CONTROL SYSTEM FOR A SEALED COOLANT FLOW FIELD FUEL CELL
POWER PLANT HAVING A WATER RESERVOIR
Technical Field
[0001] The present disclosure relates to fuel
cells that are suited for usage in transportation
vehicles, portable power plants, or as stationary power
plants, and the disclosure especially relates to a system
and method for controlling relative humidity within
reactant streams passing through a fuel cell.
Background Art
[0002] Fuel cells are well known and are commonly
used to produce electrical current from a hydrogen-rich
fuel stream and an oxygen-containing oxidant stream to
power electrical apparatus. Fuel cells are typically
arranged in a cell stack assembly having a plurality of
fuel cells arranged with common manifolds and other
components such as controllers and valves, etc. to form a
fuel cell power plant. Many such power plants utilize a
"membrane electrode assembly" ("MEA") that includes a
"proton exchange membrane" ("PEM") as an electrolyte
secured between opposed anode and cathode catalysts and
support materials.
[0003] In such a fuel cell power plant of the
prior art, it is well known that many difficulties are
associated with long-term operation of the plant. In
particular, fuel cell power plants that include a coolant
system that directs a coolant fluid through a sealed
coolant flow field in thermal exchange with the EA to
remove heat generated during operation of the fuel cells
must carefully control a relative humidity of reactant
gas streams passing adjacent the MEA indirectly by
controlling the temperature of the reactant gases. (For
purposes herein a "sealed coolant flow field is to mean
that fluids cannot pass between the coolant flow field
and adjacent fuel cell components.) If the relative
humidity is too high, water generated at the cathode
catalyst during operation of the cell will accumulate as
a liquid instead of evaporating into the reactant stream.
This produces flooding which slows down or completely
interrupts flow of the reactant stream and results in
poor or disrupted fuel cell operation.
[0004] In contrast if the relative humidity of
the reactant streams is too low, moisture within the PEM
electrolyte within the MEA will evaporate into the
reactant streams resulting in drying of the PEM. This
slows transfer of protons through the PEM which in turns
interferes with electricity production. Drying of the
PEM also results in deterioration of the PEM so that
gaseous reactant breakthrough of torn or disrupted
membranes is possible. This not only deteriorates fuel
cell performance, but also poses a risk of mixing of
reactant gases that could lead to combustion of the
gasses.
[0005] Consequently, there is a need for a fuel
cell power plant having sealed coolant flow fields that
efficiently maintains relative humidity of reactant
streams passing through fuel cells of the plant.
Summary
[0006] The disclosure is directed to a control
system for a fuel cell power plant for generating
electrical current from oxidant and hydrogen-rich fuel
reactant streams. The system controls movement of water
out of a reactant stream within a fuel cell and into a
water reservoir whenever the relative humidity of the
reactant stream is greater than 1.00. Whenever the
reservoir is full at which point excess water exits the
cell. (Relative humidity of "1.00" means one-hundred
percent of the capacity of the reactant stream to hold
gaseous water is utilized.) Whenever the relative
humidity of the reactant stream is less than 1.00, the
system controls movement of water out of the water
reservoir and into the reactant stream and PE (proton
exchange membrane electrolyte) within the fuel cell.
Additionally, the control system coordinates the relative
humidity of the reactant stream with the power output of
the fuel cell. In particular, water in the form of water
vapor moves from the water reservoir into the reactant
stream whenever power generated by the fuel cell is
between about eighty percent and about one-hundred
percent of a maximum power output of the fuel cell. And,
water moves from the reactant stream into the available
(empty) volume of the water reservoir whenever power
produced by the fuel cell is less than about seventy-five
percent of the maximum power output of the fuel cell.
(For purposes herein, the word "about" is to mean plus or
minus ten percent) .
[0007] The control system includes at least one
fuel cell including a membrane electrode assembly ("MEA")
having a proton exchange membrane ("PEM") disposed
between an anode catalyst surface and an opposed cathode
catalyst surface. An anode flow field is defined in
fluid communication with the anode catalyst surface and
with a fuel inlet line for directing flow of the
hydrogen-rich fuel reactant stream from the fuel inlet
line adjacent the anode catalyst surface and out of the
anode flow field through an anode exhaust as an anode
exhaust stream. A cathode flow field is also defined in
fluid communication with the cathode catalyst surface and
with a source of the oxidant for directing flow of the
oxidant stream from an oxidant inlet line adjacent the
cathode catalyst and out of the cathode flow field
through a cathode exhaust as a cathode exhaust stream.
