Darrieus - how big is too big?

[MountainMan]

I could sure use some expert wisdom from those that have built some of these things.  What would I run into first in terms of structural difficulties as I make a Darrieus with a larger and larger radius and taller "wings".

The siting will be the top of my 10,000 gallon steel water tank.  Only up in the air about 15 feet plus whatever I add in the way of a tripod mount or whatever.  I know, not ideal to put something near a "building", and not ideal to have it that close to the ground, but trust me, I have my reasons.

As I let my mind go nuts with the design, I can imagine the "arms" that come out from the hub to be made from something lightweight and easy to come by like maybe 10 foot long 3/4" or 1" diameter copper pipe.  There could be a center pole rising up from the center of the hub maybe three or four feet tall that I could attach guy wires to that would go out and down to the ends of the arms to give more vertical strength to the arms.

No clue what I would make the "wings" out of, but with such a large diameter, I can imagine maybe four foot tall wings to keep it in proportion.

Anybody have any idea how to calculate the forces such a beast would put on the arms near the hubs when it is turning and working against the back torque from a properly sized alternator?  I can imagine the static forces for designing something and just make educated guesses about how strong something needs to be to just stand there, but something tells me that as a genny of any size starts to grow, the other forces that come into play from centripetal forces and from applying power to the hub become more important than the static forces.  I don't have any experience from which to draw on this.  Can't translate my 40 years of building things that just sit there into a SWAG at the type of strength needed to deal with something in motion.

If we were to assume for the moment that the other parts of this thing could withstand a 40 mile an hour wind, and that it could produce 1000 watts at some wind speed below that, some calculations I got from some web research gives me approximately 70ft. lbs. of torque at 100 rpm = 1KW.  70ft. lbs. doesn't sound like a whole lot of torque.  With a ten foot long pole, that's like hanging a 7 pound weight (less than a gallon of water) on the end of it.  I can easily imagine a 10 foot copper pole withstanding the weight of a gallon of water hanging on the end of it.  Am I off in the weeds here?  Does the pole need to withstand several times this much force at the hub in the arrangement I've described?

What other areas of such a design are likely to need some sort of additional support or stiffening?

thanks,

jp

[inode buddha]

No idea how to calculate it at the moment (I would have to look it up).

BUT years of turning wrenches shows me that most cylinder head bolts can take 70 ft-lbs no problem. The grade 8 (expensive) ones. Those are usually 3/8 or 7/16 dia. Structural steel rods are not graded in the same way; a lo-carbon structural is usually only rated to 36,000 psi. This is the stuff you can pick up at any hardware store or home improvement center in the form of cold-rolled rod or all-thread.

Copper is rated way lower, especially for fatigue loads IIRC. Maybe only 10% of steel.

I would consider 5/8 or 3/4 cold-rolled rounds to allow some margin, and only use copper for slip-rings or whatever. In other words, I wouldn't use copper for anything structural. Lord knows a 3/4 steel round is cheap enough anyhow. Doing it like that, it probably wouldn't even matter what you make the wings out of -- though I would still try for something lightweight.

[MountainMan]

There you go.  People tend to use materials they are familiar with.  I'm an electronics geek (so I like copper), and I've done a little bit of plumbing.  Most materials tend to come in 8 foot lengths, so I was trying to think of some things that are easy to find in 10 foot lengths.  First thing that came to mind is copper pipe.

You're probably absolutely right about using steel.  Where do you go to buy a 10 foot long steel pipe?  I desparately need to take a shop class and learn about this sort of thing in order to do any serious experimentation.  I can solder and sweat but i've never wielded a welder in my life.

Where does a grown man go to take a shop class?

jp

[MelTx]

  Hey M.Man sometimes the jr collages have courses you can take.Also the Tech high schools have adult classes at night.I took a HVAC course a while back,lot of fun.

