Advanced Energy Engineering :- Wind Energy.pptx

AEE Module 2 part 2
Wind Energy

1. MET 458 ADVANCED ENERGY ENGINEERING Module-III

2. Introduction

3. During night time air over lands cools rapidly than water over offshore land causing breeze as shown below.

4. Moreover, the wind gets stronger on the top of the rises or in the valleys oriented parallel to the direction of the dominant wind, whereas it slows down on uneven surfaces, such as towns or forests, and its speed with respect to the height above ground is influenced by the conditions of atmospheric stability.

5. History

6. Renewable Energy  Installed capacity in India

7. Wind power plant Wind flow is created as an effect of solar heat, which creates low and high-pressure regions on the earth due to heating. Thus wind energy is rightly an indirect form of solar energy. The flowing wind is used to rotate the wind turbine , which is also known as windmill . Wind turbines are usually located at the sea shore or in the sea where there is availability of wind. For electric power generation, the average wind speed required is 5 m / s .

8. Operation

9. Sources of wind Winds are natural phenomena in the atmosphere and have two different origins viz., planetary winds and local winds .   Planetary winds – Planetary winds are caused by solar heating of the earth's surface near the equator than near the north or south poles. This causes warm tropical air to rise and flow through the upper atmosphere towards the poles and cold air from the poles to flow back to the equator nearer to the earth's surface. The direction of motion of the planetary winds is affected by the rotation of the earth. Local winds – Local winds are caused by un-equal heating and cooling of land and water, and also by hills and mountain sides. During the day warmer air over land rises upwards and colder air from lakes, ocean, forest areas, flows towards warmer zones.

10. WIND SPEED DISTRIBUTION Wind speed is the most critical data needed to appraise the power potential of a candidate site. The wind is never steady at any site. It is influenced by the weather system, the local land terrain, and its height above the ground surface. Wind speed varies by the minute, hour, day, season, and even by the year. Therefore, the annual mean speed needs to be averaged over 10 year or more.

11. Size and Applications

12. Working Principle of Wind turbine Wind turbines operate on a simple principle. Wind is merely air in motion. Wind turbines convert kinetic energy from the wind that passes over the rotors into electricity. The kinetic energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity.

13. Components of wind turbine Hub – The blades are attached to the hub . Rotor – Blades and hub together is called the rotor . Rotor is attached to the slow speed shaft. Nacelle – Nacelle is the cover housing that houses all of the generating components in a wind turbine, including the generator, gearbox, drive train, and brake assembly. Tower – The tower of the wind turbine carries the nacelle and the rotor . Towers may be made from steel or concrete. Gears – Gears connect the low-speed shaft attached to the hub to the high-speed shaft attached to the generator and increase the rotational speed.

14. Parts of Wind Turbine Anemometer: Measures the wind speed and transmits wind speed data to the controller. Blades: Lifts and rotates when wind is blown over them, causing the rotor to spin. Most turbines have either two or three blades. Brake: Stops the rotor mechanically, electrically, or hydraulically, in emergencies.

15. Wind turbine Power Plant Operation

16. Lift and drag

17. Aerodynamic principle of wind turbines All the wind turbines work on two physical principles (or combination of these two) in blade designs by which energy is extracted from the wind. These principles are either ( i ) drag principle or (ii) lift principle . Blade designs operate on either the principle of drag or lift.

18. For measurement of wind speed, the basic sensors used are anemometers and for measurement of direction, wind vanes are used. The most commonly used anemometer is rotating cup anemometer . In this type, a vertical shaft supports a cup assembly. The cup rotates about the vertical axis in proportion with the incoming wind speed. The calculation of the power of the wind energy ( P t ) is based on the kinetic energy of moving air molecules. According to Betz' law , wind power , Therefore, wind speed ( V ) is the most important parameter, as wind energy ( P t ) is proportional to the cube of wind speed . P t = Power available in the wind, = Density of air, A = Swept area, and V = Velocity of wind.

