Why are wind turbines with less blades better?

[Q-reeus]

I tend to agree with the thrust if not every fine detail of:
https://www.spectator.co.uk/2017/05...or-green-and-they-provide-zero-global-energy/
Consequently, dwelling on advantages of two versus three versus n blades or similar windmill 'optimization criteria' seems more than a little pointless.

 

[Write4U]

In view of the many "many bladed" windmills that can be seen on farms for say, pumping water, the number of blades seems to depend on the use and prevailing wind conditions.

An interesting answer may be found here: https://www.quora.com/Why-is-3-the-...es-on-a-wind-turbine-instead-of-say-5-or-more.

 

[iceaura]

An interesting answer may be found here:

There are some problems with that.
First:

To address the top answer here - there is no way the blade from a modern wind turbine will be sweping through its own wake from one rotation before. This would only be possible at a speed which is far beyond the optimum tip speed ratio of the rotor.

That is not clear. The upwind effects of a rotor's deflection of the airstream reach quite a ways - iirc the still air equivalent of at least half the diameter of the swept area (the reach of the "ground effect" that floats a skimming frisbee) and the blade tips are often spinning much faster than three times the wind speed. So even a one-bladed windmill will feel a little bit - with a two or three bladed mill the width of the blade blockage needs attention for that reason.

Second:

Western mills are used to pump water, and driving a mechanical pump needs a lot of torque. In this case rotational speed is secondary and what you are really after is creating as much torque as possible, i.e. a rotor with lots of blades is the best choice for the job.

A modern pump mill designed to extract as much power as possible from wind would not use many blades like that. It would instead gear down to deliver torque (or more likely use a variable transmission, convert to electricity and wire to the pump, or the like). That would produce more torque at low wind speeds than many less efficient blades turning slowly.

The reason for the many-blade low speed operation is mechanical simplicity when maximum power at higher wind speeds is not a consideration - https://www.homepower.com/articles/wind-power/equipment-products/pumping-water-wind . You could get more power - more torque, more everything - by introducing sophistications like transmissions and higher speed operation with fewer and longer blades, but the cost both of purchase and maintenance blows up rapidly, and durability suffers.

Meanwhile, many-blades lose less efficiency when there is little weight penalty during operation (invariant flow), less penalty for creating upstream effects from the blocking (incompressible, confined channel upstream). etc. https://en.wikipedia.org/wiki/File:J85_ge_17a_turbojet_engine.jpg

 

[Write4U]

That would produce more torque at low wind speeds than many less efficient blades turning slowly.

I doubt a wind turbine could compete with an old Dutch wind-mill as far as torque is concerned. Surface area produces torque. But I understand, there are many other considerations, such as durability and maintenance. Sails need to be furled when not in use.

And it seems that that even in water three or four bladed propellers provide the most efficient thrust v resistance.