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

[daestrom]

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.

And I proved that it is *more* than just the blade face area that is
extracting energy. What's your point?

You think more area will extract more energy? Perhaps, but more area
will also increase the losses so the net output is not improved. And
that's why thousands of engineers around the world that design these
things for a living have compromised at three blades and not a larger
number. Guess they didn't have your 'insight' to revolutionize the
industry.

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?

You've misunderstood my position. I'm saying that adding more area will
not increase the power output very much more than you already get with
three blades. That's all I'm saying.

You seem to think that putting more blades on will somehow increase the
power output by a significant margin, but all you have is 'seat of the
pants' idea, no facts to back it up.

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.

Pressure is not measured in square inches.

And the differential pressure across the unit is a big deal. Study the
Betz limit derivation and you'll find it right in the middle of
everything. Home fans accelerate just the air column through the fan
and as soon as it exits the fan a large fraction spins off in eddies and
recirculates. This is because the fan not only increases the dynamic
pressure (caused by the velocity of the air), but also the static pressure.

Commercial ventilation fans used in ductwork develop even more dP across
them. Have to in order to get air to flow through the duct-work.

Centrifugal pumps are a similar machine and similar design tradeoffs.
You want a lot of flow at low head, go with axial flow (wind-turbine or
fan). You want a high head but not so much flow, go with a volute
design (centrifugal blower style). You want even more head, go
multi-stage (jet-engine compressor).

Why is it so hard for you to understand that wind turbines are very low
head, high flow devices and that is a major dictator of their design?

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, the camber improves the lift-to-drag ratio by *increasing* lift.
Not reducing drag. If a flat blade had the same lift as a cambered one
for a given angle of attack, then the Wright brothers and all the
aeronautical engineers since then have wasted a lot of time making
cambered wing designs.

BZZZT. I'm sorry, you're wrong. But thank you for trying.

Yes, I know that.

So you just stated that the blades would be pull forward just for sake
of arguing.

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

Which they are, except they have a twist along their length to account
for the change in 'apparent wind' angle caused by the faster tangential
speed as you move further from the axis.

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.

Again you don't seem to understand how the speed of rotation and the
wind speed combine to have the air approaching the blade nearly head on,
almost the same angle as an air plane wing.

Look at a blade up close and you'll see that the wind doesn't 'push' on
the curved side, that would make the thing rotate in the other
direction. It blows against the 'flat' side (what would be considered
the bottom of an airplane wing) at a very shallow angle and flows over
the back, curved side (top of an airplane wing) in a smooth (ideally
laminar) layer.

daestrom

 

[daestrom]

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.

It's no mystery to airplane designers. There is a large low-pressure
zone created above the wing that extends upward from the wing surface
several feet.

The same is true for the blades of a turbine except the area extends
around in a circle, trailing behind the blade.

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.

An assumption that you keep making but haven't any proof.

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 *still* don't get it. The blade is arranged so the angle of attack
of the incoming air is slightly below 'horizontal' if it were a
conventional wing. The air impacting the 'bottom' or flat side of the
blade is deflected and that creates some force of rotation. But the air
traveling over the 'top' or cambered side creates a low pressure region
and that 'pulls' the blade in the same direction. Both effects are
working together to create a torque on the axle to turn the system.

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.

We're not 'confusing it', we're trying to explain to you that you seem
to have the idea that the wind approaches the 'wing' at some angle other
than the truth.

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.

Different plane wings, for different speeds of flight use some of both
of these phenomenon.

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.

Nonsense. You're 'vector math' is assuming simple collision/deflection
of the air with a stationary plate. Like a jet of water impacting a
flat plate.

45 Degrees will get you the most force on the plate, *IF THE PLATE ISN'T
MOVING*. Start moving the blade at right angles to the incoming air and
the vectors all change. If the tangential speed is the same as the wind
speed, a 45 degree angle will extract *ZERO* energy from the wind. This
is because the apparent wind speed to the blade is now sqrt(2) times
higher, but coming in at a 0 degree angle and flowing over both sides of
the plate equally with no deflection anymore.

And this is very important to consider in a turbine blade design unless
you expect your blades will never move.

