Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 14
Meeting the Energy Needs of Future Warriors 2 Technology Alternatives This chapter discusses the results of the committee’s assessments of power source options for the three power regimes. It describes the assumptions about each regime, key terms and metrics, and standards for the selection of appropriate power source technologies in the context of 24- and 72-hr missions. The chapter concludes with committee findings on appropriate science and technology (S&T) objectives for the Army to undertake for the near, medium, and far terms. Detailed discussion of all technologies covered in this chapter, including the factors considered in assigning technology readiness levels (TRLs), sources, and references, are contained in Appendix D (Source Technologies). Appendix C defines measures of performance and other terms. The NRC report Energy-Efficient Technologies for the Dismounted Soldier (NRC, 1997) described and assessed most of the energy sources considered in the current study; readers who are not familiar with basic battery, fuel cell, and hybrid technologies are encouraged to use it as a reference. Table 2-1 provides an overview of all power source technologies considered in the current study and notes the technologies that have emerged since the earlier report was published. This chapter describes and compares power source alternatives for three power regimes for missions up to 72 hours. The regimes are 20-W average, 50-W peak; 100-W average, 200-W peak; and 1- to 5-kW average. Requirements data on all missions possible for each regime do not exist, so assumptions were made regarding likely applications and power demands in each regime. These assumptions enable power source options from the ARL/CECOM workshop to be compared with options and conclusions from the present study. The statement of task specifies average power requirements and periodic peak power requirements but not duty cycles. Since duty cycle will significantly affect the choice of individual or hybrid power sources, the committee makes quantitative comparisons between power source alternatives for the average loads mentioned above. Power sources that can accommodate the peak loads are discussed, but no quantitative comparisons are possible without duty-cycle details. Chapter 3 addresses in detail how peak loads can be handled and discusses their impact on the overall power system. ASSUMPTIONS The committee made certain assumptions about the electronics applications included within the three regimes called out in the statement of task. These assumptions are consistent with the baseline ARL/CECOM Workshop. The 20-W regime was assumed to cover power solutions for computers, radios, sensors, displays, and especially, the electronics subsystems of the Land Warrior (LW) ensemble. Table 2-2 lists devices now being considered for inclusion in the Objective Force Warrior-Advanced Technology Demonstration (OFW-ATD). The list does not include an evaporative cooler estimated to demand 10 W, which is assumed to require a separate power source in the near term. Power solutions in this regime, which currently uses logistics-intensive and costly disposable batteries, are of primary importance to the Army. The Army will continue to support a wide variety of electronics in this regime, and new power system solutions are urgently needed to enhance both logistics and combat effectiveness. The single application envisioned at the ARL/CECOM Workshop for the 100-W regime was a portable recharger for rechargeable batteries. The committee also considered solutions for the laser target designator—current versions of which demand an average 100 W (peak power up to 180 W)—and microclimate cooling to be in this regime. Finally, the committee assumed that the 1- to 5-kW regime includes power-intensive capabilities, such as the exoskeleton.
OCR for page 15
Meeting the Energy Needs of Future Warriors TABLE 2-1 Overview of All Power Source Alternatives Power System State of the Art, 1997a State of the Art, 2003 Item Considered Scaling Laws Impact on Soldier Power Primary battery (includes metal/air) Mature. Up to 800 Wh/kg in lowspecific-power configurations Mature. SOA not significantly advanced beyond NRC (1997) report. Energy density. Safety. Power density. Environmental impact. Known Heavy, one-time use. Current battery of choice for combat missions. Potential for use in hybrids. Secondary battery Mature. Li ion: 100 Wh/kg in development. Mature in commercial applications. Li ion: 140 Wh/kg available; 200 Wh/kg in development. Energy density. Cycle life. Power density. Safety and cost. Known Stand-alone energy supply for many missions. Can be used in hybrid mode for high-energy missions. Fuel cell (hydrogen) Exploratory development. Many systems at laboratory scale. Power levels to 150 W considered. Beta prototypes with various hydrogen sources tested in field. Power to 150 W. Fuel reformers. Water management. Safety. Known New capability; potential for use in hybrid system. Less weight. Cost savings. Requires new battlefield fuel. Fuel cell (methanol) Emerging. Not considered. Beta prototypes developed at power levels of 20 to 50 W. 20% efficiency. Fuel and fuel crossover. Catalyst. Cost. Known New capability. Less weight. Cost savings. Requires new battlefield fuel. Fuel cell (solid oxide) Emerging. Not considered. Emphasis on small sizes. Laboratory prototypes in 20-W range. Research in high-capacity designs. High temperature. Materials. Integration and systems. Known New capability. Less weight. Easier to utilize battlefield fuels. More efficient. Internal combustion Some versions mature. Hobby application sizes coupled to generators. No commercial products on market. Commercial applications with motor-alternator combinations in 30 to 100 W/kg range. Efficiencies greater than 20% in 500-W sizes. Emerging modified hobby engines operate on diesel. Fuels. Vibrations. Life. Known Inexpensive technology. Potential for high-energy missions. Can probably be made to function with JP fuels. Current role as battery charger. External combustion (includes Stirling) Not considered. 100 W/kg specific power demonstrated for motor-alternator with efficiency of 29%. System efficiencies projected to be >20%. Laboratory 35- to 50-W systems available for beta prototypes; 1- to 2-kW beta prototypes available with ~20% system efficiencies. System-specific power appears to be around 30 W/kg. Fuels. Specific power. System-specific energy. Signatures. Known New stealth capability. Inexpensive technology. Can be made to operate on JP fuels. Potential for high-energy missions. Microturbine Emerging. Considered promising. Not considered owing to lack of progress in producing workable systems. Fuels. Specific power. System-specific energy. Materials. Cost. Unknown
