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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 07:11am 13 Feb 2009
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A windmill that is loaded correctly to the wind energy curve will not exhibit this type of behaviour. This will happen with a mill that has high blade inertia,[bad for a windmill in general], or is underloaded in parts of the wind energy spectrum. A visible sign of underloading or high rotor inertia is poor windmill, wind direction tracking. This may best be seen in wind where the windmill in not subject to furling.
Overloading of the rotor usually is seen as a really stable windmill, where the rpm does not change much with windspeed and the mill tracks the wind direction well.
Gordon.
become more energy aware
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 08:59pm 13 Feb 2009
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[QUOTE
A windmill that is loaded correctly to the wind energy curve will not exhibit this type of behaviour.
I currently have two mills one of which I purposly overloaded and one which is underloaded by having open circuit coils, and this type of behaviour can be observed. I also pointd out that overloading the blades by not providing the neccesary wind relief path thru the middle can drive the mill towards the Betz limit (maybe) eg by having overlarge cord dimensions at the blade root extending over the hub toward the pivot area also causes similar effects to be exhibited as an alternator overload. The net outcome being a reduction in power produced.....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331
Posted: 11:36pm 13 Feb 2009
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looks like i,ll have to get the finger out then...
for fun i,ll add two strakes ...somethink like two very thin ribs that ring the blade extending out about an inch , i suppose the outer one could be much thinner and both could follow the arc of the blade ...the scientific test will be then ..is the prop quieter on not
to push the boat out a bit further i,ve epoxied some trace wire to the back of the blade at its thickest point about half way down from the tipEdited by niall1 2009-02-15niall
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 02:07am 14 Feb 2009
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This sounds like what I would describe as a paddle wheel effect.
This is not always a bad result. This is a useful way of controlling the upper power levels. My own windmill blades have this type of maximum rpm limiting control. The steeper blade angle, up to 45degrees close to the blade hub, provides sufficient drag at high rpm to allow my mill to furl in strong wind gusts >20m/s with no alternator loading. Normal alternator loaded operation at the <10m/s windspeeds does not seem to be affected.
Gordon.become more energy aware
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 11:17am 14 Feb 2009
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Thoughts re Betz limit…
1/ A moving cylinder of air passing thru turbine blades contains kinetic energy.
2/ To remove the energy from the wind to make power it is necessary to slow the wind down to rotate our blades which in turn rotates our generator.
3/ Betz limit suggests a maximum slowing of about 2/3 of wind speed beyond which power available actually decreases to zero if the wind could be completely stopped.
4/ The complex and frustrating aim of achieving maximum possible power generation is to come as close to slowing the wind down to the Betz limit as possible.
5/ It is possible to have a turbine matched precisely to a generator power curve and yet be nowhere near the optimum Betz number for maximum power generation. Symptoms are high speed, erratic tracking behaviour in an offset hub, yaw furling mill and or lack of or inconsistent furling and erratic acceleration deceleration. The turbine is moving towards the Betz limit but is not being sufficiently loaded by the generator to remove the energy from the wind.
6/ A turbine with blades that are too few or too narrow will be unable to reach Betz limit before making its maximum possible power and the converse is also true.
Freewheeling turbine and Betz limit…
Assuming a perfect blade in a perfect cylindrical wind tube world;
A blade with an arbitrary tip incident angle of 12 degrees has a theoretical tip speed ratio of about 7. However assuming no friction or drag/ stall etc. The theoretical TSR approaching the Betz limit of slowing the wind by about 2/3 would be three times that or 21. This would appear impossible because the blades would calculated to be in a state of severe negative wind incident angle.
Not at all impossible; here’s how it happens:
As the turbine spins up the blades present an increasing effective disc profile to the wind which begins to slow. This in turn steepens the incident wind angle to keep the incident wind at the optimum angle of lift. This slowing wind would seem to actually want to make the blade run slower, but this slowing of the wind is more than offset by the increase in pressure at the front of the blades creating a greater lift producing pressure differential and perhaps even forcing the incident wind at a higher velocity over the blades so increasing lift and the turbine develops ever increasing torque as the wind incidence angle keeps reducing and the pressure keeps increasing until the wind has slowed to the Betz limit, where the increasing pressure has no more positive effect to counteract the effect of the reduced wind speed. At this point the turbine would be incapable of making any power at all and you could slow it down with a feather.
In the real world a freewheeling TSR of 15 would appear to be the maximum realizable, which is a slowing of the wind by about ˝. This was probably measured in a good gust!
Nevertheless it is this torque that enables such TSR’s that is available to us to make maximum power at lower TSR’s.
Limitations to such an endeavour are such things as poor blade design, poor alternator matching, turbulence, drag, delaminating stall, friction, wind loss thru and around the blades and of course the Betz 100 percent effect at the hub.
Small turbines usually operate at only about 1/3 Betz limit whereas some large turbines may operate as high as ˝. The reason for this is not the subject being addressed here.
