Energy Superconductivity
Irresistible Power
The initial euphoria about high-temperature superconductors has given way to the sober light of reality. Ten years ago, their ability to transmit electricity without loss was celebrated as a revolution in the power supply industry. Now, the first commercial applications are finally emerging.
Superconducting ribbon: A machine produces flat cable from brittle, superconducting ceramic (detail right)
Hans-Peter Krämer peers through the inspection hole of a heavy metal cylinder. Inside, small bubbles rise steadily from a hand-sized circuit board bearing a spiral-shaped printed conductor. Suddenly there's a bang and the liquid begins to boil. A few seconds later, the clouds part and everything has returned to normal. Krämer is satisfied; the experiment has been a success.
A project engineer at Siemens in Erlangen, Germany, Krämer is working on a new type of current limiter for high-voltage systems. If a short circuit occurs, such a limiter could interrupt the network instantaneously to prevent damage to cables, transformers and motors. Nothing spectacular, one might say, but there's more. The spiral-shaped printed conductor is actually a ceramic superconductor made of yttrium-barium-copper oxideYBCO for short. Submerged in liquid nitrogen (- 196 °C), this superconductor can transmit up to 50 A of current with no resistance whatsoeverand therefore at no loss. However, as soon as the current rises above this level, superconductivity breaks down, electrical resistance skyrockets, and the heat generated brings the liquid nitrogen to a boil. After a short cooling-down period, the current limiter is again ready for use. The amazing thing is that while it would take copper conductors the thickness of a pencil to transmit such high currents, the printed conductors on the circuit board are less than 0.001 mm thick.
The current limiter from Erlangen is the first promising candidate for commercial use of superconductors in the power supply industry. The phenomenon of superconductivity was first discovered in metals some 90 years ago. But the real breakthrough only came 15 years ago, when IBM researchers Georg Bednorz and Alexander Müller made the first high-temperature superconductors (HTSC) from ceramic materials. Today, the record for the highest transition temperaturei.e. the temperature below which a superconductor loses its electrical resistancelies at around 135 K (- 138 °C). This means liquid nitrogen can be used for cooling. At a cost of five cents per liter, it's far more economical than the liquid helium (6 per liter) employed to cool metallic superconductors.
High-temperature superconductors have very appealing properties. "Superconductors are simply No. 1 when it comes to efficiency," says Heinz-Werner Neumüller, head of Superconductivity Research at Siemens in Erlangen. Moreover, today's cooling systems operate so economically that they only slightly reduce the efficiency of the superconductors. Similarly, while copper cable with a cross-section of 1 mm² will conduct a current of only 2 A, high-temperature superconductors of the same size manage up to 3,000 A. And if engineers can manage to combine the granules of the high-temperature superconductor material in a better wayby selectively mixing in other elements, for examplethis figure will increase even further.
Yet an end to the world's energy problems is not yet in sight. At present, superconductors of the metallic, cryogenic type are the norm. Such superconductors, which must be cooled with liquid helium to a temperature of - 269 °C, are found in the magnets used for magnetic resonance imaging (MRI) systems. This technology was developed more than 20 years ago and today generates sales of 2.4 billion per year worldwide. On a larger scale, cryogenic superconducting technology is also found in the particle accelerators used in the field of high-energy physicsat DESY in Hamburg, for example. HERA, a 6.3-km in circumference underground ring, consists of massive superconductive magnets, supplied with liquid helium from cooling systems as tall as a house. But now, following years of disappointment, researchers are once again optimistic that high-temperature superconductors will be a commercial breakthrough. In the U.S., HTSC frequency filters are now increasingly used in cell phone base stations. According to Neumüller, most of the technical problems have now been solved, and the first applications for the power supply industry should also be appearing on the market by 2005. Today, for example, any length of superconductive wire can be produced to a consistent quality from brittle ceramics. Indeed, annual production already amounts to around 800 km.
Two sticking points remain, however. First, any power supply systems using superconductors need to be low maintenance and long lasting. The aim is to achieve a maintenance interval of three years and a lifespan of 30. "We've made plenty of progress," says Neumüller, pointing out that, for example, so-called pulse tube refrigerators reliably cool superconductors without using any moving parts. The second problem is high price. Wire made of a ceramic superconductor costs around ten times as much as standard cable. Yet as recently as 1996, it was even five times more expensive. Moreover, once larger quantities are produced, the price will fall furtherby a factor, experts say, of 2.5 by 2004, which will be low enough to launch the first large-scale production.
Current limiters could also be used to link power networks without the need for expensive auxiliary safety systems. And with HTSC cable, which requires much less space, it would not be necessary to build new transmission routes. Dr. Michael Frank, who is currently testing a synchronous motor with a superconductive winding in Erlangen, reports that his motor has a voltage stability far superior to anything achievable with a standard generator working under load. The big problem, explains Neumüller, is persuading the customer of the benefits.
Other countries are further down the road to implementation. For example, both Japan and the U.S. are currently investing over $40 million a year in the development of superconductors for the power supply industry. Germany, by contrast, lags behind with a mere $9 million.
Electric motor with an icy heart: Siemens researchers in Erlangen operate a synchronous motor with a superconducting rotor winding. The current density is around ten times that of a copper coil
With its massive energy requirements and densely populated urban areas, Japan in particular is set to profit from such technology. In Tokyo, 10 GWh of electricity is consumed per square kilometer every yearten times more than in Berlin. And with Tokyo's energy needs growing by 2 % a year, new cable is urgently needed. But there's no more room underground. That's why the city plans to replace its 275-kV cables, which now run through pipes up to three meters thick, with HTSC cables threaded through pipes only 15 centimeters in diameter, including cooling sheath. In another development, Sumitomo-Electric is testing a 100-m-long cable rated at 115 MW. And in late 2001 Italian cable producer Pirelli teamed up with U.S.-based American Superconductor to set up a similar test in Detroit, Michigan.
Siemens has developed a 1.1-MW transformer with 6 km of HTSC cable coils. A prototype of the transformer, which has been designed for locomotives used on regional rail lines, will be tested in the next three years. Although expensive, it only weighs 2.4 thalf the weight of a conventional unitand has an efficiency of 99 % as compared to 92 %, which translates into annual savings of 340 MWh. "Within three to four years, customers will have recouped the higher costs associated with the unit," says Neumüller. And the environment benefits, tooby 180 t less CO2 per train each year.
Bernd Müller