Wind Energy Conversion Systems - IIT Bombaynpsc2008/NPSC_CD/Data/Tutorial 2/Wind... · Wind Energy...

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Wind Energy Conversion Systems Lecture Notes by Prof. Shireesh B. Kedare Adjunct Assistant Professor ENERGY SYSTEMS ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY Powai, Mumbai 400076, INDIA Contents 1. Background.................................................................................................................................................. 2 2. Introduction ................................................................................................................................................. 4 3. Historical aspects ......................................................................................................................................... 5 4. Wind machine parameters ........................................................................................................................... 6 5. C P - λ characteristics for wind machines ..................................................................................................... 7 6. Aerodynamic theories for wind machines ................................................................................................. 10 7. Airfoil section ............................................................................................................................................ 11 8. Wind analysis ............................................................................................................................................ 14 9. Wind machine mechanics .......................................................................................................................... 16 10. Power output of wind machine ................................................................................................................ 20 11. Annual Energy Output (AEO) for the site of interest.............................................................................. 21 12. Wind machine design .............................................................................................................................. 23 13. Controls ................................................................................................................................................... 25 14. Siting........................................................................................................................................................ 25 15. Economics ............................................................................................................................................... 26 Bibliography .................................................................................................................................................. 27

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  • Wind Energy Conversion Systems

    Lecture Notes by

    Prof. Shireesh B. Kedare Adjunct Assistant Professor

    ENERGY SYSTEMS ENGINEERING

    INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY Powai, Mumbai 400076, INDIA

    Contents 1. Background..................................................................................................................................................2 2. Introduction .................................................................................................................................................4 3. Historical aspects.........................................................................................................................................5 4. Wind machine parameters ...........................................................................................................................6 5. CP - characteristics for wind machines .....................................................................................................7 6. Aerodynamic theories for wind machines .................................................................................................10 7. Airfoil section............................................................................................................................................11 8. Wind analysis ............................................................................................................................................14 9. Wind machine mechanics..........................................................................................................................16 10. Power output of wind machine................................................................................................................20 11. Annual Energy Output (AEO) for the site of interest..............................................................................21 12. Wind machine design ..............................................................................................................................23 13. Controls ...................................................................................................................................................25 14. Siting........................................................................................................................................................25 15. Economics ...............................................................................................................................................26 Bibliography ..................................................................................................................................................27

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    1. Background Wind energy is a large renewable energy source. Global wind power potential is of the order of 11,000 GW. It is about 5 times the global installed power generation capacity. This excludes offshore potential as it is yet to be properly estimated. About 25,000 MW is the global installed wind power capacity. It is about 1% of global installed power generation capacity. Wind produces about 50 billion kWh per year globally with the average utilization factor of 2000 hours per year. Global wind power growth trends from 1980 to 1995 are shown in Figure 1 and the country wise details of installed wind power capacity from 1998 to 2001 is given in Table 1.

    Figure 1 : Global wind power growth trends from 1980 to 1995

    Table 1 : Installed wind power capacity from 1998 to 2001

    Country Installed

    capacity, Dec. 1998

    (MW)

    Installed capacity,

    Dec. 1999 (MW)

    Installed capacity,

    Dec. 2000 (MW)

    Installed capacity,

    Dec. 2001 (MW)

    Growth rate 2001-2002

    (%)

    3 year average

    growth (%)

    1. Germany 2874 4442 6107 8734 43% 45%2. USA 2141 2445 2610 4245 63% 26%3. Spain 880 1812 2836 3550 25% 59%4. Denmark 1420 1738 2341 2456 5% 20%5. India 992 1035 1220 1456 19% 14%6. Italy 197 277 424 700 65% 53%7. Netherlands 379 433 473 523 11% 11%8. U.K. 338 362 425 525 24% 16%9. China 200 262 352 406 15% 27%10. Greece 55 158 274 358 31% 87%11. Japan 25 ? 140 357 155% ? Total Global 9476 12964 17062 24927 35% 34%

    Wind Force 12, European Wind Energy Association and Green Peace, http://www.choose-positive-energy.org/docs/wind-force-12-2002.pdf, 2002

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    Wind energy is economically viable renewable energy source today. It costs about 5 cents per kWh globally. It is the fastest growing energy market today with the growth rate of 35% per year. In India, gross potential and technical potentials are estimated to be 45,195 MW and 12,8375 MW respectively. Figure 2 shows distribution of wind power potential in India. It indicates wind power density in W/m2.

