Designing, Building and Testing a Darivonius VAWT

[MagnetJuice]

I'm starting this topic for the purpose of encouraging discussion from Forum members and hopefully guests that will join the Forum to share their ideas. The goal would be the gathering of ideas and suggestions that can lead to the design, construction and testing of a practical VAWT.

This VAWT should be:

•   Easy to build
•   Using mostly inexpensive and commonly found materials
•   Safe to use and operate
•   Robust to withstand strong winds

As I said before in another thread, a HAWT mounted on a high tower would be ideal to produce power from the wind. Not everybody has the land or the money to pay for and erect a tall tower. There are millions of people around the world that have strong surface winds and would like to be able to make a little power, and for them, a VAWT would be ideal.

What choices do we have? There is the Darrieus, patented by Georges Jean Marie Darrieus, a French engineer, in 1926. And the Savonius, invented by the Finnish engineer Sigurd Johannes Savonius in 1922 and patented in 1925.



The Darrieus needs fast winds to start turning and then they spin so fast that it is very difficult to control the speed. They usually end up self-destroying. The Savonius turns too slow to produce any power with the home made axial flux alternators that we build today.

The ideal design should be a combination of the H-Darrieus and the Savonius. By incorporating a Savonius in the center of the H-Darrieus, it will start spinning in lower winds. Then, depending on the size and shape of the Savonius wings, it would have enough drag to keep it from over-speeding, even if the wind continues to increase in speed.

Here is an image of what I envision this VAWT could look like. I am sure that other members here will have their own ideas. We can take the best features from different designs and after discussion, incorporate them into one final system that we can test.



I am willing to build the final design to experiment with and gather data. Then I will build a suitable alternator for it and run tests to collect electrical output.

I will build a powerful fan to turn it. That way I don't have to depend on the wind and the fan speed can be controlled using a variable voltage transformer (VARIAC).

I would like to make the blades on the outside able to pivot, so the angle can be adjusted about 20 degrees in or out. By changing the angle of the blade, it could be possible to help it catch the wind so it starts spinning with low winds. In addition to that, by changing the blade angle, the top speed can be controlled

I cannot build anything very big because I don't have a lot of space and my backyard is small.

I have a 1-1/4 diameter x 36 inches long solid shaft and bearings. That should be big enough to build a small VAWT to gather some useful data.

I would like this topic to be similar to a brainstorming session, where different ideas flow. During brainstorming, all ideas are welcomed and no idea is too silly or too crazy. The flow of many ideas is what stimulates people to think outside the box and be creative.

I thank you all in advance for helping with this.

Ed

[MattM]

Combining two types makes no logical sense.

[DamonHD]

There are lots of designs nominally combining the two for their different characteristics.

For example, top "Darrieus and Savonius" search result for me:

https://www.amazon.co.uk/Darrieus-generator-EOLO-3000-windgenerator/dp/B071RQ5Q17

See also, for example:

https://scholar.google.co.uk/scholar?q=Darrieus+Savonius+NREL

Rgds

Damon

[Adriaan Kragten]

I doubt the idea that the Savonious rotor can be used to slow down the Darrieus rotor at high wind speeds and a relatively low generator load. The diameter of the Savonious rotor is normally chosen that large that both rotors have the same rotational speed if they run at the optimal tip speed ratio. The optimal tip speed ratio of a H-Darrieus rotor is about 4.2 (see public report KD 601). The optimum tip speed ratio of a Savonious rotor is about 0.9 (see public report KD 599). So both rotors run about at the optimum tip speed ratio if the diameter of the Darrieus rotor is a factor 4.2 / 0.9 = 4.66 larger than the diameter of the Savonious rotor and if the generator load is large enough.

If the generator load falls off, both rotors will run about at the unloaded tip speed ratio. I think that the ratio in between the unloaded tip speed ratio and the optimal tip speed ratio is somewhat larger for the Savonious rotor than for the Darrieus rotor. So just the opposite will happen. If the Darrieus rotor runs unloaded, the Savonious rotor will still supply some positive torque and this means that the Darrieus rotor will work as a brake for the Savonious rotor.

