The world is making the transition towards renewable energy. In June the European Union agreed on a 32% target for renewable energy by 2030 . The variable nature of solar and wind energy will, however, always be a problem. Can technology based on high-temperature superconductors help us reach sustainable energy generation?
Since the 1911 discovery of superconducting materials, which can transport current with zero electrical resistance, we have been repeatedly trying to exploit this fascinating property. Ideas of lossless energy transportation have flashed before our eyes for over a hundred years but have seemingly always been out of grasp. Unfortunately, there’s no such thing as a free lunch and superconductors have to be cooled, usually with liquid helium (-269 °C). That was until the discovery of high-temperature superconductors in 1986, requiring only liquid nitrogen (-196 °C), a much more affordable and abundant coolant.
In fact, metallic low-temperature superconductors (LTS, those which are superconducting below about 20 K and so cooled with helium) have actually already been in use in some fields for a number of decades in the form of magnets. The lossless current transport allows for the construction of electromagnets, which can produce strong magnetic fields and are used in MRI and NMR devices as well as in particle accelerators such as the LHC at CERN.
So where have high-temperature superconductors been hiding all this time?
Unlike low-temperature superconductors, high-temperature superconductors (HTS) do not lend themselves as easily to being manufactured into wires. HTS, for example, YBa2Cu3O7-x (YBCO), are brittle ceramics and, due to their large crystallographic anisotropy, demand a high level of grain alignment. Simply put, these materials only transport large currents when they exist as an almost perfect crystal. How, then, could it ever be possible to make wires out of such a material?
Manufacturers have found a way. Over the last decade, several companies around the world have been producing high-temperature superconducting wire. The strategy is to deposit a thin film of YBCO on a flexible metallic tape. The YBCO is given excellent biaxial texture, or alignment of the crystal grains, either through deposition of a texturing buffer layer between the tape and the HTS or by thermomechanical treatment of the metal tape before coating with YBCO.
These so-called coated conductors can transport thousands of amperes of current in a layer of YBCO only a few microns thick—around one hundred times thinner than a human hair. What’s more, because the wires show no electrical resistance there is no resistive heating and, once they have been cooled, require very little cooling power during operation. For applications where we need very large currents, copper wires become both too massive and require too much cooling power, making HTS wire a very attractive alternative.
Where are high-temperature superconducting wires currently being put to use?
Energy is one of the major areas in which HTS is making an impact. Experimental fusion reactors such as JET, Wendelstein 7-X and the largest and most ambitious fusion project to date – ITER – all use LTS technologies. High-temperature superconductors, however, not only allows for more affordable liquid nitrogen cooling but alternatively, if cooled using helium, can produce much greater magnetic fields than LTS. Recently, a small UK company, Tokamak Energy, has started putting this into action and are currently developing smaller scale tokamak fusion reactors employing HTS electromagnets .
Wind energy is also an area where high-temperature superconducting wires are coming into their own. An EU project, EcoSwing, is currently creating the world’s first superconducting wind turbine in Denmark. The HTS-based generator has just passed preliminary tests at the Fraunhofer Institute for Wind Energy and Energy System Technology in Bremerhaven, Germany and is planned to be installed in a functioning wind turbine on the Danish coast by the end of 2018 . The generator will provide a 40% reduction in generator weight, allowing for the trend in larger and more powerful wind turbines to continue without the associated construction problems of lifting a more than 200 tonne nacelle into the air.
One of the more developed applications for HTS wires is that of power transmission cables. Through encasing multiple layers of HTS wire in lines of flowing liquid nitrogen, superconducting cables can be made with a similar diameter to existing copper cables, yet which can transport currents multiple times greater. These cables have the advantage of much more efficient energy transport which works at lower and thus safer voltages. In Essen, Germany, a one-kilometre long HTS cable has been supplying around 10,000 homes with power since 2014 . Importantly, superconducting cables are being seen as one of the key technologies that will enable a full transition to renewable energy .
So how does the future look for high-temperature superconductors?
All of these projects are more or less still showcasing the possibilities that HTS can offer. The final challenge in the long road towards seeing more superconducting materials in energy applications is reducing cost: the complex manufacturing process make HTS wires expensive to produce. The price of high-temperature superconducting wire has, however, been sinking steadily and is approaching the price range of copper wire (30 – 80 US$/kAm) . This is being driven via investment in greater mass-production of HTS wire.
Can high-temperature superconductors finally be the solution to generating and transporting stable and more efficient renewable energy? Yes, it can. With an increasing number and size of public and private projects making use of HTS wire, manufacturers are on the road to delivering wire at a cost comparable to or possibly even below that of copper.
 R. Bartunek, “EU agrees 32 percent renewables target for 2030”, Reuters, June 14, 2018 [Online]. [Accessed June 22, 2018].
 P. J. Ray, “Structural investigation of La2−xSrxCuO4+y – Following
staging as a function of temperature,” Master Thesis, Niels Bohr Institute,
Faculty of Science, University of Copenhagen, 2015.
 K, Huang, “Modern high field clinical MRI scanner. (3T Achieva, the product of Philips at Best, the Netherlands.),” Wikimedia Commons, March 27, 2006 [Online]. [Accessed June 22, 2018].
 Y. Park, M. Lee, H. Ann, Y Hyuck Choi and H. Lee, “A superconducting joint for GdBa2Cu3O7−δ-coated conductors,” NPG Asia Materials, vol. 6, p. e98 (2014)
 Tokamak Energy, “Hotter than the centre of the Sun: UK prototype reaches 15 million degrees,” Tokamak Energy, June 6, 2018, [Online]. [Accessed July 4, 2018].
 F. Grotelüschen, “Supraleitendes Windrad besteht den Test im Probelauf,” Deutschlandfunk, June 12, 2018, [Online]. [Accessed June 22, 2018].
 EcoSwing, [Online]. [Accessed July 5, 2018].
 “New Superconductor Technology for the Transmission Grid,” I-Connect 007, July 5 2018, [Online]. [Accessed July 5 2018]
 J. McMahon, “6 Things That Have To Happen For A Clean-Energy Future, According To Utility Execs,” Forbes, July 19, 2017, [Online]. [Accessed July 5 2018].
 D. Williams, “Nexans success in Essen may see roll-out in other cities,” Power Engineering International, July 1, 2016, [Online]. [Accessed July 5 2018].
 M. Noe et al., “Common Characteristics and Emerging Test Techniques for High Temperature Superconducting Power Equipment,” CIGRE Brochure, 664 (2015).
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