Читать книгу Wind Power Basics - Dan Chiras - Страница 33

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 blade length, results in a large increase in swept area. As an example, a wind generator with an 8-foot blade has a swept area of 200 square feet. A wind generator with a 25 percent longer blade, that is, a 10-foot blade, has a 314 square-foot swept area. Thus, a 25 percent increase in blade length results in a 57 percent increase in swept area and, theoretically, a 57 percent increase in electrical production.