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

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 2.05.2 2.05.2.1 2.05.2.2 2.05.2.3 2.05.2.4 2.05.2.5 2.05.3 2.05.3.1 2.05.3.2 2.05.3.3 2.05.4 2.05.4.1 2.05.4.2 2.05.4.2.1 2.05.4.2.2 2.05.5 References Further Reading Introduction Evolution of Modern Wind Turbines Growth in Installed Capacity Increase in Turbine Size Improvements in System Performance Advances in the Control and Power Transmission Systems Economic Evolution Basic Principles Power Available in the Wind Power Coefficient, Torque Coefficient, and Tip Speed Ratio Airfoil Lift and Drag Classifications of Wind Turbines Horizontal Axis Wind Turbines Vertical Axis Wind Turbines Darrieus rotor Savonius rotor Rotor Performance Curves Nomenclature A wind rotor swept area exposed to the wind stream CD drag coefficient CL lift coefficient CP power coefficient of the rotor CP max maximum power coefficient of the rotor CT torque coefficient of the rotor D airfoil drag force F thrust force experienced by the rotor L airfoil lift force m air mass N rotational speed of the rotor P theoretical power of the rotor 93 93 94 95 96 98 100 100 100 102 102 104 104 106 107 109 110 111 111 PT power developed by the turbine rotor R rotor radius t temperature T theoretical torque of the rotor TT torque developed by the rotor V wind velocity normal to the rotor plane VR resultant velocity VT tangential velocity due to the blade’s rotation Z elevation of the site α angle of attack λ tip speed ratio ρ density of air 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 Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00205-5 93 94 Wind Turbines: Evolution, Basic Principles, and Classifications 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 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 [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] 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%, 250 000 Installed capacity (MW) 200 000 150 000 100 000 50 000 1994 1996 1998 2000 2002 2004 Year Figure Growth in cumulative wind power capacity from 1996 to 2010 2006 2008 2010 2012 Wind Turbines: Evolution, Basic Principles, and Classifications 95 40 35 Growth rate (%) 30 25 20 15 10 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Year Figure Annual growth rate of wind power over the last decade 44 189 1097 2397 58 641 2008 86 075 Africa and Middle East Europe North America Asia Latin America and Caribbean Pacific region Figure 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 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 777 550 and 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 Figure Today’s largest commercially available wind turbine has a rotor of 127 m diameter and 12 668 m2 swept area [5] The rated 96 Wind Turbines: Evolution, Basic Principles, and Classifications Cumulative capacity (MW) 500 000 000 000 Reference Moderate Advanced 500 000 000 000 500 000 2005 2010 2015 2020 2025 2030 2035 Year Figure Projected growth of wind power in the next 20 years 7000 140 6000 120 Rotor diameter 100 4000 80 3000 60 2000 40 1000 20 1975 1980 1985 1990 1995 Year 2000 2005 2010 Rotor diameter (m) Power (kW) Power 5000 2015 Figure 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) 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 The trend is derived from a compilation of capacity factor data Wind Turbines: Evolution, Basic Principles, and Classifications 97 40 Capacity factor (%) 35 30 25 20 15 Pre-1998 1998–99 2000–01 2002–03 Year 2004–05 2006 Figure 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 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 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 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 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 Hub height (m) 100 80 60 40 20 0 20 Figure Variations in hub height with rotor diameter 40 60 80 Rotor diameter (m) 100 120 140 98 Wind Turbines: Evolution, Basic Principles, and Classifications Increase in velocity compared with 10 m height 2.2 Smooth terrain (roughness height 0.005) Cultivated land (roughness height 0.1) Complex terrain (roughness height 0.75) 1.8 1.6 1.4 1.2 20 40 60 80 100 120 140 160 Height (m) Figure 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 illustrates this shift in the