Wells Wind Turbines vs. Savanious Wind Generators

The non-working side of a Savonius turbine produces damping.

Really good post, thanks.

Lots of common sense can also earn GAs!!

The Betz limit is 59.3%. If a wind turbine fills a wind tunnel, it can exceed this limit because it is not in free air that can go around the wind turbine and slow down behind the wind turbine. I believe that the Wells turbine is used inside of a tube, which is the equivalent of being in a wind tunnel, and filling the wind tunnel. In wind tunnels, care must be taken to account for blockage effects that can artificially raise the measurement of wind turbine efficiency.

thanks

the wells turbine does achieve 60% efficiency.BUT......this is not the device efficiency (OWC). To turn this turbine air is needed. the waves are concentrated into a rectangular duct pointing upwards. as the waves slosh up and down in this 'chamber' to induce osscilating air flow,, there is a high loss of energy at the water-air interface,there is a damping effect as well. I am looking at a design which will not involve air. My idea is to simply get the energy from the water itself. I think i will take my chances with the savanious rotor. And also like you said, the economics plays a vital role.

Ram.

If you use a Savonius in a duct with water running through the rotor, I can't see why it wouldn't work quite well, although the drag on the return (against the flow) side would be exaggerated and would probably penalize your efficiency.

This ducted Savonius can have a support bearing each side, making it quite rugged.

You can also taper the duct to increase water velocity and reduce size of rotor needed.

An alternative would be to use a normal water turbine with ducts fitted with non return valves to ensure the water only ran one way through the turbine. This should give quite good hydraulic efficiency.

If you make a prototype, please keep us posted on how well it works. It sounds interesting.

Would a Savonius work better (but not from all directions unless it was mounted so that it could swivel to allw the wind/water from any direction....) See diagram.

If the cowling could swivel around the Savonius, as suggested, it would tend to improve the efficiency as the drag from the other side would be drastically reduced (Please correct me if my thinking is wrong) AND it would be easy to shut down the Savonius by turning it "out of the wind/water currents" or by putting a "Throttle" in the intake........

It should be dead easy to build the casing to swivel to take the wind from any direction....

I wonder if this is a "first" for a Savonius? Can anyone help me on that score?

Hi Andy Germany

For a Savonius, shrouding would reduce drag when water is going up, but increase it when the water is going down.

Swivelling the duct, as you suggested would allow this to be changed so that it works well in the other direction, but would the increased efficiency be worth the extra complexity? I'm not sure.

If the wave column was used to generate a reversing air flow, then the complexity of the shroud would not be worth it. If the rotor was placed in water flow, the extra density of water would allow some real "thud".

Perhaps using a parabolic blade instead of a circular one on the Savonius would reduce the drag in the reverse direction and make it unnecessary to resort to the swivel. Of course care would be needed not to increase the wetted area enough to undo the benefit of the more streamlined back shape of the blade.

I should know how to calculate the drag but have forgotten. Might be good for me to "hit the books" and relearn.

If we assume a turbine 1m diameter by 1m wide and 10m/s water flow speed, half the flow area available, the flow is 2500l/s, dynamic head available is about 5m.

Hydraulic power available (ie power at 100% efficiency) about 600kW. If we have 20% efficiency, shaft power available is around 120kW. (A Wells turbine, at considerably greater expense and complexity, would allow about 360kW to be generated.) (These days I'm so rusty my calcs should be regarded as dubious and checked thoroughly)

I suspect the simplicity, low cost and low maintenance of the Savonius would make it a better proposition.

It should be possible to have a duct contracting enough to speed the wave induced water flow to the design 10m/s, which means a simple, cheap and comparatively small machine could generate respectable power.

Excess dynamic head could be used to progressively bypass surplus water during storms etc to prevent overspeeding and damage to the turbine.

At 10m/s water flow, the Savonius would be doing about 200 rpm, so torque would be high.

It should be possible to incorporate large numbers of these into a concrete breakwater, turning a static civil structure into a significant power source.

Where there is a significant tidal range, this may add a little to the output, but probably not much.

If the impeller blades were constructed of a flexible rubber-like material,( think of a fabric reinforced rubber) and suspended from an inner and outer support that spun, the directional changes in water flow would work equally in either direction. Would the direction of rotation need to be the same, or could it be reversed?

Have you looked at the Kelland VAWT (see Fig. 12 in US Patent 4456429 and related claim 10). It rotates in the same direction without regard to direction of fluid flow. It's another variation on the Darrieus VAWT. Main advantages are easier self-starting and smoother output.

The photo shows a prototype of the Kelland VAWt built in the early 80s. Gorlov has since developed a similar design and has adapted it to water flow. Information on the Gorlov version can be found here at: http://www.gcktechnology.com/GCK/pg2.html

(submitted at the risk of being accused of self promotion )