wind energy explained theory design and application second edition pdf
The technology of extracting energy from the wind has evolved dramatically over the last few
decades, and there have, up until now, been relatively few attempts to describe that technology
in a single textbook.
The lack of such a text, together with a perceived need, provided the
impetus for writing this book.
The material in this text has evolved from course notes from Wind Energy Engineering, a
course which has been taught at the University of Massachusetts since the mid-1970s.
These notes were later substantially revised and expanded with the support of the US Department of
Energy’s National Renewable Energy Laboratory (NREL).
In this, the second edition of this
textbook, we have again added new material to reflect the rapid worldwide expansion of wind
engineering in the 21st century.
This book provides a description of the topics which are fundamental to understanding the
conversion of wind energy to electricity and its eventual use by society.
These topics span a
wide range, from meteorology through many fields of engineering to economics and environmental concerns.
The book begins with an introduction which provides an overview of the
technology, and explains how it came to take the form it has today.
the wind resource and how it relates to energy production.
Chapter 3 discusses aerodynamic
principles and explains how the wind’s energy will cause a wind turbine’s rotor to turn. Chapter
4 delves into the dynamic and mechanical aspects of the turbine in more detail, and considers
the relation of the rotor to the rest of the machine.
Chapter 5 provides a summary of the
electrical aspects of wind energy conversion, particularly regarding the actual generation and
conversion of the electricity. Next,
Chapter 6 presents a summary of wind turbine materials and
Chapter 7 discusses the design of wind turbines and the testing of wind turbines.
Chapter 8 examines wind turbine and wind system control.
Chapter 9 discusses siting of wind
turbines and their integration into electrical systems both large and small.
Next, Chapter 10
gives a detailed summary of wind turbine applications.
Chapter 11 concerns the economics of
wind energy. It describes economic analysis methods and shows how wind energy can be
compared with conventional forms of generation.
Chapter 12 describes the environmental
aspects of wind energy generation. Finally, a new appendix (C) has been added. This appendix
provides an overview of some of the data analysis techniques that are commonly used in wind
turbine design and use.
This book is intended primarily as a textbook for engineering students and for professionals
in related fields who are just getting into wind energy.
It is also intended to be used by anyone
with a good background in math and physics who wants to gain familiarity with the subject. It
should be useful for those interested in wind turbine design per se.
For others, it should provide
Introduction: Modern Wind
Energy and its Origins
The re-emergence of the wind as a significant source of the worlds energy must rank as one of
the significant developments of the late 20th century.
The advent of the steam engine, followed
by the appearance of other technologies for converting fossil fuels to useful energy, would seem
to have forever relegated to insignificance the role of the wind in energy generation.
In fact, by
the mid 1950s that appeared to be what had already happened.
By the late 1960s, however, the
first signs of a reversal could be discerned, and by the early 1990s it was becoming apparent that
a fundamental reversal was underway.
That decade saw a strong resurgence in the worldwide
wind energy industry, with installed capacity increasing over five-fold. The 1990s were also
marked by a shift to large, megawatt-sized wind turbines, a reduction and consolidation in wind
turbine manufacture, and the actual development of offshore wind power (see McGowan and
Connors, 2000). During the start of the 21st century this trend has continued, with European
countries (and manufacturers) leading the increase via government policies focused on
developing domestic sustainable energy supplies and reducing pollutant emissions.
To understand what was happening, it is necessary to consider five main factors. First of all
there was a need.
An emerging awareness of the finiteness of the earths fossil fuel reserves as
well as of the adverse effects of burning those fuels for energy had caused many people to look
for alternatives. Second, there was the potential.
Wind exists everywhere on the earth, and in
some places with considerable energy density. Wind had been widely used in the past, for
mechanical power as well as transportation.
Certainly, it was conceivable to use it again. Third,
there was the technological capacity.
In particular, there had been developments in other fields,
which, when applied to wind turbines, could revolutionize they way they could be used. These
first three factors were necessary to foster the re-emergence of wind energy, but not sufficient.
There needed to be two more factors, first of all a vision of a new way to use the wind, and
second the political will to make it happen. The vision began well before the 1960s with such
individuals as Poul la Cour, Albert Betz, Palmer Putnam, and Percy Thomas.
