First build. Savonius style VAWT

[Elemental madness]

I couldn't find a new members introduction thread so as this is my first post I'll use this as my introduction.

I'm from South Africa, I spent 5 years off grid on solar power starting out with only two 7ah batteries and a 100w panel powering some very dim LED strips. Building up the system to what was almost comfortable. Needless to say it was a huge learning curve.

Luckily we are back on grid power, though energy generation still excites me.

After many years of being interested in wind power I finally decided to build a small prototype with the main purpose being to learn more about wind power. So far the build has been a lot of fun, and although there are many obstacles ahead in this build, I'm sure I will learn a lot that will hopefully help me in building a larger more functional turbine.

I chose to build a VAWT due to the way the wind behaves where I am. We have relatively constant wind all year, but it is very turbulent, changes direction constantly and often reaches very high speeds for short periods of time.

I am aware that the VAWTs and especially the Savonius style turbines are not very efficient, but I have no real need for extra power and this is all purely for fun.

So far I have a semi functional spinning thing.

The bearings and axle are a temporary solution just to see if the design will spin.

The generator will be my next project and I'll update the thread as the build progresses.

[Elemental madness]

Had a couple of pictures attached. They aren't showing for some reason.

[Bruce S]

Welcome aboard.

That certainly is an interesting 'mill.

Because there are people that still log into on dial-up, we have posting limits for people with less than 20 posts.
There's also a max of 5 pictures per post.
IF the vid's link did not stay attached try it again and let us know if you get away errors.

Cheers
Bruce S

[Elemental madness]

Welcome aboard.

That certainly is an interesting 'mill.

Because there are people that still log into on dial-up, we have posting limits for people with less than 20 posts.
There's also a max of 5 pictures per post.
IF the vid's link did not stay attached try it again and let us know if you get away errors.

Cheers
Bruce S

hi and thank you for the feedback.

I tried posting the video again but I keep getting a message telling me that I cannot post external links although the video does show in the preview menu.

[TechAdmin]

Welcome to the forum!

Yes, we do have limitations for external links to prevent spambots from just linking to some random garbage after somehow getting past our anti-spam measures. I'm sorry for the inconvenience, however, I bumped your post count up by 20 so that disabled your restriction - feel free to post the link to the video.

Thank you for your understanding

[Elemental madness]

Welcome to the forum!

Yes, we do have limitations for external links to prevent spambots from just linking to some random garbage after somehow getting past our anti-spam measures. I'm sorry for the inconvenience, however, I bumped your post count up by 20 so that disabled your restriction - feel free to post the link to the video.

Thank you for your understanding

Completely understandable. Appreciate the speedy assistance.
Video has been added to original post.

[topspeed]

That looks very cool. Savonius can reach 15% efficiency...yours looks like it could nudge that.

[Elemental madness]

The idea for 3 blades came from a study I read that indicated that 3 blades would result in a slightly better tip speed ratio and higher RPM when compared to 2 and 4 blade models. 

As far as I understand is that the main problem with the savonius when trying to generate electricity is the low RPM. (Other than low efficiency that is)

If the design also helps increase the efficiency of the turbine then it would be a great bonus.

[electrondady1]

welcome to the forum .
i build vertical axis mills.
yours looks good and sturdy.
what is the swept area ?
what is the dia. of your center shaft?
you can ether go direct drive or or  run a pulley system to bring the rpm of the alternator up.

[Elemental madness]

welcome to the forum .
i build vertical axis mills.
yours looks good and sturdy.
what is the swept area ?
what is the dia. of your center shaft?
you can ether go direct drive or or  run a pulley system to bring the rpm of the alternator up.

Awesome it would be great to get advice from someone more knowledgeable on the subject.

Swept area is .54 m2 or 5.8 sq ft.

It is 680 mm high and 800 mm diameter.

Center shaft is 12 mm or 1/2 inch.

The shaft is temporary and was just to match the bearings I had on hand. Will probably use a 5/8 inch 16 mm shaft.

I get about 60-70 rpm on average at ground level though I hope to push this up when lifting it up to 3m/ 10 ft.  Can't really give accurate estimates on the wind speeds.

Based on my limited knowledge and rough calculations I'm hoping for 50watts at 10m/s winds.


I would much prefer to go direct drive, but the speeds look to be a bit low for that so a pulley system will likely be my only option.

