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Why are wind turbine blades long and skinny instead of short and fat? - Page 7

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Posted by Bob F on April 14, 2010, 3:55 am
 


Energy Guy wrote:


Have you considered the effect of the blade on air that is not directly hitting
the blade? Is the air an inch from the blade affected? 2 inches? A foot? 2 feet?
If so, then the blade is also affected by that air. And therefore, the blade, as
it sweeps around, may well be extracting energy from all of the air going
through it.



Lift to drag ratio is critical to efficient operation. The more drag you have,
the less energy is left to generate power. So a good airfoil design is very
important. Also, an airfoil has more lift than a flat shape, so again, can
develop more power, since the lift is what makes it turn.


Really? The blade must have a small enough angle of attack to prevent stalling.
If it stalls, energy is wasted in increased drag and lost lift. Sure, a farm
pump may get away with this, but it is far from optimal for power production.
Remember, the angle of attack is lessened by the speed of the blade through the
air.

All my comments relate to horizontal axis windmills. Vertical axis windmills
depend on drag, it seems.

Note the performance graph for the horizontal axis unit with airfoil blades
compared to others on the following report.
http://www.copernicusproject.ucr.edu/ssi/Science%20Fair/5/Optimizing%20Windmill%20Blade%20Efficiency.doc

Another article about blade design.
http://www.mh-aerotools.de/airfoils/windmill.htm

And more technical.
http://digital.library.okstate.edu/OAS/oas_pdf/v56/p121_124.pdf

This one is a little more fun.
http://www.kidwind.org/Presentations/BladeDesign%208.13.09.pdf




Posted by Energy Guy on April 14, 2010, 2:07 pm
 


Bob F wrote:


Have you considered the math that I posted (which you did not quote)
which clearly showed that the GE turbine was only 34% efficient at
extracting the realistic available power from the wind that it's exposed
to?


What is this new, mysterious force that you are inventing?

Does it have a name?

I just proved to you, mathematically, that the GE turbine is only 34%
efficient, so therefore it obviously IS NOT extracting anywhere near
100% of the practical or useable power that is available to it given the
area that occupies.

So you don't have to invent any new mystery force to explain the power
that the blades *are not* capturing.


I agree with that.  But whether or not a blade has a flat profile vs a
cambered profile is a different issue  vs the area of the blade, which
brings us right back to why use a narrow blade if a wide blade can
capture more wind force and convert it into rotational motion.

You don't question the fact that sailing ships have the largest
sail-area they can structurally manage, because more sail area = more
wind energy capture and conversion into motion.


The lift component of the camber of a wind-energy blade does not aid in
increasing the force vector in the direction of forward rotation.  It
decreases the drag vector in the direction that opposes forward
rotation.

You people keep confusing the lift vector from the camber of an airfoil
as used by a plane in cruise flight with the angle of attack that a
wind-blade uses when it's rotating.

A wind-blade has an angle of attack with respect to the wind in a manner
similar to a plane that's taking off from a runway.  The plane is
gaining altitude more because of this angle of attack than it is because
of the camber of it's wing cross-section.  But during cruise flight,
it's not an efficient way to maintain altitude if you maintain this
angle of attack, so this angle is reduced and the camber provides the
necessary lift to maintain constant altitude.


A wind blade with a 45 degree angle of attack is theoretically the most
efficient at converting the force of a wind vector into perpendicular
rotational motion.  It doesn't matter how long the blade is, or how fast
it's moving.  45 degrees will always give you the most rotational
force.  Vector math will tell you that.

If you reduce the angle because you want to minimize drag or turbulance,
then you might do that, and you might find that a shallower angle (25
degrees?  30 degrees?) might give you more net power (after drag is been
considered) but to have a blade with a 5 or 10 degree angle of attack is
absurd - because your goal is to not simply swing a blade in a circle
with as little drag as possible (what's the point of that?) - the goal
is to have the wind "push" the blade out of the way, and a blade does
not get pushed out of the way if it's angle of attack is low (or high -
if you consider the direction of the wind in relation to the orientation
of the blade).

A blade that is oriented at a 45 degree angle to the wind will get
pushed by the wind.  A blade that is 90 degrees will not.

If you could stand on the leading edge of a wind blade as it's rotating,
and you think you are experiencing a wind that's blowing into you, you
would be correct.  But that is a fake wind.  It's not real.  It's caused
because the blade is turning and is being pushed through the air.  That
"wind" is pushing back against the blade's leading edge as it's
turning.  It's a drag - it is opposing the blade's forward motion.  You
don't want it to be there.  It does you no good.  You can't extract
energy from it.  You can only do things to help the blade move through
it.  That's where the lift component of the airfoil camber comes in.  It
helps to reduce this drag force.  

Some people here think that a glider is somehow pulled forward by it's
wings and that a wind blade is also pulled forward because of this
mysterious force as this fake wind flows into the leading edge as it
rotates.

There is no pulling.  There is only drag.  Wings don't pull airplanes or
gliders forward, and a rotating wind blade is also not pulled forward by
the sake of or as a consequence of the fake wind that it is created by
it's own rotation.

Posted by Scott on April 14, 2010, 3:27 pm
 

On Wed, 14 Apr 2010 10:07:15 -0400, in alt.energy.homepower, Energy Guy


I take it you are a self-educated man, am I right?


Posted by Bob F on April 14, 2010, 3:55 pm
 

Scott wrote:

Home schooled maybe?



Posted by Bob F on April 14, 2010, 3:55 pm
 

Energy Guy wrote:

And how efficient is your proposed design?


It is a really advanced concept called air pressure. If you increase it one
place, it causes movement of nearby air, increasing pressure farther from the
source, with the effect lessening with distance.

You don't really think that only the molecules of air that hit the wing are
involved in giving it enough lift to fly? The pressure, or lack thereof, of air
more distant from the wing affects the pressure of the molecules that actually
lift the wing.


Extracting 100% is impossible.

If you did, the air on the back of the blade would not be moving, and no more
air would get through the blade.


I didn't invent air pressure.



Ever notice that they don't put one sail directly leeward of the 1st.


And you keep forgetting that the aerodynamic lift of the airfoil has a
rotational component that increases the rotational force.


The angle of attack increases the pressure on the bottom of the wing. The
airfoil shape decreases the pressure on the top of the wing. The difference
between these two pressures causes the lift.


Force, maybe. Power, no. You need speed to get power.


Nonsense. Noone here thinks that.


Wings produce the force that keeps the glider moving forward. The lift they
produce has a forward component as the glider descends, just like the wheel on a
car rolling down a hill does. A wind blade takes advantage of its lift the same
way.Part of the lift is in the rotation direction.


OK. You know everything, and all the windmill designers in the world are
absolute idiots.

Is this really what you think?



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