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Wind velocity (in x direction) downwind of the trajectory of 80m long tethers, outer half aerodynamically shaped
Lars Bäckström
University Lecturer in Energy Technology Umeå University
Department of Applied Physics and Electronics
901 87 Umeå Sweden lars.backstrom@umu.se
www.tfe.umu.com
Fusing Kite and Tether into one Unit
Lars Bäckström1 , Tom Guilloux2 1 Umeå University 2 ESTACA, Laval We propose to fuse the tether and kite together into one unit, resulting in an aerodynamically shaped airborne fly-ing tether with high aspect ratio. With this concept we turn the aerodynamic tether into an energy harvester that eliminates the need of a separate kite.
Many airborne wind energy systems under development use high speed crosswind kite systems with some kind of soft or hard wing that is connected to the ground by one or more tethers. In these systems it is well known that the drag forces of the fast moving tethers have large negative impact on power output and also limits altitude. By using an aerodynamic profile for the tether we can 1) reduce the drag forces and 2) produce lift forces perpen-dicular to the tether and its motion.
How can these lift forces be used for energy harvesting? The lift force is perpendicular to the tether, but the tether itself can only take forces in its own direction. Our so-lution to this problem is to balance the lift force with a centrifugal force, which with correct distribution of mass gives a resulting force in the direction of the tether. The suggested flight path for the flying tether can be de-scribed as a cone with its rotational axis pointing about 50 degrees upwards in the downwind direction. With this proposed arrangement we can have a ground based ver-tical axis generator as illustrated in figure or it can be air-borne and tethered to the ground by a cable.
What airfoils could be used in this concept?
A requirement for the airfoil used is that the pitching
torque produced should stabilize the angle of attack. For that we need an airfoil with high positive pitching mo-ment and place the center of mass close to the leading edge. It is also desirable that the airfoil have high lift to drag ratio. The airfoils we have developed for this have Cmaround 0.05 and L/D ratio around 100 at Reynolds 106.
The flight path of the tether can be controlled by moving weights inside the tether, tilting the generator or control surfaces. This concept may also be used for traction or lifting.
Wind
Conical path of flying tether and resulting forces. Blue are lift forces, green centrifugal forces and red the resultning tether forces.
