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PICList Thread
'[EE] Vertical axis wind turbine'
2006\03\17@075308 by olin piclist

face picon face
A while ago we had a discussion of wind turbine designs.  That made me
remember a few ideas I had a while ago.  They are difficult to explain in
words, so I didn't try at the time.  For some reason I was thinking about
this last night so I created some pictures to demonstrate one of the design
concepts.

A single high resolution picture is as
http://www.embedinc.com/wind/wind.jpg.  An animation of it rotating due to
wind is http://www.embedinc.com/wind/wind.gif.  An animation showing how it
reacts to wind direction changes is at
http://www.embedinc.com/wind/windd.gif.

The two green plates are one assembly, and the rotation of this assembly is
where the power is taken from.  The center gear is kept oriented so that the
one marked tooth is pointing in the direction the wind is going.  The
gearing keeps the flat paddles always oriented flat to the wind on the right
side and edge on to the wind on the left side, thereby providing the
different wind force on the two sides that makes the whole assembly rotate.


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Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@083352 by PY2NI TERRA

flavicon
face

   A very clever aproach indeed, congraulations. I just wonder how bad
those gears could impact the the overral improved  efficiency, nevertheless
I take my hat off to you.

Horta


> A while ago we had a discussion of wind turbine designs.  That made me
> remember a few ideas I had a while ago.  They are difficult to explain in
> words, so I didn't try at the time.  For some reason I was thinking about
> this last night so I created some pictures to demonstrate one of the
design
> concepts.
>
> A single high resolution picture is as
> http://www.embedinc.com/wind/wind.jpg.  An animation of it rotating due to
> wind is http://www.embedinc.com/wind/wind.gif.  An animation showing how
it
> reacts to wind direction changes is at
> http://www.embedinc.com/wind/windd.gif.
>
> The two green plates are one assembly, and the rotation of this assembly
is
> where the power is taken from.  The center gear is kept oriented so that
the
> one marked tooth is pointing in the direction the wind is going.  The
> gearing keeps the flat paddles always oriented flat to the wind on the
right
> side and edge on to the wind on the left side, thereby providing the
> different wind force on the two sides that makes the whole assembly
rotate.
>


2006\03\17@090152 by olin piclist

face picon face
PY2NI_TERRA wrote:
> A very clever aproach indeed, congraulations. I just wonder how bad
> those gears could impact the the overral improved  efficiency,
> nevertheless I take my hat off to you.

The design does have the drawback of rather a large number of moving parts.
However note that the wind power is not transferred over those gears.  The
wind pushes the paddles which rotate the whole assembly.  The gears only see
whatever torque is on the paddles from uneven left/right wind loading on
each paddle, primarily due to the wind shadow effect of upstream paddles.

So to answer your question, simple spur gears are relatively efficient and
since the main power isn't transferred over them anyway, I don't think these
gears have much of an impact on efficiency.  Far greater losses will come
from the fact that the paddles on the left side still have some wind
resistance, and this force directly subtracts from the "useful" force of the
paddles on the right side that are flat to the wind.

Once nice thing that just happened to work out in this design is that the
paddles at front and back are oriented 45deg to the wind such that they push
left and right in the right direction.  This means the paddles are producing
useful torque over well more than half the rotation of the assembly.  At
some point the angles work out so that the forward torque from the sideways
"sail" force is exactly ballanced by the opposite torque from the drag
force.  I don't know how to calculate that, but the angle over which the
paddles on the left are a liability has got to be considerably smaller than
180deg.  And since they are fairly edge-on to the wind during this time,
that liability has got to be relatively small.

Another way of looking at this is that the total wind intercept area is a
rectangle (from the wind's point of view).  At some point along that
rectangle there is a vertical line where to the right the wind forces
produce useful work and to the left are a liability decreasing the overall
efficiency.  I think that line is well left of center, and that efficiency
is "reasonable" compared to other wind turbines in that a good fraction of
the wind over the intercept area is used efficiently to produce output
power.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@095124 by Russell McMahon

face
flavicon
face
>... wind turbine designs ... a few ideas I had a while ago.

> http://www.embedinc.com/wind/wind.jpg.  An animation of it rotating
> due to
> wind is http://www.embedinc.com/wind/wind.gif.  An animation showing
> how it
> reacts to wind direction changes is at
> http://www.embedinc.com/wind/windd.gif.

Looks brilliant to me :-) !!!
Gearing in the primary path should be avoided, but for something with
gears etc this looks very clever.
Could be done with chains/sprockets or V belts.

It's not obvious without further inspection that the blade angle is
always optimum for its position but they do seem to be doing pretty
much the right thing.

When are you going to build one ? :-)


       Russell McMahon

2006\03\17@095245 by Josh Koffman

face picon face
On 3/17/06, Olin Lathrop <spam_OUTolin_piclistTakeThisOuTspamembedinc.com> wrote:
> A while ago we had a discussion of wind turbine designs.  That made me
> remember a few ideas I had a while ago.  They are difficult to explain in
> words, so I didn't try at the time.  For some reason I was thinking about
> this last night so I created some pictures to demonstrate one of the design
> concepts.

I've always been a big fan of the Darrieus Lift style. My favourite is
the eggbeater variant. Check out a picture on this page:
http://www.awea.org/faq/vawt.html - that particular one belongs to the
US Department of Energy. There are significant disadvantages to this
style of turbine, some detailed on the page linked above. I believe
this style can also have issues starting up - you need to actually
give it a boost to get it spinning before the wind power takes over. I
might be wrong though. I do vaguely remember watching a TV show about
these once and they person being interviewed was talking about the
amount of torque these generate. Apparently shearing the 2" bolts in
the various bits of the machinery was a real issue. I haven't really
studied this, I just think they are cool.

By the way Olin, what did you use to generate your images? They look cool!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2006\03\17@102249 by Wouter van Ooijen

face picon face
>... wind turbine designs ... a few ideas I had a while ago.

The problem with these 'drag' type mills is what to do whith the slow
rotation? Generators need a much higher rotation frequency, so you'll
end up with gears anyway. Lift mills (partially) avoid this by rotating
much faster.

