breathing systems. From a systems perspective, the most mature technologies are PEM and DMFC, which require hydrogen (PEM) or methanol (DMFC). The advantage of the SOFC and the engine systems is their ability to operate on logistics fuels. SOFC offers the potential for the quietest system of the three operating on JP-8; however, some fuel processing is required before the fuel-cell stack. Small hydrocarbon fuel reformers under development (Altex) could be integrated with the SOFC, or integral CPOX reactors could be used. The latter have been demonstrated with butane (AMI). Small engines can utilize JP-8 type fuels directly; their minimal impact on logistics makes them very attractive. The reliability of these small engines remains to be determined.

Peak power in these systems could be provided by the prime power source, depending on the duty cycle. If the latter is too demanding, capacitors or lithium ion batteries could supplement the prime power source for the 200-W peak.

Findings for 100-W Average Power Regime
  • Fueled systems become more attractive as power demand increases.

  • Li ion batteries have the rate capability to power laser designators.

  • Energy conversion technologies can reduce significantly the mass of 100-W systems that operate for 24 hours or longer.

  • JP-8-fueled systems appear to weigh the least; however, these systems are not yet at high TRLs, and they may be more massive, have higher thermal and acoustic signatures, and be less reliable than other options.

  • Non-JP-8-fueled systems offer significant performance advantages over batteries without some of the compromises of JP-8-fueled systems; however, the logistics burden will be greater to supply these fuels.

Science and Technology Objectives

S&T objectives consistent with the committee’s selection of alternatives in the 100-W regime are listed below. The objectives are listed in priority order, along with key development issues to be resolved.

The three near-term objectives are these:

  • Develop smart hybrid systems with fuel cells and high-power batteries or electrochemical capacitors.

  • Develop small fuel processors for logistics fuels, methanol, ammonia, and other viable fuels. Key issues: thermal management, coking, sulfur removal, gas stream cleanup, start-up and load following, shutdown, packaging, and interfaces with energy converter.

  • Evaluate DMFC and PEM systems for various specific missions. Key issues: modeling the capability of these systems with respect to loads, mission profiles, and operational and logistical constraints; overcoming technical issues as mentioned above.

There are two mid-term objectives:

  • Develop small engines. Key issues: balance-of-plant (fuel delivery, vaporization, atomization, control system); integrating and packaging complete systems for field evaluation; validating performance scaling laws; assessing reliability and failure modes.

  • Develop solid oxide fuel cells. Key issues: fuel processing, sulfur tolerance.

The sole far-term objective is this:

  • Develop high-specific-energy, air-breathing batteries. Key issues: validating advanced battery concepts such as Li/air (TRL 3) and C/air (TRL 2); testing and evaluating thermally self-sustaining C/air systems; increasing the rate capability of Li/air by a factor between 5 and 10.

1- to 5-kW Average Power

System mass versus total energy is plotted in Figure 2-6 for three power source systems of TRL 9:0.9 kW, 1.2 kW, and 2 kW. The plot was normalized to total energy delivered as these power sources have different power ratings. The conclusions below apply to power levels up to 5 kW. The Honda gasoline generator and the Mechron 2-kW diesel generator are commercial products (TRL 9). The latter is the generator supplied by the Project Manager for Mobile Electric Power (PMMEP) to the Army. The Ballard Nexa 1.2-kW PEM fuel cell is being packaged as a commercial product (TRL 9). A 70 percent efficient diesel fuel reformer and a 37 percent efficient fuel cell system were assumed in order to generate the Nexa fuel cell plot.

Mass was not estimated for the reformer, because no data were available. This reformer mass would need to be added to the points for the PEM fuel cell in order to obtain the mass for the total system. While the mass of the reformer is unknown, it would have to be 44 kg to result in a system having the same mass as the 2-kW diesel unit. It is anticipated that a reformer capable of supplying sufficient hydrogen to a 1-kW PEM fuel cell would have much less mass than this. The PEM fuel cell with an efficient diesel reformer could have an overall efficiency of 26 percent (0.7 × 0.37).

A system has not yet been demonstrated, so the Stirling engine was not included in Figure 2-6. Sunpower offers a 1-kW prototype Stirling engine having a dry mass of 32 kg,

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