Volume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications

Volume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications vVolume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications Volume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications Volume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications Volume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications Volume 2 wind energy 2 05 – wind turbines evolution, basic principles, and classifications

S Mathew, University of Brunei Darussalam, Gadong, Brunei Darussalam

GS Philip, KCAET, Malapuram, Kerala, India

© 2012 Elsevier Ltd All rights reserved

2.05.1 Introduction

2.05.2 Evolution of Modern Wind Turbines

2.05.2.1 Growth in Installed Capacity

2.05.2.2 Increase in Turbine Size

2.05.2.3 Improvements in System Performance

2.05.2.4 Advances in the Control and Power Transmission Systems

2.05.2.5 Economic Evolution

2.05.3 Basic Principles

2.05.3.1 Power Available in the Wind

2.05.3.2 Power Coefficient, Torque Coefficient, and Tip Speed Ratio

2.05.3.3 Airfoil Lift and Drag

2.05.4 Classifications of Wind Turbines

2.05.4.1 Horizontal Axis Wind Turbines

2.05.4.2 Vertical Axis Wind Turbines

2.05.4.2.1 Darrieus rotor

2.05.4.2.2 Savonius rotor

2.05.5 Rotor Performance Curves

References

Further Reading

A wind rotor swept area exposed to the wind stream R rotor radius

CD drag coefficient t temperature

CL lift coefficient T theoretical torque of the rotor

CP power coefficient of the rotor TT torque developed by the rotor

CT torque coefficient of the rotor VR resultant velocity

D airfoil drag force VT tangential velocity due to the blade’s rotation

F thrust force experienced by the rotor Z elevation of the site

L airfoil lift force α angle of attack

N rotational speed of the rotor ρ density of air

P theoretical power of the rotor Ω angular velocity

2.05.1 Introduction

During its transition from ancient ‘windmills’ to modern electricity generating ‘wind turbines’, the wind energy conversion technology has undergone significant changes Turbines of various shapes and sizes, working on different design principles, were introduced by researchers and inventors during the course of this development In this chapter, we will briefly describe this evolution of the modern wind energy conversion technology This is followed by discussions on the basic principles governing the wind energy conversion process and classifications of wind turbines

2.05.2 Evolution of Modern Wind Turbines

While looking back at the history, we can see that the wind energy conversion technology has undergone three distinct stages of development From the inception of the technology through the invention of grain grinding windmills by the Persians in 200 BC to the popular wind pumps of the eighteenth century, it was the era of ancient windmills By the 1800s, engines powered by steam and gas started getting popular and the use of wind machines was restricted only for remote applications, where a steady and reliable supply of power is not critical Several such systems served the power needs of remote areas during the eighteenth century

The next phase of development began with the introduction of electricity generating wind ‘turbines’ in the early 1900s The first wind electric generator was constructed in Denmark in 1890 and a utility-scale system was installed in Russia by 1931 Though efforts in similar direction were made in different parts of the world, the interest in wind energy gradually declined due to the popularity of diesel generators, which were considered to be more convenient and economic in those times Though the restriction

in oil supply during the First and Second World Wars prompted us to reconsider the wind energy option, it sustained only for a short period and the interest in wind energy declined gradually, till the oil shock in the 1970s

With the oil crisis in 1973, the world recognized the importance of energy independence, and as a result, activities for wind energy development were once again revived on a global scale A number of research and development programs were instantiated during this period, and several turbine prototypes of different sizes and shapes were built Research was focused on developing efficient, reliable, and cost-effective systems by modifying all the hardware components – right from the rotor to the control systems Some of the milestones in this development are the MOD series turbines by the National Aeronautics and Space Administration (NASA) [1] and the Vertical Axis Darrieus turbines developed by Sandia National laboratories [2] Research and development activities in this area were further accelerated as a result of the increased environmental consciousness These efforts gave birth to the next phase of wind energy development – the era of modern wind turbines In this section, we will restrict our discussions to the evolution of modern wind turbines, giving emphasis to

