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Energy-Efficient Technologies for the Dismounted Soldier Appendix C Energy Source Technologies BATTERIES Although batteries in general represent a very large, mature product class in commercial production, enormous improvements in specific power, specific energy, and cycle life (for rechargeable batteries) have been made in the past decade (Space Power Institute, 1990, 1992b). Much of the driving force for the technical improvements has come from the rapid growth of portable computers, cellular telephones, and other communication devices. However, very few of these improvements have been of direct benefit to communication devices used by the Army. Battery production worldwide is approximately $40 billion (Salkind, 1996) with U.S. production at about $11 billion. Military purchases are only a small percentage of the total, and there appears to be little interest among large manufacturers in producing military batteries. Improving the specific energy (available energy from a fixed mass) and energy density (available energy from a fixed volume) of batteries have been commercial goals. But because most commercial devices require only a few AA cells, weight reduction has been second in importance to energy capacity. As shown in Figure C-1, the capacity of AA nickel alkaline (NiCd and NiMH) batteries has risen from 0.4 Ah to 1.2 Ah in the past 20 years. Very fast recharging (in less than 1 hour) has also become available. Lithium rechargeable systems in the same size packaging have approximately the same capacity, but at much higher voltages, resulting in cells with higher specific energy. However, so far lithium rechargeable cells cannot be recharged quickly. Improvements continue to be made. Among the Army's options for keeping pace with these rapid changes is the adaptation of commercially available cells. Current military battery systems could be replaced by systems with different voltage characteristics as long as the new system volume is the same or smaller. This should be possible with new, more efficient techniques for DC-DC conversion, which would eliminate the problem of Army communication devices being locked into using power sources with particular voltage levels.
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Energy-Efficient Technologies for the Dismounted Soldier FIGURE C-1 Chronological improvements in the capacity of AA nickel batteries. The performance characteristics and production levels of the common primary, secondary, and special battery systems considered in this report are listed in Tables C-1, C-2, and C-3. Systems Likely to Meet the Needs of the Dismounted Soldier Of the more than 30 rechargeable battery systems in commercial production or in advanced development, only seven or eight seem likely to meet the military goals of availability in small sealed cells with appropriate levels of safety, reliability, and low temperature and high temperature performance. These few systems are described in this section, with estimates of their present performance levels and estimates of what might be achieved in five and ten years. The research needed to achieve the listed goals is also briefly described. Although a low temperature requirement of -40°C is still listed in some Army documents, the committee was informed that this temperature requirement was principally for storage. For operations, the committee assumed a minimum temperature requirement of -25°C but even this may be unrealistically low and may disqualify otherwise practical systems. The systems likely to provide the desired combination of compactness, specific energy, and specific power fall into two categories: rechargeable alkaline electrolyte systems (nickel-metal hydride, nickel-zinc, MnO2-zinc) and rechargeable lithium electrode systems (lithium metal anodes, lithium intercalating anodes, lithium alloy anodes [including the tin oxide type]).
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-1 Summary of Primary Battery Data Theoretical Working Battery System Anode Cathode Voltage Ah/kg Wh/kg Voltage Wh/kg Wh/l Production Valuea Lechlanche (zinc-carbon) Zn MnO2 1.6 224 358 1.5 85 165 vl Magnesium Mg MnO2 2 271 758 1.75 100 195 vs Alkaline Zn MnO2 1.6 224 336 1.25 125 330 vl Mercury Zn HgO 1.34 190 255 1.3 100 470 vvs Silver (silver-zinc) Zn Ag2O 1.5 180 288 1.45 120 500 ss AgO 1.85 270 445 (2 plateaus) 140 650 Zinc-air Zn O2(air) 1.65 658 1,066 1.25 500 1,050 1 Aluminum-air Al O2(air) 2.7 2,980 8,046 1.1 300 240 vs Lithium Systems Sulfur dioxide Li SO2 3.1 379 1,175 2.8 260 415 1 Thionyl chloride Li SOCl2 3.66 407 1,489 3.3 320 700 1 Sulfuryl chloride Li SO2Cl2 3.9 360 1,405 3.7 450 900 vvs Manganese dioxide Li MnO2 3.5 286 1,001 2.8 230 550 vl Carbon monofluoride Li (CF)x 3.1 703 2,180 2.5 250 600 l Iron disulfide Li FeS2 1.8 725 1,304 1.4 130 400 l a Key: vl = $1 billion l = $100 million to $1 billion s = $10 million to $100 million vs =< $10 million vvs = < $2 million Improvements MnO2 cathode material improvements can increase nonlithium system capacity by as much as 15 percent. Improvements in separator material and technology can increase stability and rate of all primary cells. Air electrode improvements can increase power capability of air cathode systems. Safety for all lithium battery systems can be improved with improvements in separators. Packaging technology can increase specific energy of Li/MnO2 technology. MnO2 cathode material improvements can increase capacity and discharge rate in lithium systems. Focus Chemistries Zn, Mg/MnO2, and Li/FeS2 are commercial market driven. Zinc-air, Li/MnO2, and Li/CTx are areas of interest for the government because they have either high specific power or high specific energy or both.