19. Classification of wind turbines

20. Classification of wind turbines Horizontal axis wind turbine ( HAWT ) –The horizontal axis machines have to face the direction of the wind in order to generate power. In addition to being parallel to the ground, the axis of blade rotation is parallel to the wind flow. Vertical axis wind turbine ( VAWT ) – In vertical-axis wind turbines, the orientation of the spin axis is perpendicular to the ground. A vertical axis wind turbine can catch wind in all directions . So, a vertical axis machine need not be oriented with respect to wind direction . This means that unlike a HAWT, no yawing mechanism (adjusting the nacelle about the vertical axis to bring the rotor facing the wind) is needed for a VAWT. Because the shaft is vertical , the transmission and generator can be mounted at ground level allowing easier servicing and a lighter weight, lower cost tower.

21. Horizontal axis wind turbines (HAWT) Commonly found horizontal axis wind turbines are multi-blade type , sail type and propeller type . Both the multi-blade and sail-type wind turbines run at low speeds of 60 to 80 rpm . The propeller type has two or three aerofoil blades and run at speeds of 300 to 400 rpm .

22. Horizontal Wind Turbine Operation

23. Vertical axis wind turbines (VAWT) The basic vertical axis designs are the Darrieus type , which has curved blades and efficiency of 35%, and the Savonius type having the efficiency of 30%. Savonius type uses drag forces to create rotation of the shaft. Savonius windmill consists of a hollow circular cylinder sliced in half, the two halves being fixed to a vertical axis with a gap in between Darrieus type uses lift forces to create the rotation of the shaft. Darrieus type requires much less surface area. It is shaped like an egg beater and has two or three blades shaped like aerofoils .

24. SAVONIUS WIND TURBINE Savonius turbines are one of the simplest turbines. Aerodynamically, they are drag-type devices , consisting of two or three scoops. Looking down on the rotor from above, a two-scoop machine would look like an "S" shape in cross section. Because of the curvature, the scoops experience less drag when moving against the wind than when moving with the wind.

25. DARRIEUS WIND TURBINE The Darrieus wind turbine is a type of vertical axis wind turbine (VAWT) used to generate electricity from the energy carried in the wind. The turbine consists of a number of aerofoils usually—but not always—vertically mounted on a rotating shaft or framework.

26. GIROMILL WIND TURBINE A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting. The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used. Giromill VAWTs are also self-starting.

27. Upwind and downwind machines In an upwind machine are those machines that have rotor facing the wind. In these machines the wind meets the rotor first and then leaves from the direction in which the nacelle is located. In a downwind machine, the rotor is located downwind of (behind) the tower as shown in the figure. This means the nacelle comes first in the path of the wind and then the blades.

28. Horizontal axis machines Axis of the turbine is parallel to the wind direction Example : Ducted rotor

29. As the blades are turned by the wind, centrifugal forces pull air from the hallow tower through the blade tips At the same time pressure difference between tips of the rotor and blade pedestal also draws air up through semi vacuum created in the tower As the air passes through the tower and passes through the turbine and gives energy to it No direct coupling between rotor and power generating equipment

30. Vertical axis machines Axis of the wind turbine is perpendicular to the motion of the winds Torque is produced by the pressure difference between concave and convex surfaces of the half facing the wind and by recirculation effects on the convex surfaces It found to be heavy weight per unit power output It requires external input power for starting

31. Nomenclature of WECS An aerofoil is the cross-section view of an aircraft wing. Angle of attack-Angle between chord and relative airflow is known as angle of attack Upstream -away from leading edge in the air flow direction Downstream -away from trailing edge in the air flow direction Swept area- area covered by the rotating rotor Solidity - ratio of blade area to swept area Cut in speed- at which wind turbines start operates

32. Cut out speed- above which wind turbines stops due to high winds Yaw control- it keeps the axis of the turbine in wind direction

33. Advantages of Vertical axis turbine

34. Disadvantages of Vertical axis turbine

35. Comparing HWT and VWT Parameter HWT VWT Tip speed ratio High and hence noisy. Low and hence less noisy. Application Large scale electricity generation. Small scale electricity generation. Yawing Yawing is required, as HWTs are dependent of wind direction. Yawing is not required, as VWTs are independent of wind direction, but are affected by wind speed. Torque Low. More at lower wind speeds. Maintenance Difficult. Easier, as heavy components can be located at the ground level. Stability More stable and hence large sized turbines can be constructed. Less stable.

