Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 119

visualization, thermographic stress visualization, and acoustic emission. Two new facilities, one in
Massachusetts and the other in Texas, for testing the blades up to 70 m length are in the process of
development [42]. New blade designs, 12 m long for constant rpm, 100 kW unit, using carbon fila-
ments for more strength at less weight, were fabricated and then tested [43]. All the blades survived
the specified test loads and two designs exceeded it significantly.
Of course in the final analysis, it is energy produced by the wind turbine at the most economical
$/kWh. A rotor design study considered four basic configurations: upwind three blades, upwind two
blades, downwind three blades, and downwind two blades [44]. The cost of energy was estimated
with improvements, as compared to baseline turbines of 750 kW, 1.5 MW, and 3.0 MW. Two of the
conclusions were that the cost of energy would be reduced by up to 13%, which was small relative
to the magnitude of the load reduction, and more than 50% of the cost of energy was unaffected by
rotor design and system loads.
6.8.2 R EST OF THE S YSTEM
For large wind turbines, the most common configuration is three blades made from FRPs, upwind,
drive train, asynchronous generator, and tubular steel tower. The driver is the rotor, and these dynamic
loads are transferred to the rest of the system: drive train, generator, and tower. The difference between
variable- and constant-rpm operation is that part of the wind loads can be absorbed by inertia of the
rotor in variable-rpm operation. This reduces the severity of the loads for the drive train and generator.
Computer codes are available for the prediction of the wind turbine loads and responses. The
NWTC has a tool kit for creating wind turbine models [45] for input into a multibody dynam-
ics code (commercial). FAST (Fatigue, Aerodynamics, Structures, and Turbulence) can be used
to model two- and three-bladed horizontal-axis wind turbines. The code models the wind turbine
as a combination of rigid and flexible bodies. For example, two-bladed, teetering-hub turbines are
modeled as four rigid bodies and four flexible ones. The rigid bodies are the earth, nacelle, hub,
and optional tip brakes (point masses). The flexible bodies include blades, tower, and drive shaft.
The model connects these bodies with several degrees of freedom: tower bending, blade bending,
nacelle yaw, rotor teeter, rotor speed, and drive shaft torsional flexibility. The flexible tower has two
modes of vibration, and the blades have two flapwise modes and one edgewise mode. Flutter is the
coupling between blade flap and edge modes of vibration, and it was actually used as a method of
overspeed control for a 300 W wind turbine. The blades were constructed from carbon filaments,
formed in an injection mold, so the high strength allowed flutter. In all other cases, when a blade
enters flutter, it generally will fail within a short time period.
All wind turbines and blades have natural frequencies (modes) of vibration. The models predict
the modes, and they can also be found experimentally. So operation, especially constant-rpm opera-
tion, needs to avoid the major modes, for example, the natural frequency of guy wires for vertical-
axis wind turbines. For constant-rpm operation, the drive train may incorporate a torque damper.
Monitoring of acoustic emissions can be used to determine future problems in the drive train,
thereby reducing costs by preventive maintenance.
There are various towers for wind turbines—pole, guyed pole, pole or guyed pole with gin pole—so
operation and maintenance can be at ground level ( Figures 6.18 and 6.19 ) , guyed lattice, lattice, and
tube towers. Most towers are made of steel, although concrete has been used and fiberglass is being
considered. Primarily lattice towers were used in the early wind farms in California; however, the
later, large wind turbines with hub heights from 50 to 100 m used tubular steel towers. So far, the
record hub height is a 160 m lattice tower for a 2.5 MW wind turbine, constructed by Fuhrländer.
Towers have to be strong enough to support the weight on the tower top and to resist the move-
ment of the wind forces trying to push the tower over, which during operation at rated power in
high winds can be quite large. There are different foundations, and a lot of rebar and concrete are
required for the large wind turbines. Examples of foundations are pier and bell (at the bottom) for
each leg for lattice towers; different type of anchors, primarily piers for guyed pole towers; and for