FOR SHIP PROPULSION
LCDR Whitcomb received a BS in Nuclear Engineering from the University of Washington, Seattle, WA in 1984; an MS in Electrical Engineering and Computer Science from MIT in 1992; a Naval Engineer degree from MIT in 1992; and a Ph.D. in Mechanical Engineering from the University of Maryland in 1998. His current position is as an Associate Professor of Naval Architecture and Marine Engineering at MIT in Cambridge, MA. Until recently hc was a Program Officer in the Ship Structures and Systems Science and Technology Division (ONR 334) at the Office of Naval Research. He was also the Systems Engineering Manager for the Power Electronic Building Block (PEBB) project team at ONR. His naval service has included tours onboard USS SCAMP (SSN 588) and ship work coordinator at the Supervisor of Shipbuilding, Conversion, and Repair, Groton, CT.
The January 2000 announcement made by Secretary of the Navy Richard Danzig to select electric drive for use within the 0021 program sets the stage for the largest technological leap forward in naval ship propulsion since steam engines replaced sails. The commitment to use electric drive will pay huge dividends to the Navy in several ways, but the most outstanding and technologically promising reward will be when high temperature superconductor technology is first incorporated into a ship or submarine in the form of an electric motor.
Recent commercial development in superconducting motors provides an opportunity for meeting a critical need for powering future naval ships and submarines. As part of the spectrum of electrical technologies available to support future all-electric Navy flexibility, superconducting motors offer an opportunity to extend mission performance through improved efficiency at reduced power system size and weight, in addition to meeting acoustic performance requirements. This opportunity can now be realized by leveraging the recent advances in superconducting technology.
A recent statement related to the Submarine Force of the future listed high temperature superconductivity as “promising” to future submarine performance improvement1. Although reduced acoustic signatures are as vital for the Submarine Force as ever, new requirements dictate the need for a multi-objective approach to submarine design considering mission flexibility, which results in the need for designing in operational flexibility. This fact is borne out by the declaration by the Submarine Force that the four pillars of connectivity, payload, sensors, and platform have been defined to guide submarine development for the next generation. Electric technologies provide a high degree of design, construction, and operational flexibility to meet the challenges of the near future.2 The surface ship community is also pursuing electric technologies to achieve acoustic quieting and mission flexibility goals for the SC-21 family.
Superconducting machines have advantages related to conventional machines due to two main factors, lossless current transport and the ability to create large magnetic fields based on this high current1. These factors lead directly to improved efficiency and reduced size, as measured by both weight and volume. Further, they can lead to lower first as well as life cycle costs.
In the commercial and industrial sector, the efficiency translates itself into several tangible results such as energy savings, size reduction, and operational cost reductions. For instance, of the 1,683 x 109 kilowatt-hours (kWh) of electricity consumed annually in the United States, 64% (1,081×109 kWh) is transformed to shaft power by electric motors3. Even a small increase in motor efficiency would result in substantial energy savings each year. Since there are approximately 50 million motors in operation in the industrial and commercial sector alone, with more than 1 million of these greater than 5 Hp, there is tremendous potential for many new applications of superconducting motor usage5.
For the military sector, the weight and volume reductions are equally important, since most military machines must be transported or are used for transportation powering. The superconducting phenomenon and related engineering for application to practical devices is presented, including a critical look at some key technology developments that are now enabling superconductivity to be applied to ships in the near future.
Superconductivity
Superconductivity is a fascinating phenomenon that encompasses much more than just simple zero DC resistance at very low temperatures4. To completely describe the effect and its applications requires discussion of aspects from such disciplines as classical physics, quantum mechanics, thermodynamics, materials science, and electrical engineering among others. This broad overview will serve as an introduction to the salient points of basic superconductivity. The major aspects associated with superconductivity are temperature dependence and diamagnetism.
Temperature dependence is the most familiar aspect of the superconducting phenomenon. As the temperature of a superconducting material is lowered, it eventually reaches a critical temperature, Tc, where direct current (DC) resistance becomes zero. Different materials transition to the superconducting state at different Tc’s, from 0.01 K for tungsten to above 100 K for some of the higher temperature superconductors (HTS). The rate of transition is also important, since this determines some of the behavior of the material in real applications. When a material becomes superconducting, normal electrons pair together to become super electrons known as Cooper pairs. These Cooper pairs are bound together with an amount of energy defined as 2 triangle, which is on the order of 103 eV. Cooper pairs are responsible for the transport of electric current with zero DC resistance. The density of these pairs is inversely dependent upon temperature, the lower the temperature the more super electrons form from the normal electrons.
