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8 Electric Power and Propulsion Introduction The panel focused on those electric power generation, storage, and propulsion technologies that, when applied as a system, will support the electrification of ships, submarines, and land-based vehicles. It surveyed the status of key elements of power systems such as energy storage devices, electric power recovery subsystems, and battery technologies. Industrial development is ongoing in most of these areas, and current trends suggest steady improvement in capacity, density, modularity, and reliability. The naval forces can take advantage of commercial developments by actively monitoring the commercial marine power and propulsion sector and by applying a top-down systems engineering process to fleet capability upgrades. A recent naval planning document1 outlined a development path for modernizing the power and propulsion capabilities of future surface ships and submarines. The plan does the following: Takes a systems approach, allowing appropriate trades for optimization of performance within the constraint of affordability; Takes a design approach in requiring modularity, commonality, and use of commercial technology for economy of scale wherever appropriate; 1 Advanced Surface Machinery Programs Division, Ship Research Development and Standards Group, Engineering Directorate (SEA 03R2). 1996. ''A Strategy Paper on Power for U.S. Navy Surface Ships," Naval Sea Systems Command, Arlington, Va., April 8.
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Has initiated, and is following, a development path for those basic power generation, distribution and control, and propulsion technologies that are critical for shipboard implementation; Has initiated a path whereby this capability can be used to backfit existing hulls in a hybrid configuration and be fully electric in new construction platforms; and Provides a path for developing those critical technologies and beginning the integration of both hybrid and fully electric systems into the fleet during the next decade. The concept appears to be sound, the enabling technologies are achievable, and the opportunity is now available to initiate a major change in the generation and use of electric power aboard ship. The benefits to the naval forces could be significant and far reaching. Advances in materials used in electromagnetic machinery, such as high-field permanent magnets and high-temperature superconductors, present opportunities for substantial increases in power density and efficiency. The electric ship approach is enabled by the ongoing revolution in electronics where advancements in solid-state semiconductor technology are being applied to power semiconductor devices. The emergence of direct electric conversion technologies, such as fuel cells, offers potential for fuel-efficient, low-emission, and low-noise sources of electrical power. Within the plan, the Department of the Navy's integrated propulsion system (IPS) architecture will allow incorporation of developing technologies such as permanent magnet (PM) electric machines, fuel cells, and the power electronic building block (PEBB) architecture into future ship designs as programmed, preplanned replacements for the core technology in the first-generation IPS modules. Use of technology-independent module interface standards will facilitate technology insertion with minimum impact on the ship design and construction process. IPS with electric propulsion motors allows for consideration of integrated motor and propulsor concepts. The steerable podded propulsor provides the greatest potential for meeting naval hydrodynamic and hydroacoustic requirements while also being competitive with conventional propulsors on commercial ships. The benefits of ship electrification are as follows: Combat systems effectiveness. Integrated electric power and propulsion systems enable design flexibility that, in turn, will facilitate the optimization of topside arrangements for maximum combat system effectiveness. Survivability. The elimination of gear trains and propeller shafts, together with the flexibility and modularity of electric systems, will enable graceful degradation and rapid reconfiguration of vital systems, thus enhancing survivability. Signature reduction and quieting. By eliminating the mechanical link between the power plant and the propulsor (i.e., propeller), electric drive will
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enable reduction of noise and vibration by allowing acoustic isolation of the engine generators. Improved operational flexibility and reliability. With the power station concept, all power will be supplied by a set of prime movers that provide power to propulsion, ship service, and other designated loads. This approach will provide the flexibility to shift power between propulsion, ship service, and other electrical loads, which cannot be done with a mechanical drive ship, and can enable improved speed control and steaming efficiency and the elimination of less efficient controllable pitch propellers in favor of fixed pitch types. Increased flexibility and adaptability. Integrating electric power and propulsion systems will provide flexibility in servicing other loads such as environmental controls (air conditioning) and launch and recovery systems, electric armor, high-power advanced electronics, and electric weapons. More space available. Eliminating gears and shafts from the propulsion system will make more space available for other uses. Also, the prime mover will no longer be tied to the propeller shaft line, and the power sources can be distributed throughout the ship as necessary. Reduced manning. Digital control and automation, which will be an integral part of ship electrification, will reduce the requirements for human operators (in machinery spaces, for example). Reduced logistics. Common power and propulsion modules can be used across the fleet. Reduced costs. Commercial technology appears likely to be available for many of the system elements. Life-cycle cost and fuel-consumption savings. Overall fuel efficiency can be improved if a ship is able to operate at varying speeds over a substantial part of its operational profile, and variable speed propulsion will be enabled by electric drive. In general, the same line of reasoning that applies to surface ship power and propulsion systems also applies to submarines. Optimization of submarine systems will be driven by stealth, safety, power density, and other requirements that differentiate surface and undersea vehicles. Submarine power and propulsion system technologies of the future will include the following: Very low harmonic motor controllers; High-power-density and high-performance solid-state inverters and converters and the components thereof; Electrical substitutes for systems and components that now rely on fluid transport for energy and actuation, e.g., electric actuators, electromagnetic launchers, and thermoelectric coolers; Motors and generators with very low acoustic and magnetic signatures, including versions that can operate submerged in seawater at high pressure; and
