Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 117

FIGURE 6.17 Blade cross sections for megawatt wind turbine.
The material for blades is predominantly FRPs, for both the spar and the blade skin, which also sup-
ports the load. For the large wind turbine blades the construction is quite different than airplane wings.
One basic concept is two glass fiber shells attached to two rigid beams, or a glass fiber shell with one
beam (Figure 6.17). The technology, from design to process, is discussed by LM Glasfiber, the world's
leading supplier of blades [38]. There has also been a switch from wood to FRP blades for small wind
turbines. Blades of composite wood laminate, 6.5 m long, have been successful on a 50 kW unit.
Sandia Laboratories is concentrating on the aerodynamic and structural design of wind turbine
blades [39]. Topics include adaptive structures, thick airfoils, material and fatigue, manufacturing
research, design tools and applications, and sensors and nondestructive inspection. Reports in all of
these areas are listed. Another aspect of the program is the long-term inflow and structural test, a
joint project of Sandia and NREL to collect experimental inflow and turbine response data. One of
the instrumented wind turbines is a GE 1.5 MW unit.
One of the main concerns is the procedure and mechanism for the shutdown for overspeed. If
there is a lost of load, for example, the utility transmission line goes down due to an ice storm, dur-
ing high winds with the wind turbine operating at rated power, then the power of the rotor has to be
controlled with 5-10 s. If the condition results in so much power that it can not be controlled, even
with the application of a mechanical brake, then the unit will self-destruct or a few high wind speed
shutdowns will place so much stress on the drive train that it has to be replaced. For light-weight
blades on wind turbines operating at constant rpm the time period is 4-5 s. The Alternative Energy
Institute and USDA, Agricultural Research Service, have installed and tested over sixty prototype
and first-production wind turbines, from 50 W to 500 kW. Almost all the units had some kind of
failure within 1 year, and some of the failures resulted in loss of the rotor or even the destruction of
the unit. When a rotor is in a runaway condition, the only thing you can do is get up wind and wait
a while.
If a mechanical brake is part of the system for overspeed control, then it needs to be on the
low-speed shaft, because if it is on the high-speed shaft and the drive train fails, then the brake is
useless. It does not have to be a mechanical failure; for example, a 500 kW VAWT was lost because