BTW I think I have seen this type of mill before, but I don't recall
where.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\17@102334 by olin piclist

face picon face
Russell McMahon wrote:
> Gearing in the primary path should be avoided,

It is.  Note that the gears are NOT in the power train.  The power is taken
from the rotation of the whole assembly, signified by the green platters in
the images.  The purpose of the gearing (fancier than I would like too) is
to keep the paddles oriented properly for the wind direction.  A wind vane
would be attached to the center gear, although that is not shown.  Perhaps I
should add one to make that more clear.  For now imagine the marker on that
gear always pointing in the direction of the wind.

> It's not obvious without further inspection that the blade angle is
> always optimum for its position but they do seem to be doing pretty
> much the right thing.

I don't know what optimum is for all points in the circle, but the paddles
are oriented in what seems the intuitive optimum angle at the four points of
right, back, left, and front (from the wind's point of view).  At right the
paddle is totally flat to the wind for maximum wind resistance, and at left
totally in line for minimum wind resistance.  At the front and back points
they are exactly 45deg to the wind which produces a sideways force in the
direction of rotation.  In between is going to come out as it comes out, but
I'm figuring must be pretty good if it's interpolating between optimum
points every 90deg.

> When are you going to build one ? :-)

As "spare" time permits, which isn't much.  I really should be getting to
the Windows driver for my new USB PIC programmer.  A problem retrieving the
extra PC from someone who borrowed it to use as the driver test platform
gave me a breather.  I've actually had this idea for a few years, but
something else always seemed more urgent than building a model.  Beside, I'm
sure someone out there has done this already.  If not, then that will give
me considerable incentive to build a working model.

I've got other ideas for wind turbines too.  For example, a horizontal axis
design with traditional "propeller".  The difference is that the power
takeoff is a vertical rotating shaft, and that torque on that shaft does NOT
cause any torque on the wind-orientation motion of the top assembly.  Again,
I imagine this has been done too.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@105013 by olin piclist

face picon face
Josh Koffman wrote:
> By the way Olin, what did you use to generate your images? They look
> cool!

Thanks.  I used my "slide making" program.  It started in the early 1980s as
a way of making computer images containing text for writing to slides using
a film recorder.  That sounds simple, but you soon realize you need to
control this parameter, then another, then save/restore state as you make
something temporarily yellow for example, then have global state like margin
location, etc, etc.  It's been thru two major rewrites with the last about
10 years ago.  Now it's a full blown interpreter with variables,
subroutines, intrinsic 2D and 3D primitives, conditionals, loops, built in
spline handling, vector and matrix math, and a few other things.  It's still
useful for writing 2D text to images too.  The entire code for generating
all the images is 304 lines long, and a good chunk of that is comments.

I don't mind releasing it if others want to use it.  I converted the source
file to be HTML viewable and put it at http://www.embedinc.com/wind/s.sl.htm
so you can have a look if you want.  You can probably guess what most
statements are doing.  Please keep in mind this was a quick hack I banged
together yesterday afternoon and was never meant to be publicly viewed (I
guess I should know better).  Yes, I know there is some sloppy programming
in there.  Now you get to see how ugly my code looks when I think I'm
writing a one-off that nobody else will ever see.


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consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@105518 by Michael Rigby-Jones

picon face


{Quote hidden}

There is a design called a cycloturbine that functions in a broadly simmilar manner, though the motion is not achieved through gears but rather through cranks and linkages.  However rather than relying on drag from flat panels, the cycloturbine uses proper aerofoil shaped blades to use lift to rotate it which is generaly more efficient IIRC.  Olins design could doubtless be adapted to use aerofoils though.

Regards

Mike

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2006\03\17@105844 by olin piclist

face picon face
Olin Lathrop wrote:
> I converted the
> source file to be HTML viewable and put it at
> http://www.embedinc.com/wind/s.sl.htm so you can have a look

Oops, should have been http://www.embedinc.com/wind/w.sl.htm.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@141637 by M. Adam Davis

face picon face
Very nice!  Here's a similar windmill:

From:
http://www.thebackshed.com/Windmill/photos2.asp

Overview picture:
http://www.thebackshed.com/Windmill/images/115-1566_img.jpg

Closeup of offset bearing with wind vane:
http://www.thebackshed.com/Windmill/images/115-1564_img.jpg

I see that it's easy to change the angle of the blades on yours - this
version may be more limited and difficult to adjust, but requires
fewer moving parts.

-Adam


On 3/17/06, Olin Lathrop <.....olin_piclistKILLspamspam.....embedinc.com> wrote:
{Quote hidden}

> -

2006\03\17@141900 by M. Adam Davis

face picon face
And a more complete page on this type of windmill:

http://www.windstuffnow.com/main/darrieus_type.htm

-Adam

On 3/17/06, M. Adam Davis <EraseMEstienmanspam_OUTspamTakeThisOuTgmail.com> wrote:
{Quote hidden}

2006\03\17@144812 by olin piclist

face picon face
M. Adam Davis wrote:
> Very nice!  Here's a similar windmill:
>
> From:
> http://www.thebackshed.com/Windmill/photos2.asp
>
> Overview picture:
> http://www.thebackshed.com/Windmill/images/115-1566_img.jpg
>
> Closeup of offset bearing with wind vane:
> http://www.thebackshed.com/Windmill/images/115-1564_img.jpg
>
> I see that it's easy to change the angle of the blades on yours - this
> version may be more limited and difficult to adjust, but requires
> fewer moving parts.

I hadn't thought of pivoting the blades via a linkage, but that is the
second reference to that kind of wind turbine someone has pointed out.  It
looks like the blades are in their primary driving position at front and
back with them at an angle for the wind to push them sideways.  That's
actually a rather different concept from mine where the main drive comes
from direct pushing of a flat surface downwind.  I wonder if either method
is inherently more efficient?  I would guess not, but that each has a
different speed/torque tradeoff.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@145621 by olin piclist

face picon face
M. Adam Davis wrote:
> And a more complete page on this type of windmill:
>
> http://www.windstuffnow.com/main/darrieus_type.htm

That's like the other one as you said, and the author states it's probably
mostly lift and maybe a little drag.  It's interesting he called this a
Darrieus type.  At least this one I can understand how it works by pivoting
the blades differently during the rotation.  However I've seen the egg
beater style called a Darrieus and I haven't seen a good explanation for how
those work.  The blades are thin and curved so there would be no way to tilt
them as this one does.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@155208 by Wouter van Ooijen

face picon face
> The blades are thin and curved so there would be
> no way to tilt them as this one does.

