Savonius Wind Turbine
VAWTs (Vertical Axis Wind Turbines) come in two flavors: lift- and drag- based designs. Lift based designs can usually output much more power, more efficiently, than drag-based designs.
A Savonius wind turbine is an example of a drag-based design. Invented by the Finnish engineer S J Savonius in 1922, it can be made with different types of scoops, e.g., buckets, paddles, sails, or oil drums. Looking down on the rotor from above, a two-scoop machine would look like an “S” shape in cross section.
Because the design is simpler than other types of wind turbines, it finds application in low maintenance situations. Design is simplified because no pointing mechanism is required to allow for shifting wind direction, unlike horizontal axis turbines, and the turbine is self-starting. While previously rejected for generating electricity because of a slow speed, a Savonius wind turbine can be useful for grinding grain, pumping water, and other tasks because it yields high torque.
The reason for a discourse on Savonius wind turbines is that Jon Ricker on Yahoo! Groups awea-wind-home on Sun Dec 22, 2002 8:58 pm noted:
Terra-Aqua Moya has four (or five) vertical axis turbines about a mile off of Route 25 on the Wyoming and Colorado border. Although the property is off limits to the public, I visited it last summer; it was an interesting site. The product development director answered a few very technical questions for me. They have a patented vertical blade shaped diffuser near the blade area, which looked similar to a series of Savonius blades. A trapezoid augmenter ramps the wind flow to the patented vertical blade shaped diffuser before it approaches the blades.
Associated Press recently carried a story (Tiny URL) from the Jackson Hole Star-Tribune about a design breakthrough in vertical axis wind turbines announced Friday, November 4 by Terra-Aqua Moya of Cheyenne, WY. TMA has claimed that this patented design is 43 to 45 percent efficient.
The TMA VAWT is closer to the theoretical maximum efficiency of 59 percent than efficiencies of of prop-driven, horizontal axis wind turbines (HAWTs) that are typically 25 to 40 percent. The new VAWTs are more efficient and less costly than propeller-driven machines and earlier TMA versions due to the addition of vertical airfoils — similar to airplane wings — that surround the turbine’s curved, vertical blades. The interaction between the airfoils and blades produces low pressure which actually accelerates wind flow.
One reason that TMA can claim that their new VAWT is less costly; the generator can be located on a shaft at the bottom of the tower rather than mounted in the nacelle. Solar Navigator informs that “the rotor attaches to the nacelle, which sits atop the tower and includes the gear box, low- and high-speed shafts, generator, controller, and brake. A cover protects the components inside the nacelle. Some nacelles of very large HAWTs are large enough for a technician to stand inside while working.”
The new TMA wind turbines are characteristically VAWTs in that they are lower to the ground and slower than HAWTs. TMA sees these characteristics as advantages.
At a maximum of 96 feet tall, the turbines can be placed in industrial areas where taller propeller turbines are not allowed. The speed of the blades is very low, making them less noisy and less dangerous to birds, officials said. In fact, they said no dead or injured birds have ever been found at the sites of their test models west of Cheyenne.
The article reported that TMA “plans to sell turbines generating from 500 watts to 1 megawatt. The smaller ones are portable and can be used by farmers, the military and remote cabin owners.” Thus, the TMA VAWTs are another example of micro power generation. The promotional article mentioned nothing about how the mechanical design accommodates pulsating torque problems experienced by other VAWTs, nor was there any mention of the type of power electronics utilized.
Note: TSR (Tip Speed Ratio) is the ratio of the speed of the wind to the speed of the tips of the blades of a wind turbine. According to the Iowa Energy Center Wind Energy Manual, lift-type wind turbines. i.e., the lift provided by the airfoil is the force driving the shaft, have maximum tip-speed ratios of around 10, while drag-type wind turbines have ratios that are approximately one. Therefore, Terra-Aqua Moya can be justifiably proud of a VAWT design that is more efficient than HAWT designs, particularly since the Iowa Energy Center Wind Energy Manual states that “some experimental wind turbines have incorporated an added structural design feature… intended to increase the amount of wind passing through the blades. However, these augmenters do not increase the energy capture of the machine enough to justify the added cost of employing them.”
3 Comments
In noting the TMA announcement Open Source Energy Network observes that “the design creates pull on the back side” contributing to 40 plus wind conversion efficiencies.
Jerry Halstead observes, “Having called Wyoming home on a few occasions I can vouch that is truly is an ideal place for wind generators. They have a saying that snow never melts in Wyoming: it's either blown into Nebraska or worn out by the wind on the way.”
By digging into the TMA patent the Watt has more about their cross-flow wind turbine; and, a commenter posts about Barco WM, a Spanish R&D group has been developing a similar vertical axis wind turbine design.
“Their design features four flat arms rotating around a central vertical axis. Each arm is made up of a number of panels which lie perpendicular to the wind to offer the greatest resistance to the wind when they are located in the front rotation area and then turn parrallel to the wind to offer practically no resistance when they are in the generator?€™s return area.”
“Like (the TMA) design, they claim lower cut-in speeds and significantly higher max wind speeds with a greater efficiency than normal horizontal axis turbines (~40%). The higher max speeds are important for high wind areas as the power curve of turbines typically rises exponentially with wind speed until it reaches a cut off (max) speed at which the blades are rotated parallel to the wind and locked to prevent damage. Barco's design can run in very high winds (they even claim hurricane-force winds) and will continue to generate electricity. The combination of higher efficiency, lower cut-in speed and much higher max speed means these turbines should be able to generate significantly more megawatt-hours annually than horizontal axis designs.”
“Also, like the above design, they do not need to rotate to track the wind like horizontal turbines as any wind direction works the same.”
“The final advantage of vertical axis designs (which is a big one) is that the generator for these turbines can be installed at the ground rather than up in the nacelle on top of a tower like horizontal designs. There is simply little one can do to miniaturize or make lighter the dense coil of wires and large magnets that the generators consist of. Having to suspend all this weight in the air atop a tower is a significant engineering issue, necessitates large, expensive and very strong towers and puts a limit to the maximum size horizontal axis turbines can grow to. Vertical axis turbines avoid this issue as they can place the generator on the ground.”
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[...] Speaking of stimulating, I have been musing over the coupling of a VAWT with an EMAFER. A VAWT as you already should know is a Vertical Axis Wind Turbine. However, VAWTs suffer from (said sotto voce) “the Intermittency Problem“, which is where, as previously suggested and Energy Storage Council endorsed, we could apply a stationary EMAFER, for improving the power quality (voltage stabilization) of energy to the Grid. [...]