Ideal Path for Wind Turbine Blade
Doesn’t it seem like the ideal path for a wind turbine blade would have the entire blade moving at “tip speed”, spending the majority of its time traveling in a direction that is orthogonal to the direction of the wind?
Description of the Machine
(Note that the word “power cable” in the diagram below does not refer to an electrical cable, but rather to a cable that carries the mechanical power to the pulley wheels at the generators.)
Drag on Moving Cable
How much aerodynamic drag does a moving cable add to the system compared to the drag of a strut or the low speed part of a troposkein Darrieus blade? Consider an H Rotor supported by a strut. The instantaneous velocity of a given radial location on the strut is at right angles to its longitudinal dimension. For this reason, it is able to leave a semi-evacuated space behind it as it moves. This space is filled with turbulent air. But the velocity of a given cylindrical cross-section of a cable is along its longitudinal dimension. Because there is always more cable following any cylindrical cross section, it is impossible for the cable to leave a semi-evacuated space in its wake. The only semi-evacuated, turbulated space it can leave is on its downwind side, and the force thus produced cannot drag down the system because it is orthogonal to the velocity of the cable. The only aerodynamic drag that is possible is that drag that is created by the cable’s ability to “stick to the air around it”. I’m not an aerodynamicist, but I’m guessing this might turn out to be pretty small. Also, the moving cable in the machine presented here has small diameter because it moves at “tip speed”, and because it is only required to carry the torque at tip speed. Because tip speed is high, torque will be low, and the cable will have small diameter.
One obvious disadvantage of the oblong path is the need to yaw the track. We can eliminate this problem by providing the turbine with a polygonal path that approximates a circle:
Airfoils Could be Slowed to Go Around Tower
To avoid inflicting excessive centrifugal force on the blades as they go around the tower, maybe the control system could sense the point at which blades are nearing towers, and slow them down somewhat as they make their way around the tower. Alternatively, mechanical support could be provided on the opposite side of the airfoil to prevent excessive bowing due to centrifugal force:
Low Drag Airfoil Variation
The airfoils need to have gaps near the regions where the load carrying cables penetrate them. This is necessary to accomodate the mechanical supports for the apparatus that steers the airfoils around the tower. But if the airfoils carry a small generator, like the generator that rubs against a bicycle tire in order to power a light for the bicycle, then these gaps could be filled with streamlined shapes during the long path between the two towers. The streamlined shapes would be pulled out of the way as the airfoil makes its way around a tower. The actuating mechanism is powered by electricity that is generated from the small generator just described that is located inside the blades.
Alternatively, maybe the streamlining shapes could be pushed out of the way by some mechanical means as the airfoil goes around a tower.