36. Performance of wind turbines The tip-speed ratio is the ratio of the rotational speed of the blade to the wind speed . The larger this ratio, the faster the rotation of the wind turbine rotor at a given wind speed. Lift-type wind turbines have maximum tip-speed ratios of around 10, while drag-type ratios are approximately 1. The coefficient of performance is defined as the ratio of the power delivered by the rotor, P , to the maximum power available, P t , in the wind and is given by the following expression. It is seen that the values of tip speed for the multi-blade and Savonius types are much lower than the values for the propeller and the Darrieus types. It is also seen that the highest values of C p are obtained with the propeller type.

37. Power Solar-wind hybrid energy system is the combined power generating system consisting of wind turbines and solar energy panel. It also includes a battery which is used to store the energy generated from both the sources. Using this system, power generation by wind turbines when wind source is available and generation from PV module when light radiation is available can be achieved. Both units can be generated power when both sources are available.

38. Storage of wind energy

39. Wind Energy Farms

40. Wind Resource Surveys Three types of wind survey projects were undertaken during 1985 by MNRE with Indian Institute of Tropical Meteorology 1 st category was of a wind monitoring project to determine windy locations using 20m mast and a microprocessor based measuring instrument, to generate data for wind power development 2 nd category constituted wind mapping projects, based on 5m mast, to establish wind regime in a given area on an extensive basis 3 rd category projects covered complex terrain studies in hilly and mountainous regions to find wind flow in mountain passes and over undulated terrains

41. Assessment of Wind Availability from Meteorological Data Meteorological data is used to evaluate: To identify the areas where highest wind speeds are available To measure Mean Annual Wind Speeds (MAWS) and their variability form year to year To record Monthly Mean Wind Speeds to indicate wind regimes for the area Measurement of daily mean wind speeds to understand their variation during different seasons The MAWS is an approximate index of wind potential at a site. Mean Monthly Wind Speed provides a comprehensive pattern about variability in wind energy during the course of the year.

42. Daily Mean Wind Speed is the average of winds during 24 hours of the day. A threshold speed of 15kmph is the lowest speed needed to operate the wind electric generators. Wind power classification

43. Estimation of Wind Energy Potential Wind speed extrapolation : Wind Speed data are recorded by data loggers at a height of 10m and 20m. Wind speed increases with height as per power law. Since WEGs are installed at a greater height, it is necessary to extrapolate the mean wind speed measured at one level to higher levels. Methods of calculation: Based on wind data of a specific site using frequency distribution Based on type of wind energy generator Based on Weibull factors of wind data and WEG characteristics.

44. Equations used for Calculations Based on wind data: Power law index is calculated from the equation: Wind power density is calculated from the equation: Based on wind energy generator (WEG): Machine capacity factor(CF): Ratio of average power output of a turbine during a month or a year to the rated power output

45. 2. Capacity utilisation factor (CUF): Capacity factor on the basis of WEG characteristics and using Weibull factors. Energy likely to be generated is calculated using the power curve of the WEG and above equations, based on frequency distribution.

46. Wind Resource Assessment in India Centre for Wind Energy Technology (C-WET), Chennai conducts wind energy assessment in India in coordination with state nodal agencies. India’s wind power potential has been assessed at 45000MW. But potential for the grid-interactive wind power is less, around 15000MW.

47. Technical Planning of a Wind Power Project Phase I : land availability, characteristic location and landscape profile : the proposed capacity of the project determines land requirement. Accessibility to wind project site: Approach roads to the site are needed for the transportation of wind turbine and electrical parts, civil construction materials, etc Soil characteristics : Soil investigation of the proposed site has to be carried out for foundation and earthing designs State grid : Grid must be available to pump generated electricity to the electricity board grid. Ambient conditions at the proposed site: Temperature, relative humidity, corrosion factor, sand and salt concentration in air, etc would affect the WEG performance.

48. Phase II : Micro- siting of the wind electric generators (WEGs) : siting is necessary to optimise the power output Visual inspection of the land helps in understanding the topography of the terrain. WEGs are located at highest level of the land in the region of least turbulence. Array efficiency should normally be above 95%, which depends on specific configuration and orientation. Minimum loss due to shadow effect should be ensured A schematic layout of a 10MW wind power plant having 50 nos. of 225kW WEGs is given:

49. Annual Energy Output The power curve of a 225 kW WEG as a function of wind speed distribution pattern ( follows the Weibull probability density function):

50. Annual generation at wind farms with different wind speeds: The capacity of a wind generator is optimised to suit the site by having theoretical energy projections. A right choice of WEG reduces the generation cost.

