From %cmsa.Berkeley.EDU@calypso-2.oit.unc.eduTue Jan 23 15:35:28 1996 Date: Sun, 21 May 1995 04:47:14 GMT From: Kerry Miller <%cmsa.Berkeley.EDU@calypso-2.oit.unc.edu> To: london@sunsite.unc.edu Subject: Re: Any Experince with Savonius Rotor Windmills? Hugh wrote (citing Joergen), > |> Advantages is low cost, standard equipment/materials, and the > |> interesting possibility to build a Savonius into the existing design of a > |> standard house, even in a city (chimney-mount if not to heavy).... > > Another claimed advantage is that the gearbox and generator > are at the bottom of the rotor [often on the ground], with easy > access for installation or maintenance. The more reliable the > generator the less advantage. > > |> ... For an experiment, start up with Polyethylene > |> bottles or ping-pong balls cut in half. You will then be able to do some > |> experiments. If you want an anemometer, go with bottles. But a rotor of any size reveals that it is essential to have *both* bearings securely anchored. Guys won't do it. The problem is, you're dealing with a three-dimensional dynamic balance problem (bad enough in ~2D: ever try to balanvce your own tires?) and the rectangular cross section of a cylindrical Savonius is made to order for a design that throws itself out of balance once its rotating. Take your oil drum, cut it in half, fasten the halves to hubs (offsetabout a foot) and string it up on a shaft. Then wait and watch the central space *open up* and youll be able to see the shaft . What this tells you is that centripetal forces are deforming the surface -- in the section, from a vertical line to a catenary curve. But the distortion is different on the downwind side from the upwind side: i.e., out of balance. Once the thing starts throwing out, it's only a matter of time before the guys give way, or the shaft fatigues or the whole thing topples over -- taking your chimbley with it. Having damped your enthusiasm, here's a possible solution: hang the axis *horizontally*.: both bearings are equally rigid (or floppy if you want to look at it that way) plus you can draw your power off from *both* ends. The axis is of course transverse to the prevailing winds. (Dont worry about omnidirectionality. That was S's big idea, because he thought it would work as a sail for water craft which go all directions - but show me a place that has good consistent wind from all directions and I'll show you a place called Oz.) The rotor will respond well to wind even 45 off-axis, in any case. Secondly, forget the drums; make your own windcatcher with (doubly) curved skins. Fabric over ribs is one way to at least approximate this, with the further advantage that when the wind finally takes it (nothing manmade yet that's put up against nature stays put forever) its only a few yards of cloth blowing into the neighbors yard rather than a serious piece of metal plate. The comments about loss of efficiency in gearing up for generation are true enough -- but there are other things to do with power (e.g. pumped water storage or hydraulic swashplate compression) and anyway, whatever happened to the Orbidrive (TM) cam type boxes that were marketed as having 1000:1 ratios at 97% eff? If you really want electrics, wind your own rotor: at a radius of two or 3 ft, something will come of it (providing of course the stators are on the same floppy/rigid mountings...), tho at this point I'm not speaking from experience ;-) kerry miller astingsh@ksuvm.ksu.edu / / * Reduce Reuse Repair Recycle Reject Remember. * / / Re: Any Experince with Savonius Rotor Windmills?