control options The number of pitch-controlled turbines available in the market is 4.50 Ratio of pitch to stall designs 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 1997 1998 Figure Increase in pitch-regulated turbine designs 1999 2000 2001 Year 2002 2003 2006 Wind Turbines: Evolution, Basic Principles, and Classifications 99 almost 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 Fixed speed Two speed Variable speed 75.79 Figure 10 Market shares of fixed, two speed, and variable speed turbines 100 2.05.2.5 Wind Turbines: Evolution, Basic Principles, and Classifications 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 in Figure 11 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 4500 4000 Cost (USD kW–1) 3500 Historic cost Projected cost 3000 2500 2000 1500 1000 1970 1980 1990 2000 Year Figure 11 Historic and projected capital cost of wind energy projects 2010 2020 2030 2040 Wind Turbines: Evolution, Basic Principles, and Classifications 101 0.1200 0.1000 USD kWh–1 0.0800 0.0600 0.0400 0.0200 0.0000 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Year Figure 12 Unit cost of wind energy generation V A V Figure 13 A wind turbine interacting with wind stream E ¼ mV 2 ½1Š where m is the mass of air interacting with the turbine, which is basically volume times the density of air The volume of air interacting with the turbine per unit time is given by AV Hence, the energy contained in the air stream per unit time – i.e., the power – is expressed as P ¼ ρAV ½2Š 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 −0:034 353:049 t ρ¼ e ½3Š t 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 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 102 2.05.3.2 Wind Turbines: Evolution, Basic Principles, and Classifications 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, Cp ẳ 2PT AV ẵ4 where PT is the power output of the turbine rotor Similarly, the theoretical thrust force experienced by the rotor (F) can be expressed as F ¼ ρAV 2 ½5Š If R is the rotor radius, then the corresponding torque T is given by T ¼ ρAV R ½6Š 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, CT ¼ 2TT ρAV R ½7Š 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, λ¼ R 2NR ẳ V V ẵ8 where 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 Cp ¼ 2PT 2TT Ω ¼ ρAV ρAV ½9Š Dividing the above equation by eqn [7], we get Cp RΩ ¼ ¼λ V CT ½10Š 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 Figure 15 illustrates such an airfoil placed in a flow stream Near the leading edge of the airfoil, the flow will get separated with 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 Wind Turbines: Evolution, Basic Principles, and Classifications 103 Leading edge Chord line Trailing edge Angle of attack Chord length Figure 14 Sectional view of an airfoil L Flow F D Figure 15 Flow around an airfoil If CL and CD are the lift and drag coefficients, then the lift force L is given by L ¼ CL ρAV 2 ẵ11 D ẳ CD AV 2 ½12Š and the drag force D can be expressed as For a given airfoil, the lift and drag forces are influenced by the angle at which the airfoil is oriented toward the wind flow This angle, which is the angle between the undisturbed wind direction and the chord of the airfoil, is known as the angle of attack (Figure 14) The effect of angle of attack on airfoil lift is shown in Figure 16 As we can see, lift increases with the angle of attack and reaches its maximum at a certain angle With any further increase in angle of attack, the lift is drastically reduced This is because, if the angle of attack is increased beyond a certain limit, the flow enters an excessively turbulent region and the boundary layers get separated from the airfoil At this region, lift force decreases and drag force is rapidly built up, resulting in the stall of the blade 14 1.6 12 1.4 1.2 0.8 CL 0.6 CL/CD 0.4 0.2 Figure 16 Lift–drag characteristics of an airfoil 10 15 Angle of attack 20 25 CL Lift / Drag 10 104 Wind Turbines: Evolution, Basic Principles, and Classifications L D VT VR α V Figure 17 Forces on a rotating blade In case of a wind turbine operating on lift principle, the lift force contributes to the energy conversion whereas the drag is a parasitic component Hence, in a given flow, we would like to place the blades at an angle for which the CL/CD ratio is the maximum