DUAL OR TWIN ROTOR CONCEPT

[Usman]

I was wondering how a dual rotor wind turbine would function, presuming the yaw mechncism is an active one (motorized).The intentions are space-saving and savings on tower, foundations etc. Presuming that the dual rotor turbine is in fact two turbines on a single tower consisting of two independent rotors, drive-trains, generators etc. They only share the turbine chassis-frame and tower. I know that a couple of successful trials have taken place recently. Such a project:

1. - Eotheme from Switzerland (www.eotheme.com)
   
   

I am particularly interested in the Swiss company's concept.

Usually in such a concept, the two rotors turn in the opposite direction to each. The 2nd rotor usually suffers inefficiency, but, I think it's efficiency can be improved by adjusting one or more of the following parameters:

A) No. of blades on the 1st and/or the 2nd rotor e.g. two blades on the 1st and three to five blades on the 2nd rotor.

B) Size of Blades, the 2nd rotor being larger than the 1st rotor.

C) Increasing the horizontal distance between the two rotors. (However, this is not always possible and usually always infeasible).

D) Adjusting the rotor's angle with respect to each other e.g. 1st rotor being at 97 degrees horizontal to the ground and the 2nd at 87 degrees.

E) Such a concept is more applicable to larger 3KW+ turbines (slow turning rotors) rather than the smaller ones that turn at fairly high speeds and have more vibration and alignment issues.

I know that the 1st rotor usually steals away most of the power in the prevailing winds and the 2nd rotor experiences, perhaps 40% of the originally prevailing winds, but I think this could be improved, in fact, if designed in a clever way, supersede the efficiency of the 1st rotor. i.e. by decreasing the size of the 1st rotor, lets say 40% than the 2nd rotor, so that the wind may be augmented/amplified before approaching the 2nd rotor, and the 2nd rotor could experience 100-130%() increase in the ambient winds! But this would be accomplished if vibration-issues are carefully handled.

Does anyone have a good engineering or at least an Imagineering to such a concept?

My proposed design:

The 1st rotor consisting of two blades, 30% smaller than the 2nd rotor, angled at 95 degrees to the ground, with independent drive train, including the generator: and the 2nd rotor being 30% larger and consisting of three blades, angled at 80 degrees to the ground. The yaw mechanism in such a turbine has to be active.

Any suggestions or comments?

Thanks.  

[luckeydog]

this website has a lot of info on this subject.

http://www.dualrotor.com

I like the idea i think it has it's good and some downfalls

But I think the good out weighs the downfalls

[vawtman]

I thought hawts were so efficient ,so why would you need 2 or more aligned to the wind?Honestly i think these ideas are stupid and wonder how they would react when things get wacky.An opinion.

[luckeydog]

Great inventions have come from stupid ideas.

so to call any idea stupid, Is stupid.

.

[jimjjnn]

Stupid ideas are in the same category as stupid questions.The only stupid questions are the ones that are never asked.

Lots of great things have happened due to stupid ideas.

[coldspot]

My Imagineering to such a concept.

My proposed design:

A normal dual rotor air gap main turbin,

but for the tail or vain, a mini jumbo using a stepper motor and clear lexan for the outer bottom half of the wind block for the blades. This tail would be a normal gravity furling set-up but also have wires running back to tower top.

 This extra power could be used for LED lights only or added to the total.

 The tail I've built, one small one as a test but didn't have a place to test and a few parts have been borrowed for other things.

LOL

$0.02

[TomW]

jim;

Not to argue the point, but....

There is always a "but..."

Just because you think it up does not mean it "works". If any idea cannot stand up to some critical dissection then I guess its not such a good idea? Or something.

Personally, I feel there are lots of stupid ideas, stupid questions and stupid supporters of them. I usually try to either stay out of it or just state facts [as I see them]. Then we get to the laziness factor. If your question or idea has been tried or asked many times previous then it is probably an "ignorant question" which I tend to lump in with "stupid". We all know ignorance can be cured but stupid is forever.

In this case, show me a real one that has been in service awhile and I may alter my opinion, but until then, I reserve the right to remain skeptical on unproven hardware.

Frankly this site he posted in the original story looks like a wizard with PhotoShop cooked it up.