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New technologies for motors and generators such as superconducting magnets, cryogenic coolers, current collectors, high-field permanent magnets, liquid cooling, and active noise control. A HTS high-torque drive motor can be one-fourth the size of a permanent magnet motor, and its efficiency can be high enough to compete with existing reduction gear/steam turbine systems. HTS motors and generators can also be quieter because the high-density magnetic fields produced allow elimination of iron cores, simplification of armature designs, and elimination of other components that create noise. The Marine Corps relies heavily on power and propulsion technology for its vehicle power systems. The requirements for higher water speed amphibious vehicles, signature reduction, adaptability to electric power, future electric weapons, and reduction of vehicle weight all lead to the need for an integrated electric power system for future land and amphibious vehicles. Key technologies for land and amphibious vehicles mirror those described for surface ship systems but are driven by a different set of requirements. These include the following: Pulsed-power systems including energy storage pulse-forming networks and very high power solid-state switching; Flywheel energy storage; High-power-density engines, including electric drive systems with a unit size of several hundred horsepower; and Electrical steering, suspension, and actuators. In conclusion, the panel believes that electric power science and technology holds great promise for major technical progress and payoff in the future. The remainder of this chapter is organized to first discuss electric power generation and storage technologies and then focus on the electric ship concept and associated technologies. Electric Power Generation and Storage Technology Introduction Warfare in the early 21st century will involve the use of advanced technologies and combat systems with ranges, lethality, and detection capabilities surpassing anything known in contemporary warfare. In the battle zone surrounding and including the battle group and expeditionary forces, there will be requirements for electrical power at levels from tens of watts for surveillance and communication, to kilowatts for radar and for a variety of electric motors, to hundreds of kilowatts for field-base power demand and multi-megawatts for advanced
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weaponry, countermeasures, and propulsion systems. Mobility of all system elements will be essential for survival and success. Therefore, although fuel supply may dominate the total system operational weight for nonnuclear power, mobile weapons and sensors should be powered by supplies that have high specific power, are compact and quiet, and have minimal signatures in the full electromagnetic and acoustic spectra. Nonpropulsive power generation requirements and implementations will be integrated into a single system with propulsive power generation. This, and the continued deployment of special electric loads, will determine the requirements for power distribution, storage, and conditioning. The concept of an all-electric, or more electric, ship is gaining momentum in both U.S. and foreign navies. Simply stated, this concept embodies the use of electrical means for all power needs in lieu of other means such as mechanical, pneumatic, and hydraulic. This then provides an opportunity to standardize components and systems, reduce signatures, and better integrate the power and propulsion systems for all vehicles. The move toward increased reliance on electricity is also being applied to air vehicles and to Marine Corps amphibious and land vehicles. Versatility in electric power availability will enable new varieties of weapons and ship systems. Some proposed new weapons present especially rigorous power supply requirements. Electrothermal, chemical, and electromagnetic guns and high-powered laser or microwave directed-energy weapons, for example, require large amounts of power over very short time periods, as do electric rail aircraft launchers (electric catapults), which are technically similar to electromagnetic guns. Sophisticated power generation, distribution, and control systems will enable local-zone power distribution and conditioning, which will enable lower platform signatures and localized platform damage control. An order-of-magnitude evolution in size and power requirements for specialized sensors and unmanned systems presents new application requirements for power storage systems. A general schematic for the type of power system addressed by the panel is presented in Figure 8.1. A prime electric power source feeds directly, or via a load-leveling or storage stage, into a conditioning stage to alter the electrical parameters for optimum reliable power distribution to a load. The panel believes that the following three technologies will require special attention and support if the full potential of integrated electric power and propulsion systems is to be realized: Power generation, including advanced fuel-efficient turbogenerator power units for continuous power and for pulsed or short-duration power generation applications and direct electrochemical and electrothermal generators such as fuel cells; Power conditioning and distribution, including passive components, such as capacitors, inductors, and transformers, and active devices, such as power
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FIGURE 8.1 Schematic of a generic power and propulsion system. electronic switches and inverters for continuous as well as pulsed or short-duration, high-power conditioning and distribution; and Energy storage, including primary and secondary electrical energy storage such as batteries, flywheels, and generator combinations, and magnetic energy storage devices. Table 8.1 lists the technology areas covered by the panel. Technology forecasts are provided in the sections that follow. TABLE 8.1 Electric Power Technology Areas Electric Power Generation Energy Storage and Recovery as Electric Power Nuclear-electric generators Primary batteries Turboshaft engine generators Rechargeable batteries Piston-engine generators Superconducting magnets Fuel cells Flywheels H2 Pumped liquids Other Compressed gases Explosive/magneto-hydrodynamic Thermal storage Power conditioning Bus power Slow power Fast power SOURCE: Adapted from the Board on Army Science and Technology, 1993, "Electric Power Technology for Battle Zones," STAR 21: Strategic Technologies for the Army of the Twenty-First Century, National Academy Press, Washington, D.C., Table 43-1, pp. 539–540.
