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Energy-Efficient Technologies for the Dismounted Soldier 3 Energy Sources and Systems The highly mobile and automated Army of the future will rely on an increasing variety of "smart" ordnance and equipment. Technological sophistication, however, will be accompanied by the need for advanced energy technology formatted for particular users. Battery type, fuel type, autonomy time, absolute power, reliability, commonality, and the cost of power systems will shape the battlefield as much as advances in communications, computing, sensing, lethality of weapons, and protection. Numerous studies have recognized the need for major advances in battlefield power technology (Space Power Institute, 1990, 1992a, 1992b, 1994). Mobility for the dismounted soldier demands energy storage density in a compact package that can satisfy power requirements of up to 100 W. In addition there is at least one requirement for more than 100 W to support a microclimate cooling suit that would ensure a modicum of comfort under biological, chemical, or nuclear attack. New energy sources and systems must meet these requirements without increasing the vulnerability of the dismounted soldier to detection by the growing array of sophisticated sensors on the battlefield. The basic types of military energy systems have not changed appreciably since the Second World War. The motor generator and the battery are still the primary sources of energy on the battlefield. Batteries are the workhorse of energy storage for the dismounted soldier and have improved steadily through the years. But they have reached the point at which improvements of more than a factor or two in most critical parameters cannot be made safely. Using new materials and chemistries, batteries are approaching explosives in terms of energy density. Hence, safety is now an issue from the perspectives of both storage and usage. Other long-standing issues related to using batteries in large-scale military operations are cost, reliability, maintenance, and availability. Equipment for the dismounted soldier must be both compact and rugged. Availability of fuel, specific energy, specific power, minimal signature (electronic, thermal and acoustic), simplicity of operation, and environmental impact are also major considerations. The mass a soldier can carry on a mission is already approaching its limits forcing a trade-off of bullets or food for batteries to power electronics.
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Energy-Efficient Technologies for the Dismounted Soldier Advanced power technology is multidisciplinary. The ultimate utility of a particular technology may depend not only on the fundamentals of the device itself, but also on the electrode technology or the availability of the fuel needed to power it. As a consequence, the Army is exploring ways to manage and increase mission times utilizing whatever technology can be invented. Any reduction in energy demand for the same capability immediately translates into increased mission time for the same mass or the same mission time with reduced mass. The Army can optimize its use of energy and power by: employing more energy-dense power sources using systems optimized for minimal energy and power requirements maximizing the use of available energy (e.g., use of automated controllers and chargers that carefully meter energy and place noncritical functions in sleep mode while maintaining power for critical functions) To date, fueled energy systems are low in specific energy and specific power except when they are used for long run times when fuel mass dominates system mass. Scaling to small sizes is only now being understood. Recent advances in fueled technologies, such as proton exchange membrane (PEM) fuel cells, thermophotovoltaics, alkali-metal thermal-to-electrical converters (AMTEC), and microturbines, appear to be capable of producing higher specific energy and specific power than the fueled systems currently in use (motor generator sets). Furthermore, these technologies appear to maintain their favorable characteristics when they are scaled to small sizes. A technology that can be deployed in the field rarely has the same capabilities as laboratory prototypes or theoretical models. Some obstacles are fundamental, but some can be overcome through appropriate research and development (R&D), innovations, and skillful engineering. The power level of a fueled system is determined solely by the converter. The specific power of a converter decreases with the addition of fuel, in direct contrast to a battery, in which the specific power stays constant as energy is added. The use of a fueled system does not eliminate the need for intermediate storage because fueled systems cannot operate when they are submerged unless they carry a stored oxidant. The intermediate store must also be rechargeable. Of the numerous choices for intermediate storage, only the ones most relevant to the dismounted soldier are discussed in this report. Figure 3-1 lists the range of storage media available for consideration within the framework of this study. Note the enormous difference between energy sources based on the chemical bond and those associated with the nucleus. Although there are political, safety, and technological barriers to using nuclear energy to provide or store power for the dismounted soldier, the committee could not ignore its vast potential. Human- powered systems used by some foreign armies and by U.S. special forces were also considered.
