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Why are wind turbine blades long and skinny instead of short and fat? - Page 3

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Posted by Bob F on April 12, 2010, 6:10 am

Energy Guy wrote:

Clearly, you are way smarter than all the designers of wind turbines in this
world. Why they haven't come to you for your superior knowledge is beyond

A number of people here have suggested good reasons your theory is wrong. But
clearly, you aren't. It is obvious that designers of cheap home fans know way
more than the designers of multi-million dollar wind turbines, and that their
ideas will solve America's energy crises, with your help.

Thank you for correcting the errors of those stupid engineers, who clearly don't
have your intuitive insight into wind turbine design.

Posted by Energy Guy on April 12, 2010, 2:42 pm

In fan design, there is a measurement known as the "solidity factor".

If a fan blade has a width of FB inches at it's tip or end, and if the
blade is BL inches in length (from the center of rotation to the tip of
the blade), and if there are N blades, then the solidity ratio is:

Solidity ratio = (FB x N) / (BL x 2 x pi)

The work done by a fan blade increases the further out you go from the
center of the fan out to the tip of the fan blade and is proportional to
the solidity ratio.  The wider that a blade is at any given radius, the
more work it can do.  The further away that a blade section is from the
center of rotation, the more work it can do compared to a section that
is closer to the center of rotation.

One way to increast the solidity ratio is to simply increase the number
of blades (N).

If a blade is tapered (wider near the center of rotation, narrower at
the tip) it will have a changing solidity ratio (higher at the center,
smaller at the tip) but can have equal work done by every blade section
as a function of distance from center of rotation.

Blades that are wider at the tip have an increased solidity ratio and
ability to do work as a function of radius compared to a straight or
tapered blade.

The work performed by a fan blade at any radial position is a function

a) Blade chord width (solidity ratio)

b) blade or airfoil twist

c) tangential velocity squared

Posted by Guido on April 12, 2010, 5:48 pm

Energy Guy wrote:

Apples and Oranges. Moving air with a fan is not the same as moving a
fan with air. Like trying to move water up hill instead of down takes
entirely different thinking.

But the way you seem to be thinking, if we shine light on a light bulb,
it should produce electricity. But it clearly doesn't. I am sure there
are other factors that need to be considered as to why fat blades aren't
used. Things like the weight and the friction that weight causes, drag
coefficients, cost to Watt ratios, etc..., that makes the skinny blades
preferable, or they'd be using the fat blades instead.

I won't insult you like many do here, and in fact, applaud your
curiosity and courage to ask why in a public forum where ridicule seems
to be relished.

Then again, there is nothing so hidden as the absolute obvious ;-)

Posted by Scott on April 13, 2010, 1:58 am

On Tue, 13 Apr 2010 00:48:43 +0700, in alt.energy.homepower, Guido

As has been mentioned, the shape of a windmill blade is rooted in the same
design sensibilities as the wing of a sailplane -- to extract wind energy as
efficiently as possible.  In a sailplane, efficiency yields better climbs,
greater range, longer glides, etc., all desireable traits.  In a windmill,
efficiency is simply a way to maximize your return on investment.

Some of the same principles apply to fans.  The prop on the front of my
Cessna 172 is certainly designed to develop as much thrust as possible from
the available power, and sure enough it's got long skinny airfoils.  Quiet,
it is not.

I would submit for consideration that in e.g. a residential or commercial
fan (commercial = think office, not factory), one of the prime
considerations would be NOISE.  In these applications, a fan that operates
near silently is much preferable to a fan that moves more CFMs per watt but
makes a lot of noise doing it.  I'm thinking that the wide blades (and low
RPMs) we see in these applications is as much about noise control as
anything else.

Posted by Energy Guy on April 13, 2010, 3:33 am

Scott wrote:

That is, more or less, completely wrong.

When a glider is moving forward through the air, 99% of the air flow
that the wings experience is parallel or in the same plane as the wing.
Wings generate lift - not propulsion.  A glider has no engine, and the
wings don't give the glider any propulsive forward thrust.  Gravity is
what causes a glider to move forward (after it has been released by a
tow plane).

A glider gains altitude (or potential energy) by flying into and with
thermal updrafts along the sides of hills or mountains.  If those did
not exist, then a glider can do only one thing:  Lose altitude while
moving forward.

In a wind turbine, wind flows directly into the face of the blades -
perpendicular to the blades - not in the same plane as the blades.

If turbine blades were to function as wings, then they would generate a
lift force that would operate perpendicular to the surface of the blade
in the same way that a wing generates a lift force that operates on the
top surface of the wing and in a direction that is perpendicular to the
wing surface.  Such a lift force is, naturally, desirable for a wing,
but would perform no function for a wind turbine blade.  The blade wants
to see a force operating in the plane of rotation, which happens when it
converts the perpendicular wind force into a tangential force because of
it's twisted shape.  

Noise represents energy loss caused by turbulance or non-laminar flow.

A fan that is quiet is also a fan that moves air efficiently, without
energy loss caused by noise.

Noise for wind turbines is also an important consideration, and
especially since phenomena such as infrasonic noise is becoming a factor
in public acceptance of these large wind plants.

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