The 'eggbeater' style vertical rotors work on lift, not drag. The rotors
have a vertical-areoplane-wing form, creating lift when the wind flows
around. The advantage of lift is that the rotor can move faster than the
wind flowing around it, drag is limited to the wind speed.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\17@162455 by Herbert Graf

flavicon
face
On Fri, 2006-03-17 at 21:54 +0100, Wouter van Ooijen wrote:
> > The blades are thin and curved so there would be
> > no way to tilt them as this one does.
>
> The 'eggbeater' style vertical rotors work on lift, not drag. The rotors
> have a vertical-areoplane-wing form, creating lift when the wind flows
> around. The advantage of lift is that the rotor can move faster than the
> wind flowing around it, drag is limited to the wind speed.

Only problem with them as I understand it is they can't start on their
own, they have to be spun up to a certain speed before they are good to
go.

The benefit of them of course is they can use wind from any direction
parallel to the ground.

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

2006\03\17@164526 by olin piclist

face picon face
Wouter van Ooijen wrote:
> The 'eggbeater' style vertical rotors work on lift, not drag. The
> rotors have a vertical-areoplane-wing form, creating lift when the
> wind flows around. The advantage of lift is that the rotor can move
> faster than the wind flowing around it, drag is limited to the wind
> speed.

Yes, I understand that, but there has to be a change in airfoil profile or
angle of attack between the front and back.  Something has to preferentially
push the blade one way when it's at front, then the other way when you flip
it 180deg.  Unless they are counting on the wind shadow effect of the front
blade on the back, but that doesn't sound too efficient.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@165440 by olin piclist

face picon face
Herbert Graf wrote:
> Only problem with them as I understand it is they can't start on their
> own, they have to be spun up to a certain speed before they are good
> to go.

Yes, but again what produces the net torque?  Let's say the blade is set up
so that when it's at the front it pushes to the right.  When that same blade
gets around to the back it has now pivoted 180deg.  If it's just a flat
blade, it will now be pushing again to the right, counter acting the torque
from when in front.  Something has to make it push either to the left, or
not as hard to the right when it's in back.  The windmill in the picture
achieved this by pivoting the blade back and forth.  Mine does this by
rotating the blades at half the speed of the turbine.  I don't see how the
eggbeater type can do either of those.  Do these blades have some strange
airfoil shape that has different lift when flipped 180deg?  Are they relying
on the wind shadow of the front blades reducing the torque produced by the
back blades?  Different warping when pushed on from opposite directions?
Something else?

The fact that it doesn't work until it gets started makes me think there is
a wind shadow effect in there somewhere.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@172217 by Roy

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face
part 1 986 bytes content-type:text/plain; (decoded 7bit)

Looks good,

What do you use to make this program execute?

_______________________________________

Roy Hopkins
Tauranga
New Zealand
_______________________________________


> I don't mind releasing it if others want to use it.  I converted the
> source
> file to be HTML viewable and put it at
> http://www.embedinc.com/wind/s.sl.htm
> so you can have a look if you want.  You can probably guess what most
> statements are doing.  Please keep in mind this was a quick hack I
banged
> together yesterday afternoon and was never meant to be publicly viewed
(I
> guess I should know better).  Yes, I know there is some sloppy
programming
> in there.  Now you get to see how ugly my code looks when I think I'm
> writing a one-off that nobody else will ever see.
>

--
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.385 / Virus Database: 268.2.5/284 - Release Date:
17/03/2006




part 2 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2006\03\17@172729 by Wouter van Ooijen

face picon face
> > The 'eggbeater' style vertical rotors work on lift, not
> drag. The rotors
> > have a vertical-areoplane-wing form, creating lift when the
> wind flows
> > around. The advantage of lift is that the rotor can move
> faster than the
> > wind flowing around it, drag is limited to the wind speed.
>
> Only problem with them as I understand it is they can't start on their
> own, they have to be spun up to a certain speed before they
> are good to
> go.

Note that you will often see them with a small double-oil-drum type
rotor (drag tupe) as an intergral part, taking care of the startup.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\17@172729 by Wouter van Ooijen

face picon face
> > The 'eggbeater' style vertical rotors work on lift, not drag. The
> > rotors have a vertical-areoplane-wing form, creating lift when the
> > wind flows around. The advantage of lift is that the rotor can move
> > faster than the wind flowing around it, drag is limited to the wind
> > speed.
>
> Yes, I understand that, but there has to be a change in
> airfoil profile or
> angle of attack between the front and back.  Something has to
> preferentially
> push the blade one way when it's at front, then the other way
> when you flip
> it 180deg.  Unless they are counting on the wind shadow
> effect of the front
> blade on the back, but that doesn't sound too efficient.

I am no expert, IIRC there is no deformation going on, the blades have a
teardrop-shaped cross-section. I think the active part is when the
blades are at the upwind and lowwind sides, with both sides contributing
a pull in the same rotational direction. The other two quardrants do not
contribute. With a drag style rotor like yours the active part is the
downwind-moving one of these quardrants.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\17@173805 by Wouter van Ooijen

face picon face
> Yes, but again what produces the net torque?

http://en.wikipedia.org/wiki/Darrieus_wind_turbine

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\17@173948 by Herbert Graf

flavicon
face
On Fri, 2006-03-17 at 16:54 -0500, Olin Lathrop wrote:
> Herbert Graf wrote:
> > Only problem with them as I understand it is they can't start on their
> > own, they have to be spun up to a certain speed before they are good
> > to go.
>
> Yes, but again what produces the net torque?  Let's say the blade is set up
> so that when it's at the front it pushes to the right.  When that same blade

Not sure, never really looked into it, all I know is they do work (they
built one once on Scrapheap Challenge (a.k.a. Junkyard Wars)). When they
explained it on the show it made sense to me, but I no longer remember
the details.

I do know that the blades have an aerofoil shape, and that they can spin
faster then the wind speed.

Considering the predominance of the standard blade type windmill for
commercial generation, I guess there's some advantage of the blade type
windmill over the "egg beater" windmill for large scale farms.

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

2006\03\17@175108 by olin piclist

face picon face
Wouter van Ooijen wrote:
>> Yes, I understand that, but there has to be a change in
>> airfoil profile or
>> angle of attack between the front and back.  Something has to
>> preferentially
>> push the blade one way when it's at front, then the other way
>> when you flip
>> it 180deg.  Unless they are counting on the wind shadow
>> effect of the front
>> blade on the back, but that doesn't sound too efficient.
>
> I am no expert, IIRC there is no deformation going on, the blades
> have a teardrop-shaped cross-section. I think the active part is when
> the blades are at the upwind and lowwind sides, with both sides
> contributing a pull in the same rotational direction.