OCR for page 16
Meeting the Energy Needs of Future Warriors Power System State of the Art, 1997a State of the Art, 2003 Item Considered Scaling Laws Impact on Soldier Power Thermoelectric Some versions mature. Low potential. Best system efficiency on order of 5%; converter efficiencies projected to 10%. Insufficient progress to consider for current applications. Progress in new high-ZT materials makes technology worth watching for long term. Efficiency. Materials-specific power. System-specific energy. Known Not applicable owing to low efficiency. Possible niche application in small sizes. Thermo-photovoltaic (TPV) 20% TPV cells demonstrated. System projections to 20%. Not considered owing to lack of progress in systems. Known Nuclear isotope Limited consideration. Rejected owing to cost,safety,environmental considerations,and lack of infrastructure. Not considered. Safety. Environmental impact. Cost. Public acceptance. Known Alkali metal thermal-to-electric converter Speculative technology. Systems projection to 500 W/kg. Not considered owing to lack of progress. Known Energy harvesting; solar Some versions mature. Considered for low-capacity niche applications. Known Driver for reducing power demand. NOTE: SOA, state of the art; Li ion, lithium ion; JP, jet propellant; ZT, thermoelectric figure of merit. aNRC, 1997. FIGURES OF MERIT The governing figure of merit used to discriminate among and, in the final analysis, to rank-order the technologies was total system mass (as estimated from the specific energy of the underlying technology). Other figures of merit used to evaluate the technologies and systems are described in detail in Appendix C. The committee estimated technology readiness levels (TRLs) to determine the systems worthy of consideration. Definitions for the nine TRLs are also contained in Appendix C. It is important to note that technical figures of merit for many of the emerging power sources were not available, and in a few instances, the information was not considered reliable enough. Many of the technologies evaluated were in various states of development, and the committee made some assumptions about expected system characteristics and performance. These assumptions and/or extrapolations are documented so that the reader can better judge the relative merits and risks of the technology options. POWER SOURCE SOLUTIONS Batteries represent the ideal solution for soldier power and energy applications. Only when every effort has been made to conserve and manage energy and it is found that batteries cannot meet requirements should air-breathing systems, such as fuel cells or small engines, be considered. A heavy price is paid when these nonbattery options are used, including the requirement for continuous airflows, sensitivity to contaminants, temperature restrictions, possible orientation dependence, acoustic and thermal signatures, nonstandard fuels, surface and exhaust temperature, and exhaust gas contamination. That being said, there are mission requirements today for soldiers that exceed the reasonable capabilities of battery technology, and in these cases air-breathing alternatives are emerging to meet these needs. The rationale used to compare alternatives provided a framework for selecting power source options for the different power/energy regimes. The analyses considered all that is presently known about existing and emerging power source performance, and the technologies selected for further consideration by the Army are those that will most likely meet mission requirements with respect to specific power and specific energy. It is always possible that for particular missions, other factors, such as acoustic and thermal signature, operating temperature, fuel, orientation dependence, logistics, etc. will be more important than the specific power and specific energy of the system. Ultimately, the Army will
OCR for page 17
Meeting the Energy Needs of Future Warriors TABLE 2-2 Devices in 20-W Regime Planned for Objective Force Warrior (OFW)-Advanced Technology Demonstration Function Power Demanda (W) Communications Soldier radio 7.8 Squad radio 6.2 UAW/robotic vehicle 6 Computer displays Handheld flat panel 7.05 Helmet-mounted 0.5 Integrated sight—module display 3 Sensors 9.5 Computer 17.42 Total 57.97 aBreakdown of OFW numbers: • Soldier radio, 7.80 W (JTRS numbers are not available; assumed the same as Stryker MBITR radio); • Squad radio, 4.40 W (communications processor card) + 0.60 W (WLAN card) + 0.60 W (VoIP processor) + 0.60 W (WLAN antenna); • UAW/robotic vehicle, 3 to 10 W for como-crypto interface (Brower, 2003); • Handheld flat panel, 6.30 W + 0.75 W (handheld keyboard and cable); • Helmet-mounted display, 0.50 W; • Integrated sight display, 3 W (HIA module including breakaway connection to body PAN); • Sensors, 2.15 W (thermal weapons sight) + 1.10 W (daylight video sight) + 4.00 W (multifunction laser) + 1.50 W (GPS) + 0.25 W (dead reckoning module) + 0.50 W (microphone/speaker assembly); and • Computer, 2.10 W (computer assembly) + 10.9 W (computer processing card) + 3.42 W (PAN body hub) + 1.00 W (PAN weapon hub). NOTE: UAW, universal access workstation; JTRS, Joint Tactical Radio System; MBITR, multiband intra/inter team radio; WLAN, wireless local area network; VoIP, Voice over Internet Protocol; HIA, high integration actuator; PAN, primary area network; and GPS, Global Positioning