[Elemental madness]

Here are some pictures showing the design. I have the design in pdf and dwg if anyone is interested.

Some aspects were changed during the building process to simplify the build. 

Can give measurements of the final build if anyone wants them.

The last picture is simply an idea for fins to gather more wind and block some of the oncoming wind that hits the advancing blade.

[topspeed]

I couldn't find a new members introduction thread so as this is my first post I'll use this as my introduction.

I'm from South Africa, I spent 5 years off grid on solar power starting out with only two 7ah batteries and a 100w panel powering some very dim LED strips. Building up the system to what was almost comfortable. Needless to say it was a huge learning curve.

Luckily we are back on grid power, though energy generation still excites me.

After many years of being interested in wind power I finally decided to build a small prototype with the main purpose being to learn more about wind power. So far the build has been a lot of fun, and although there are many obstacles ahead in this build, I'm sure I will learn a lot that will hopefully help me in building a larger more functional turbine.

I chose to build a VAWT due to the way the wind behaves where I am. We have relatively constant wind all year, but it is very turbulent, changes direction constantly and often reaches very high speeds for short periods of time.

I am aware that the VAWTs and especially the Savonius style turbines are not very efficient, but I have no real need for extra power and this is all purely for fun.

So far I have a semi functional spinning thing.

The bearings and axle are a temporary solution just to see if the design will spin.

The generator will be my next project and I'll update the thread as the build progresses.

You have to do some study...the pitch controlled H-darreius turbines seem to very efficient...but many aren't especially if the pitch control fails or isn't there at all.

[Ungrounded Lightning Rod]

Interesting rotor.

Aerodynamics is 'way counter-intuitive, so it's hard to design without cut-and-try or supercomputer-level simulaton.  Nevertheless...

The outward-pointing inner ends of the blades look like they'd direct some of the airflow around the blades "the wrong way", so it would fight, rather than help, the rotation.

Have you tried a variation where they are pulled more to the center and end inside, rather than outside, each-other?

[Ungrounded Lightning Rod]

Darrieus designs are more efficient, but suffer from insanely high cyclic loads that tend to destroy them after a short time.  I don't know of any but maybe some Gorlov helical designs that have survived for long enough to provide decent power.  (I know the catenary "eggbeater"s, the only darrieus that was tried in California's Livermore area, were shut down and torn down years ago, leaving nothing but a plethora of three-blade horizontal-axis designs in commercial service.)

Savonius based designs are less efficient, but seem to me somewhat underrated, for a couple of reasons.

One is that there's a chart circulating - of efficiency of different turbine designs at various tip speed ratios - that had the labels of the savonius and the american multiblade ("patent") windmills swapped.  The basic two-hemicylinder savonius is more efficient than the patent, and the patent windmill is eminently practical and has pumped water - and sometimes generated electricity - across the great US great plains for 170 years or so.

Another is that the Savonius presents a rectangle, not a circle, to the wind.  So its swept area is greater for a given diameter.  I once worked out that, using the Sandia Savonius rotor's claimed efficiency, a rotor only 12% taller than its diameter, would collect the same energy as a good horizontal axis machine of the same diameter. 

The Ugrinsky rotor is claimed to have an efficiency about as good as a good horizontal axis machine - which would be the ideal, as they are approaching the Betz limit.  But I have yet to see any serious analysis of the design and (as our member Adriaan Kragten points out) wind tunnel tests can easily overrate horizontal axis designs.  They tend to restrict the spreading of the airflow, forcing more through the rotor than they'd experience in free air.

An upside to Savonius designs is that they're very sturdy and, at their low speed, don't tend to fly apart (or chop up birds).  A corresponding downside is that they're HEAVY.  On the other hand, they're simple and can be made with easily available materials.  (Auto/trailer wheel bearings can support a LOT of spinning weight.)

For converting the slow rotation to a fast spin of a generator shaft, I've always considered v-belts to be a viable option.  They're easy to obtain in many sizes, and only lose a few percent of the energy if they're kept properly adjusted.  You may be able to find a giant pulley on a salvaged large-buiding blower.

[Elemental madness]

I couldn't find a new members introduction thread so as this is my first post I'll use this as my introduction.

