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Cretan sail wind turbine - efficient or not?
By jamesjones, Section Wind Posted on Thu Oct 19, 2006 at 08:04:20 PM MST
A cheap design

I am just posting this to see if anybody here has had any experience with a sail based turbine, as shown in the picture below. (This is just a garden ornament that cost about $10, but runs really nicely). I am interested in producing this type of turbine because I don't have the workshop equipment, nor the skills, to manufacture blades. I understand that the efficiency of this design is lower than a blade based turbine, but the important thing is cost per watt, and I presume that the sails will be really cheap to produce, and much cheaper to replace than blades.
Although thousands of these are in use all over the world, I'm having a hard time finding much actual research on them - do any of you guys know of any?

Cretan sail wind turbine - efficient or not? | 29 comments (29 topical)

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#1)
by dinges on Thu Oct 19, 2006 at 03:32:40 PM MST

Windgenerators are useless. Wind generating systems are useful.

"but the important thing is cost per watt."

And the other important thing is to look at the entire system:

generator
blades
tower
lines
battery bank (if used)
inverter (if used).

You think that, in this grander scheme of things, the cost of the blades is a significant amount? My estimate is that it will be negligent, compared to cost of batteries, tower and inverter (if used).

Yes, the blades will probably be cheap and easy to produce. And less efficient. The rest of your system, however, has a much greater impact on 'cost per watt' than the blade. So, in effect, the blades are relatively cheap, but they do have a major impact on efficiency. Seems to me like the wrong place to start cutting corners.

Just my humble opinion. But one which I value greatly.

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#3)
by jamesjones on Thu Oct 19, 2006 at 04:16:56 PM MST

I brought up this design after reading about some of the workshops that have been held, where it took a very long time to create the blades. I have no experience in this and no hope of doing it with my current setup, so sails are the only way for me. It wasn't so much that they were cheaper, just more feasible for me (and probably some other home builders). Do you have any figures for the relative efficiency of blades versus this sail design? What if I can get the same power output by building this design twice as large as the equivalent blade design, with this design being ten times easier for me to build twice as big, than a blade design would be?

But I totally see what you mean re the total cost - the rest of the system is way more expensive than the blades or sails themselves.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#7)
by Garry on Thu Oct 19, 2006 at 06:00:03 PM MST

James,
Google Princeton sail wings or check Jack Parks "The Wind Power Book" for details.
Garry

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#2)
by stephent on Thu Oct 19, 2006 at 04:14:52 PM MST

Think of the word "panemone".
Google knows of it.

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#4)
by jamesjones on Thu Oct 19, 2006 at 04:34:54 PM MST

This isn't a panemone.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#5)
by stephent on Thu Oct 19, 2006 at 05:20:01 PM MST

Ok--granted, but there's been a bunch of those used for centuries for power of one sort or another. (turn it vertically to the wind--it still turns but not as fast--less sail area facing the wind--now it's a panemone).
The "Dutch" type windmills still use "sail" types on some.
They worked 1000 + years ago and still will.
The sail "blades" will be mostly just drag based torque and will turn slower then the more aerodynamically shaped "hard" blades....much slower.
So build it.

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#6)
by wdyasq on Thu Oct 19, 2006 at 05:31:18 PM MST

The Dutch blades worked on the lift principle, not drag.

Just keeping facts as facts.

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#8)
by Flux on Fri Oct 20, 2006 at 12:51:18 AM MST

The Cretan mill as you show it is reasonably efficient when rigged properly but is slow with tsr not much over one and with a sharp peak at best tsr. You will need about 6 times speed increase over conventional blades.

The best source of information is Roger Calvert's book called wind power principles or something similar.

There was a CAT publication on a Cretan mill at one time, may still be available.

It is possible to use sail blades in a different way with a tsr of about 4. This was used by Trimble Windmills on their contra rotating design.

