Keiichi Okai

Fully superconducting rotating machines, which have superconducting field and armature windings, attract attention because they achieve high efficiency and high-power density. In the design of a fully superconducting rotating machine, it is very important to estimate the AC losses in the armature windings to which AC magnetic fields and AC currents are applied. Therefore, we measure the AC losses of an armature winding using an AC loss measurement equipment, calculating the AC loss from the difference between the mechanical inputs obtained with and without the superconducting coil present. The rotation of the permanent magnet rotor causes transient phenomena due to the change in magnetic field distribution. In addition, temperature distributions occur along the coil. In this study, we conduct experiments and perform a numerical analysis to evaluate the AC loss characteristics of an MgB₂ coil under a rotating magnetic field, taking into account the magnetic field distributions. We evaluate the validity of the AC loss measurement using this equipment by comparing the measurement and the numerical analysis results.

Japan's strategy to import overseas-produced liquefied hydrogen faces an economic hurdle due to liquefaction costs. Efficiently tapping into the cold heat produced by liquid hydrogen is pivotal, with high-temperature superconducting (HTS) devices offering a promising solution. Commercializing a liquid hydrogen-cooled HTS generator that operates alongside a hydrogen gas turbine holds significant promise. This setup eliminates the need for a refrigerator by utilizing cold heat, leading to notable efficiency improvements. A national project is underway to develop liquid hydrogen-cooled HTS generators, with the ultimate objective being to commercialize a 600 MW-class generator. A 10 kW-class demonstration system should be developed and verified in the project. A notable technical challenge in applying HTS coils to large-diameter, high-speed rotating field coils is their mechanical fragility. For the 600 MW system, the anticipated compressive stress due to centrifugal force is approximately 50 MPa. A technology is being developed to realize high-strength coils five times more robust than conventional coils. Mechanical simulations revealed a degradation mechanism, in which the innermost and outermost wire edges of conventional coils are delaminated. Coil casing reinforcement proved effective in reducing the stress in the superconducting layer while maintaining the high current density of the coil. Designing a liquid hydrogen-cooled HTS field coil necessitates a comprehensive understanding of the coil's transport behavior when subjected to liquid hydrogen immersion cooling. Energizing tests for the field coils of the demonstration system were conducted; the HTS coils continued to be energized without thermal runaway, even at a generated voltage of 0.8 mV.

The benefit of Boundary Layer Ingestion (BLI) was evaluated by wind tunnel test and analysis. Using a simple engine/airframe model which is composed of an airfoil based on NACA0012, an electric fan and an electric motor, wind tunnel test was conducted. Net streamwise force and electric power were mainly measured. Based on the power balance method originally described by Drela, the concept of conversion efficiency was added to separate demerit of BLI. By using conversion efficiency, mechanical power was changed to electrical power and making it easy to understand benefit of BLI. This analytical method was fit with experimental data and well represent BLI benefit. At cruise condition in 40m/s, BLI configuration needs lower 7.66% electrical power than nonBLI configuration.