> I worked thru the equations of getting 
> power from pure drag, and if I didn't mess up it shows that 
> maximum power from an object moving downwind results from the 
> object's speed being 1/3 the wind speed.  If you apply this 
> to the wind power equation for a square flat plate 
> (coefficient of drag about 1.17), then you get about 17% 
> efficiency (again if I didn't mess up).  That is the power 
> that can be taken from a square flat plate being pushed 
> downwind compared to the total power in the wind.  The 1.17 
> figure assumes a square flat plate in free air, meaning the 
> air flows around all sides and only truly stalls in the 
> center.  This coefficient of drag can be higher if air can be 
> prevented from flowing around all sides giving a larger stall 
> or near-stall area as is possible with my design.

I would love to see how you made that calculation. I don't question it, I'm
just curious how you calculated it.

> 
> I don't know how 17% compares to other wind turbine designs 
> or the theoretical maximum that can be taken from wind (there 
> is a sort of Carnot efficiency for wind, you can never get it all).

I've been told that the maximum theoretical efficiency of a windmill is 35%.
No idea how that was calculated, but it is just about twice what you came up
with so I wonder if there is a doubling going into their calculation.

> Also as you noted, efficiency in terms of fraction of energy 
> removed from the wind isn't the point anyway for small scale 
> use.  Efficiency in watts/$ is more relevant, including $ for 
> maintenance.

Absolutely. And different designs may be better matches to different
situations.

> As for the airplane analogy, that doesn't make any sense.  
> Airplanes are trying to accomplish different things.  For one 
> thing drag is very important because it directly counters 
> thrust and therefore reduces efficiency.
> However for a wind turbine, much of the drag only results in 
> greater forces on a bearing.  This increases cost, but has 
> nothing to do with energy conversion efficiency.


The amount of air that is acted upon by a wing is greater than that of a
flat plate. This allows the same size device to act as if it were larger. 

Or at least that is how I understood it. I have no real knowledge in this
area.

---
James.
 

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