U.S. patent application number 11/155803 was filed with the patent office on 2005-12-01 for fuel cell system shunt regulator method and apparatus.
Invention is credited to Barton, Russell H., Sexsmith, Michael, Wardrop, David S..
Application Number | 20050266283 11/155803 |
Document ID | / |
Family ID | 21782831 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050266283 |
Kind Code |
A1 |
Wardrop, David S. ; et
al. |
December 1, 2005 |
Fuel cell system shunt regulator method and apparatus
Abstract
A fuel cell system for powering a work load includes a fuel cell
stack and a shunt regulator having a threshold detection;
transistorized power switching element, and a dump load. The
threshold detection element identifies when an abnormally high
voltage rises. The power switching element routes power from the
high voltage buss to the dump load. The dump load acts as an
electrical energy sink, and may provide dissipated energy to the
fuel cell stack in the form of heat. The switching element can also
shunt power to the dump load when a digital control signal is set,
for example at startup or during cold start conditions.
Inventors: |
Wardrop, David S.;
(Vancouver, CA) ; Sexsmith, Michael; (North
Vancouver, CA) ; Barton, Russell H.; (New
Westminster, CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Family ID: |
21782831 |
Appl. No.: |
11/155803 |
Filed: |
June 17, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11155803 |
Jun 17, 2005 |
|
|
|
10017483 |
Dec 14, 2001 |
|
|
|
Current U.S.
Class: |
429/432 ; 337/15;
429/440; 429/444; 429/454 |
Current CPC
Class: |
H01M 8/04559 20130101;
H01M 8/0494 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/023 ;
429/026; 429/013; 337/015 |
International
Class: |
H01M 008/04; H01H
079/00 |
Claims
1. A fuel cell stack assembly for providing power to a working
load, comprising: a first set of fuel cells; a first threshold
detector responsive to an stack terminal voltage across the first
set of fuel cells; a first transistor coupled for activation via
the first threshold detector; and a first dump load, wherein the
first transistor is responsive to the stack terminal voltage across
the first set of fuel cells to selectively couple the first dump
load in parallel with the first set of fuel cells when the stack
terminal voltage across the first set of fuel cells exceeds a
threshold voltage and to uncouple the first dump load when the
stack terminal voltage across the first set of fuel cells is below
the threshold voltage.
2. The fuel cell stack assembly of claim 1, further comprising: a
second set of fuel cells; a second threshold detector responsive to
an stack terminal voltage across the second set of fuel cells; a
second transistor coupled for activation via the second threshold
detector; and a second dump load, wherein the second transistor is
responsive to the stack terminal voltage across the second set of
fuel cells to selectively couple the second dump load in parallel
with the second set of fuel cells when the stack terminal voltage
across the second set of fuel cells exceeds a threshold voltage and
to uncouple the second dump load when the stack terminal voltage
across the second set of fuel cells is below the threshold
voltage.
3. The fuel cell stack assembly of claim 1 wherein the dump load is
positioned upstream downstream from the fuel cells in an air flow
for providing heat to the fuel cells.
4. The fuel cell stack assembly of claim 1 wherein the dump load is
positioned proximate the fuel cells for providing heat thereto.
5. The fuel cell stack assembly of claim 1, further comprising: a
capacitance electrically coupled across the dump load.
6. The fuel cell stack assembly of claim 1, further comprising: an
inductance electrically coupled in series between the first set of
fuel cells and the dump load.
7. The fuel cell stack assembly of claim 1 wherein the first
transistor is an n-channel field effect transistor.
8. The fuel cell stack assembly of claim 1 wherein the first
transistor is a p-channel field effect transistor.
9. The fuel cell stack assembly of claim 1 wherein the first
transistor is one of an n-channel bipolar junction transistor and a
p-channel bipolar junction transistor.
10. A shunt regulator for a fuel cell stack having a high voltage
bus for providing power to a work load, comprising: a transistor
responsive to a fuel cell stack terminal voltage on the high
voltage bus; and a dump load selectively coupleable to the high
voltage bus in parallel with the fuel cell stack by the transistor
while the fuel cell stack terminal voltage exceeds a threshold
voltage.
