Post from Jeffrey Winters:
If you spend enough time digging around hobbyist and do-it-yourself websites–guilty as charged–you’ll inevitably run across dirt simple plans for building a vertical-axis wind turbine. Many of those plans tout the ability of the VAWT to turn in very low winds. That makes for a nice display, but it winds up being fairly worthless for capturing energy, since the power of the wind is proportional to the cube of the velocity. Half the speed means one-eighth the power.
The other problem is that while building a Savonius rotor is easy–basically, it’s a pair of half cylinders mounted on an axis–its efficiency is low. That’s because while the wind produces torque pushing on the concave side of one half-cylinder, it’s cancelling some of that torque by also pushing on the convex side of the other cylinder.
There are, as you might imagine, DIY solutions to that problem, and two of them were tested late last year in ASME’s Journal of Solar Energy Engineering. Binyet Emmanuel and Wang Jun of the Fluid Machinery Department of Huazhong University of Science and Technology in Wuhan used CFD modeling to test whether tripling the number of blades or placing a cowling or vanes around the rotating blades would cut down on negative torque and increase the turbine’s coefficient of power.
The results were interesting. The six-bladed Savonius rotor performed only marginally better than the traditional two-bladed one, maxing out at 30 percent efficiency rather than 25 percent. But by adding fixed vanes that funnel the wind into the concave side of the rotors, maximum efficiency jumped to nearly 50 percent.
The researchers cautioned that real-world efficiencies would be a little lower, but it’s gratifying to see that some of those backwoods engineers crafting gear from plywood and sheet aluminum might be onto something.