CL–CD characteristic of an airfoil is shown in Figure 16 In this case the lift–drag ratio reaches its maximum for an angle of attack of 5° Now consider the blades of a wind turbine under rotating condition (Figure 17) In addition to the free wind stream velocity V, the blade section is subjected to the tangential velocity VT due to the blade’s rotation Hence, the velocity experienced by a point at the section would be the resultant of V and VT This is represented by VR in the figure, which has drag force D in line with and lift component L perpendicular to VR Under this condition, the angle of attack α is the angle between ‘VR’ and the chord line of the airfoil For a given rotational speed, the tangential velocity VT at different sections of the blade varies with the distance of the section from the rotor hub Hence, the angle at which the resultant velocity approaches the rotor would also be different along the blade section, being steeper at the root of the blade As we have seen, the CL/CD ratio for an airfoil is the maximum at a particular angle of attack To maintain this optimum attack angle throughout the blade sections, the blade may be twisted along its length 2.05.4 Classifications of Wind Turbines Based on the axis of rotation, wind turbines are classified as horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs) 2.05.4.1 Horizontal Axis Wind Turbines HAWTs have their axis of rotation horizontal to the ground and almost parallel to the wind stream Most of today’s commercial wind turbines fall under the HAWT category (Figure 18) Figure 18 An offshore wind farm with three bladed horizontal axis wind turbines Retrieved November 2011, from http://en.wikipedia.org/wiki/ Wind_turbine, © Hans Hillewaert, http://creativecommons.org/licenses/by-sa/3.0/ Wind Turbines: Evolution, Basic Principles, and Classifications Gear box Hub Main shaft Housing Blade 105 Generator Brake High-speed shaft Tower Figure 19 Sectional view of a HAWT Constructional features of a typical HAWT are shown in Figure 19 HAWTs work predominantly on lift principle As the wind stream interacts with the rotor blades, lift force is generated as explained in the previous section, causing the rotor to rotate The rotational speed varies with the design features and the size of the rotor For a typical MW-sized turbine, this could be as low as 16 rpm [5] The low-speed main shaft transmits this rotation to the high-speed shaft through the gearbox (there are direct drive turbines also, which not have a gearbox in the transmission line) The speed is enhanced by the gear trains to match with the higher speed requirement of the generator The generator then converts the mechanical energy to electrical energy There are a series of control systems in between for yaw alignment, power regulation, and safety A detailed description of these systems and their working principles are included in later chapters The number of rotor blades in a HAWT varies depending on the application for which they are used and wind regimes in which they are expected to work Based on the number of blades, HAWT rotors can be classified as single bladed, two bladed, three bladed, and multibladed Some of these classifications are shown in Figure 20 The major advantage of a single-bladed rotor is the saving in blade materials, making them comparatively cheaper It should be noted that the rotor accounts for 20–30% of the cost of a modern wind turbine Moreover, as the blade area exposed to the flow would be minimum for the single-bladed designs, the drag losses on the blade surface also would be lower Single-bladed designs are not very popular due to problems in balancing and visual acceptability Two-bladed rotors also have these drawbacks, but to a lesser extent Most of the modern wind turbines employed for electricity generation have three-bladed rotors The loading pattern for these rotors is relatively uniform and they are visually more acceptable (a) (b) (c) Figure 20 Single bladed (a), two bladed (b), and multibladed (c) turbines Retrieved November 2011, from http://en.wikipedia.org/wiki/ Wind_turbines_design Source: (a) Viterna, (b) NASA, and (c) Thomas Conlon, Iron Man Windmill Co Ltd., http://creativecommons.org/licenses/by-sa/3.0/ 106 Wind Turbines: Evolution, Basic Principles, and Classifications Wind turbines