At any given temperature, there is a critical current density (Jc) at which a superconductor goes back to being a normal conductor since the electron pair absorbs enough energy to split into two normal conducting electrons. Therefore Jc is a function of temperature. The maximum critical current density is an extrinsic property of the material that can be affected somewhat by micro structural changes, such as voids and grain boundaries, in material processing, especially in the HTS materials.
Diamagnetism is the expulsion of the magnetic flux within the superconducting material. Flux expulsion, also known as the Meissner effect, manifests itself in superconducting materials by expelling all magnetic flux from the interior of the material. In other words, when in the presence of an applied external magnetic field, B = 0 within the material bulk. The flux-affected states of the superconductor are actually thermodynamic phases in the H-T (magnetic field – temperature) plane. Vortex formation occurs as the flux enters the bulk material volume as a regular triangular array of vortices. These vortices have a center of normally conducting material with a radius on the order of the coherence length. This normal region is formed due to the increase in kinetic energy of the Cooper pairs. As the distance from the center of the vortex decreases, the energy increases to the point where it exceeds the pair’s binding energy and they split into two normally conducting electrons. The material between the vortices remains superconducting. As the applied field is increased, more and more vortices enter the material and the vortex density increases. The vortices interact with each other and with applied currents and magnetic fields through Lorentz-like forces. These interactions cause the vortices to move, which dissipates energy. Since superconductors are used to eliminate energy losses, the vortices must be held in place so efficiency is not compromised. This is accomplished by pinning the vortices. The superconducting material is processed such that normal conducting regions of approximately the same size as the vortex diameter are placed at regular intervals in the lattice structure. The vortices will find a lower energy state when their cores align with the pinning sites. This reduced energy state holds the vonices in place until the applied field or the applied current exceeds the value at which the vortices begin to become unpinned. Vortex movement causes a sudden increase in the energy dissipated by the material which may cause the material to come out of the superconducting state.
This sudden transition from the relatively lossless superconducting state to a normal energy dissipating state is known as quenching. Most low Tc materials exhibit quench type behaviors, while most HTS materials are not as susceptible to sudden quench due to shallow transition temperature slopes and high specific heats. Quenching can be caused by any of the parameters on which superconductivity is dependent such as excessive magnetic field, overheating, or high current densities. The effects of quenching are mitigated by constructing superconducting wires with copper or silver matrices surrounding them. Thus if a quench occurs, the metal carries current for short periods of time and carries the heat away quickly to keep the superconductor cool. Since Jc is a function of temperature, as temperature increases, Jc decreases and the material comes closer to losing superconductivity. Thus quench has a positive feedback on itself. The metal matrix helps mitigate the onset of thermal runaway.
Materials
Development of materials for superconducting applications presents challenges in electrical and mechanical areas. Electrically, there are two basic types of superconducting materials, type I and type II. These materials each possess the two key properties of superconductors, zero DC resistance and diamagnetism. The applied flux interaction is the differentiating feature between the two. Ginzburg-Landau theory defines a critical value to determine if a material is type I or II. Type I materials have one phase boundary, Hc(T), below which the superconducting state is favored (and the Meissner effect is exhibited) and above which the normal conducting state is favored. In other words if the applied magnetic field at a given temperature exceeds Hc. the superconducting effect is destroyed and the material becomes a normal conductor. Type II superconductors exhibit two phase transitions, Hc1(T), and Hc2(T). At any given temperature below Hc1 the material acts to expel flux while above Hc2 the material becomes a normal conductor. In the region in between, with Hc1 < Happlied < Hc2 , these materials superconduct in the mixed or vortex state. In the vortex state, some of the applied field penetrates the interior of the material.
It is the field penetration, while still in a superconducting state, which allows type II superconductors to carry higher current density in large applied fields. An applied field penetrates a type I material only slightly, so the externally applied current flows in a thin surface layer. This means the current density exceeds Jc at low driving current. In type II materials, vortices penetrate deep into the material volume. Since it is the current density that is critical to high power superconducting applications, increasing material volume available to current transport increases the total current which can be transported. So it is the type II materials that are applicable for use in high current, large magnetic field applications such as motors, generators, energy storage, and transmission devices. For example, the critical current density on the overall conductor area Jc, required for motors ranges from 0.1 to 0.25 x 104 A/mm2 @ Hc of 2-3 T[7]. The current density limit for bulk material currently ranges from about 4 x 103 A/cm2 @ Hc of 6 T to 7.5 x 104 A/mm2 @0T1.
