High temperature superconductors Fig. 1 䊳 Second-generation conductors are pending industrial implementation 䊳 Greater grid security thanks to fault current limiters 䊳 Concepts for future electricity grids with superconducting cables 䊳 Industrial heating processes use up to 40% less energy while improving the production workflow Second-generation high-temperature superconductor (right) conducts the same rated current of 200 A without losses as the illustrated copper cable (left). A pprox. 100 years ago, the physicist H.K. Onnes discovered that mercury suddenly loses its electrical resistance below a temperature of 4.2 Kelvin. Subsequently, superconducting metal alloys with transition temperatures of up to 23 K were developed. In the last 30 years, this facilitated pioneering developments, primarily in medical technology, but also in the physical research. However, considering its versatility, superconductivity long remained a niche technology. The main reason is the difficulty of maintaining such low temperatures. As only helium is liquid below 23 K, it is the only possible coolant. However, it is expensive and difficult to handle. Also, a significant amount of energy is required to liquefy it. The response in 1986 was appropriately positive when the physicists J.G. Bednorz and K.A. Müller developed materials which became superconductive at temperatures as high as 35 K. By contrast to the already known metals and alloys, these materials were surprisingly ceramic substances. Just one year later, the new materials achieved transition temperatures above 77 K. This allowed liquid nitrogen, a widely-available, inexpensive industrial product, to be used as a coolant. This greatly expands the possible areas of application, and it also makes energy technology applications, and in particular effi- ciency technologies, possible. However, there is still need for further development to turn the brittle ceramic materials into superconductive wires or bands in the required quality and quantity at acceptable costs. Pilot projects in electricity grids and for motors and generators and applications in industrial processes show that significant energy savings are implemented compared with conventional technology. There is often a secondary benefit, which makes superconductivity particularly attractive compared with conventional technology. For example, superconductors are smaller, lighter and more efficient than conventional conductors, and also allow improvements in processes in some cases. The potential of the technology has been identified around the world. Countries such as the USA, Japan, Korea, China and India have increased their research activity. Germany is among the front runners, having offered significant research subsidies in the nineties. Medium-sized companies in particular can offer highly-developed products and system solutions. In order to maintain and expand this good position, the German Federal Ministry of Economics and Technology promotes the development of the technology and its applications in numerous projects. 䊳 HTS wires and bands Fig. 4: Layer architecture of 2G conductors Fig. 2: Manufacturing 1G conductors Protective layer HTS layer Buffer layer Extruding Rolling Currently, two compounds with the technical and commercial potential for manufacturing HTS wires are known. Both are ceramic oxide materials with copper oxide surfaces as a common basis: They are bismuth-(lead)-strontium-calcium-copper-oxide ((Bi,Pb)2Sr2Ca2Cu3O10+x or BSCCO) with a transition temperature of Tc=110 K and yttrium-barium-copper-oxide (YBa2Cu3O7-x) with Tc=92 K (YBCO). Producing flexible and long HTS conductors using these two materials presents researchers with a variety of challenges. The materials are brittle and fragile. They only achieve the required flexibility in sufficiently small dimensions, and in combination with a metallic matrix or a metal substrate. The orientation of the crystal structure (texture) also affects the current conductivity via multiple scales and grain boundaries of the crystalline structure limit the rated currents which can be reached. Finally, the microstructure also determines the magnetic field strength until which the HTS material remains superconductive. In energy technology applications in particular, this critical magnetic field is often the deciding factor. While industrial manufacturing processes are available for the first generation, which featured embedded long thin silver fibres as metals, the development is focusing to an increasing extent on different thin-film technologies for the band conductors of the second generation. Annealing The first generation – Wires First generation HTS wires (1G) are made with BSCCO using the powder-in-tube method: The initial material is poured into a malleable silver tube as a fine powder, and the tube is then extruded to a thin wire. The diameter reduces from approx. 35 to 2 mm. The powder particles adapt to the thinner wire cross-section. In turn, several of these individual wires are combined in a silver tube and extruded to a multi-filament wire. In a subsequent multi-stage annealing process in an oxygen atmosphere and at temperatures of up to 900 °C, the superconductive phase is formed as finest crystallite in the parallel filaments, which are each separated by the metallic sleeve material. Multiple rolling phases between the heat treatments ensure that the crystallite and conductive CuO surfaces are aligned in parallel. 