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Weight, drag, and stability. The more blades you have, the more power is required to turn the rotor. With three blades, you're getting the most rotations out of the broadest wind speed range. More blades actually decreases power production.

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I totally saw a video about this. Here it is.

https://www.youtube.com/watch?v=WGKIjojADmg

More blades produce more power and more wind passing across the blades produces more power but more blades blocks more wind from passing. Then math led to the optimum design which you can imagine is what we build as modern wind turbines.

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The power output does not scale linearly. The blades interfere with each other, so the more blades you put on a turbine, the less power each blade generates. Same reason nobody builds biplanes anymore - one long wing is more efficient than two shorter ones, one on top of another.

In fact, two blades would be more efficient (but less stable), and there are even examples of single blade turbines out there.

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You're extracting energy from the flow of wind, and there is a finite amount of energy to be extracted.

It's easier to think of the analogy of a hydro turbine in a river. The water clearly has mass and velocity, which gives it energy. As the water makes the turbine blades spin, you extract some of the energy from the water and turn it into rotational energy of the blades. Thus, the water behind the turbine has less energy and will be moving slower. If I put another turbine behind it, the water that hits it will have far less energy to extract.

Where it gets complicated is that the water behind a turbine blades will have lower pressure than the water in front of it, because energy has been extracted. As you know, a low pressure fluid wants to flow towards a high pressure fluid, so this would theoretically induce flow 'backwards' through the turbine. In reality, it just acts as a resistive force against the fluid flowing through turbine. The more energy you extract, the more the fluid 'behind' the blades will resist the flow.

This actually leads to some interesting math: to maximize efficiency, a wind turbine should only extract 50-60% of the energy flowing across its rotor area. Thus, if you can make one set of blades extract 50-60%, then an additional set of blades is unnecessary and will only add cost and complexity.

Edited to add a source for the last point on the math: https://en.wikipedia.org/wiki/Betz%27s_law

This is correct, it's something called solidity, ratio of number of blades, diameter and chord length. Generally the lower you go the more efficient, though there is some lower limit.

Thanks for the fantastic explanation

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Um, pressure drives flow from regions of high pressure to regions of low pressure.

Also, Betz's law isn't a law to maximize efficiency. It tells you that because fluid has to leave the back of the wind turbine, it has to have some energy. It can't move at zero velocity. It's impossible to extract 100% of the kinetic energy from the flow for this reason. Betz used a simplified but reasonably accurate model of how a wind turbine works to derive a limit on exactly how much kinetic energy can actually be extracted, even from a perfectly efficient turbine, to be compatible with the fact that the air has to go somewhere.

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