Burton T. (et. al.) Wind energy Handbook

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and the potential of wind energy to help limit climate change. In 1997 the Commis-

sion of the European Union published its White Paper (CEU, 1997) calling for 12

percent of the gross energy demand of the European Union to be contributed from

renewables by 2010. Wind energy was identiﬁed as having a key role to play in the

supply of renewable energy with an increase in installed wind turbine capacity

from 2.5 GW in 1995 to 40 GW by 2010. This target is likely to be achievable since at

the time of writing, January 2001, there was some 12 GW of installed wind-turbine

capacity in Europe, 2.5 GW of which was constructed in 2000 comp ared with only

300 MW in 1993. The average annual growth rate of the installation of wind

turbines in Europe from 1993–9 was approximately 40 percent (Zervos, 2000). The

distribution of wind-turbine cap acity is interesting with, in 2000, Germany account-

ing for some 45 percent of the European total, and Denmark and Spain each having

approximately 18 percent. There is some 2.5 GW of capacity installed in the USA of

which 65 percent is in California although with increasing interest in Texas and

some states of the midwest. Many of the California wind farms were originally

Figure 1.5 Wind Farm of Six Pitch-regulated Wind Turbines in Flat Terrain (Reproduced by

permission of Wind Prospect Ltd., www.windprospect.com)

HISTORICAL DEVELOPMENT 5

constructed in the 1980s and are now being re-equipped with larger modern wind

turbines.

Table 1.1 shows the installed wind-power capacity worldwide in Janua ry 2001

although it is obvious that with such a rapid growth in some countries data of this

kind become out of date very quickly.

The reasons development of wind energy in some countries is ﬂourishing while

in others it is not fu lﬁlling the potential that might be anticipated from a simple

consideration of the wind resource, are complex. Important factors include the

ﬁnancial-support mechanisms for wind-generated electricity, the process by which

the local planning authorities give permission for the construction of wind farms,

and the perception of the general population particularly with respect to visual

impact. In order to overcome the concerns of the rural population over the environ-

mental impact of wind farms there is now increasing interest in the development of

sites offshore.

1.2 Modern Wind Turbines

The power output, P, from a wind turbine is given by the well-known expression:

P ¼

rAU

where r is the density of air (1:225 kg=m

), C

is the power coefﬁcient, A is the rotor

swept area, and U is the wind speed.

The density of air is rather low, 800 times less than that of water which powers

hydro plant, and this leads directly to the large size of a wind turbine. Depending

on the design wind speed chosen, a 1.5 MW wind turbine may have a rotor that is

more than 60 m in diameter. The power coefﬁcient describes that fractio n of the

power in the wind that may be converted by the turbine into mechanical work. It

has a theoretical maximum value of 0.593 (the Betz limit) and rath er lower peak

Table 1.1 Installed Wind Turbine Capa-

city Throughout the World, January 2001

Location Installed capacity

(MW)

Germany 5432

Denmark 2281

Spain 2099

Netherlands 444

UK 391

Total Europe 11831

California 1622

Total USA 2568

Total World 16461

Courtesy of Windpower Monthly News Magazine

6 INTRODUCTION

values are achieved in practice (see Chapter 3). The power coefﬁcient of a rotor

varies with the tip speed ratio (the ratio of rotor tip speed to free wind speed) and is

only a maximum for a unique tip speed ratio. Incremental improvements in the

power coefﬁcient are continually being sought by detailed design changes of the

rotor and, by operating at variable speed, it is possible to maintain the maximum

power coefﬁcient over a range of wind speeds. However, these measures will give

only a modest increase in the power output. Major increases in the output power

can only be achieved by in creasing the swept area of the rotor or by locating the

wind turbines on sites with higher wind speeds.

Hence over the last 10 years there has been a continuous increase in the rotor

diameter of commercially available wind turbines from around 30 m to more than

60 m. A doubling of the rotor diameter leads to a four-times increase in power

output. The inﬂuence of the wind speed is, of course, more pronounced with a

doubling of wind speed leading to an eight-fold increase in power. Thus there have

been considerable efforts to ensure that wind farms are developed in areas of the

highest wind speeds and the turbines optimally located within wind farms. In

certain countries very high towers are being used (more than 60–80 m) to take

advantage of the increase of wind speed with height.