1G conductors based on BSCCO are reliable and durable. The established manufacturing process currently guarantees sufficient availability. However, the material costs are high, as the wires have a silver content of 60% and higher. If all crystal axes are not absolutely perfectly aligned, causing a reduction in conductivity, in particular in magnetic fields above a temperature of 50 K, this reduces the areas of application. Fig. 3: Current capacity in magnetic fields 1 bar 106 FCL Cable Generators, motors Transformers Jc [A/cm2] Coating rate Performance (current density) 5 MA/cm2 μm/s 0.01 bar 2 MA/cm2 104 103 2G HTS conductor [77 K] nm/s 10-7 bar 1G HTS conductor [77 K] 102 2 The second generation – Bands HTS band conductors of the second generation (2G) achieve higher current densities, are more suitable for magnetic field applications due to the different material class (YBCO instead of BSCCO), and make lowcost mass production possible. However, only two American companies currently offer significant quantities, but European and in particular German companies plan to establish production lines. The coated conductors are based on a layer architecture: On a metallic base band, first ceramic buffer layers and then the actual superconductor layer are precipitated. The German Federal Ministry of Economics and Technology subsidises various processes for industrial production in three large-scale joint projects. Physical methods like sputtering, laser ablation or electron beam evaporation create layers with maximum performance. THEVA GmbH has already succeeded in exceeding a current density of 1000 A/mmÇ with electron beam evaporation. However, as the physical processes take place in a high vacuum, lower coating rates are reached than with chemical processes such as chemical solution deposition (CSD). Metal organic chemical vapour deposition (MOCVD), which only requires a moderate vacuum, takes the middle ground. Fig. 5: Comparison of coating processes2 Pressure 105 Metal substrate BINE projektinfo 06/10 5 CSD 10 15 20 MOCVD PVD 䊳 Application research A variety of research projects, demonstration projects and studies, some of which are funded by the German Federal Ministry of Economics and Technology, are investigating the broad range of uses of HTS superconductors. These are among others the following: ■ A 4 MW ship drive motor with a great torque at low rotational speeds is currently being tested by Siemens. ■ When renovating a hydroelectric power plant in Hirscheid, an HTS generator with higher output capacity replaces a conventional generator of the same size. This saves conversion in the building, which is listed for protection. ■ Converteam and Zenergy are developing generators for wind power plants and hydroelectric power plants, including a gearless 8 MW wind turbine which is 75% lighter than the conventional technology. ■ Oswald develops motors in the medium power range for vehicles, generators and tool machines. ■ German companies (Nexans and NKT) are major participants of cable projects at an international level. As an example, two developments will be introduced which offer users other key advantages in addition to higher energy efficiency, represent an economic solution and are interesting for a large number of potential users. Industrial heat treatment In industrialised countries, 1 to 5% of the overall electricity consumption is used for material heating in extruding plants. A newly developed superconductive magnetic heater consumes approx. 40% less energy than an induction furnace fired with alternating current and around 60% less energy than a gas furnace of comparable power. The system investment for a magnetic heater is recouped through increased productivity and reduced energy costs in less than two years on average. The developers of the system, Zenergy Power GmbH and Bültmann GmbH, won the German 2010 Innovation Prize for Climate and the Environment. The magnetic heater operates on the eddy current brake principle. The system contains a superconductive magnetic coil powered with direct current. The extrusion billets are rotated by two electromotors in the field of the magnetic coil. The braking effect of the magnetic field must be overcome, which causes the rotating material to be heated. Short-circuit resistance of electricity grids Superconductive fault current limiters increase the short-circuit resistance and reliability of electricity grids and can also reduce the investment costs. During short circuits, current levels occur in electricity grids which can exceed the rated current many times. The grid must be designed for such loads, i.e. must be short-circuit resistant, to prevent major damage. Relative to normal operation, this results in oversizing, which is primarily reflected in the investment costs. In the past, grid failures were common, as a result of short circuits in the grid. Fault current limiters which restrict the level of short-circuit current can make a significant contribution to increasing the security, availability and reliability of grids. Ideal FCLs have a low alternating current resistance (impedance) in normal operation, rapid and effective current limiting in the event of failures and automatic reusability readiness. Previous measures to restrict short-circuit currents are used in medium voltage and either result in permanent increases in impedance during normal operation, or they must be replaced every time they are triggered (e.g. fuses or Is-limiters, which disconnect the current path via an explosive charge). To date, there were no fault current limiters for the high-voltage grid – even conventional solutions from medium-voltage systems cannot be applied here. By contrast, superconductive FCLs fulfil all requirements of an ideal fault current limiter. They take advantage of the fact that the superconductive state transitions to a normal conductive state above the maximum current density. Thus, the superconductor instantly establishes a high resistance, which restricts the current efficiently to a designed value, regardless of how high the expected shortcircuit current is. Thus, the FCL functions without external signals and is intrinsically safe. It reactivates automatically after a brief cooling phase without further maintenance. There is great interest in the technology. In Germany, four companies are currently developing superconductive fault current limiters, some of which feature different principles. Nexans SuperConductors devices are the first in commercial use in public electricity grids in England and Germany. One of these has protected the electricity supply of coal pulverisers and breakers against short circuits in the Saxon lignite power station in Boxberg since the end of 2009. The operator is of the opinion that the