1.1 Wind Energy Analysis. This looks mainly at operation of wind turbines

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1. WIND ENERGY ANALYSIS 1

2. Lift and Drag Force A wind turbine output of varies with the wind's speed through the rotor. The "rated wind speed" is the wind speed at which the "rated power" is achieved( Near Maximum Point) At lower wind speeds, the power output drops off sharply. Using the power curve, it is possible to determine the power average speed prevalent at a site. 2

3. The lift principle

4. The Air Craft Lift

5. 5 Wind Turbine Blade The wind passes over both surfaces of the airfoil shaped blade, but different speeds causing pressure difference The pressure differential results in a force, called aerodynamic lift. The tip- speed is the ratio of the rotational speed of the blade to the wind speed. The larger this ratio, the faster the rotation There is also force, a "drag" force perpendicular to the lift force impedes rotor rotation. A wind turbine designed so that the blade can have a relatively high lift- to- drag ratio. This ratio can be varied along the length of the blade to optimize the turbine’s energy output at various wind speeds.

6. Drag and Lift Drag Devises The drag devises utilise the force that acts perpendicular to the wind direction. Lift Devises In may many bodies such as airfoils, transverse plates, the force resulting from interception of the air, does not only have drag force D aligned to the direction of the air, but also a force components perpendicular to the flow. D 2 The drag coefficient is a propositional constant describes the aerodynamic quality of the body. It takes the value of 1.11 for circular plate, 1.10 flat plate and 0.34 semi- sphere (open back) D = c 1 A p v 2 L 2 6 L = c 1 p Av 2

7. Aerofoil Theory The Pressure Differential Between Top and Bottom Surfaces Results in a Force, Called Aerodynamic Lift; In an Aircraft Wing, This Forces Causes the Airfoil to ”Rise," Lifting the Aircraft Off the Ground; “Since the Blades of a Wind Turbine are Constrained to Move in a Plane With the Hub as Its Center, the Lift Force Causes Rotation About the Hub”

8. Energy From the Wind The energy is extracted from the wind depends on the kinetic energy of the wind. p = the density of the air . • E = 1 mv 2 2 m = mass of the air passing the machine in a unit time. v = velocity of the air. Changein time Power = Change in kinetic energy m = p Av P = 1 ( p Av ) v 2 = 1 p Av 3 2 2 . A 2 8 P = 1 p v 3

9. The Turbine Dimensioning :Betz Power in the wind Up stream velocity u o Turbine velocity u 1 Down Stream u 2 The variation of areas, is max down stream The force or thrust on a turbine is the rate of change of momentum. . . F = mv – mv 2 . . T P = F v 1 = m ( v – v 2 ) v 1 2 2 9 1 . P wind = 2 m ( v – v 2 ) ( v – v ) v = 1/ 2( v 2 – v 2 ) = 1/ 2( v – v )( v – v ) 2 1 2 2 2

10. Power from Turbine The interference factor a . 2 1 1 1 1 A v ( v – v ) – ( 2 v – v )] = 2 p . 2 1 1 T P = p A v [ v 2 2 1 v + v v = a = ( v – v 1 ) v 2 2 . 1 T A (1 – a ) v [ v – (1 – a ) v ] P = 2 p 2 1 3 1 2 . A v ] = [4 a (1 – a ) ]( p . p C = [4 a (1 – a ) 2 ] 10

11. 11 The Power From Wind On differentiation with respect to a, the maximum Cp value occurs at a=1/3 The Cp max = 16/27= 0.59

12. Thrust on Turbines 2 2 2 2 2 1 1 1 2 1 2 1 + gz + v p + gz + v p = p p + gz + 1 v 2 = Cons tan t 2 p p 2 2 2 2 ( u – u ) o 2  p = p – p = 2 u 2  p = 2 12 2 A p u F thust max = . F Axil = m ( u – u 2 ) = p A 1 u 1 * 2 au 2 = A p u 2 * 4 a (1 – a ) = change in momentum

13. 13 The Thrust on Turbine • The term is the force hitting a solid disk. The actual turbine The actual force The maximum value of C F would be 1, when a = ½; equivalent to u 2 = 0, therefore maximum power extraction by the Betz criterion occur when a = 1/3 and C F = 8/9 As the wind speed increases. The thrust force may reach a level where by the wind energy converting system may not sustain the trust. 2 will experience a fraction of this force will be experienced by the turbine. Axial force coefficient C F . A p u 2 F Actual = C F 1

14. 14 The speed of rotation: Tip- speed ration λ It is important to determine the optimum rotation speed of the blade about the hub for a particular wind speed. At a very slow rational speed it will allow the wind to pass unperturbed : rotor will be inefficient. At very high rotational speed the rotor will appear solid to the wind. At some value between very slow and very fast is the optimum rotational speed for transmission of power to the rotor. The tip speed ratio λ is defined as the ratio of outer blade tip speed u t to unperturbed wind speed u .

