|Fig. 1: Timeline of the development of high-temperature superconductors.  (Source: S. McLaughlin)|
Superconductors seem like the stuff of science fiction, but they are very real. Below a certain critical temperature, these materials have no resistance. Not practically no resistance, mind you; it is exactly zero. They also completely expel all magnetic fields from their interiors, which among other things allows them to float above permanent magnets as if by magic. Figure 1 shows the timeline of development of high temperature superconductors; scientists are coming closer to a superconductor that can be used at room temperature. The mind abounds with applications of such a material: lossless power transmission, levitating trains, and more efficient electronics are the obvious stuff. However, physics and economics give pause to these dreams; it is important to understand the real limitations of these sorts of plans before we get carried away with ourselves.
Superconductivity occurs when three effects fall into a "goldilocks zone." If their temperature is not too hot, the magnetic field around them not too strong, and the current through them not too high, they will collapse into a superconducting state.  These phenomena therefore place constraints on the practical use of superconductors. The most restrictive of these is temperature; after all, the day-to-day environment of humans is relatively free of strong magnetism and currents, but full of warmth. Developing and understanding a room temperature superconductor has been the goal of scientists for decades, and thus far that dream seems a ways off. Superconductors are divided into two types, Type-I and Type-II. Type-I superconductors behave like normal metals over a certain magnetic field, while Type-II superconductors have an in-between state where they can allow some magnetic field penetration without completely losing their superconductivity. To drastically oversimplify, Type-I superconductors are generally well understood by physicists, while the others are not. Unfortunately, high-temperature superconductors are going to need to be members of the second type. Type-I superconductors consist primarily of pure metals with fairly low Tc's: the highest among them is that of lead at 7°K (-266°C). Meanwhile, Type-II superconductors have been observed at temperatures as high as 150°K (-123°C). These materials tend be incredibly complex ceramic compounds like YBa2Cu3O7-x and Hg1.4Ba2Ca2Cu3O8+δ, which are difficult to synthesize and often contain rare, expensive and sometimes even toxic elements.  It is important that we consider these limitations as we consider a world with a room temperature superconductor.
The most obvious place to apply a high-Tc superconductor is in power transmission. Power is currently transmitted from where it is created to where it is needed via conventional conductors. The power lost in these wires is proportional to their length and the square of the current traveling through them. As an example, California lost about 8% of power to transmission in 2010.  These transmission wires carry power at a high voltage because this allows them to transmit at a lower current and therefore, lower loss. Their voltage cannot be increased indefinitely, as at a certain point charge can jump to the ground, and this becomes a greater source of loss.
It would be great if we could reduce this loss to zero via the use of superconductors. However, it is unlikely this would be of much benefit. For one, it's clear that current transmission is already very efficient; there would have to be a significant economic advantage to removing that 8% loss. Installing new, room temperature superconducting cables would have to be worth the power they would save. Currently, most powerline cables are made of aluminum, which costs less than a dollar per pound.  It's impossible to estimate the cost of our imaginary superconductor, but it is pretty reasonable to assume it will be far more expensive than aluminum, especially given this material's other applications. High-Tc superconductors are generally very complex compounds that are difficult to synthesize. There is another detail that will likely make superconducting power transmission unlikely. If the high-Tc superconductor has a critical temperature lower than summer air temperatures, the lines will need some form of refrigeration. Otherwise, the 0 resistance line will suddenly have resistance and the results would be catastrophic. Keeping outdoor power lines cool in the Arizona summer will likely cost more energy than is currently lost by conventional cables! The only application that may be reasonable is transmission over very long distances. There are still the same drawbacks as listed above, but the power transmission loss from conventional cables would be most significant in this special case. The dream of covering the Sahara with solar panels and sending the power around the world is unfortunately a bit far-fetched.
The more appealing use of this technology is in power storage. Superconductors are the closest thing to perpetual motion that exist in nature. Current in a loop of superconducting cable will cycle forever. Loops like these could replace conventional chemical batteries, which are surprisingly inefficient. Lithium ion batteries have, on average, a charge/discharge efficiency of about 90%.  As energy production shifts more and more to renewables, energy storage is increasingly more important. A high-Tc superconductor would allow for efficient storage (and transport) of power. Batteries are also much easier to keep refrigerated if necessary, and there are greater efficiency gains to be had. Superconducting batteries are the real energy gain from high-Tc superconductors.
There are, however, limits to this approach. A back of the envelope calculation reveals that this approach may not completely revolutionize the energy economy. Energy stored in a superconducting battery as described above effectively stores energy in a magnetic field generated by its circulating current. However, as mentioned above, a certain critical magnetic field/ current will destroy superconductivity. Therefore, there is a fundamental limit to how much energy can be stored in such a battery. As an example, a magnetic field of 2 Tesla (a very high critical field) stores ~ 2 MJ per cubic meter. Meanwhile, gasoline stores 30 GJ of energy per cubic meter, more than 10,000 times as much! Therefore, it is unlikely that high-Tc superconductors will revolutionize energy storage en masse. However, their almost lossless storage will likely replace chemical batteries in most applications.
|Fig. 2: A modern Maglev train. (Source: Wikimedia Commons)|
A room temperature superconductor would likely cause dramatic changes for energy transmission and storage. It will likely have more, indirect effects by modifying other devices that use this energy. In general, a room temperature superconductor would make appliances and electronics more efficient. Computers built with superconductors would no longer get hot, and waste less energy. As mentioned above, it is unlikely that this new material will be affordable or easy to manufacture, so it is unlikely it will be ubiquitous in new devices right away, if ever. They will likely first be implemented in high-end goods, and where safety is a concern. If we could rewire a home's internal wires with this new material, electrical fires would be a thing of the past.
One of the most exciting uses of superconductors is not their lack of resistance, but their expulsion of magnetic fields. The Meissner effect allows superconductors to float above a permanent magnet, balanced by the magnetic field. This effect is already in used to make trains that levitate above the tracks, and they are the fastest trains in the world. Figure 2 shows an example of such a train already in use today. For the most part, they are not very popular due to their high cost. A room temperature superconductor would make the construction of these trains much easier, and would enable new, more energy efficient transport. It would also be possible to turn more mundane transit systems like subways into levitating systems.
A proper room temperature superconductor is likely a long way away. However, it is worth considering that its impact would be. Though the benefits of such a material are often stated, the practical applications may not be as amazing as claimed. Energy transport with superconductors may not be practical; levitating trains and energy storage may be the real benefit.
© Sean McLaughlin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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