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).

But just like a wing in level flight, a blade with a shallow angle of
attack can be 'lifted' around the circle of its rotation. And with
laminar flow over the back side of the blade (where the camber is), you
reduce drag quite a lot.

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.

On the contrary, that is *exactly* the air that is flowing over the
surfaces of the blade. It is very *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.

This is 'apparent wind' Any sailor will tell you that as your boat
picks up speed and adds vectorily to the wind's own speed, you have to
trim the sails to a new angle to maintain optimum performance.

Similarly, the mounting angle of the blade in a turbine has to consider
not only the wind speed but the tangential speed of the blade. The
combination will have the air meeting the leading edge of the blade at a
shallow angle of attach and flowing over both sides.

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

No one has made such a suggestion except you, you keep dragging this
'red herring' out and it's nonsense.

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.

Again, no one has suggested it is pulled 'forward', whatever direction
that is. The blade is pulled *around* the axis of rotation by this lift
force.

If we use 0 degrees as the reference and have it pointing toward the
direction the wind is coming from, and we have an 'apparent wind' on the
blade of 45 degrees because the tangential speed (at right angle to the
wind direction) is the same as wind speed, then the 'lift' force
developed by the camber on the blade is at an angle of 135 degrees. If
the flat side happens to be angled at 45 degrees to the reference, it
will not deflect the air at all and create no torque. But a real blade
design would have the flat face at something like 50 to 55 degrees from
the reference direction so it does deflect the air 5-10 degrees.

The 'lift' force can be broken down into two vectors, one 70% in the
same direction as the wind (180 degrees from the reference) and the
other 70% in the direction of rotation. Not against rotation, in the
same direction of rotation. So 70% of the 'lift force' is applied to
the machine to help it rotate. While 70% of the 'lift force' is acting
along the axis of rotation and must be resisted by the strength of
materials. None of the 'lift force' is working against rotation.

There is no pulling. There is only drag.

BZZZT, sorry, wrong again. The orientation of the 'wing' guarantees
that the 'lift' it develops will be at least partially in the direction
of rotation.

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.

Wings pull airplanes and gliders 'up' and the same forces 'pull' blades
'around' on a turbine. Your constantly restating these fake straw-man
arguments are a waste of time.

daestrom

 

[daestrom]

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.

Careful, his '100%' is 100% of the Betz limit, not 100% of all the
energy in the wind. He's at least got that right.

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


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

Actually, there is an interesting characteristic. On a typical sloop
rig, the foresail is often larger than the triangular area between mast
and fore-stay. 150% is common. When sailing close to the wind, much of
this sail is 'behind' the main sail.

This works well because the air hitting the face of the foresail is
deflected back along its length and that funnels it right along the back
side of the main sail. So the speed of the air along the back of the
main sail is increased by this oversized fore sail. And that generates
a lower pressure region on the back side of the main sail so it produces
more thrust (dare I say 'lift' :-/).


(of course now 'guy' is going to claim that's his whole idea and why
don't designers do this with wind turbines, sigh...)

The thrust is just about perpendicular to the boom (bottom of the sail)
and so is not in the direction of straight 'forward'. But it decomposes
into two vectors, one in the direction of movement and one sideways.
The keel resists the sideways force and the forward component moves the
boat.

The wind turbine analogy is that 'thrust' is created by the low pressure
that is created by the air flowing over the cambered surface. This
thrust is at some angle with the axis of rotation. It decomposes into
two vectors, one perpendicular to the axis, causing the blades to rotate
around, the other pushing the blades in the direction of the wind and
that is resisted by the blade structure and hub.

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

Exactly right. He doesn't seem to understand how the blades are
oriented and which 'direction' the 'lift' force is pointing with respect
to the shaft.

daestrom

 

[Energy Guy]

And I proved that it is *more* than just the blade face area that
is extracting energy.

The energy available in wind currents is expressed as a function of the
volume of air (a cubic meter) and it's mass (1.23 kg per cubic meter)
and it's velocity (m/s, to the power 3).