If the diameter of the Savonious rotor is chosen larger than according to the ratio 4.66, the Savonious rotor can work as a brake if the Darrieus rotor runs unloaded. However, it should certainly be chosen not that large that the Savonious rotor works as a brake when the Darrieus rotor runs at the optimum tip speed ratio. The power which can be dissipated in a braking Savonious rotor is only limited as the swept area of the Savonious rotor is much smaller than the swept area of the Darrieus rotor. So the rotational speed of the Darrieus rotor will be reduced only a little by a braking Savonious rotor and this small reduction of the rotational speed will not be enough to make the Darrieus rotor safe at high wind speeds and at a low generator load.

A problem with a H-Darrieus rotor is that it needs a certain minimum blade chord to get Reynolds numbers which are high enough to realise drag/lift ratio's which are small enough (see KD 601). So it is useless to test everything on a small model.

[MagnetJuice]

Adriaan, thank you for you input. I respect and value your opinions.

You said:

“The optimal tip speed ratio of a H-Darrieus rotor is about 4.2”




I need to be able to understand about H-Rotors and TSR.  Here is my question.

In the picture above, is the optimal tip speed ratio of 4.2 for A, B or for both?

Ed

[SparWeb]

MJ,
The answer to your question is "both".  But getting to that answer exposes a pet-peeve of mine.

I have some doubts about some of the statements that have been made up to now.
Firstly, any idea that there is one "ideal TSR for a Savonius/Darrieus rotor" is just bunk.  There's actually a wide range where it's OK and it depends on the driveshaft loading and details in the airfoils/cups on it.
Even considering the high skill and scientific interests on Fieldlines there's not much difference between TSR=4.0 and TSR=4.4 when it comes to actually getting reliable measurements of windspeed and RPM simultaneously and well correlated.  It would be a shame if people got so focused on the math problems, without first getting a clear scope of what they want to accomplish, and choose design features such as "Power output", "Means of shut down" and "Means of self-starting" and "wind speed limit" before picking a specific set of blades and justifying whatever they picked.

Combining rotors is not a bad idea, it's just more difficult than doing single rotors. 
Below is an old pic of my attempt to combine Sav and Dar into a self-starting VAWT.  This one was about 12" x 24".  It worked as far as that goal went, but I got stuck looking for ways to scale it up into something that produced a useful amount of power.  I only wanted the Savonius in the middle to contribute to starting, but not be in the way when the Darrieus blades got turning.  I found a "clutch bearing" that allowed the Savonius to turn the entire machine when the Darrieus was stopped, but not brake the Darrieus if it wanted to turn faster than the Sav.  If the Savonius locked to the Darrieus shaft, then it would act as a brake in most wind speeds, not just strong ones.



Every VAWT I built had problems.  The one in this picture was going to need a lot of "babying" for the clutch bearing to work long-term.  It was hard to keep the friction down, which was necessary to let the Darrieus speed up to its optimal TSR.  I was going to need a different solution if I wanted this machine to resist the weather and deliver good power, especially if I scaled it up.

[JW]

Even considering the high skill and scientific interests on Fieldlines there's not much difference between TSR=4.0 and TSR=4.4 when it comes to actually getting reliable measurements of windspeed and RPM simultaneously and well correlated.

That is a very interesting way to look at it, but fieldlines is not a benchmark. At least how I see it.

We have to be careful, we are just enthusiasts building machines, that cant be said everywhere. There is not a commercial aspect... Not that the commercially aspect is bad    

JW

[MattM]

If only there was a way to do some kind of plate bearing, kind of like one of those spherical bridge bearings, to give your axle more support while not increasing drag too much.

[MagnetJuice]

I agree; trying to use conventional math for an unconventional design like this could be frustrating and the results unpredictable.

The outer blades of this machine will not be true airfoils. Only approximations that are inexpensive and easy to construct. A negative camber could be easily produced on the lower surface of the blade if necessary. That could be done by using a thicker and a thinner panel for the sides of the blade. As I said before, the outer blades I want to be able to pivot so the angle can be changed. That will give us a way to experiment to find out how different angles affect RPM at different speeds.

I figured that with the 36-inch shaft, we can have blades that are 40 inches long. The width of the rotor we will determine later. That should be a good size to play with.

The three sections in the center are not a true Savonius either. The only purpose of those center blades will be to catch the low winds to get the mill spinning. I can use polyethylene or PVC sheets for those blades. We just need to make guesstimates as to the size and positioning of the center blades and go with it.

