Cell Power Plants
Each of the three fuel cell power plants is reusable and restartable.
The fuel cells are located under the payload bay area in the forward
portion of the orbiter's midfuselage.
The three fuel cells operate as independent electrical power
sources, each supplying its own isolated, simultaneously operating
28-volt dc bus. The fuel cell consists of a power section, where
the chemical reaction occurs, and an accessory section that controls
and monitors the power section's performance. The power section,
where hydrogen and oxygen are transformed into electrical power,
water and heat, consists of 96 cells contained in three substacks.
Manifolds run the length of these substacks and distribute hydrogen,
oxygen and coolant to the cells. The cells contain electrolyte
consisting of potassium hydroxide and water, an oxygen electrode
(cathode) and a hydrogen electrode (anode).
The accessory section monitors the reactant flow, removes waste
heat and water from the chemical reaction and controls the temperature
of the stack. The accessory section consists of the hydrogen and
oxygen flow system, the coolant loop and the electrical control
Oxygen is routed to the fuel cell's oxygen electrode, where it
reacts with the water and returning electrons to produce hydroxyl
ions. The hydroxyl ions then migrate to the hydrogen electrode,
where they enter into the hydrogen reaction. Hydrogen is routed
to the fuel cell's hydrogen electrode, where it reacts with the
hydroxyl ions from the electrolyte. This electrochemical reaction
produces electrons (electrical power), water and heat. The electrons
are routed through the orbiter's EPDC subsystem to perform electrical
work. The oxygen and hydrogen are reacted (consumed) in proportion
to the orbiter's electrical power demand.
Excess water vapor is removed by an internal circulating hydrogen
system. Hydrogen and water vapor from the reaction exits the cell
stack, is mixed with replenishing hydrogen from the storage and
distribution system, and enters a condenser, where waste heat
from the hydrogen and water vapor is transferred to the fuel cell
coolant system. The resultant temperature decrease condenses some
of the water vapor to water droplets. A centrifugal water separator
extracts the liquid water and pressure-feeds it to potable tanks
in the lower deck of the pressurized crew cabin. Water from the
potable water storage tanks can be used for crew consumption and
cooling the Freon-21 coolant loops. The remaining circulating
hydrogen is directed back to the fuel cell stack.
The fuel cell coolant system circulates a liquid fluorinated
hydrocarbon and transfers the waste heat from the cell stack through
the fuel cell heat exchanger of the fuel cell power plant to the
Freon-21 coolant loop system in the midfuselage. Internal control
of the circulating fluid maintains the cell stack at a normal
operating temperature of approximately 200 F.
When the reactants enter the fuel cells, they flow through a
preheater (where they are warmed from a cryogenic temperature
to 40 F or greater); a 6-micron filter; and a two-stage, integrated
dual gas regulator module. The first stage of the regulator reduces
the pressure of the hydrogen and oxygen to 135 to 150 psia. The
second stage reduces the oxygen pressure to a range of 62 to 65
psia and maintains the hydrogen pressure at 4.5 to 6 psia differential
below the oxygen pressure. The regulated oxygen lines are connected
to the accumulator, which maintains an equalized pressure between
the oxygen and the fuel cell coolant. If the oxygen's and hydrogen's
pressure decreases, the coolant's pressure is also decreased to
prevent a large differential pressure inside the stack that could
deform the cell stack structural elements.
Upon leaving the dual gas regulator module, the incoming hydrogen
mixes with the hydrogen-water vapor exhaust from the fuel cell
stack. This saturated gas mixture is routed through a condenser,
where the temperature of the mixture is reduced, condensing a
portion of the water vapor to form liquid water droplets. The
liquid water is then separated from the hydrogen-water mixture
by the hydrogen pump/water separator.
The hydrogen pump circulates the hydrogen gas back to the fuel
cell stack, where some of the hydrogen is consumed in the reaction.
The remainder flows through the fuel cell stack, removing the
product water vapor formed at the hydrogen electrode. The hydrogen-water
vapor mixture then combines with the regulated hydrogen from the
dual gas generator module, and the loop begins again.
