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Wind energy converted to mechanical and electrical energy.
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    Presents  A sample chapter from the title Wind & Solar Power Renewable Energy Technologies Website: www.idc-online.com E-mail: idc@idc-online.com       Fundamentals of Wind Energy   Objectives After reading this chapter you will be able to: ã   Understand the nature of wind resource ã   Study the mechanics of wind ã   Carry out measurements of wind speed and direction ã   Understand the local effects on wind flow ã   Carry out wind assessment at a potential site 6.1 Introduction Wind energy system is a conversion system, which converts the energy of the wind into mechanical or electrical energy. This energy can be used in different ways such as to pump water or grind grain or to power homes and businesses. This system uses wind turbines which convert the kinetic energy of the wind into mechanical energy with the help of propeller like blades. This mechanical  power is then converted into electrical energy with the help of a generator. Historically, wind energy was being used as early as 5000 BC. In the US the usage of windmills started during the end of 19th century in the west. These wind mills were primarily used for pumping water for farms and ranches. By 1910 many European countries too started producing wind energy. In order to design and install a wind energy system it is very important to understand the nature and the characteristics of wind resources. It is also very important to analyze the site selection since  poor site selection can result in low wind speed, which means less available energy to convert. This largely affects the output of the wind energy system. In this chapter let us understand the characteristics of wind resources, which will help the student in selecting the site for designing the wind energy system. 6.2 The wind resource 6.2.1 Wind energy is an indirect form of solar energy. How is wind generated? The earth’s surface comprising of deserts, oceans and hilly terrain absorb the solar energy in different magnitudes. Due to this the earth’s surface gets unevenly heated The regions around equator, at 0° latitude are heated more by the sun than the rest of the globe. Since the hot air is  78   Wind and Solar Power lighter than the cold air, it rises in the sky to the height of approximately 10 km and starts spreading towards the North and the South. If the earth did not rotate, this hot air will just turn up at the  North pole and the South pole and then gradually return to the equator. However, due to the rotation of the earth, the wind is deflected towards right in the northern hemisphere and towards the left in the southern hemisphere. This bending force is known as Coriolis force. For example this  phenomenon can also be observed in railroad tracks. Imagine the track going from the south pole to the north pole. In the northern hemisphere the track on the eastern (right) side will wear out faster than the one on the other side. This bending (Coriolis) force affects the formation of the global winds. It prevents the hot air rising at the equator from moving too far. At the latitude of about 30 degrees in both the hemispheres the air begins to sink down thus forming a high pressure zone in these areas. At the equator there will  be a low pressure zone near the ground level due to the rising hot air, and at the poles there will be a high pressure area due to the cooling of the air. Following are the results of the direction of the wind at various latitudes. Prevailing Wind Directions Latitude Direction 90-60°N NE 60-30°N SW 30-0°N NE 0-30°S SE 30-60°S NW 60-90°S SE The predominant winds in the Northern hemisphere are called north easterly trade winds while in the southern hemisphere they are called south easterly trade winds. The winds discussed above are also called geostrophic winds. They depend largely on the temperature differences on the earth. They are not notably affected by the uneven surface of the earth. The geostrophic wind is found at altitudes above 1000 meters above ground level. At the lower altitudes (up to 100 meters) the earth’s surface roughness and obstacles come into picture. The winds at this level are called as surface winds. In the following figure we can see how wind flows over the earth’s surface. Figure 6.1
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