I'm from South Africa, I spent 5 years off grid on solar power starting out with only two 7ah batteries and a 100w panel powering some very dim LED strips. Building up the system to what was almost comfortable. Needless to say it was a huge learning curve.

Luckily we are back on grid power, though energy generation still excites me.

After many years of being interested in wind power I finally decided to build a small prototype with the main purpose being to learn more about wind power. So far the build has been a lot of fun, and although there are many obstacles ahead in this build, I'm sure I will learn a lot that will hopefully help me in building a larger more functional turbine.

I chose to build a VAWT due to the way the wind behaves where I am. We have relatively constant wind all year, but it is very turbulent, changes direction constantly and often reaches very high speeds for short periods of time.

I am aware that the VAWTs and especially the Savonius style turbines are not very efficient, but I have no real need for extra power and this is all purely for fun.

So far I have a semi functional spinning thing.

The bearings and axle are a temporary solution just to see if the design will spin.

The generator will be my next project and I'll update the thread as the build progresses.

You have to do some study...the pitch controlled H-darreius turbines seem to very efficient...but many aren't especially if the pitch control fails or isn't there at all.

I have looked at the H-darreius design, admittedly not in great detail. My main concern especially with the variable pitch design is the complexity of the build.

Factoring in the cost of the extra parts needed for the pitching mechanism, design and prototyping time, decreased reliability due to complexity and higher speeds.

It seemed to me that I could simply up scale a Savonius quite a bit to get the required generating capacity as the parts and materials are easily available and quite cheap. I also am not restricted in any way for the size of the mill as I have 3 hectares of land and no neighbours close by.

I could be very wrong though and there is a simple way around my problems.

If you know of a working design for the H-Darrieus that is reliable and more cost effective then I would love to hear about it. The design really does intrigue me and it could be very fun building one.

[topspeed]

Darrieus designs are more efficient, but suffer from insanely high cyclic loads that tend to destroy them after a short time. I don't know of any but maybe some Gorlov helical designs that have survived for long enough to provide decent power.  (I know the catenary "eggbeater"s, the only darrieus that was tried in California's Livermore area, were shut down and torn down years ago, leaving nothing but a plethora of three-blade horizontal-axis designs in commercial service.)

Savonius based designs are less efficient, but seem to me somewhat underrated, for a couple of reasons.

One is that there's a chart circulating - of efficiency of different turbine designs at various tip speed ratios - that had the labels of the savonius and the american multiblade ("patent") windmills swapped.  The basic two-hemicylinder savonius is more efficient than the patent, and the patent windmill is eminently practical and has pumped water - and sometimes generated electricity - across the great US great plains for 170 years or so.

Another is that the Savonius presents a rectangle, not a circle, to the wind.  So its swept area is greater for a given diameter.  I once worked out that, using the Sandia Savonius rotor's claimed efficiency, a rotor only 12% taller than its diameter, would collect the same energy as a good horizontal axis machine of the same diameter. 

The Ugrinsky rotor is claimed to have an efficiency about as good as a good horizontal axis machine - which would be the ideal, as they are approaching the Betz limit.  But I have yet to see any serious analysis of the design and (as our member Adriaan Kragten points out) wind tunnel tests can easily overrate horizontal axis designs.  They tend to restrict the spreading of the airflow, forcing more through the rotor than they'd experience in free air.

An upside to Savonius designs is that they're very sturdy and, at their low speed, don't tend to fly apart (or chop up birds).  A corresponding downside is that they're HEAVY.  On the other hand, they're simple and can be made with easily available materials.  (Auto/trailer wheel bearings can support a LOT of spinning weight.)

For converting the slow rotation to a fast spin of a generator shaft, I've always considered v-belts to be a viable option.  They're easy to obtain in many sizes, and only lose a few percent of the energy if they're kept properly adjusted.  You may be able to find a giant pulley on a salvaged large-buiding blower.

Maybe this can be dealt with ?

I am pulling 79 G:s already at 5.5 m/s wind and I have tested it to run insanely fast at 10 m/s...and it is still in tact.

[topspeed]

I couldn't find a new members introduction thread so as this is my first post I'll use this as my introduction.

I'm from South Africa, I spent 5 years off grid on solar power starting out with only two 7ah batteries and a 100w panel powering some very dim LED strips. Building up the system to what was almost comfortable. Needless to say it was a huge learning curve.