[0008] A sealed coolant flow field is secured in
thermal exchange relationship with at least one of the
anode and the cathode flow fields for directing a coolant
fluid from an inlet of a coolant loop through the coolant
flow field and out of the flow field through an outlet of
the coolant loop. The coolant loop includes a coolant
loop pump, a coolant loop flow rate controller, and a
coolant loop heat exchanger for controlling a temperature
of the coolant fluid within the coolant flow field.
Additionally, a water reservoir is secured in fluid
communication with at least one of the anode flow field
and the cathode flow field and is secured in fluid
isolation from the sealed coolant flow field. The water
reservoir is configured for moving water out of the
reservoir and into the reactant stream passing through
the at least one anode and cathode flow fields, and for
moving water out of the reactant stream passing through
the at least one anode and cathode flow fields and into
the reservoir.
[0009] A relative-humidity sensor is secured in
communication with the reactant stream passing through at
least one of the anode flow field and the cathode flow
field. A relative-humidity controller is also secured in
communication between the relative-humidity sensor and at
least one of: a fuel inlet valve secured to the fuel
inlet line; an oxidant inlet valve secured to the oxidant
inlet line; an oxidant blower secured to the oxidant
inlet line; an anode exhaust valve secured to the anode
exhaust; a cathode exhaust valve secured to the cathode
exhaust; the coolant loop pump; coolant loop flow rate
controller; and, the coolant loop heat exchanger. The
relative humidity sensor may also be a relative humidity
sensor means for determining a relative humidity of at
least one of the reactant streams passing through the at
least one of the anode flow field and the cathode flow
field. Therefore, the relative humidity sensor means may
be an apparatus for sensing relative humidity, or may be
any apparatus or combinations of apparatus capable of
performing the described function, such as sensors
measuring fuel cell operating parameters, and using the
sensed measurements to calculate the relative humidity of
at least one of the reactant streams.
[0010] The relative-humidity controller is
configured to selectively control at least one of: a
pressure of the reactant streams; a flow rate of the
reactant streams; a temperature of the coolant in the
sealed coolant flow field; a flow rate of the coolant
fluid passing through the sealed coolant flow field; so
that water moves from the water reservoir into the
reactant stream and PEM whenever power generated by the
fuel cell is between about eighty percent and about onehundred
percent of a maximum power output of the fuel
cell, and so that water moves from the reactant stream or
PEM into the water reservoir whenever power produced by
the fuel cell is less than about seventy-five percent
(75%) of the maximum power output of the fuel cell as
long as reservoir volume is available. In an alternative
embodiment, the control system may include a de-rate
power function if the reservoir becomes exhausted while
operating above 80% of the maximum power output of the
fuel cell. In a preferred embodiment, the water
reservoir includes pores defined in or adjacent to a
separate porous body secured in fluid communication with
at least one of the anode and the cathode flow fields .
[0011] In operation of the control system, as
demand for electricity increases by a load, such as by an
electric motor of a transportation vehicle, current
density generated by the fuel cell increases and
therefore heat generation increases in the MEA (membrane
electrode assembly) causing the temperature to rise in
the MEA and within the flow fields. This increased
temperature causes the relative humidity within the
reactant streams to decline , which results in water held
in the water reservoir evaporating into the reactant
streams to increase the relative humidity. The relativehumidity
controller is configured to maintain the
relative humidity of the reactant streams within at least
one of the anode and cathode flow fields to be greater
than 1.00 until the power output of the fuel cell reaches
about seventy-five percent of the maximum power output of
the fuel cell . While the relative humidity of the
reactant streams is greater than 1.00, water produced by
the fuel cell moves into the water reservoir until it is
full, and excess water is removed from the fuel cell
within the cathode exhaust stream. As the fuel cell
power output increases to between about eighty percent
and one-hundred percent of the maximum potential power
output, the temperature increases and the relative
humidity decreases so that water held in the water
reservoir evaporates into the reactant streams .
[0012] This evaporation of water held within the
water reservoir also aids in cooling the reservoir and
the adjacent MEA. The higher relative humidity and
evaporative cooling of the reactant streams will also
allow operating the fuel cell power plant with a higher
coolant exit temperature at a high current density. This
will decrease a parasitic power demand on the fuel cell
power plant to cool itself and reduce radiator size.