Russell
Edited by RevUpWind 2009-02-16....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 11:58am 14 Feb 2009
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Hi Russell,
Most of the large 1MW size mills operate at 50% extraction of the wind energy. Betz limit is approx 59%. This means that these windmill operate at 84% of Betz limit, and not 50% as you suggest.
The Lakota carbon blades, that I have on my AxFx mill have approx 25-28% calculated wind energy extraction, based on averages of wind energy calculated from recorded weather station windspeeds and simultaneous power to the load, across a 3 to 10m/s windspeed range. This equates to 42-47% of Betz limit. I am happy with approx half of a theoretical maximum, given that my alternator has very high efficiency. My comparison testing of a F&P and AxFx windmills in close proximity has prompted me to eventually replace my F&P windmill with another AxFx windmill.
On another note, I believe oztules has noticed that on his windmill, [very close to the ground] that the air exiting behind the blades is slowed considerably. He may wish to add to this.
Gordon.
become more energy aware
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 10:26pm 14 Feb 2009
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[QUOTE
Most of the large 1MW size mills operate at 50% extraction of the wind energy. Betz limit is approx 59%. This means that these windmill operate at 84% of Betz limit, and not 50% as you suggest.
Hi Gordon,
I stand corrected
Ye ....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 10:35pm 14 Feb 2009
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[QUOTE
On another note, I believe oztules has noticed that on his windmill, [very close to the ground] that the air exiting behind the blades is slowed considerably. He may wish to add to this.
I agree with his observation as it fits with the theory that the whole column of air has a nett slow down. Too close to the Betz limit and a turbine tail may become less effective.
Sorry for using vague approximations. Just trying to keep a mind bending mental exersize more simple. Perhaps at some point one could post a copy of the Betz theory on this thread. (copyright?)
Russell ....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225
Posted: 02:37am 15 Feb 2009
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If two anemometer's were mounted one in front and one behind the blades along the axis at about half the swept radius.The difference in wind speed could be used to detirmine the energy extraction.Another thread previously discussed different ways of making the anemometers with LED's or detection circuits from car manifold pressures devices.
The rear mounted anemometer could be trialled at different radius's to see where the greatest extraction is occurring.
Or we could just hire Oztules for the detector.
Anthony
Remove space between T ID for link to work.Edited by Jarbar 2009-02-16"Creativity is detirmined by the way you hold your tounge".My Father
"Your generation will have to correct the problems made by mine".My Grandfather.
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 06:07am 15 Feb 2009
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I agree with his observation as it fits with the theory that the whole column of air has a nett slow down.
Just a further note; At high wind velocities the whole wind column spins in a vortex and the wake wind flares. Both of these factors help keep the wind incident angle low and the blades out of stall.
Betz limit is a maximum attainable power coefficient of .5926 at the moment. Some are suggesting that a Cp of .89 is achievable.
See http://www.inderscience.metapress.com/index/W1H1YYDTEQW8UYJU .pdf
Russell ....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225
Posted: 07:36am 15 Feb 2009
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Place a piece of chicken koop wire the same area as swept behind the blades and attach wool pieces as tell tails at regular intervals across the grid pattern.Although a bit of work it would give a very visual indication of whats happening downwind and the wake of the blades at different speeds.If I had a HAWT I'd try it myself.Meanwhile I'll read the link from Russell.
Anthony"Creativity is detirmined by the way you hold your tounge".My Father
"Your generation will have to correct the problems made by mine".My Grandfather.
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 08:33am 15 Feb 2009
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....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 08:35am 15 Feb 2009
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Hi I just altered an obvious mistake in my ramblngs. Edited by RevUpWind 2009-02-16....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 10:05am 15 Feb 2009
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Hi Gordon,
Nacelles have been the most proven performer in this area of enhancement. In fact with these perforance beyond the Betz limit has been observed.
The wind at the tips has long been known to flip over the end of the blades so to speak and cause tangental wind differential along the length of the blade.
This is because of the differential caused by the wind at the front of the bade sliding towards the tip. This causes the front surface wind to take an angular path across the blade with respect to the wind flowing over the rear of the blade. This will effect the drag but not cause wind delamination, but because of the vector angle change caused by more drag to the resultant angle of lift, on some blades running at unpowercurved TSR's this could actually be an advantage. (Hey I coined a big word!)
Big rod in the fire? Perhaps?
The tip device I have in grey matter is to improve negative TSR change in turbulent winds. (Remember my talking about blades going into stall on decellerating wind.) If you have got the mill generator overloaded you may not observe this problem so I contend your mill is not working as close as it could to the Betz limit.
Small mills have the disadvatage of being much more affected by turbulence. That is probably why they operate at lower Betz numbers.
I will try to obtain empirical results that support a real world benefit, after all who wants a b.. great nacelle up in the sky.
We are the little folk; keep it small and keep it simple.
I can talk?
As we all know that B... wind just won't keep still and strong.
