VAWT "shading"

[MountainMan]

...due to the strong winds and lack of interfering objects.

Still, not satisfied with average performance, I began to mess with the design (really easy to do when it's in your head).  Not knowing much about this stuff yet, my design was the simple "cut a barrel in half and face the two halves in opposite directions" type.  With this sort of a rotor, one half is always being "pushed" by the wind (i.e. the "cupped" half), and the other half is fighting it's way back "upwind".  Obviously this is an oversimplification -- otherwise such a rotor would never spin, as the two sides would always cancel each other out.

I'm thinking that if there was a simple shield to keep the wind off of that "returning" side of the rotor, it should increase the power output and/or efficiency of the unit.  In my head this was true, and the sucker started spinning about twice as fast.

Now I'm hoping somebody who has knowledge of how this stuff works in the real world can tell me if this is just "strange physics" going on in my head, or if the idea has been tried in the real world and if it doesn't work, why not.

thanks,

jp

P.S.  I'm starting to get dizzy, can somebody tell me how to make this thing stop?  I tried putting my fingers in my ears to stop the wind but that didn't work.  8^)

[electrondady1]

yes it works , they have been doing that shading trick since anchient times.

[MountainMan]

Thanks electrondady1.  What is it usually called?  I just searched here for "shading", and only found my post.

thanks,

jp

[RP]

Try google for shield and either savonius or darrieus for other example of this.

Also consider that even better than shadowing or shielding the upwind side is to use the shield as a guide to push the unwanted wind over into the downwind side.  In effect you can make a funnel scoop thing that'll pick up all the air hitting the front of the mill and guide it all into the appropriate half of the turbine.

[electrondady1]

try "shrouded rotor"

[Ungrounded Lightning Rod]

In effect you can make a funnel scoop thing that'll pick up all the air hitting the front of the mill and guide it all into the appropriate half of the turbine.

A thing to remember about shrouds on a VAWT is that they must pivot around the mill axis to point into the wind - and they funnel the air into an off-center postion.  So there is a wind force trying to rotate them out of position - in the direction opposite the rotor's motion if it's a savonius.

(One thing you can do is have a number of vanes on your shroud and let it rotate, too, and get power off THAT.  B-) ).

Not an issue for your horizontal-axis savonius.  (Instead it will try to tilt the tower.  If you're pointing at the prevailing wind you're fine.  If you're pivoting the whole structure you'll have to use bearings that can take significant side force.)

Speaking of savonius variants:  I saw on along US 101 quite a few miles north of San Francisco and Marin, maybe ten years back.  Somebody had built a house with a many-bladed savonius rotor about the size of a merry-go-round on top of the garage.  The blades were 55 galon drums cut in half and painted a variety of bright colors.  Looked like a giant squirrel-cage rotor.

At one point the house was up for sale and the wife and I dropped in - but nobody was there to show us around or tell us about the mill.  We pulled up to the house and discovered that it made quite a bit of noise - bearing rumble mainly.  (I assume it had wheels under the rotor.)  Some time later we went by and it was no longer turning.  I presume the new owner decided the power wasn't worth the maintainence and vibration of the house.  (Another example of the "don't mount the mill on the house if you want any sleep unless you're already living next to an airport" rule.)

[windstuffnow]

  You can also search for "diffuser" as it would apply to wind turbines.  If you design the diffuser properly you can collect from the total area of the turbine forcing it through the downwind side.  This will increase wind speed through the collection side.  It will require a method of seeking the wind direction.

[Electric Ed]

I've considered  trying to build something like this, but the materials used would have to be light-weight and strong, with good quality (tapered roller) bearings such as auto wheel bearings.

Biggest problem would be assembling the thing up on a tower.

Ed

[MountainMan]

Ungrounded Lightning Rod:  Yes, the "Horizontal VAWT on the garage" is exactly what I had in mind, only designed to be less visible.  I'm interested by your statements about the noise.  Is it usually the case that high quality bearings will solve this sort of rumble?  Seems likely.  Since you didn't mention other types of noise, is it safe to suppose that it was fairly quiet other than the bearing rumble?