11. The shunt regulator of claim 10, further comprising: a
capacitance electrically coupled across the dump load.
12. The shunt regulator of claim 10 wherein the dump load comprises
a resistor.
13. The shunt regulator of claim 10 wherein the transistor is one
of an n-channel field effect transistor and a p-channel field
effect transistor.
14. The shunt regulator of claim 10 wherein the transistor is one
of an n-channel bipolar junction transistor and a p-channel bipolar
junction transistor.
15. A shunt regulator for a fuel cell assembly including a fuel
cell stack, the shunt regulator comprising: load dumping means for
dissipating excess power from the fuel cell stack as heat; and
transistorized threshold detection and switching means for
determining when an stack terminal voltage across the fuel cell
stack exceeds a threshold level and for selectively electrically
coupling the load dumping means in parallel across the fuel cell
stack in response to the transistorized threshold detection means
detecting the stack terminal voltage exceeding the threshold
level;
16. A method of operating a fuel cell stack, comprising:
determining a voltage across at least a portion of a fuel cell
stack; determining whether the determined voltage exceeds a
threshold voltage; and selectively operating a transistorized
switch to place a dump load across the fuel cell stack when the
determined voltage across at least the portion of the fuel cell
stack exceeds the threshold voltage.
17. The method of claim 16, further comprising: providing heat
dissipated by the dump load to the fuel cell stack.
18. The method of claim 16, further comprising: determining whether
a digital logic control signal is set; and selectively operating
the transistorized switch to place the dump load across the fuel
cell stack when the digital logic control signal is set while the
determined voltage does not exceed the threshold voltage.
19. A method of operating a fuel cell stack, comprising:
determining a voltage across a power bus of a fuel cell stack;
determining whether the determined voltage exceeds a threshold
voltage; and selectively operating a transistorized switch to place
a dump load across the power bus of the fuel cell stack while the
determined voltage exceeds the threshold voltage.
20. The method of claim 19, further comprising: providing heat
dissipated by the dump load to the fuel cell stack.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/017,483 filed Dec. 14, 2001, now
pending.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention is generally related to fuel cell systems,
and more particularly to controlling the open circuit voltage of a
fuel cell stack.
[0004] 2. Description of the Related Art
[0005] Electrochemical fuel cells convert fuel and oxygen to
electricity. Solid polymer electrochemical fuel cells generally
employ a membrane electrode assembly ("MEA") which includes an ion
exchange membrane or solid polymer electrolyte disposed between two
electrodes typically comprising a layer of porous, electrically
conductive sheet material, such as carbon fiber paper or carbon
cloth. The MEA contains a layer of catalyst, such as a finely
comminuted platinum, at each membrane electrode interface to induce
the desired electrochemical reaction. In operation, the electrodes
are electrically coupled to conduct electrons through an external
circuit. Typically, a number of MEAs are electrically coupled in
series to form a fuel cell stack having a desired nominal power
output to power a load.
[0006] Fuel cell stacks are often sized to meet a variety of
different voltage and current constraints imposed by the variety of
work loads, the size and shape of the fuel cell and the number of
fuel cells. While it is possible to scale a specific fuel cell
system design for each application, it is not always commercially
economical. It is often desirable to connect fuel cell stacks to
loads which require different voltage ranges than those naturally
provided by the fuel cell stack. Since the voltage of the stack is
a function of the current it varies from its zero current voltage
(open circuit voltage or OCV) to its peak load voltage. This range
of voltages may cause three problems in matching the needs of the
loads.
[0007] First, since efficiency requirements make it desirable to
match the fuel cell voltage with the load voltage at the nominal
operating load, the zero current voltage may exceed the allowable
range in voltage of the work load circuit (note, all electrical
components have a maximum rated voltage). The work load should be
protected from exposure to an overvoltage.
