U.S. patent application number 11/558383 was filed with the patent office on 2007-04-19 for method and apparatus for multiple mode control of voltage from a fuel cell system.
Invention is credited to Martin T. Pearson.
Application Number | 20070087231 11/558383 |
Document ID | / |
Family ID | 21782722 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070087231 |
Kind Code |
A1 |
Pearson; Martin T. |
April 19, 2007 |
METHOD AND APPARATUS FOR MULTIPLE MODE CONTROL OF VOLTAGE FROM A
FUEL CELL SYSTEM
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. Thus, the fuel cell
system operates 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
as the load pulls even more current. 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, BC) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 5400
SEATTLE
WA
98104
US
|
Family ID: |
21782722 |
Appl. No.: |
11/558383 |
Filed: |
November 9, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10017462 |
Dec 14, 2001 |
7144646 |
|
|
11558383 |
Nov 9, 2006 |
|
|
|
Current U.S.
Class: |
429/9 ; 320/101;
429/431; 429/432; 429/471 |
Current CPC
Class: |
H02J 13/0062 20130101;
Y02E 60/00 20130101; H01M 8/0491 20130101; H01M 8/04917 20130101;
H01M 16/006 20130101; H02J 3/382 20130101; H02J 13/00018 20200101;
Y04S 10/123 20130101; H02J 13/00016 20200101; H02J 2300/30
20200101; H02J 13/00012 20200101; Y02E 60/10 20130101; H01M 8/04373
20130101; Y04S 40/124 20130101; H01M 8/04888 20130101; H02J 13/0079
20130101; H02J 3/381 20130101; H01M 8/04567 20130101; H02J 1/082
20200101; Y02E 40/70 20130101; H01M 8/04119 20130101; H02J 3/387
20130101; Y02E 60/50 20130101; H01M 8/04589 20130101; H01M 8/04597
20130101; H02J 2300/20 20200101; H01M 8/04007 20130101; H01M
8/04992 20130101; Y02E 60/7838 20130101; Y04S 10/12 20130101 |
Class at
Publication: |
429/009 ;
429/023; 320/101 |
International
Class: |
H01M 16/00 20060101
H01M016/00; H01M 8/04 20060101 H01M008/04; H02J 7/00 20060101
H02J007/00 |
Claims
1. A fuel cell system, comprising: 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; and 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.
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 first diode having an anode and a cathode, the anode
coupled to receive the battery charging current error; a second
diode having an anode and a cathode, the anode coupled to receive
the battery voltage error; and a third diode having an anode and a
cathode, the anode coupled to receive the stack current error,
where the cathode of each of the first diode, the second diode and
the third diode are coupled to one another to form an analog OR
circuit.
6. 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; a first diode having an anode and a cathode,
the anode coupled to the battery charging current error integrator;
a second diode having an anode and a cathode, the anode coupled to
the battery voltage error integrator; and a third diode having an
anode and a cathode, the anode coupled to the stack current error
integrator, where the cathode of each of the first diode, the
second diode and the third diode are coupled to one another to form
an analog OR circuit; a level shifter electrically coupled between
the analog 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.
7. The fuel cell system of claim 1 wherein the series pass element
comprises a field effect transistor.
8. 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.
9. 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; and 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.
10. The fuel cell system of claim 9 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.
11. The fuel cell system of claim 9 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.
12. The fuel cell system of claim 9 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; 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.
13. The fuel cell system of claim 9 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.
14. The fuel cell system of claim 9, further comprising: a battery
charging current sensor; a battery voltage sensor; and a stack
current sensor.
15. The fuel cell system of claim 9, 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.
16. 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 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; and 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.
17. The fuel cell system of claim 16 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.
18. The fuel cell system of claim 16 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.
19. The fuel cell system of claim 16, 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; and 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.
20. The fuel cell system of claim 16, 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; and 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.
21. The fuel cell system of claim 16, 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; and 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.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a division of U.S. patent application
Ser. No. 10/017,462, filed Jun. 19, 2003, now pending, which
application is incorporated herein by reference in its
entirety.