You can have your ideas, I want to sse hardware in use. For this site [fieldlines] and the DIY type these "innovations" are probably beyond our reach in a guys garage. Not to mention highly unproven.

Just my opinion.

If I had any sense I would have put this story in the dungeon. Next folks will be pointing to sell samms dingbat ideas. That was another guy giving windpower a bad name with his "sky snake" and inflated claims.

Plus, most of these "cool new technology links" come from brand new users and that leads me to think they are either from the companies being pimped or have some vested interest in said company. Most have no actual product, just ideas and a need for investors. Improper use of this forum, IMHO. Just being honest here. The opposition may open fire ...

[end rant]

T

[asheets]

Seems like this concept would be simple enough to test on a smaller scale...  grab a dual-shaft Ametek, 2 sets of 2-bladed PVC hubs, and a couple of flex shafts.  Use a standard jerry-bladed Ametek as a control and diff the two in terms of performance.

From a meteorological stance, this concept is fairly well proven (at least from a measuring standpoint).  Look up the term "trivane anemometer".

[whatsnext]

The swiss mill is really nothing like the Selsam mill, which actually makes sense, other than the idea of using more blades. The swiss mill could be made much simpler by ditching the counter rotating mechanism and just putting a six blade prop on any normal mill. Selsam's idea actually takes advantage of the economy of using smaller blades by increasing the swept area the small blades see.

John........

[Usman]

But what you think if the swiss mill's 2nd rotor was to have it's own drive train, generator,  it would surely spin and produce some power, no matter 30-40% less compared to the 1st rotor, keeping in view the 2nd rotor is 20-25% larger than the 1st one?

It may even do better than the 1st rotor, perhaps due to the wind amplification factor produced by the 1st smaller rotor.

I suppose wouldn't really know until try it out in realtime...Just imagineering now!

[whatsnext]

What do you think if we had a thousand mills in a row or even a million? Wouldn't the "wind amplification factor" eventually produce enough power to cause world peace? There is no increase in swept area so why would you think you'd get any extra power or even enough to overcome all the mechanical losses?

John...

[Usman]

I agree, but the concept under disucssion here is a turbine with completely independent drive trains (shaft, brake, generator, gearing etc), not sharing the generator to increase the torque. unless, if we develop a drive-train so that the 2nd rotor turns the generator body in a direction opposite to that of the 1st rotor, which is turning the shaft. Slip rings would need to be added in this case to tap the power out from the generator. Complicated and prone to more mechanical failures.

What you mentioned about the trivane effect is interesting. I thinkk that would simplify the conceptulization of this dual rotor wind turbine. It would be useful to briefly describe this "trivane" phenomenon, as the google search brough up some complex stuff.

I really think this would work, and prototyping of such a wind turbine is not hard to accomplish in a modest workshop. The size and no. of blades and their sequence of which rotor comes on 1st is also not hard to try, only one piece of three and another of two bladed hub or five bladed hub with removeable root extenders, and a random sequence of trying them on two wind turbines stacked back to back would show required results. However, measuring amps/volts at a wind speed and storing the historical data is important part of this project, so as to compare the output performce of each set.

[Usman]

We are imagining not one generator with dual shafts, but TWO INDEPENDENT WIND TURBINES on the same nacelle or chassis frame turning counter to each other with 1st rotor havving smaller size blades as well as smaller number of blades, say 2 with respect to the 2nd larger rotor with more blades, say 3 or 5, to avoid turbulance, each rotor having it's own swept area.

[asheets]

I also considered at one point 2 sets of 3 blade contrarotating props -- 1 turning the rotor, and 1 turning the stator in the opposite direction.  You'd need brushes and goodness knows what else to accomplish that, though.

A trivane anemometer (or, more accurately, a trivane aerovane) is simply a device that measures the 3-dimensional velocity and direction components of wind.  If somebody were "REALLY" hard-core about capturing as much power as possible from wind, then you'd use one of these to determine the exact direction the wind was coming from, then point the wind turbine in exactly that direction (for instance, 10 degrees W from magnetic north, 7 degrees up from the horizon). A place with lots of onshore/offshore breezes would be a good candidate for something like this.