      H.F. has got a small stick welder about $100 that would do most every thing a shade tree tincker needs.                 MelTx

[MountainMan]

That's what I thought, but I've been watching all the catalogs they mail me and...no joy.  Seems like "how to use a computer" has taken over that curriculum.  I'm thinking maybe when I re-retire I'm gonna work at a welding shop for a few weeks just to learn a little bit about metal.  So many things I want to mess around with require basic metal working and welding skills.

jp

[inode buddha]

Mtn man, steel is usually sold in 20 foot sections from the commercial suppliers, its either 20 or 40 or 60 feet long. They charge you extra for every time you want them to cut it. The stuff you get at a hardware or home center is way over-priced by about 8 times. If it was me, I would fish around at a local scrap-yard with a (hack-saw and a ruler) and save big. Truck axles even. I got nothing against copper because I used to do radios -- but I just think it's the wrong thing for the job you are asking.

[electrondady1]

 hi mt.man, learning to weld will be a great asset to you . i learned back in high school . it's a real power trip. the ultimate hot glue gun. i have a 360 amp ac stick welder now, soon i will purchase a small wire feed mig. a night school course would probably be best.

[RobC]

My advice is buy a welder and a book on welding and start practicing. Also you must have a friend or two who can weld and would help you get the hang of it. I taught my self to weld when I was teenager. The hard part is deciding what kind of equipment to buy. The easist to use is a wire feed machine a good one is expensive though around 1600 to 1800 dollars or you could buy a simple AC stick machine and get by. Just beware the cheap imports they aren't that great. RobC

[Norm]

   Ever wonder why most of us stick to Savonius

type VAWTs?  What you have in mind is best left

to scientific teams with engineers metalurgists

and top notch welders....

   Just by reading some postings you'll start to

realize (as I have)....I can imagine that a 4

foot diameter Darrius and the same diameter and

height Savonius spinning in a 20mph wind are 2

different animals indeed  ...picture yourself

in the near vicinity of a runaway diesel??

   There's more to welding than being able to

run a 'perfect bead' as more than one instructor

had told us.

                 So please be careful!

                     ( :>) Norm.

[windstuffnow]

  I've built several smaller ones in different configurations.  This is simply my opinion... if you don't have an engineering background you shouldn't build a large darrieus.   You'll need to examine every aspect of the assembly and make absolutly sure everything is within tolerance ( its only as strong as its weakest link ).  

  Many people have been hurt by these things and some have been killed.

The formula for centrifugal force is...

F =.000341 x W x R x n^2

where

F = force in lbs

W = Weight of revolving bodies

R = distance from center axis ( radius)

n = revolutions per minute

For instance... a 4 ft diameter machine with a blade that weighs 5 lbs traveling at 500 rpm

.000341 x 5 x 2 x 500^2 = 852.5 lbs

As you can see things could easily get out of hand quickly if a strut attatching the wing let loose.   The blade traveling at 500 rpm is actually doing about 70 mph.  At the point of release the amount of stored energy in that blade can cause some extreem damage.

Be carefull !

.

[electrondady1]

mt.man how about a photo of your water tank . include dimentions and total height.

[wiredwrong]

I too have a stick welder and taught myself to weld as a teen, my welds were never very pretty, and seldom held like I thought they should until I found this http://www.mig-welding.co.uk/  since I read this website my welding has improved greatly, I can even weld very thin metal without problems, somtimes all it takes is a couple of really good hints to make things "click". Even tho this site is about mig welding I was able to use a couple of techniques learned here to improve my skills with a stick. I hope that this site helps someone else improve their skills as a welder.

I am not related or paid by the site, its just a site I found and want to share.

[jlt]

[jlt]

[MountainMan]

Thanks Ed,

Isn't it also true that the larger it is the slower it turns in the same wind?  I was counting on that, and was "ass-uming" that this aspect of a HAWT was also true of a VAWT.  

If I understand your formula, with the centrifugal force being a factor of the rpm squared, wouldn't we expect the linear reduction in rpm for a large VAWT to produce a second order reduction in force?  That would be half canceled out by the first order increase due to larger radius.  Seems  like we are left with a first order (i.e. linear) reduction in centrifugal force as the radius of a VAWT goes up.

