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It's possible to build a wind turbine with two blades or five blades or even one blade, but the vast majority of modern windmills have three blades. And there are three good reasons for that. The first reason for three blades comes from physics. All else being equal, the more blades a windmill has, the more torque it can generate. But also, the more drag it has, because each additional blade has to cut through the wind, and drag grows quickly the faster the windmill is spinning. So, in general, many bladed windmills work better at lower rotational speeds relative to wind speed. On the other hand, a windmill with fewer blades generates less torque, but also
experiences less drag. So, fewerb bladed windmills tend to work better at higher rotational speeds. Here's a graph showing the efficiencies of turbines with different numbers of blades, where the x-axis shows how fast they're spinning relative to the wind, and the y-axis shows the efficiency of the turbine. The efficiency for a five blade turbine is highest at low speeds, while a oneb blade turbine works better at high rotational speeds. You can see that the highest possible efficiency of any turbine comes at intermediate speeds on this curve here, which corresponds to, you guessed it, three blades. But there's more to a turbine than efficiency. A super high efficiency windmill is no use if it falls apart
after just a little while. So the second reason why windmills have three blades comes from engineering. The problem is that windmills have to be strong enough to withstand the many different bending and rotational and vibrational forces necessary for their operation. For example, when the blades of a two-blade wind turbine are aligned with or opposite the pole, it can create an unbalanced twisting force under the wrong wind conditions. This can lead to large forces pushing the pole to bend forwards or backwards or twist side to side. There are similar issues with other even numbers of blades, including for our purposes a single blade rotor since it only has a counterwe on the other side and nothing to balance the
unequal wind forces. If the rotor has an odd number of blades like three or five, the fact that blades don't come in pairs means that wind loads are more evenly distributed across the face of the rotor and so the bending and twisting forces are greatly reduced. So in order to use two blades, the materials and engineering of the tower, hub, and blades need to be more robust, which costs more and negates many of the other cost benefits of using fewer blades. Also, when comparing three and five blade designs with the two-blade design, the lower rotational speeds of turbines with more blades means that all the moving parts experience less wear over time. So to minimize unwanted twisting
forces from the wind, we want an odd number of blades. And more blades cause less wear and tear, but more blades cost more money. If you took the blades from two fiveblade turbines, you'd have enough to make three-blade turbines plus one left over. Taking all these engineering factors into account, three blades it is. The third reason for windmills to have three blades comes from human comfort. If you want to build a windmill, you need people to be okay with you building a windmill. So how it looks and how it sounds matters. Three-blade rotors are generally considered more visually pleasing than a two-blade equivalent. One possible reason for this is that the space taken up by a three-blade rotor in the horizontal and vertical directions
changes only slightly over time, whereas a two-blade rotor switches back and forth and back and forth between mostly horizontal and mostly vertical and generally looks more choppy. Even at rest, people prefer the symmetry of three blades over two. Three look pretty good, while two can look off balance or awkward depending on how they're oriented relative to the tower. Wind turbines are also noisy and basically all of the noise comes from the turbulence that forms as the blades cut through the air. So, the least noisy windmills are the slowest. Looking again at the efficiency graph, we see that a three-blade rotor achieves the same or better level of efficiency at slower rotational speed than a two-blade rotor,
which means for a given wind speed, threeblade windmills can run more quietly, though they are by no means silent. Obviously, we've glossed over some details, but essentially that's it. Three good reasons that windmills have three blades. Physics says that three-blade rotors are most efficient. Engineering says that for the same cost, three-blade rotors are less likely to break than two-b blade ones. And human comfort says they're not noisy, headless stick figures. The amount of electricity we can generate with wind power and other forms of clean energy is amazing.
Just look at this chart from the last 25 years. So, on September 21st, we're going to celebrate that progress and help push for even more in a series of events around the world. Rallies, solar installations, parades, all sorts of stuff. It's called Sunday. You know, the sun does power the wind. And if you want to learn more about it or attend an event or even host your own, please check out sunday.ear. Together we can make the sun rise on our clean energy future. That's Sunday.