[0008] Second, in some applications, fuel cells are electrically
connected to other energy sources such as power grids, battery
systems or electrical generators (such as in a vehicular
application with regenerative braking systems). In these
applications, it is possible (usually under fault conditions) for
these other energy sources to raise the voltage across the fuel
cell stack terminal higher than the maximum allowable fuel cell
stack voltage. If this stack voltage is exceeded, the fuel cell
stack could be damaged by several types of failure including
insulation breakdown, electrochemical corrosion or reverse
reaction. Thus, the fuel cell stack should be protected from this
voltage condition. While a series diode may block any currents
flowing into the stack thus preventing damage, a series diode
results in a constant parasitic waste of power.
[0009] Third, in typical applications, fuel cell stacks are
separated from the rest of the load circuit by a switch or set of
switches. When these switches are closed there is typically an
in-rush of electrical current into the load circuit to charge up
its inherent capacitance. The in-rush current can be very large and
can damage several types of electrical components. Damage modes
include blown fuses, contactor arcing and overheated components. It
is thus typical to have a voltage matching circuit that either
raises the voltage on the load side of the circuit or lowers the
voltage on the supply (i.e., fuel cell stack) side of the
switch.
BRIEF SUMMARY OF THE INVENTION
[0010] A shunt regulator can reduce the voltage across the stack
terminals to protect the work load component from being exposed to
an overvoltage by detecting the high voltage and sinking current
into the dump resistor, thus lowering the voltage at the stack
terminal. A shunt regulator can also route energy form other energy
sources away for the fuel cell stack to protect the fuel cell stack
from an excessive voltage condition by detecting the threshold
voltage and then dumping the excess energy through the dump
resistor, avoiding the constant parasitic waste of power. A shunt
regulator can further be used to lower the stack voltage to the
desired level, for example when switching the load onto the fuel
cell stack. Thus, a shunt regulator with an appropriate threshold
voltage and a digital override circuit could solve the above
recited problems. A single circuit could thus be used, dramatically
reducing cost and volume while increasing the reliability of the
fuel cell system since there are fewer parts to fail.
[0011] In one aspect, a fuel cell stack assembly includes: a first
set of fuel cells, a first threshold detector responsive to an
stack terminal voltage across the first set of fuel cells, a first
transistor coupled for activation via the first threshold detector,
and a first dump load wherein the first transistor is responsive to
the stack terminal voltage across the first set of fuel cells to
selectively couple the first dump load in parallel with the first
set of fuel cells when the stack terminal voltage across the first
set of fuel cells exceeds a threshold voltage and to uncouple the
first dump load when the stack terminal voltage is below the
threshold voltage. The fuel cell stack assembly may also include a
second set of fuel cells, a second threshold detector responsive to
an stack terminal voltage across the second set of fuel cells, a
second transistor coupled for activation via the second threshold
detector, and a second dump load, wherein the second transistor is
responsive to the stack terminal voltage across the second set of
fuel cells to selectively couple the second dump load in parallel
with the second set of fuel cells when the stack terminal voltage
exceeds a threshold voltage and to uncouple the second dump load
when the stack terminal voltage is below the threshold voltage. The
dump load may be positioned proximate the fuel cells and/or
upstream from the fuel cells to supply heat to the fuel cell stack.
A capacitance may be electrically coupled across the dump load
and/or an inductance may be electrically coupled in series between
the fuel cells and the dump load.
[0012] In another aspect, a shunt regulator includes: a transistor
responsive to a fuel cell stack terminal voltage on the high
voltage bus and a dump load selectively coupleable to the high
voltage bus in parallel with the fuel cell stack by the transistor
while the fuel cell stack terminal voltage exceeds a threshold
voltage.
[0013] In a further aspect, a method of operating a fuel cell stack
includes: determining a voltage across at least a portion of a fuel
cell stack, determining whether the determined voltage exceeds a
threshold voltage, and selectively operating a transistorized
switch to place a dump load across the fuel cell stack when the
determined voltage across at least a portion of the fuel cell stack
exceeds the threshold voltage.
