Aeronautical Engineering/Savonius wind turbine vs. three blade prop.
QUESTION: I recently read an article in Popular Science Mag (P.S. Oct. 2014 Iss). about Alteros's Buoyant Airborne Turbine (BAT).
My question is: what is the difference in the power to weight ratio when a comparison is drawn between a savonius(1) and a typical horizontal axis wind turbine (HAWT)of the standard three propeller configuration with a swept area diameter of less than 8 feet factoring in the following assumption: a) an equal area wind contact working surface and b*) the same weight of construction for each device
where b* must also (if an inherent design requirement) the weight of any superstructure used to support the distal end of the savonius design. Use the engineering results data from the standard "S" type savonius construction from the article below if no other source of similar design/construction information is available to you please.
1) see reference: "Optimization of savonius rotor Vertical Axis Wind Turbine for use in water pumping systems in Honduras" (M.I.T.)publication (shown below)as a possible guide:). http://dspace.mit.edu/bitstream/handle/1721.1/40927/212409044.pdf
I attempted to download the document for you, but the file format of .pdf is not allowed. Why is this?
ANSWER: Hi Steve - Short of doing an engineering study for you, which is beyond this job description, I have made a few rough estimates for you.
I found that a three-bladed horizontal wind turbine 3.2 m diameter will generate about 2 kW and weigh 80 kg (without tower).
A Savonius turbine is estimated to produce a max power: P = 0.36 hrv^3 Watts where h is the Savonius height (m), r is radius (m), and v is wind speed (m/s). So a machine with the same area as the above horizontal wind turbine would have h = 2.8 m and r = 1.4 m giving a power of 1.4 kW in a 10 m/s wind. The weight could be all over the map depending on material chosen, so I will leave that for you. If you find a weight, you can get power-to-weight ratio. I assume the tower weight would be roughly similar for the two systems.
Note that the wind speeds over which the Savonius operates is probably quite limited (fig. 6 of your reference) due to inertia and friction in the machinery. They typically operate at low rpm and are optimum at low wind speeds. So, you really need to compare the machines over a typical wind day, which will probably give advantage to the horizontal wind turbine.
The Savonius is an interesting concept nonetheless.
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QUESTION: Not really a question but a thank you and another comment. Thank you for giving me the answer to a question I did not know to ask (about the rotational operating speeds most efficient for the savonius). For had I known that bit of engineering knowledge, I would not have considered asking the question that I did. No point putting a slow speed wind turbine design way up in the air where the wind speeds are too high! Duh!
It was interesting though that based on equal swept areas, the savonius was around 2/3 the watt generator capacity as the HAWT design. I wonder what happens when you "overspeed" a savonius, does it shake itself apart, or simply self-limit due to aerodynamic stall (is that the right word to use?
The Savonius is a drag device, so it never stalls in the usual sense of the word. But the drag force, which is greater on one side of rotation than the other is not steady. When the cups are perpendicular to the wind the drag is maximum, when the cups are parallel to the wind the driving force is minimum. Of course you can smooth this out by adding more cups to the perimeter. But as the wind speed increases, the mass or inertia in the cups make it harder for the drag force to accelerate the cups and keep the speed up. If the mass was very small, it might be possible to operate at a high wind speed as in an anemometer. But for structural reasons, Savonius turbines are often fairly heavy. The other factor is that the Savonius is taking power out of the air which has to go somewhere. It goes into heat in the bearings and, if the device is attached to an electrical generator, it goes into the power of the turbine. That also tends to slow down the system. At some maximum wind speed, the device cannot absorb more power because it can't dissipate it fast enough so it will stop accelerating or it will overheat and fail.