Wind Power Basics. Dan Chiras

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 Bubble. The turbulence bubble extends upwind, downwind, and even above the clutter. The bubble shifts when wind direction changes.

Fig. 2.10: Treelines and Wind Speed. Siting a wind turbine in an open area surrounded by trees is possible. As explained in the text, the top of the tree line is the effective ground level. The turbine needs to be mounted well above the trees to perform optimally.

      If your home or business is in an open field surrounded by trees, the wind turbine needs to be well above the tree line (Figure 2.10). Remember, too, to account for growth of trees over the 20- to 30-year life span of your wind system when determining tower height.

The Mathematics of Wind Power

      To understand how important it is to mount a wind turbine on a tall tower, consider a simple mathematical equation. It’s called the power equation and is used to calculate the power available from the wind. This equation shows us that three factors influence the output of a wind energy system: (1) air density, (2) swept area, and (3) wind speed — all explained shortly.

      The power equation is: P = ½ d x A x V³.

      P stands for the power available in the wind (not the power a wind generator will extract — that’s influenced by efficiency and other factors). Density of the air is d. Swept area is A. Wind speed is V.

      Air Density

      Air density is the weight of air per unit volume, which varies with elevation. As a general rule, anticipate a decrease in the air density of about 3 percent per 1,000 feet (300 meters) increase in elevation. As a result, air density doesn’t affect the power available from the wind until elevation reaches 2,500 feet (760 meters) above sea level. At 3,000 feet (about 910 meters) above sea level, the air density is 9 percent lower than at sea level. At 5,000 feet (about 1,525 meters) air density declines by about 15 percent.

      Air density is also a function of relative humidity, although the difference between a dry and humid area is usually negligible.

      Temperature also affects the density of air. Warmer air is less dense than colder air. Consequently, a wind turbine operating in cold (denser) winter winds would produce slightly more electricity than the same wind turbine in warmer winds blowing at the same speed.

      Although temperature and humidity affect air density, they are not factors we can change. Installers must be aware of the reduced energy available at higher altitudes, however, so they don’t create unrealistic expectations for a wind system.

      Although density is not a factor we can control, wind installers do have control over a couple of other key factors, notably, swept area (A) and wind speed — both of which have a much greater impact on the amount of power available to a wind turbine and the electrical output of the machine than air density.

      Swept Area

      Swept area is the area of the circle that the blades of a wind machine create when spinning. It is a wind machine’s collector surface. The larger the swept area, the more energy a wind turbine can capture from the wind. Swept area is determined by blade length. The longer the blades, the greater the swept area. The greater the swept area, the greater the electrical output of a turbine. As the equation suggests, the relationship between swept area and power output is linear. Theoretically, a ten percent increase in swept area will result in a ten percent increase in electrical production. Doubling the swept area doubles the output.

      When shopping for a wind turbine, always convert blade length to swept area, if the manufacturer has not done so for you (they usually do). Swept area can be calculated using the equation A = π · r².

      In this equation, A is the area of the circle, the swept area of the wind turbine. The Greek symbol is pi, which is a constant: 3.14. The letter r stands for the radius of a circle, the distance from the center of the circle to its outer edge. For a wind turbine, radius is usually about the same as the length of the blade.

      Because swept area is a function of the radius squared, a small increase in radius, or