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Energy-Efficient Technologies for the Dismounted Soldier FIGURE 3-1 Specific energy and specific power for various energy storage media. In general, the higher the specific power, the lower the specific energy. Note that 1 MJ/kg is equivalent to 277 Wh/kg. The committee investigated some technologies that are not discussed in this report because they would be inconsistent with soldier mobility requirements and thus not good candidates to support future dismounted soldier systems. For example, the large masses associated with containers and cyrogens needed for inductive energy storage (possibly including superconductors) would restrict soldier operations. Similarly, thermal storage and conversion of waste heat, while technically feasible, would require "phase change material," which would add to the mass burden. Detailed descriptions of the energy and power systems surveyed in this chapter are contained in Appendix C. Table 3-1 summarizes their characteristics. For each system, the table includes assessments of the state of the art, the potential for improvement, key issues, scaling laws, the impact on dismounted soldier operations, hostile signature and suppression potential, fuel requirements, and autonomy time. Every system included in Table 3-1 has some drawbacks. For example, recent advances in the development of hybrid power systems with a fueled system
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Energy-Efficient Technologies for the Dismounted Soldier TABLE 3-1 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 Thermophoto voltaics 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 Special 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 for primary energy storage could be revolutionary for the dismounted soldier but may require alternative (nonlogistic) fuels for use on the battlefield. Primary batteries cannot provide the requisite mission energy for the energy budgets projected today without creating a significant safety hazard. Primary batteries also pose a significant environmental hazard, which is likely to increase as newer chemistries become available. The main hazards for batteries are explosive rupture, toxic and corrosive electrolytes, and environmental pollution (if they are not recovered). Inevitably, there will be trade-offs among safety, energy, and power, which must be carefully considered for each system and mission. It would be highly desirable for soldiers to have a secondary battery with the specific energy and specific power of current primary batteries. This would lessen environmental impact because the need to recycle them would be less frequent. Even a high specific energy rechargeable battery with limited life, say 50 charge/discharge cycles, would greatly relieve supply problems and ease the problem of ultimate 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 units. Batteries require that the oxidant and reductant be in close proximity. In fueled systems, the energy dense fuel is in a separate enclosure. Except for fueled systems that use hydrogen as a fuel, they use rather involatile fuels, which can burn rapidly but will probably not explode. The fuels themselves are housed in external tanks, which could be subject to penetration and subsequent burning if the penetrant were energetic enough to cause ignition. Primary batteries will be used in military systems for the foreseeable future, but they will also continue to present problems with disposal, inventory, safety, and availability. Wherever possible, the Army will replace them. The dismounted soldier will probably ultimately use rechargeable batteries that have higher specific power and energy (which would match or exceed those available from current primary batteries) coupled with "personal" charges. The rechargeable batteries would have enough lifetime for many missions and could be returned to the inventory after recharging. For long missions, the primary store, which is envisioned as fueled storage, could be replenished with fuel, preferably, but not necessarily, a battlefield fuel. All of the fueled systems described in this chapter have the potential for long life and thousands of refuelings. With the exception of microturbines, all of them are at a stage at which advanced development is possible. Coupled with a suitable rechargeable battery with similar cycle capability, these fueled systems would reduce the inventory necessary to maintain readiness. The primary logistical concern would become fuel. The environmental impact associated with the disposal of batteries would also be greatly reduced because fueled systems would not have to be recycled after each mission. High specific energy rechargeable batteries are also important to commercial industry. It is possible, therefore, that the military battery will become
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Energy-Efficient Technologies for the Dismounted Soldier more readily available, guaranteed by the commercial uses of the same technology. Smart chargers, ''fuel gauges," and power management techniques will also be forthcoming from the commercial sector. Thus, commercial off-the-shelf (COTS) systems on the battlefield could become the norm rather than the exception. ALTERNATIVE TECHNOLOGIES Primary and alternative technologies for meeting power requirements below 100 W are discussed in the following sections. Combined with the detailed descriptions contained in Appendix C, this survey provides a basis for an engineering database of the most promising technologies. Sections include rechargeable batteries, fueled systems, nuclear energy sources, human-powered systems, photovoltaic technology, thermophotovoltaics, electrochemical capacitors, and hybrid systems. Rechargeable Batteries Although battery technology is relatively mature (Cairns, 1992; McLarnon and Cairns, 1989), evolutionary changes in practical batteries are likely to continue, even in the long term. In the future, batteries will be safer, last longer, and be predominantly rechargeable. The specific energy of rechargeable batteries will more than double and will approach or exceed the specific energy of the best primary batteries. Lithium chemistries are the most energetic and will continue to be pursued in both the commercial and military sectors (Arthur D. Little, Inc., 1996; Megahed et al., 1994). Rechargeable lithium polymer batteries are being actively developed by both sectors because they promise both high specific energy and high specific power. Laboratory prototypes have specific energy on the order of 200 Wh/kg and specific power on the order of 200 W/kg. The growth potential in practical features for these technologies is a factor of two or more, but safety will be an issue. The number of charge-discharge cycles for the laboratory prototypes is approximately 300, with growth potential to 600 or more. The environmental issues associated with lithium chemistry will remain, however, and prices will be high as long as the civilian market for them is small. Coupled with a charger system, rechargeable lithium batteries offer enormous potential for the Army. Major improvements (more than 20 percent) in the battery and hybrid systems discussed in this chapter can be achieved by improving processing technology, active material composition and morphology, reinforcing components, electrolytes, and key components that limit cycle life and cycle rate, such as separators.