Chemical stability challenges include the need to account for susceptibility to absorbing water and losing oxygen, which eliminates the superconducting property. Mechanical properties needed to be addressed, since the new HTS materials are ceramics and therefore brittle with low tensile strength. The mechanical issues have been effectively addressed through strengthened matrix and steel lamination.
Recent Developments
Although low temperature superconductivity has been known and applied for many years, it was the discovery of HTS materials, that began the superconducting transition in the temperature range of 77 – 100 K, that has allowed for more widespread application. The ability to operate at higher temperatures removes a major obstacle in the practical use of superconductivity, the necessity of a liquid helium refrigeration system. The first HTS compounds discovered, such as YBa2Cu307-x (Y123), become superconducting above liquid nitrogen temperature. Newer compounds, such as Bi 2223 are being used in the manufacture of electrical conductors and a variety of magnets. This offers the potential to reduce operating costs due to reduced refrigeration requirements.
Recent innovations by American Superconductor Corporation (ASC) have shown promise as engineered solutions to both parts of the superconducting application challenge, the ability to use the HTS materials in real applications and the development of mechanically simple refrigeration mechanisms. ASC has developed superconducting wires that have overcome technological problems associated with physical and material HTS properties that until now have prevented their use in practical applications. The electrical properties include the critical current density, Jc, and critical magnetic field intensity, Hc. Average Jc performance of 14,000 A/cm2 at 77 K in self-field has been reported by ASC for long lengths of wire. Jc performance of 70,000 A/cm2 has been achieved for short length research and development samples , ASC has overcome the chemical and mechanical problems by developing robust materials, such as Bi 2223, that have been designed into long conductors having good chemical stability and mechanical properties for use in high voltage electric utility transmission cables and large scale alternating current (AC) machines. They have also developed and demonstrated low power cryocoolers in naval superconducting magnets for mine sweeping applications. This cryocooler could also work in superconducting motor applications.
Superconductive Motor Design
The advantages of using superconductivity in commercial motors are attractive, since the size and weight are reduced to as little as 1/3 of conventional motors while the losses are halved. In the past, the major disadvantages of superconductivity applied to AC and DC machines related to the thermal management. This included increased capital cost due to the necessary refrigeration and increased complexity of motor design and upkeep due to incorporation of the cryogenic cooling system. Other factors include the reliability in the low temperature thermal support systems.
AC machines are the most common in a wide range of industrial applications. The application of superconductivity still retains the basic configuration and operation of conventional AC motors. A superconducting magnet creates a magnetic field high enough that iron teeth are not needed to enhance the magnetic flux, either in the rotor or the stator. This means that the current densities in the active regions are not limited by iron saturation. The stator only requires the use of back iron acting primarily as a shield to keep magnetic flux inside the machine. A resulting HTS air core configuration, with higher flux density, is significantly smaller and lighter than conventional AC synchronous motors.
The lack of iron teeth in the rotor and stator provides additional benefits. First, it eliminates the need for winding slots, thus reducing the major source of cogging torques that lead to radiated noise. The reduction of iron also decreases the inherent armature (stator) reactance, resulting in improved machine dynamic performance. The lack of iron leaves more room in the stator and rotor structure for winding conductors, thus increasing the power density and efficiency. Dynamic characteristics will be different in the superconducting motor. The lack of iron will lower synchronous and subtransient reactances, resulting in lower torque angles during operation, thus improving transient stability. This stability improvement should be realized even with the reduced air core rotor inertia.
HTS motors can be designed for direct line start. The AC synchronous motors being designed today by ASC will be capable of self-start by switching on full vollage at zero speed. The asynchronous field projected by the stator starting currents on the rotor will be intercepted by an electromagnetic shield surrounding the HTS windings. The superconducting windings are kept open circuited during this period, and are energized with DC when the motor reaches near synchronous speed. Analyses performed by ASC during the development of commercial and marine propulsion motors have shown that AC losses in the HTS windings are very small, and will not damage the HTS winding. The 200 HP HTS synchronous developmental motor was started in this mode without load. Calculations are currently being made to support future commercial and marine propulsion AC synchronous motors.
The successful design of HTS motors benefits naval propulsion system engineering. The development of practical HTS materials removed the most significant economic roadblock, which is the cost of cooling earlier materials to 4K. Large scale motor demonstrations are planned including a 1000 HP motor. The initial commercial investment impetus is in the regime of large ( > 5,000 Hp) industrial and commercial motors. Thus the first of the new HTS motors would likely incorporate technology on a scale such that it demonstrates technologies applicable to larger naval ship propulsion motors.
