The Army has begun development on a recharger technology in limited production using fuel-cell prototypes. Unfortunately, the fuel cell alternatives use hydrogen, propane, or methanol fuels, and the Army would much prefer not to have a new battlefield fuel in the inventory. As an alternative to fuel cells, CERDEC funded research on two types of systems that could use standard battlefield fuels—small external combustion Stirling energy systems and small internal combustion engine converter systems.
External combustion engines such as steam engines and Stirling cycle engines have been in use since about 1800 but seem, except in a few embodiments, to have mostly been relegated to history, because internal combustion engines and electrical power from an ever-expanding grid were more efficient. Their primary advantage lies on the fact that the thermal process is steady state, which allows combustion optimization and energy recuperation. Further, steady-state combustion inherently has a lower acoustic signature than internal impulsive combustion. It is possible to operate two separate free-piston versions of Stirling engines such that all vibration is canceled, resulting in an extremely quiet system.
Early versions of Stirling engines employed exotic materials and had low specific power even though they were efficient converters of thermal energy to electricity. In recent years, however, advances in materials have led to the development of components with sufficient high-temperature properties that interest in Stirling technology has emerged as a viable energy converter for some applications. It is currently a viable candidate for deep space exploration1 and shows promise for battlefield (NRC, 2004) and commercial applications such as co-generation.2 Since the Stirling converter requires only a heat source, it is inherently multifuel-capable and has been demonstrated with a range of energy sources, from nuclear to heavy distillates. The Stirling engine can be made in a range of sizes with no loss of efficiency. For example, since about 1990, free-piston Stirling engines have been successfully demonstrated over power levels from 40 W to 25 kW. Stirling engine technology offers the potential for unique military systems that have extremely long life ( >10 years) of continuous operation, unprecedented reliability in a military environment, and extremely simple and elegant mechanical functionality—the motor alternator comprises only one or two moving parts, neither of which is in contact with the other. Since 2005, there has been steady progress in taking the NASA investment in Stirling technology for deep space missions and adapting it for use as a battlefield energy source (CERDEC-funded Defense Advanced Research Project Agency (DARPA)