Additionally, operating the fuel cells at a maximum
relative humidity at low current output can be controlled
by changing oxidant reactant stream flow rates, reactant
stream pressures, etc. to prevent flooding by excess
water.
[0013] The water reservoir defines an adequate
volume to retain sufficient water for maintaining the
relative humidity of the reactant streams at or about
1.00 during anticipated high power demands placed on the
fuel cell. In other words, a particular fuel cell, such
as a fuel cell for a transportation vehicle, or a fuel
cell for a stationary power plant, will have a
predictable duration of a high-power output. The water
holding volume defined by the water reservoir is
structured to maintain the relative humidity of the
reactant streams at or about 1.00 during such
predetermined durations of a high-power output . A
preferred water holding volume defined by the water
reservoir is an adequate volume to maintain the relative
humidity of the reactant streams at or about 1.00 for
about five minutes . After the predetermined duration of
high power output, such as the five minute duration the
control system may initiate a de-rate function to return
the fuel cell to operating at less than about seventyfive
percent (75%) of the maximum power output of the
fuel cell. The reservoir or reservoirs of the present
control system further assist fuel cell operation by
wicking water from reactant flow fields, and generally
making water removal easier. The present disclosure also
includes methods of operating a fuel cell power plant to
maintain efficient relative humidity of reactant streams
passing through fuel cells of the power plant as
described above, and as described in more detail below.
[0014] Accordingly, it is a general purpose of
the present disclosure to provide a control system for a
sealed coolant flow field fuel cell power plant having a
water reservoir that overcomes deficiencies of the prior
art .
[0015] It is a more specific purpose to provide a
control system for a sealed coolant flow field fuel cell
power plant having a water reservoir that efficiently
maintains optimal relative humidity of reactant streams
within fuel cells of the power plant.
[0016] These and other purposes and advantages of
the present a control system for a sealed coolant flow
field fuel cell power plant having a water reservoir will
become more readily apparent when the following
description is read in conjunction with the accompanying
drawings .
Brief Description of Drawing
[0017] Figure 1 is a simplified schematic
representation of a control system for a sealed coolant
flow field fuel cell power plant having a water reservoir
of the present disclosure.
[0018] Figure 2 is a graph showing relative
humidity of a reactant stream exiting a fuel cell as a
function of voltage, current density and heat.
[0019] Figure 3 is a graph showing a change in
the relative humidity of the reactant stream of the
Figure 2 graph in response to an increase in operating
temperature of a fuel cell .
Description of the Preferred Embodiments
[0020] Referring to the drawings in detail, a
control system for a sealed coolant flow field fuel cell
power plant having a water reservoir is shown in FIG. 1
and is generally designated by the reference numeral 10.
he system 10 is applied to a fuel cell power plant 12
for generating electrical current from oxidant and
hydrogen-rich fuel reactant streams. The system 10
controls movement of water 14 out of a reactant stream
16A, 16B and into a water reservoir 18A, 18B whenever a
relative humidity of the reactant stream 16A, 16B is
greater than 1.00. (Relative humidity of "1.00" means
one-hundred percent of the capacity of the reactant
stream 16A, 16B to hold gaseous water is utilized.)
[0021] Whenever the relative humidity of the
reactant streams 16A, 16B is less than 1.00, the system
10 controls movement of water 14 out of the water
reservoir 18A, 18B and into the reactant streams 16A,
16B. Additionally, the control system 10 coordinates the
relative humidity of the reactant streams 16A, 16B with a
power output of a fuel cell 20. In particular, water 14
moves from at least one of the water reservoirs 18A, 18B
into at least one of the reactant streams 16A, 16B
whenever power generated by the fuel cell 20 is between
about eighty percent and about one-hundred percent of a
maximum power output of the fuel cell 20. Water 14 moves
from at least one of the reactant streams 16A, 16B into
at least one of the water reservoirs 18A, 18B whenever
power produced by the fuel cell 20 is less than about
seventy-five percent of the maximum power output of the
fuel cell 20. (For purposes herein, the word "about" is
to mean plus or minus five percent) . In a preferred
embodiment the fuel cell 20 would be configured so that
during 50% of its operating time and preferably during
75% of its operating time, water 14 moves from at least
one of the reactant streams 16A, 16B into at least one of
the water reservoirs 18A, 18B. The control system 10 and
fuel cell 20 would therefore be structured so that the
fuel cell 20 produces less than about 75% of its maximum
power output during the 50% of its operating time and
preferably during 75% of the operating time of the fuel
cell 20.