    Figure 2 : Wind power density distribution in India (in W/m2 at 30 m hub height)

    Major efforts towards installation of wind machines in India started in the late 1980s and about 1,700 MW wind power capacity is installed by September 2002. Installed wind power capacity growth in India is shown in Figure 3.

    Figure 3 : Installed wind power capacity growth in India

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    India is ranking 5th in the world based on the installed wind power capacity. It is producing about 2 billion kWh per year from wind with the average utilization factor of 1175 hours per year. Cost of energy from wind is about Rs.4 to 5 per kWh in India. India has also started exporting 1 MW capacity machines. 2. Introduction Wind is an indirect solar energy source. Its characteristics can be summarized as follows.

    It is environmentally clean source of energy. It is a dilute source of energy. It is perennially available. Its availability is unpredictable. Data are available about its availability pattern around the day for different months of the year.

    Wind Machine : The machine that converts kinetic energy in the wind into usable form of mechanical energy (usually shaft power). Wind machines are typically used for mechanical applications like water pumping, grinding, woodcutting, or for AC or DC power generation in grid connected or isolated mode. Details of different types of wind machines are given in Table 2. Figure 4 shows typical wind machines.

    Table 2 : Different types of Wind Machines

    Type of machine No. of Blades

    Axis of rotation

    Rotor position wrt tower

    Starting torque

    Rotor speed

    Power

    Propeller machine 2 or 3 Horizontal Upwind or downwind

    Moderate Fast Electrical

    Multi-bladed machine 6 to 24 Horizontal Upwind High Slow MechanicalSavonius machine 2 or 3 Vertical - Very high Slow MechanicalDarrius machine 2 or 3 Vertical - Very low Fast Electrical

    Study of wind machines is called Molinology. It cuts across various fields including Meteorology, Aerodynamics, Machine Design, Structural Design, Materials Technology, Power Engineering, Reliability Engineering, Instrumentation and Controls Engineering. Three bladed propeller deign has evolved as the commercial Wind Energy Conversion System today. Designs of 225 kW, 450 kW, 650 kW or 1 MW are installed commercially in India. Design and performance details of a typical 1 MW unit installed by Suzlon are given in Table 3.

    Table 3 : Details of a typical 1 MW Wind Energy Conversion System

    1. Rotor diameter 61 m 2. Blade length 30 m 3. Rotor speeds 13 RPM / 22 RPM 4. Tower height 60 m 5. Wind speed range 3 m/s to 25 m/s 6. Power control Active blade pitching 7. Type of generator Induction generator 8. Generator Speed 1000 rpm / 1500 rpm 9. Generator rating 1 MW 10. Annual Energy Output 1.8 to 2 GWh 11. Machine cost About Rs. 4.2 Crores

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    Work on MOD series in US is now leading to the development of 3.2 MW 2-bladed propeller machine. It will have diameter of around 92 m and will have tower height of about 60 to 70 m.

    Figure 4 : Typical wind machine designs

    3. Historical aspects Some milestones in the history of wind machines are given in Table 4. Conceptual understanding of aerodynamic shape and position of center of forces or zero moment reduced the structural problem of supporting the blade. This was in the second decade of twentieth century. This is an important mile-stone in the history of wind machines. Thereafter, longer blades of aerodynamic shape could be designed and used.

