This is an interesting topic for sure.
- Soft kite durability is an issue still unsolved. I dont think a power plant where the kites need replacement every few months is likely to be economic viable, nor perhaps environmentally friendly
- Soft kites will never approach a rigid wing’s fidelity in actuator precision and perhaps also the magnitude of actuated forces. Because a soft kite will change shape. Soft kites need to be passively stable («need») because of this. History has shown that evolving soft kite design is hard and time consuming.
Rigid kites truck vs racecar this is true and a big design limitation making rigid power kite design fundamentally different than aircraft design. Understanding this may probably initiated the recent paper on high Cl biplane wings. Quite a few of the rigid wing actors show elements of high Cl design if you look closely already.
A sailplane can be made to have a L/D ratio of, say 15. By applying advanced aerodynamic analysis, an increase in this number to eg. 20 would directly translate to a faster plane with longer range. Any added weight translates to larger flying speeds. This is manageable.
For kites the story is a bit different. You can have the same L/D 15 wing, then add the tether to get a wing with total L/D 7. Now increase the L/D to 20, and your total L/D is still around 7. Tether drag is dominating kite drag.
You can alleviate this by having a larger wing or higher Cl or both, in which case the wing will easily break the tether unless carefully managed.
The tether drag may be approximted by
F_d = 1/4 (1/2 rho Cd length diameter v^2)
This may be interpreted as tether drag being 1/4 of drag compared to all of the tether moving at the kite’s speed. Though we have the 1/4 factor, we still have F_d proportional to length and v squared.
How high can a rigid wing kite fly without tether drag dominating? You’d have to do the exact numbers for your own rig, but 200 meter is proven (?) fine, and 2 km is too much tether for most I believe.
The tether diameter scales with area not diameter, so for very large kites, you can go higher without as large issues. If your design uses many tethers, the issue is compounded.
Noe back to the truck vs racecar argument. If you instead use a huge soft kite with L/D of only 4, the velocity of the kite is much smaller. In this way you get rid if the bulk if the v^2 part of the tether drag equation. In short you get the same pull wasting less energy on tether drag
Having said this, I am not convinced that this argument would out rigid wing designs in the infeasible bracket. It just pans out as stuff you need to design your rig for, and in the end, limitations in performance.