The exact formula is 0.5 x air density (kg/m3) x area (m2) x wind speed
(m/s)^3

At a wind speed of 12 meters per second, it takes 0.083 seconds for our
hypothetical 1 cubic meter of wind to pass by (or through) a wind
turbine blade.

The GE 1.5 MW turbine has blades that are 38 meters long, and a typical
rated RPM of about 20 (or .333 rotations per second). In one second,
the tip of one blade will travel 80 meters (total circumference is 242
meters). In 0.083 seconds the blade tip will travel 6.64 meters.
Naturally, other sections of the blade closer to the hub will travel a
shorter distance during this time.

This gives the blades an apparent larger area then their actual physical
area.

What's your point?

That if fans with wide blades are efficient at making air move, then the
converse must also be true (that air can efficiently make wide blades
turn).

You think more area will extract more energy? Perhaps, but more
area will also increase the losses so the net output is not
improved. And that's why thousands of engineers around the world
that design these things for a living have compromised at three
blades and not a larger number.

Can you show mathematically that a thin, long blade is more efficient at
being rotated by an air current compared to a short wide blade?

How do we know that the large wind turbines that we see in current
operation aren't a comprimize that take other factors into consideration
- such as aesthetics, impact on wildlife, humans, extreme weather, etc?

 

[daestrom]

The energy available in wind currents is expressed as a function of the
volume of air (a cubic meter) and it's mass (1.23 kg per cubic meter)
and it's velocity (m/s, to the power 3).

The exact formula is 0.5 x air density (kg/m3) x area (m2) x wind speed
(m/s)^3

At a wind speed of 12 meters per second, it takes 0.083 seconds for our
hypothetical 1 cubic meter of wind to pass by (or through) a wind
turbine blade.

The GE 1.5 MW turbine has blades that are 38 meters long, and a typical
rated RPM of about 20 (or .333 rotations per second). In one second,
the tip of one blade will travel 80 meters (total circumference is 242
meters). In 0.083 seconds the blade tip will travel 6.64 meters.
Naturally, other sections of the blade closer to the hub will travel a
shorter distance during this time.

This gives the blades an apparent larger area then their actual physical
area.


That if fans with wide blades are efficient at making air move, then the
converse must also be true (that air can efficiently make wide blades
turn).


Can you show mathematically that a thin, long blade is more efficient at
being rotated by an air current compared to a short wide blade?

How do we know that the large wind turbines that we see in current
operation aren't a comprimize that take other factors into consideration
- such as aesthetics, impact on wildlife, humans, extreme weather, etc?

I'd bet you the first ones built were a matter of energy production and
cost. Nothing else. The design hasn't changed much since then, only
incremental improvements in blade shape, turbine control systems and
material science.

As far as proving 'mathematically', I'd also bet that wind tunnel tests
were extensive and the design you see today was the result of such
tests. Wind turbines were tested in wind tunnels long before modern
computer simulations.

 

[Ulysses]

That is my understanding. From what I have read about wind turbine designs
is the narrow blades are capable of turning faster than the air is moving
whereas shorter, wider blades are not.

If the goal of a house fan is to generate as much air flow as possible
with as small a motor as possible (and possibly as weak a motor as
possible) then the result is fan blades that are wide and fat. When it
comes to making air move, blades that are wide and fat seem to be used
in more situations vs blades that are long and thin.

So it would seem. However, I have owned many 20" box fans over the years
and the newests ones have narrower blades, *apparently* move more air, and
use less power. Also, the difference in power consumption between the Low
and High settings is very small.

Wide and fat blades seem to be efficient at making air move.

Why isin't the converse true - that wide and fat blades are more
efficient at being moved by air?

Wide blades *can* be used but are likely to be more limited because once
they reach their maximum speed that's it--they can't go any faster because
of their own wind resistance. I can imagine certain low-wind applications
where they might actually have some advantages.

Look at a jet engine compressor. The blades are thin, but there are
many of them, resulting in a frontal surface area that is mostly blade.

If you look at the old farm windmills of yesteryear they had many small
blades and could start to turn in low wind. They also seem to have been
very durable. Many farms used them to produce electricity (usually 36
volts, I think).