Superconductivity: the phenomenon exhibited by certain substances of conducting electrical current without resistance when cooled to low temperatures. (Webster 1343) Heike Kamerlingh Onnes first discovered this property in 1911 when an amount of mercury (Hg), supercooled with liquid helium (He) to 20 Kelvin (K), demonstrated no measurable resistance to electrical current. However, at that time, practical implementations did not seem plausible because of the extraordinary amounts of energy necessary to obtain the coolant, liquid helium, and the extreme low temperatures needed to achieve a superconductive state. Up until 1986, scientists believed that the critical temperature (Tc) for superconductors was 20 K and below. However, that year, IBM scientists in Zurich discovered compounds that were superconducting at temperatures up to 35 K (Ashley 59). Only one year later, at the University of Houston, Dr. Paul Chu discovered a compound that would display superconducting properties at 94 K. This was a major discovery because liquid helium would no longer be needed to cool the superconducting substances. Nitrogen (N), which exists in liquid state up to 77 K, could now be used in place of He. This change from N to He was a major benefit because its liquid temperature is achieved with much less energy and N is much more available than He.
This new generation of superconductors, aptly dubbed 'High Temperature Superconductors' (HTS), provided new hope for industrial and commercial uses. Most HTS's are ceramic in nature when compared to earlier 'Low Temperature Superconductors' (LTS) which were metallic. The companies at the forefront of this emerging technology include Pirelli Cable Corporation , Electric Power Research Institute (EPRI), Southwire Company, and American Superconductor. Also aiding in the continuing research of superconductors is the National Renewable Energy Laboratory (NREL) , the Oak Ridge National Laboratory (ORNL), and the Department of Energy (DOE) which funds most of the research.
These organizations are currently researching two specific cable designs; each with its own benefits and drawbacks. One type has the actual HTS material enclosed in a cryogenic environment, which, in turn is surrounded by a conventional room-temperature dielectric. A more popular design is one in which there are two concentric HTS conductors used. The first type is suitable for pipe 'retrofitting'. While less HTS tapes are used in the room-temperature dielectric model, there is a relatively high percentage of electric and thermal loss. Therefore, while the initial costs are low, the overall lifetime operating costs are high compared to the other model. This model, unlike the room-temperature one, uses many expensive HTS tapes, making the initial cost higher. Once in operation, the initial costs can be justified by the fact that they are much more efficient. The cryogenic cables can carry a higher current, have smaller dimensions, and lower power losses than their room-temperature counterparts (Rahman 32).
While these two designs are different, their main component, the HTS tape, is the same. The HTS tapes are the actual superconducting substance with a covering called a sheathing attached. There are four main materials that can be used to produce HTS: yttrium-barium -copper-oxide (YBCO), bismuth-strontium-calcium-copper-oxide (BSCCO), thallium-barium -calcium-copper-oxide (TBCCO), or mercury-barium-calcium-copper-oxide (HBCCO). YBCO standard wire thickness can support 1,200,000 A/cm2 at 75 K and 0 Tesla (T). YBCO's structure is very resistant to current loss from microscopic malformations, compared to its counterparts. BSCCO wire yields either 44,000 or 74,000 A/cm2 at 77 K and 0 T depending on manufacturing process. However BSCCO is anisotropic which means that its manufacture is much more complicated due to the fact that the particles must be properly aligned in order for optimal current flow. TBCCO wire current density is 68,000 A/cm2 at 77 K and 0 T. This compound along with HBCCO have not been fully researched due to their toxic nature. Most implementations of HTS today use YBCO or BSCCO conductors (Balachandran 147).
Benefits
The science of HTS is one of great importance because electricity accounts for 36% of the total energy used in the United States. By the year 2020 consumer demand for electricity will increase by 50%. It is paramount to find a more efficient means of energy transport to reduce unnecessary energy loss. This is why HTS cables may very well be the answer to the future energy problems. Compared to conventional copper wire structures, HTS cables retain twice as much energy from resistance losses. Overall, the current capacity is 3 to 6 times more than that of conventional methods (Ashley 62). If HTS cables were to be used in place of conventional wiring, the space needed to transport the same amount of power would be magnitudes smaller, and if the pre-existing conduits were altered to run HTS cables, the increased energy supplied by these 'pipes' would benefit all involved.
HTS cables also provide increased stability over conventional counterparts. The cables are less prone to electrical spikes and surges, because of their underground placement, thus protecting all components attached to the affected electrical grid. In addition, HTS wiring structures are quite compatible with future add-on HTS structures allowing for the implementation of the industry concept of 'deferred expansion'.
The combined benefits of HTS wiring will also ease stress placed upon the environment caused by power production facilities. If there is less loss of energy in the transport, then less energy would have to be produced, and hence there is less emission. Current transformers use an environmentally unfriendly, oil coolant to increase performance, however, by using HTS wiring the nitrogen cooling process can be interfaced and integrated with the transformers, creating a less hazardous leak potential. The underground wiring will also require less above ground space, thus saving trees and nearby obstructions.
