Evolution of Modern Wind Turbines Part B: 1988 to 2008 - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 174

Figure 3-41. Schematic view of the component layout within the nacelle of a typical
modern wind turbine.
Recent Developments in Wind Power Plant Operation
Turbine Performance
The power performance of modern commercial wind turbines has improved dramatically
over the past 20 years. Rotor systems can now capture about 80 percent of the theoretically
extractable energy in the low stream. This has been made possible partly through the design
of custom airfoils for wind turbines. In fact, it is now commonplace for turbine manufactur-
ers to have special airfoil designs for each individual turbine design. These special airfoils
attempt to optimize aerodynamic eficiency at low wind speeds and limit aerodynamic loads
in high winds. They also attempt to minimize sensitivity to blade fouling , which is caused
by dirt and bugs that accumulate on blade leading edges and can greatly reduce aerodynamic
eficiency. Although rotor design methods have improved signiicantly, there is still much
room for improvement [Tangler 2001].
As the power performance of commercial turbines has increased with time during the
past decade, resulting capacity factors have also slowly increased. Capacity factor is deined
as the ratio of the annual average power output divided by the rated power of the genera-
tor. Table 3-5 presents capacity factors from the Lawrence Berkeley National Laboratory
database versus the commercial operation start date [Wiser and Bollinger 2008]. These data
show that turbines that began commercial operation prior to 1998 have an average capacity
factor of about 22 percent, whereas the turbines that began commercial operation after that
show an increasing capacity factor trend, reaching close to 35 percent in 2004-05. Much
of this increase can be attributed to increases in turbine size and power, plus installation on
taller towers reaching into more-energetic winds. This increasing trend in capacity factors is
expected to continue over time.