Next-generation aircraft are required to be zero-emission from the viewpoint of global environmental conservation. The aviation industry itself has declared net zero CO2 emissions in 2050 by 2021. Electric propulsion of aircraft represented by NASA's N3-X and Airbus' Zero-e has been proposed as the most promising candidate, and the R&D began in the 2010s in Europe and the United States. As a common concept of electric propulsion systems, the fuel is liquid hydrogen, the hydrogen gas turbine turns a generator, and the generated power is supplied to the motor in real time to turn the fan and obtain thrust. Here, if a conventional normal conducting machine, which is a lump of iron and copper, is applied to the generator and motor, it is too heavy and results in opposite effects. A general industrial induction motor weighs 2 tons at 300 kW and has a power density of 0.1-0.2 kW/kg. A single generator and motor for aircraft must achieve an output density of 15-20 kW/kg. Therefore, it is essential to develop a superconducting propulsion system consisting of a superconducting generator, a superconducting motor, a superconducting cable, a low-temperature inverter, and so on. Cooling of the superconducting propulsion system utilizes the cold heat of the liquid hydrogen fuel.
In Japan, as a NEDO project, in 2019, R&D of an electric propulsion system for aircraft using superconducting technology was started. We are conducting R&D of an electric propulsion system cooled with subcooled liquid nitrogen. As the first step of the NEDO project, we designed and prototyped a 400 kW fully superconducting synchronous machine using REBCO superconducting tapes, and succeeded in a rotation test as a synchronous motor up to 460 rpm. The rotational speed of 460 rpm was restricted by the output voltage limits of the used bipolar power supply. The stationary armature is cooled by subcooled liquid nitrogen at 65-70K, and the rotor, which is a rotating field winding, is cooled by helium gas at around 60K. This time, in parallel with the fully superconducting synchronous machine, we have also developed a cooling system that uses heat exchange between liquid hydrogen and liquid nitrogen. For convenience, liquid helium was used as an alternative to liquid hydrogen in the tests. In the future, we will conduct a rotation test as a motor with a rated speed of up to 2500 rpm, an output test as a synchronous generator, an actual load test using a 500 kW normal conducting synchronous machine and an inverter, and a rotation test under low pressure and low temperature simulating an aircraft environment.
This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).