Luckily we are back on grid power, though energy generation still excites me.

After many years of being interested in wind power I finally decided to build a small prototype with the main purpose being to learn more about wind power. So far the build has been a lot of fun, and although there are many obstacles ahead in this build, I'm sure I will learn a lot that will hopefully help me in building a larger more functional turbine.

I chose to build a VAWT due to the way the wind behaves where I am. We have relatively constant wind all year, but it is very turbulent, changes direction constantly and often reaches very high speeds for short periods of time.

I am aware that the VAWTs and especially the Savonius style turbines are not very efficient, but I have no real need for extra power and this is all purely for fun.

So far I have a semi functional spinning thing.

The bearings and axle are a temporary solution just to see if the design will spin.

The generator will be my next project and I'll update the thread as the build progresses.

You have to do some study...the pitch controlled H-darreius turbines seem to very efficient...but many aren't especially if the pitch control fails or isn't there at all.

I have looked at the H-darreius design, admittedly not in great detail. My main concern especially with the variable pitch design is the complexity of the build.

Factoring in the cost of the extra parts needed for the pitching mechanism, design and prototyping time, decreased reliability due to complexity and higher speeds.

It seemed to me that I could simply up scale a Savonius quite a bit to get the required generating capacity as the parts and materials are easily available and quite cheap. I also am not restricted in any way for the size of the mill as I have 3 hectares of land and no neighbours close by.

I could be very wrong though and there is a simple way around my problems.

If you know of a working design for the H-Darrieus that is reliable and more cost effective then I would love to hear about it. The design really does intrigue me and it could be very fun building one.

Yes there is a simple way around it. 

In savonius..has anyone tried expanding surfaces in tail wind section of the cycle ?

[Elemental madness]

Interesting rotor.

Aerodynamics is 'way counter-intuitive, so it's hard to design without cut-and-try or supercomputer-level simulaton.  Nevertheless...

The outward-pointing inner ends of the blades look like they'd direct some of the airflow around the blades "the wrong way", so it would fight, rather than help, the rotation.

Have you tried a variation where they are pulled more to the center and end inside, rather than outside, each-other?

What you are saying does make sense, I'll definitely look into pulling the  inner ends in, maybe building a few small prototypes with varying curves.

[Elemental madness]

Darrieus designs are more efficient, but suffer from insanely high cyclic loads that tend to destroy them after a short time.  I don't know of any but maybe some Gorlov helical designs that have survived for long enough to provide decent power.  (I know the catenary "eggbeater"s, the only darrieus that was tried in California's Livermore area, were shut down and torn down years ago, leaving nothing but a plethora of three-blade horizontal-axis designs in commercial service.)

Savonius based designs are less efficient, but seem to me somewhat underrated, for a couple of reasons.

One is that there's a chart circulating - of efficiency of different turbine designs at various tip speed ratios - that had the labels of the savonius and the american multiblade ("patent") windmills swapped.  The basic two-hemicylinder savonius is more efficient than the patent, and the patent windmill is eminently practical and has pumped water - and sometimes generated electricity - across the great US great plains for 170 years or so.

Another is that the Savonius presents a rectangle, not a circle, to the wind.  So its swept area is greater for a given diameter.  I once worked out that, using the Sandia Savonius rotor's claimed efficiency, a rotor only 12% taller than its diameter, would collect the same energy as a good horizontal axis machine of the same diameter. 

The Ugrinsky rotor is claimed to have an efficiency about as good as a good horizontal axis machine - which would be the ideal, as they are approaching the Betz limit.  But I have yet to see any serious analysis of the design and (as our member Adriaan Kragten points out) wind tunnel tests can easily overrate horizontal axis designs.  They tend to restrict the spreading of the airflow, forcing more through the rotor than they'd experience in free air.

An upside to Savonius designs is that they're very sturdy and, at their low speed, don't tend to fly apart (or chop up birds).  A corresponding downside is that they're HEAVY.  On the other hand, they're simple and can be made with easily available materials.  (Auto/trailer wheel bearings can support a LOT of spinning weight.)

For converting the slow rotation to a fast spin of a generator shaft, I've always considered v-belts to be a viable option.  They're easy to obtain in many sizes, and only lose a few percent of the energy if they're kept properly adjusted.  You may be able to find a giant pulley on a salvaged large-buiding blower.

