WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–15
which in turn is proportional to air pressure. A given wind speed therefore produces less
power from a particular turbine at higher elevations, because the air pressure is less. A wind
turbine located at an elevation of 1000 m above sea level will produce only about 90 % of
the power it would produce at sea level, for the same wind speed and air temperature. There
are many good wind sites in the United States at elevations above 1000 m, as can be seen
by comparing Fig. 8 with a topographical map of the United States. Therefore this pressure
variation with elevation must be considered in both technical and economic studies of wind
power.
Figure 9: Pressure variation with altitude for U.S. Standard Atmosphere
The air density at a proposed wind turbine site is estimated by finding the average pressure
at that elevation from Fig. 9 and then using Eq. 6 to find density. The ambient temperature
must be used in this equation.
Example
A wind turbine is rated at 100 kW in a 10 m/s wind speed in air at standard conditions. If power
output is directly proportional to air density, what is the power output of the turbine in a 10 m/s wind
speed at Medicine Bow, Wyoming (elevation 2000 m above sea level) at a temperature of 20 o C?
From Fig. 9, we read an average pressure of 79.4 kPa. The density at 20 o C = 293 K is then
ρ = 3.484(79.4)
293
=0.944
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001