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Energy-Efficient Technologies for the Dismounted Soldier
Appendix CEnergy Source Technologies
BATTERIES
Although batteries in general represent a very large, mature product class in commercial production, enormous improvements in specific power, specific energy, and cycle life (for rechargeable batteries) have been made in the past decade (Space Power Institute, 1990, 1992b). Much of the driving force for the technical improvements has come from the rapid growth of portable computers, cellular telephones, and other communication devices. However, very few of these improvements have been of direct benefit to communication devices used by the Army.
Battery production worldwide is approximately $40 billion (Salkind, 1996) with U.S. production at about $11 billion. Military purchases are only a small percentage of the total, and there appears to be little interest among large manufacturers in producing military batteries.
Improving the specific energy (available energy from a fixed mass) and energy density (available energy from a fixed volume) of batteries have been commercial goals. But because most commercial devices require only a few AA cells, weight reduction has been second in importance to energy capacity. As shown in Figure C-1, the capacity of AA nickel alkaline (NiCd and NiMH) batteries has risen from 0.4 Ah to 1.2 Ah in the past 20 years. Very fast recharging (in less than 1 hour) has also become available. Lithium rechargeable systems in the same size packaging have approximately the same capacity, but at much higher voltages, resulting in cells with higher specific energy. However, so far lithium rechargeable cells cannot be recharged quickly. Improvements continue to be made.
Among the Army's options for keeping pace with these rapid changes is the adaptation of commercially available cells. Current military battery systems could be replaced by systems with different voltage characteristics as long as the new system volume is the same or smaller. This should be possible with new, more efficient techniques for DC-DC conversion, which would eliminate the problem of Army communication devices being locked into using power sources with particular voltage levels.
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Energy-Efficient Technologies for the Dismounted Soldier
Appendix C Energy Source Technologies
BATTERIES
Although batteries in general represent a very large, mature product class in commercial production, enormous improvements in specific power, specific energy, and cycle life (for rechargeable batteries) have been made in the past decade (Space Power Institute, 1990, 1992b). Much of the driving force for the technical improvements has come from the rapid growth of portable computers, cellular telephones, and other communication devices. However, very few of these improvements have been of direct benefit to communication devices used by the Army.
Battery production worldwide is approximately $40 billion (Salkind, 1996) with U.S. production at about $11 billion. Military purchases are only a small percentage of the total, and there appears to be little interest among large manufacturers in producing military batteries.
Improving the specific energy (available energy from a fixed mass) and energy density (available energy from a fixed volume) of batteries have been commercial goals. But because most commercial devices require only a few AA cells, weight reduction has been second in importance to energy capacity. As shown in Figure C-1, the capacity of AA nickel alkaline (NiCd and NiMH) batteries has risen from 0.4 Ah to 1.2 Ah in the past 20 years. Very fast recharging (in less than 1 hour) has also become available. Lithium rechargeable systems in the same size packaging have approximately the same capacity, but at much higher voltages, resulting in cells with higher specific energy. However, so far lithium rechargeable cells cannot be recharged quickly. Improvements continue to be made.
Among the Army's options for keeping pace with these rapid changes is the adaptation of commercially available cells. Current military battery systems could be replaced by systems with different voltage characteristics as long as the new system volume is the same or smaller. This should be possible with new, more efficient techniques for DC-DC conversion, which would eliminate the problem of Army communication devices being locked into using power sources with particular voltage levels.
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Energy-Efficient Technologies for the Dismounted Soldier
FIGURE C-1 Chronological improvements in the capacity of AA nickel batteries.
The performance characteristics and production levels of the common primary, secondary, and special battery systems considered in this report are listed in Tables C-1, C-2, and C-3.
Systems Likely to Meet the Needs of the Dismounted Soldier
Of the more than 30 rechargeable battery systems in commercial production or in advanced development, only seven or eight seem likely to meet the military goals of availability in small sealed cells with appropriate levels of safety, reliability, and low temperature and high temperature performance. These few systems are described in this section, with estimates of their present performance levels and estimates of what might be achieved in five and ten years. The research needed to achieve the listed goals is also briefly described.
Although a low temperature requirement of -40°C is still listed in some Army documents, the committee was informed that this temperature requirement was principally for storage. For operations, the committee assumed a minimum temperature requirement of -25°C but even this may be unrealistically low and may disqualify otherwise practical systems.
The systems likely to provide the desired combination of compactness, specific energy, and specific power fall into two categories: rechargeable alkaline electrolyte systems (nickel-metal hydride, nickel-zinc, MnO2-zinc) and rechargeable lithium electrode systems (lithium metal anodes, lithium intercalating anodes, lithium alloy anodes [including the tin oxide type]).