It was continued
by Johannes Juul, E. W. Golding, Ulrich H€utter, and William Heronemus, but soon spread to
others too numerous to mention.
At the beginning of wind’s re-emergence, the cost of energy
1.1 Modern Wind Turbines
A wind turbine, as described in this book, is a machine which converts the power in the wind
into electricity. This is in contrast to a ‘windmill’, which is a machine which converts the winds
power into mechanical power.
As electricity generators, wind turbines are connected to some
These networks include battery-charging circuits, residential scale power
systems, isolated or island networks, and large utility grids. In terms of total numbers, the most
frequently found wind turbines are actually quite small – on the order of 10 kW or less. In terms
of total generating capacity, the turbines that make up the majority of the capacity are, in
general, rather large – in the range of 1.5 to 5 MW.
These larger turbines are used primarily in
large utility grids, at first mostly in Europe and the United States and more recently in China and
A typical modern wind turbine, in a wind farm configuration, connected to a utility
network, is illustrated in Figure 1.1.
The turbine shown is a General Electric 1.5 MW and this
manufacturer had delivered over 10 000 units of this model at the time of writing of this text.
To understand how wind turbines are used, it is useful to briefly consider some of the
fundamental facts underlying their operation.
In modern wind turbines, the actual conversion
process uses the basic aerodynamic force of lift to produce a net positive torque on a rotating
shaft, resulting first in the production of mechanical power and then in its transformation to
electricity in a generator.
Wind turbines, unlike most other generators, can produce energy only
in response to the resource that is immediately available
1.1.1 Modern Wind Turbine Design
Today, the most common design of wind turbine, and the type which is the primary focus of this
book, is the horizontal axis wind turbine (HAWT).
That is, the axis of rotation is parallel to the
ground. HAWT rotors are usually classified according to the rotor orientation (upwind or
downwind of the tower), hub design (rigid or teetering), rotor control (pitch vs. stall), number of
blades (usually two or three blades), and how they are aligned with the wind (free yaw or active
The principal subsystems of a typical (land-based) horizontal axis wind turbine are shown in
Figure 1.3. These include:
. The rotor, consisting of the blades and the supporting hub.
. The drive train, which includes the rotating parts of the wind turbine (exclusive of the rotor);
it usually consists of shafts, gearbox, coupling, a mechanical brake, and the generator.
. The nacelle and main frame, including wind turbine housing, bedplate, and the yaw system.
. The tower and the foundation.
. The machine controls.
. The balance of the electrical system, including cables, switchgear, transformers, and
possibly electronic power converters.
Wind Characteristics and Resources
This chapter will review an important topic in wind energy: wind resources and characteristics.
The material covered in this chapter can be of direct use to other aspects of wind energy which
are discussed in the other sections of this book. For example, knowledge of the wind
characteristics at a particular site is relevant to the following topics:
. Systems design – system design requires knowledge of representative average wind
conditions, as well as information on the turbulent nature of the wind and extreme wind
events. This information is used in the design and selection of a wind turbine intended for
a particular site.
. Performance evaluation – performance evaluation requires determining the expected
energy productivity and cost effectiveness of a particular wind energy system based on the
. Siting – siting requirements can include the assessment or prediction of the relative
desirability of candidate sites for one or more wind turbines.
. Operations – operation requirements include the need for wind resource information that
can be used for load management, operational procedures (such as start-up and shutdown),
and the prediction of maintenance or system life.
The chapter starts with a general discussion of wind resource characteristics, followed by
a section on the characteristics of the atmospheric boundary layer that are directly applicable
to wind energy applications.
The next two sections present a number of topics that enable
one to analyze wind data, make resource estimates, anticipate extreme wind speeds, and
determine wind turbine power production from wind resource data, or from a limited amount of
wind data (such as average wind speed).
Next, a summary of available worldwide wind
resource assessment data is given followed by a discussion of wind prediction and forecasting.
The section after that reviews wind resource measurement techniques and instrumentation.
The chapter concludes with a summary of a number of advanced topics in the area of wind