As the world struggles to reduce carbon emissions, air traffic is increasing by 5 percent each year. Ultimately, fully electric civil aircraft are planned, but hybrid-electric propulsion is expected to enter service first. Hybrid-electric distributed propulsion could give massive reductions in fuel consumption and emissions, but would require greatly improved electric motors with much higher power-to-weight ratio and efficiency. The advanced superconducting motor experimental demonstrator (ASuMED), funded by the European Union, is developing a new fully superconducting motor with a power-to-weight ratio of 20kW/kg and an efficiency that’s better than 99 percent. Only superconducting motors have the potential to give such high efficiencies combined with this power-to-weight ratio. They will be a key enabler for the electrification of commercial aircraft as well as improving efficiency within industrial and power generation applications. ASuMED has now produced a 1MW superconducting motor that will undergo testing early next year.
The Superconducting Advantage
All electric motors dissipate some energy as heat and therefore have less than 100 percent efficiency. Losses are caused by mechanical friction, electrical resistance within the windings, eddy current effects and hysteresis. The most efficient synchronous motors can achieve efficiencies of 99 percent, but this requires large windings to keep electrical resistance low. Achieving a very high efficiency with a high power-to-weight ratio (light weight) means that resistance needs to be dramatically reduced.
In ordinary metallic conductors, resistance decreases slightly as the temperature is reduced, but even at close to absolute zero there is significant resistance. When a superconductor is cooled below its critical temperature, its resistance suddenly disappears. It is then able to transmit an electrical current without any loss; a loop of superconductor can store a charge by allowing the current to flow around it indefinitely. Typically, this critical temperature is close to absolute zero, which requires cooling using liquid helium, with a boiling point of -269°C (4 K), something that is generally not practical. However, so-called high-temperature superconductors have critical temperatures that can be more readily achieved with liquid nitrogen, which has a boiling point of -196°C (77 K).
Using superconducting materials for motor windings has the potential to allow highly efficient motors,since electrical resistance is eliminated. They are also able to achieve very high power-to-weight ratios, since large coils are not required to reduce resistance and the superconducting materials can carry extremely high current density. These properties can be achieved in relatively small individual motors, potentially enabling distributed propulsion, which has aerodynamic advantages. Allowing small propulsion units to be located freely around the aircraft will give designers far greater freedom to create new aircraft with enhanced efficiency.
The ASuMED Project
The ASuMED project is being funded by the European Union to help achieve its Flightpath 2050 objectives of reducing CO2 by 75 percent, NOx and particulates by 90 percent, and noise by 65 percent compared to 2000. The project, which began in May 2017, is being led by the German electric motor manufacturer Oswald Elektromotoren. Project partners include Karlsruhe Institute of Technology, Aschaffenburg University of Applied Sciences, the University of Cambridge, Netherlands-based cryogenics specialist Demaco, the Slovakian Institute of Electrical Engineering, Air Liquide of France, and Russian-based superconductor specialist SuperOx.
The project aims to demonstrate a 1 MW motor running at 6,000 rpm, with a power-to-weight ratio of 20 kW/kg and motor efficiency of better than 99.9 percent. Due to the power required to run the cryogenic cooling system, the combined efficiency should be better than 99 percent.
“The 1-megawatt motor is only to demonstrate that the superconducting technology works in principle, and we can scale it up if necessary to 10 megawatts or more. Underwing designs would require a few large motors, while for distributed propulsion there would need to be more motors—but smaller ones," explained Eva Berberich of Oswald Elektromotoren.
The demonstration motor will be synchronous, meaning that it must operate at a constant speed, synchronized to the AC power supply. Synchronous motors achieve the highest efficiencies. It will be fully superconducting, meaning that both the rotor and the stator will use superconducting materials. The rotor will use superconducting tapes arranged in stacks, while the rotor will use superconducting coils. This allows for high current densities that will generate extremely high flux density, leading to the required high power-to-weight ratio. Oswald’s expertise in motor design was utilized in designing these components.
Another critical aspect of the superconducting motor, not found in a conventional motor, is the cryostat and cooling systems. The stator cryostat is based on a capillary system that uses liquid hydrogen as the cooling fluid. The rotor rejects approximately 150 W of heat and is cooled using a forced gaseous helium system. These systems have been integrated to achieve the highest possible efficiency.
The rotor cryostat was particularly challenging because of the cryogenic operating temperatures, the cooling requirements, and the rotating parts, which include a rotary seal. After considering a number of possible heat transfer mechanisms, a forced convection-based system was selected. This uses the forced circulation of gaseous helium. The rotor cooling system is provided with helium at 25 K. The superconducting stacks in the rotor operate at a temperature of between 27 and 35 K. The delta-T, temperature differences between the superconducting stacks and the coolant, is therefore just 2 to 10 K.
Other important aspects of the demonstrator include the specialized power-control electronic hardware and software, which must deal with the unusual characteristics of superconductive windings while maintaining high dynamic speed and torque control. The controller includes an inverter with fail-safe features aimed at airworthiness requirements.
With a motor built, the project is now in its final stage with tests planned to take place at Oswald’s Miltenberg factory near Frankfurt in February or March 2020. These tests will be carried out on the motor only, without a propulsor, although under guidance from Rolls-Royce, Airbus and Siemens.
With such an incredibly high power-to-weight ratio, combined with extremely high efficiency, superconducting motors look set to be a vital ingredient in the electrification of aviation. This technology is also likely to improve efficiency in other types of electric vehicle and power generation.