Engineering background yes, but an electron head, not a mechanics of materials guy.  So point well taken.

thanks,

jp

[stevesteve]

That is an amazing machine!!

The long arms would give high torque but do look a bit odd on top of the rather smart drum assembly. Also they would give a hell of a force on one side of the towerbut if it works I can't fault it.

Is that slowly trundling round pumping water?

[Ungrounded Lightning Rod]

Isn't it also true that the larger it is the slower it turns in the same wind?  I was counting on that, and was "ass-uming" that this aspect of a HAWT was also true of a VAWT.

Same tip speed, lower RPM.

If I understand your formula, with the centrifugal force being a factor of the rpm squared, wouldn't we expect the linear reduction in rpm for a large VAWT to produce a second order reduction in force?

Yes, force per unit of mass goes down.  But when you build bigger you have to build heavier - so it comes back another way, via the square-cube law.  Keep it in proportion and when you double the radius you divide the force per unit mass by two and multiply the mass by 8.  So your force went UP by a factor of four.  You've also increased the cross-section of your supports by a factor of four and that brings you right back to where you started.

The power you're collecting is proportional to the swept area, which goes up with the square of the radius if you keep everything in proportion.  That energy gets stored as momentum until you suck it out via the genny - or released when the mill comes apart.

Since your mass is up by a factor of r^3 and the speeds are all the same, your momentum and stored energy are both up by a factor of r^3.  That'e far worse than the r^2 of the power - the extra factor of r comes from the mill taking longer to spin up.  Make it bigger and the energy released when it fails gets bigger cubed.  This gets out of hand very fast.

Figure that power goes up with the square of the radius and danger goes up with the cube.  That argues for limiting the size of the mill.  B-)

The problem with a darrieus (and other VAWTs) is that, unlike a HAWT it flexes every revolution.  If you don't design it just right this leads to fatigue, which leads to breakage.  Since it has more force per unit mass trying to tear it apart than something slower (like a savonius) AND a flimsier structure, breakage is more likely to lead to violent disintegration.  Then, since it has more energy stored in its mass, disintegration is a bigger deal if/when it occurs.

[MountainMan]

ULR,

Thanks for your analysis.  It is not falling on deaf ears.  I understand that I don't yet understand all of the ramifications of building a large darius.  That's the point of this diary, and it is serving that purpose well.

That said, I'm gonna have to take issue with your physics.  Square qube law does not apply to this contraption in the way that  you suggest.  That law describes how the enclosed volume (and mass) of something goes up with larger size.  Specifically, I've seen it used as an explanation for why an elephant needs big ears to cool it down.  It generates heat in its volume, which goes up as the cube of its "radius", but dissipates the heat over its surface, which only goes up as the square of its "radius".  Thus, the bigger an animal gets, the more out of proportion its cooling system gets.  Thats why many of the larger animals that are still around are water oriented animals.  Water cooling is more efficient.

In my case, I would have to say that as the linear dimensions of this contraption grows, the mass increases roughly with the square of the "scale", as the increasing scale is not describing an enclosed volume, but rather the length and width of some pieces out at the ends. If one takes precautions to reduce the weight (especially the weight that is out at the ends of the arms) as one scales it up, it should be possible to keep the weight of the relevant components (those out near the radius) quite low.  Certainly not "small darius weight times radius qubed".

This might be a good reason for making my 10 foot arms in two sections, with something stronger and heavier near the hub (steel), and something lighter for the "outer" half of each arm (maybe 3/4" copper or some sort of aluminum extrusion).  It also suggests that the "wings" should be made in some sort of aircraft style, with light weight spars and a lightweight skin material.  Or perhaps sytrofoam wings with an epoxy skin.  That both reduces the likelihood of launching them at 100 mph and reduces the danger that they pose if they do leave the contraption in a hurry.

thanks,

jp

[MountainMan]

From Underground Lightning Rod:

"The power you're collecting is proportional to the swept area, which goes up with the square of the radius if you keep everything in proportion. "

I've been wondering about this. I agree that this would be the proportion for a HAWT.  The wind being "slowed down" by a HAWT covers basically the area that the blades rotate through.  The wind is coming at the blades of a HAWT "on axis".