In the past a number of studies were undertaken to determine the ‘optimum’ size

of a wind turbine by balancing the complete costs of manufacture, installation and

operation of various sizes of wind turbines against the revenue generated (Molly

et al., 1993). The results indicated a minimum cost of energy would be obtained with

wind turbine diameters in the range of 35–60 m, depending on the assumptions

made. However, thes e estimates would now appear to be rather low and there is no

obvious point at which rotor diameters, and hence output power, will be limited

particularly for offshore wind turbines.

All modern electricity-generating wind turbines use the lift force derived from the

blades to drive the rotor. A high rotational speed of the rotor is desirable in order to

reduce the gearbox ratio required and this leads to low solidity rotors (the ratio of

blade area/rotor swept area). The low solidity rotor acts as an effective energy

concentrator and as a result the energy recovery period of a wind turbine, on a good

site, is less than 1 year, i.e., the energy used to manufacture and install the wind

turbine is recovered with in its ﬁrst year of operation (Musgrove in Freris, 1990).

1.3 Scope of the Book

The use of wind energy to generate electricity is now well accepted with a large

industry manufacturing and installing thousands of MWs of new capacity each

year. Although there are exciting new developments, particularly in very large

wind turbines, and many challenges remain, there is a considerable body of estab-

lished knowledge concerning the science and technology of wind turbines. This

book is intended to record some of this knowledge and to present it in a form

suitable for use by students (at ﬁnal year undergraduate or post-graduate level) and

by those involved in the design, manufacture or operation of wind turbines. The

overwhelming majority of wind turbines presently in use are horizontal-axis, land-

SCOPE OF THE BOOK 7

based turbines connected to a large electricity network. These turbines are the

subject of this book.

Chapter 2 discusses the wind resource. Particular reference is made to wind

turbulence due to its importance in wind-turbine design. Chapter 3 sets out the basis

of the aerodynamics of horizontal-axis wind turbines while Chapter 4 discusses their

performance. Any wind-turbine design starts with establishing the design loads and

these are discussed in Chapter 5. Chapter 6 sets out the various design options for

horizontal-axis wind turbines with approaches to the design of some of the important

components examined in Chapter 7. The func tions of the wind-turbine controller are

discussed in Chapter 8 and some of the possible analysis techniques described. In

Chapter 9 wind farms and the development of wind-energy projects are reviewed

with particular emphasis on environmental impact. Finally, Chapter 10 considers

how wind turbines interact with the electrical power system.

The book attempts to record well-established knowledge that is relevant to wind

turbines, which are currently commercially signiﬁcant. Thus, it does not discuss a

number of interesting research topics or where wind-turbine technology is still

evolving rapidly. Although they were investigated in considerable detail in the

1980s, vertical-axis wind turbines have not proved to be commercially competitive

and are not currently manufactured in signiﬁcant numbers. Hence the particular

issues of vertical-axis turbines are not dealt with in this text.

There are presently some two billion people in the world without access to mains

electricity and wind turbines, in conjunction with other generators, e.g., diesel

engines, may in the future be an effective means of providing some of these people

with power. However, autonomous power systems are extremely difﬁcult to design

and operate reliably, particularly in remote areas of the world and with limited

budgets. A small autonomous AC power system has all the technical challenges of

a large national electricity system but, due to the low inertia of the plant, requires a

very fast, soph isticated control system to maintain stable operation. Over the last 20

years there have been a number of attempts to operate autonomous wind–diesel

systems on islands throughout the world but with only limited success. This class

of installation has its own particular problems and again, given the very limited

size of the market at present, this specialist area is not dealt with.

Installation of offshore wind turbines is now commencing. The few offshore wind

farms already installed are in rather shallow waters and resemble land-based wind

farms in many respects using medium sized wind turbines. Very large wind farms

with multi-megawatt turbines located in deeper water, many kilometres offshore,

are now being planned and these will be constructed over the coming years.

