CHAPTER 1 Fundamentals of wind energy WIT Press

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4 Wind Power Generation and Wind Turbine Design, Wind energy represents a mainstream energy source of new power generation. and an important player in the world s energy market As a leading energy technol. ogy wind power s technical maturity and speed of deployment is acknowledged. along with the fact that there is no practical upper limit to the percentage of wind. that can be integrated into the electricity system 1 It has been estimated that the. total solar power received by the earth is approximately 1 8 1011 MW Of this. solar input only 2 i e 3 6 109 MW is converted into wind energy and about. 35 of wind energy is dissipated within 1000 m of the earth s surface 2 There. fore the available wind power that can be converted into other forms of energy is. approximately 1 26 109 MW Because this value represents 20 times the rate of. the present global energy consumption wind energy in principle could meet entire. energy needs of the world, Compared with traditional energy sources wind energy has a number of bene. fits and advantages Unlike fossil fuels that emit harmful gases and nuclear power. that generates radioactive wastes wind power is a clean and environmentally. friendly energy source As an inexhaustible and free energy source it is available. and plentiful in most regions of the earth In addition more extensive use of wind. power would help reduce the demands for fossil fuels which may run out some. time in this century according to their present consumptions Furthermore the. cost per kWh of wind power is much lower than that of solar power 3. Thus as the most promising energy source wind energy is believed to play a. critical role in global power supply in the 21st century. 2 Wind generation, Wind results from the movement of air due to atmospheric pressure gradients. Wind flows from regions of higher pressure to regions of lower pressure The. larger the atmospheric pressure gradient the higher the wind speed and thus the. greater the wind power that can be captured from the wind by means of wind. energy converting machinery, The generation and movement of wind are complicated due to a number of fac. tors Among them the most important factors are uneven solar heating the Coriolis. effect due to the earth s self rotation and local geographical conditions. 2 1 Uneven solar heating, Among all factors affecting the wind generation the uneven solar radiation on the.
earth s surface is the most important and critical one The unevenness of the solar. radiation can be attributed to four reasons, First the earth is a sphere revolving around the sun in the same plane as its. equator Because the surface of the earth is perpendicular to the path of the sunrays. at the equator but parallel to the sunrays at the poles the equator receives the great. est amount of energy per unit area with energy dropping off toward the poles Due. to the spatial uneven heating on the earth it forms a temperature gradient from the. equator to the poles and a pressure gradient from the poles to the equator Thus hot. air with lower air density at the equator rises up to the high atmosphere and moves. WIT Transactions on State of the Art in Science and Engineering Vol 44 2010 WIT Press. www witpress com ISSN 1755 8336 on line,Fundamentals of Wind Energy 5. towards the poles and cold air with higher density flows from the poles towards the. equator along the earth s surface Without considering the earth s self rotation and. the rotation induced Coriolis force the air circulation at each hemisphere forms a. single cell defined as the meridional circulation, Second the earth s self rotating axis has a tilt of about 23 5 with respect to its. ecliptic plane It is the tilt of the earth s axis during the revolution around the sun. that results in cyclic uneven heating causing the yearly cycle of seasonal weather. Third the earth s surface is covered with different types of materials such as vegeta. tion rock sand water ice snow etc Each of these materials has different reflecting. and absorbing rates to solar radiation leading to high temperature on some areas e g. deserts and low temperature on others e g iced lakes even at the same latitudes. The fourth reason for uneven heating of solar radiation is due to the earth s. topographic surface There are a large number of mountains valleys hills etc on. the earth resulting in different solar radiation on the sunny and shady sides. 2 2 Coriolis force, The earth s self rotation is another important factor to affect wind direction and. speed The Coriolis force which is generated from the earth s self rotation deflects. the direction of atmospheric movements In the north atmosphere wind is deflected. to the right and in the south atmosphere to the left The Coriolis force depends on. the earth s latitude it is zero at the equator and reaches maximum values at the. poles In addition the amount of deflection on wind also depends on the wind. speed slowly blowing wind is deflected only a small amount while stronger wind. deflected more, In large scale atmospheric movements the combination of the pressure gradient.