[0022] The control system 10 includes at least
one fuel cell 20 including a membrane electrode assembly
22 ("MEA") having a proton exchange membrane 23 ("PEM" )
disposed between an anode catalyst surface . 24 and an
opposed cathode catalyst surface 26 of the MEA. An anode
flow field 28 is defined in fluid communication with the
anode catalyst surface 24 and with a fuel inlet line 30
for directing flow of the hydrogen-rich fuel reactant
stream from a fuel storage source 32 through the fuel
inlet line 30 adjacent the anode catalyst surface 24 and
out of the anode flow field 28 through an anode exhaust
34 as an anode exhaust stream. A cathode flow field 36
is also defined in fluid communication with the cathode
catalyst surface 26 and with an oxidant source 38 (such
as the atmosphere, or an oxidant storage container 38)
for directing flow of the oxidant reactant stream from an
oxidant inlet line 40 adjacent the cathode catalyst
surface 26 and out of the cathode flow field 36 through a
cathode exhaust 42 as a cathode exhaust stream.
[0023] A sealed coolant flow field 44 is secured
in thermal exchange relationship with at least one of the
anode flow field 28 and the cathode flow field 36 (as
shown in FIG. 1 ) for directing a coolant fluid from a
coolant inlet 46 of a coolant loop 48 through the sealed
coolant flow field 44 and out of the flow field 44
through a coolant outlet 50 of the coolant loop 48. The
coolant loop 48 also includes a coolant loop pump 52,
possibly a coolant flow rate control valve 53 for
controlling a flow rate of the coolant fluid passing
through the coolant loop 48 and coolant flow field 44,
and a coolant loop heat exchanger 54 for controlling a
temperature of the coolant fluid passing through the
coolant flow field 44 .
[0024] At least one water reservoir 18A, 18B is
secured in fluid communication with at least one of the
anode flow field 28 and the cathode flow field 36 and is
also secured in fluid isolation from the sealed coolant
flow field 44. The water reservoir 18A, 18B is
configured for moving water out of the reservoir 18A, 18B
and into a reactant stream 16A, 16B located in the at
least one anode flow field 28 and cathode flow field 36,
and for moving water out of the reactant stream 16A, 16B
located in the at least one anode flow field 28 and
cathode flow field 36 and into the reservoir 18A, 18B.
For example, if the reactant stream is an oxidant
reactant stream 16A passing through the cathode flow
field 36, the water reservoir 18A secured adjacent the
cathode flow field 36 transfers water 14 into and out of
the oxidant reactant stream 16A depending upon the
relative humidity of the stream 16A. FIG. 1 shows the
reservoirs 18A, 18B adjacent opposed surfaces of the MEA
22, however, the reservoirs 18A, 18B may be located in
different locations provided they remain in fluid
communication with at least one of the reactant streams
16A, 16B.
[0025 ] A relative-humidity sensor 60 may be
secured in communication with the reactant stream passing
through at least one of the anode flow field 28 and the
cathode flow field 36. Preferably the relative-humidity
sensor 60 is secured in communication with the cathode
exhaust stream within the cathode exhaust 42, as shown
schematically in FIG. 1 . The relative-humidity sensor 60
may a Sensirion SHT10 available from www.sensirion.com,
or any structure known in the art and capable of sensing
the relative humidity of a reactant stream 16A, 16B and
communicating the sensed relative humidity.
[002 6] As described above, the relative humidity
sensor 60 may also be a relative humidity sensor means 60
for determining a relative humidity of at least one of
the reactant streams 16A, 16B passing through the at
least one of the anode flow field 28 and the cathode flow
field 36. Therefore, the relative humidity sensor means
60 may be an apparatus for sensing relative humidity as
described above, or may be any apparatus or combinations
of apparatus capable of performing the described
function, such as measuring devices (not shown) for
measuring fuel cell operating parameters, and for using
the sensed measurements to calculate the relative
humidity of at least one of the reactant streams.
