Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 206

Large-Scale Horizontal-Axis Wind Turbines

As the dominant size turbine category today, large-scale wind turbines offer advantages
that include

- the ability to extract more wind energy per unit of land area

- improved aerodynamic performance, because of higher Reynolds numbers associ-

ated with larger blade chord dimensions

- lower sensitivity of larger blades to dirt, rain, and insects, because of larger blade

thickness dimensions

- potential economies of scale for maintenance and some components, such as control

system cost per unit of installed power

While all HAWTs share many similar design considerations, some of these may be more
critical for multi-megawatt machines and must be carefully addressed in the design process.
These include

- handling, storing, and transporting components in the shop and to the site

- availability of cranes and limitations on lift weights and heights

- dependence on aerodynamic control of speed and power

- provisions for personnel access and safety, including access inside the rotor

- good exterior condition with minimum maintenance

- site selection to avoid electromagnetic interference

- structural design for weather extremes and long-term (30- year) life

- grid compatibility and power quality

- quality control to high standards for steel weldments, castings, and forgings

The configuration of the General Electric .5-MW wind turbine shown in Figure 4-
is typical of the modern commercial wind turbines. Table 4-3 contains a summary of
characteristics of representative commercial large-scale wind turbines. The majority of cur-
rent large-scale HAWTs have a rotor with three blades attached to a rigid hub, with integral
full-span blade pitch control. Other typical features include a rotor that is located upwind of
the support tower. These design features, characterized by the early Danish HAWT designs,
have demonstrated good reliability and performance over time. Soft tubular steel towers ( i.e. ,
with lowest tower bending natural frequency less than the number of blades per rotor revolu-
tion) and variable speed drive trains follow the design approach of early U.S. prototype wind
turbines.
Two-bladed wind turbines are also commercially available and promote the cost advan-
tage of elimination of one blade and lower weight throughout the system for a given energy
output. Teetered hubs (rotor hubs containing a hinge permitting upwind/downwind motion
of the blades) are used on two-bladed rotors to reduce or eliminate cyclic loads into the drive
train and tower. The Nordic Windpower turbine shown in Figure 4-2 is an example of this
configuration.