As for a working RPM, I think that we should aim to keep it under 450 RPM with 30 MPH (13M/S) winds. Does that sound reasonable? Some type of mechanical brake would be handy to have too. That way we can stop it for servicing, or to lock it down if a storm is forecast that we know can exceed those winds.

I think a mill like this should be OK for low to mid winds. For winds like MaryB of 70 plus MPH, that would be a totally different kind of animal. If we are successful with this one, I would love to try to design something for very high winds. I already have some ideas. It would be good exercise for some of us with rusty brains. Good for our self-esteem too.

I was going to build a strong fan using a 1/3 HP motor, but today I saw a nice 30 inch industrial fan on Craigslist. That should be powerful enough and save me some work.

I'm ordering bearings for the 1-1/4 shaft too. I have some bearings already but they are ball bearings. The one's I want to order are tapered roller bearings that can take a heavier load. I found some made by Timken for the John Deere small tractors and the prices are very good.

So keep the wild crazy ideas coming, I want to get this built before Christmas.

Ed

[Adriaan Kragten]

Adriaan, thank you for you input. I respect and value your opinions.

You said:

“The optimal tip speed ratio of a H-Darrieus rotor is about 4.2”


(Attachment Link)

I need to be able to understand about H-Rotors and TSR.  Here is my question.

In the picture above, is the optimal tip speed ratio of 4.2 for A, B or for both?

Ed

The optimum tip speed ratio of a Darrieus rotor has nothing to do with the ratio in between the diameter D and the height H of a Darrieus rotor. The optimum tip speed ratio of a Darrieus rotor is defined in the same way as it is defined for a HAWT. It is the ratio in between the tip speed and the undisturbed wind speed V for which the power coefficient Cp is maximal. If you look at the design formula's for a HAWT which are given in my public report KD 35, you see that the blade angle beta and the chord c decrease at increasing design tip speed ratio. But for a H-Darrieus rotor with a symmetrical airfoil, the blade angle is zero. So only the chord and the number of blades can be varied for a certain rotor diameter.

In report KD 601 I have tried to find a theory for the optimum tip speed ratio for a H-Darrieus rotor with a symmetrical airfoil and I found that it is about 4.2. I also found a formula which gives the chord for a certain number of blades and a certain rotor diameter. I know no other theory which gives you these values. I advice to build a Darrieus rotor according to the geometry which I have found and to measure in the field if a rotor according this geometry really has an optimum tip speed ratio of about 4.2 and an acceptable maximum Cp of about 0.35.

[Bruce S]

Would it also be able to set a few realistic goals?

What do we want to call useful power?
Voltage
Current
Wattage

Good place to start when we get to the gen build

Cheers
Bruce S
 

[MagnetJuice]

Yes Bruce, the swept area of the windmill could be about 6.65 Square Ft. (.61 Sq M). That is assuming that it will be 24 inches wide, but that can change. It depends on what the more knowledgeable gurus here will determine. That has the potential to yield about 130 Watts with 20 MPH (9 m/s) winds. That could put 9 Amps into a 12 Volts battery. I could built an alternator that can output about 250 Watts. I will build the alternator in a way that the air gap can be adjusted easy. That way I can open the gap if needed to keep the blades from stalling and to match the windmill output to the alternator.

One of my building criteria is to build everything without spending a lot of money and using common materials. That is why I'm planning on using used saw blades for the magnet rotors and inexpensive ferrite magnets. I can use two saw blades per magnet rotor. I hope that would be enough metal to keep the magnets from saturating the steel.

I want this to be a 12 Volt system because I have a 100 AH deep-cycle battery, some 12 Volt Golf cart batteries and 12 Volt inverters that I can use. Anybody else that builds something like this can go with 24 Volts if they want to. For 48 Volts, I think that the windmill should be larger than this one.

Ed

[Adriaan Kragten]

Yes Bruce, the swept area of the windmill could be about 6.65 Square Ft. (.61 Sq M). That is assuming that it will be 24 inches wide, but that can change.

