U.S. patent application number 10/017461 was filed with the patent office on 2003-06-19 for fuel cell system multiple stage voltage control method and apparatus.
This patent application is currently assigned to Ballard Power System Inc.. Invention is credited to Pearson, Martin T..
Application Number | 20030111977 10/017461 |
Document ID | / |
Family ID | 21782717 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030111977 |
Kind Code |
A1 |
Pearson, Martin T. |
June 19, 2003 |
FUEL CELL SYSTEM MULTIPLE STAGE VOLTAGE CONTROL METHOD AND
APPARATUS
Abstract
A fuel cell system determines each of a battery charging current
error, a battery voltage error, and a stack current error. The fuel
cell system regulates current through a series pass element in
response to a greater of the determined errors, operating in three
modes: battery voltage limiting mode, stack current limiting mode
and battery charging current limiting mode. Additionally, there can
be a fourth "saturation" mode where the stack voltage V.sub.S drops
below the battery voltage V.sub.B. A voltage difference across the
series pass element is compared to a desired condition such as a
saturation level, and a partial pressure of a reactant flow to the
fuel cell stack adjusted based on the determined amount of
deviation limiting the energy dissipated by the series pass
element. Individual fuel cell systems can be combined in series
and/or parallel to produce a combined fuel cell system having a
desired output voltage and current.
Inventors: |
Pearson, Martin T.;
(Burnaby, CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Assignee: |
Ballard Power System Inc.
Burnaby
CA
|
Family ID: |
21782717 |
Appl. No.: |
10/017461 |
Filed: |
December 14, 2001 |
Current U.S.
Class: |
320/101 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/0491 20130101; H01M 8/04798 20130101; Y02E 60/10 20130101;
H01M 8/04119 20130101; H01M 16/006 20130101; H01M 8/04888 20130101;
H01M 8/04597 20130101; H01M 8/04007 20130101; H01M 8/04917
20130101; H01M 8/04992 20130101; H02J 7/34 20130101; H01M 8/04753
20130101; H02J 1/082 20200101; H02J 2300/30 20200101; H01M 8/04589
20130101; H01M 8/04567 20130101; H01M 8/04373 20130101 |
Class at
Publication: |
320/101 |
International
Class: |
H02J 007/00; H01M
010/46 |
Claims
We/I claim:
1. A fuel cell system, comprising: a fuel cell stack having a
number of fuel cells; a battery having a number of battery cells
electrically couplable in parallel across the fuel cell stack; a
series pass element electrically coupled between at least a portion
of the fuel cell stack and a portion of the battery; a regulating
circuit for regulating current through the series pass element in
response to a greater of a battery charging current error, a
battery voltage error and a stack current error; a reactant
delivery system for delivering reactant to the fuel cells, the
reactant delivery system including at least a first control element
adjustable to control a partial pressure in a flow of a reactant to
at least some of the fuel cells; and a control circuit coupled to
receive signals corresponding to a voltage on an input side and an
output side of the series pass element and configured to determine
a deviation of a voltage difference across the series pass element
from a desired operational condition based on the received signals,
the control circuit further coupled to control the at least first
control element based on the determined deviation.
2. The fuel cell system of claim 1 wherein the regulating circuit
comprises: a battery charging current error integrator having a
first input coupled to receive a battery charging current signal
and a second input coupled to receive a battery charging current
limit signal; a battery voltage error integrator having a first
input coupled to receive a battery voltage signal and a second
input coupled to receive a battery voltage limit signal; and a
stack current error integrator having a first input coupled to
receive a stack current signal and a second input coupled to
receive a stack current limit signal.
3. The fuel cell system of claim 1 wherein the regulating circuit
comprises: a charge pump; and a level shifter coupled between the
charge pump and the series pass element.
4. The fuel cell system of claim 1 wherein the regulating circuit
comprises: an OR circuit.
5. The fuel cell system of claim 1 wherein the regulating circuit
comprises: a battery charging current error integrator having a
first input coupled to receive a battery charging current signal
and a second input coupled to receive a battery charging current
limit signal; a battery voltage error integrator having a first
input coupled to receive a battery voltage signal and a second
input coupled to receive a battery voltage limit signal; a stack
current error integrator having a first input coupled to receive a
stack current signal and a second input coupled to receive a stack
current limit signal; an OR circuit having an input side and an
output side, the input side coupled to the battery charging current
error integrator, the battery voltage error integrator, and the
stack current error integrator; a level shifter electrically
coupled between the OR circuit and the series pass element; and a
charge pump coupled to supply a charge to the series pass element
via the level shifter.
6. The fuel cell system of claim 1 wherein the series pass element
comprises a field effect transistor.
7. The fuel cell system of claim 1 wherein at least a portion of
the battery is electrically coupled in parallel with at least a
portion of the fuel cell stack.
8. The fuel cell system of claim 1 wherein the control circuit
comprises a first comparator coupled to receive the first and the
second voltages, and a second comparator coupled to receive the
voltage difference from the first comparator and value
corresponding to the desired operational condition.
9. The fuel cell system of claim 1 wherein the control circuit
comprises a first comparator coupled to receive the first and the
second voltages, and a second comparator coupled to receive the
voltage difference from the first comparator and value
corresponding to the desired operational condition, wherein the
desired operational condition is between approximately 75 percent
and 95 percent of a saturation level for the series pass
element.
10. A fuel cell system, comprising: a number of fuel cells forming
a fuel cell stack; a number of battery cells forming a battery; a
series pass element; a blocking diode electrically coupled between
the fuel cell stack and the series pass element; a regulating
circuit for regulating current through the series pass element in
proportion to at least a greater of a difference between a battery
charging current and a battery charging current limit, a difference
between a battery voltage and a battery voltage limit, and a
difference between a stack current and a stack current limit; a
reactant delivery system for delivering reactant to the fuel cells,
the reactant delivery system including at least a first flow
regulator adjustable to control a partial pressure in a flow of a
reactant to at least some of the fuel cells; and a control circuit
coupled to receive signals corresponding to a voltage across the
series pass element and to provide a control signal to at least the
first control element mathematically related to a voltage
difference across the series pass element.
11. The fuel cell system of claim 10 wherein the regulating circuit
comprises: a battery current integrator having a first input, a
second input and an output, the first input coupled to receive a
battery current value and the second input coupled to receive a
battery current limit value; a battery voltage integrator having a
first input, a second input and an output, the first input coupled
to receive a battery voltage value and the second input coupled to
receive a battery voltage limit value; a stack current integrator
having a first input, a second input and an output, the first input
coupled to receive a stack current value and the second input
coupled to receive a stack current limit value; and an OR circuit
coupled to the output of each of the battery current integrator,
the battery voltage integrator and the stack current integrator to
select the greater of a value on each of the respective
outputs.
12. The fuel cell system of claim 10 wherein the regulating circuit
comprises: a level shifter electrically coupled between the OR
circuit and the series pass element; and a charge pump coupled to
provide current to the series pass element through the level
shifter.
13. The fuel cell system of claim 10 wherein the regulating circuit
comprises: a battery current integrator having a first input, a
second input and an output, the first input coupled to receive a
battery current value and the second input coupled to receive a
battery current limit value; a battery voltage integrator having a
first input, a second input and an output, the first input coupled
to receive a battery voltage value and the second input coupled to
receive a battery voltage limit value; a stack current integrator
having a first input, a second input and an output, the first input
coupled to receive a stack current value and the second input
coupled to receive a stack current limit value; an OR circuit
coupled to the output of each of the battery current integrator,
the battery voltage integrator and the stack current integrator; a
level shifter coupled to the OR circuit to receive the greater of
the value on each of the outputs; and a charge pump coupled to the
series pass element through the level shifter.
14. The fuel cell system of claim 10 wherein the regulating circuit
comprises a microprocessor programmed to regulate the current
through the series pass element by: integrating a difference
between a battery current and a battery current limit; integrating
a difference between a battery voltage and a battery voltage limit;
integrating a difference between a stack current and a stack
current limit; selecting a greater of the integrated differences;
and applying a control signal to the series pass element
proportional to the greater of the integrated differences.
15. The fuel cell system of claim 10, further comprising: a battery
charging current sensor; a battery voltage sensor; and a stack
current sensor.
16. The fuel cell system of claim 10, further comprising: a battery
charging current sensor; a stack current sensor; battery voltage
sensor; a battery temperature sensor; and a temperature
compensation circuit coupled to the battery temperature sensor to
produce a battery voltage limit that is temperature
compensated.
