Superconducting Power Lines

[f95toli]

Originally posted by: BrownTown
Well liquid hydrogen needs a much lower temperature, so it would be harder to obtain. not to mention the fact that liquid hydrogen has a relatively low energy density, and the conversion between gas to liquid and back to gas would probably be rather efficient. if you wanted to transmit energy across long distances electric lines are a far more efficient energy carrier then liquid hydrogen.

Well yes, but the basic idea would be to use the liquid hydrogen as fuel for cars. And since hydrogen is much colder than liquid nitrogen it would be possible to significantly increase the current density of the superconducting wire (which would just be running inside the pipeline); i.e the same network would be used for fuel and electricity.
As far as I remember this idea came out of a study commisioned by DOE and the main author was Paul Chu (a pretty famous guy, now a the Texas Center for Superconductivity).

That said, it would require HUGE investments in infrastructure and safety would be major concern (liquid hydrogen isn't exactly safe to handle).
I have heard a few talks about this ideas at various conferences, but it is not very likely to happen.

 

[Mark R]

Interesting.

The main advantage of superconductors at the moment seems to be size - superconducting cables are much smaller and the big interest is in using them to upgrade dense power grids where where the costs of conventional conductors would be prohibitive. E.g. in dense city centers, where primary power cables are buried in underground conduits. It may be impossible or impractical to upgrade the conduit to take uprated cables.

In terms of efficiency, current superconductor technology is considerably less good. The big problem is the cooling, which is very energy intensive, as heat must constantly be removed the superconductor: both thermal leakage through the thermal insulation (and through the splices where the conductors exit the cryostat) and resistive losses in the superconductors (and conventional conductors). Initial studies have suggested an energy loss of about 3-4x that of conventional conductors, so initial installations will not be for energy effiency reasons, but for installation cost reasons.

It's very unlikely we would move from 3 phase to single phase. 3 phase has several big advantages: 1. There is constant generation and delivery of power - this means that 3 phase generators run smooth (as do 3 phase motors), whereas single phase motors necessarily give pulsed torque (which cause vibration and premature wear)
2. You can use smaller conductors with 3 phase, because of current cancellation in the neutral conductor. You can use 4 cables to deliver 3 phase power, but if the power was single phase, it would require 6 cables (or 2 cables of 3x the area - actually about 4x the area due to cooling requirements). This is less of a problem with superconductors, but superconductors have a maximum current rating too - the difference is that if you exceed it, the cable loses superconducitivity. (This is the basis of the superconducting fault current limiter mentioned in the article).

It's possible that HVDC links will be increasingly used. They have several interesting advantages, although the one that's most interesting for superconductors is that the use of DC will abolish the resistive losses in the superconductor, and drastically decrease the cooling requirement.

 

[PolymerTim]

I am curious to see what the final size difference is after the cooling sheath is included. The picture comparison in the article doesn't seem quite fair since they have the copper cable with nearly all its insulation and then the superconductor with apparently no cooling sheath. Am I seeing that right. I don't no how much power the copper cable shown can handle, but if the first picture of the three superconducting cables going underground is comparable, then they van get just as large.

Can anyone clarify just how much size difference there is? I think that article just doesn't give enough info to prove the claim. The picture comparison looks lie a typical marketing comparison to me.

 

[silverpig]

Originally posted by: Mark R
Interesting.

The main advantage of superconductors at the moment seems to be size - superconducting cables are much smaller and the big interest is in using them to upgrade dense power grids where where the costs of conventional conductors would be prohibitive. E.g. in dense city centers, where primary power cables are buried in underground conduits. It may be impossible or impractical to upgrade the conduit to take uprated cables.

In terms of efficiency, current superconductor technology is considerably less good. The big problem is the cooling, which is very energy intensive, as heat must constantly be removed the superconductor: both thermal leakage through the thermal insulation (and through the splices where the conductors exit the cryostat) and resistive losses in the superconductors (and conventional conductors). Initial studies have suggested an energy loss of about 3-4x that of conventional conductors, so initial installations will not be for energy effiency reasons, but for installation cost reasons.

It's very unlikely we would move from 3 phase to single phase. 3 phase has several big advantages: 1. There is constant generation and delivery of power - this means that 3 phase generators run smooth (as do 3 phase motors), whereas single phase motors necessarily give pulsed torque (which cause vibration and premature wear)
2. You can use smaller conductors with 3 phase, because of current cancellation in the neutral conductor. You can use 4 cables to deliver 3 phase power, but if the power was single phase, it would require 6 cables (or 2 cables of 3x the area - actually about 4x the area due to cooling requirements). This is less of a problem with superconductors, but superconductors have a maximum current rating too - the difference is that if you exceed it, the cable loses superconducitivity. (This is the basis of the superconducting fault current limiter mentioned in the article).

