Page 366 - Wind Energy Handbook

340 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

6.4.5 Visual considerations

There is a consensus that turbines are more disturbing to look at the faster they
rotate.

6.5 Number of Blades

6.5.1 Overview

European windmills traditionally had four sails, perhaps because pre-industrial
techniques for attaching the sail stocks to the shaft lent themselves to a cruciform
arrangement in which the stocks for opposite sails formed a continuous wooden
beam. By contrast the vast majority of horizontal axis wind turbines manufactured
today have either two or three-blades, although at least one manufacturer used to
specialize in one-bladed machines. As the latter are relatively unusual, considera-
tion of them will be restricted to Section 6.5.7, and the rest of Section 6.5 will
concentrate on two- and three-bladed machines.
In comparing the relative merits of machines with differing numbers of blades,
the following factors need to be considered:

• performance,

• loads,
• cost of rotor,
• impact on drive train cost,
• noise emission,

• visual appearance.

Some of these factors are strongly influenced by rotational speed and rotor solidity,
and the ideal relationship between these parameters and the number of blades is
considered in the next section. Section 6.5.3 investigates alternative two-bladed
derivatives of a realistic three-bladed baseline design and compares their relative
energy yields and notional costs. Section 6.5.4 reviews the differences in loading
imposed by two- and three-bladed rotors on the supporting structure, and Section
6.5.5 considers the constraint on rotational speed imposed by noise emission. Visual
appearance is considered briefly in Section 6.5.6.

6.5.2 Ideal relationship between number of blades, rotational speed
and solidity

The effect of the number of blades on the blade chord and rotational speed of a
machine optimized for a particular wind speed is given by Equation (6.8):