System. SOURCE: Adapted from Erb, 2003, and Brower, 2003. need to make the final decision based on trade-offs to suit specific mission requirements. The statement of task includes peak power requirements for two of the three power regimes of interest. How peak power is handled by the energy source will depend on the duty cycle. In general, hybrid systems can enable high-efficiency operation over an entire power spectrum of operation provided that the requirement for a separate peak power source warrants the additional weight and volume. If a separate battery is chosen to meet the minimum and peak power demands, it must be capable of delivering the desired power and part of the total energy. The energy converter portion of the hybrid must provide the average power and all of the balance of the total energy. This includes energy sufficient to fully recharge the battery during the nonpeak or low-power operating portion of the duty cycle. There are three important issues that need to be addressed when making comparisons between figures of merit for the various power sources. The first is encountered when energy storage and energy conversion devices, e.g., batteries and fuel cells, are to be compared. Reasonable comparisons can be made if, and only if, the total energy content, including converter, fuel, and fuel tank, of the energy conversion device is compared with an energy storage device, such as a battery pack, having an equal amount of stored energy. In other words, the total energy produced by each system must be measured under the identical load conditions (power profile) to obtain an accurate comparison between the two. A second issue is related to technology maturity. For many emerging technologies, fully packaged systems are not available. The system dry weight, including the fuel tank, the quantity of fuel, the energy content of the fuel, and the energy conversion efficiency, are all needed to compute performance metrics. Efficiency data are available for some emerging technologies. If the quantity and energy content of the fuel are known, all that is needed is the dry weight of the optimized system for a specific mission requirement. Reasonable estimates of system dry weights can be inferred from breadboards, brassboards, prototypes, and commercial products. In the latter case, allowances need to be made to account for differences between commercial and military priorities, e.g., the weight may be unimportant to a commercial customer whereas it is critical to the soldier. Hence, the commercial product is not optimized for weight, and the specific energy of the technology may be underestimated. To make meaningful comparisons among alternatives, energy conversion system dry weights were estimated based on assumptions that are explained in Appendix D with references.
OCR for page 18
Meeting the Energy Needs of Future Warriors The third issue relates to battery comparisons. Performance specifications for batteries are given for specific cell sizes and discharge rates. The specific energy data quoted in this study are valid for the discharge rates under consideration. However, there will be a packaging penalty (weight and volume) for battery packs. For example, a lithium ion laptop computer battery may be in a square configuration 2.5-cm thick, but in reality there are eight cylindrical 18650-Li ion cells inside this package. The performance specifications of the 18650-cell should be discounted to account for this. A good rule of thumb is to deduct 15 percent from the cell performance figures. Fueled systems, which are in various stages of development, can be used to replace batteries or supplement them as part of a hybrid system. For systems supplying more than about 1 kWh, fueled systems offer a significant mass advantage over batteries. Figure 2-1, taken from the 1997 report, illustrates this point. It can be seen that the battery mass is directly proportional to mission energy requirement. In contrast, for fueled systems, mass comprises the fuel (including fuel tank) mass, which is a function of the mission energy requirement, and the energy converter mass, which is a function not of the mission energy requirement but of the mission power requirement. The y intercept in the figure is the dry mass of the energy converter and the slope is the product of the energy content of the fuel and the system energy conversion efficiency. These issues are explained in detail in the earlier report (NRC, 1997). ANALYSIS OF ALTERNATIVES The known performance of state-of-the-art lithium/ manganese dioxide and lithium/carbon monofluoride (Li/ MnO2 and Li/(CF)x) primary batteries and lithium ion (Li ion) rechargeable battery technologies was compared with that of promising energy conversion technologies. The interpretation of data for new technologies was intentionally conservative, with every effort made to use performance data obtained from completely packaged systems. In some cases, projections were made from subsystem data if system data were unavailable. Assumptions and references used to make these projections are documented in Appendix D. Low-TRL concepts (e.g., lithium/air, carbon/air) were not compared if too many assumptions were needed to predict system-level performance. Similarly, low-performance technologies—that is, those that were known not to exceed lithium battery performance—were not included in the analyses. Plots of total system mass including fuel versus 24- and 72-hr mission durations were developed for alternatives in each power regime (Figures 2-2 to 2-5). The corresponding numerical data are included in Tables 2-3 and 2-4. The technologies were rated on their ability to provide both average and peak powers for a given regime. The battery mass needed to produce the equivalent amount of energy was calculated from cell data. Because the typical 15 percent mass penalty for packaging these cells into battery packs was not included, battery performance is slightly overestimated in these charts FIGURE 2-1 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 5 W. PEMFC is proton exchange membrane fuel cell. SOURCE: NRC, 1997.
