four BB 590 NiCd batteries. Because these requirements are far in excess of the other OFW requirements, the energy efficiency of such systems should be of special interest to the Army. Power requirements for new laser designating devices could easily override other considerations and control the selection of a centralized energy source for the soldier system.
The Army did not provide its concept for fielding the devices, and the power demands were not included in the power allocation for the OFW. Clearly, reducing power demand should be a major consideration in designing laser designators. The committee expects that the energy efficiency of such devices can be improved, but it did not know enough about such devices to recommend specific improvement techniques.
Although soldier microclimate management efforts have been under way since the early 1990s, they remain in an early stage of development. Specifications, including duty cycles and energy use data, for the many prospective systems being contemplated by the Army vary considerably, and the committee was unable to determine whether the solutions proposed are particularly energy efficient. In this report, the committee does document its observations on the preliminary design information provided by the Army program manager (Masadi, 2003). A variety of system approaches—from ice cooling systems to vapor compression and absorption refrigeration—have been tried.
The basic difficulty with management of the dismounted soldier’s microclimate is the large power requirement for such an effort. A dismounted soldier doing very light work such as guard duty has a work rate between 100 and 175 W. Light work such as cleaning a rifle has a work rate of 125 to 325 W. Moderate work such as foxhole digging has a work rate between 325 and 500 W. Heavy work such as emplacement digging has a work rate above 500 W. Since the human body is on the order of 18 percent efficient, these work rates would require cooling rates five times greater.
The power required for microclimate cooling is much greater than the power required for other functions of the dismounted soldier. Because of these excessive power requirements, microclimate cooling for the OFW will be limited to providing ventilation for soldiers clad in protective clothing, whereby a ventilator moves air into and through the soldier’s protective clothing to provide modest improvements in comfort.
This effort is being accomplished in concert with modifications to soldier uniforms that provide passages near the skin in which air can flow to provide evaporation and transport of perspiration. This air movement can be provided with minimum pressure drops for filtering toxic or noxious inputs to the system or modest pressure drops to afford airflow within the soldier’s uniform. For the OFW program, a power budget of 10-12 W has been allocated to accomplish ventilation and cooling tasks. It is expected that a separate specific power source will be provided for the ventilation system.
The present approach is to think of providing soldier microclimate cooling in three variations. The first variation, described in the two preceding paragraphs, is passive cooling designed into a porous uniform. The second variation would provide active cooling by ventilating the soldier’s uniform with continuous airflow from a small fan. The third variation would provide active cooling with a mechanical refrigeration device. The second and third variations are not likely to be available until significant development has been completed. This means that active cooling will not be available until well after the OFW system is completed.
Researchers at the University of Wisconsin surveyed a broad range of possible alternatives for use in active cooling of the dismounted soldier. They also looked at potential power sources for the alternative systems. Their analysis used performance characteristics of large, state-of-the-art, optimized cooling system components, even though such components do not exist on the scale required for individual soldier use. When such components are scaled, there might be significant degradation of system performance.
Vapor compression systems for cooling, which might be made available soonest, were among the most effective. Other systems that might have comparable effectiveness were in early development and had correspondingly low TRLs. The vapor compression systems were projected to be capable of a coefficient of performance of 3.8 and would thus consume approximately 80 W on a continuous basis to handle a modest 300 W body heat load.
Forced ventilation systems are the most energy-efficient cooling system, but their capabilities are limited. This type of microclimate cooling is only effective under favorable conditions of temperature and relative humidity. Because of the effectiveness of ventilation systems and their modest power requirements (10 to 12 W) they are being developed before active microclimate cooling systems. Even this relatively low power level may require a separate power source, in addition to those sources envisioned for the other soldier electronics.
If active microclimate cooling is to be pursued, much higher power levels will be required. Based on the power demands for microclimate cooling options explored in the University of Wisconsin survey, an energy-dense fueled system will probably be required, even for advanced systems.
The Defense Advanced Research Projects Agency (DARPA) is developing an exoskeleton prototype as part of a human performance augmentation program that is focused