The oxygen from the dual gas regulator module flows directly
through two ports into a closed-end manifold in the fuel cell
stack, achieving optimum oxygen distribution in the cells. All
oxygen that flows into the stack is consumed, except during purge
Reactant consumption is directly related to the electrical current
produced: if there are no internal or external loads on the fuel
cell, no reactants will be used. Because of this direct proportion,
leaks may be detected by comparing reactant consumption and current
produced. An appreciable amount of excess reactants used indicates
a probable leak.
Water and electricity are the products of the chemical reaction
of oxygen and hydrogen that takes place in the fuel cells. The
water must be removed or the cells will become saturated with
water, decreasing reaction efficiency. With an operating load
of about 7 kilowatts, it takes only a few minutes to flood the
fuel cell with produced water, thus effectively halting power
generation. Hydrogen is pumped through the stack, reacting with
oxygen and picking up and removing water vapor on the way. After
being condensed, the liquid water is separated from the hydrogen
by the hydrogen pump/water separator and discharged from the fuel
cell to be stored in the ECLSS potable water storage tanks.
If the water tanks are full or there is line blockage, the water
relief valves open at 45 psia to allow the water to vent overboard
through the water relief line and nozzle. Check valves prevent
water tanks from discharging through an open relief valve. An
alternate water delivery path is also available to deliver water
to the ECLSS tanks if the primary path is lost.
For redundancy, there are two thermostatically activated heaters
wrapped around the discharge and relief lines to prevent blockage
caused by the formation of ice in the lines. Two switches on panel
R12, fuel cell H 2 O line htr and H2O relief htr , provide the
flight crew with the capability to select either auto A or auto
B for the fuel cell water discharge line heaters and the water
relief line and vent heaters, respectively.
Thermostatically controlled heaters will maintain the water line
temperature above 53 F, when required. The normal temperature
of product water is approximately 140 to 150 F. The thermostatically
controlled heaters maintain the water relief valve's temperature
when in use between 70 to 100 F. Temperature sensors located on
the fuel cell water discharge line, relief valve, relief line
and vent nozzle are displayed on the CRT.
If the potassium hydroxide electrolyte in the fuel cell migrates
into the product water, a pH sensor located downstream of the
hydrogen pump/water separator will sense the presence of the electrolyte,
and the crew will be alerted by an SM alert and display on the
During normal fuel cell operation, the reactants are present
in a closed-loop system and are 100 percent consumed in the production
of electricity. Any inert gases or other contaminants will accumulate
in and around the porous electrodes in the cells and reduce the
reaction efficiency and electrical load support capability. Purging,
therefore, is required at least twice daily to cleanse the cells.
When a purge is initiated by opening the purge valves, the oxygen
and hydrogen systems become open-loop systems; and increased flows
allow the reactants to circulate through the stack, pick up the
contaminants and blow them out overboard through the purge lines
and vents. Electrical power is produced throughout the purge sequence,
although no more than 10 kilowatts should be required from a fuel
cell being purged because of the increased reactant flow and preheater
Fuel cell purge can be activated automatically or manually by
the use of fuel cell switches on panel R12. In the automatic mode,
the fuel cell purge heater switch is positioned to GPC . The purge
line heaters are turned on to heat the purge lines to ensure that
the reactants will not freeze in the lines. The hydrogen reactant
is the more likely to freeze because it is saturated with water
vapor. Depending on the orbit trajectory and vehicle orientation,
the heaters may require 27 minutes to heat the lines to the required
temperatures. The fuel cell current is checked to ensure a load
of less than 350 amps, due to limitations on the hydrogen and
oxygen preheaters in the fuel cells. As the current output of
the fuel cell increases, the reactant flow rates increase, and
the preheaters raise the temperature of the reactants to a minimum
of minus 40 F in order to prevent the seals in the dual gas regulator
The purge lines from all three fuel cells are manifolded together
downstream of their purge valves and associated check valves.
The line leading to the purge outlet is sized to permit unrestricted
flow from only one fuel cell at a time. If purging of more than
one cell at a time is attempted, pressure could build in the purge
outlet line and cause a decrease in the flow rate through the
individual cells, which would result in an inefficient purge.
When the fuel cell purge valves 1, 2 and 3 switches are positioned
to GPC, the fuel cell GPC purge seq switch is positioned to start
and must be held until the GPC purge seq talkback indicator indicates
gray (in approximately three seconds). The automatic purge sequence
will not begin if the indicator indicates barberpole. The GPC
turns the purge line heaters on and monitors the temperature.