For the life of me, I can't find a picture of a trivane online.  However, there are two ways to visualize one:

1. take three of these -- http://www.uwsp.edu/geo/faculty/ritter/images/atmosphere/instruments/aerovane_NSSL_small.jpg -- stick one pointing north parallel to the horizon, another one pointing west parallel to the horizon, and another one pointing straight up.  You'd have to use a bit of triganometry to determine the exact wind vector.
   
2. take one of these, put it on a ball joint, and balance it perfectly.  It should have +- 90 degrees of roll, +- 90 degrees of yaw and +/- 180 degrees of pitch.  Properly set up, you will get the exact wind vector just by looking at it.
   

[whatsnext]

Then have it because you have a good imagination. Just don't ask me to loan you any money because the answer will be NO. Really, you should try the thousand in a row model. If two is good more will be better.

[Flux]

Trimble Windmills ( uk 1970?) Used 3 and 5 blade rotors based on sailwings.

It worked perfectly well and may have been justified by the extra effective rotational speed with the permanent magnets of the time. The slip rings and messy bearing arrangement were a nuisance.

Flux

[Usman]

The concept of one wind rotor turning the generator's rotor and the 2nd one turning the stator in the counter direction is very efficient, econmically as well as technically, but in real world, is not easily achieveable.

With magnets getting cheaper these days, it's probably more efficient to just install another generator.

It would be interesting to hear your ideas as how you imagined your dual-rotor counter rotating turbine to operate? advantages & disadvantages?

 

[Ungrounded Lightning Rod]

A trivane anemometer (or, more accurately, a trivane aerovane) is simply a device that measures the 3-dimensional velocity and direction components of wind.

The professor at the U of Michigan that was building them back in '64 or '65 called them "vector anemometers".

[vawtman]

Luckeydog

 Thanks for confidence boost.Im probably the king of bad ideas.Maybe the s word was a little harsh.But i dont mind being called it even if its turns true or not.

 Learn from it and move on.

[Ungrounded Lightning Rod]

The professor at the U of Michigan that was building them back in '64 or '65 called them "vector anemometers".

Hmmm... Now that I look at the linked image it may not be the same device.

His was three styrofoam helical blades on three axes - one vertical, one N/S, one E/W, each with a tachometer generator (small DC motor with great bearings).  The three blades were mounted each about a foot from a tiny central metal cube that sat on the top of the pole.

[coldspot]

as how my imagined dual-blade set counter rotating turbine.

The concept of one wind blade set turning the generator's front magnet plate and the 2nd one turning the rear magnet plate in the counter direction.

While the stator stays in place, no slip-rings required.

But this would still be hard to have an auto furling.

I'm still thinking that the tail turbine idea is better!

$0.02

[mustang19432001]

Lets also keep in mind the concept of wind shadows. By this I mean as the wind strikes the turbine it slows down and spreads out. This happens even before it reaches the turbine. Just like in stock car racing you see a number of cars bumper to bumper the front car takes the brunt of the wind and the airflow goes across the tops of every car behind it. Geese are another perfect example, the lead goose takes the brunt of the wind.

Now with a smaller turbine in front and a larger one in back the larger turbine requires more wind to move the greater mass of the turbine than that of the smaller one. In addition with the larger turbine partially in the wind shadow of the smaller one you might end up with a descent small turbine and a larger one with turbulant slow moving wind to turn it.

Another way to look at it is the foils most every tractor trailer truck has. Slightly different topic, but similar concept. The trailer is in the wind shadow of the truck to increase the aerodynamics of a vehicle that has the aerodynamics of a brick.

As you can probably tell I like to use examples, haha.

Air flow in itself is an even smooth motion, granted there are no obsticles in its path. So when you start adding obsticles you start seeing turbulance. Turbulance is when there is less to no fluid flow, it takes up more space. So when wind strikes an obsticle the wind not only goes around, but the air behind it expands... when the wind passes the obsticle the turbulance pushes the wind out farther, hence the wind shadow.

OK... I've rented on long enough. There's a possibility I may not be entirely accorate, or I may not be precise enough. If the dual rotor idea works, not only will I be surprised, but I'll be impressed. Like anything else we prove or disprove theories with experimentation. If you think it'll work, and you can prove me wrong, more power to you. Good luck. And yes I'm new to posting so if I've insulted anyone I offer my appologies.

[TomW]

Mustang;

And yes I'm new to posting so if I've insulted anyone I offer my appologies.