I think these blades ( if you can call them that) work fairly well in low wind areas but flap about and fail early in high winds. Don't forget the massive increase in tower loading.
Flux

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#9)
by Flux on Fri Oct 20, 2006 at 01:40:27 AM MST

Windpower Principles, Calvert.
http://www.cat.org.uk/shopping/searchx.tmpl?subdir=shopping&init=1&selected=Windpower&re init=1

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#10)
by jamesjones on Fri Oct 20, 2006 at 03:32:44 AM MST

Many thanks for the information, I will try to get those books. I was wondering if there has been any recent research (i.e. last 20 years) that I could access through an online database? I have a friend who can access loads of journals for their university course, so I thought maybe I could look some up online to save time, and get a more up to date look at things.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#11)
by jamesjones on Fri Oct 20, 2006 at 03:38:20 AM MST

Thanks for your info. Why would there be a massive increase in tower loading? I would have thought that if they were less efficient than blades, then (presumably) more of the wind's energy would pass through unused, so there would be less load on the tower, or am I missing something?

The garden ornament version I have is up and running, and seems to spin at high RPM, though I don't have a similar sized blade turbine to compare it to.

I was going to use the same material as yacht sails are made from (possibly even used yacht sails) so I think they should stand up quite well. If I remember correctly, yacht sails will fray along one edge as they are continuously 'whipping', but in a turbine, the sails (I presume) won't be experiencing this because they are able to move with the wind, or at least, perpendicular to it, thus mitigating the effect of 'whipping'. (This is purely conjecture on my part, I admit!)

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#12)
by Flux on Fri Oct 20, 2006 at 03:51:58 AM MST

The thrust on these will be similar to a solid disc, as it is for the multiblade wind pump. The thrust on a high speed rotor seems to be nearer Cp x disc loading.

You will have no furling to reduce the loading in high winds.

I have never tried these things, I would find it infinitely more difficult to make than carving blades. If you have access to yacht materials and the means to work them why not try. I always find the low tech methods difficult compared with normal methods but everyone to his own choice.
Flux

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#13)
by jamesjones on Fri Oct 20, 2006 at 05:40:16 AM MST

I thought that I could furl the sails by putting in a spring of some type in each of the radial spokes, so as the pressure on the sails increased, the outer rope would increase in length, which would change the shape of the sail and allow more wind through unused. Or use some kind of semi-elastic rope/material that would naturally stretch when the wind got too strong?
I was going to buy a used yacht sail for around $20 on Ebay and use that.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#14)
by nolsson on Fri Oct 20, 2006 at 11:48:16 AM MST

If you like the idea of using sailing tech for wind turbines, you might find this design interesting:

"http://www.michaelbradley.info/lowtech/windmotor.html"

Nat

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#16)
by jamesjones on Fri Oct 20, 2006 at 03:21:39 PM MST

Thanks for the link Nat. Why do you think he made the wind-motor so tall and thin, instead of squarish in dimensions? I would have thought it would have been better to reduce the height and increase the width.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#21)
by nolsson on Mon Oct 30, 2006 at 12:33:32 PM MST

My only guess would be that the wind motor was adapted from sailing vessels with traditionally tall masts to take advantage of the increase in wind speed with altitude. I have no expertise (educational or practical) to back me up there.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#18)
by jamesjones on Fri Oct 20, 2006 at 04:06:14 PM MST

Flux, what did you mean by this bit: "You will need about 6 times speed increase over conventional blades."?

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#19)
by Flux on Sat Oct 21, 2006 at 10:46:30 AM MST

Typical blades run at tsr 6 or similar. If you run the sails with tsr 1 then you need to increase speed to the same type of alternator. It probably becomes prohibitively expensive to design alternators for 30 rpm cut in.
flux

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#15)
by hvirtane on Fri Oct 20, 2006 at 01:37:09 PM MST

Hi,

I'm not at home now so I cannot
give very exact knowledge
concerning written resources.

But there are some very good books
available concerning sail wings
as used with water pumping wind machines.
Especially John Furze from Denmark
(on this forum 'JF') has collected
lots of knowledge about such literature.
He has got some really nice
photos about the original
machines as still to be seen
in Crete. I hope that he would
publish some of his pictures here.

I've mentioned some books
earlier here on this forum
as knowledge gained from John,
because of his research.

