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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Executive Summary The U.S. Army envisions that many of its future combat vehicles will feature a hybrid electric power system containing a diesel or turbine generator that will supply electric power to operate the vehicle subsystems, including electric drive and weapons systems. In military hybrids, pulsed power and continuous power must operate together without interference. Pulsed power is required for high-power lasers, an electrothermal chemical (ETC) gun, high-power microwave weapons, electromagnetic armor, and other systems. Elements of the continuous power system include prime power (diesel or turbine), generator, motors, converters, power distribution systems, storage, fault protection, safety systems, and auxiliary power connections. While some of the technologies required to support combat hybrid vehicle power systems are in hand, many technical challenges remain. In 1997, the Defense Advanced Research Projects Agency (DARPA) initiated the Combat Hybrid Power System (CHPS) program, whose goal is to develop and test a full-scale hybrid electric power system for advanced combat vehicles. To achieve that goal, the program has developed a 100 percent hardware-in-the-loop System Integration Laboratory (SIL)—a reconfigurable laboratory using state-of-the-art hardware and software. In support of this effort, DARPA requested that the National Research Council (NRC) convene a committee of experts to undertake the following task: Address the key issues for emerging technologies in the development of the combat hybrid power system components. The technologies to be addressed include permanent magnet technology for hub motors, Li-ion batteries, and high-temperature, wideband gap materials. Other such emerging technologies may also be addressed. On August 26 and 27, 2002, the NRC Committee on Assessment of Combat Hybrid Power Systems convened a data-gathering workshop in San Jose, California. The committee targeted the three emerging technology areas specified in the statement of work: Advanced electric motor drives and power electronics, Battery technologies for military electric and hybrid vehicle applications, and High-temperature, wideband gap materials for high-power electrical systems. In addition, the committee determined that three additional emerging technologies should also be addressed: High-power switching technologies, Capacitor technologies, and Computer simulation for storage system design and integration.

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Tables ES-1 through ES-6 summarize the results of the committee’s analysis of the technical challenges, performance metrics, and research priorities associated with these six areas. TABLE ES-1 Advanced Electric Motor Drives and Power Electronics System/Component Technical Challenge Performance Metric R&D Priorities Electric motors for traction Simulation of drive cycles/mission profiles to establish torque-speed requirements of the electrical drive Changes in component (e.g., motor) design parameters quantitatively linked to changes in overall system performance Expansion of current research to validate models and link motor design programs with power electronics and drive simulation programs   Optimizing auxiliary power unit, battery, and other energy storage device characteristics to meet the torque-speed requirements of the drive     Motor and inverter technology development to meet wide constant horsepower speed range without impacting the size of inverters     Comparison of various power train configurations, e.g., wheel motors, axle motors with and without gearboxes and transmissions     ”Apple to apple” comparison of internal permanent magnet motors and inverters with induction and other motors and inverters for traction drive  

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Materials for electric motors Electrical losses in copper windings and iron in magnetic materials Low loss even at high frequencies Low loss materials that can be readily manufactured in laminar form   Buried permanent magnet rotors for large machines Retention of remnant flux density and energy product characteristics Techniques for injecting magnetic materials into the rotors, and curing and magnetizing them on-site Power devices and inverters Inverters that operate with high efficiency at higher power High-current, high-voltage switching characteristics Development of wideband gap materials such as SiC   Improving device cooling   Development of thermal management systems with phase transition and other materials to remove heat quickly from the power devices and inverters and improve transient performance   Reducing electromagnetic interference (EMI)   Integration of SiC diodes with insulated gate bipolar transistor hard switched inverters to reduce reverse recovery transients to yield low EMI and high efficiency comparable to soft switching inverters DC bus capacitors Keeping the voltage ripple within specified limits Size, efficiency, operating temperature, and ripple current carrying capacity Fundamental research on materials to meet these requirements DC/DC converters High ratio voltage conversion at high power Performance at high power, EMI/electromagnetic compliance shielding, and packaging size Design tools for optimizing the combination of devices required and other characteristics, e.g., switching frequency Integrated thermal management systems Adequate cooling for both motors and power electronics Cooling efficiency, packaging size Development of high thermal conductivity materials such as graphite foams and silicon carbide, in combination with phase transition fluids

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities TABLE ES-2 Battery Technologies System/Component Technical Challenge Performance Metric R&D Priorities Advanced battery concepts Validation of batteries in vehicle applications Specific power Specific energy Triple the power and energy with nanomaterials technology and new chemistries   Safety   Increased safety (eliminate flammable materials; better packing for isolation, containment, venting; thermally stable materials; diagnostics/ prognostics integrated in pack; eliminate ground fault and arcing; improved materials that reduce gassing)   Battery management (state of health, state of charge, power availability, life prediction, temperature management, diagnostics, and prognostics)   Electrode/electrolyte interface Voltage drop caused by limited chemical reactivity at the interface   Advanced electrode/electrolyte materials with high surface reactivity   Increased electrode surface area by increased matrix porosity or perhaps application of nanomaterials Electrolyte Voltage drop caused by mass transfer overpotential   Electrolytes with high concentrations of reactant species and low ion transfer resistance Connectors and terminals Ohmic resistance of materials Minimized resistance Low-resistance materials

