Number of blades on a wind turbine

by Ray
(Kentucky)

I see from some of your other pages that the "swept area" - the area covered by the turbine blades - is very important.

If a turbine has 6 blades as opposed to three does the power it can harness increase?

what about 4?


Thanks
Ray

ANSWER:

Hi Ray – thanks for your question. The best number of blades to have on a turbine is a question which is often asked in forums and discussion groups, particularly by people who want to build their own turbine. I'm not sure whether you fall into that group or not, but in any case, it is a subject that has been researched quite thoroughly by the makers of commercial turbines as well.

I'll start a the bottom, so to speak.

In essence, the ideal turbine would have one blade. You lower the production and material costs and it has a high lift-to-drag ratio. However, it is difficult to balance & usually requires a counter-balancing weight to be attached. They also suffer from aesthetics and noise problems. Such turbines, although many were built, have never stood the test of time.

Two blade turbines give you an increase in efficiency over one blade but also suffer from aesthetic and noise problems. Many diy'ers build two blade turbines because they can save on the production costs.
and they are easier and cheaper to build than three bladed turbines.

Over time the favored configuration has become three blades, the most common feature of modern commercial turbines. Three-bladed turbines have shown themselves to be more efficient, more aesthetically pleasing and better balanced and therefore less noisy.

And now to your question...

Increasing the number of blades beyond three is going to lead to a loss of efficiency due to the greater drag. You could have the most efficient air foil ever made but all you will achieve is a loss of efficiency.


My previous comments are directed towards someone who wants a turbine to generate a reasonable amount of electricity so as to reduce their power bill.

There is , however, one situation where you might consider having more blades and that is if you want to have a turbine which is used maybe for pumping water on your ranch. What you need there is a greater amount of torque and this is provided by an increase in the number of blades. Think of the old style windmills that dotted the American countryside – many blades and they fulfilled their pumping purpose very well indeed. But note, speed was sacrificed for torque and speed is important when it comes to a wind turbine used to generate electricity for the family home.

To return to your question – yes - swept area is a key factor in how much power you can extract from the wind and a three-bladed machine is going to give the best results. Increasing the blade numbers will lead to a los of efficiency and therefore less power will be extracted from the wind. And that means less electricity generated and a smaller reduction on your power bill.

Cheers

Ian
BlueRam Group

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