Environmental Engineering Reference
In-Depth Information
Figure 7.18
A U.S. map showing areas graded by the annual average wind energy flux
ρ
V
3
/
2, with darker
areas indicating higher wind energy flux. (Data from Elliot, D. L., C. G. Holladay, W. R. Borchet, H. P.
Foote, and W. F. Sandusky, 1987.
Wind Energy Resource Atlas of the United States
. DOE/CH 10093-4.
Golden: Solar Energy Research Institute.)
speed
V
d
. When the wind speed is higher than
V
d
, the turbine can deliver much more power (since
P
varies approximately as
V
3
) but the electric generator cannot absorb this additional power without
overheating and burning out, so the rotor pitch angle is adjusted to deliver only the design power
output to the generator. On the other hand, at wind speeds less than
V
d
, much less power can be
be generated, by a factor of about
V
d
), and the electrical output is reduced below the design
value. The economical optimum design is one that maximizes the ratio of the average electrical
power output to the capital cost of the wind turbine installation, given the wind speeds available
at the site. For efficient designs used at desirable sites, the ratio of time-averaged power output to
the rated output, called the
capacity factor,
is about 25-30%.
A desirable site for a wind power installation is one having a high average wind speed. Sites
are commonly rated by the average value of the energy flux per unit area,
(
V
/
2.
20
High values
of wind energy flux require smaller, less costly turbine rotors for a given electrical power output.
Figure 7.18 identifies U.S. regions with high wind energy levels. These are located mostly at higher
elevations and in mountainous regions, as well as in coastal areas.
At any one site, wind speeds are quite variable in magnitude and duration. Daytime wind speeds
are higher than nighttime levels because of daytime solar heating of the atmosphere. Average wint
er
season wind speeds exceed those of the summer. While the time-averaged annual wind speed
V
approximately measures the availability of wind energy, a more detailed knowledge of wind speed
statistics is needed for an accurate assessment of a site's wind energy potential and the selection of
V
3
ρ
/
20
Site energy fluxes are classified in one of five levels: class 1 (0-100 W/m
2
), class 2 (100-200 W/m
2
), ...,
class 5 (more than 400 W/m
2
).