For ship propulsion applications, the superconducting motor improvements in weight and volume are as important as the increase in efficiency. Most commercial motors are used in high speed, low torque applications, where the superconducting motor weight and volume savings are not as important as efficiency improvements. For naval ship propulsion, however, the motors must be low speed and high torque, resulting in high weight when conventional type motors are installed. The lower weight and volume superconducting motors will be able to operate in the low speed ranges, less than 200 RPM, to directly drive the propeller, while greatly reducing weight and volume over conventional motors. The size reduction is important both for arrangement considerations in tight after areas of the ship, in exterior propulsion implementations, and in the possible reduction of overall ship displacement. The low weight also helps reduce moments caused by large distances from the longitudinal center of gravity, leaving greater design flexibility for the naval architect to achieve ship balance. Recent ASC preliminary design of a 25 MW, 120 RPM motor show 5 to 10 fold decrease in size and weight as compared with conventional motors.
American Superconductor Air Core AC Synchronous Motor
The new American Superconductor HTS conductor materials have made the transition from science to engineering and are now available to begin construction of superconducting motors today. The United States Navy Office of Naval Research (ONR) and the Naval Research Laboratory (NRL) have recently initiated programs to design and build superconducting ship propulsion motors that take advantage of these HTS engineering advancements. American Superconductor is currently under contract to design and build a superconducting motor directed specifically towards eventual use in ship propulsion.
The superconducting air core AC synchronous design takes full advantage of commercial developments in motor design, as well as the factors of low volume, weight, and high efficiency, making the motor attractive for shipboard applications.10 11 12 The initial implementation has a copper stator winding and superconducting rotor coils. Modular construction of the winding assemblies simplifies the design, production, testing, and maintenance of the motor10 11 12. As new conductors are created, they can easily be developed, manufactured, mass-produced, and tested in parallel with existing coils, so that when higher performance is ready for insertion, the motor can be marketed taking advantage of nonrecurring engineering costs of the design of the balance of components. The assemblies can be bench tested co full voltage and current before installation, improving reliability.
Heat transfer is probably the biggest challenge in the motor design. The most important development pointing to a successful implementation in this area is the use of extremely low power, low weight, high reliability, mechanically simple cryocoolers instead of high power, high weight, complicated cryogenic cooling systems used in earlier systems. A cryocooler is a small refrigeration unit that uses gas flow through a regenerator bed to achieve conductive cooling. ASC has used cryocoolers in many HTS magnets including a mine hunting application. A cryocooler suitable for a 5000 HP motor requires only about 6 kW to keep the rotor at the requisite 30 K during operation. Cryocooler maintenance only consists of yearly change out of the compressor discharge stream filter. The HTS motor has no special shock considerations.
The motor system is predicted to be robust enough for at-sea operations, even in a loss of cooling condition. A loss of cooling is predicted to allow operation at continually reducing power, allowing time for corrective actions. Most importantly, the motor is small and light with a high efficiency for a propulsion motor. Overall the motor diameter of a 25 MW, 120 RPM motor will be approximately 90 inches by 65 inches with a specific weight of 1.5 lbs/HP and an efficiency of 97. 7 percent.
Configurations have been completed through conceptual design for a baseline rating of 25 MW at 120 RPM. American Superconductor, with it partner Rockwell/ Reliance, built and tested a 200 HP motor in 199710 11 12. This industrial team is scheduled to test a 1000 HP, 1800 RPM motor designed for commercial application in early 2000, and a similar 5000 HP motor in early 2001. ASC is also working to deliver a 5000 HP motor to the Navy’s Office of Naval Research in 2001, and is designing a 35,000 HP motor for future ship application.
Conclusions
Design of electric drive for ship propulsion applications has languished for years since a small burst of activity in the early 1970’s. The renewed involvement of the United States government agencies, such as ONR and NRL, in motor research and development (R&D), along with the major commercial involvement from American Superconductor, is encouraging. The market for commercial superconducting motors is very large, and can only help the possibility of dual use application within the military. The combination of high tech R&D and commercial product development of modularly designed motor magnets should produce a viable technological result that will make sense in both military and commercial aspects.
In addition, NA VSEA and CDNSWC are in the development of integrated power systems, integrating electric power production, distribution, and control that will lead to new all-electric ship designs in both surface and submarine platforms. This bodes well for the superconducting motor for future shipboard application, so the Navy can take full advantage of the reduced size and improved efficiency along with high torque, low speed operation required for ship propulsion.
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