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Energy-Efficient Technologies for the Dismounted Soldier Fueled Systems Fueled systems, which are in various stages of development, have the potential to replace or augment batteries in the Army inventory. For energy requirements greater than about 1 kWh, fueled systems offer a clear mass advantage (see Appendix C). Their fuel costs are several orders of magnitude lower than those of the current primary batteries, and they are less harmful to the environment. In addition, fueled systems are reusable and can generate continuous power with refueling. Using small fueled systems may require a new battlefield fuel, such as hydrogen, methanol, propane or natural gas. Using hydrogen or methanol would eliminate or drastically reduce the need for primary batteries and would also produce drinkable water. Fuel Cells Enormous progress has been made in the development of small fuel cells that can replace batteries in some applications (Courtesy Associates, 1990, 1992, 1994, 1996; Kinoshita and Cairns, 1994, Kinoshita et al., 1988; Gottesfeld et al., 1995). Fifty-watt hydrogen PEM fuel cells with a form factor the same as that of the Army BA-5590 battery have been built and demonstrated. Recently, a similar fuel cell was built to fit in the battery cavity of a standard Army field radio. In addition to silent operation and minimal thermal signature, this fuel cell is free of the logistics chain associated with primary batteries because it uses hydrogen, a fuel that can be obtained directly or indirectly from many source materials. The principal by-product of hydrogen fuel cells is water. Several demonstrators with power levels of 50 to 300 W are being evaluated in the field. Figure 3-2 illustrates the relationships between fuel cells and batteries. The fuel cell is not a viable candidate in terms of specific energy for mission energies less than about 0.75 kWh. For these low-energy missions, the battery is still preferable because of the irreducible mass penalty for the fuel cell converter. However, most missions described by the Army for the Land Warrior ensemble require energy greater than 1 kWh. For very high energy missions, fuel cells can provide an order of magnitude improvement over the comparable mass of batteries because only the mass of the fuel expended needs to be added to increase the available energy. The fuel cell technology shown in Figure 3-2 is state of the art. Expected improvements in fuel storage and the introduction of methanol fuel cells promise to increase the specific energy of fuel cells. The single most important problem with the deployment of fuel cells is the need for hydrogen as a battlefield fuel. The decision to use a new fuel on the battlefield will depend on the savings in the reduction of, or elimination of, the need for primary batteries, which now perform the same function. There are several ways to produce hydrogen for battlefield use. Reformers are available for the JP fuels, methane, propane, butane, and natural gas, all common, readily
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Energy-Efficient Technologies for the Dismounted Soldier FIGURE 3-2 Graph showing the "crossover" points for battery and fuel cell power systems as functions of available energy and system mass. The assumed system power level is 50 W. PEMFC is proton exchange membrane fuel cell. obtainable fuels from commercial sources. Hydrogen can be stored in hydrides and produced by chemical reaction. It can also be stored either as a compressed gas in high pressure tanks or as a liquid at cryo temperatures. The development of advanced fuel cells that oxidize methanol directly is progressing. Current efforts are focused on the development of membrane materials that will dramatically reduce the tendency for methanol to cross the membrane (fuel crossover decreases efficiency and limits the life of the device). The power density of direct methanol fuel cells is lower than optimal for military applications. Excellent progress is being made toward solving both of these problems. In terms of technology demonstration, direct methanol fuel cells are three to five years behind the hydrogen-fueled PEM fuel cells. Current laboratory cells are approximately 35 percent efficient and have demonstrated modest life. Efficiencies as high as 45 percent are projected. Methanol, which is only about one-sixth as energy dense as hydrogen, also produces drinkable water and carbon dioxide gas. Methanol is liquid at room temperature and can be readily manufactured from coal, natural gas, and wood by-products. The current cost of methanol is about 40 cents per gallon.
