Rotor Basics I often get inquiries from people who are interested in installing wind turbines and have
come across an advertisement for an inexpensive device with a very small rotor that will generate incredibly large amounts of electricity relative to its cost. They are completely taken in by claims of a breakthrough technology offering the promise of never-before-seen efficiencies. Unfamiliar with the nuances of small wind technology, compounded by a misunderstanding of wind resource and siting, consumers are understandably confused. The rotor of a wind turbine is made up of the blades that spin and capture energy in the wind that passes through them. While some rotors are traditional horizontal axis devices typically sporting two or three blades, others are vertical axis systems of various blade configurations, and still others are hybrids of these two orientations. Regardless, it is the rotor that extracts the kinetic energy in the wind and converts it to rotational momentum used to drive an electric generating device. Math Basics It is well understood with other renewable technologies that the size of your collector determines the amount of renewable energy that you can collect and convert to some useful purpose. Lets use solar water collectors as an example. One four-foot by eight-foot solar water collector has an area of 32 square feet (4 x 8 = 32). It can collect only the amount of sunlight that falls on it, no more. The collector is limited in the amount of hot water that it can generate based on the amount of sunlight that it collects. If we double the area exposed to the sunlight by adding a second solar collector, we double the amount of sunlight that can be collected, resulting in a doubling of the amount of hot water that can be generated. This is pretty straightforward: the amount of solar energy that can be extracted from the sunlight is proportional to the size of the solar collector used. Simple stuff! The same holds true for a wind turbine. A small rotor can only extract small amounts of kinetic energy out of the wind and generate small amounts of electricity. The amount of energy that can be extracted at a given wind speed is proportional to the size of the rotorperiod. No magic can happen beyond the simple mathematics of the swept area of a wind turbines rotor. Swept area Swept area is defined as the area of wind intercepted by the turbines rotor. The only way to extract more energy at a given wind speed is to increase the area that the rotor sweeps. Increasing rotor area is quite easily accomplished: simply increase the length of the blades that make up the rotor. The results of increasing blade length are quite dramatic due to the fact that the area of a circle is proportional to the square of the radius on the circle. In the case of a wind turbine, the radius is the length of one blade. As can be seen in the diagram below, doubling the length of the blades results, not in a doubling of the swept area, but a four-fold increase in the amount of wind that the rotor can capture and convert to rotational momentum used to drive the generator.
