Suppose you had viable room temperature electrical superconductors today.
Suppose you had transmission lines, electric motors, and alternators/dynamoes using superconductors.
Efficiency-wise, would there be any difference between AC and DC power?
At first thought I’d say no. The advantage of AC for long distance transmission is that you can greatly reduce line losses by using high voltage.
DC transmission has a lot of advantages. No corona or radiation loss. All of the current generated goes to produce power, there is no power factor (voltage-current phase lead or lag) as with AC.
The advantages (and disadvantages) of DC should apply to superconductors.
The main reason we use AC is that with late 19th and early 20th century technology, it’s a heck of a lot easier to make an efficient system compared to DC. The most important thing to remember here is that the main source of inefficiency is the line losses.
If you eliminate the line losses by using superconductors, you’ve just also eliminated the main reason we crank the voltage up as high as we can. All of your substations go away, as do all of the transformers that feed every few houses. The total loss in the lines is now zero. Doesn’t matter if it’s AC or DC.
Once you get the power into your house, you need to convert it into something that your devices can use. For this purpose, AC is better than DC. DC to DC transformers have certainly come a long way since the early 20th century (yes, they did have them back then - couple a DC generator to a DC motor and you’ve got a DC transformer), but it’s still not as efficient as an AC transformer.
All the stuff about corona and wire spacing and insulation distances all goes away. With superconducing transmission lines, you don’t need to boost the voltage. You can deliver power at 12 volts if you want. You don’t need huge overhead high voltage lines. A tiny thin piece of wire with only the barest amount of insulation around it can carry enough power to light a city. The wire size is going to be dictated mostly by what you need for physical strength so that the wires don’t constantly get broken.
AC wins, but not by much.
If you can create an ideal DC to DC transformer (hey, we’re making superconductors, so why not?) then it’s a tie.
Don’t you still have capacitive loss with AC?
On average, residential loads are slightly inductive, due mostly to motors (hair dryer, refridgerator, clothes washer/dryer, vacuum cleaner, etc). A little extra capacitance on the system is actually going to improve things by bringing the power factor closer to unity, but the natural capacitance of the lines and such isn’t enough. The power companies intenionally add capacitance to the lines to balance out the inductive loads. Once you get the power factor balanced to unity, what is really happening is that the capacitances in the system is storing charge while the inductances are releasing charge, and when the capacitances are releasing charge the inductances are storing it, so that the overall net effect on the generators is zero. As long as you balance the power factor, the generators don’t have to supply extra current, so it doesn’t affect the efficiency.
I suppose you could make the argument that the equipment used to switch the capacitors on and off the line requires some energy, but that’s going to be pretty small compared to the energy lost in DC/DC conversion throughout a DC system.
As long as we have a hypothetical, room-temperature superconductor let’s just hypothetically have all home appliances run at the same voltage? That way we need not have voltage converters at all.
You also need the hypothetical room-temperatue superconductor that is capible of carying high current. It does not do a lot of good to have power lines of 12V if they can only carry a few amps Amps. You need them to carry tremendous current or you can step up and down the voltages like we do now.
Current carrying capacity is limited by heating of the wires. With superconductors (nonohmium wire) there isn’t any heating of the wires. Low resistance connections would be the limiting factor, but if we can invent nonohmium we can certainly figure out how to make superium for connections.
The thesis for my PhD in physics was in the field of superconductivity (specifically, Josephson junctions). It’s been a few years and I’m no longer in the field; nevertheless, I know whatof I speak:
First, superconductors are not idealized, lossless conductors. Under some limited conditions they come close, but be very suspicious of any conclusions based on that assumption.
All superconducting materials have a critical current. This is the maximum d.c. current it can carry before the superconductivity breaks down. At currents below the critical, you have no voltage and essentially lossless transmission.
Above the critical current, you will have both a supercurrent and a conventional current. There will be a voltage and the conventional current will have losses. This is risky, because those losses heat the material and can raise the temperature above the critical temperature of the material. If you go above the critical temperature, you lose all superconductivity! In that event, if the current is not quickly reduced, the (now greatly increased) resistive heating will probably destroy the material.
The above explanation is for d.c. current-driven superconductors. A.c. is more complicated. The a.c. current creates magnetic flux. In a “perfect” superconductor the flux can flow freely, causing losses. Not good! A practical superconductor for energy transmission will have embedded flaws (compare to the doping of semiconductors) to “pin” the flux. As long as you have enough pins for the flux, your transmission will be lossless. Too many (because your a.c. current is too high amplitude or too high frequency) and there’ll be losses.
To answer the OP–in practice superconductors can sustain a higher d.c. rather than a.c. current and this superconducting transmission lines will be d.c.
And it would seem that they would have supersensitive and superfast control systems to keep the current below the critical value by load shedding or some other means.
See, if you just wait long enough a specialist in the subject will show up.
Is it current or current density? That is, if 50 mA is the critical current for #40 wire would it also be that for #10 wire?
Definitely. Failsafes typical involve superconducing fuses and conductive cladding. The fuses are what you’d expect–easily replaceable circuit segments with lower critical current.
The cladding is typical silver or copper surrounding and in continuous electrical contact with the superconducting line. Under usual conditions, the current will stay almost entirely in the superconductor. If the superconductor goes normal, current will pop out into the cladding since most superconductors are poor conductors. As a bonus, the cladding is also a good thermal conductor.
Good point. The bulk superconductor has a critical current density (A/m[sup]3[/sup]). Once it’s shaped into a particular configuration, you can talk about a particular device’s critical current (A). Typical superconducting transmission lines are made of many braids. For example, a silver wire with a superconducting core; tapes instead of wires are common too. The total critical current will be the sum of the component critical currents in parallel.
My PhD has now served its purpose. 
Even if DC would be more efficient, we’d likely still stick with AC, since it’d be difficult to change over all of the infrastructure at the consumer end that’s currently built to take AC. Eventually, the change might be made, but not until after a long transition period filled with adapters and devices with two plugs.
I expect only high-power transmission lines would ever be converted over to superconductors. Only there is efficiency the overriding concern. A.c. is just too convenient to replace at the consumer level.
And worth every single hour and every single penny spent on it, too.
It is worth noting that solid state rectifiers and alternators have now reached the point that it is feasible to transform your AC into high voltage, rectify it, transmit it as DC and then convert it to AC to step it down for local transmission and use. Hydro-Quebec does this on a large scale. Much of its power is transmitted at about 3/4 of a megavolt DC for the 1000 miles or so between James Bay and Montreal. So, I am quite certain that they would use DC if they had room temperature (or even near room temperature) superconductors. It is always advantageous to send less current.
(BUMP)
Would a Magnetic Flywheel count as an electric motor/electric generator transformer? Apparently, an MF is already the ideal electrical storage device.
I referenced Josephsons in one of my Star Trek fanfics, I mention for the heck of it.