I greatly appreciate the time taken for the write-up. There is a lot of valuable info and points for me to do research on.

I havn't heard of the Ugrinsky rotor, but it does sound promising.

I have also come across multiple people stating that the wind tunnel tests are biased and does not accurately portray real world conditions, especially related to gusts that do not follow the same path as the main wind direction as well as when very turbulent winds are present.

I also like the lower speed factor, it does come with it's own drawbacks and limitations. I'm yet to see how difficult they are to overcome.

Weight is an issue and really restricts the tower height, but as you say the simplicity makes the building materials rather cheap.

I havn't thought of the trailer wheel bearing, that could really help carry the weight when up scaling the design.


V belt does seem like the way to go, they are quiet, cheap, and efficient and will probably be what I'll use, at least for prototyping.

[Elemental madness]

Darrieus designs are more efficient, but suffer from insanely high cyclic loads that tend to destroy them after a short time. I don't know of any but maybe some Gorlov helical designs that have survived for long enough to provide decent power.  (I know the catenary "eggbeater"s, the only darrieus that was tried in California's Livermore area, were shut down and torn down years ago, leaving nothing but a plethora of three-blade horizontal-axis designs in commercial service.)

Savonius based designs are less efficient, but seem to me somewhat underrated, for a couple of reasons.

One is that there's a chart circulating - of efficiency of different turbine designs at various tip speed ratios - that had the labels of the savonius and the american multiblade ("patent") windmills swapped.  The basic two-hemicylinder savonius is more efficient than the patent, and the patent windmill is eminently practical and has pumped water - and sometimes generated electricity - across the great US great plains for 170 years or so.

Another is that the Savonius presents a rectangle, not a circle, to the wind.  So its swept area is greater for a given diameter.  I once worked out that, using the Sandia Savonius rotor's claimed efficiency, a rotor only 12% taller than its diameter, would collect the same energy as a good horizontal axis machine of the same diameter. 

The Ugrinsky rotor is claimed to have an efficiency about as good as a good horizontal axis machine - which would be the ideal, as they are approaching the Betz limit.  But I have yet to see any serious analysis of the design and (as our member Adriaan Kragten points out) wind tunnel tests can easily overrate horizontal axis designs.  They tend to restrict the spreading of the airflow, forcing more through the rotor than they'd experience in free air.

An upside to Savonius designs is that they're very sturdy and, at their low speed, don't tend to fly apart (or chop up birds).  A corresponding downside is that they're HEAVY.  On the other hand, they're simple and can be made with easily available materials.  (Auto/trailer wheel bearings can support a LOT of spinning weight.)

For converting the slow rotation to a fast spin of a generator shaft, I've always considered v-belts to be a viable option.  They're easy to obtain in many sizes, and only lose a few percent of the energy if they're kept properly adjusted.  You may be able to find a giant pulley on a salvaged large-buiding blower.

Maybe this can be dealt with ?

I am pulling 79 G:s already at 5.5 m/s wind and I have tested it to run insanely fast at 10 m/s...and it is still in tact.

I am keenly following your thread. Very exciting to see such a high performance vertical mill.
 Very different approach to the savonius and will be interesting to see how it holds up with time.

The fact that you have such a small load on currently, and it has not yet come apart is very promising for when it is properly loaded.

[topspeed]

Darrieus designs are more efficient, but suffer from insanely high cyclic loads that tend to destroy them after a short time. I don't know of any but maybe some Gorlov helical designs that have survived for long enough to provide decent power.  (I know the catenary "eggbeater"s, the only darrieus that was tried in California's Livermore area, were shut down and torn down years ago, leaving nothing but a plethora of three-blade horizontal-axis designs in commercial service.)

Savonius based designs are less efficient, but seem to me somewhat underrated, for a couple of reasons.

One is that there's a chart circulating - of efficiency of different turbine designs at various tip speed ratios - that had the labels of the savonius and the american multiblade ("patent") windmills swapped.  The basic two-hemicylinder savonius is more efficient than the patent, and the patent windmill is eminently practical and has pumped water - and sometimes generated electricity - across the great US great plains for 170 years or so.

Another is that the Savonius presents a rectangle, not a circle, to the wind.  So its swept area is greater for a given diameter.  I once worked out that, using the Sandia Savonius rotor's claimed efficiency, a rotor only 12% taller than its diameter, would collect the same energy as a good horizontal axis machine of the same diameter. 