OCR for page 19
Meeting the Energy Needs of Future Warriors TABLE 2-3 Comparison of Soldier Power/Energy Sources for 20-W Average Power Missions of 24 and 72 Hours Technology Mission Length (hr) System Mass (kg) Power (W) Total Energy (Wh) Specific Energy (Wh/kg) TRL Stirling (JP-8) 24 0.82 20 480 588 2 PEM/H2 (5,000 psi) 24 1.15 30 720 626 7 PEM/NaBH4 24 1.16 20 480 414 6 Li/(CF)x (SOA, primary) 24 1.54 20 946 614 8 Li/MnO2 (SOA, primary) 24 1.71 20 480 280 8 IC (JP-8) 24 2.00 50 1,200 600 4 DMFC 24 2.04 20 480 235 6 Li/MnO2 (LW, primary) 24 2.46 20 480 195 9 SOFC (butane) 24 2.68 20 480 179 4 Li ion (SOA, rechargeable) 24 2.82 20 480 170 9 Li ion (LW, rechargeable) 24 3.31 20 480 145 9 Stirling (JP-8) 72 1.20 20 1,440 1,200 2 PEM/H2 (5,000 psi) 72 2.09 30 2,160 1,033 7 Li/(CF)x (SOA, primary) 72 2.35 20 1,440 614 9 PEM/NaBH4 72 2.59 20 1,440 556 6 IC (JP-8) 72 3.00 50 3,600 1,200 4 DMFC 72 3.01 20 1,440 478 6 SOFC (butane) 72 3.90 20 1,440 369 4 Li/MnO2 (SOA, primary) 72 5.14 20 1,440 280 8 Li/MnO2 (LW, primary) 72 7.37 20 1,440 195 9 Li ion (SOA, rechargeable) 72 8.47 20 1,440 170 8 Li ion (LW, rechargeable) 72 9.94 20 1,440 145 9 NOTES: Table is sorted by system mass. TRL, technology readiness level; JP-8, jet propellant 8; PEM, proton exchange membrane; SOA, state of the art; IC, internal combustion; DMFC, direct methanol fuel cell; LW, Land Warrior; SOFC, solid oxide fuel cell. TABLE 2-4 Comparison of Soldier Power/Energy Sources for 100-W Average Power Missions of 24 and 72 Hours Technology Mission Length (hr) System Mass (kg) Power (W) Total Energy (Wh) Specific Energy (Wh/kg) TRL IC (JP-8) 24 2.70 100 2,400 889 4 Stirling (JP-8) 24 4.00 100 2,400 600 2 SOFC (JP-8) 24 4.24 150 3,600 849 4 PEM/H2 6% 24 6.21 100 2,400 386 7 Li/(CF)x (SOA) 24 7.69 100 2,400 614 8 Li/MnO2 (SOA) 24 8.57 100 2,400 280 8 DMFC 24 11.40 150 3,600 316 6 Li/MnO2 (LW) 24 12.31 100 2,400 195 9 Li ion (SOA) 24 14.12 100 2,400 170 8 Li ion (LW) 24 16.55 100 2,400 145 9 IC (JP-8) 72 5.70 100 7,200 1,263 4 Stirling (JP-8) 72 6.00 100 7,200 1,200 2 SOFC (JP-8) 72 7.42 150 10,800 1,456 4 PEM/H2 6% 72 10.93 100 7,200 659 7 SOA Li/(CF)x 72 11.70 100 7,200 614 8 DMFC 72 18.60 150 10,800 581 6 Li/MnO2 (SOA) 72 25.71 100 7,200 280 8 Li/MnO2 (LW) 72 36.92 100 7,200 195 9 Li ion (SOA) 72 42.35 100 7,200 170 8 Li ion (LW) 72 49.66 100 7,200 145 9 NOTES: Table is sorted by system mass. TRL, technology readiness level; JP-8, jet propellant 8; IC, internal combustion; SOFC, solid oxide fuel cell; PEM, proton exchange membrane; SOA, state of the art; DMFC, direct methanol fuel cell; LW, Land Warrior.
OCR for page 20
Meeting the Energy Needs of Future Warriors FIGURE 2-2 24-hr mission at 20-W average power. FIGURE 2-3 72-hr mission at 20-W average power.
