Wind Speed and Energy Distributions - Wind and Solar Power Systems - page 52

n

1

∑

V = V (4-31)

avg i

n

i = 1

As seen earlier, for assessing the wind power, the rmc speed is what
matters. The rmc equivalent of the digital data logging is as follows:

n

1

∑

3

V = V (4-32)

3

rmc i

n

i = 1

The above equation does not take into account the variation in the air mass
density, which is also a parameter (although of second order) in the wind
power density. Therefore, a better method of collecting the wind power data
is to digitize the yearly average power density as follows:

n

1
2

∑

3

P =

1 ρ

⋅ V (4-33)

rmc i i

n

i =

where n = number of observations in the averaging period

ρ i = air density (kg/m3), and

V i = wind speed (m/s) at the ith observation time.

4.6.7 Effect of Height

The wind shear at ground surface causes the the wind speed increase with
height in accordance with the expression

α

=⋅

h
h

2 V

2

(4-34)

1

1

where V 1 = wind speed measured at the reference height h 1

V 2 = wind speed estimated at height h 2 , and

α = ground surface friction coefficient.

The friction coefficient is low for smooth terrain and high for rough ones
( Figure 4-15 ) . The values of for typical terrain classes are given in Table 4-3 .

The wind speed does not increase with height indefinitely. The data col-
lected at Merida airport in Mexico show that typically the wind speed
increases with height up to about 450 meter height, and then decreases
( Figure 4-16 ) 1 . The wind speed at 450 meters height can be four to five times
greater than that near the ground surface.

α