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities TABLE ES-3 High-temperature, Wideband Gap (WBG) Materials System/Component Technical Challenge Performance Metric R&D Priorities Bulk SiC Improvement of material quality and substrate diameter Low defect density Processing to exploit advantages of 4H-SiC (1120) a-plane crystal orientation Metal-semiconductor contacts Improve ohmic contact fabrication processes Contact stability under extreme conditions Improvement of science and technology of implantation, implantation activation, and metal-semiconductor metallurgy in wideband gap devices and materials Device packaging Development of packaging that can accommodate the high-temperature, high-power characteristics of wideband gap devices while providing high rates of heat removal Stability, heat removal rate For SiC devices, development of processes for high-resistivity poly SiC with a matched coefficient of thermal expansion Bulk GaN and AlN Improvement of substrate material quality Low defect density Fundamental processing research to control defects in bulk GaN and AlN

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities TABLE ES-4 High-power Switching Technologies System/Component Technical Challenge Performance Metrics R&D Priorities Power converters Higher power densities, switching frequencies, and greater reliability High power density Manufacturing simplicity Processes for integration of distributed components with active devices   Reduced design and verification cycle times Design tools for three-dimensional thermal management, packaging, system design, and manufacturability Power electronics for pulse energy storage Effective decoupling of pulse loads from the power distribution system High current density High level of decoupling Development of storage system interfaces with bimodal (slow and fast) power transfer capability   Development of interfaces with flexibility to tailor output voltage/current waveforms to requirements of weapons systems Power distribution network Mission-critical systems that degrade gracefully under fault conditions Level of functionality under unplanned faults and component failures Fundamental understanding of factors affecting system stability   Dynamic models of power converter interactions at the DC bus   Controls that mitigate instabilities on the DC bus

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities TABLE ES-5 Capacitor Technologies Component/System Technical Challenges Performance Metrics R&D Priorities Polymer film capacitors Films with improved dielectric properties Dielectric constant Dielectric withstand New polymer films with increased dielectric constant and dielectric withstand similar to biaxially oriented polypropylene   Filled polymer films: either inorganic filler to improve dielectric strength, high dielectric constant filler to increase dielectric constant, or high dielectric polymer filler to reduce volume within the film, resulting in a combination of increased operating field and increased dielectric constant Ceramic capacitors Lack of understanding of aging/failure mechanisms   Research on aging/failure mechanisms under high-temperature, high-field conditions   Dielectrics with improved properties Dielectric constant Dielectric withstand Research to improve high energy density, high-temperature ceramic dielectrics   Improved operating electric field Operating field Ceramic-polymer composites or other technologies that reduce the free volume within the ceramic Double layer capacitors Lack of understanding of aging and degradation processes at high temperature   Investigate role of impurities in the carbon electrodes and interactions among the electrodes, electrolyte, and separator   Improvement of properties of electrolytes, increase in cell voltage, and reduction of equivalent series resistance Cell voltage equivalent series resistance     Predictability of performance over time Stability of properties Materials and processes that achieve reproducible cell characteristics that are stable over time, or age uniformly

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities   over time   Reduction of current densities Effective electrode surface area Research into materials and manufacturing processes that increase the effective surface area of electrodes TABLE ES-6 Computer Simulation for the Design of Storage Systems and Components Component/System Technical Challenges Performance Metrics R&D Priorities CHPSET tool set Validation against available hardware Accurate simulation of hardware performance Validation using data from the Systems Integration Laboratory and possibly hybrid HMMWV and Scout vehicles Cooling airflow Modeling cooling effectiveness and cooling airflow, especially through combat grillwork Resemblance of emulation hardware to notional, demonstrator-level hardware Emulation of environmental factors Emulation using grillwork hardware Linkage of CHPSET codes Effective information transfer between system designers and component designers Fidelity of vendor-supplied models Development of a common, expanded solid model database   Difficulty of modeling hardware provided by vendors Compatibility of models with CHPSET tools Vendors encouraged to provide solid models of their hardware, validated at the numeric, component, and system levels Incorporation of CHPSET tools in a virtual battlefield environment Understanding of power management during the various modes of operation Successful integration of a CHPSET model into a higher-level simulation Integration of CHPSET models into the Joint Modeling and Simulation System (JMASS) Consideration of environmental factors in CHPSET Need for realistic mission-related resistance data Successful incorporation of NATO Reference Mobility Model (NRMM) data into CHPSET Explore use of NRMM and related software tools such as a route analysis tool kit to generate input data for CHPSET User options in CHPSET code Need for comparative analysis capability involving other vehicle options Executable code user friendliness Expand executable CHPSET code to include additional user options such as parallel hybrid and conventional vehicles, with appropriate user documentation

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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Incorporation of CHPSET codes into failure modes and effects analysis (FMEA) Enhancement of system reliability and mitigation of effect of component failures Risk priority numbers Identification of potential failure modes Design-specific, skid-mounted hardware emulators of Future Combat System Enhancement of emulator fidelity Resemblance of emulation hardware to notional, demonstrator-level hardware Development of design specifics for notional, demonstrator-level systems

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