The one oxygen line temperature sensor must register at least
69 F and the two hydrogen line temperature sensors 79 and 40 F,
respectively, and be verified by the GPC before the purge sequence
begins. If the temperatures are not up to minimum after 27 minutes,
the GPC will issue an SM alert and display the data on the CRT.
When the proper temperatures have been attained, the GPC will
open for two minutes and then close the hydrogen and oxygen purge
valves for fuel cells 1, 2 and 3 in that order. Thirty minutes
after the fuel cell 3 purge valves have been closed (to ensure
that the purge lines have been totally evacuated), the GPC will
turn off the purge line heaters. This provides sufficient time
and heat to bake out any remaining water vapor. If the heaters
are turned off before 30 minutes have elapsed, water vapor left
in the lines may freeze.
The manual fuel cell purge would be initiated by the flight crew
using the switches on panel R12. In the manual mode, the three
fuel cells must be purged separately. The fuel cell purge heater
switch is positioned to on for the same purpose as in the automatic
mode, and the flight crew verifies that the temperatures of the
oxygen line and two hydrogen lines are at the same minimum temperatures
as in the automatic mode before the purge sequence is initiated.
The fuel cell purge valves 1 switch is positioned to open for
two minutes, and the flight crew observes that the oxygen and
hydrogen flow rates increase on the CRT. The fuel cell purge valves
1 switch is then positioned to close , and a decrease in the oxygen
and hydrogen flow rates is observed on the CRT, indicating the
purge valves are closed. Fuel cell 2 is purged in the same manner
using the fuel cell purge valves 2 switch. Fuel cell 3 is then
purged in the same manner using the fuel cell purge valves 3 switch.
After the 30-minute line bakeout period, the fuel cell purge heater
switch is positioned to off.
In order to cool the fuel cell stack during its operations, distribute
heat during fuel cell starting, and warm the cryogenic reactants
entering the stack, the fuel cell circulates a coolant-fluorinated
hydrocarbon-throughout the fuel cell. The fuel cell coolant loop
and its interface with the ECLSS Freon-21 coolant loops are identical
in fuel cells 1, 2 and 3.
Where the coolant enters the fuel cell, the temperature of the
F-40 coolant returning from the ECLSS Freon-21 coolant loops is
sensed before it passes through a 75-micron filter. After the
filter, two temperature-controlled mixing valves allow some of
the hot coolant to mix with the cool returning coolant to prevent
the condenser exit control valve from oscillating. The condenser
exit control valve adjusts the flow of the coolant through the
condenser to maintain the hydrogen-water vapor exiting the condenser
at a temperature between 148 and 153 F.
The stack inlet control valve maintains the temperature of the
coolant entering the stack between 177 and 187 F. The accumulator
is the interface with the oxygen cryogenic reactant to maintain
an equalized pressure between the oxygen and the coolant (the
oxygen and hydrogen pressures are controlled at the dual gas regulator)
to preclude a high-pressure differential in the stack. The pressure
in the coolant loop is sensed before the coolant enters the stack.
The coolant is circulated through the fuel cell stack to absorb
the waste heat from the hydrogen/oxygen reaction occurring in
the individual cells. After the coolant leaves the stack, its
temperature is sensed and the data transmitted to the GPC, to
the fuel cell stack temp meter through the fuel cell 1, 2, 3 switch
located below the meter on panel O2, and to the CRT display. The
yellow fuel stack temp C/W and the backup C/W alarm lights on
panel F7 and the SM alert light will be illuminated if fuel cell
and stack temperatures exceed certain limits: below 172.5 F or
above 243.7 F. The hot coolant from the stack flows through the
oxygen and hydrogen preheaters, where it warms the cryogenic reactants
before they enter the stack.
The coolant pump utilizes three-phase ac power to circulate the
coolant through the loop. The differential pressure sensor senses
a pressure differential across the pump to determine the status
of the pump. The fuel cell pump C/W light on panel F7 will be
illuminated if fuel cell 1, 2 or 3 coolant pump delta pressure
is lost. The SM alert light also will be illuminated, and a fault
message will be sent to the CRT. If the coolant pump for fuel
cell 1, 2 or 3 is off , the backup C/W alarm light will be illuminated,
and a fault message will be sent to the CRT. The temperature-actuated
flow control valve downstream from the pump adjusts the coolant
flow to maintain the fuel cell coolant exit temperature between
190 and 210 F. The stack inlet control valve and flow control
valve have bypass orifices to allow coolant flow through the coolant
pump and to maintain some coolant flow through the condenser for
water condensation, even when the valves are fully closed due
to the requirements of thermal conditioning.