However, the technology of offsh ore wind-energy projects is still evolving at too

rapid a pace for inclusion in this text which attempts to present established engin-

eering practice.

References

CEU, (1997). ‘Energy for the future, renewable sources of energy – White Paper for a

Community Strategy and Action Plan’. COM (97) 559 ﬁnal.

INTRODUCTION

Freris, L. L. (ed.), (1990). Wind energy conversion systems. Prentice Hall, New York, US.

Golding, E. W. (1955). The generation of electricity from wind power. E. & F. N. Spon (reprinted

R. I. Harris, 1976).

Molly, J. P. Keuper, A. and Veltrup, M., (1993). ‘Statistical WEC design and cost trends’.

Proceedings of the European Wind Energy Conference, pp 57–59.

Putnam, G. C. (1948). Power from the wind. Van Nostrand Rheinhold, New York, USA.

Spera, D. A. (1994) Wind-turbine technology, fundamental concepts of wind-turbine engineering.

ASME Press, New York, US.

Zervos, A. (2000) ‘European targets, time to be more ambitious?’. Windirections, 18–19.

European Wind Energy Association, www.ewea.org.

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.

REFERENCES 9

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 understan ding 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 vegeta-

tion 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 velocitie s 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.

On time-scales sho rter than a year, seasonal variations are much more predict-

able, although there are large variations on shorter time-scales still, which althou gh

reasonably well understood, are often not very predictable more than a few days

ahead. These ‘synoptic’ variations are associated with the passage of weather

systems. Depending on location, there may also be considerable variations with the

time of day (diurnal variations) which again are usually fairly predictable. On these

time-scales, the predictability of the wind is important for integrating large amounts

of wind power into the electricity network, to allow the other generating plant

supplying the network to be organized appropriately.

On still shorter time-scale s of minutes down to seconds or less, wind-speed

variations known as turbulence can have a very signiﬁcant effect on the design and

performance of the individual wind turbines, as well as on the quality of power

delivered to the network and its effect on consumers.

Van der Hoven (1957) constructed a wind-speed spectrum from long- and short-

term records at Brookhaven, New York, showing clear peaks corresponding to the

synoptic, diurnal and turbulent effects referred to above (Figure 2.1). Of particular

interest is the so-called ‘spectral gap’ occurring between the diurnal and turbulent

peaks, showing that the synoptic and diurnal variations can be treated as quite

distinct from the higher-frequency ﬂuctuations of turbulence. There is very little

energy in the spectrum in the region between 2 h and 10 min.

2.2 Geographical Variation in the Wind Resource

Ultimately the winds are driven almost entirely by the sun’s energy, causing differ-

ential surface he ating. The heating is most intense on land masses closer to the equator,

and obviously the greatest heating occurs in the daytime, which means that the region

of greatest heating moves around the earth’s surface as it spins on its axis. Warm air

rises and circulates in the atmosphere to sink back to the surface in cooler areas. The

resulting large-scale motion of the air is strongly inﬂuenced by coriolis forces due to

the earth’s rotation. The result is a large-scale global circulation pattern. Certain

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

10 days 4 days 24 h 10 h 2 h 1 hr 30 min 10 min 3 min 1 min 30 s 10 s 5 s

Synoptic peak

Diurnal peak

Turbulent peak

Spectrum, f.S(f)

Frequency, log (f)

Figure 2.1 Wind Spectrum Farm Brookhaven Based on Work by van der Hoven (1957)

THE WIND RESOURCE

identiﬁable features of this such as the trade winds and the ‘roaring forties’ are well

known.

The non-uniformity of the earth’s surface, with its pattern of land masses and

oceans, ensures that this global circulation pattern is disturbed by smaller-scale vari-

ations on continental scales. These variations interact in a highly complex and non-

linear fashion to produce a somewhat chaotic result, which is at the root of the day-

to-day unpredictability of the weather in particular locations. Clearly though, under-

lying tendencies remain which lead to clear climatic differences between regions.

These differences are tempered by more local topographical and thermal effects.

