Multiple rotors on a wind turbine

In summary: Having a long axial shaft that extends a long way past the bearing housings would have greater stresses - huge stresses I would think if 3 were mounted like the model pictured. One ahead and one behind the gearbox, paired on a fixed shaft would be the simplest high strength arrangement but independent rotation would probably be needed. But would two deliver more power than a single rotor? As I understand it a wind turbine's maximum wind energy to rotation efficiency is near 60%, but closer to... I don't know, that's a difficult question to answer. I think it would depend on the design and how the additional rotors were configured.
  • #1
unseeingdog
16
2
Could multiple, smaller rotors on a wind turbine (on the same shaft) be more effective than a single, larger rotor? Why do most wind turbines have only one big rotor? I was thinking about doing a science fair project on this topic, but I have been unable to find a conclusive answer to this question online.
^AB355758EFFE2B61D070B204BC0257AF8CFF9EC77199633E78^pimgpsh_fullsize_distr.jpg
 
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  • #2
What do you mean by multiple rotors? Can you make a sketch and post it here using the UPLOAD button?
 
  • #3
anorlunda said:
What do you mean by multiple rotors? Can you make a sketch and post it here using the UPLOAD button?
Done. I've uploaded the sketch. Sorry if it's a little ugly though, I was just trying to show what I meant.
 
  • #4
unseeingdog said:
Done. I've uploaded the sketch. Sorry if it's a little ugly though, I was just trying to show what I meant.

Try again, I see no sketch. After uploading you must press the FULL IMAGE button.
 
  • #5
I can think of at least 2 reasons that they are not configured that way. Can you maybe figure out what those two issues would be? :smile:
 
  • #6
anorlunda said:
Try again, I see no sketch. After uploading you must press the FULL IMAGE button.
Weird, I saw the image and now it's gone. It shows 3 rotors mounted coaxially.
 
  • #7
anorlunda said:
Try again, I see no sketch. After uploading you must press the FULL IMAGE button.
Ok, now it should be there
 
  • #8
berkeman said:
I can think of at least 2 reasons that they are not configured that way. Can you maybe figure out what those two issues would be? :smile:
Not really... Perhaps the rotation of each rotor would slow the others down?
 
  • #9
Think about it. Is is likely that the world's best wind turbine designers have not already considered all possible variations on design and selected the best. It would be very hard for me or you to think of something they have not already considered and rejected.

Here's some advice for your project. Do a project on all the designs that have been considered and rejected. What are the criteria for "best" design? Use this wikipedia article as a source. https://en.wikipedia.org/wiki/Wind_turbine_design

You can also be scientific and explain Betz's Law as part of your project. https://en.wikipedia.org/wiki/Betz's_law as part of your project. That's the real reason why multiple rotors aren't good. The designers are already close to Betz's Law performance without multiple rotors.
 
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  • #10
unseeingdog said:
Not really... Perhaps the rotation of each rotor would slow the others down?
@anorlunda said it pretty well. The "shadowing" effect was my first issue with the coaxial propeller design proposal. My 2nd issue involves contrasting a single larger propeller/turbine versus multiple smaller ones (ignoring shadowing). Either in the Wikipedia link or in your other reading, have you seen an issue with smaller faster turbines versus larger slower turbines? It's a bit subtle, but very important in the real world.

Another approach for your science fair project might be to prototype several different turbine designs and test them with a simple fan-driven wind tunnel. There are some pretty weird turbine designs that could make it a fun and visual presentation in your Science Fair presentation...

https://i.pinimg.com/originals/41/76/b0/4176b0726d042beba72f40db51a1ab43.jpg
4176b0726d042beba72f40db51a1ab43.jpg
 
  • #11
These types of ideas have been tried and any small gains they deliver are not worth the engineering challenges; better to put that second and third rotor on their own towers, sited to avoid anything upwind or downwind that affect wind flow. I'm not familiar with the specific issues, but I suspect air turbulence immediately behind a wind rotor would be very significant. The overall wind speed would be reduced as well as being more turbulent. Any rotor sited directly behind another is going to have reduced output - optimal spacings for towers are meant to ensure they spend the least time in line with each other. Wind speeds are affected ahead of an obstruction as well as behind it so the first rotor's output will probably be reduced just by putting a second rotor behind it. Close spaced and they would effectively be like a single rotor with more blades; more blades have been tried but don't deliver better performance; two or three blade rotors work the best, but whilst you don't see them with 10 or 20 blades, you can find single blade wind turbines and they have good efficiency.

