The Wiki page says the refrigeration cost is negligible. Which fits with intuition; given that the electrical losses are near zero, the refrigeration just needs to keep up with losses through insulation. The structure and superconductor costs dominate.
And sure, for certain applications the very high power density is nice. But that’s a pretty small niche. Li-Ion is sufficient for grid smoothing. Railguns and the like would benefit, though.
I thought natural gas turbines were direct combustion (or whatever it’s called), not steam turbines? Shouldn’t they be a hair more efficient as a result?
Yeah, SMES has been an application being developed for as long as I can remember, but has never taken off as a product. Though there are companies out there that will sell you one if you have the money.
The issue of refrigeration isn’t necessarily efficiency (in this case the amount of power “used” in storing a certain amount of power), but the fear that the cooling will fail at some inconvenient time (cryophobia is a nice trenchant description).
[sort-of off-topic]
Does anybody remember those flywheel energy devices for wrist-strengthening exercises? One brand was called a “Dynabee” but there are other brands. It’s a hand-held hollow plastic ball with a flywheel mounted inside. You start the flywheel spinning, then hold it in your hand and move your wrist in circles. The movement causes the wheel to spin faster and faster, but the energy required to do so causes resistance to the movement which exercises your wrist. You could get it going so fast that the bearings would start whistling.
[/sort-of off-topic]
Okay, here’s some questions arising out of absolute total ignorance on this subject, so be gentle please.
Does superconductivity truly mean totally zero resistance? Or merely very very low resistance?
Once you get a current going in a superconductor, what would make it stop? Only an external load with a resistance? (And would that external load be necessarily necessary if you wanted to extract any useful work from your superconductor?)
Is there any kind of danger that once started, a current in a superconductor could run out of control, with unpleasant results? Or is this just absurd doomsday thinking (and if so, why)?
Comments like the following lead me to ask if we are on the verge of talking about (near?)-perpetual motion mochines!
That’s fair. As noted, the effects from loss of cooling can be… energetic. So you probably want backups upon backups.
Though it should be noted that “room-temperature superconductor” doesn’t necessarily have the literal meaning. It’s more that it doesn’t require cryocooling. I’ve seen 0 C as a threshold. You’d need a much higher transition temperature before you can be comfortable eliminating the backups for your cooling systems.
There are basically two different types of natural gas power plants. The type that I am more familiar with is when they take an old coal fired plant and swap out the burner for a natural gas burner, leaving the rest of the equipment (steam turbine, generator, etc) in place. This is a lot cheaper than building a new plant.
The second type is the more expensive option of actually building a new plant, with a new gas turbine generator. It is more efficient, but is also more costly to build. So more money up front, but more efficiency in the long term.
Back when I worked in a power plant, nuke plants provided the most power in the U.S., with coal fired plants a relatively close second. There has been a push to eliminate coal and nukes in the past several decades, so natural gas plants have risen from being a very small percentage of U.S. power to now being the number one provider (roughly 40% IIRC). Nukes are now second, at about half that (20%), and coal is just behind nukes.
Switching coal plants to natural gas has been a very cost effective way of ditching coal for the last couple of decades, so there are a lot of those types of plants around. I don’t know of too many new natural gas plants, though I do know of a couple. But then I don’t work in the power industry any more either so I don’t know what the percentage of each type of plant is.
Yes. Using a rule of thumb of half to no more than 2/3 Tc for stable operation with no change in superconducting properties and close to maximum currents and magnetic fields, I threw out a 500 Kelvin (200 C) Tc as an ideal goal.
People don’t generally realize that one of the main attractions of superconducting power systems is that as one scales up in the amount of power involved, they offer lighter, smaller systems than conventional systems, even accounting for refrigeration.
An interesting case is the “electric airplane”, which is currently a hot topic. Using high temperature superconducting motors and generators is the lowest size and weight solution for large planes (think airliners). Of course, the worst case of coolant loss at 30,000 feet might give one pause…
Superconductivity means the resistance = 0. That is how energy can be stored indefinitely as a current in a superconducting coil, with no applied voltage.
That question has a definite answer:
Is this a potential hazard in MRI machines? Have there been any unfortunate events with patients during MRI exams? If so, were they at least fast and merciful?
No danger to the patients. A “quench” can happen where the MRI magnet loses superconductivity, but this results in the coolant (liquid helium) turning to gas and being vented outside. It’s an expensive event but the energy in the magnet isn’t enough to make it outright explosive.