• growth in wind power capacity,

• increase in unit size,

• improvements in system performance,

• technological advances in control and power transmission systems, and

• economical competitiveness

2.05.2.1 Growth in Installed Capacity

One of the major milestones in the evolution of wind energy conversion technology is the significant increase in the installed wind power capacity Time series evolution of wind energy capacity from 1996 to 2010, based on studies of the Global Wind Energy Council (GWEC) [3], is shown in Figure 1 The cumulative installations have increased from 6100 to 194 390 MW during this period The rates of growth of wind power installations for the past 10 years are shown in Figure 2 [3] It could be seen that wind energy could register an average growth rate of 27.4% over the last decade This is an impressive achievement, which makes wind the fastest growing energy source in the world

A region-wise wind power scenario is displayed in Figure 3 [3] Considering the installations by 2010, the major contribution to this impressive status comes from Europe Germany (27 214 MW) and Spain (20 676 MW) are the leaders in this region With capacity additions over 19 000 MW during the last three consecutive years, Asia is emerging as one of the significant players in wind power development

The two emerging economies, China and India, share the credit for this growth For example, in 2010, China added 16 500 MW

to its wind power capacity to reach a total of 42 287 MW This enabled China to exceed the cumulative installations of the United States (40 200 MW) and become the world leader in wind energy utilization

The major driving force behind this rapid growth in global wind power deployment is the environmental commitments and emission reduction targets set by different countries For example, under the Kyoto protocol, China and the United States (responsible for 22.30% and 19.91% of the global emission, respectively) have a CO2 reduction target of 40% and 17%,

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Year

0

50 000

100 000

150 000

200 000

250 000

Figure 1 Growth in cumulative wind power capacity from 1996 to 2010

1097

58 641

86 075

2008

44 189 2397

Africa and Middle East Asia Europe Latin America and Caribbean North America Pacific region

0

5

10

15

20

25

30

35

40

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Year

Figure 2 Annual growth rate of wind power over the last decade

Figure 3 Region-wise distribution of global wind power capacity by the end of 2010

respectively, by 2020 [4] Other countries also have similar commitments Wind energy, with its clean, mature, and economically competitive technology, would be the obvious choice among the renewables for meeting these environmental obligations As a result, economic incentives of different kinds are being offered for catalyzing wind-based clean energy generation in these countries Other factors like concern over energy security and creation of the so-called green collar jobs are also in favor of the wind power sector

The current boom in the wind power sector is expected to continue in the coming years as well For example, GWEC, in collaboration with Greenpeace International and the German Aerospace Centre (DLR), examined the future growth potential of wind power [4] Three scenarios, namely, reference, moderate, and advanced, were considered for the projections The reference scenario took only the existing policies and measures in the energy sector into account whereas the moderate scenario considered the current and planned policy measures to support the renewables The advanced scenario brings out the highest possible level to which the global wind industry can grow under the most favorable situations

Results of this analysis are shown in Figure 4 In the next 20 years, the global cumulative wind power capacity is expected to reach 572 733 MW under the reference scenario Corresponding growth expected under the moderate and advanced scenarios are

1 777 550 and 2 341 984 MW, respectively Under these predictions, share of wind energy in the global power generation is found to vary between 4.9% and 5.6% under the reference, 15.1–17.5 under the moderate, and 18.8–21.8 under the advanced scenarios Hence, under favorable conditions, wind energy is going to be a major player in the global energy market, meeting a fifth of the world’s power demand

2.05.2.2 Increase in Turbine Size

During the evolution of modern wind turbines, the unit size of the machine has been considerably increased From small systems limited to a few kW capacity, wind turbines have grown to gigantic machines of MW class This scaling-up trend is shown in

