I remember reading once about jet streams, and I found them very interesting. Jet streams are high-speed air flows that happen on high altitudes in certain locations of the World.

The idea is a turbine, that is supported by ballons. It is tethered to the ground and remains somewhat in position. 

It uses the high speed currents to generate energy that is sent through a cable to ground, and from there transmission lines take it the rest of the way to the grid.

Now the first thing that came to my mind is that, this thing is going to twist and starts spinning like crazy. It would be necessary to have some system to keep its orientation. Something in the design that made air flow in such a way to produce restaurative forces.

Now, I have not considered the roughly 8-km cable, can a cable be suspended this long?