Page 403 - Wind Energy Handbook

7

Component Design

7.1 Blades

7.1.1 Introduction

A successful blade design must satisfy a wide range of objectives, some of which
are in conflict. These objectives can be summarized as follows:

(1) maximize annual energy yield for the specified wind speed distribution;
(2) limit maximum power output (in the case of stall regulated machines);
(3) resist extreme and fatigue loads;
(4) restrict tip deflections to avoid blade/tower collisions (in the case of upwind
machines);
(5) avoid resonances;
(6) minimize weight and cost.

The design process can be divided into two stages: the aerodynamic design, in
which objectives (1) and (2) are satisfied, and the structural design. The aerody-
namic design addresses the selection of the optimum geometry of the blade external
surface – normally simply referred to as the blade geometry – which is defined by
the aerofoil family and the chord, twist and thickness distributions. The structural
design consists of blade material selection and the determination of a structural
cross section or spar within the external envelope that meets objectives (4) to (6).
Inevitably there is interaction between the two stages, as the blade thickness needs
to be large enough to accommodate a spar which is structurally efficient.
The focus of Section 7.1 is on blade structural design. After a brief consideration
of the aerodynamic design in Section 7.1.2, practical constraints on the optimum
design are noted in Section 7.1.3 and forms of blade structure surveyed in Section
7.1.4. An overview of the properties of some potential blade materials is given in
Section 7.1.5 and the properties of glass-fibre reinforced plastic (GFRP) and
laminated wood are considered in more detail in Sections 7.1.6 and 7.1.7. Governing
load cases are considered in Sections 7.1.8 with reference to both stall- and pitch-