Please see for example the discussion:
http://www.fieldlines.com/story/2003/4/3/95557/80233

I think that especially
as water pumpers sail wings are great.
You can use them for generating
electricity if you'll make
an axial flux generator with large
magnet rotors or by using a gearbox or...

- Hannu

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#17)
by jamesjones on Fri Oct 20, 2006 at 03:23:32 PM MST

Thanks for the info. Hannu.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#20)
by elvin1949 on Wed Oct 25, 2006 at 01:19:00 AM MST

Morning James
If i remember right popular mechanics did an
artical on a downwind wind genny with cloth [sailwing]blades.
They had a pic on the front cover.It was sometime in the 80's.Will take a while to find it.[had to move house was full.IT'S IN THE OTHER HOUSE]
later
elvin

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#22)
by Kwazai on Mon Oct 30, 2006 at 12:45:36 PM MST

I built a couple (non- power generating) using bicycle wheels and plastic from grocery bags and from trash bags and duct tape. a 55 gallon bag cut in half fits a 26' diameter wheel pretty well(savonius bucket like). the grocery bag plastic would turn in real low winds(1 m/s or less). the 55gal. bag one (much like a 2-bucket savonius) took a little more wind(2.5 to 3 m/s-best guess). The 55gallon bag one is still working as the grocery bags degrade in sunlight pretty quickly. It is somewhat self limiting and hits max speed well before the 18mph storm winds I see in my back yard (don't get many windy days). anyway. fyi.
Mike

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#23)
by Kwazai on Mon Oct 30, 2006 at 12:51:04 PM MST

p.s. I've got a 5.5 mb mpeg if you'd want to see it spinning.
drop me an email.
Mike

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#24)
by Kwazai on Mon Oct 30, 2006 at 01:04:07 PM MST

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#26)
by jamesjones on Wed Nov 01, 2006 at 08:15:16 AM MST

Thanks for that picture, excellent stuff!

I was just thinking that it would be easier for a lot of people to construct a sail based Cretan wind turbine, than to carve blades. What irks me is that so little research seems to have been done into the Cretan design - I'd like to see some published research on the effect of the number of sails, the effect of the size of the sails, how taut or loose they are, and things like that. There clearly IS the ability for large sails to capture a large amount of the energy in the wind, just look at ships throughout history, that used nothing but sails, and weighed hundreds of tons.
Some people here have mentioned that though the torque is high, the rotational speed will be slow. The same applies the bigger you make a bladed HAWT - the rotational speed slows down. So both will require gearing.

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#27)
by Kwazai on Wed Nov 01, 2006 at 08:41:53 AM MST

I tried to model it after a savonius that had no down side- efficiency should be better than a savonius and startup speed should be lower too. The parafoils/sails are basically a drag type foil. I never did find an optimum number of sails- the more sails the higher starting speed after three sails- basically the wind is turned at 90 degrees with four sails/foils. I would assume that three sails/foils is probably optimal (120 dgrees apart) from what I've seen of the savonius research (sandia).
I had also seen one built with bicycle wheels and venetian(sp.) blinds.looked like an old timey farm mill.
Mike

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#28)
by ghurd on Wed Nov 01, 2006 at 08:59:59 AM MST

Maybe Willib's?
http://www.otherpower.com/images/scimages/2965/my_turbine.JPG
http://www.otherpower.com/images/scimages/2965/my_turbine3.bmp
G-
Ghurd.info
[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#29)
by Kwazai on Wed Nov 01, 2006 at 11:50:38 AM MST

thats the one. I was wondering how he'd hooked up a genny to it. I've collected some bicycle generators and a coupla dryer belts if I ever get back to playing with mine.
there is bound to be some static wind pressure/open area ratio relationship for max speed on em- just the math eludes me.
Mike

[ Parent ]

Re: Cretan sail wind turbine - efficient or not? (3.00 / 0) (#25)
by Kwazai on Mon Oct 30, 2006 at 01:16:13 PM MST

http://www.grc.nasa.gov/WWW/K-12/airplane/kiteincl.html

Cretan sail wind turbine - efficient or not? | 29 comments (29 topical)

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