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    Table 4 : Historical development of Wind Energy Conversion System

    Period Machine Application 640 AD Persian wind mills Grinding, etc Before 1200 AD Chinese sail type wind mill Grinding, water pumping, etc 12th century AD Dutch wind mills Grinding, water pumping, etc. 1700 AD Dutch w/mill to America 1850 to 1930 AD American Multi-bladed Water pumping, 35 VDC power 1888 AD Brush wind turbine; Dia.17m, Tower 18.3m 12 kW Electric power 1925 AD Jacobs 3 bladed propeller

    Dia.5m, 10-20m/h, 125 to 225 rpm 0.8 to 2.5 kW at 32 VDC

    1931 AD Yalta Propeller, Russia; 2 bladed, dia.100 ft 100 kW 1941 AD Smith-Putnam Propeller

    2 bladed, dia.175ft, 30 m/h, 28 rpm 1250 kW

    1925 AD Savonius Machine Mechanical or Electrical power 1931 AD Darrius Electrical power 1980s AD 2 bladed propeller (Commercially available) 225 kW 2000 AD HAWT, VAWT 400-625kW, 1.2-3.2 MW

    4. Wind machine parameters Power developed by the wind machine is mainly affected by wind speed, u, area swept by the rotor, AS, density of air, , rotational speed of the machine, , radius of the rotor, R, number of blades, B and total blade area. It is also affected by lift and drag characteristics of the blade profile. Lift and drag forces acting on a blade element is shown in Figure 5. Application of dimensional analysis evolves following parameters for characterizing wind machines.

    Figure 5 : Lift and drag forces acting on a blade element

    Coefficient of Performance, CP = P / ( AS u3) Where,

    P = Power output at rotor shaft (W) AS = Area swept by the machine (m2) = Air density (kg/m3) u = Undisturbed wind speed (m/s) Tip speed ratio, = R / u

    R = Rotor radius (m), = rotational speed of the rotor (rad/s) Solidity, = blade area / AS = (average chord x blade length x number of blades) / AS

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    Energy content of the wind The quantity ( AS u3) in the denominator of the definition of CP may be identified as power in the wind. It is proportional to the cube of wind speed, u as the volume of bottle is to its dimensions. If u = 15 kmph, power in the wind is 42 W/m2. For u = 36 and 90 kmph, it is 583 and 9102 W/m2. 5. CP - characteristics for wind machines Drag translator device Let a drag translator device shown in Figure 6 be subjected to wind with speed U. This is a device that utilizes the drag force of the wind acting on it to develop power. Let the device be moving at speed v under the wind forces acting on it in the direction same as that of the wind.

    Figure 6 : A drag translator device

    Then, the relative speed of the wind as seen by the device is (U v) and the drag force of the wind acting on a unit length of the drag translator device is given by,

    D = CD ( c (U-v)2) Where, CD = drag coefficient of the drag translator device c = width or chord of the drag translator device And the power developed, P by the drag translator device is given by (D.v) or P = CD ( c (U-v)2) v Coefficient of Performance, CP can then be written as, CP = CD (U - v)2 v / U3 Maximizing this with respect to (v/U) gives, (v/U) = and CP,max = (4/27) CD. With CD = 1.5, CP,max = 0.22. Lift translator device On the similar lines, let a drag translator device shown in Figure 7 be subjected to wind at speed U. This is a device that primarily utilizes the lift force of the wind acting on it to develop power. Let the device be moving at speed v under the wind forces acting on it in the direction perpendicular to that of the wind.

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    Figure 7 : A lift translator device

    Let the chord of the device be inclined at angle with respect to the direction of the wind. Then, the relative speed of the wind as seen by the device is Vr = (U2 + v2) and the driving force of the wind acting on a unit length of the lift translator device is given by, F = L cos D sin = ( c Vr2) [CL cos CD sin ] CL = lift coefficient of the lift translator device CD = drag coefficient of the lift translator device c = width or chord of the lift translator device And the power developed, P by the lift translator device is given by (F.v) or P = ( c Vr2) [CL cos CD sin ] v Coefficient of Performance, CP can then be written as, CP = [CL cos CD sin ] Vr2 v / U3 = [CL cos CD sin ] [1 + (v/U) 2] (v/U) Simplifying after substituting cos and sin in terms of (v/U) and maximizing CP with respect to (v/U) gives, (v/U) = (CL/CD) and CP,max = (2/9) CL (CL/CD) { (4/9) (CL/CD) 2 + 1 } With CL = 1 and (CL/CD) = 10, CP,max = 15. It is obvious from these results that the lift device has higher speed ratio and CP,max than those for the drag device. Betz limit on CP for actuator disc in the stream-tube For ideal rotor disc or non-participating actuator disc in the stream-tube of air flow as in Figure 8, maximum limit for power extraction can be derived under the following assumptions.

