A new power revolution is going on in orbit – that of electrical propulsion for satellites. An ion thruster is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions with electricity. As the ionised particles escape from the aircraft, they generate a force moving in the other direction. Power supplies for ion thrusters are usually electric solar panels, but at sufficiently large distances from the sun, nuclear power is used.
Recent years have witnessed a rising interest in electric propulsion (EP), not only for applications to satellites but also for future interplanetary travel Comparing EP performance to that of classical chemical thrusters. the latter have lsp in the range 300-500 sand the former in the range 400-8000 s. implying a potentially dramatic reduction in propellant consumption for a given mission. Propellant mass saved by using EP can go toward either heavier payloads, or to perform a given mission with smaller and less expensive launch vehicles
Michael Patterson, senior technologist for NASA’s In-Space Propulsion Technologies Program compared ion and chemical propulsion with “Tortoise and the Hare”. “The hare is a chemical propulsion system and a mission where you might fire the main engine for 30 minutes or an hour and then for most of the mission you coast.” “With electric propulsion, it’s like the tortoise, in that you go very slow in the initial spacecraft velocity but you continuously thrust over a very long duration — many thousands of hours — and then the spacecraft ends up picking up a very large delta to velocity.”
A seeond advantage of EP systems over chemical thrusters is their thrust modulation, making them ideal for attitude control, station
keeping, and orbit adjustment. On the other hand, EP systems need considerable electrical power to produce reasonable thrust at high lsp
and efficiency. This large power requirement implies a mass penalty [mostly due to solar panels and power conditioning units (PCU).
The limits imposed by the electrical power system are mirrored in the maximum available thrust, which is of the order of millinewtons
IO newtons (depending on applications). To overcome such limits. and to make EP more appealing for future interplanetary missions.
superconducting (SC) materials could be utilized.
The materials called Superconductors have unique properties including, Zero resistance to direct current; Extremely high current carrying density; Extremely low resistance at high frequencies; Extremely low signal dispersion; High sensitivity to magnetic field; Exclusion of externally applied magnetic field; Rapid single flux quantum transfer; and Close to speed of light signal transmission.
Superconductor Based Equipment provides many benefits. Superconductivity brings sensitivity, accuracy and performance advantages beyond the theoretical limits of conventional electronics technology. Additionally, in large scale superconducting systems, when all the necessary cryogenic components are included, size and weight reductions of 50-70% are achieved versus conventional equipment.
Superconductor Applications in Electric propulsion
Today, superconductivity has a variety of practical applications, including the following:
- Electrical power transmission cables
- Magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machines
- Particle accelerators and magnetic fusion devices
- Some electric motors and generators
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. About half of all electric power is consumed by electric motors. Because of this large electric power requirement, even a 0.1-0.2 % global enhancement of efficiency contributes to a tremendous aggregate energy saving and CO2 emission reduction.
One way to design high efficiency motors is to adopt superconducting technology providing zero electrical resistance. Zero resistance and high current density have a major impact on electric power transmission and also enable much smaller or more powerful magnets for motors, generators, energy storage, medical equipment and industrial separations.
Thus, a world-wide competition of R&D for superconducting motor/generator is under progress, and global cooperation will accelerate progress of this challenging and beneficial technology towards a commercial reality.
Superconductors could also pave the way for “almost lossless storage” of energy, meaning we wouldn’t have to worry about the degradation that plagues today’s batteries. We could move electricity from a renewable energy farm on one side of a continent to another without losing any energy in the process.
However, superconductivity’s applications in the aerospace industry have yet to be fully explored. Airbus UpNext is looking to change that with its latest ground-based demonstrator project ASCEND.
Using experiments and digital simulations, Siemens’ eAircraft said it has proved the feasibility of a high power-density superconducting generator that could be used in a hybrid-electric propulsion system for future short-haul commercial aircraft. The milestone comes as Siemens conducts laboratory tests of the 2-megawatt SP2000 conventional, non-superconducting electric motor that will power Airbus’ E-Fan X hybrid-electric regional aircraft demonstrator in 2021.
Revolutionary Marine Ship Propulsion
Within the past 20 years ship designers have begun to adopt electric propulsion systems which are hailed as the most important design change since the adoption of the diesel engine in the 1920s. Among large Commercial Ocean going vessels, nearly 100% of all new ships are electrically propelled. This includes many large cruise ships such as the Queen Mary 2. In 2002, the U.S. Navy announced it would migrate towards an all-electric fleet.
The large size and heavy weight of conventional copper-based electrical propulsion motors and generators have been a barrier to broad adoption of electric propulsion. For these reasons superconductor-based ship drive motors will revolutionize electrically propelled ships. HTS motors and generators are much smaller and lighter; operating prototypes are one-third the size and weight of their copper wound conventional counterparts and run quieter with less vibration.
In Navy vessels quieter and less vibration means a lower probability of early detection. In addition the elimination of rotor losses results in much higher efficiency, especially under partial load conditions, where many ships operate for the great majority of their operating hours. This translates into a longer cruising range and greater fuel economy. Smaller and lighter motor assemblies could also enable electric ships to use shallower ports and result in greater maneuverability.