Problems
Certain implementations of HTS wiring are appearing in devices such as motors, generators, fault-current limiters, and cellular phone base stations, all of which are aimed at increasing efficiency of the specific device. HTS use as a means of long distance energy transport presents some problems which are unique to this type of technology. The grains of the HTS powder must be aligned properly in order for the electric current to flow. An ill-begotten microstructure directly weakens the macrostructure properties and greatly reduces the superconductive property. Another problem is that the BSCCO is stable only in a very narrow temperature range and is therefore precise control is needed in the creation of the substance (Balachandran 147). A major flaw of YBCO is that it forms weak microstructural links and those imperfections act as barriers to the flow of the current.
Not only are there problems intrinsically present in the materials, but difficulties arise in the manufacturing process. It is hard to manufacture the HTS tapes with a high enough critical density yet be in the temperature range that would produce the superconductive property in the material. The higher the density, the higher the temperature used in the manufacturing; and if the temperature is too low, the density will not be high enough (Larbalestier 736). Another problem is that the manufacturing process is very expensive. The materials of the tape are not readily available and must be chemically produced in a very controlled environment to ensure the purity of the substance and in order to ensure that the superconductive properties will be present. Also, the mechanical properties of the HTS tapes themselves are not conducive to industrial manufacturing at the present time. When they are wound or processed, they bend and twist which cause macroscopic flaws that lessen their superconductive properties. A technique has not yet been perfected to produce tapes that are without flaws.
Finally, a problem arises in the covering of the tape, or the 'sheath'. Silver (Ag) is used for this process because it is compatible with the ceramic of the superconductor (it does not react with it), it is highly ductile and can be shaped around the superconducting material, it is permeable to O2, it conducts electricity, and although unproven, it is hypothesized that it aids in the alignment of the grains (Balachandran 148). Silver is expensive and not very strong. Therefore, great quantities of it are necessary to protect the ceramic core. Because of the amount of silver needed, there is not as much room for the superconductive ceramic powder and less power can be transferred. While it would be beneficial to find a different substance to use to coat the core, it is difficult to find another material that is cheaper and stronger and still fulfill all of the other requirements (i.e. conduct electricity, is highly ductile, and permeable to O2, etc.).
After the manufacturing of the HTS, applications are hampered due to a series of problems. In order to change the power system of a city to HTS cables, they would have to not only clean the oil out of the existing pipes in order to retrofit them, but they would have to re-route the power from other parts of the city in order to keep the daily routine going. Also, the joining of one HTS cable to another is a dilemma. They do not solder well because of the HTS properties involved and the insulation at the joint is not very tight and that can cause leaks in the insulation which leads to energy dissipation. To ensure that it does not occur, more insulation is needed. With all of the added difficulties that arise, the initial price of the installation of the HTS cables increases. Thus, they become unattractive because of the cost.
Once the cables are in place, situations still arise that are unique to this kind of operation. The temperature at which the cables operate range from 60-80 K which is expensive and difficult to maintain without heat transfer. Also, if breaks occur in the cable, there would be a loss of coolant and energy would be lost. The insulation deteriorates due to thermal/chemical aging and/or water seepage (Rahman 32). Most interesting though, when a magnetic field is present, it causes the flux lines in the HTS to move and hamper the current flow. The flux lines are really electromagnetic forces that can hamper the flow of the electricity. Therefore, if the flux lines are hampered, the electricity flow is affected (Balachandran 148).
Solutions
While the problems seem daunting, there are solutions that are being worked on. By the year 2010, it is projected that the superconductivity worldwide market will be $45 million (Balachandran 145). Therefore, it is safe to say that scientists see the problems not as roadblocks, but merely as obstacles that can be overcome. Some of the techniques that they have suggested include what Argonne National Laboratory is researching. They are using process which uses Argon to synthesize a phase of BSCCO which would allow precise control of the powder make-up which would then directly affect the current carrying capability. ORNL is using an aerosol spray manufacturing technique to produce superconductive powder with properties such as narrow distribution of particle size which causes the electricity to flow better, and overall homogeneity of the particles which would in turn increase current flow.
Also, the sheathing problem, which seemed daunting is being tackled. One technique is dispersion strengthening which leads to stronger sheathing and more of the space can be used for the actual HTS material which means that more current can pass through the wires. Also, the thinner sheaths mean that less silver needs to be used which in turn helps to keep the costs down. Another way that the sheathing problem is being approached is through the use of silver alloys. They have the same benefits as the above sheaths, but do directly lower the manufacturing costs. A great deal of silver is still needed but these sheaths will be stronger and thus efficient. Because of that, the overall costs will be lower.
Superconducting cables used in place of conventional wiring is an exciting prospect. While there are still many problems that scientists have to deal with such as the cost, the idea is still exciting. Superconducting cables would be more efficient, better for the environment and more powerful. The challenges that the scientists need to overcome are not as distant as they once were. One day, instead of copper wiring, HTS cables will be used and people will be wondering how they ever got along with out them.
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