Description of technology
The greatest impact of fuel savings will come from technologies that augment the efficiency of the ship’s propulsion system. Superconductor-based machines have been used in advanced naval prototypes since the 1970s but it was the discovery of high temperature superconductors (HTS) in 1986, operable at liquid nitrogen temperatures that made them serious candidates for high performance marine applications. One issue to consider is the generation of heat in HTS wire due to internal losses: whereas losses under dc power operation are negligible, they become larger as the frequency increases and are significant enough at power frequencies (50-60 Hz) to compromise the benefits of the superconductor, especially because their effect is amplified by the refrigeration penalty. This has generally limited the use of HTS to the parts of electric machines carrying dc current.
The electric machine type that has become the most popular recipient of HTS technology is the one known as synchronous machine. In its most common version, the rotating member (rotor) wound with coils carrying dc current (field winding) establishes in the air gap a dc magnetic field. As this field rotates in the bore of the stationary member (stator), it interacts with the windings located therein (armature windings) to generate an output ac voltage at their terminals (generator operation) or, if the armature windings themselves are independently powered with ac current, to generate a torque on the shaft (motor operation). Thus, the dc field winding of a synchronous machine is a natural application for HTS wire.
One obvious benefit of using HTS wire in the field winding is the elimination of the electrical conduction losses, as the HTS operates in a virtually lossless regime, with consequent improvement of the machine’s efficiency. An even greater advantage, however, results from the ability of the superconducting field winding to carry much larger currents than normal conductors and, thus, to generate much larger fields than in conventional machines. This allows the operation at higher power density with consequent reduction of machine size and weight for the same power rating.
Of course, the use of HTS wire requires that the field winding be maintained at cryogenic temperatures. This entails the use of a host of auxiliary devices not normally found in a conventional machine: a cryogenic refrigerator, a suitable rotating coupling to transfer the cooling fluid to the spinning rotor, extensive thermal insulation in the rotor including a vacuum jacket, and a suitable ac shield to protect the field winding from time varying magnetic flux components (Figure 1). All these items detract from the theoretical gains in both size and efficiency, but the auxiliary overhead scales less than proportionally to the rated power: the end result, therefore, is more favorable the larger the power rating of the machine. It must be added that, although more complex, a HTS rotor is well within the present state of the art thanks to progress in the technology and reliability of cryogenic apparatus. The widespread use of superconducting magnets in magnetic resonance imaging (MRI) systems, however, has reduced the cost and increased the reliability of cryogenic cooling systems.
In a study for Northrop Grumman conducted by J. Herbst and K. Davey of the Center for Electromechanics of the University of Texas, the conclusion was reached that the crossover point where a HTS-based machine becomes more advantageous than a conventional one lies in the range between 5 MW and 10 MW of power per machine(Figure 2). It is not surprising, therefore, that among the leading installations of HTS machines in recent years, are a 5 MW and a 36.5 MW synchronous motors by American Superconductor Co. (Figures 3 and 4).