51. SITE SELECTION OF WIND POWER PLANT High, exposed sites. Not suitable sites in highly populated residential areas. Avoid roof mounted turbines. Power transmission loss Distance between the turbine and the nearest obstacle Connection with national power grid

52. Advantages of Wind Power Environmental No air pollution No greenhouse gasses No water needed for operations Resource Diversity & Conservation Domestic energy source Inexhaustible supply Small, dispersed design reduces supply risk Cost Stability

53. Economic Development Expanding Wind Power development brings jobs to rural communities Increased tax revenue Purchase of goods & services Wind turbines can be used for both distributed generation or grid interactive power generation using on-shore or off shore technologies. Ranges of power producing turbines are available. Micro-turbines are capable of producing 300 W to 1MW and large wind turbines have typical size of 35 kW –3 MW . It can be made available easily in many off-shore, on-shore and remote areas; thus, helpful in supplying electric power to remote and rural areas. It is a non-polluting and environment friendly source of energy.

54. It is an important renewable and sustainable source of energy, available free of cost. The scope of wind resource, globally, is enormous and is less dependent on latitude than other solar based renewable energy technologies. Power generation is cheaper as there is no shortage of input cost and recurring expenses are almost nil.

55. Disadvantages of wind power It has low energy density. Electricity production depends on- wind speed, location, season and air temperature. Hence various monitoring systems are needed and may cost expensive. High percentage of the hardware cost (for large wind turbine) is spent on the tower designed to support the turbine It is variable, unsteady, irregular, intermittent, erratic and sometimes dangerous. Wind turbine design, manufacture and installation have proved to be complex due to widely varying atmospheric conditions in which they have to operate.

56. Wind farms can be located only in vast open areas in locations of favorable wind. Generally, such locations are away from load centers. The appearance of wind turbines on the landscape and their continual whirling and whistling can be irritating.

57. Environmental impacts of wind power Conventional technologies have regional and global impacts due to their emissions , however the impacts of wind energy systems are local Erosion Bird and bat kills Visual impacts Noise

58.  THE POWER IN THE WIND Wind mills converts the kinetic energy of the wind to mechanical energy. The total power of the wind stream is equal to the time rate of kinetic energy. Kinetic energy = 1/2 mv 2 ...(9.1) The amount of air passing in unit time, through an area A. with velocity V = AV. Mass m = PAV ...(9.2) where 'p' is the density of air particles (kg/m²) Kinetic energy per unit volume = From equation (9.1) and (9.2) Kinetic energy = PAV³ Watts Total power P, = PAV³ Watts ...(9.3)

59. All this power cannot be extracted because for this wind velocity would have to be reduced to zero which means that the wind mill would accumulate static air around it which would prevent the wind mill operation. It is clear that the power output of a windmill varies as cube of the wind velocity. directly proportional to wind density and directly proportional to area of stream A So, the wind mill produces maximum power at high wind velocity. Wind velocities below 5 m/s and above 25 m/s are not suitable for wind turbine. At lower speed, very large turbine-rotor is required and at higher wind speed, the stresses on turbine blade and shaft are very high. From equation (9.5) wind power is proportional to the intercept area. Thus windmill with a large swept area has higher power. Normally, area is circular with diameter D, thus

60. A = ( Π * D 2 )/4 then P t = ½ PV3 Π D 2 /4 = 1/8 ρ Π D 2 V 3 From e quation (9.6) it is clear that power is proportional to the square of the diameter of swept area. The combined effects of wind speed and rotor diameter variation shown in Fig. 9.3.

61. Most commonly used wind turbine is horizontal axis, propeller type. Consider this wind turbine. Let a = Inlet plane b = Exit plane P₁ = Incoming wind pressure V = Incoming wind velocity P = Wind pressure at exit from blades C V= Wind velocity at exit from blades (m/s) v = Specific volume = 1/ ρ V e is less than Vi because Kinetic enregy extracted by the turbine.Assuming no energy loss and no change in air density. The pressure and velocity changes are plotted as shown in fig 9.4

62. Applying total energy equation Similarly, for exit area The wind velocity decreases from a to b, because kinetic energy is converted to mechanical work. Therefore, V i > V a V b > V e P a > P i P b <P e

63. From equations (9.8) and (9.9) Assume that at exit end away from the turbine at e, can be assumed to ambient i.e.. P = P₁ V₁ = V = V₁ then from equation (9.10). If 'A' is the projected area of wind mill perpendicular to the wind stream, the axial force F, is given by

64. Axial force also equal to the change of momentum Equating the equations (9.12) and (9.13)

65. Now consider the total thermodynamic system bounded by i and e. The general energy equation now reduces to the steady flow work w' and kinetic energy terms.

