Author Topic: Parabolic profile for VAWT blades (Read 10721 times)

thyristor · « **on:** January 26, 2008, 04:16:35 PM »

I want to make a Savonius VAWT and have read quite a lot about it on the web and on wind power forums, but nowhere could I find someone who claims to have made one that can output some real electrical energy. Plenty small experimental ones, though. Question: Anyone out there that can confirm or deny that having a blade curvature that is parabolic will be better than a semi-circular curvature?

Thanks!

obelix · « **Reply #1 on:** January 26, 2008, 10:34:10 AM »

Hello,

I was a french froggie and so don't blame me for my poor english !!

I found on the net a paper on the two blades system upsizing the "benesh" blades.

I think you can read it before making your own ....

The link was:

http://www.energy.ca.gov/research/innovations/eisg_final_reports/index.html

and then:

DEVELOPMENT OF OPTIMUM DESIGN CONFIGURATION AND PERFORMANCE FOR VERTICAL AXIS WIND TURBINE.

It was very interesting, because it encompass the double mode of vawt:

Drag + Lift and study the implementation of the lift side !!

Hope this helps!!!

clarsondd · « **Reply #2 on:** January 26, 2008, 01:16:47 PM »

this is the most efficent savonius blade

feral air · « **Reply #3 on:** January 26, 2008, 04:30:37 PM »

IMO, the trick with s-vawts is in the size - I don't think the profile changes things that drastically...

According to the book I'm looking at, a 10sqft barrel Savonius should be capable of producing 0.238HP in a 30mph wind. That's only 177 watts....and that's BEFORE you start counting the losses (friction, etc).

Using that as the starting point and multiplying our way up, if the vawt were 100sqft it could produce 1.77kw in a 30mph wind....again, before losses. Maybe I should add an "in theory" as well, for good measure.

With that in mind, I don't think changing the profile on a 10sqft vawt will get you much. 0.24HP instead of 0.238HP? Honestly, I dunno...

It seems to me that making it HUGE will have a more dramatic effect on the output though. take it easy

thyristor · « **Reply #4 on:** January 27, 2008, 05:43:10 AM »

Thanks for the link, obelix! Also thank you to all for your contributions. It is appreciated.

thyristor · « **Reply #5 on:** January 27, 2008, 05:46:54 AM »

It would still be interesting to hear from someone who is actually generating electricity using a savonius. Anyone out there with success?

feral air · « **Reply #6 on:** January 27, 2008, 09:54:27 PM »

On this board somewhere (I think it was here), someone claimed to have built a double barrel s-vawt that made 50-60w. Of course that's all I remember though...bummer.

I'd like to see an s-vawt that generates some electricity too so I'm putting some time and money on it...

I'm aiming for at least 96sqft, forget barrels. If it looks like I can I might go up to 144sqft. I got the first 48sqft section done in a weekend...it's a start. Maybe by the end of summer she'll be up and generating something.

take it easy

clarsondd · « **Reply #7 on:** January 28, 2008, 01:27:49 AM »

this person is:

http://www.fieldlines.com/story/2007/12/12/9444/1180

wooferhound · « **Reply #8 on:** January 28, 2008, 06:01:29 PM »

WindStuffNow ia making over 50 watts here

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

feral air · « **Reply #9 on:** January 29, 2008, 04:16:41 PM »

That's not the thread I was thinking of. The one I was thinking of was a bit older and it may have just been a comment in a vawt-related thread, not a story about the s-vawt itself. take it easy

feral air · « **Reply #10 on:** January 29, 2008, 04:17:41 PM »

woof, that's not a Savonius. :p

chris psmith · « **Reply #11 on:** January 31, 2008, 12:22:46 PM »

Well, can't claim 'making electricity' yet, but getting close, 2m benesh / 0.5m diameter, hope to be at generating stage next weekend.

thyristor · « **Reply #12 on:** January 31, 2008, 12:55:04 PM »

That looks like a beautiful turbine! I'll be waiting anxiously to hear about your findings/progress! Thanks for sharing.

fcfcfc · « **Reply #13 on:** January 31, 2008, 04:52:59 PM »

Hi: I read this one myself, about a dozen times. I actually used it as a beginning design idea for my VAWT design, that I will be wind tunnel testing a prototype of. An excellent article...