[0027 ] A relative-humidity controller 62 is also
secured in communication with the relative-humidity
sensor 60 through a first communication line 64. (By the
phrase "communication line", it is meant that described
components communicate with each other through any
apparatus known in the art, such electric wires, optical
fibers, electromagnetic waves, mechanical valves actuated
by human operators in response to visual indicia, etc.,
all of which are represented by the hatched line 64 and
other hatched lines described below.) The relativehumidity
controller may be any controller means known in
the art for performing the described functions, such as
for example a computer, electro-mechanical controls,
hydraulic controls responding to electro-mechanical or
radio wave signals, a human operator responding to visual
or audio signals, etc. The relative-humidity controller
62 is also secured in communication with at least one of
the following components. The controller 62 may be
secured through a second communication line 66, with a
fuel inlet valve 68 secured to the fuel inlet line 30;
through a third communication line 70 with an oxidant
inlet valve 72 secured to the oxidant inlet line 40;
through a fourth communication line 74 with an oxidant
blower 76 secured to the oxidant inlet line 40; through a
fifth communication line 78 with the coolant loop 48
coolant control valve 43, coolant loop pump 52 and/or
coolant loop heat exchanger 54; through a sixth
communication line 80 with a cathode exhaust valve 82
secured to the cathode exhaust 42; through a seventh
communication line 84 with an anode exhaust valve 86
secured to the anode exhaust 34; and, through an eighth
communication line 88 with a reactant exhaust gas flow
rate controller secured to a reactant exhaust gas recycle
line, such as a reactant exhaust gas flow rate controller
89 secured to an anode exhaust gas recycle line 91. The
anode recycle line 91 is secured between the anode
exhaust 34 and the fuel inlet line 30, and the reactant
exhaust gas flow rate controller may be a blower 89, an
ejector, or any other apparatus capable of controlling a
rate of flow of recycling a fuel cell reactant exhaust
gas .
[0028] The relative-humidity controller 62 is
configured to selectively control at least one of: a
pressure of the reactant streams 16A, 16B within the
anode flow field 28 and/or the cathode flow field 36; a
flow rate of the reactant streams 16A, 16B flowing
through the anode flow field 28 and/or the cathode flow
field 36; a temperature of the coolant fluid in the
sealed coolant flow field 44; a flow rate of the coolant
fluid passing through the sealed coolant flow field 44;
so that water 14 moves from the water reservoir 18A, 18B
into the reactant stream 16A, 16B whenever power
generated by the fuel cell 20 is between about eighty
percent and about one-hundred percent of a maximum power
output of the fuel cell 20, and so that water 14 moves
from the reactant stream 16A, 16B into the water
reservoir 18A, 18B whenever power produced by the fuel
cell 20 is less than about seventy-five percent of the
maximum power output of the fuel cell 20.
[0029] The relative-humidity controller 62 may
control one or more of the aforesaid parameters in
response to information received from the relativehumidity
sensor 60, or the controller 62 may simply be
tuned to control one or more of the parameters in
response to other operating characteristics of the fuel
cell power plant 12. Fo example, if the power plant 12
is to serve as a stationary power plant and operate at a
near steady-state of current output, the variables
primarily effecting relative humidity would be ambient
air conditions. Therefore, a relative-humidity sensor 62
apparatus may not be needed, and the fuel cell power
plant 12 may be tuned by other relative humidity sensor
means described above to produce varying relative
humidity of the reactant streams 16A, 16B by other sensed
information, or simply in response to fuel cell current
output .
[0030] preferred embodiment, the
reservoir 18A, 18B includes pores defined in a
hydrophobic anode porous body 90 secured in fluid
communication with the anode catalyst surface 24 of the
MEA 22, and pores defined in a hydrophobic cathode porous
body 92 secured in fluid communication with the cathode
catalyst surface 26 of the MEA 22. An alternative water
reservoir 18A or water reservoirs 18A, 18B may include
alternative structures (not shown) that are capable of
performing the water transfer into and out of the
reactant streams 16A, 16B described above, such as
hollow fibers, hydrophilic assemblies in the gas
diffusion layers.
