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by PerpetualAFK on February 27th, 2007

PerpetualAFK

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Why are windmill blades so slim? Wouldn't a large fan-like blade be better? I guess there's a problem with the windmill tipping over, but is that the only reason?

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  • by lloydweaver on July 7th, 2008

    lloydweaver

    The torque to a windmill is created by the velocity change (angle change) as the wind passes by the blade. The theoretical energy that can be had is about 50% of the total air volume, about a 30% velocity change of the whole air mass. It’s no problem to slow down the air mass 30% with a large 3-blade turbine. They are now made large in diameter to reduce the rpm to 20 or so at full power so as not to kill birds, and 3-bladed to reduce the pulsation impacts as it passes by the tower. Given the large diameter of the blades, it doesn’t take a lot of pressure, even on the small blade area of these large turbines, to produce the approximate million foot-pounds of torque that a 3-MW turbine will need at 20 rpm. Large turbines in class 5 winds can achieve about a 40% capacity factor, that is, a 3 MW turbine would average 0.40 x 3 or 1.2 MW average output continuously. Interestingly, wind turbines have only about 10% of the footprint of photovoltaic power systems and so wind power will likely be the major source of “solar” energy, if not totally dominate. Wind power is also potentially very low cost electrical energy, and it would be simple to install enough wind turbines to charge every commuter car on the road every day if they were efficient all-electric vehicles. We are just getting stated in wind power technology; it will get a lot cheaper and over time a lot more “light air” efficient.

    Comments

    • Thanks for the good answer.

      It still seems astonishing that the incremental efficiency gain by increasing the number to, say, five, is not worthwhile. On an intuitive level, it seems like a lot of air is going without doing work.

      AlwaysHope

      by AlwaysHope on July 15th, 2009

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