Page 355 - Wind Energy Handbook

6

Conceptual Design of Horizontal-

Axis Turbines

6.1 Introduction

Within the general category of horizontal axis wind turbines for grid applications
there exists a great variety of possible machine configurations, power control
strategies and braking systems. This chapter looks at the different areas where
design choices have to be made, and considers the advantages and disadvantages
of the more conventional options in each case. Inevitably there are situations in
which decisions in one area can impact on those in another, and some of these are
noted.
Alongside these discrete design choices there are several fundamental design
parameters, such as rotor diameter, machine rating and rotational speed, which also
have to be established at the start of the design process. Continuous variables such
as these lend themselves to mathematical optimization, as described in the opening
sections of the chapter.

6.2 Rotor Diameter

The issue of what size of turbine produces energy at minimum cost has been
fiercely debated for a long time. Protagonists of large machines cite economies of
scale and the increase in wind speed with height in their favour. From the other
camp, the ‘square-cube law’, whereby energy capture increases as the square of the
diameter, whereas rotor mass (and therefore cost) increases as the cube, is advanced
as an argument against.
In reality, both arguments are correct, and there is a trade-off between economies
of scale and a variant of the ‘square-cube law’ which takes into account the wind
shear effect. This trade-off can be examined with the help of simple cost modelling,
which is considered next.