Superconductivity – a lean solution to a big problem?

A well-known saying coined by the science fiction author R. A. Heinlein is: ‘There is no such thing as a free lunch’. Loosely translated, this means: ‘Nothing is free’. This also applies to the conduction of electricity in classic conductors, e.g. made of copper or aluminium. Superconductors, on the other hand, do not seem to recognise R. A. Heinlein's proverb and conduct electricity without losses.

What makes superconductivitiy so special?

The phenomenon that electrons cannot move freely through the conductor on a microscopic level due to scattering processes is perceived as electrical resistance on a macroscopic level. Part of the electrical energy is lost as heat. How great the resistance of the conductor is depends on its geometric dimensions (length and cross-section) and its material-dependent specific resistance. This is why the thin wire of a light bulb glows, whereas our house wiring hardly heats up at the same power.

Superconductors, on the other hand, conduct electrical current without resistance. Their main application is in high-performance magnets for medicine and research and in energy transmission. Due to the loss-free current conduction, very high electrical power can be transmitted through a cable with an extremely small cross-section, unlike in conventional cables. The fact that superconductors save a significant amount of space makes them interesting for electricity suppliers.

Comparison of expansion of conventional power lines with superconductor lines.

Why we still use 'normal' cables

One small but important detail about superconductors has not yet been mentioned: they need it cool. And by cool, we don't mean the northern German summer, but temperatures of -196 °C and colder. Superconductors only show their superconducting effect below their transition temperature. This temperature, also known as the ‘critical temperature’ Tc, is specific to each superconducting material. Two frequently commercially used representatives are magnesium diboride (MgB2; TC = 39 K [-234.15 °C]) and rare-earth barium copper oxide (ReBCO), materials whose best-known representative is probably yttrium barium copper oxide, or YBCO for short (YBa2Cu3O7-x; Tc = 92 K [-181.15 °C]). In order to maintain the superconducting effect, permanent cooling with cryogenic liquid gases such as nitrogen (77 K; -195.8 °C), hydrogen (21.15 K; -252 °C) or helium (4.2 K; -269 °C) is necessary. For energy transport, the superconducting cables are traditionally flushed with liquid nitrogen as a cooling medium. The cable can be encased in a thermal insulation layer so that the nitrogen does not vaporise immediately.

Isn't production incredibly expensive?

As a superconductor can transmit around 100 times the current density in the same cross-section as copper cables, the material costs are hardly significant. At present, the price of superconductors is therefore mainly determined by the manufacturing costs and, with further scaling, the material costs could fall below those of copper. Although superconducting cables are not yet mass products, they could be economically superior to classic underground cables in the future.

The importance of superconductors for the energy transition

The high current density, which can be transmitted in a comparatively small cross-section, makes it possible for the cable routes to be significantly narrower with the same capacity. For comparison: A 6.4 GW overhead line requires a corridor around 125 metres wide, whereas an underground cable system is only 13-22 metres wide and the superconductor only needs 5.5 metres. The line width is a very important issue in the construction of power lines for several reasons.

For example, it is often not possible to retrofit or upgrade conventional routes in conurbations due to space constraints, but a superconducting route can be realised due to its small space requirement. This was demonstrated as an example in the AmpaCity project and will now be realised over 12 km across Munich - a world record.

But superconductors are also justified in rural areas. The narrow line width limits the impact on the natural environment. Particularly in sensitive natural areas such as the Wadden Sea, superconductors offer an ecologically better alternative to the large number of conventional cable routes that are required to connect offshore wind farms. In addition, reduced cable route widths are also advantageous in terms of right of way and public acceptance.

Network expansion using underground (left) and overhead cables (centre) requires far more space than laying superconductors (right) with comparable transmission capacity.

In addition to the structural aspects, the energy transition also harbours other challenges for our energy supply. On the electricity supply side, we are seeing increasing challenges for grid stability and regulation with the expansion of renewable energies and the dismantling of base load-capable power plants. Due to the increased use of renewable energies, energy generation is becoming increasingly decentralised and intermittent. As a result, large amounts of energy have to be transported from the production site to the grid and to consumers.

This affects the transmission grids in particular. For example, depending on the weather situation, wind power from the north has to be transported to consumers in the south or solar power from there to the north. Around 6 % of the electrical energy provided is lost as line losses. With gross electricity generation in Germany totalling 569 TWh in 2022, that is over 34 TWh lost in the transmission grid. Even if one conservatively assumes that superconductors are only 50% more effective due to the power required for cooling, that is still a huge 17 TWh. This corresponds to 13% of the energy consumed by private households in 2022 or a potential saving of around 7.5 million tonnes of CO2 or the annual CO2 emissions of 2.2 million vehicles. Of course, superconducting cables cannot be used everywhere now, but it shows the scale of the savings that the technology makes possible.

Conclusion

Superconductors are an exciting alternative to traditional copper cables and offer enormous structural advantages, particularly in cable construction in confined and vulnerable environments. Due to their physical properties, they can massively reduce transmission losses and help to reduce CO2 emissions caused by power generation.

EurA AG is working on this topic as part of the ZIM network ‘SupraHEET’. The aim of the network is to drive forward the development of superconductivity in combination with liquid hydrogen as a cooling medium instead of liquid nitrogen (see blog article ‘Superconductor technology - what's behind it?’). In addition to the higher electrical transmission capacity, hydrogen also provides a chemical energy carrier. The synergy from the simultaneous transport of electrical and chemical energy creates new application possibilities and contributes to economic efficiency. 

Have we aroused your interest? Please do not hesitate to contact us: Our experts Dr Knut Behnke and Dr Jochen Lach will be happy to discuss the topics of superconductors and renewable energies with you at any time.

Text: Jochen Lach

Header picture: NKT Cables Group
  Category picture on the left: Amprion 
  Category picture on the right: Westenergie: 'Supraleitung Baustelle 2014'