If practical superconductors were available

[silverpig]

Originally posted by: Nathelion
Please correct me if I'm wrong, but isn't there a limit to how much current a given superconductor can take before it stops being superconducting?

Yes, but you can just make the superconductor thicker and it will conduct more of course

 

[Cogman]

Originally posted by: silverpig

Originally posted by: Nathelion
Please correct me if I'm wrong, but isn't there a limit to how much current a given superconductor can take before it stops being superconducting?

Yes, but you can just make the superconductor thicker and it will conduct more of course

Isn't the number absurdly high as well? Like you said, make the super conductor a little thicker and viola, you have a much more capacity.

 

[KIAman]

Originally posted by: Nathelion
Please correct me if I'm wrong, but isn't there a limit to how much current a given superconductor can take before it stops being superconducting?

That's interesting that there would be a limit to the amount of current a super conductor would be able to transmit.

I had the impression that resistance and the resulting heat caused physical limits to normal conductors and without resistance, a superconductor would not be so constrained.

What would be the next physical limitation? The number of electrons flowing?

 

[bobsmith1492]

Originally posted by: KIAman

Originally posted by: Nathelion
Please correct me if I'm wrong, but isn't there a limit to how much current a given superconductor can take before it stops being superconducting?

That's interesting that there would be a limit to the amount of current a super conductor would be able to transmit.

I had the impression that resistance and the resulting heat caused physical limits to normal conductors and without resistance, a superconductor would not be so constrained.

What would be the next physical limitation? The number of electrons flowing?

There is a limitation to current - it depends on the diameter of the cable though I don't know the exact limiting mechanism.

Superconductors are used in massive-current (thousands of amps) surge limiters or circuit breakers. A surge of current causes the superconductor to stop superconducting, limiting the current flow and giving an easily-detectable fault signal.

 

[Mark R]

Originally posted by: KIAman

Originally posted by: Nathelion
Please correct me if I'm wrong, but isn't there a limit to how much current a given superconductor can take before it stops being superconducting?

That's interesting that there would be a limit to the amount of current a super conductor would be able to transmit.

I had the impression that resistance and the resulting heat caused physical limits to normal conductors and without resistance, a superconductor would not be so constrained.

What would be the next physical limitation? The number of electrons flowing?

The problem in superconductors is the magnetic field generated by the current flow. Where a magnetic field line passes touches the superconductor it generates a 'vortex' which is an area where superconductivity is lost.

The higher the current, the higher the magnetic field strength created by the current, which in turn means more closely packed field lines and vortices. When there are only a few vortices, cooper paired electrons are able to go around them in superconducting fashion. However, as the number of vortices increases, eventually they fill the superconductor meaning only resistive current flow is possible.

 

[yh125d]

Make the world's WORST electric radiant heater



Do I win?

 

[Ken g6]

I suppose that depends on your definition of practical. We have three things that might qualify right now:

1. Low temperature superconductors
Benefits:
- Easy to make, with well-known metals.
- Malleable (flexible)
- Ductile (so they can be formed into wires)
- Relatively insensitive to magnetic fields (so they can be used in maglev trains, MRIs, etc.)
Problems:
- They only operate at liquid-helium temperatures. (Some operate in the range of liquid hydrogen, so if we run out of helium, we might be alright.)

2. "High-temperature" superconductors
Benefits:
- Can use liquid nitrogen cooling, which is much cheaper than liquid helium.
Problems:
- Require unusual, sometimes costly materials.
- Are ceramics, so they aren't very malleable or ductile.
- Relatively sensitive to magnetic fields (so they aren't completely superconducting when carrying large currents, or when formed into (useful) electromagnets.)

3. Carbon nanotube wires
"...A perfect metallic nanotube should be a ballistic conductor.... Although a ballistic conductor does have some resistance, this resistance is independent of its length.... Indeed, only a superconductor (which has no electrical resistance whatsoever) is a better conductor." (1)
Benefits:
- Works at room temperature! Wikipedia says they're stable up to 750 C in air.
- Cheap materials (e.g. carbon)
- Probably malleable, if not ductile
- Probably not sensitive to magnetic fields
Problems:
- Not a superconductor; just a very low-resistance conductor. "...nanotubes are predicted to have a minimum resistance of about 6500 Ohms, independent of their length." That's quite large, but that's per-nanotube; and "it would take several million lying side by side to cover an inch..." (2)
- Can't store current in a loop (probably, although I wonder what would happen if a single nanotube were made into a ring).
- Hard to manufacture and manipulate (so far).

So to be "practical":
- How much does it have to be cooled, if at all? Dry-ice temperature? Regular ice temperature?
- How cheap must the materials be?
- How easy must the manufacturing process (for the material and/or for wires) be?
- How much magnetic field tolerance is required?