The Ugrinsky rotor is claimed to have an efficiency about as good as a good horizontal axis machine - which would be the ideal, as they are approaching the Betz limit.  But I have yet to see any serious analysis of the design and (as our member Adriaan Kragten points out) wind tunnel tests can easily overrate horizontal axis designs.  They tend to restrict the spreading of the airflow, forcing more through the rotor than they'd experience in free air.

An upside to Savonius designs is that they're very sturdy and, at their low speed, don't tend to fly apart (or chop up birds).  A corresponding downside is that they're HEAVY.  On the other hand, they're simple and can be made with easily available materials.  (Auto/trailer wheel bearings can support a LOT of spinning weight.)

For converting the slow rotation to a fast spin of a generator shaft, I've always considered v-belts to be a viable option.  They're easy to obtain in many sizes, and only lose a few percent of the energy if they're kept properly adjusted.  You may be able to find a giant pulley on a salvaged large-buiding blower.

Maybe this can be dealt with ?

I am pulling 79 G:s already at 5.5 m/s wind and I have tested it to run insanely fast at 10 m/s...and it is still in tact.

I am keenly following your thread. Very exciting to see such a high performance vertical mill.
 Very different approach to the savonius and will be interesting to see how it holds up with time.

The fact that you have such a small load on currently, and it has not yet come apart is very promising for when it is properly loaded.

Yes thanks...I am currently testing the 3 phase BAFANG 8FUN 250 WATT hub motor....and just hand cranked 18.25 volts and 0.82 amps. Seems that my self made rectifier takes at least 15-25% of the output.

I figure it is ideal for the high wind conditions for the existing wings. On slower breeze the wings have to be a tad bigger for it. The planet gear takes a heavy toll...ie the starting forces are quite significant.

[Elemental madness]

topspeed as far as I know the H-Darrieus is not self starting and if it does self start it will have very low torque at low RPM. I've been wondering about those e bike motors as generators as they are pretty hard to get locally where I am, at least affordably that is.


15-25% is a lot of loss through a rectifier, I have seen 3 phase models of the ones you have pictured for roughly the same price as the single phase ones. Might be the easiest way to minimize loss.

[electrondady1]

so often in windmill discussions the idea of wind mill efficiency of different designs  comes up.
the way i like to think of drag mills is, they may only capture 15% of the energy of that swept area,
 but it is 15% of an infinite resource .
they are easy to duplicate and easy to make larger

[Elemental madness]

so often in windmill discussions the idea of wind mill efficiency of different designs  comes up.
the way i like to think of drag mills is, they may only capture 15% of the energy of that swept area,
 but it is 15% of an infinite resource .
they are easy to duplicate and easy to make larger

Makes sense, cost to increase the size is actually very little on these types of mills. If space is no issue then there is really no reason to not go bigger instead of adding complexity to gain a bit in efficiency. Even on quite a large version the footprint remains quite small.


I must admit though that it is fun to push the limits on designs even if only to see what is possible.

[topspeed]

topspeed as far as I know the H-Darrieus is not self starting and if it does self start it will have very low torque at low RPM. I've been wondering about those e bike motors as generators as they are pretty hard to get locally where I am, at least affordably that is.


15-25% is a lot of loss through a rectifier, I have seen 3 phase models of the ones you have pictured for roughly the same price as the single phase ones. Might be the easiest way to minimize loss.

I was now informed the loss is only 2 % in my rectifier.

I got my hub motor for free..used..they don't mend them here in bike shops. It was very messy inside...I cleaned it and put new lubricant.

My old did start by itself...and I figure I wanna keep that feature in it.

[Adriaan Kragten]

In 2009 I did some reseach to measurements of Savonius rotors available on the Internet. This research is presented in my public report KD 599 which can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports. At point 2.2 of this report you find the reference to the report which I think, is the best report about Savonius rotors which can be copied for free. A conclusion in this report is that a Savonius rotor must have two buckets.

