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Comment by ReptileMan | Hacker News Reader

The second to holy grail is temperature above liquid nitrogen at around normal pressure with cheap and easy to obtain coolant. It will enable big things like really high voltage intercontinental transmission lines etc.

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We've already got there with YbCO. But just barely, which means useful amounts of current and magnetic field density bring it out of the critical region.

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.

The energy could be very low - after all we are really good at insulation.

They are of the same order of magnitude...

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...

The actual trade-off would have to be far better insulation e.g. double wall vacuum insulated stainless pipe. With much higher upfront costs.

So now you have two problems. Not only liquid nitrogen but also vacuum :)

This is my own ignorance, but what determines the power carrying capacity of a wire besides melting from the resistance? Could you transmit a lot more wattage through that same line when superconducting?

...and how that compares to the energy loss you'd have from resistance in a regular cable.

The energy loss (in the sense of where the heat ends up) is at the refrigeration plant. The cable itself extracts heat from the environment. So SC cables make sense for underground cables, where heat buildup is a problem.

We've had that for decades.

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