Vertical Axis Wind Turbines
There are two different kinds of Vertical Axis Wind Turbines. The two different designs are the Savonius wind turbines and the air foil Darrieus wind turbine. The Darrieus wind turbine uses lift force from the wind to spin the wind turbine, similar to an airplane’s wings during take-off. The Darrieus has around the same efficiency as a propeller type Horizontal Axis Wind Turbine (around 35%), but wears quickly from the stress caused by the from the lift force against the airfoils. The force of stress against the airfoils can be equivalent to the full force of the wind at a head on angle of attack. The wind turbine spins from the airfoils pushing the axel in opposite directions, causing rotation. If there is too much lift force the wind turbine will wear and sometimes break. The Darrieus wind turbine is a high efficiency low reliability wind turbine.
The Savonius wind turbine is very different from the Darrieus wind turbine. The Savonius wind turbine is a drag type wind turbine. A drag type wind turbine rotates from one blade having more drag then the other blade(s). This has been accomplished by curving the wind turbine blades. The blade with more drag spins the rotor, while the other blade(s) not in use lower efficiency from being pushed through the full force of the wind. Because of the blade(s) not in use, the highest accomplished efficiency in a drag type wind turbine is just over 15%.
Savonius Wind Turbine’s Large Surface Area
Although the Savonius wind turbine isn’t very efficient in common wind speeds (6 knots+) it is the highest efficiency wind turbine in very low in speeds. The Savonius wind turbine is very efficient in low wind speeds because it has the largest surface area, making it catch the most wind. The Horizontal Axis and Darrieus wind turbine are almost still in wind speeds below 4 knots because the rotors don’t have enough wind to create torque to spin the rotor. The Horizontal Axis and Darrieus wind turbine tend to have the greatest efficiency in higher wind speeds, while the Savonius wind turbine should be used in low wind speeds.
The large surface area of the Savonius wind turbine also gives it the greatest torque. The force of the torque created by the Savonius wind turbine is equivalent to the force of the wind subtracted by the weight of the rotor and the aerodynamic drag from the second blade. The Savonius wind turbine property of having high torque gives it many capabilities other then producing electricity. The Savonius wind turbine can be use for pumping water, sawing wood, or any other mechanical uses. The large surface area of the Savonius wind turbine gives it high torque, allowing the wind turbine to have many different capabilities.
Savonius Wind Turbine Reliability
The Savonius wind turbine is most commonly used because of its reliability. The Savonius wind turbine automatically starts rotating no matter the direction of the wind; it takes a long time to show wear, and is sturdy at almost any wind speed. The characteristics of reliability the Savonius wind turbine has, give it jobs most wind turbines aren’t capable of doing. Some jobs that the Savonius wind turbine does because of its reliability are creating electricity for ocean navigation , measuring wind speed for any purpose, creating electricity in tough conditions, keeping birds off boats and many other uses. Reliability is a very important characteristic of the Savonius wind turbine.
Savonius Wind Turbine Cost Effectiveness
The Savonius wind turbine is and isn’t cost effective. The initial cost of the Savonius wind turbine is cheap, causing the wind turbine to be cost effective on a short term basis. The efficiency of the wind turbine lowers the cost effectiveness of the Savonius wind turbine and may break even with other wind turbines over a long term basis. The cost effectiveness of a wind turbine is usually looked over a five to ten year basis, which is over a long period of time. If someone designs a Savonius wind turbine that can achieve close to a Horizontal Axis or Darrieus wind turbines efficiency under the same cost, the Savonius wind turbine could become the most overall cost effective wind turbine. This achievement would be a great green technology business model that may enlarge the market for Savonius wind turbines significantly.
Force of the Wind
The force of air for wind turbines depends on three variables, the density of the air, the diameter of the turbine blades squared, and the velocity of the wind to the third power. The force of the wind also has two constants, which are Pi and the numbers needed to convert the force of the wind into a unit (mph, kmh, fps, ECT…) which can be multiplied into the equation.
