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 wind turbines often installed on short towers by cost-conscious homeowners. Buffeted by turbulent winds, wind machines hunt around on the top of their towers, constantly seeking the strongest wind, starting and stopping repeatedly. This decreases the amount of electricity a turbine generates.
Fig. 2.8: Ground Clutter and Turbulence. Trees, houses, barns, silos, billboards, garages and other structures are referred to as ground clutter. They create turbulence. Like eddies behind rocks in streams, the turbulent zone contains a fluid (air) that swirls and tumbles, moving in many directions. Turbulence reduces the harnessable energy of the wind and causes more wear and tear on wind machines, damaging them over time.
Turbulence also causes vibration and unequal forces on the wind turbine, especially the blades, that may weaken and damage the machine. Turbulence, therefore, increases wear and tear on wind generators and, over time, can destroy a turbine. The cheaper the turbine, the more likely it will be destroyed in a turbulent location. A homeowner may find that a machine he or she had hoped would produce electricity for ten to twenty years only lasted two to four years.
Turbulence is to a wind machine like potholes to your car.
— Robert Preus, Abundant Renewable Energy
When considering a location to mount a wind turbine, be sure to consider turbulence-generating obstacles such as silos, trees, barns, houses and other wind turbines. Proper location is the key to avoiding the damaging effects of turbulence. Turbulence can also be minimized by mounting a wind turbine on a tall tower. In sum, then, mounting a wind generator on a tall tower offers four benefits: (1) it situates the wind generator in the stronger higher-energy-yielding winds, substantially increasing electrical production, (2) it raises the machine out of damaging turbulent winds, (3) it decreases the wind turbine’s maintenance and repair requirements, and (4) it increases the wind turbine’s useful lifespan substantially, perhaps tenfold. Longer turbine life means less overall expense — and more electricity from your investment.
As shown in Figure 2.9, all obstacles create a downstream zone of turbulent air, or “turbulence bubble.” It typically extends vertically about twice the height of the obstruction and extends downwind approximately 15 to 20 times the height of the obstruction. A 20-foot-high house creates a turbulence bubble that extends 40 feet above the ground and 300 to 400 feet downwind. As illustrated in Figure 2.9, the turbulence bubble also extends upwind — about two times the object’s height. In this case, the upwind bubble extends about 40 feet upwind from the house. The upstream portion of the bubble is created by wind backing up as it strikes the obstacle — much like water flowing against a rock in a river.
To avoid costly mistakes, installers recommend that wind machines be mounted so that the complete rotor (the hub and the blades) of the wind generator is at least 30 feet (9 meters) above the closest obstacle within 500 feet (about 150 meters), or a tree line in the area, whichever is higher (Figure 2.10). Don’t listen to those who recommend lesser heights. Many unhappy customers will attest to that!
Fig. 2.9: Turbulence