OCR for page 21
Meeting the Energy Needs of Future Warriors FIGURE 2-4 24-hr mission at 100-W average power. FIGURE 2-5 72-hr mission at 100-W average power.
OCR for page 22
Meeting the Energy Needs of Future Warriors for the state-of-the-art primary Li/MnO2 and Li/(CF)x and rechargeable Li ion batteries. The packaged LW batteries (Li/MnO2 and Li ion) are included as examples of the state of the art in the tables and charts. The committee determined science and technology objectives for Army investment in the near term (2010), medium term (2015), and far term (beyond 2015) based on the technologies selected and overall results of its comparison of alternatives in each regime. Far-term objectives are also discussed in Chapter 6. Energy per unit of system mass, i.e., specific energy, served as the primary basis for selecting technologies for the Army to pursue. To be considered a viable alternative, a technology had to have demonstrated a TRL that would allow the committee to estimate its performance in a power/energy source system. Because of this, the Army will need to conduct detailed trade studies (e.g., specific energy versus logistics, signature, cost) to determine if a selected power source technology is actually suited for a particular application and/or mission. The technical characteristics of the technologies that are documented in Appendix D to this report should facilitate such trade studies. Ideally, simulated models that incorporate equipment inventory, load profiles, mission duration, and environmental conditions should be used to determine the best overall power solution for a given application. 20-W Average Power Current power source development goals are listed in Table 2-5. The 12-hr goals are certainly achievable with primary batteries, but they are still a stretch for rechargeable cells. The committee believes that incremental improvements can be made in near-term programs to help rechargeable cells achieve the 12-hr mission goals. On the other hand, there is no battery capable of meeting the 72-hr mission goal. Batteries have shown a continuous, steady increase in energy density for the last 40 years, from about 30 to 300 Wh/kg. The possibilities for further improvement are good and need to be pursued aggressively, but the pace of improvement is likely to continue to be slow compared with other areas of technology development. Thus, in order for the Army to meet its 72-hr goal in the near term, it must consider investing in both long-term, relatively high-risk programs such as Li/air and shorter-term hybrid and nonbattery systems. Total system mass versus 24- and 72-hr mission lengths is plotted for a 20-W average power mission in Figures 2-2 and 2-3. Numerical data are presented in Table 2-3. The battery technologies chosen for comparison are primary Li/MnO2 (state-of-the-art (SOA) and LW versions), primary SOA Li/(CF)x (Eagle-Picher LCF-112, DD cell) and rechargeable lithium ion (SOA and LW versions). The data for proton exchange membrane/sodium borohydride (PEM/ NaBH4) (Lynntech) and direct methanol fuel cell (DMFC) (Ball) data are based on complete system demonstrations (TRL 6). The proton exchange membrane/hydrogen (PEM/ H2) data are based on a packaged 30-W system (Ball, TRL 7). The solid oxide fuel cell (SOFC) point is based on projections from breadboard testing of a butane-fueled system (AMI, TRL 4). The internal combustion (IC) (D-Star, TRL 3-4) and Stirling (Sunpower, TRL 1-2) points are projections based on demonstrated engine performance and balance-of-plant (BOP) estimates; for example, the Sunpower motor-alternator is at TRL 4, but lack of BOP technologies reduces the overall system TRL to 1-2. Modest gains over rechargeable batteries are achievable with some energy conversion systems for the 24-hr mission. While the Stirling engine appears to be the lightest option, further development is necessary to validate this point due to the relatively low TRL level for this system technology. The most attractive candidates that have been demonstrated at relatively high TRL levels are PEM/H2 systems. With the advent of small, lightweight stacks (Protonex), the performance could improve even further. It should be noted that the PEM/H2 data are for a 30-W system that includes a Li ion battery, which could be used for brief peak loads. This system has the highest specific energy for the 24-hr mission when one considers the total energy delivered. If hydrogen is unacceptable, DMFC could be developed further and could outperform primary batteries. Because of their modest specific power, nine Li/(CF)x cells will be required to meet the 20-W power demand. These nine cells provide the lightest battery option and will deliver 20 W for 47 hours, nearly twice as long as the other batteries or systems considered. Energy conversion systems become more attractive at the 72-hr mission length where there is a potential for reducing the total mass by a factor of 4 or 5. They are all TABLE 2-5 Power Source Development Goals for Soldier Systems Load (W) (average/peak) Mission Time of 12 hr Mission Time of 72 hr Required Weight (kg) 20/50 240a 1,440a 1.0 100/200 300a 1,800a 4.0 aThese numbers are calculated specific energy in Wh/kg. SOURCE: Pellegrino, 2003.
