In the UK, 8% of electricity generated is lost in transmission, totalling roughly 180TWh, equivalent to 15.5 million tonnes of oil. By reducing or virtually eliminating energy loss through transmission, we could significantly reduce our energy consumption and thus our carbon footprint, while also making the cost of energy cheaper. This could be possible with superconductors.
A superconductor is a material with exactly zero resistance. A material becomes a superconductor when cooled below its transition temperature, which for most superconductors is below -200°C. Superconductors are not rare; almost half(57/118) of all elements become superconductors once cooled below their transition temperature. Superconductors are divided into two groups: Type 1, which only function at low temperatures, typically below 200°C, and Type 2, which function above 200°C.
In a Type I superconductor, when an electron moves through the lattice, the positively-charged nuclei are attracted to it and move closer. This leads to a strong positive charge around the electron. This positive charge attracts the next electron towards it, causing it to move forward. The two electrons form a Cooper pair, an extremely weak interaction. This interaction is broken at higher temperatures by the increased movement of the nuclei, meaning that the electrons cannot remain as a Cooper pair above the transition temperature, and therefore superconductivity no longer works. The Cooper pair leads to zero resistance as it acts as a single particle composed of two fermions. As explained in my previous blog post, a particle composed of an even number of fermions is a boson, and therefore the Cooper pair acts like a boson. As bosons, unlike fermions, can occupy the same energy level in the same space at the same time, this allows all of the Cooper pairs to occupy the lowest energy level, where they cannot emit and lose energy anymore, leading to zero energy loss. The Cooper pairs interact with each other, in effect forming a larger boson, which does not stop moving through the lattice unless all of its constituent particles collide with nuclei at the same time, in effect making the resistance that it experiences zero.
Type II superconductors are much less well understood, which is unfortunate, as they have much more potential, with some being able to be used at close to room temperature, although no room-temperature superconductor (a superconductor with a transition temperature of >0°C) exists yet, with highly pressurised lanthanum decahydride having the highest proven transition temperature, of -23°C. Type II superconductors are characterised by an intermediate state above the transition temperature where the material shows both superconducting and non-superconducting properties.
Due to the Meissner effect, the superconductor expels magnetic fields from it when being cooled below the transition temperature. If the magnetic field is of sufficient strength, then the conversion to a superconducting state is prevented, and the magnetic field is not expelled. A complete Meissner effect is only observed with Type I superconductors in a cylindrical or ellipsoid shape, although a partial Meissner effect can be achieved with other Type I or Type II superconductors in other shapes. In Type II superconductors, in the intermediate temperature range above the transition temperature, but below the point at which it does not expel any magnetic flux, the material allows partial penetration by a magnetic field, only through certain points, known as vortices.
Flux pinning is achieved when a material, either a Type II superconductor, or a Type I superconductor of sufficiently low thickness, is penetrated by a magnetic field, and then held in place relative to the source of the magnetic field. This is because the magnetic fields are restricted to remaining in the vortices of the material, and therefore when the magnet is moved, the superconductor also moves, to maintain the correct position for the magnetic field to remain in the vortices.
The Meissner effect can be applied to create high-speed trains, by eliminating friction with the ground. In this design, superconducting rails are placed on the ground and magnets are placed on the underside of the train, or vice-versa. Using this technology, in 2015 a new railway speed record was achieved by the Japanese L0 Series train, which reached a speed of 603km/h. As a result of advances in technology, the Japanese company JR Central has begun construction of a commercial superconducting maglev (SCMaglev) railway between Tokyo and Nagoya, roughly 286km long, which is estimated to be in operation by 2027. JR Central is also negotiating with the US for permission to build a maglev railway in the Northeast, with a planned opening in 2028.
Energy transmission by superconductors, although currently lagging behind SCMaglev applications, has been making progress recently. Most notably, in 2014, the world's first superconducting power line was opened in Essen, connecting two transformer stations, as part of the AmpaCity project. As well as lowering energy loss during transmission to zero, superconducting wires negate the need for transformers, as high voltage transmission is not needed to reduce power loss, when power loss has already been minimised. Promising materials for superconducting wires include YBCO(Yttrium Barium Copper Oxide) compounds, some of which have a transition temperature above the boiling point of liquid nitrogen, allowing them to be more easily cooled than superconductors with a lower transition temperature. However, due to the high cost of manufacturing and replacing regular wires with superconducting wires, it is likely that a conversion, if it happens, will be limited to large power cables used by thousands of people, and not smaller personal cables.
Sources:
Introduction to Superconductivity: Second Edition, Michael Tinkham
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