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Energy-Efficient Technologies for the Dismounted Soldier Heat Engines and Electromechanical Energy Converters Options for converting the energy stored in fuels to electricity include fuel cells, thermoelectric and thermophotovoltaic sources, and heat engines with electromechanical energy converters. Of these options, conventional heat engines represent the most mature technology, with high converter operating efficiencies and power densities (Space Power Institute, 1992b). An unconventional microturbine system in the very early stages of development also holds significant promise (Tan et al., 1997). Heat engines can be classified in a number of ways, but perhaps the distinction between internal and external combustion engines is the most appropriate discriminator for the dismounted soldier. Both convert the heat of combustion of an appropriate fuel to mechanical energy in the form of a rotating or oscillating shaft. This mechanical energy can, in principle, be converted to electrical energy by either rotating or reciprocating electrical generators with magnetic or electric fields. In sizes appropriate for the dismounted soldier (50 to 250 W), internal combustion engines are the most mature heat engine technology. The impulsive nature of thermodynamic energy conversion, however, leads to noise and vibration problems, as well as difficulty in reliably restarting on demand. Efforts are under way to produce a microturbine technology by applying microfabrication technology to the development of a gas microturbine that can be used as a compressor or as a driver for an alternator. This project, which is based on fabrication techniques for silicon-carbide microelectronics, promises economical microturboalternators with high specific power. By 2015, prototype microturbines machined from silicon may have been developed, which will provide 1,000 Wh in a package weighing as little as 0.25 kg. If this high risk effort is successful, it would provide extremely attractive specific power and energy for the dismounted soldier. Potential problems associated with heat engines include difficulties of starting, thermal and acoustic signatures, vibration, the generation of toxic or hazardous combustion products, and the inability to operate in all orientations or to operate under water (without air). Thermoelectric Conversion Thermoelectrics technology is mature and has been used in numerous space applications for power and a broad array of applications for cooling, in both the civil and military sectors. Thermoelectric generators use the Peltier effect to produce electricity from any heat source (Rowe, 1988). The efficiency of these devices is determined by the temperature of the heat source, the rejection temperature, and the materials comprising the thermoelectric elements. Thermoelectrics is a mature and proven technology, and thermoelectric devices are extremely reliable
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Energy-Efficient Technologies for the Dismounted Soldier (years in space), have few moving parts, and are inherently silent. At present, the maximum efficiency attainable from thermoelectrics is on the order of 8 to 9 percent in a laboratory device. The efficiency is less than 5 percent in practical situations. Advanced programs are researching new materials that could improve the efficiency for practical power systems to 10 percent. The commercial infrastructure is in place to manufacture and market large thermoelectric units for remote applications. The maximum power levels for these units are on the order of 100 W. Thermoelectric power systems are capable of using standard Army multifuel. A 500 W unit was projected to weigh 20 kg unfueled and to have an efficiency on the order of 9 percent (Bass et al., 1994). Although thermoelectric power systems are silent, they do have substantial thermal signatures. Alkali-Metal Thermal-to-Electrical Energy Converter (AMTEC) The "sodium heat engine," or AMTEC, is capable of converting thermal energy from any heat source to electricity with estimated efficiencies as high as 35 percent (Ivanenok and Hunt, 1994). In the last decade, much progress has been made in the development of the relevant materials technology and in understanding the basic physics of single cells. Serious efforts have been made worldwide to reduce the technology to practice. Civilian applications in automobiles, self-powered home gas appliances, and space power have been investigated. The most interest is being shown at present in using AMTEC for deep space probes where the heat source is nuclear. The principle of energy conversion used in the AMTEC is independent of the heat source and, because modest laboratory efficiencies have been obtained, it is a good candidate for Army applications in the 50 to 500 W range. Power sources using AMTEC are multifuel-capable, and scaling to small sizes is reasonably well understood. Like all fueled systems that are combustion driven, AMTEC ejects the thermal energy that is not converted at a relatively high temperature. AMTEC cells use liquid sodium in the energy conversion process. The amount per cell is less than one gram and is usually confined to a wick structure. NASA has chosen AMTEC as the conversion mechanism for the "Fast Pluto Flyby" spacecraft power system. In laboratory cells, reliability has been demonstrated to thousands of hours, and laboratory efficiencies of 20 percent have been recorded. At the single cell level, AMTEC converters are well understood. The primary outstanding technical issues are: long-term materials degradation and poisoning of the alkali-metal loop techniques for effectively and efficiently making parallel and series arrays that minimize heat loss efficient external liquid combustion and recuperation in small systems system demonstrators