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Power Generation The concept of the electric ship is a powerful enabler for new concepts in ship design and operation. Independent placement of propulsion and power generation elements and the integrated approach to electric power distribution will rely for their efficiency on new developments in prime power generation, conditioning, and distribution. Electric propulsion assumes continuous generation capacity that will also provide electric power for ship auxiliary functions from the most mundane application to the more exotic ones, such as catapults and electric weapons. It will often be necessary to provide auxiliary power generation for backup, or for specific subsystems, or for zonal requirements. Aircraft and launch and recovery systems, electric armor, high-power advanced electronics, and electric weapons all require pulsed-power systems whose average power for the duration of output ranges to hundreds of megawatts. The mass and volume of generators in this class are half power unit and half power conditioning. The physical decoupling of power and propulsion subsystems afforded by electric drive will be a powerful enabler for compact, versatile, and powerful future ships. Continuous Power Generators For nonnuclear operation even the most efficient internal combustion engines consume their own weight in fuel in about 10 hours. Therefore, fuel storage determines overall operational system weight, and fuel supply will dominate logistics considerations. For mobile electrical generators and power conditioners, substantial weight reduction can be achieved by increasing the generating and distribution frequency from the present 60-Hz standard up to the 400-Hz range.2 Of the continuous power generation systems listed in Table 8.1, some are already in use by the naval forces. Nuclear-electric generators have potential for significant advances in specific power output. Although the pressurized water nuclear power plants currently in use do not hold much promise for greatly improved power density, a new generation of high-power-density nuclear power plants could be developed but would require substantial R&D investment. A development program could build on past R&D programs that have focused on compact, high-output reactor designs. Turboshaft engine-generators, which represent another important class of continuous power generation systems, are currently used in military helicopters and some specialized boats. They are also being incorporated into mobile electric 2 Board on Army Science and Technology. 1993. STAR 21; Strategic Technologies for the Army of the Twenty-First Century, National Academy Press, Washington, D.C.
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power units, primarily by the Army. They burn jet fuel and have rotational speeds in excess of 20,000 rpm. The high rotational speed could be used to advantage in a high-frequency, lightweight, direct-coupled electric generator operating at 24,000 rpm (400 Hz) or higher. Typically, however, they are geared down to 60-Hz alternators, and these gears increase weight and noise. The panel evaluated the various continuous and pulsed/short-duration power generation, power-conditioning, and energy-storage technologies by creating figures of merit in the near term (year 2000) and far term (year 2020). The selection of certain technology areas for inclusion in this report was based in part on the potential to achieve high values for figures of merit in the future. The figures of merit include functionality, mobility, supportability, manpower requirements, and cost. A significant improvement in any of these figures of merit could justify adoption of the technology by the Department of the Navy. The panel believes that, of the various technologies evaluated, turboshaft-engine-driven alternator systems have the best potential for practical, continuous improvement in all figure of merit areas. To realize this potential, the Department of the Navy should support the development of advanced turboshaft engines such as in the DOD/NASA Integrated High-Performance Turbine Engine Technology (IHPTET) program and carry out an aggressive development effort in advanced lightweight alternators, power-conditioning control systems, and unit integration and modularization. The electric power systems driven by turboshaft engines have the best potential for evolving into compact, lightweight units with reasonable noise control. Systems with these characteristics will be required in the future, particularly for the intermediate power ranges from 50 kW up to the megawatt level. The most significant factors controlling the specific power of an integrated unit are engine rotating speeds, generator frequency, and generated voltages. To a first approximation, the physical size of an engine is proportional to torque, and increasing the speed increases the shaft power proportionately with only a modest change in weight. The weight advantage gained by high-speed operation can be lost, however, by coupling through a gear box to heavy low-frequency generators and power conditioners. The present standard of 60-Hz generators requires that either engines with speeds greater than 3,600 rpm must be geared down to 3,600 rpm or the high-frequency alternating current (ac) that is directly generated must be rectified to direct current (dc) and then converted back to 60-Hz ac with an inverter.3 Both approaches introduce weight and cost into the system. Design optimization studies performed for the international space station suggest a different overall approach: Let the alternator rotate at the turbine speed and produce power at that frequency. Turbine speeds of 24,000 rpm or higher are 3 Board on Army Science and Technology. 1993. STAR 21: Strategic Technologies for the Army of the Twenty-First Century, National Academy Press, Washington, D.C.
