Why do wind turbines have three blades? Why not just two?
I will go down in history as the person who asked the last question on AskSam, the NH Public Radio series in which reporter / podcaster Sam Evans-Brown answers listeners’ questions. Sam quits radio to become an advocate for clean energy.
My question was: why do all wind turbines look the same? You can read or listen to his response here. I pretty much knew the answer, but Sam went into more detail than I knew and even tracked down the man responsible for the design. Very cool!
But he didn’t answer a question: why three blades? You want the minimum number possible to keep construction costs down – more blades don’t automatically produce more power due to increased drag, despite what many alternative energy DIYers think – and a single blade would be so out of balance. that it would unbalance the turbine. BUT why not use two, which would be well balanced?
Fortunately, I have a friend, Earle Rich, who worked on wind turbines in their early days, including Altamont Pass Wind Farm in California, the first large wind farm on the continent. That was back when this problem was solved through design and experimentation and here is his answer:
Their simplest answer is that three blades will have a uniform thrust on the generator when it tilts into the wind. A two-bladed machine, when turning on the tower to face the wind or downwind tracks, will have maximum yaw resistance when the blades are parallel to the ground and minimum resistance when vertical. This extreme change will cause the tower to periodically torque, which is hard on ground mounting and guy lines, hard on nacelle bearings and other parts.
Three blades are a bit more difficult to balance, with the root, center, and tip distributing the weight all having to be considered. But, the benefits far outweigh the added cost of an additional blade and mounting hub.