For Navy vessels the smaller propulsion motors provides additional space and weight allowances for high power combat radars and additional missiles. A prototype 36.5 MW HTS ship drive motor that has been fully lab tested is sitting in the Philadelphia Naval Yard awaiting incorporation into a Navy vessel. This is an equivalent power replacement for a conventional ship drive at one-third the weight and size. U.S. Navy should implement a demonstration of the benefits of this major HTS ship drive propulsion system that is so close to realization.
The research will explore the use of magnets to create a light-weight and energy-efficient propulsion system to help satellites maintain or change orbit.
Collaboration between New Zealand’s Victoria University of Wellington, particularly the Faculty of Engineering’s Robinson Research Institute, and Australia’s UNSW Canberra will focus on the use of superconducting magnets in satellites.
The leader of the project and Director of the Robinson Research Institute explained that the magnets will be used as they aim to create a light-weight and energy-efficient propulsion system to help satellites maintain or change orbit. With this system, the satellites can use solar power to provide thrust, rather than carrying chemical propellants from Earth that they burn in space. Moreover, these magnets can also potentially be used to protect satellites from radiation, capture space junk, and store energy, thereby, proving to be a very versatile technology.
In particular, scientists from Robinson Research Institute will be investigating the thermal management of cryogenic superconducting magnets. They will be looking at the best way to use cryogenic technology in order to keep the superconducting magnets cold. The magnets have to be kept cold, and it has to be done efficiently, otherwise, the energy efficiency benefits of this technology are negated. This is a crucial feature of this technology. The magnet system needs to be carefully designed to make sure the energy needs of all parts of the satellite are met in an energy-efficient way.
Scientists from the Institute have a world-wide reputation for the development of superconducting devices for transportation, space, and energy applications. This work will build on their existing experience and make sophisticated design tools available to the commercial partners. It should greatly accelerate the adoption of advanced magnet technology in space applications such as propulsion, control systems, and radiation shielding.
Fully Superconducting Motor Prepares for Testing
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.
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 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 in 2020.
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 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.
ASCEND – a first step towards cryogenic electric propulsion in aircraft
One of the major challenges of scaling up electric propulsion to larger aircraft is the power-to-weight ratio. In other words, today’s electrical systems simply do not meet the necessary power requirements without adding excess weight to the aircraft. But high-temperature superconducting technologies are emerging as a promising solution to this technical conundrum, notably by increasing power density in the propulsion chain while significantly lowering the mass of the distribution system.
Airbus reports that it has created the ASCEND program to demonstrate that an electric- or hybrid-electric propulsion system complemented by cryogenic and superconducting technologies can be more than 2 to 3 times lighter than a conventional system—through a reduction in cable weight and a limit of 30kW/kg in power electronics—without compromising a 97% powertrain efficiency.
A ground demonstrator will be built by Airbus subsidiary UpNext (Ottobrunn, Germany) targeting a minimum of 50% reduction in powertrain weight and electrical losses while increasing efficiency by 5-6% compared to conventional technologies. Composite materials reportedly will be considered for cryogenic insulation.
The three-year demonstrator project aims to show
To achieve this objective, ASCEND features a 500kW powertrain consisting of the following components:
A superconducting distribution system, including cables and protection item
Cryogenically cooled motor control unit
A superconducting motor
A cryogenic system
“With the ASCEND demonstrator, we’ll adapt ground-based cryogenic and superconducting technologies to a fully electric powertrain to confirm their potential at aircraft level,” explains Ludovic Ybanez, Head of the ASCEND demonstrator. “Integrating these components will not only be a world first, but also an essential step towards future full-scale tests and flying demonstrators.”
Liquid hydrogen to cool conventional technologies
In addition to optimising the weight of the distribution system, another objective of ASCEND is to significantly increase the power density of the propulsion chain. This is a key consideration, as increasing the power of current electrical aircraft systems from a few hundred kW to the MW required for a fully electric aircraft is no easy feat. Simply put, more power increases weight and installation complexity, and generates more heat.
However, if a cold source at 20°K (-253.15°C), such as liquid hydrogen, is available on board, it can be used to cool the electrical systems. The superconducting components can then work to significantly improve the power density of the electric-propulsion systems.
Airbus is already looking into how liquid hydrogen could be used as fuel for an internal combustion engine or fuel cell as part of its ZEROe pre-programme. The ASCEND demonstrator will thus complement this research by providing additional insight into how cryogenic and superconducting technologies can support an ultra-efficient electric- and/or hybrid-electric propulsion system for future aircraft.
ASCEND will test and evaluate solutions that could be adapted to turboprop, turbofan and hybrid propeller engines by the end of 2023. It will support Airbus’ decision-making process for the type of propulsion system architecture required for future aircraft. ASCEND is also expected to support performance improvements on existing and future propulsion systems across the entire Airbus portfolio, including helicopters, eVTOLs, as well as regional and single-aisle aircraft.
“With the ASCEND demonstrator, we’ll pave the way for a real breakthrough in electric propulsion for future aircraft,” says Sandra Bour Schaeffer, Airbus UpNext CEO. “The importance of this work can’t be understated: cryogenic and superconducting technologies could be key enablers to enhancing the performance of low-emission technologies, which will be essential to achieving our ambitious decarbonisation targets.”
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