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Energy-Efficient Technologies for the Dismounted Soldier Nuclear Energy Sources Nuclear energy sources are capable of more than 1,000 times the energy density of energy sources based on chemical bonds (see Figure 3-1) (Space Power Institute, 1992c). However, releasing nuclear energy in a controlled way is extremely difficult. Because of possible health and environmental hazards, the problems inherent in using nuclear materials on a battlefield are formidable and could only be addressed in a large and expensive program. This is unfortunate because deep space probes using nuclear sources have demonstrated lifetime and reliability characteristics far exceeding those of other energy sources. Nuclear power sources could extend autonomy time to months or years instead of hours. Nuclear isotopes offer the most potential for Army applications. The energy density of radioisotopes is enormous, and they are extremely cost effective per watt-hour delivered. The fundamental physics of isotope-powered systems, however, dictate that the production of energy by an isotope cannot be turned on and off; once the isotope is manufactured, it begins decaying. This circumstance places interesting constraints on the shelf life and fuel infrastructure necessary to maintain readiness. Isotope systems are extremely reliable and have been the system of choice for deep space probes. (Indeed, they have made them possible.) Except for a few isotopes, massive shielding is necessary to protect biological systems that must survive in close proximity to the devices. Several small units designed for miniprobes of the lunar and Martian surfaces may in fact be of interest to the terrestrial power community. These innovative designs may be mass and volume efficient because fuel would not have to be encapsulated to survive inadvertent reentry. Other innovations in insulation, such as a proposed modified radioisotope thermal generator (MOD-RTG) for space applications, can also provide important mass savings. Miniature heat engines, such as Stirling, AMTEC, and thermophotovoltaics (TPV), some with efficiencies as high as 30 percent, could be used terrestrially with the possibility of repair and replacement. High efficiency reduces the need for nuclear materials. Power systems based on nuclear isotopes have been niche technologies confined primarily to space probes, underwater power systems, and use in remote terrestrial locations. Nevertheless, there is a long list of potential applications for isotope systems if sufficient material were available at a reasonable cost and the public bias against the use of nuclear materials could be overcome. The best systems have specific power on the order of 8 W/kg and specific energy greater than 100,000 Wh/kg. Although the specific energy of a nuclear source is phenomenal, the specific power is poor. Efficient thermal converters, such as the AMTEC, could improve specific power. "Self-powered chips," in which the power levels are relatively small and the amount of radioactivity per chip is as low as a home "smoke detector," are also being investigated. Outstanding technical issues deal with system studies rather than fundamental research. The Army should keep informed of developments in nuclear
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Energy-Efficient Technologies for the Dismounted Soldier technologies, including high efficiency converters and fuel technologies and "beta voltaic" systems intended for integration on microchips. Human-Powered Systems Using human powered systems to meet Army power needs may appear to be new and innovative, but human-power has been used for electrical/mechanical systems for decades (Starner, 1996). The hand-cranked portable generator used by Army Special Forces is a good example. Another example is the small "flashlight," which is energized by squeezing a lever. For purely mechanical conversion, the Apollo astronauts took with them to the moon a rotary shaver that had a small flywheel energy store, which was activated by pulling a cord. It is possible to generate up to 100 W in this fashion. Devices of this type are not passive, and they effectively immobilize the individual while power is being generated. Except for the references cited in Appendix C, there does not appear to be any research aimed at exploiting the energy associated with body motion by converting it to electricity. It is possible through sophisticated energy management and through low power electronics to reduce the dismounted soldier's demand for energy to a level at which it may be possible for the soldier to produce enough electrical power to provide a substantial amount of the electrical energy he or she needs. This would require converting to electricity some of the energy expended by the soldier during everyday activities. The human body stores an enormous amount of energy. The average person consumes between 2,000 and 3,000 calories per day, which is the equivalent of approximately 2,200 to 3,300 Wh. Clearly the amount of energy consumed by an individual is sufficient to power electronic devices if a suitable method could be found for converting a small fraction of that energy to electricity. Moving the limb and striking the heel while walking or running are potential sources of power, as long as the energy requirement is only a few watts. But physical activity is intermittent, so there would have to be a storage mechanism. Rechargeable batteries, electrochemical capacitors, pneumatics, springs, and flywheels are candidates discussed in Appendix C. And conversion of human power to electricity would still require a generator of some sort. Although the idea is intriguing, at present it is impossible to estimate system performance in units such as Wh/kg and W/kg. This is an area for further research. Photovoltaic Technology Like many of the technologies discussed in this report, photovoltaics are mature in many applications and have been used in the commercial domain for many years (IEEE, 1996). Specialized space applications use more efficient cells
