Russell McMahon wrote:
> What is more surprising is the comment on efficiency.

I think some of this dicussion of efficiency is misplaced and his analogy
invalid.  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 don't know how 17% compares to other wind turbine designs or the
theoretical maximum that can be taken from wind (there is a sortof Carnot
efficiency for wind, you can never get it all).

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 maintenence.

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.


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