Performance metrics of electrical conductors for aerospace cryogenic motors, generators, and transmission cables

摘要

In this work we evaluate the performance of various electrical conductors, both normal-state and super-conducting, at cryogenic temperatures with an eye towards motor, generator, and power transmission applications for use in all-electric and more-electric aircraft. In addition to MgB2 and YBCO superconductors, we consider high purity Cu and Al as well as carbon-nanotube (CNT) yarn. We analyze the normal-state conductors for current carrying capacity at room temperature as well as LN2 (77 K) and liquid hydrogen temperatures (20 K), where appropriate. To parameterize these materials for aerospace applications we have explored various options for defining current capacity metrics for these conductors at cryogenic temperatures. Paralleling definitions for ambient environment conductors, we first define electrical current capacity in terms of a limiting temperature rise for (i) windings directly immersed in a pool-boiling cryogen environment, and (ii) the case of thermal conduction though an epoxy winding. While neither metric is fully satisfactory, their implications are important. These results are then compared to a current capacity criterion developed in terms of a specified loss generation limit, which turns out to be a more meaningful approach. The results for these normal-state conductors operating at cryogenic temperatures are then compared to superconducting MgB2 and YBCO. After this, we consider two cases for overall system-level benefit in terms of power density, one for the case where the size of the cryogenic cooling apparatus is included, and a second case where the cooling is "free" (e.g., when liquid hydrogen is already onboard). We find total system weight reductions for the cryocooled case only for the superconducting options. If time varying magnetic fields are present, superconductor filament size must be kept small; an expression for limiting filament size is developed.