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Energy-Efficient Technologies for the Dismounted Soldier than those widely available on a commercial basis. But the cost of these cells is orders of magnitude higher than the cost of cells generally available. Photovoltaics are also already being used by the Army. At the earth's surface, the power incident from the sun is on the order of one kilowatt per square meter of surface. Conversion at a modest efficiency rate makes this a major energy source that is "there for the taking." The successful harvesting of solar energy depends on the development of a viable, affordable photovoltaic cell technology. In general, costs in dollars per watt have precluded large scale commercial exploitation, although the U.S. Department of Energy has funded large demonstration projects capable of producing megawatts of electrical power. One problem with photovoltaic technology is that the systems can produce power only in daylight and only on clear days. Furthermore, for optimal production, units must "track" the sun. The Army currently uses solar battery chargers in desert operations. These arrays can be folded and can produce enough power to charge several batteries. Several thousand solar battery chargers were used in the Desert Storm operation. Planar arrays with specific power as high as 60 W/kg have been produced for space applications. In general, the conversion efficiency is a function of the cell type and ranges from 10 to 30 percent. Systems studies of photovoltaic chargers could lead to the development of personal chargers, in appropriately sunny climates, that would function like fueled systems. The following issues are still outstanding: bandgap tailored photovoltaics that could function with both artificial light sources and sunlight manufacturing technology that would reduce costs innovative system demonstrators Thermophotovoltaics Recent advances in the technology associated with TPV (thermophotovoltaics) suggest that power systems can be built that range from a few watts of power to more than 500 W (Benner et al., 1994, 1995). Improvements in photovoltaics and emitters, in terms of reliability, size, weight, and energy efficiency, will translate immediately into increased capability and, perhaps, lower cost. TPV is a multidisciplinary technology. For example, solid-state converters must be combined with a radiant element, which is heated from a fossil-fueled combustion source. Recovering the energy remaining in combustion gases is vital for efficiency. The utility of TPV may depend not on the fundamentals of the device itself, but on whether it can be mass produced from affordable materials, whether it can be made robust and reliable enough to function in a hostile environment, whether it can be engineered into a package with minimal signature, and whether it will enhance the capability of soldiers in the field.
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Energy-Efficient Technologies for the Dismounted Soldier The emphasis to date has been on demonstrating capability, materials, and processes for laboratory devices. Packaging, which is only now being addressed, should clarify many of the obstacles that must be overcome before devices that can be put in the field are possible. Very little attention has been paid so far to demonstrating full systems or establishing engineering parameters, such as figures of merit. Performance specifications and the range of parameters for each component are not known. Efforts to establish an infrastructure are emerging. Major potential applications in the military are auxiliary power units (APUs), battery chargers, and direct battery replacement. TPV technology is available to build systems with efficiency values on the order of 10 percent, and the most optimistic projections for efficiency values are on the order of 30 percent. Until more emphasis is placed on recuperation, however, it is not possible to predict the specific power with any degree of confidence. But specific powers greater than 100 W/kg appear to be reasonable. Electrochemical Capacitors Traditional capacitors, in general, have high specific powers but are incapable of high specific energies. Conversely, batteries have high specific energies but are incapable of extremely high specific powers. For many applications, a power source must have the best attributes of both, that is, high specific energy and high specific power. In recent years, classes of devices called "electrochemical capacitors" have emerged that have some attributes of both batteries and capacitors (Florida Educational Seminars, 1996). In specific power and specific energy, they fall between classical batteries and capacitors. They have specific energies on the order of 10 to 20 percent of "good" batteries, and specific powers at least an order of magnitude better than conventional batteries. Compared with conventional capacitors, they have specific power an order of magnitude lower but specific energy an order of magnitude higher. Although commercial electrochemical capacitors are already on the market, none to date has the requisite internal parameters appropriate for the communications systems envisioned in this report. With successful development of the technology, energy use may become more efficient; pulsed digital communications can reduce the demand for energy while increasing the life of a primary battery or the time between recharges for a secondary battery. Laboratory prototypes of electrochemical capacitors that can meet many of the criteria for battlefield use have been produced, but they are handmade, and the technology for mass producing them with acceptable and reproducible results has yet to be developed. The technological issues that must be resolved before electrochemical capacitors can be successfully integrated into the electronic devices envisioned for the future Army are listed below: understanding the physical phenomena limiting the specific energy, specific power, internal series resistance, internal parallel resistance, degradation mechanisms, temperature dependent phenomena, and the productive life time of electrochemical capacitors
