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W i nd T urbine Generator (WTG) Components 171

Lightning protection system, with lightning receptors all

along the length of the blade connected to conductors that
ground the lightning.
Sensors in the blade to monitor stress, strain, acoustic emis-

sions, and other signals.
Overspeed control mechanism like pitchable tips. Modern

large turbines do not have this feature.
There are two predominant methods for the manufacturing of large
blades: Epoxy prepregmolding and vacuum-assisted resin transfer
1
molding (VARTM). In the epoxy prepreg molding, fiberglass impreg-
nated with epoxy is laid out in layers and placed in a mold. The layers
are pressed and then cured at elevated temperature. In the VARTM,
fiberglass is laid in a pre-form and placed in a closed mold. In this
mold, epoxy resin is sucked in using vacuum and then cured to form
a blade. VARTM has resulted in a simpler process, although it is still
time intensive. Application of epoxy resin on such a large structure
without imperfections, like air pockets and without resin-rich pockets,
is challenging. These imperfections cause stress concentrations lead-
ing to fatigue failure. The percentage of material by weight in a blade
is in Table 9-1.
Blades of large utility-scale turbines are 35-plus meters in length.
The total weight of a 40-m blade can be 5,700 kg. (See Tables 9-5 to 9-8
for weight of blades of five commercial utility-scale turbines.) One of
thelargestmass-producedbladesisa61.5-mbladefora125-mrotordi-
ameter offshore turbine with a weight of 18 tons. For this LM Glasfiber
blade, about 30% reduction in weight was achieved with combination
3
of carbon and glass fibers. Carbon fiber-reinforced plastics are lighter
weight, possess about three times the stiffness of glass fiber-reinforced
plastics, and possess significantly better fatigue properties.

Component of Blade Weight, %
Fiberglass 51
Epoxy resin 33
Bonding adhesive 7.5
Sandwich core 4
Miscellaneous—bolts and lightning protection 4.5

Source: From Ashwill, T. Blades: Trends and Research Update. [Online] 5 12, 2008.
http://www.sandia.gov/wind/2008BladeWorkshop/PDFs/Mon-02-Ashwill.
pdf.
TABLE 9-1 Weight of Components in a Blade