Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 86

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

2 3 4 5 6 7 8 9 10

Average Wind Speed, m/s

FIGURE 5.16 Estimated annual energy production based on annual average wind speed.

Notice the large difference in the answers for the two examples, which could be related to two
factors: generator size is too large for rotor size, or the wind regime is low, that is, the wind map
value is low. With this estimate of energy production, the wind map value should be selected or
estimated for the hub height of the wind turbine, especially when estimating energy production for
large wind turbines.

5.7.3 M ANUFACTURER ' S C URVE
Manufacturers assume a Rayleigh distribution for the wind speed at 1 m/s intervals and then cal-
culate the annual energy production at standard density using the power curve for their wind tur-
bine at a selected hub height. An example graph of the annual energy production versus average
wind speed is given for a 1 MW wind turbine (Figure 5.16). Notice the average wind speed at your
location should be somewhat close to the hub height. At 10 m height, the average wind speed was
around 6 m/s for the High Plains of Texas (1,100 m elevation), and at 50 m height, the wind speed
was 8.2 m/s. So, from the graph, a wind speed of 8.2 m/s means the turbine should produce around
2,800,000 kWh/year.

5.8 CALCULATED ANNUAL ENERGY
If the wind speed histogram or wind speed distribution is known from experimental data, then a
good estimation of energy production can be calculated from the histogram and the power curve for
the wind turbine. Manufacturers will supply power curves for their wind turbines, and most of the
power curves are available online. For each interval (a bin width of 1 m/s is adequate), the number of
hours at that wind speed is multiplied by the corresponding power to find the energy. These values
are added together to find the energy production for the total number of hours ( Table 5.1 ) . This is the
method that wind farm developers use to estimate the energy production. Wind speed histograms
should reflect annual values, not the value for part of a year or even 1 year, which could be above or
below the annual values. A 1-year histogram could be adjusted to annual values if long-term regional
data are available. Two to 3 years of wind speed data, averaged to an annual histogram, will suffice.
Wind speed histograms and power curves have to be corrected to the same height and adjusted
for air density due to location of the data compiled for the power curve. So when the density correc-
tion is made from 1.2 to 1.1 kg/m 3 for the Texas Panhandle and an availability of 98% is assumed,
that reduces 3,061,000 kWh/year to 2,750,000 kWh/year.