Selsam multi-rotor wind turbine
by TreeHugger 
on 04. 5.05
Design and innovation do matter. One sees everyone doing things the same way- big supposedly bird-chomping view despoiling wind turbines (note the terminology, eye!) and then the creative genius comes up with a different way of building a turbine that is smaller, cheaper and more effective. A little work on the aesthetics and treehugger looks forward to turbine farms that you can barely see from the next county.
It appears that by using a lot of smaller turbines the engineering becomes much more manageable and the resistance caused by the size and weightof the components is reduced significantly. Treehugger really liked the creativity of their analysis of the benefits of multiple units versus single units:
How many wheels does your car have?
Not just one? What about trucks?
How many cylinders in your engine?
More cylinders run smoother - and have more surface area per unit mass.
How many cars in a train? Not just one? Why?
It's much easier on the track. Economy of scale - repeating units...
How many sails on a clipper ship? Not just one? Why not?
How many tiles around your bathtub? Not just one huge thick tile? Better surface to volume ratio with multiple small tiles? Just checking...
How many tiles on your roof? Not just one big, thick tile? Better surface area to mass ratio with multiple smaller, thinner tiles? I see.
Why do we look up and see a flock of geese, rather than just one giant goose? (run!) Why do ostriches and all other birds over a certain size have to walk everywhere?
How many engines on a large airplane? How many props? Why not just one big prop?
How many loops of wire in a generator? Not just one? Why not? More power?
How many fingers do you have? How many Toes? How many teeth?
How many leaves on a plant?
Now how many rotors should a wind turbine have?
good points, all! via :: Worldchanging :: Selsam by {LA}
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How many brains do you have? Would a lot of tiny brains work better?...
While the multiple turbine idea may actually work the "creative analysis" logic is just crazy non-science. Most wind turbine projects are already using multiple (single) units spaced far apart.
Here's some more "creative analysis"...
How many wheels does your car have?
How many wheels does a bicycle have?
Which is more energy efficient?
The bicycle of course, but does that relate in any way to a multiple bladed wind turbine?
Let's not give up logic and reason.
Seriously. That's terrible logic. Now, if for cost to build, this produces more electricity, that's great - but I suspect it does not, given that the market is moving to larger and larger rotors *because* they produce more power.
Nocar's point is great, and correct, I think, in terms of efficiency, but here's a counterpoint: for non-commerical or small applications, e.g. if you're building your own turbine to power your house, which is easier to maintain? One giant set of blades with a lot of mass whirling around, or a bunch of smaller, lighter blades that are easier to stop, fix, and put back into operation?
Jacob, you do have a point. You can more easily work with smaller blades, although I don't know that this design allows for taking a single rotor offline. In most cases, though, a farm owner doesn't maintain their turbines themselves - an outside agency whose job it is to maintain turbines comes in and does it for them.
If one stops spinning on one extreme of the lateral support, the torque on the main upright will become a problem. Two on one side out and it could become debilitating. Also need to consider the added torque induced by lateral member against the earth plane. Its entire surface is at the level of the highest wind flux. Both added torques mean more substantial upright needed. There is no free design lunch.
Is there an analysis of mutliple rotor design somewhere. In the Power formula does the multiple rotot design just increase the value of Cp so that it is closer to the Betz limit of 0.59 or is this a way to get more than that as Dayna Walker's project suggests with her figures of 50.717 Watts with a 28cm rotor in a 6.41 m/s wind?
P = 0.5xCpx3.14xRxRx1.225xVxVxV
P = 0.5x0.59x3.14x0.28x0.28x1.225x6.41x6.41x6.41
= 23.43 Watts
See observation 14 in: http://www.alumni.ca/~walk4d0/sf11.html
and
http://www.alumni.ca/~walk4d0/sf12.html
for the corresponding wind speed.
What I forgot was to multiply the whole equation by the number of rotors.
Thanks for the nice coverage:
Here's a simple equation to baffle the world's finest minds:
More rotors = More power.
No need to make it any more complicated.
Doug Selsam
http://www.superturbine.net
why do gas and steam turbines have many blades, as opposed to a big propeller?
Certainly I wish Doug well with his quest.
One doesn't really encounter the betz limit here, as each blade is in a different stream. And the utilization of structural members is increased because of the higher axle speed. If you consider that the best wind is at the highest altitude, it makes sense to move as much of the system up, and avoid long blades which come down - aside from which they are noisy. That being said, there are issues, and the most obvious is bird blending - as slowing the rotors was done on purpose to avoid killing birds. But good luck and congratulations.
Benjamin Gatti
Doug a friend M. Gilman told me about you I think, but I am not sure you are him. If so let me know. If not tell me why in the world don't people use a foil or turbine that uses the total force of the wind by deflecting the energy 90 deg. It seems to me the propeler type foil I see all over is good like as on a toy airplain pushed through the air you might get one rev. per foot and nearly zero torque as apposed to the simple pin wheel that might get fore + rev's per foot and some "finger on the blade" torque and if it has to do with the load pulling tower out of the ground isn't that were the the power really comes from "wind to ground" thats where it is measured. I would like some one tell why "drag" applies when it comes to wind mills. nobody wonts it windy behind their sail now do thay. So how does some one expect to pull a load on their generator with out pulling on the ground. lift drag raito I know is allways a consern but really do we need to put an air foil off a U-2 on the thing? It is time to get some REAL power out of thies things so lets load them up.
A Quick reply to T.K. Hall
There really are few improvements to the airfoild which have not been tried. One could use more blades, but s it turnes out, its better to put the additional blades on a new tower where they can get clean air, than to have more than 3 blades. Three are used to deal with vibration and centrifugal effects. There is a problem with the pinwheel, and that is if you try to hard to stop the wind, the wind will simply go around - thus the Betz limit of 59% suggests the maximum energy which can be extracted.