With a VAWT, the wind you are slowing down would seem to be more like the diameter of the VAWT times the height of the VAWT.  Still an area, but a rectangular area that describes the area of wind that we are trying to slow down.  If this is correct, the power for a VAWT only goes up linearly with radius, not as the square of radius, unless the height of the wings is increased in proportion to the radius. (not necessarily a given).

If this is correct, then I should be able to experiment with a large diameter darius with dinky little wings on it.  Then gradually scale up the size of my wings as I get some experience with the critter.  I like the idea of long "arms" on the device because for a given power output that will give me a slower speed and theoretically less noise.

My limited (OK non-existent) experience with windmills leaves me uncertain about something though.  If I make a darius with long arms and dinky wings, does that mean slow turning in general, or does it just mean I will need really high wind speed to get it to turn at all, and then off she goes at high speed.  Is this merely a question of friction and inertia?  Or does it involve that still mysterious (to me) ether of windmill science called TSR?

I guess I have put it off long enough.  I need to understand this mysterious quantity that seems to be everywhere in this science of windmills.  What the fork is TSR, and where can I read a really good explanation of all the ways that it influences the design of windmills?

thanks,

jp

[windstuffnow]

  MM,

    TSR is simply the tip speed ratio.  That is, the speed the tip travels in relation to the wind.   When someone refers to a TSR of 7 then the tips are traveling at 7 x faster than the wind speed.  It doesn't have anything to do with power per say, it relates to the speed or rpm the machine will run.  In relation to power, the speed at which they run affects the torque.  A low TSR will produce higher torque at lower rpm's, which is generally the case with most VAWT's - the acception of course is the darrieus.  The darrieus needs to run in the range of a TSR of 3 to 4 to produce good or reasonable efficiency.  This is quite fast for a vertical machine.  There used to be several Mw Darrieus type units around the world and I believe there are only two left that I'm aware of... one in Texas and one in Canada that are still running.  The rest of them had some type of catastrophic failure.  One of which failed prematurly because of a maintenance mishap.  When it let loose it took down a tower and a fairly large building.

   If this is your first turbine, and don't take this wrong, this may not be a good turbine to "practice" with.  Even smaller ones you can see the wings change shape in a stiff wind.  Some of the small 2 ft diameter models I played with were downright scarry when in a 20 mph wind.

   Have fun but play safe !

.

[MountainMan]

OK Ed,

You've convinced me this is not a good starting point (maybe not a good ending point either).

Taking that as a given, I need to understand what aspect of the design and construction of a darrius makes it operate so much differently than one of the other types of VAWT.  Is it the fact that you don't have a solid top and bottom plate?

When you look at different VAWT designs, that seems to be about the only big difference; other than that its mostly the shapes of the airfoils.  Some of them are clearly drag machines that just sort of catch some air in a "cup".  But others I've seen here have airfoil shaped "wings" that appear to have a significant "lift" factor.  Just trying to understand why a darreous is so different from the "lifty" ones that have the more standard "top and bottom disk with stuff in between" VAWT topology.

thanks,

jp

[windstuffnow]

  The darrieus is strictly a lift driven machine like most airfoil bladed HAWT's and require a fair amount of speed to make them operate properly. Also, the darrieus needs a bit of a "push" to get it going, they don't start by themselves reliably.   Machines like the Savonius are strictly drag machines and require the wind to "push" on the blades.  Two different ways to extract power from the wind.

  The wind offers the same amount of power flowing through them at a given windspeed for a given area, the real difference is their efficiencies and speed.  There are alot of variations out there for the VAWT's but none that I've seen really offer the efficiency that the HAWT's do.... until now of course. I'll elaborate on that at a later date.

   The simplest to build is the sav and parts can be almost anything from drums to sheet metal formed over plywood.  A little creativity can go along way when building one.

