U.S. patent application number 11/392062 was filed with the patent office on 2006-08-03 for electric power plant with adjustable array of fuel cell systems.
Invention is credited to Martin T. Pearson.
Application Number | 20060172162 11/392062 |
Document ID | / |
Family ID | 29553651 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060172162 |
Kind Code |
A1 |
Pearson; Martin T. |
August 3, 2006 |
Electric power plant with adjustable array of fuel cell systems
Abstract
An electric power plant includes an array of fuel cell systems.
The fuel cell systems are electrically couplable in series and/or
parallel combinations to provide a variety of output powers, output
current and/or output voltages. The fuel cell systems are "hot
swappable" and redundant fuel cell systems may automatically
replace faulty fuel cell systems to maintain output power, current
and/or voltage.
Inventors: |
Pearson; Martin T.;
(Burnaby, CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Family ID: |
29553651 |
Appl. No.: |
11/392062 |
Filed: |
March 29, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10440025 |
May 16, 2003 |
|
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11392062 |
Mar 29, 2006 |
|
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60436759 |
Dec 27, 2002 |
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Current U.S.
Class: |
429/431 ;
429/432; 429/454; 429/515; 429/900 |
Current CPC
Class: |
H01M 8/04947 20130101;
H01M 16/003 20130101; H01M 8/04917 20130101; H01M 8/04089 20130101;
H01M 8/0494 20130101; H01M 8/04007 20130101; H01M 8/249 20130101;
H01M 8/0488 20130101; Y02E 60/50 20130101; H01M 8/0491 20130101;
H01M 8/04753 20130101 |
Class at
Publication: |
429/022 ;
429/038 |
International
Class: |
H01M 8/04 20060101
H01M008/04; H01M 8/24 20060101 H01M008/24 |
Claims
1. An electric power plant comprising: an array of fuel cell
systems, the array comprising, a power bus, a plurality of fuel
cell systems electrically couplable in series to the power bus,
each of the fuel cell systems comprising a fuel cell stack, an
electrical storage device electrically coupled in parallel with at
least one of the fuel cell stacks, a plurality of switches
selectively operable to electrically decouple a respective one of
the fuel cell systems from the power bus, each of the switches
responsive to an operating condition of the respective one of the
fuel cell systems, and at least a first redundancy switch
selectively operable to electrically couple a respective first one
of the plurality of fuel cell systems in series to the power bus,
the first redundancy switch responsive to an operating condition
different from the operating condition of the respective first one
of the plurality of fuel cells; an oxidant supply system for
supplying an oxidant gas to at least one of the fuel cell stacks;
and a fuel supply system for supplying a fuel gas to at least one
of the fuel cell stacks.
2. The power plant of claim 1 wherein the array further comprises
an additional number of redundancy switches, each of the additional
number of redundancy switches selectively operable to electrically
couple a respective one of the fuel cell systems to the power bus
in response to an operating condition different from the operating
condition of the respective one of the fuel cell systems.
3. The power plant of claim 1 wherein each of the fuel cell systems
further comprise a number of sensors for determining a set of
operating parameters of the respective fuel cell system and a
processor configured to determine an operating condition of the
respective fuel cell system based on the determined set of
operating parameters, and wherein the operating condition that the
first redundancy switch is responsive to is the operating condition
determined by at least one of the processors.
4. The power plant of claim 1 wherein the operating condition that
the first redundancy switch is responsive to is at least one of a
power output of the power bus, a voltage across the power bus and a
current output of the power bus.
5. The power plant of claim 1 wherein the operating condition that
the first redundancy switch is responsive to is at least one of a
desired nominal power output of the power supply system, a desired
nominal voltage output of the power supply system, and a desired
nominal current output of the power supply system.
6. The power plant of claim 1, further comprising: a controller
communicatingly coupled to receive a set of operating parameters
from each of the plurality of fuel cell systems, the controller
configured to determine an operating condition of the fuel cell
systems based on the received sets of operating parameters, and
wherein the operating condition that the first redundancy switch is
responsive to is the operating condition determined by the
controller.
7. The power plant of claim 1 wherein the electrical storage device
is at least one of a battery and an ultracapacitor.
8. An electric power plant comprising: at least two fuel cell
systems, each of the fuel cell systems comprising a fuel cell
stack, an electrical storage device electrically coupled in
parallel with the fuel cell stack, a series pass element
electrically coupled between the fuel cell stack and the electrical
storage device, and regulating means for regulating a series pass
element electrically coupled between the fuel cell stack and the
electrical storage device; means for selectively electrically
coupling the at least two fuel cell systems in series; an oxidant
supply system for supplying an oxidant gas to at least one of the
fuel cell stacks; and a fuel supply system for supplying a fuel gas
to at least one of the fuel cell stacks.
9. The power plant of claim 8 wherein the means for selectively
electrically coupling the at least two fuel cell systems in series
comprises a controller responsive to a request for at least one of
a desired power output, a desired current output and a desired
voltage output of the power supply system.
10. An electric power plant comprising: an array of fuel cell
systems, each of the fuel cell systems comprising a fuel cell
stack, an electrical storage device electrically coupled in
parallel with the fuel cell stack, a first series pass element
electrically coupled between the fuel cell stack and the electrical
storage device, and first regulating means for regulating the first
series pass element electrically coupled between the fuel cell
stack and the electrical storage device; an oxidant supply system
for supplying an oxidant gas to at least one of the fuel cell
stacks; and, a fuel supply system for supplying a fuel gas to at
least one of the fuel cell stacks.
11. The power plant of claim 10 wherein the array comprises a power
bus and a first arm, the first arm comprising a first plurality of
the fuel cell systems electrically couplable to the power bus and
electrically couplable in series to each other.
12. The power plant of claim 11 wherein the first arm of the array
includes at least one redundant fuel cell system.
13. The power plant of claim 11 wherein the array further comprises
a second arm comprising a second plurality of the fuel cell
systems, wherein the second plurality of the fuel cell systems is
electrically couplable to the power bus and electrically couplable
in series to each other; and the second arm is electrically
couplable in parallel to the first arm.
14. The power plant of claim 13 wherein at least one of the first
and second arms of the array includes at least one redundant fuel
cell system.
15. The power plant of claim 10 wherein the electrical storage
device comprises a storage battery or an ultracapacitor.
16. The power plant of claim 10 wherein the fuel supply system
comprises hydrogen storage equipment.
17. The power plant of claim 10 wherein the fuel supply system
comprises pressurized hydrogen tanks.
18. The power plant of claim 10, further comprising a rectifier
electrically couplable to the electrical storage device and an AC
power source.
19. The power plant of claim 10, further comprising an inverter
electrically couplable to the array.
20. An electric power plant comprising: a power supply system, the
power supply system comprising a power bus, a first and second fuel
cell system, a first switch selectively operable to electrically
couple the first fuel cell system in series in the power bus, and a
second switch selectively operable to electrically couple the
second fuel cell system in series in the power bus; an oxidant
supply system for supplying an oxidant gas to at least one of the
fuel cell stacks; a fuel supply system for supplying a fuel gas to
at least one of the fuel cell stacks; and means for selectively
electrically coupling the at least two fuel cell systems in series
in the power bus, wherein each of the fuel cell systems comprises a
fuel cell stack and an electrical power storage device electrically
coupled in parallel with the fuel cell stack, the electrical power
storage device comprising at least one of a battery and an
ultracapacitor electrically coupled to a charging current limiter.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. Patent Application
No.10/440,025, filed May 16, 2003, now pending, which application
claims the benefit under 35 U.S.C. .sctn. 119(e) of U.S.
Provisional Patent Application No. 60/436,759, filed Dec. 27, 2002,
both of these applications being incorporated herein by reference
in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This disclosure generally relates to power supplies, for
example, fuel cell systems, and to electrical power storage
devices, for example, batteries and/or ultracapacitors.
[0004] 2. Description of the Related Art
[0005] Fuel cells are known in the art. Fuel cells
electrochemically react a fuel stream comprising hydrogen and an
oxidant stream comprising oxygen to generate an electric current.
Fuel cell electric power plants have been employed in
transportation, portable and stationary applications.
[0006] Stationary and portable applications include distributed
power generation, back-up power, peak power, and uninterruptible
power supply (UPS) systems. Distributed power generation relates to
providing electrical power to residential, commercial and/or
industrial customers instead of, or as a supplement to, the utility
power grid. Power plants in such applications typically operate
continuously. They are particularly suited to situations where the
power grid is not available or sufficiently reliable. Peak power
systems are intended to supplement the power grid, providing
electrical power intermittently during periods of peak use when
sufficient grid power may not be available or when the rate charged
by the utility increases. Back-up power and UPS systems provide
electrical power during periods when the power grid, or other
primary power source, is unavailable.
[0007] In addition, UPS systems must be able to provide power to
the consumer substantially continuously, i.e., they must be
"instant on" so that the loss of grid power does not result in an
interruption of power supply to the consumer. Consumers who rely on
electronic equipment, for example, cannot tolerate even minor
interruptions in power supply. In this regard, the Information
Technology Industry Council has issued guidelines for voltage
dropouts, which are not to exceed 20 milliseconds. In this context,
a voltage dropout includes both severe RMS voltage sags and
complete interruptions of the applied voltage.
[0008] Conventional back-up power and UPS systems employ
rechargeable battery banks for supplying electric power when the
power grid is interrupted. For applications where a relatively
short run time is acceptable, battery banks may be the sole source
of back-up power. Where longer run times are required, however,
such systems also employ a generator to supply power. In this case,
the battery banks provide immediate power until the generator can
come online.
[0009] Valve regulated lead acid (VRLA) batteries are most often
employed in the battery banks. The number of batteries depends on
the required run time. For lower power applications (2-7.5 kW), run
times of 15 minutes or less are common; other systems employing
batteries alone may require run times of 4-8 hours, or more.
Current limits are set on re-charging of batteries to avoid
damaging them. In practice, VRLA batteries are recharged at a
6.times.-10.times. rate, that is, the time to fully re-charge the
batteries is six to ten times longer than their run time.