17. A circuit for a fuel cell system having a fuel cell stack and a
battery, the control circuit comprising: a series pass element
electrically coupleable between at least a portion of the fuel cell
stack and a portion of the battery; a regulating circuit for
regulating current through the series pass element in response to a
greater of a battery charging current error, a battery voltage
error and a stack current error; and a control circuit coupled to
receive signals corresponding to a voltage on an input side and an
output side of the series pass element and configured to determine
a deviation of a voltage difference across the series pass element
from a desired operational condition based on the received signals
and to produce a control signal based on the determined
deviation.
18. The circuit of claim 17 wherein the regulating circuit
comprises: a battery charging current error integrator having a
first input coupled to receive a battery charging current signal
proportional to a battery charging current, a second input coupled
to receive a battery charging current limit signal proportional to
a battery charging current limit, and an output to supply a battery
current error signal proportional to a difference between the
battery charging current and the battery charging current limit; a
battery voltage error integrator having a first input coupled to
receive a battery voltage signal proportional to a battery voltage,
a second input coupled to receive a battery voltage limit signal
proportional to a battery voltage limit, and an output to supply a
battery voltage error signal proportional to a difference between
the battery voltage and the battery voltage limit; a stack current
error integrator having a first input coupled to receive a stack
current signal proportional to a stack current, a second input
coupled to receive a stack current limit signal proportional to a
stack current limit, and an output to supply a stack current error
signal proportional to a difference between the stack current and
the stack current limit; and an OR circuit coupled to the output of
each of the error integrators to select a greater one of the error
signals from the error integrators.
19. The circuit of claim 17 wherein the series pass element
comprises a transistor having a first terminal, a second terminal
and a control terminal, the first and the second terminals
coupleable between the fuel cell stack and the battery, and wherein
the regulating circuit comprises: a level shifter coupled to
receive the greater of the battery charging current error, the
battery voltage error and the stack current error; and a charge
pump coupled to the control terminal of the transistor by way of
the level shifter.
20. A circuit for a fuel cell system, comprising: a series pass
element; a blocking diode electrically coupled in series with the
series pass element; a regulating circuit coupled to the series
pass element to regulate a current through the series pass element
in proportion to at least a greater of a difference between a
battery charging current and a battery charging current limit, a
difference between a battery voltage and a battery voltage limit,
and a difference between a stack current and a stack current limit;
and a control circuit coupled to receive signals corresponding to a
voltage across the battery and to provide a control signal
mathematically related to a difference between the voltage across
the battery and a defined desired voltage across the battery.
21. The circuit of claim 20, further comprising: a battery charging
current sensor; a battery voltage sensor; and a stack current
sensor.
22. The circuit of claim 20 wherein the regulating circuit
comprises: a battery current integrator having a first input, a
second input and an output, the first input coupled to receive a
battery current value and the second input coupled to receive a
battery current limit value; a battery voltage integrator having a
first input, a second input and an output, the first input coupled
to receive a battery voltage value and the second input coupled to
receive a battery voltage limit value; a stack current integrator
having a first input, a second input and an output, the first input
coupled to receive a stack current value and the second input
coupled to receive a stack current limit value; and an OR circuit
coupled to the output of each of the battery current integrator,
the battery voltage integrator and the stack current integrator to
select the greater of a value on each of the outputs.
23. The circuit of claim 20 wherein the regulating circuit
comprises: a battery current integrator having a first input, a
second input and an output, the first input coupled to receive a
battery current value and the second input coupled to receive a
battery current limit value; a battery voltage integrator having a
first input, a second input and an output, the first input coupled
to receive a battery voltage value and the second input coupled to
receive a battery voltage limit value; a stack current integrator
having a first input, a second input and an output, the first input
coupled to receive a stack current value and the second input
coupled to receive a stack current limit value; an OR circuit
coupled to the output of each of the battery current integrator,
the battery voltage integrator and the stack current integrator to
select the greater of a value on each of the outputs; a level
shifter coupled to the OR circuit to receive the greater of the
value on each of the outputs; and a charge pump coupled to the
series pass element through the level shifter.
24. The circuit of claim 20 wherein the series pass element
comprises a field effect transistor.
25. A circuit for a fuel cell system, comprising: a battery
charging current sensor; a battery charging current error
integrator having a first input coupled to the battery charging
current sensor to receive a battery charging current signal
proportional to a battery charging current, a second input coupled
to receive a battery charging current limit signal proportional to
a battery charging current limit, and an output to supply a battery
current error signal proportional to a difference between the
battery charging current and the battery charging current limit; a
battery voltage sensor; a battery voltage error integrator having a
first input coupled to the battery voltage sensor to receive a
battery voltage signal proportional to a battery voltage, a second
input coupled to receive a battery voltage limit signal
proportional to a battery voltage limit, and an output to supply a
battery voltage error signal proportional to a difference between
the battery voltage and the battery voltage limit; a stack current
sensor; a stack current error integrator having a first input
coupled to the stack current sensor to receive a stack current
signal proportional to a stack current, a second input coupled to
receive a stack current limit signal proportional to a stack
current limit, and an output to supply a stack current error signal
proportional to a difference between the stack current and the
stack current limit; an OR circuit coupled to the output of each of
the battery current error integrator, the battery voltage error
integrator and the stack current error integrator; a series pass
element having a pair of terminals for selectively providing a
current path and a control terminal coupled to the OR circuit for
regulating current through the current path in proportion to a
greater of the battery current error signal, the battery voltage
error signal and the stack current error signal; and a control
circuit coupled to receive signals corresponding to a voltage on an
input side and an output side of the series pass element and
configured to determine a deviation of a voltage difference across
the series pass element from a desired operational condition based
on the received signals and to produce a control signal based on
the determined deviation.
26. The circuit of claim 25 wherein the regulating circuit
comprises a number of discrete integrators.
27. The circuit of claim 25 wherein the regulating circuit
comprises a microprocessor.
28. The circuit of claim 25, further comprising: a temperature
compensation circuit coupled to the battery temperature sensor to
produce a battery voltage limit that is compensated for
temperature.
29. A circuit for a fuel cell system, comprising: means for
determining a greater of a difference between a battery charging
current and a battery charging current limit, a difference between
a battery voltage and a battery voltage limit, and a difference
between a stack current and a stack current limit; series pass
regulating means for regulating a flow of stack current through a
blocking diode in proportion to the determined greater difference;
means for determining a difference between a voltage difference
across the series pass regulating means and a desired a desired
operational condition of the series pass regulating means; and
means for controlling a partial pressure of at least one reactant
flow in proportion to the determined difference between the voltage
difference across the series pass regulating means and the desired
operational condition of the series pass regulating means.
30. The circuit of claim 29, comprising: integrating means for
determining the difference between the battery charging current and
the battery charging current limit; integrating means for
determining the difference between the battery voltage and the
battery voltage limit; and integrating means for determining the
difference between the stack current and the stack current
limit.
31. The circuit of claim 29 wherein the means for determining a
difference between a voltage difference across the series pass
regulating means and a desired a desired operational condition of
the series pass regulating means includes first comparator means
for comparing a first voltage on an input side of the series pass
regulating means and a second voltage on an output side of the
series pass regulating means, and second comparator means for
comparing the voltage difference across the series pass regulating
means with a value corresponding to a desired percentage of a
saturation value for the series pass regulating means.
32. A method of operating a fuel cell system, comprising: supplying
current at a number of output terminals from at least one of a fuel
cell stack and a battery electrically coupled in parallel with the
fuel cell stack; in a first stage, regulating a current through a
series pass element in proportion to at least a greater of a
difference between a battery charging current and a battery
charging current limit, a difference between a battery voltage and
a battery voltage limit, and a difference between the stack current
and the stack current limit; and in a second stage, adjusting a
partial pressure of a reactant flow to at least a portion of the
fuel cell stack to maintain a series pass element at a desired
saturation level.
33. The method of claim 32 wherein the first stage and the second
stage occur during a same time.
34. The method of claim 32, further comprising: determining a first
voltage on an input side of the series pass element; determining a
second voltage on an output side of the series pass element;
determining a difference in voltage across the series pass element
from the first and the second voltages; and determining an amount
of deviation of the difference in voltage across the series pass
element from a value corresponding to the desired saturation level,
and wherein adjusting a partial pressure of a reactant flow to at
least a portion of the fuel cell stack to maintain the series pass
element at the desired saturation level includes adjusting the
partial pressure of the reactant flow based on the determined
amount of deviation.