A diameter of 24 inches = about 0.61 m is much too small for a Darrieus rotor. The Reynolds values at low wind speeds will then be very low and this means that a symmetrical airfoil will stall already at low angles of attack. You have the largest angles of attack when the blade is at the front and at the back side (see KD 601 figure 2 position 1 and 7) and that is the position where most of the power for a Darrieus rotor is generated. So a large part the power generated by the rotor will be dissipated in airfoil drag for the part of the rotation where the blade is stalling. If you have this small rotor diameter in mind, you can better use only a Savonious rotor as a Savonious rotor has a large chord and is therefore less sensible to low Reynolds values.

The rotational speed for only a Savonius rotor will be rather low at low wind speeds and the average torque will be rather high. This means that you need a rather big and expensive PM-generator which can generate enough power at low rpm but that's the consequence when you chose for a VAWT.

[CraigM]

Going back to tip speed ratio for a bit.

TSR is the ratio of wind speed to blade tip speed for a HAWT, or for a VAWT, the outer diameter rotation speed. TSR of 10 means in a 10 mph wind the blade tip or outer diameter is traveling at 100 mph. A TSR of 4.2 as Mr. Kragten suggest would travel at 42 mph in a 10 mph wind.

Now think of rotor diameter and how that affects a given TRS. At this point it's best to use metric and instead of mph we will use meters/second or m/s.

Use this formula to determine rpm;
m/s * tsr * 60 / circumference (circumference is diameter * pi (3.14). Or 5 m/s (11.2 mph) * 4.2 * 60 / (.61 (2 feet) * 3.14) or 5 * 4.2 * 60 / 1.915 = 658 rpm

658 rpm! That seems pretty fast to me. Scale the size up a bit to 3 feet in diameter (.91 meter) and all else being the same the rpm drops to 441 rpm. Something to consider when thinking of matching an alternator.

Seat of the pants thinking tells me the aspect ratio (ratio of length to chord) will play into this as well. If designed for higher wind speeds the blades would be high aspect ratio (long with shallow cord) to reduce drag... but high aspect ratio may hamper self starting. In low winds a low aspect ratio may help to self start. Where is the middle ground compromise? I don't know.

After reading Mr. Kragtens KD 601 report about adjusting angle of attack this has me thinking as well. Need to sleep on it for now.

Thanks much,
CM

[Adriaan Kragten]

Some more about stalling. A problem with a stalling airfoil is that there is a large area in which there is hysteresis in the Cl-alfa curve. This means that if you start at a small angle of attack alpha and if you increase the angle, stalling may start at an angle of about 12°. But if the angle is large and the airfoil is stalling, stalling will only end when the angle is reduced to a much smaller value, may be about 8°. The difference in between both angles is the area of attack for which there is hysteresis in the Cl-alpha curve. This area becomes larger as the Reynolds value is lower.

Very nice research to this effect for low Reynolds values has been done by F. W. Schmitz for the Gottinger 625 airfoil. This research is presented at page 3-70A in report R-343-D: "Catalogue of Aerodynamic Characteristics of Airfoils in the Reynolds number range 10^4 - 10^6". This report has recently be made public on the website of one of the senior members of this forum. At this page, both the Cl-alpha and the Cl/Cd curves are given (Cl is called Ca and Cd is called Cw). In the Cl/Cd curves, it can be seen that the Cd values are very much higher for a stalling airfoil than for a not stalling airfoil.

Calculation of the rotational speed for a small Darrieus rotor says nothing about the Reynolds number. The rotational speed can be rather high but the Reynolds value can still be low if the chord is small and if the wind speed is low. For determination of the Reynolds value, you have to use formula 5 out of KD 601.

[DamonHD]

Here's the link should you want it:

http://www.earth.org.uk/catalogue-of-aerodynamic-characteristics-of-airfoils.html

Rgds

Damon

[Mary B]

Some ideas in this video from wind gen kits... first part is just assembly of a kit he no longer sells, end is about setting the angle of attack. And it could give you ideas for mounting the blades...

https://youtu.be/O6kfVlwsZ5M?t=460

[MagnetJuice]

When I started this topic, I was hoping to get some useful and helpful advice and that is exactly what I'm getting. Thank you very much everybody.

If 24 inches for the diameter is not big enough, then we have to go bigger. I would still want to use the 3 feet long shaft and bearings that I have, so we have to go wide. That would be OK because that will increase the torque and gives more power at a reasonable RPM. I would like to stay under 300 RPM when it is running at optimum TSR, could that be possible? I was thinking of doing the testing on my back deck, but if it has to be bigger then it would be better to do the testing out in the back yard. And by using a powerful fan to spin it, I have full control of the speed.

