PVC Style blades -- guidance needed

[SpmP]

Good Day All,

 I am hoping that the gurus of the ingenious PVC blade can help me out a bit.

I am in a 10m/s coastal site, with a solid downwind turbine base, hoping (needing) to get ~1500W in this wind.

 I have a fixed material: 150mm OD aluminium quarter rounds with flat section at either end (usually cut these off)

 I have a 1.8m diameter bladeset atm, but with three bladed hub is only making ~200W in ~12m/s, so much dignity lost 8(

 I can modify my hubs to take 2,3, or 5 blades.

 Driving PM F&P dual generators, non rewired, will reduce voltage with power electronics (SMPS)

 Before I ruined my tower trying to raise it without a gin pole I tried to increase the angle of attack for the blades to ~15deg. at the tip, I think putting htem in permenant stall.

 I an intrigued with the genius of using a piece of pipe to change the twist by altering the arc width, very nice, does this mean I get a fixed TSR for a fixed diameter of both turbine and pipe?

 Anyway, if Zub, or WooF or OTher T&Eexperts are around and could lead me through getting rel power out of this setup, my hurt male pride could be restored.

Cheers

Jasper

[RogerAS]

Jasper,

I can't comment on you blade issue, but Zubbly is no longer with us.

Is there a reason you don't want to carve wooden blades?

..

[taylorp035]

1.8m diameter is about 5.9 ft.  200 watts out of a 5.9ft turbine with pvc blades is awesome...  I wouldn't be disappointed.

If you want a 1500watt turbine, you are going to need a larger diameter, 12+ ft diameter or about 4 meter diameter.

Or... you could have 22 m/s wind to hit 1500 watts approximately.  

To be honest, I have a hard time seeing a cut PVC pipe that it 2m long that doesn't bend too much.  I hope you can prove me wrong.

[SpmP]

Sorry to hear about Zubbly.

 I dont have access to the wood, or the tools atm, and would like to follow this through and see how far I can get with it.

Cheers

[SpmP]

I'me using aluminium, so it should be able to get 2m without bending too badly, about 5mm thick.

You say 1.8 with PVC and 200W is awesome, is that because PVC blades typically suck?

I wondered if filling in the curved side with expanding foam to form the straight side of the airfoil could help at all. Probably with a bog/fiberglass capping. Would that make a difference?

 Has anyone made PVC blades and got good power out of them (i.e >1kW)

Cheers

[ChrisOlson]

I don't recommend PVC blades.  I had one come apart on a 6.8' turbine and it's lucky somebody didn't get killed by the shards that fly when they fail.

Further, PVC blades don't give you an airfoil that creates any lift.  They are pure drag blades and will rarely run much beyond a TSR of 2 loaded if they have any size.  If you could see one run in a wind tunnel with trace smoke you'd see the problem - the cup creates a low pressure area in BACK of the blade when it tries to run faster than the wind speed.  The blade wastes all its energy dragging that low pressure area behind it.

I'd make them out of wood, and if you can't do that buy a set of WindMax blades or something.  I have a mold that I spent many hours making, and I lay up my blades with carbon fiber composite with a foam core and epoxy resin.  I use a NACA 4415 airfoil, and on a set of blades designed for 1.5 kW I'd use 10.8' diameter, 7.2" cord at the root, 2.625" at the tip, 13.7° angle of attack at the root and 1.8° at the tip.

If you want a 1.5 kW turbine at wind speeds under 24 mph you're going to need at least 10.8' blades.  The formula is:

Watts / ( .00508 x windspeed^3 x turbine efficiency) = (swept area)

Wind speed is in mph in that formula, and the efficiency, once you figure generator losses, a good set of blades and the Betz Limit, line run etc., will be around 30% (.3 in the formula).  The formula will give you the swept area of the turbine in square feet.

--

Chris

[snake]

if you will add many blades,this will increase torque and will decrease speed and fewer blades will increase speed and will decrease torque.try 2 blades,maybe you will get more rpm and power.

[ChrisOlson]

Two blades will increase rotational speed due to less rotor drag.  But it won't increase the amount of power extracted from the wind - it will reduce it because more of the available wind energy passes thru the turbine rather than being converted to kinetic energy.  You simply can't cover the swept area with two blades, even at higher rotational speed, as well as you can with three.  Two blade designs also have a problem with gyroscopic imbalances when the rotor yaws.