The coolant (that which is not made to bypass) exits the fuel
cells to the fuel cell heat exchanger, where it transfers its
excess heat to be dissipated through the ECLSS Freon-21 coolant
loop systems in the midfuselage.
In addition to thermal conditioning by means of the coolant loop,
the fuel cell has internal startup and sustaining heaters. The
2,400-watt startup heater is used only during startup to warm
the fuel cell to its operational level. The 1,100-watt sustaining
heaters normally are used during low power periods to maintain
the fuel cells at their operational temperature.
Two 160-watt end-cell electrical heaters on each fuel cell power
plant were used to maintain a uniform temperature throughout the
fuel cell power section. As an operational improvement, the end-cell
electrical heaters on each fuel cell power plant were deleted
due to potential electrical failures and were replaced by fuel
cell power plant coolant (F-40) passages. This permits waste heat
from each fuel cell power plant to be used to maintain a uniform
temperature profile for each fuel cell power plant.
The hydrogen pump and water separator of each fuel cell power
plant were also improved. To minimize excessive hydrogen gas entrained
in each fuel cell power plant's product water, modifications were
made to the water pickup (pitot) system. The centrifugal force
of high-velocity water flowing around the pitot tube's bends separates
the hydrogen gas and water. Pitot pressure then expels the hydrogen
gas into the hydrogen pump's inlet housing though a bleed orifice.
A current measurement detection system was added to monitor the
hydrogen pump load for each fuel cell power plant. Excessive load
could indicate improper water removal, which could lead to flooding
of the fuel cell power plant and eventually render that power
The start/sustaining heater system for each fuel cell power plant
was also modified. The modification was required specifically
for fuel cell power plant No. 1, mounted on the port, or left,
side. The No. 1 fuel cell power plant start/sustaining heater
system added heat to that fuel cell power plant's F-40 coolant
loop system during the startup of the power plant. Because of
its orientation, any entrained gas in the coolant could enter
the heater and become trapped at the heater elements. This would
result in overheating of the heater elements, which could vaporize
the F-40 coolant, causing heater failure and extensive damage
to the fuel cell power plant. The F-40 coolant loop flow system
within the start/sustaining heater of each fuel cell power plant
was modified to prevent a gas bubble from developing or being
trapped at the heater elements, preventing the loss of the start/sustaining
A stack inlet temperature measurement was added to each fuel
cell power plant. The temperature measurement was added to the
in-flight system to provide full visibility of the thermal conditions
of each fuel cell power plant (similar to the existing stack exit
and condenser exit temperatures of each fuel cell power plant).
The product water from all three fuel cell power plants flows
to a single water relief control panel. The water can be directed
from the single panel to the ECLSS potable water tank A or to
the fuel cell power plant water relief nozzle. Normally, the water
is directed to water tank A. In the event of a line rupture in
the vicinity of the single water relief panel, water could spray
on all three water relief panel lines, causing them to freeze
and prevent fuel cell power plant water discharge.
The product water lines from all three fuel cell power plants
were modified to incorporate a parallel (redundant) path of product
water to ECLSS potable water tank B in the event of a freeze-up
of the single water relief panel. In the event of the single water
relief panel freeze-up, pressure would build up and relieve through
the redundant paths to water tank B. Temperature sensors and a
pressure sensor installed on each of the redundant water line
paths transmit data via telemetry for ground monitoring.
A water purity sensor (pH) was added at the common product water
outlet of the water relief panel to provide a redundant measurement
of water purity. A single measurement of water purity in each
fuel cell power plant was provided previously. If the fuel cell
power plant pH sensor failed, the flight crew was required to
sample the potable water.