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-1 Summary of Primary Battery Data
Theoretical
Working
Battery System
Anode
Cathode
Voltage
Ah/kg
Wh/kg
Voltage
Wh/kg
Wh/l
Production Valuea
Lechlanche (zinc-carbon)
Zn
MnO2
1.6
224
358
1.5
85
165
vl
Magnesium
Mg
MnO2
2
271
758
1.75
100
195
vs
Alkaline
Zn
MnO2
1.6
224
336
1.25
125
330
vl
Mercury
Zn
HgO
1.34
190
255
1.3
100
470
vvs
Silver (silver-zinc)
Zn
Ag2O
1.5
180
288
1.45
120
500
ss
AgO
1.85
270
445
(2 plateaus)
140
650
Zinc-air
Zn
O2(air)
1.65
658
1,066
1.25
500
1,050
1
Aluminum-air
Al
O2(air)
2.7
2,980
8,046
1.1
300
240
vs
Lithium Systems
Sulfur dioxide
Li
SO2
3.1
379
1,175
2.8
260
415
1
Thionyl chloride
Li
SOCl2
3.66
407
1,489
3.3
320
700
1
Sulfuryl chloride
Li
SO2Cl2
3.9
360
1,405
3.7
450
900
vvs
Manganese dioxide
Li
MnO2
3.5
286
1,001
2.8
230
550
vl
Carbon monofluoride
Li
(CF)x
3.1
703
2,180
2.5
250
600
l
Iron disulfide
Li
FeS2
1.8
725
1,304
1.4
130
400
l
a Key: vl = $1 billion
l = $100 million to $1 billion
s = $10 million to $100 million
vs =< $10 million
vvs = < $2 million
Improvements
MnO2 cathode material improvements can increase nonlithium system capacity by as much as 15 percent.
Improvements in separator material and technology can increase stability and rate of all primary cells.
Air electrode improvements can increase power capability of air cathode systems.
Safety for all lithium battery systems can be improved with improvements in separators.
Packaging technology can increase specific energy of Li/MnO2 technology.
MnO2 cathode material improvements can increase capacity and discharge rate in lithium systems.
Focus Chemistries
Zn, Mg/MnO2, and Li/FeS2 are commercial market driven.
Zinc-air, Li/MnO2, and Li/CTx are areas of interest for the government because they have either high specific power or high specific energy or both.
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-2 Summary of Rechargeable Portable Battery Data
Theoretical
Working
Battery System
Negative Electron
Positive Electron
Voltage
Ah/kg
Wh/kg
Voltage
Wh/kg
Wh/l
Production Valuea
Estimated Life (Cycles)
Lead-acid
Pb
PbO2
2.1
83
175
2.0
35–50
85
vl
400
Nickel-iron
Fe
NiOOH
1.4
224
313
1.2
35–60
70
vs
500
Nickel-cadmium
Cd
NiOOH
1.35
181
244
1.2
35–52
75
vl
600
Nickel-zinc
Zn
NiOOH
1.73
215
372
1.6
65–80
150
s
400
Silver-zinc
Zn
AgO
1.85
283
524
1.5
90–150
180
vs
100
Nickel-hydrogen
H2
NiOOH
1.5
269
434
1.4
55–60
60
S
600
Nickel-metal hydride
Mhx 1.2 to 2 w/o H
NiOOH
1.35
206
278
1.2
55–70
120
vl
800
Silver-cadmium
Cd
AgO
1.4
227
318
1.2
60–80
110
vvs
200
Zinc-bromineb
Zn
Br Complex
1.85
139
258
1.55
70
60
vvs
400
Alkaline manganese
Zn
MnO2
1.6
224
330
1.2
55
250
vl
15
Zinc-air
Zn
O2 (air)
1.6
658
1,085
1.15
110
130
vs
25
Lithium Systems
LiMn2O4
Li
Mn2O4
4
143
510
3.7
140
300
vs
250
LiNiO2
Li
NiO2
4.2
137
575
3.6
155
325
res
—
LiCoO2
Li
CoO2
4.2
178
750
3.7
95
235
vs
250
Li/organosulfide
Li
R-S-S-R
3
~300
~900
2
200 est
300 est
res
300
Li/organosulfide
Li
(CS)x
2
~400
~800
2
200 est
300 est
res
300
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Energy-Efficient Technologies for the Dismounted Soldier
Theoretical
Working
Battery System
Negative Electron
Positive Electron
Voltage
Ah/kg
Wh/kg
Voltage
Wh/kg
Wh/l
Production Valuea
Estimated Life (Cycles)
LiMn2O4
Li+C
Mn2O4
4/3
102
356
3.7
70–100
170
res
—
LiNiO2
Li+C
NiO2
4.2/3
100
360
3.6
70–100
170
res
—
LiCoO2
Li+C
CoO2
4.2/3
100
360
3.7
70–100
170
1
1,000
Polymer
Li+C
Mn2O4
4/3
102
358
3.0
150 est
300 est
vvs
300
Large iron sulfides
Ll(Al)
FeS/FeS2
1.33/1.73
285/345
459/514
1.3/1.6
100/180
200/350
res
~1,000
a Key: vl = $1 billion
l = $100 million to $1 billion
s = $10 million to $100 million
vs = < $10 million
vvs = < $2 million
res = research
b Not portable.
Improvements
Charger and charging methods can improve cycle life and safety of rechargeable cells.
Improvements in NiOOH and separator technology can increase capacity of all nickel systems.
Improvements in metal hydride anode can increase the energy by nearly 2 times (Mhx 1.2 to 2 w/o H).
Material improvements can increase cycle life of rechargeable alkaline battery.
Material improvements can increase cycle life of rechargeable zinc-air battery.
Air cathode improvements can increase power capability and cycle life of the zinc-air system.
Safety for all rechargeable lithium batteries can be improved with improvements in separators.
Anode material improvements for lithium ion and lithium polymer batteries can increase the specific energy and safety.
Cathode material improvements can increase specific energy of all lithium batteries.
Focus Chemistries
Nickel-metal/hydride, alkaline, and zinc-air are market driven; thus, unique military requirements may be overlooked.
Lithium systems focus on military-unique requirements.
Zn, Mg/MnO2, and Li/FeS2 market driven.
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-3 Summary of Data on Reserve, Thermal, and High Temperature Rechargeable Batteries
Battery System
Anode
Cathode
Working Voltage
Wh/kg
Wh/l
Estimated Life (Cycles)
Reserve
Water activated
Mg or Zn
CuCl
1.5–1.6
65
125
(Not rechargeable)
MnO2
1.5–1.6
65
125
AgCl
1.5–1.6
125
250
Others
Spin activateda
Pb
PbO2
1.5
(Not rechargeable)
Zn
AgO
1.4
Li
SOCl2
3.5
Li
FeS2
1.8
Electrolyte introduction-activated
Zn
AgO or Ag2O
1.6
50
160
(Not rechargeable)
Li
V2O5
3.3
50
100
Li
SO2
3
120
200
Li
SOCl2
3.5
150
300
Thermal batteries
Ca
CaCrO4
2.4
30
40
(Not rechargeable)
Mg
V2O5
2.5
Li
FeS2
1.8
40
100
High temperature rechargeable batteries
Lithium-iron-sulfide
Li
FeS
1.3
100
200
700
FeS2
1.6
180
350
1,000
Sodium-sulfur
Na
S
2.1
170
250
100–2,000
Sodium-nickel chloride
Na
NiCl2
2.58
90
160
600–1,000
a These batteries are not designed to be weight or volume efficient.
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-4 Nickel Metal Hydride Battery Systems
Present Advantages
Present Disadvantages
5 Years
10 Years
Higher specific energy than NiCd
Lower specific power than NiCd
Higher rate capability, 25% more capacity per volume
40% capacity improvement per volume
Rapid recharge at room temperature
Poor charge retention, 5% per week loss at room temperature
Charge loss reduced to 2% per week at room temperature
—
Long cycle life
Poor thermal stability
Poor overcharge recombination kinetics
Lower vapor pressure alloys
Lower vapor pressure alloys
Maintenance free
—
—
—
Rechargeable Alkaline Electrolyte Systems
Most anode battery systems can be assembled with various cathodes and electrolytes in combinations described in the Tables C-4 through C-15. These tables present a summary of the candidates likely to meet the future power requirements of the dismounted soldier. Each table summarizes the advantages and disadvantages of each chemistry, as well as technological projections of what can be accomplished in five and ten years.
Improvements in nickel metal hydride battery systems are shown in Table C-4. The anticipated improvements will require sustained research in the following areas:
metal hydride alloys for better thermal stability
cathode materials with improved volumetric efficiency (e.g., nanostructured, fibrous, and higher valence materials)
charge profile with optimum charging, overcharge recombination kinetics
better separators
Improvements in rechargeable alkaline manganese dioxide battery systems are shown in Table C-5. To achieve the projected improvements, it will be necessary to research the following areas in depth:
materials for better cycle life and low temperature performance (nanostructured, catalytic MnO2, improved carbons and graphites)
improved cellophane (or other separator) for higher rate performance
optimal recharging profile
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-5 Rechargeable Alkaline Manganese Dioxide (RAM) Battery Systems
Present Advantages
Present Disadvantages
5 Years
10 Years
Low cost
Lower specific power
Improved rate
Improved cycle life
Maintenance free
Poor cycle life
Improved cycle to cycle capacity
Improved low temperature operation
Good charge retention
Decreasing capacity with cycle life and depth of discharge
—
—
Poor low temperature performance
—
—
Improvements in metal zinc battery systems are shown in Table C-6. To achieve the projected improvements, major research will be needed in the following areas:
cathode materials for improved volumetric efficiency (e.g., nanostructured, fibrous, higher valence)
lightweight current collectors for the nickel electrode
charge profile for optimal charging, overcharge recombination kinetics
better separators, microporous membranes, and cellulosic films
complex electrolytes for improved cycle life
TABLE C-6 Nickel Zinc (NiZn) Battery Systems
Present Advantages
Present Disadvantages
5 Years
10 Years
Higher specific energy than NiCd
Poor overcharge recombination kinetics
Higher specific power, 10% more capacity per volume
20% specific energy improvement per volume
Maintenance free
—
—
—
Rapid recharge
Moderate charge retention; 2% per week at room temperature
Charge loss reduced to 1% per week at room temperature
—
Moderate cycle life
—
Improved separator and electrolytes; 500–800 cycles
Improved separator and electrolytes; 800–1000 cycles
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-7 Lithium Batteries with Lithium Metal Anode Structures
Present Advantages
Present Disadvantages
5 Years
10 Years
Highest energy and power capability
Safety
Improved safety and cycle life through improved electrolytes
—
Poor cycle life
No tolerance to overcharge and overdischarge
Rechargeable Lithium Systems
Lithium systems offer the most promise in terms of specific energy (energy per unit weight). Lithium chemistry, however, raises serious safety and environmental concerns. Even though lithium systems as presently fabricated have no tolerance to overcharging or overdischarging, lithium batteries offer enormous promise as energy sources for the dismounted soldier. Lithium systems can be categorized by the type of components (anode, electrolyte, separator, cathode); each component can be used with a variety of other components to produce a complete cell. Tables C-7 through C-9 characterize lithium battery technologies in terms of their anode structure and materials.
Table C-7 shows improvements in lithium batteries with lithium metal anode structures. To achieve the projected improvements, research will be needed in the following areas:
Charge control in order to eliminate safety concerns
Electrolyte and separator development to improve charge morphology
Management of the film on lithiums surface for improved cycle life
Lithium intercalating anodes include carbon or graphite (LiCx); tin, aluminum, and other metals; and silicon and other nonmetals are shown in Table C-8 To achieve the projected improvements, research will be needed in the following areas:
Improved binders for improved stability of electrode
Materials research to increase rate capability and specific energy
Lighter weight host materials for lithium cathodes
Improved reversibility of positive electrode materials through new preparation methods
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-8 Lithium Batteries with Lithium Intercalated Anode Structures
Present Advantages
Present Disadvantages
5 Years
10 Years
Safer than lithium metal anodes
Rate limiting electrode; no tolerance for overdischarge or overcharge
—
—
Long cycle life
Reduced power and specific energy as compared to lithium metal
Improved power and specific energy through materials improvements
Improved power and specific energy through materials improvements
Reduced low-temperature performance
Material and electrolyte improvements
Material and electrolyte improvements
Some voltage penalty over pure lithium
Lightweight host materials for lithium electrode
—
Lithium alloy anodes include aluminum (LixAl); ternary alloys with manganese; and other lithium alloys such as silicon alloys are shown in Table C-9. To achieve the projected improvements, research will be needed in the following areas:
Materials research to increase rate capability and specific energy
Charge control in order to eliminate safety concern
Electrolyte and separator development to improve charge morphology
Lithium batteries can also be characterized with respect to electrolytes. Tables C-10 and C-11 project the developments and necessary research and development over the next ten years.
TABLE C-9 Lithium Batteries of Lithium Alloy Anode Structures
Present Advantages
Present Disadvantages
5 Years
10 Years
Increased power density as compared to lithium carbon
Reduced specific energy as compared to lithium metal
Improved specific power and specific energy through materials improvements
Improved specific power and specific energy through materials improvements
Voltage penalty
Material and electrolyte improvements
Material and electrolyte improvements
No tolerance of overcharge and overdischarge
—
Increased tolerance of overcharge
Rate limiting electrode
—
—
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-10 Lithium Batteries with a Liquid Organic Electrolytes
Present Advantages
Present Disadvantages
5 Years
10 Years
Mixed organic stable at high voltages
Volatile and flammable
Material improvements to reduce flammability
Material improvements to reduce flammability
High conductivity
Requires stable separator; presently microporous polyolefins
Improved conductivity through salt research
Improved conductivity through salt research
Some toxicity
Less toxic materials
Less toxic materials
No tolerance to overcharge and overdischarge
—
—
Table C-10 shows improvements in lithium batteries using liquid organic electrolytes. To achieve the projected improvements, research will be necessary in:
Materials research to identify stable nonflammable electrolytes
Charge control in order to eliminate safety concerns
Electrolyte and separator development to improve charge morphology
Electrolyte salt investigation.
Table C-11 shows improvements in lithium batteries using liquid organic electrolytes. To achieve the projected improvements, research will be necessary in:
Materials research to identify higher conductivity electrolytes
Charge control in order to eliminate safety concerns
Electrolyte development to improve charge morphology
Electrolyte salt investigation
Lithium/polymer interface reactions (a rise in cell impedance on standing and/or cycling has been observed)
TABLE C-11 Lithium Batteries with Polymer Gel Electrolytes
Present Advantages
Present Disadvantages
5 Years
10 Years
Stable at high voltages
Low conductivity
—
—
Polymer electrolyte and separator
—
Material improvements improving conductivity
Material improvements improving conductivity
Encapsulates volatile and flammable electrolytes
—
Improved conductivity through salt research
Improved conductivity through salt research
No tolerance of overcharge and overdischarge
—
—
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-22 Summary of Electrochemical Capacitor Technology
Construction
Performance
Status
Name
Electrode Configuration
Electrolyte
Energy Density (kJ/kg)
Energy Density (kJ/I)
Resistance (ohms/cm2)
Maximum Power (W/kg)
Cost
Voltage
Typical Capacitance(F)
Largest Unit(J)
Basis for Projection
Readiness Level
NEC Supercap
bipolar carbon/carbon composite
sulfuric acid
4.7
6.8
0.16
4
low
15
470
55k
manufacturer specification
commercially available
NEC FY
bipolar carbon
sulfuric acid
1.2
1.98
45
—
low
5
2.2
—
manufacturer specification
commercially available
NEC FE
bipolar carbon
sulfuric acid
0.036
0.65
1.9
—
low
5
1.5
—
manufacturer specification
commercially available
Panasonic
spiral wound, single-cell carbon
organic
7.9
10.4
7
2.7
low
3
470 1,500
6.7k
commercial device
commercially available
Evans
prismatic carbon
sulfuric acid
0.72
1.8
1
—
low
11
—
40k
manufacturer specification
commercially available
Seiko Instruments
polyacene polymer, button cell
organic
6.84
17.6
12
—
—
5
2.5
—
manufacturer specification
commercially available
Pinnacle Research Institute
bipolar pseudocap using mixed oxides (Ru, Ta)
sulfuric acid
18
50.4
102
2
high
100
0.01
15k
manufacturer test data
custom order
46.8
144
<102
med
theoretical lab projections
Maxwell/
bipolar carbon/
KOH
4.32
7.2
0.1–0.2
1.7
med
28
12
6k
engineering
custom order
Auburn
metal composite
organic
22
32.4
1.5
3
med
3
2,700
12.5k
prototypes
SAFT
bipolar carbon
organic
10.4
15.8
15
1.2
low
3
175
—
engineering prototype
custom order
ARL
bipolar hydrous RuO2
sulfuric acid
96 (active material only)
18.7 (active material only)
—
10
high
5
2.72
34
lab cell
—
Livermore National Laboratory
bipolar aerogel carbon particulate
KOH
3.6
5.4
—
—
med
1
35
—
lab cells
—
Sandia National Laboratory
bipolar synthetic, activated carbon
aqueous
5.0
6.1
0.35
1
med
1
3.5
—
lab cells
—
Los Alamos National Laboratory
bipolar conducting polymer on carbon
solid organic
36–72
—
—
—
low
—
—
—
theoretical lab projections
—
Technautics Hypercap
bipolar pseudocap, Ag-anode, C-cathode
Solid RbAg4I5
1.98
12.6
>1
—
—
0.6
—
—
manufacturer test data
custom order
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-23 Most Promising Component Technologies for Hybrid Systems
Prime Source
Intermediate Storage Unit
Fueled system
High power density rechargeable battery
Battery
Electrochemical capacitor
Solar photovoltaic
Regenerative fuel cell
Nuclear
Flywheel
Metal-air battery
Superconducting inductor
The demand for electrical power in any system is rarely constant. Typically, the demand is cyclic, with the peak demand far exceeding the average power requirements. Because power sources rarely have both high specific energy and high specific power simultaneously, designers have typically designed power systems to meet the maximum demand to ensure adequate energy for the worst case. Thus, systems may be heavier than necessary, or planners may be forced to plan shorter missions or to resupply the primary energy sources. If the differences between the peak and average demands are large, it is advantageous to combine a high specific energy, low-specific-power source with a low-specific-energy, high specific power intermediate store to provide load leveling, which would meet the demand with substantial mass savings or with longer operational times for the same mass.
Many combinations of energy sources and intermediate storage are in use today, such as portable x-ray machines, photoflash units, electric cars, and portable cardiac defibrillators. Usually these are battery-capacitor systems. However, the principle could be applied equally well to a number of prime source-intermediate storage technologies. The most promising component technologies for the dismounted soldier, are listed in Table C-23.
Any combination of a primary power source and intermediate storage unit is capable of producing a power train suited to pulsed operation. A limited number of combinations are described here. The Army is already investing in solar photovoltaic-battery systems, which have been proven in combat.
Fueled System and Battery Hybrid
All fueled systems will probably be hybrids of one kind or another. Any fueled system operated in the battlefield environment will be subject to conditions under which it will be difficult or impossible to operate. Examples are submersion, extreme dust, and closed or confined spaces where exhaust fumes would be harmful to humans. Even in less extreme situations, a battery may still have to provide initial start-up for the fueled system. Depending on the climate and type of system, the battery may have to provide power for preheating the fuel or system, for initial pump power, or for control power.
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Energy-Efficient Technologies for the Dismounted Soldier
TABLE C-24 High Specific Power Batteries for Hybrid Systems
Chemistry
Current Status
Future
Specific Power (W/kg)
Specific Energy (Wh/kg)
Specific Power (W/kg)
Specific Energy (Wh/kg)
Present Cycle Life (Cycles)
State of Development
Nickel
Aqueous
100–200
40–52
150–250
52–70
500–1,000
available
Aqueous (future)
200–500
25–36
250–1,000
30–40
400–800
possible
Bipolar
—
—
200–400
60–80
—
under development
Pb-acid
Bipolar
200
25–45
—
—
300
available
Bipolar (future)
—
—
300
45–60
300
possible
Thin foil
1,000
5
—
—
300
under development
Data described in the section on fueled systems indicate that they are five to ten times more energy dense than batteries for the same mission profile. Therefore, the fueled system, not the battery, provides practically all of the overall mission energy requirement.
For the fueled system-battery combination, there are at least three battery chemistries that warrant further consideration: nickel-cadmium; lithium; and lead-acid. Tables C-24, C-25, and C-26 show the specific energy, specific power, and
TABLE C-25 Commercial and Developmental High Specific Energy Batteries as Energy Sources in Hybrid Systems
Chemistry
Current Status
Future
Specific Power (W/kg)
Specific Energy (Wh/kg)
Specific Power (W/kg)
Specific Energy (Wh/kg)
Present Cycle Life (Cycles)
Future Cycle Life (Cycles)
State of Development
Li-ion/CoO2
100
100
150
150
1,000
2,000
commercially available
Li-ion/Mn2O4
70–100
70–100
150
150
300
600+
available soon
Li(c)/polymer/Mn2O4
150
150 (est)
200+
200
300
600+
prototype soon
Li(c)/polymer/(CS)x
200a
200a
400
300
300
600+
prototype soon
Li(c)/polymer/S
200b
400b
400
600
research
research
research phase
a Prototype
b Laboratory cells
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TABLE C-26 Potential Fueled Systems for Hybrid Power Systems
System
Specific Power (W/kg)a
Specific Energy (Wh/kg)
Status
PEM fuel cell
28
571
prototype
Thermophotovoltaic
25
520
research phase
Alkali-metal thermal-to-electrical converter (AMTEC)
16
1,040
research phase
a Calculated for 2 kg of fuel.
state of development of battery technologies that could be intermediate stores for a fueled-system battery hybrid (Arthur D. Little, 1996). As shown in the tables, a system consisting of an AMTEC and a 0.5 kg lithium-polymer battery would provide 5.3 kWh of energy, with peaks of 100 W, for a total mass of 5 kg. The battery pack could provide 100 Wh of energy without recharging. In some scenarios, this might correspond to an hour or more of operating capability. A lithium-polymer battery pack that could provide the total energy would have a mass of 26 kg. It is impossible to estimate the weight of the associated electronics and packaging that would be necessary to use this technology in a practical scenario. There is, however, almost a factor of five difference in mass (5 kg to 26 kg) for the same available energy.
An AMTEC-NiCd system designed to perform the same functions would have a similar mass. The weight of a NiCd system for the total energy requirement alone would be on the order of 170 kg. The NiCd battery could meet the total energy demand for about 20 minutes, but the NiCd battery would have to be recharged more often than the lithium—polymer battery. In any case, the total energy available would be dominated by the energy in the fuel. It is assumed that the AMTEC is 20 percent efficient in converting the heat of combustion of JP-8 to usable electricity. For this system, the pulse time would be on the order of hours depending on the scenario. In general, a status monitor for the battery would determine its state-of-charge and command the fueled system to maintain an acceptable level automatically. The individual soldier would have override capability. In most scenarios, the fueled system could maintain an intermediate store at 90 percent or more most of the time.
Battery and Electrochemical Capacitor
A battery-capacitor combination for an energy storage system would exploit the high specific power of a capacitor and the high specific energy of a battery. For this system, the time scale of the peak power delivery intervals would
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be shifted from the tens of minutes or hours required for the fueled-battery system to minutes or less for the battery-capacitor system. This combination offers a better system for brief pulses of high power, the kind anticipated for portable digital telephones. Meeting this requirement with a battery alone would require a battery that could provide high power pulses at 8 to 10 times normal capacity and would still have maximum life and adequate operational time between charges. Using a capacitor to meet the peak power requirement would provide better operating performance, longer battery life, and better low temperature operation while lowering life cycle costs and a smaller, lighter weight package. Figure C-13 shows a generic power-time profile for a pulsed digital communications system.
In a recent paper, J.R. Miller (1996) developed a simple simulation of a 1 Ah lithium battery in parallel with an experimental electrochemical capacitor. The battery had an open circuit voltage of 4.1 V and an internal resistance of 0.1 ohms. The parameters assumed for the electrochemical capacitor were a capacitance of 1.28 F and an internal resistance of 0.069 ohms. For a repetitive pulse train of 8.3 ms at 10 A spaced by 90 ms, the battery alone was able to provide 12 minutes of operation. The battery-capacitor combination was able to power the system for 61 minutes, an improvement of roughly a factor of five. Simple circuit models were developed that can be used to predict the performance
FIGURE C-13 Typical power-time profile for pulsed digital communications devices.
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TABLE C-27 Energy Storage Media That Could Be Used in Hybrid Systems
Storage Media
State of Art
Energy Density (Wh/kg)
Practical Limit to Specific Power (Wh/kg)
Key Issues
Scaling Laws
Impact
Storage Time
Batteries
highly developed
180–360
~400
electrodes; electrolytes; seals; safety; corrosion
known
major/ enabling
years
Capacitor
highly developed
0.25–1.00
~8.00
molecular engineering of film; manufacturing technology; thermal stability; electrical breakdown
known
enabling for some systems concepts
minutes
Film Foil
Paper Foil
Ceramic
highly developed
~0.30
> 3.00
large area samples; electrical breakdown; manufacturing technology
known
enabling for some systems concepts
moderate
Electrolytic
highly developed
< 0.5
>0.75
large surface area material; suitable oxides; electrolytes
known
minimal
minutes
Chemical double layer
developing
~7.00
>12.00
large surface area materials; electrolytes; equivalent series resistance/equivalent parallel resistance; seals
known
major
minutes
Magnetic
advanced
> 15.00
strength of materials limited
advanced composites; low resistivity materials
known
minimal
milliseconds
Inertial
highly developed for some applications
100.00
> 300.00
high strength materials; gyroscopic effects; safety
known
minimal
hours/days
Thermal
evolving
sensible heat depends on dT
absolute temperature dominated >5000
materials compatibility; high strength materials; high specific heat
known
uncertain
days/weeks
of battery-capacitor combinations accurately. In a similar experimental study, Merryman and Hall (1996) showed that the power train mass for an electrically actuated thrust vector control system for the space shuttle could be reduced by 59 percent when a battery-capacitor combination was used.
Table C-27 is a compilation of the characteristics of energy storage media that could possibly be used in hybrid systems. (For completeness, the table includes some media not covered in the text.)
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Key Research Issues
Pulsed power techniques have been used extensively in the high-power regimes. Numerous laboratory demonstrations of hybrid systems, typical of systems appropriate for the dismounted soldier, have been performed. To date, there have been no field tests to determine the utility of using hybrid systems for human-portable power. To optimize the design, information on the power demand time history for a variety of mission profiles will be necessary. Given this data, a hybrid system can be designed for the worst case scenario that maximizes the available energy. The key issues are developmental and consist of:
development of computer models for predicting performance as a function of mission profile
development of laboratory prototypes
obtaining reliable field data for the development of energy utilization profiles of the various soldier subsystems
SUMMARY
Table C-28 summarizes the energy and power systems discussed in this appendix. The development of hybrid systems with a fueled primary store would be revolutionary. However, each of the technologies described in Table C-28 has drawbacks. Primary batteries cannot provide the requisite energy for the projected energy budgets of dismounted soldier systems without becoming unstable and creating a significant safety hazard. Primary batteries also pose a significant environmental hazard that will probably increase as new chemistries become available. The primary hazards of batteries are explosive rupture, toxic and corrosive electrolytes, and environmental pollution if they are not recovered. Inevitably, trade-offs among safety, energy, and power considerations will have to be carefully assessed for any system or mission. A secondary battery with the specific energy and specific power of primary batteries would be highly desirable. If this technology were available, the environmental restrictions would be lessened because less frequent recycling would be required. Even a high specific energy rechargeable battery with limited life (say, 50 charge/discharge cycles) would greatly lessen the current problems of supply and disposal.
Any system energetic enough to be considered a major advance for the Army will undoubtedly also be dangerous. Batteries are both energy storage systems and converters in the same unit, and battery safety is closely related to the oxidants and reductants. Consequently, if batteries are designed toward the margin, they have a tendency to explode.
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TABLE C-28 Technology Summary of Energy Systems
Power System
State of the Art
Potential for Improvement
Key Issues
Scaling Laws
Impact on Dismount Soldier
Hostile Signature
Suppression Potential
Fuel Required
Autonomy Time
Primary battery
Mature
Moderate
Energy density Safety Power density Environmental impact
Known
Longer mission Less weight Disposability
Minimal
Excellent
None
Hours/days
Secondary battery
Mature
Moderate
Energy density Cycle life Power density
Known
New capability Cost savings Less weight
Minimal
Excellent
None
Hours
Thermophotovoltaics
Emerging
Excellent
Requires cooling Efficiency Lifetime Ruggedization
Uncertain
New capability Cost savings Longer mission
Thermal
Moderate
Multifuel
Days/weeks
Fuel cells (hydrogen)
Exploratory development
Excellent
Fuel Water management Safety
Known
New capability Less weight Cost savings
Thermal
Excellent
Hydrogen
Days/weeks
Fuel cells (methanol)
Emerging
Excellent
Fuel and fuel crossover Catalyst
Uncertain
New capability Cost savings Less weight
Thermal
Excellent
Methanol
Days/weeks
Alkali-metal thermal-to electrical converters
Emerging
Excellent
Liquid metal Membranes Pumps/wicks Ruggedization
Uncertain
New capability Less weight Cost Savings
Thermal
Moderate
Multifuel
Days/weeks
Nuclear isotope
Limited
Excellent
Safety Environmental impact Cost Public acceptance
Known
New capability Autonomy
Thermal Nuclear
Moderate
Special
Month/years
Internal combustion
Some versions mature
Moderate to excellent
Fuels Vibration Life
Uncertain
Cost savings Less weight
Thermal Acoustic
Moderate
Multifuel (Some Special)
Days/weeks
Microturbine
Emerging
Excellent
Safety
Uncertain
New capability
Acoustic
Difficult
Days/weeks
Thermoelectric
Some versions mature
Moderate to excellent
Efficiency Materials Coupling
Known
New capability Less weight
Thermal
Moderate
Multifuel
Days/weeks
Human-powered
Nonexistent
Excellent
Conversion mechanisms
Unknown
New capability Cost saving Autonomy
Minimal
Excellent
Food
Weeks
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In fueled systems, the energy dense fuel is in a separate enclosure and is slowly exposed to the oxidant so that only the fuel that is in the converter at any given time is subject to inadvertent catastrophic failure. With the exception of hydrogen, all of the other fuels are rather involatile, that is, they can burn rapidly but will probably not explode. Fuels are housed in external tanks, which would be subject to penetration and burning if the penetration were energetic enough to ignite them.
Primary batteries will be used in military systems for the foreseeable future. There will, however, continue to be problems associated with their disposal, inventory, safety, and availability, and wherever possible, they should be replaced. The logical evolution of the Army power system for the dismounted soldier is toward a rechargeable battery with improved specific power and energy that would meet or exceed the power available with current primary batteries coupled with a ''personal" charger that contains the primary store of energy for the mission. For many missions, the rechargeable battery alone would have enough energy. In those cases, the battery would be returned to the inventory after being recharged. For longer missions, the primary store would be fueled by a standard battlefield fuel. All of the fueled systems described in this appendix offer the possibility of long life with thousands of refuelings, and all of them are at a stage at which advanced development is possible. Coupled with a suitable rechargeable battery with similar cycle capability, these systems would dramatically reduce the inventory necessary to maintain combat readiness. The primary logistic consideration would be—as it is now—fuel supplies. Because batteries could be recharged many times, recycling after each mission would not be necessary, which would greatly reduce their adverse environmental impact.
High specific energy rechargeable batteries are becoming increasingly important in the commercial sector, which could provide the Army with a secure, high volume, guaranteed source of batteries. "Smart" chargers and power management circuitry will also be forthcoming from the commercial sector.
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