In the development of future airborne megawatt-class power generation, it is important to minimize both the size and the weight of the system. The primary means of increasing the power density within the generator, as for all rotating machinery such as motors and alternators, is to maximize the magnetic flux density. This can be achieved by using a higher current-carrying capacity wire to increase the ampere-turns in the windings without adding more turns via a longer length of wire. This has already been accomplished through the incorporation of superconducting wire in magnetic resonance imaging (MRI) magnets used in the medical field.
Another beneficial effect of incorporating superconductors into power systems is to increase the overall operation efficiency, thereby lowering parasitic heat losses, which can become substantial for higher-power systems. A common misconception is that superconductor usage requires large amounts of cryogenic fluids in complicated coolant systems. Advancements in refrigeration systems eliminate this need, allowing for the use of more compact, higher-efficiency cryocoolers. The cooling needs of a cryogenic system depend on the design of the system and, in particular, the heat losses of the insulation components and electrical devices.
In addition to cryogenic systems, a new class of high-temperature superconducting (HTS) wire, made from an yttrium barium copper oxide (YBCO) coated conductor, is available. The wire typically takes the form of a thin, flat tape, as opposed to a round wire. The YBCO wire allows a much higher operating temperature than the previous generations of superconducting wire made from the bismuth strontium calcium copper oxide (BSCCO) family, thereby requiring a significantly smaller cryocooler to function. Depending on the magnetic field of the application, the operational temperature of YBCO is typically 20-40 K higher than for BSCCO wires.
Recent efforts by the Air Force have been advancing power technologies using superconductors for airborne high-power applications (HPA). Large onboard demands for electrical power are projected for future military aircraft, making it necessary to develop not only suitable power generators, but power distributors and conditioning technologies as well.
The Megawatt-level Electric Power System (MEPS) program was begun to develop and test superconducting power systems for airborne HPA. The objective for the MEPS generator was to demonstrate HTS machine designs yielding power ratios in excess of the Air Force’s initial goal of 4.0 kW/lb. Using this figure as a starting point, future systems could be driven to much higher power ratios, since the initial machine configuration was a homopolar inductor alternator (HIA). A prototype one-megawatt generator produced 1.3 MW output at its design speed of 10,000 rpm, and achieved 97% overall efficiency, even taking into account cryocooler losses.
Another superconductor candidate for HPA is the gyrotron magnet. A gyrotron is a high-field magnet necessary to generate high-power electromagnetic radiation. Similar to the MRI magnet, this can be accomplished with superconducting wire, but uses older, low-temperature superconductors (LTS). Developing an HTS magnet with the newer HTS wire to replace the LTS windings could substantially reduce the refrigeration load. The new YBCO conductor operates at 60-77 K (as opposed to 4.2 K for LTS wire), and requires a cryocooler that is more than an order of magnitude smaller (by output) than that used for LTS materials.
For airborne applications, operating voltages are typically fixed at 270V to minimize arc discharges at lower atmospheric pressures. However, this also causes problems with power supplies and power electronics. The output or operating power of a device is known from basic principles, specifically Ohm’s law (P = IV, where I is the applied current, and V is the operating voltage). Thus, it is not practical to increase the operating voltage to increase the power output substantially for airborne applications, as would be typical for ground-based transmission systems. Consequently, it would only be practical to increase the operating current. Since voltage will not be increased, the device design may benefit by reducing the amount of electrical insulation needed.
The energy densities afforded by superconductor power transmission devices over their copper counterparts are tremendous, which demonstrates how higher-current-density wires can be incorporated into power systems, thus greatly reducing the size and weight required for airborne applications. Also, heat losses can be substantially reduced. It should be noted that these improvements are realized for power transmission between devices operating at 50-77 K, such as superconducting generators and gyrotron magnets, as described above. If one of the devices was to operate at room temperature, a significant number of additional high-current power leads would be required to deliver the equivalent electrical power as at 77 K.
This work was done by LaMarcus Hampton, Paul N. Barnes, T. J. Haugan, George A. Levin, and Edward B. Durkin of the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. AFRL-0188
This Brief includes a Technical Support Package (TSP).
Compact Superconducting Power Systems for Airborne Applications
(reference AFRL-0188) is currently available for download from the TSP library.
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