The electrical control unit located in each fuel cell power plant
is the brain of the power plants. The ECU contains the start up
logic, heater thermostats, and 30-second timer and interfaces
with the controls and displays for fuel cell startup, operation
and shutdown. The ECU controls the supply of ac power to the coolant
pump, hydrogen pump/water separator, the pH sensor, and the dc
power supplied to the flow control bypass valve (open only during
startup) and the internal startup and sustaining heaters. The
ECU also controls the status of the fuel cell 1, 2, 3 ready for
load and coolant pump P talkback indicators on panel R1.
The nine fuel cell circuit breakers that connect the three-phase
ac power to the three fuel cells are located on panel L4, and
the fuel cell ECU receives its power from an essential bus through
the FC cntlr switch on panel O14.
The fuel cell start/stop switch on panel R1 for each fuel cell
is used to initiate the start sequence or stop the fuel cell operation.
When this switch is held in its momentary start position, the
ECU connects the three-phase ac power to the coolant pump and
hydrogen pump/water separator (allowing the coolant and the hydrogen-water
vapor to circulate through these loops) and connects the dc power
to the internal startup and sustaining heaters and the flow control
bypass valve. The switch must be held in the start position until
the coolant pump P talkback shows gray in approximately three
to four seconds, which indicates that the coolant pump is functioning
properly by creating a differential pressure across the pump.
When the coolant pump P talkback indicates barberpole, it indicates
the coolant pump is not running.
The ready for load talkback for each fuel cell will show gray
after the 30-second timer times out and the stack-out temperature
is above 187 F (which can be monitored on panel O2 in conjunction
with the 1, 2, 3 switch located beneath the fuel cell stack out
temp meter). This indicates that the fuel cell is up to the proper
operating temperature and is ready for loads to be attached to
it. It should not take longer than 25 minutes for the fuel cell
to warm up and become fully operational, the actual time depending
on the fuel cell's initial temperature. The ready for load indicator
remains gray until the fuel cell start/stop switch for each fuel
cell is placed to stop, the FC cntlr switch is placed to off ,
or the essential bus power is lost to the ECU.
The startup heater enable/inhibit switch on panel R12 for each
fuel cell provides the crew control of the off/on status of the
startup heaters during fuel cell startup. The inhibit position
allows the startup heaters to remain off and would be used only
when immediate power is required from a shutdown fuel cell.
Fuel cell 1, 2 or 3 dc voltage and current (amps) can be monitored
on the dc volts and dc amps meters on panel F9, using the fuel
cell volts/amp rotary switch to select a specific fuel cell.
The fuel cells will be on when the crew boards the vehicle, and
the vehicle is powered by the fuel cells and load sharing with
the ground support equipment power supplies. Just before lift-off
(T minus three minutes and 30 seconds), the GSE is powered off
and the fuel cells take over all of the vehicle's electrical loads.
Indication of the switchover can be noted on the CRT display and
the dc amps meter. The fuel cell current will increase to approximately
220 amps; the oxygen and hydrogen flow will increase to approximately
4 and 0.6 pound per hour, respectively; and the fuel cell stack
temperature will increase slightly.
Fuel cell standby consists of removing the electrical loads but
continuing operation of the fuel cell pumps, controls, instrumentation
and valves, with the electrical power being supplied by the remaining
fuel cells. A small amount of reactants is used to generate power
for the fuel cell internal heaters.
Fuel cell shutdown, after standby, consists of stopping the coolant
pump and hydrogen pump/water separator by positioning that fuel
cell start/stop switch on panel R1 to the stop position. If the
temperature in the fuel cell compartment beneath the payload bay
is lower than 40 F, the fuel cell should be left in standby instead
of being shut down to prevent it from freezing.
Each fuel cell power plant is 14 inches high, 15 inches wide
and 40 inches long and weighs 255 pounds.
The voltage and current range of each is 2 kilowatts at 32.5
volts dc, 61.5 amps, to 12 kilowatts at 27.5 volts dc, 436 amps.
Each fuel cell is capable of supplying 12 kilowatts peak and 7
kilowatts maximum continuous power. The three fuel cells are capable
of a maximum continuous output of 21,000 watts with 15-minute
peaks of 36,000 watts. The average power consumption of the orbiter
is expected to be approximately 14,000 watts, or 14 kilowatts,
leaving 7 kilowatts average available for payloads. Each fuel
cell will be serviced between flights and reused until each accumulates
2,000 hours of on-line service.