[0031] In operation of the control system 10, as
demand for electricity increases by a load (not shown) ,
current density generated by the fuel cell 20 increases
and therefore heat produced by the MEA 22 increases and
transfers to the anode flow field 28 and the cathode flow
field 36 causing temperatures to rise. This increased
temperature causes the relative humidity within the
reactant streams 16A, 16B to decline, which results in
water 14 held in the water reservoir 18A, 18B evaporating
into the reactant streams 16A, 16B to increase the
relative humidity of the reactant streams 16A, 16B. The
relative-humidity controller 62 is configured to maintain
the relative humidity of the reactant streams 16A, 16B
within at least one of the anode and cathode flow fields
26, 36 to be greater than 1.00 until the power output of
the fuel cell 20 reaches about seventy-five percent of
the maximum power output of the fuel cell 20. While the
relative humidity of the reactant streams 16A, 16B is
greater than 1.00, water produced by the fuel cell 20
moves into the available volume in the water reservoir
18A, 18B, and any excess water 14 is removed from the
fuel cell 20 within the cathode exhaust stream exiting
the cathode exhaust 42. As the fuel cell 20 power output
increases to between about eighty percent and one-hundred
percent of a predetermined fuel cell maximum potential
power output, the temperature within the fuel cell 20
increases and the relative humidity therefore decreases
so that water 14 held in the water reservoir 18A, 18B
evaporates into the reactant streams 16A, 16B.
[0032] The water reservoir 18A, 18B may also
define an adequate water-retention volume to retain
sufficient water for maintaining the relative humidity of
the reactant streams 16A, 16B at or about 1.00 during
anticipated high power demands placed on the fuel cell
20. A particular fuel cell 20, such as a fuel cell for a
transportation vehicle (not shown) , or a fuel cell for a
stationary power plant (not shown) , will have a
predictable, predetermined duration of a high-power
output. The water holding volume defined by the water
reservoir 18A, 18B is dimensioned to retain an adequate
volume of water 14 to maintain the relative humidity of
the reactant streams 16A, 16B at or about 1.00 during
such predetermined durations of a high-power output. A
preferred water holding volume defined by the water
reservoir 18A, 18B is an adequate volume to maintain the
relative humidity of the reactant streams at or about
1.00 for about five minutes. After the predetermined
duration of high power output, such as the five minute
duration, the control system may initiate a de-rate
function to return the fuel cell to operating at less
than about seventy-five percent (75%) of the maximum
power output of the fuel cell .
[0033] FIG. 2 presents a simulated polarization
curve on a graph that shows a change in relative humidity
of a cathode stream exiting a fuel cell in response to
increasing current density. The fuel cell performance
simulated in FIG. 2 yields the heat generation rate in
FIG. 2 for a three hundred and twenty square centimeter
PEM with a fixed temperature of sixty-five degrees
Celsius of a coolant fluid passing through a sealed
coolant flow field adjacent the simulated fuel cell
cathode flow field. The relative humidity at steady
state is represented by plot line 100; the fuel cell heat
is represented by plot line 102; the fuel cell voltage is
represented by plot line 104; the temperature of the
coolant fluid at the coolant outlet is represented by
plot line 106. FIG. 2 shows that the relative humidity
of the reactant stream remains above 1.0 at a current
density (as shown on the horizontal axis) of about 1.5
amps per square centimeter ("A/cm2" ) . The relative
humidity then declines below 1.0 at a current density
greater than 1.5 A /cm2 . As described above, when the
relative humidity of the reactant stream falls below a
1.0 whenever the fuel cell is operated at current
densities above 1.5 A/cm 2, water evaporates out of the
fuel cell giving rise to decreased hydration of the PEM
resulting in decreased fuel cell performance and harmful
hydration cycling of the PEM
[0034] FIG. 3 shows the same plot lines described
in FIG. 2 for a simulated fuel cell performance wherein
the flow of the coolant fluid through the sealed coolant
flow field is divided by 1.5 so that the flow rate is
approximately two- thirds of the flow rate shown in FIG.
2 . This causes an increase in the temperature in the
coolant fluid leaving the fuel cell, and more importantly
causes the relative humidity, shown in plot line 100 in
FIG. 3 , to decline below 1.0 at about 1.1 A/cm 2 .
Therefore, the fuel cell performance shown in FIG. 3
demonstrates that controlling coolant fluid temperature
by changing coolant fluid flow rates is one of several
ways that may be utilized by the present relativehumidity
controller 62 to maintain a relative humidity of
reactant streams passing through a fuel cell at or above
1.0 as the current density or power output of the fuel
cell increases to exceed eighty percent of a maximum
current output of the fuel cell.