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achievable and efficient. When directly coupled, 24,000 rpm yields ac power at 400 Hz, which is the current aviation standard. Using these higher frequencies for power generation can reduce the weight of the active components in the system by a factor of about six, compared with a 60-Hz system. Solid-state converters could then be used to shift the frequency to 60 Hz for distribution. Because of the ease of converting dc to any desired frequency, it is feasible to consider dc distribution in many cases. The voltages used for power generation and distribution also have leverage with respect to high specific power. The required conductor cross section, and hence the weight, decreases as the square of the voltage, so operating voltage is an important parameter.4 The practical voltage limit (currently about 1 kV) for mobile systems is set primarily by the reliability limits of semiconductor devices. Improvements in high-voltage semiconductor devices could permit more system-optimized voltages to be used up to the Navy standard bus transmission level of 5 kV. As the technologies evolve over the next 20 years, the specific power of high-rpm engines should approach 10 kW/kg. Similar progress is anticipated for high-rpm alternators with stationary armatures and separately excited rotating-magnetic-flux generating units. The weights of these generators are almost inversely proportional to their rotating speed (rpm). Thus, they could be designed for 400-Hz operation and directly coupled to the gearbox output shaft at 24,000 rpm. Although some special generator units have been operated at specific powers as high as 20 kilowatts of electric power output per kilogram (kWe/kg), generators built to military standards will more generally achieve 10 kWe/kg. At high-frequency operation, power-conditioning components are small, lightweight, and highly efficient, particularly when series resonant conversion is used in converters. Advanced technology should yield systems with power densities in the range of 15 to 30 kW/kg. Power densities as high as 100 kW/kg are conceivable for power conditioners when integrated-circuit fabrication techniques are used to manufacture integrated converters. Figure 8.2 shows how the operating time affects the total specific power (power/weight ratio) for existing and estimated future mobile electric power units. The diagonal limiting line in the figure represents the maximum specific energy available from combustion of the JP-8 fuel that will be used in the Navy's diesel engines or turbine engines. An overall efficiency of 39 percent is assumed from maximum fuel heating value to conditioned electric power. The ordinate gives the specific power based on the total system weight: kilowatt-hours divided by the weight of the total integrated system, including engine, generator, power conditioner, integration factor, and fuel consumed. For many engagements 4 Power (P) = V2/R, where V is the voltage and R is the resistance. In a cylindrical cable of cross-section area A. R is inversely proportional to A. Therefore P is proportional to V2A. For a given power, the required area A is inversely proportional to V2.
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FIGURE 8.2 Specific power versus operating time for advanced turbogenerators. SOURCE: Adapted from the Board on Army Science and Technology, 1993, "Electric Power Technology for Battle Zones," STAR 21: Strategic Technologies for the Army of the Twenty-First Century, National Academy Press, Washington, D.C., Figure 43-1 p. 547.
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in the future, actual operating times may be relatively short (hours to hundreds of hours). Therefore the specific power of the hardware units will be critical to mobility and initial operational capability. Fuel cells hold promise for zonal power generation. They are a particularly efficient and clean method of utilizing hydrocarbon fuels and are appropriate for medium-sized power generation units (1 to 4 kW). Hydrogen-based fuel cells electrochemically oxidize hydrogen with oxygen (or air) and directly convert chemical energy to electric energy. The product of this combustion is water, and this combined with the extremely low signature of the cell make it a very attractive power alternative for Navy and Marine Corps operations. Because they are essentially isothermal devices and therefore not limited by heat engine thermodynamics, they can in principle have very high energy densities (watt-hours per kilogram), limited only by the specific free energy change in converting hydrogen and oxygen to water. The great obstacle to hydrogen-based fuel cells for mobile, battle-zone applications is the volumetric problem of storing hydrogen. Although hydrogen produces a great deal of energy on a weight basis, it produces very little energy on a volume basis, even when stored at high pressure. Today hydrogen is the only fuel that can be electrochemically oxidized in a fuel-cell system at practical rates. The ideal system would directly convert the free energy of combustion of other fuels, such as hydrocarbons, into electric energy. Other fuels are used in prototype fuel-cell systems by either externally or internally reforming them to a mixture of hydrogen and carbon dioxide, then using the hydrogen electrochemically in the fuel cell. A major breakthrough in electrocatalysis would be required before other fuels could be electrochemically oxidized at useful rates. Hydrogen fuel cells are impractical for at-sea applications because of the space required to store hydrogen, but hydrocarbon fuel conversion designs would be directly applicable to Navy use. Most units under development (primarily in the automotive industry) require higher-quality fuels than standard Navy distillates. Although there is no current evidence of an impending technical breakthrough in this area, the promise is such that near-term programs to develop converters for standard Navy fuels should be encouraged. Pulsed-power Generators Pulsed-power generation systems can be divided into two major subsystems as illustrated in Figure 8.3: (1) energy storage or prime-power generation, followed by (2) power conditioning. The panel considered five classes of pulsed-power generation as listed below: Open-cycle, combustion-driven gas turbine generators. These systems burn fuel in excess air to produce hot combustion gases, which are then expanded through a turbine to drive a generator, producing the prime power. They have