due to the uneven solar radiation and the Coriolis force due to the earth s self. rotation causes the single meridional cell to break up into three convectional cells. in each hemisphere the Hadley cell the Ferrel cell and the Polar cell Fig 1. Each cell has its own characteristic circulation pattern. In the Northern Hemisphere the Hadley cell circulation lies between the equa. tor and north latitude 30 dominating tropical and sub tropical climates The hot. air rises at the equator and flows toward the North Pole in the upper atmosphere. This moving air is deflected by Coriolis force to create the northeast trade winds. At approximately north latitude 30 Coriolis force becomes so strong to balance. the pressure gradient force As a result the winds are defected to the west The air. accumulated at the upper atmosphere forms the subtropical high pressure belt and. thus sinks back to the earth s surface splitting into two components one returns to. the equator to close the loop of the Hadley cell another moves along the earth s. surface toward North Pole to form the Ferrel Cell circulation which lies between. north latitude 30 and 60 The air circulates toward the North Pole along the. earth s surface until it collides with the cold air flowing from the North Pole at. approximately north latitude 60 Under the influence of Coriolis force the mov. ing air in this zone is deflected to produce westerlies The Polar cell circulation lies. between the North Pole and north latitude 60 The cold air sinks down at the. WIT Transactions on State of the Art in Science and Engineering Vol 44 2010 WIT Press. www witpress com ISSN 1755 8336 on line,6 Wind Power Generation and Wind Turbine Design. North Pole,Polar cell,Polar easterlies,Ferrel cell. Westerlies,Hadley cell,Trade winds,South Pole,Figure 1 Idealized atmospheric circulations. North Pole and flows along the earth s surface toward the equator Near north lati. tude 60 the Coriolis effect becomes significant to force the airflow to southwest. 2 3 Local geography, The roughness on the earth s surface is a result of both natural geography and. manmade structures Frictional drag and obstructions near the earth s surface gen. erally retard with wind speed and induce a phenomenon known as wind shear The. rate at which wind speed increases with height varies on the basis of local condi. tions of the topography terrain and climate with the greatest rates of increases. observed over the roughest terrain A reliable approximation is that wind speed. increases about 10 with each doubling of height 4, In addition some special geographic structures can strongly enhance the wind.
intensity For instance wind that blows through mountain passes can form moun. tain jets with high speeds,3 History of wind energy applications. The use of wind energy can be traced back thousands of years to many ancient. civilizations The ancient human histories have revealed that wind energy was. discovered and used independently at several sites of the earth. WIT Transactions on State of the Art in Science and Engineering Vol 44 2010 WIT Press. www witpress com ISSN 1755 8336 on line,Fundamentals of Wind Energy 7. 3 1 Sailing, As early as about 4000 B C the ancient Chinese were the first to attach sails to. their primitive rafts 5 From the oracle bone inscription the ancient Chinese. scripted on turtle shells in Shang Dynasty 1600 B C 1046 B C the ancient. Chinese character i e sail in ancient Chinese often appeared In. Han Dynasty 220 B C 200 A D Chinese junks were developed and used as. ocean going vessels As recorded in a book wrote in the third century 6 there. were multi mast multi sail junks sailing in the South Sea capable of carrying 700. people with 260 tons of cargo Two ancient Chinese junks are shown in Figure 2. Figure 2 a is a two mast Chinese junk ship for shipping grain quoted from the. famous encyclopedic science and technology book Exploitation of the works of. nature 7 Figure 2 b illustrates a wheel boat 8 in Song Dynasty 960 1279. It mentioned in 9 that this type of wheel boats was used during the war between. Song and Jin Dynasty 1115 1234, Approximately at 3400 BC the ancient Egyptians launched their first sailing. vessels initially to sail on the Nile River and later along the coasts of the. Mediterranean 5 Around 1250 BC Egyptians built fairly sophisticated ships to. sail on the Red Sea 9 The wind powered ships had dominated water transport. for a long time until the invention of steam engines in the 19th century. 3 2 Wind in metal smelting processes, About 300 BC ancient Sinhalese had taken advantage of the strong monsoon.
