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Forum Index : Windmills : How Many Blades?

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pntrbl
Newbie

Joined: 12/07/2008
Location: United States
Posts: 32
Posted: 04:12am 20 Jul 2008
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Still lurking and learning here, but googling my way around the internet I came across this:

nov55.com/wdm.html

Seems to contradict just about everything I've learned from Hugh Piggot .... but what do I know?

Anything to it?

SP
 
CraziestOzzy

Senior Member

Joined: 11/07/2008
Location: Australia
Posts: 135
Posted: 04:36am 20 Jul 2008
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I do not proclaim to know much about the dynamic interaction of wind on blades attached to a windmill of any description...in fact I know next to nothing.
The article has some valid points raised, but lacks more than one reference...so I will ignore his arguments and focus on his concept of using more blades.
Providing that stress can be overcome in a relatively high wind event (a reference to Coulomb types of stress is worth reading), I think the idea of more blades is valid and worth trying. Edited by CraziestOzzy 2008-07-21
http://cr4.globalspec.com/member?u=25757

http://www.instructables.com/member/OzzyRoo/
 
Dinges
Senior Member

Joined: 04/01/2008
Location: Albania
Posts: 510
Posted: 08:29am 20 Jul 2008
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I'd highly recommend reading this '73 interview with Marcellus Jacobs, father of the small windturbine industry in the USA who nearly single-handedly electrified rural USA. I think you'll find it will answer your questions:

http://www.motherearthnews.com/UnCategorized/1973-11-01/The- Plowboy-Interview.aspx

It's an enjoyable interview, makes for pleasant reading. And it'll teach you a thing or two about what worked and what didn't. Many things have already been tried and rejected in windturbine design. Usually for good reasons. No need to reinvent the wheel again.
Edited by Dinges 2008-07-21
 
robbo

Regular Member

Joined: 25/03/2007
Location: Australia
Posts: 71
Posted: 10:15am 20 Jul 2008
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Hey Dingoes,
Thats a really cool thread. about the history of wind plants, etc. Lots of interesting ideas.
Recommended reading.
"the Earth was not given to us, by our fathers, rather, it is lent to us by our children".
 
pntrbl
Newbie

Joined: 12/07/2008
Location: United States
Posts: 32
Posted: 03:24pm 20 Jul 2008
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Well that settles the issue for me. 3 blades it is and thanx for an enjoyable Sunday morning read.

SP
 
oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 10:28am 23 Jul 2008
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Jacobs is the king of power generation windmills, and his views certainly need to be taken into account, but as with everything to do with power from wind, there are no hard and fast rules... except "nothing describes the power of a mill more than the swept area"... after that it is open slather.

If you are using an alternator that you build, three blades is the best to design around.
If you are forced to use some other alternator, then it may well be that you find a different combination of diameter and blade count works better.

I think it was FandPwithPVC who found that 4 or 5 blades were better for his purposes, and outperformed his three blade arrangement for his F&P in his circumstances.

Breezy 5.5 (http://www.prairieturbines.com/) found that after extensive testing 4 blades did what they were after driving a 60hz alternator from a 60rpm blade set.



On the extreme end of things, Claus Nybroe from Windmission http://www.windmission.dk/index.htm found that for his particular combination of induction generator:rpm:diam, that lots was better and achieved a quite high rotor efficiency.



Windmission:
"In the theoretical windmill literature the maximum obtainable rotor efficiency is 0.6 . This is called the Betz efficiency, named after the man who calculated it on the basis of a rotor with an "infinite" number of blades.

The rotor efficiency is also named Cp, and in real life the highest obtainable Cp values are around 0.5.
The 12 bladed Windflower rotor, for example, operates with a measured maximum Cp value of 0.47, which is very good. "

His blades are as about as good as it gets, they are very quiet (250rpm I think) with good startup torque to drive a high ratio gearbox... a nice combination and a .47cp

So it is not all black and white, and this should be in the back of your mind at all times when making hard and fast decisions. Different alternators require different drive characteristics.

Turbex with a zillion blades works very well with a cp of .35 from memory. I think the South Africans did a bit better than Jacobs managed with the same type of setup.

http://www.turbex.co.za/Documents/Generator.html
5000 watts worth looks like this (18' rotor) Nice and slow (48rpm or so) and quiet.



and they go up to 20000watts. Not too shabby eh?

Of course, sometimes the blade number is not the most important detail to sweat over and some of the finer points need to be addressed:


(http://bob.gudgel.org/)

In general, 3 is the magic number... but with wind, not much is set in concrete.....it all depends on the alternator setup.
messy really ...isn't it?




..........oztulesEdited by oztules 2008-07-25
Village idiot...or... just another hack out of his depth
 
vawtman

Senior Member

Joined: 14/09/2006
Location: United States
Posts: 146
Posted: 09:50pm 23 Jul 2008
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right on Oz
 
Dinges
Senior Member

Joined: 04/01/2008
Location: Albania
Posts: 510
Posted: 11:39pm 23 Jul 2008
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Oz,

I'm a lazy bastard. I dislike carving 3 blades (wood is yucky, steel is nice), let alone 30+ as in that last picture of yours...

3 blades is a compromise, as 2 blades tend to have yawing problems and single-blades have balancing issues. But I promise, the day the single-blade turbine is DIY-able, I'll build my turbine with just one blade! No more need to make 3 identical blades anymore (let alone 30+) .

As I said, lazy...
 
oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 12:08am 24 Jul 2008
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Dinges,
I'm right with you on this one. ( my wife tells me I'm a lazy slob too )

The wood is sitting on the floor at the moment. We cut 4 "chunks" of 8" x 2" pine (8' long) out of a fallen pine tree. lots of knots and not pretty, but will work I think.

It was green at the time, and I have carefully and methodically done absolutely nothing for the last 4 months at least. (waiting for it to dry out is my excuse... and I'm sticking to it) The wood seems to be a whole lot lighter now, and I'm in serious danger of having to actually do something soon as my excuse for doing nothing is drying up.

Alas, I can see a shed covered in wood bits.... and I just can't get excited about it.

If you find a way of solving the dynamic inbalance of a single blade that is easy for slack peasants (like me), I'm all ears.

I can see that soon I will be covered in wood shavings... and whats worse I will probably find a way to stuff it up and have to start the whole process again.


.........oz(I really don't want to carve these things)tules


Edited by oztules 2008-07-25
Village idiot...or... just another hack out of his depth
 
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