A normal 2-bucket Savonius rotor is partly a drag and partly a lift machine. Pure drag machines are the less efficient wind turbines you can make. Information about drag machines is given in my public report KD 416. A normal Savonius rotor works partly as a lift machine because there is a flow through the rotor. However, if you use three buckets and especilally if the buckets touch each other in the centre, no flow is possible and then the windmill becomes a pure drag machine with a maximum Cp of only about 0.05. A well designed Savonius rotor can have a maximum Cp of about 0.2.

A normal 2-bucket Savonius rotor has a torque coefficient which depends very much on the position of the blades with respect to the wind direction. For some position, it is almost zero. The main reason why people design 3-bucket Savonius rotors is that the torque is fluctuating less than for a 2-bucket Savonius rotor. However, the torque fluctuation of a 2-bucket Savonius rotor can also be flattened by using two Savonius rotors which are rotated 90° with respect to each other. This option also has the advantage that the wake pattern of the upper rotor has a positive influence on the wake pattern of the lower one, resulting in an increase of the Cp with about 10% because of the so called meandering effect (see KD 599 point 2.3).

[Elemental madness]

In 2009 I did some reseach to measurements of Savonius rotors available on the Internet. This research is presented in my public report KD 599 which can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports. At point 2.2 of this report you find the reference to the report which I think, is the best report about Savonius rotors which can be copied for free. A conclusion in this report is that a Savonius rotor must have two buckets.

A normal 2-bucket Savonius rotor is partly a drag and partly a lift machine. Pure drag machines are the less efficient wind turbines you can make. Information about drag machines is given in my public report KD 416. A normal Savonius rotor works partly as a lift machine because there is a flow through the rotor. However, if you use three buckets and especilally if the buckets touch each other in the centre, no flow is possible and then the windmill becomes a pure drag machine with a maximum Cp of only about 0.05. A well designed Savonius rotor can have a maximum Cp of about 0.2.

A normal 2-bucket Savonius rotor has a torque coefficient which depends very much on the position of the blades with respect to the wind direction. For some position, it is almost zero. The main reason why people design 3-bucket Savonius rotors is that the torque is fluctuating less than for a 2-bucket Savonius rotor. However, the torque fluctuation of a 2-bucket Savonius rotor can also be flattened by using two Savonius rotors which are rotated 90° with respect to each other. This option also has the advantage that the wake pattern of the upper rotor has a positive influence on the wake pattern of the lower one, resulting in an increase of the Cp with about 10% because of the so called meandering effect (see KD 599 point 2.3).

Wow there really is a lot of useful information to take in. Thank you for taking the time to compile the reports. Great website btw, I'll be spending a lot of time reading through your reports.

It makes a lot of sense regarding the two bucket design and stacking them out of phase completely eliminates my main concern with savonius namely only having two "power strokes"

[DamonHD]

It probably still makes sense to have the whole device taller than it is wide, but should each section of a stacked pair be taller than wide Adriaan?

I'd be slightly worried bending moments on the structure if it is very tall and thin.

Rgds

Damon

[Adriaan Kragten]

I can't remember now if there is a certain optimum ratio in between the diameter D and the height H of a 2-bucket Savonius rotor. But if you read the reports which I have found (if they are still available) you might find it.

[MagnetJuice]

Adriaan, the link that you have on report KD 599 doesn't work any longer.

The referenced PDF deals with the test of TWO and Three-Bucket Savonius Rotors by Sandia Labs.

It is still available here:

www.vawt.om2cm.sk/sites/default/files/2or3savonius.pdf

Ed

[CraigM]

Ed, thanks for finding that report, I was hoping it was still available.

On page 31 the 2 bucket Savonius is said to be superior to the 3 bucket. Also references height and the importance of having an air gap in the center.

Good read
CM

[Elemental madness]

Here is another interesting report relating to number of blades, though the designs used are not true Savonius with overlapping buckets, but rather purely drag based. This is the report I originally read that inspired my 3 bladed design.

https://www.researchgate.net/publication/277338974_An_Experimental_Study_on_the_Performance_of_Savonius_Wind_Turbines_Related_With_The_Number_Of_Blades

[topspeed]

Here is another interesting report relating to number of blades, though the designs used are not true Savonius with overlapping buckets, but rather purely drag based. This is the report I originally read that inspired my 3 bladed design.

https://www.researchgate.net/publication/277338974_An_Experimental_Study_on_the_Performance_of_Savonius_Wind_Turbines_Related_With_The_Number_Of_Blades

Very interesting......you might make history here.