It's possible that HVDC links will be increasingly used. They have several interesting advantages, although the one that's most interesting for superconductors is that the use of DC will abolish the resistive losses in the superconductor, and drastically decrease the cooling requirement.

Superconductors work with 0 resistance... ie no joule heating and no thermal losses.

You can make insulation pretty bloody efficient. It's more complicated yes, but it'll have large advantages in high density areas (cities).

 

[CycloWizard]

This isn't really related, but I was told by an electrical engineer that most power lines are aluminum rather than copper because Al is much less dense and nearly as conductive. Is this really true? It makes sense that the high voltage lines that everyone seems to think are the primary candidates for replacement are copper, but in general, is Al or Cu more prevalent?

 

[RideFree]

Originally posted by: Stiganator
Liquid Nitrogen is not 1.00/gal, its like 5 cents/liter.

I'll bet you Googled it & did not realise that it was a 1994 price.

 

[DrPizza]

Originally posted by: CycloWizard
This isn't really related, but I was told by an electrical engineer that most power lines are aluminum rather than copper because Al is much less dense and nearly as conductive. Is this really true? It makes sense that the high voltage lines that everyone seems to think are the primary candidates for replacement are copper, but in general, is Al or Cu more prevalent?

I'm not sure that's exactly the reason, or the only reason. But it's one of the major reasons that boils down to: it's more cost effective to use aluminum than copper.

 

[KIAman]

The only issue I see is their risk mitigation dealing with line breaks and such. Once the line loses coolant and rises in temperature, the resistance goes up which could cause quite a bad chain reaction.

 

[BrownTown]

Originally posted by: DrPizza

Originally posted by: CycloWizard
This isn't really related, but I was told by an electrical engineer that most power lines are aluminum rather than copper because Al is much less dense and nearly as conductive. Is this really true? It makes sense that the high voltage lines that everyone seems to think are the primary candidates for replacement are copper, but in general, is Al or Cu more prevalent?

I'm not sure that's exactly the reason, or the only reason. But it's one of the major reasons that boils down to: it's more cost effective to use aluminum than copper.

also, high voltage power lines aren't just aluminum either, they are usually a steel core surrounded by aluminum. The skin effect means that it doesn't really matter what the inside of the line is made of (for overhead ground wire they sometimes put fiber optic cable at the center).

 

[Mark R]

Originally posted by: silverpig
Superconductors work with 0 resistance... ie no joule heating and no thermal losses.

With DC currents, yes. With AC currents, there is a small, but finite resistance, and therefore thermal losses and ohmic heating - the mechanism is to do with interactions of the dyamic magnetic field and the super-conductor.

At 60 Hz, the resistivity of a superconductor is a few orders of magnitude less than a conventional conductor, but at the current densities proposed, and the low COP of cryocoolers, removing it is a non-negligible part of the energy budget of such a transmission line.

 

[PowerEngineer]

Originally posted by: BrownTown

Originally posted by: DrPizza

Originally posted by: CycloWizard
This isn't really related, but I was told by an electrical engineer that most power lines are aluminum rather than copper because Al is much less dense and nearly as conductive. Is this really true? It makes sense that the high voltage lines that everyone seems to think are the primary candidates for replacement are copper, but in general, is Al or Cu more prevalent?

I'm not sure that's exactly the reason, or the only reason. But it's one of the major reasons that boils down to: it's more cost effective to use aluminum than copper.

also, high voltage power lines aren't just aluminum either, they are usually a steel core surrounded by aluminum. The skin effect means that it doesn't really matter what the inside of the line is made of (for overhead ground wire they sometimes put fiber optic cable at the center).

All true. The primary driving force is that aluminum is far cheaper than copper. Virtually all high voltage power lines constructed over the past 50 years have aluminum conductors.

 

[Mark R]

Originally posted by: CycloWizard
This isn't really related, but I was told by an electrical engineer that most power lines are aluminum rather than copper because Al is much less dense and nearly as conductive. Is this really true? It makes sense that the high voltage lines that everyone seems to think are the primary candidates for replacement are copper, but in general, is Al or Cu more prevalent?

Pretty much it.

It comes down to cost efficiency. Copper cables are much heavier for the equivalent resistance. The problem then is one of how do you build towers capable of supporting the weight of the cables - the reduced weight of aluminium cables meant enormous cost savings for the towers, making it the overall cheaper solution.

The most common solution at present is steel cored aluminum cables. The steel provides structural support to the Al, although at the cost of some weight.

There are some new technologies around, which are being marketed to power companies. One is carbon fiber cored aluminium. The carbon fiber provides structual support, but with lower weight, so more aluminium can be used, lowering the overall resistance. Additionally, the carbon fiber has a lower thermal expansion coefficient, so the maximum operating temperature can be higher (allowing higher currents). Normal Al/steel cables are limited to about 90 C due to thermal expansion, but some manufacturers are claiming safe operating temperatures up to 250 C for their carbon fiber core cables.