[0010] These conventional power supply systems have several
significant disadvantages. For example, particularly in
applications requiring extended battery run time (e.g., >4 hr),
VRLA battery banks are large and heavy. A large battery bank
requires a significant amount of indoor floor space for
installation, which can be expensive. In addition, the weight of
the battery bank may require indoor floor space with increased
loading capacity, further increasing costs. Environmental
regulations relating to the storage and operation of VRLA batteries
also add to increased installation costs. Operating and maintaining
a generator further adds to the cost and complexity of systems
employing them.
[0011] Back-up power and UPS systems employing fuel cell electric
power plants have also been described. The described systems have
several disadvantages relating to the supply of reactants to the
fuel cells, the time it takes for the fuel cells to produce full
power, and their surge demand capacity, for example.
[0012] Fuel cell output is proportional to the amount of reactants
supplied. On start-up, there is typically a delay until the fuel
cells reach full operating power. For this reason, back-up or UPS
systems solely employing fuel cells are inadequate for some
applications because they are not "instant on". One approach has
been to keep the fuel cells in such systems continuously running,
either supplying power to the load or in a low output "stand-by"
mode. While this approach improves response time, it exacerbates
hydrogen storage issues by significantly increasing hydrogen
consumption. In addition, operational lifetime of the power plant
may be adversely affected compared to systems where the power plant
is operated intermittently.
[0013] Fuel cells can be damaged if the load requirements exceed
their maximum output. Thus, in power plants solely employing fuel
cells, the rated output of the fuel cell stack is generally matched
to the expected peak load. In applications where transient load
increases are significantly higher than normal load requirements,
this necessitates a larger size and output fuel cell stack than
required for normal operation in order to deal with surge demand.
This, in turn, undesirably increases the cost of the power
plant.
[0014] In most practical applications, it is desirable to maintain
an approximately constant voltage output from the fuel cell stack.
One approach is to employ a battery electrically coupled in
parallel with the fuel cell system to provide additional current
when the demand of the load exceeds the output of the fuel cell
stack and to store current when the output of the fuel cell stack
exceeds the demand of the load, as taught in commonly assigned U.S.
Pat. No. 6,841,275, issued Jan. 11, 2005, and U.S. Pat. No.
6,573,682, issued Jun. 3, 2003, and in commonly assigned U.S.
Patent Publication 2003/0113599, published Jun. 19, 2003.
[0015] The many different practical applications for fuel cell
based power supplies require a large variety of different power
delivery capabilities. In most instances it is prohibitively costly
and operationally inefficient to employ a power supply capable of
providing more power than required by the application. It is also
costly and inefficient to design, manufacture and maintain
inventories of different power supplies capable of meeting the
demand of each potential application (e.g., 1 kW, 2 kW, 5 kW, 10
kW, etc.). Further, it is desirable to increase the reliability of
the power supply, without significantly increasing the cost. Thus,
a less costly, less complex and/or more efficient approach to fuel
cell based power supplies is desirable.
BRIEF SUMMARY OF THE INVENTION
[0016] An electric power plant is provided. In one aspect, the
present power plant comprises: a power supply system, the power
supply system comprising a power bus and first and second fuel cell
systems; an oxidant supply system for supplying an oxidant gas; and
a fuel supply system for supplying a fuel gas. Each of the fuel
cell systems comprises a fuel cell stack; the first and second fuel
cell systems further comprising an electrical storage device
electrically coupled in parallel with at least one of the stacks.
Each fuel cell system further comprises a switch selectively
operable to electrically couple the fuel cell system in series in
the power bus.
[0017] In another aspect, the present electric power plant
comprises: an array of fuel cell system; an oxidant supply system
for supplying an oxidant gas; and a fuel supply system for
supplying a fuel gas. The array comprises: a power bus; a plurality
of fuel cell systems electrically couplable in series to the power
bus, each of the fuel cell systems comprising a fuel cell stack; an
electrical storage device electrically coupled in parallel with at
least one of the stacks; a plurality of switches selectively
operable to electrically decouple a respective one of the fuel cell
systems from the power bus, each of the switches responsive to an
operating condition of the respective one of the fuel cell systems;
and, one or more redundancy switches selectively operable to
electrically couple one of the fuel cell systems to the power bus,
the redundancy switch(es) responsive to an operating condition
different from the operating condition of the respective fuel cell
stack.
[0018] In a further aspect, the electric power plant comprises: at
least two fuel cell systems; means for selectively electrically
coupling the at least two fuel cell systems in series; an oxidant
supply system for supplying an oxidant gas; and a fuel supply
system for supplying a fuel gas. Each of the fuel cell systems
comprises: a fuel cell stack; an electrical storage device
electrically coupled in parallel with the fuel cell stack; a series
pass element electrically coupled between the fuel cell stack and
the electrical storage device; and regulating means for regulating
the series pass element electrically coupled between the fuel cell
stack and the electrical storage device.
[0019] In still another aspect, the electric power plant comprises:
an array of fuel cell systems; an oxidant supply system for
supplying an oxidant gas; and a fuel supply system for supplying a
fuel gas. Each of the fuel cell systems comprises: a fuel cell
stack; an electrical storage device electrically coupled in
parallel with the stack; a series pass element electrically coupled
between the stack and the electrical storage device; and regulating
means for regulating the series pass element electrically coupled
between the stack and the electrical storage device.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0020] 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.
[0021] FIG. 1 is a schematic diagram of a hybrid fuel cell system
powering a load, the hybrid 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, the fuel cell system for use with an
illustrated general embodiment of the invention.
[0022] FIG. 2 is a schematic diagram of a power supply system
powering a load, the power supply system including a number of
individual hybrid fuel cells systems forming a one-dimensional
array of fuel cell systems electrically couplable in series to
provide a desired power at a desired voltage and a desired current
to the load.
[0023] FIG. 3 is a schematic diagram of a power supply system
including a number of fuel cell systems forming a two-dimensional
array of fuel cell systems electrically couplable in a variety of
series and parallel combinations.
[0024] FIG. 4 is a schematic diagram illustrating a number of the
fuel cell systems of FIG. 3 electrically coupled in a series
combination to provide a desired output power at a first output
voltage and a first output current.
[0025] FIG. 5 is a schematic diagram illustrating a number of the
fuel cell systems of FIG. 3 electrically coupled in a parallel
combination to provide the desired output power at a second output
voltage and a second output current.
[0026] FIG. 6 is a schematic diagram illustrating a number of the
fuel cell systems of FIG. 3 electrically coupled in a series and
parallel combination to provide the desired output power at a third
output voltage and a third output current.
[0027] FIG. 7 is a flow diagram of a method of operating the power
supply system of FIGS. 2 and 3 according to one exemplary
embodiment which comprises replacing a faulty fuel cell system with
a redundant fuel cell system.
[0028] FIG. 8 is a flow diagram of an optional step for inclusion
in the method of FIG. 7.
[0029] FIG. 9 is a flow diagram of an optional step for inclusion
with the method of FIG. 7.
[0030] FIG. 10 is a flow diagram showing a method of operating the
power supply system of FIGS. 2 and 3 according to an additional or
alternative exemplary embodiment including electrically coupling a
number of fuel cell systems in a determined series and/or parallel
combination to produce at least one of a desired power, voltage and
current output.
[0031] FIG. 11 is a schematic diagram of a hybrid fuel cell system
powering a load, the fuel cell system having a fuel cell stack, a
series pass element, a regulating circuit or controller for
controlling current flow through the series pass element, and an
ultracapacitor based circuit as an electrical power storage device
that simulates a battery (ultracapacitor battery simulator).
[0032] FIG. 12 is a schematic diagram of an alternative
ultracapacitor based circuit suitable for use in the fuel cell
system of FIG. 11.
[0033] FIG. 13 is a schematic diagram of a further alternative
ultracapacitor based circuit suitable for use in the fuel cell
system of FIG. 11.
[0034] FIG. 14 is an electrical schematic diagram of an
ultracapacitor based circuit comprising a string of ultracapacitors
electrically coupled in series, a linear mode charging current
limiter, and a bypass diode.
[0035] FIG. 15 is an electrical schematic diagram of the
ultracapacitor based circuit of FIG. 14 where the charging current
limiter further comprises a pair of Darlington coupled transistors
to limit power loss.
[0036] FIG. 16 is an electrical schematic diagram of the
ultracapacitor based circuit of FIG. 15 where the charging current
limiter further comprises circuitry to cut off the charging current
in the event of an over voltage condition.
[0037] FIG. 17 is an electrical schematic diagram of the
ultracapacitor based circuit of FIG. 16 where the charging current
limiter further comprises circuitry to cut off charging current
when a desired voltage is obtained across the ultracapacitors.
[0038] FIG. 18 is a flow diagram of one illustrated method of
operating a hybrid fuel cell system.
[0039] FIG. 19 is a schematic diagram of a power system comprising
one or more rectifier arrays, fuel cell hybrid module arrays,
ultracapacitor battery simulator arrays, flywheel battery simulator
arrays and/or rechargeable batteries.
[0040] FIG. 20 is a schematic diagram of one illustrated embodiment
of a fuel cell hybrid module array suitable for use with the power
system of FIG. 19.
[0041] FIG. 21 is a schematic diagram of a power supply system
including a number of fuel cell systems forming a two-dimensional
array of fuel cell systems electrically coupled in series and
parallel to provide at least N+1 redundancy.
[0042] FIG. 22 is a schematic diagram of a power supply system
including a number of fuel cell systems forming a two-dimensional
array of fuel cell systems electrically coupled in series and
parallel to provide multiple voltage levels with at least N+1
redundancy.
[0043] FIG. 23 is a schematic diagram of a power supply system
including a number of fuel cell systems forming a two-dimensional
array of fuel cell systems electrically coupled in series and
parallel to provide multiple bipolar voltage levels with at least
N+1 redundancy.
[0044] FIG. 24 is a schematic illustration of a conventional DC
back-up power or UPS system.