66. This is the condition for maximum power V opt is the optimum exit velocity. Put the value of V e opt in equation 

67. The maximum efficiency also called power coefficient is given by where P toal is the total power in the wind stream and put this value from equation (9.5). From equation (9.21) it is clear that maximum efficiency of a propeller type turbine is 59%. The factor 59 is known as Betz limit. Actual efficiency is less than the maximum efficiency. Actual efficiency is generally 50% to 70% of maximum efficiency. n = Actual efficiency = 0.6 × 0.59 = 0.354.

68. Therefore, the actual efficiency is approximately 35%. Fig. 9.6 shows the power coefficient for different types of rotors verses the tip speed ratio (which is the ratio of tip (peripheral) velocity of rotor to the wind speed).

69. FORCES ON BLADES AND TORQUE OF WINDMILL The torque or circumferential force causing the rotation of the wind turbine shafts depends on the turbine rated power output and rotor angular velocity. Thus T = Torque (N) ω = Angular velocity of turbine wheel (m/s) N = Speed of the wheel D = Diameter of turbine wheel (m)

70. Efficiency = P/P t P = η * P t Put the value of P, from equation (9.6) From the equation 9.22 The value of the torque will be maximum at maximum efficiency. Put the value of maximum efficiency from (9.21)

71. Axial force is given by equation (9.12) Put the value of A in above equation

72. Maximum axial force will occur at maximum efficiency. The condition for maximum efficiency is V e = 1/3 V i Put this value in equation (9.27) From equation (9.28) it is clear that the axial force is directly proportional to the square of the diameter. Therefore, there is an upper limit of diameter of the wheel. The ratio of peak rated wind velocity to average wind velocity is an important parameter which governs the overall performance of the windmill system. For a generator of a given rated power, a low peak to average velocity ratio requires a large rotor windmill while a high peak to average velocity ratio requires a small rotor windmill. A large rotor mill is more expensive but gives greater average output and, therefore, a balance between the two is necessary.

73. 1. A propeller type, horizontal shaft wind turbine having following wind characteristics. Speed of wind 10 m/s at 1 atm and 15 °C. The turbine has diameter of 120 m and its operating speed is 40 rpm at maximum efficiency. Calculate ( i ) the total power density in the wind stream (ii) the maximum obtainable power density assuming eta = 40 % (iii) total power produced (in kW) and (iv) the torque and axial thrust.

74. Solution. where, Air density p= P RT P= Pressure of air, Pat T = Temperature of air, K R= Gas constant R = 0.287 KJ/kg K 1atm = 1.01325 * 10 ^ 5 * P * a T= 15+ 273 288 K

75. POWER DURATION AND VELOCITY DURATION CHARACTERISTIC OF WIND Fig. 9.6 shows the velocity duration and power duration curve wind power per unit area is proportional to cubic power of wind velocity. These curves are for a certain height from the ground. These curves are useful in deciding the design wind speed, once the type of windmill is decided and performance is known, these data also useful in estimating the actual energy output of the plant.

76. The number of hours for which mean wind velocity of a particular value is available is called the frequency duration curve, shown in Fig. 9.7

77. For the design of windmill following wind speeds are associated ( i ) Cut-in speed (V). Wind speed at which wind-turbine starts delivering shaft power or in other words, the speed below which the windmill does not operate. (ii) Design speed (V). The speed for which rotor is designed. (iii) Rated wind speed. It is speed at which the wind turbine generator delivers rated power. At this speed plant output is maximum. (iv) Cut-out wind speed or Furling speed (V). During storms, necessary to cut out the power conversion of wind turbine by furbing the wind turbine blades. The speed at which power conversion is cut out is called cut out wind speed or furling speed.

78. The area abcde represents the annual energy output from ideal plant. Area "ABCD' represents the output obtainable from ideal plant, if the windmill operate at design speed. The ratio of area abcde to the area ABCD is the annual load factor of the plant.