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Acknowledgements
A large number of individuals have assisted the authors in a variety of ways in the
preparation of this work.

In particular, however, we would like to thank David
Infield for providing some of the content of Chapter 4, David Quarton for scrutinising and commenting on Chapter 5, Mark Hancock, Martin Ansell and Colin
Anderson for supplying information and guidance on blade material properties
reported in Chapter 7, and Ray Hicks for insights into gear design.

Thanks are also
due to Roger Haines and Steve Gilkes for illuminating discussions on yaw drive
design and braking philosophy, respectively, and to James Shawler for assistance
and discussions about Chapter 3.
We have made extensive use of ETSU and Risø publications and record our
thanks to these organisations for making documents available to us free of charge
and sanctioning the reproduction of some of the material therein.
While acknowledging the help we have received from the organisations and
individuals referred to above, the responsibility for the work is ours alone, so
corrections and/or constructive criticisms would be welcome.
Extracts from British Standards reproduced with the permission of the British
Standards Institution under licence number 2001/SK0281.

Complete Standards are
available from BSI Customer Services.

(Tel þ44 (0) 20 8996 9001).

 

 



1.1 Historical Development
Windmills have been used for at least 3000 years, mainly for grinding grain or
pumping water, while in sailing ships the wind has been an essential source of
power for even longer. From as early as the thirteenth century, horizontal-axis
windmills were an integral part of the rural economy and only fell into disuse with
the advent of cheap fossil-fuelled engines and then the spread of rural electrification.

The use of windmills (or wind turbines) to generate electricity can be traced
back to the late nineteenth century with the 12 kW DC windmill generator
constructed by Brush in the USA and the research undertaken by LaCour in
Denmark. However, for much of the twentieth century there was little interest in
using wind energy other than for battery charging for remote dwellings and these
low-power systems were quickly replaced once access to the electricity grid became
available. One notable exception was the 1250 kW Smith–Putnam wind turbine
constructed in the USA in 1941.

This remarkable machine had a steel rotor 53 m in
diameter, full-span pitch control and flapping blades to reduce loads.

Although a
blade spar failed catastrophically in 1945, it remained the largest wind turbine
constructed for some 40 years (Putnam, 1948).
Golding (1955) and Shepherd and Divone in Spera (1994) provide a fascinating
history of early wind turbine development.

They record the 100 kW 30 m diameter
Balaclava wind turbine in the then USSR in 1931 and the Andrea Enfield 100 kW
24 m diameter pneumatic design constructed in the UK in the early 1950s. In this
turbine hollow blades, open at the tip, were used to draw air up through the tower
where another turbine drove the generator.

In Denmark the 200 kW 24 m diameter
Gedser machine was built in 1956 while Electricite´ de France tested a 1.1 MW 35 m
diameter turbine in 1963. In Germany, Professor Hutter constructed a number of
innovative, lightweight turbines in the 1950s and 1960s. In spite of these technical
advances and the enthusiasm, among others, of Golding at the Electrical Research
Association in the UK there was little sustained interest in wind generation until
the price of oil rose dramatically in 1973.
The sudden increase in the price of oil stimulated a number of substantial
Government-funded programmes of research, development and demonstration.

In the USA this led to the construction of a series of prototype turbines starting with
the 38 m diameter 100 kW Mod-0 in 1975 and culminating in the 97.5 m diameter
2.5 MW Mod-5B in 1987. Similar programmes were pursued in the UK, Germany

Bibliography
Eggleston, D. M. and Stoddard, F. S., (1987). Wind turbine engineering design.

Van Nostrand
Rheinhold, New York, USA.
Gipe, P., (1995). Wind energy comes of age. John Wiley and Sons, New York, USA.
Harrison, R., Hau, E. and Snel, H., (2000). Large wind turbines, design and economics. John Wiley
and Sons.
Johnson, L., (1985). Wind energy systems. Prentice-Hall.
Le Gourieres, D., (1982). Wind power plants theory and design. Pergamon Press, Oxford, UK.
Twiddell, J. W. and Weir, A. D., (1986).

Renewable energy sources. E. & F. N. Spon.

2
The Wind Resource
2.1 The Nature of the Wind
The energy available in the wind varies as the cube of the wind speed, so an
understanding of the characteristics of the wind resource is critical to all aspects of
wind energy exploitation, from the identification of suitable sites and predictions of
the economic viability of wind farm projects through to the design of wind turbines
themselves, and understanding their effect on electricity distribution networks and
consumers.
From the point of view of wind energy, the most striking characteristic of the
wind resource is its variability.

The wind is highly variable, both geographically
and temporally. Furthermore this variability persists over a very wide range of
scales, both in space and time.

The importance of this is amplified by the cubic
relationship to available energy.
On a large scale, spatial variability describes the fact that there are many different
climatic regions in the world, some much windier than others. These regions are
largely dictated by the latitude, which affects the amount of insolation. Within any
one climatic region, there is a great deal of variation on a smaller scale, largely
dictated by physical geography – the proportion of land and sea, the size of land
masses, and the presence of mountains or plains for example.

The type of vegetation may also have a significant influence through its effects on the absorption or
reflection of solar radiation, affecting surface temperatures, and on humidity.
More locally, the topography has a major effect on the wind climate. More wind
is experienced on the tops of hills and mountains than in the lee of high ground or
in sheltered valleys, for instance. More locally still, wind velocities are significantly
reduced by obstacles such as trees or buildings.
At a given location, temporal variability on a large scale means that the amount
of wind may vary from one year to the next, with even larger scale variations over
periods of decades or more. These long-term variations are not well understood,
and may make it difficult to make accurate predictions of the economic viability of
particular wind-farm projects, for instance

3
Aerodynamics of Horizontal-Axis
Wind Turbines
To study the aerodynamics of wind turbines some knowledge of fluid dynamics in
general is necessary and, in particular, aircraft aerodynamics.

Excellent text books
on aerodynamics are readily available, a bibliography is given at the end of this
chapter, and any abbreviated account of the subject that could have been included
in these pages would not have done it justice; recourse to text books would have
been necessary anyway.

Some direction on which aerodynamics topics are necessary for the study of wind turbines would, however, be useful to the reader.
For Sections 3.2 and 3.3 a knowledge of Bernoulli’s theorem for steady, incompressible flow is required together with the concept of continuity. For Sections 3.4
and 3.10 an understanding of vortices is desirable and the flow field induced by
vortices.

The Biot–Savart law, which will be familiar to those with a knowledge of
electric and magnetic fields, is used to determine velocities induced by vortices. The
Kutta–Joukowski theorem for determining the force on a bound vortex should also
be studied.

For Sections 3.5, 3.6 and 3.7 to 3.10 a knowledge of the lift and drag of
aerofoils is essential, including the stalled flow and so a brief introduction has been
included in the Appendix at the end of this chapter.