Advanced Superconducting Motor Experimental Demonstrator Addresses Environmental Concerns

by Ana Perez, project engineer, Demaco Holland BV,

The continuous growth of air traffic entails an increase of fossil fuel consumption and emissions, each representing one of the major environmental issues of the present day.

For this reason, the European ACARE Flightpath project has established emission targets to be achieved by 2050, including a reduction in CO2 by 75 percent, NOx and particulates by 90 percent and noise by 65 percent as compared to 2000.

Conventional aircraft configurations will not be sufficient to meet these requirements but many agencies and researchers are already developing advanced systems. Included here are distributed propulsion systems used in combination with superconductive machines, a mixture resulting in Hybrid-Electric Distributed Propulsion technology (HEDP), itself seen as the breakthrough technology to achieve these ambitious goals.

The consortium of the Advanced Superconducting Motor Experimental Demonstrator (ASuMED) is sponsored by the European Union under the Horizon 2020 program and will develop, build and test the first fully superconductive (SC) motor for aerospace applications. The motor will be able to achieve the power densities and efficiencies required by HEDP for future large civil aircraft. The prototype is expected to be in the range of 1 MW power with thermal losses lower than 0.1 percent at cryogenic temperatures.

The ASuMED motor consists of SC stator coils and SC stacks on the rotor. The motor features a dual-cryostat concept in which two separate cryostats for rotor and stator are combined. This solution provides the opportunity to use the most suitable technology for both stator and rotor and thus meet the most demanding requirements. Demaco Holland BV (CSA CSM), with its cryogenic expertise, is responsible for developing the rotor cryostat and supporting the consortium with cryogenic knowledge.

The rotor cryostat design is to a large extent determined by a cooling concept that is particularly challenging because of the cryogenic temperatures, the cooling requirements and the rotary parts—which include a rotary seal.

The required cooling power is defined by the magnetic losses (heat generation in the SC rotor stacks during operation) and the heat leak into the system. The cooling fluid used in the rotor cryostat is helium, which will enter the rotor cryostat at 25 K and 2 bar(a). The rotor pole is the housing for the cryostat, meaning that the cooling circuit is built inside the rotor.

A number of options, based on different heat transfer mechanisms, were considered for the rotor cooling system topology. A conduction-based system was the first design concept to be investigated. Its cooling concept required the use of two fluids: the helium cooling fluid, which is circulated through the system, and a second fluid that remains stationary.

In this way, the stationary fluid was the heat sink for the Sc rotor stacks and was in turn cooled by the helium cooling fluid. A preliminary heat transfer analysis showed that the achieved cooling power was not sufficient and a conduction-based system is therefore not a feasible solution for the rotor cooling system.

A forced convection-based system was then proposed as a way of improving the heat transfer in the system. A preliminary heat transfer analysis on this configuration showed that the system has the potential to achieve the required cooling power as the forced convection in the system is realized by the forced circulation of the cooling gas.

This is the so-called Externally Controlled Rotor Cooling System for the ASuMED motor, in which gaseous helium is circulated to remove the heat generated in the SC rotor stacks. The concept is supported by a detailed flow and heat transfer analysis, using the finite element method.

The ASuMED motor demonstrator is currently in the detailed design stage. Apart from the rotor cooling system, Demaco is responsible for the design and manufacture of the conditioning equipment for the demonstrator. This system will provide the motor with helium at the required temperature. The initial tests of the demonstrator’s cooling concept will take place at Demaco this year, while the final tests of the complete demonstrator are expected by early 2020.