[0014] In yet a further aspect, a method of operating a fuel cell
stack includes: determining a voltage across a power bus of a fuel
cell stack, determining whether the determined voltage exceeds a
threshold voltage, and selectively operating a transistorized
switch to place a dump load across the power bus of the fuel cell
stack while the determined voltage exceeds the threshold
voltage.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0015] In the drawings, identical reference numbers identify
similar elements or acts. Sizes and relative positions of elements
in the drawings are not necessarily drawn to scale. For example,
the shapes of various elements and angles are not drawn to scale,
and some of these elements are arbitrarily enlarged and positioned
to improve drawing legibility. Further, the particular shapes of
the elements as drawn, are not intended to convey any information
regarding the actual shape of the particular elements, and have
been solely selected for ease of recognition in the drawings.
[0016] FIG. 1 is a schematic diagram of a fuel cell system
providing power to a work load, the fuel cell system including a
fuel cell stack and a shunt regulator having a threshold detector,
a dump load and a transistorized switch.
[0017] FIG. 2 is an electrical schematic of the fuel cell system of
FIG. 1 providing power to the work load, further illustrating one
embodiment of the shunt regulator including an N-channel JFET as
the transistorized switch.
[0018] FIG. 3 is a graph showing a relationship of a shunt current
through a dump load with respect to a voltage across the fuel cell
stack, for the fuel cell system of FIG. 1.
[0019] FIG. 4 is an electrical schematic showing a p-channel JFET
for use as an alternative transistorized switch in the circuit of
FIG. 2.
[0020] FIG. 5 is an electrical schematic showing an n-channel
MOSFET for use as an alternative transistorized switch in the
circuit of FIG. 2.
[0021] FIG. 6 is an electrical schematic showing a p-channel MOSFET
for use as an alternative transistorized switch in the circuit of
FIG. 2.
[0022] FIG. 7 is an electrical schematic showing a npn bipolar
junction transistor for use as an alternative transistorized switch
in the circuit of FIG. 2.
[0023] FIG. 8 is an electrical schematic showing a pnp bipolar
junction transistor for use as an alternative transistorized switch
in the circuit of FIG. 2.
[0024] FIG. 9 is a schematic diagram of an alternative embodiment
of the fuel cell system powering the work load, employing a number
of shunt regulators, each associated with a respective portion of
the fuel cell stack.
[0025] FIG. 10 is a flow diagram showing an exemplary method of
operating the fuel cell system of FIGS. 1-9.
DETAILED DESCRIPTION OF THE INVENTION
[0026] In the following description, certain specific details are
set forth in order to provide a thorough understanding of the
various embodiments of the invention. However, one skilled in the
art will understand that the invention may be practiced without
these details. In other instances, well-known structures associated
with fuel cells, fuel cell stacks and fuel cell systems have not
been shown or described in detail to avoid unnecessarily obscuring
descriptions of the embodiments of the invention.
[0027] Unless the context requires otherwise, throughout the
specification and claims which follow, the word "comprise" and
variations thereof, such as, "comprises" and "comprising" are to be
construed in an open, inclusive sense, that is as "including, but
not limited to."
[0028] FIG. 1 shows a fuel cell system 10 providing power to a work
load 12 according to an illustrated embodiment of the invention.
The work load 12 typically constitutes the device to be powered by
the fuel cell system 10, such as a vehicle, appliance, computer
and/or associated peripherals. An impedance and conductance
J.sub.LOAD is associated with the work load 12. Although the fuel
cell system 10 is not typically considered part of the work load
12, in some embodiments portions of the fuel cell system 10 such as
the control electronics may constitute a portion or all of the work
load 12. The work load 12 is electrically coupled to the fuel cell
system 10 via a contactor 13, which typically permits the selective
electrical coupling and decoupling of the work load 12 to the fuel
cell system 10.