[0035] The present disclosure also includes
methods of operating the sealed coolant flow field fuel
cell power plant 12 to maintain efficient relative
humidity of reactant streams 16A, 16B passing through the
fuel cell 20. A method of so operating the fuel cell
power plant 12 includes controlling at least one of: a
pressure of the reactant streams 16A, 16B within the
anode flow field 28 and/or the cathode flow field 36; a
flow rate of the reactant streams 16A, 16B flowing
through the anode flow field 28 and/or the cathode flow
field 36; a temperature of the coolant fluid in the
sealed coolant flow field 44; a flow rate of the coolant
fluid passing through the sealed coolant flow field 44;
so that water 14 moves from the water reservoir 18A, 18B
into the reactant stream 16A, 16B whenever power
generated by the fuel cell 20 is between about eighty
percent and about one-hundred percent of a maximum power
output of the fuel cell 20, and so that water 14 moves
from the reactant stream 16A, 16B into . the water
reservoir 18A, 18B whenever power produced by the fuel
cell 20 is less than about seventy-five percent of the
maximum power output of the fuel cell 20.
[0036] The method of operating the fuel cell
power plant 12 may also include storing an adequate
amount of water 14 in the water reservoir 18A, 18B for
maintaining the relative humidity of the reactant streams
16A, 16B at about 1.00 during predetermined durations of
a high-power output by the fuel cell 20. The storing of
the adequate amount of water 14 may also include storing
a volume of water 14 that is adequate to maintain a
relative humidity of the reactant streams 16A, 16B at or
about 1.0 during a five minute duration of high-power
output by the fuel cell 20.
[0037] While the above disclosure has been
presented with respect to the described and illustrated
embodiments of the control system 10 for a sealed coolant
flow field fuel cell power plant 12 it is to be
understood that the disclosure is not to be limited to
those alternatives and described embodiments.
Accordingly, reference should be made primarily to the
following claims rather than the forgoing description to
determine the scope of the disclosure.

CLAIMS
What is claimed is:
1 . A control system (10) for a fuel cell power plant
(12) for generating electrical current from an oxidant
reactant stream (16A) and a hydrogen-rich fuel reactant
stream (16B) , the system (10) comprising:
a . at least one fuel cell (20) including a
membrane electrode assembly (22) having a
proton exchange membrane (23) disposed between
an anode catalyst surface (24) and an opposed
cathode catalyst surface (26) of the assembly
(22), an anode flow field (28) defined in fluid
communication with the anode catalyst surface
(24) and with a fuel inlet line (30) for
directing flow of the hydrogen-rich fuel
reactant stream (16B) from the fuel inlet line
(30) adjacent the anode catalyst surface (24)
and out of the anode flow field (28) through an
anode exhaust (34) as an anode exhaust stream,
a cathode flow field (36) defined in fluid
communication with the cathode catalyst surface
(26) and with a source (38) of the oxidant for
directing flow of the oxidant reactant stream
(16A) from an oxidant inlet line (40) adjacent
the cathode catalyst surface (26) and out of
the cathode flow field (36) through a cathode
exhaust (42) as a cathode exhaust stream;
b . a sealed coolant flow field (44) secured in
thermal exchange relationship with one of the
anode flow field (28) and the cathode flow
field (36) for directing a coolant fluid from a
coolant inlet (46) of a coolant loop (48),
through the coolant flow field (44) and through
a coolant loop outlet (50) , the coolant loop
configured for controlling a temperature of the
coolant fluid within the coolant flow field
(44);
at least one water reservoir (18A, 18B) secured
in fluid communication with at least one of the
anode flow field (28) and the cathode flow
field (36) and secured in fluid isolation from
the sealed coolant flow field (44), the water
reservoir (18A, 18B) configured for moving
water from the reservoir (18A, 18B) and into
the reactant stream (16A, 16B) located in the
at least one anode and cathode flow fields (28,
36), and for moving water (14) from the
reactant stream (16A, 16B) located in the at
least one anode and cathode flow fields (28,
36) and into the at least one reservoir (18A,
18B) ; and,
a relative-humidity controller (62) secured in
communication with the fuel cell (12) for
selectively controlling at least one of a
pressure of the reactant streams (16A, 16B) , a
flow rate o f the reactant streams (16A, 16B) , a
temperature of the coolant fluid within the
sealed coolant flow field (44), so that water
(14) moves from the water reservoir (18A, 18B)
into the reactant gas streams (16A, 16B)
whenever power generated by the fuel cell (20)
is between about eighty percent and about onehundred
percent of a predetermined maximum
power output of the fuel cell (20), and so that
water (14) moves from the reactant gas streams
(16A, 16B) into the water reservoir (18A, 18B)
whenever power produced by the fuel cell (20)
is less than about seventy-five percent of the
predetermined maximum power output of the fuel
cell (20) .
