Weights and Efficiencies of Electric Components of a Turboelectric Aircraft Propulsion System

The benefits that turboelectric propulsion may offer transport aircraft due to the flexibility of electrical distribution of power have been discussed by various authors and are briefly summarized. Estimates of the weights and efficiencies of the electric components, based on approximate sizing models for fully superconducting motors and generators and on aggressive estimates for cryocoolers and inverters, were presented at ASM2009 in a baseline turboelectric system study. An SBIR study has since predicted that the apparently aggressive cryocooler weight and performance estimates (5 lb/input-hp at 30% of Carnot efficiency) can be met. Another SBIR study has predicted that a cryogenically cooled inverter can exceed the 10 hp/lb specific power (including cooler) and the 98.8% efficiency (including cooler) that were assumed in the baseline study. The inverter with its cooler constituted half of the baseline electrical system weight. On the other hand, new estimates for the superconducting motors and generators, based upon inclusion of additional components in the sizing models and more complete ac loss models, predict somewhat heavier and slightly less efficient motors and generators. The lighter, more efficient inverter roughly offsets the adverse motor and generator changes for machines wound with a hypothetical future high temperature superconductor with low ac losses. For such a material, the weight of the entire electric system changes very little, and the already high efficiency improves slightly, from the ASM2009 estimates. For machines wound with the intermediate temperature superconductor MgB2, the system weight increases about 25%, because MgB2 must operate much colder. The modeling and performance estimates of each component are discussed and compared to the baseline estimates and to the current state-ofthe-art. The dependence of motor and generator weights and efficiencies upon some important design parameters are presented in an appendix. These results quantify the benefits of technology development of lighter cryocoolers and of superconductors with lower ac loss. A zero-th order estimate of the benefits of lower fan pressure ratio and of boundary layer ingestion, offset by the electric system weight and inefficiency penalties from the added components, gives a net fuel burn reduction of about 9%, before iterating and resizing.