technology will Fig. 6: Magnetic heater Fig. 7: Fault current limiter in the Boxberg power station provide a significant gain in health and safety and plant reliability. If the design proves itself, such fault current limiters could protect the entire internal electricity supply of power station against short-circuit currents. Superconductive FCLs are suitable for new power stations, as well as for extensions such as retrofitting CO2 separating systems. The project won the Energy Masters Award 2010. Superconductive fault current limiters for high-voltage systems are currently in development, but not yet in practical use. They should also facilitate the construction of more efficient grid structures: For example, 110 kV sub-grids, each connected by a 400 kV transformer, can be coupled using FCLs. This coupling means that one of the two transformers can be omitted (n-l principle), thus avoiding both the investment costs and losses, which reduces operating costs. BINE projektinfo 06/10 3 Prospects PROJECT ORGANISATION PROJECT ADDRESSES • Adelwitz Technologiezentrum GmbH (ATZ), Arzberg, Germany ▼ ▼ A new generation of high temperature superconductors is in the starting blocks and will open the technology for a wide range of applications. While the first generation of HTS wires was 60% silver, which made it almost prohibitively expensive, the new HTS band conductors require hardly any expensive raw materials. Various manufacturing processes are being improved or developed, and taken from the laboratory phase to industrial pilot production with the support of the German Federal Ministry of Economics and Technology. However, the production capacities remain low. A twokilometre pilot section for a medium-voltage superconductive cable, currently being considered by power company RWE, would currently require the entire global annual production of HTS band conductors. Depending on the market penetration of HTS components, experts estimate the future demand for HTS wire at between 10,000 km per annum in the next few years to 500,000 km in five to ten years. If the expected significant cost decrease as a result of mass production is achieved, high temperature superconductors could play an increasingly important role in power supply and efficiency technologies. For example, it facilitates lossless power cables, more compact motors and generators with greater efficiency and efficient heating processes and frictionless superconductive bearings for industrial applications. The technology is considered particularly important for the adjustment of the electricity grid to the changing, increasingly decentralised generation structures. A current study documents significant advantages, in conurbations in particular. More compact and lighter transformers and cables can be implemented, while reduced impedance increases grid stability, and they can be designed with active short-circuit current limiting. Superconductive magnetic energy storage systems can compensate for power and voltage fluctuations in the grid. The great current-bearing capacity facilitates new grid topologies, in which individual grid levels are no longer necessary. Overall, the use of various HTS components can help save energy in urban grids. Using Cologne as an example, the study predicts an annual CO2 emission reduction of 60,000 tonnes due to lower grid losses. Pilot large-scale grid applications are pending implementation. For example, plans are underway in the USA to connect two large and one small electricity grids totalling 5 GW via superconductive high-voltage direct current cables. The “Tres Amigas” project is intended to facilitate the integration of renewable energy sources into the power supply and also improve the grid stability in the USA. The range of German institutions and commercial companies researching this area is unique in Europe and compares well with leading technology nations such as the USA and Japan. This means that Germany is well positioned to take on a leading role in superconductive energy technology, similar to the success it has enjoyed with many environmental technologies and renewable energy sources such as wind and solar power. ADDITIONAL INFORMATION Internet • www.ivsupra.de • Bruker HTS GmbH, Hanau, Germany • GTT Gesellschaft für Technische Thermochemie und -physik mbH, Herzogenrath, Germany • Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany • Nexans SuperConductors GmbH, Hürth, Germany • Oswald Elektromotoren GmbH, Miltenberg, Germany • Theva Dünnschichttechnik GmbH, Ismaning, Germany • Zenergy Power GmbH, Rheinbach, Germany Picture credits • Background photo, p. 1, Figs. 1-6: Zenergy Power GmbH, Rheinbach, Germany • Fig. 7: Nexans SuperConductors GmbH, Hürth, Germany • Background photo, p. 4: Theva Dünnschichttechnik GmbH, Ismaning, Germany Service • This Projektinfo brochure is also available as an online document at www.bine.info under Publikationen/Projektinfos. Additional information in German, such as other project addresses and links, can be found under “Service”. ■ Project Funding Federal Ministry of Economics and Technology (BMWi) 11019 Berlin, Germany Project Management Organisation Jülich Research Centre Jülich Dr. Claus Börner 52425 Jülich, Germany ■ Project Number 0327431H 0327429B, C, D, E IMPRINT ■ ISSN 0937 – 8367 ■ Publisher FIZ Karlsruhe 76344 Eggenstein-Leopoldshafen Germany ■ Copyright Text and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you. ■ Editor Dr. Franz Meyer BINE Information Service Energy Expertise BINE Information Service provides information on energy-efficient technologies and renewable energy: Using a combination of free brochures, the BINE web site (www.bine.info), and a newsletter, BINE shows how innovative research ideas hold up in practice (in German). BINE is an information service by FIZ Karlsruhe, which is promoted by the Federal Ministry of Economics and Technology (BMWi). FIZ Karlsruhe, Büro Bonn Kaiserstrasse 185 – 197 53113 Bonn Germany Tel.: +49 228 92379-0 Fax: +49 228 92379-29 bine@fiz-karlsruhe.de www.bine.info 4 BINE projektinfo 06/10 KERSTIN CONRADI · Mediengestaltung, Berlin, Germany 䊳