> > > What is the can halves going to be used for?

> > When I first saw Russells request I thought he was making a mini-barbeq=
ue,
> > maybe for doing spit roast mouse or something ...
> >
> > No knowing what he is likely to want to cook, what with all his visits =
into
> > the Asian region ... ;)))))

Denny  saith:
> Funny, my first thought was a really tiny Savonius Rotor...
> http://en.wikipedia.org/wiki/Savonius_wind_turbine

Denny gets the olive wreath.
Looking for an "appropriate technology" material for micro turbine
where cost is crucial.
While Coke cans may not be as common in some developing country areas
as other materials, they have widespread availability and very
standardised construction.
I thought it worth throwing out the query for input first as,
regardless of how good they prove strength wise, there re definitely
applications for them in this role.


Whether material will be able to be strong enough is tbd.
Will be trying shortly.

Calculations suggest it may do better than may be expected.

Low diameter has two interesting effects.

Low width means that great length is needed for modest area.
Mass can be low and a very long string or rotors using "cord" (wire -
rope -  yak's   wool -  ...) may allow a several meter long rotor to
be utilised. Potential problems multiply with thought :-)

A pure drag WT has a max tip speed to wind speed ratio of 1 and
probably less. Savonius is NOT pure drag and can manage TSR of up to
a.x where x varies with profile and more.
1.2 ish for 1/2 barrels .
1.5 - 2.0 maybe for Benesh airfoil and magic.

At TSR =3D 1:1 the OD travels at wind speed.
So the smaller the dia and circumference the greater the RPM per wind speed=
..
 A Coke can rotor will be ~~~~=3D 100 mm dia or 314mm circ or about 3.2
revs/second per m/s of wind speed or 190 rpm per m/s.
Say 200 RPM per m/s
So in a 5 m/s wind you get 1000 RPM at TSR of 1.

High RPM is of great advantage when trying to build small alternators
with a minimum cost on magnetics.

A coke can rotor doing 15 revs per second =3D 1000 RPM would be a sight
to behold !!! :-).
Centrifugal force gets interesting :-).
A 1 metre length of Savonius should produce 0.5 to 1.5 Watts at 5 m/s

P =3D~~~~~~ 600 x A x z x (V/10)^3

A area m^2
Z efficiency
V wind speed m/s

1 metre x 0.1m =3D 0.1 m^2
Z =3D 0.1 to start. Savoinius can get 0.1 - 0.2 done well.

600 x 0.1 x 0.1 x (5/10)^3 ~=3D 0.75 Watt.

3 metres in a "string" - pivot at top to guy to tree etc.
Base to ground with "guy" or a weight.
Say 2 Watts at 5 m/s.

Designing it to stall at above some max speed (probably no more than
above) is needed, or else.

Placing 1 metre lengths of these in a frame with appropriate spacing
covers the whole area. You get a rectangular WT :-)
Quite simple frames MAY suffice.

I'll be playing games with PVC and other pipes as well.
NB: Low pressure PVC pipe and much high pressure shatters nastily when
over stressed. Use at high TSR needs careful thought.

Quick BOTE calc
E&OE
Force per metre on one side at 5 m/s ~=3D 3 Newton =3D ~300 gram
Mean radiius =3D 50 mm.
Torque =3D 0.3 x 0.05 =3D 0.015 kg.m =3D 1.5 kg. cm (to mix units)
That's a useful amount of starting torque!
That's on one blade and the other will partially oppose it but still
shows why Savonius is a good self starter.
NB E&OE 5:20am :-)


          Russell



Anon

            Russell



where
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