7000 140

1975 1980 1985 1990 1995 2000 2005 2010 2015

Year

2 500 000

2 000 000

1 500 000

1 000 000

500 000

0

Year

Figure 4 Projected growth of wind power in the next 20 years

Figure 5 Increase in turbine size over the years

capacity of the machine is 7.5 MW Several bigger designs are under planning and development stages The 10 MW turbine planned

by the Norwegian industry is an example [6]

There are many factors favoring the scaling up of the size of wind turbines The most obvious one is the economic advantages Cost of the wind turbines, per unit size ($ kW−1), can be considerably reduced by scaling up the system size It is a fact that the expenditures on many components, systems, and services (e.g., safety features, electronic circuits, investments in R&D, and production manpower) do not scale up at the same rate as that of turbine size Similarly, a single higher capacity turbine is easier

to maintain than a number of smaller turbines contributing to the same capacity Thus, scaling-up of the system is one of the major factors contributing to the unit cost reduction in wind turbine technology in recent years

Environmental factors also favor the scaling up of wind turbines Due to the larger rotor size, bigger turbines are designed to run slower to keep the optimal tip speed ratio For example, the 7.5 MW turbine mentioned above runs at a speed of 5–12 rpm whereas a

330 kW unit of the same design rotates at 18–45 rpm Lower rotational speed minimizes the risk of avian mortality, which is one of the major environmental concerns raised against wind farms Similarly, aerodynamic noise can also be minimized by reducing the operating speed

2.05.2.3 Improvements in System Performance

The process of converting wind into electrical energy has also become more efficient during the course of time Improvements in the wind turbine capacity factor over the years are shown in Figure 6 The trend is derived from a compilation of capacity factor data

Capacity factor (%)

40

35

30

25

20

15 Pre-1998 1998–99 2000–01 2002–03 2004–05 2006

Year Figure 6 Improvement in the capacity factor

from 170 US wind farms between 1983 and 2006 [7] An impressive improvement in the capacity factor can be observed from the figure For example, comparing the weighted average capacity factor during 2004–06, the study indicated an improvement of

33–35% by 2007 In general, projects at high wind resource areas could attain a capacity factor above 40% during 2007 The Hawaii

2 project which showed a capacity factor of 45% is a good example The major reasons for this impressive improvement in wind farm performance are (1) higher hub heights, (2) improvements in siting, and (3) technological advances in the control and power transmission systems

With the increase in wind turbine size, the diameter of the rotor also has increased considerably in recent years For example, today’s most popular 2 MW class turbines have diameters around 90 m, with slight variations depending upon the design Taller towers are required to accommodate rotors of this size A general trend in the variations in hub height with the rotor diameter, brought out in a study by Garrad Hassan [8], is shown in Figure 7 The relationship between the rotor diameter (D) and hub height (h) can be expressed as h = 2.7936D0.7663

Apart from meeting the technical requirements, taller towers enable the turbine to capture the stronger wind spectra available at higher elevations Wind velocity increases with height due to wind shear reduction For example, the increase in wind velocity with height under three terrain conditions is shown in Figure 8 It should be noted that, even a small increase in the velocity could result

in significant change in energy capture due to the cubic velocity–power relationship Hence, contemporary wind turbines are capable of extracting more power from a given site due to the higher hub height

Recent advances in computer modeling made it possible to have a better understanding on the wind resource available at the wind farm sites These computer models could not only identify potential locations for wind farm activities but also describe the variations in wind power availability within the site by incorporating the effects of elevation, topography, and ground cover

120

100

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60

40

20

0

0 20 40 60 80 100 120 140

Rotor diameter (m) Figure 7 Variations in hub height with rotor diameter

1 1.2 1.4 1.6 1.8

2 2.2

Smooth terrain (roughness height 0.005) Cultivated land (roughness height 0.1) Complex terrain (roughness height 0.75)

0 20 40 60 80 100 120 140 160

Height (m) Figure 8 Increase in wind velocity with height

conditions Further, the effect of wake and other losses experienced by a turbine due to the presence of other turbines in the farm also could be modeled with some level of accuracy These models, when used in conjunction with the available ground measure­ ments and geographic information, could yield optimal options for micrositing the turbines