35. A method of operating in a fuel cell system, the method
comprising: determining a battery charging current error;
determining a battery voltage error; determining a stack current
error; regulating current through a series pass element in response
to a greater of the battery charging current error, the battery
voltage error and the stack current error; determining a voltage
difference across the series pass element; determining an amount of
deviation of the determined voltage difference from a desired
operational condition of the series pass element; and for at least
one reactant flow to at least a portion of the fuel cell stack,
adjusting a partial pressure of the reactant flow based on the
determined amount of deviation.
36. The method of claim 35 wherein, determining a battery charging
current error includes integrating a difference between a battery
charging current and a battery charging current limit over time;
determining a battery voltage error includes integrating a
difference between a battery voltage and a battery voltage limit
over time; and determining a stack current error includes
integrating a difference between a stack current and a stack
current limit over time.
37. The method of claim 35, further comprising: selecting the
greater of the battery charging current error, the battery voltage
error and the stack current error; level shifting the selected one
of the battery charging current error, the battery voltage error
and the stack current error; and applying the level shifted
selected one of the battery charging current error, the battery
voltage error and the stack current error to a control terminal of
the series pass element.
38. The method of claim 35, further comprising: determining a
temperature proximate a battery; determining a battery voltage
limit based at least in part on the determined temperature; and
integrating a difference between a battery voltage and the
determined battery voltage limit over time to determine the battery
voltage error.
39. The method of claim 35, further comprising: selectively
coupling charge from a charge pump to a control terminal of the
series pass element in response to the greater of the battery
charging current error, the battery voltage error and the stack
current error.
40. The method of claim 35, further comprising: selectively
coupling charge from a charge pump to a control terminal of the
series pass element in response to the battery charging current
error at a first time, the battery voltage error at a second time
and the stack current error at a third time.
41. The method of claim 35, further comprising: determining a first
voltage on an input side of the series pass element; and
determining a second voltage on an output side of the series pass
element.
42. The method of claim 35, further comprising: determining a first
voltage on an input side of the series pass element; and
determining a second voltage on an output side of the series pass
element, and wherein determining a voltage difference across the
series pass element includes determining the difference between the
first and the second voltages.
43. The method of claim 35 wherein determining an amount of
deviation of the determined voltage difference from a desired
operational condition of the series pass element includes
determining a difference between the determined voltage difference
and a value corresponding to a percentage of a saturation level of
the series pass element, where the percentage is between
approximately 75 percent and approximately 95 percent.
44. The method of claim 35 wherein adjusting a partial pressure of
the reactant flow based on the determined amount of deviation
includes adjusting a partial pressure of a flow of fuel to at least
a portion of the fuel cell stack and adjusting a partial pressure
of a flow of oxidant to at least the same portion of the fuel cell
stack.
45. The method of claim 35, further comprising: holding a pressure
of the at least one reactant flow approximately constant while
adjusting the partial pressure of the at least one reactant
flow.
46. A method of operating in a fuel cell system, the method
comprising: determining a difference between a battery charging
current and a battery charging current limit; determining a
difference between a battery voltage and a battery voltage limit;
determining a difference between a stack current and a stack
current limit; regulating a current through a series pass element
in proportion to at least a greater of the difference between the
battery charging current and the battery charging current limit,
the difference between the battery voltage and the battery voltage
limit, and the difference between the stack current and the stack
current limit; determining a voltage difference across the series
pass element; determining an amount of deviation of the determined
voltage difference from a desired operational condition of the
series pass element; and for at least one reactant flow to at least
a portion of the fuel cell stack, adjusting a partial pressure of
the reactant flow based on the determined amount of deviation.
47. The method of claim 46, further comprising: selecting the
greater of the battery charging current error, the battery voltage
error and the stack current error; level shifting the selected one
of the battery charging current error, the battery voltage error
and the stack current error; and applying the level shifted
selected one of the battery charging current error, the battery
voltage error and the stack current error to a control terminal of
the series pass element.
48. The method of claim 46, further comprising: determining a
temperature proximate a battery; determining the battery voltage
limit based at least in part on the determined temperature.
49. The method of claim 46, further comprising: selectively
coupling charge from a charge pump to a control terminal of the
series pass element in proportion to the greater of the battery
charging current error, the battery voltage error and the stack
current error.
50. The method of claim 46, further comprising: selectively
coupling charge from a charge pump to a control terminal of the
series pass element in proportion to the battery charging current
error at a first time, the battery voltage error at a second time
and the stack current error at a third time.
51. A fuel cell system, comprising: a voltage bus; a first fuel
cell stack electrically couplable across the voltage bus; a first
battery electrically couplable across the voltage bus; a first
series pass element electrically coupled in series on the voltage
bus between at least a portion of the first fuel cell stack and a
portion of the first battery; a first regulating circuit for
regulating current through the first series pass element in
response to a greater of a battery charging current error, a
battery voltage error and a stack current error; a first reactant
delivery system for delivering reactant to the first fuel cell
stack, the reactant delivery system including at least a first
control element adjustable to control a partial pressure in a flow
of a reactant to at least some of the fuel cells of the first fuel
cell stack; a first control circuit coupled to receive signals
corresponding to a voltage on an input side and a voltage on an
output side of the first series pass element and configured to
determine a deviation of a voltage difference across the first
series pass element from a desired operational condition based on
the received signals, the first control circuit further coupled to
control the at least first control element based on the determined
deviation; a second fuel cell stack electrically couplable across
the voltage bus; a second battery electrically couplable across the
voltage bus; a second series pass element electrically coupled in
series on the voltage bus between at least a portion of the second
fuel cell stack and a portion of the second battery; a second
regulating circuit for regulating current through the second series
pass element in response to a greater of a battery charging current
error, a battery voltage error and a stack current error; a second
reactant delivery system for delivering reactant to the second fuel
cell stack, the reactant delivery system including at least a
second control element adjustable to control a partial pressure in
a flow of a reactant to at least some of the fuel cells of the
second fuel cell stack; and a second control circuit coupled to
receive signals corresponding to a voltage on an input side and a
voltage on an output side of the second series pass element and
configured to determine a deviation of a voltage difference across
the second series pass element from a desired operational condition
based on the received signals, the second control circuit further
coupled to control the at least second control element based on the
determined deviation.
52. The fuel cell system of claim 51 wherein the second fuel cell
stack, the second battery and the second series pass element are
electrical coupled in series with the first fuel cell stack, the
first battery and the first series pass element.
53. The fuel cell system of claim 51 wherein the second fuel cell
stack, the second battery and the second series pass element are
electrical coupled in parallel with the first fuel cell stack, the
first battery and the first series pass element.
54. The fuel cell system of claim 51, further comprising: a third
fuel cell stack electrically couplable across the voltage bus; a
third battery electrically couplable across the voltage bus; a
third series pass element electrically coupled in series on the
voltage b us between at least a portion of the third fuel cell
stack and a portion of the third battery; a third regulating
circuit for regulating current through the third series pass
element in response to a greater of a battery charging current
error, a battery voltage error and a stack current error; a third
reactant delivery system for delivering reactant to the third fuel
cell stack, the reactant delivery system including at least a third
control element adjustable to control a partial pressure in a flow
of a reactant to at least some of the fuel cells of the third fuel
cell stack; and a third control circuit coupled to receive signals
corresponding to a voltage on an input side and a voltage on an
output side of the third series pass element and configured to
determine a deviation of a voltage difference across the third
series pass element from a desired operational condition based on
the received signals, the third control circuit further coupled to
control the at least third control element based on the determined
deviation.
