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Wind energy
The document discusses wind energy, highlighting its generation from solar energy and the factors influencing wind formation such as uneven heating of the earth. It explains the mechanics of wind turbines, including how they extract kinetic energy and the concept of the Betz limit, which states that turbines can achieve a maximum efficiency of 59%. Additionally, it covers the configuration of wind turbines, the importance of wind speed, the design of wind farms, and the advantages and disadvantages of wind energy.
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Wind energy
- 1. Chapter 7 Wind energy By:MESFIN MEGRA.
- 2. WIND POWER –WHAT IS IT? All renewable energy (except tidal and geothermal power), ultimately comes from the sun. The earth receives 350 W/m2 from the sun. About 2% of this energy (= 7 W/m2) is converted to wind energy. 12/19/2017 2
- 3. WIND POWER –WHAT IS IT? Cont.... Differential heating of the earths surface and atmosphere induces vertical and horizontal air currents that are affected by the earth’s rotation and contours of the land. 12/19/2017 3
- 4. Uneven heating: Intensity ofsolar energy is varies due to the angle of the sun (the equator vs the poles). Land heats up faster than water does, but also loses heat faster. These difference in air temperature across the globe can create wind. 12/19/2017 4
- 5. Uneven heating: Con…. 35%of wind energy (2.45 W/m2) is dissipated in the first kilometers above the Earth’s surface and available for turbine. 12/19/2017 5
- 6. Available power: Over aperiod of one year, the wind energy (E) is approximately. 𝐸 = 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 ∗ 𝐸𝑎𝑟𝑡ℎ′ 𝑠 𝑆𝐴 ∗ 𝑆𝑒𝑐𝑜𝑛𝑑𝑠 𝑝𝑒𝑟 𝑦𝑒𝑎𝑟 = 2.45 𝑊/𝑚2 5.1 𝑥 1014 𝑚2 3.2 𝑥 107 𝑠 = 4.0 𝑥 1022 𝐽 Which is 200 times larger than our energy consumption (2.0 𝑥 1020 𝐽). 12/19/2017 6
- 7. Maximum Power Extracted Calculatethe power extracted from wind. Calculate kinetic energy, 𝐾𝐸 = 1 2 𝑚𝑣2 of air passing through the rotor of the wind turbine. Measure mass of the air travelling through area of circle swept out by rotor blades in time ∆𝑡. 12/19/2017 7
- 8. Con…. Time t =0: Time ∆𝑡: 𝑣. ∆𝑡 Figure 1: At time t=0, mass of air is just about to pass through the loop, but ∆𝑡 later, the mass of air is passed through the loop. The mass of this piece of air is the product of its density 𝜌, area A, and length 𝑣. ∆𝑡. A 12/19/2017 8
- 9. Con…. From this youcan find the mass. M𝑎𝑠𝑠 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 ∗ 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝜌 ∗ 𝐴 ∗ 𝑣 ∗ ∆𝑡 𝑣 𝑖𝑠 𝑡ℎ𝑒 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟 𝜌 𝑖𝑠 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟 1.23 𝑓𝑜𝑟 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑇 𝑎𝑛𝑑 𝑃 ∆𝑡 𝑖𝑠 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡𝑖𝑚𝑒 𝑓𝑜𝑟 𝑎 𝑢𝑛𝑖𝑡 𝑎𝑖𝑟 𝑡𝑜 𝑝𝑎𝑠𝑠 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝐴 𝑖𝑠 𝑎𝑟𝑒𝑎 𝑠𝑤𝑒𝑝𝑡 𝑏𝑦 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒𝑠, 𝑛𝑜𝑡 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒 𝑎𝑟𝑒𝑎 12/19/2017 9
- 10. Con…. Therefore the kineticenergy, KE, is found to be. 𝐾𝐸 = 1 2 𝑚𝑣2 = 1 2 𝜌𝐴∆𝑡𝑣3 While the power of the wind passing through the loop is. 𝑃 = 1 2 𝜌𝐴∆𝑡𝑣3 ∆𝑡 = 𝟏 𝟐 𝝆𝑨𝒗 𝟑 12/19/2017 10
- 11. Con…. But turbines can’textract all of the kinetic energy of the wind. Why not? The above expression is true for a single wind turbine in constant wind condition. 12/19/2017 11
- 12. Actual Power Extractedfrom the Wind The actual power extracted by the rotor blades is the difference between the upstream and the downstream wind powers. 𝑃 𝑜 = 1 2 𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑 . (𝑉2 − 𝑉𝑜 2 ) 𝑉 = 𝑢𝑝𝑠𝑡𝑟𝑒𝑎𝑚 𝑤𝑖𝑛𝑑 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑉𝑜= 𝑑𝑜𝑤𝑛𝑠𝑡𝑟𝑒𝑎𝑚 𝑤𝑖𝑛𝑑 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑃𝑜 = 𝑚𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑜𝑢𝑡𝑝𝑢𝑡 12/19/2017 12
- 13. Con…. The mass flowrate of air through the rotating blades is, therefore, derived by multiplying the density with the average velocity. That is M𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 = 𝜌. 𝐴. 𝑉 + 𝑉𝑜 2 12/19/2017 13
- 14. Con…. The mechanical powerextracted by the rotor, which is driving the electrical generator is therefore: 𝑃𝑜 = 1 2 𝜌. 𝐴. 𝑉 + 𝑉𝑜 2 . (𝑉2 − 𝑉𝑜 2 ) 12/19/2017 14
- 15. Con…. The above expressioncan be algebraically rearranged: 𝑃𝑜 = 1 2 𝜌. 𝐴. 𝑉3 . 1 + 𝑉𝑜 𝑉 1 − 𝑉𝑜 𝑉 2 2 12/19/2017 15
- 16. Con…. The power extractedby the blade is customarily expressed as a fraction of the upstream wind power as follows: 𝑃𝑜 = 1 2 𝜌. 𝐴. 𝑉3 . 𝐶 𝑝 where 𝐶 𝑝 = 1 + 𝑉𝑜 𝑉 1 − 𝑉𝑜 𝑉 2 2 12/19/2017 16
- 17. Con…. For a givenupstream wind speed, the value of Cp depends on the ratio of the downstream to the upstream wind speeds that is 𝑉𝑜 𝑉 . The plot of power coefficient versus ( 𝑉𝑜 𝑉 ) shows that Cp is a single, maximum value function 12/19/2017 17
