The tether was indeed medium voltage, but the motors and controllers were not, through use of a fairly novel stacked architecture.
There were still hard problems to solve there, but ultimately the power system worked well, and out performed it's original design targets. It was not responsible for the projects demise.
There isn't a large amount of energy stored in the tether catenary or elasticity. Any hardware to recoup it probably wouldn't pay for itself.
Offshore, there can be a lot of energy stored in buoy motion, and utilizing this can help, but probably in a resonant way that doesn't require additional hardware.
"Rigid" is a relative term. The tether already utilized a carbon fiber core. It doesn't get much stiffer, short of going to exotic fibers or more of them, both of which degrade performance in other ways.
The optimum is going to be closer to a strength limited tether than an overly stiff one.
The Airborne Wind Energy section is general, and discusses energy kite performance in a broad way - the tether drag was well predicted and wasn't a surprise for our prototype.
The inability to turn the necessary tight paths and some degraded aero performance are responsible for most of the performance miss. Also hugely concerning was the struggle to saturate power at high winds.
It is important to separate and highlight tether drag in the context of the purported big benefit of airborne wind turbines accessing higher, faster winds. While they do indeed access higher winds, they do so via tether length and elevation angle, both of which carry losses. The net effect in almost all scenarios isn't a win.
System scale is a crucial component of a cost competitive energy system. I lay out the argument for this in the early part of my section - Airborne Wind Turbine Performance - of the released documentation.
A 100 kW system may prove useful to some niche, but I don't think can be competitive with the utility energy market at large, even if you give away the system for free.
At the larger scales, these electronics are not meaningful cost drivers at all.
pretty sure there is a top level avionics layout in there.