Читать книгу Wind Power Basics - Dan Chiras - Страница 34

Although swept area is more important than density, wind speed is even more important in determining the output of a wind turbine. That’s because the power available from the wind increases with the cube of wind speed. This relationship is expressed in the power equation as V³ or V x V x V — wind speed multiplied by itself three times.

Consider an example: Suppose that you mount a wind machine 18 feet (5.5 meters) above the ground surface on the grasslands of Nebraska. Suppose that the wind is blowing at eight miles per hour (3.6 meters per second). A friend, who knows how important it is to mount a wind machine on a tall tower, installs an identical wind turbine on a 90-foot (27-meter) tower. When the wind is blowing at 8 mph where your turbine flies at 18 feet, an anemometer on your friend’s 90-foot tower indicates that the wind is blowing at 10 miles per hour. Wind speed is 25 percent higher. What’s the difference in available power?

Shopping Tip

Because swept area is such an important determinant of the output of a wind turbine, we strongly recommend focusing more on the swept area of a wind turbine than on its rated power — at least until the industry can come up with a standardized way of measuring and reporting rated power.

The power available in the wind can be approximated by multiplying the wind speeds by themselves three times. (Units aren’t important for this comparison.) For the lower turbine the result is 8 x 8 x 8 or 512. The power available to the wind turbine mounted on a 90-foot tower is 10 cubed or 10 x 10 x 10 which is 1,000. Thus, a two-mile-per-hour increase in wind speed, a paltry 25 percent increase, doubles the available power. Put another way, a 25 percent increase in wind speed yields an increase of nearly 100 percent. The important lesson is that because power is function of V³, a small increase in wind speed results in a very large increase in the power available to a wind turbine. This can result in a very large increase in the electrical output of a wind turbine.

Although winds are out of our control, homeowners can affect the wind speed at their wind turbines by choosing the best possible site and by installing their machines on the tallest towers.