Hugh Burn asks why wind turbines cannot comprise hollow hemispheres rotating in a horizontal plane, like large anemometers (NCE last week).
Conventional rbines transmit a constant stress to the pylon at any given wind speed.
Unless there is a significant difference in wind speed between the lower and upper extremities of the blades, the horizontal forces do not vary as the blades rotate.
A hemispheric cup turbine, or one based on aerofoil section blades which was proposed some years ago, presents an ever-changing profile to the wind. This causes variations in stress according to the orientation of the rotor at any instant in time, and in turn leads to cyclic loading and consequent oscillation of the pylon. I believe that is why the aerofoil design never got beyond the prototype stage.
Paul Hayward (M), Rowan House, Viewlands, Silkstone Common, Barnsley, S75 4QP