If you increase the number of blades beyond three, you start losing ground in how much power you can make because of rotor drag.  That's why you get less rpm with four or more blades than you get with three.  Turbines with, say, six blades, as you mention, will create lots of rotational torque, but then the generator has to be designed to run at slower speed to reach cut-in at a reasonable rpm, or you have to use a geared step-up drive on the generator.

There's a reason that engineers design the majority of commercial grid-tied wind turbines with three blades.  Other designs have been tried, but none match the efficiency of the three bladed turbine running at a loaded TSR of 6-7.

--

Chris

[ghurd]

I agree with Taylor.  200W is not so bad.

Not because "PVC style blades" suck, but because 1.8M is not very big.  And I figure there are other blade problems.

It is too big for blades made of PVC.  PVC is not strong enough for that size.

Jerry does well with 4' dia PVC, but I think that's about the limit, IMHO.

Can you measure the output amps vs. wind speed?

If the amps and wind increase to a certain point, then the amps stay about the same while the wind increases, the blades are stalled.

That would be sort of like the blades are not powerful enough to turn the 2 F&Ps.

The blades sound quite narrow to have the angles correct, and have good torque.

The 6" / 150mm pipe makes for skinny blades at that length.

The design of PVC blades is very important.

Change anything much, and they do not work worth a hoot.

Too skinny and they have no torque (hard starting, and prone to stall).

Too wide and they can not get moving as fast as they should.

A common problem with well made PVC blades is they are connected to the hub at a self defeating angle, with the trailing edge continuing away from the hub and into the wind.

I use the original angles.  Deviating much makes them not work very great.

Notice the bolts are in line with the trailing edge. (on a flat hub)

http://www.fieldlines.com/story/2003/11/11/172721/53

My best guess is they run loaded, when well matched to the PMA, at somewhere around 4.5 TSR.

If they were purely drag based, they would run loaded below 1.

I bet you are stalled.

G-

[Beaufort]

What is the weight of your blades with the foam core and carbon fiber laminate?  Sounds like a very advanced setup.

[wooferhound]

Hey there

 PVC blades are fun to get started with and they are quick and easy to make, but they aren't the best choice if you really want some serious power. They are the perfect 'starter' blades and great for experimenting with stepper motors. Hitting the tower seems to be the most common failure. They flex a lot so leave plenty of clearance to the tower.

Good Luck !!

[ChrisOlson]

4.22 lbs for a 43" blade (7.5' diameter mounted to the turbine flex plate), 4.84 lbs for a 51" blade (8.8' diameter mounted), 6.51 lbs for a 63" blade (10.8' diameter mounted).

Those are the only three molds I have at present.  The molds are quite expensive - I machine them out of billet aluminum on a CNC mill using the coordinates downloaded from NASA's airfoil database.

It may sound more advanced than the average homebuilder's setup, and it probably is.  But I have a shop set up to work with metal so it's not really all that advanced when you have the proper equipment.  If I tried to build a blade out of wood it would be a dismal failure because I don't have the first clue when it comes to workworking.

--

Chris

[Beaufort]

Thanks for the info.  I'm always researching blade materials and was curious at how that construction compares to others in terms of weight.  I just weighed one of my Windmax 8.5' dia blades at 5.02 lbs (chopped fiberglass, terrible finishing work on these Chinese things).  Compared to 4.84 lbs for your 8.8' dia blades, I would have expected more of a weight savings with foam core CF.

[ghurd]

Why worry about the blade weight so much?

G-

[SpmP]

fieldlines-1

@ghurd

When I altered the "angle of attack" i.e rotated the blades to increase tip alpha, I notice the blades seemed to be in permenant stall... I will be careful to remount them with the trailing edge parallel to the plane of rotation.

I dont think the blades deviate too greatly from the plans, unless I totally misunderstand them:

 69mm Radius id (not 70mm)

 700mm long (mounted 200mm from center of the hub)

 45deg. root

 12deg tip

at 1.6m long what diameter pipe would you need? would 150mm just not cut it? (damn!)

@ChrisOlson

 Have you actually seen PVC blades in a wind tunnel? wow. sounds like you cant get a tsr>1

 Thanks for the suggestion. What are you basing your root and tip cord, and angle of attack on?

 hmmm I think my power calculation is a bit off. Converting to imperial and outputting watts (you poor bas*(rds having to use such a muddle of units) I get very diffent figures. Using your formula I get 467W in 10m/s(22mph) with 1.8m diameter

and 1578W using mine... hmm

P=0.5*rho*SweptArea*V^3*0.3

 rho=1.23

dammit. I was hoping that you would all say... PVC;s great just.. blah. Yea I suck t working wood. havnt much experience in fiberglass.