If we need to go bigger and wider, then the shaft will have to be supported not only at the bottom, but also at the top. We will have to build a frame to support it. If a frame like this is used, then this design could be scaled-up to produce more power just by using a longer shaft to make it taller. The frame will be something like this picture.



Considering that I'm limited in space to do this testing, what would be the minimum width for the rotor that we can use that will generate some useful power? In addition, is there any recommendations for the characteristics of the outer airfoil wings as to a particular profile?

When calculations are made, keep in mind that I want to incorporate some type of Savonius-like blades in the center. That is important to me because I want this design to be self-starting.

Again, thank you everybody.

Ed

[Adriaan Kragten]

In chapter 3 of KD 601 I give the geometry of a H-Darrieus rotor for which I think that the Reynolds number is high enough at low wind speeds. This rotor has a radius R = 1 m, so D = 2 m, a height H = 1.5 m so the swept rotor area is 2 * 1 * 1.5 m = 3 m^2. The rotor has three blades under 120° with a chord of 200 mm. The airfoil is the NACA 0015. The blade angle beta is 0°. The airfoil geometry is given in figure 1 of KD 601. The Reynolds number isn't depending on the height H of the rotor. So the height can be chosen larger or smaller than 1.5 m. However, a blade has tip losses at both blade ends and these losses are more relevant if the height is smaller. I think that the miniumum height is about 1 m. So if you make a rotor with a diameter of 2 m, there is a good chance that it works nicely (if you find a way to start the rotor and to limit the maximum rotational speed at high wind speeds).

The estimated Cp-lambda curve is given in fig. 3 of KD 610. You have to use the curve "fixed blades" if the rotor isn't equipped with a pitch mechanism. The P-n curves and the optimum cubic line of the rotor can be derived from the Cp-lambda curve using the method as given in chapter 8 of public report KD 35. However, the formulas given in this chapter are valid for a HAWT with a swept rotor area of pi * R^2. For a VAWT with a radius R and a heigt H, the swept rotor area is 2 * R * H. So the formulas for P (4.1) and for the optimum cubic line (8.1) have to be modified for the correct swept area if you use them for a VAWT.

In the P-n graph you can read the rotational speed n and the mechanical power P depending on the wind speed and the tip speed ratio at which the rotor is turning. The optimum load is the load which is following the optimum cubic line as the Cp is maximal for this line.

A Savonious rotor should not be positioned in the heart of the Darrieus rotor because the wake behind the Savonious rotor will completely destroy the flow around the blade of the Darrieus rotor when it moves at the backside. The best places seem to be at the top and at the bottom of the Darrieus rotor if you use two 2-bucket Savonious rotors which are 90° rotated with respect to each other.

[oneirondreamer]

Hello Magnet Juice,


I've got what might be some pretty controversial idea's around high solidity VAWT's, especially how they function at very low Reynolds numbers where viscosity is much more important than inertia. 

Quite a few years back I learned a lot about alternator design here, posting as oneirondreamer, however I've lost access to the email that I used back then, so wasn't able to reactive the account.  I see now that with my new account I can't post links, which is too bad as I've got some good youtube video's and instructables.    In youtube you can search Andrew Rokeby-Thomas.   

My suggestion is that you first consider the classic NREL funded research by Blackwell in the 70's, but I suggest first reading a paper by Ian Ross, "wind tunnel blockage correction factors for high solidity VAWT's" or something close to that.    You should be able to find links to them on my site ...artinventing.... though I haven't even looked at my site in a while.

After reading those you will likely recognize that the classic savonius design failed because it's likely between 5-15% efficient, and the 15% is probably quite optimistic.   There are no independent measurements, of non wind tunnel results, that suggest higher than 10%, that I've been able to find.

That said, Blackwell reported that a 3 bucket rotor had higher peak torque, but at a significant loss of rpm vs the 2 bucket, with the 2 bucket having a reported Cp of somewhere around 0.21 and the 3 bucket having around half that.   

So, if you are determined to use a Savonius for startup of your Darius, then a two bucket is likely better than a 3. 


