Author Topic: VAWT rotor questions & suggestions (Read 9380 times)

pguillem · « **on:** July 12, 2013, 03:10:23 AM »

Hi there folks
HereÂ´s a challenge for powerful minds.

I have a 2KW H-type VAWT with the following characteristics:

Alternator: 48V 3-phase AC (i think its DELTA)
Rotor diameter: 2 m
Blade Height: 2,60 m
Chord length: 0,29 m
Blades: 5
Blade Shape: NACA 0018
Blade area: 3,77 m2
Swept Area: 5,26 m2

Start speed: 3 m/s
Average wind: 11 m/s
Measured TSR: 1.3

Offcourse, the damn thing is only producing 300 watts (54V - 5 amps) at most... and i strongly believe i have a rotor/alternator missmatch. The alt is indeed 2KW (verified) so i focused on the blades... I fine tuned the pitch of the blades to the best angle by trial and error and it climbed from 80 RPM to 130 RPM.

I verified the alternator on a test and it needs to run at 220 RPM to produce 2KW, which i will never reach with this blades unless a hurricane passes over my house. So I figured if i re-build the blades (make them 30% bigger) i might just give the alternator what it needs to run on its full potential.

Having 11 m/s... how would you guys modify the rotor size, blade size and chord length in order to deliver 220 RPM to that thing? I need to achieve a 40% increase in TSR while mantaining the torque and inertia....

Should i build a rotor with lower solidity? or increase the total blade area to get more power (torque) delivered to the alternator?

IÂ´m well aware the problem of low solidity blades is that they have a higher TSR while unloaded, but when loaded they will drop to half the TSR and deliver less torque.... and this is a big, fat, torque-hungry alternator.... So iÂ´m frankly stuck trying to figure this out.

I spent the last 4 days reading about blade design (mostly incomplete theory) and NOBODY seems to have clear concepts, or a specific set of equations and specific guidelines to design Vawt blades proportional to an alternator.

Any ideas?

Pedro

Bruce S · « **Reply #1 on:** July 12, 2013, 10:51:35 AM »

Need to head out for an interview, however let me try to help just a little.
VAWTs are mostly slow turning machines, 220RPM can be hard to attain, however if you need to turn faster smaller diameter, taller height blades will help with this.
Gearing is not out of the question and there are several posts about this from GoVertical in the wind section.
Also Ed Lenz over at www.windstuffnow.com has extensive knowledge on this very subject and his Lenz2 has been copied by even large scale ventures (PACWIND) though didn't even give him so much as a nod much less money.
You might drop over there, grab his email address and ask him the same thing. His website also has tons of info. Ed has been a long time and highly respected forum member. He may(when not out building stuff) see this post and respond.
Also there are others on here who still build and use VAWTs (Tecker, Electrondaddy1) I'm sure they'll post when they have the chance.
Gotta go!
Best of Luck
Bruce S

karlb · « **Reply #2 on:** July 12, 2013, 11:06:44 AM »

Have you seen this site? : http://usvawt.com/cgi/windpower.cgi
Not sure if it is exactly what you are looking for but it may help.

I will be attaching the generator I have built to a VAWT in the next week or so.

gww · « **Reply #3 on:** July 12, 2013, 05:24:09 PM »

Besides this site, I have seen govertical posting recently on the site "thebackshed".
gww

pguillem · « **Reply #4 on:** July 13, 2013, 12:44:43 AM »

Dear Bruce, Karl and G.
Thanks for taking the time to write some lines.

1) IÂ´ll study EdÂ´s designs and papers to look for some light on the matter.

2) Thanks Karl for the link to the vawt calculator, we have been indeed using it as reference table for some time. Very good tool. The downside is the calculations are limited to showing what the vawt should be producing vs the power available in the swept area; there is no data on how you should design the mill in order to achieve a certain % of efficiency, which is exactly what iÂ´m looking for.

In the meantime, iÂ´ve made some math... Instead of making each blade 30% bigger in all proportions, i guess i could modify only the "blade height" longer without modifying the current rotor diameter and chord length of the blade.

In theory this mod would decrease rotor solidity and increase the blade area... it should give in 50% more torque and add a little more speed simultaneously.

In order to do this, i have quoted the forging of a HUGE 6 meter long replica of a single blade... which i will cut in 10 equal pieces and then tie 2 pieces to each existing blade.

The new blade area should sum 5,51 meters against the current 3,77.

Am i making sense?

Pedro

pguillem · « **Reply #5 on:** July 13, 2013, 03:10:39 AM »

Ok, new findings...