A quenching MRI did once kill all the iPhones in a hospital (they recovered eventually).
Another thing: a quench can be induced to save the patient. If someone screwed up and got a large magnetic object too close to the MRI, it could get pulled toward the patient and pin/crush them. The operator would hit the quench button to turn off the magnet if the patient was in danger.
I saw a photo of that once, although I don’t think there was a patient present. Somebody brought some furniture item into the room (a kitchen chair or some such?) and it got sucked right into the tunnel.
What matters is the thermodynamic efficiency as dictated by Carnot.
\eta_{max} = 1 - {T_C\over T_H}
where T_H is the temperature of the hot side, and T_C is the temperature of the cold side as an absolute temperature.
A steam turbine has the advantage that it can exhaust into a near vacuum created by the condensers, the temperature of the condensers setting a very low exhaust temperature (T_C) for the heat engine, even though the input temperature is not all that high. A steam turbine allows for huge expansion of the gas, grabbing every last bit of energy it can before the water decides it no longer wants to be gas.
A gas turbine burns hot (T_H), but its exhaust temperature (T_C) is still pretty hot, limiting its efficiency. It is much harder to get big expansion of exhaust gas, particularly as it exhausts into the atmosphere, so T_C remains high.
A combined cycle gas turbine can get efficiencies of near 60% by heating water for a steam turbine with the gas turbine exhaust. But the capital cost is much higher.
As I recall, there was a news item about someone who accidentally had an oxygen bottle sucked into an MRI.
When higher temperature superconductors were being discovered, someone suggested a ring maybe 1/4 mile diameter could store enough energy to power a medium-sized city. This would act like a battery, for the output from variable sources like wind and solar. They suggested embedding it in bedrock due to the force of the magnetic field.
You might even have a small gizmo like this in your house - but the question would be, what would , say, 100kWh stored and released as heat in one second look like? You sure as heck wouldn’t want one of those things exposed to a house fire, assuming higher temps cancel the superconducting - suddenly, at the threshold temperature. Worse yet for an industrial facility.
Superconductivity does (so far as we can tell) mean truly zero resistance, but resistance isn’t the only thing that can stop a current. If your superconducting wire is at all curved (which it will be, if it’s a closed loop), then current going through it will produce electromagnetic radiation, which will remove energy from the current. You can make this radiative loss very small, by making your loop very large, but you can’t eliminate it entirely.
And the current won’t run out of control. If you start a certain amount of current in a loop, then it’ll stay that same amount of current, without decreasing (aside from the above-mentioned radiative loss), but neither will it increase.
Compare to a mechanical flywheel. You can store energy in it by spinning it. Spin it up, and it’ll keep on spinning. If the bearings are very good, then it’ll spin for a very long time. If you extrapolate to perfect bearings, then it’ll spin forever. But it won’t speed up, unless there’s something speeding it up.
I don’t think this is right. It’s been many years since I thought about this, but I believe the issue is that superconductors are not perfect conductors. The effects, including zero resistance, are derived from the properties of the Bose condensation, not mean free path. In this case, it requires that if the condensate is going to give up any energy, that energy has to be greater than the pair breaking energy (the energy gap). A DC circulating superfluid can’t give up “small amounts of electromagnetic radiation”. It has to come in chunks that, energetically, are pretty significant at the temperature of the superconductor.
I should see if there have been any persistent current (in superconductors, normal metal exhibit persistent current as well, but that’s a different phenomena) experiments or theory published in the past few years.
As I understood, for a steady current, the work required is in setting up a magnetic field when the current is injected. Then, unless certain (conducting) objects are moving through field and have current induced in them, the whole is steady state. It’s not like AC, constantly constructing and reversing the magnetic field. (Which is why DC transformers don’t work, but DC generators or motors, involving moving parts, can.)
A loop of superconductor (or any conductor) with constant current will not radiate. It is counterintuitive, because if there were just a single electron, it would radiate. But when you spread a bunch of them evenly along a conductor, they do not.
You will get radiation if the current changes, though, as it might if you’re driving the superconductor like an antenna. In that case the energy in the static EM field won’t be quite what you put into it.
It’s not often, any more, that I’m corrected on a matter of physics. But looking over it, you’re correct. Thank you, most sincerely.