Yes, that's what I meant by front and back.  But you're saying there is such
a thing as an airfoil shape that at some orientation it pushes to the right,
and when it's flipped 180deg it pushes to the left?  That sounds a little
hard to believe or at least it doesn't sound like it would be a major effect
if there is one.

> With a drag style rotor like yours the
> active part is the downwind-moving one of these quardrants.

Look at mine carefully and you'll see that's not true.  The right side
definitely works on drag effect.  But, the blades are oriented exactly 45
degrees to the wind when at front and back giving a sideways push in the
direction of motion in both cases.  Only the left "quadrant" doesn't
contribute, and in fact adds some counter-drag.  The high angle of attack
(45deg) at the front and back means that the free running speed there is
about the same as the wind speed, which is definitely true of the right side
which works on pure drag.  I therefore expect my turbine to have high torque
and low speed, and be relatively efficient.


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Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\17@190536 by VULCAN20

picon face
This is a very good design.  But it does have one fault to it.  All wind
power devices need to have a way to stop and create as low of wind
resistance as possible.  This is necessary for many reasons.  
Maintenance or high winds that would cause the unit to self destruct.

Bob

Olin Lathrop wrote:

{Quote hidden}

2006\03\17@200337 by Dave Tweed

face
flavicon
face
VULCAN20 <@spam@VULCAN20KILLspamspamcharter.net> wrote:
> This is a very good design.  But it does have one fault to it.  All
> wind  power devices need to have a way to stop and create as low of
> wind resistance as possible.  This is necessary for many reasons.  
> Maintenance or high winds that would cause the unit to self destruct.

Not a problem. Just rotate the center gear out of its optimum position
for the current wind direction, or even "pop" it up out of the plane of
the other gears and let the paddles freewheel into idle positions.

-- Dave Tweed

2006\03\17@200811 by Jeff Latta

picon face
Olin Lathrop wrote:
> Yes, I understand that, but there has to be a change in airfoil profile or
> angle of attack between the front and back.  Something has to preferentially
> push the blade one way when it's at front, then the other way when you flip
> it 180deg.  Unless they are counting on the wind shadow effect of the front
> blade on the back, but that doesn't sound too efficient.
>  
There's a picture and explanation at
http://en.wikipedia.org/wiki/Darrieus_wind_turbine that shows how this
works.

Wind turbine design is actually quite complex.  There's a trade-off
between power, torque and speed range.
If your machine is drag based you can expect its efficiency to be quite
low, in the order of 10% like the Savonius rotor turbines.  If you
change the angle of the air flow by deflecting it 45 degrees you can
expect high torque, an efficiency of around 30% but a very narrow
operating range, like traditional water pumping windmills.  Modern three
bladed electrical generating turbines can achieve efficiencies of 50%
but are characterized by low-torque and high operating speed.  IIRC one
of the characteristics of high efficiency machines was minimizing the
rotation imparted to the air stream.  As I said, it's all about
trade-offs.  Note too the maximum attainable efficiency is 59 % (Google
Betz limit).

For anyone wanting to build a wind turbine, here's an equation to help
you size it.
Power = 0.5 * Rho * A * Cp * V^3
Power -  Watts
Rho - air density, 1.2 kg/m^3
A - wind area captured by the device, m^2
V - wind speed, m/s
Cp - Coefficient of Performance (same as efficiency).

Once you start doing some calculations it becomes apparent that you need
either a large area or a high wind speed, or both, to generate any
meaningful power.  For example, a 12 m^2, 40% efficient turbine in a 20
km/h wind will produce 500 Watts.
That's a 4 x 3 metre cross section.  Consider how much money it would
cost to build a machine of that size and look at how much energy it will
produce.  Now that you are not under any false illusions, build the
machine.  :-)
Notwithstanding the above, I do believe there's a future in wind energy.

--Jeff Latta


2006\03\17@204215 by VULCAN20

picon face


Dave Tweed wrote:

>VULCAN20 <KILLspamVULCAN20KILLspamspamcharter.net> wrote:
>  
>
>Not a problem. Just rotate the center gear out of its optimum position
>for the current wind direction,
>
With this design that is not possibly. where ever you place the wind
direction vane  you will still have some panel at an angle to the wind.

> or even "pop" it up out of the plane of
>the other gears and let the paddles freewheel into idle positions.
>  
>
The paddles are balanced (both sides the same length or area) so they
will not weather vane. Just letting them spin will create a tremendous  
wind resistance.  After the wind drops you would have to go up and re
time the unit before it would operate properly again.

>-- Dave Tweed
>  
>

2006\03\17@213331 by Lee McLaren

flavicon
face
How about a servo / stepper motor to position the vanes? I know it will
use some electric power but  it would have been used mechanically
anyway. Drop the power and let the stepper float to the off position and
then when ready to go power up and resync (spin against a stop and step
back to the righ position?) and go.

regards

Lee McLaren

VULCAN20 wrote:
{Quote hidden}

2006\03\17@213339 by James Newtons Massmind

face picon face
Brilliant! Not just the idea, but the animation that shows how the system
works. I've added it to my little page on wind power at
http://www.massmind.org/techref/other/windmills.htm with full credit to you.
I hope that is ok?

Of course the paddles should be replaced with airfoils so that the maximum
speed of rotation will actually exceed the wind speed. This is what the
design at
http://www.thebackshed.com/Windmill/photos2.asp does.

The biggest problem I see with wind power is that A) the wind is way up
there in the air and gets (much) slower as you approach the ground B)
anything that you lift way up into that good wind can fall / fly a long way
and C) if it is heavy, it's going to do a lot of damage when it hits the
ground.

So I'm more interested in windmills that use light-weight cloth type
materials.

The version I designed (see the page above) is actually similar to your
design, except that it uses sails instead of solid paddles and the air
pressure on the driven (leading or running) sail is used to pull the
trailing sail into a tight trim.

The other advantage is that when the wind speed gets too high, the sails
luft and so the unit will self-regulate it's rotational speed. In fact, it's
useless as a high speed, electricity generating mill. It can really only be
used as a high power, low speed system and so requires lots of gearing to
generate power.

