Researchers from New Zealand’s Robinson Research Institute at the Victoria University of Wellington have developed an HTS dynamo as a wireless current supply for superconducting field coils that are intended for use on a hybrid commercial aircraft. The device would reduce the cost of keeping the superconductor at cryogenic temperatures.
Over the past six years, the Robinson team has focused on developing and optimising HTS dynamo devices as current sources for superconducting magnets. This work was initially supported by two Ministry of Business Innovation & Employment (MBIE) ‘Smart Ideas’ grants.
The work is currently being carried out under the auspices of a five-year $6 million MBIE ‘Endeavour’ grant for a project entitled “Ultra-high Speed Superconducting Machines for Hybrid-electric Aircraft.” The research group has also been awarded a Royal Society ‘Marsden’ grant, led by Chris Bumby, which will investigate the intriguing underlying physics behind these devices.
System Would Decouple Power and Thrust
According to Robinson research scientist James Storey, the MBIE program has a number of components:
- A new concept HTS machine designs including: AC Homopolar, Induction and Wound-rotor architectures;
- Novel subsystem components including flux pump exciters and HTS bearings, quench detection, and high-saturation-field soft ferromagnets;
- Computational tools to model and predict superconducting AC loss in ultra-high speed HTS machines.
“Hybrid turbo-electric aircraft, which employ both conventional jet turbines and electric motors, are the focus of significant international effort by the large international aerospace corporations,” Storey said. “In such a system, a jet turbine drives an electric generator, which powers one or more electric motors.
“This allows so-called distributed propulsion to be realised, where a number of smaller electrically powered fans can be placed very close to, or semi-recessed within, the fuselage. In doing so, the fans can ingest the slower-moving boundary-layer air, resulting in efficiency advantages - slower air into the fan produces higher thrust. In addition, since power and thrust are decoupled, it is possible to run the jet engine at its most efficient operating point for longer, instead of throttling up and down.”
Reducing Heat Load Halves Cooling System Size
Scientists recognize that power densities of more than 15 kW/kg are required to make hybrid electric propulsion systems feasible for commercial aircraft. HTS motors and generators are considered as the only technology that can enable the step change in power density to be realized.
“Robinson Institute has focused on developing a key novel component which enables the production of truly lightweight superconducting generators and motors,” Storey noted. “The HTS dynamo is a fully superconducting compact, lightweight, excitation current source for the DC HTS field windings required in these machines.
“This very low power device breaks the thermal link between the cryostat and current leads for the field windings. The resulting reduction in total heat load enables the overall cryogenic cooling system to be halved in size, thus greatly reducing the overall system weight and footprint.”
Storey explained how Robinson’s dynamo reduces the cooling costs of the superconductor: “Electromagnets made from HTS are typically connected to room-temperature power supplies via copper current leads. These current leads provide a direct path for heat to leak into the superconductor, and are typically the largest source of heat leak in the system.
“This heat must then be removed by the cryogenic cooling system. The superconducting dynamo provides a means of wirelessly energizing a dc current in the electromagnet, thus eliminating these copper current leads. This significantly reduces the overall heat load, in turn allowing a smaller cooling system to be used.”
Current Builds up to 700 amps
The system operates by attaching small rare earth Nd-Fe-B permanent magnets to a steel rotor, which passes the magnets over superconducting tape. Each time the magnet passes over the tape, it generates a current, which can build up to about 700 amps. For comparison, cryocooled HTS coils fed by current leads typically operate at 100-200 amps.
“The field achieved by a superconducting winding is determined by the current injected into the coil and the total number of turns,” Bumby commented. “In principle, there is no limit to the maximum field, which can be achieved using our dynamo, as the device pumps current rather than flux. However, there are limits on both the voltage and current which the device can output, such that the maximum achieved field is actually limited by the HTS magnet coil, that is, the critical current and inductance of that coil.”
Storey noted that an HTS dynamo is currently being tested as a brushless exciter within a prototype generator in South Korea: “That project is a collaboration with Changwon National University. One of our dynamos has been installed within a prototype 10 kW wind turbine generator.
“We are awaiting successful cool-down of the Changwon system in order for tests to be carried out. Changwon uses a liquid-neon based cooling system.”