This chapter summarizes key findings and provides the study recommendations. As requested by the statement of task, these include recommended priorities for investment in compact high-power and energy-dense source technologies for each of the regimes considered as well as specific recommendations in areas of hybrid systems, low-power electronics, power management and distribution, battlefield recharging, and predictive modeling.
The recommendations are presented in roughly the same order as requested by the task statement: beginning with findings in relevant technology areas (Recommendations 1-5), followed by findings for each of the power regimes (Recommendations 6-8), and ending with findings addressing specific topics (Recommendations 9-12). Recommendations 11 and 12 are considered overarching and are highlighted to show their overall importance.
POWER SOURCE TECHNOLOGIES
The committee formulated recommendations based on its evaluations of power source technologies in the regimes defined by the ARL/CECOM workshop. As detailed in Chapter 2, technology readiness levels (TRLs) were estimated for power solutions with the highest potential and then used as a basis for developing science and technology objectives and for making recommendations on technologies applicable to each target regime that are worthy of future Army investment.
Battery and Fuel-Cell Development
Batteries are the generic solution for soldier power. They will be an integral part of hybrid and stand-alone energy sources for the foreseeable future. The challenge is to make them smaller, lighter, cheaper, more energy-dense, more reliable, and with no sacrifice of safety. There is much commercial interest in achieving these ends, but these developments are designed for consumer electronics and are years away from being adapted as standards for the battlefield.
Recommendation 1: The Army should focus on batteries with a specific energy of 300 Wh/kg and higher for insertion into future versions of the Land Warrior (LW) ensemble. It should continue to promote and support innovative approaches to disposable and rechargeable batteries that can be adapted for military use. To select the best candidates for a given application, the Army should explore the trade-off space that exists between lifetime (measured in terms of charge-discharge cycles), specific power, specific energy, safety, and cost.
Fuel cells are the focus of intense interest by the military, primarily because of their potential as instantly “rechargeable” energy sources that can meet specific energy requirements for high electrical loads and long mission. Like metal-air batteries, fuel cells are air-breathing devices that cannot operate when submerged in water. Future acceptance of fuel cells on the battlefield will be determined to a great degree by logistics, because current prototypes are fueled by the nonstandard logistics fuels (methanol and hydrogen).
Recommendation 2: The Army should evaluate the applicability of small-scale, portable fuel processors capable of reforming the Army-standard fuels for use in proton exchange membrane (PEM) fuel cells or solid oxide fuel cells (SOFC). Scaling laws should be determined and cost/benefit analyses should be performed to determine whether there are power levels and/or mission durations that make such reformers an attractive alternative.
The Army must determine whether an alternative, nonstandard fuel source (such as methanol, hydrogen, or ammonia) is logistically acceptable. A proper analysis of trade-offs would permit decision makers to make an
informed judgment on whether the operational advantages outweigh added logistics complexity and costs. Ideally, this would include testing in line units (even if only at the squad level) under representative field conditions. It would also save the Army money otherwise invested in research on fueled system alternatives that do not make logistical or operational sense.
Recommendation 3: The Army should immediately conduct a comprehensive and definitive analysis of the operational and logistical implications of fielding nonbattery solutions as power sources for dismounted soldiers. This should include consideration of operational benefits, logistical limitations, and life-cycle costs, as well as considerations of safety and risk. It should develop models of competing energy sources, including fuel cell systems, and use them in simulations of battlefield operations. The data can then be combined with estimates of system costs to conduct cost/benefit analyses that would either support the consideration of non-standard-fueled fuel cell systems or eliminate them from consideration.
Several internal and external combustion engine prototypes have been demonstrated and show potential for military applications. Microturbine systems have not as yet demonstrated the capability to provide a net positive power output. Stirling engines use standard logistics fuel and could serve as a power source for battery rechargers or to meet anticipated requirements for high-demand microclimate cooling and exoskeletal applications. All small internal combustion engine systems now available have distinctive acoustic and heat signatures that would restrict their utility in combat. Stirling engines are inherently quiet but have significant thermal signatures.
Recommendation 4: The Army should adjust the focus of internal combustion engine development to demonstrate net power outputs and balance-of-plant systems appropriate to specific Army applications. Heavy emphasis should be placed on developing packaged systems with reduced heat and noise signatures. Once power output capabilities are demonstrated, the development should focus on improving system efficiencies.