And it still requires a heavy frame so it isn't something I want to lift up
high in the air.

I've been turning over in my mind a sort of dirigible with sail "wings" that
would be turned about its axis by the sails and use a single mooring line
looped over a pulley at the nose. It would pull up on one side of the line
and let out the other. On the ground, the pull up side of the line would
turn a pulley on a generator. The "let out" side of the mooring line would
run over a pulley some distance from the generator side which would be
spring loaded to maintain tension on the loop.

The killer on that design is how you keep the thing from turning in the air
and tangling the lines, while at the same time allowing it to turn into the
wind.

I put a sketch up at
http://www.massmind.org/images/member/jmn-efp-786/wind/flymill.jpg

---
James Newton, massmind.org Knowledge Archiver
spamBeGonejamesspamBeGonespammassmind.org 1-619-652-0593 fax:1-208-279-8767
All the engineering secrets worth knowing:
http://techref.massmind.org What do YOU know?




> {Original Message removed}

2006\03\17@221706 by Dave Tweed

face
flavicon
face
VULCAN20 <TakeThisOuTVULCAN20EraseMEspamspam_OUTcharter.net> wrote:
> Dave Tweed wrote:
> > Not a problem. Just rotate the center gear out of its optimum position
> > for the current wind direction,
>
> With this design that is not possibly. where ever you place the wind
> direction vane  you will still have some panel at an angle to the wind.

You're not visualizing it correctly. If you rotate the center gear 90
degrees in either direction from the "optimal" position, you get a
symmetrical arrangement of panels that generates no net torque.

> > ... or even "pop" it up out of the plane of
> > the other gears and let the paddles freewheel into idle positions.
>
> The paddles are balanced (both sides the same length or area) so they
> will not weather vane.

Yeah, I thought about that. With some sort of rotary dampers, though, I
think they would tend to find a low-energy state and stay there.

> After the wind drops you would have to go up and re time the unit
> before it would operate properly again.

The gears don't have to be on the top as Olin showed them.

In any cae, there are lots of windmill designs that have no "inherently
safe" state and rely on brakes to stop things when necessary. Take the
Darrieus "eggbeater", for example. It isn't that big of a deal.

-- Dave Tweed

2006\03\18@005132 by Russell McMahon

face
flavicon
face
I passed Olin's email on to a friend who is interested in the
alternative energy field and he had some somewhat surprising
comments - copied below.

Clever as it undoubtedly is, I'm not surprised that people have
thought of this or similar before - I am constantly thinking up things
and almost as often finding that someone has thought of them already.
I keep a file of all such ideas. One of these days ... .

What is more surprising is the comment on efficiency. Also, I'm not
sure how important this is even if true. It's usually not absolute
efficiency that counts but power provided per $ in a given situation.
There are some situations where enrgy available is limited and
efficiency counts but this is not usual, expecially on smaller or one
off system. ie if you have a windfarm on 100 acres then there will be
a maximym number of windmills you can fit and best utilisation of
available land and wind may well be important. If you want to make a
steady 1 kW for your backwoods hideaway then the cost of doing this is
liable to matter more than does how much wind you need to utilise to
do it.

       RM

____________

Ken says:



Russell,

This is a very old idea.  I saw a model one at least 15 years ago
demonstarted at the "inventors corner" which used to be (and may still
be)
part of the Easter Show.


Ross's cousin Duncan will tell you that it essentially operates on the
principle of drag rather than lift (as do vane/propellor type turbines
including the Darius rotor VWT).  As such it will have abysmal
performance
by comparison  - think of trying to build an aircraft where vertical
forces
are created by flat inclined surfaces rather than an airfoil.

Moving the orientation of the vanes helps but not enough to compete
with an
airfoil-based design.

Regards,

Ken Mardle

2006\03\18@033917 by Wouter van Ooijen

face picon face
> Yes, that's what I meant by front and back.  But you're
> saying there is such
> a thing as an airfoil shape that at some orientation it
> pushes to the right,
> and when it's flipped 180deg it pushes to the left

The shape is asymmetrical, so when it is at the back it automatically
'points' in the opposite direction compared to at the front (still in
the same direction rotation-wise).

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\18@115618 by olin piclist

face picon face
James Newtons Massmind wrote:
> Brilliant! Not just the idea, but the animation that shows how the
> system works. I've added it to my little page on wind power at
> http://www.massmind.org/techref/other/windmills.htm with full credit
> to you. I hope that is ok?

Of course.

> Of course the paddles should be replaced with airfoils so that the
> maximum speed of rotation will actually exceed the wind speed. This
> is what the design at
> http://www.thebackshed.com/Windmill/photos2.asp does.

This design doesn't work that way.  It is a drag versus lift design.  The
right side is essentially a flat plate being pushed forwards by the wind.
When the paddles are at front and back they are at 45deg to the wind.  I
also don't see a good reason why running faster than the wind speed is
inherently better.  High torque at low speed is not inherently better or
worse than low torque at high speed.  It depends on the desired appliation,
like pumping water, making a vaccuum, generating electricity, etc.  And of
course there are various ways to convert along the torque/speed tradeoff
scale.

> The biggest problem I see with wind power is that A) the wind is way
> up there in the air and gets (much) slower as you approach the ground
> B) anything that you lift way up into that good wind can fall / fly a
> long way and C) if it is heavy, it's going to do a lot of damage when
> it hits the ground.

This means you want to build it next to someone else's house.

> The version I designed (see the page above) is actually similar to
> your design, except that it uses sails instead of solid paddles and
> the air pressure on the driven (leading or running) sail is used to
> pull the trailing sail into a tight trim.

I've never heard of that idea before.

> I've been turning over in my mind a sort of dirigible with sail
> "wings" that would be turned about its axis by the sails and use a
> single mooring line looped over a pulley at the nose.

And that idea is new to me too.  I'm not sure how ultimately practical these
are, but I think it's good to come up with new ideas and explore them.
Others may think up refinements in the future or a particular problem may
come along where that set of tradeoffs happens to fit well.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\18@121650 by olin piclist

face picon face
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.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\18@133746 by Peter

picon face

On Fri, 17 Mar 2006, Olin Lathrop wrote:

> Wouter van Ooijen wrote:
>> The 'eggbeater' style vertical rotors work on lift, not drag. The
>> rotors have a vertical-areoplane-wing form, creating lift when the
>> wind flows around. The advantage of lift is that the rotor can move
>> faster than the wind flowing around it, drag is limited to the wind
>> speed.
>
> Yes, I understand that, but there has to be a change in airfoil profile or
> angle of attack between the front and back.  Something has to preferentially
> push the blade one way when it's at front, then the other way when you flip
> it 180deg.  Unless they are counting on the wind shadow effect of the front
> blade on the back, but that doesn't sound too efficient.