Seems around here stating the facts as you see them that go contrary to any "new idea" is frowned on by some but encouraged by many others. I say bring it on, if the idea cannot stand up to some scrutiny then its likely a waste of time anyway! I, too, like to use examples explaining things. You picked a couple good ones most of us have at least observed casually. Semi rigs do the same thing let one truck bust the wind then others [sometimes long strings] slip in behind and avoid that wall of air on the nose. I know from experience the engine needs to make lots less power once you slip into the sweet spot. Another thing to notice is when a big rig goes by you can see the turbulence in the road dust, rain or just the wind as it goes by.

By all means, please continue with reality check postings.

Cheers.

TomW

[asheets]

I still can't find any pictures online (how hard could it possibly be to find a pic outside of a hardcopy textbook !!!!), but I did look up the names of the big 3 used by the NWS:

1. The Climet Axiometer
   
2. The Meteorology Research Vectorvane
   
3. The R.M. Young/Gill Bivane
   
   

I think this -- http://130.226.17.201/extra/web_docs/capel/photo_03.gif -- is a picture of a vectorvane, but it is missing the horizontal axis wings.

[finnsawyer]

"Lets also keep in mind the concept of wind shadows. By this I mean as the wind strikes the turbine it slows down and spreads out."

Not exactly true. The wind doesn't slow down.  The presence of an object does cause a region of higher pressure in front of the object.  This, in turn causes the air flow to change direction, and actually speed up as it flows around the object.  Nature does this to keep the air mass moving and not stalling.  The faster moving air as it passes the object creates a lower pressure region behind the object, which in turn causes air to swirl behind the object.  This swirling air, in turn, lowers the pressure behind the object, which results in the familiar force on the object that we experience.

As far as the wind turbine is concerned, the hub will exhibit this effect, but there is no reason to assume the working part of the blades needs to.  The Betz Limit implies that the air leaving the turbine must have a velocity 74% or higher of that of the unimpeded incident air.  Any slow down of the air before it hits the blades will show up as a drop in efficiency.  So, what is the best efficiency that one can achieve with airplane style blades?  While there is room for this type of an efficiency loss when going from 35% as found here to the Betz Limit, one must first account for all other losses, such as due to drag or tip vortices.  

Actually, the most direct way to determine if there is a drop in wind speed just before the blades would be to measure air pressure just before the air reaches the blades in a wind tunnel scenario.  

   

[Usman]

Mustang,

Your comments were very useful, at least they lead to a very satisfactory follow up response from GeoM. What GeoM says, in my opinion, is in line with thermofluids principles rather than personal opinions or imagineering.

I still think that the 2nd rotor would loose only 25-35% efficiency advantage, being on the back of the 1st rotor, but that may also change if we increase or decrease the distance in between the two rotors.

Any further comments would be appreciated.

[Usman]

Mustang,

Your comments were very useful, at least they lead to a very satisfactory follow up response from GeoM. What GeoM says, in my opinion, is in line with thermofluids principles rather than personal opinions or imagineering.

I still think that the 2nd rotor would loose only 25-35% efficiency advantage, being on the back of the 1st rotor, but that may also change if we increase or decrease the distance in between the two rotors.

Any further comments would be appreciated.

[Usman]

GeoM,

Very useful information. So how we may draw up a conclusion on this? What efficiency drop you may judge for the 2nd rotor? What could be donw to improve it's efficiency?

As I mentioned on, shuffling the:

1. no. of blades; as an example: the 2nd rotor may have three blades as the 1st one, or may even have five blades to handle turbulent flow of wind prevailing past the 1st rotor or vice versa, or another sequence of arrangement
   
2. size on the two rotors; the 1st may be smaller than the 2nd rotor or both similar sized! This may also include the front turbine to be larger in power-rating than the back one or vice versa. So, the 1st turns slower than the 2nd, causing less turbulence to the 2nd rotor, or vice versa, or any other set of arrangement.
   
3. mounting (tilt) angle of the rotor; both being parallel-aligned to each other, or tilted against or in parallel to each other, or any other  arrangement.
   
4. Different power ratings on the 1st and the 2nd rotor, this is already explained in bullet point 2.
   
   

One or all of these may achieve effective gains in efficiency. Also, the distance in between the two rotors i.e. the length of the nacelle may play important role. But this may not be cost-effective to increase.

