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Comment by joshuahedlund | Hacker News Reader

I'm gathering from the article and comments here and there that "room temperature superconductivity" would be an awesome breakthrough that would lead to lots of awesome things, but I'm not familiar with this sort of thing. Can you guys give some examples of why this would be so awesome?

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One simple answer: lossless power transmission over long distance. A measurable percentage of our electrical generation is lost in powerlines or transformers without doing any useful work.

Zero loss is only true for DC transmission. It's true, loss is lower than copper at 50-60Hz AC but radiative losses don't magically disappear. You're also limited in how much current you can carry since superconductivity breaks down in sufficiently high magnetic fields. For all that room temperature superconductors are revolutionary because of the jump in power density in things like motors and yes power lines.

But didn't we only use AC because it was more efficient? So we switch back, and turn it back into AC for our houses, until we can switch over completely.

AC becomes less efficient (financially and electrically) at more than 1 Megavolt. The big big transmission lines in the United States actually use DC because the AC transformers would have to be so large, and the losses at that high of a voltage are huge (and that 1 MV generators get to be fairly expensive).

NPR made a really awesome visualization of the United States grid (http://www.npr.org/templates/story/story.php?storyId=1109973...) that shows the lines used to transmit power from generating sources.

Adding to that high voltage more than anything is the benefit of AC.

The weight of the conductor is a big problem for long runs of power lines so if you double the AC voltage the weight of the conductor can decrease by 25%. DC over 1500 V isn't really possible/efficient (some limit of generators) but AC can be many hundreds of thousands of volts.

It appears some of my electronics training is sinking in, I put it to good use!

What do you mean by doubling the AC voltage to decrease the weight by 25%? If I continue doubling, will I eventually not need a conductor?

The true power of AC is the fact that the energy is transmitted through the electric and magnetic fields (Poynting vector). DC needs to push everything down a little copper tube. When you start oscillating things though, the effective area of your conductor increases greatly (you start using the air as a transmission medium). This is why you can do things like this (http://hacknmod.com/hack/field-of-fluorescent-tubes-powered-...).

I'm just going by my textbook, I'm still new to this.

I understand what you're saying about AC creating an emf and pushing itself through a conductor via magnetism and frequency. The skin effect would play a big role in this too. As I say I'm still new to this, I know enough to be dangerous.

Here's the exact quote in case you are interested:

"The weight of a conductor required to transmit a given amount of power a given distance with a fixed loss varies inversely as the square of the transmission voltage."

Ah ha! That's actually kind of cool.

Power's given as: P = V^2 * R R is given as: R = rhoL/(pir^2) Substituting: and clumping constants: P = (V^2 / r^2) * (rho*L/pi) Weight is proportional to r^2, we can see that voltage and weight are inverses of one another, and increasing the voltage for a fixed amount of power decreases the radius necessary to transmit it, decreasing the weight.

I really wish I still had my lecture notes about the energy flow through free space, this is the best I could find online - http://amasci.com/elect/poynt/poynt.html.

Graph 0.75^(log X) : http://www.google.com/search?q=0.75%5E+log+x

Anyway, DC current is still mostly a magnetic interaction. Actual electrons rarely get above a few cm/s.

>DC over 1500 V isn't really possible/efficient

High-voltage DC uses voltages in the hundreds of kV. It is also the most efficient way of sending serious amounts of power over long distances (even before superconductors). Just as required cable thickness decreases with AC voltage, it has to increase with current. HVDC gives allows for smaller cabling.

That is an excellent link. Thank you for sharing it.

No. We are using AC because it is much easier to convert voltages with AC (by using a transformer) than with DC. Generator can't give very high voltage, it is in the range of a few kilovolts, but that voltage can't be efficiently transported over long distances due to heat losses (yes avoidable with superconductivity), and super high (100s of kilovolts) voltage can't be used by consumers, so when delivered, it has to be driven down to 100s of volts (in a few stages).

Superconductivity will change all of that (maybe in a 100 years or so since existing power infrastructure is worth hundreds of billions if not trillions and is a pain to replace).

Ok, so we only use AC because it lets us use more efficient voltages. I think we understand each other.

Also, tying back into what you said about efficiency: AC power can be much more efficient due to this easy transformation. The power dissipated across a resistor (which is a reasonably good model for power lines) is directly proportional to the square of the current passing through it. Thus, by stepping power up to high voltages, you can drastically cut the current and hence resistive losses in the line.

...but this whole thread was prefaced on room temperature superconductors. Hypothetically at least, this high voltage advantage wouldn't matter if you could have 0 transmission losses with DC at low voltages.

The point anovikov was making is that AC is not intrinsically more efficient. In fact, it is less efficient over very long distances dues to radiative losses. We don't use AC because it lets us use more efficient voltages today. We use AC because 100 years ago we didn't have the technology to boost direct current to high voltages.

OK, so if we had been able to just blast DC down the lines at 100kV or whatever, that would have been better?

I would imagine this might also affect strategy regarding power plant placement.

Exactly, one of the big barriers to renewable energy now is that the sources of it, like areas with more wind or sunshine, are very far from the main consumers, like large east coast cities in the US.

On a global scale, with no transmission loss, Egypt could use the Sahara to export solar energy to Japan.

For example, super conductors can 'freeze in' magnetic fields. This means, that the super conductor can essentially only move along the field lines. ( Found this video [1], which shows the effect.) With the same effect you can 'levitate' objects, for example a bookshelf. So there would be endless possibilities for novelty toys.

[1] https://www.youtube.com/watch?v=TeS_U9qFg7Y

Won't regular electromagnet work the same way, just constantly use energy?