2 . The control system (10) of claim 1 further
comprising a relative-humidity sensor (60) means secured
in communication with the reactant stream (16A, 16B)
passing through the at least one of the anode flow field
(28) and the cathode flow field (36) , and also secured in
communication with the relative-humidity controller (62)
for communicating sensed information about the relative
humidity of the reactant streams (16A, 16B) to the
controller (62) .
3 . The control system (10) of claim 2 , wherein the
relative-humidity sensor (60) means is secured in
communication with the cathode exhaust (42) .
4 . The control system (10) of claim 1 wherein the
relative-humidity controller (62) is also secured in
communication at least one of a fuel inlet valve (68)
secured to the fuel inlet line (30), an oxidant inlet
valve (72) secured to the oxidant inlet line (40) , an
oxidant blower (76) secured to the oxidant inlet line
(40) , an anode exhaust valve (86) secured to the anode
exhaust (34), a cathode exhaust valve (82) secured to the
cathode exhaust (42) , the coolant loop (48) .
5 . The control system (10) of claim 1 , wherein the
water reservoir (18A, 18B) further comprises pores
defined in at least one of a cathode porous body (92)
secured in fluid communication with the cathode catalyst
surface (26) of the membrane electrode assembly (22) and
an anode porous body (92) secured in fluid communication
with the anode catalyst surface (24) of the membrane
electrode assembly (22) .
6 . The control system (10) of claim 1 , wherein the
water reservoir (18A, 18B) defines a water-retention
volume dimensioned to retain an adequate volume of water
(14) to maintain the relative humidity of the reactant
streams (16A, 16B) at or about 1.00 during a
predetermined duration of power output of the fuel cell
(20) that is between about eighty percent and about onehundred
percent of a predetermined maximum power output
of the fuel cell (20) .
7 . The control system (10) of claim 6 , wherein the
water reservoir (18A, 18B) defines a water-retention
volume that is adequate to maintain a relative humidity
of the reactant streams 16A, 16B at or about 1.0 whenever
the predetermined duration of the power output of the
fuel cell (20) that is between about eighty percent and
about one-hundred percent of a predetermined maximum
power output of the fuel cell (20) is about five minutes.
8 . A method of operating the sealed coolant flow field
fuel cell power plant (12) of claim 1 to control
relative-humidity levels of reactant streams (16A, 16B)
passing through the fuel cell (20) , the method
comprising:
a . controlling at least one of: a pressure of the
reactant streams (16A, 16B) within at least one
of an anode flow field (28) and a cathode flow
field (36) ; a flow rate of the reactant streams
(16A, 16B) flowing through at least one of the
anode flow field (28) and the cathode flow
field (36) ; a temperature of a coolant fluid
passing through a sealed coolant flow field
(44) ; a flow rate of the coolant fluid passing
through the sealed coolant flow field (44);
b . so that water (14) moves. from a water reservoir
(18A, 18B) into the reactant stream (16A, 16B)
whenever power generated by the fuel cell (20)
is between about eighty percent and about onehundred
percent of a maximum power output of
the fuel cell (20), and so that water (14)
moves from the reactant stream (16A, 16B) into
the water reservoir (18A, 18B) whenever power
produced by the fuel cell (20) is less than
about seventy-five percent of the maximum power
output of the fuel cell (20) .
9 . The method of claim 8 further comprising, retaining
an adequate volume of water (14) within the water
reservoir (18A, 18B) to maintain the relative-humidity of
the reactant streams (16A, 16B) at about 1.00 during a
predetermined duration of power output of the fuel cell
(20) that is between about eighty percent and about onehundred
percent of a predetermined maximum power output
of the fuel cell (20) .
10. The method of claim 8 , wherein controlling water to
move from the reactant stream (16A, 16B) into the water
reservoir (18A, 18B) comprises controlling water to move
water into pores defined in at least one of a cathode
porous body (92) secured in fluid communication with the
cathode catalyst surface (26) of the membrane electrode
assembly (22) and an anode porous body (90) secured in
fluid communication with the anode catalyst surface (24)
of the membrane electrode assembly (22) .