This article highlights large-scale applications of high-temperature superconductors (HTS), based on copper-oxide ceramics, which have begun to occur in the United States. A number of major electric-power application projects have been undertaken as partnerships between the US Department of Energy and various companies. Superconductors are particularly appropriate for electric power applications because of the total lack of resistance in direct current applications and very low losses in alternating current. The low losses allow use of much higher current densities than can be achieved in normal conducting metals, such as copper or aluminum. An HTS transmission line has been operating at the Southwire Co. in Carrollton, GA, since January 5, 2000. Since then, the three 100-foot-long above-ground cables have supplied 1250 A at 12.4 kV to three manufacturing plants at the Southwire headquarters. During its first year of operation, the line supplied more than 5000 hours of operation at 100% load. Southwire’s HTS cables lose only about 0.5% of power during transmission, compared to 5 to 8 percent lost by traditional cables.
Approximately 15 years after the discovery of the first high-temperature superconductors, based on copper-oxide ceramics, large-scale applications of these materials are now beginning to occur in the United States. A number of major electric-power application projects have been undertaken as partnerships between the U.S. Department of Energy and various companies.
Superconductors are particularly appropriate for electric power applications because of the total lack of resistance in direct current applications and very low losses in alternating current. The low losses allow use of much higher current densities than can be achieved in normal conducting metals, such as copper or aluminum. The higher density results in devices that are more efficient and take up less volume than conventional electrical devices of the same power rating. Recent advances in the performance and reliability of cryogenic refrigeration, especially the commercialization of pulse-tube cryocoolers, have greatly enhanced many applications of superconductors.
Most of the applications that use superconductivity employ wires. For many years, the materials of choice for these wires were the relatively ductile NbTi, or the more brittle Nb3Sn, both commonly called low-temperature superconductors because they typically require liquid helium cooling, or cryocoolers operating below 10 K, to maintain their superconductivity. Now, several families of higher-temperature materials can enter the superconductive stage at the temperature of liquid nitrogen, which is 77 K. Conductors of one family are seeing service in applications, and another family is rapidly being developed for possible use in the near future.
The present workhorse of HTS wire is the bismuth copper oxide family, which may be in the form of Bi2Sr2Ca2Cu30x (where "x" is not necessarily known) or in Bi2Sr2Cal CU20x' These HTS wires are fabricated inside a silver-alloy metallic sheath and undergo a number of thermal and mechanical treatments before they become good superconductors.
A disadvantage of the bismuth HTS is that its critical current density is relatively low at liquid nitrogen temperatures in magnetic fields that are typically used in electric power applications. However, in the 20 to 30 K temperature range, the engineering current densities that this HTS can provide are useful in many applications.
The rapid development of second-generation wire, made by depositing thin films of yttrium- based copper oxide on metallic substrates , is expected to further accelerate commercial use of superconductors. This HTS material can deliver substantial current in a magnetic field at liquid - nitrogen temperatures . Many research groups have demonstrated that in a thin-film form , it can transmit more than 1 MA/ cm2 of current , more than 1,000 times the current density that flows through household wiring.
Of course, depositing thin films that remain contiguous over long lengths is very challenging, and present effort s are devoted toward making the fabrication of these wires more rapid and less expensive. Ultimately, this second generation of HTS wire is expected to have an engineering current density that is about an order of magnitude higher than the first-generation bismuth HTS.
An HTS transmission line has been operating at the Southwire Co. in Carrollton, Ga ., since Jan. 5, 2000. Since then , the three 100-foot-long above ground cables have supplied 1,250 A at 12.4 kV to three manufacturing plants at the Southwire headquarters. During its first year of operation, the line supplied more than 5,000 hours of operation at 100 percent load . Southwire's HTS cables lose only about 0.5 percent of power during transmission, compared to 5 to 8 percent lost by traditional cables. The cables also deliver added power density, about three to five times more than traditional power cables.
