Environmental Engineering Reference
In-Depth Information
Chapter 7
Blade Design, Manufacture, and Testing
7.1 Introduction
Most small turbines do not have pitch adjustment of the blades whereas most large
turbines do. The reasons are mainly associated with cost and complexity, but they
make starting performance a subject worthy of a whole chapter. The development
of blade design in this chapter assumes no pitch adjustment. There are three main
results from
Chaps. 4
-
6
that must be kept in mind in considering blade design. First,
power extraction occurs mainly on the outer part of any turbine blade. Large blades
usually have a linear taper approximating the theoretical optimum 1/r dependence
of the chord near the tip. The deviations closer to the hub are of little consequence.
Second, starting torque is generated mainly near the hub. Thirdly, thick aerofoil
sections are not to be used for small turbine blades as their low-Re performance is
generally very poor at high angles of attack. The first two results are encouraging as
they suggest that a reasonable compromise between high efficiency and good
starting is possible.
Section 7.3
shows that to be the case, in the context of a
numerical optimisation method described in the next section. The third result is
cause for concern because thick sections are used near the hub of large blades for
structural strength (in combination with the circular root attachment that allows
pitch adjustment). It is for these reasons that the biggest difference between the
shape of optimal blades for large and small turbines will occur in the hub region.
7.2 Optimisation Method
Designing a wind turbine blade for the single criterion of maximum power
extraction is straightforward: once the operational tip speed ratio and aerofoil
section(s) have been chosen, Eqs.
5.12
and
5.13
fix the chord and twist
1
at least
1
Note that the large differences between (
5.12a
,
b
) and (
5.13a
,
b
) occur near the hub where little
power is produced.
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