OCR for page 23
Meeting the Energy Needs of Future Warriors significantly better than rechargeable lithium ion and, with development, will be much better than primary Li/MnO2. A caveat is that if these systems are being used as part of an overall hybrid system (battery plus additional electronics), additional weight and volume will have to be added to the total system mass calculation. This will be the case for air-independent operation and for duty cycles that cannot be accommodated by the prime power source. Depending on the duty cycle, a lithium ion battery or capacitor can handle the 50-W peak requirement. The PEM and DMFC demonstration data depicted in Figure 2-3 show much better performance than batteries for the 72-hr mission. There are opportunities for significant mass reduction in both of these systems, which will make these comparisons even better. It should be noted that these systems are more complex than batteries alone and require that attention be paid to such things as start-up, fuel and oxidant control, water management, shutdown, and storage below freezing. The PEM system data shown in Figures 2-2 and 2-3 represent fully packaged systems. Other hydrogen sources for PEM should be evaluated in the context of a complete system as they become available. In general, any hydrogen storage or generation concept that yields over 6 percent hydrogen storage based on the total mass of the system should be assessed for Army use. One interesting hydrogen generation alternative is ammonia cracking. Ammonia can be stored at reasonable pressures, thus reducing the mass of the storage container and valve. The reformate is a mixture of hydrogen and nitrogen and does not contain the sulfur and carbon monoxide contaminants found in hydrocarbon reformate. To yield pure hydrogen with no nitrogen dilution, ammonia reforming would be implemented with a hydrogen separation membrane, which would reject the nitrogen. These systems are under current development (MesoFuel). Recent work in small hydrocarbon reformers (Altex) shows promise for integration with small fuel cells. These systems are better suited for integration with SOFCs as the relatively high temperature of the reformate is more compatible with SOFC than with PEM stacks. SOFCs and small engines have the advantage of operating on energy-dense hydrocarbon fuels that are readily available in the field, but SOFCs are more sensitive to contaminants in the fuel than engines. While SOFC is probably the least developed of these technologies in this power range, significant weight reductions are likely to be achieved in the future. It should be noted that the SOFC point is based on a breadboard system with a catalytic partial oxidation (CPOX) reformer. SOFC has a much lower acoustic signature than IC engines but is less rugged and will require accommodation for shock and vibration. This has been achieved for the packaged PEM and DMFC systems constructed by Ball Aerospace, and similar techniques could be applied to SOFC systems. AMI has demonstrated that full power can be obtained for a 20-W SOFC stack in less than 3 minutes, which is a major achievement for this technology. And, unlike large fuel cells (greater than 1 kW), these small versions can be stopped and started multiple times without detriment (see Appendix D). MEMS advances are likely to play a role in the future development of fuel cell and other power systems. They will enable the miniaturization of balance-of-plant (BOP) components, including integrated fuel processors, for many of the conventionally fabricated systems now under consideration. The contribution of MEMS to BOP is more critical to soldier power solutions than its contribution as a prime generator of energy. The mass projected for small engines considered in this regime required some speculation on BOP components and scaling laws, with more assumptions for the Stirling data than for the IC data. Note that the D-STAR engine is a 50-W device, so that while it is heavier than some of the other options, the specific energy is among the highest (Table 2-3) because more energy is produced in a given time. Both IC and Stirling engines are capable of operation on logistics fuels—that is, fuels (such as JP-8) that are readily available in bulk on the battlefield. D-STAR has made considerable progress on suppressing acoustic signature, which is a potential drawback for this technology. Stirling engines with minimal acoustic signature have been demonstrated. Findings for 20-W Average Power Regime Energy conversion systems are somewhat better than batteries for the 24-hr mission but are more complex to operate. However, mission length is easily extended by fuel addition, and the longer the mission the more competitive energy conversion systems become. An added benefit is the ability to continue a mission (provided enough fuel is left) even if resupply is not forth-coming in 24 hours. Energy conversion systems are currently one fourth as massive as rechargeable batteries for longer missions (72+ hours). From a logistics perspective, prepackaged fuels can be treated as battery packs as long as appropriate safety and handling procedures are developed. Such prepackaged fuels will enhance the attractiveness of fueled energy conversion alternatives. Some energy conversion technologies in the 20-W power range may be capable of operating on bulk logistics fuels. Logistics issues are as important as performance in determining which power source to use for soldier systems. Science and Technology Objectives Science and technology (S&T) objectives consistent with the committee’s selection of alternatives in the 20-W regime are listed below. The objectives are listed in order of
OCR for page 24