The blade that faces the wind the 'right' way (aerofoil-wise speaking)
generates lift. The other does not, but neither makes a lot of drag
because they are aerodynamically efficient (thin chord). These are
Darreius rotors and they do not start by themselves, they have a starter
motor or a small Savonius turbine built into the axle to start them. The
foils generate torque only for a part of the rotation.

But the most efficient turbine is the two-blade 'propeller' type, by
far.

Peter

2006\03\18@134533 by Peter

picon face

On Fri, 17 Mar 2006, Olin Lathrop wrote:

> Herbert Graf wrote:
>> Only problem with them as I understand it is they can't start on their
>> own, they have to be spun up to a certain speed before they are good
>> to go.
>
> Yes, but again what produces the net torque?  Let's say the blade is set up
> so that when it's at the front it pushes to the right.  When that same blade
> gets around to the back it has now pivoted 180deg.  If it's just a flat
> blade, it will now be pushing again to the right, counter acting the torque

It is a regular aerofoil optimized for the calculated airspeed. If you
look at the lift/drag vs. AoA curve of an aerofoil you will see a lot of
lift and relatively little drag. If the foil is used 'backwards' (the
case for the receding blade in a 2-blade vertical Darreius) the lift is
nowhere near that which appears in 'forward' 'flight' because the
profile is all wrong. Thus the blade that generates lift wins. The ratio
between lift and drag can be 5:1 for part of the rotation.

A vertical Darreius can be made easily by gluing two aerofoils (foam,
cardboard, whatever) onto the edge of a turntable style base, at a
certain AoA. In fact if the platter has enough inertia one wing half is
enough. Thin, short chord aerofoils are preferred. A wire should be
attached from the blade tip to an anchor on the platter to prevent
centrifugal force from tearing the blade out.

Peter

2006\03\18@135345 by Lars Bloch Gravengaard

flavicon
face

----- Original Message -----
From: "Peter" <RemoveMEplpspamTakeThisOuTactcom.co.il>
To: "Microcontroller discussion list - Public." <piclistEraseMEspam.....mit.edu>
Sent: Saturday, March 18, 2006 7:37 PM
Subject: Re: [EE] Vertical axis wind turbine


>
> But the most efficient turbine is the two-blade 'propeller' type, by
> far.
>

Are you sure ? Why is that 'ALL' wind turbines is the 3 blade type !

Lars

2006\03\18@140257 by olin piclist

face picon face
Peter wrote:
> But the most efficient turbine is the two-blade 'propeller' type, by
> far.

And why is 2 blades more efficient than some other number, like 3 or 4 or
more?

By the way, I've changed the web info on my concept.  There is now a real
web page at http://www.embedinc.com/wind instead of just the images.  I also
added a wind vane to show how direction is handled and increased the size of
the animated images a bit.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\18@141913 by Lars Bloch Gravengaard

flavicon
face

----- Original Message -----
From: "Lars Bloch Gravengaard" <EraseMEmail1spamlg-web.dk>


{Quote hidden}

http://www.windpower.org/en/tour/design/concepts.htm

"Two-bladed wind turbine designs have the advantage of saving the cost of
one rotor blade and its weight, of course. However, they tend to have
difficulty in penetrating the market, partly because they require higher
rotational speed to yield the same energy output. This is a disadvantage
both in regard to noise and visual intrusion. Lately, several traditional
manufacturers of two-bladed machines have switched to three-bladed designs."

"Yes, one-bladed wind turbines do exist, and indeed, they save the cost of
another rotor blade! If anything can be built, engineers will do it.
One-bladed wind turbines are not very widespread commercially, however,
because the same problems that are mentioned under the two-bladed design
apply to an even larger extent to one-bladed machines."

The bedst :-)

"The Danish Three-Bladed Concept
Most modern wind turbines are three-bladed designs with the rotor position
maintained upwind (on the windy side of the tower) using electrical motors
in their yaw mechanism. This design is usually called the classical Danish
concept , and tends to be a standard against which other concepts are
evaluated. The vast majority of the turbines sold in world markets have this
design."

Lars

2006\03\18@152030 by James Newtons Massmind

face picon face
> 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 would love to see how you made that calculation. I don't question it, I'm
just curious how you calculated it.

>
> 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 sort of Carnot efficiency for wind, you can never get it all).

I've been told that the maximum theoretical efficiency of a windmill is 35%.
No idea how that was calculated, but it is just about twice what you came up
with so I wonder if there is a doubling going into their calculation.

> 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
> maintenance.

Absolutely. And different designs may be better matches to different
situations.

> 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.


The amount of air that is acted upon by a wing is greater than that of a
flat plate. This allows the same size device to act as if it were larger.

Or at least that is how I understood it. I have no real knowledge in this
area.

---
James.


2006\03\18@152722 by Rolf

face picon face
Hmmm. I fly model planes, and regularly there are discussions about
propeller efficiency. It appears that 2 bladed props are more efficient
than 3, and that 1 is better than two. The problem is that there are
pragmatic issues with mounting 1-bladed props.... un-even thrust and a
corkscrewing motion of the plane behind it (even when counterbalanced
correctly). Also the landing gear needs to be really long.

http://www.hartzellprop.com/engineering/engineering_faqs.htm

http://www.pilotsofamerica.com/forum/archive/index.php/t-3247.html

I was about to shelve this mail as not adding much to the discussion
until I cam across the following. Really fascinating article (and mostly
on topic):

http://www.windmission.dk/workshop/Marcellus%20Jacobs.htm

Enjoy

Rolf

Lars Bloch Gravengaard wrote:
> {Original Message removed}

2006\03\18@152844 by James Newtons Massmind

face picon face
Olin said:
> This design doesn't work that way.  It is a drag versus lift
> design.  The right side is essentially a flat plate being
> pushed forwards by the wind.

Yes, but I believe it COULD work that way.

