Is there some napkin math available on the net for a transmission line with a nitrogen-cooled high-temp superconductor (Tc > 90K) and thick thermal insulation? I mean for the energy required per km too keep it cooled below Tc.
Big high voltage transmission lines lose ~200 watts per meter in resistive losses when under full load.
The electrical energy to keep something 1 meter long at liquid nitrogen temperatures is also ~200 watts, assuming 8 inches of insulation.
The resistive losses go down with the square of the power transmitted - so they fall to zero rapidly when not under full load. Cooling losses stay approximately constant.
Therefore, I suspect a liquid nitrogen cooled superconducting cable wouldn't work out financially.
Math only correct to an order of magnitude...