Having a long axial shaft that extends a long way past the bearing housings would have greater stresses - huge stresses I would think if 3 were mounted like the model pictured. One ahead and one behind the gearbox, paired on a fixed shaft would be the simplest high strength arrangement but independent rotation would probably be needed. But would two deliver more power than a single rotor? As I understand it a wind turbine's maximum wind energy to rotation efficiency is near 60%, but closer to 40% in practice. 40% of the less than 60% of wind energy left, when some of that initial 40% will be lost and the second rotor is in turbulent air, reducing the efficiency... I would be surprised if there is much overall improvement. Conversely I would not be surprised if there was no improvement or even a reduction in efficiency.
 
  • #12
With a bit more searching I did find attempts at twin rotor wind turbines - counter-rotating ones - with claims of a 40% gain in output from the same swept area over single rotors, so my final remarks look incorrect. However they have not found any commercial success. It's always tricky to equate commercial success with technical superiority - there are so many other factors, including management and financing that can see better technologies outcompeted - but it does look like the efficiency gains from twin rotors aren't enough to displace the simpler single rotor designs.
 
  • #13
unseeingdog said:
Could multiple, smaller rotors on a wind turbine (on the same shaft) be more effective than a single, larger rotor? Why do most wind turbines have only one big rotor? I was thinking about doing a science fair project on this topic, but I have been unable to find a conclusive answer to this question online.
Counter rotating rotors would be a possibility because it reduces the minimum wind speed nescessary for a given rpm, but then one has two rotors instead of a rotor and stator. Think about induction generators and getting current from a rotor vs a stator.

Wind turbines are unducted fans, so there are losses at the tips that increase with speed.

Two fans, either vertical or horizontal are potentially practical, but one must understand the stage losses and interactions (e.g., potential instabilities, tip losses, flutter, . . . ) to ensure stability over the range of flow.
 
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  • #14
There is only so much energy available to be captured. The larger the swept area the better.

Remember that extracting energy causes the air to slow down. You can try and use multiple rotors but the air can't come to a complete stop - It has to get out of the way for the next lot.
 
  • #15
Astronuc said:
Counter rotating rotors would be a possibility because it reduces the minimum wind speed nescessary for a given rpm, but then one has two rotors instead of a rotor and stator. Think about induction generators and getting current from a rotor vs a stator.

Wind turbines are unducted fans, so there are losses at the tips that increase with speed.

Two fans, either vertical or horizontal are potentially practical, but one must understand the stage losses and interactions (e.g., potential instabilities, tip losses, flutter, . . . ) to ensure stability over the range of flow.

For what it's worth, I did find a lot of wind turbines that are ducted fans.
 
  • #16
CWatters said:
The larger the swept area the better.

I believe that is the dominant factor in wind turbine design. It is cheaper to put up two simple turbines and get 2x power than spending more money and getting less reliability for a more complex design that might only give 1.5x more power.

The KISS principle in engineering is frequently underestimated.
 
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  • #17
By using a large rotor and a small rotor, a dual-rotor design can be superior to a single rotor.The blades of most modern turbines are very large and as a result the inner section of the blade is optimized for structure, not aerodynamics. There is energy in the wind that is not being extracted due to this compromise. A much smaller rotor, that is optimized for aerodynamics and placed co-axially with the larger rotor can help to recover some of this energy. In addition, this design can also help to mitigate the wake losses in turbine farms because it promotes mixing in the wake.
 
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  • #18
well my 2 c - "outside" of the issues with the blades, the gearbox / construction would be a nightmare... $ & $ ... as it is the gearbok is a significant cost and reliability point.
 
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  • #19
The question comes down to
  1. Designing a more efficient wind turbine.
  2. Designing a more power efficient wind power generator.
  3. Designing a more cost efficient wind power generator.
They can lead to drastically different answers.
 