    No frictional (viscosity) losses Axi-symmetric flow, that is, absence of any vortex or rotational flow

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    Figure 8 : Actuator disc in the stream-tube

    Using continuity principle and momentum equation expression can be derived for thrust acting on the actuator disc and the power developed as wind flow retards across the rotor disc. CP is maximum (= 16 / 27 = 0.593) for an optimum value of flow retardation, a i.e., for an optimum value of . This limit is called Betz limit. CP - characteristics for practical wind machines CP - curves for typical wind machines are shown in Figure 9. Please note the positions of the curves for Savonious machines and multi-bladed machines. Contrary to belief, the Savonious machines have been shown to have high CP of the order of 0.3.

    Figure 9 : Cp- curves for practical wind machines

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    Effect of solidity (range 0.03 to 1)

    Torque increases as solidity increases Wake in the downwind flow increases as solidity increases Maximum power decreases as wake increases

    Hence, maximum power decreases as solidity increases Effect of tip speed ratio

    Tip speed ratio increases as solidity decreases Maximum power increases as tip speed ratio increases

    6. Aerodynamic theories for wind machines Equation for CP as function of for axi-symmetric disc rotor considering wake effect

    CP = (16/27) exp(-0.3538 -1.2946) Equation for CP as function of for rotor considering wake as well as drag effect with = CD / CL (Blade Element Theory)

    CP = (16/27) { exp(-0.3538 -1.2946) - } where, = CD / CL = Ratio of drag coefficient and lift coefficient for the airfoil Equation for maximum power coefficient, CP,max as function of with = CD / CL and B = number of blade

    CP, max = (16/27) { B0.67 / [1.48 + (B0.67 0.04) + 0.0025 2 ] [(1.92 2 B)/ (1 + 2 B)] } where, B = number of blade This equation plotted for different combinations of number of blades, B and ratio of drag coefficient and lift coefficient for the airfoil, is shown in Figure 10.

    Figure 10 : CP, max as function of for different combinations of B and

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    7. Airfoil section Airfoil is profiled to have very small drag. A typical airfoil having 200 mm chord and 15% thickness has drag equivalent to a wire of 1mm diameter. It is characterized by coefficient of lift, CL and coefficient of drag, CD that are function of angle of attack. Refer Figure 11. CL and CD are defined as,

    CL = L / ( A u2); CD = D / ( A u2)

    Figure 11 : Definition of chord and angle of attack for an airfoil section

    Different types of airfoil sections are shown in Figure 12a and typical lift and drag characteristics are shown in Figure 12b. Value of CL can go as high as 1.2 after which an airfoil stalls, typically at the angle of attack 150. Minimum value of CD / CL is typically in the range of 0.01 to 0.1.

    Figure 12a : Types of airfoils

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    Figure 12b : Typical curves of Cl vs and Cl vs Cd

    SERI blade sections The design requirements for airfoil sections of a wind machine are different from those of airplanes. In wind machines fouling is a major problem because once the machine goes on line it has to run round the day and round the year preferably without any break. Also the blades of wind machine are inaccessible once installed and cannot be easily cleaned. Further, dirt deposited on the blades affect the performance of the blade and in turn that of the machine significantly as shown in Figure 13.

    Figure 13 : Effect of dirt on blade on the performance of the machine

    Another consideration is the thickness of the blade at root. Typical NACA sections, common for airoplanes, used for wind machine are shown in Figure 14a. They have high CL and high (CL/CD) ratio. But, they are comparatively thinner. Structurally, a thick blade at root will always be easier to design to take loads. Based on these considerations, blades have been designed by Solar Energy Research Institute (now called National Renewable Laboratory NREL) in the USA. A set of these blades used for tip, middle span and root is shown in Figure 14b.

