Power System

State of the Art, 1997a

State of the Art, 2003

Item Considered

Scaling Laws

Impact on Soldier Power


Some versions mature.

Low potential.

Best system efficiency on order of 5%; converter efficiencies projected to 10%.

Insufficient progress to consider for current applications.

Progress in new high-ZT materials makes technology worth watching for long term.


Materials-specific power.

System-specific energy.


Not applicable owing to low efficiency.

Possible niche application in small sizes.

Thermo-photovoltaic (TPV)

20% TPV cells demonstrated. System projections to 20%.

Not considered owing to lack of progress in systems.




Nuclear isotope

Limited consideration.

Rejected owing to cost,safety,environmental considerations,and lack of infrastructure.

Not considered.


Environmental impact.


Public acceptance.



Alkali metal thermal-to-electric converter

Speculative technology.

Systems projection to 500 W/kg.

Not considered owing to lack of progress.




Energy harvesting; solar

Some versions mature.

Considered for low-capacity niche applications.



Driver for reducing power demand.

NOTE: SOA, state of the art; Li ion, lithium ion; JP, jet propellant; ZT, thermoelectric figure of merit.

aNRC, 1997.


The governing figure of merit used to discriminate among and, in the final analysis, to rank-order the technologies was total system mass (as estimated from the specific energy of the underlying technology). Other figures of merit used to evaluate the technologies and systems are described in detail in Appendix C. The committee estimated technology readiness levels (TRLs) to determine the systems worthy of consideration. Definitions for the nine TRLs are also contained in Appendix C.

It is important to note that technical figures of merit for many of the emerging power sources were not available, and in a few instances, the information was not considered reliable enough. Many of the technologies evaluated were in various states of development, and the committee made some assumptions about expected system characteristics and performance. These assumptions and/or extrapolations are documented so that the reader can better judge the relative merits and risks of the technology options.


Batteries represent the ideal solution for soldier power and energy applications. Only when every effort has been made to conserve and manage energy and it is found that batteries cannot meet requirements should air-breathing systems, such as fuel cells or small engines, be considered. A heavy price is paid when these nonbattery options are used, including the requirement for continuous airflows, sensitivity to contaminants, temperature restrictions, possible orientation dependence, acoustic and thermal signatures, nonstandard fuels, surface and exhaust temperature, and exhaust gas contamination. That being said, there are mission requirements today for soldiers that exceed the reasonable capabilities of battery technology, and in these cases air-breathing alternatives are emerging to meet these needs.

The rationale used to compare alternatives provided a framework for selecting power source options for the different power/energy regimes. The analyses considered all that is presently known about existing and emerging power source performance, and the technologies selected for further consideration by the Army are those that will most likely meet mission requirements with respect to specific power and specific energy. It is always possible that for particular missions, other factors, such as acoustic and thermal signature, operating temperature, fuel, orientation dependence, logistics, etc. will be more important than the specific power and specific energy of the system. Ultimately, the Army will

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