Also, despite all the opinions to the contrary, I'm considering a ground level siting also.  In particular, under a deck that will be terraced into the hill.  True, it won't get as good of numbers as a tower mounted mill (or anywhere near it), but if you were standing there on that hill with your mustache being parted by the wind, I think you would agree that it will turn, and will produce some power, and would not be visible from the frequented parts of the property.  My main concern about the whole idea at this point is the noise.

thanks for everyone's comments.

jp

[Ungrounded Lightning Rod]

Is it usually the case that high quality bearings will solve this sort of rumble?

No clue what would be available to handle this.  Since it's low and enormous diameter you'd probably want to support it with a track and wheels, rather than just a rod at the center.

Since you didn't mention other types of noise, is it safe to suppose that it was fairly quiet other than the bearing rumble?

Hard to tell since I couldn't get close enough to it.  The bearing rumble was conducted to the building so it was what was heard.

Since it's a savonius variant drag turbine I'd expect some vortex peeling at some point around the rotor.

I'm considering a ground level siting also.  In particular, under a deck that will be terraced into the hill.

That sounds like a good approach for this sort of turbine.

If you make the rotor a little smaller than the deck the deck will shield most of the high frequency components of the noise from the people on it.  (You can add some guides to the corners of the deck to funnel air to the turbine, giving it a cross section closer to that of the deck than that of the rotor proper.)

If the deck and the rotor have separate foundations and no connection (except maybe through a length of buried conduit with a right-angle joint underground) you should have little conducted bearing noise to the deck.

[hvirtane]

A nice drawing.

I've been thinking about

a similar construction

some years ago.

Does anybody know

about real experiments

with these?

It also came to my mind that

the shroud could be used

as a 'furling system'. When

the wind speed reaches a certain

limit the tail would start moving

the shroud to 'shadow'

more the opposite side of the

rotor.

- Hannu

[rotornuts]

My biggest issue with a shroud is the fact that you are potentially "capturing" air in the shroud requiring the blades to push the captured air through the shrouded portion and also you are eliminating potential sources of air to help relieve "pressure drag" on the upwind side. Perhaps a perferated shroud would help but I don't consider it likely. Overall I can't visualize how a shroud would provide a benefit over a well designed unit.

Just my opinion.

Mike

[nomad01]

JP,

I asked myself the very same question about two weeks ago (had the same problem with my head spinning).  This is what I came up with (plan view):

Place two removable walls, say sheets of 3/4 inch plywood, at 45 degree angles to the direction of the prevailing winds that day.  Say the weather man says expect winds out of the south, as in the above diagram - place one end of a wall near the southern point of your VAWT, angled out in a south easterly direction; place the other wall starting 90 degrees clockwise from the first wall (in this case starting at the western point and extending south west).  What you have is a 90 degree funnel channeling wind into the entrance, and blocking wind at the exit.  I figure a pair of walls that are each the length of the diameter of your VAWT could channel up to four times the airflow into your scoops, while doing its blocking thing (all subject to Bernoulli's principals and whats-his-name's coefficient, and other things I don't understand).  

Of course, that means you would have to physically move the walls, probably at least once a day after hearing a wind report. If you make 12 prearranged slots, each corresponding to the points of a compass, that your walls can slide into, that should cover all your wind conditions and ease some of the pain involved.  The slots could maybe be concrete molds, or upright poles to hold the ends of the walls in place.  The whole thing would have to be pretty low to the ground.

Anyway, my head stopped spinning after I diagrammed this out on graph paper.  

nomad01    

[Shannon]

Nice to see the input for your project.

If you are looking for some real life experiments you can see one at

www.me.dal.ca/~dp_03_7/finalreport.pdf

This will give you some insight.

Shan