 The aluminium blades dont seem to flex at all, but I suppose they must, being downwind they flex away from the pole tho. Is filling in the rear not going to decrease the drag at all?

Given that I still want to attach these blades to my current hub, I cant help thinking wood will be too heavy, not that it matters too much once they are spinning.  

 Is fiberglass inappropriate for any reason? too heavy? too weak?

@Woof, how are you matching the length/width/twist for even power with PVC blades? Unless I have it completely wrong, I see you set the root/tip angles 45deg/7deg, but how then do you determine, given the radius of the pipe, the length that will result in even power from tip to root?

Thanks all for the feedback.

Jasper

[ChrisOlson]

I think the WindMax blades are hollow, though, right?  My blades are solid, albeit with the dense foam core.  When I lay them up it's quite time consuming and it's a job that you have to stay at from start to finish.  You can't walk away and come back to it.

I hate working with epoxy resins because of their short pot life.  But I tried PVC, aluminum, and thought about wood.  Then decided to try making blades out of carbon fiber composite.  It took several tries before I learned how to do it.

Even with a nice mold there's a lot of finishing work to do on them before they're ready for paint.  It's not a deal where you get a perfect blade out of the mold every time.  But it's the only way I've found that I have to build good blades that perform.  I've thought about buying the WindMax blades but I think that what I build is better because I spend the time to make sure every one is perfect before it gets painted.  My rough estimate is that it costs about double for materials what a set of those 8.5 WindMax blades cost.

But, like I said, I learned how to build carbon fiber composite blades out of necessity because I couldn't get PVC blades to put out any power.  And aluminum is too prone to fatigue and failure.

--

Chris

[ChrisOlson]

@SpmP

  Have you actually seen PVC blades in a wind tunnel? wow. sounds like you cant get a tsr>1

 Thanks for the suggestion. What are you basing your root and tip cord, and angle of attack on?

I haven't seen PVC blades in a wind tunnel in person, but someplace I saw a little video clip on the internet of some college engineering students fiddling with a small PVC turbine and they had it in a wind tunnel.

I've gotten a TSR of up to 4 with PVC blades, but they don't stay there once you put a load on them.  They just don't develop the lift and spin free like a real blade.

I base my angle of attack and cords on my own experience/experimentation, and using NASA's extensive database of airfoil testing information.  I use the NACA 4415 airfoil because it's a simple airfoil to form and it's been highly successful on wind turbines.  Newer turbines use more advanced airfoils, but the efficiency gain with those more complicated airfoils, I think, doesn't pay off on small wind turbines because most of the more advanced airfoils are made for large, very slow turning turbines that use variable pitch hubs.

--

Chris

[ChrisOlson]

This is a photo of my 43" carbon fiber blades (7.5' on the turbine hub).  I'm getting ready to lay up the third blade in this set.  The photo was taken with a cell phone camera so you'll have to excuse the poor quality of the picture.

http://picasaweb.google.com/christopher.w.olson/WindTurbine#5426809108459415890

These blades are optimized for lower wind speeds and they run at a loaded TSR of about 6.5 in a 18-20 mph wind.  I've gotten 430 watts into the batteries with these blades in 18 mph wind.

This set of blades is for a friend that just build a wind turbine and needed a set of blades for it.

There's only two holes in the hub mount but they actually bolt to the turbine hub with three bolts.  I put the blades on the hub and measure the distance between the tips before I drill the third hole.

This is a photo of one of my 10.8' blades. I've gotten over 2.0 kW out of these blades @ 24 mph wind speed.

http://picasaweb.google.com/christopher.w.olson/WindTurbine#5424581787155901730

--

Chris

[KarlJ]

get onto www.thebackshed.com and do Gordons CAP doubler

mod, for most of us mere mortals toying with F&P's like me

they have the potential to double the output of your combo.

100S stators work best and the 80 in 2X7C are good 48V solutions that work well

with the CAP's, ther eare also solutions for 24V but 12V is somewhat of a black

art as we have moved away from this config, more drawbacks than wins.

