Overview Of Superconducting Motors And Generators: Potential Applications, Benefits And Disadvantages
Introduction
Superconducting materials and its potential applications and benefits to the real world is not a new idea, with many having ideas of how it can be utilise ever since the inception of the BSC theory. With the development in material science and engineering, these ideas are finally becoming possible. As with any technology, there is bound to be some advantages that can be capitalised on, and also disadvantages it has intrinsically and when compared to current technology.
There are generally two types of superconductors currently, namely High Temperature Superconductors (HTS) and Low Temperature Superconductors (LTS). However, there is no defined limit on the temperature to classify a superconducting material to be either HTS or LTS, with some defining it above or below 30 Kelvin, the upper limit for temperature by BCS theory, while some defines it as above or below 77 Kelvin, the boiling temperature of liquid nitrogen. In this essay, HTS and LTS shall follow the latter classification for discussion.
Operating principles of superconducting motors and generators
Superconducting motor and generator works in similar ways to a normal motor and generator respectively, as the fundamental working principles are the same. Motors will convert the input electrical energy into mechanical energy, whereas a generator will convert input mechanical energy into electrical energy. From the rotation of either the stator or rotor, a changing magnetic field is created, and by Faraday’s Law of Electromagnetic Induction it will generate either a force or electrical current depending on the system.
Comparing a normal generator or motor to a superconducting one, there will be some differences in the set up and construction of the machinery, however, the operating principles and underlying science remains unchanged. Mainly the differences is the usage of superconducting material in replacing the usual copper wire winding and the addition of a coolant and cooling system, such that the critical temperature is reached for superconductivity in the wires. There are two types of superconductors out on the market currently, with the main difference being the critical temperature for the material to achieve superconductivity. Superconductivity is the phenomenon when a material’s electrical resistance becomes zero, and almost all magnetic flux is prevented from passing through the object. In order to achieve this phenomenon, the object has to be cooled below its critical temperature. Therefore, a cooling system is installed, usually using liquid nitrogen or liquid helium as the coolant to provide the extremely low temperature required. With that, the main requirements has been met, and hence other supporting structures needs to be added and modified to allow for the higher current flow, such as high current metal brushes, structural support capable of withstanding the temperature. Factoring all the essential parts, the superconducting generator and motor will then be able to produce the desired output, with either the input energy of a rotating drive shaft or a current flow producing a magnetic current.
- Quieter operation
- Compact and light-weight
- Highly efficient
- Lower cost incurred
Superconducting motors and generators are much quieter when put in comparison against normal motors and generators. Traditional motors and generator generate sound from sources like time harmonics, permeance harmonics and MMF harmonics, which in turn produces an electromagnetic force on the system, causing sound waves to be created and subsequently emitted. This noise is reduced in a superconducting motor or generator, as the noise is usually generated from the iron teeth of the armature coils, a component that is not required in a superconductor. With the removal of the iron core teeth, the reduction in vibration also helps to reduce the overall total noise output by the machine.
For the same power output, the superconducting motors are much smaller and more compact compared to their non-superconducting counterparts. The number of parts in a superconducting generator or motor is greatly reduced, mainly due to the smaller wire required to handle the same workload, removal of iron core teeth and lastly the compact liquid cooling system. Superconductors are able to handle the same amount of current in a smaller volume of wire as there is negligible electrical resistance, and the power loss from heating of wires due to current flow is greatly reduced as a result. With the usage of HTS superconducting material such as BSCCO and YBCO, it can result in a reduction of 50% in size and weight. As the current flow is much higher, the magnetic field strength is therefore much stronger which permits the removal of iron cores without heavily impacting efficiency and performance. Lastly, the cooling system will become centralised, with liquid nitrogen or liquid helium as the coolant. This removes the need for air ducts, fans and cooling water systems. As a result, it creates a motor or generator that will require lesser space and also be lighter.
The efficiency of superconducting motors and generators are also much higher. In general, the greatest loss of power is from resistance from the wires as it generates sound, heat and light as wasted energy. As such, the phenomenon of superconductivity means that the resistance of the wires becomes zero, and therefore there will be no power loss from the wires. With this technology implemented, it makes it possible to have an efficiency of over 90%, a huge improvement from the current efficiency rating of 80% to 85%. Additionally, the efficiency is maintained across a broad range of load, thus the advantage is capitalised constantly.