55. The fuel cell system of claim 5 1, further comprising: a third
fuel cell stack electrically couplable across the voltage bus; a
third battery electrically couplable across the voltage bus; a
third series pass element electrically coupled in series on the
voltage bus between at least a portion of the third fuel cell stack
and a portion of the third battery; a third regulating circuit for
regulating current through the third series pass element in
response to a greater of a battery charging current error, a
battery voltage error and a stack current error, wherein the second
fuel cell stack, the second battery and the second series pass
element are electrical coupled in series with the first fuel cell
stack, the first battery and the first series pass element and
wherein the third fuel cell stack, the third battery and the third
series pass element are electrical coupled in series with the first
and the second fuel cell stack, the first and the second battery
and the first and the second series pass element; a third reactant
delivery system for delivering reactant to the third fuel cell
stack, the reactant delivery system including at least a third
control element adjustable to control a partial pressure in a flow
of a reactant to at least some of the fuel cells of the third fuel
cell stack; and a third control circuit coupled to receive signals
corresponding to a voltage on an input side and a voltage on an
output side of the third series pass element and configured to
determine a deviation of a voltage difference across the third
series pass element from a desired operational condition based on
the received signals, the third control circuit further coupled to
control the at least third control element based on the determined
deviation.
56. The fuel cell system of claim 51, further comprising: a third
fuel cell stack electrically couplable across the voltage bus; a
third battery electrically couplable across the voltage bus; a
third series pass element electrically coupled in series on the
voltage bus between at least a portion of the third fuel cell stack
and a portion of the third battery; a third regulating circuit for
regulating current through the third series pass element in
response to a greater of a battery charging current error, a
battery voltage error and a stack current error, wherein the second
fuel cell stack, the second battery and the second series pass
element are electrical coupled in series with the first fuel cell
stack, the first battery and the first series pass element and
wherein the third fuel cell stack, the third battery and the third
series pass element are electrical coupled in parallel with the
first and the second fuel cell stack, the first and the second
battery and the first and the second series pass element; a third
reactant delivery system for delivering reactant to the third fuel
cell stack, the reactant delivery system including at least a third
control element adjustable to control a partial pressure in a flow
of a reactant to at least some of the fuel cells of the third fuel
cell stack; and a third control circuit coupled to receive signals
corresponding to a voltage on an input side and a voltage on an
output side of the third series pass element and configured to
determine a deviation of a voltage difference across the third
series pass element from a desired operational condition based on
the received signals, the third control circuit further coupled to
control the at least third control element based on the determined
deviation.
57. A fuel cell system combination, comprising: a voltage bus; a
first fuel cell system having a first fuel cell stack and a first
battery electrically coupled in parallel across the voltage bus;
and a second fuel cell system having a second fuel cell stack and a
second battery electrically coupled in parallel across the voltage
bus.
58. The fuel cell system of claim 57 wherein the first fuel cell
stack has a first fuel cell polarization curve and the first
battery has a first battery polarization curve, the first battery
polarization approximately matching the first fuel cell
polarization curve, and wherein the second fuel cell stack has a
second fuel cell polarization curve and the second battery has a
second battery polarization curve, the second battery polarization
approximately matching the second fuel cell polarization curve.
59. The fuel cell system of claim 57, further comprising: means for
approximately matching a polarization curve of the first fuel cell
stack and a polarization curve of the first battery; and means for
approximately matching a polarization curve of the second fuel cell
stack and a polarization curve of the second battery.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention is generally related to fuel cell systems,
and more particularly to controlling an output voltage of the fuel
cell system.
[0003] 2. Description of the Related Art
[0004] Electrochemical fuel cells convert fuel and oxidant 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, typically in the form
of finely comminuted platinum, at each membrane electrode interface
to induce the desired electrochemical reaction. In operation, the
electrodes are electrically coupled for conducting electrons
between the electrodes through an external circuit. Typically, a
number of MEAs are electrically coupled in series to form a fuel
cell stack having a desired power output.
[0005] In typical fuel cells, the MEA is disposed between two
electrically conductive fluid flow field plates or separator
plates. Fluid flow field plates have flow passages to direct fuel
and oxidant to the electrodes, namely the anode and the cathode,
respectively. The fluid flow field plates act as current
collectors, provide support for the electrodes, provide access
channels for the fuel and oxidant, and provide channels for the
removal of reaction products, such as water formed during fuel cell
operation. The fuel cell system may use the reaction products in
maintaining the reaction. For example, reaction water may be used
for hydrating the ion exchange membrane and/or maintaining the
temperature of the fuel cell stack.
[0006] Stack current is a direct function of the reactant flow, the
stack current increasing with increasing reactant flow. The stack
voltage varies inversely with respect to the stack current in a
non-linear mathematical relationship. The relationship between
stack voltage and stack current at a given flow of reactant is
typically represented as a polarization curve for the fuel cell
stack. A set or family of polarization curves can represent the
stack voltage-current relationship at a variety of reactant flow
rates.
[0007] In most applications, it is desirable to maintain an
approximately constant voltage output from the fuel cell stack. One
approach is to employ a battery in the fuel cell system to provide
additional current when the demand of the load exceeds the output
of the fuel cell stack. This approach often requires a separate
battery charging supply to maintain the charge on the battery,
introducing undesirable cost and complexity into the system.
Attempts to place the battery in parallel with the fuel cell stack
to eliminate the need for a separate battery charging supply raises
additional problems. These problems may include, for example,
preventing damage to the battery from overcharging, increasing
efficiency, as well as the need for voltage, current, or power
conversion or matching components between the fuel cell stack,
battery and/or load. A less costly, less complex and/or more
efficient approach is desirable.
BRIEF SUMMARY OF THE INVENTION
[0008] In one aspect, a fuel cell system includes: a fuel cell
stack, a battery, a series pass element electrically coupled
between at least a portion of the fuel cell stack and a portion of
the battery, a regulating circuit for regulating current through
the series pass element in response to a greater of a battery
charging current error, a battery voltage error, and a stack
current error, a reactant delivery system for delivering reactant
to the fuel cells, the reactant delivery system including at least
a first control element adjustable to control a partial pressure in
a flow of a reactant to at least some of the fuel cells, and a
control circuit coupled to receive signals corresponding to a
voltage on an input side and an output side of the series pass
element and configured to determine a deviation of a voltage
difference across the series pass element from a desired
operational condition based on the received signals, the control
circuit further coupled to control the at least first control
element based on the determined deviation. The fuel cell system may
include a battery charging current error integrator having a first
input coupled to receive a battery charging current signal and a
second input coupled to receive a battery charging current limit
signal. The fuel cell system may also include a battery voltage
error integrator having a first input coupled to receive a battery
voltage signal and a second input coupled to receive a battery
voltage limit signal. The fuel cell system may further include a
stack current error integrator having a first input coupled to
receive a stack current signal and a second input coupled to
receive a stack current limit signal. The fuel cell system may
additionally include an OR circuit for selecting a greater of the
battery charging current error, the battery voltage error and the
stack current error.
[0009] In another aspect, a fuel cell system includes: a number of
fuel cells forming a fuel cell stack, a number of battery cells
forming a battery, a series pass element, a blocking diode
electrically coupled between the fuel cell stack and the series
pass element, a regulating circuit for regulating current through
the series pass element in proportion to at least a greater of a
difference between a battery charging current and a battery
charging current limit, a difference between a battery voltage and
a battery voltage limit, and a difference between a stack current
and a stack current limit, a reactant delivery system for
delivering reactant to the fuel cells, the reactant delivery system
including at least a first flow regulator adjustable to control a
partial pressure in a flow of a reactant to at least some of the
fuel cells, and a control circuit coupled to receive signals
corresponding to a voltage difference across the series pass
element and to provide a control signal to at least the first
control element mathematically related to a voltage difference
across the series pass element.
[0010] In yet another aspect, a circuit for a fuel cell system
includes a series pass element electrically coupleable between at
least a portion of the fuel cell stack and a portion of the
battery, a regulating circuit for regulating current through the
series pass element in response to a greater of a battery charging
current error, a battery voltage error and a stack current error,
and a control circuit coupled to receive signals corresponding to a
voltage on an input side and an output side of the series pass
element and configured to determine a deviation of a voltage
difference across the series pass element from a desired
operational condition based on the received signals and to produce
a control signal based on the determined deviation.
[0011] In a further aspect, a circuit for a fuel cell system
includes a series pass element, a blocking diode electrically
coupled in series with the series pass element, a regulating
circuit coupled to the series pass element to regulate a current
through the series pass element in proportion to at least a greater
of a difference between a battery charging current and a battery
charging current limit, a difference between a battery voltage and
a battery voltage limit, and a difference between a stack current
and a stack current limit, and a control circuit coupled to receive
signals corresponding to a voltage across the serried pass element
and to provide a control signal mathematically related to a voltage
difference across the series pass element.
