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Why are wind turbine blades long and skinny instead of short and fat? - Page 6

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Posted by Energy Guy on April 11, 2010, 2:29 pm
 


Jim Wilkins wrote:


http://www.appletreeblog.com/wp-content/2007/11/song-class-submarine.jpg

Exactly.  Look at the total area occupied by the blades as a percentage
of the total swept area.

Like this:

http://www.blogcdn.com/www.luxist.com/media/2007/04/fan.jpg

And look how many blades this aircraft engine has:

http://www.hoise.com/CEC/99/articles/CEC/LV-TN-12-98-4.html

If you look at the Lift Equation (how a wing or turbine blade generates
lift) and Betz's Law (Gives the maximum energy that can be derved from a
given swept area - called the "Betz Limit") you will see that indeed the
more blades the better.   Three blades will give you no where near the
Betz Limit.  the problems with more blades are noise and mechanical
complexity in the hub to feather all the blades in high wind.

Then there are designs like these:

http://www.youtube.com/user/spiralairfoil#p/u/4/gO3WK89BB5Q

http://www.youtube.com/user/spiralairfoil#p/u/1/4vSD8z4fzNg

http://www.youtube.com/user/spiralairfoil#p/u/6/AJtCJnN7lXc

Wind plants should be optimized to harvest wind energy during times of
very low wind speed, as is typical of the very hot "dog-days" of summer,
when energy requirements are high (for residential and commercial air
conditioning) and ambient wind speeds are very low.

Posted by daestrom on April 11, 2010, 2:15 pm
 


Energy Guy wrote:

Not true.  This is a common misconception, that only the face of the
blade interacts.  As the wind in front of a blade slows/changes
direction, the wind between the blades is also interacting.  Just
indirectly.  How much this 'in between' air interacts is a function of
the fluid properties, speed, and pressure drop through the blading
(front/back of turbine).


Turbine blading looks that way because there is a much larger pressure
change across each row of blading.  As the pressure drop/rise across a
row of blading rises, there is more turbulence between wind directly in
front of a blade and wind flowing between blades.  This turbulence means
the air 'between blades' is not able to transfer as much of its energy
to the air 'in front of blades'.

But adding more blades also adds more drag.  In a windmill, where it
isn't enclosed, you have very little pressure difference from front to
back of the swept area.  So there is a lot less turbulence of the wind
and more energy from 'between blade' air can transfer to 'front of
blade' air.  And fewer blades means more of total energy comes to output
shaft and is not wasted on overcoming drag.

If typical winds were a lot higher, and air was a lot denser, a
different design *would* work better.  But wind is what it is, so the
tri-blade design is one of the most common.


This depends on 'making air move'.  Are you trying to get a lot of air
to move 1-2 mph, or a small amount of air 7-10 mph?  And just how
efficient do you need the fan to be?  The manufacturer doesn't pay
operating costs, they pay only construction cost.  And the end user
might rather have a 'stiff breeze' blowing on their face than the whole
house ventilated by one fan.

Larger fans for moving air in ducts have fewer blades, and when a high
differential pressure is needed, often are centrifugal blower types with
many, many blades.

Function decides form.


When the blade is turning, the air flows over the surface just like an
aircraft wing.  It's a combination of the wind blowing and the blade
turning.  That's why the blade has a 'twist' in it.  The part out near
the end is traveling around the circle faster because it is further away
from the center.  So for a given wind speed, the apparent wind
approaching the blade near the tip is faster but at a much shallower
angle.  To maintain a good 'angle of attack', the blade must be twisted.


Nope.  'Lift' does not pull the wing of an airplane directly forward, it
pulls it upward (i.e. it is what 'lifts' the airplane off the ground).
When climbing, the force (perpendicular to the wing) is actually tilted
towards the back of the plane somewhat.  So increasing the wing's angle
of attack to the air increases lift but also increase the drag that
tends to slow the plane down.  Therefore you have to apply power when
climbing to maintain air speed.

On a windmill blade this force works at an angle that pulls the windmill
blade forward slightly but mostly 'around' in the circle, creating
torque.  It's *generally* perpendicular to the line fore-aft through the
wing/blade profile.  And since the profile is at a complicated angle to
the rotor shaft, the 'lift' force is also at a compound angle.  The
total 'lift' force splits between some portion turning the rotor and the
rest tending to warp the blade (has to be considered in blade
design/strength).

Modern sailboat sails are another good example of this.  When going
towards the wind at a 45 degree angle (sailing NE when the wind comes
from due N), the sail bulges out and creates an air foil.  When properly
trimmed, the air flows along both sides of the sail cloth with little
turbulence.  Some of the thrust generated from the sail comes from
re-directing air flow into a new direction (SSW in my example instead of
due S).  And some of the thrust is actually a 'lift' force created by
the air foil.  This force is more due east.  But the boat's keel won't
let it slide due east so this force is split between south-east against
the keel and north-east making the boat move forward.

daestrom

Posted by Energy Guy on April 11, 2010, 3:14 pm
 

daestrom wrote:


When you have those huge wind plants, a lot of air is going to flow
right between the relatively slow-moving blades without touching them.
There's a lot of potential energy that sailing right between those 3
narrow blades.


The manufacturer wants to build a fan that

- moves the most air relative to the swept area.  Which means he
  wants to optimize the CFM of the fan relative to the size of
  the fan.  A smaller fan means a smaller package, smaller
  packaging box, more boxes per shipping container, etc etc

- moves the most air relative to the power capacity of the motor.
  a weaker motor is a cheaper motor, and also most likely a
  lighter motor, making the entire fan lighter and less expensive
  to package and ship.


Now reverse all the above.

