Power System

State of the Art, 1997a

State of the Art, 2003

Item Considered

Scaling Laws

Impact on Soldier Power

Thermoelectric

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.

Efficiency.

Materials-specific power.

System-specific energy.

Known

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.

 

Known

 

Nuclear isotope

Limited consideration.

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

Not considered.

Safety.

Environmental impact.

Cost.

Public acceptance.

Known

 

Alkali metal thermal-to-electric converter

Speculative technology.

Systems projection to 500 W/kg.

Not considered owing to lack of progress.

 

Known

 

Energy harvesting; solar

Some versions mature.

Considered for low-capacity niche applications.

 

Known

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.

FIGURE D-1 Schematic cross section of a battery.

in cameras, though the Li/MnO2 2/3A cell is the more popular choice for cameras due to its lower cost. As summarized in Table D-2, Li/(CF)x has higher theoretical specific energy than Li/MnO2 cells (2120 Wh/kg vs. 900 Wh/kg) and an open circuit potential (OCV) of about 3.2 V. The theoretical OCV, based on free-energy calculations, is about 4.5 V. The difference between theoretical and practical OCV values has been discussed by Whittingham (1975). A comparison of the practical performance of Li/(CF)x vs. that of Li/MnO2 is shown in Table D-2.

In spite of the much higher theoretical specific energy in a Li/(CF)x cell, (CF)x is much lighter than MnO2 (2.5 g/cc vs. 4.5 g/cc) and gives comparable practical energy performance in commercial small cells. During the discharge of the cell, the carbon monofluoride in the positive electrode changes from a poor conductor to a more conductive amorphous carbon when discharged. Thus, the reaction efficiency increases with discharge. Li/(CF)x cells are known for their high-temperature performance (as high as 150°C according to Panasonic coin cells data), long shelf life (>10 years), and



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