[0012] In yet a further aspect, a circuit for a fuel cell system
includes a battery charging sensor, a battery charging current
error integrator, a battery voltage sensor, a battery voltage error
integrator, a stack current sensor, a stack current error
integrator, an OR circuit coupled to the output of each of the
battery current error integrator, the battery voltage error
integrator and the stack current error integrator, a series pass
element having a pair of terminals for selectively providing a
current path and a control terminal coupled to the OR circuit for
regulating current through the current path in proportion to a
greater of the battery current error signal, the battery voltage
error signal and the stack current error signal, and a control
circuit coupled to receive signals corresponding to a voltage on an
input side and an output side of the series pass element and
configured to determine a deviation of a voltage difference across
the series pass element from a desired operational condition based
on the received signals and to produce a control signal based on
the determined deviation.
[0013] In even a further aspect, a method of operating a fuel cell
system includes: supplying current at a number of output terminals
from at least one of a fuel cell stack and a battery electrically
coupled in parallel with the fuel cell stack, in a first stage,
regulating a current through a series pass element in proportion to
at least a greater of a difference between a battery charging
current and a battery charging current limit, a difference between
a battery voltage and a battery voltage limit, and a difference
between the stack current and the stack current limit, and in a
second stage, adjusting a partial pressure of a reactant flow to at
least a portion of the fuel cell stack to maintain a series pass
element at a desired saturation level.
[0014] In even a further aspect, a method of operating a fuel cell
system includes: determining a battery charging current error,
determining a battery voltage error, determining a stack current
error, regulating current through the series pass element in
response to a greater of the battery charging current error, the
battery voltage error, and the stack current error, determining a
voltage difference across the series pass element, determining an
amount of deviation of the determined voltage difference from a
desired operational condition of the series pass element, and for
at least one reactant flow to at least a portion of the fuel cell
stack, adjusting a partial pressure of the reactant flow based on
the determined amount of deviation. Determining the battery
charging current error may include integrating a difference between
a battery charging current and a battery charging current limit
over time. Determining the battery voltage error may include
integrating a difference between a battery voltage and a battery
voltage limit over time. Determining the stack current error may
include integrating a difference between a stack current and a
stack current limit over time. The method may also include
selecting the greater of the battery charging current error, the
battery voltage error and the stack current error, level shifting
the selected one of the errors, and applying the level shifted
error to a control terminal of the series pass element. The method
may further include determining a temperature proximate the battery
and determining the battery voltage limit based at least in part on
a determined temperature.
[0015] In still a further aspect, a method of operating a fuel cell
system includes: determining a difference between a battery
charging current and a battery charging current limit, determining
a difference between a battery voltage and a battery voltage limit,
determining a difference between a stack current and a stack
current limit, regulating a current through a series pass element
in proportion to at least a greater of the difference between the
battery charging current and the battery charging current limit,
the difference between the battery voltage and the battery voltage
limit, and the difference between the stack current and the stack
current limit, determining a voltage difference across the series
pass element, determining an amount of deviation of the determined
voltage difference from a desired operational condition of the
series pass element, and for at least one reactant flow to at least
a portion of the fuel cell stack, adjusting a partial pressure of
the reactant flow based on the determined amount of deviation.
[0016] In yet still a further aspect, a combined fuel cell system
includes two or more individual fuel cell systems electrically
coupled in series and/or parallel combinations to produce a desired
current at a desired voltage.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0017] In the drawings, identical reference numbers identify
similar elements or acts. The 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.
[0018] FIG. 1 is a schematic diagram of a fuel cell system powering
a load, the fuel cell system having a fuel cell stack, a battery, a
series pass element, a first stage including a regulating circuit
for controlling current flow through the series pass element and a
second stage including a controller employing a voltage difference
across the series pass element to reduce the energy dissipated by
the series pass element via control of reactant partial pressure in
accordance with an illustrated general embodiment in the
invention.
[0019] FIG. 2 is a schematic diagram of the first stage of the fuel
cell system of FIG. 1.
[0020] FIG. 3 is an alternative embodiment of the first stage of
the fuel cell system, employing a microprocessor as the regulating
circuit.
[0021] FIG. 4 is a flow diagram of an exemplary method of operating
the first stage of the fuel cell system of FIGS. 2 and 3.
[0022] FIG. 5 is an electrical schematic diagram of the second
stage of the fuel cell system of FIG. 1.
[0023] FIG. 6 is a flow diagram of an exemplary method of operating
the second stage of the fuel cell system of FIG. 5.
[0024] FIG. 7 is a graphical representation of the polarization
curves for an exemplary fuel cell stack, for five exemplary partial
pressures.
[0025] FIG. 8 is a schematic diagram of an alternative embodiment
of the fuel cell system of FIG. 1, in which portions of the fuel
cell stack are interconnected with portions of the battery.
[0026] FIGS. 9A-9F are a series of graphs relating stack, battery
and load currents, battery and bus voltages and load resistances of
the fuel cell system, where the fuel cell stack is sufficiently
powering the load without draining or recharging the battery.
[0027] FIGS. 10A-C are a series of graphs relating stack, battery
and load current over time for the fuel cell systems, where the
battery supplies current to the load to cover a shortfall from the
fuel cell stack and the fuel cell stack later recharges the
battery.
[0028] FIG. 11 is a schematic diagram of a number of the fuel cell
systems of FIG. 1, electrically coupled to form a combination fuel
cell system for powering a load at a desired voltage and
current.
DETAILED DESCRIPTION OF THE INVENTION
[0029] 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, batteries and fuel cell systems
have not been shown or described in detail to avoid unnecessarily
obscuring descriptions of the embodiments of the invention.
[0030] 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."
[0031] Fuel Cell System Overview
[0032] FIG. 1 shows a fuel cell system 10 providing power to a load
12 according to an illustrated embodiment of the invention. The
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. While the fuel cell system 10 is not
typically considered part of the load 12, portions of the fuel cell
system 10 such as the control electronics may constitute a portion
or all of the load 12 in some possible embodiments.
[0033] 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, represented
by arrow 9, such as hydrogen and air via a reactant supply system
16. The reactant supply system 16 may include one or more reactant
supply reservoirs or sources 11, a reformer (not shown), and/or one
or more control elements such as one or more compressors, pumps
and/or valves 18 or other reactant regulating elements. Operation
of the fuel cell stack 14 produces reactant product, represented by
arrow 20, typically including water. The fuel cell system 10 may
reuse some or all of the reactant products 20. For example, as
represented by arrow 22, some or all of the water may be returned
to the fuel cell stack 14 to humidify the hydrogen and air at the
correct temperature and/or to hydrate the ion exchange membranes
(not shown) or to control the temperature of the fuel cell stack
14.
[0034] The fuel cell stack 14 can be modeled as an ideal battery
having a voltage equivalent to an open circuit voltage and a series
resistance R.sub.S. The value of the series resistance R.sub.S is
principally a function of stack current I.sub.S, the availability
of reactants, and time. The series resistance R.sub.S varies in
accordance with the polarization curves for the particular fuel
cell stack 14. The series resistance R.sub.S can be adjusted by
controlling the availability of reactants 9 to drop a desired
voltage for any given current, thus allowing an approximately
uniform stack voltage V.sub.S across a range of stack currents
I.sub.S. The relationship between the reactant flow and the series
resistance R.sub.S is illustrated in FIG. 1 by the broken line
arrow 13. However, simply decreasing the overall reactant and
reaction pressures within the fuel cell system 10 may interfere
with the overall system operation, for example interfering with the
hydration of the ion exchange membrane and/or temperature control
of the fuel cell stack. To avoid these undesirable results, the
fuel cell system 10 may adjust the reactant partial pressure, as
explained in more detail below.
[0035] The fuel cell stack 14 produces a stack voltage V.sub.S
across a high voltage bus formed by the positive and negative
voltage rails 19a, 19b. The stack current I.sub.S flows to the load
12 from the fuel cell stack 14 via the high voltage bus. As used
herein, "high voltage" refers to the voltage produced by
conventional fuel cell stacks 14 to power loads 12, and is used to
distinguish between other voltages employed by fuel cell system 10
for control and/or communications (e.g., 5V). Thus, high voltage
and is not necessarily "high" with respect to other electrical
systems.
