The superconductor cable designed by Nexans has three layers of superconductive material and two channels for liquid nitrogen cooling

World's longest superconductor cable works without a hitch

The world’s longest superconductive cable and the first to have been integrated into an urban electricity grid has celebrated six months of flawless operation, fuelling hopes of the energy industry for future lossless energy transport.

Part of the AmpaCity project, the 1km-long 10kV high-temperature ceramic-based superconductive cable was integrated into the inner city grid in Essen, Germany, replacing a ten-times thicker 110kV copper cable.

During a briefing in Essen the project partners praised the trial, saying the 15cm diameter cable not only allows transmission of as much electricity as the previously used ten-times thicker copper cable but also enables simplification of the city grid scheme, reducing the number of transformer stations required by 40 per cent.

“During the first 180 days, the AmpaCity cable performed with 100 per cent reliability and we hope that when we complete two years of testing, it will still be somewhere around 99.9 per cent,” said Frank Merschel, project manager for new technologies at German utility company RWE, which runs the grid in Essen.

“We plan to extend the test until 2016, to see that the system performs well in various weather conditions. If the results are good, we may consider a more widespread implementation of superconductor technology as part of our electricity network.”

During the first six months, the project partners, including RWE, cable manufacturer Nexans and Karlsruhe Institute of Technology, learned many valuable lessons but encountered surprisingly few problems.

“We had to switch off the cable only once for a very short period of time during the first 180 days of operation,” said Oliver Sauerbach, RWE’s head of grid planning in the Ruhr region. “It was during the power outages caused by the Ela storm, which disabled the cooling system of the superconductor. However, the data later showed we could have left it on as the temperature wasn’t raising that fast.”

The cable, made of three layers of bismuth strontium calcium copper oxide, is cooled down to minus 207°C to achieve the superconductive state, which completely eliminates electrical resistance allowing for 100 per cent efficient energy transmission without losses.

The 15cm diameter cable, developed by Nexans, includes an inner and outer channel through which liquid nitrogen flows providing efficient cooling. The structure is insulated from its outer shell by a layer of vacuum to prevent thermal energy transfer to the surrounding environment.

“The liquid nitrogen flows through the 1km-long cable from the cooling station through the outer channel and returns back through the inner path,” Sauerbach explained. “That’s 2km to complete the circle there and back to the cooling station, after which the liquid nitrogen returns three to five Kelvin warmer than when it left. About 2.5 cubic metres of liquid hydrogen are circulating inside the cable at any given time.”

High-temperature superconductors (HTS), such as that used in the AmpaCity project, achieve superconductive properties at around minus 200°C. Before the discovery of the first HTS in the 1980s, it was thought superconductivity could only exist near absolute zero (-273°C).

The discovery of HTS by German scientists Georg Bednorz and Alex Müller in 1987 opened new possibilities for the practical use of superconductivity as it allowed using the cheaper and quite easily storable liquid nitrogen as a coolant instead of helium needed to achieve absolute zero.

Bednorz, who together with his colleague Müller received the Nobel Prize for Physics for his work, was present at the AmpaCity 180-day review in Essen, expressing optimism that superconductors could revolutionise power transmission in the not-so-distant future, minimising transmission losses and contributing to cutting carbon emissions.

He said that integrating the superconducting technology with an efficient cooling system was likely the greatest lesson to be learned from the Essen trial.

According to Frank Schmidt, head of Nexans’ Superconductor Division, the price of the superconductive cable was only twice as high as that of a similarly powerful copper cable. The whole system, Nexans said, can perform more efficiently than a conventional one, reducing operational cost by €10m over a projected 40-year lifetime.

“With a copper cable system with all its transformer stations, twice as much energy is lost than we need to power the cooling system of the superconductive cable,” Schmidt said.

“The fact that the superconductive cable is considerably thinner than a copper-based one with the same performance makes the installation much easier as you don’t need to dig such a big trench. That’s especially valuable in an urban environment as it minimises disruption plus all the land that can be freed for other purposes when we get rid of the transformer stations.”

According to Schmidt, Nexans' existing technology was used in the AmpaCity project which enabled the project’s €13.5m budget to be kept tightly under control.

“The technology is basically ready for the market,” Schmidt said. “In the future, we would like to install longer cables, up to 3km long, and continue with tests in the city environment, which we believe could benefit the most from the technology."

He said using superconductors for long-distance energy transport may not be cost-effective in this stage of development, as multiple cooling stations would have to be installed along the way, inflating the overall price.

The AmpaCity project was co-funded by the German Federal Ministry of Economics and Energy, RWE, Nexans and the Karlsruhe Institute of Technology.

Over the first 180 days of its operation, the AmpaCity cable delivered about 20 million kilowatt-hours of energy to customers in Essen, powering approximately 10,000 households.

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