According to my math, my current rotor has a solidity of 73%!!! which is a lot for a VAWT...

I found this VERY good paper
http://www.cscjournals.org/csc/manuscript/Journals/IJE/volume4/Issue5/IJE-192.pdf

Before i do anything fancy, since the solidity is a function of the chord length, number of blades and diameter.... iÂ´ll try reducing my solidity down to 44% by removing 2 blades from my rotor. That shoud give the rotor higher stall on specific angles (it wonÂ´t start on low winds) but should produce a higher TSR according to the theory.

IÂ´ll report my findings.

tecker · « **Reply #6 on:** July 13, 2013, 08:50:27 AM »

I suspect the RPM is the main Problem . If you unload the stator You can set up some E+I*R ratios to turn the rotor combination Add more blades to increase the darius RPM use an inner S rotor to start in low winds Use a MPPT ,controller or pwm to find the sweet spot
Good luck .If you want some performance feed back Link in some video . Most will get a feel for what kind of stage your in .

pguillem · « **Reply #7 on:** July 13, 2013, 09:47:36 PM »

Thanks tecker. IÂ´ll have that in mind.

Tomorrow i will disassemble the blades from the shaft and drill the corresponding holes at a 120 degrees from each other, so the 3 blades fit at the correct angle. Since i have 5 blades, they are 72 degrees appart.

You can see the rotor starting up: here
http://www.youtube.com/watch?v=Hb6cRzzO-r4

(watch the volume... i also recorded the wind at 11 m/s.. OOPS)

On the video you can actually see the pitch modification i performed to the blades from the original design in order to double the RPMs. This was applied where the blade meets the arms (10 degrees).

electrondady1 · « **Reply #8 on:** July 14, 2013, 08:39:08 AM »

so is the vawt a kit you purchased?

GoVertical · « **Reply #9 on:** July 14, 2013, 06:39:04 PM »

Hi, what is the internal resistance of your stator? You may need a MPPT or a buck converter with a adjustable duty cycle.

GoVertical · « **Reply #10 on:** July 14, 2013, 06:45:33 PM »

Hi, your average wind speed is over 20 MPH?

pguillem · « **Reply #11 on:** July 15, 2013, 01:51:48 AM »

Hi all.

1) yes. We sadly purchased this kit from a chinese company and have been learning the science of wind power for a year, trying to make it work the way it should. Now we know they don't know the word "warranty" and they don't really understand the engineering behind what they copy. Long story short, we bought a 2kw vawt that produces 300 watts.

It was only usd 4600 including 12 batteries, alternator, blades and charge controller, which is good.

The 10KW inverter however we bought from a dominican company, VERY good quality industrial stuff indeed for nearly 1800 USD. The brand is FONDEUR. So we at least have a hell of a UPS that keeps our home running for 60 hours.

2) MPPT and buck-converter are built into the charge controller. We can modify the admitance value to show more or less load to the alternator... Its not automatic, but it does the job.

3) we also bought the Guhrd controller from Glen Hurd. Which we wanted to try.

4) yes, we live next to the beach and we have almost constant nothern wind at 11 m/s

pguillem · « **Reply #12 on:** July 15, 2013, 01:57:40 AM »

About the internal resistance of the alternator... I have no clue. Is there a practical way to measure / calculate this value?

We disassembled the alternator and measured all details. Could anybody guide me into calculating its internal resistence?

electrondady1 · « **Reply #13 on:** July 15, 2013, 07:26:14 AM »

according to the us vawt calculator
@11.62 m/s and 5.2 sq m. swept area, a lowly drag mill ( the kind i build) should be able to produce 979.81 watts.
between those readings and the rpm figure's your getting it sounds like your 5 blade Darrius mill is not going into lift at all.
messing with buck converters and such won't change that.

since your not afraid to experiment to find more power i suggest you completely flip the blades around so the lifting surface is on the outside rather than the inside .
of course this will change the direction of rotation.
your wings are quite non symmetrical
i can't recall seeing such a curvy profile used on a set of blades with the lifting surface on the inside.
if it doesn't work your can always put them back.

electrondady1 · « **Reply #14 on:** July 15, 2013, 08:32:19 AM »

you can find "internal resistance" by using an ohm meter.
on three phase delta, measure the resistance between any two of the three wires coming out of the alternator.
they should be the same reading.
if you have four wires coming out it' wired in star.
you need to find out the rpm of your mill
can you operate it unloaded for a bit? just to see if it can go into lift ?
or if the alt is dragging it down

GoVertical · « **Reply #15 on:** July 15, 2013, 02:02:50 PM »

Hi, nice location. I agree with ED1, run the turbine unloaded to see if it will overspin. If it does then the problem is with the alternator.

http://www.midnitesolar.com/pdfs/classicDevUpdate3.pdf
This is the only MPPT controller that I know of for wind turbines.