with more rotor blades (say 6, 8, 12, 18, or even more) are also available, which are usually used for specific applications like water pumping For example, wind-powered water pumping system with piston pumps requires high starting torque to overcome the initial load imposed by the water column on the piston For such systems, starting torque demand goes up to 3–4 times that of the running torque demand [13] As the starting torque increases with solidity (the ratio between the actual area of the blades and the swept area of a rotor), rotors with more number of blades (high solidity) are preferred for such applications However, high-solidity rotors work at low tip speed ratios and hence are not recommended for wind electric generators Similarly, their efficiency would also be lower as aerodynamic losses increase with solidity Further, a HAWT can have upwind- or downwind-type rotors An upwind turbine has its rotor fixed in front of the unit, directly facing the incoming wind stream (Figure 21) In contrast, the downwind turbines have their rotors positioned at the back side, leaving the nacelle to face the wind first The major advantage of upwind rotors is that they not suffer from the tower shadow effect However, upwind rotors are to be placed at some distance from the tower and a yaw mechanism is essential to keep the rotor always facing the wind On the other hand, downwind machines are more flexible and may not require the yaw mechanism This makes these designs relatively cheaper But, as the rotors are placed at the leeward side of the tower (see Figure 21), there may be uneven loading on the blades as they pass through the shadow of the tower There are several aerodynamic theories put forth for defining the performance of HAWTs Some of the basic theories are the axial momentum theory, blade element theory, and the blade element momentum (BEM) theory The most widely applied aerodynamic analysis for HAWT is based on the BEM theory Detailed discussions on these theories are presented in the corresponding Chapter of this Volume HAWTs have the following distinct advantages: • They are the most stable and commercially accepted design Today, most of the large – grid-integrated – commercial wind turbines work on three-bladed horizontal axis designs • They have a relatively lower cut-in wind velocity and higher power coefficient resulting in higher system efficiency and energy yield • There are possibilities of using taller towers to tap the better wind potential available at higher elevations This would be a distinct advantage at sites with strong wind shear where the velocity at higher levels could be significantly higher • There is greater control over the angle of attack, which can be optimized through variable blade pitching This results in better system output under fluctuating wind regimes • There is easy furling by turning the rotor away from the wind direction However, HAWTs have some inherent drawbacks as well: • HAWTs require yaw drives (or tail mechanism in case of small turbines) to orient the turbine toward wind • The heavy units of generator and gearbox are to be placed over the tall tower, which requires stronger structural support This makes the HAWTs more complex and expensive • Taller towers make installation and maintenance more difficult and expensive • Again, the taller mast height can make HAWT visible even from longer distances, which may aggravate problems related to the visual impact of wind farms 2.05.4.2 Vertical Axis Wind Turbines Wind turbines, which have their axis of rotation vertical to the ground and almost perpendicular to the wind direction, are called VAWTs (Figure 22) VAWTs have some distinct advantages over the HAWTs Upwind Figure 21 Upwind and downwind turbines Downwind Wind Turbines: Evolution, Basic Principles, and Classifications 107 Figure 22 A vertical axis Darrieus wind turbine Retrieved November 2011, from http://en.wikipedia.org/wiki/File:Darrieus-windmill.jpg Source: Aarchiba/http://creativecommons.org/licenses/by-sa/3.0/ • • • • VAWTs can receive wind from any direction They not require yaw drives to get oriented toward the wind The gearbox and generator can be positioned near ground level, simplifying the structural requirement of the tower Maintenance is easier as it can be done at ground level VAWTs not