- 18. Con…. It has amaximum value of 0.59 (Betz limit) when the ( 𝑉𝑜 𝑉) is one-third (0.33). The maximum power is extracted from the wind at speed ratio, when the downstream wind speed equals one-third of the upstream speed. 𝑃𝑚𝑎𝑥 = 1 2 𝜌. 𝐴. 𝑉3 . 0.59 12/19/2017 18
- 19. Con…. The theoretical maximumvalue of Cp is 0.59. in practical designs, the maximum achievable Cp is below 0.5. 12/19/2017 19
- 20. Efficiency Wind turbines arenot 100% efficient. Power = efficiency * max. power extracted 𝑃𝑜 = 𝜂 1 2 𝜌. 𝐴. 𝑉3 = 𝜂 1 2 𝜌. 𝑉3 . 𝜋 𝑑 2 2 = 1 8 𝜂. 𝜌. 𝑉3 . 𝜋𝑑2 12/19/2017 20
- 21. Factor affecting windpower harnessing The power produced by a wind turbine depends on: Rotor area Air density Wind speed Wind shear 12/19/2017 21
- 22. Con…. Wind shear isa difference in wind speed and direction over a short distance and is caused by mountains, coastline, and weather pattern. Air density increases with colder temperatures, decreased with altitude, and decreased humidity Wind speed increases the farther you get away from ground. 12/19/2017 22
- 23. Con…. As you gethigher off the ground, the air speed increases. The rotor are tilted slightly upwards so that each part of the rotor is exposed to the same speed 12/19/2017 23
- 24. Con…. Wind turbines aremost efficient when wind moves uniformly in the same direction. Turbulence is caused by buildings, trees, and land formations (wind break). The edge of a continental shelf, high ground and tundra have low turbulence and are the best locations to build a turbine 12/19/2017 24
- 25. Feasibility Local wind speedis also an important factor since power ∝ (𝑤𝑖𝑛𝑑 𝑠𝑝𝑒𝑒𝑑)3 . The local wind speed should be, on average, at least 7 m/s at 25 m above the earth’s surface in order to make harnessing wind from it worthwhile. 12/19/2017 25
- 26. Demand and Dependability Windis not locally predictable in the short term, and so its use should be limited to only fulfill 5- 15% of the total energy demand of the area. Setting up turbines in several locations makes wind energy more reliable. 12/19/2017 26
- 27. Con…. The available poweris averaged. Globally there is always a relatively constant amount of wind energy being harnessed at any one moment. 12/19/2017 27
- 28. Configuration of WindTurbine 12/19/2017 28
- 29. HAWT’s vs VAWT’s HAWT’s Advantage Variablepitch Tall tower- higher wind speeds Steady angle attack Disadvantage Tall tower/large blade- difficult to install High visibility Yaw control is necessary 12/19/2017 29
- 30. HAWT’s vs VAWT’scont.… VAWT’s Advantage Smaller support structure Yaw control not needed Generator components located on the ground Less noise compared to HAWT’s Disadvantage Cyclic loading makes fatigue failure more likely Lower wind speed due to shorter structure 12/19/2017 30
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- 32. Description of adouble-fed WTG 12/19/2017 32
- 33. 1. Blade 2.Pitch Drive 3.Hub 4.Generatorrotor 5.Generator stator 6.Yaw drive 7.Wind sensors 8.Service hoist 9.Top Box 10.Main frame 11.Nacelle cover 12.Tower Nacelle 12/19/2017 33
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- 35. Wind Farms Cities andcountries need huge wind farms to satisfy their energy needs. To optimize energy production in a wind farm, Turbines are spread 5 – 9 rotor diameter apart in the prevailing wind direction and, 3 – 5 rotor diameter apart in perpendicular direction 12/19/2017 35
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- 37. Wind Turbine Control Ingeneral, wind turbines are designed to operate when the incident wind is high enough to generate electricity and to shut down when the wind speed exceeds 25 to 30 m/s. 12/19/2017 37
- 38. Yaw Control The yawcontrol continuously orients the rotor in the direction of the wind. It can be as simple as the tail vane or More complex on modern towers, using wind direction sensor and electric or hydraulic drive motors, to orient the rotor 12/19/2017 38
- 39. speed Control The speedcontrol methods fall into the following categories. No speed control: here the wind systems are designed to withstand the extreme wind speed. Yaw and tilt control: here the rotor axis is shifted out of the wind direction when wind speed exceeds the design limit. 12/19/2017 39
- 40. Cont.…. Pitch control: itchanges the pitch of the blade with the changing wind speed to regulate the rotor speed. Stall control: here when the wind speed exceeds the safe limit on the system, the blades are shifted into a position such that they stall. 12/19/2017 40
- 41. Advantage vs disadvantage Doesn’t produce hazardous wastes. Use less space than traditional power stations. Arguments against include fears of damages From collapsing turbines Noise Unreliable power source A less attractive skyline Unnecessary high bird fatality 12/19/2017 41
- 42.