A fan that doesn't take much energy to produce a given CFM will also be
a fan that doesn't take much CFM to turn a generator to produce energy.


But still have a higher blade-area to swept-area ratio then the massive
commercial wind generators.


An aircraft wing is designed to generate lift.  

If wind power blades were exactly like aircraft wings, then they would
have no twist, would be perfectly flat relative to the plane of
rotation, and wouldn't turn at all if wind was flowing directly
(perpendicularly) into their top (curved) surface.

If you start turning such a blade arrangement using external power, the
blades will experience a bernoulli force that would tend to bend the
blades forward (the same way that a plane wing wants to bend upward and
results in lifting the plane).


Re-read what I said.  I did not say that lift pulls airplane wings
forward.

I said that if you take an airplane wing (or 3 wings) and mount them
vertically on a pinwheel, then their rotation would generate a force
that would pull the wings forward (forward means out of the plane of
rotation).  


This analogy with a plane wing or an airplane should not consider the
specific cases of take-off and landing, but instead should consider the
situation the wings are in during cruise flight, when the attitude of
the plane and wings are configured for low drag (and essentially no
angle of attack).  That is what the wing is really designed for.
Optimal low drag and optimal lift for cruise flight.

That particular mode of operation of aircraft wings (cruise flight) is
of no use for the blades of a wind power plant.  

For an aircraft in cruise flight, there is no transverse wind flowing
across the wings (ie no air flow in the direction perpendicular to
forward travel).  Airplane wings want to slice through the air - and not
be affected by vertical drafts in air (or cause any such vertical drafts
or flows).  Air currents that are perpendicular to the wings result in
turbulance and are not desired and the wings are not designed to either
use this turbulance or create or cause it.

But wind power plants *want* to see air flows that are perpendicular to
the direction of blade travel.

Naturally, the goal of a wind power plant is have it's blades turn, and
(if you imagine the blades rotating in a plane parallel with the ground)
then they might look like the wings of an airplane moving forward, but
the dynamics of that apparent forward motion shares nothing with the
dynamics of an airplane wing that is moving forward.

Posted by daestrom on April 13, 2010, 10:37 pm
 

Energy Guy wrote:

Okay, try looking at it this way.  A GE 1.5 MW wind turbine gets
1,500,000 watts from the wind when it blows at 14 m/s

http://www.gepower.com/prod_serv/products/wind_turbines/en/downloads/GEA14954C15-MW-Broch.pdf

A 14 m/s wind has energy density of about 1631 watts per m^2.

It's a basic law of physics that if you extract all the energy from each
kg of air, the air won't have any velocity left.  And if that were to
happen, the air would not be moving.  Betz calculated the absolute
maximum power you can extract from a wind turbine at 59% of the total
power flowing.

That turbine has three blades that are about 38 m long and about 1 m
wide.  So that's a 'blade area' of just 114 m^2.  So it's developing
over 13,000 watts per m^2 of blade area.

Pretty neat trick since the wind only has 1,631 watts per m^2.

So there's a whole lot of wind that is *not* 'sailing right between
those 3 narrow blades.'


<snip>


And still work off much large differential pressures than windmills.
Still apples / oranges.


That's why only an idiot would orient the blades as you suppose.  The
blades are oriented so that the apparent wind flows along both sides of
the airfoil, not just 'flat'.

Blades are twisted because the outer end of the blade is moving much
faster than the inner part, so the vector sum of the blade's speed and
the wind speed always create an 'angle of attack' that has the air
flowing smoothly over the air foil.

Near the hub where tangential speed is low, the blade is angled to face
nearly straight into the wind.  Near the tip, where the blade tip speed
is high, the combination of blade speed and wind speed cause the air to
meet the blade at a very shallow angle with respect to the axis so the
blade is twisted to nearly the same shallow angle.


And that is *not* how wind turbine blades are angled.  So your statement
is pointless.  If you take your 'airplane wing' and turn it 90 degrees
to your mounting so the wind is flowing over both surfaces just like the
air flows over an airplane's wing, now the 'lift' generated by the air
foil doesn't pull it 'forward' but 'around' the axis of your pinwheel.


Turn the wing sideways so the 'lift' turns the axle and you'll see that
it makes a lot more sense.

daestrom

Posted by Energy Guy on April 14, 2010, 2:37 am
 

daestrom wrote:
 

So using a radius of 38 meters, we have a total area of 4,536 square
meters that we've devoted to energy extraction.

A 14 m/s breeze has an energy density of 1631 watts per square meter.
So we have a total potential of 7.4 MW of energy.  Taking 59% of that is
4.37 MW.  And what is our GE generator doing?  It's extracting 1.5 MW,
or 34% of the realistic maximum energy, while 66% is sailing right
through those long skinny blades.


Pretty neat trick to not do what I did - which is to consider the total
swept blade area.  Like I said, I've just proved that 2/3 of the
extractable energy is sailing right between these skinny blades.


Even to a non-engineer, it's obvious that a lot of wind *will* sail
right between those blades.  Why isin't that obvious to you?


Wrong.  Wind speed is wind speed.  Differential pressure (when expressed
in terms of square inches or meters) is of the same scale, whether we're
talking about a huge blade or a house fan.


A propeller blade (and I suppose a wind-generator blade) will have a
cross-section that looks like the profile of a wing (camber) for only
one reason:  To improve the lift-to-drag ratio.  A flat profile blade
will have a similar lift-to-alpha ratio as a cambered profile (alpha =
angle of attack).


Yes, I know that.

I'm responding to the argument that a wind blade is shaped like the wing
of a plane (specifically - a glider).

My point is that aircraft wings are designed for cruise flight, where
the angle of attack is very small - if not zero.

A wind blade will not have such a small angle of attack.  Quite the
opposite.

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