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-2 Summary of Rechargeable Portable Battery Data Theoretical Working Battery System Negative Electron Positive Electron Voltage Ah/kg Wh/kg Voltage Wh/kg Wh/l Production Valuea Estimated Life (Cycles) Lead-acid Pb PbO2 2.1 83 175 2.0 35–50 85 vl 400 Nickel-iron Fe NiOOH 1.4 224 313 1.2 35–60 70 vs 500 Nickel-cadmium Cd NiOOH 1.35 181 244 1.2 35–52 75 vl 600 Nickel-zinc Zn NiOOH 1.73 215 372 1.6 65–80 150 s 400 Silver-zinc Zn AgO 1.85 283 524 1.5 90–150 180 vs 100 Nickel-hydrogen H2 NiOOH 1.5 269 434 1.4 55–60 60 S 600 Nickel-metal hydride Mhx 1.2 to 2 w/o H NiOOH 1.35 206 278 1.2 55–70 120 vl 800 Silver-cadmium Cd AgO 1.4 227 318 1.2 60–80 110 vvs 200 Zinc-bromineb Zn Br Complex 1.85 139 258 1.55 70 60 vvs 400 Alkaline manganese Zn MnO2 1.6 224 330 1.2 55 250 vl 15 Zinc-air Zn O2 (air) 1.6 658 1,085 1.15 110 130 vs 25 Lithium Systems LiMn2O4 Li Mn2O4 4 143 510 3.7 140 300 vs 250 LiNiO2 Li NiO2 4.2 137 575 3.6 155 325 res — LiCoO2 Li CoO2 4.2 178 750 3.7 95 235 vs 250 Li/organosulfide Li R-S-S-R 3 ~300 ~900 2 200 est 300 est res 300 Li/organosulfide Li (CS)x 2 ~400 ~800 2 200 est 300 est res 300
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Energy-Efficient Technologies for the Dismounted Soldier Theoretical Working Battery System Negative Electron Positive Electron Voltage Ah/kg Wh/kg Voltage Wh/kg Wh/l Production Valuea Estimated Life (Cycles) LiMn2O4 Li+C Mn2O4 4/3 102 356 3.7 70–100 170 res — LiNiO2 Li+C NiO2 4.2/3 100 360 3.6 70–100 170 res — LiCoO2 Li+C CoO2 4.2/3 100 360 3.7 70–100 170 1 1,000 Polymer Li+C Mn2O4 4/3 102 358 3.0 150 est 300 est vvs 300 Large iron sulfides Ll(Al) FeS/FeS2 1.33/1.73 285/345 459/514 1.3/1.6 100/180 200/350 res ~1,000 a Key: vl = $1 billion l = $100 million to $1 billion s = $10 million to $100 million vs = < $10 million vvs = < $2 million res = research b Not portable. Improvements Charger and charging methods can improve cycle life and safety of rechargeable cells. Improvements in NiOOH and separator technology can increase capacity of all nickel systems. Improvements in metal hydride anode can increase the energy by nearly 2 times (Mhx 1.2 to 2 w/o H). Material improvements can increase cycle life of rechargeable alkaline battery. Material improvements can increase cycle life of rechargeable zinc-air battery. Air cathode improvements can increase power capability and cycle life of the zinc-air system. Safety for all rechargeable lithium batteries can be improved with improvements in separators. Anode material improvements for lithium ion and lithium polymer batteries can increase the specific energy and safety. Cathode material improvements can increase specific energy of all lithium batteries. Focus Chemistries Nickel-metal/hydride, alkaline, and zinc-air are market driven; thus, unique military requirements may be overlooked. Lithium systems focus on military-unique requirements. Zn, Mg/MnO2, and Li/FeS2 market driven.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-3 Summary of Data on Reserve, Thermal, and High Temperature Rechargeable Batteries Battery System Anode Cathode Working Voltage Wh/kg Wh/l Estimated Life (Cycles) Reserve Water activated Mg or Zn CuCl 1.5–1.6 65 125 (Not rechargeable) MnO2 1.5–1.6 65 125 AgCl 1.5–1.6 125 250 Others Spin activateda Pb PbO2 1.5 (Not rechargeable) Zn AgO 1.4 Li SOCl2 3.5 Li FeS2 1.8 Electrolyte introduction-activated Zn AgO or Ag2O 1.6 50 160 (Not rechargeable) Li V2O5 3.3 50 100 Li SO2 3 120 200 Li SOCl2 3.5 150 300 Thermal batteries Ca CaCrO4 2.4 30 40 (Not rechargeable) Mg V2O5 2.5 Li FeS2 1.8 40 100 High temperature rechargeable batteries Lithium-iron-sulfide Li FeS 1.3 100 200 700 FeS2 1.6 180 350 1,000 Sodium-sulfur Na S 2.1 170 250 100–2,000 Sodium-nickel chloride Na NiCl2 2.58 90 160 600–1,000 a These batteries are not designed to be weight or volume efficient.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-4 Nickel Metal Hydride Battery Systems Present Advantages Present Disadvantages 5 Years 10 Years Higher specific energy than NiCd Lower specific power than NiCd Higher rate capability, 25% more capacity per volume 40% capacity improvement per volume Rapid recharge at room temperature Poor charge retention, 5% per week loss at room temperature Charge loss reduced to 2% per week at room temperature — Long cycle life Poor thermal stability Poor overcharge recombination kinetics Lower vapor pressure alloys Lower vapor pressure alloys Maintenance free — — — Rechargeable Alkaline Electrolyte Systems Most anode battery systems can be assembled with various cathodes and electrolytes in combinations described in the Tables C-4 through