> When the paddles are at front and back they are at 45deg to
> the wind.  I also don't see a good reason why running faster
> than the wind speed is inherently better.  High torque at low
> speed is not inherently better or worse than low torque at
> high speed.  It depends on the desired application, like
> pumping water, making a vacuum, generating electricity, etc.
>  And of course there are various ways to convert along the
> torque/speed tradeoff scale.

Very true. I mention the desire for higher speed only because that is what
most people want: Fast rotation to drive a generator.

{Quote hidden}

<grin> Ah, you see right through me. The real question is "how can I build a
windmill on my 1/4 acre suburban lot." The options are low to the ground and
heavy or high in the air and very, very light.

> > The version I designed (see the page above) is actually similar to
> > your design, except that it uses sails instead of solid paddles and
> > the air pressure on the driven (leading or running) sail is used to
> > pull the trailing sail into a tight trim.
>
> I've never heard of that idea before.

I think with some levers or cams in between the lines that connect the two
sides to each other, the efficiency could be improved. The idea is to ensure
no matter the angle of the sail to the wind, it is property trimmed. Just
like a sail boat. There is a patent for that (a set of sail boats around the
rim of a merry-go-round) but the trimming of the sails was managed by a
complex set of levers and poles. I think something simpler could be
developed, much like your gear train.

> > I've been turning over in my mind a sort of dirigible with sail
> > "wings" that would be turned about its axis by the sails and use a
> > single mooring line looped over a pulley at the nose.
>
> And that idea is new to me too.  I'm not sure how ultimately
> practical these are, but I think it's good to come up with
> new ideas and explore them.

Keeps me from thinking about sex all the time.

> Others may think up refinements in the future or a particular
> problem may come along where that set of tradeoffs happens to
> fit well.
>

Which is why the web is so great. I put all my crazy ideas up and sometimes
people comment on them and bring them one step closers to reality.
http://www.massmind.org/techref/ideas.htm

---
James.


2006\03\18@153633 by Peter

picon face
Olin Lathrop <olin_piclist <at> embedinc.com> writes:

>
> Peter wrote:
> > But the most efficient turbine is the two-blade 'propeller' type, by
> > far.
>
> And why is 2 blades more efficient than some other number, like 3 or 4 or
> more?

I don't know the details. Any aerodynamics manual says so. 2 blades is most
efficient (1 blade may be even more efficient). All the others lag this. Going
from 2 to 3 blades etc. changes the power density not the efficiency.
2-blade top efficiency is given as >75%. Even two 2-bladed props one after the
other (tandem) are less efficient than one 2-bladed one. It is something to do
with the disturbed air (non-laminar flow) I think.

Google has a lot of material on this, usually applying to aircraft propellers,
not wind turbines. Here is a good start (the abstract is relevant):

http://naca.central.cranfield.ac.uk/report.php?NID=1544

the top page is also full of gems (from 1939!):

http://naca.central.cranfield.ac.uk/

I was looking for this page some days ago and could not find it ... hmm.

Peter


2006\03\18@153956 by Wouter van Ooijen

face picon face
> > But the most efficient turbine is the two-blade 'propeller'
> type, by
> > far.
> >
>
> Are you sure ? Why is that 'ALL' wind turbines is the 3 blade type !

IIRC the number of blades (for N>0) does not affect the maximum
efficiency, but it determines the optimum rotation frequency for a given
windspeed. Somthing like: optimal is [tip linear speed] * [number of
blades] = [some constant] * windspeed. So less blades means higher
frequency. Most commercial windmills generate electricity, which
requires a high frequency to drive the generator. So less blades == less
gearboxing = less losses etc. 1 blade is impractical, 2 can be done, 3
is much more stable than two, 4 does not add much beyond 3. So most
designs are 2 or 3. Except for the american 'slow runners' with lots of
blades, which are not used to generate electricity (used for water
pumping?).

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\18@161410 by Peter

picon face
Peter <plp <at> actcom.co.il> writes:


> 2-blade top efficiency is given as >75%. Even two 2-bladed props one after the
> other (tandem) are less efficient than one 2-bladed one. It is something to do

Correction: 2-blade has >85% efficiency and I have seen 89% quoted

Peter



2006\03\18@162009 by Peter

picon face

Ok, now I know why your design was familiar. You nearly reinvented (or
repurposed) the Voith-Schneider drive:

http://www.voithturbo.de/file_download.php?file=applications/documents/document_files/659_e_am_vsp_safty_en.pdf

(which uses a cleverer system than gears). There are also wind turbines using
the same principle. The center cam is set by a wind vane.

Peter

2006\03\18@174432 by olin piclist

face picon face
James Newtons Massmind wrote:
>> 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).
>
> I would love to see how you made that calculation. I don't question
> it, I'm just curious how you calculated it.

I got your message first thing after coming back from the recycling center.
My notes are gone, but I'll try to reconstruct the logic.  I'm sure
everything I did is on some professor's or NASA web site, but I sometimes
like to derive the answer for myself to get better insight.  If nothing else
it tells you the limits beyond which the stock answer doesn't apply.
Unfortunately this is all too often missing from descriptions and the "well
known" answer gets applied in circumstances where it is no longer correct.
I think a little of that is going on in the discussion of lift here.

First we'll derive the optimum speed for getting the most power from an
object moving directly downwind.  The force (F) on an object is proportional
to the wind speed (W) squared.  For simplicity we'll asume we are using
units where the proportionality constant is 1, so we have:

 F = W**2

If the object is moving downwind with speed S, then the force on it is

 F = (W - S)**2

to compensate for the reduced apparent wind speed by moving downwind.  The
power (P) that can be taken from the object's motion is the force on the
object times it's speed:

 P = SF = S(W - S)**2

As a sanity check we can see that the power is zero both when S=0 (not
moving) and when S=W (moving exactly with the wind) as expected.  In other
words, to get power you want to move fast, but if you move too fast your
apparent wind goes down and you lose power.

