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SuperconductorsSuperconductor Developments | Superconductor Applications | Superconducting Magnetic Energy Storage Systems | Further Information | References |Superconductor DevelopmentsThe capacity of superconducting materials to handle large currents with no resistance and extremely low energy losses is a huge benefit over competing technologies. The relatively recent developments in this field are impressive and will play a part in more efficient energy systems. 
Figure 1 A superconducting metal. (courtesy of Tom & Robs Superconducting Website)
Superconductors are generally divided into two categories; Low-Temperature Superconductors (LTS or type I), and High-Temperature Superconductors (HTS or type II). LTS must be cooled to just above absolute zero (-269°C) and can be utilised as storage devices that provide power conditioning and power backup and are used by some electricity utilities. HTS only have to be cooled to -173°C are preferred to LTS technologies as they do not have problems of maintaining such low temperatures (EERE, 2005). The applications of superconductors are diverse. They are being used to improve the efficiency of motors, generators, transmission lines, transformers, and energy storage technologies. Using an integrated power system incorporating superconductor technologies can reduce the amount of power that is needed to be generated to supply the same demand, as well as allow many new applications, such as magnetic levitating trains (See Figure 2). Superconducting Magnetic Energy Storage (SMES) is a new technology that is used to regulate power fluctuations and maintain the stability of the grid when large changes in load occur. SMES systems store energy in a magnetic field created by the flow of direct current in a coil of superconducting material that has been cryogenically cooled. A superconducting material enhances storage capacity. In low-temperature superconducting materials, electric currents encounter almost no resistance. The challenge is to maintain that characteristic without having to keep the systems quite so cold (NREL, 2006).
Superconductor ApplicationsThe following table shows a list of (potential) applications for superconducting technologies;
Table 1 Table derived from EERE.
Figure 2 Maglev trains in Japan (courtesy of Maglev R&D at the Railway Technical Research Institute)
One of the applications for SMES systems are with renewable energy technologies that produce large fluctuations in power generation. There are electricity utility applications for SMES for MW sized systems that are being used in Northern Wisconsin. This installation uses a string of distributed SMES units, deployed to enhance stability of a transmission loop. The transmission line is subject to large, sudden load changes due to the operation of a paper mill, with the potential for uncontrolled fluctuations and voltage collapse. Besides stabilising the grid, the six SMES units also provide increased power quality to customers served by connected feeders. Several 1 MW units are used for power quality control in installations around the world. (EERE, 2005). SMES power is available almost instantaneously and very high power outputs are provided for a brief period of time, with no loss of power, and there are no moving parts. However the energy content of SMES is short-lived and the technology used to cool the SMES system down can cause issues (NREL, 2006). This technology is used in the high-speed magnetic-levitated trains. In addition to this application, superconductors are also being developed for use in microelectronics and communications (NREL, 2006).
Superconducting Magnetic Energy Storage SystemsA typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2-3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%. Due to the energy requirements of refrigeration, and the limits in the total energy able to be stored, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality. If SMES were to be used for utilities it would be a diurnal storage device, charged from baseload power at night and meeting peak loads during the day. The high cost of superconductors is the primary limitation for commercial use of this energy storage method (Wikipedia, 2006).
Further InformationSuperconductivity of Electrical Systems Maglev R&D - Railway Technical Research Institute RISE Information Portal Applications ReferencesEERE, 2005. “Super Conductivity” (Online) http://www.eere.energy.gov/EE/power_superconductivity.html (Accessed May 24 2006). NREL, 2006. “Superconducting Magnetic Energy Storage” (Online) http://www.eere.energy.gov/de/supercon_magnetic.html (Accessed May 24 2006). |
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