The number and variety of wind turbine designs available has never been greater. With so many choices, it seems like we should be seeing all manner of wind turbines busily spinning and generating electricity. Yet too many sit idle—generating little, if any, electricity. That’s because a wind-electric system must encompass much more than just the turbine.
Besides a turbine (aka wind generator), you’ll need an adequate wind resource, a properly sized tower, a suitable site, and the wherewithal to maintain and watch over the system.
A wind turbine’s blades are fixed to a hub. Together, they make up the rotor. The rotor turns in the wind, converting the kinetic energy in moving air (wind) into rotational momentum to spin an electrical generator. The rotor is the collector for a wind turbine, harvesting wind energy, which is then converted into electricity by the generator.
Hundreds of different rotor designs have been invented, tried, and discarded over the past eight decades. Regardless, there are still all sorts of claims about unprecedented efficiencies of various new turbine designs. So how can you sort out fact from fiction?
Let’s use a simple analogy to explain the concept of the rotor as a collector, using a solar hot water collector. A 4- by 8-foot collector is capable of collecting a certain amount of sunlight and converting that sunshine into a certain amount of hot water. If we double the size of the collector, it makes sense that the system would now be able to collect twice the sunlight and generate twice as much hot water.
The bigger the renewable energy collector, the more energy it is exposed to that can be collected—and the more output the system will generate. The area of the wind that the rotor intercepts is called the swept area. Just as with solar collectors, increasing the swept area of the rotor increases the amount of wind the turbine can intercept and convert (to electricity). There is no circumventing this concept; it’s just simple mathematics. Doubling the diameter of the rotor results in a four-fold increase in the swept area—and potentially four times the electricity for any given wind speed.
Wind is the fuel for the wind turbine. The more fuel the wind turbine has available to it, the more electricity it will generate. Wind fuel has two components, both equally important. One is the quantity of wind available. The other is the quality of the wind passing through the rotor. Let’s look at these separately.
The equation that defines how much power is available to any wind turbine rotor is P = 1/2dA V3, where P is the power in the wind, d is the density of the air (affected by both elevation above sea level and air temperature), A is the swept area of the rotor, and V is the wind velocity (speed). For a given wind turbine and site, swept area and density are constants. As such, the power available in the wind is approximately the cube of the wind speed.
So, the power that is in the wind (P) is proportional to the cube of the wind speed, or V3. The interesting thing about this relationship is the effect of increasing wind speed. For example, a 2.5 mph difference in wind speed—say, from 10 to 12.5 mph, is an increase of only 25% in wind speed. However, since P ~ V3 (not just V), the numerical increase in the power equation attributable to wind speed is nearly 100%: 10 × 10 × 10 = 1,000, while 12.5 × 12.5 × 12.5 = 1,953. So even small increases in wind speed result in very large increases in power available in the wind that can be converted to electricity.