Any practical tips & suggestions would be helpful.

Thanks.

[rotornuts]

Usman,

I'm not inclined to deride the idea of multiple rotors sharing the same axis of rotation but I don't see the point.

I can't help but think a properly designed single rotor plane is more than enough to accomplish let alone synchronizing multiple rotors.

If said rotors are not matched to the conditions they incur then an inefficiency will be realized as a matter of simple physics let alone aerodynamics. The idea of separate shafts on the same plane helps solve the problem but I think the best solution is to not do it.

I can imagine the complexity one would have trying to predict the behavior of the air as the down wind rotor will see it and envisioning the turbulent state I don't imagine any real efficiency coming form it.

All this comes from a guy who has advocated single blade rotors and horizontals so I don't generally dismiss seemingly ridiculous ideas that easily but I put this one in the  I'm not even interested in trying it category.

Mike

[finnsawyer]

The speed of the air leaving a wind turbine at the Betz Limit can be no less than 74% that of the incident air speed.  For a turbine at 35% efficiency it can be .866 times that of the incident air.  That doesn't leave much room for spilling air due to a build up of pressure in front of the blade plane.

The fundamental problem with the second rotor is the same one that leads to the Betz Limit: The air leaving the turbine can not pile up.  Going from .866 to .74 times the incident air speed could work with 35% efficient turbines, as the second turbine would then lower the wind speed to the desired 74%.  If the turbines are more efficient than 35% then speeding up the air before it reaches the turbines, say via a Venturi effect, would allow a much larger overall drop in speed.  For turbines operating at the Betz Limit the incident air speed would need to be increased by 35%.

Is the flow turbulent behind the blades?  If operation was at the Betz Limit one would have to say no, as all the air leaving the turbine must have exactly the right velocity everywhere.  How does the operation of a 35% efficient turbine differ from this.  Probably not very much.  In any case putting the second turbine right behind the first would minimize any effects.  For instance, the blades of the second turbine would tend to break up the tip vortices of the first.  Any differences in the speed of the air coming off of the first turbine that might cause turbulence would also tend to be averaged out by the effects of the blades of the second turbine.

Note: I tried to post this yesterday, to no avail.

[chadking]

This is my first post here, though I've been visiting fieldlines and otherpower for several years.  I see you received mixed responses on the details of the setup mentioned.  I have not worked hands on with counter rotating wind turbines, but I have investigated and worked with counter rotation on propellers for rc toys, small UAVs and aircraft.  There are indeed pro's and con's to such a setup, especially on aircraft where weight is critical.  But when used properly, counter rotating props exhibit substantial efficiency and controlability benefits over a single propeller in aircraft.

--------

Contra-rotating (or Counter-rotating) propellers use a second propeller rotating in the opposite direction immediately 'downstream' of the main propeller so as to recover energy lost in the swirling motion of the air in the propeller slipstream. Contra-rotation also increases power without increasing propeller diameter and provides a counter to the torque effect of high-power piston engine as well as the gyroscopic precession effects, and of the slipstream swirl. However on small aircraft the added cost, complexity, weight and noise of the system rarely make it worthwhile.

-  http://en.wikipedia.org/wiki/Propellor

--------

As my interests in wind energy have increased, I've been realizing all the similarities between aircraft and wind turbines...and through my many late night searches across the internet, I've came across some interesting related information.  

Take a look at this PDF:

http://www.jstage.jst.go.jp/article/jsmeb/49/2/450/_pdf

Like everything, you have to be careful what you take from someone analyzing their own creation, and there's some hype in there.  But it seems to offer some benefits.  They use a co-axial style, counter rotating setup.  One drive shaft runs inside the other, and each powers it's own generator. The second prop is rotating in the opposite direction as the first one, and it's cashing in on the swirling motion of the air as it interacts with the first prop.

Someone responded to your post proposing you try 1000 props in a row.  This is not what is being suggested here.  I am just looking at the advantages of a second prop turning the opposite direction of the first.  Nor did anybody say they wanted to get more power out of the air than is possible with a given swept area.  We are just looking to increase the % of power that we pull from that given swept area.