Hills and mountains result in local regions of increased wind speed. This is partly

a result of altitude – the earth’s boundary layer means that wind speed generally

increases with height above ground, and hill tops and mountain peaks may ‘project’

into the higher wind-speed layers. It is also partly a result of the acceleration of the

wind ﬂow over and around hills and mountains, and funnelling through passes or

along valleys aligned with the ﬂow. Equally, topogr aphy may produce areas of

reduced wind speed, such as sheltered valleys, areas in the lee of a mountain ridge

or where the ﬂow patterns re sult in stagnation points.

Thermal effects may also result in considerable local variations. Coastal regions

are often windy because of differential heating between land and sea. While the sea

is warmer than the land, a local circulation develops in which surface air ﬂows from

the land to the sea, with warm air rising over the sea and cool air sinking over the

land. When the land is warmer the pattern reverses. The land will heat up and cool

down more rapidly than the sea surface, and so this pattern of land and sea breezes

tends to reverse over a 24 h cycle. These effects were important in the early

development of wind power in California, where an ocean current brings cold

water to the coast, not far from desert areas which heat up strongly by day. An

intervening mountain range funnels the resulting air ﬂow through its passes,

generating locally very strong and reliable winds (which are well correlated with

peaks in the local electricity demand caused by air-conditioning loads).

Thermal effects may also be caused by differences in altitude. Thus cold air from

high mountains can sink down to the plains below, causing quite strong and highly

stratiﬁed ‘down slope’ winds.

The brief general descriptions of wi nd speed variations in Sections 2.1 to 2.5 are

illustrative, and more detailed information can be found in standard meteorological

texts. Section 9.1.3 describes how the wind regimes at candidate sites can be

assessed, while wind forecasting is covered in Section 2.9.

Section 2.6 presents a more detailed description of the high-frequency wind ﬂuctua-

tions known as turbulence, which are crucial to the design and operation of wind

turbines and have a major inﬂuence on wind turbine loads. Extreme winds are also

important for the survival of wind turbines, and these are described in Section 2.8.

2.3 Long-term Wind speed Variations

There is evidence that the wind speed at any particular location may be subject to

very slow long-term variations. Although the availability of accurate historical

LONG-TERM WIND SPEED VARIATIONS 13

records is a limitation, careful analysis by, for example, Palutikoff, Guo and

Halliday (1991) has demonstrated clear trends. Clearly these may be linked to long-

term temperature variations for which there is ample historical evidence. There is

also much debate at present about the likely effects of global warming, caused by

human activity, on climate, and this will undoubtedly affect wind climates in the

coming decades.

Apart from these long-term trends there may be considerable changes in windi-

ness at a given location from one year to the next. These changes have many causes.

They may be coupled to global climate phenomema such as el nin

o, changes in

atmospheric particulates resulting from volcanic eruptions, and sunspot activity, to

name a few. These changes add signiﬁcantly to the uncertainty in predicting the

energy output of a wind farm at a particular location during its projected lifetim e.

2.4 Annual and Seasonal Variations

While year-to-year variation in annual mean wind speeds remains hard to predict,

wind speed variations during the year can be well characterized in terms of a

probability distribution. The Weibull distribution has been found to give a good

representation of the variation in hourly mean wind speed over a year at many

typical sites. This distribution take s the form

F(U) ¼ exp 



(2:1)

where F(U) is the fraction of time for which the hourly mean wind speed exceeds

U. It is characterized by two parameters, a ‘scale parameter’ c and a ‘shape

parameter’ k which describes the variability about the mean. c is related to the

annual mean wind speed

U by the relationshi p

U ¼ cˆ(1 þ 1 =k)(2:2)

where ˆ is the compl ete gamma function. This can be derived by consideration of

the probability density function

f (U) ¼

dF(U)

¼ k

k1

exp 



(2:3)

since the mean wind speed is given by

U ¼

Uf(U)dU (2:4)

A special case of the Weibull distribution is the Rayleigh distribution, with k ¼ 2,

which is actually a fairly typical value for many locations. In this case, the factor

14 THE WIND RESOURCE