[0045] FIG. 25 is a schematic illustration of a conventional AC
back-up power or UPS system.
[0046] FIGS. 26 and 27 are schematic illustrations of a
conventional 400 A 4 hour power supply employing VRLA battery banks
and a comparable embodiment of the present power plant,
respectively.
[0047] FIGS. 28 and 29 are schematic illustrations of a
conventional 400 A 8 hour power supply employing VRLA battery banks
and a comparable embodiment of the present power plant,
respectively.
DETAILED DESCRIPTION OF THE INVENTION
[0048] 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, electrical power storage devices
such as batteries, flywheels, and super- or ultracapacitors,
reactant delivery systems, temperature control systems and fuel
cell systems have not been shown or described in detail to avoid
unnecessarily obscuring descriptions of the embodiments of the
invention. The terms supercapacitor and ultracapacitor are used
interchangeably throughout the description and claims.
[0049] 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."
Fuel Cell System Overview
[0050] FIG. 1 shows a hybrid fuel cell system 10 providing power to
a load 12 for use in an illustrated embodiment of the invention.
The load 12 typically constitutes the device to be powered by the
hybrid fuel cell system 10, such as a vehicle, appliance, computer
and/or associated peripherals. While the hybrid fuel cell system 10
is not typically considered part of the load 12, portions of the
hybrid fuel cell system 10 such as the control electronics may
constitute a portion or all of the load 12 in some possible
embodiments.
[0051] The fuel cell system 10 comprises 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 comprise 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.
[0052] 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.
[0053] 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.
[0054] The hybrid fuel cell system 10 comprises an electrical power
storage device such as a supercapacitor and/or 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.
[0055] 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 comprises other diodes, as well as fuses or other surge
protection elements to prevent shorting and/or surges.
Fuel Cell System Control Stages
[0056] The fuel cell system 10 comprises 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 and fuel
cell stack 14 from damage.
[0057] 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).
[0058] 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.
[0059] The details of the first and second stages are discussed in
detail below.
First-Stage Overview, Series Pass Element Regulator
[0060] With continuing reference to FIG. 1, the first stage of the
fuel cell system 10 comprises 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 comprises 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, for example, 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.
[0061] The first stage of the fuel cell system 10 comprises a
number of sensors for determining the various operating parameters
of the fuel cell system 10. For example, the fuel cell system 10
comprises a battery charge current sensor 36 coupled to determine a
battery current I.sub.B. Also for example, the fuel cell system 10
comprises a fuel cell stack current sensor 38 coupled to determine
the stack current I.sub.S. Further for example, the fuel cell
system 10 comprises a battery voltage sensor 40 for determining a
voltage V.sub.B across the battery 24. Additionally, the fuel cell
system 10 may comprise 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.
[0062] The first stage of the fuel cell system 10 may comprise 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 comprise a fast off circuit 17 for quickly shutting down to
prevent damage to the fuel cell stack 14, for example if a problem
occurs in the reactant supply system of the stack, where load must
be removed quickly to prevent damage to the stack, or if a problem
occurs with the second stage control.
Second Stage Overview. Reactant Partial Pressure Controller
[0063] The second stage of the fuel cell system 10 comprises 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.
[0064] 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.
Power Supply System
[0065] FIG. 2 shows one embodiment of a power supply system 50
including a one-dimensional array 52 of a fuel cells systems,
collectively referenced as 10, that are electrically couplable in
series to positive and negative voltage rails 56a, 56b,
respectively, that form a power bus 56 for supplying power to the
load 12. A respective diode, collectively referenced as 58, is
electrically coupled between the positive and negative outputs of
each of the fuel cell systems 10. The illustrated power supply
system 50 comprises a number M+1 fuel cell systems, which are
individually referenced as 10(1)-10(M+1), the number in the
parenthesis referring to the position of the fuel cell system 10 in
the array. The ellipses in FIG. 2 illustrate that the power supply
system 50 may comprise additional fuel cell systems (not explicitly
shown) between the third fuel cell system 10(3) and the M.sup.th
fuel cell system 10(M). One or more of the fuel cell systems (e.g.,
10(M+1)) may serve as a "redundant" fuel cell system, being
electrically coupled in series on the power bus 56 as needed, for
example, when one of the other fuel cell systems 10(1)-10(M) is
faulty or when the load 12 requires additional power or
voltage.
[0066] The power supply system 50 may employ one or more fault
switches such as a contactor or transistor 60, that can
automatically disconnect a respective fuel cell system 10 in the
event of a fault or failure. For example, the fault transistor 60
may open upon a fault or failure in the fuel cell system's 10 own
operating condition or upon a fault or failure in the operating
condition of the power supply system 50.
[0067] The power supply system 50 may employ one or more redundancy
switches, such as a contractor or transistor 62, that can manually
or automatically electrically couple a respective fuel cell system
10(M+1) to the power bus 56 based on a condition other than the
fuel cell system's 10(M+1) own operating condition. For example,
where another fuel cell system 10 is faulty, the redundancy
transistor 62 may close to electrically couple the redundant fuel
cell system 10(M+1) to the power bus 56 to maintain the power,
voltage and current to the load 12. Also for example, where a
higher output power is desired, the redundancy transistor 62 may
close to electrically couple the redundant fuel cell system 10(M+1)
to the power bus 56 to adjust the power, voltage and current to the
load 12.
[0068] While manual operation may be possible, the power supply
system 50 may comprise control logic 64 for automatically
controlling the operation of the redundancy switch (e.g.,
transistor 62).
[0069] The control logic 64 may receive an input from one or more
of the other fuel cell systems 10(1)-10(M), the input relating to
an operating condition of the respective fuel cell system
10(1)-10(M) (i.e., "connect on failure of Unit 1 through M"). For
example, the control logic 64 may receive voltage, current and/or
power measurements related to the fuel cell stack 14 and/or
electrical power storage 24 of the fuel cell system 10. Such
measurements may include, but are not limited to, stack current
I.sub.S, stack voltage V.sub.S, battery current I.sub.B, and
battery voltage V.sub.B, and/or temperature. Also for example, the
control logic 64 may receive logical values relating to the
operating condition of various systems of the fuel cell system 10,
including, but not limited to, an ambient hydrogen level, an
ambient oxygen level, and a reactant flow. In this respect,
reference is made to commonly assigned U.S. Pat. No. 6,887,606,
issued May 3, 2005.
[0070] Additionally, or alternatively, the control logic 64 may
receive an input from other components of the power supply system
50, such as voltage and current sensors coupled to determine a
voltage or current at various points on the power bus 56. For
example, the control logic 64 may receive a voltage reading
corresponding to the voltage across the power bus measured at a
"top" of the one-dimensional array 52, allowing the control logic
64 to indirectly detect a fault in one or more of the fuel cell
systems 10 by detecting a measurement below an expected threshold
value (i.e., "connect if V.sub.X<M.times.24V"). The threshold
for detecting a fault condition may be predefined in the control
logic 64 or may be set by a user or operator via a user interface
66 such as analog or digital controls, or a graphical user
interface on a special purpose or general purpose computer.
[0071] Additionally or alternatively, the control logic 64 may
receive an input from the user or operator via the user interface
66 which may comprise a set of user controls to set operating
parameters such as power, voltage, and or current thresholds, to
set desired parameters such as desired power, desired voltage or
desired current nominal values, to provide electrical configuration
information, to provide switching signals, and/or to signals to
override the automatic operating aspects of the control logic 64.
The user interface 66 may be remote from the remainder of the power
supply system 50. The control logic 64 can be embodied in one or
more of hardwired circuitry, firmware, micro-controller,
application specific processor, programmed general purpose
processor, and/or instructions on computer-readable media.
[0072] Where the output voltage of the fuel cell systems 10 can be
tightly controlled, such as under the first and/or second stage
operation discussed above, the series coupling of the fuel cell
systems 10 is possible. Thus any desired number of fuel cell
systems 10 may be electrically coupled in series to realize any
integer multiple of voltage output of the individual fuel cell
system 10. For example, where each fuel cell system 10 produces 24
volts across the rails 19a, 19b, three fuel cell systems
10(1)-10(3) are electrically couplable to produce 72 volts across
the power bus 56. More generally stated, a number M of fuel cell
systems 10 can be electrically coupled in series to produce M times
the nominal fuel cell system voltage across the power bus 56.
Additionally, the series coupling renders the position of the
redundant fuel cell system 10(M+1) in the one-dimensional array 52
unimportant.
[0073] FIG. 3 shows a two-dimensional array 68 of fuel cell systems
10, arranged in a number M of rows and a number N of columns for
powering the load 12 via the power bus 56. The fuel cell systems 10
are individually referenced 10(1,1)-10(M,N), where the first number
in the parenthesis refers to a row position and the second number
in the parenthesis refers to a column position of the fuel cell
system 10 in the two-dimensional array 68. The ellipses in FIG. 3
illustrate that the various rows and columns of the two-dimensional
array 68 may comprise additional fuel cell systems (not explicitly
shown). The diodes 58, fault and redundancy switches 60, 62,
respectively, control logic 64, and user interface 66 have been
omitted from FIG. 3 for clarity of illustration.
[0074] Each of the fuel cell systems 10(1,1)-10(M,N) is
individually couplable to the power bus 56 to provide a variety of
desired output power, voltage or current. The fuel cell systems
10(1-M,1),10(1-M,2),10(1-M,3)-10(1-M,N) in each column 1-M are
electrically couplable in series to one another. The fuel cell
systems 10(1,1-N), 10(2,1-N), 10(3,1-N)-10(M,1-N) in each row 1-N
are electrically couplable in parallel to one another. From FIG. 3
and this description, one skilled in the art will recognize that
the two-dimensional array 68 permits the series coupling of fuel
cell systems 10 to adjust an output power of the power supply
system 50 by adjusting an output voltage. One skilled in the art
will also recognize that the two-dimensional array 68 permits the
parallel coupling of fuel cell systems 10 to adjust the output
power of the power supply system 50 by adjusting an output current.