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 an output voltage of the fuel
cell system.
[0004] 2. Description of the Related Art
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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 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, the need for
voltage, current, or power conversion or matching components
between the fuel cell stack, battery and/or load, as well as the
use of blocking diodes resulting in system inefficiency. A less
costly, less complex and/or more efficient approach is
desirable.
BRIEF SUMMARY OF THE INVENTION
[0009] 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, and 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. 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.
[0010] 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, and 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.
[0011] In yet another aspect, a control 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 and 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.
[0012] In a further aspect, a control circuit for a fuel cell
system includes a series pass element, a blocking diode
electrically coupled in series with the series pass element, and 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.
[0013] In yet a further aspect, a control 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, and 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.
[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, and 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 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, and 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.
[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 having a
fuel cell stack, a battery, series pass element, and a regulating
circuit for controlling current flow through the series pass
element in accordance with an illustrated general embodiment in the
invention.
[0019] FIG. 2 is a schematic diagram of an alternative embodiment
of the fuel cell system that employs a microprocessor as the
regulating circuit.
[0020] FIG. 3 is a diagram illustrating the relative positions of
FIGS. 4A-4E.
[0021] FIGS. 4A-4E are an electrical schematic diagram of the fuel
cell system of FIG. 1.
[0022] FIG. 5 is a flow diagram of an exemplary method of operating
the fuel cell system of FIGS. 1 and 2.
[0023] FIG. 6 is a schematic diagram of a number of the fuel cell
systems of FIGS. 1 and 2, electrically coupled to form a
combination fuel cell system for powering a load at a desired
voltage and current.
DETAILED DESCRIPTION OF THE INVENTION
[0024] 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.
[0025] 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."
[0026] 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, in some aspects portions
of the fuel cell system 10 such as the control electronics may
constitute a portion or all of the load 12.
[0027] 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 16. The reactant
supply system 16 may include one or more reactant supply reservoirs
(not shown), a reformer (not shown), and/or one or more control
elements (not shown) such as one or more compressors, pumps and/or
valves or other reactant regulating elements. Operation of the fuel
cell stack 14 produces reactant products, such as water. The fuel
cell system 10 may reuse some or all of the reactant products, for
example to humidify the hydrogen and air at the correct temperature
and/or to hydrate the ion exchange membranes (not shown).
[0028] 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 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. This
relationship is illustrated in FIG. 1 by the broken line arrow 17.
The fuel cell system 10 may control the reactant partial pressure
to regulate the availability of the reactants.
[0029] The fuel cell stack 14 produces a voltage V.sub.S across a
high voltage bus 19 formed by the positive and negative voltage
rails 19a, 19b (FIG. 6). A stack current I.sub.S flows to the 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 (e.g., 5V). Thus, high voltage
and is not necessarily "high" with respect to other electrical
systems.
[0030] The fuel cell system 10 includes a battery 22 electrically
coupled in parallel with the fuel cell stack 14 on the high voltage
bus 19 to power the load 12. The open circuit voltage of the
battery 22 is selected to be similar to the full load voltage of
the fuel cell stack 14. The internal resistance R.sub.B of the
battery 22 is selected to be much lower than the internal
resistance of the fuel cell stack 14. Thus, the battery 22 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 bus voltage will be the open
circuit voltage of the battery 22 minus the battery discharging
current multiplied by the value of the battery's internal
resistance R.sub.B. The smaller the internal resistance R.sub.B of
the battery 22, the smaller the variations in bus voltage.
[0031] Since the battery 22 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 22.
[0032] A reverse current blocking diode D1 is electrically coupled
between the fuel cell stack 14 and the battery 22 to prevent
current from flowing from the battery 22 to the fuel cell stack 14.
The fuel cell system 10 may also include a reverse shorting diode
D2 electrically coupled in parallel with the battery 22 and load 12
to prevent reverse shorting. The fuel cell system 10 may also
include a fuse 28 electrically coupled in series with the load 12
to protect the fuel cell system 10 against power surges. The fuel
cell system 10 may further include a ground 30.