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range suitable for ship propulsion. This propulsion motor module will conform to the IPS interface specifications and standards and will serve as a demonstration of technology insertion in an open-architecture system. Other machinery technology under development both by the military and commercial sectors includes homopolar motors and superconducting machines. Although the two concepts are usually linked together, homopolar motors can be excited by either conventional or superconducting magnets, and superconducting magnets can be used in both homopolar and ac motors. As with the PM motors, homopolar and superconducting motors would ideally be developed to conform to the IPS interface specifications and standards. The IPS architecture allows for extension of the specifications and standards if warranted either by commercial demand or by critical Navy-unique requirements (e.g., acoustics). IPS with electric propulsion motors allows consideration of integrated motor/propulsor concepts. The advanced modular propulsor, for example, incorporates steering into a propulsion module that replaces the existing appendage arrangement of exposed shafting, struts, and rudder. The goal of the modular propulsor program is to define a module or family of modules based on a common set of parts that can provide the propulsion requirements for Navy and commercial ships. Affordability, which is the key to commercial viability, must include equipment acquisition and operating costs, propulsor hydrodynamic efficiency, and ship effects such as producibility enhancement and revenue-earning capacity of any space freed up by moving the motor and shaftline outside the hull. The hydrodynamic efficiency issue is complicated because Navy surface combatants must be designed for 30-plus knots maximum speed, whereas they spend most of their time operating at cruise speeds of one-half to two-thirds of maximum. For Navy applications, there is the additional consideration of hydroacoustic performance—for this effort the requirement was to be at least as good as today's surface combatants (e.g., DDG-51). The steerable podded propulsor provides the greatest potential for meeting Navy hydrodynamic and hydroacoustic requirements while also being competitive with conventional propulsors on commercial ships. Past Department of the Navy R&D efforts on podded propulsors have adopted both improved efficiency and noise reduction as design objectives. Model tests on a 1/20th-scale DD-963 resulted in 10 to 20 percent reduction in fuel consumption. Later efforts focused on significant reductions in both propulsor and propulsion machinery noise and developed full-scale pod conceptual designs based on liquid-cooled synchronous motors driving counterrotating and postswirl propellers via epicyclic gears. Scale-model tests were conducted with geometrically similar pods on a scale model of the PG-100 (an intermediate-scale demonstration using the PG-100 was planned, but not completed). Results confirmed hydrodynamic performance projections. Also, as part of the same overall program, scale counterrotating propellers were tested in open water in the Navy's large cavitation channel, with results confirming the projections for significant increase in cavitation inception speed over fleet
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propellers. Most recently, ASMP has been tracking the progress of Kvaerner Masa/ABB on their Azipod joint commercial venture. The Azipod is a steerable pod housing an electric motor driving an external propeller and is being marketed on the basis of reduced ship construction costs and improved ship maneuvering and has been demonstrated up to the 15-W power level for an icebreaker application. Counterrotating versions of the Azipod design are planned. This is a prime example of an opportunity for the Navy to leverage a commercial development. The objective of the current modular propulsor effort was to identify technology options for providing a family of cost-effective modular propulsors for military and commercial use. The approach was to conduct a survey of future commercial and military ships and then develop a strategy for producing a common family of standardized propulsor modules. Based on the performance and affordability criteria provided by the Navy, the steerable pod propulsor emerged as the preferred concept. It was noted, however, that if reduction in propulsor noise under all operating conditions were required, then ducted propulsors (both in-hull and external configurations) should be considered. It is anticipated that motor developments under the IPS program will support pod-mounted configurations; therefore, the modular propulsor effort will concentrate on the rest of the equipment needed to produce the steerable pod module. This would include a full-scale, at-sea demonstration of propulsor performance. Power Control Standard Monitoring Control System The standard monitoring and control system (SMCS) will integrate the sensing, transmission, interpretation, and display of hull, mechanical, and electric (HM&E) parameters necessary for machinery control, condition monitoring and assessment, signature management, and damage control. The system design is consistent with the total-ship integrated command and control (IC2) architecture and supports and enhances the proposed integrated survivability management system (ISMS) and the integrated condition assessment system (ICAS). SMCS offers the potential to reduce acquisition costs and to introduce a standard system for application across multiple platforms, taking advantage of open systems architectures and commercial industry standards. It will also provide the necessary architecture to support critical imperatives for embedded readiness assessment, mission planning and training, and condition-based maintenance. The IPS system monitoring and control subsystem consists of the software necessary to implement power management, fault response, and system human-computer interface. This subsystem is composed of system-level control software (PCON-1) and of zonal-level control software (PCON-2). The system monitoring and control subsystem is assumed to reside on a computational and
