Blade Design, Manufacture, and Testing - Small Wind Turbines: Analysis, Design, and Application - page 138

blades. Higher volume production often uses a closed-mould process, such as
some form of resin-transfer moulding (RTM). RTM (usually) requires a core
which is (usually) made separately and then placed in the mould along with the
fibreglass. The mould is then closed and resin injected under pressure. If the mould
is sealed correctly there is little trimming required. It was to make RTM easier that
the leading edge of the blade in Fig. 7.6 was made straight.
There are many variations possible on the themes of vacuum infusion and resin
transfer moulding, such as light-RTM, vacuum assisted RTM etc. and it is possible
to make laminates using pre-impregnated or ''prepreg'' reinforcement. Further-
more, it is possible to form the core at the same time as the laminate surface. There
is also a considerable choice in the resins that can be used. The most common for
large blades are vinylester, polyester, and epoxy resins supplied by companies like
Huntsman 4 and Hexion. 5 Makers of small blades should seriously consider using
resins developed for large blades as long as they comply with guidelines such as
GL [ 9 ]. If used properly any large-blade resin will have material properties that are
appropriate for small blades. Epoxy resins tend to more expensive but the dif-
ference in terms of the total material costs for a blade is likely to be small. They
also tend to have the longest shelf life and superior fatigue properties. Polyester
resins are usually easier to handle, but have a higher shrinkage and curing
temperature. The blades in Fig. 7.6 were made using vinylester. However the
shrinkage caused ''print through'' as the blade surface conformed to the shape of
the reinforcement immediately below it rather than the mould surface, so an epoxy
resin was substituted for later blades. The key resin properties are the viscosity,
which determines how well it flows and wets out a mould, pot life, which limits the
time during which a blade can be made, and the glass transition temperature,
which determines the curing temperature. GL [ 9 ] requires a glass transition tem-
perature of at least 65C, and higher than any operational temperature. The GL
guidelines also specify that the material safety factors for composite small blades
must take account of environmental degradation and operating temperatures.
Safety factors are further described and used in Chap. 9 .
It is one of life's ironies that the resins used for wind turbine blades are all
derivatives of petroleum. This has lead to considerable interest in sustainable
alternatives, of which timber is obviously one. Other sustainable materials such as
bamboo are being investigated as alternatives to current reinforcements, [ 10 ].
Developments could also include more widespread use of timber spars for small
blades, perhaps using Paulownia, a low density Asian timber with a straight grain
that grows rapidly in plantation. 6
A wide range of gelcoats can be used when moulding blades to give a good
surface finish. 7 Some resins are more suited than others to gelcoats: epoxy resins,

4 http://www.huntsman.com/advanced_materials/ and click on ''Wind Energy'' (accessed 2 Sept
2010).
5

http://www.hexionchem.com/Industry/wind_energy.aspx?id=8174 (accessed 2 Sept 2010).

6

http://www.worldpaulownia.com/ (accessed 2 Sept 2010).