[0033] The fuel cell system 10 includes a series pass element 32
electrically coupled between the fuel cell stack 14 and the battery
22 for controlling a flow of current I.sub.S from the fuel cell
stack 14 to the battery 22 and the load 12. The fuel cell system 10
also includes a regulating circuit 34 coupled to regulate the
series pass element 32 based on various operating parameters of the
fuel cell system 10.
[0034] The fuel cell system 10 includes a number of sensors for
determining the various operating parameters. For example, the fuel
cell system 10 includes a battery charge current sensor 36 coupled
to determine the 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 22.
Additionally, the fuel cell system 10 may include a battery
temperature sensor 42 positioned to determine the temperature of
the battery 22. While 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.
[0035] The regulating circuit 34 includes 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 corresponding to the greater of the determined
errors.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 regulating circuit 34 also includes a charge pump
66 for providing a voltage to a control terminal 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.
[0041] The fuel cell system 10 may include a soft start circuit 18
for starting the charge pump 66 which slowly pulls up the voltage.
The fuel cell system 10 may also include a fast off circuit 20 for
quickly turning off the charge pump 66 to prevent damage to the
battery 22, for example when there is no load or the load 12 is
drawing no power.
[0042] FIG. 2 shows an alternative embodiment 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.
[0043] 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.
[0044] FIG. 3 shows the arrangement of the drawings sheets
comprising FIGS. 4A-4E. FIGS. 4A-4E are an electrical schematic
illustrating one embodiment of the fuel cell system of FIG. 1.
[0045] A power bus includes positive and negative rails 100, 102,
respectively, that electrically couple the battery 22 in parallel
with the fuel cell stack 14. (The battery 22 is illustrated in
FIGS. 4A-4E as first and second battery portions B1, B2,
respectively.) The load 12 is selectively coupled across the
positive and negative rails 100, 102 in parallel with the fuel cell
stack 12 and battery portions B1, B2.
[0046] The reverse current blocking diode D1 is electrically
coupled between the fuel cell stack 14 and the battery portions B1,
B2. The series pass element 32 (FIG. 1) can take the form of a
field effect transistor ("FET") Q1, electrically coupled between
the blocking diode D1 and the battery portions B1, B2. The reverse
shorting diode D2 is electrically coupled in parallel with the load
and the battery portions B1, B2 to prevent forward biasing of the
FET Q1. The fuse F1 is electrically coupled in series with the load
to provide protection from surges.
[0047] The stack current sensor 38 (FIG. 1) can take the form of a
first Hall effect sensor A1 coupled between a source of the FET Q1
and the node formed by the load 12 and the battery portions B1, B2.
The first Hall effect sensor A1 includes three poles, the third
pole being coupled to ground and the first and second poles
electrically coupled to the battery charging current and stack
current error integrators as described below. The battery charging
current sensor 36 (FIG. 1) can take the form of a second Hall
effect sensor A2 coupled between the battery portions B1, B2 and
the node formed by the load 12 and the battery portions B1, B2. The
second Hall effect sensor A2 includes three poles, the third pole
being coupled to ground and the first and second poles electrically
coupled to the battery charging current and stack current error
integrators as described below. A fuse F2 is electrically coupled
between the first and second Hall effect sensors A1, A2.
[0048] As illustrated in FIG. 4E, the battery charging current
error integrating circuit 44 includes battery charging current
error integrator U1d and a voltage divider formed by resistors R31,
R13 R17 and R21. The non-inverting terminal of the battery charging
current error integrator U1d is electrically coupled to the second
pole of the second Hall effect sensor A2. The inverting terminal of
the battery charging current error integrator U1d is electrically
coupled to the first pole of the first Hall effect sensor A1
through the voltage divider formed by resistors R31, R13 R17 and
R21. The inverting terminal of the battery charging current error
integrator U1d is also coupled to the output terminal of the
battery charging current error integrator U1d through the capacitor
C9.