The total project cost of superconducting generator and motor, which will include the procurement, shipping, maintenance and other aspects, will likely be lower than the cost of a non-superconducting generator and motor. Due to the smaller size and lesser material required, the initial costs of manufacturing, shipping and installation will be cheaper. Over the service lifetime, the lowered RPM in a superconductor will result in lower stress on the equipment, reducing the overall downtime and the number of parts that needs to be replaced. With lower number of parts to maintain and run, the manpower requirements will also drop, which means lesser time and money is spent on a single resource, lowering cost of operation.
Reliability and maintenance issues
Reliability and maintenance ease is important to any warship, for it will have an impact on the down time and operational readiness of that particular ship. With superconducting materials in generators and motors, it is critical that the system is functional at all times for the machine to work. However, the cooling system has been noted to have long term reliability issue. This would mean repairs needs to be carried out if possible, or the complete replacement of the system. With extreme temperature and gases, maintenance would also become more difficult, due to the increased risks and safety concerns of technicians. As such, it might not be easy for preventive maintenance works to be carried out smoothly and quickly, further increasing down time.
Feasibility in operational environment and meeting operational requirements
Superconductors are a very advance technology available in this world, as such, not every country will have the required expertise to repair and replace damaged or worn out parts. While experts can be flown over within the matter of hours, replacement parts would not be as easily accomplished, due to the weight and size of them. Additionally, the superconductors require certain conditions be met, which would be affected by external factors such as ambient temperature and humidity. Out in the operational environment such as open seas, sea state will be a huge factor that needs to be taken into account. Therefore, supporting systems needs to be put in place, as these factors may affect the superconductors in their ability to carry out their job leaving the ship without power.
Out at sea, there is greater difficulty to conduct emergency repairs to ensure survivability and meeting the command priority. While it may be possible to conduct the mitigation factors at times, when this is not possible, it may cumulate in a change in command priority and mission failure.
More efficient usage of space
With superconducting motors and generators, it is possible to reduce the size of the machinery by a factor of 2 and more, which would have a large impact on the space allocation for a warship. Most warships have redundancy for critical equipment, such as engines and generators. With this level of space saving, it enables ship designers to create smaller and slimmer ships with the same level of propulsion. This opens up the choice of either having a shorter breadth or more compartment space in the ship for different purposes.
Impact on stealth and detection
In addition to reducing the space needed for superconducting motors and generators, there is also no need for an air cooling system. As a result, air ducts and openings can be removed from the ship entirely. This would permit lesser edges for radar to detect the ship, lowering the ship’s radar signature. With a smaller radar signature and smaller ship, it allows for more stealthy ships and covert operations to be carried out. This can also mean a longer underwater time for submarines, as there can be more space catered to batteries, and with lesser losses incurred. Any marine engine will require fuel and combustion, creating heat and thus making an area of the ship hotter. When a superconducting motor or generator is added, there will be a need for a cooling system, which will at least be 100K and below. The huge contrast in temperature will be very obvious in a thermal camera, making it easier to detect by that means. Therefore, the ship might have to take temperature difference into account, to reduce the heat signature it produces to improve stealth capability from thermal detectors.
Structural changes and adaptations needed
With liquid nitrogen or liquid helium on board the ship, they would have to be stored in a strong container that is capable of both withstanding the pressure and the vibrations of a missile hit. This is to prevent leakage, which if it occurs, the coolant will quickly evaporate into gas due to their boiling point, and pressurising the water tight compartment. Ultimately, it will cause problems like an explosion if the pressure is too great for the compartment to handle. Therefore, mitigating factors must be employed, such as stronger and thicker shielding may be employed to prevent a leakage, or having something like a pressure relief valve. However, considerations such as the importance of generators and motors must be taken into account when inventing a solution. Additionally, the solution cannot completely negate the benefits of using superconductors. For example, thicker walls might be a possible solution, but it would mean more weight, which will affect the overall advantages.
Most materials become brittle when it reaches extremely low temperatures, which will affect a warship as it is mainly made of steel. Steel is a material that will go through a ductile-to-brittle transition. With the cold temperatures from the cooling system of the superconductor, the steel may become brittle. Having brittle items on a warship will weaken its structural integrity and lead to possible structural failure or machineries to spoil.
Conclusion
Superconducting motors and generators hold great promise, and are likely to be used in many future warships due to its advantages. However, this is still a developing technology, with many issues that needs to be ironed out in order for it to meet the stringent needs of the military and the harsh operating conditions. With the smaller and lower number of parts needed to make a warship with superconducting generators and motors, it will reduce the lead time for such ships, making it very attractive to navies that are looking to upgrade their fleet. When the technology is ready, it will have a significant impact on ship designs, opening up more choices for navies around the world to choose and develop a ship with better capabilities.