Theory is it helps unload the alternator, allowing it to run out of stall,

halves the cutin voltage and increases the top end due to reasons we cant explain!

I have a Phillm dual stator with 3m (10') rotor with two 100S staors one in star the other delta with caps and I'm hoping for 1KW+.

only time will tell, still putting up the pole.....

check it out http://www.thebackshed.com/windmill/forum1/forum_posts.asp?TID=2300&PN=1

[scoraigwind]

This is all very interesting and it confirms what I suspect about PVC blades.  Easy to make but not a good enough airfoil to run fast.  Aluminium would scare me.  I would expect it to crack at the fixings and then fly off in ugly sharp pieces.  (At least that has been my limited experience with the material.)

My reason for commenting here is to try to persuade people not to use the term 'angle of attack' to describe the angle of the blade.  The angle of the blade (relative to the plane in which it rotates) can be called the blade angle or is sometimes called the pitch of the blade (beta).  This is Not the angle of attack (alpha).

I have done some notes on this (about 12 years back) that you can see at this site

http://users.aber.ac.uk/iri/WIND/TECH/WPcourse/index.html

See page 4 in particular.

Increasing the blade angle will reduce the angle of attack.  Reducing the blade angle increases the angle of attack, and this can produce stall if the blade is running too slowly and is maybe being dragged down by something that prevents it reaching its best speed.  This applies to an almost flat blade.

If the blade angle is 15 degrees or so then the rotational speed will be very low, because the tip speed ratio will be limited to a maximum of about 2.  This is not stall - in fact it is the opposite.  Stall is when the blade angle is too flat and the angle of attack is therefore too large.  In this case the blade angle is too large and the angle of attack soon drops to zero as the blade begins to spin.

I hope this helps.

[ghurd]

Hi Hugh,

It sounds like I am using the term 'stall' incorrectly, in a mostly different context.

What is the term for when a turbine hits say 2A easily, then will just not go higher?

Example of what I mean, though this starts at the solution.

http://www.fieldlines.com/story/2009/11/30/194624/37

PVC type/shaped blades do some crazy things.

I am not sure their operating parameters stay the same as they work.

TLG blades are sort of the same, with the trailing edge section a little flattened out, which looks (to me) like they should have a bit better pseudo airfoil and run a bit faster.  From the end they look like more of a J than a C, if you know what I mean.

TLG claims 12.73 TSR for the Tri-Nando AL (yes, I know).

But the Boast Booster's apparently run slower than Jerry's.  Power output matched up at 10 vs 15MPH, and 15 vs 20MPH, depending on the wiring configuration.

From here,

http://www.fieldlines.com/story/2009/9/19/3199/30084

The whole issue keeps me confused!

G-

[ChrisOlson]

Yes, you're right on this Hugh.  Angle of attack is the angle that the airfoil runs in relation to the relative wind, or the direction of the airflow that the blade sees, and not the relation from plane of rotation.  I should change my terminology to use blade angle, or probably more properly, pitch angle.

--

Chris

[SpmP]

Well... I packed out the blade mounting the opposite direction to when I was trying to increase my "angle of attack", and WOW, now they spin. I have to get some real load on them now, but a major prima facia improvement. Maybee it will turn into power.

 Thanks Hugh for the corrections on terminlology. I suspected that I was misusing the term hence the speechmarks

 I am amazed at how well the aluminium blades have hung on. I had my bad welds onto the blade mounts break and the hub rip apart, but the blades were still attached to thier mounts. Embedded nice and deep in the ground. The mount holes were ripped tho.

 For my ignorance: What is fast? I feel that Chris may have it summed up: Great unloaded speed, but they die with a load on. Still more testing, time will tell. I need to get an anemometer up there so i have something to compare it all with. Hopefully a friend will be getting me through C and AVR stuff, so I can convert my butterly to do all this for me.

Something I noticed when I was bench testing F&P's was that I got maximum power out for a given rpm when load was taken at half the open circuit voltage. Is this a well known phenomenon or anything?

Thanks KarlJ! Thats def. something to sink the teeth into. Your setup looks pretty nice (yea the cour is important. Mines SHINY silver ie. grey). It took me a week to get my anchor holes etc right, so I can guarantee they can take ~1.5Ton. Sands amazing stuff, when its not wet, vibrated, or has air flowing through it.