The equation for the force of the wind:
An interesting fact you might have realized from the equation is that when the wind speed doubles the force of the wind becomes eight times as powerful. This means if you use a Savonius wind turbine and somehow take the blade not in use out of the wind’s drag, the rotor of the wind turbine theoretically could spin eight times faster when the wind speed doubles. This is part of the reason why there is such a significant difference between 30 and 60 mph wind speeds.
Efficient Designs of Vertical Axis Wind Turbines
For the Savonius and the Darrieus wind turbines there are many variations that improve the efficiency of the wind turbine. Here are the variations of VAWT’s.
Darrieus Wind Turbine Variations
Cycloturbine Wind Turbine Design
The Cycloturbine is a Darrieus type wind turbine where each blade is “pitched” at an angle, allowing the wind turbine to always have an angle of attack. Although the wind turbine always has an angle of attack with the wind, it doesn’t always start by itself and sometimes needs an external force, like an electric motor, to get the wind turbine to start spinning and generate electricity. The Cycloturbine is very efficient, but wears from the lift force against the rotor blades.
Helical Bladed Wind Turbine
The most recent variation of the Darrieus wind turbine is helical blades. Helical blades are curved and usually tilted at a 60 degree angle. These wind turbines are the most efficient and wear less than the Cycloturbine. The wind turbine still uses the lift method of other Darrieus wind turbines and tends to be lighter. The only problem with this wind turbine is the initial cost, which takes many years to break even with other Darrieus wind turbines. This wind turbine is currently the best in overall efficiency for wind turbines.
Savonius Wind Turbine Variations
Basic Savonius Wind Turbine
A basic Savonius wind turbine with curved blades above is one of the most basic wind turbines. The Basic Savonius can be as simple as two cups attached to a rod that is standing vertically. The wind turbine in the picture above would have an efficiency of around 6-10% depending on the materials. Basic Savonius wind turbines are the least costly.
Two Scoop Savonius
The two scoop Savonius has around the same efficiency of a basic Savonius, but is more durable. The two scoops take extreme wind speeds (130mph+) to start to wear. These wind turbines are commonly used in areas where the wind turbine cost and reliability are more important than efficiency.
Combination Savonius and Darrieus
The combination Savonius and Darrieus wind turbine would seem more efficient then the Darrieus itself, but really isn’t. The efficiency of this wind turbine is a little less than 30% because of the drag of the Savonius blades. Although this wind turbine is less efficient, the purpose of the wind turbine is to get more energy out a small surface area. This wind turbine has a large amount of torque and is capable of producing great amounts of electricity.
Three Blade Savonius
The three blade Savonius can be a little more efficient than two blade Savonius. The three blade Savonius can start spinning automatically in any situation. The three blade Savonius is known for self starting the easiest of any VAWT.
Half Savonius, Half Darrieus
The Half Savonius, Half Darrieus blade wind turbine uses both drag and lift force to spin the turbine. The blades of the turbine are very aerodynamic, limiting the drag for the Savonius half of the blade out of use. The Darrieus half is a little less efficient then it normally would be. The website says the wind turbine is still going through experimental stages and the efficiency is not yet known.
· High torque
· Low cost
· Rotates at low wind speeds (4kts<)
· Low mounted (Good for residential)
· Low mounted (easier maintenance)
· Bird friendly ( low to the ground and easier to be spotted by birds)
· Low mounted (less wind)
· Long term cost effectiveness
· Difficult to install in towers
· Can’t spin faster that the approaching wind speed
History of Savonius and Darrieus Wind Turbines
Savonius wind turbines have been around ever since the ninth century to pump water and crush grain in Persia. In the very early 1900s a man named Sigurd J. Savonius re-introduced the wind turbine and called it the Savonius Wind Turbine. The Savonius wind turbine was revolutionary it was not only reliable, but easy to build. At the time, it was used to produce a small amount of electricity without having to pay for the electricity. Sigurd J. Savonius made the Savonius wind turbine quiet and the choice for residential use. Since Sigurd J. Savonius, his wind turbine hasn’t changed much.
Darrieus wind turbine was patented in 1931 by G. J. M. Darrieus. The wind turbine was re-invented in 1970 by the National Research Council of Canada. In 1974 Sandi research labs built a 5 meter diameter Darrieus that has been continuously researched to become cheaper.
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