winds to provide furnaces with sufficient air for raising the temperatures inside. furnaces in excess of 1100 C in iron smelting processes This technique was. capable of producing high carbon steel 10, Figure 2 Ancient Chinese junks ships a two mast junk ship 7 b wheel. WIT Transactions on State of the Art in Science and Engineering Vol 44 2010 WIT Press. www witpress com ISSN 1755 8336 on line,8 Wind Power Generation and Wind Turbine Design. The double acting piston bellows was invented in China and was widely used. in metallurgy in the fourth century BC 11 It was the capacity of this type of. bellows to deliver continuous blasts of air into furnaces to raise high enough tem. peratures for smelting iron In such a way ancient Chinese could once cast several. tons of iron,3 3 Windmills, China has long history of using windmills The unearthed mural paintings from the. tombs of the late Eastern Han Dynasty 25 220 AD at Sandaohao Liaoyang City. have shown the exquisite images of windmills evidencing the use of windmills in. China for at least approximately 1800 years 12, The practical vertical axis windmills were built in Sistan eastern Persia for. grain grinding and water pumping as recorded by a Persian geographer in the. ninth century 13, The horizontal axis windmills were invented in northwestern Europe in 1180s.
14 The earlier windmills typically featured four blades and mounted on central. posts known as Post mill Later several types of windmills e g Smock mill. Dutch mill and Fan mill had been developed in the Netherlands and Denmark. based on the improvements on Post mill, The horizontal axis windmills have become dominant in Europe and North. America for many centuries due to their higher operation efficiency and technical. advantages over vertical axis windmills,3 4 Wind turbines. Unlike windmills which are used directly to do work such as water pumping or. grain grinding wind turbines are used to convert wind energy to electricity The. first automatically operated wind turbine in the world was designed and built by. Charles Brush in 1888 This wind turbine was equipped with 144 cedar blades. having a rotating diameter of 17 m It generated a peak power of 12 kW to charge. batteries that supply DC current to lamps and electric motors 5. As a pioneering design for modern wind turbines the Gedser wind turbine was. built in Denmark in the mid 1950s 15 Today modern wind turbines in wind. farms have typically three blades operating at relative high wind speeds for the. power output up to several megawatts, Kites were invented in China as early as the fifth or fourth centuries BC 11 A. famous Chinese ancient legalist Han Fei Zi 280 232 BC mentioned in his book. that an ancient philosopher Mo Ze 479 381 BC spent three years to make a kite. with wood but failed after one day flight 16, WIT Transactions on State of the Art in Science and Engineering Vol 44 2010 WIT Press. www witpress com ISSN 1755 8336 on line,Fundamentals of Wind Energy 9.
4 Wind energy characteristics, Wind energy is a special form of kinetic energy in air as it flows Wind energy can. be either converted into electrical energy by power converting machines or directly. used for pumping water sailing ships or grinding gain. 4 1 Wind power, Kinetic energy exists whenever an object of a given mass is in motion with a trans. lational or rotational speed When air is in motion the kinetic energy in moving. air can be determined as,E k 12 mu 2 1, where m is the air mass and u is the mean wind speed over a suitable time period. The wind power can be obtained by differentiating the kinetic energy in wind with. CHAPTER 1 Fundamentals of wind energy Wei Tong Kollmoren g Corporation ginia Vir USA able solutions to the global climate change and energy crisis The utilization of wind power essentially eliminates emissions of CO 2 SO 2 NO x and other harmful wastes as in traditional coal fuel power plants or radioactive wastes in nuclear power plants By further diversifying the energy supply

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