C-15. These tables present a summary of the candidates likely to meet the future power requirements of the dismounted soldier. Each table summarizes the advantages and disadvantages of each chemistry, as well as technological projections of what can be accomplished in five and ten years. Improvements in nickel metal hydride battery systems are shown in Table C-4. The anticipated improvements will require sustained research in the following areas: metal hydride alloys for better thermal stability cathode materials with improved volumetric efficiency (e.g., nanostructured, fibrous, and higher valence materials) charge profile with optimum charging, overcharge recombination kinetics better separators Improvements in rechargeable alkaline manganese dioxide battery systems are shown in Table C-5. To achieve the projected improvements, it will be necessary to research the following areas in depth: materials for better cycle life and low temperature performance (nanostructured, catalytic MnO2, improved carbons and graphites) improved cellophane (or other separator) for higher rate performance optimal recharging profile
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-5 Rechargeable Alkaline Manganese Dioxide (RAM) Battery Systems Present Advantages Present Disadvantages 5 Years 10 Years Low cost Lower specific power Improved rate Improved cycle life Maintenance free Poor cycle life Improved cycle to cycle capacity Improved low temperature operation Good charge retention Decreasing capacity with cycle life and depth of discharge — — Poor low temperature performance — — Improvements in metal zinc battery systems are shown in Table C-6. To achieve the projected improvements, major research will be needed in the following areas: cathode materials for improved volumetric efficiency (e.g., nanostructured, fibrous, higher valence) lightweight current collectors for the nickel electrode charge profile for optimal charging, overcharge recombination kinetics better separators, microporous membranes, and cellulosic films complex electrolytes for improved cycle life TABLE C-6 Nickel Zinc (NiZn) Battery Systems Present Advantages Present Disadvantages 5 Years 10 Years Higher specific energy than NiCd Poor overcharge recombination kinetics Higher specific power, 10% more capacity per volume 20% specific energy improvement per volume Maintenance free — — — Rapid recharge Moderate charge retention; 2% per week at room temperature Charge loss reduced to 1% per week at room temperature — Moderate cycle life — Improved separator and electrolytes; 500–800 cycles Improved separator and electrolytes; 800–1000 cycles
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-7 Lithium Batteries with Lithium Metal Anode Structures Present Advantages Present Disadvantages 5 Years 10 Years Highest energy and power capability Safety Improved safety and cycle life through improved electrolytes — Poor cycle life No tolerance to overcharge and overdischarge Rechargeable Lithium Systems Lithium systems offer the most promise in terms of specific energy (energy per unit weight). Lithium chemistry, however, raises serious safety and environmental concerns. Even though lithium systems as presently fabricated have no tolerance to overcharging or overdischarging, lithium batteries offer enormous promise as energy sources for the dismounted soldier. Lithium systems can be categorized by the type of components (anode, electrolyte, separator, cathode); each component can be used with a variety of other components to produce a complete cell. Tables C-7 through C-9 characterize lithium battery technologies in terms of their anode structure and materials. Table C-7 shows improvements in lithium batteries with lithium metal anode structures. To achieve the projected improvements, research will be needed in the following areas: Charge control in order to eliminate safety concerns Electrolyte and separator development to improve charge morphology Management of the film on lithiums surface for improved cycle life Lithium intercalating anodes include carbon or graphite (LiCx); tin, aluminum, and other metals; and silicon and other nonmetals are shown in Table C-8 To achieve the projected improvements, research will be needed in the following areas: Improved binders for improved stability of electrode Materials research to increase rate capability and specific energy Lighter weight host materials for lithium cathodes Improved reversibility of positive electrode materials through new preparation methods
