Which method do you think is best?
4. Which two factors affect how air density varies with height?
5. A meteorological mast measures average wind speeds of 7.1 m/s at a height of
58.5 m and 6.2 m/s at 32.2 m.
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