Here's some more information regarding counter rotating wind turbines:

--------

Counter-rotating horizontal-axis turbines

Counter rotating turbines can be used to increase the rotation speed of the electrical generator. As of 2005, no large practical counter-rotating HAWTs are commercially sold. When the counter rotating turbines are on the same side of the tower, the blades in front are angled forwards slightly so as to avoid hitting the rear ones. If the turbine blades are on opposite sides of the tower, it is best that the blades at the back be smaller than the blades at the front and set to stall at a higher wind speed. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower. To reduce sympathetic vibrations, the two turbines should turn at speeds with few common multiples, for example 7:3 speed ratio. Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds.

Appa designed and demonstrated a contra rotor wind turbine in FY 2000-2002 funded by California Energy Commission. This study showed 30 to 40% more power extraction than a comparable single rotor system. Further it was observed that the slower the rotor speed better the performance. Consequently Megawatt machines benefit most.

--------

There are also some other dual rotor setups I'm sure some of you are aware of...these are two roters on a single shaft, sparated by a good distance, and the shaft is angled.  This effectively increases swept area, and the makers claim that the disturbed air coming from the top of one prop gives a similar boost effect to the bottom of the next prop, since the those parts of the props are travelling in opposite directions.  Here's some more info:

---------

Co-axial, multi-rotor horizontal-axis turbines

Two or more rotors may be mounted to the same driveshaft, with their combined co-rotation together turning the same generator -- fresh wind is brought to each rotor by sufficient spacing between rotors combined with an offset angle alpha from the wind direction. Wake vorticity is recovered as the top of a wake hits the bottom of the next rotor. Power has been multiplied several times using co-axial, multiple rotors in testing conducted by inventor and researcher Douglas Selsam, for the California Energy Commission in 2004. The first commercially available co-axial multi-rotor turbine is the patented dual-rotor American Twin Superturbine from Selsam Innovations in California, with 2 propellers separated by 12 feet. It is the most powerful 7-foot diameter turbine available, due to this extra rotor.

http://dualrotor.com/

---------

Alright, that's enough info for tonight. I haven't had time to do any testing myself yet, but it seems like a niche that could use some more exploring.  I hope this sheds some light in that direction.

As is said a lot around here...Have fun!

Chad

[TomW]

Chad;

I was kind of with you right up until you brought that huckster SelSam into it. Oh and cited Wickedpedia as a source of honest info.

The guy has set back windpower a good ways with his rediculous claims and car salesman attitude.

He is one of those people I would not believe if he said water was wet and the sky was blue.

Looks to me like you signed up about 2 seconds before this post and all the veneer is simply to cover the fact that you are pimping for Dougie.

Phht.

No Cheer.

T

[chadking]

Sorry to throw that in there then.  I have just seen that design pop up a couple times, and it is a definitely a dual rotor setup to compare to, which is what I thought this thread was about. I believe counter rotating blades are a way to increase the effeciency pulled from the air. I have no idea about the Selsam thing, but they use similar terminology on their write ups so I thought I'd would include it for dicussion.

I have no idea who Dougie is..LoL.    

I did sign up right before I made the post..I finally found a thread that I felt I could contribute some valid info to, on a subject I feel has merit, and figured I'd waited long enough to make a post.  

Say whatever you want about Wikipedia.  I cited it because it did a good job of stating what I wanted to say.  It is definitely not perfect, but neither is anything else.  And it does hold an enormous amount of information, much of it valid.

----

Wikipedia vs. Britannica

For its study, Nature chose articles from both sites in a wide range of topics and sent them to what it called "relevant" field experts for peer review. The experts then compared the competing articles--one from each site on a given topic--side by side, but were not told which article came from which site. Nature got back 42 usable reviews from its field of experts.

In the end, the journal found just eight serious errors, such as general misunderstandings of vital concepts, in the articles. Of those, four came from each site. They did, however, discover a series of factual errors, omissions or misleading statements. All told, Wikipedia had 162 such problems, while Britannica had 123.

That averages out to 2.92 mistakes per article for Britannica and 3.86 for Wikipedia.

- http://news.com.com/Study+Wikipedia+as+accurate+as+Britannica/2100-1038_3-5997332.html

---

No harm no foul, and I know we live in a cynical world, but lighten up, man. :-) What you see is what you get. I don't use veneer.  

Peace,

Chad