EDIT: ok, nevermind, it won't.

http://en.wikipedia.org/wiki/Magnetic_levitation http://en.wikipedia.org/wiki/Earnshaw%27s_theorem

Earnshaw's theorem only applies to static electromagnetic fields. An electromagnet coupled with some sensors and a computer can give feedback to make stable levitation. But, as you said, it would constantly use energy.

I don't know how much heat in CPUs is based on transmission resistance vs something inherent in the operations done (something analogous to how a Carnot heat engine with 100% theoretical efficiency is impractical) but if you have a circuit with no or significantly less power loss you could blow up Moore's law since if I recall correctly the limiting factor is heat.

As the wires get smaller and smaller and smaller, resistance becomes a huge problem, and you wind up needing a crapload of repeaters and wasting a ton of power.

Consider that r= (rho * L / A), and that you need a ~4nm tantalum sheath around copper wire to keep it from poisoning silicon. What happens to r when the width starts getting really small? 40nm - (4 + 4) = 32nm, 10nm - (4+4) = 2nm, so for a 4x process shrink we've seen a 16x reduction in area assuming height remains constant and that the cross section of the wire is roughly rectangular (neither of those assumptions is perfect).

So yes, superconductors would be nice.

The trend I've seen in Intel processors is that every time they jump down a manufacturing size (eg: 32 vs. 22nm), the power usage (per flop, at least) has gone down. Why's that?

Smaller transistors take less energy to switch. The problem with heat dissipation is not that the amount of energy wasted goes up, it is that the [i]ratio[/i] of energy wasted goes up. There is a point where the increase in resistance will nullify the decrease in switching costs, it is at this point where we would need superconductors.

Use asterisks for italics.

Smaller gates mean lower capacitance, so you need to drive less current into them to switch.

The nice thing about CMOS is that you only need to flow current when switching them (at other times current is blocked by an "OFF" transistor somewhere in the logic), so resistance doesn't matter as much as it would for a motor or a light.

This is not entirely true, in an ideal world a CMOS device just sitting there not switching would use basically no power, but in actual application there is some leakage (both form not being fully off and some current jumping from the gate). This leakage gets worse as the transistors get smaller and when you have billions of transistors even the tiniest amount of leakage adds up, the result is leakage current can be a significant amount of the power used by a modern processor.

IIRC the voltage can drop with the size, and power loss drops in proportion to the V^2

Have you seen this picture on Wikipedia?

http://en.wikipedia.org/wiki/File:CERN-cables-p1030764.jpg

The killer applications would probably be power transmissions (as saulrh mentions) and magnets. It would definitely help make MRI's cheaper and probably smaller, and I think it would probably help a lot with electric motors.

Easy long distance power transmission might also help with renewables (think large solar plants in the Sahara or New Mexico with superconducting lines to bring the power without loss to where we want it)

Have you seen back to the future? Hover boards: http://en.wikipedia.org/wiki/Magnetic_levitation_device#Supe...

Since everyone is talking about power transmission like a bunch of boring old farts, I'll just leave this here:

http://www.youtube.com/watch?v=Ws6AAhTw7RA

I believe you can also store energy in a super conducting loop (the current just keeps going around forever.)

So maybe we'd get a new type of battery if we have room temp superconductors. I haven't been able to find any power density figures on such a device though, so I'm not sure if it would be an improvement over Lion batteries. Anyone know?

They already have such things. They are called Superconducting Magnetic Energy Storage (SMES): http://en.wikipedia.org/wiki/Superconducting_magnetic_energy...

They're currently used to regulate power grids from unexpected load changes. Superconductors are good at absorbing and releasing large amounts of power very quickly (since there's no resistance), so they are ideal for situations like this.

There is a still a big energy cost in cooling these things though.

In addition to the things stated, electric motors and generators can become much more efficient.

Aren't they already 95% efficent. Not to much room for improvement.

More amps - your motor is going to melt down. More copper - your motor is going to be very big.

With superconducting coil, you could make a very small but infinitely powerful electric motor.

Very cool. Thank you.

CPU's and GPU's that don't get hot. No more need for cooling systems, no limits on hardware design due to need for temp control, smaller computers and laptops and more powerful mobile devices, and longer battery life.

Are superconductors also really good at transmitting heat? (I read that in a Niven book, I don't know if it's true).

If they are there would be a lot of applications on that front as well.

A search for "superconductor heat pipe" says Niven is right and superconductors can be used for super-fast cooling.

A superconducting wine glass that fits onto a superconducting cold spot on the table. Or a superconducting sleeping bag for those hot mosquito nights. (The sleeping bag also levitates on magnetic lines so you can sleep on air.)

> Or a superconducting sleeping bag for those hot mosquito nights.

http://en.wikipedia.org/wiki/Mosquito_net works pretty well...

Unfortunately, electrical superconductors are terrible conductors of heat. Metals conduct heat well because the conduction electrons occassionally scatter off of a thermal vibration, are accelerated, and the travel a good long distance before scattering again.

But in a superconductor, the conduction electrons are bonded to each other by quantum mechanics magic. It takes a lot of energy to break one loose, more than you can get from heat. In fact, if the material was hot enough to scatter many of the electrons (the critical temperature), it would stop superconducting. Basically, superconding electrons are a perfect mirror for heat.

I have some hope for composite materials made with superconductors. Long nanowhiskers might be able to serve as antennas that carry infrared signals from point A to point B.

Medical imagining (in the Hospital) and imagining of biologically interesting molecular structures (such as small proteins).

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