I did some back of the napkin calculations to compare a TRPT with Makani’s 5 MW design from the FAA application.
The Makani rig produces 5 MW for a wing weighing 10 ton and a tether weighing 3.6 ton. The wingspan is 65 meter. Tether is 1 km, looping radius is 265 m.
I suggest replacing this with a 6 kite TRPT, with carbon fiber spacers in the shaft. The six kites have wingspan 27 meter, and the flying radius is thus reduced to the equivalent looping radius 108 m. The shaft diameter is 72 meters (one third of the looping radius, a number chosen at whim). The spacers will be looking like “star of david”, and six of these spacers are deemed necessary to support the hollow shaft. 6 rods of approximately 60 meters are needed for each spacer assembly.
From the paper [2] we look at the optimum weight of the hollow spars for the supports. A value of \phi = 10 and \gamma = 18^{^o} is chosen (again on whim) and some calculations later (eq 17 in [2]) I arrive at a total weight of the spacers of 1360 kg. This is very small compared to the weight of Makani’s specified conductive tether.
Now if this is even close to correct, the TRPT is quite interesting because it does use a lot less airspace, is unaffected by gravity slowdown, requires less control input and still provides a way of energy transfer to the ground.
If more precise calculations were made, I would assume the tether length would have to be reduced to approximately 400 m for the TRPT to account for extra tether drag using six tethers rather than one.
- [1] http://www.energykitesystems.net/FAA/FAAfromMakani.pdf
- [2] Design of laminated composite cylindrical shells under axial compression P.M. Weaver*