FIGURE 2-6 System mass versus total energy.

but this does not include the circulating cooling system or the burner and other ancillaries. An efficiency of 28 percent is claimed for thermal energy in to electricity out.

Stirling is inherently low in acoustic signature (less than 65 dBa at 1 meter) but has two sources of thermal signature. The motor-alternator must be kept at a rejection temperature on the order of 100°C, which is comparable to the temperature of operation of PEM fuel cells. The second source of thermal signature is the exhaust gas from the combustion process, which is likely to be hotter. The temperature is determined by the thermal recuperator and whatever mitigation scheme can be employed to further cool the exhaust gas.

Diesel and gasoline generators in this regime are highly developed (TRL 9). The Honda generator (19 percent efficient) has excellent performance and is light, but it operates on gasoline and has a significant acoustic signature (59 dBa at rated load). The Mechron 2-kW generator (16 percent efficient) in current use by the Army operates on diesel fuel and has a dry mass of 56 kg and an acoustic signature of <77 dBa at 7 meters. Specifications are readily available on the company Web sites.

Findings for the 1- to 5-kW Average Power Regime
  • Fuel cells offer lower signatures than IC engines.

  • Stirling engines offer potentially low thermal and acoustic signatures.

Science and Technology Objectives

S&T objectives consistent with the committee’s selection of alternatives in the 1-5-kW regime are listed below:

  • Near-term objectives. (1) Develop lightweight, efficient, 1- to 5-kW engines that operate on logistics fuel (key issues: tribology, reliability, integrating combustion sources with Stirling engines, and reducing system mass) and (2) develop lightweight logistics fuel reformers.

  • Mid-term objective. Integrate logistics fuel reformers with lightweight PEM fuel cells.

  • Far-term objective. Develop SOFCs. Integrate logistics fuel reformers with SOFCs.

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