My textbook says that the windmill cannot extract all the kinetic energy of the wind available. Even in theory, the best that can be done is to extract about 60 percent of it. (While in practice, because of blade design and gearing friction it comes 10 15 percent). Why so?
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Conversion of Energy from one form to another, energy lost due to friction.
that comes under the practical aspect, doesn't it? i mean, why can't even theory say around 90 or hundred percent can be extracted? or, what is the difference between the pratical aspect and the theoretical aspect?
yeh one thing i must tell u is that the distance from blade to ground is high so when energy travels in such long pipes there is energy losses u must know
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I think while calculating theoretically we assume lot of things to be perfect and constant, I haven't had much exposure in Physics as I am only a high school graduate. But while solving problems in physics like the projectile motion, I have to generally assume the air drag to be negligible and ignore it, while in the real world air drag can not be ignored, it has a major effect on the projectile and it's trajectory. I guess same must be with windmills.
Simply put: A windmill is a centrifugal turbine. A centrifugal turbine has to allow some of the air to flow through it for it to make rotational energy. This fact requires that some of the energy of the wind pass through without being harnessed by the windmill.
I am not saying that those people ignore the air drag but lets think about the blades. In a wind-farm they must have hundreds of windmills making each blade to be exact replica of the theoretical model must be impossible. This is just one of the parts of the windmill, what about the others parts? And also the ground isn't even, air flow isn't constant, change in temperature. There must be several other factors you can think of which vary with time and are very difficult to replicate in the theoretical model.