One skilled in the art will further recognize that the
two-dimensional array 68 permits the series and parallel coupling
of fuel cell systems 10 to adjust the output power of the power
supply system 50 by adjusting both the output current and the
output voltage. Thus, for the illustrated embodiment where each
fuel cell system produces, for example, 1 kW at 24 volts and 40
amps, a maximum output power of N.times.M kW is possible. One
skilled in the art will further recognize that the one- and
two-dimensional array structures discussed herein refer to
electrically couplable positions relative to one another, and do
not necessary require that the fuel cell systems 54 be physically
arranged in rows and/or columns.
[0075] FIGS. 4-6 illustrate three different electrical
configurations of the fuel cell systems 10 of the two-dimensional
array 68 of FIG. 3, to produce a desired output power, for example
4 kW where each fuel cell system 10 is capable of providing 1 kW at
24 volts and 40 amps. In particular, FIG. 4 shows one illustrated
example employing four of the fuel cell systems 10(1,1)-10(4,1)
from the first column of the two-dimensional array 68 electrically
coupled in series to provide 4 kW of power at 96 volts and 40 amps.
FIG. 5 shows an illustrated embodiment of four of the fuel cell
systems 10(1,1)-10(1,4) of a first row of the two-dimensional array
68 electrically coupled in parallel to provide 4 kW of power at 24
volts and 160 amps. FIG. 6 shows an illustrated example employing
four fuel cell systems 10(1,1), 10(1,2), 10(2,1), 10(2,2) of the
two-dimensional array 68, where two pairs of series coupled fuel
cell systems 10(1,1), 10(2,1) and 10(1,2), 10(2,2) are electrically
coupled in parallel to produce 4 kW of power at 48 volts and 80
amps. One skilled in the art will recognize from these teachings
that other combinations and permutations of electrical couplings of
the fuel cell systems 10 of the two-dimensional array 68 are
possible.
Reactant Supply Systems
[0076] The oxidant gas can be pure oxygen or an oxygen-containing
gas, such as air. In the former case, the oxidant supply system may
include a stored oxygen supply; in the latter case, air may be
supplied to the stack at ambient or higher pressure. Where higher
pressure operation is desired, gas compression equipment, including
compressors, blowers, pumps, boosters, or ejectors, may be
employed. Single- and multi-stage compression may be employed, as
desired.
[0077] The fuel supply system includes hydrogen storage equipment
for storing the hydrogen fuel supplied to the stacks. The fuel
supply system may include a hydrocarbon fuel source and a fuel
processing subsystem comprising a reformer. The fuel processing
subsystem converts the fuel to a hydrogen-rich reformate gas that
is supplied to the stacks.
[0078] Alternatively the hydrogen fuel may be substantially pure
hydrogen. The type of hydrogen storage is not essential to the
present power plant. For example, hydrogen may be stored as a
pressurized gas or a liquid, if desired. Alternatively,
solid-hydrogen storage media may be employed, including metal
hydride (e.g., nickel metal hydride), chemical hydride (e.g.,
borohydrides) or carbon nanomaterials. Low pressure hydrogen gas
storage suffers from relatively low volumetric and gravimetric
energy densities, but is relatively inexpensive and simple to
implement. As the pressure of the stored hydrogen increases,
volumetric and gravimetric energy density increases. Metal hydrides
exhibit superior volumetric energy densities, but their weight
results in significantly inferior gravimetric energy densities
compared to other hydrogen storage approaches. Associated
temperature regulating equipment--metal hydrides are typically
cooled to facilitate hydrogen adsorption and heated to facilitate
hydrogen release--and (optionally) gas pressurizing equipment can
also add cost and complexity to the overall power plant. Liquid
hydrogen storage exhibits good volumetric and gravimetric energy
densities, but the associated temperature regulating equipment
required to maintain cryogenic storage also adds cost and
complexity to the power plant. In addition, liquid hydrogen storage
equipment experience evaporative losses ("boil-off") over time.
Thus, the choice of hydrogen storage equipment for a given
application balances various factors, including the size and weight
of the equipment, cost and complexity of operation. Persons skilled
in the art will be aware of such considerations and can readily
select suitable hydrogen storage equipment for a given
application.
[0079] In some embodiments, the pressure of hydrogen in the
hydrogen storage equipment exceeds the operating pressure of the
stacks. For example, the stacks may operate at ambient pressure
while the hydrogen storage system comprises compressed hydrogen
tanks, which can store hydrogen at pressures of up to 10,000 psi
(700 bar) or more. The hydrogen storage system may therefore
comprise means for reducing the pressure of the hydrogen fuel from
a storage pressure to a stack operating pressure. The selection of
pressure reducing means is not essential to the present power
plant, and any suitable pressure reducing means, including reducing
valves, expanders, differential pressure regulators or expanded
lines, may be employed.
[0080] Either or both of the incoming reactant streams may be
humidified before being directed to the stacks. The means for
humidifying the reactant stream(s) is not essential to the present
power plant and operating method, and persons skilled in the art
can readily select suitable such means for a given application. For
example, the reactant stream(s) may be humidified in a membrane
exchange humidifier that also receives the reactant exhaust from
the stacks. Alternatively, a fine stream of water may be injected
into the reactant stream(s). As a further example, the reactant
stream(s) may be humidified by contact with hot water. Other
suitable such means, including enthalpy wheels or pressure swing
adsorption (PSA) units, will be apparent to persons skilled in the
art. Humidification of the reactant streams is not required,
however. For example, ambient air may be supplied to the stack
without humidification.
System Redundancy
[0081] As discussed previously, one or more of the fuel cell
systems in the present power plant (e.g., 10(M+1)) may serve as a
"redundant" fuel cell system. As will be apparent to persons
skilled in the art, the concept of redundancy may be applied to
various other systems of the present power plant, as well.
[0082] For example, it has been disclosed that the fuel cell
systems may further comprise an electrical storage device
electrically coupled in parallel with the fuel cell stack across a
high voltage bus to power the load. Alternatively, such an
electrical storage device may be electrically coupled in parallel
with the fuel cell stacks of a group of fuel cell systems. By way
of illustration, in the array 68 of FIG. 3, each group of series
connected fuel cell systems 10 may include one or more electrical
storage devices parallel coupled to all or a portion of the
constituent fuel cell stacks. Thus, each "arm" of the array may
comprise one or more such electrical storage devices.
[0083] As another example, where pressurized operation is desired,
the fuel cell systems of the array, or groups of them, may share
common gas compression equipment; or each fuel cell system may have
its own gas compression equipment. As further examples, similar
considerations apply with respect to the redundancy of
humidification equipment, gas supply and manifolding equipment, and
control systems.
[0084] From a balance-of-plant perspective, common systems reduce
cost and complexity. On the other hand, redundant systems can
desirably increase reliability. Multiple systems may also provide a
greater degree of control of individual fuel cell systems or
portions of the array. The system configuration and degree of
redundancy is not essential to the present power plant, and persons
skilled in the art can readily select a suitable system
configuration for a given application.
Operation
[0085] FIG. 7 shows a method 100 of operating the power supply
system 50 according to one exemplary illustrated embodiment, which
is discussed with reference to FIG. 2. The method 100 may be
embodied in the control logic 64, discussed above.
[0086] In step 102, the control logic 64 electrically couples a
number M of fuel cell systems 10(1)-10(M) in series on the power
bus 56 by selectively operating appropriate ones of the switches
60, 62. In step 104, the control logic 64 determines if there is a
fault. For example, the control logic 64 may determine whether any
of the parameters of one of the fuel cell systems 10(1)-10(M) is
outside of an acceptable range, or exceeds, or falls below, an
acceptable threshold. As discussed above the control logic 64 may
receive voltage, current and/or power measurements related to the
fuel cell stack 14 and/or electrical power storage 24 of the fuel
cell system 10. Additionally, or alternatively, the control logic
64 may receive logical values relating to the operating condition
of various systems of the fuel cell system 10. Additionally, or
alternatively, the control logic 64 may receive an input from other
components of the power supply system 50, such as voltage and
current sensors coupled to determine a voltage or current at
various points on the power bus 56. The control logic 64 can
comprise comparison circuitry such as a comparator, or instructions
for comparing the received values to defined range and/or threshold
values, for example, ensuring that the total voltage across the
power bus 56 is above a defined threshold or within a defined
range. Alternatively, or additionally, the control logic 64 can
rely on a set of logical values returned by the individual fuel
cell systems 10(i)-10(M), such as a "1" or "0" corresponding to one
or more operating conditions of the respective fuel cell system
10(1)-10(M).
[0087] If there is no fault, the method 100 returns to step 104,
performing a monitoring loop. If there is a fault, the control
logic 64 electrically couples the redundant fuel cell system
10(M+1) in series on the power bus 56 in step 106, for example, by
sending an appropriate signal to the corresponding redundant switch
such as by applying a signal to a gate of the redundant transistor
62. The fuel cell systems 10(1)-10(M+1) are "hot swappable" so the
power supply system 50 does not have to be shutdown.
[0088] In optional step 108, the control logic 64 electrically
decouples the faulty fuel cell system, for example 10(3), from the
power bus 56, for example, by sending an appropriate signal to the
corresponding fault switch such as by applying a signal to a gate
of the fault transistor 60. In optional step 110, a user or service
technician replaces the faulty fuel cell system 10(3) in the array
52 of the power supply system 50. The replacement fuel cell system
10 may serve as a redundant fuel cell system for a possible
eventual failure of another fuel cell system 10.
[0089] FIG. 8 shows an optional step 112 for inclusion in the
method 100. In step 112, an additional fuel cell system 10 is
electrically coupled in series on the power bus 50 with one or more
of the fuel cell systems 10(1)-10(M). For example, where the faulty
fuel cell system 10(3) has been replaced, the replacement fuel cell
system may be electrically coupled in series to increase the power
output of the power supply system 50.
[0090] FIG. 9 shows an optional step 114 for inclusion in the
method 100. In step 114, an additional fuel cell system 10 is
electrically coupled in parallel on the power bus 52 with one or
more of the fuel cell systems 10(1)-10(M). From this description,
one skilled in the art will recognize that the method 100 may
employ any variety of series and/or parallel combinations of fuel
cell systems 10.
