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Vertical Axis Wind Turbine
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Vertical Axis Wind Turbine.
With the increasing interest in alternative energy production methods, wind has become one of the most actively investigated sources of power. And while wind turbines have been used for decades for this type of energy production, the vast majority has been of the horizontal axis variety. However, the vertical axis wind turbine is quickly becoming popular alternative and as such, many new designs have been produced in recent years.
Currently, there are two basic categories when it comes to the vertical axis wind turbine; the Savonius rotor design and the Darrieus model. Both designs use a vertically oriented drive shaft, but the method of harnessing the wind power is slightly different. The Savonius version uses what are essentially “scoops” that capture the wind, forcing the drive shaft to rotate accordingly. This method, while capable of generating electricity, is fairly inefficient, maxing out at the 10-15 percent range.
The Darrieus model, by comparison, uses what essentially looks like an egg beater to generate shaft rotation. This design uses two air foils that are each attached at two points, the top and the bottom of the drive shaft. These air foils create a lift force when wind blows across them, much like the wings on an airplane. This lift force is what causes the shaft to rotate, thereby creating electricity, and doing so at a much higher rate of efficiency than the Savonius design.
The vertical axis wind turbine does have some disadvantages over horizontally-oriented designs. First, a vertical axis design tends to undergo more wear and tear than a typical horizontal design; largely due to the effects of centrifugal force on the blades. On a horizontal design, most of the stresses inherent in their operation are focused on the part of the blade closest to the hub. On these designs, this is where the blades are strongest. However, on a vertical design, most of the stress is focused on the center part of the blades, where they are weakest. This can be somewhat overcome by using a more beefed up design, but by doing so the turbine becomes heavier, and therefore less efficient.
The other disadvantage is that the vertical axis wind turbine cannot be built as tall as a horizontally-oriented design. The reason for this is that there are two bearings used for a vertical design, one on the top and one on the bottom of the drive shaft. In most cases, the bottom bearing can be supported by the weight of the machine; however, the top bearing must be supported by guide cables. This creates two problems. First, the taller the turbine, the more support it requires and therefore the farther out the guide cables much reach in order to be effective, which causes problems with available space. Some people argue that a shorter vertical turbine can be used without the guide cables; but this creates the second problem. Wind speeds decrease as you get closer to the ground, and as such, short turbines don't always receive enough wind power to generate any substantial amount of electricity.
Improvements in design might make the vertical axis wind turbine more viable in the future, but for now they are largely relegated to small-scale duties.
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