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  • #20
unseeingdog said:
Could multiple, smaller rotors on a wind turbine (on the same shaft) be more effective than a single, larger rotor? Why do most wind turbines have only one big rotor? I was thinking about doing a science fair project on this topic, but I have been unable to find a conclusive answer to this question online.
Two turbines rotating in the same sense is equivalent to one turbine with twice as many blades. The rpm of an efficient turbine or propeller is inversely proportional to the number of blades. Less blades are needed at higher rpm to “slice” the air sufficiently to extract optimum energy. The minimum stable system has three blades, which also minimises gearbox weight. The limiting speed is then determined by the blade tip velocity becoming supersonic.

There is already a problem of blade noise or pressure waves reflecting from the support tower and back to the same or another blade. When two counter rotating turbines are used, the two helical “dirty air” disturbances of opposite sense cross at a regular rate. That produces an oscillation in the blades that then flex to the point of collision and destruction.
 
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  • #21
Hi Any idea how much money has been spent in total R&D? Who is considered the tec leader in wind turbine design? How much work has NASA done that is directly related to this?
 
  • #22
scott mcfee said:
How much work has NASA done that is directly related to this?
Wind turbines do not work in space so NASA would let the US Department of Energy handle wind generation.

Google search has no problem finding answers to your questions. Try it ...
https://www.ieawind.org/long-term%20reports/IEA%20Long%20Term%20R_D_Approved%20July%2023%202013.pdf
http://www.windpowerengineering.com/featured/business-news-projects/next-gen-wind-technologies-get-36-9-billion-rd-investment-2026/
 
  • #23
American Wind Energy Association is a good place to start searching for R&D news.
http://www.aweablog.org/tag/rd/
If you're interested in wind energy i recommend attending their annual conference, it's a huge huge trade show.

NREL dot gov is another. If you're around Denver stop in at their visitor center. .http://visitgolden.com/project/national-renewable-energy-laboratory-nrel-visitor-center/

https://www.nrel.gov/docs/gen/fy02/31958.pdf
upload_2017-9-27_8-39-52.png


DOE's wind R&D newsletter https://energy.gov/eere/wind/wind-rd-newsletter
 
  • #24
Baluncore said:
Wind turbines do not work in space so NASA would let the US Department of Energy handle wind generation.

Google search has no problem finding answers to your questions. Try it ...
https://www.ieawind.org/long-term%20reports/IEA%20Long%20Term%20R_D_Approved%20July%2023%202013.pdf
http://www.windpowerengineering.com/featured/business-news-projects/next-gen-wind-technologies-get-36-9-billion-rd-investment-2026/

NASA actually does a lot of aeronautics research including wind turbines.
 
  • #25
RandomGuy88 said:
NASA actually does a lot of aeronautics research including wind turbines.
Has NASA been involved with wind turbines in the last quarter of a century ?

Wikipedia suggests NASA was managing some projects for the DoE only from 1974 to 1992.
https://en.wikipedia.org/wiki/NASA_wind_turbines
 

What are multiple rotors on a wind turbine?

Multiple rotors on a wind turbine refer to the use of more than one rotor or blade set on a single wind turbine tower. This is done to increase the amount of energy that can be harvested from the wind, as multiple rotors can capture wind from different directions and at different speeds.

How do multiple rotors affect the efficiency of a wind turbine?

Multiple rotors can significantly increase the efficiency of a wind turbine by allowing it to capture more wind energy. With more blades, a wind turbine can harvest energy from a wider range of wind speeds and directions, resulting in a higher overall energy output.

What are the advantages of using multiple rotors on a wind turbine?

Some advantages of using multiple rotors on a wind turbine include increased energy production, improved performance in low wind conditions, and lower maintenance costs. Additionally, multiple rotors can also make the wind turbine more visually appealing and easier to integrate into the surrounding landscape.

Are there any disadvantages to using multiple rotors on a wind turbine?

While there are many advantages to using multiple rotors on a wind turbine, there are also some potential disadvantages. These may include increased complexity and cost of construction, as well as potential noise pollution and interference with bird and bat populations. Proper planning and design can mitigate these issues.

What is the future outlook for using multiple rotors on wind turbines?

The use of multiple rotors on wind turbines is still a relatively new concept, but it has shown promising results in terms of increasing energy production and efficiency. As technology continues to advance, it is likely that we will see more widespread use of multiple rotors on wind turbines in the future as a way to harness even more renewable energy from the wind.

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