Other major factors that contributed to the improvement in capacity factor are the recent technological advances in the control and power transmission system designs These are discussed as a separate section below

2.05.2.4 Advances in the Control and Power Transmission Systems

Power developed by modern wind turbines is regulated either by pitch or by stall control mechanisms In pitch control, the angles of individual blades are changed to adjust the angle of attack, thereby controlling the driving lift force and thus the power Stall-controlled blades are aerodynamically profiled in such a way that, when the wind speed exceeds a certain limit, the angle of attack increases This changes the flow pattern over the top side of the blade from laminar to turbulent Thus the lift force is spoiled

at the desired level and power is regulated Stall control can be either active or passive Even a combination of pitch and stall control concepts is employed in some of the designs

Though the stall-controlled designs were preferred by the industry in the earlier days, pitch-controlled turbines dominate the market today Figure 9 illustrates this shift in the control options The number of pitch-controlled turbines available in the market is

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

1997 1998 1999 2000 2001 2002 2003 2006

Year Figure 9 Increase in pitch-regulated turbine designs

almost 4 times that of the stall-controlled machines The major reason is that the pitch-controlled machines offer better output power quality This is a definite advantage, especially when the grid presence of wind-generated electricity becomes significant On the other hand, concerns about the stall-induced vibrations, vibrations at the edge of the rotor blades, and aerodynamic losses make the option of stall regulation less attractive, especially for today’s multimegawatt machines

Today’s commercial wind turbines can operate either at fixed or at variable speed modes The variable speed option can be continuously variable or two-staged variable types The share of these three designs among the large wind turbine market (1 MW and above) is shown in Figure 10 It can be seen that most of the large turbines adopt the variable speed design today The industry adopts several methods to achieve variable speed operation The ‘traditional method’ allows the rotor to rotate

at varying speeds in tune with the fluctuations in the wind velocity, and the rotor power is transmitted to the generator through suitable gear drives Electrical energy is thus produced at variable frequency depending upon the speed of the rotor Before feeding the electricity to the grid, it is further conditioned by power electronic controllers, which modify the frequency as per the grid requirement Another approach is to use direct drive systems, which have the capability to operate at a wide range of speeds Variable speed systems, which are becoming popular these days, employ doubly-fed induction generators (DFIG) In turbines with DFIG, the stator winding is directly connected to the grid, whereas the rotor winding is fed through a converter, which can vary the electrical frequency as desired by the grid Thus the electrical frequency is differentiated from the mechanical frequency, which allows the variable speed operation possible The advantage of this approach is that, as only a fraction of the power passes through the converter, its size can be reduced approximately by a third Thus, the costs and losses can be reduced considerably

The major advantage of the variable speed option is its ability for better power capture In contrast with fixed speed turbines which can operate at peak efficiency point at only one wind speed, the variable speed turbines can be designed to have peak performance at a wide range of wind speeds Thus, considering the frequent fluctuations in wind velocity at a site, the variable speed turbines produce more energy and yield better capacity factor Further, with well-designed electronic controllers, the variable speed option can give better power quality, thus improving the ‘grid friendliness’ of wind-generated electricity If the rotational speed is kept constant, while transmitting a large amount of power at higher wind speeds, the torque and load levels experienced by the transmission system would be significantly high Hence, another factor favoring the choice of variable speed drive is the reduction in drive train loads

In recent years, there has been a significant trend in the industry toward the direct drive machines The major attractions in avoiding gears in the power train are as follows:

• Gearboxes are considered as the most failure-prone component of a wind turbine It requires constant care and maintenance Several instances of premature failures of the gearboxes have been reported

