Superconductor Applications

Superconducting Transmission Lines

Since 10% to 15% of generated electricity is dissipated in resistive losses in transmission lines, the prospect of zero loss superconducting transmission lines is appealing. In prototype superconducting transmission lines at Brookhaven National Laboratory, 1000 MW of power can be transported within an enclosure of diameter 40 cm. This amounts to transporting the entire output of a large power plant on one enclosed transmission line. This could be a fairly low voltage DC transmission compared to large transformer banks and multiple high voltage AC transmission lines on towers in the conventional systems. The superconductor used in these prototype applications is usually niobium-titanium, and liquid helium cooling is required.

Current experiments with power applications of high-temperature superconductors focus on uses of BSCCO in tape forms and YBCO in thin film forms. Current densities above 10,000 amperes per square centimeter are considered necessary for practical power applications, and this threshold has been exceeded in several configurations.

Power Applications, High Tc

Power applications of high temperature superconductors would have the major advantage of being able to operate at liquid nitrogen temperature. The biggest barrier to their application has been the difficulty of fabricating the materials into wires and coils. Current development focuses on BSCCO and YBCO materials.

Fault-Current Limiters

High fault-currents caused by lightning strikes are a troublesome and expensive nuisance in electric power grids. One of the near-term applications for high temperature superconductors may be the construction of fault-current limiters which operate at 77K. The need is to reduce the fault current to a fraction of its peak value in less than a cycle (1/60 sec).

A recently tested fault-current limiter can operate at 2.4 kV and carry a current of 2200 amperes. It was constructed from BSCCO material.

Superconducting Motors

Superconducting motors and generators could be made with a weight of about one tenth that of conventional devices for the same output. This is the appeal of making such devices for specialized applications. Motors and generators are already very efficient, so there is not the power savings associated with superconducting magnets. It may be possible to build very large capacity generators for power plants where structural strength considerations place limits on conventional generators.

In 1995 the Naval Research Laboratory demonstrated a 167 hp motor with high-Tc superconducting coils made from Bi-2223. It was tested at 4.2K and at liquid neon temperature, 28K with 112 hp produced at the higher temperature.

Superconducting Maglev Trains?

While it is not practical to lay down superconducting rails, it is possible to construct a superconducting system onboard a train to repel conventional rails below it. The train would have to be moving to create the repulsion, but once moving would be supported with very little friction. There would be resistive loss of energy in the currents in the rails. Ohanian reports an engineering assessment that such superconducting trains would be much safer than conventional rail systems at 200 km/h.

A Japanese magnetically levitated train set a speed record of 321 mi/h in 1979 using superconducting magnets on board the train. The magnets induce currents in the rails below them, causing a repulsion which suspends the train above the track.