[0036] The fuel cell system 10 includes a battery 24 electrically
coupled in parallel with the fuel cell stack 14 across the rails
19a, 19b of the high voltage bus to power the load 12. The open
circuit voltage of the battery 24 is selected to be similar to the
full load voltage of the fuel cell stack 14. An internal resistance
R.sub.B of the battery 24 is selected to be much lower than the
internal resistance of the fuel cell stack 14. Thus, the battery 24
acts as a buffer, absorbing excess current when the fuel cell stack
14 produces more current than the load 12 requires, and providing
current to the load 12 when the fuel cell stack 14 produces less
current than the load 12 requires. The voltage across the high
voltage bus 19a, 19b will be the open circuit voltage of the
battery 24 minus the battery discharging current multiplied by the
value of the internal resistance R.sub.B of the battery 24. The
smaller the internal resistance R.sub.B of the battery 24, the
smaller the variations in bus voltage.
[0037] An optional reverse current blocking diode D1 can be
electrically coupled between the fuel cell stack 14 and the battery
24 to prevent current from flowing from the battery 24 to the fuel
cell stack 14. A drawback of the reverse current blocking diode D1
is the associated diode voltage drop. The fuel cell system 10 may
also include other diodes, as well as fuses or other surge
protection elements to prevent shorting and/or surges.
[0038] Stages
[0039] The fuel cell system 10 includes two control stages; a first
stage employing a series pass element 32 and a regulating circuit
34 for controlling current flow through the series pass element 32,
and a second stage employing a controller 28 for adjusting reactant
partial pressures to control the series resistance R.sub.S of the
fuel cell stack 14. The first and second stages operate together,
even simultaneously, in cooperation with the parallel coupled
battery 24 to achieve efficient and continuous output voltage
control while protecting the battery 24 from damage.
[0040] The first stage is a relatively fast reacting stage, while
the second stage is a slower reacting stage relative to the first
stage. As discussed above, the battery 24 provides a very fast
response to changes in load requirements, providing current to the
load 12 when demand is greater than the output of the fuel cell
stack 14 and sinking excess current when the output of the fuel
cell stack 14 exceeds the demand of the load 12. By controlling the
flow of current through the series pass element 32, the first stage
ensures that the battery 24 is properly charged and discharged in
an efficient manner without damage. By controlling the reactant
partial pressures, and hence the series resistance R.sub.S, the
second stage controls the efficiency of the fuel cell stack 14
operation (i.e., represented as the particular polarization curve
on which the fuel cell is operating). Thus, the second stage limits
the amount of heat dissipated by the series pass element 32 by
causing more energy to be dissipated via the fuel cell stack 14 (i
e., via less efficient operation).
[0041] Where the fuel cell stack 14 dissipates energy as heat, this
energy is recoverable in various portions of the fuel cell system,
and thus can be reused in other portions of the fuel cell system
(i.e., cogeneration). For example, the energy dissipated as heat
may be recycled to the fuel cell stack 14 via an airflow, stack
coolant, or via the reactants. Additionally, or alternatively, the
energy dissipated as heat may be recycled to a reformer (not
shown), other portion of the fuel cell system 10, or to some
external system. Additionally, limiting the amount of energy that
the series pass element 32 must dissipate, can reduce the size and
associated cost of the series pass element 32 and any associated
heat sinks.
[0042] The details of the first and second stages are discussed in
detail below.
[0043] First Stage Overview, Series Pass Element Regulator
[0044] With continuing reference to FIG. 1, the first stage of the
fuel cell system 10 includes the series pass element 32
electrically coupled between the fuel cell stack 14 and the battery
24 for controlling a flow of current I.sub.S from the fuel cell
stack 14 to the battery 24 and the load 12. The first stage of the
fuel cell system 10 also includes the regulating circuit 34 coupled
to regulate the series pass element 32 based on various operating
parameters of the fuel cell system 10. The series pass element 32
can take the form of a field effect transistor ("FET"), having a
drain and source electrically coupled between the fuel cell stack
14 and the battery 24 and having a gate electrically coupled to an
output of the regulating circuit 34.
[0045] The first stage of the fuel cell system 10 includes a number
of sensors for determining the various operating parameters of the
fuel cell system 10. For example, the fuel cell system 10 includes
a battery charge current sensor 36 coupled to determine a battery
current I.sub.B. Also for example, the fuel cell system 10 includes
a fuel cell stack current sensor 38 coupled to determine the stack
current I.sub.S. Further for example, the fuel cell system 10
includes a battery voltage sensor 40 for determining a voltage
V.sub.B across the battery 24. Additionally, the fuel cell system
10 may include a battery temperature sensor 42 positioned to
determine the temperature of the battery 24 or ambient air
proximate the battery 24. While the sensors 36-42 are illustrated
as being discrete from the regulating circuit 34, in some
embodiments one or more of the sensors 36-42 may be integrally
formed as part of the regulating circuit 34.
[0046] The first stage of the fuel cell system 10 may include a
soft start circuit 15 for slowly pulling up the voltage during
startup of the fuel cell system 10. The fuel cell system 10 may
also include a fast off circuit 17 for quickly shutting down to
prevent damage to the battery 24, for example when there is no load
or the load 12 is drawing no power.
[0047] Second Stage Overview, Reactant Partial Pressure
Controller
[0048] The second stage of the fuel cell system 10 includes the
controller 28, an actuator 30 and the reactant flow regulator such
as the valve 18. The controller 28 receives a value of a first
voltage V.sub.1 from an input side of the series pass element 32
and a value of a second voltage V.sub.2 from an output side of the
series pass element 32. The controller 28 provides a control signal
to the actuator 30 based on the difference between the first and
second voltages V.sub.1, V.sub.2 to adjust the flow of reactant to
the fuel cell stack 14 via the valve 18 or other reactant flow
regulating element.
[0049] Since the battery 24 covers any short-term mismatch between
the available reactants and the consumed reactants, the speed at
which the fuel cell reactant supply system 16 needs to react can be
much slower than the speed of the electrical load changes. The
speed at which the fuel cell reactant supply system 16 needs to
react mainly effects the depth of the charge/discharge cycles of
the battery 24 and the dissipation of energy via the series pass
element 32.
[0050] First Stage Description, Series Pass Element Regulation
[0051] FIG. 2 shows a one embodiment of the regulating circuit 34,
including components for determining a battery charging current
error, stack current error and battery voltage error, and for
producing an output to the series pass element 32 corresponding to
the greater of the determined errors.
[0052] The regulating circuit 34 includes a battery charging
current error integrating circuit 44 and a battery charging current
limit circuit 46 for determining the battery charging current
error. The battery charging current limit circuit 46 provides a
battery charging current limit value to the inverting terminal of
the battery charging current error integrating circuit 44, while
the battery charging current sensor 36 provides a battery charging
current value to the non-inverting terminal. A capacitor C9 is
coupled between the inverting terminal and an output terminal of
the battery charging current error integrating circuit 44. The
battery charging current limit error integrating circuit 44
integrates the difference between the battery charging current
value and the battery charging current limit value.
[0053] The regulating circuit 34 includes a stack current error
integrating circuit 50 and a stack current limit circuit 52 for
determining the stack current error. The stack current limit
circuit 52 provides a stack current limit value to the inverting
terminal of the stack current error integrating circuit 50, while
stack current sensor 38 provides a stack current value to the
non-inverting terminal. A capacitor C8 is coupled between the
inverting terminal and an output terminal of the stack current
error integrating circuit 50. The stack current error integrating
circuit 50 integrates the difference between the stack current
value and the stack current limit value. The limiting effect of the
second stage on the stack current limit is represented by broken
line arrow 53.
[0054] The regulating circuit 34 includes a battery voltage error
integrating circuit 56 and a battery voltage set point circuit 58.
The battery voltage set point circuit 58 provides a battery voltage
limit value to the inverting terminal of the battery voltage error
integrating circuit 56, while the battery voltage sensor 40
provides a battery voltage value to the non-inverting terminal. A
capacitor C7 is electrically coupled between the inverting terminal
and the output terminal of the battery voltage error integrating
circuit 56. The battery voltage error integrating circuit 56
integrates the difference between the battery voltage value and the
battery voltage set point value.
[0055] The regulating circuit 34 may also include a temperature
compensation circuit 62 that employs the battery temperature
measurement from the battery temperature detector 42 to produce a
compensation value. The battery voltage set point circuit 58
employs the compensation value in determining the battery voltage
set point value.
