In the mid-term (5-10 years), the basic technologies associated with the fuel-cell types in Table I-2 have already undergone extensive development and aside from reducing the cost of manufacturing, the biggest single gain that can be expected is to develop a reliable, sulfur-tolerant jet propellant (JP) fuel reforming capability that could be integrated into these fuel cells. Note that both liquid propane and JP fuels have approximately the same specific energy but that liquid propane has about half the energy density in Whr/liter that JP has. Both have roughly 2.25 times the specific energy of methanol, a difference that increases when it is necessary to use a methanol/water mixture for operation at higher temperatures.

As with any of the fueled systems, there are two issues associated with warm-up time and with operation of the small fuel cells as part of a hybrid system that the Soldier can carry: immersion in water and contaminant ingestion. DMFC and RMFC are currently being built for limited deployment in Afghanistan and in other active military operations. All three of the main types discussed above are at a TRL of 6-7 and are in various stages of testing. As mentioned, their widespread introduction into the inventory will also require the introduction of a new “logistics fuel,” which does not seem to be practical at this time.

In the far term (10-20 years), the technology using hydrocarbon fuels will continue to mature and to achieve marginal increases in performance. However, given that hydrogen has a specific energy of about 40,000 Whr/kg, roughly 3.25 times that of JP and liquid propane gas, major technological advances in a highly competitive civilian hydrogen economy could drive increases in system-specific energy greater than JP fuel systems. Note that the energy density in Whr/liter is extremely poor, forcing trade-off between mass and volume for specific hydrogen storage technologies.

COMBUSTION SYSTEMS

In two earlier studies, Energy-Efficient Technologies for the Dismounted Soldier, (NRC,1997) and Meeting the Energy Needs of Future Warriors, (NRC, 2004), fueled electrical energy sources were considered in the size range relevant to the dismounted Soldier. In general, the conclusions from those studies show that fueled sources have significant potential for providing a reliable source of energy for the Soldier. In both studies, conversion efficiencies in the 10-50 percent range (fuel heat value to usable energy) were possible, but the level of technical development for suitable fuels was at a low TRL.

The Army is rapidly moving to implement rechargeable battery technology as the Army standard as the specific energy of rechargeable batteries improves and the costs of primary batteries and of their delivery to theater falls. While delivery of a single battery to theater is approximately the same no matter what the type, rechargeable batteries will cost about 5 times as much to manufacture. Given that they can undergo the charge/discharge cycle 200 times or more—a primary battery is used only once—it is obvious that the basic cost and delivery cost savings are



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