• Gears are expensive and add significantly to the system cost

• Gears contribute to energy losses during transmission

• Drive trains with heavy gears demand stronger towers to support their weight

Enercon, with its time-proven direct drive technology, has the major market share in the direct drive sector The current design trend

is to develop direct drive turbines with permanent magnet generators (PMG), resulting in higher efficiency and reliability Reduction

in cost and weight are the major challenges Some innovative concepts like superconducting drive trains and continuously variable transmissions with fluid drive systems are also being proposed to improve reliability and performance of wind turbine drive trains [9]

9.47

14.74

75.79

Fixed speed Two speed Variable speed

Figure 10 Market shares of fixed, two speed, and variable speed turbines

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2.05.2.5 Economic Evolution

One of the reasons behind the wide acceptance of wind energy as a clean energy alternative is its economic competitiveness With the technological evolution, wind energy is much cheaper than other renewable options like solar PV and small hydroelectric power plants [10] At sites with good wind resource potential, the cost of wind-generated electricity is even comparable with the energy generated from traditional sources like nuclear, coal, and natural gas Hence, wind energy is no more a technology for demonstra­ tion that survives on subsidies and incentives Today, it is an economically viable energy option

Historical and projected capital investments for wind energy projects are shown inFigure11 The historic data are based on the average cost estimates from 36 US wind farms, with a total capacity of 4079 MW [7] The projections are based on a study by the GWEC [4] During the period 1980–2000, a steady decline in the unit cost of the projects can be observed For example, the cost kW−1

dropped by USD 2700 kW−1 during this period, to reach its lowest level in 2000 The major reason for this decline is the scaling up of the turbine size, as we discussed earlier Better engineering approaches in the production and installation of turbines also contributed to this cost reduction

However, the average project costs have shown an increasing trend in recent years This is mainly due to the increase in turbine prices (which covers almost 50% of the total project cost [10]) during this period Recently, turbine prices have gone

up mainly due to the increase in the prices of the raw materials, requirements for highly sophisticated turbine designs, and the shortage in the supply of sensitive components Future projections indicate that this trend will continue for some more years, after which a slight decline in the project cost can be expected For example, the cost kW−1 may drop to USD 1775 by 2030, from 2010’s level of USD 1937 kW−1 In a more optimistic scenario, this drop can even be up to USD 1600 kW−1 by 2030 [4] Interestingly, in spite of the recent increase in the capital cost of wind energy projects, cost of wind-generated electricity keeps on showing a declining trend as in Figure 12 As the generation cost is influenced by a number of site-specific factors (e.g., the wind resource available at the site), the cost of generation cannot be simply generalized for all the wind farms However, a general trend can be derived from the figure It can be seen that the unit generation cost has declined by almost half during the last 10 years This trend is expected to be continued in the coming years as well The major contributing factor toward this is the steady improvement

in the performance of wind farms as we have seen in the earlier sections

2.05.3 Basic Principles

Wind results from the movement of large quantities of air mass over the Earth’s surface Hence, the basic form of energy contained in

a wind stream is the kinetic energy A wind turbine interferes with the free wind flow, allowing its blades to extract kinetic energy from wind, which is then transformed to mechanical or electrical forms depending on our end use In this section, we will introduce the basic principles governing this energy transfer

2.05.3.1 Power Available in the Wind

Consider a wind rotor with a swept area A exposed to the wind stream of velocity V normal to the rotor plane (Figure 13) The kinetic energy in the stream is given by

Figure 11 Historic and projected capital cost of wind energy projects

Year 0.0000

0.0200 0.0400 0.0600 0.0800 0.1000 0.1200

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

V

Figure 12 Unit cost of wind energy generation

Figure 13 A wind turbine interacting with wind stream

1

where

with the turbine per unit time is given by

2 where ρ is the density of air

From the above expression, we can see that the power is proportional to the cube of the wind velocity As a result, even a slight variation in the wind velocity can result in significant changes in the power available

Another factor influencing the power available is the density of air Density of air is influenced by the temperature and elevation

of the location For any given site at an elevation Z and temperature t, air density can be approximated as [11]