[0091] FIG. 10 shows a method 130 of operating the power supply
system 50 according to an additional, or alternative, illustrated
embodiment, which is discussed with reference to the
two-dimensional array 68 of FIG. 3. Thus, the power supply system
50 may employ the method 130 in addition to, or alternatively from,
the method 100.
[0092] In step 132, the control logic 64 determines at least one of
a desired power, voltage and current output from the power supply
system 50. The desired values may be defined in the control logic
64 or the control logic 64 may receive the desired value(s) from
the user or operator by way of the user interface 66. In step 134,
the control logic 64 determines an electrical configuration of
series and/or parallel combinations of a number of fuel cell
systems 10(1,1)-10(M,N) to provide the desired power, voltage
and/or current. In step 136, the control logic 64 operates a number
of the redundant switches such as a transistor 60 (FIG. 2, only one
shown) to electrically couple respective ones of fuel cell systems
10(1,1)-10(M,N) into the determined electrical configuration.
[0093] The above description shows that any number of fuel cell
systems 10 are electrically couplable in series and/or parallel
combinations to form a combined power supply system 50 for powering
the load 12 at a desired voltage and current.
[0094] The fuel cell systems 10 can take the form of any of the
fuel cell systems discussed above, for example, the fuel cell
system 10 illustrated in FIG. 1. As discussed above, the power
supply system 50 takes advantage of a matching of polarization
curves between the fuel cell stacks 14 and the respective
electrical power storage 24 to allow series coupling of fuel cell
systems. 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 electrical power storage 24 from a desired voltage across the
electrical power storage 24. A further approach includes
controlling a partial pressure of one or more reactant flows based
on a deviation of an electrical storage charge from a desired
electrical storage charge. The electrical power storage charge can
be determined by integrating the flow of charge to and from the
electrical power storage 24. Other approaches may include phase or
pulse switching regulating or control schemes. Reasons for
employing a series configuration include the cost advantage, and
the configuration having the highest efficiency at the full output
power point if the stack voltage equals the battery float voltage
at that point, e.g., efficiency can exceed 97% in a 24V system with
no R.F. noise problem. While the fuel cell systems 10 are
illustrated having two stages, in some embodiments the power supply
system 50 may incorporate one or more fuel cell systems 10 having
only one of the stages, either the first or the second stage.
[0095] FIG. 11 shows another embodiment of a hybrid fuel cell
system 10 operable to power an external load 12. In contrast to the
previously discussed embodiments, the fuel cell system 10 of FIG.
11 employs an ultracapacitor battery simulator circuit 200 as an
electrical power storage device 24 (FIG. 1), the ultracapacitor
battery simulator circuit 200 being configured to simulate a
battery.
[0096] The fuel cell system 10 may comprise one or more internal
loads 202, which represent the various active components of the
fuel cell system 10, for example, processors, sensors, indicators,
valves, heaters, compressors, fans, and/or actuators such as
solenoids. These internal loads 202 are typically referred to as
the "balance of system" or "balance of plant" (BOP). The internal
loads 202 are electrically coupled to receive power from the fuel
cell stack 14 via the power bus 19a, 19b. The fuel cell system 10
may also comprise one or more current sensors 204 and voltage
sensors 206.
[0097] The ultracapacitor battery simulator circuit 200 comprises a
number of ultracapacitors C.sub.1-C.sub.n electrically coupled in
series between the rails 19a, 19b of the voltage bus. A charging
current limiter 208 is electrically coupled in series with the
ultracapacitors C.sub.1-C.sub.n to limit charging current to the
ultracapacitors C.sub.1-C.sub.n. A bypass diode D.sub.2 is
electrically coupled across the charging current limiter 208 to
provide a path for discharge current which bypasses the charging
current limiter 208. A reverse charging diode D.sub.3 prevents the
ultracapacitors C.sub.1-C.sub.n from charging in the reverse
direction, for example, when connected in series with other
electrical power storage devices 24 or hybrid fuel cell systems
10.
[0098] A number of surge diodes D.sub.S are electrically coupled
across respective ones of the ultracapacitors C.sub.1-C.sub.n. The
surge diodes D.sub.S equalize the voltage across each of the
ultracapacitors C.sub.1-C.sub.n during charging, and thus may limit
the voltage across any ultracapacitor C.sub.1-C.sub.n to the surge
rating of the ultracapacitor C.sub.1-C.sub.n. For example, typical
ultracapacitors C.sub.1-C.sub.n may have a working voltage of
approximately 2.5 volts. As illustrated, the ultracapacitors
C.sub.1-C.sub.n may be connected in series to achieve higher
working voltages. Thus, for example, four surge diodes D.sub.S
electrically coupled across respective ultracapacitors
C.sub.1-C.sub.n may limit the voltage across the respective
ultracapacitor C.sub.1-C.sub.n to approximately 2.8 volts, which is
the typical surge rating of the ultracapacitors
C.sub.1-C.sub.n.
[0099] The bypass diode D.sub.2 is selected such that if the
voltage on the capacitor bank (i.e., the series coupled
ultracapacitors) rises above the point where all of the
ultracapacitors C.sub.1-C.sub.n have approximately 2.8 volts across
them, and all surge diodes D.sub.S turn ON, the voltage drop across
the current limiter 208 will rise to limit the current through the
surge diodes D.sub.S and prevent a short circuit.
[0100] FIG. 11 shows the charging current limiter 208 and bypass
diode D.sub.2 positioned at one end of the string of
ultracapacitors C.sub.1-C.sub.n. FIG. 12 shows the charging current
limiter 208 and bypass diode D.sub.2 positioned at the other end of
the string of ultracapacitors C.sub.1-C.sub.n. FIG. 13 shows the
charging current limiter 208 and bypass diode D.sub.2 positioned
between the ends of the string of ultracapacitors C.sub.1-C.sub.n.
Thus, it is apparent that the charging current limiter 208 and
bypass diode D.sub.2 may be positioned at either end, or anywhere
in the string of ultracapacitors C.sub.1-C.sub.n.
[0101] FIG. 14 shows one embodiment of the charging current limiter
208 in the form of a linear mode charging current limiter. The
charging current limiter 208 comprises a charging current limiting
transistor Q.sub.1, feed back transistor Q.sub.2. first resistor
R.sub.1, second resistor R.sub.2 and Zener diode D.sub.4. The
charging current limiting transistor Q.sub.1 comprises a pair of
active terminals (e.g., collector and emitter) and a control
terminal (e.g., base), the active terminals electrically coupled in
series with the ultracapacitors C.sub.1-C.sub.n. The feedback
transistor Q.sub.2 comprises a pair of active terminals (e.g.,
collector and emitter) and a control terminal (e.g., base), the
active terminals electrically coupled between rails 19a, 19b and
the control terminal electrically coupled to the emitter of the
charging current limiting transistor Q.sub.1. The first resistor
R.sub.1 is electrically coupled between the control terminal of the
feedback transistor Q.sub.2 and one rail 19b of the voltage bus.
The second resistor R.sub.2 and Zener diode D.sub.4 are
electrically coupled between the control terminal of the charging
current limiting transistor Q.sub.1 and the other rail 19a of the
voltage bus.
[0102] In use, the linear mode charging current limiter 208 passes
charging current when the terminal voltage V.sub.1-V.sub.0 is above
some defined threshold voltage. When a voltage greater than the sum
of the Zener voltage of Zener diode D.sub.4 and the voltage
required to turn ON the charging current limiting transistor
Q.sub.1 (e.g., approximately 0.7 volts) is applied to the terminals
of the ultracapacitor battery simulator circuit 200, current will
begin to flow into the control terminal of the charging current
limiting transistor Q.sub.1. This causes current to flow into the
collector of the charging current limiting transistor Q.sub.1, and
begins charging the bank of ultracapacitors C.sub.1-C.sub.n. When
the current from the emitter of the charging current limiting
transistor Q.sub.1 is sufficiently high to cause approximately 0.7
volts across the first resistor R.sub.1, the feedback transistor
Q.sub.2 begins to turn ON. This reduces the current through the
charging current limiting transistor Q.sub.1. In this way, the
charging current of the bank of ultracapacitors C.sub.1-C.sub.n is
limited to approximately 0.7 volts divided by the value of the
first resistor R.sub.1. For example, if the first resistor R.sub.1
is approximately 0.175 ohms, then the charging current would be
limited to approximately 4 amps.
[0103] The circuit configuration of FIG. 14 also minimizes the
current drawn from the ultracapacitors C.sub.1-C.sub.n when storing
charge (i.e., when not on float charge). When a voltage less than
the sum of the Zener voltage of Zener diode D.sub.4 (e.g.,
approximately 24 volts) and the voltage required to turn ON the
charging current limiting transistor Q.sub.1 (e.g., approximately
0.7 volts) is applied to the terminals of the ultracapacitor
battery simulator circuit 200, no current will be drawn from the
ultracapacitors C.sub.1-C.sub.n, which will consequently maintain
their charge for relatively long intervals.
[0104] FIG. 15 shows another embodiment of the charging current
limiter 208, adding a pair of transistors, pair of resistors and
diode, coupled in a Darlington circuit configuration Q.sub.3, to
the embodiment of FIG. 14. The Darlington circuit Q.sub.3 is
electrically coupled between the base of the charging current
limiting transistor Q.sub.1 and the Zener diode D.sub.4, reducing
the current I.sub.D3 flowing through the second resistor R.sub.2
and the Zener diode D.sub.4. While slightly more complicated than
the embodiment of FIG. 14, the configuration of FIG. 15 reduces
power lost through the second resistor R.sub.2 and Zener diode
D.sub.3.