[0056] The regulating circuit 34 also includes an OR circuit 64 for
selecting the greater of the output values of the error integrators
44, 50, 56. The OR circuit 64 can take the form of three diodes
(not shown) having commonly coupled cathodes. The anode of each of
the diodes are electrically coupled to respective ones of the error
integration circuits 44, 50, 56.
[0057] The regulating circuit 34 also includes a charge pump 66 for
providing a voltage to a control terminal (e.g., gate) of the
series pass element 32 by way of a level shifter, such as an
inverting level shifter 68. The inverting level shifter 68 provides
a linear output value that is inverted from the input value.
[0058] FIG. 3 shows an alternative embodiment of the first stage of
the fuel cell system 10, employing a microprocessor 70 as the
regulating circuit. This alternative embodiment and those other
alternatives and alternative embodiments described herein are
substantially similar to the previously described embodiments, and
common acts and structures are identified by the same reference
numbers. Only significant differences in operation and structure
are described below.
[0059] The microprocessor 70 can be programmed or configured to
perform the functions of the regulating circuit 34 (FIG. 1). For
example, the microprocessor 70 may perform the error integration
for some or all of the battery charging current, stack current and
battery voltage values. The microprocessor 70 may store some or all
of the battery charging current limit, stack current limit and/or
battery voltage limit values. The microprocessor 70 may also
determine the temperature compensation based on the battery
temperature value supplied by the battery temperature detector 42.
Further, the microprocessor 70 may select the greater of the error
values, providing an appropriate signal to the control terminal of
the series pass element 32.
[0060] FIG. 4 shows an exemplary method 100 of operating the first
stage of fuel cell system 10 of FIGS. 1, 2 and 3. The method 100
repeats during operation to continually adjust the operating
parameters of the fuel cell system 10.
[0061] In step 102, the battery charging current sensor 36 (FIGS.
1-3) determines the value of the battery charging current I.sub.B.
In step 104, the battery charging current error integrating circuit
44 (FIG. 2) or microprocessor 70 (FIG. 3) determines the value of
the battery charging current error.
[0062] In step 106, the stack current sensor 38 (FIGS. 1-3)
determines the value of the stack current. In step 108, the stack
current error integrating circuit 50 (FIG. 2) or microprocessor 70
(FIG. 3) determines the value of the stack current error.
[0063] In step 110, the battery voltage sensor 40 (FIGS. 1-3)
determines the value of the voltage V.sub.B across the battery 24.
In optional step 112, the battery temperature sensor 42 determines
the temperature of the battery 24 or the ambient space proximate
the battery 24. In optional step 114, the temperature compensation
circuit 62 (FIG. 2) or microprocessor 70 (FIG. 3) determines the
value of the battery voltage limit based on determined battery
temperature. In step 116, the battery voltage error integrating
circuit 56 (FIG. 2) or microprocessor 70 (FIG. 3) determines the
value of the battery voltage error.
[0064] The fuel cell system 10 may perform the steps 102, 106 and
110 in a different order than described above, for example
performing step 106 before step 102, or performing step 110 before
step 102 and/or step 106. The sensors 36, 38, 40, 42 may perform
the steps 102, 106, 110, 112 at the same time or approximately at
the same time so as to appear be operating in parallel. Thus, the
enumeration of the above acts does not identify any specific
sequence or order.
[0065] In step 118, the OR circuit 64 (FIG. 2) or an OR circuit
configured in the microprocessor 70 (FIG. 3) determines the greater
of the determined errors values. The OR circuit may be hardwired in
the microprocessor 70, or may take the form of executable
instructions. In step 120, the charge pump 66 (FIG. 2) produces
charge. While not illustrated, the embodiment of FIG. 3 may also
include a charge pump, or the microprocessor 70 can produce an
appropriate signal value. In step 122, the level shifter 68 (FIG.
2) or microprocessor 70 (FIG. 3) applies the charge as an input
voltage to the control terminal of the series pass element 32
(FIGS. 1-3) in proportion to determined greater of errors
values.
[0066] The first stage of the fuel cell system 10 thus operates in
essentially three modes: battery voltage limiting mode, stack
current limiting mode, and battery charging current limiting mode.
For example, when the battery 24 is drained, the fuel cell system
10 will enter the battery charging current mode to limit the
battery charging current in order to prevent damage to the battery
24. As the battery 24 recharges, the fuel cell system 10 enters the
battery voltage limiting mode, providing a trickle charge to the
battery 24 in order to maintain a battery float voltage (e.g.,
approximately 75%-95% of full charge) without sulfating the battery
24. As the load 12 pulls more current than the fuel cell stack 14
can provide, the fuel cell system 10 enters the stack current
limiting mode. Additionally, there can be a fourth "saturation"
mode where, as the load 12 pulls even more current, the stack
voltage V.sub.S drops below the battery voltage V.sub.B. The
battery 24 will discharge in this "saturation" mode, eventually
entering the battery charging current limiting mode when the
battery 24 is sufficiently drained, as discussed above.
[0067] Second Stage Description, Reactant Partial Pressure
Control
[0068] FIG. 5 illustrates the second stage in further detail, which
employs a voltage difference across the series pass element 32 as
the operating condition.
[0069] In particular, the controller 28 includes a first comparator
90 that receives the value of the first voltage V.sub.1 from the
input side of the series pass element 32 and the value of the
second voltage V.sub.2 from the output side of the series pass
element 32. The first comparator 90 produces a process variable
.DELTA.V corresponding to a difference between the first and second
voltages V.sub.1, V.sub.2.
[0070] The controller 28 also includes a second comparator 92 that
receives the process variable .DELTA.V from the first comparator 90
and a set point. The comparator 92 compares the process variable
.DELTA.V to the set point and produces a first control voltage CV1.
The set point reflects the desired maximum operating level of the
series pass element 32, and may typically be between approximately
75% and approximately 95% of the saturation value for the series
pass element 32. A set point of 80% of the saturation value is
particularly suitable, providing some resolution in the circuitry
even when the fuel cell stack 14 is operating under a partial
load.
[0071] The comparator 92 supplies the resulting control variable
CV1 to the actuator 30 which adjusts the compressor or valve 18
accordingly. The valve 18 adjusts the reactant partial pressure to
the fuel cell stack 14, which serves as a second control variable
CV2 for the fuel cell system 10. As noted above, controlling the
reactant partial pressure adjusts the internal resistance of
R.sub.S of the fuel cell stack 14, as well as adjusting the power
output of the fuel cell stack 14. The first and second comparators
90, 92 may be discrete components or may be implemented in a
microprocessor, microcontroller or other integrated circuit.
[0072] The controller 28 may also include logic 94 for controlling
various switches, such as a first switch 96 that electrically
couples the battery 24 in parallel with the fuel cell 14, and
second switch 98 that electrically couples the load 12 in parallel
with the fuel cell stack 14 and the battery 24.
[0073] FIG. 6 illustrates an exemplary method 200 of operating the
second stage of the fuel cell system 10, of FIGS. 1 and 5. In step
102, the battery 24 is electrically coupled in parallel with the
fuel cell stack 14. In step 204, the load 12 is electrically
coupled to the battery 24 and fuel cell stack 14. In step 206, at
least one of the fuel cell stack 14 and battery 24 supplies current
to the load 12. The fuel cell stack 12 supplies the current to the
load 12 where the fuel cell stack 14 is producing sufficient
current to meet the demand of the load 12. Excess current from the
fuel cell stack 14 recharges the battery 24. The battery 24 may
supply a portion or even all of the power to the load 12 where the
fuel cell stack 14 is not producing sufficient power to meet the
demand.
[0074] In step 208, the second stage of the fuel cell system 10
determines the first voltage V.sub.1 on the input side of the
series pass element 32. In step 210, the second stage of the fuel
cell system 10 determines the second voltage V.sub.2 on the output
side of the series pass element 32. The order of steps 208 and 210
are not important, and can occur in any order or even at a same
time.
[0075] In step 212, the first comparator 90 determines the
difference between the first and the second voltages V.sub.1,
V.sub.2. In step 214, the second comparator 92 compares the
determined difference .DELTA.V to the set point. In step 216, the
second stage of the fuel cell system 10 adjusts a partial pressure
of at least one reactant flow to the fuel cell stack 14 via the
actuator 30 and valve 18 based on the determined amount of
deviation. For example, fuel cell system 10 may adjust the partial
pressure of the hydrogen, the partial pressure of the oxidant
(e.g., air), or the partial pressure of both the hydrogen and the
oxidant. As discussed above, by varying the partial pressure of
fuel and/or oxidant, the value of the internal series resistance
R.sub.S inherent in the fuel cell stack 14 can be varied to control
the voltage that is dropped at any given stack output current. By
varying the partial pressure in such a way, the maximum voltage
dropped across the series pass element 32 can be reduced.
