The Darrieus Turbine is a type of wind power generator. Technically, it's a VAWT, which stands for 'Vertical Axis Wind Turbine.' It looks like large eggbeater stuck in the earth. Patented in the U.S. in 1931 by its inventor, the Frenchman Georges Jeans Mary Darrieus, it is in widespread use for the direct generation of electric power from wind. The Darrieus Turbine differs from the familiar bladed-rotor windmill in its orientation. Whereas the bladed-rotor windmill faces into the wind, with the axis of rotation parallel to the ground, the Darrieus Turbine spins around an axis that is perpendicular to the ground. This means that the stress of the wind's direct force does not have to be handled at the top of a tall tower, with the resulting leverage problems (see below). Also, the moving parts of both the rotor and the electrical generation can be placed at ground level (or even under it) making maintenance easier and the structure simpler to properly brace.

There are other advantages. The Darrieus Turbine is airfoil driven, which means that its rotational speed is not tightly linked to the windspeed. Wind across the airfoils produces a constant rotational force on the turbine, accelerating it to operating speed. This is in contrast to 'drag' based wind systems, where the speed of rotation is closely linked to windspeed. This optimizes the Darrieus Turbine for constant speed operation in variable winds, which is good for electrical power generation, at the expense of torque which is more important to mechanical wind devices such as pump windmills.

Starting a Darrieus Turbine requires it to be spun up, since the Turbine itself usually doesn't have enough torque to start itself; it is usually done by reversing the generator and using it as a motor. Smaller scale versions sometimes have additional wind elements attached which can be used as drag elements at low speeds, producing enough torque to spin up the turbine before being moved to a neutral position.

There are two major 'types' of Darrieus mechanisms. They differ on how they handle the centrifugal force imposed on the blades of the turbine. One, called the 'squirrel cage' variant, consists of two disks at top and bottom, with the airfoils running straight up and down between their rims. This allows the centrifugal force to be handled by the relatively sturdy construction of the disks. The other, called the 'eggbeater' variant, has the blades curved into 'C' shapes, attached to the central rotor at top and bottom and bowed out in the middle. This allows for a lighter weight rotor mechanism and hence is more efficient; the forces are handled at the attachments at top and bottom. These can be more complex if the blades are variable-geometry. Eggbeater variants are usually built with two or three blades, whereas the squirrel cage variants usually have more due to the ability of the disks to handle greater lateral loading. Eggbeater variants can offload their lateral stress to guy wires stretched from the top of the structure to the ground, which is much more efficient than building structural load-bearing elements.

SharQ asks, appropriately, what the disadvantages are of the Darrieus turbine, since they're not that common. There are a few. For one, they're a patented design, which means they're probably more expensive to build. For another, they have their sensitive mechanisms down at ground level, as well as spinning blades which come much closer to the ground than other designs - these both mean that access control is a much more significant problem for them than for regular tower-dwelling turbines. In addition, the requirement to spin them up means that they're not great for use in areas with highly variable winds, and make them more difficult to operate automatically/autonomously.

The largest Darrieus Turbine in the world is (I believe) presently in Canada, at Cap-Chat Quebec. It is almost 100 meters in height. Although capable of producing nearly 4 MW of power, it is being operated at reduced speed to reduce maintenance load and is producing around 2.5 MW. Note that the basic design of the Darrieus turbine can be (and has been) used in hydropower - i.e., underwater, with water flow rather than airflow to turn the rotors!

Bad ASCII Art illustrations:

Standard Wind turbine - high levered force on structure:

                                       
                                       |
                                 |== \ |
                                 |  ==D>   <--- Wind Direction (force)
                                 |== / |
                              /   |    |
     Lever force on tower    /    |
                            /     |
                           L      |

Darrieus Wind turbine (eggbeater) - smaller mechanical loads on structure, force at base of rotor near ground:

                     
                    _|_  
                   / | \
                  /  |  \
                 |   |   |     <--- Wind Direction (force)
                 |   |   |   
                  \  |  /
                   \ | /
                    ===
                   =====

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