[0049] Also as illustrated in FIG. 4E, the stack current error
integrating circuit 50 includes a stack current error integrator
U1c and a voltage divider formed by resistors R30, R12, R16 and
R20. The non-inverting terminal of the stack current error
integrator U1c is electrically coupled to the second pole of the
first Hall effect sensor A1. The inverting terminal of the stack
current error integrator U1c is electrically coupled to the first
pole of the second Hall effect sensor A2 through the voltage
divider formed by resistors R30, R12, R16 and R20. The inverting
terminal of the stack current error integrator U1c is also coupled
to the output terminal of the stack current error integrator U1c
through capacitor C8.
[0050] As illustrated in FIG. 4D, the battery voltage error
integrating circuit 56 includes a battery voltage error integrator
U1b and a voltage divider formed by resistors R23, R29. The
non-inverting terminal of the battery voltage error integrator U1b
is electrically coupled to the second pole of the first Hall effect
sensor A1. The inverting terminal of the battery voltage error
integrator U1b is electrically coupled to the battery voltage set
point circuit 58, which is formed by U4 and resistors R15,R19,R27.
The battery voltage set point circuit 58 is electrically coupled to
the temperature compensation circuit 62 (FIG. 1), which include
integrated circuits U6 and U3. The inverting terminal of the
battery voltage error integrator U1b is also coupled to the output
terminal of the battery voltage error integrator U1b through
capacitor C7.
[0051] The OR circuit 64 is formed by the diodes D10, D11, D12
having commonly coupled cathodes. The anode of each of the diodes
D10, D11, D12 is electrically coupled to a respective one of the
error integrators U1b, U1c, U1d.
[0052] The level shifter 68 includes a transistor such as the
bipolar transistor Q2, resistors R8, R26 and diode D5. The resistor
R8 electrically couples an emitter of the transistor Q2 to ground.
A collector of the transistor Q2 is electrically coupled to the
gate of the field effect transistor Q1 and a base of the transistor
Q2 is electrically coupled to the output of the OR circuit 64. The
diode D5 prevents the gate of the FET Q1 from being pulled down too
low.
[0053] The charge pump 66 includes an integrated circuit U2, zener
diodes D3, D6 capacitors C1, C3, C2, C4, resistors R2, R3 and
diodes D7, D8. The output of the charge pump 66 is electrically
coupled to the collector of the transistor Q2 and to the gate of
the FET Q1. The bipolar transistor Q2 sinks charge from the charge
pump 66 to the grounded resistor R8 in proportion to the output of
the OR circuit 64 (i.e., diodes D10, D11, D12) to control the
voltage applied to the gate of the FET Q1. Thus, the amount of
stack current I.sub.S that the FET Q1 passes is proportional to the
greater of the battery charging current error value, stack current
error value and battery voltage error value (i.e., outputs of the
error integrators U1b, U1c, U1d, respectively).
[0054] Additionally, the fuel cell system 10 can include a low
battery voltage regulating circuit 72. The low battery voltage
regulating circuit 72 includes an integrator U1a, bipolar
transistor Q3, integrated circuit U5, diode D9, resistor R6, and a
voltage divider including resistors R10, R14, R18, R24. An output
of the integrator U1a is electrically coupled to the base of the
transistor Q3 through the diode D9 and resistor R6.
[0055] Further, the fuel cell system 10 can include a number of
subsystems. For example, the fuel cell system 10 may include a
system power subsystem 74 for powering the control circuitry of the
fuel cell system 10. A start command subsystem 76 starts operation
of the fuel cell system in response to an operator selection after
the start command subsystem determines that the fuel cell system 10
is ready for operation, for example after checking parameters such
as temperature. A control subsystem 78 for controlling operation of
the fuel cell system 10. A hydrogen valve control subsystem 80 for
controlling the flow of hydrogen to the fuel cell stack 14.
[0056] An isolation circuit 82 prevents shorting of the battery 22
through the communications ports. The isolation circuit 82 includes
an isolated converter SA3 electrically coupled to a standard
connector DB-25. A battery charging subsystem 84 allows the battery
22 to be recharged independently of the fuel cell stack 14. The
battery charging subsystem 84 includes a battery recharging circuit
SA2, fuse F5 and switch S2. A second reverse shorting diode D4 and
a fuse F4 are electrically coupled between the positive rail 100
and the battery charger SA2, in parallel with the battery portions
B1, B2.