require pitch control for synchronous applications The major disadvantages of VAWTs are the following: • In general, VAWTs are not self-starting (except for drag-driven designs like the Savonius rotor) and hence not suitable for stand-alone applications This may not be a serious issue for grid-integrated systems as the generator can be used as a motor to push-start the system • As the tower height is low and the rotor is close to the ground, these turbines may not be able to utilize better wind at higher elevations • As the rotor completes its rotation, the blades have to pass through aerodynamically dead zones Hence the overall system efficiency may not be very impressive • The support structure may require guy wires which may cause inconveniences during installation and maintenance Though there are several types of VAWTs having different shape variations, we will limit our discussions to two major designs: Darrieus and Savonius turbines 2.05.4.2.1 Darrieus rotor Darrieus turbine concept was proposed by Georges Jean Marie Darrieus in 1931 In the original design, the blades were shaped like egg beaters or troposkein (turning rope) to minimize the bending stress experienced by the blades A typical Darrieus rotor is shown in Figure 22 and its operating principle is illustrated in Figure 23 The rotor has airfoils vertically mounted on a central shaft These symmetrically arranged airfoils are set to be aligned (at zero angle) with the mounting structure When the rotor starts its rotation, along with the incoming wind velocity, the blades would experience airflow due to the spinning of the rotor Hence, the velocity experienced by a blade would be the resultant of these two velocities as shown in the figure Thus, between the resultant wind flow and the chord of the blade, a positive angle of attack is experienced A net lift force is created as shown in the figure and the rotor is forced to rotate in a forward direction From the above discussion, it is evident that the angle of attack changes with the blade’s rotation With the changes in angle of attack, the lift force also would change Hence, the power developed by the blades would vary in a sinusoidal pattern with peaks at two opposite points (at the front and back of the turbine) This cyclic power pattern makes the design more complicated Further, it should be noted that for deriving the driving force, the rotor must already be spinning, and under stationary conditions, rational forces could not be developed This is the reason why Darrieus rotors are generally not self-starting Sometimes, high-solidity drag devices like Savonius rotors are used in combination with Darrieus rotors to make them self-starting (Figure 24) 108 Wind Turbines: Evolution, Basic Principles, and Classifications Lift Lift Wind Resultant velocity Wind velocity due to rotation Figure 23 Working principle of Darrieus rotors Retrieved November 2011, from http://en.wikipedia.org/wiki/File:Darrieus-windmill.jpg Source: GRAHAMUK, http://creativecommons.org/licenses/by-sa/3.0/ Figure 24 A Darrieus rotor coupled with Savonius rotor, Retrieved November 2011, from http://en.wikipedia.org/wiki/File: Taiwan_2009_JinGuaShi_Historic_Gold_Mine_Combined_Darrieus_Savonius_Wind_Turbines_FRD_8638.jpg Source: Hsu, http://creativecommons org/licenses/by-sa/3.0/ Another challenge is protection against high wind speeds As is evident, the rotor cannot be turned away from wind as it could be in the case of a HAWT Due to the typical shape of the blades, pitch control is also not possible Mechanical braking is the only option, though it is not a completely safe solution A variation of the Darrieus rotor with straight blades is shown in Figure 25 These straight-bladed Darrieus designs are often called Giromill or H turbines One advantage of this design is that as blade pitching is possible, overspeed protection is easier However, due to the intensive centrifugal forces, the loads acting on the blade would be very high Further, the H rotors require long spokes to connect the blades with the hub and these spokes have to be aerodynamically shaped to avoid excessive drag forces Cycloturbines, helically bladed VAWTs, and Musgrove rotors are some variations of the Darrieus concept Wind Turbines: Evolution, Basic Principles, and Classifications 109 Figure 25 