Siting - Wind Energy: Renewable Energy and the Environment - page 180

TABLE 9.2
Rating of Visual Impact of Small Wind Turbines

Score
Neighbor Public

Negligible 0-3 0-3

Minimal 3-6 3-9

Moderate 6-9 9-14

Significant 9-12 14-18

viewpoint and 18 for the public viewpoint. If the score (Table 9.2) is below the significant range, the
wind turbine is unlikely to have a visual impact unless it is close to and at the center of a scenic view.
The score is only a general indicator for visual impact of small wind turbines. Wind turbines will be
visible, at least from some viewpoints, as they will be above surrounding trees. In the Plains areas
with few trees, small wind turbines will be noticeable from 1 to 3 km, the same as the trees around
a farmhouse. Notice that there are comparable-height towers, such as cell phone towers, towers for
lights at highway interchanges, radio towers, and the long rows of towers for utility transmission
lines. The difference is that those towers do not have moving rotors.

9.2 WIND FARMS
For wind farms, long-term data are a necessity, and data should be collected on site for 2 to 3 years.
Then the questions are: What is the long term annual variability? and How well can you predict the
energy production for a wind farm? The siting of turbines over an area the size of a wind farm, about
5-20 km 2 , is termed micrositing. Thus, the wind turbines should be located within the wind farm
to maximize annual energy production, which gives the largest financial return. Array losses have
to be considered in the siting process.

9.2.1 L ONG -T ERM R EFERENCE S TATIONS
To determine if data from a historical site are adequate to describe the long-term wind resource
at another site, the analysis should be done rigorously. Simon and Gates [18] recommend that
the annual hourly linear correlation coefficient be at least 0.90 between the reference site and
off-site data. Remember to take into account wind shear if the heights are different at the two
locations. If the two sites are not similar in wind speed and direction trends and do not have
similar topographic exposure, then they will probably not have that correlation value. Long-
term reference stations should be considered in all locations in the world where there is wind
power potential. These stations should continue to collect data even after a wind farm has been
installed. Not only will this improve siting of wind farms, but it will provide reference sites for
delineating the wind resource for single or distributed wind turbines in that region. As wind
turbines have increased in size, the hub heights are higher, and because in most locations wind
speed increases with height, there is a need for reference stations to collect data at least at 50 m,
and if possible to 100 m.

9.2.2 S ITING FOR W IND F ARMS
The number of met stations and the time period for data collection to predict the energy produc-
tion for a wind farm vary depending on the terrain and the availability of long-term base data