What IS the current in a superconductor measured as?

[Stoney3K]

the term is layman (from brick layer? not sure of the etymology.) but layman's terms are usually thought to be an oversimplification, and on the contrary that is an excellent analogy! that is _exactly_ how it is, and a much better description than _i_ did!

What can I say, I watched a lot of Stargate Atlantis, and picked up some traits from McKay along the way. More specifically, the tendency to spam tedious paragraphs of technobabble only to finish them with a simple, understandable English analogy.

I did have a bit of a brain-fart here, because frictionless flywheels are pretty much the holy grail of energy storage when it comes to mechanical engineering. Since SC loops are the electrical equivalent, how efficient would they be in acting as a 'magnetic capacitor', capable of storing energy like a battery would?

Especially when critical temperatures would be going up more, this could have some serious impact on the way we power our cars, laptops and cellphones. Chemical batteries are kind of evil.

[erblo]

If you trust wikipedia: http://en.wikipedia.org/wiki/SMES

They are very efficient, up to 95% energy out/energy in, but also expensive.

[Stoney3K]

If you trust wikipedia: http://en.wikipedia.org/wiki/SMES

They are very efficient, up to 95% energy out/energy in, but also expensive.

I guess the main losses (and cost) are being caused by the cryogenic system needed to keep the SC coil at temperature. Get that out of the loop (something not infeasible with superconductors at 'normal' refrigeration temperatures) and the cost go down a LOT.

A superconductor which goes critical at 0C or even -10C can be maintained by your regular household freezer (or a simple glycol loop cooling) or any similar system on board a vehicle or even spacecraft. If you're creative enough, you would probably even be able to do it with a TEC for mobile applications, since the amount of heat flux is fairly minimal.

The Wikipedia article focuses mostly on SMES for utility storage, not as replacement for batteries where only a few dozen kJ of energy needs to be stored.

[mdeminico]

That poses another question: How are you supposed to calculate the energy stored in a superconducting loop, when it has effectively zero resistance?

Inductance. Or equivalently the energy of the magnetic field produced by the current.

That's the question I have... the energy of the magnetic field is determined by the *current* going through it, so how the heck do you measure that?

[mdeminico]

What can I say, I watched a lot of Stargate Atlantis...

Was it Stargate, or some other show, where the intro & promos said "Imagine being transported 30,000 light years into the future..." I remember that from over a decade ago and still think it was the most stupid thing I've ever heard.

[happyjack27]

What can I say, I watched a lot of Stargate Atlantis...

Was it Stargate, or some other show, where the intro & promos said "Imagine being transported 30,000 light years into the future..." I remember that from over a decade ago and still think it was the most stupid thing I've ever heard.

"fast? it did the kessel run in 21 parsecs!"

[mdeminico]

A superconductor which goes critical at 0C or even -10C can be maintained by your regular household freezer (or a simple glycol loop cooling) or any similar system on board a vehicle or even spacecraft. If you're creative enough, you would probably even be able to do it with a TEC for mobile applications, since the amount of heat flux is fairly minimal.

The problem with maintaining it at those higher temperatures is that the critical current tends to go downhill as the temperature rises. In other words, the higher the critical temperature of the material, typically (or always, I'm not sure) the higher the critical current.

Presumably that's why they maintain them with liquid helium, because they need that temperature to get the amount of current through it to perform their task, plus some for a buffer. Really high temperature materials could possibly use liquid nitrogen, because they probably figure they'll *never* need so much current that liquid helium temperatures would be required.

[happyjack27]

from what i read in the wikipedia article the cost of cooling is relatively minimal. the main impedement seems to be the maximum current, and that is mostly limited by the mechanical strength of the materials - too high of a magnetic field and it'll break under its own internal pressure. most of the issues in the "technical challenges" section seem to stem from the maximum current issue. (e.g. anything mentioning a 100 mile coil)

[happyjack27]

did some research on compressive strength on materials and all i found was concrete. also, that decreasing grain size is the way to go. not sure how one would decrease grain size in a superconducting cable. but you can make a concrete "guide" on the inside of the toroid to resist compression. e.g. with a cross-section that would look something like: )> <(

also, simply giving it an aspect ratio closer to a sphere would probably help.

[hanelyp]

That's the question I have... the energy of the magnetic field is determined by the *current* going through it, so how the heck do you measure that?

Measuring the current through a conductor without a series load resistor? Simple, measure the magnetic field produced. The Hall effect or magneto-resistive effect could be used.

[Stoney3K]

...most of the issues in the "technical challenges" section seem to stem from the maximum current issue. (e.g. anything mentioning a 100 mile coil)

Most of the issues also focus on utility storage of energy, not for movable applications.

I hope you can imagine that a 100 mile coil buried underground would be a bit useless if you want to power a car or laptop with it.

Given your average laptop battery, which is about 12V/4Ah, the amount of energy you need to store for that would be 172kJ (or 48 Wh). That's a few orders of magnitude different from the MWh-scale devices they're testing now.

A coil a few inches across can be cooled effectively with a TEC assembly, which could also help in cooling the rest of the machine's components.