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networking infrastructure that is external to IPS. Currently, this external infrastructure is SMCS compliant. Designing the IPS control software to be independent of the host hardware as much as possible should enable the exploitation of future advances in computers and networks. Intelligent Ship Control Support of reduced manning on future ships requires safety critical automation systems that can be trusted to operate reliably under all conditions, including battle damage. They must possess sufficient system integrity and fault tolerance to enable significant reduction in the ship's crew. The SMCS provides an affordable, modular, distributed, open-architecture, machinery-monitoring and control system based on commercial and dual-use standards and technology, but requires enhanced automation, integration, system integrity, and fault tolerance to satisfy future affordability and manning goals. ONR has an ongoing program to develop and demonstrate the required integrated ship control (ISC) automation technology base. This automation will support both low manned and advanced machinery ship concepts. ISC will fully support highly integrated ship operation and vital combat system support within emerging U.S. Navy dual-use and open-architecture standards for communications and computer resources. Future Impact on the Navy The Advanced Surface Machinery Program employs a systems engineering approach that maintains flexibility and minimizes investment until technologies are demonstrated, evaluated, and brought together for optimum total-ship cost-effectiveness. Affordability assessments have been made of individual program elements, but it is in the context of the full set of ASMP elements, integrated first as a total machinery system and further as part of the total-ship concept, that the most significant gains are achieved. Figure 8.7 indicates the system improvements and expected fuel savings. These fuel savings are based on equal shaft power requirements and do not take into account any change in hull drag that may result from resizing the ship. ASMP has conducted affordability assessments that take into account ship impact costs as well as component cost and ship producibility effects as part of overall ship acquisition cost. Operating and support costs included manning, maintenance, and training in addition to fuel. ASMP systems engineering has followed a design policy of common modules and standard components and interfaces and is developing modules to perform each of the critical functions of naval ship machinery systems. A family of modules is being defined that meets the range of projected fleet requirements at minimum fleet life-cycle cost. It employs a common module interface standard
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FIGURE 8.7 Integrated power system (right) advantage: Operators can determine how best to use the power, and the system saves 15 to 19 percent fuel in gas turbine surface combatants. SOURCE: Adapted from Krolick, C.F., 1996, ''Advanced Surface Machinery Programs (ASMP)," presented to the Panel on Technology by the Naval Sea Systems Command, Arlington, Va., May 2. to facilitate rapid and affordable introduction of new technology, as driven by industrial market forces and future warfighting requirements. Although the current emphasis is on reducing the cost of meeting existing levels of performance, it is also necessary to assess the ability to deliver enhanced performance as required to meet new threats. For example, the fuel efficiency advantages of ICR/IPS can provide operational flexibility for a notional eastern Mediterranean transit and station-keeping mission by increasing transit speed by 7 knots, increasing days on station by 9 days, or increasing gauge by 1,100 nautical miles, while keeping fuel load constant compared with the baseline. Developments Needed A concurrent engineering approach is necessary that identifies modular-technology building blocks that can be used in various combinations to cover the range of possible requirement sets and then engages a development process with three overlapping phases for those modules to minimize risk and financial exposure. Development of test hardware and systems and component technologies at less than full scale would constitute the first phase. This would provide the technical basis for definition of the complete set of modules needed to support the range of projected fleet requirements, including the many alternatives being considered for the next-generation surface combatant. Application of the systems engineering process would lead to identification of a small subset that would actually be developed and demonstrated in the second phase. These would be selected on the basis of minimizing the risk of producing ship equipment that would meet some as yet unspecified requirements during the full-scale engineered development phase.
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Time Scale for Development/Deployment ASMP has recognized the need to get products into the fleet at the earliest opportunity. The goal of rapid deployment can sometimes conflict with the systems engineering approach of optimizing the total-ship design. There are two ways to address this dilemma. Designers can anticipate where technology is headed and plan for the phased introduction of advanced components as they become available. In addition, the use of open architecture and modular designs will facilitate the insertion of technology upgrades. ASMP has followed this approach with introduction of the zonal electrical distribution and SMCS architectures in DDG-51 class ships. The zonal electrical system required no technology development, and the SMCS may wind up using only the architecture, software, and local level hardware from the advanced development model (ADM) by taking advantage of its openness in choice of supervisory-level host hardware. Figure 8.8 provides a schedule of the ASMP products and their fleet transition possibilities. IPS Architecture and Core Technology The IPS architecture and core technology have their maximum impact on new-design ships and would be hampered in forward-fit applications by the extensive nonrecurring engineering costs and limited opportunity for taking advantage of arrangement flexibility. The first surface combatant new design opportunity is the SC-21 (FY 2003 leadership award). Completion of FSAD will enable FIGURE 8.8 Integrated propulsion system. SOURCE: Adapted from (1) Advanced Surface Machinery Programs Office, 1996, "A Strategy Paper on Power for U.S. Navy Surface Ships," draft, Naval Sea Systems Command, Arlington, Va., Figure A-1,April 8, and (2) Krolick, C.F., 1996, "Advanced Surface Machinery Programs (ASMP)," presented to the Panel on Technology by the Naval Sea Systems Command, Arlington, Va., May 2.
