Wind Characteristics - Wind Energy: Renewable Energy and the Environment - page 41

25 50

0

8.0

30

330

6.0

300 60

4.0

2.0

270 90

0.0

240

120

210 150

180

FIGURE 3.11 Annual average wind direction at 25 and 50 m height, 10° sectors, Dalhart, Texas, April 1996-2000.

Previously wind speed data at U.S. National Weather Service (NWS) stations were recorded
on a strip chart and the observer estimated a wind speed over 1 to 2 min each hour. Wind speed
data along with pressure, temperature, and other climatological data were put on magnetic tape
for every hour. The National Weather Service converted to automated surface observation systems
as of 1993-1994. Wind speed and direction are sampled at 1 Hz, averaged over 5 s, and rounded.
Then a 2 min running average is calculated from the twenty-four 5 s samples. Data on CD-ROMs,
data downloaded to a computer through the Internet, and data sheets of monthly summaries can be
purchased ( http://lwf.ncdc.noaa.gov/oa/ncdc.html ) .
If the wind speeds are known, then the average wind power or average wind energy per unit area can
be estimated for any convenient time period, usually months, seasons, or year. When more than 1 year
of data are available, then the year data or month data are averaged to obtain annual values by year or
month. The wind power per area is referred to as the wind power potential or wind power density:

3

N

P
A
N

N

0 . 5 R vA

A

N
£

££
j

j j j 3

0 . 5 R v

(3.9)

jj

P
A

avg 1 j 1 j 1

N N

where N is the number of observations.
Average values of temperature and pressure can be used to calculate an average density, and then
the average power/area can be calculated for the available wind speed data. The result will be fairly
accurate since the pressure and temperature will not vary over the month or year nearly as much as
the wind speeds.

P
AN v j
j

0.5 R

N

£

(3.10)

avg avg 3

1