Many different blade designs and configurations have been tried over the years, and most don’t work terribly well. There are good reasons that support the successful designs. Look around—from sailboat wind generators to utility-scale machines—and you’ll see the results of decades of engineering development. More information on aerodynamics can be found in many places. Hugh Piggott’s book, Windpower Workshop, offers technically accurate, concise explanations.
Wind is a cubic resource. The power available in the wind increases as the cube of the wind speed. If we shovel gravel twice as fast, we get twice as much gravel. If the atmospheric heat engine shovels moving air twice as fast, we get eight times as much energy (23, or 2 × 2 × 2).
This helps us understand wind generator design and siting. At the low end of the scale, it’s important to know that anyone making significant performance claims at, say, 4 miles per hour is either clueless about the physics of wind or trying to pull the wool over your eyes. Do the math: Four times four times four is 64; 10 times 10 times 10 is 1,000. There just isn’t very much energy at very low wind speeds. A wind generator doesn’t start doing much until at least 10 mph.
On the other end of the scale, it’s important to remember the forces we are dealing with. Forty times forty times forty is 64,000. That’s 64 times more energy than a 10 mph wind carries. You can see why wind turbines worth buying will protect themselves (govern) at 25 mph to 30 mph, shedding the rare high winds so they can stay alive for the next reasonable wind.
So there’s a practical limit to how high of winds a wind generator is designed to capture. There’s also a physical limit of how much of any wind you can capture. The Betz theorem states that you can only harvest a maximum of 59.3% of the wind before any attempt to take more will decrease what you get.
Think of it this way. A knife blade sticking up will take almost no energy out of the wind (only a little, due to friction). On the other end of the scale, a brick wall will try to take all of the energy out of the wind, blocking its path completely, but it won’t turn any of it into useful motion. In the first case, the wind just passes by the “collector.” In the second case, the wind backs up behind the collector, slows down, goes around, and just doesn’t do any useful work.
Somewhere in between, there’s a sweet spot where we can capture some of the energy in the wind without slowing it down too much. We need to allow enough moving air through the collector to ensure adequate flow, and not back up too much wind against our collector, diminishing the return. Betz says that the maximum is about 60%. In real-world applications, a well-designed, modern utility-scale turbine might hit 50%, while residential turbines fall more in the 20% to 40% range.
Once we’ve taken our portion of the energy of that moving air and turned it into spinning motion in a shaft, we still need to spin the alternator. That shaft is spinning in a certain speed range, generally peaking at 150 to 900 rpm, depending on the machine’s size and its design. (Low-rpm machines capture the same amount of energy from the wind as faster-turning ones, but the slower pace incurs less wear and tear and makes less noise.)
We can use a generator that works in the same speed range as the spinning shaft coupled to the blades (called the “rotor”). These are “direct-drive” machines, and they are the simplest and most efficient, if the blades and generating device are well-matched.