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.

Small Engines

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.



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