The cap mod sort of begs the question as to how far the SMPS thing can be taken, whether theres any gains in leveraging the fact a generator is also a whole lot of inductors. (sorry havnt thought about it really, just an off the top of the head)

 I wonder if the inductance/capacitance in the wires will affect things much...

great! progress is being made. Pride is slowy being restored.

I am quite amazed by this whole process. I told my neighbour that the wind turbine was more about turning peoples minds to the awesome resource(s) that are around them, than producing power. The response has been overwhelming. I think everyone who can see it has been thinking about, commenting, inventing...etc. An overwhelming response really. So moral of the story, dont put up a fully working turbine, it needs to show improvement, etc. you get the idea.

[Noam]

I have designed PVC blades and generated an excell file with all the design parameters. I have built a 2400 set of blades for Hugh Piggot 2400 machine. It is up and running for couple of month now. I measured TSR of about 7. The cut in is quite similar to the the one with the wood blades. In the near future I shall make a complete comparison between the PVC blades and the wood blades by Hugh with a monitoring system and share all the data with you.

At my place strong wind are rare so we shall have to wait for a while untill a complete set of data is available. any way I have measured more than 25A at 24V from this set. I also have made the XFOIL calculations for the blade. It turns out that they have a lift/drag of about 75.

They are easy to make. they require a UV protection coat.

The only problem with it is:

You can not extend the blade length too much, probably 1.5 m is the longest, also you should know that the PVC industry is highly intoxicating thing. so try and use only scrap material.

I'm willing to free share the design spreadsheet with you.

[SpmP]

Thank you. (thank you thank you thank you)

 Thats great!

I am using 3mm aluminium, which is also a dangerous industry, but the breakdown products are not nearly as bad.

 I think I will see if I can push 2m. I have been warned tho.

It feels inevitable that I will need to learn the patience required to make wooden blades at some point.

When I have the data logger thing and anemometer (when its fixed, after it fell with the tower...) up I will be able to give some useful data.

[KarlJ]

The PVC / ALuminium blades we are using out here in Aus

are not structural members only airfoil sections, the structural member

is the Steel tube that the blades mount on.

(ie the steel tube runs inside the blades in the 1" hole made for 1" steel tube.

A solid section of bar is plug welded into the tube for the first 25% of the blade giving it strength where it needs it.

Only failures so far are from testing their limits and not using sufficient steel tube or just going too big (16') and 500rpm without considering the forces applied to the tube.

Plain old steel inside is fine and will take longer than most mills to break from fatigue cycling.

[wooferhound]

What Diameter of PVC pipe are you working with.

I personally would not use less than 6 inch but I have made some with 4, 3 and 2 inch pipes.

[scoraigwind]

Yes those posts are certainly confusing.  

The simple fact is that a blade with too flat a pitch (low blade angle) running loaded at too low a speed tends to stall.  Often this is manifest as you describe - produces a couple of amps in a very low wind but fails to produce decent power as the wind rises.  You can identify this problem because when you disconnect the wires it speeds up dramatically.  (You may be able to reconnect and see good output for a while even).

The other thing that limits output is the opposite: too large a blade angle.  This will not stall the blades.  These blades will run too slow because they work at too low tip speed ratio.  They will not cut in in low winds.  If you disconnect the wires it will make no difference.  The blade angle is large and so the angle of attack is small, even at low speeds.  This kind of blade is good for getting started against a heavy magnetic drag or belt-drive friction but it's not going to run fast.  It will not stall.  It will not give you high enough rpm either.

I hope this helps to clarify the issue of blade angle and stall.

[SpmP]

How can I get the spreadsheet etc. off you? pm, email...

cheers

[SpmP]

Being uniform extrusions, do these blades get noisy at high speed? Is there much real world data around? Anyone in NZ with them?

 Cheers

[Noam]

In general the PVC pipe diameter is at least 1/5 of the blade length. I made 1.2 meter blade to run Hugh 2400 turbine with 20 CM pipe diameter. Also it is recommended not to exceed 1/4 of the pipe circumference. My design is based on the blade cord width given by Hugh. from there I calculate the required angle along the blade for given TSR and angle of attack. XFOIL calculations predict lift over drag of 75 around 4 deg. and this is what I use. If you have a place I could post for you the spreadsheet I would do it.

Bye. Noam

[Noam]

mail your mail to noam.dotan(at)gmail.com

[ghurd]

I would like to see it too, but I can't decipher that address.

ghurd1 at yahooo,comm

G-