SIEMENS

Dutch physicist Heike Kamerlingh Onnes had no idea he was launching a scientific revolution when he became the first person to liquefy helium back in 1908. The process he used resulted for the first time in temperatures only two degrees (two Kelvin) above absolute zero (-273 degrees Celsius). Through his cryogenic experiments in 1911, he discovered that the electrical resistance of mercury dropped suddenly to a barely measurable value at a temperature of four Kelvin (K). Superconductivity — the loss-free transmission of electrical energy — had been discovered.

Although it would take 46 more years to develop a theory explaining this phenomenon, scientists nonetheless soon realized its potential. Superconductivity not only offered the possibility of transporting large amounts of electricity over great distances without losses, it could also be used to generate strong magnetic fields, develop extremely precise measurement techniques, and make energy systems more efficient and powerful. But one major problem remained. Complex and expensive cooling technology with the inert gas helium seemed to be the only way to achieve the transition temperature — i.e. the point at which the superconducting effect first occurs. Superconductors were therefore simply too cost-intensive for most industrial companies.

But this changed in 1986, when two physicists, Alex Müller from Switzerland and Georg Bednorz from Germany, discovered a ceramic compound, lanthanum barium copper oxide, that becomes superconductive at 35 K. They received the Nobel Prize in 1987 for their work. Inspired by this so-called high-temperature superconductor (HTS), researchers around the world began searching for substances with even higher transition temperatures. The HTS record is currently held by mercury thallium barium calcium copper oxide, whose transition temperature is 138 K. The discovery of yttrium barium copper oxide (transition temperature: 92 K) in 1987 made it possible to cool with liquid nitrogen at 77 K. Unlike liquid helium, liquid nitrogen is a coolant that is easy and relatively inexpensive to produce. However, until HTS technology can be used on a broad scale, technically complex, highquality- superconductors will continue to dominate the market. Such superconductors can be found today in imaging systems manufactured by Siemens, such as magnetic resonance tomographs (MRT). Here, superconducting wires are made of a niobium-titanium alloy. Thanks to the powerful electric currents flowing through them, these superconducting magnets generate very strong magnetic fields of several tesla — stronger than those created by HTS. Stronger magnetic fields in an MRT result in better signal- to-noise ratios and sharper images.

Ship Propulsion with Superconductors. The first commercial HTS-based applications are gradually emerging in Siemens’ Industry and Energy Sectors. Working with Siemens’ Marine Solutions and Large Drives business units, researchers at Siemens Corporate Technology (CT) have developed an HTS ship propulsion unit whose rotor displays no electrical losses although the superconductors in the rotor coils have a current density 100 times greater than that of copper coils. This makes it possible to reduce weight and volume by up to 50 percent; since fewer materials are used, costs are also significantly lower. This is important for ship operators, whose propulsion systems are subject to size limitations.

CT researchers are also examining the use of HTS current limiters at high voltage facilities. These limiters can automatically and rapidly protect power grids in the event of short circuits, thus preventing damage to cables, transformers, and generators. Another research focus is on HTS coils that can cut power plant generator losses in half. HTS cables in a generator rotor must withstand centrifugal accelerations 5,000 times greater than the acceleration due to the Earth’s gravity. They also must be reliably cooled to 33 K. In February 2011 a project for building an HTS test rig for such a power plant generator application was launched with funding from Germany’s Ministry of Economics and Technology. The project is being coordinated by CT and carried out in cooperation with the Karlsruhe Institute of Technology (KIT). The project’s long-term objective is to develop a prototype HTS generator with an output of several hundred megawatts.

Despite all these projects and successes, the potential of superconductors is far from exhausted. Scientists are sure that Onnes’ discovery will be the foundation of many future applications, from generators and motors to current limiters and MRTs. So here’s to another “cool” 100 years!