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-8 Lithium Batteries with Lithium Intercalated Anode Structures Present Advantages Present Disadvantages 5 Years 10 Years Safer than lithium metal anodes Rate limiting electrode; no tolerance for overdischarge or overcharge — — Long cycle life Reduced power and specific energy as compared to lithium metal Improved power and specific energy through materials improvements Improved power and specific energy through materials improvements Reduced low-temperature performance Material and electrolyte improvements Material and electrolyte improvements Some voltage penalty over pure lithium Lightweight host materials for lithium electrode — Lithium alloy anodes include aluminum (LixAl); ternary alloys with manganese; and other lithium alloys such as silicon alloys are shown in Table C-9. To achieve the projected improvements, research will be needed in the following areas: Materials research to increase rate capability and specific energy Charge control in order to eliminate safety concern Electrolyte and separator development to improve charge morphology Lithium batteries can also be characterized with respect to electrolytes. Tables C-10 and C-11 project the developments and necessary research and development over the next ten years. TABLE C-9 Lithium Batteries of Lithium Alloy Anode Structures Present Advantages Present Disadvantages 5 Years 10 Years Increased power density as compared to lithium carbon Reduced specific energy as compared to lithium metal Improved specific power and specific energy through materials improvements Improved specific power and specific energy through materials improvements Voltage penalty Material and electrolyte improvements Material and electrolyte improvements No tolerance of overcharge and overdischarge — Increased tolerance of overcharge Rate limiting electrode — —
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-10 Lithium Batteries with a Liquid Organic Electrolytes Present Advantages Present Disadvantages 5 Years 10 Years Mixed organic stable at high voltages Volatile and flammable Material improvements to reduce flammability Material improvements to reduce flammability High conductivity Requires stable separator; presently microporous polyolefins Improved conductivity through salt research Improved conductivity through salt research Some toxicity Less toxic materials Less toxic materials No tolerance to overcharge and overdischarge — — Table C-10 shows improvements in lithium batteries using liquid organic electrolytes. To achieve the projected improvements, research will be necessary in: Materials research to identify stable nonflammable electrolytes Charge control in order to eliminate safety concerns Electrolyte and separator development to improve charge morphology Electrolyte salt investigation. Table C-11 shows improvements in lithium batteries using liquid organic electrolytes. To achieve the projected improvements, research will be necessary in: Materials research to identify higher conductivity electrolytes Charge control in order to eliminate safety concerns Electrolyte development to improve charge morphology Electrolyte salt investigation Lithium/polymer interface reactions (a rise in cell impedance on standing and/or cycling has been observed) TABLE C-11 Lithium Batteries with Polymer Gel Electrolytes Present Advantages Present Disadvantages 5 Years 10 Years Stable at high voltages Low conductivity — — Polymer electrolyte and separator — Material improvements improving conductivity Material improvements improving conductivity Encapsulates volatile and flammable electrolytes — Improved conductivity through salt research Improved conductivity through salt research No tolerance of overcharge and overdischarge — —
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-22 Summary of Electrochemical Capacitor Technology Construction Performance Status Name Electrode Configuration Electrolyte Energy Density (kJ/kg) Energy Density (kJ/I) Resistance (ohms/cm2) Maximum Power (W/kg) Cost Voltage Typical Capacitance(F) Largest Unit(J) Basis for Projection Readiness Level NEC Supercap bipolar carbon/carbon composite sulfuric acid 4.7 6.8 0.16 4 low 15 470 55k manufacturer specification commercially available NEC FY bipolar carbon sulfuric acid 1.2 1.98 45 — low 5 2.2 — manufacturer specification commercially available NEC FE bipolar carbon sulfuric acid 0.036 0.65 1.9 — low 5 1.5 — manufacturer specification commercially available Panasonic spiral wound, single-cell carbon organic 7.9 10.4 7 2.7 low 3 470 1,500 6.7k commercial device commercially available Evans prismatic carbon sulfuric acid 0.72 1.8 1 — low 11 — 40k manufacturer specification commercially available Seiko Instruments polyacene polymer, button cell organic 6.84 17.6 12 — — 5 2.5 — manufacturer specification commercially available Pinnacle Research Institute bipolar pseudocap using mixed oxides (Ru, Ta) sulfuric acid 18 50.4 102 2 high 100 0.01 15k manufacturer test data custom order 46.8 144 <102 med theoretical lab projections Maxwell/ bipolar carbon/ KOH 4.32 7.2 0.1–0.2 1.7 med 28 12 6k engineering custom order Auburn metal composite organic 22 32.4 1.5 3 med 3 2,700 12.5k prototypes SAFT bipolar carbon organic 10.4 15.8 15 1.2 low 3 175 — engineering prototype custom order ARL bipolar hydrous RuO2 sulfuric acid 96 (active material only) 18.7 (active material only) — 10 high 5 2.72 34 lab cell — Livermore National Laboratory bipolar aerogel carbon particulate KOH 3.6 5.4 — — med 1 35 — lab cells — Sandia National Laboratory bipolar synthetic, activated carbon aqueous 5.0 6.1 0.35 1 med 1 3.5 — lab cells — Los Alamos National Laboratory bipolar conducting polymer on carbon solid organic 36–72 — — — low — — — theoretical lab projections — Technautics Hypercap bipolar pseudocap, Ag-anode, C-cathode Solid RbAg4I5 1.98 12.6 >1 — — 0.6 — — manufacturer test data custom order