[0076] FIG. 7 illustrates exemplary polarization curves for the
fuel cell stack 14, corresponding to five different reactant
partial pressures. Stack voltage V.sub.S is represented along the
vertical axis, and stack current I.sub.S represented along the
horizontal axis. A first curve 59 represents the polarization at a
low reactant partial pressure. Curves 61, 62, 63 and 65 represent
the polarization at successively increasing reactant partial
pressures. A broken line 69 illustrates a constant nominal output
voltage of 24 volts. Vertical broken lines 71, 723, 75, 77, 79
illustrate the stack current corresponding to the 24 volt output
for the respective partial pressure curves 59, 61, 63, 65, 67.
[0077] Battery Portions/Fuel Cell Portions Interconnected
Embodiment of Fuel Cell System
[0078] FIG. 8 shows a further embodiment of the fuel cell system 10
where the where portions of the battery 24 are interconnected with
portions of the fuel cell stack 14.
[0079] In particular, the fuel cell stack 14 can include a number
of groups or portions 14a, 14b, . . . 14n which are interconnected
with respective groups or portions of the battery 24a, 24b, . . .
24n. While illustrated as one battery cell 24a, 24b, . . . 24n to
each set of fuel cells 14a, 14b, . . . 14n, the fuel cell system 10
can employ other ratios of battery cells to fuel cells.
[0080] The fuel cell system 10 can include a capacitor, such as a
super-capacitor 140, electrically coupled in parallel across the
load 12. The fuel cell system 10 of FIG. 8 may be operated in
accordance with the methods 100 and 200 of FIGS. 4 and 6.
[0081] While not illustrated in FIG. 8, separate control elements
such as valve 18, controller 28, and/or actuator 30 can be
associated with respective ones the sets of fuel cells 14a, 14b . .
. 14n.
[0082] Currents Voltages and Resistance of Fuel Cell System and
Load
[0083] FIGS. 9A-9F show a series of graphs illustrating the
relationship between various currents, voltages, and resistance in
the fuel cell system 10 in single phase AC operation where the fuel
cell stack is sufficiently powering the load without draining or
recharging the battery. The various graphs of FIG. 9A-9F share a
common, horizontal time axis.
[0084] FIG. 9A is a graph 150 illustrating the actual stack current
I.sub.S and the average stack current I.sub.S-AVG as a function of
time. FIG. 9B is a graph 152 illustrating the actual battery
current I.sub.B as a function of time. FIG. 9C is a graph 154
illustrating the actual battery voltage V.sub.B and the average
battery voltage V.sub.B-AVG as a function of time. FIG. 9D is a
graph 156 illustrating the actual current through the load I.sub.L
and the average load current I.sub.L-AVG as a function of time.
FIG. 9E is a graph 158 illustrating the actual load resistance
R.sub.L as a function of time. FIG. 9F is a graph 160 illustrating
an AC voltage V.sub.ac across the load 12 as a function of
time.
[0085] FIGS. 10A-10C show a series of graphs illustrating the
relationship between various currents, voltages, and resistance in
the fuel cell system 10 in single phase AC operation where the
battery supplies current to the load to cover a shortfall from the
fuel cell stack and the fuel cell stack later recharges the
battery. The various graphs of FIGS. 10A-10C share a common,
horizontal time axis.
[0086] FIG. 10A is a graph 162 illustrating the stack current
I.sub.S as a function of time. FIG. 10B is a graph 164 illustrates
the battery current I.sub.B as a function of time. FIG. 10C is a
graph 166 illustrating the load current I.sub.L as a function of
time. As can be seen from FIGS. 10A-10C, as the load 12 increases
demand, the battery 24 supplies current to make up for the
shortfall from the fuel cell stack 14. As the load 12 decreases
demand, the fuel cell stack 14 recharges the battery 24 until the
battery 24 returns to the float voltage.
[0087] Fuel Cell Systems as Component Blocks of Combined Fuel Cell
System
[0088] FIG. 11 shows a number of fuel cell systems 10a-10f,
electrically coupled to form a combined fuel cell system 10g, for
powering the load 12 at a desired voltage and current. The fuel
cell systems 10a-10f can take the form of any of the fuel cell
systems 10 discussed above, for example the fuel cell systems 10
illustrated in FIGS. 1 and 2.
[0089] The combined fuel cell system 10g takes advantage of a
matching of polarization curves between the fuel cell stacks 14 and
the respective batteries 24. One approach to achieving the
polarization curve matching includes the first stage regulating
scheme generally discussed above. Another approach includes
controlling a partial pressure of one or more reactant flows based
on a deviation of a voltage across the battery 24 from a desired
voltage across the battery 24. A further approach includes
controlling a partial pressure of one or more reactant flows based
on a deviation of a battery charge from a desired battery charge.
The battery charge can be determined by integrating the flow of
charge to and from the battery 24. Other approaches may include
phase or pulse switching regulating or control schemes.
[0090] As an example, each of the fuel cell systems 10a-10f may be
capable of providing a current of 50A at 24V. Electrically coupling
a first pair of the fuel cell systems 10a, 10b in series provides
50A at 48V. Similarly electrically coupling a second pair of the
fuel cells systems 10c, 10d in series provides 50A at 48V.
Electrically coupling these two pairs of fuel cell systems 10a, 10b
and 10c, 10d in parallel provides 100A at 48V. Electrically
coupling a third pair of fuel cells systems 10e, 10f in series
provides an 50A at 48V. Electrically coupling the third pair of
fuel cell systems 10e, 10f in parallel with the first pair of
series coupled fuel cell systems 10a:10b and the second pair of
series coupled fuel cell systems 10c:10d, provides 150A at 48V.
[0091] FIG. 11 shows only one possible arrangement. One skilled in
the art will recognize that other arrangements for achieving a
desired voltage and current are possible. A combined fuel cell
system 10g may include a lesser or greater number of individual
fuel cell systems 10a-10f than illustrated in FIG. 11. Other
combinations of electrically coupling numbers of individual fuel
cell systems 10 can be used to provide power at other desired
voltages and currents. For example, one or more additional fuel
cell systems (not shown) can be electrically coupled in parallel
with one or more of the fuel cell systems 10a-10b. Additionally, or
alternatively, one or more additional fuel cell systems (not shown)
can be electrically coupled in series with any of the illustrated
pairs of fuel cell systems 10a:10b, 10c:10d, 10e:10f. Further, the
fuel cell systems 10a -10f may have different voltage and/or
current ratings. The individual fuel cell systems 10a -10f can be
combined to produce an "n+1" array, providing a desired amount of
redundancy and high reliability.
[0092] 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. Additionally or alternatively, the fuel
cell system 10 can interconnect portions of the fuel cell stack 14
with portions of the battery B1, B2. The fuel cell system can
employ various other approaches and elements for adjusting reactant
partial pressures. The various embodiments described above can be
combined to provide further embodiments. U.S. patent application
Ser. No. 09/______, entitled "METHOD AND APPARATUS FOR CONTROLLING
VOLTAGE FROM A FUEL CELL SYSTEM" (Attorney Docket No. 130109.436);
and U.S. patent application Ser. No. 09/______, entitled "METHOD
AND APPARATUS FOR MULTIPLE MODE CONTROL OF VOLTAGE FROM A FUEL CELL
SYSTEM" (Attorney Docket No. 130109.442), both filed concurrently
with this application, are incorporated herein by reference in
their entirety. Aspects of the invention can be modified, if
necessary, to employ systems, circuits and concepts of the various
patents, applications and publications to provide yet further
embodiments of the invention. For example, the fuel cell system 10
can additionally, or alternatively control the reactant partial
pressure as a function of the either the battery voltage V.sub.B,
current flow to and from the battery 24 or battery charge, as
taught in U.S. patent application Ser. No. 09/______, entitled
"METHOD AND APPARATUS FOR CONTROLLING VOLTAGE FROM A FUEL CELL
SYSTEM" (Attorney Docket No. 130109.436).
[0093] 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.
* * * * *