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I have read (and to some degree understood) that if one were to increase the number of blades for a wind turbine, it would be reasonable to reduce its rotational speed. In my mind I'd associate this with a change in transmission ratio between rotor and generator. So it would be a kind of design decision, aimed to maximize efficiency.

Now I have heard a claim which sounds stronger, namely that everything else being equal (which in my opinion includes the size and shape of the blades and the transmission ratio of the gearbox), increasing the number of blades would lead to a decrease in rotational speed. Is this true?

I find this hard to believe. I can see how increasing the number of blades would increase the amount of turbulent air each blade is passing through in the wake of its preceding blade. Due to this, each blade would contribute considerable less angular momentum, but is it really enough to over-compensate the higher number of such contributions? Am I missing some effect here? I guess drag for the whole rotor would increase, but drag per blade should be about the same.

I am not interested in problems of structural integrity which would likely result from just putting more blades on an existing design. So perhaps I should have formulated this the other way round: will removing two blades from a well-designed four-blade turbine automatically make the remaining blades rotate faster? Again disregarding the kinds of problems two-blade designs in particular do entail.

The related question Wind generators - why so few blades? discusses the number of blades in actual turbines, and its top answer compares four shorter blades against three longer ones, presumably fixing the total amount of material instead of the shape of each blade. Which probably makes sense in practice in terms of the power-to-cost ratio.

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MvG
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2 Answers2

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I have never heard of the relationship you mention. Quite the opposite, increasing the number of blades is known to increase aerodynamic efficiency (largely as a result of tip speed). I cannot think, off the top of my head, any reason the system wold speed up, and looking at typical sources I don't see blade count anywhere.

...and there it is, the claim at least. But this is directly at odds with the efficiency stated elsewhere so I don't give this much credence.

Update: I have been looking for references on this, and found a few hand-waving ones. The advantage to smaller number of blades appears to be blamed on the tip avoiding its own wake. However, to me, this makes no sense: even in near-zero wind speed the wake should be far behind the blade before it hits the "next one". NASA built one in the 80's and there was a little research on it in Europe, but that's it, everyone since then has been three blades. That seems to be the sweet spot.

Maury Markowitz
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Caveat : not my field, this is based on memory from a few years ago.

AIUI the relationship is actually between solidity (proportion of the disc that consists of blade) and rotational speed. If solidity is lower, then it takes longer at the same speed to sweep the same area. The optimum speed and solidity will depend on many factors, but I think the misunderstanding in the question is that this is about design optimisation - ie a turbine designed with high solidity will tend to be designed to rotate slower - not about "if I remove a blade from a turbine while it's running, it will speed up".

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