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-23 Most Promising Component Technologies for Hybrid Systems Prime Source Intermediate Storage Unit Fueled system High power density rechargeable battery Battery Electrochemical capacitor Solar photovoltaic Regenerative fuel cell Nuclear Flywheel Metal-air battery Superconducting inductor The demand for electrical power in any system is rarely constant. Typically, the demand is cyclic, with the peak demand far exceeding the average power requirements. Because power sources rarely have both high specific energy and high specific power simultaneously, designers have typically designed power systems to meet the maximum demand to ensure adequate energy for the worst case. Thus, systems may be heavier than necessary, or planners may be forced to plan shorter missions or to resupply the primary energy sources. If the differences between the peak and average demands are large, it is advantageous to combine a high specific energy, low-specific-power source with a low-specific-energy, high specific power intermediate store to provide load leveling, which would meet the demand with substantial mass savings or with longer operational times for the same mass. Many combinations of energy sources and intermediate storage are in use today, such as portable x-ray machines, photoflash units, electric cars, and portable cardiac defibrillators. Usually these are battery-capacitor systems. However, the principle could be applied equally well to a number of prime source-intermediate storage technologies. The most promising component technologies for the dismounted soldier, are listed in Table C-23. Any combination of a primary power source and intermediate storage unit is capable of producing a power train suited to pulsed operation. A limited number of combinations are described here. The Army is already investing in solar photovoltaic-battery systems, which have been proven in combat. Fueled System and Battery Hybrid All fueled systems will probably be hybrids of one kind or another. Any fueled system operated in the battlefield environment will be subject to conditions under which it will be difficult or impossible to operate. Examples are submersion, extreme dust, and closed or confined spaces where exhaust fumes would be harmful to humans. Even in less extreme situations, a battery may still have to provide initial start-up for the fueled system. Depending on the climate and type of system, the battery may have to provide power for preheating the fuel or system, for initial pump power, or for control power.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-24 High Specific Power Batteries for Hybrid Systems Chemistry Current Status Future Specific Power (W/kg) Specific Energy (Wh/kg) Specific Power (W/kg) Specific Energy (Wh/kg) Present Cycle Life (Cycles) State of Development Nickel Aqueous 100–200 40–52 150–250 52–70 500–1,000 available Aqueous (future) 200–500 25–36 250–1,000 30–40 400–800 possible Bipolar — — 200–400 60–80 — under development Pb-acid Bipolar 200 25–45 — — 300 available Bipolar (future) — — 300 45–60 300 possible Thin foil 1,000 5 — — 300 under development Data described in the section on fueled systems indicate that they are five to ten times more energy dense than batteries for the same mission profile. Therefore, the fueled system, not the battery, provides practically all of the overall mission energy requirement. For the fueled system-battery combination, there are at least three battery chemistries that warrant further consideration: nickel-cadmium; lithium; and lead-acid. Tables C-24, C-25, and C-26 show the specific energy, specific power, and TABLE C-25 Commercial and Developmental High Specific Energy Batteries as Energy Sources in Hybrid Systems Chemistry Current Status Future Specific Power (W/kg) Specific Energy (Wh/kg) Specific Power (W/kg) Specific Energy (Wh/kg) Present Cycle Life (Cycles) Future Cycle Life (Cycles) State of Development Li-ion/CoO2 100 100 150 150 1,000 2,000 commercially available Li-ion/Mn2O4 70–100 70–100 150 150 300 600+ available soon Li(c)/polymer/Mn2O4 150 150 (est) 200+ 200 300 600+ prototype soon Li(c)/polymer/(CS)x 200a 200a 400 300 300 600+ prototype soon Li(c)/polymer/S 200b 400b 400 600 research research research phase a Prototype b Laboratory cells
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-26 Potential Fueled Systems for Hybrid Power Systems System Specific Power (W/kg)a Specific Energy (Wh/kg) Status PEM fuel cell 28 571 prototype Thermophotovoltaic 25 520 research phase Alkali-metal thermal-to-electrical converter (AMTEC) 16 1,040 research phase a Calculated for 2 kg of fuel. state of development of battery technologies that could be intermediate stores for a fueled-system battery hybrid (Arthur D. Little, 1996). As shown in the tables, a system consisting of an AMTEC and a 0.5 kg lithium-polymer battery would provide 5.3 kWh of energy, with peaks of 100 W, for a total mass of 5 kg. The battery pack could provide 100 Wh of energy without recharging. In some scenarios, this might correspond to an hour or more of operating capability. A lithium-polymer battery pack that could provide the