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    Figure 14a : NACA Airfoils Figure 14b : SERI Airfoils

    The comparative performance of wind machine with NACA and SERI blades when they are clean and fouled (dirty) is shown in Figures 15a and 15b.

    Figure 15a : Comparative performance of wind machine with clean NACA and SERI blades

    Figure 15b : Comparative performance of wind machine with fouled NACA and SERI blades

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    8. Wind analysis Wind data collection and analysis suitable for wind energy system siting and designing started since 1980 in India. The annual average and monthly average for July of mean hourly wind speeds for India at 10m above ground are shown in Figures 16 and 17. Hourly mean wind speed Wind speed is typically measured at 10 and 20 m height for 2 second duration at 1 minute interval. These data are used for calculating frequency distribution. The Hourly Mean Wind Speed is calculated by averaging the 1 minute data for the hour. Frequency distribution Power in the wind cannot be estimated unless its frequency distribution is known. This is recorded as hourly mean value and plotted for the site of interest. Typical frequency distribution, f(u) as function of wind speed u and cumulative frequency, F(u) also as function of wind speed u are shown in Figure 18.

    Figure 16 : Mean annual wind speed for India

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    Figure 17 : Mean monthly wind speed for India for July

    Figure 18 : Frequency distribution curve and Figure 19 : Weibull density function f(u) for cumulative frequency scale parameter c = 1

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    Weibull distribution The wind speed frequency distribution curve is closely represented by Weibull distribution as given below. Frequency function in s/m : f(u) = (k / c) (u / c)(k-1) exp [-(u / c)k]

    Scale factor in m/s, c > 1; Shape factor, k > 0; wind speed in m/s, u >0.

    This function has maximum at, u / c = [ (k-1) / k ](1/k) Effect of k on frequency density curve is shown in Figure 19. Factors c and k can be related to mean wind speed, umean and standard deviation, n-1 for the site of interest. c / umean = 1.12 for 1.5 < k < 3 and, k = ( n-1 / umean)-1.086 Elaborate curve fitting into the wind data at the site can also be done to find c and k. For this, cumulative frequency data is very useful. When integrated, Weibull distribution leads to cumulative frequency of occurrence of wind up to speed u given as,

    F(u) = 1 exp [ - (u / c)k ] Power density, Pd in the wind in W/m2, energy pattern factor, EPF and energy content, EC of the wind in Wh/m2/yr, defined below serve as useful parameters for assessing prospective wind energy site and are calculated from the data for the site of interest. Pd = f(u) u3 du ; EPF = Pd / ( umean3) = (u3)mean / (umean)3 ; EC = 8760 x Pd where, is air density at the site in kg/m3. Rayleigh distribution Rayleigh distribution has been commonly used for analyzing wind frequency distribution. It is a special case of Weibull distribution with k = 2 and c = (2 umean / ). It is given by, Frequency function in s/m : f(u) = ( u / 2 umean2) exp [-(/4) (u/umean2)] Power Law Index Variation in the wind speed, u along the height, z above ground level is given by the Power Law Index, as below. Value of is typically 0.14 or 1/7. Hence, it is called 1/7th Power Law.

    u1 / u2 = ( z1 / z2 )

    9. Wind machine mechanics Power developed by wind machine vs its speed of rotation for different wind speed can be calculated for the given machine from its Cp- curve and is shown in Figure 20. Constant speed lines are marked on the graph.

    0

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    Figure 20 : Shaft power output as function of speed of rotation for 17 m Sandia Darrius

    Starting and operating point for wind machine Refer Figure 21 that shows torque developed by a wind machine as a function of its rotational speed.

    Starting of the machine (propeller type) : Machine starts and stabilizes at high under no load Operating condition : Machine stabilizes at appropriate matching with load conditions

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    Figure 21 : Torque developed by a wind machine as a function of its rotational speed

    Fixed speed and variable speed machines

    Variable speed machines run at different RPM at different wind speeds. Load varies with machine RPM, e.g., reciprocating pump attached as load.

    Constant speed machines are designed to run at constant RPM. Load varies to match the power generated or blade angle is changed (pitched) to change the power developed.

    Two speed machines are designed to run at two different constant RPMs. They develop lower power at low wind speeds running at lower RPM. They develop higher power at high wind speeds running at higher RPM.