[0105] FIG. 16 shows a further embodiment of the charging current
limiter 208, that adds over voltage circuitry that cuts off the
charging current in an over voltage situation, to the embodiment of
FIG. 15. The over voltage circuitry comprises an over voltage
transistor Q.sub.4, over voltage Zener diode D.sub.5, and over
voltage resistor R.sub.3. The emitters of the over voltage
transistor Q.sub.4 and feed back transistor Q.sub.2 are commonly
coupled, and the collectors of the over voltage transistor Q.sub.4
and feed back transistor Q.sub.2 are also commonly coupled. The
over voltage Zener diode D.sub.5 and over voltage resistor R.sub.3
are electrically coupled between the base of the over voltage
transistor Q.sub.4 and one rail of the voltage bus.
[0106] When the terminal voltage of the bank of ultracapacitors
C.sub.1-C.sub.n exceeds a sum of the Zener voltage of the Zener
diode D.sub.5 (e.g., approximately 30 volts) and the voltage
required to turn ON the over voltage transistor Q.sub.4 (e.g.,
approximately 0.7 volts), the over voltage transistor Q.sub.4 turns
OFF both the feedback transistor Q.sub.2 and charging current
limiting transistor Q.sub.1, thus preventing further charging
current from entering the ultracapacitors C.sub.1-C.sub.n. Although
the over voltage cutoff is not a feature inherent in batteries, it
is desirable in a hybrid fuel cell system to account for the rise
in voltage of the fuel cell stack 14 in no load conditions (e.g.,
open circuit voltage or OCV). The embodiment of FIG. 16 also has
the advantage of limiting the heat produced by the charging current
limiting transistor Q.sub.1 and consequently the size of any
associated heat sink.
[0107] FIG. 17 shows yet a further embodiment of the charging
current limiter 208, that adds circuitry to cut off charging
current when the bank of ultracapacitors C.sub.1-C.sub.n reaches a
desired voltage, to the embodiment of FIG. 16. The circuitry
comprises a voltage setting transistor Q.sub.5, voltage setting
Zener diode D.sub.6, and voltage setting resistor R.sub.4. Active
terminals of the voltage setting transistor Q.sub.5 are
electrically coupled between the base of the over voltage
transistor Q.sub.4 and the rail of the voltage bus. The voltage
setting Zener diode D.sub.6 and voltage setting resistor R.sub.4
are electrically coupled between the base of the voltage setting
transistor Q.sub.5 and the bank of ultracapacitors C.sub.1-C.sub.n.
The embodiment of FIG. 17 has the advantage of limiting the heat
produced by the charging current limiting transistor Q.sub.1 and
consequently the size of any heat sink associated. The embodiment
of FIG. 17 also saves power and improves overall system
efficiency.
[0108] The embodiments of FIGS. 11-17 are compatible with, and
complimentary to, previously discussed concepts, any may also be
employed with black start techniques discussed in commonly assigned
U.S. Pat. No. 7,011,902, issued Mar. 14, 2006.
[0109] In the embodiments of FIGS. 11-17, the fuel cell system 10
behaves as a two-mode power supply. The output is controlled by two
settings: 1) an output current limit; and 2) an output voltage
limit. When the load resistance is high enough to draw a current
lower than the output current limit, the fuel cell system 10 acts
as a constant voltage source to set the output voltage limit. When
the load resistance is low enough to draw a current higher than the
output current limit at the output voltage limit set point, the
fuel cell system 10 acts as a constant current source set to the
output current limit. Charging current limiting is handled by the
ultracapacitor battery simulator circuit 200, rather than via the
series pass element 32 (FIG. 1) in battery charging current limit
mode discussed in reference to FIG. 1. It would be advantageous to
incorporate the charging current limiting in other electrical power
storage device circuitry, even where the electrical power storage
device comprises a battery rather than an ultracapacitor, since
this would prevent the in rush of current when, for example, a dead
or discharged battery is plugged into a system with charged
batteries.
[0110] For a fuel cell system 10 employing a Ballard Nexa.TM. fuel
cell stack, the output voltage limit would be set at or below the
open circuit voltage (OCV) of the fuel cell stack 14 (e.g.,
approximately 54.8 volts), and the output current limit would be
set such that the fuel cell stack current limit and the fuel cell
system's thermal limits were not exceeded. For example, if the
output power limit is 1.3 kW, the output current limit would be
approximately 23.7 amps.
[0111] The ultracapacitor battery simulator circuit 200 acts as a
DC/DC converter. The balance of plant 202 (FIG. 11) is typically
run on 24 VDC derived from the output of the ultracapacitor battery
simulator circuit 200, rather than directly from the stack
voltage.
[0112] The ultracapacitor battery simulator circuit 200 may have an
input voltage range of 55 volts (at OCV) to 25.5 volts (at full
load). If the input voltage (i.e., stack voltage) falls below 25.5
volts, the ultracapacitor battery simulator circuit 200 may lower
its output current limit to the point where the input voltage does
not go any lower. If the input current (i.e., stack current) rises
to 48 amps, the ultracapacitor battery simulator circuit 200 may
lower its output current limit to the point where the input current
would not any higher.
[0113] FIG. 18 shows a method 300 of operating a fuel cell system
10 employing an ultracapacitor battery simulator circuit 200
according to one illustrated embodiment. In step 302, charging
current is supplied, for example, from the fuel cell stack 14. In
step 304, the charging current limiting transistor Q.sub.1 and
feedback transistor Q.sub.2 limit charging current supplied to the
ultracapacitors C.sub.1-C.sub.n below a charging current limit
threshold. In step 306, the surge diodes D.sub.S limit the voltage
across each of the ultracapacitors C.sub.1-C.sub.n. In step 308,
the over voltage transistor Q.sub.4 stops the supply of charging
current to the ultracapacitors C.sub.1-C.sub.n in the event of an
over voltage condition. In step 310, the voltage setting transistor
Q.sub.5 stops the supply of charging current to the ultracapacitors
C.sub.1-C.sub.n if the desired voltage across the bank of
ultracapacitors C.sub.1-C.sub.n has been attained. In step 312, the
ultracapacitors C.sub.1-C.sub.n, discharge via the bypass diode
D.sub.2, bypassing the charging current limiter 208.
[0114] FIG. 19 shows a power system 500 for supplying power to a
load 12 via rails 556a, 556b of a DC bus. The power system 500
receives power from a power grid 502, typically in the form of
three-phase AC power. The power system 500 comprises one or more
rectifier arrays 504 (1)-504(n), which receive the AC power from
the power grid 502 and rectify the power. The rectified power may
be supplied to the load 12 via the DC bus 556a, 556b. The array of
rectifiers 504(1)-504(n) serves as a primary source of DC power to
continuously power the load 12, and to recharge a variety of
electrical power storage devices 24.
[0115] The power system 500 includes an array of one or more fuel
cell hybrid modules 510(1)-510(n). The array of fuel cell hybrid
modules 510(1)-510(n) provide continuous backup power to the load
12 via the DC bus 556a, 556b, for example, in the event of an
interruption of the power grid 502.
[0116] The power system 500 may also include an array of one or
more ultracapacitor battery simulators 200(1)-200(n) that may store
energy for load bridging and providing surge (i.e., demand) power.
Additionally, or alternatively, the power system 500 may include a
fly wheel battery simulator 506, that may store energy for load
bridging and providing surge power. The fly wheel battery simulator
506 may employ circuitry similar to that described for the
ultracapacitor battery simulator 200. Additionally, or
alternatively, the power system 500 may include one or more
rechargeable batteries 508 that store energy for load bridging and
providing surge power. These electrical power storage devices may
supply power to the load 12 via the DC bus formed by rails 556a,
556b.
[0117] FIG. 20 shows a fuel cell hybrid module array 510, suitable
for use in the power system 500 of FIG. 19. The fuel cell hybrid
module array 510 includes first and second fuel cell stacks 14
electrically coupled in series and associated balance of plant 202,
regulator 517 (e.g., series pass element 32 and regulating circuit
34 of FIGS. 1 and 11) and an ultracapacitor battery simulator array
200. The fuel cell hybrid module array 510 may also include a
electrical power storage device 509, such as ultracapacitor battery
simulators 200 (1)-200(n), fly wheel battery simulator 506, or
rechargeable batteries 508 of FIG. 19. The ultracapacitor battery
simulator arrays 200 provide dynamic response for the fuel cell
hybrid modules, supplying and absorbing current quickly in response
to load requirements, while stacks 14 and balance of plants 202
respond more slowly. Electrical power storage device 509 provides
energy for load bridging and providing surge power. This
configuration may permit a smaller number of ultracapacitors to be
used in battery simulator arrays 200 than would be the case if they
were also required to provide load bridging and surge capacity. In
some embodiments, in turn, this may result in an overall reduction
in the number of ultracapacitors employed in the power plant.
[0118] Auxiliary devices, such as hydrogen supply solenoid-valves
210 (or ventilation fans or flow switches (not shown)), can be
powered from center bus 556a. One or more equalizing circuits may
be employed to aid in system startup by balancing the load to
provide a reference. The equalizing circuits may take the form of a
string of resistors 212 between bus 556a, 556b and 556c. Other
active or passive means of balancing the load on center bus 556a
may also be employed, if desired, such as an active controller that
shares a load to maintain a particular voltage level.
[0119] FIG. 21 shows a two-dimensional array 468 of fuel cell
systems 10, arranged in a number M of rows and a number N of
columns to form a power system for powering one or more loads 12
via the power bus 456a, 456b. The fuel cell systems 10 are
individually referenced 10(1,1)-10(M,N) where the first number in
the parentheses refers to a row position and the second number in
the parentheses refers to a column position of fuel system 10 in
the two-dimensional array 468. The ellipses in FIG. 21 illustrate
that various rows and columns of the two-dimensional array 468 may
comprise additional fuel cell systems (not explicitly shown). While
not illustrated, other multi-dimensional arrays of fuel cell
systems 10 are also possible, for example, three-dimensional arrays
of fuel cell systems 10.