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Energy-Efficient Technologies for the Dismounted Soldier development of a series of laboratory prototypes for evaluation in hybrid power systems development of high-voltage electrolytes development of low cost materials for use in both chemical double layer capacitors and pseudocapacitors Hybrid Systems In the future, hybrid power sources are likely to be the "technology of choice" for providing power and energy for the Army. Hybrid systems combine the advantages of very high specific energy sources capable of maintaining the base load with very high specific power sources capable of providing peak power when needed. This configuration will greatly enhance power and energy capabilities and will require small portable package. Hybrid systems can be used for everything from personal battery chargers to replacements for conventional field generators. Digitization places special demands on the power systems envisioned for the dismounted soldier. The best digital transmission is pulsed, requiring higher peak powers in times of battle. The demand for energy is cyclic. But it is the exception rather than the rule for a power source to have both high specific energy and high specific power simultaneously. To ensure that adequate energy is available for the worst case, power system designers typically size systems to meet the maximum demand. As a result, either systems are heavier than necessary or mission planners may be forced to shorten missions or resupply the prime energy source for a given mission. If there are large differences between peak and average demands, it is advantageous to combine a high specific energy, low-specific-power source with a low-specific-energy, high specific power intermediate store. This "load leveling" would enable the soldier to meet the demand with substantial mass savings or longer operational times for the same mass. Many possible hybrid systems are discussed in detail in Appendix C. In general, fueled systems with rechargeable batteries for the intermediate store are extremely promising. The fueled system could provide long-term average power with a high specific energy rechargeable battery, thus providing higher power levels for hours at a time. Lithium polymer batteries coupled with fuel cells appear to be a particularly attractive combination, although any of the combinations discussed in Appendix C would be acceptable. On a shorter time scale, a hybrid system would have to provide short bursts of power for pulsed communications or digital image transmission. 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. With this combination, the intervals between peak power deliveries would be reduced from
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Energy-Efficient Technologies for the Dismounted Soldier tens of minutes for the fueled-battery system to minutes or less for the battery-capacitor combination. As described in Appendix C, the addition of a capacitor to provide peak power improves performance, increases battery life, and improves low temperature operation while lowering life cycle costs and requiring a smaller, lighter package. POWER FOR MICROCLIMATE COOLING In the future, the dismounted soldier may have to function in environments that pose serious biological or chemical risks. The approach to personal protection in these environments is to provide the soldier with an overgarment that presents an impermeable barrier to biological and chemical agents. But an impermeable barrier will also impede the evaporation of perspiration, thereby severely limiting the amount of heat that can be removed naturally from the body. Overgarment protection leads to intense heat stress with even modest activity. A soldier's metabolic heat generation is roughly 100 W at rest and as much as 1,000 W during an endurance march. Therefore, an impermeable overgarment will require some mechanism for removing heat. One solution is to provide the garment with supplemental cooling powered by an external energy store. A minimal level of cooling requires approximately 300 to 400 W. Numerous technologies have been investigated for the cooling system (Raskovich, 1993). The most promising seems to be a vapor compression/expansion cycle similar to the one used in conventional air conditioning and refrigeration. Cooling based on thermoelectrics is inefficient, costly, and heavy. Improvement by a factor of two or more in thermoelectrics will be necessary before thermoelectric coolers will be competitive with vapor cycle methods. For a modest coefficient of performance for the cooling system, the compressor electrical power needs to be in the range of 100 to 150 W. Microclimate cooling is the most demanding power requirement for the dismounted soldier. There are three options for delivering energy. The first is heat engines that can directly drive the compressor in a vapor cycle. Heat engines in the form of small internal and external combustion devices have been investigated