[0029] The fuel cell system 10 includes a fuel cell stack 14
composed of a number of individual fuel cells electrically coupled
in series. The fuel cell stack 14 receives reactants such as
hydrogen and air from a reactant supply system (not shown). The
reactant supply system may include one or more reactant supply
reservoirs, a reformer, and/or one or more control elements such as
compressors, pumps and/or valves. Operation of the fuel cell stack
14 produces reactant products, such as water. The fuel cell system
10 may reuse some reactant products, for example to humidify the
hydrogen, air and/or ion exchange membrane, or to control the stack
temperature. The fuel cell stack 14 produces a voltage V.sub.STACK
across a positive rail 16 and a negative rail 18 of a high voltage
bus formed by the rails 16, 18. A stack current I.sub.STACK flows
to the work load 12 from the fuel cell stack 14. As used herein,
high voltage refers to the voltage produced by conventional fuel
cell stacks 14 to power work loads, and is used to distinguish
between other voltages employed by fuel cell control system. Thus,
high voltage is not necessarily "high" with respect to other
electrical systems.
[0030] The fuel cell system 10 includes a shunt regulating circuit
(identified by broken line box 20) for dissipating power when the
stack voltage V.sub.STACK exceeds a threshold voltage V.sub.T. The
shunt regulating circuit 20 includes a dump load 22, a threshold
detector 24 and a transistorized switch 26 for electrically
coupling and uncoupling the dump load 22 across rails 16, 18 of the
high voltage bus in parallel with the fuel cell stack 14. The shunt
regulating circuit 20 may electrically couple to the rails 16, 18
on the fuel cell stack 14 side of the contactor 13, where the load
side requires protection.
[0031] The threshold detector 24 is electrically coupled across the
rails 16, 18 of the high voltage bus to determine the stack voltage
V.sub.STACK. The threshold detector 24 can also receive a digital
logic control signal 28. The threshold detector 24 can be
responsive to the digital logic control signal 28 to shunt a
portion of the stack current I.sub.STACK through the dump load 22
even if the voltage threshold V.sub.T is not exceeded. The digital
logic control signal 28 may be supplied by a control system,
microprocessor or micro-controller (not shown) associated with the
fuel cell system 10. The digital logic control signal 28 may be
supplied when a temperature T.sub.S of the fuel cell stack 14 is
below optimal and additional heating is required. This digital
logic control signal 28 may be supplied just prior to activation
(either upon closing or particularly upon opening) the load
disconnect contactor 13 to ensure that the voltage on the stack
side of the contactor remains within the specification.
[0032] The dump load 22 can be located proximate the fuel cell
stack 14 to provide heat to the fuel cell stack 14. Additionally or
alternatively, the dump load 22 can be positioned upstream in an
airflow (illustrated by arrow 30) to cause heat dissipated by the
dump load 22 to warm the fuel cell stack 14. A fan or other air
circulating mechanism (not shown) can supply the airflow 30. The
dump load 22 has different transient (i.e., short term) and
continuous energy capacities. The continuous rating of the dump
load 22 can be sized to match the amount of energy that must be
absorbed to bring the stack voltage V.sub.STACK within acceptable
OCV tolerances. A dump load 22 having a high transient sinking
capability is desirable, but must be balanced against competing
concerns such as cost, size, and ability of the fuel cell system 10
to dissipate heat.
[0033] FIG. 2 shows an embodiment of the shunt regulating circuit
20. The dump load 22 includes a resistive element such as a
resistor 32 for thermally dissipating excess power when shunted
across the high voltage bus formed by rails 16, 18. The dump load
22 may also include a capacitance, such as a capacitor 34,
electrically coupled in parallel with the resistor 32 to lower the
dynamic impedance of the dump load 22 and/or improve the transient
absorption capability. Additionally, the dump load 22 may include
an inductance, such as an inductor 36, electrically coupled in
series with the fuel cell stack 14 and the resistor 32, to alter
the dynamic impedance a load side transient would see on the path
back to the fuel cell stack 14.
[0034] The threshold detector 24 can include a Zener diode 38 and a
voltage divider composed of a first resistor 40 and second resistor
42 coupled between the positive and negative rails 16, 18,
respectively, forming the high voltage bus.
