Superconductivity is a physical phenomenon exhibited by certain materials in which at temperatures below a material-specific critical temperature the materials lose their ohmic resistance making them in principle able to conduct electric current without loss. The discovery of high-temperature superconductors in 1987 resulted in an astonishing increase in the critical temperature from around 4 K before the discovery to around 100 K afterwards. In other words, the distance from the critical temperature to absolute zero increased by a factor of 25. Roughly put, one could say that the use of superconductors in applications suddenly became about 25 times easier. For instance, for so-called high-temperature superconductors, the refrigerant medium is liquid nitrogen, which is far cheaper to produce than the liquid helium previously required. But 100 K is still -173 °C and the effort required to maintain this temperature is large. But this effort may well be worthwhile, particularly in applications that exploit another beneficial property of superconductors – their ability to carry current densities approximately one hundred times greater than those in metals, where current densities are limited by thermal effects. Semiconductors are used to generate extremely powerful magnetic fields for research in nuclear physics and for medical diagnostics. They are also used in the construction of lighter machines for applications in which volume or weight are of crucial importance. For a long time many of these highly specialized applications delivered behind-the-scenes benefits that remained generally unknown to the wider public. An industry association has now been established in Germany that is working to promote superconducting applications and improve public recognition of these technical developments. Applications include a drive system for a naval vessel and an 8 MW wind turbine. Superconducting short-circuit current limiters also look set to revolutionize power network engineering. Until recently the demands for a vanishingly small network impedance during normal operations and for a sufficiently large impedance in the event of a short-circuit appeared incompatible and a compromise solution was needed. It now seems that it is possible in principle to meet both demands and a number of systems are currently undergoing practical testing. In addition to the critical temperature another important parameter of any superconductor is its saturation current density, called quench, that is the current density at which superconductivity suddenly collapses just as suddenly in fact as it appears. The remarkably simple solution to this problem involves a conventional metallic conductor (usually made of copper) that surrounds the superconductor and that carries the current for the very short period until the short-circuit has ceased with the current limited by the ohmic resistance of the metallic conductor.
Meanwhile the idea has finally popped up to merge this component with a superconducting transformer. Hereby this might become a reasonable approach, while the transformer alone is not, since it exhibits too low losses to save more on these than the cooling consumes.
After all, the numerous reports in recent years of the potential of superconductors to save energy should, be viewed with a healthy degree of scepticism. The power network components that we have been discussing such as extra-high-voltage underground cables and large transformers already have efficiencies significantly above 99%, in fact a high-power transformer (≈800 MVA) exhibits an efficiency of 99.75% at full load and 99.8% at half load. In grids such as those in Germany, Austria and Switzerland no more than 5% of the electrical energy is lost along the path between the power generating station and the domestic outlet socket – and most of that 5% is lost in the heavily branched low-voltage distribution network. Distribution transformers have efficiencies of ‘only’ 98.5% at full load and 99.0% when operating at half load.  Even if copper losses at half load are a quarter of their value under full load conditions, the energy needed to cool the transformer down to the cryogenic temperatures of a superconductor remains unchanged. A (relatively large) distribution transformer with a rated output of, say, 1 MVA and losses of 15 kW (or significantly less than 5 kW when operating at half-load) would have to be maintained at a temperature of 100 K in order for any sort of energy savings to be made. And even then, only the copper losses would be eliminated, not the iron losses that actually contribute substantially to the transformer’s life-cycle costs.
Calculations have shown that for an extra-high-voltage underground cable a positive energy balance would be achieved at transmission powers of 5 GW and above. That corresponds to the total power output from four nuclear power plant blocks. But a cable of this type does not exist as there is simply no demand for it at present and there is unlikely to be any demand in the future. The model calculation is thus purely academic and of no real practical utility.
Whereas with underground and undersea cables, an »efficiency« always needs to be referenced to the respective length, since a metre of cable will always have the same amount of losses at equal current and voltage. According to latest information, the latest product in this area is characterized as having a loss level < 5% – even so with a transmission power magnitude of 2.6 GW and a length of 1500 km!
There have also been reports of energy savings of ‘up to 50%’ if the wind turbine mentioned above is fitted with a superconducting generator. First of all, the expression ‘up to’ is usually of no practical worth as it only ever specifies one extremum, while the other extremum in the opposite direction and the average value are never mentioned. Secondly, what is meant here is, of course, a reduction in the losses, which translates to an energy saving of about 1% of the energy generated. Wind turbines typically operate at full load for only a relatively few number of hours per year. It is all the more important then to recall that the copper losses increase with the square of the load, but that the cooling for the superconducting material is a permanent requirement and has to be maintained even during windless periods as the duration of such periods is unpredictable. It is also worth noting that one could also save about 90% of the power losses using conventional copper conductors were these conductors cooled from the usual operating temperature to cryogenic temperatures. The temperature dependence of the ohmic resistance of copper would effectively allow us to create a ‘90% superconductor’ – but nobody would ever do this, because it is simply not worth it. Finally, we note that superconductivity functions only fully with direct electric current, and is only partially present with alternating currents. Attempts to use superconductors directly to avoid ohmic losses and thus save energy are well suited to newspaper reports or political sound bites, but they tend to be compromised by practical realities. Superconductors do though offer extremely interesting applications in areas where copper and silver conductors cannot be used. Returning to the wind turbine discussed above, the generator can be made smaller and lighter by using superconducting materials and this opens up new performance categories that would unattainable with a conventional electric generator, as a conventional generator would be so heavy that no crane is currently available that could lift it into place. A fact that is generally not mentioned too prominently in the relevant press releases.