counter rotating vertical axis wind turbine

[Vertical]

The benefit of a counter rotating vertical axis wind turbine has been demonstrated over a single rotating vertical impeller wind turbine when using slow wind.

The results were found to be as high as six times more output. No investigation has been done for motors or impellers rotating at optimal speeds.

Having one VAWT positioned above the other and in the middle between the two VAWT's a PMG positioned in such a way that one VAWT rotates the Rotors while at the same time having the other VAWT counter rotate the Stator…

For example, if the wind allows a rotation of 50 RPM…then what we obtain is 50 RPM for the rotor and 50 RMP for the stator, but because the Stator is rotating counter clock wise the end result is 100 RPM…

Our plan is to build a 26 inch rotor like the one below… what worries us is the overheating of the PMG…
The 6 meter blades we are using are part of a 5 blade, 6 meter diameter wind turbine, which is already in production and rated to produce 10 KW at a wind speeds of 11m/s… so what we would like to see is two sets of this VAWT one on top of the other and try to achieve more than double the Kw's …hopefully 50 Kw…
Any suggestions for the PMG assembly…

[joestue]

yep, the efficiency at low rpms is about directly proportional to the rpm.. and at higher rpms the power is proportional to rpm but the efficiency stops increasing due to iron and eddy current losses.. given that you have none.. you find the power and efficiency are both steeply increasing with rpm.

then again you need slip rings to pull that off... why not just use a gear box?

[Flux]

This has been done with a contra rotating HAWT ( Trimble windmills and possibly others) There is a fair increase in complexity with extra bearings and slip rings but anything to increase effective rotational speed is worthwhile if you can solve the mechanical difficulties.

Flux

[electrondady1]

for several years there was a vertical forum that attracted a lot of builders . one fellow built a double, counter rotating lenz2 mill.
 he found in lesser winds, one section would rotate and in higher winds both would rotate.
from your description it sounds like a type of darrius design.
it would be a mechanical challenge.
from a practical stand point if you run two 6mx6m mill segments in tandem
12mx6m and drive the alternator with a belt you could fine tune the alternator /mill combination.

[lifer]

I guess the only good reason to use counter rotating rotor/stator is to push the resulting generator RPMs over 1000 or something (i.e. 800 rpm rotor + 800 rpm stator) to avoid mechanical stress (centrifugal force effect on magnets).

To increase total RPM from 50 to 100 it's far more easier to use a transmission belt (especially for VAWTs). Moreover, you have the posibility to further increase (more than double) the resulting RPM by choosing various transmission ratios (pulleys).

 

[Vertical]

It is my understanding that when you use a transmission belt you loose torque... or you loose speed depending on the combination... the wind VAWT is high toque low RPM in winds of 10 m/s... the reason why we think of using a double one VAWT on top of the other is so hat we don't loose torque... the questions remains in regards of what would be the ideal Stator/Rotor combination

[lifer]

Like you just said, a VAWT is a high torque/low rpm machine. By using a transmission belt (or gears) you get a low torque/high rpm machine (HAWT type) so you could match it with a regular axial flux generator.

Another option is to increase the generator rotor/stator diameter. I'm not saying that counter rotating turbines it's not an option but might not be the optimal one (though it's aestethically pleasant to see such a form of kinetic art).

[Vertical]

I still think that we loose power... any use of a belt and we are loosing power since we want to pull as much power as possible I would think that the best approach would be to design a PMG that uses all that power with out diluting any of it by the use of belts and gears...

[Vertical]

it might be interesting to read the following study from Manchester University... it is for low winds... but never the less it quite interesting to see the results they obtained...

http://www.academia.edu/340999/Design_and_Development_of_a_Vertical_Axis_Micro_Wind_Turbine

[lifer]

The efficiency of a transmission belt could reach 95-97%. Those 3-5% losses could be compensated by a 5% larger turbine (or could be assumed as negligible).

By splitting the turbine in two (the counter rotating situation) you also lose at least 3-5% by using more bearings, slip rings and so on. Not mentioning (again) the possible interferences (air flow) between two close counter rotating turbines.

(thanks for the link, I'm starting to read that study) 

[Vertical]

I understand now... it does make sense to use a gear box and a low torque PMG...

[Flux]

This is always a compromise, everything is with wind. Transmission efficiencies are high at full load to the point of being negligible. Near cut in the loss will be much greater, a chain drive is best, timing belt is reasonable and a "v" belt will be poor, gears are big problems for wind and best avoided.

Yes the extra bearings will add a bit of loss and so will the slip rings but if we are looking at an air gap alternator I think this arrangement will do better in the region near cut in. If there is useful power there then fine, a few % of near nothing is not likely to be worth chasing.

If  this is an iron cored alternator the core loss will increase with speed so the gain is less so the chain drive may compete, belt probably not.

The low speed alternator will be way more expensive for the same power rating and efficiency so you have to decide where the trade off is. The implication seems to be that the gain is only in light winds so the high wind efficiency may not matter for a very low wins site.

I am not convinced that wind energy below 6mph is worth chasing but that is not my concern. For realistic winds even with a HAWT a chain transmission is well worth considering for many conditions, its main snag is complexity rather than efficiency in real wind areas.

Flux

[lifer]

If only chain transmission were not so noisy (I had to use belts just because of this).

(I have no intention of hijacking this thread but I have to ask you, Flux, if you have any clue about the strange (or not?) behaviour of my generator (I described the issue in "maximum RPM" thread). Thanks in advance for any advice.)

[Vertical]

Dear Lifer I think you are located in a low wind area...if so please read the results of the study from Manchester University about low wind..I provide that link above in the post...

[lifer]

We rather have mixed winds around here.. mostly low/moderate winds indeed but quite often twe have strong gusts and gales (especially during winter).

I have no choice but to harvest those winds as I'm living off grid from few months now and it happens to have few days of cloudy/foggy skies when my solar (PV) array (8 x 250W) is almost useless.

[lifer]

That study seems pretty inconclusive to me. The author itself wrote this:

Even though the power output of CR-VAMWT was 6 times higher than the total of the two separate wheels' power outputs it has to be due to the power curve of the generator which indicates that increasing rotation speeds the output increases drastically in the beginning.

However, we couldn't reach the design speed of the generator which would have given us better information as it was assumed that the speed increase in the motor with counter rotating ability would be only two times less than if we had had single ones. Therefore the power output could be estimated proportional to that ratio.

It's fun to build a one-watt wind turbine (like in the experiment above) but when it comes to scale it up 1000-2000 times.. I already have some problems with my 1.6m x 2m Lenz2 VAWT in high winds (I have to use few more guy wires to lower the vibrations).