Lots of Fun!

.

[rotornuts]

MountainMan, I wouldn't really consider building a so called true to style Darrius. If you look at Ed's lastest design I think this is the the most promising of the recent designs I've seen. I have to say Ed's three blade vertical has alot of features that promise to make it a very good machine. I recently scaled up my Savonius variation to a 12" x 26" rotor and I'm mildly amused that it still works but in order to achive the diameters required to provide sufficient torque, material consumption Vs. efficiency are going to leave it in the dust compared to Ed's unit. I'm considering going back to the open center design to permit efficient use of materials at sufficient diameters. Two of the significant features of Ed's design is the wide cord blade and the rounded leading edge. I have always believed in the fat leading edges on the verticals and the wide cord is a natural considering the speed of operation, couple that with the cupped  collection side that presents a wide collection surface leading to a small cup that doesn't present as much drag as a traditionally shaped collector and I think we may be seeing the shape of things to come. Of course There's always more than one way to skin a cat but at the moment I'm looking over my shoulder at what the guy beside me is doing.

Bravo Ed!

Mike

[MountainMan]

Nuts,

Ed has already talked me back to sanity and out of building a large Daerieous (or however  you spell the darn thing).

My current plan is to watch his VAWT exploits and eventually build one of his to scale to learn, then scale it up just past the limits of sanity.  8^)

jp

[electrondady1]

mt.man , thats why i was curious about the dia. of your water tank. it would probably make an excellant platform.

[rotornuts]

AT the moment I'm considering a three blade variation using someting along the lines of a GOE 435 profile. I doesn't matter if I copy it exactly as it was designed for linear flight rather than in and out of the wind. It looks like a good starting point so I'll model it. I'm thinking about a closed profile because I'm still extreamely curious if Ed's success is in the cupped backs on the blades. I know Ed has done some experiments with closing the wings which resulted in power reductions but I'd like to put it to rest completely if I can. That way the focus can be placed on improved cupped wing designs if that's where it's headed. Like Ed I'll also shoot for about a 30% solidity which is I'm sure a portion of the low wind speed success.

Here's a GOE 435

Mike

P.S. Don't wait for Ed to finish up his design because if I've learned anything about Ed it's that he will continue to improve this design for as long as he can imagine an improvement. So take what is there so far and start your own experiments. It's truely fun and rewarding to fly your own tinkering. There is nothing to loose and everything to learn.

[MountainMan]

The water tank is a 10,000 gallong metal tank.  It is approx. 12 feet in diameter and roughly 13 feet tall at the "peak".  It sits near the high point of my property and up at the top of it there is essentially nothing obstructing the wind in any direction.  It has a few good tabs and other attachment points that I could easily affix a short tripod style "tower" to.  Happily, it is also located in an area of the property that has a good spot for a "power shed", so small transmission losses.  I think it's the best spot I have for my first RE attempt.

Sorry, don't have  any pictures of it at the moment.

thanks,

jp

[MountainMan]

Nuts,

If you make the blades in the shape of an airfoil, doesn't it become a darreous "by deffinition"?  A "lift" machine instead of a drag machine?

thanks,

jp

[inode buddha]

Two piece arms, yeah. Car companies are good at composite bonding, just take a close look at a newer Corvette.

If I was going to try it in the cheap, I would get some long leaf springs from a truck and flatten them out. I figure what the hey, the factory already tapered them for me!

[rotornuts]

While a Darrius is indeed a lift based machine there are lots of lift based Verticals. I don't really like the use of "brand" names to ID turbines but it's really all we have so when a design looks similar to a unit that has been given a name, either that of the inventor of the original design or that of a research institute or company, we call it a Darrius or a Savonius or a ropatec or a Gorlov etc. ED's unit uses both drag and lift  but it's not a Darrius or a Savonius or a Gorlov.

