the latter--1980's. A natural gas supply is the rec-
compared to the more widely used electrical power
generation systems. With few exceptions, very
ommended fuel; however, bottled gas can be pro-
limited data are to be found on reliability of fuel
vided. Most units are provided electric-powered re-
former units; however, if necessary the hydrogen
cells in service. Fuel cells offer some advantages to
the designer of hardened facilities and their devel-
gas can be obtained by use of the destructive distil-
lation of coal and wood, which is an expensive proc-
opment should be watched.
ess. These units are being tested in a cogeneration
(2) Of the fuel-cell types that have been most
mode, with the thermal energy being used for
extensively tested, the hydrogen-oxygen fuel cell
space heating and domestic hot water heating.
used in the Apollo space program is of the type
best suited to closed-cycle operation and possibly
(6) For a relatively low-powered system and a
moderately long endurance period, say 10 kW and
the only specific system design for which reliability
60 days, the fuel-cell system space requirement
data are available. The Apollo fuel-cell system was
based on the Apollo system and a packing factor of
designed for maximum power-to-weight ratio and
3
0.2 would be only about 300 ft ; the storage space
maximum efficiency and is, consequently, relative-
at 50 percent overall system efficiency for liquid
ly costly (on the order of 0,000 to 0,000/kW
hydrogen and oxygen will be substantially greater.
at 1976 costs). The system is packaged in units
For liquid gas storage in shock-isolated dewars,
rated at about 1.4 kW d.c. output at 27 to 31 V.
with reasonable allowances for ullage, replacement
The claimed efficiency, based on the low heat value
of "boil off" at 30-day intervals, and a packing fac-
of the hydrogen fuel, is nearly 90 percent. Based
tor that allows for pumps, plumbing, and rattle-
on the high heat value, which is of prime interest
space, the estimated fuel-cell system gas storage
from the standpoint of waste-heat generation, the
space requirements would be on the order of 1500
efficiency would be about 75 percent.
3
3
ft , or about 104 ft /MWh. The ratios of shock-
(3) Limited data from larger hydrogen-oxygen
isolated weight and shock-isolated space appear to
fuel-cell systems indicate that increased power de-
be 100:1 and 50:1, respectively, in favor of the fuel-
mand of auxiliary systems will reduce the overall
cell system over the battery power system.
system efficiency. Based on an average inverter ef-
(7) The direct capital costs of the fuel-cell sys-
ficiency of 85 percent, the probable overall, aver-
tem plus fuel and oxygen supply for the system
age thermal efficiency is on the order of 50 percent.
identified (14.4 MWh at 10 kW) would be competi-
Hydrogen-oxygen fuel cell systems have an advan-
tive with a battery-powered system. Further ma-
tage over all power sources except batteries in that
jor reductions in fuel-cell costs are entirely proba-
their part-load efficiency does not decline below
ble, but substantial reductions in battery costs are
rated-load efficiency until the load falls to about 20
less likely.
percent of rated load. If hydrogen and oxygen are
stored as liquids, problems associated with excess
d. Combined fuel-cell and battery systems.
boil-off and long-term storage of cryogenics should
Where power profiles call for standby periods at
not be overlooked.
low-power level interspersed with shorter periods
(4) Cost considerations make it unlikely that
of substantially higher power demand, there may
hydrogen-oxygen fuel-cell inverter systems will be
be considerable advantage in combined fuel-
used to meet very high peak power demands, but
cell/battery systems. Together fuel cells and
such systems may be applicable to long term, low-
batteries would support the peak loads and the
level power profiles. They are readily adaptable to
fuel-cell system would support the standby load
closed-cycle operation since the only waste efflu-
and recharge the batteries during low power
ents are heat and water and the cell operating tem-
demand.
peratures are high enough (about 500F) to permit
(1) In a single example, doubling the peak out-
put of the fuel-cell system discussed above, with
final water temperature of nearly 212F for an
the same total energy output (14.4 MWh), would
unpressurized heat sink.
nearly double the first cost of the system. How-
ever, if the peak output were to be doubled to 20
(5) Phosphoric acid fuel cell power plants have
kW for 8-hour in 24-hour by use of a battery in par-
been developed and are currently being tested. A
allel with the fuel-cell system, the battery recharge
40 kW phosphoric-acid-type fuel cell has been de-
requirement (at 80 percent discharge/ charge effi-
veloped under joint sponsorship of the U.S. De-
ciency) would absorb about 62.5 percent of the fuel-
partment of Energy (DOE) and the utility industry
cell system capacity during the remaining time,
represented by the Gas Research Institute (GRI).
This fuel cell is costly (approximately 0,000 for
leaving 37.5 percent or 3.75 kW of the net fuel-cell
system output for the facility's load during a
a 40 kW unit based on a test project). Phosphoric
acid fuel cells should be commercially available in
16-hour day. With the same liquid hydrogen and
2-5
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