|Speaker:||Donald U. Gubser, Naval Research Laboratory|
|Topic:||“Superconductivity: An Emerging, Energy Efficient Technology”|
The 2158th meeting of the Philosophical Society of Washington was called to order at 8:18 PM in the Powell Auditorium of the Cosmos Club. President Haapala was in the chair. President Haapala introduced the speaker for the evening, Donald Gubser who is the superintendent of the material science and technology division of the Naval Research Laboratory. The speaker is a graduate of the University of Illinois with a PhD in Physics. Mr. Gubser addressed the society on superconductivity, an emerging energy efficient technology.
Super conductivity is defined by the behavior, electrical resistance, of a material as a function of temperature. As the temperature drops electrical resistance drops. In a super conductor the electrical resistance drops to zero at a material specific low temperature above zero Kelvin. Magnetic fields can destroy the superconducting properties of a material. A super conducting material in its superconductive state excludes magnetic fields from its interior. Electrons can form bound pairs which act as a large object deforming the crystal lattice. The deformed lattice allows the electron “object” to move through the lattice without resistance. Apparently strong magnetic fields prevent the lattice from deforming because in the presence of strong magnetic fields superconductors do not exclude the magnetic field and superconductivity does not occur.
Super conducting materials are usually binary alloys. Nobelium-Titanium is a ductile alloy which can be drawn into fine wires. It was first used for the superconducting magnets in research and for the particle accelerators. It is now used in the magnets of the Magnetic Resonance Imaging machines (MRI.) A Wisconsin utility is investigating the use of these superconducting magnets for the storage of electrical energy in SMES. It takes energy to keep things cold and the measure of the ability of a device with a given unit of power to keep something cold is the coefficient of performance. To keep superconductors cold enough for superconducting behavior requires coefficients of performance four orders of magnitude higher than the home refrigerator. Super conductors would be a lot more useful if they could work at a higher temperature. Betnaur Mueller raised the superconductivity threshold by a factor of ten by using a material with five components. The response to the dramatic announcement of high temperature superconductivity included the quickest Nobel Prize in history.
NSA, DoE and DoD have all increased funding for research into high temperature superconductivity. However it is not at all clear how to deal with this new material, Many of the components are toxic, the alloy is brittle not ductile. To be useful in engineering it will be necessare to be able to draw it into wire. The new high temperature materials exhibit different responses to temperature. So far at least three different critical magnetic field levels have been identified which restrict or eliminate superconductivity. Superconductive behavior in the new materials is non-isotropic. The lattice structure of the alloy must be crystolographically aligned. A way was eventually found to make the material into wire. The powdered alloy was added to a silver matrix, heated, extruded and rolled into wire. The improvements in performance have been evolutionary. The new high temperature superconductor can carry ten times more current than a copper wire of the same cross section. Potential new products being developed with the new wire include, for Power distribution systems: (1) Transmission lines, (2) Energy storeage flywheels, and (3) fault current limiters.
Sixteen percent of the power generated in the U.S. is used by motors. While there are more electronic devices in use their needs are for power quality not for large amounts of current. The potential energy savings of superconducting transmission lines is calculated to be equivalent to 382 million barrels of petroleum, the equivalent of 41 days of imports. For electric motors, AC synchronous motors the U.S. Navy is planning to have several in use by the year 2005. The Navy requirements are for high power, low rpm propulsion motors which are lighter and quieter than existing motors. A 25 megawatt 30,000 horsepower main propulsion motor is now being built by American Semiconductor Inc. They expect to reduce the motor size and weight by a factor of two and the size and weight of generators by a factor of five. The multi-pole design maintains high efficiency at low rotational speeds and eliminates the requirements for a gear box. This saves on maintenance costs and eliminates wear and noise from the power train. General Electric is interested in building increased efficiency electric power generators. They are working on ways to rebuild existing generators with superconducting coils to increase their generating capacity without having to replace the entire generator or change its foot print.