Meeting the Energy Needs of Future Warriors importance, along with the key development issues to be resolved. There are eight near-term objectives: Develop batteries for the 24-hr mission with specific energies greater than 300 Wh/kg. Key development issues: low-cost, safe routes for the synthesis of carbon monofluoride (CF)x; custom electrode formulation and materials optimization to support higher rates with minimum impact on specific energy; lithium sulfur (Li/S) rechargeable battery (see Appendix D). Develop smart hybrid systems with batteries and energy conversion power sources. Key issues: predictive models of sink demands; duty cycles; air-breathing and air-independent operational modes. Develop generic modeling capabilities. Key issues: model materials properties through system integration; model steady-state and transient power source/sink behavior; establish control algorithms that optimize energy use. Develop BOP components for small fuel-cell systems. Key issues: reduce parasitic power, reduce size and weight, increase reliability, decrease costs. Develop small fuel processors for logistics fuel, methanol, ammonia, and other viable fuels. Key issues: thermal management; coking; sulfur removal; gas stream cleanup; start-up and load following; shutdown; packaging; interfaces with energy converter. Develop and field-test PEM/H2 systems. Key issues: logistics impact of packaging hydrogen fuel; greater than 6 percent hydrogen storage/generation technologies; high-performance stacks. Develop and field-test DMFC hybrid systems. Key issues: logistics impact of packaging methanol fuel; low-crossover membranes; low catalyst loadings; high-activity catalysts; low-temperature storage and start-up. Conduct battlefield-relevant safety testing of alternative solutions (H2, MeOH, ammonia, JP-8, and Li batteries). There are two mid-term objectives: Develop rapid start-up, compact SOFC operating on low-sulfur logistics fuel or surrogates. Key issues: coke formation both inside and outside the stack; metal corrosion; integration with fuel processors; sulfur tolerance and/or removal. Develop complete small IC and Stirling engine systems with low signatures operating on JP-8 or diesel fuels. Key issues: bearings; BOP technologies—for example, fuel vaporization/atomization, control, acoustic and thermal signatures. There are, as well, three far-term objectives: Develop high-specific-energy, air-breathing battery systems. Key issues: validate advanced battery concepts such as Li/air (TRL 3) and C/air (TRL 2); test and evaluate thermally self-sustaining C/air systems. Develop microelectromechanical system (MEMS) components for power technologies. Key issues: evaluate impact of incorporating MEMS in power systems; establish performance metrics and cost analysis for MEMS-based components; integrate MEMS components with conventionally fabricated components in complete systems. Develop SOFC systems that operate on high-sulfur fuels. 100-W Average Power Total system mass for a 100-W average power mission is plotted against 24- and 72-hr mission lengths in Figures 2-4 and 2-5, respectively. The numerical data are given in Table 2-4. The battery technologies chosen for comparison are identical to those chosen for the 20-W case. The DMFC (Giner, TRL 7) data are based on a 150-W packaged system that has not been optimized for low mass. In addition, the DMFC produces 10,800 Wh (150 W for 72 hr) at 17 percent efficiency. To produce the same number of watt-hours as the comparable technologies in the figure—that is, 7,200 Wh—would require 3.4 kg less fuel. A significantly lower mass for this system should be possible considering that a 100-W system would be lighter and that the efficiency of this system is not state of the art. A 30 percent total system efficiency has been demonstrated for DMFC by others (Ball, 20-W DMFC). The PEM/H2 6 percent data are derived from the performance of a packaged, field-tested 100-W system (Ball, PPS100) and assume a 6 percent hydrogen storage/generation system. (Hydrogen stored at 5,000 psi with a safety factor of 2.25 will yield 6 percent hydrogen based on total mass.) As in the 20-W case, the IC data (projection based on scaling of the DSTAR 50-W engine/generator set, TRL 4) and the Stirling data (projection based on scaling of a Sunpower engine/generator set, TRL 1-2) are based on assumptions about the BOP items and scaling laws. All of the energy conversion systems perform better than rechargeable batteries for the 24-hr mission at 100 W average power. Energy converters that employ JP-8 fuels (the SOFC and the IC and Stirling engines) are the best performers and are significantly less massive than even the best primary batteries in this regime. For the 72-hr mission, energy conversion systems are an attractive alternative to batteries and could offer fivefold to tenfold mass reductions. As stated previously, there are significant operational constraints when using these air-
OCR for page 25
Meeting the Energy Needs of Future Warriors breathing systems. From a systems perspective, the most mature technologies are PEM and DMFC, which require hydrogen (PEM) or methanol (DMFC). The advantage of the SOFC and the engine systems is their ability to operate on logistics fuels. SOFC offers the potential for the quietest system of the three operating on JP-8; however, some fuel processing is required before the fuel-cell stack. Small hydrocarbon fuel reformers under development (Altex) could be integrated with the SOFC, or integral CPOX reactors could be used. The latter have been demonstrated with butane (AMI). Small engines can utilize JP-8 type fuels directly; their minimal impact on logistics makes them very attractive. The reliability of these small engines remains to be determined. Peak power in these systems could be provided by the prime power source, depending on the duty cycle. If the latter is too demanding, capacitors or lithium ion batteries could supplement the prime power source for the 200-W peak. Findings for 