Can you post the name or link of the VAWT you purchased?

tecker · « **Reply #16 on:** July 17, 2013, 07:33:22 PM »

What you made try is a tear drop foil

pguillem · « **Reply #17 on:** July 22, 2013, 06:29:39 PM »

Thanks all for the new suggestions.
We will restart the investigations on thursday and present the results on the forum.

The link to the design i purchased is here:
http://www.richuan.com.cn/product/713669658-210850173/Low_RPM_600W_1KW_2KW_3KW_5KW_10KW_20KW_Wind_Turbine_Generator_for_sale.html

We had never seen a non symetrical model of the blade either, but we assumed that their engineers wanted to use such shape to drive the lift force vector pushing inwards rather than outwards to the shaft. Does this makes any sense?

We will test a couple of more things with this model and, if unsuccessful, we will switch to a 3KW HAWT rotor which they have agreed to send as warranty for USD 500. This is 1.9 meters per blade. TSR of 6. They claim a 30% efficiency, which matches the Cp to extract 2.7 kw under 11m/s at that swept area.

tecker · « **Reply #18 on:** July 24, 2013, 10:08:47 AM »

I made a small rotor with a similar foil several years back and modified it over the winter the blade I made out of a wire spool . I settled in on an s826 that was based on a 11" diameter. I was getting drag upwind . I ended up filling in the foil until it was symmetrical. The symmetrical blades hade power and speed that was just not there with the 865 foil . They were also scalable with three blades

pguillem · « **Reply #19 on:** July 24, 2013, 10:48:35 AM »

Thank you Tecker. That is very nice information to know.

We had been discussing the shape earlier this year but, since our expertise is Power Electronics and Computer science (my dad and me), we have not much background deducing its aerodynamics... we preety much assumed the chinese had copied a "working" airfoil from somebody else and what you are saying makes a lot of sense, given that our TSR is 1.2 or JUST a little over the wind speed at 11 m/s, which would suggest it is behaving like a DRAG machine rather than a LIFT machine.

In your experience, would you suggest a specific NACA airfoil for this application?
We have a bunch of companies in town that will re-manufacture our blades for 35 bucks per meter or something, we could give it a try.

Pedro

tecker · « **Reply #20 on:** July 24, 2013, 12:50:49 PM »

You should be able to fill in the foil with foam board cut the foam and glass the foil . There are many running rotors with the tear dropfoil
Most are at 120 degrees . Mold a sanding jig on the wide foil for a finish sander foam and glass clean up

pguillem · « **Reply #21 on:** July 24, 2013, 01:28:27 PM »

Ok, just to double ckeck i got this right:

1) are you suggesting we don't build new blades but instead "complete" the tear shape of the current blades with foam?

2) where is the lift pressure produced if the shape is symmetrical? I understand that in order to create lift, the upper surface must be a bit larger than the lower surface.

Any comments?

pguillem · « **Reply #22 on:** July 26, 2013, 02:33:14 PM »

Well, after experimenting for the day, hereÂ´s the results:

1) After measuring the alternatorÂ´s internal resistance: it gave 0.5 ohms among all pairs of phases. ItÂ´s very low.
2) Switching from 5 to 3 blades produces the same amount of power.

Next step is to turn the blades symmetrical.
Any suggestions?

tecker · « **Reply #23 on:** July 28, 2013, 09:16:47 AM »

Given that you paid good money for the blades . You can cover them with packing tape and glue on some foam board to fashion the blade and paint the foam for test .
Again lay some tape on the convexed side and put several wide strips of gauze dipped in poly to start a finish sander for the foam .
The other option is to divert the wind around the upwind arc of the rotor work cycle . There's 40 to 60 degrees of upwind exposure that produces drag before ingaugeing the down wind cycle .

GoVertical · « **Reply #24 on:** July 28, 2013, 09:44:18 PM »

Hi, I tried researching the PMG and I was not able find any other info. Does your system have the brake option?

electrondady1 · « **Reply #25 on:** July 29, 2013, 07:25:19 AM »

i have purposely staid away from lift machines for simplicity's sake
arbitrarily changing the shape of your blades is just a shot in the dark unless you have done real research into airfoil profiles.
have you identified your present airfoil?
does your windmill get into lift if it's unloaded?

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Author Topic: VAWT rotor questions & suggestions (Read 9380 times)