Environmental Engineering Reference
In-Depth Information
recent three decades, remarkable advances in wind turbine design have been
achieved along with modern technological developments. It has been estimated
that advances in aerodynamics, structural dynamics, and micrometeorology may
contribute to a 5% annual increase in the energy yield of wind turbines [35].
Various wind turbine concepts have been developed and built for maximizing
the wind energy output, minimizing the turbine cost, and increasing the turbine
effi ciency and reliability.
5.1 Wind turbine classifi cation
Wind turbines can be classifi ed according to the turbine generator confi guration,
airfl ow path relatively to the turbine rotor, turbine capacity, the generator-driving
pattern, the power supply mode, and the location of turbine installation.
5.1.1 Horizontal-axis and vertical-axis wind turbines
When considering the confi guration of the rotating axis of rotor blades, modern
wind turbines can be classifi ed into the horizontal-axis and vertical-axis turbines.
Most commercial wind turbines today belong to the horizontal-axis type, in which
the rotating axis of blades is parallel to the wind stream. The advantages of this
type of wind turbines include the high turbine effi ciency, high power density, low
cut-in wind speeds, and low cost per unit power output.
Several typical vertical-axis wind turbines are shown in Fig. 6. The blades of the
vertical-axis wind turbines rotate with respect to their vertical axes that are perpen-
dicular to the ground. A signifi cant advantage of vertical-axis wind turbine is that
the turbine can accept wind from any direction and thus no yaw control is needed.
Since the wind generator, gearbox, and other main turbine components can be set
up on the ground, it greatly simplifi es the wind tower design and construction, and
consequently reduces the turbine cost. However, the vertical-axis wind turbines
must use an external energy source to rotate the blades during initialization.
Because the axis of the wind turbine is supported only on one end at the ground,
its maximum practical height is thus limited. Due to the lower wind power effi -
ciency, vertical-axis wind turbines today make up only a small percentage of wind
turbines.
5.1.2 Upwind and downwind wind turbines
Based on the confi guration of the wind rotor with respect to the wind fl owing
direction, the horizontal-axis wind turbines can be further classifi ed as upwind
and downwind wind turbines. The majority of horizontal-axis wind turbines being
used today are upwind turbines, in which the wind rotors face the wind. The main
advantage of upwind designs is to avoid the distortion of the fl ow fi eld as the wind
passes though the wind tower and nacelle.
For a downwind turbine, wind blows fi rst through the nacelle and tower and
then the rotor blades. This confi guration enables the rotor blades to be made more
fl exible without considering tower strike. However, because of the infl uence of the
distorted unstable wakes behind the tower and nacelle, the wind power output
Search WWH ::
Custom Search