[0120] The two-dimensional array 468 of FIG. 21 is similar to that
of FIG. 3, however, comprises links 490 electrically coupling the
fuel cell systems 10 forming a row (e.g., 10(3,1),10(3,2),10(3,3),
. . . 10(3,N)) for providing at least N+1 redundancy. The
two-dimensional array 468 may omit the diodes 58, fault and
redundancy switches 60, 62, and other elements of the previously
discussed embodiments. The links 490 provide redundancy, preventing
the failure of a single fuel cell system 10 from eliminating an
entire voltage string (column). For example, without the links 490,
if fuel cell system 10(2,1) was to fail, then fuel cell systems
10(1,1), 10(3,1) through 10(M,1) would be unavailable. The links
490 prevent the loss of any individual fuel cell system 10 in a
column from hindering the ability to fully supply the load 12. As
discussed below, the links 490 may be tapped or may form taps, to
produce desired potentials on the rails of voltage buses.
[0121] FIG. 22 shows an embodiment of the two-dimensional array 468
capable of providing multiple voltage levels with at least N+1
redundancy. A second column of fuel cell systems 10(1,2), 10(2,2),
10(3,2) . . . 10(M,2) can supply 40 amps at M.times.24 volts to a
first load 12 a via a voltage bus form by taps or rails 456a, 456b.
A third column of fuel cell systems 10(1,3), 10(2,3) can supply 40
amps at 48 volts to a second load 12b via a second voltage bus
formed by taps or rails 456b, 456c. A third column of fuel cell
systems 10(1,4) can supply 40 amps at 24 volts to a third load 12c
via a voltage bus formed by taps or rails 456b, 456d. If the load
requires more current, additional columns of fuel cell systems 10
can be added between the rails of the corresponding voltage bus.
Thus in the exemplary system, current can be increased in multiples
of 40 amps by adding fuel cell systems 10 to the array 468.
[0122] A first column of fuel cell systems 10(1,1), 10(2,1), 10(3,
1) . . . 10(M,1) provides redundancy for each of the other fuel
cell systems 10 in the two-dimensional array 468. The number of
fuel cell systems 10 in the first column is equal to the number of
fuel cell systems 10 in the largest column of the array 469 to
ensure at least N+1 redundancy. By employing a single column of
fuel cell systems 10(1,1)-10(M,1), redundancy is provided to each
of the other columns, without the need to provide specific fuel
cell systems for each column. This obtains at least the desired N+1
redundancy with fewer fuel cell system 10 then in previously
described embodiments.
[0123] FIG. 23 illustrates another embodiment of a two-dimensional
array 468 of fuel cell systems 10 suitable for supplying multiple
bipolar voltage levels with redundancy. The second column of fuel
systems 10(-2,2), 10(-1,2), 10(1,2), 10(2,2), 10(3,2), 10(4,2),
10(5,2) is capable of supplying 40 amps at 120 volts to the first
load 12a via a voltage bus formed by taps or rails 456a, 456b. A
third column of fuel cell systems 10(-2,3), 10(-1,3), 10(1,3),
10(2,3) is capable of supplying 40 amps at +48 volts to the second
load 12b via a voltage bus formed by taps or rails 456a, 456c, or
supplying 40 amps at -48 volts to a third load 12C via a voltage
bus formed by taps or rails 456a, 456d. A fourth column of fuel
cell systems 10(-1,4), 10(1,4) is capable of supply 40 amps at +24
volts to a fourth load 12d via voltage bus formed by taps or rails
456a, 456e, or supplying 40 amps at -24 volts to a fifth load 12e
via voltage bus formed by taps or rails 456a, 456f. Again, a first
column of fuel cell systems 10(-2,1), 10(-1,1), 10(1,1), 10(2,1),
10(3,1), 10(4,1), 10(5,1) provides at least N+1 redundancy to all
the remaining fuel cell systems 10 in the array 468.
[0124] While not illustrated, the array 468 may employ one or more
equalizing circuits to aid in system startup by balancing the load
to provide a reference. The equalizing circuits may be as described
in relation to FIG. 20, above. Where the fuel cell systems 10
employ ultracapacitors, for example, equalizing devices for the
intermediate voltages across any number of series connected fuel
cell systems 10 may be added to improve the source impedance
(stiffness) of the intermediate buses.
[0125] The embodiment of FIG. 23 is particularly suitable for
providing power conditioning and/or power backup in telephone
related applications, such as telephone switching offices which
typically employ 24 volts for wireless communications such as
Personal Communications Services (PCS) and microwave repeater
stations, 48 volts for traditional communications via wire
(Wireline), and 120 volts DC for switching operations and
substations.
EXAMPLES
[0126] FIGS. 24 and 25 are schematic illustrations of conventional
back-up or UPS systems. In FIG. 24, power grid 502 normally
supplies power to rectifier 504 and other AC loads such as
lighting, HVAC, and other equipment. Rectifier 504 converts the AC
power from the grid to DC power and directs it to DC power
distribution panel 512. From panel 512 power may be directly
supplied to compatible voltage DC loads. Power may also be directed
to DC/DC converter 514, which converts it to a different voltage
for use by other DC loads; or to inverter 516, which converts the
power to an AC source for use by AC loads. If the grid fails, power
is supplied by battery bank 518 to panel 512. Battery bank 518 will
be sized to provide adequate power to critical loads for the
desired amount of time. Standby generator 520 is brought online by
switch 152 to provide power if the grid is interrupted for an
extended period.
[0127] In FIG. 25, power from the grid is normally supplied from AC
power distribution panel 512 to AC loads directly, to -48 VDC loads
via rectifier 156, and/or to other DC loads via AC/DC converter
158. If the grid fails, power is supplied by battery bank 518 to
inverter 154, which converts the DC source to an AC source. As in
FIG. 24, standby generator 520 is brought online to provide power
if the grid is interrupted for an extended period.
[0128] FIGS. 26-29 are schematic illustrations of conventional VRLA
power supply systems and embodiments of the present power
plant.
[0129] The systems illustrated in FIGS. 26 and 27 are capable of
providing power at 400 A/48 VDC for 4 hours (400 A 4 hr back-up).
In FIG. 26, the power grid normally supplies load power via
rectifier 504. When the power grid is interrupted, VRLA batteries
530 in battery bank 518 provide load power. Once the grid is
restored, it supplies power to rectifier 532 in order to recharge
battery bank 518.
[0130] The embodiment of FIG. 27 comprises an array 468 of fuel
cell systems 10 configured to provide 400 A/48 VDC power. Array 468
has ten sets of two fuel cell systems: the two fuel cell systems in
each set electrically coupled in series, with each set electrically
coupled in parallel. Each fuel cell system 10 comprises a 47-cell
PEM fuel cell stack producing 1 kW at 24 volts and 40 amps,
electrically coupled in parallel to two 12 VDC VRLA batteries. When
grid power is interrupted, hydrogen from hydrogen supply 540 is
supplied to the fuel cells of array 468 via supply line 542; air is
also supplied to the fuel cells from an appropriate air supply (not
shown). Electric power generated by array 468 is then supplied to
the load. Once grid power is restored, it supplies power to
rectifier 504 in order to recharge the batteries of fuel cell
systems 10, if required. In FIG. 27, hydrogen supply 540 is a 100
gal. (380 l) 3600 psi (25 MPa) hydrogen tank, which holds
sufficient hydrogen for array 468 to continuously supply load power
for 4 hours.
[0131] The dimensions, weight and footprint of the 400 A 4 hour
systems of FIGS. 26 and 27 are summarized in Table 1. The size and
weight of battery bank 518 is based on currently-available VRLA
battery banks. The battery bank rating used in the comparison is
based on a 10.times. recharge rate and assumes a constant current
load. The rectifier rating is based on the load current plus a 0.1
C battery charging current. Front clearance refers to area between
adjacent walls and the back and sides of the power supply required
by safety regulations. TABLE-US-00001 TABLE 1 Comparison of 400
A/48 VDC 4 hour back-up power systems VRLA Battery Bank Fuel Cell
Array Battery Rating 3000 A-hr Array Rating 400 A @ 48 VDC Battery
Size Width: 93 in (236 cm) Array Size Width: 48 in (122 cm) Depth:
21 in (53 cm) Depth: 36 in (91 cm) Height: 72 in (183 cm) Height:
84 in (213 cm) Battery Weight 13,725 lbs. (6020 kg) Array Weight
2000 lbs (910 kg) Footprint 13.6 ft.sup.2 (1.3 m.sup.2) Footprint
12.0 ft.sup.2 (1.1 m.sup.2) Floor Loading 979 lbs./ft.sup.2 167
lbs./ft.sup.2 (149 kgf/m.sup.2) (25.3 kgf/m.sup.2) Rectifier Rating
700 A @ 54 VDC Rectifier Rating 410 A @ 54 VDC Rectifier Size
Width: 48 in (122 cm) Rectifier Size Width: 24 in (61 cm) Depth: 21
in (53 cm) Depth: 36 in (91 cm) Height: 72 in Height: 72 in (183
cm) (183 cm) Rectifier 7.0 ft.sup.2 (0.65 m.sup.2) Rectifier 6.0
ft.sup.2 (0.56 m.sup.2) Footprint Footprint Front Clearance Width:
141 in (358 cm) Front Clearance Width: 72 in (183 cm) Depth: 36 in
(91 cm) Depth: 36 in (91 cm) Clearance 35.3 ft.sup.2 (3.3 m.sup.2)
Clearance 18.0 ft.sup.2 (1.7 m.sup.2) Footprint Footprint Total
Footprint 55.9 ft.sup.2 (5.2 m.sup.2) Total Footprint 36 ft.sup.2
(3.3 m.sup.2)
[0132] The systems illustrated in FIGS. 28 and 29 are configured to
provide 400 A/48 VDC power for 8 hours (400 A 8 hr back-up). Array
468 is as described in FIG. 27. Because of the longer run time,
hydrogen storage 540 in FIG. 29 comprises 2.times.100 gal. (380 l)
3600 psi (25 MPa) hydrogen tanks.