[0035] As illustrated in FIG. 2, the transistorized switch 26 may
take the form of an n-channel junction field effect transistor
("JFET") 44. The n-channel JFET 44 has a drain coupled to the
positive voltage rail 16 through the dump load 22 and a source
coupled to the negative voltage rail 18. The n-channel JFET 44
includes a gate electrically coupled to the voltage divider formed
by resistors 40, 42. One skilled in the art may favor n-channel
transistors which tend to be relatively inexpensive and have good
triggering capabilities.
[0036] FIG. 3 shows the relationship 45 between current shunted
I.sub.SHUNT through the dump load 22 and the stack voltage
V.sub.STACK. The shunt current I.sub.SHUNT is essentially zero
until the voltage threshold V.sub.T is reached, at which point the
shunt current I.sub.SHUNT rapidly rises. Thus, at high voltage bus
voltages less than the voltage threshold V.sub.T, the shunt
regulating circuit 20 presents essentially no load, while at
voltages greater than the threshold voltage V.sub.T, the shunt
regulating circuit 20 appears as a load and will absorb (i.e.,
sink) electrical energy and thus lower the high voltage bus
voltage.
[0037] FIG. 4 shows an alternative embodiment of the transistorized
switch 26 in the form of a P-channel JFET 50.
[0038] FIG. 5 shows another alternative of the transistorized
switch 26 in the form of an n-channel metal oxide semiconductor
field effect transistor ("MOSFET") 52.
[0039] FIG. 6 shows a further alternative of the transistorized
switch 26 in the form of a p-channel MOSFET 54.
[0040] FIG. 7 shows a further alternative of the transistorized
switch 26 in the form of a npn bipolar junction transistor 56.
[0041] FIG. 8 shows a further alternative of the transistorized
switch 26 in the form of a pnp bipolar junction transistor 58.
[0042] FIG. 9 shows an alternative embodiment of the fuel cell
system 10, employing individual threshold detectors 24a, 24b-24n,
each associated with a respective portion of the fuel cell stack
14a, 14b-14n. The illustrated embodiment employs an AND/OR or
nonexclusive OR circuit 60 to couple an excessive voltage condition
signal from any of the individual threshold detectors 24a, 24b-24n
to the transistorized switch 26 for shunting the dump load 22
across the rails 16, 18 of the high voltage bus. In a further
alternative, the embodiment of FIG. 7 can eliminate the OR circuit
60 by employing respective transistorized switches 26 and/or dump
loads 22 associated with each of the threshold detectors 24a,
24b-24n.
[0043] In typical use, a respective shunt regulating circuit 20 may
be located across each individual fuel cell row in the fuel cell
stack 14. However, a single shunt regulating circuit 20 could span
across an entire module (e.g., one regulating circuit for one or
more cell rows), or could be implemented as many shunt regulating
circuits 20, each across a respective fuel cell stack grouping with
as few as a single cell. One skilled in the art may recognize that
employing a respective shunt regulating circuit 20 for each
individual cell row provides a good match to commonly available
electrical components and to the dump load (i.e., heater)
availability at conventional fuel cell system operating levels. The
use of four or six shunt regulating circuits 20 (e.g., one per row)
in a fuel cell stack system 10 also provides a measure of
redundancy.
[0044] FIG. 8 shows an exemplary method 100 of operating the fuel
cell system 10. In step 102, the fuel cell system 10 determines the
voltage V.sub.STACK across the fuel cell stack 14. In step 104, the
fuel cell system determines whether the determined voltage exceeds
the threshold voltage V.sub.T. If the determined voltage exceeds
the threshold voltage V.sub.T, the fuel cell system 10 selectively
operates the transistorized switch 26 in step 106, to electrically
coupled the dump load 22 across the fuel cell stack 14 while the
determined stack voltage V.sub.STACK exceeds the threshold voltage
V.sub.T. If the determined voltage does not exceed the threshold
voltage, the fuel cell system determines whether the digital logic
control signal 28 is "set" or "ON" in step 108. If the digital
logic control signal 28 is not "set" or "ON," the fuel cell system
10 opens the transistorized switch 26 in step 110, to electrically
uncouple the dump load 22 from across the fuel cell stack 14. If
the digital control logic signal is "set" or "ON," the fuel cell
system 10 selectively operates the transistorized switch 26 in step
106, to electrically couple the dump load 22 across the fuel cell
stack 14. The method 100 is repeated while the fuel cell system 10
is operating.