[0057] The fuel cell system 10 includes a number of other discrete
components such as capacitances K1, K2, K3, diode D14, resistors
R5, R22, R25, and switch S1, as illustrated in the electrical
schematic of FIGS. 4A-4E. Since the battery portions B1, B2 appear
as large capacitances to the control circuitry, the fuel cell
system's control circuitry includes a 90 degree phase shift of all
outer control loops.
[0058] FIG. 5 shows an exemplary method 100 of operating the fuel
cell systems 10 of FIGS. 1 and 2. The method 100 repeats during
operation of the fuel cell to continually adjust the operation of
the fuel cell system 10.
[0059] In step 102, the battery charging current sensor 36 (FIGS. 1
and 2) determines the value of the battery charging current
I.sub.B. In step 104, the battery charging current error
integrating circuit 44 (FIG. 1) or microprocessor 70 (FIG. 2)
determines the value of the battery charging current error.
[0060] In step 106, the stack current sensor 38 (FIGS. 1 and 2)
determines the value of the stack current. In step 108, the stack
current error integrating circuit 50 (FIG. 1) or microprocessor 70
(FIG. 2) determines the value of the stack current error.
[0061] In step 110, the battery voltage sensor 40 (FIGS. 1 and 2)
determines the value of the voltage V.sub.B across the battery 22.
In optional step 112, the battery temperature sensor 42 determines
the temperature of the battery 22 or the ambient space proximate
the battery 22. In optional step 114, the temperature compensation
circuit 62 (FIG. 1) or microprocessor 70 (FIG. 2) determines the
value of the battery voltage limit based on determined battery
temperature. In step 116, the battery voltage error integrating
circuit 56 (FIG. 1) or microprocessor 70 (FIG. 2) determines the
value of the battery voltage error.
[0062] 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.
[0063] In step 118, the OR circuit 64 (FIG. 1) or an OR circuit
configured in the microprocessor 70 (FIG. 2) 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. 1) produces
charge. While not illustrated, the embodiment of FIG. 2 may also
include a charge pump, or the microprocessor 70 can produce an
appropriate signal value. In step 122, the level shifter 68 (FIG.
1) or microprocessor 70 (FIG. 2) applies the charge as an input
voltage to the control terminal of the series pass element 32
(FIGS. 1 and 2) in proportion to determined greater of errors
values.
[0064] 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 22 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 22. As the battery 22
recharges, the fuel cell system 10 enters the battery voltage
limiting mode, providing a trickle charge to the battery 22 in
order to maintain a battery float voltage (e.g., approximately
75%-95% of full charge) without sulfating the battery 22. 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 22 will discharge in
this "saturation" mode, eventually entering the battery charging
current limiting mode when the battery 22 is sufficiently drained,
as discussed above.
[0065] The above described approach reduces the possibility of cell
reversal since the stack voltage V.sub.S is clamped to the battery
voltage V.sub.B. If a cell reversal is detected, a switch can
automatically disconnect the fuel cell stack 14 from the battery
22. The battery 22 would continue to power the load 12 while the
fault clears. The above described approach may eliminate the need
for voltage, current or power conversion or matching components
between the fuel cell stack 14, battery 22 and/or load 12.
[0066] FIG. 6 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.
[0067] For 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 and second
pairs of fuel cell systems 10a:10b, 10c:10d, 10e:10f provides 150A
at 48V.
[0068] FIG. 6 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. 6. 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.
[0069] 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. Commonly assigned U.S.
patent application Ser. No. 10/017,470, entitled "METHOD AND
APPARATUS FOR CONTROLLING VOLTAGE FROM A FUEL CELL SYSTEM"; and
U.S. patent application Ser. No. 10/017,461, entitled "FUEL CELL
SYSTEM MULTIPLE STAGE VOLTAGE CONTROL METHOD AND APPARATUS"; 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.
[0070] 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.
* * * * *