total energy would have a mass of 26 kg. It is impossible to estimate the weight of the associated electronics and packaging that would be necessary to use this technology in a practical scenario. There is, however, almost a factor of five difference in mass (5 kg to 26 kg) for the same available energy. An AMTEC-NiCd system designed to perform the same functions would have a similar mass. The weight of a NiCd system for the total energy requirement alone would be on the order of 170 kg. The NiCd battery could meet the total energy demand for about 20 minutes, but the NiCd battery would have to be recharged more often than the lithium—polymer battery. In any case, the total energy available would be dominated by the energy in the fuel. It is assumed that the AMTEC is 20 percent efficient in converting the heat of combustion of JP-8 to usable electricity. For this system, the pulse time would be on the order of hours depending on the scenario. In general, a status monitor for the battery would determine its state-of-charge and command the fueled system to maintain an acceptable level automatically. The individual soldier would have override capability. In most scenarios, the fueled system could maintain an intermediate store at 90 percent or more most of the time. Battery and Electrochemical Capacitor A battery-capacitor combination for an energy storage system would exploit the high specific power of a capacitor and the high specific energy of a battery. For this system, the time scale of the peak power delivery intervals would
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Energy-Efficient Technologies for the Dismounted Soldier be shifted from the tens of minutes or hours required for the fueled-battery system to minutes or less for the battery-capacitor system. This combination offers a better system for brief pulses of high power, the kind anticipated for portable digital telephones. Meeting this requirement with a battery alone would require a battery that could provide high power pulses at 8 to 10 times normal capacity and would still have maximum life and adequate operational time between charges. Using a capacitor to meet the peak power requirement would provide better operating performance, longer battery life, and better low temperature operation while lowering life cycle costs and a smaller, lighter weight package. Figure C-13 shows a generic power-time profile for a pulsed digital communications system. In a recent paper, J.R. Miller (1996) developed a simple simulation of a 1 Ah lithium battery in parallel with an experimental electrochemical capacitor. The battery had an open circuit voltage of 4.1 V and an internal resistance of 0.1 ohms. The parameters assumed for the electrochemical capacitor were a capacitance of 1.28 F and an internal resistance of 0.069 ohms. For a repetitive pulse train of 8.3 ms at 10 A spaced by 90 ms, the battery alone was able to provide 12 minutes of operation. The battery-capacitor combination was able to power the system for 61 minutes, an improvement of roughly a factor of five. Simple circuit models were developed that can be used to predict the performance FIGURE C-13 Typical power-time profile for pulsed digital communications devices.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-27 Energy Storage Media That Could Be Used in Hybrid Systems Storage Media State of Art Energy Density (Wh/kg) Practical Limit to Specific Power (Wh/kg) Key Issues Scaling Laws Impact Storage Time Batteries highly developed 180–360 ~400 electrodes; electrolytes; seals; safety; corrosion known major/ enabling years Capacitor highly developed 0.25–1.00 ~8.00 molecular engineering of film; manufacturing technology; thermal stability; electrical breakdown known enabling for some systems concepts minutes Film Foil Paper Foil Ceramic highly developed ~0.30 > 3.00 large area samples; electrical breakdown; manufacturing technology known enabling for some systems concepts moderate Electrolytic highly developed < 0.5 >0.75 large surface area material; suitable oxides; electrolytes known minimal minutes Chemical double layer developing ~7.00 >12.00 large surface area materials; electrolytes; equivalent series resistance/equivalent parallel resistance; seals known major minutes Magnetic advanced > 15.00 strength of materials limited advanced composites; low resistivity materials known minimal milliseconds Inertial highly developed for some applications 100.00 > 300.00 high strength materials; gyroscopic effects; safety known minimal hours/days Thermal evolving sensible heat depends on dT absolute temperature dominated >5000 materials compatibility; high strength materials; high specific heat known uncertain days/weeks of battery-capacitor combinations accurately. In a similar experimental study, Merryman and Hall (1996) showed that the power train mass for an electrically actuated thrust vector control system for the space shuttle could be reduced by 59 percent when a battery-capacitor combination was used. Table C-27 is a compilation of the characteristics of energy storage media that could possibly be used in hybrid systems. (For completeness, the table includes some media not covered in the text.)
