Wind Power Basics. Dan Chiras

expect, but from the northeast to southwest. Why?

      Because they are “deflected” by the Earth’s rotation.

      In reality, the Earth’s rotation doesn’t deflect winds. It makes it appear as if the winds have been deflected. The apparent deflection in wind direction in the tropics is a planetary sleight of hand, an illusion produced by the rotation of the Earth on its axis. To understand this phenomenon, consider a simple example. Imagine that you board a plane leaving the North Pole. The pilot plots a course that will take you due south toward an airfield on Sri Lanka just south of India. If the pilot flies the plane due south the entire trip, however, the plane will end up somewhere over the Arabian Sea because of the Earth’s rotation.

As shown in Figure 2.5, as the plane travels south, the Earth rotates beneath it. The Earth rotates eastward. If you plot the flight path of the plane it appears to have been deflected. In reality, it only looks that way.

      The apparent deflection of the plane’s path is the Coriolis effect. In the Northern Hemisphere, the deflection is to the right of the direction of travel. In the Southern Hemisphere, the deflection is to the left.

      Winds flowing north or south also appear to be deflected thanks to the Coriolis effect. The south-flowing trade winds, for instance, appear to flow from northeast to southwest.

      In the temperate zone, as shown in Figure 2.4c, the low-level north-flowing winds that sweep across the surface of the Earth flow across the North American continent not from south to north but from the southwest to northeast. These are the prevailing south-westerly winds that blow across the Great Plains of North America. Many a wind farm and many a small wind operation depend on them.

Fig. 2.5: The Coriolis Effect. The rotation of the Earth causes an apparent deflection in the path of winds. This can be understood by observing the flight of a plane that begins at the North Pole and heads south toward Sri Lanka. The plane appears to veer off course. It hasn’t. The Earth’s rotation makes it look that way.

Wind from Storms

      Winds are often associated with storms. Storms, in turn, are produced when high-pressure and low-pressure air masses collide. High and low pressure zones move across the continents.

      Low-pressure air masses originate in the tropics. They are created by the huge influx of solar energy in these regions. Huge masses of low-pressure air frequently break away and migrate northward, sweeping across the North American continent.

      High-pressure air masses originate in the North and South poles, regions of more or less permanent cold, high-pressure air. Like warm tropical air, huge masses of cold Arctic air also break loose and drift southward, sweeping across the Northern Hemisphere.

      High-pressure and low-pressure air masses, often measuring 500 to 1,000 miles in diameter, move across continents. The movement of high- and low-pressure air masses across continents is steered by prevailing winds and by the jet stream (high altitude winds). As these air masses collide, they produce an assortment of weather, often accompanied by winds. As with all other forms of wind, storm winds are created by differences in pressure between high- and low-pressure air masses. The greater the difference in pressure between a high-pressure air mass and a “neighboring” low-pressure air mass, the stronger the winds. In some cases, these winds contain an enormous amount of energy.

Friction, Turbulence and Smart Siting

      Wind does not flow smoothly over the Earth’s surface. It encounters resistance, known as friction. This results in a phenomenon called ground drag. Ground drag is caused by friction when air flows across a surface.

      Friction is the force that resists movement of one material against another. You create friction, for example, when you rub your hands together. When wind flows across land or water, friction occurs. This reduces the speed at which air moves over a surface.

      Ground drag due to friction, however, varies considerably, depending on the roughness of the surface. The rougher or more irregular the surface, the greater the friction. As a result, air flowing across the surface of a lake generates less friction than air flowing over a meadow. Air flowing over a meadow generates less friction than air flowing over a forest.

      Friction extends to a height of about 1,650 feet (500 meters). However, the greatest effects are closest to Earth’s surface — the first 60 feet over a relatively flat, smooth surface. Over trees, the greatest effects occur within the first 60 feet (18 meters) above the tree line.

Friction has a dramatic effect on wind speed at different heights. For instance, a 20-mile-per-hour wind measured at 1,000 feet above land covered with grasses flows at 5 miles per hour 10 feet above the surface. It then increases progressively until it breaks loose from the influence of the ground drag or friction. Figure 2.6 shows the difference in wind speed at 50 meters (165 feet) to 5 meters (16.5 feet). Figure 2.7 compares wind speed over a grassy area to wind speed over a forest, a significantly rougher surface. Notice that the wind speed increases more rapidly above the forest.

      Ground drag dramatically influences wind speed near the surface of the ground where residential wind generators are located. Because the effects of friction decrease with height above the surface of the Earth, savvy installers typically mount their wind machines on towers 80 to 120 feet high (24 to 37 meters), or even as high as 180 feet (55 meters) in forested regions, so their turbines are out of the most significant ground drag. At these heights, the winds are substantially stronger than near the ground. As discussed shortly, a small increase in wind speed can result in a substantial increase in the amount of power that’s available from the wind and the amount of electricity a wind generator produces. Mounting a wind turbine on a tall tower therefore maximizes the electrical output of the machine. Placing a turbine on a short tower has just the opposite effect. It places the generator in the weaker winds and is a bit like mounting solar panels in the shade.

Fig. 2.6: Effect of Ground Drag. Winds move more slowly at ground level due to friction. Friction diminishes with height, so wind speed increases.

Fig. 2.7: Wind Speed vs. Height. These graphs compare the wind speed over a grassy area and a forest. As you can see, a forest virtually eliminates ground-level winds. As a result, the effective ground level shifts upward. Note that the wind speed increases more rapidly with height over a forest than over a grassy area. Wind turbines placed well above the tree line can avail themselves of powerful winds.

Another natural phenomenon that affects the output of most wind turbines is turbulence. Turbulence is produced as air flowing across the Earth’s surface encounters objects, such as trees or buildings. They interrupt the wind’s smooth laminar flow, causing it to tumble and swirl, the same way rocks in a stream interrupt the flow of water (Figure 2.8). Rapid changes in wind speed occur behind large obstacles and winds may even flow in the direction opposite to the wind. This highly disorganized wind flow is referred to as turbulence.

      Turbulent wind flows wreck havoc on wind machines, especially the less expensive, lighter-weight