One thing to be careful of with non egg beater style VAWT's is that the aspect ratio, that is the height to width ratio changes the picture allot. Taller and thinner is better than short and squatty... The more dependent the turbine is on drag VS lift, the more this is important. I find it incredible that "no one" out there has a VAWT producing kilowatts of power..??.. I don't think this is really the case. Most likely the people that have them just are not into discussion boards and such. They have them, it works, end of story...

feral air · « **Reply #14 on:** February 01, 2008, 10:58:19 AM »

Is that the same one you talk about here? If so, I think you'll be a bit disappointed...

0.5sq meters is 5.38sqft. According to the book I've got (if I'm reading it correctly!), 5sqft will give you about 0.14HP (=88w) at the shaft in 30mph wind. Of course, what you can get out of it as electricity will be something less than that.

If I was you I wouldn't expect more than 50w from 30mph wind. That assumes a total loss of about 40w, which could be too little or too much, I dunno.

The figures from the book are for a "Simple Savonius" so you might see a little more power than that but at that size I would just write it off.

To get 500w from 10mph wind you'll need to go way bigger. 60sqft might get you close. take it easy

chris psmith · « **Reply #15 on:** February 01, 2008, 11:54:45 AM »

I think at 60sq ft the side of my garage may go with it in a good gust, kowing my luck the benesh would stay still and the garage would spin round!

With my calcs:

Power 0.5x1.22(air density)x0.5(swept area)x13cubed(wind speed for 30mph)x0.3(efficiency) 201w

Same calcs gave 20W/13mph 66W/20mph 1113W/51mph

The rating for the same pmg on a hawt is 26W at 3m/s wind vs my vawt at 31W with 7m/s wind, so i need wind of about 14mph to generate.

All in all, I don't think thats too bad, can argue up and down for the efficency, savaonius seems to be about 0.25, benesh claima 0.35, so picked 0.3.

With the gusty and high winds we get where I am, the vawt seems like a good choice, don't need to mount high to get out of turbulance etc etc.

Of course, my calcs could be a pile of old mince, but also convinced that any atempt from me to make blades to spin at a couple of thousand rpm would result in a small explosion and bits of blade scattered around the neighbourhood, in a 25mph wind my benesh is doing about 280rpm (with no load connected), much safer speed for my engineering!!!

Chris

feral air · « **Reply #16 on:** February 01, 2008, 01:59:34 PM »

lol, the mental image of your garage spinning about your vawt is just too cartoony not to chuckle.

Anyway, I don't claim to understand the math but that just seems off to me. The book I'm working from isn't very clear though...

In fact, the whole section is convoluted. He starts with a 10sqft barrel vawt and then decides that its radius is 8ft. Ok, I can deal with that. But then he switches from 30mph wind to talking about the cup's speed being 10mph and then switches again to 5mph in the last paragraph.

He concludes with this;

At a cup speed of 5mph, the working cup will be developing a force of 35lb and the dragging cup a force of 17.5 lb for a net difference of 17.85lb. The machine would have a torque (neglecting friction) of 142.8 foot-pounds and would be developing a power of 0.238 horsepower. It would be turning at 8.75rpm.

So I guess a 10sqft barrel vawt with an 8' diameter would develop 0.238HP if the cups are moving at 5mph. Is that right? What would it take windspeed-wise to get the cups moving at 5mph? 30mph wind?

I guess I'm lost. take it easy

chris psmith · « **Reply #17 on:** February 02, 2008, 01:16:27 AM »

If he has an 8ft radius, and a 10sq ft rotor, then his blades are either 15 inches high (if that is the swept area) or 7.5 inches if that is total area, giving it the benefit of the doubt and assuming that the swept area, then a savonius 16 feet across by 15 inches high would still be a very odd design!

I think my formula is right, so for a savonius drop the efficiency to 0.25, change the swept area to 10sq foot but convert to sq meters then play around with the wind speed in m/s to see what it would do.

The point of the Benesh profile is that it makes better use of the blade design to increase speed as well as reduce drag, hence the reason for greater efficiency.

We have 48mph winds forecast for Sunday so I am goint to try and get it finished today, that the only real way to tell what power I can get from the thing.