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introduction of IPS in noncombatant applications in FY 1998, which would lessen the fleet introduction cost and logistics support burden on the SC-21 class. Introduction of IPS on aviation platforms will follow a two-step approach that straddles the SC-21 time frame. Although the ultimate objective is to provide a full-up IPS configuration for the CVX (FY 2006 award) that addresses all potential electrical loads including electric propulsion and electromagnetic catapult, the near-term CVN-77 (FY 2002 award) presents a transitional platform for introduction of a partial IPS configuration that addresses ship-service electrical loads only. The upfront systems engineering will define the CVX IPS configuration and identify those features that can be cost effectively demonstrated on CVN-77. Maximum utilization will be made of IPS SC-21 modules that will minimize development and fleet introduction and support costs and lead to identification of any additional modules needed to meet specific CVX requirements. These new modules would then be developed and qualified under the appropriate R&D program. For the large-deck amphibious ship (LHX) application, IPS offers arrangement flexibility for locating machinery relative to the well deck and engine operational flexibility, important when ship-service load is a large fraction of total installed power. Again, maximum reuse will be made of modules from previous ship applications. Power Generation The ICR gas turbine and its derivative, power generation module-1 (PGM-1) is envisioned as the main power unit for the IPS fleet starting with the SC-21. Selection of technology for any lower-power-rated PGMs for application to SC-21 will be driven by the requirements that emerge from the SC-21 cost and operational-effectiveness analysis (COEA) process. While these requirements are being finalized and the selection criteria developed and prioritized, it will be necessary to establish a technology base for the competitive selection process. Commercial medium- to high-speed diesels are readily available in the power range of interest and could be deployed in the FY 2003 time frame. In any case, IPS diesel-based PGMs are likely to be identified to support the commercial end of the applications spectrum. Advanced cycle gas turbines in the 1- to 4-MW range with fuel efficiency appreciably better than the existing DDA-501 are not commercially available at this time, although some commercial and DOE funding is going into their development. The technology exists and adequate time may be available to develop and qualify a new Navy engine from a commercial core in time for SC-21, assuming that a requirement emerges in the next year or so. The present ASMP strategy is to initiate militarization of a commercially available engine starting in FY 2000, based on the overall fleet requirements at that time. Fuel cell developments are being supported with commercial and government funding, but largely for utility or land transportation applications that do not
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use Navy distillate as fuel. The Navy should continue to ensure demonstration of the fuel-processing system at a scale adequate to level the playing field with the alternative technologies in time of the SC-21 selection process. Also, if commercial or other government-funded efforts will not provide a full-scale stack with the characteristics required by naval force applications in time for demonstration with the fuel processor, then the Department of the Navy should consider investing in accelerating development of the stack. As fuel cell technology matures, it may achieve power density and costs that make it competitive as a larger-power-rated PGM as early as the 2010 time frame. Other direct electrical conversion technologies such as thermionics and thermophotovoltaics will begin to emerge in the 2010 time frame as part of a cogeneration scheme using conventional engines. Power Distribution/Conversion Direct current zonal electric distribution systems, as with IPS in general, will have the greatest impact in a new design, but the power-conversion equipment can be approached in a way that allows their introduction in the ac ZEDS on new-construction DDG-51 and/or as part of the partial IPS configuration demonstrated in CVN-77. As part of this early introduction of power conversion equipment, first- or second-generation PEBB devices may be used. This will be driven by commercial viability of these early PEBB products that will encourage investment in production capacity. The dc ZEDS for SC-21 and CVX will be fully integrated with the auxiliary and combat system equipment, and generation-three or -four PEBBs will likely be employed in the power conversion equipment. By 2010, the PEBB will be fully utilized in the implementation of an autonomic dc ZEDS. Generation-five to generation-seven PEBBs will include high-voltage applications such as solid-state switchgear for the IPS 4160V bus or for the rectifiers and inverters for a higher voltage dc bus. Pulse-power Systems These systems could support back-fit, forward-fit, and new design, since the ETC gun concepts conform to existing weapons modules. However, gun and associated prime power system development has been delayed such that the SC-21 now appears to be the earliest target for these systems. Pulse loads are more likely for the CVX. By 2010, high-voltage, pulse-duty PEBBs could be employed in the pulse-power rectifiers. Propulsion Motor Although IPS core technology can support existing ships, the PM motor and a PEBB-based propulsion power converter are candidates for the SC-21, particularly
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in the lower displacement variants where power density has a greater value. Steerable pod propulsors are also viable for SC 21, most likely in conjunction with PM motor technology. Homopolar and superconducting motor technology will be available by 2010, if commercial development of high-Tc superconducting machinery is successful or if Navy-unique critical requirements (e.g., acoustics) drive their development. Power and Propulsion For Submarines Future submarine power and propulsion technology will inevitably move toward increased reliance on electricity. Advanced dc power distribution using high-speed, turbine-driven rectified alternators feeding solid-state inverters through a dc distribution bus is already targeted to the new nuclear-powered submarine (SSN). Electric propulsion offers an affordable opportunity to enable new propulsor concepts that can improve signature. Studies over the past few years indicate that integration of the electric-propulsion and ship-service power-generation functions at the prime mover can result in reduced ship size and displacement because of fewer power plant components and improvements in their arrangement. Studies have also shown that eliminating centralized hydraulic systems and transporting energy electrically to the point of use will result in up to 70 tons of weight savings and potential cost reduction. In the future, this concept might be extended to other systems, such as heating, ventilation, and air conditioning. In general, power and propulsion technologies for surface ships are also applicable to submarines. However, the optimization of submarine systems will be driven by stealth, safety, power density, and other requirements that differentiate surface and undersea vehicles. The technologies that may be required for submarine power and propulsion systems of the future will include the following: Very low harmonic motor controllers; High-power-density, high-performance, solid-state inverters and converters and the components thereof; Motors and generators with very low acoustic and magnetic signature, including versions that can operate submerged in seawater at high pressure; New technologies for motors and generators such as superconducting magnets, cryogenic coolers, current collectors, high-field permanent magnets, liquid cooling, and active noise control; and Electrical substitutes for systems and components that now rely on fluid transport for energy and actuation, including electric actuators, electromagnetic launchers, and thermoelectric coolers.