Hybrid Power Systems
From a simple energetics point of view, hybrids offer enormous advantages for longer mission times. They also can provide a way to overcome the disadvantage of an air-breathing power source. Assuming that hybrids can be packaged to meet battlefield logistics and soldier operating requirements, they have the potential to replace batteries as the ultimate rechargeable energy source for soldier electronics. For fueled systems, efficient conversion at a modest 20 percent of the lower heating value (LHV) of the fuel leads to specific energy factors 2 to 5 times better than those of the best primary batteries.
Hybrids enable a system to be optimized for both high energy and high power demands. Some combinations, such as the battery-battery and battery-electrochemical capacitor hybrids, are air-independent and impervious to dust and moisture. Others that combine an air-breathing power source (e.g., metal-air battery, fuel cell, small engine) with a battery pose a problem for soldiers. To be acceptable, a fueled hybrid must be smart—that is, it must be capable of sensing and reacting to its environments so as to allow the unit to operate under water and to protect the unit from destruction.
Modeling is critical to the design of acceptable hybrid systems. The OFW-ATD program is collecting data to characterize Land Warrior power demands; it is possible that these data could also serve as a basis for modeling constructs to resolve soldier power issues.
Recommendation 5: The Army should refine duty-cycle estimates for the Land Warrior suite of electronics so as to enable the development of high-fidelity models incorporating soldier usage patterns and other details of interactions between power sources and soldier electronics. These estimates are essential for developing smart hybrid systems that can react to the environment for the future LW as well as for developing energy-efficient systems to meet unforeseen Army mission requirements.
Technologies for Target Regimes
While many commercial energy sources exist, they are motivated by a consumer market and are not developed in sizes commensurate with the broad spectrum of Army needs. The committee assessed technologies with high potential in each of the target regimes and determined science and technology (S&T) objectives for the near term (3-5 years), mid-term (5-10 years), and far term (beyond 10 years) based on a realistic appraisal of their current state of technology readiness. Table 7-1 lists the S&T objectives and indicates the relative risk (low, medium, or high) associated with each objective. Key research issues for each objective are enumerated in Chapter 2.
The committee was specifically requested in its task statement to select and prioritize power source alternatives in each of the target regimes.
20-W Average with 50-W Peak
Recommendation 6a: As its first priority in the 20-W target regime, the Army should support development of batteries with specific energies greater than 300 Wh/kg (e.g., Li/(CF)x, Li/S, Li/air, C/air) in sizes commensurate with LW requirements.
TABLE 7-1 Science and Technology Objectives for the Near Term, Mid-Term, and Far Term, in Three Power Regimes
Recommendation 6b: The Army should develop smart hybrid systems capable of air-independent operation and the 50-W peak load. These hybrid systems must be developed with the aid of duty-cycle analysis and modeling. Key to this is an evaluation of the limits of battery-battery hybrid system performance as well as methods for packaging or sealing air-breathing hybrid systems.
Recommendation 6c: If the Army determines that a nonstandard fuel source is acceptable for battlefield use by dismounted soldiers (see Recommendation 2 above), it should develop PEM and SOFCs as complete systems with the hydrogen storage or generation subsystem yielding at least 6 percent by weight hydrogen, including all components. In this context, the Army should investigate
methods of reforming methanol, ammonia, butane, and liquid hydrocarbon fuels and should evaluate whether the development of direct methanol fuel cell (DMFC) systems would be less complex than fuel-processing approaches.
Recommendation 6d: As a final priority in the 20-W regime, and for the far term, the Army should develop and evaluate small engines that operate on standard logistics fuels.
100-W Average with 200-W Peak
Recommendation 7a: As its first priority in the 100-W target regime, the Army should develop smart hybrid systems capable of air-independent operation that can accommodate total energy requirements. The emphasis should be placed on fueled systems (small engines, fuel cells) capable of operating on standard logistics fuels.
Recommendation 7b: The Army should support development of high-specific-energy batteries for niche applications, such as laser designators.
1- to 5-kW Average
Recommendation 8a: As its top priority in the 1- to 5-kW regime, the Army should continue to develop lightweight engines with high specific power that operate on standard logistics fuels. It should investigate Stirling engines, as they are fuel versatile and offer significant acoustic signature reduction.
Recommendation 8b: For the 1- to 5- kW regime, the Army should develop the ability to process standard logistics fuels as needed for emerging high-specific-power PEM and solid oxide fuel cells.
SOLDIER SYSTEM ELECTRONICS
Considering OFW as the third generation, the average power has been estimated at 20 W and the peaks at 60 W for three successive generations of LW electronics. Power savings made possible by technology improvements in later electronics designs, primarily in the computer processors, have been traded for improved combat effectiveness as well as to allow the use of plug-and-play architecture to support future evolution. While the desire for such flexibility is understood, the approach comes at a high energy cost and restricts the use of more energy-efficient design solutions.
Energy-Efficient Technologies (NRC, 1997) determined that a Land Warrior averaging only 2 W would be possible if commercial design approaches, including system-on-a-chip (SoC) technology, could be applied to developing the soldier system. Use of SoC design techniques could reduce power by more than one order of magnitude for the digital computing and communications processing, making consumption negligible in comparison to that of analog sensors and displays. There has been no effort in this direction in spite of the recommendations of that earlier study. Special efforts are also needed to reduce the power demand of analog portions of the OFW electronics, particularly the communications devices.
The OFW-ATD Program has until the end of 2004 to integrate and demonstrate the third-generation LW. The length of the program, especially the technology time horizon, is too short to allow developing a SoC solution. The program is also constrained to using off-the-shelf components and cannot take advantage of true spiral development in evolving the soldier system. Consequently, it is unable to build upon the early LW program and evolve a SoC to meet new needs and requirements. In the evolution of commercial cell phones, the SoC approach has allowed each generation to enhance subsequent generations, bringing new capabilities at consistently lower cost in power. The committee determined that such a system would be easily powered by batteries already available. The cost savings from using fewer batteries would easily pay for any increases in program costs.
Both the LW acquisition program and the OFW-ATD program rely on separate Army programs to develop and acquire the component electronics. These other programs do not have the incentive to develop or procure electronics using commercially proven design approaches to reduce energy consumption, such as were described in Chapter 5.
Incentive is the key here. The Army buys things from companies oriented to the defense market but has provided these companies with few or no incentives to develop energy-efficient products. The committee believes that neither the LW acquisition program nor the OFW-ATD programs are large enough or have long enough development horizons to deal effectively with power issues. Power is a long-term concern that is drowned out by the Army’s relatively near-term objectives.
As tempting as it may be for the Army to simply continue its use of outdated design techniques, a different strategy is required to design the equipment that the soldier must carry as compared with equipment that is carried on vehicles or other mobile or fixed platforms. Consider that there are important differences between what is required to design a smart cell phone and what was required to design an office telephone or a home computer. Just as cell phone users have special requirements, the soldier is a unique platform on which must be built a complex electronics system. For these reasons, it is important for the Army to increase its investment in Land Warrior electronics sufficient to begin a customized SoC approach to the development of these systems. In addition to reducing soldier energy needs, achieving energy efficiency for these electronics will resolve a myriad of problems now associated with the integration of disparate systems.
The Army acquisition system is impaired in its ability to focus on soldier power issues because it does not take into account the logistics costs of providing power on the battlefield when computing the true life-cycle costs of soldier electronics. The Army should take advantage of the new power-reduction designs and techniques that are well-known in commercial industry, especially in light of the high stakes involved for future soldiers on the battlefield.
Recommendation 9: The Army should make realistic estimates of the life-cycle cost, including reasonable logistics costs, of providing power on the battlefield and use such estimates in determining how much to invest in future Land Warrior design and development. Additional funding to extend the technology horizon of the program would enable a design solution that optimizes low-energy applications.
Power for Soldier Communications
Power and duty-cycle estimates for the LW soldier radio have not been refined for at least 5 years, even though communications technology has improved considerably. Wireless communications is the most power-hungry of soldier electronics applications and offers the potential for large reductions in energy requirements for the future warrior.
The importance of focusing on communications-electronics was emphasized in Energy-Efficient Technologies (NRC, 1997), but the Army has yet to pay attention. Five years later, the power performance planned by OFW-ATD for the Joint Tactical Radio System (JTRS) soldier radio is based on a rough equivalence with the MBITR radio, hardly the cutting edge of energy-efficient radios.
Power reduction has been given not nearly enough priority in the development of communications. Contracts that specify goals for power are not working. In fact, the added cost and risk of development serve as disincentives to reduce power demand. A thorough analysis of the communication solutions, mission scenarios, and resultant power demands is needed to determine if the power demand goals of the OFW program can be met.