Chris

chris psmith · « **Reply #18 on:** February 02, 2008, 01:27:16 AM »

Interesting that www.RaceCom.com are claiming 2.5kw from a vawt measuring 2ft by 4ft, nice design.

fcfcfc · « **Reply #19 on:** February 02, 2008, 01:32:26 PM »

Hi: 2m high and .5m in diameter would be 1 m^2 sweep not .5, correct..??.. So your 88 watts would become 176 watts.... Still not allot of power but twice is better than not twice...

chris psmith · « **Reply #20 on:** February 03, 2008, 07:39:45 AM »

No, you take half of the diameter, so 0.25x2. I get 200w by my calcs, not 88. Only way to be sure will be to measure it, i have it finished now, so just need some wind...

fcfcfc · « **Reply #21 on:** February 03, 2008, 02:49:37 PM »

Hi:

P = 0.5 x rho x A x Cp x V^3 x Ng x Nb

P = .5 x 1.225 x 1 x .35 x 13^3 x .75 x .75 = 264 watts

I used .75 for a gear box eff and .75 for a gen eff and .35 for a turbine eff including beltz at 30MPH wind (13m/sec) and a turbine area of 1m^2 (.5 x 2)

That might be a bottom number in which you might do better....

.....Bill

vawtman · « **Reply #22 on:** February 03, 2008, 03:04:59 PM »

Hi Bill

My opinion is you and others are taking this WAY too far.Its not a math competition

Just build with your current knowledge and have fun!

fcfcfc · « **Reply #23 on:** February 03, 2008, 03:22:37 PM »

Hi: Well YES have fun, but its also fun too see how close your estimates can be to the real thing ESPECIALLY when you yourself are putting allot of hard work into it...

vawtman · « **Reply #24 on:** February 03, 2008, 03:53:28 PM »

Hi Bill

Does anyone really know what the real thing is?What would happen if Betz and the other geezers gave up back in the day?

fcfcfc · « **Reply #25 on:** February 03, 2008, 04:22:53 PM »

Ok, its Super Bowl Sunday. You have probably had a few to many, so I will let that one just blow away in the wind... I hope your VAWT works great!! We need some proven ones out there that people are willing to show and talk about, in all different sizes...

vawtman · « **Reply #26 on:** February 03, 2008, 04:56:29 PM »

feral air · « **Reply #27 on:** February 12, 2008, 01:45:38 PM »

So is it finished? Anything new?

I'm still working on my monster s-vawt and saving for parts...nothing too exciting yet.

MattM · « **Reply #28 on:** February 12, 2008, 07:38:28 PM »

A 2 blade savonius just doesn't look very efficient. That blade out of the wind just doesn't look like it should be avoiding much pressure about a 1/4 of the revolution. Wouldn't 3 or 4 blades work better for a savonius?

Compare:

thyristor · « **Reply #29 on:** February 17, 2008, 03:49:44 AM »

Any further progress on the power generation? I would like to learn how much power you get out of your design?

Thanks

feral air · « **Reply #30 on:** February 17, 2008, 12:49:28 PM »

A 2 bladed s-vawt will channel some of the wind over to, and out, the other side - this is an extra (if slight) push. Giving it a benesh profile should also add a bit of lift to the equation.

You can't take a 20sqft 4 bladed design and compare it to a 10sqft 2 bladed design, look at the power coming out and then say the 4 bladed version is more efficient. For measuring efficiency you need to put them on even ground.

A 10sqft, 2 bladed s-vawt should outperform a 10sqft 4 bladed s-vawt.

feral air · « **Reply #31 on:** February 17, 2008, 12:59:57 PM »

I think he has the cups arranged like an anemometer. That would make a typical s-vawt a bit more efficient than his model (in theory).

I still don't think the profile makes much difference though, not compared to the size. An itty-bitty benesh isn't going to put out that much more than an itty-bitty 'Simple Savonius'. If you scale it up (30+ sqft?) the extra efficiency might start amounting to something though. take it easy

MattM · « **Reply #32 on:** February 17, 2008, 06:47:56 PM »

You make a lot of sense. I hadn't considered the area as the rating for the vawt. I was thinking more that it would be rated in terms of diameter and height.

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Author Topic: Parabolic profile for VAWT blades (Read 10721 times)