[0133] The dimensions, weight and footprint of the 400 A 8 hour
systems of FIGS. 28 and 29 are summarized in Table 2. The
comparative data is based on the same assumptions given for the
data in Table 1. TABLE-US-00002 TABLE 2 Comparison of 400 A/48 VDC
8 hour back-up power systems VRLA Battery Bank Fuel Cell Array
Battery Rating 5333 A-hr Array Rating 400 A @ 48 VDC Battery Rating
104 A-hr Battery Size Width: 165 in (420 cm) Array Size Width: 48
in (122 cm) Depth: 21 in (53 cm) Depth: 36 in (91 cm) Height: 72 in
(183 cm) Height: 84 in (213 cm) Battery Weight 23,585 lbs. (10,700
kg) Array Weight 2000 lbs (910 kg) Footprint 24.1 ft.sup.2 (2.2
m.sup.2) Footprint 12.0 ft.sup.2 (1.1 m.sup.2) Floor Loading 979
lbs./ft.sup.2 (149 kgf/m.sup.2) 167 lbs./ft.sup.2 (25.3
kgf/m.sup.2) Rectifier Rating 933 A @ 54 VDC Rectifier Rating 410 A
@ 54 VDC Rectifier Size Width: 72 in (183 cm) Rectifier Size Width:
24 in (61 cm) Depth: 21 in (53 cm) Depth: 36 in (91 cm) Height: 72
in (183 cm) Height: 72 in (183 cm) Rectifier 10.5 ft.sup.2 (0.98
m.sup.2) Rectifier 6.0 ft.sup.2 (0.56 m.sup.2) Footprint Footprint
Front Clearance Width: 237 in (602 cm) Front Clearance Width: 72 in
(183 cm) Depth: 36 in (91 cm) Depth: 36 in (91 cm) Clearance 59.3
ft.sup.2 (5.5 m.sup.2) Clearance 18.0 ft.sup.2 (1.7 m.sup.2)
Footprint Footprint Total Footprint 93.9 ft.sup.2 (8.7 m.sup.2)
Total Footprint 36 ft.sup.2 (3.3 m.sup.2)
[0134] As previously mentioned, there are environmental concerns
relating to current VRLA battery-based power supplies.
Environmental regulations relating to the storage and operation of
the batteries increases the cost of the power supply. Furthermore,
the risk of liability for hazardous/toxic site clean-up in the
event of an accidental discharge of battery components is
significant.
[0135] As shown in Tables 1 and 2, even embodiments of the present
power plant incorporating VRLA batteries significantly reduce the
costs and potential environmental liability associated with current
VRLA battery-based power supplies. For example, the power supply of
FIG. 27 requires 95.4% less batteries than the conventional system
of FIG. 26. Between the power supplies of FIGS. 28 and 29, there is
a 97.4% reduction in the number of VRLA batteries employed. Of
course, in other embodiments of the present power supply, VRLA
batteries may be eliminated entirely.
[0136] Tables 1 and 2 also show the reduction in weight and
footprint of the present power supply compared to conventional VRLA
battery-based power supplies. The embodiments of FIGS. 27 and 29
represent an area savings of 19.9 ft.sup.2 (1.9 m.sup.2) and 57.9
ft.sup.2 (5.4 m.sup.2), respectively, compared to the conventional
systems. For many point-of-presence applications, where the costs
of housing the power supply can reach or exceed $US
650.00/ft.sup.2, the smaller footprint of the present power plant
alone can provide significant cost savings. Additional cost savings
may be realized due to the smaller size and footprint of the
rectifier required for the embodiments of the present power plant
compared to the conventional power supplies.
[0137] The data in Tables 1 and 2 does not take into account the
dimensions and footprint of the hydrogen storage associated with
the power plants of FIGS. 27 and 29. This is because the hydrogen
storage does not have to be situated with the rest of the power
plant. In conventional power supplies, the batteries are both the
energy storage device and the electrical power source. As a
practical matter, the battery banks must be situated close to the
power distribution panel and/or load, since power losses in DC
systems increase dramatically With the distance from the power
source. In the present power plant, the energy storage device is
decoupled from the electrical power source; hence, the hydrogen
storage equipment may be placed any desired distance from the fuel
cell array. Thus, it is not necessary to store the hydrogen storage
equipment indoors with the rest of the present power plant and,
therefore, to include it in the footprint analysis.
[0138] The decoupling of energy storage and power supply in the
present power plant may provide significant advantages over current
battery systems. Hydrogen storage equipment could be placed
outside, in an out-building or in an underground facility, for
example. In certain telecom applications, for example,
communications equipment is often situated on the roof of a
building. Most building codes will not permit a VRLA battery bank
to be installed on the roof, so the back-up power supply must be
installed some distance from the equipment. As mentioned earlier,
this arrangement can result in significant power losses in
providing power from the batteries to the load(s). With the present
power plant, on the other hand, the fuel cell array may be
installed on the roof, because of its lower floor loading, and
hydrogen could be supplied from hydrogen storage equipment located
any distance from the equipment, without an attendant power
loss.
[0139] Furthermore, a low-cost underground facility could be used
for the hydrogen storage equipment. Indeed, it may be possible to
simply bury hydrogen tanks, for example, near the facility housing
the fuel cell array: this is because hydrogen does not contaminate
groundwater, but will percolate out of the soil in case of a leak.
The fact that hydrogen does not pollute groundwater means that
expensive containment vessels, such as required for diesel or other
fuel tanks, are not required. Thus, hydrogen storage may be less
costly and more environmentally friendly than other options.
[0140] In fact, conventional power supplies, such as shown in FIGS.
26 or 28, could be upgraded to an embodiment of the present power
supply by replacing the battery bank with a suitably sized fuel
cell array and hydrogen storage. Existing rectifiers employed for
recharging the batteries could be eliminated or used to increase
the output of the power supply. Take, for example, the conventional
400 A 4 hour back-up power supply of FIG. 26. Because the fuel cell
array 468 of the present power plant does not require rectifier 532
for recharging, two rectifiers 504 may be employed to supply load
power, if desired. Thus, the power supply of FIG. 26 could be
upgraded to a 800 A power supply. Furthermore, by suitably
selecting the capacity of the hydrogen storage, the upgraded system
could supply back-up power for 4-8 hours, or more. At the same
time, the upgraded system would have a smaller footprint.
[0141] While the illustrated embodiments are described as -48 VDC
systems, the present power plant is not limited to such systems.
For example, the present power plant may be configured like array
468 of FIG. 23 in order to supply DC power at 24, 48 and 120 V, or
otherwise configured to supply DC power of any desired voltage(s).
Similarly, the present power plant may be configured to provide AC
power. For example, by substituting rectifier 504 in FIG. 27 or 29
with an inverter, the illustrated embodiments would be suitable for
use as an AC power supply system.
Conclusion
[0142] The disclosed embodiments provide a "building block" or
"component" approach to the manufacture of power supply systems,
allowing a manufacturer to produce a large variety of power supply
systems from a few, or even only one, basic type of fuel cell
system 10. This approach may lower design, manufacturer and
inventory costs, as well as providing redundancy to extend the mean
time between failures for the resulting end user product (i.e., the
power system). This approach may also simplify and reduce the cost
of maintenance or repair.
[0143] Although specific embodiments of, and examples for, the
power supply 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 10 including other types of fuel cell stacks 14 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 electrical power
storage device, such as cells of the battery, flywheel, or
ultracapacitor bank 24. The fuel cell system 10 can employ various
other approaches and elements for adjusting reactant partial
pressures, or may operate without regard to partial pressure. The
various embodiments described above can be combined to provide
further embodiments.
[0144] Generally, the present power plant may be employed in a
back-up power or UPS system for a range of applications, including,
but not limited to: [0145] 1. Network server farms: LAN/WAN
equipment such as hubs and routers. [0146] 2. Communications: CATV,
radio, telecommunications storage systems and/or servers, wireless
base stations, microwave repeater stations, radar tracking systems.
[0147] 3. Computer rooms: small and mid-range servers, large
enterprise servers, data storage systems, network computer
clusters, internet data centers. [0148] 4. Desktop/Workstations:
stand-alone PCs, workstations and computer peripherals. [0149] 5.
Industrial/Commercial: process control equipment, medical
equipment, laboratory instrumentation, traffic management systems,
security equipment, point of sale equipment.
[0150] In addition, the present power supply may also be used in
peak power or distributed power applications.
[0151] The present power plant and operating method provide for a
system that is smaller and lighter than conventional power supply
systems employing VRLA batteries. The present power plant also
provides for "instant on" operation with individual fuel cell
systems that are "hot swappable".
[0152] All of the U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification and/or listed in the Application Data Sheet,
including but not limited to U.S. patent application Ser. No.
09/916,240, filed Jul. 25, 2001, now U.S. Pat. No. 6,887,606,
issued May 3, 2005; U.S. patent application Ser. No. 10/017,470,
filed Dec. 14, 2001, now U.S. Pat. No. 6,841,275, issued Jan. 11,
2005; U.S. patent application Ser. No. 10/017,462, filed Dec. 14,
2001, now U.S. 2003/0113599, published Jun. 19, 2003; U.S. patent
application Ser. No. 10/017,461, filed Dec. 14, 2001, now U.S. Pat.
No. 6,573,682, issued Jun. 3, 2003; U.S. patent application Ser.
No. 10/388,191, filed Mar. 12, 2003, now U.S. Pat. No. 7,011,902,
issued Mar. 14, 2006; U.S. patent application Ser. No. 10/440,034,
filed May 16, 2003, now U.S. 2004/0009380, published Jan. 15, 2004;
U.S. patent application Ser. No. 10/440,451, filed May 16, 2003,
now 2004/0229095, published Nov. 18, 2004; U.S. patent application
Ser. No. 10/440,025, filed May 16, 2003, now U.S. 2004/0126635,
published Jul. 1, 2004; U.S. patent application Ser. No.
10/440,512, filed May 16, 2003, now U.S. Pat. No. 6,838,923, issued
Jan. 4, 2005; and U.S. Provisional Patent Application Ser. No.
60/436,759, filed Dec. 27, 2002; 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.
[0153] 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 claimed, 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.
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