CHARACTERIZING THE OBSERVED WIND RESOURCE - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 138

In these equations, N i refers to the number of records in direction sector i , N
is the total number of records in the data set, WPD i is the average wind power
density for direction sector i , and WPD is the average wind power density for all
records. Figure 10-5 contains a typical plot showing both frequency and percent of
energy.
10.2 QUESTIONS FOR DISCUSSION
1. Describe some situations in which the annualized mean wind speed could differ
significantly from the observed mean speed. Explain why the annualized mean
is usually a better representation of the wind resource.
2. Explain the meaning of WPD. Using the Internet, find one or more references
that describe the wind resource of a site or region in terms of WPD, and compare
the estimates you find with corresponding estimates of mean wind speed. Is there
a one-to-one correspondence between WPD and mean speed?
3. From the perspective of a wind project developer, rank the wind resource char-
acteristics described in this chapter from most to least important. Why did you
choose this order?
4. Why is it important to maintain a height ratio of 1.5 or greater between two
levels for wind shear measurement? Suppose the relative speed error is 1.2%
and the two heights are 50 and 40 m. What is the shear contribution to the speed
error at 80 m?
5. Name a type of wind speed frequency distribution not represented well by the
Weibull function.
6. Suppose the observed wind speed frequency distribution at a site has a k value
of 1.3. Would you say the winds at this site are more or less variable than a site
with a k value of 2.3? Assume the two sites have the same value of A ,9m/s.
Is a wind speed of 12 m/s more or less likely to occur at the first site compared
to the second? Which site is likely to generate more energy?
7. Why do you think it is useful to include more than just the frequency by direction
in a wind rose chart? Would you prefer to include mean speed or percent energy
by direction? Explain why.
8. The effects of water vapor have been ignored in Equation 10.9. Describe how
accounting for water vapor would affect the air density estimate.
9. What impact does air density have on the available kinetic energy in the wind?
For the same speed frequency distribution, would a high air density or low air
density site produce more energy for the same mean wind speed? Why?
10. Why is knowing the wind rose important for designing the instrumentation
on a meteorological mast? Is it also important for designing the turbine
layout?