Superconductivity comes in from the cold

Superconductivity – moving from laboratory concept to commercial reality in a remarkably short space of time – is now set to hit the market. Mark Fletcher reports Once the realm of science fiction, before moving in earnest into the laboratory, high temperature superconductivity (HTS) is going to have a big impact on the world of motors when the major players finally get their prototypes out of the laboratory and into the field on a regular basis. Initial trial applications show promising results which confirm the technology’s future in industry, propulsion and power applications. The concept of superconductivity has been around for years but it is only recently that the necessary complementary technology has advanced to such a stage that the whole package becomes commercially viable – especially at more realistic temperatures. To highlight what type of impact these advances will have it is necessary to put them into context with regard to existing, proprietary technology. According to the US Department of Energy, motors account for over 70% of the energy consumed within the manufacturing sector – a figure which equates to greater than 55% of the total energy production in the US. And within these consumption figures large motors (greater than 1,000hp) consume 25% of the total energy generated. If a 5MW motor (approximately 6,500hp) which runs 24/7 was to experience a 1% increase in efficiency it would save over 430,000kWh per year. The technology still requires an extra bit of effort and infrastructure as the high temperature tag is a tad misleading – it is an expression derived from a temperature relative to absolute zero (-273 degrees C). At this figure the electrical resistance of superconducting materials disappears, unfortunately the liquid helium needed to reach this low is both time consuming and costly to manufacture. However, within the realm of high-temperature superconductors, the electrical resistance disappears at around –190 degrees C, thanks to the use of much cheaper liquid nitrogen. As this temperature conduction becomes virtually loss free, greater efficiency is achieved and higher magnetic field strengths are possible. The higher transition temperature significantly reduces the cooling effort required resulting in a more commercially viable approach. Hand in hand with all of the performance gains comes a significant drop in motor size when compared with similarly powered, copper-based motors. A superconducting motor can offer the same performance in an envelope a third to a quarter the size and at a third of the weight. This smaller size also demands less raw materials and hence the unit costs can be expected to be lower than the equivalent conventional motor. There is also no need for the iron ‘teeth’ used in existing motor designs to conduct the magnetic field. Late last year Siemens’ Research Centre at Erlangen claimed to have “started up the first motor in Europe to have a winding made of high temperature superconducting material”. Figures gleaned from the research promise a power output more than double that of conventional motors of a similar size with copper windings, while losses are halved. Rockwell Automation has also demonstrated its own model through the successful running of a 1,000hp motor. This particular motor was developed with the aid of several partners, one of which was American Superconductor Corporation, which supplied the coils and the cryogenics. The company tells us that the estimated market for large industrial motors is over $1.2bn annually with electric marine propulsion, growing strongly at a current level of $250m, with some studies indicating a quadrupling of the market in the next decade. The firm also goes into some detail related to the benefits on offer from HTS motors. Aside from the manufacturing and running costs and the decreases in size and weight, there is also the question of increased stability as HTS motors are inherently more electrically stable than conventional motors during transients because they operate at smaller load angles – 15deg vs. 70deg – and have a much higher peak torque capability (roughly 300%). The result of this is that the motors can withstand large transients or oscillatory torques without losing synchronous speed and they do not require rapid field forcing during fast load changes as is often the case with conventional models. One of the major factors stalling the earlier introduction of this technology is the wiring. Superconducting wire can now carry 100,000A/cm² – an impressive figure in comparison to copper’s 600A/cm². The first generation wire which showed promise for mass production used what is known as powder-in-tube technology. It comprised a silver powder-filled tube which was then pulled into a wire, but as you can imagine, it proved costly to produce in both labour and materials. A second generation technology was then produced at Los Alamos National Laboratory which used special chemistry and manufacturing processes to deposit a thin film of the superconductor onto a metal substrate. It is envisioned that this wire should be available for as little as $10/kAm by 2004. In theory the wire can carry as much as 1million A/cm² but this is reined in to 100,000A/cm² due to the substrates being used. Superconductivity clearly has massive potential and is certainly not ‘blue sky’ technology. Commercial applications are already under test and the expected influx into general industry is only a year or two away. As the saying goes “watch this space”. Electrical potential Motors are not the only beneficiaries of superconductivity research. The wiring, the crux behind the efficiency gains, can be used in one or more guises in other electrical applications, one of the most promising being transformers. As with motors, size will be the obvious immediate benefit, other benefits include the ability to carry twice the normal current during peak loading conditions (although less efficiently) and reduced losses – down to a few hundred watts in a 30MVA transformer instead of the 100-150kW offered by conventional units. Other uses under investigation include power transmission infrastructure, fault current limiters and energy storage using superconductive magnets for flywheel bearings and superconductive magnetic energy storage systems (SMES). The first application of an SMES system has increased capacity by 15% while reducing voltage instability. MF Image courtesy of American Superconductor