A Darrius would properly have the blades mounted to the hub at the top and bottom and curved out to a maximum diameter at half it's height, hence the eggbeater nickname. An H rotor is very similar to what ed has done but as the H suggests it's blades are mounted to a single hub at half thier height. A Savonius is a grossly overused term to describe anything that looks even remotely like the original design even if it doesn't look or work anything like the original design. If we followed this kind of logic for terming what someone has built then the closest fit for Ed's unit for example would be a "three bladed Gorlov" though I think that would be incorrect because the details say that the "Lenz Turbine" doesn't operate the same way as a Gorlov. If "Looks" dictate name thought a Gorlov it would be. Interestingly now that Ed has publicised his machine units that come along in the future that are similar in appearance regardless of differences in there operating principle will likely be dubbed a Lenz Turbine (around here anyway).

We need to be carefull how we term Verticals because this comparmentalizing them into a few narrow catagories is creating a fog with regards to how they operate. One needs to learn what's really going on with each design to avoid confusion. I understand though that the understanding is not that easy.

Mike

[Ungrounded Lightning Rod]

Hi again.

I see the others have already talked you out of trying a darreius.  But I'll answer some of your questions and make some comments anyhow to clarify some of the things I said.

Square qube law does not apply to this contraption in the way that you suggest. ...

In my case, ... as the linear dimension[] grows, the mass increases roughly with the square of the "scale", as the increasing scale is not describing an enclosed volume, but rather the length and width of some pieces out at the ends.

I was talking from the assumption that you were keeping things in proportion as you scaled up (which would actually make it flimsier as you got bigger, like a mouse scaled-up to elephant size).

If one takes precautions to reduce the weight (especially the weight that is out at the ends of the arms) as one scales it up, it should be possible to keep the weight of the relevant components (those out near the radius) quite low.  Certainly not "small darius weight times radius qubed".

But as it gets bigger the length of the unsupported span grows.  So you need more strength in the blade to support itself.  Sorry, at LEAST cube.  (And yes I'm assuming you're scaling height with radius, as you wondered about in another part of the thread.)

My point was that you were being fooled by the fact that the centripital forces per unit of spinning mass decrease with scale given the constant TSR.  Since your blades get bigger to support themselves, staying at least as proportionally thick, your forces still go up with size - and if you stay in proportion with the blades, staying in proportion with the supporting arms makes them thicken correctly to just compensate for the extra force from the extra spinning mass.

One of the problems with the darreius is the design tends to run to long hunks of blade with little support, being stressed by a flexing force whose frequency varies with the wind speed.  The result is that at some wind speed the blades tend to resonate, vastly increasing the amount they flex.  Damping resonances before they destroy things is one of the toughest problems in the design of structures, and getting it wrong leads to things like the Tacoma Narrows Bridge disaster.  (They still show the "Galloping Gertie" film in mechanical engineering classes as an object lesson of how bad things can go wrong.)  Apparently such runaway resonances is what caused several of the Darrieus projects to fail.

But the drag-type VAWTs aren't all THAT inefficient.  The Sandia Savonius gets almost 2/3s of Betz (and Ed thinks he's doing better).  So just build a Sandia half again as tall, or 23% bigger in both radius and height, and you'll beat even the best Darrieus design.

(There's an efficiency vs. TSR graph circulating that claims savonius rotors are worse.  But that's because the guy who did it accidentally swapped the labels for the savonius and prarie/"patent" windmill curves, and it's been reprinted that way ever since.)

[electrondady1]

i don't think it looks much like a gorlov mike but i agree with every thing else you said. wasn't gorlov trying to overcome the torque pulses by curving the blades in a siral?

mt man , a 12' dia. vertical is very do-able

( well , i'm hoping in about three years)

if your's was the same height as the tank 13' it would look nice. sculptural. it would be powerfull too! you could gear drive or run  alternators off it's cercumferance.  you might want to start with some thing smaller while your getting the hang of welding.

[rotornuts]

Electro. That's kind of my point. I agree that Ed's unit has straight blades rather than helical blade like a gorlov but if I recall the original savonius wasn't helical either yet my helical would be called a savonius despite this fact.

Mike