100-W Average Power Regime Fueled systems become more attractive as power demand increases. Li ion batteries have the rate capability to power laser designators. Energy conversion technologies can reduce significantly the mass of 100-W systems that operate for 24 hours or longer. JP-8-fueled systems appear to weigh the least; however, these systems are not yet at high TRLs, and they may be more massive, have higher thermal and acoustic signatures, and be less reliable than other options. Non-JP-8-fueled systems offer significant performance advantages over batteries without some of the compromises of JP-8-fueled systems; however, the logistics burden will be greater to supply these fuels. Science and Technology Objectives S&T objectives consistent with the committee’s selection of alternatives in the 100-W regime are listed below. The objectives are listed in priority order, along with key development issues to be resolved. The three near-term objectives are these: Develop smart hybrid systems with fuel cells and high-power batteries or electrochemical capacitors. Develop small fuel processors for logistics fuels, methanol, ammonia, and other viable fuels. Key issues: thermal management, coking, sulfur removal, gas stream cleanup, start-up and load following, shutdown, packaging, and interfaces with energy converter. Evaluate DMFC and PEM systems for various specific missions. Key issues: modeling the capability of these systems with respect to loads, mission profiles, and operational and logistical constraints; overcoming technical issues as mentioned above. There are two mid-term objectives: Develop small engines. Key issues: balance-of-plant (fuel delivery, vaporization, atomization, control system); integrating and packaging complete systems for field evaluation; validating performance scaling laws; assessing reliability and failure modes. Develop solid oxide fuel cells. Key issues: fuel processing, sulfur tolerance. The sole far-term objective is this: Develop high-specific-energy, air-breathing batteries. Key issues: validating advanced battery concepts such as Li/air (TRL 3) and C/air (TRL 2); testing and evaluating thermally self-sustaining C/air systems; increasing the rate capability of Li/air by a factor between 5 and 10. 1- to 5-kW Average Power System mass versus total energy is plotted in Figure 2-6 for three power source systems of TRL 9:0.9 kW, 1.2 kW, and 2 kW. The plot was normalized to total energy delivered as these power sources have different power ratings. The conclusions below apply to power levels up to 5 kW. The Honda gasoline generator and the Mechron 2-kW diesel generator are commercial products (TRL 9). The latter is the generator supplied by the Project Manager for Mobile Electric Power (PMMEP) to the Army. The Ballard Nexa 1.2-kW PEM fuel cell is being packaged as a commercial product (TRL 9). A 70 percent efficient diesel fuel reformer and a 37 percent efficient fuel cell system were assumed in order to generate the Nexa fuel cell plot. Mass was not estimated for the reformer, because no data were available. This reformer mass would need to be added to the points for the PEM fuel cell in order to obtain the mass for the total system. While the mass of the reformer is unknown, it would have to be 44 kg to result in a system having the same mass as the 2-kW diesel unit. It is anticipated that a reformer capable of supplying sufficient hydrogen to a 1-kW PEM fuel cell would have much less mass than this. The PEM fuel cell with an efficient diesel reformer could have an overall efficiency of 26 percent (0.7 × 0.37). A system has not yet been demonstrated, so the Stirling engine was not included in Figure 2-6. Sunpower offers a 1-kW prototype Stirling engine having a dry mass of 32 kg,
OCR for page 26
Meeting the Energy Needs of Future Warriors FIGURE 2-6 System mass versus total energy. but this does not include the circulating cooling system or the burner and other ancillaries. An efficiency of 28 percent is claimed for thermal energy in to electricity out. Stirling is inherently low in acoustic signature (less than 65 dBa at 1 meter) but has two sources of thermal signature. The motor-alternator must be kept at a rejection temperature on the order of 100°C, which is comparable to the temperature of operation of PEM fuel cells. The second source of thermal signature is the exhaust gas from the combustion process, which is likely to be hotter. The temperature is determined by the thermal recuperator and whatever mitigation scheme can be employed to further cool the exhaust gas. Diesel and gasoline generators in this regime are highly developed (TRL 9). The Honda generator (19 percent efficient) has excellent performance and is light, but it operates on gasoline and has a significant acoustic signature (59 dBa at rated load). The Mechron 2-kW generator (16 percent efficient) in current use by the Army operates on diesel fuel and has a dry mass of 56 kg and an acoustic signature of <77 dBa at 7 meters. Specifications are readily available on the company Web sites. Findings for the 1- to 5-kW Average Power Regime Fuel cells offer lower signatures than IC engines. Stirling engines offer potentially low thermal and acoustic signatures. Science and Technology Objectives S&T objectives consistent with the committee’s selection of alternatives in the 1-5-kW regime are listed below: Near-term objectives. (1) Develop lightweight, efficient, 1- to 5-kW engines that operate on logistics fuel (key issues: tribology, reliability, integrating combustion sources with Stirling engines, and reducing system mass) and (2) develop lightweight logistics fuel reformers. Mid-term objective. Integrate logistics fuel reformers with lightweight PEM fuel cells. Far-term objective. Develop SOFCs. Integrate logistics fuel reformers with SOFCs.
Representative terms from entire chapter: