Soldier requirements for power are changing as fast as new electronics are being developed. In addition to communications and computers, a myriad of applications for the dismounted soldier of the future will require portable energy, including such things as laser-designators, chemical-biological sensors, uniform ventilators, and exoskeletal enhancements. This report assesses power/energy sources, low-power electronics, and power management technologies and provides recommendations on energy solutions for the future soldier. The committee focused on realistic energy alternatives, concentrating on the energy source technologies about which enough data were available to support likely system concepts and to estimate essential system parameters. The report builds on technology assessments documented in Appendix D and in a previous NRC report, Energy-Efficient Technologies for the Dismounted Soldier (NRC, 1997), which will be referred to throughout this report as Energy-Efficient Technologies.
Electronics are critical to soldier combat effectiveness. Primary batteries now provide the main energy source, but the acquisition, storage, distribution, and disposal of over a hundred different battery types poses an enormous logistical challenge on the battlefield. New technologies have at the same time increased the number and variety of power-driven functions that require soldier-portable power. In the early 1980s, the Army recognized that it must approach equipping the dismounted soldier from an integrated system vantage. The concept of the soldier as a system led to a prototype of the first Land Warrior (LW) system, which combined electronics, weapons, and power sources in a single ensemble.
Dismounted soldiers act as both sensors and shooters, and the LW suite of electronics enhances combat effectiveness through increased situational awareness. Night-vision and infrared sights extend the reach of personal weapons, computer displays provide maps and locations of friendly and enemy troops, communications send and receive information about prospective targets as well as available sources of fire beyond rifle range. Suitably equipped soldiers can relay details about local targets and bring to bear virtually unlimited firepower, a capability that would have been inconceivable as recently as the first Gulf War.
But these capabilities come at a cost. Even without the LW equipment, the physical load borne by a dismounted soldier can exceed 100 pounds for certain missions. When it is fielded, the weight of the LW ensemble may add 30 pounds or more, not counting any extra batteries needed to guarantee power for the mission, which would clearly impact the soldier’s combat effectiveness.
The Army Program Executive Officer-Soldier is responsible for both the LW acquisition program and the Objective Force Warrior-Advanced Technology Demonstration (OFW-ATD) program.1 The OFW-ATD is working to integrate LW electronics using advanced concepts and to demonstrate an OFW prototype in 2004. This may serve as the basis for a future generation of LW to be fielded in the 2007-2010 time frame. In the far term, the Army envisions integrating soldier functions even more extensively by possibly embedding the electronics in a uniform made of advanced materials.
Portable power/energy sources were reviewed and the power demand was categorized in distinct regimes at the Energy and Power Workshop for the Soldier, sponsored by the Army Research Laboratory/Communications Electronics Command (ARL/CECOM). This workshop and the earlier National Research Council study, Energy-Efficient Technologies (NRC, 1997), provided the foundations for this study, which was requested by the Assistant Secretary of the
Army (Acquisition, Logistics, and Technology) to accomplish the following tasks:
Expand upon the conclusions from the ARL/CECOM Energy and Power Workshop for the Soldier, held on 15-17 October 2002, through the specification of both impact and feasibility of incorporating power management components, techniques and procedures for powering low-power electronic devices. The specific regimes from the workshop were: 20-watt average with 50-watt peak and 100-watt average with a 200-watt peak for up to 72-hr missions. Address power for high-power draw applications such as exoskeleton applications (1 to 5 kW average).
Assess electric power technologies to support soldier applications associated with future power and energy demands on the battlefield, e.g. expected OFW operational capabilities for the 2005-2025 time frame, with emphasis on alternative compact high-power and energy-dense sources, power management and distribution techniques, and low-power electronics such as asynchronous microchips, smart dust, etc. Assess technical risks and feasibility associated with each of the technologies and make recommendations pertaining to their potential efficacy and utility within the context of future OFW operational capabilities. Consider risks associated with technology development, integration of hybrid generators and sources, adaptation of commercial technologies, and battlefield logistics. Systems concepts involving appropriate power sources, power management and low-power electronics are to be specified and delineated.
Update the technologies evaluated in the 1997 NRC report on Energy-Efficient Technologies for the Dismounted Soldier including changes in individual technology development trends. Determine advantages and disadvantages for appropriate technologies in prospective application areas. Develop standard measures to facilitate comparison.
Prepare a consensus report documenting the study results and containing findings and recommendations to assist the Army in its development program. Prioritize the energy source alternatives appropriate to each application. Propose science and technology (S&T) objectives leading to the future incorporation in the Objective Force Warrior program. The report will include:
Recommendations for examined technologies with high benefit for target regimes with detailed justification for technology selection or rejection.
Recommendations for power distribution techniques for soldier systems. Applicability of low-power electronics, such as asynchronous microchips, smart dust, etc., to soldier device loads.
Recommendations for centralized vs. distributed power management for soldier systems including software/hardware techniques for control and conversion.
Applicability of examined technologies to single type sources vs. hybrid sources considering logistics, versatility, utility, environmental factors, safety, reliability, logistic infrastructure, manufacturability and availability.
Recommendation for recharging from soldier carried sources, robots (or vehicle) or fixed platforms.
Recommendations for predictive models and modeling techniques that would elucidate power use and management.
This executive summary summarizes key findings, including the science and technology (S&T) objectives in compact high-power and energy-dense source technologies for each of the regimes, and enumerates the specific recommendations contained in the study report.
Consistent with the ARL/CECOM Workshop, the committee assumed that the 20-W regime included power solutions for computers, radios, sensors, displays—all electronics subsystems of the LW ensemble. The 100-W regime included niche applications such as high-demand laser target designators and future microclimate cooling capabilities. Finally, the 1- to 5-kW regime was assumed to include the most power-intensive capabilities, such as portable power generators, rechargers for rechargeable batteries, and future exoskeleton devices.
The committee assessed and compared technologies at varying levels of technology readiness. Energy per unit of system mass, i.e., specific energy, served as the primary metric for selecting the technologies with greatest potential for Army purposes from among the many alternatives. Three important issues had to be addressed to make valid comparisons. First, the total energy produced must be measured under identical load conditions (power profile). Second, since fully packaged systems are not available for many of the emerging technologies, comparable parameters had to be estimated. Third, since batteries specify different performance specifications for different cell sizes, the committee provided varying allowances for packaging.
Fueled systems, which are in various stages of development, can be used to replace batteries as well as to supplement batteries in a hybrid system; the committee calculated standard mission energy requirements and used these to compare required masses for battery and fueled systems. Such things as fuel tank and fuel, energy content of the fuel, and energy conversion efficiency were used to compute comparable performance metrics.
Based on these considerations, the committee evaluated
and selected technologies with the greatest potential in each regime. (See Recommendations 6 through 8.) It also developed S&T objectives for the Army consistent with these recommendations, as shown in Table ES-1 for the near term (2010), medium term (2015), and far term (beyond 2015). Table ES-1 also indicates the relative risk (low, medium, or high) associated with each objective. Technologies considered as viable alternatives had to have demonstrated a level of technology readiness that would enable the committee to estimate its performance in a power/energy source system. Because of this, the Army will need to conduct detailed trade studies (specific energy vs. logistics, signature, cost, and so forth) to confirm that particular power source solutions are suited for particular applications.
TABLE ES-1 Science and Technology Objectives for the Near Term, Mid-Term, and Far Term, in Three Power Regimes
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 reliable, and more energy-dense without sacrificing safety. Fuel cells are the focus of intense interest by the military because of their potential as instantly “rechargeable” energy sources that can meet specific energy requirements for high electrical loads and long mission lengths. 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 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.
Logistical and Operational Considerations
Batteries currently add a substantial burden to the heavy load carried by the dismounted soldier. Use of disposable batteries in training and field operations has proven to be a substantial expense. Employment of rechargeable batteries for many applications promises to reduce life cycle cost but adds the cost of additional equipment and the logistics complexity of recharging in forward areas. Fueled hybrid solutions offer even greater promise than rechargeable batteries in reducing weight for longer missions. These have operational advantages and limitations but add tasks for the logistician, who would have to deal with another nonstandard fuel to be carried forward.
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. Microturbines have not to date demonstrated the ability to provide a net positive system power output. Stirling engines use standard logistics fuel (JP-8) 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
Hybrids offer enormous advantages from a simple energetics point of view for longer mission times. A hybrid power/energy system can be optimized for both high energy and high power demands. It can also provide the means to overcome the disadvantage of an air-breathing power source
by combining an air-breathing system (e.g., metal/air battery, fuel cell, small engine) with a rechargeable battery.
To be acceptable for soldier use, a power/energy source must be impervious to dust and moisture. An acceptable 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 it from destruction. Modeling is critical to the design of acceptable hybrid systems.
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 driven by the consumer market and are not developed in sizes commensurate with the broad spectrum of Army needs. The committee was specifically requested in the task statement to select and prioritize power source alternatives in each of the three target regimes. Recommendations 6 through 8 are consistent with the previous recommendations and the S&T objectives set for the Army in Table ES-1.
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.
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.
LAND WARRIOR SYSTEM
Correctly matching power source technologies (sources) with particular electronics applications (sinks) can greatly affect energy efficiency. System developers must also consider how Army logistics and operations impact the selection of power solutions for the soldier. The duty cycle is extremely important when considering a hybrid power solution. Also, dismounted soldiers who are accompanied in combat by a robotic vehicle, as envisioned by the Army for the future, will have a possible means for recharging batteries or fuel supplies that soldiers operating alone do not have.
Considering the OFW prototype as a possible third generation of LW electronics, the average power has been estimated at 20 W and the peak power at 60 W, for all three generations. The committee observed that power savings made possible by technology improvements in later elec-
tronics designs, primarily in 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 LW system 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 over an order of magnitude for the digital computing and communications processing, making it negligible in comparison with the power demand of analog, sensor, and display functions.
Commercial progress in developing low-power technology has been rapid, even outstripping Semiconductor Industry Association (SIA) roadmap estimates. Since 1997, the energy efficiency of circuits has improved by at least a factor of five. By one measure, this improvement in reducing power demand is greater than the improvement in rechargeable batteries, since time between recharges has increased only 20 percent. There are barriers to continuing improvements, the most important being the power lost due to leakage currents, but the Army has yet to avail itself of any of the gains made in past years.
Reducing power demand is an Army concern, but it is drowned out by the Army’s relatively near-term objectives to field and upgrade successive versions of LW. 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. The simple fact is that laws of physics, chemistry, and size are unlikely to produce the required near-term gains in energy, weight, and size of wearable power sources that will be needed while maintaining the current agility of the soldier. It is therefore imperative that the Army devote R&D effort to reduce the power drain in parallel with continued development and improvement of power sources.
Both the LW acquisition and the OFW-ATD programs rely on other Army programs to develop and acquire the component electronics. None of these programs have an incentive to develop or procure electronics using commercially proven design approaches to reduce energy consumption. And, because of the added cost and risk involved in development, there are actually disincentives for reducing power demand.
As tempting as it may be for the Army to continue use of traditional design techniques, a different strategy is required to design the equipment that the soldier must carry as compared with equipment for vehicles or other mobile or fixed platforms. Consider that there are major differences between what is required to design a smart cell phone and what was required to design an office telephone or 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 system-on-a-chip (SoC) approach to the development of future warrior systems. Achieving energy efficiency for these electronics will resolve a myriad of problems now associated with the integration of disparate systems in addition to reducing soldier energy needs.
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 stakes involved with 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
Wireless communications is the most power-hungry of soldier electronics applications and offers the best chance to reduce future warrior energy requirements. 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.
There is clear evidence that reductions in power demand are not a high enough priority for communications-electronics. Power and duty-cycle estimates for the LW soldier radio have not been refined for at least 5 years, even though communications technology has advanced considerably and new network-centric capabilities are planned to one day connect every soldier on the battlefield.
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 energy efficiency encourages trading off power savings achieved for new electronics. With no net reduction in power, this approach could undermine the benefits of a system-of-systems design approach to reducing power demand and could contribute further to the chasm that exists between the consumer electronics’ state of the art and Army state of the art.
Table ES-2 summarizes areas within the Land Warrior system that are key to improving energy efficiency and reducing power demand. The first column lists major components of the system, the second column lists mitigation techniques, and the third column shows the improvement possible. These are improvements that could be realized using a system approach to mitigate energy issues associated just with 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.
Future Warrior Goal
The Army envisions a future uniform-and-electronics ensemble for the Future Warrior. The committee believes that soldier electronics requiring a mere 2-W average, 5-W peak power is attainable in the far term if the recommendations of this study are fully implemented. By adopting state-of-the-art commercial design practices and incorporating energy-efficient technologies, peak power demand on energy sources can be reduced, thereby increasing the combat effectiveness of individual soldiers and extending the duration of their missions.
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 to provide reliable power sources at such low power 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
The surest way to manage power is to utilize power-down technology for devices with heavy duty cycles. This requires detailed knowledge of duty cycles for the components as used in soldier operations. Additionally, the power dissipation of components in standby mode should be reduced as much as possible. This will become an increasingly important issue in the future owing to increased leakage currents.
Rather than crude duty cycle guesses, actual measurements of dynamic loads are needed to enable simulations of the dynamic operation of LW electronics synchronized with a power source simulator. Given a mission scenario, a suite of soldier equipment, and the size or makeup of a combat team, the Army should be able to determine an optimum type, quantity, and distribution of power sources, as well as fuel requirements. Full simulation of OFW power sources and sinks would help to determine the directions that developments must take to have the most impact. Systems could then be designed using aggressive techniques tailored to each application and to the most likely soldier modes of inter-
TABLE ES-2 Techniques for Mitigating Energy Issues in Key Land Warrior System Components and Improvements That Could Be Realized
action, thus reducing power requirements for computation and communication by several orders of magnitude.
Simulations also have the potential to save development time and money, but they require high-performance computers and accurate system inputs. High-fidelity models based on experimental data can narrow the parameters of optimization and expedite the proper selection and matching of power sources. The Army has access to high-performance computing resources easily capable of supporting such important tasks. Ideally, the military should develop and acquire new equipment only on the basis of such models, so that the lifetime of the equipment can be maximized.
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; otherwise, 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.
Martin, T.L., D.P. Siewiorek, A. Smailagic, M. Bosworth, M. Ettus, and J. Warren. 2003. A case study of a system-level approach to power-aware computing. ACM Transactions on Embedded Systems 2(3): 255-276.
NRC (National Research Council). 1997. Energy-Efficient Technologies for the Dismounted Soldier. Washington, D.C.: National Academy Press.