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For Amphibious and Land Vehicles The Marine Corps relies heavily on power and propulsion technology for its vehicle power systems. The mandates for higher-water-speed amphibious vehicles, signature reduction, adaptability to electric power, future weapons, and reduced vehicle weight all lead to the need for an integrated electric power system for future land and amphibious vehicles. Key drivers will be the following: Electric propulsion to allow the prime mover/generator to supply power to both water and land propulsion trains; Electric power distribution and control to direct and condition power for propulsion, weapons, and sensors as required; and Electric power generation and energy storage to allow operation of critical systems with reduced IR and other signatures. Key technologies, as listed below, mirror those described above in this report for the surface-ship systems but are driven by the land and amphibious requirements: Pulsed-power systems including energy storage pulse-forming networks and very high power solid-state switching; Flywheel energy storage; High-power density engines; Electrical steering, suspension, and actuators; and High power density electric drives at the unit size of several hundred horsepower. For Air Vehicles The trend toward increased reliance on electricity is also evident in recent technical developments directed toward future aircraft.12 The drivers toward a more electric aircraft include the following: Reduction in aircraft weight; Improved survivability and reliability; Reduced maintenance; Reduced cost; Higher fuel efficiency; and Reduced need for ground-support equipment. 12 Office of Navel Research. 1994. Power by Wire Technology Assessment, Arlington, Va., May.
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One solution to these drivers is to replace hydraulic, pneumatic, and mechanical aircraft systems with electrical functional equivalents and to provide the means to supply the increased electrical load (while reducing other engine loads). The key technologies that support the development of more electric aircraft mirror those that enable the electric ship. In most cases, however, the electrical power levels in aircraft systems are an order of magnitude less than those in a typical ship. Key technical developments for more electric aircraft power and propulsion include the following: Combined electric starter/generator for direct engine mounting; Smart, fault-tolerant electric power distribution and control; High-temperature-capable (˜500 °C), solid-state power switches for engine-mounted electronics and power distribution control elements; and High-temperature-capable electrically powered actuators. Recommendations The panel examined a wide range of technologies, trends, and the Department of the Navy drivers that will push the development of new technologies in the future. It concluded that a focus on technologies that enable higher power density and more efficient and more effective propulsion, power generation, and distribution in all naval vehicles will be necessary to ensure future performance and affordability. The general areas of electrical power science and technology, and technology that will enable the efficient conversion of fuel to electrical power, hold great promise for major technical progress and payoff in the future. The Department of the Navy should place a high priority on the development of new all-electric ships with the associated drive, power-conditioning, and distribution systems. Investment in the following areas is warranted: Increasing switching speed, current, voltage, and operating temperature of solid-state power switching devices; Solid-state power controllers that are adaptable to multiple uses, modular, and compatible with the commercial market; Electrically driven substitutes for mechanically driven, hydraulic, pneumatic, or steam-driven actuators; Means for rapidly detecting and controlling the state of complex power distribution systems; Electric motor and generator technologies that increase power and torque density, reduce acoustic and electromagnetic signature, operate at elevated temperatures, have higher efficiency, and/or allow operation submerged in seawater; High-energy-density means for both long- and short-term electrical energy storage;
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Means for high-efficiency direct conversion of fuel to electric power; Higher-speed, more efficient engines that increase power density while reducing fuel consumption; and Higher-efficiency devices at the user end of the power system where the impact of efficiency is greatest (e.g., counterrotating propulsors).
Representative terms from entire chapter: