TM
5-858-7
cal systems offer a nearly ready-made design ap-
these cells undergo a loss of energy capacity on the
p r o a c h to closed-cycle power systems. The
order of 0.0042 percent/hour (about 3 percent in 30
required rectifier charger-inverter equipment is
days), based on the 8-hour rating, as indicated by
available off-the-shelf from a number of manufac-
the float charge current necessary to maintain a
turers. Lead-acid cell batteries are commonly used
fully charged condition. The rates of loss and
in battery systems. More than 100 years of exten-
compensating float charge are nearly constant over
sive development, and use of lead-acid cells has re-
this useful life (20 to 25 years). In comparison, the
sulted in extremely high reliability of such
open circuit energy loss rate for new antimony-
batteries when properly maintained. Except at
hardened, lead-acid cells is about eight times
very high rates of discharge, batteries generate
greater (about 0.032 percent/hr) and increases with
virtually no waste heat.
age. Whenever the specified pastattack endurance
period exceeds a few days, the 0.028 percent/hour
(1) Since lead-acid batteries are capable of
differential becomes an important consideration.
very high power output for short periods, the peak
(4) For large installations, the largest stand-
power output capability of battery systems is usu-
ard cell, rated at 8000 amp-hr (or 15 kWh) under
ally limited only by the inverter units. The rectifier
standard conditions, will maximize storage density.
units are normally designed to carry the rated in-
A 46-MWh installation of this type, including main-
verter load with sufficient excess capacity to per-
tenance access space, support structure, shock iso-
mit simultaneous battery charging at about 1/8 of
lation, and rattlespace, and fitted to a cylindrical
the 8-hour battery rating, or 1/8 of the maximum
capsule configuration, has been estimated to have a
discharge rate as limited by the inverter capacity,
packing factor of about 0.17 and to require about
whichever is smaller. With better inverter systems
3
4350 ft of installation volume per MWh of battery
operated. in a standby mode, the changeover to bat-
capacity. Based on 82 percent overall postattack
tery power on failure of the external power source
system efficiency, the space requirement wou1d be
is virtually undetectable on an oscillogram of the
3
about 5305 ft /MWh of net useful energy. At cur-
inverter a.c. output. The high reliability of the
rent (1976) design weights, the battery cell net
inverter section is obtained by the use of high qual-
weight would be about 143,000 lb/MWh. These
ity parts, oversizing, and redundancy, but at rela-
weights and volumes do not include the capsule
tively high cost in size, weight, and dollars. In-
shell itself. See TM 5--858-4 for area and volume
stalled cost, weight, and volume (exclusive of
needs of shock isolation platforms needed under
batteries) for the larger capacity systems (250 kW
various specified threats.
or more) are comparable to those for complete,
(5) The large unit weight anti space indicated
high quality, diesel-generator installations. Sim-
for large battery installations make it important to
pler, more compact, and less costly inverter sys-
recognize that the numbers quoted are intended
tems can be built, and could be adequate for some
only for preliminary estimates and apply to typical
prime-mission requirements; however, lack of reli-
(lead-acid cell) batteries. Variations in discharge
ability data could be a serious problem.
time, system efficiency, and cell operating condi-
(2) Rectifier-inverter systems are available as
tions can increase initial cell requirements.
integrated units up to about 250 KW (300 kVA) ca-
c. Fuel cell. Fuel cells convert chemical energy
pacity. These units can be paralleled to obtain out-
directly to direct current electrical energy. Indi-
puts to 3 MW or more. In high-quality systems the
vidual fuel cells usually operate at less than one
overall rectifier-inverter system efficiency will
volt, and become useful power sources only as
range from about 50 percent to 88 percent, at 25
batteries or stacks of individual cells connected in
and 100 percent, respectively, of rated load. Waste
series to produce higher voltages. In addition to
heat should probably be based on an average
the fuel cells themselves, all fuel-cell systems in-
rectifier-inverter system efficiency of no higher
clude a relatively complex array of auxiliaries to
than 80 percent. Most of the losses will be in the
provide and control fuel, oxidizer, and electrolyte
inverter section.
flow, and waste-heat rejection. Since fuel-cell out-
(3) The calcium-hardened lead-acid cell is
put is direct current, it is necessary to consider
strongly recommended to eliminate potential
fuel-cell systems in conjunction with inverters to
poisoning by gaseous antimony hydride. These are
make useful comparisons with other potential pow-
lead-acid cells in which the alloying material used
er sources considered herein. This necessarily re-
to harden the lead plates is calcium rather than an-
duces overall thermal efficiency and increases total
timony. Calcium-hardened cells are somewhat
--
power-system weight and space requirements.
higher in first cost than regular lead-acid cells, but
are longer lived, require less maintenance, and
(1) In higher power ranges no fuel-cell system
has been perfected that is cost-effective (1973)
have much lower internal losses. On open circuit
2-4
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