15. The Tip Speed Ratio The tip speed ration is one of the most important parameters for the wind turbine design The tip of the blade moves at speed R x where R is the blade radius x is the angular speed of rotation or rotational frequency. If y and α are constant, their sum ф = y + α , is constant. Thus the cotangent of is also constant. This dimensionless ratio is the tip speed ratio. R x 15 λ = u t = u u

16. The Tip Speed Ratio Tip- speed ration λ It is important to determine the optimum rotation speed of the blade about the hub for a particular wind speed. The tip speed ratio λ is defined as the ratio of outer blade tip speed u t to unperturbed wind speed u . 16

17. 17 Tip Speed Ratio and Turbine Design. Both variable and fixed speed turbines reach their maximum, rates power in strong wind. The design procedure of the typical Wind Energy Conversion system is as follows: The maximum power of the wind turbine is determined. From meteorological information, the most likely wind speed, u0, and distribution of wind speeds, is deduced for the site The length of the blades, R, is calculated to optimise annual energy production at the least cost. The most probable rate of rotation is calculated, so the blade tips have a speed λ u0, where λ usually has a value of about 7- 10. An appropriate combination of gearbox generator and power electronics is chosen, to produce electricity at 50 Hz.

18. Torque Vs Tip Speed Ratio 18

19. Types of wind turbines

20. Tip Speed ratio Verses Performance Cp

21. 21 Designing Wind Turbine Determine the application Review pervious experiences Select topology Preliminary load estimates Develop tentative design Predict the performance and Evaluate design Estimate the cost and cost of energy Refine design and Build and test prototype Design Production machine

22. 22 The Blades Generally three blades are dynamically more smoother Two blades have cyclic inertia moments against the yaw One- bladed need very high quality aerofoil and balancing it as a challenge It operates better at very high wind speeds

23. 1-2- 3 Blades Turbines 23

24. 24 The Drive Train Components : rotor, shaft, bearings, breaks gear box, and generator

25. 25 The Wind Speed The wind speed is determines the application of WECS Wind Turbines reach the highest efficiency at the designed wind speed 3- 5 m.s - 1 water pumping12- 16 m.s - 1 electricity generation. Excessive power output of the rotor must be limited to keep the power output close to rated capacity. The energy content of the wind varies with the cube of the average wind speed. An annual average wind speed of above 6 m.s - 1 defines a good wind site for electricity and 4- 5 m.s - 1 water pumping.

26. 26 Number of Blades As a blade rotates, it moves into the space occupied by a previous blade, fast- turning rotors should have few blades. Having more blades increase torque on the rotor shaft. The general rule for the optimum number of blades on a rotor depends on its function: Electricity generation requires high speed at low torque, so its rotor has few blades. Water pumping requires large torque at low speed, so its rotor has many blades. The minimum number of blade is one, which is possible with a dense counterweight ; however the rotor motion is very uneven Having two blades is common, but motion is still not steady. A three- bladed rotor has a steady motion, is quieter and is visually the most acceptable.

27. 27 Solidity In many wind turbine machines, solidity is described by giving the number of blad es. This is a ratio of the total area of the blade at any moment in the direction of the air stream, to the swept area across the air stream high solidity means the machines used for water pumping low solidity turbines are used for electricity generation. The solidity can vary from less than 0.1 for two- blade windmill to about 0.65 for windmill used in water pumping.

28. 28 Design For power per unit rotor area, under standard conditions, a wind speed of 12 m/s), will have the potential power of about 1.06 kW/m 2 50% Reduction in wind speed to 6 m/sec reduce the potential wind power to 0.132 kW/m 2 ), an 8- fold reduction. In addition to the cubic relationship with wind velocity, the theoretical power obtainable varies as the square of the rotor diameter.

29. The Power Curve . 29

30. 30 Electrical power output The power output from a WECS will be a function of the wind power available, the power coefficient, the efficiencies of the mechanical interface, and, in the case of electric generation, the generator efficiency and the efficiency of any power- conditioning equipment. P e = 1/2 ρ v 3 A η g η m η p C p Where P e = Turbine electric power output η g = Generator efficiency η m = Mechanical interface efficiency η p = Power- conditioning equipment efficiency C p = Power coefficient .

31. 31 Optimisation The majority of power produced over the year will be at wind speeds above the annual mean. systems may be designed to optimize efficiency by : Under high wind speed conditions, at the expense of sacrificing efficiency under lower wind speeds. Efficiency may be sacrificed at steady- state design load to minimize wear and lower long- term maintenance and repair requirements To reduce sound emissions. Usually, large HAWTs are designed to start running at wind speeds 3 to 5 m/s.