Since the start of the establishment of the first Navy, many means of propulsion has been utilised and has evolved throughout the years. From ancient warships that uses sail and oars to move their vessels to having steam engines during the 19th century. The 20th century seen the rise of Dreadnoughts which employ steam turbines as the main means of propulsion. In recent years, warships has evolved to using diesel-electric configuration propulsion or the nuclear propulsion to cater to increasing demand of electronically enabled weapons such as Guided Missiles. Looking at the evolution of propulsion in warships, one could deduce that the design improvements are primarily based on increasing warships’ efficiency and operational capabilities as well as their ships stealthiness.
In recent years, the US Navy has been pushing for all-electric drive propulsion. This meant that warships produces electricity that would be then distributed to propelling machinery. However, primary concern for all-electric drive propulsion is that power generators are not able to meet the demands of the propelling machinery and the combat systems of the warships.
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One such example of full-electric drive propulsion warship by US Navy would be the newly commissioned USS Zumwalt. USS Zumwalt is a 15,995 tonnes, 190m long and 24.6m wide vessel. Its power generation unit includes two Rolls-Royce MT30 Twin Spool, high-pressure ratio, marine gas turbine engines and two RR4500 Auxiliary Turbine gen-sets (ATGs) bringing the total ship power to 78 megawatts (MW). (http://www.fi-aeroweb.com/Defense/DDG-1000-Zumwalt-Destroyer.html). It is able to travel at cruising speeds of 30 knots. However, full-electric drive could be further improved using superconductor technology. This technology utilises superconducting materials in generators to produce electricity as opposed to using standard conducting materials like copper. Not only does it helps to improve the efficiency of the generator, it could be used in motors as well to ensure minimal power losses. Having superconducting generators and motors would mean that naval vessels are able to produce more power with lesser space needed and with improved stealthiness. With development of new types of electric-based weapons like Railguns and Solid State Lasers, superconducting generators could also introduce a new market of high powered generators to meet the demands of these weapons systems.
Hence, in this essay, I will discuss about the operating principles of superconducting generators and their pros and cons as well as their possible implications on navy ship design requirements.
When current flows through a normal conductor like copper wires, it passes through with resistance. This resistance causes power losses and lowers the efficiency of the circuit as the power is convert to heat and light. These energy that are convert to light and heat are otherwise useless and does not contribute to the function of the circuit in most cases. Electric current that flows through the superconductor will not have any resistance at low temperatures. This helps to increase efficiency and lowers the weight and size of the generator or motor of the same rating.
Working principle are, in most cases, similar to a conventional motor/generator however there are 2 main differences that makes it a superconductor motor/generator. Windings on both stator and rotor uses superconducting material which has zero resistance when it is cooled to its transition temperature (Tc). Superconducting materials can be classified into 2 types namely, Low-Temperature Superconductors (LTS) and High-Temperature Superconductor (HTS). The difference between the two is the critical transition temperature. Tc of HTS is more than 77K which is the boiling point of liquid nitrogen. (https://www.princeton.edu/~ota/disk2/1988/8807/880709.PDF)
Another difference would be the cooling system needed to ensure windings retain their superconducting properties. A cooling chamber is located within the hollow rotating shaft, along with a rotary seal to provide the room for cooling to occur. Extra measures are in place like vacuum jacket which provides vacuum insulation layer, a torque tube to provide insulating structure as well as a shield to prevent release of magnetic field to ensure that superconductors stay within the critical transition temperature (Tc) and maintain high efficiency. Other components may include the coil cover which supports the winding when under centrifugal force and a damper for protection from high frequency magnetic fields. (http://engineering.electrical-equipment.org/electrical-distribution/super-conducting-generators.html) ***Add in the standard operating principle of a conventional generator as well to explain the movement***
Smaller in weight and size as compared to conventional copper generator/motor
High-temperature superconductors (HTS) has higher power density capability as compared to their copper counterparts. (https://www.linkedin.com/pulse/military-application-superconductor-alfonso-walter-de-masi) Having close to zero resistance, superconductors will be able provide the same power rating with lesser weight and size. Superconductors eliminates the need for large amount of windings as it is able to produce magnetic fields needed to generate the same amount of power. This allows naval vessels to open up more space for war fighting capabilities or other modifications like storing of aircraft and UAVs or ammunition to be possible.
Lowered weight and mechanical noise and vibration due to elimination of gearbox
Having high power density, high torque allows naval vessels to have low-speed operation if fitted with superconductor motors. Low rotational speed is of value in that it permit direct drive connected to the propeller without the use of a gearbox. The purpose of a gearbox is mainly to lower rotational speeds of the prime mover to produce enough torque to drive the ship. By eliminating the gearbox, space and weight could be reduced and mechanical noise and vibration generated by the gearbox could be phased out. In addition, electric motors are much quieter than turbines or engines and exhaust for prime movers could be eliminated as well.
Having low noise and vibration signature is an important advantage for naval vessels due to their nature of work. Naval vessels have to minimise their signature to ensure that they would not be detected by the enemy vessels early. For naval vessels, the main source of noise and vibration comes from the propelling machines and auxiliary engines. Therefore, superconductor generator/motor would suit the needs of a naval vessel.
Introduction of Integrated Power System in naval vessels
It is no doubt that future naval vessels would require more electricity to power its increasing demand of newly developed weapon systems. As mentioned earlier, USS Zumwalt (DDG 1000) is a stealth destroyer with deployment of electric weapons with high power dense components like electromagnetic rail gun (underdevelopment), high power laser, high power radar. This increases the need for generator with higher power rating and smaller size as well as the need to reduce operating cost to support these weapon systems.
These could be possible by using Integrated Power System (IPS) with superconductors as medium for generator/motors and the transmission cables. In present naval vessels, shipboard propulsion systems are separated from the electrical systems. By having IPS, power generation would be combined with load busses to share the power between sources. IPS will be able to provide electrical energy for propulsion, auxiliary systems and combat systems. Superconducting is a viable option for this configuration as it provides high power output for power generation and low resistive energy losses for transmission cables to high powered weapon systems.
Increasing payload percentage of full load displacement for smaller vessels
In Figure 1.1, one can observe the trend of percentage of payload which are the mission system onboard the vessel to the ship’s total displacement. Smaller ships have much smaller percentages of payload to full load displacement ratio as compared to larger vessels. For vessels that are 450ft or 138m, 1 ton of mission payload requires 9 tons of ship. Superconductors are able to provide the power generation capability to tradeaway shipboard machinery weight which contributes to full load displacement with mission payload. This would have much larger impact on smaller vessel as the curve can be observed to be rising exponentially.
This is relevant for the Singapore Navy as our need for small vessels to conduct littoral missions are high and saving weight to improve multi-mission capabilities is of utmost importance. However, this requires the superconducting technology to be advance and cheap enough to be less than the cost of building the 9 tons additional ship.
Figure 1.1 Payload percentage of full load displacement vs Ship Length (ft)
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Low operational temperature
High-Temperature Superconductors requires a cooling system consisting of liquid nitrogen in order to retain its superconductor properties. Reason is that superconductors only function at zero resistance at temperature lower than its critical temperature. Critical transition temperature (Tc) of HTS must be less than 77K (-196.15oC). Cooling system is very expensive as it has to maintain extremely low critical temperature. This requires high maintenance as compared to a conventional copper generator.
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