The world's longest superconductor cable is providing hope for the future of the technology after successfully supplying electricity to about 10,000 households in Essen, Germany for six months.
German utility company RWE's AmpaCity project, run in partnership with cable manufacturer Nexans, aims to prove the viability of high temperature ceramic-based superconductor (HTS) cables that, thanks to having zero resistance to electrons, promise the reliable and more efficient transportation of electricity to power hungry urban areas.
Since commissioning the 1km-long 10kV underground cable on 30 April 2014, RWE has successfully integrated it into the inner city grid of Essen and used the cable to deliver around 20 million kilowatt-hours, five times as much electricity that can be transmitted by the 110kv conventional copper cable it replaced.
Frank Merschel, project manager for new technologies at RWE, said: “During the first 180 days, the AmpaCity cable performed with 100% reliability and we expect it to achieve around 99.9% over the next two years of testing.
“We plan to continue the test until 2016 to see that the system performs well in various weather conditions. If the results are good, we will look to undertake a wider implementation of superconductor technology as part of our electricity network.” This would involve supplying electricity for a 23km ring of inner city Essen by 2020.
The cable has already proved to be robust, with RWE having only switched it off once for a few minutes during the first 180 days of operation. This was due to power outages caused by the severe hail and and wind storm Ela that struck Germany, France and Belgium on 9 June.
Oliver Sauerbach, RWE’s head of grid planning in the Ruhr region said the cooling system of the superconductor cable was temporarily disabled which led to them switching it off as a precautionary measure. “As a result we have made slight adjustments to the system parameters to protect the cables from similar issues in the future,” he said.
The cable, made of three concentric layers of bismuth strontium calcium copper oxide, is cooled down to -200°C to achieve a superconductive state, reducing electrical resistance to nearly zero and allowing the transport of electricity almost without losses. The “high temperature” superconductivity of the cable is achieved thanks to research by Professor Alex Muller and Dr Johannes Georg Bednorz, which won them the Nobel prize for Physics in 1987. Before this discovery it was thought superconductivity could only be achieved at temperatures of -270°C using more expensive and less easily stored helium as a coolant.
The 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 is continuously circulating inside the cable.”
Because of the cables higher capacity it can operate at lower voltages, which not only means avoiding electrical losses but also the capital costs of step up and step down transformers typical of high current operations. In addition HTS cables do not emit magnetic fields and measure just 15cm in diameter, enabling them to be installed close to existing cables, a desirable factor when dealing with complex and crowded inner city grid systems.
Frank Schmidt, head of Nexan's Superconductor Division, said that while the HTS cables are more efficient and have practical benefits they currently costs twice as much to manufacture than conventional copper cables. However, he stressed taking a “whole system” approach when looking at the overall savings HTS cables can provide.
He referenced a study the AmpaCity partners conducted in 2013 which found that a typical urban network of 20 transformers could be reduced to 15 using superconducting cables.
The study also found that superconductor cables – despite needing a flow of liquid nitrogen to cool them – would be cheaper both to install and run over a 40-year period than conventional high voltage lines, which require high levels of maintenance as well as the additional network infrastructure, offering a projected saving of €10 million.
Schmidt said: “The product is market ready and production is already made on an industrial scale, not in a lab.
“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."
Having begun to prove the technology's viability outside the laboratory, delegations from China, France, Ghana, Japan and the US have visited RWE to conduct early stage discussions of tenders.
The €13.5m AmpaCity project was co-funded by the German Federal Ministry of Economics and Energy, RWE, Nexans and the Karlsruhe Institute of Technology.