The interconnection of the SSP satellite solar panels will require a large amount of cable. Additional cable with higher current carrying capacity will be needed to connect the output of many interconnected panels through DC/DC converters to the input of the magnetrons. Using copper cable, or even aluminum cable, would be prohibitively expensive in terms of launch costs. By taking advantage of the fact that the background temperature of space is ~4K, the use of superconducting wires will greatly increase the specific current carrying capacity. As an illustration of the advantages of using superconductors, note that the two types of cable in Figure 1 can carry the same current.
Figure 1. Copper vs. Superconductor Comparison
Most superconducting materials have transition temperatures of just a few degrees above absolute zero, but in the 1980s a new class of ‘high-temperature’ superconductors was discovered with transition temperatures of up to 100K. The firm American Superconductor has claimed that their second-generation wire – made from yttrium, barium copper and oxygen (YBCO) – is cheaper to manufacture and retains its superconducting abilities better under magnetic fields, which should speed up the acceptance of high-temperature superconductor technology in the market place. In addition to an approximately 100-fold increase in current carrying capacity when compared to copper wire of the same dimensions, a superconductor can conduct electricity with no power loss due to its zero resistance. Figure 2 shows how the resistance of a superconductor rapidly falls to zero once it reaches its critical temperature.
Figure 2. Superconducting Breakpoint
There are some drawbacks to the use of superconducting materials, such as the requirement of cryogenic cooling, the possibility of the wire quenching (a sudden loss of superconductivity), and the high cost of wire materials and construction. Given the low ambient temperature in space, only minimal cooling would be required for the superconductors to maintain their zero-resistance state. In an attempt to mitigate the quenching dangers, the superconductor is usually in filament form in a copper or aluminum matrix, which can carry some of the current should the superconductor quench for any reason. Superconducting wire costs around five times as much as a standard cable, but the price will fall once larger quantities are produced. Furthermore, American Superconductor is currently manufacturing ribbon-shaped YBCO wires that are 100m long and 4.8mm/12mm wide, which is the type of construction necessary for use on a SSP satellite. This copper laminated superconductor wire (Figure 3) has extremely high specific current carrying capacity, near perfect efficiency, and high strength and stability, making it an excellent choice for use in space.
Figure 3. Properties of Superconducting Wire
The smaller wire will be used to interconnect strings of solar panels in series, which will require an estimated 9000km per 5GW array. The larger wire will be used to connect the strings of solar panels in parallel and transmit this power to high efficiency DC/DC converters which will require an estimated 1000km per 5GW array. The larger wire will also be used to interconnect the magnetron array.
References
http://www.macdiarmid.ac.nz/news/success/tallon2.php
http://physicsworld.com/cws/article/news/25436
http://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/components/cable.htm
http://www.amsc.com/library/CPRAMP_DS_1111_r4.pdf