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Energy-Efficient Technologies for the Dismounted Soldier Key Research Issues Pulsed power techniques have been used extensively in the high-power regimes. Numerous laboratory demonstrations of hybrid systems, typical of systems appropriate for the dismounted soldier, have been performed. To date, there have been no field tests to determine the utility of using hybrid systems for human-portable power. To optimize the design, information on the power demand time history for a variety of mission profiles will be necessary. Given this data, a hybrid system can be designed for the worst case scenario that maximizes the available energy. The key issues are developmental and consist of: development of computer models for predicting performance as a function of mission profile development of laboratory prototypes obtaining reliable field data for the development of energy utilization profiles of the various soldier subsystems SUMMARY Table C-28 summarizes the energy and power systems discussed in this appendix. The development of hybrid systems with a fueled primary store would be revolutionary. However, each of the technologies described in Table C-28 has drawbacks. Primary batteries cannot provide the requisite energy for the projected energy budgets of dismounted soldier systems without becoming unstable and creating a significant safety hazard. Primary batteries also pose a significant environmental hazard that will probably increase as new chemistries become available. The primary hazards of batteries are explosive rupture, toxic and corrosive electrolytes, and environmental pollution if they are not recovered. Inevitably, trade-offs among safety, energy, and power considerations will have to be carefully assessed for any system or mission. A secondary battery with the specific energy and specific power of primary batteries would be highly desirable. If this technology were available, the environmental restrictions would be lessened because less frequent recycling would be required. Even a high specific energy rechargeable battery with limited life (say, 50 charge/discharge cycles) would greatly lessen the current problems of supply and disposal. Any system energetic enough to be considered a major advance for the Army will undoubtedly also be dangerous. Batteries are both energy storage systems and converters in the same unit, and battery safety is closely related to the oxidants and reductants. Consequently, if batteries are designed toward the margin, they have a tendency to explode.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE C-28 Technology Summary of Energy Systems Power System State of the Art Potential for Improvement Key Issues Scaling Laws Impact on Dismount Soldier Hostile Signature Suppression Potential Fuel Required Autonomy Time Primary battery Mature Moderate Energy density Safety Power density Environmental impact Known Longer mission Less weight Disposability Minimal Excellent None Hours/days Secondary battery Mature Moderate Energy density Cycle life Power density Known New capability Cost savings Less weight Minimal Excellent None Hours Thermophotovoltaics Emerging Excellent Requires cooling Efficiency Lifetime Ruggedization Uncertain New capability Cost savings Longer mission Thermal Moderate Multifuel Days/weeks Fuel cells (hydrogen) Exploratory development Excellent Fuel Water management Safety Known New capability Less weight Cost savings Thermal Excellent Hydrogen Days/weeks Fuel cells (methanol) Emerging Excellent Fuel and fuel crossover Catalyst Uncertain New capability Cost savings Less weight Thermal Excellent Methanol Days/weeks Alkali-metal thermal-to electrical converters Emerging Excellent Liquid metal Membranes Pumps/wicks Ruggedization Uncertain New capability Less weight Cost Savings Thermal Moderate Multifuel Days/weeks Nuclear isotope Limited Excellent Safety Environmental impact Cost Public acceptance Known New capability Autonomy Thermal Nuclear Moderate Special Month/years Internal combustion Some versions mature Moderate to excellent Fuels Vibration Life Uncertain Cost savings Less weight Thermal Acoustic Moderate Multifuel (Some Special) Days/weeks Microturbine Emerging Excellent Safety Uncertain New capability Acoustic Difficult Days/weeks Thermoelectric Some versions mature Moderate to excellent Efficiency Materials Coupling Known New capability Less weight Thermal Moderate Multifuel Days/weeks Human-powered Nonexistent Excellent Conversion mechanisms Unknown New capability Cost saving Autonomy Minimal Excellent Food Weeks
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Energy-Efficient Technologies for the Dismounted Soldier In fueled systems, the energy dense fuel is in a separate enclosure and is slowly exposed to the oxidant so that only the fuel that is in the converter at any given time is subject to inadvertent catastrophic failure. With the exception of hydrogen, all of the other fuels are rather involatile, that is, they can burn rapidly but will probably not explode. Fuels are housed in external tanks, which would be subject to penetration and burning if the penetration were energetic enough to ignite them. Primary batteries will be used in military systems for the foreseeable future. There will, however, continue to be problems associated with their disposal, inventory, safety, and availability, and wherever possible, they should be replaced. The logical evolution of the Army power system for the dismounted soldier is toward a rechargeable battery with improved specific power and energy that would meet or exceed the power available with current primary batteries coupled with a ''personal" charger that contains the primary store of energy for the mission. For many missions, the rechargeable battery alone would have enough energy. In those cases, the battery would be returned to the inventory after being recharged. For longer missions, the primary store would be fueled by a standard battlefield fuel. All of the fueled systems described in this appendix offer the possibility of long life with thousands of refuelings, and all of them are at a stage at which advanced development is possible. Coupled with a suitable rechargeable battery with similar cycle capability, these systems would dramatically reduce the inventory necessary to maintain combat readiness. The primary logistic consideration would be—as it is now—fuel supplies. Because batteries could be recharged many times, recycling after each mission would not be necessary, which would greatly reduce their adverse environmental impact. High specific energy rechargeable batteries are becoming increasingly important in the commercial sector, which could provide the Army with a secure, high volume, guaranteed source of batteries. "Smart" chargers and power management circuitry will also be forthcoming from the commercial sector. REFERENCES Army Materiel Command. 1992. Soldier as a System Symposium/Exposition. Proceedings of a symposium sponsored by the Army Material Command, June 30–July 1, 1992, Arlington, Virginia. Arthur D. Little, Inc. 1996. Proceedings of the Fourth International Conference on Power Requirements for Mobile Computing and Wireless Communications, Santa Clara, California, October 1996. Available from Giga Information Group, One Long water Circle, Norwell, Mass. 02061. Bass, J.C., N.B. Elsner, and F.A. Levath. 1994. The preliminary design of a 500 W thermoelectric generator. Pp. 586–591 in Proceedings of the 29th Intersociety Energy Conversion and Engineering Conference. AIAA-94-4197-CP. Reston, Virginia.: American Institute of Aeronautics and Astronautics. Benner, J.P., T.J. Coutts, and D.S. Ginley, eds. 1995. Proceedings of the Second NREL Conferences on the Thermophotovoltaic Generation of Electricity. AIP Conference Proceedings 358. Woodbury, N.Y.: AIP Press.