    Selection of operating speed is always an important design issue. Figure 22 shows power developed by a machine at different speeds of rotation. It can be seen that the machine developes more power at higher spped of rotation at higher wind speeds. However, at low wind speeds which are more frequent, a slower machine can develop higher power. Therefore, modern machines operate at two different rotational speeds with different ratings. Load matching An ideal load would be that for which the machine always develops maximum power or always operates at maximum Cp as shown in Figure 23. Any load that develops torque proportional to the square of its rotational speed can be matched with the machine requirement by appropriate gear selection.

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    Figure 22 : Power developed by a wind machine at different speeds of rotation

    Figure 23 : Torque vs rotational speed for a wind machine with ideal load

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    10. Power output of wind machine In practical machines, generator has the rating and it cannot be operated beyond this limit. Hence at higher wind speeds, wind machine blades are regulated to shed extra power. Thus the machine operates at reduced Cp as shown in Figure 24.

    Figure 24 : Cp vs u showing reduced Cp

    Mechanical and electrical power output as function of wind velocity for constant speed machine are shown in Figure 25. Idealized wind turbine output vs wind speed is shown in Figure 26.

    Figure 25 : Mechanical and electrical power Figure 26 : Idealized wind turbine output

    output as function of wind speed for vs wind speed 17 m Darrius turbine at 42 rpm

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    Following characteristic parameters for wind machine are clear from this Figure.

    Rated power of the machine, PeR is the maximum power developed by the rotor and is also the generator rating.

    Cut-in speed, uc is the minimum wind speed at which the machine starts rotating. Rated speed, uR is the minimum wind speed at which the machine develops rated power. This is

    the speed at which the blade regulation bcome active. Furling speed, uF is the maximum wind speed at which the machine develops power. The machine

    is stopped beyond this speed. Electrical power, Pe developed by wind machine as function of u may be idealized as,

    Pe = 0 for u < uc Pe = a + b uk for uc < u < uR Pe = PeR for uR < u < uF Pe = 0 for u > uF

    with, a = PeR uck / (uck - uRk ); and, b = PeR / (uRk - uck )

    11. Annual Energy Output (AEO) for the site of interest The total energy output of the machine considering the power output at any wind speed and frequency of occurrence of that wind in a year at the site of interest is AEO. Combining Pe as function of u and wind speed frequency distribution described by Weibull parameters for the site, the annual average power for the machine is given by,

    Pe, ave = PeR { [exp (-Qc) exp (-Qr) ] / (Qr Qc) exp (-Qf) } where, Qc = (uc / c)k , Qr = (uR / c)k , Qf = (uF / c)k

    Then, the Capacity Factor (CF) for the wind machine is given by, CF = Pe, ave / PeR Wind machine speed range for given site is selected such that Capacity Factor (CF) or Annual Energy Output (AEO) is maximum. The effect of rated wind speed for the machine on it CF or AEO can be studied by studying the behaviour of normalized power function, PN. Normalized power, PN is defined as,

    PN = CF (uR / c)3 Figure 27 shows PN plotted vs (uR / c) for different values of Weibull shape parameter k. Plot (a) is plotted for (uc / uR) = 0.5 and (uF / uR) = 2. Plot (b) is plotted for (uc / uR) = 0.4 and (uF / uR) = 2. These plots can be used for selecting uc, uR and uF for maximum Capacity Factor (CF) or Annual Energy Output (AEO) for c and k of the site. Selection of the ratio (uc / uR) depends upon the starting characteristics of the wind machine. This analysis can also be applied to see how much power can be generated for a wind machine of given parameters.

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    Figure 27a : PN vs (uR/c) for different values of k for (uc/uR) = 0.5 and (uF/uR) = 2.

    Figure 27b : PN vs (uR/c) for different values of k for (uc/uR) = 0.4 and (uF/uR) = 2.

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    12. Wind machine design Blade design is carried out based on aerodynamic analysis for optimum CP at design wind speed. Airfoil with 0SERI sections are used and chord and blade angles are calculated based on aerodynamic analysis. A photograph of a propeller machine blade being fabricated is shown in Figure 28 and nacelle of 3-bladed propeller machine is shown in Figure 29.