Straight-bladed Darrieus rotor Retrieved November 2011, from http://en.wikipedia.org/wiki/File:H-Darrieus-Rotor.png.jpg Source: Stahlkocher, http://creativecommons.org/licenses/by-sa/3.0/ In spite of all its inherent technical limitations, it should be noted that the Darrieus turbines are the only VAWT which are being commercially manufactured The Darrieus concept still fascinates many wind turbine inventors and different versions of this technology are being proposed 2.05.4.2.2 Savonius rotor The Savonius wind turbine is a simple vertical axis wind machine invented by Sigurd J Savonius in 1922 In its original design, it was made with two half-cylindrical blades arranged in an ‘S’ shape The convex side of one half-cylinder and the concave side of the other face the wind at the same time as shown in Figure 26 As the drag coefficient of the concave surface is more than that of the convex side, in a given wind stream, the drag force experienced by the concave half would be higher than that of the other half (refer eqn [12]) It is this difference in drag force that spins the rotor to develop mechanical power Several variations of Savonius rotors are available For example, the Savonius rotor shown in Figure 27 has two sets of rotors placed at 90° offset to smoothen out torque fluctuations during the operation Similarly, instead of cylindrical rotors, blades with elliptical bases are also tried to improve aerodynamic efficiency Another attempt to improve performance was to attach deflector augmenters with Savonius rotors [11] The augmenter shades the convex half facing the wind and directs the flow to the concave half, thereby enhancing rotor performance The major advantage of the Savonius rotor is its simplicity As no sophisticated methods or technical skills are required for its fabrication, it can even be made in small local workshops This makes it attractive for applications in less developed countries Savonius rotors have high solidity and run at low speeds As high-solidity rotors can develop high starting torque, they are suitable for applications like water pumping The major disadvantage of these rotors is low efficiency Being a drag machine working at low tip speed ratios, these rotors have a relatively lower power coefficient (however, efficiency up to 35% has been reported for some experimental rotors [11]) Further, due Figure 26 Operational principle of a Savonius rotor 110 Wind Turbines: Evolution, Basic Principles, and Classifications Figure 27 A modified Savonius rotor with elliptical blades to their structural features, these rotors are installed on shorter towers and hence stronger wind speeds at higher heights cannot be utilized Similarly, compared with the HAWT option, Savonius rotors consume more materials per unit power rating 2.05.5 Rotor Performance Curves For a given design, the efficiency of a wind rotor depends on the relative speed between the rotor and the incoming wind Hence, the performance of the rotor is generally expressed in terms of the variations in the power coefficient (CP) with the tip speed ratio (λ) As we can see from eqns [4] and [8], both CP and λ are dimensionless parameters Hence, the CP–λ relationship deduced for a particular rotor will be valid for dimensionally similar rotors of any size 0.5 Power coefficient 0.4 Savonius Horizontal axis three bladed 0.3 Dutch four blade type 0.2 Vertical axis Darrieus 0.1 Multibladed 0 Figure 28 Performance characteristics of wind rotors Tip speed ratio Wind Turbines: Evolution, Basic Principles, and Classifications 111 Based on the axial momentum theory, it can be shown that the maximum theoretical efficiency of a wind turbine operating on lift principle is 16/27 (Betz limit) On the other hand, the maximum level of efficiency that a drag turbine can attain is only 8/27 [11] This is why rotors operating on lift principles are always preferred over drag devices Typical CP–λ curves for various wind rotors are shown in Figure 28 In general, the power coefficient increases with the tip speed ratio, reaches a maximum, and then decreases with further increase in the tip speed ratio In general, high-solidity multibladed rotors, operating at lower tip speed ratios, show poor power coefficients For example, a typical value for the peak power coefficient of such rotors could be 14% at tip speed ratios less than unity Efficiency of Savonius rotors is also not very impressive Two- and three-bladed horizontal axis rotors show the highest efficiency (CP between 0.4 and 0.5) Darrieus rotors, though running at higher tip speed ratios, show a lower power coefficient compared with low-solidity propeller turbines The reasons are explained in the previous sections [14] References [1] NASA (1982) Mod-2 wind turbine system development final report DOE/NASA/0002-2, NASACR.