[0045] Thus, as described above, the shunt regulating circuit 20
may provide transient protection to the fuel cell stack 14, may aid
in cold startup conditions, and/or may provide high stack open
circuit voltage surge protection to the work load 12, while
maintaining good power economy. The shunt regulating circuit 20 may
protect both the work load 12 and the fuel cell stack 14, and may
provide protection whether the transient condition is stack
generated or load generated.
[0046] Also as described above, the shunt regulating circuit 20
absorbs only the transient/surge energy from the fuel cell stack 14
or the work load 12. Because of the threshold switching, the shunt
regulating circuit 20 will not absorb any of the normal energy and
therefore does not pose a constant parasitic power draw.
[0047] Additionally, the shunt regulating circuit 20 can dissipate
power to serve various requirements of the fuel cell system 10. For
example, the shunt regulating circuit 20 can dissipate power (i.e.,
heat) to coolant in order to assist in cold starts and/or to
facilitate work load contactor 13 or closure. As discussed above, a
high OCV condition is a common occurrence on cold starts with PEM
fuel cells. The shunt regulating circuit 20 operates to place a
work load 12 on the fuel cell stack 14 until the stack voltage
V.sub.STACK drops to an acceptable level. The electrical energy
dissipated from the shunt regulating circuit 20 can be converted
(i.e., a heater) to heat energy which can then elevate the
temperature of stack current and/or stack reactants. The shunt
regulating circuit 20 warms the fuel cell stack 14 in two distinct
ways, internally by operating the fuel cell stack 14 to source the
energy that flows to the shunt regulating circuit 20, and
externally, by returning the dissipated energy to the fuel cell
stack 14 as heat. Thus, in operation the shunt regulating circuit
20 with a heater dump load 22 may shorten cold start time.
[0048] Further, if the active device (i.e., transistorized switch
26) of shunt regulating circuit 20 fails open, the shunt regulating
circuit 20 will be disconnected, while if the active device 26
fails closed a permanent parasitic load will be imposed on the fuel
cell stack 14. In neither case will the fuel cell stack 14 be
disconnected from the work load 12. Thus, the shunt regulating
circuit 20 performs better under the fault scenario than a series
regulator.
[0049] Although specific embodiments of and examples for the fuel
cell system and method are described herein for illustrative
purposes, various equivalent modifications can be made without
departing from the spirit and scope of the invention, as will be
recognized by those skilled in the relevant art. For example, the
teachings provided herein can be applied to fuel cell systems
including other types of fuel cell stacks or fuel cell assemblies,
not necessarily the polymer exchange membrane fuel cell assembly
generally described above. Additional transient suppression devices
can be placed across the stack (i.e., faster devices with higher
threshold, lower dynamic impedance and lower power absorption
capability) without risk of their failure. The shunt regulating
circuit 20 can also include self protection devices, such as fuses
or thermal trips. While the drawings all illustrate low side
switching, the fuel cell system 10 can employ high side switching
as well.
[0050] The various embodiments described above can be combined to
provide further embodiments. Aspects of the invention can be
modified, if necessary, to employ systems, circuits and concepts to
provide yet further embodiments of the invention.
[0051] All of the above U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification and/or listed in the Application Data Sheet,
including but not limited to U.S. application Ser. No. 10/017,483,
filed Dec. 14, 2001, are incorporated herein by reference, in their
entirety.
[0052] These and other changes can be made to the invention in
light of the above-detailed description. In general, in the
following claims, the terms used should not be construed to limit
the invention to the specific embodiments disclosed in the
specification and claims, but should be construed to include all
fuel cell systems that operate in accordance with the claims.
Accordingly, the invention is not limited by the disclosure, but
instead its scope is to be determined entirely by the following
claims.
* * * * *