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From the bottom of the New Scientist article called , 'The superconductor breakthrough that could mean an energy revolution.':

Something like 10 per cent of electrical power is lost in long-distance, high-voltage cables, so making them out of superconductors would be an immediate big win.

But then, a few sentences later...

In applications such as motors and generators, they would offer a significant improvement in the power-to-weight ratio, boosting the efficiency of electric vehicles, for example.

Read more: https://www.newscientist.com/article/mg24933170-900-the-superconductor-breakthrough-that-could-mean-an-energy-revolution/#ixzz6kDwgGq8O

If even long-distance power cables only lose about ten percent, how would the much-shorter wires inside a car save so much power?

Apologies in advance if this is a stupid question.....

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  • \$\begingroup\$ They wouldn't save much. The motor is already normally over 90% efficiency, so there's less than 10% to gain. Useful but not earth shattering. \$\endgroup\$ Jan 21, 2021 at 22:40
  • \$\begingroup\$ Assuming room temperature superconductors, there would be a significant savings in the weight of the cables which would add to the overall vehicle efficiency as well as the savings in heat dissipation, \$\endgroup\$
    – Barry
    Jan 21, 2021 at 23:00
  • \$\begingroup\$ The quote says it would boost power to weight ratio, so presumably they're talking about making the car weigh less, which in turn would reduce energy consumption. \$\endgroup\$ Jan 22, 2021 at 1:21
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    \$\begingroup\$ "Continue reading... Subscribe now for unlimited access" - clickbait? \$\endgroup\$ Jan 22, 2021 at 5:33
  • \$\begingroup\$ The wires in motors are long, but they are wound around a steel core so their length is not evident. \$\endgroup\$
    – mkeith
    Jan 22, 2021 at 7:21

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They wouldn't. To address the two areas you mention:

Wiring

The wiring in electric cars is definitely an area of loss, but it's not that big a deal. The Kia Niro I recently drove did about 14kWh/100km on the open road, so roughly 15kW to cruise at highway speeds. With a 356V battery bus, that works out at around 40A battery current. At city speeds it would be closer to 10A.

At a rough estimate the battery cables are about 6mm diameter copper, so 28mm2, which has a resistance of 0.0005ohm/m. If the cables are around 4m (there and return) then that is 0.002ohms of resistance. The losses on that at 40A are 3.2W, so represent around 0.02% loss in efficiency.

As you can see that is not exactly a big issue and changing to superconductors is not going to give you any noticeable increase in range.

There might be a secondary benefit if the superconducting cables can be lighter, but this would depend entirely on the material properties and be marginal at best (unless it is a truly magical new material).

Motor

In superconductor based motors, the superconductors is used in the rotor as an extremely high strength permanent magnet. Having a stronger magnet does potentially increase power density of the motor, but the gains are not that impressive for the overall system. Note that you still need copper windings for the stator, so superconductors cannot make the motor 100% electrically efficient with current materials we know about.

For example, the Tesla Model S motor weighs about 35kg, but the entire vehicle is 2000kg. Even if the use of superconductors halved the weight of the motor, this is a trival change in the overall vehicle weight and will offer almost no improvement in range.

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Any discussion of superconductors requires note of cooling as well. Significant amounts of power and engineering are required to ensure the cables stay below their superconducting temperature. For long transmission cables, this requires insulation and cooling along the entire length sufficient to keep it well below ambient. Losses in the cooling system are quite significant, and if the cooling system fails and the superconductor heats up while conducting significant current, the portion that heats up will suddenly have nonzero resistance and quench the superconductor(explode). So now you're looking at sensors and safety measures along the entire length of the superconductor, which is a large engineered pipe now rather than a thin wire. If it's a big pipe it can't always be above ground, certainly not conveniently high in the air, and if you need access underground it's in a tunnel. All of this costs money. If they can save half of the losses of a conventional system, such a facility might eventually pay for itself, but with the newest best research candidate I could find, the hope is that it may be possible to produce the cable at an adequately low cost, and the cost improvement they're drooling over is just in the material of the semiconductor core of the cable, which is only a small part of the prohibitive installation and upkeep cost.

Looking at a superconductor car the same applies. Even a room temperature superconductor would need cooling on a hot day, and current superconductors require much more cooling than that. So now your tesla has to haul around a big refrigeration unit and if it has a permanent superconducting electromagnet in it, it must also never run out of power or the magnet will quench. Superconducting electromagnets are also decently large even at minimum size due to the cooling and also the way they must be switched, so building one into a motor in a useful way is a feat of engineering in itself, let alone to do so in a motor or set of motors that you could call compact. The newer electric motors they'd be trying to compete with use combined permanent magnet and reluctance based rotor cores to provide good torque and efficiency at a wide range of operating speeds. A superconducting magnet rotor, assuming you could build it at that size, would be unable to use reluctance and thus efficiency would probably be limited at high speed. Using superconductor field coils on the other hand would require a more advanced drive circuit, and not all of the circuit can be made of superconductor, so you haven't eliminated resistive losses entirely at the same time as you've massively increased cost and complexity.

So when is a superconductors a favorable option? For one thing they can produce very strong permanent magnets by flowing large amounts of current in a loop. This type of magnet has a valuable property in that you can change the field strength, drain, or even quench it in an emergency for a valuable safety factor when dealing with strong magnets. With a large system confined within a reasonable size facility economies of scale for cooling become better. A superconductor by nature rejects external magnetic fields, so they don't suffer long term demagnetization like permanent magnets. It would take an incredible amount of force and field strength to theoretically push a magnetic field through a superconductor, inducing a current, and doing so will cause that region of the magnet to stop superconducting and quench.

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Resistance in the wires isn't the only factor

Sumimoto Electric, a major manufacturer of wiring for cars, has a 2012 whitepaper which gives a good overview of the potential of superconductors in electric vehicles. I've formatted into a bulleted list to make it clearer:

  • A superconducting coil provides a high magnetic flux density, and therefore, delivers much higher torque than ordinary motors.
  • A superconducting motor can be used without copper loss, and an air-core superconducting motor may be developed in the future to reduce iron loss and increase motor efficiency.
  • Automobile motors need to respond to a wide range of operations from low speed driving to high speed driving and from low torque output for constant-speed cruising to high torque output for acceleration. While ordinary motors exhibit considerable copper loss and poor efficiency during high torque output, super-conductive motors provide high efficiency over a wide range.
  • The high torque feature leads to the development of a system in which the motor directly drives a shaft without variable-speed gears, and thus, results in the reduction of transmission loss caused by the gear system.

They highlight that cooling is still a challenge, adding weight and volume to the vehicle that could otherwise be used for batteries. This could be why, in more recent research, the focus seems to be on larger motors (such as for aircraft and wind turbines), where I imagine the superconducting benefits scale more significantly.

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