Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 469

Figure 8-1. Semi-permanent global wind patterns. Trade winds and Westerlies are par-
ticularly well-suited for wind energy conversion.

Since we are interested in capturing energy from the wind and not just producing power,
our best indication of the size of the local wind energy resource is its annual average wind
power density , or

p W , a = 0.5 r

ò

U 3 dt (8-2)

8760 year

where

p W,a = annual average wind power density (W/m 2 )

t = time (h)

Wind resource maps generally estimate the potential for wind energy conversion in terms
of wind power classes , as illustrated in Figure 8-2. Each class represents a range of annual
average wind power densities and equivalent mean wind speeds (Table 8-1). Areas desig-
nated as Class 3 or higher are suitable for most wind turbine applications, and it is the
challenge of the wind power engineer to capture this energy in a safe, cost-effective manner.