Since we only care about the optimum ratio of S/W for maximum power, we can
pick an arbitrary value for W in the equation.  I pick 1:

 P = S(1 - S)**2

Expanding out to show the individual S terms yields the third order
polynomial:

 P = S**3 - 2S**2 + S

Again, note that P goes to 0 when S=0 and S=1 as expected.  We are looking
for the maximum point between S=0 and S=1.  The derivative of this equation
must be zero there:

 dP/dS = 3S**2 - 4S + 1 = 0

Solving the quadratic (A=3, B=-4, C=1):

 -B +- sqrt(B**2 - 4AC)    4 +- sqrt(16 - 12)   4 +- 2
 ---------------------- =  ------------------ = ------ = 1/3 and 1
          2A                       6               6

So the two inflection points of the P cubic equation are at 1/3 and 1.
Clearly the one at 1/3 is the maximum we are looking for.  We already know
the power at 1 is 0.

Therefore, wind does the most work on an object moving downwind when its
speed is 1/3 the wind speed.

Now let's look at the maximum power we can extract from the wind using pure
drag (which is the case we calculated the optimum speed ratio for above).
According to several sources I looked at, the power in the wind (Pw) is

 Pw = .5 p A V**3

where p is the air density, A is the area, and V the wind speed.  Again
according to what looked like reputable sources, the drag force on an object
is

 F = .5 p A Cd V**2

where Cd is the coefficient of drag.  It seems this is about 1.17 for a thin
square plate oriented flat into the wind with the wind able to flow around
all sides.  The power produced by the object (Po) is the force on it times
its speed (S).  We already know the optimum speed for maximum power output
is V/3, so the maximum power (Po) that can be taken from an object moving
downwind can be calculated.  In this case the apparent wind on the object is
2V/3 and its speed (S) is V/3:

 Po = F * S = .5 p A Cd (2V/3)**2 V/3

The total effeciency at this point is:

 Po   .5 p A Cd (4/27)V**3   Cd * 4
 -- = -------------------- = ------ = 0.148 Cd
 Pw   .5 p A          V**3     27

For the case of the flat thin square with Cd=1.17 this comes out to 17%.
The equation above should be true for any rigid object moving downwind.  The
only way to change the theoretical maximum efficiency is to increase the
coefficient of drag.  The rest is physics that you don't get to mess with.
For example, a cup shape has a higher coefficient of drag, and therefore has
a higher maximum possible efficiency.

So you could get around 20-25% with a well shaped object moving linearly
down a track directly downwind at 1/3 the windspeed.  For an operating wind
turbine you generally want your materials to be reusable, so you have to
figure out how to get them back upwind at less drag then you got out of them
going down wind.  This is where my idea of rotating flat plates came from.
I get the 1.2 or so at the full area going downwind, and a much much smaller
area going upwind.  In my design it is possible to prevent the wind from
flowing around three of the sides of the plate when going downwind, thereby
creating a much larger stall area.  In the terms of the equation above, that
means the effective Cd of my paddles on the right side can be higher than
1.17.  I don't have a good feel for how much higher, but I do think that my
design is pretty efficient compared to all other drag types.

I haven't seen this analysis done for airfoils, but I suspect it gets a lot
more complicated.  Several people have pointed out that drag turbines can't
be as efficient as airfoil types, and that may well be true.  However there
are a number of factors that decide on the relative merit of wind turbines
for particular circumstances.  In most cases total wind efficiency is less
important than other attributes.  Drag turbines have some inherent
advantages too.  Since they operate at low speeds, they are much quieter and
don't kill birds which can handle things moving at 1/3 the wind speed.  The
vertical axis topology also leaves more options for using the mechanical
output since it doesn't have to be transferred over a rotating interface.
If higher speed is desired, such as for generating electricity, the heavy
transmission can be on the ground where it's weight is pretty much
irrelevant.  Also there are direct mechanical uses, like pumping water,
running a compressor, etc, where slow and high torque is desirable.

One of my other ideas is a stand alone device by the seashore that uses
mostly solar heating to make water for drinking or irrigating crops.  The
device itself has no moving parts but requires a partial vacuum to operate
efficiently.  Air that was dissolved in the seawater builds up inside and
needs to be occasionally removed.  I've thought of having a simple wind
turbine for this purpose.  Think of a device that just sits there next to
the ocean with a small wind turbine on top with a tank and spiggot at the
bottom.  The tank gets automatically filled with clean water at no
incremental cost other than maintenence of the apparatus.

Anyway, back to wind turbines.  If you see any error in my calculations,
please let me know.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\18@174549 by Peter

picon face
Been posting without reading up again. Max efficiency for wind turbine is
apparently 60%, three blades may balance differential forces better than two.

Peter


2006\03\18@175324 by olin piclist

face picon face
James Newtons Massmind wrote:
>> This design doesn't work that way.  It is a drag versus lift
>> design.  The right side is essentially a flat plate being
>> pushed forwards by the wind.
>
> Yes, but I believe it COULD work that way.

Not without a radical change that would no longer make it that basic
concept.  The basic concept is that the paddles rotate at half the speed of
the turbine.  The fact that they rotate (versus reciprocate) means they will
move thru all possible angles to the wind at some point.  It is also
inherent to the design that the downwind leg (the right "quadrant" in my
pictures) is a flat plate being driven directly downwind.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\18@175700 by olin piclist

face picon face
Wouter van Ooijen wrote:
> So most designs are 2 or 3. Except for the
> american 'slow runners' with lots of blades, which are not used to
> generate electricity (used for water pumping?).

And the large wooden ones in your country which often seem to have 4 blades.
I understand they were also used for grinding grain and pumping water (a
useful thing to be good at when you live below sea level).


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\03\19@041815 by Wouter van Ooijen

face picon face
> And the large wooden ones in your country which often seem to
> have 4 blades.

Yep

and greek/spanish-type windmills, and most other old designs.

> I understand they were also used for grinding grain and
> pumping water (a useful thing to be good at when you live
> below sea level).

Slowly rotating is good for just about everything, except the one thing
most current mills are designed for: generating electricity.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\03\19@193845 by Russell McMahon

face
flavicon
face
> But the most efficient turbine is the two-blade 'propeller' type, by
> far.

The one bladed probably wins - but the engineering gets "hard".

As long as an N  bladed turbine sweeps through an Nth of a turn in
less time than the wind takes to travel a "pitch distance" through its
blade-circle then the blade(s) have "touched" all the wind available
in the blade circle and it's then up to the engineers to optimise
things.


       RM




2006\03\20@081430 by Nicholas Wagon

picon face
I saw this on TV show a few months ago.

It might be of interest http://www.turby.nl/

The shown installation was on top of taller building to make use of the
created up(down?) drafts.



Nick Wagon

R&D Engineer

Jackson Care Technologies



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