-168007 National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio, USA [2] http://windpower.sandia.gov/other/VAWThist.pdf Sandia National Laboratories, CA Retrieved November 2011 [3] GWEC (2011) Global wind report: Annual market update 2010 Global Wind Energy Council, Brussels, Belgium [4] GWEC (2011) Global wind energy outlook 2010 Global Wind Energy Council, Brussels, Belgium [5] ENERCON (2010) Wind Energy Convertors Product Review Aurich, Germany: ENERCON GmbH [6] John V (2010) Engineers race to design world’s biggest offshore wind turbines The Guardian, 26th July 2010, p 19 [7] Wiser R and Bolinger M (2008) Annual report on U.S wind power installation, cost, and performance trends: 2007 DOE/GO-102008-2590/May 2008, U.S Department of Energy, Springfield, USA [8] http://www.wind-energy-the-facts.org/en/part-i-technology/chapter-3-wind-turbine-technology/technology-trends/hub-height.html European Wind Energy Association, Brussels, Belgium Retrieved November 2011 [9] DOE (2010) Advanced wind turbine drive train concept: Workshop report Wind and Water Power Programme DOE/GO-102010-3198/December 2010, U.S Department of Energy, Springfield, USA [10] Kaldellis JK and Zafirakis D (2011) The wind energy (r)evolution: A short review of a long history Renewable Energy 36(7): 1887–1901 [11] Mathew S (2006) Wind Energy: Fundamentals, Resource Analysis and Economics, 1st edn Berlin, Heidelberg, Germany: Springer [12] Timmer WA and vanRooy RPJOM (1993) Wind tunnel results for 25% thick wind turbine blade airfoil In: Proceedings of EWEC’93 Lubeck, Travemunde, Germany, European Wind Energy Association, Brussels, Belgium, pp 416–419 [13] Mathew S and Pandey KP (2003) Modelling the integrated output of mechanical wind pumps Journal of Solar Energy Engineering, Transactions of the American Society of Mechanical Engineers 122(4): 203–206 [14] UNFCCC (2010) Report of the conference of the parties on its fifteenth session, held in Copenhagen from to 19 December 2009 – Addendum – Part Two: Action taken by the conference of the parties at its fifteenth session United Nations Framework Convention on Climate Change, 30 March 2010, Copenhagen Further Reading [1] [2] [3] [4] [5] [6] [7] [8] Burton T, Jenkins N, Sharpe D, and Bossanyi E (2011) Wind Energy Handbook, 2nd edn Hoboken, NJ: Wiley Chiras D (2010) Wind Power Basics: A Green Energy Guide, 1st edn Canada: New Society Publishers European Wind Energy Association (2009) Wind Energy – The Facts: A Guide to the Technology, Economics and Future of Wind Power, 1st edn Oxford, UK: EarthScan Gipe P (2009) Wind Energy Basics: A Guide to Home- and Community-Scale Wind-Energy Systems, 2nd edn White River Junction, VT: Chelsea Green Publishing Manwell JF, McGowan JG, and Rogers AL (2010) Wind Energy Explained: Theory, Design and Application, 2nd edn Hoboken, NJ: Wiley Mathew S and Philip GS (2011) Advances in Wind Energy Conversion Technology, 1st edn Berlin, Heidelberg, Germany: Springer Musgrove P (2010) Wind Power, 1st edn Cambridge, UK: Cambridge University Press Spera DA (2009) Wind Turbine Technology: Fundamental Concepts in Wind Turbine Engineering, 2nd edn American Society of Mechanical Engineers, ASME Press, New York, USA  ... cumulative wind power capacity from 1996 to 20 10 20 06 20 08 20 10 20 12 Wind Turbines: Evolution, Basic Principles, and Classifications 95 40 35 Growth rate (%) 30 25 20 15 10 20 00 20 01 20 02 2003 20 04 20 05. .. of wind energy projects 20 10 20 20 20 30 20 40 Wind Turbines: Evolution, Basic Principles, and Classifications 101 0. 120 0 0.1000 USD kWh–1 0.0800 0.0600 0.0400 0. 020 0 0.0000 19 92 1994 1996 1998 20 00... (Figure 22 ) VAWTs have some distinct advantages over the HAWTs Upwind Figure 21 Upwind and downwind turbines Downwind Wind Turbines: Evolution, Basic Principles, and Classifications 107 Figure 22 A