    Figure 28 : Propeller machine blade under preparations

    Figure 29 : Nacelle of 3-bladed propeller machine Mechanical designing of gears, shafts and brakes is carried out. They have effect on minimum power required to start the machine, cut-in-speed, capacity to absorb variation in rotor torque by storing in the form of strain energy, shaft rigidity for non-useful loads and flexibility for useful loads. Structural design is carried out for blades, root, hub, support structure, tower and foundation based on static, dynamic and fatigue loading. Blade root is subjected to loads as well as moments in all three direction. Further they vary through-out the life of the machine even if it is in a shut-down condition. Soft designing techniques like coning and teetering of rotor may be used in rotor design. Selection of tower is also important in wind machine design as it slows down the wind and also imparts excitation loads on the machine. Natural frequencies of different wind machine components need to be carefully designed considering the frequency of these excitation loads.

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    The wind machine should also be designed considering the requirements of transport and installation. Stronger and comparatively straight roads are required to transport long blades and heavy equipment to the site usually in remote areas. Similarly, heavy, sophisticated and costlier material handling equipment is required to handle heavy and unwieldy machine components at site. Figure 30 shows machine installed at site.

    Figure 30 : Wind machine being installed at site.

    Design drivers for wind machine components are listed in Table 5. Designing of wind machine is highly demanding job and beats even the design difficulties of aircraft or helicopters as is evident from Figure 31.

    Table 5 : Design drivers for wind machine components

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    Figure 31 : Comparative design challenge by different machines

    13. Controls The rotor power is limited by generator rating. At high wind speeds the power is regulated by any of the following approaches.

    Pitch control : blade is pitched so as to keep angle of attack low. Stall regulation : blade is designed so that angle of attack is high in stall range at high wind speeds Active stall control : blade is pitched so as to keep angle of attack high in stall range

    Yaw control and furling is done by yaw gear at the top of the tower. Furling can also be done by blade pitching. 14. Siting Wake effects of one machine on another decides the spacing between the wind machines in a farm. Typical spacing between the machines in a wind farm is shown in Figure 32 and effect of spacing on energy loss is shown in Figure 33. Grid connectivity, accessibility are important considerations in selection and design of wind machine site. Other considerations are reducing noise, transmission disturbance and visual disturbance.

    Figure 32 : Typical spacing for wind farm

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    Figure 33 : Effect of spacing on energy loss

    Typical modern wind farm is shown in Figure 34 and off-shore wind machine installation is shown in Figure 35. Wind machine farm in the hills of District Satara, Maharashtra, India is shown in Figure 36.

    Figure 34 : Typical modern wind farm Figure 35 : Off-shore wind machine installation 15. Economics The latest designs of propeller type wind machines are available in Indian market in 250, 450, 600 or 1000 kW size and cost Rs.4 to 4.2 Crore/MW. They are economic if they run for 1200 to 1500 hours annually.

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    Wind Energy Conversion Systems Prof. S.B.Kedare 27

    Figure 36 : Wind machine farm in the hills of District Satara, Maharashtra, India

    Bibliography Johnson G.L., Wind Energy Systems, Prentice Hall, 1985. Ministry of Non-conventional Energy Sources (MNES), Wind Power Development in India Towards Global Leadership, MNES, Government of India, October 2002. Rangarajan S., Wind Energy Resource Survey in India, Vol.5, Indian Institute of Tropical Meteorology, Bangalore, 1998 Spera D.A. (Ed.), Wind Turbine Technology : Fundamental Concepts of Wind Turbine Engineering, ASME Press, New York,1994. Sukhatme S.P., Solar Energy, 2nd Ed., Tata McGraw-Hill Publishing Co.Ltd., N.Delhi, 1996. http://www.windpower.org, Danish Wind Industry Association, Copenhagen V, Denmark

    .o0o. For any comments, contact : Prof. S.B.Kedare, Energy Systems Engineering, IIT, Powai, Mumbai 400076, INDIA sbk _ 030416