Russell
....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
vawtman Senior Member Joined: 14/09/2006 Location: United StatesPosts: 146
Posted: 02:41pm 15 Feb 2009
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It's pretty cool standing inside my 8'x8' turbine.When the blades are rotating ~120rpms there is virtually no breeze just tinsy pulses has the blades go by.
wind-pirate Senior Member Joined: 01/02/2007 Location: CanadaPosts: 101
Posted: 07:56pm 15 Feb 2009
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This comes to mind .. How would this affect the ability of the tail to follow the wind.
My big problem is the winds swirl badly. My blades are searching for wind always.
THE Pirate.
stealing wind & solar energy is fun
niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331
Posted: 03:01am 16 Feb 2009
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hi wind -pirate
my machine suffers the same problem ..always speeding up ...slowing down (its in a village low lying area ) ...i often wonder if it was set up as a downwind machine would it behave any different niall
DaViD Senior Member Joined: 14/01/2009 Location: United StatesPosts: 120
Posted: 03:41pm 17 Feb 2009
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What I found on the net. about betz limit according to MICHAEL KLEMEN really interesting stuff.
Perfect Turbine Performance!
Wind Turbines are limited by what is called the Betz law. Simply put, if you capture 100% of the energy available in the wind, you stop the wind. Obviously, the wind will stop flowing through such a turbine. The opposite of that is that if you don't capture any energy in the wind, you don't need a turbine. The wind is able to flow around any major obstruction. The Betz limit says that essentially, if you capture 59.6% of the energy in the wind, that is the best compromise between stopping the air and forcing it to go around your machine. You need to maintain the flow of air, that's the compromise any wind machine must make whether it is a horizontal axis (traditional style turbine) or vertical axis turbine, with many blades or few, or any such combination. It's covered by the Betz limit.
I've calculated the energy you can capture per square meter and per square foot for given wind speeds. There's power curve data, and energy production data to make your life as easy as possible. Find the swept area of your machine, multiply by the appropriate value, and see if it's even possible! I've compared my GOOD wind turbine columns with production data from 4 reputable manufacturers' turbines, using their data. NONE of them are as good as my GOOD turbine columns. Those are based on an average efficiency of 35%. It is not terribly likely that you'll find a machine that is more efficient than suggested by this column. If it's too good to be true, it may very well be! There are too many caveats in acquiring data that can be fudged to make things look better than they are in reality. If you have questions, ask me!
First, here is the Energy Per Month (given in kWh) that a perfect turbine could produce (Betz limit), and what a good real turbine could produce. This table assumes a Rayleigh distribution about the mean wind speed. So, if you have a turbine that captures 10 square feet, in a 10 mph wind, look at the 10 mph row, find the "Good Turbine" per ft^2 value of 2.078. Multiply it by 10 because you have 10 square feet. If the manufacturer is claiming it can put out more than 20.78 kWh/month, it is probably too good to be true. Next, find the Betz limit value of 3.502. Multiply it by 10. If the manufacturer claims you can generate more than 35 kWh/month, they have just broken the laws of physics. It is impossible. There's something wrong with their data. That's how you can use this information.
Energy Per Month (kWh)
Wind Speed
mph -[m/s]
Betz Limit
per m^2
Good Turbine
per m^2
Betz Limit
per ft^2
Good Turbine
per ft^2
5 - [2.24]
4.47
2.65
0.415
0.246
6 - [2.68]
7.99
4.74
0.742
0.440
7 - [3.13]
12.98
7.70
1.206
0.715
8 - [3.58]
19.66
11.66
1.826
1.083
9 - [4.02]
28.02
16.63
2.604
1.545
10 - [4.47]
37.70
22.36
3.502
2.078
11 - [4.92]
47.95
28.45
4.455
2.643
12 - [5.36]
57.96
34.38
5.384
3.194
13 - [5.81]
67.01
39.75
6.226
3.693
14 - [6.26]
74.68
44.30
6.938
4.116
What follows is a set of power curve calculations using the Betz limit (a perfect turbine), and a good, real turbine. Just like in the above graph, you can locate the wind speed. Here, it is possible a manufacturer may beat the "good" turbine data by a small margin. These turbines can be more efficient than this for a wind speed or two, but over the entire range, they won't reach this kind of performance. Once again, in a 10 mph wind, a 10 square foot rotor would likely put out 17.5 watts. A perfect turbine would put out 29.5 watts. If the turbine you are looking at does better than that, there's something wrong with their data.
If your not living on the edge your taking up to much space!
oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 08:59pm 17 Feb 2009
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Maybe a link would have been better
..........oztulesVillage idiot...or... just another hack out of his depth
DaViD Senior Member Joined: 14/01/2009 Location: United StatesPosts: 120
Posted: 09:29pm 17 Feb 2009
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Sorry, just thought it would save time. I had to download a complete magazine just to get this article in pdf format DaveIf your not living on the edge your taking up to much space!
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The Back Shed's forum code is written, and hosted, in Australia.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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is the prop quieter on not