�

Z

353:049 t

ρ

�

According to the International Standard Atmosphere (ISA), density of air at sea level and at 15 °C can be taken as 1.225 21 kg m−3 Hence, the air density may be taken as 1.225 for most of the practical calculations This low density of air (e.g., 1/816th of water) makes wind a rather diffused form of energy Hence, large-sized systems are required for wind energy generation

Equation [2] shows that the power is directly proportional to the rotor area This indicates that, by doubling the rotor diameter, the power could be enhanced by 4 times Along with this, the unit cost of generation can be reduced significantly by increasing the size of the wind turbine This justifies the current trend in the wind power industry to develop huge turbines of several MW capacity

2.05.3.2 Power Coefficient, Torque Coefficient, and Tip Speed Ratio

Equation [2] gives us the theoretical power contained in a wind stream Rotor of a wind turbine can receive only a fraction of this power The efficiency with which a rotor can extract the kinetic energy of the passing wind stream depends on many factors like the profile of the rotor blades, blade arrangement and setting, and variations in the velocity This efficiency is commonly known as the power coefficient of the rotor Thus, the power coefficient (CP) can be defined as the ratio of actual power developed by the rotor to the theoretical power available in the wind stream So,

2P

where PT is the power output of the turbine rotor

Similarly, the theoretical thrust force experienced by the rotor (F) can be expressed as

1

2

If R is the rotor radius, then the corresponding torque T is given by

1

The above expression represents the maximum theoretical torque The ratio between the actual torque developed by the rotor (TT) and this theoretical torque is known as the torque coefficient (CT) Thus,

2T

The velocity of the tip of the rotor relative to the wind velocity is a critical factor deciding the power coefficient and thereby the power output of a wind turbine If the rotor blades move too slowly and wind velocity is high, a considerable portion of the incoming wind stream may pass through the blade gaps, without interacting with the blades On the other hand, if the rotor is rotating too fast and the wind velocity is low, the wind stream may get deflected from the rotor and thus the energy may be lost due

to turbulence and vortex shedding Hence, for maximum energy extraction, a dynamic matching between the rotor and wind velocities is essential

The ratio between the velocity of the rotor tip and the wind velocity is termed as the tip speed ratio (λ) Thus,

RΩ 2πNR

where Ω is the angular velocity and N is the rotational speed of the rotor in rad s−1 The power and torque coefficients of a rotor vary significantly with λ For every rotor, there is an optimum λ at which the energy transfer is most efficient and thus the power coefficient is the maximum (CP max)

As we have seen in eqn [4], the power coefficient is given by

2P

Cp ¼ T 2T

Dividing the above equation by eqn [7], we get

CT V Thus, the tip speed ratio is given by the ratio between the power coefficient and torque coefficient of the rotor

2.05.3.3 Airfoil Lift and Drag

For the efficient transfer of energy from the wind stream to the turbine, modern wind turbines have rotors made up of airfoil-shaped blades At the earlier stages of the technology development, airfoils from the aviation industry were adopted for wind turbine applications However, currently, custom-made airfoils capable of working under a wide range of Reynolds’ number and having better stall characteristics are being used in the turbines The DU airfoil series developed by the Delft University of Technology [12] is

a good example A sectional view of a typical airfoil, indicating its major features, is shown in Figure 14

streamlines above and below the airfoil Obviously, due to the typical curvature of the airfoil, particles following the upper streamlines would experience a higher velocity as they have to travel a longer distance and join back with the particles moving through the lower streamline at the same time As per Bernoulli’s theorem, to keep the total head above and below the airfoil the same, the increase in velocity should be compensated for with a reduction in pressure Hence, a pressure drop is experienced at the upper surface of the airfoil, casing a lift force L as shown in the figure Similarly, the fluid would also exert a drag force D on the airfoil The net force experienced by the airfoil F would be the resultant of these lift and drag forces