In order to better understand this thing we call " the wind" we will need to understand a few simple concepts. The average wind speed determined for a specific location, is proportional to its height above the ground raised to the power 0.14. This implies that the speed of the wind 20 feet above the ground will be about 20 percent higher than that of the wind at a height of 5 feet.
The kinetic energy of 1 cubic foot of air (in joules) is equal to 0.5D times V(squared), where D is air density (about 0.1 pound per cubic foot near the ground at sea level), and V is the average wind speed (in feet per second). The power of the wind (in watts) is the product of wind energy (E), wind speed (V), and the area perpendicular to the direction of the wind that is swept by that is swept by any type of machanical device (A is feet squared) and is equal to 0.5D(A3 cubed"). If the wind power goes up by the cube of the average speed, then doubling the speed increases the available power by approximately a factor of eight. Thus, locations with sustained winds are superior to those with mild winds. It is important to note that the height above the ground is a major consideration for the development of wind mills. In order to generate electricity of any economic value wind mills are set on the top of tall towers and have blades 150 feet spans.
Early societies would capture the wind flowing within 100 feet of ground level, and most of the windmills built prior to this time had spans of less than 30 feet. Wind flows also exhibits large temporal and spatial variations. Even in windy places, wind speed averages will fluctuate by a factor of 30 percent. Terrain irregularities and other structures present, may easily cause an increase or decrease in the average speed of the wind by one-half within a radius of 100 feet. Modern electrical generating windmills can be conveniently located at exceptionally windy sites, to enhance electricity production.
Wind machine can not extract all of the available wind power as this would require the machine to completely stop all air movement and would cause the air to accumulate at the point of conversion. The maximum extractable power is equal to nearly 60 percent, of the kinetic energy potential. Actual performance is less than that for modern wind machines, and it averaged between 20 and 30 percent for pre-industrial windmills. Taking this information into account a wood-and-canvas tower mill with a blade diameter of 60 feet could theoretically extract about 112 kW from an air stream with a velocity of 30 feet/s but would deliver less than 40 kW of useful power. Keeping all this information in mind start thinking about how you would improve on windmill design. This is very critical since overall design affect cost and maintenance. Large windmills play havoc on bird populations and can cause major damage to any structures in the vicinity. Keep in mind that the higher the elevation the less dense the air becomes requiring a stronger wind speed.
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