 

[Born2bwire]

They have a long ways to go with CNT, the maximum ballistic length is on the order of millimeters.

 

[silverpig]

Originally posted by: Born2bwire
They have a long ways to go with CNT, the maximum ballistic length is on the order of millimeters.

Millimeters already? The MFP in most high quality crystals is hundreds of microns, and that's at low temperature.

 

[NutherWorkingDrone]

Originally posted by: bobsmith1492

Originally posted by: KIAman

Originally posted by: Nathelion
Please correct me if I'm wrong, but isn't there a limit to how much current a given superconductor can take before it stops being superconducting?

That's interesting that there would be a limit to the amount of current a super conductor would be able to transmit.

I had the impression that resistance and the resulting heat caused physical limits to normal conductors and without resistance, a superconductor would not be so constrained.

What would be the next physical limitation? The number of electrons flowing?

There is a limitation to current - it depends on the diameter of the cable though I don't know the exact limiting mechanism.

Superconductors are used in massive-current (thousands of amps) surge limiters or circuit breakers. A surge of current causes the superconductor to stop superconducting, limiting the current flow and giving an easily-detectable fault signal.

A thicker cable is not the answer, as even superconducting cables are subject to the skin effect. Back around 1990 I visited Brookhaven National Lab where they were building the initial segments of the SSC. The magnets used superconducting cables composed of 5 micron diameter filaments, and the cable was about 2 inches diameter (IIRC after 19 years).

As for likely uses, a similar discussion years ago decided that besides medical MRIs the first use was going to be electrical transformers in substations. In a substation you have a large collection of transformers that could benefit from a centralized super-critical cooling system. With sufficient insulation, once you cool the transformers down to SC levels then it takes little energy to keep them at the required temp; remember, SCs don't generate heat due to resistance. The power grid consumes ~6% of generated power due to distribution & transmission loss, so the math worked out that SC transformers had the most bang for the buck.

Having said all this, I am at a loss to explain either the economics or science that prevent our substations from being large SC installations.

 

[Eeezee]

Power transmission losses are enormous, so that'd be the first application right there.

 

[OfficeLinebacker]

Originally posted by: Cogman
I think power generation would become much more centralized, after all, why build 1000 plants everywhere when you can build 100 high power plants in the central of a country and have all the engineers you could want in a close vicinity.

Cos the enemy would know exactly where to point their missles!

However, if you really could centralize, why do it in Nebraska when you could do it in Saskatoon or Fairbanks or even near one of the poles? That also would presumably save money.

 

[Jabbernyx]

Ultra high speed logic using Josephson junctions?

 

[TecHNooB]

I swear, you're all so freakin smart. Do you all have PhDs or something? Need to get smarter..

 

[Mark R]

Originally posted by: NutherWorkingDrone
With sufficient insulation, once you cool the transformers down to SC levels then it takes little energy to keep them at the required temp; remember, SCs don't generate heat due to resistance. The power grid consumes ~6% of generated power due to distribution & transmission loss, so the math worked out that SC transformers had the most bang for the buck.

Having said all this, I am at a loss to explain either the economics or science that prevent our substations from being large SC installations.

Actually, SC cables do produce heat due to electrical resistance in AC circuits - e.g. transformers. The resistance of superconducting cables is 0 for DC, but not AC - although even for AC it is very small. However, in terms of the thermal budget for a practical superconducting power line or transformer, resistive losses do significantly erode into it, although they are smaller than thermal leakage (at least for power lines).

In the case of superconducting transformers, leakage is less as a component of the losses - and resistive losses are of a similar size to leakage - typically around 200 W each for a 1 MVA transformer - which would requires about 3 kW (consumption) of cryoplant to replenish the liquid nitrogen.

When you consider that a conventionally conducting 1 MVA transformer would have total losses less than 10 kW - you can see, the concept of the Superconducting transformer is considerable effort for only marginal gains. This is a technology that would likely only be seen at the very largest power plant transformers, or distribution transformers - e.g. those with power ratings of over 1000 MVA.

 

[Jabbernyx]

Originally posted by: TecHNooB
I swear, you're all so freakin smart. Do you all have PhDs or something? Need to get smarter..

*sheepish grin* Working towards one

 

[Born2bwire]

Originally posted by: silverpig

Originally posted by: Born2bwire
They have a long ways to go with CNT, the maximum ballistic length is on the order of millimeters.

Millimeters already? The MFP in most high quality crystals is hundreds of microns, and that's at low temperature.

Yeah my bad. I just got around to typing up my notes on this this afternoon and saw that I had written down that DC ballistic transports were on the order of micrometers, at least when that review paper was published in 2007.

 

[sdifox]

Isn't most of the power loss in the step up and step down stage as opposed to the actual line loss? Assuming the power requirements of superconductors are negligible, you can eliminate the step up and step down stages.