Why are wind turbines with less blades better?

Discussion in 'General Science & Technology' started by Edont Knoff, Apr 4, 2018.

  1. Q-reeus Valued Senior Member

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  3. Write4U Valued Senior Member

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  5. iceaura Valued Senior Member

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    There are some problems with that.
    First:
    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:
    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
     
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  7. Write4U Valued Senior Member

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    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.
     
    Last edited: Apr 8, 2018

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