Power distribution lines in urban areas are often routed through tunnels which tend to become filled to capacity with distribution cables as demand grows. Use of superconductor cables offers the ability to carry three to five times more power through the same cross sectional area which could meet future demand for some time without having to dig up the streets to enlarge the tunnels. Power distribution cables do have to be replaced from time to time. If ten percent of the existing distribution cables were replaced by superconducting cable the nation could save 100,000 barrels of oil per year. A demonstration project high voltage superconducting cable built by South Wire Corporation had been carrying current continuously for 30 months, two thirds of the time at full power. The Pirelli corporation is now building superconducting underground distribution cables for the commercial power grids to retrofit existing tunnels. In one installation the cable was pulled through three bends of 90 degrees. The cable was not a commercial success and did not carry as much current as planned. But this was because of a failure of the vacuum system. The superconducting wire material did not fail in this application. Several other commercial installations of superconducting cables are planned over the next two to three years. Transformers are another application where superconductivity would offer real advantages. A 30 MVA transformer weighs 48 tons and includes 23,000 liters of flammable oil as coolant. A cryogenic superconducting transformer of the same capacity could be built weighing only 24 tons and requiring no flammable or toxic oils for coolant. Liquid Nitrogen would be the refrigerant. Gaseous Nitrogen is non-toxic raising the possibility that such transformers could be sited in the basements of habitable buildings. They would be much easier to transport and more environmentally benign.
Superconducting fault current limiters are possible because they use the change in resistance of the high temperature superconductinr wire. When an accidental short to ground occurs the increased current heats the wire enough to cause it to develop resistance thus limiting the current it could carry. The energy storage device for shifting power production from off peak to peak hours employs a flywheel spinning in a vacuum. The flywheel is completely levitated by high temperature superconducting magnets eliminating bearing wear as a source of loss of energy. The Bowing corporation is building one of these devices. They are not without engineering challenges because when they fail they fail catastrophically and must be placed in an adequate containment structure to retain the shrapnel. A ten kilowatt hour model has been built and is being tested. A thirty five kilowatt hour model is in the works.
Other applications noted were a magnetic separation to remove magnetic impurities from Kaolin clays developed by DuPont and the MRI now installed in numerous hospitals and medical centers to perform non invasive visualization of body organs and soft tissue. The current MRI models use the low temperature superconductor NbTi . The current world market for MRI instruments as three billion dollars per year. The Oxford Instrument Company builds them on an assembly line. A newer model the open MRI also uses the low temperature superconductor material. It could be smaller, quieter and lighter if it were made with the high temperature super conductor material.
High temperature superconductors now cost around $200 a per meter for 1 Kiliamp capacity. The goal is to get it down around $50 per meter per Kiliamp. For comparison copper costs $1 per meter per Kiliamp but will only carry one tenths the current. The temperature range for the new materials BSCCO ~35 K to 70 K and with YBCO ~70 K. The goal is a material which will be superconducting at ~100 K. The prognosis for commercial applications of superconductors from 2003 to 2010 is that they will slowly enter the market in niches where their performance can not be matched with any existing non superconducting material especially applications where size and weight are more important constraints than cost. A larger market is forseen after 2015 when cryogenics have become more reliable and cost effective.
The President thanked the speaker for the Society. Mr. Gubser kindly agreed to answer questions from the floor. The questions included:
Q- Instead of looking for a 100 K semiconductor material why not develop better insulation? Wouldn't that be more cost effective?
A- Yes it is being worked on – but not by me.
Q- Has anyone looked at the application of high voltage DC application?
A- Yes it will work but that is not a large a market.
Q- As the current goes up in a superconducting transformer wouldn't there be a risk of developing hot spots?
A- They seem to be fail safe because of the current limiting behavior above the superconducting threshhold. At least they fail gracefully in small systems. That is the reason why Japan is switching their MagLev train from low temperature superconductors to high temperature superconductors because they fail more gracefully.
Q- What about the cost of superconducting motors?
A- Superconducting motors are 5 to 10 times more expensive than conventional electric motors at present.
President Haapala adjurned the meeting for the social hour at 9:58 PM. He made the usual announcements and appeal for restrained behavior regarding carrying alcoholic beverages outside the auditorium.
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