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Energy-Efficient Technologies for the Dismounted Soldier Florida Educational Seminar. 1996. Proceedings of the Sixth International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Boca Raton, Florida. Boca Raton: Paumanok Publications, Inc. Halpern, G. 1997. Personal communication from Gerald Halpern, NASA Jet Propulsion Laboratory, with M.F. Rose, member of the Committee on Electric Power for the Dismounted Soldier, January. IEEE (Institute of Electrical and Electronics Engineers). 1996. Proceedings of the 20th–25th Photovoltaics Specialists Conferences. Piscataway, N.J.: Institute of Electrical and Electronics Engineers. Ivanenok, J.F., R.K. Sievers, and T. Hunt. 1993. High Power Density AMTEC . Pp. 861–865 in Proceedings of the 28th Intersociety of Energy Conversion and Engineering Conference, Atlanta, Georgia. Reston, Virginia: American Institute of Aeronautics and Astronautics. Ivanenok, J.F., and T.H. Hunt. 1994. High voltage terrestrial AMTEC. Pp. 900–909 in Proceedings of the 29th Intersociety Energy Conversion and Engineering Conference, Monterey, Calif.. Paper no. AIAA-94-3903-CP. Reston, Virginia: American Institute of Aeronautics and Astronautics. Merryman, S.A., and D.K. Hall. 1996. Chemical double layer power source for electromechanical thrust vector control actuator. Journal of Propulsion and Power 12(1): 89–94. Miller, J.R. 1996. Battery-capacitor power source for digital communication applications: simulations using advanced electrochemical capacitors. Pp. 246–255 in Proceedings of the Symposium on Electrochemical Capacitors. F.M. Delnick and M. Tomkiewicz, eds. Proceedings Volume 95-29. Pennington, N.J.: Electrochemical Society. Morton, D. 1952. Human Locomotion and Body Form. Baltimore: The Williams and Wilkins Co. NTSE. 1992. Nuclear Technologies for Space Exploration. (NTSE '92). Proceedings of a conference held in Jackson Hole, Wyoming, August 16–19. 3 vols. Grange Park, Illinois: American Nuclear Society, Idaho Section. Raskovich, E., ed. 1993. Front End Analysis of Soldier Individual Power Systems. USA-BRDEC-TR//2541. Ft. Belvoir, Va.: Belvoir Research, Development and Engineering Center. May. Rose, M.F., C. Johnson, E. Owens, and B. Stevens. 1993. Limiting factors for carbon-based chemical double layer capacitors. Journal of Power Sources 47: 303–313. Rowe, D.M., ed. 1988. Proceedings of the First European Conference on Thermoelectrics. Stevengate, Hertsfordshire, U.K.: Peter Peregrinus, Ltd. Salkind, A.J. 1996. Estimate by A.J. Salkind, presented to the Battery Division of the Electrochemical Society at its annual meeting, San Antonio, Texas, October 6. Schuller, M. 1997. Personal communication from Dr. Michael Schuller, U.S. Air Force, Phillips Laboratory, to M.F. Rose, member of the Committee on Electric Power for the Dismounted Soldier. Space Power Institute. 1990. Mobile Battlefield Power Workshop. M.F. Rose, ed. Results of a workshop held in Durham, North Carolina, October 30–November 1, 1990. 2 vols. Sponsored by the Army Research Office, Contract No. DAAL03-86-D-001. Auburn, Alabama: Space Power Institute. Space Power Institute. 1992a. RTG Power Applications Workshop. M.F. Rose, ed. Results of a workshop held in Park City, Utah, March 22–25, 1992. Sponsored by the Army Research Office, the U.S. Department of Energy, and the Jet Propulsion Laboratory. Auburn, Alabama: Space Power Institute. Space Power Institute. 1992b. Prospector III: High Energy Density—High Power Density Power Sources RD Workshop. M.F. Rose, ed. Results of a workshop held in Auburn, Alabama, May 26–28, 1992. Sponsored by the Army Research Office. Auburn, Alabama: Space Power Institute. Space Power Institute. 1992c. Prospector IV: Small Engines and Their Applicability to the Soldier System Workshop. M.F. Rose, ed. Results of a workshop held in Durham, North Carolina, November 10–12, 1992. Sponsored by the Army Research Office. Auburn, Alabama: Space Power Institute.
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