As the rapid growth of urban areas increases demand for electricity and limits the space available for overhead and underground cable installations, the ability of HTS cables to transmit more power using the same amount of space as traditional cable will be increasingly important. HTS cables can be used underground in areas where more power is needed but space for additional lines is not available. HTS cables may also play a future role in improving the efficiency of the electrical gr id. Over the past decade, as long-distance wheeling of electricity has increased , the average losses of the electrical grid have increased from 7.3 percent to about 11 percent, according to the Energy Information Administration.
Another HTS cable project is under construction at Detroit Edison, where a 2,400-A HTS cable will be installed. This cable was manufactured by Pirelli, using 18 miles of HTS wire that were made by American Superconductor Corp. of Westborough, Mass. This will be the first project in the world to retrofit a superconducting power cable system into an operating utility substation. The three-phase, 2,400-A, 24-kV superconducting cable circuit, after being installed in Detroit Edison's Frisbie substation, is designed to transmit 100 MVA of power, serving approximately 14,000 customers. The three HTS cables will replace nine conventional cables in three copper- conductor cable circuits. This will leave six ducts open, available for additional HTS cables or for installation of other assets, such as Internet connections.
As part of the retrofit, the cable will have to be pulled through existing ducts 10 cm in diameter, with several 90- degree bends. Mechanical engineers designed a cable that could withstand such treatment while using a relatively brittle HTS as a crucial component. In addition to DOE and private support, this project receives support from the Electric Power Research Institute in Palo Alto, Calif.
Testing an Electric Motor
Rockwell Automation/ Reliance Electric of Cleveland has been testing a 750-kW HTS electric motor. The motor's stationary stator winding is made of copper, as customary, but the rotor winding is HTS, the copper in its field coils replaced by a bismuth tape produced by American Super conductor. The HTS coils are cooled by gaseous helium at a temperature of about 30 K.
Already, this early prototype has an efficiency of just above 97 percent, which is the performance of most conventional large electric motors. Reasonable improvements in the HTS motor technology are expected to take the motor efficiency to 98. 5 percent, reducing the losses of such motors to half that of conventional motors with the same rating, even after accounting for the input power required for the cooling system. Plans are now under way to construct a 3,700-kW (5,000-hp) motor, the last step in scale-up to what is believed to be a commercially viable size.
Flywheels, Pellets, and Bearings
Another DOE-funded project, led by the Boeing Co. Phantom Works of Seattle, uses HTS crystals to stably levitate a large 10-kWh flywheel rotor based on HTS bearings. The device uses no superconductive wires. The bearing is composed of a permanent magnet and a number of bulk HTS pellets . The pellets are single domain (essentially a single crystal, but with doping impurities scattered throughout the crystal structure). The low losses of the HTS bearings should make the fly wheels very efficient. Both a 3-kW and a 100-kW motor/ generator are envisioned for the Boeing project, so that HTS flywheels can either act as diurnal energy storage or as uninterruptible power supplies.
Waukesha Electric of Waukesha, Wis., is leading a project to develop HTS transformers. Superconducting transformers are expected to be smaller and more efficient than conventional transformers of the same rating and not require any flammable oil for cooling. A single-phase I-M V A HTS transformer has already been constructed and tested. Work is now being done on a three-phase prototype that can be operated from 5 to 10 MVA, as an intermediate step to the 30- to 60-MVA commercial size. The I-MVA coil, core, and tank were built essentially at full 30-MV A scale. HTS wire for this project was supplied by Intermagnetics General Corp. of Latham, N.Y.
Magnetic resonance imaging. has been a commercial medical technology for many years. It uses superconducting magnets to generate high magnetic fields over large volumes to image the body noninvasively.
Commercial MRI magnets have so far used low-temperature superconductors , either NbTi or Nb3Sn, and are usually cooled in a liquid-helium bath. In the early years of this technology, the liquid helium would evaporate from the system and have to be refilled at various intervals. The medical practitioners using the MRI put up with this inconvenience only because diagnostic information they were receiving could not be gained by any other means. What they really wanted was a system in which the helium never had to be refilled and the cryogenic system was essentially invisible to the user.
Now, improvements to the magnet insulation, systematics, and current leads, and increased efficiency in cryocoolers have brought MRI technology to the point that once the magnet is initially filled with helium, it never requires refilling. Such transparency to cryogenics is a goal to which other superconductivity applications will need to aspire.
In addition to the numerous activities in the United States, similar projects are under way in Europe and Asia. None of the HTS applications currently being pursued would be considered for commercial use if not for the successful parallel development of cryocoolers.
Cryocoolers are refrigerators that can achieve the low temperatures required for superconducting applications. High-temperature superconductors have to be cooled to about 20 K. The performance of a cryocooler is measured primarily by three parameters: the cooling capacity (in watts), the minimum temperature at which the cooling capacity is available, and the efficiency, which determines the electrical power input.
Since ordinary vapor-compression cycles are not suitable for cryogenic applications, due to the limitations on the saturation temperature of the refrigerant, cryocoolers are ordinarily based on cycles in which the refrigerant does not undergo a phase change.
A notable exception, however, is the Joule-Thomson cryocooler, which produces cooling by allowing a pressurized gas to expand isenthalpically across a valve. The minimum temperature of a JT cryocooler is limited by the saturation temperature of the gas, which is 87 K for argon, 77 K for nitrogen, 27 K for neon, and 4 K for helium. Both open and closed-cycle JT cryocoolers have been developed, with cooling capacities ranging from milliwatts to several watts.
A variety of regenerative, closedcycle cryocoolers are also available, including the Stirling, the Gifford McMahon, the Vuilleumier, and the pulse-tube systems. In each case, a compressor drives the refrigerant, which is normally helium gas, in an oscillating manner. Crucial to the performance of these cryocoolers is the effectiveness of the regenerator, which is a compact heat exchanger that allows the refrigerant to exchange heat with a porous medium.
Small as a Soda Can
Thousands of Stirling cryocoolers are being manufactured each year, in sizes as small as a typical soda pop can. The Gifford-McMahon cycle easily lends itself to multistaging, making it possible to achieve temperatures as low as 15 or 20 K, and even lower with an added JT stage.
The Vuilleumier cryocooler is attractive because of its high reliability and low acoustical noise.
The pulse-tube cryocooler, which in its simplest form consists of a gas-filled tube that is closed at one end and has a piston at the other, offers arguably the greatest promise for long-tern1 reliability because of its simplicity and lack of moving parts at its cold end.
A variant of the pulse-tube cryocooler is the thernmoacoustic refrigerator, which uses sound waves to achieve the compression and the expansion of the working fluid. Temperatures as low as 4 K have been achieved with either single- or multistage pulse-tube cryocoolers.
Improving the understanding and performance of pulse-tube cryocoolers is one of the most active fields of research today in cryogenic technology. Several such coolers have been developed in support of space missions, where high reliability and low weight are primary concerns. One, produced by TRW Inc. in Redondo Beach, Calif., provides 1.5 W of cooling at 115 K, requires 38.2 W of input power, and weighs only 2.3 kg, not including the control electronics. Indeed, this is an example of the potential miniaturization of cryocoolers- particularly pulse tubes-that will make it easier to incorporate cryocoolers in a variety of applications, especially superconducting electronics.
There has been recent interest in several types of miniature (also called mesoscale) cryocoolers, including those based on the sorption and reverse-Bray ton cycles, but pulse-tube cryocoolers are a prime candidate for miniaturization. One approach is to use semiconductor processing technology to fabricate tiny pulse tubes out of silicon, but a severe drawback to this approach is the high thermal conductivity of silicon, which leads to unacceptably high parasitic heat losses
A challenge for any miniature cryocooler is the development of a satisfactory small-scale compressor. Assuming that the design and fabrication hurdles can be overcome to produce a low-cost, efficient miniature cryocooler, we will likely see a vast increase in the number of consumer and industrial superconducting systems in the marketplace.