as a way to provide mechanical energy directly to a compressor. Internal combustion devices could be used, but they have acoustic and thermal signatures and produce significant vibrations. Internal combustion motors that have been investigated were taken from the hobby industry or from commercial garden tools. Laboratory prototypes have had limited success. The Stirling engine, an external combustion device, is quieter than an internal combustion engine, modestly efficient, multifuel capable, and relatively inexpensive. The primary drawback of the Stirling motor is low power density. The primary advantages of Stirling engines are reliability, low acoustic signature, lower thermal signature, and multifuel capability. A Stirling duplex cooler/generator under construction is designed to produce 100 W of electric power and 300 W of cooling power. The device is in the laboratory stages, and the
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Energy-Efficient Technologies for the Dismounted Soldier goal is to produce a device with a mass of 9 kg. The laboratory version is projected to be on the order of 13.5 kg. The second option is to use the electrical energy from a prime source to drive a motor/compressor for the vapor cycle. The fueled systems described in Appendix C appear to be the preferred method of providing the necessary 100 to 150 W of compressor power. Assuming that maximum cooling is needed for six hours, the energy required would be 0.9 kWh for a 150 W electrical demand. A PEM fuel cell point design is being constructed and evaluated that incorporates an oxidizer to prevent fuel cell contamination by the chemicals. The projected mass for this design is 4.5 kg, not including the motor/compressor and distribution system for the coolant. Doubling the fuel load would increase the time to 12 hours and increase the mass by 1 kg. A third option is batteries that could drive the motor/compressor. Current primary batteries (with 150 Wh/kg specific energy) would have a mass of about 6 kg for a 0.9 kWh (six hour) mission and approximately 12 kg for a 1.8 kWh (12 hour) mission. Technology Forecast In the near term, demonstrators capable of powering the microclimate suit will be produced and evaluated on the laboratory scale. Significant improvements in the mass of the system may be possible with new materials and innovative designs. The mass of the system for 12 hours of cooling should be less than 10 kg. In the long term, advanced thermoelectrics should make the thermoelectric cooler competitive with vapor cycle coolers. Thermoelectric coolers would have a minimum number of moving parts, generate little acoustic and thermal signature, and would be easily repairable. Power systems to drive the thermoelectric arrays will continue to improve and become much less of a factor in the total system mass. Fuel cells and microturbines are the most likely candidates for powering thermoelectric microclimate coolers. Key Research Issues Microclimate cooling presents the most challenging power problem for the dismounted soldier. As mission times increase, the mass of battery powered systems quickly escalates to values not compatible with the soldier's load. Fueled systems offer the best solution to the problem. Outstanding issues are: development of high specific power, high specific energy fueled systems development of lightweight motor/compressor components development of new thermoelectric materials with greater efficiency, lower cost, and lower mass improved fuel storage and utilization
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Energy-Efficient Technologies for the Dismounted Soldier FINDINGS Fueled hybrid systems promise revolutionary advances in providing energy to the dismounted soldier, which can dramatically improve logistics and extend mission times. Several fueled energy sources with greater than an order of magnitude improvement in specific energy (compared to primary batteries) are in laboratory demonstration phases and represent low to moderate risk. Fueled systems must be environmentally robust, be able to operate independently of orientation, use common battlefield fuels where possible, be submersible, and operate automatically. Fueled systems will not eliminate the need for a secondary storage unit. Except for special cases, fueled systems will be hybrids consisting of a fueled converter, a rechargeable battery, and a second electrochemical store in the form of a high specific power battery or capacitor. Hybrid systems can optimize both power and energy requirements. No order-of-magnitude improvements in batteries are anticipated. High specific power, high specific energy, secondary batteries, and electrochemical capacitors will be essential for future dismounted soldier systems. Missions of more than 1 kWh will require nonbattery power systems/sources or recharging. Electrochemical capacitors are key elements in hybrid systems. The Army Communications-Electronics Command (CECOM), Army Research Office (ARO), the Army Research Laboratory (ARL), and DARPA have been active in establishing the feasibility of fueled systems. It is time to move from laboratory components to field trials. The survey of technologies in this study provides a good basis for an engineering database of promising energy sources and systems technologies. In addition to tracking technology development, a database could be used to develop high fidelity computer models of power/energy systems for use in dismounted soldier simulations. The models could take into account soldier systems parameters as well as load profiles for realistic battlefield scenarios. Given this modeling capability, both conceptual and laboratory prototypes of power systems could be built and evaluated.
Representative terms from entire chapter: