Wind Energy

The Natural Phenomenon

Earth's winds are generated by seasonal and geographic variations in planetary heating, resulting in the formation of weather systems that redistribute heat around the globe. Wind can be used to perform work because air has mass, and thus, as it moves it has kinetic energy. The amount of kinetic energy (KE) possessed by a given volume of air with mass (M) and velocity (V) is described by the relationship KE = 0.5MV2.

Earth's winds are dissipated by their interaction with the planet's surface, which acts as a source of friction. Because of friction, winds on our planet are typically stronger and more consistent the higher one travels into the atmosphere. For the same reason, near-surface winds are typically higher in areas where there are few surface obstructions, such as on the ocean, treeless plains, and glacial plateaus. The twin influences of altitude and terrain can sometimes result in dramatic spatial variations in average wind speed over short distances. In addition, the wind speed varies considerably over time, so wind is considered an intermittent energy resource.

Generating Electricity From Wind

Although in the past wind energy was used to propel sailing ships, pump water, and grind grain, it is currently used almost exclusively to generate electric power. Turbines used to generate electricity come in many shapes and sizes. The most common modern design uses three vertically oriented blades to drive a shaft oriented in the horizontal direction, usually termed a three-blade, horizontal-axis turbine. Other varieties exist (e.g., different types of vertical-axis turbines) but are comparatively rare. Modern wind turbines are typically measured by their ability to generate electric power, termed rated capacity. The rated capacity is the maximum amount of power that the turbine can instantaneously produce under ideal conditions. Modern turbines range in power output from several hundred watts (W) to several megawatts (MW). One MW of electricity-generating capacity can serve 200 to 300 average homes. Today, the nation with the largest installation and rated capacity of wind turbines, or “wind farms,” is Germany, with the United States and Spain a distant second and third.

The capacity of a wind turbine is determined to a large degree by the length of its blades and the area they sweep, termed the swept area (A). The power (P) in the area swept by the blades is determined by the relationship P = 0.5ρAV3, where p equals the density of air (in kilograms per cubic meter, kg/m3) at sea level. To arrive at the actual rated capacity of a wind turbine, additional coefficients must be added to describe the efficiency of the turbine. Turbines used for commercial power generation often top 400 feet from the base of the tower to the tip of the blades, allowing for longer blades and a larger swept area as well as giving them access to stronger, less turbulent winds away from the ground.

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