Most of the considerable public attention, and more of the modest government funding on high temperature superconductor technologies for power systems, has centered on superconducting materials, wires, and device development. However, superconductors operate only at very low temperatures, typically below the boiling point of liquid nitrogen. I wanted to share a summary of an op-ed piece by Michael J. Gouge, Applied Superconductivity Group, Oak Ridge National Laboratory, that appeared recently in Superconductor Week discussing the need for an increased national focus on cryogenic technologies.
Michael J. Gouge writes:
Superconducting-based systems on the electric grid require three enabling technologies: the superconducting wire or tape, a cryogenic cooling system, and high-voltage cryogenic dielectrics. Most of the R&D effort is going into development of the superconductor, second generation (YBCO) tape in case of HTS. This is understandable as there are no grid-based applications without a cost effective and capable superconductor.
Little resources, however, are being applied to development of the other two areas: cryogenic cooling and high voltage dielectrics. In part, this is due to the perception that there are available systems and materials that can be made to work, at least in short term demonstrations. Lurking below the surface, however, are several technical issues that need to be addressed for utility acceptance of HTS grid devices.
In the cryogenics area the three major issues are reliability, efficiency and cost. In the most important area, reliability, the performance of cryogenic systems to date has ranged from 95 to 99% reliable. This needs to improve significantly to 99.5 to 99.9% if the HTS devices are to be seamlessly inserted into the US grid.
The needed improvements in efficiency are also substantial. Present closed-cycle cryogenic cooling systems have thermodynamic efficiencies from 10 to 15% of ideal Carnot efficiency. This needs to about double if the overall HTS system efficiency gains due to superconductivity are to be realized.
Finally, the cost per watt of cooling needs to be reduced by a factor of 2 to 4, depending on the application. This can best be done with the economy of scale that should come from a large production base.
If HTS products are to be routinely accepted on the grid in the next 5-10 years, the level of effort in cryogenics and cryogenic dielectrics R&D by the government and industrial sectors should be expanded to address and solve the issues discussed above.