Recommendation 10: The Army should make energy efficiency a first-order design parameter whenever specifying system performance parameters in its contracts. It should provide monetary incentives as needed to reduce power demand in all its procurements for soldier electronics, especially for communications.
The OFW focus on increasing combat effectiveness rather than saving energy encourages trading off power savings achieved for new electronics. With no net reduction in power, this focus can undermine the benefits of system-of-systems design and contribute further to the chasm that exists between the state of the art in consumer electronics and in Army electronics.
Table 7-2 summarizes areas that are key to improving energy awareness and reducing power demand within the Land Warrior system. The first column lists major components of the system, the second column lists techniques for improving energy awareness, and the third column shows improvements that could be realized by using a system approach to mitigating energy issues associated with just the communications and computation functions of the Land Warrior.
To make progress toward providing adequate power for soldiers on the battlefield, the Army must shift its focus from providing energy to reducing energy demand, and it must do the hard job of developing a realistic mission profile. Recommendations based on these findings are considered of overarching importance in successfully confronting the issues of soldier power.
TABLE 7-2 Techniques for Mitigating Energy Issues in Key Land Warrior System Components and Improvements That Could Be Realized
Future Warrior Goal
The Army envisions a future uniform-and-electronics ensemble for the LW of the future. The committee believes that soldier electronics requiring a mere 2 W average power, 5 W peak power is attainable in the far term if the recommendations of this study are fully implemented. Using a 200 Wh/kg battery, available within the next few years, such a system could operate continuously for about 100 hr in a 1 kg package. However, concepts for powering the reduced needs of future soldiers should take advantage of likely reductions in the scale and distribution of power demand and consider options such as energy-harvesting technologies to provide reliable power at such low levels.
Recommendation 11: The Army should aim for a future soldier system capable of no more than 2-W average power, 5-W peak power. Achieving this will free the soldier from worries about power shortages on the battlefield and greatly enhance combat effectiveness.
Determining Energy Needs
Modeling has the potential to be a tool that saves time and money in developing efficient portable electronic systems if accurate system input can be supplied. Modeling can complement experimental data as it narrows down the parameters of optimization. The data ultimately need to be verified with experimental data, but the modeling can expedite the selection of a power source. Ideally, the military should develop and acquire new equipment based on recommendations and considerations from power sources modeling in order to maximize the lifetime of the equipment.
Substantial power reduction can be achieved through management techniques that power down unused components. Additionally, the power dissipation of components in standby mode should be reduced as much as possible; this will become increasingly important as silicon technology continues to lead to increased leakage currents. Actual measurements of the varying loads (rather than crude duty-cycle guesses) will allow simulating the dynamic operation of LW electronics in concert with a power source simulator.
At the highest level of simulation, given a range of mission scenarios, a suite of soldier equipment, and the size and makeup of combat teams, the Army should be able to determine optimum types, quantities, and distribution of power sources (and their fuel and recharging requirements). At the lowest level, the Army should be able to perform comprehensive analysis of every element and subelement in the entire system. Such an analysis must extend all the way from the leakage, clocking structure, and power-down capabilities on individual chips to the duty cycle on the laser designator, display, or radio, and everything in between. Engineers and scientists who are well versed in all of the modern technologies for very low power SoC design need to be sought out and used in this important effort to characterize the soldier requirement.
Full simulations of OFW power sources and sinks would also help to determine the directions that developments must take to have the most impact on power. While models based on experimental data can be used to expedite the proper selection and matching of power sources, higher order models could be used in simulations to tailor soldier applications to the most likely soldier modes of interaction, thus reducing power requirements for computation and communication. The Army has ready access to high-performance computing resources that are capable of supporting such important tasks, and such simulations can go a long way toward improving energy efficiency in military electronics.
Recommendation 12: The Army should develop a modeling capability for soldier equipment that includes power sources and also enables detailed simulation, verification, and analysis of power requirements for given operational parameters.
Ensuring adequate power for soldier systems is by no means a simple problem; if it were, the Army would not have asked the National Academies to do this study. It is a multidimensional challenge, and the solutions are found by considering not only energy sources but also energy sinks and energy management. The good news is that solutions exist in all regimes to satisfy known power requirements, and major breakthroughs in power/energy source technologies are not needed. To satisfy the needs of future warriors on the battlefield, the Army must move power to the forefront of considerations in developing and acquiring soldier electronics, especially communications. It also must invest in the means to analyze power requirements so as to take advantage of reductions that can only be achieved by efficient power management.