I've recently published my construction notes for my very unusual looking Savonius type turbine on the instructables website, along with my plans for a 3D printed Axial Flux  alt (150mm magnet circle, 8 poles, 6 serpentine windings in 3 sets of two, wired in series).   

It's inspired by Barnaby Wainfan's faucetmobile , an unusual flying wing made from a series of flat planes, as well as my previous research into high solidity VAWTS.

I have not tested it yet, and hope to be doing that soon.   I'll be posting an instructable on the testing. 

A previous VAWT of mine 1 m did by 2.25m tall, based on similar idea's was tested by a consulting engineer, paid by potential investors, and found to be likely 24-29% efficient (Cp 0.24-0.29) as measured by Proney Brake dynamometer on a moving platform test bed (not wind tunnel), and stationary in measured wind on the same platform.   You can find the letter from the engineer on my website artinventing  .   I haven't found a credible 3rd party report of another VAWT of similar sized doing anywhere as well.   Darius of similar sizes (around 1m diameter), like the wind spire, and UGE/Gorlovs in testing by 3rd parties top out at around 17% in air (much better in water due to higher Reynolds numbers).   It's true that big Darius turbines, as reported by the Sandia Labs "A VAWT retrospective" found Cp's up to around 44%, but at small sizes, the Reynolds numbers are so poor, and dynamic stall such a large factor at that cord length, 11-17 is as good as commercial manufacturers are getting. 

My old website was ....artturbine...  Sadly that company was taken over by some investors who couldn't agree on anything other than they could turn it into a product without me.   It imploded about a year later, after a few years of big bills from an engineering company that was happy to jump in over their heads. 

I've got what might be some pretty controversial idea's around high solidity VAWT's, especially how they function at very low Reynolds numbers where viscosity is much more important than inertia. 

[JW]

When I started this topic, I was hoping to get some useful and helpful advice and that is exactly what I'm getting. Thank you very much everybody.

JW

[MagnetJuice]

Drew, welcome back again, hopefully you don't lose your login info this time.

I looked at some of your videos and your website. Very interesting design. I also looked for and saved the PDF's with the information that you suggested. I'll review it soon.

Your “controversial” ideas won't be controversial here. That is because we are always open to breakthroughs and new information, and we know that technology is not static. So feel free to discuss anything that you want.

Nice to have you here,

Ed

[DamonHD]

Yep, as long as you are not ignoring the laws of physics or off-topic, you're very welcome!

Rgds

Damon

[MagnetJuice]

Yes, Damon is correct. What I meant by discussing “anything”, is anything related to the topic at hand. In this case, VAWT's design, and operational parameters. For example, solidity and Reynolds numbers.

Ed

[oneirondreamer]

I've created a new topic under the newby section, for me to answer questions about my new design.   

I believe strongly in the laws of physics, though I did have a moment where some test results made me question Betz, I quickly found that my results were due to my mathematical inadequacies.   

I also agree that extraordinary claims require solid repeatable evidence.    This does leave me in a bit of a pickle, as I don't have the resources to fund a conventional research project.   As well of course there is the reality that our planet is loosing it's ability to cope with our species and anything that can help needs to get into the market as quickly as possible. 

Thus my plan to conduct the next phase of my research publicly, and to share it widely.    I have filed a USPTO provisional on it, and what I'd like to see (if it works as I hope), is that it get turned into an open source design.    A central authority could issue licences to manufacturers certifying that it's products are of the most recent research, and that manufacturers have met minimum standards, and could use the funds from licensing to continue research.   

Thanks for your time and attention.   
Best Wishes,
Drew Rokeby-Thomas

[JW]

Hey there

I have filed a USPTO provisional on it, and what I'd like to see (if it works as I hope), is that it get turned into an open source design.

A provisional patent is not published, my last patent I had a no public disclose app. I wish it was that the application did publish. My other three patents have not paid maint fee so there are in the open source as of now.

JW   

[oneirondreamer]

Hey there

I have filed a USPTO provisional on it, and what I'd like to see (if it works as I hope), is that it get turned into an open source design.

A provisional patent is not published, my last patent I had a no public disclose app. I wish it was that the application did publish. My other three patents have not paid maint fee so there are in the open source as of now.

JW

It's my understanding that provisional patents are only published if they are used as the basis for a utility application, otherwise they are not published.   I've filed to have priority, in case someone else were to try to patent it from under me, because it's pretty cheap and easy to file a provisional and because it seems like a good idea :-) .   

Once I've done some more research on the shape, I'll publish more about it.    I'm just about finished my truck mounted test system, so I should have a power curve soon.   

Thanks for your interest, I hope it'll be worth your time.

Drew

[MagnetJuice]

A Savonious rotor should not be positioned in the heart of the Darrieus rotor because the wake behind the Savonious rotor will completely destroy the flow around the blade of the Darrieus rotor when it moves at the backside. The best places seem to be at the top and at the bottom of the Darrieus rotor if you use two 2-bucket Savonious rotors which are 90° rotated with respect to each other.

What about placing 2 cups at the bottom close to the ends, as in this image. Would that be better than in the center?



I would prefer not to place it on the center top if I don't have to.

I also found this photo of a hybrid Darrieus-Savonius that has been operating in Taiwan for a few years.

I wonder if anybody has information on this turbine.


Ed

[oneirondreamer]

These combined rotors have been around for a long time, I think since the 70s, and I've never seen any data on them.   Absence of evidence for them working doesn't mean they don't work, but it does mean they don't likely work as if they did have good performance results then a manufacturer would likely be waving them around.

I agree with earlier posts that conclude the mixing of 2 different turbines, with very different operating conditions, doesn't seem likely to result in a good working system.  From what I've been able to see it doesn't look like Darius turbines really work well until they get to sizes where the blade chord is greater than 20cm or so.    I'd love to see good 3rd party data on a small Darius system, but the fact that it's hard to find says something too.

[Adriaan Kragten]

My report KD 599 about Savonius rotor has been changed. At point 2.8, a link has been added to the report: "Wind tunnel blockage corrections: an application to vertical-axis wind turbines" from Ian J. Ross. This report shows that wind tunnel tests for VAWT's give much too high Cp values because of tunnel blockage if the model is too large with respect to the cross sectional area of the wind tunnel. This report supports my experience that scale models of wind turbines have to be measured in front of an open wind tunnel such that the wake can expand just as it also happens in real wind. Another option is to position the rotor high enough above a car which drives at a certain speed when the wind speed is zero.

[oneirondreamer]

I was surprised to see that a Darivonius VAWT is commercially available, and not only that it's performance has been certified by SWCC, so it's performance numbers should be reliable!    It's quite interesting to see that it's actually achieving Cp 0.2  .    It's also interesting to see how much power is drawn when it's not in operation.      I expected it to be around Cp 0.1   

http://smallwindcertification.org/wp-content/uploads/2019/07/Summary-Report-18-02-20190510.pdf

It still seems like it'll suffer from the reversing stresses on it's Darius blades, and probably fail at the blade roots, but interesting

[Adriaan Kragten]

What about placing 2 cups at the bottom close to the ends, as in this image. Would that be better than in the center?

If you do it this way, you are not adding a Savonious rotor but a pure drag machine to the Darrieus rotor. A pure drag machine has a much lower maximum Cp than a Savonius rotor (about 0.05) and the optimum tip speed ratio is also much lower (about 0.15). Drag machines are explained in my public report KD 416.

If the buckets are placed at the given position in your picture, they will result in only a very low rotational speed for a certain wind speed which might be too low for the Darrieus rotor to reach the tip speed ratio where the Cq of the Darrieus rotor becomes positive. But even if the Darrieus rotor would start, the drag machine will run at a tip speed ratio much higher than the unloaded tip speed ratio for a drag machine and so the drag machine will work as an aerodynamic brake for the Darrieus rotor.

[Adriaan Kragten]

In my public report KD 416 I have derived that the maximum Cp of a pure drag machine is about 0.05. I have used half hollow spheres as drag bodies because the difference in between the drag coefficients, depending if the hollow or the convex side of the cups is facing the wind, is maximal for these geometries. The drag coefficients are respectively 1.42 and 0.38 so the difference is 1.04.

If you use half hollow cylinders, the difference in between the drag coefficients is smaller and the maximum Cp is therefore even lower. The drag coefficient of a half hollow cylinder depends on the ratio H/d in between the height H and the diameter d of the cylinder and is smaller as H/d is smaller. This is because of tip losses at both open ends of the cylinder.