U.S. patent application number 11/214577 was filed with the patent office on 2006-01-05 for method of operating a fuel cell system with integrated feedback control.
This patent application is currently assigned to Ener1, Inc.. Invention is credited to Victor Gurin, Peter Novak.
Application Number | 20060003201 11/214577 |
Document ID | / |
Family ID | 34739063 |
Filed Date | 2006-01-05 |
United States Patent
Application |
20060003201 |
Kind Code |
A1 |
Gurin; Victor ; et
al. |
January 5, 2006 |
Method of operating a fuel cell system with integrated feedback
control
Abstract
A recirculating reagent fuel-cell includes an ion-exchange
membrane interposed between an anode and cathode anode to form a
membrane/electrode assembly (MEA), the MEA interposed between a
fuel gas diffusion layer and an air (oxidant gas) diffusion layer.
An air and fuel flow network are provided having an input portion
for supplying reagent and an output portion for removing reagent
after electrochemical reaction. At least one of the air flow
network and fuel flow network includes a recirculation loop, the
recirculation loop feeding back a portion of the fuel or air after
electrochemical reaction to their respective input portion. The air
flow network can include a water vapor condenser where water formed
on the cathodes in proportion to the external load on the fuel cell
stack is extracted and the fuel flow network can include an
evaporator, where water is fed to the evaporator in the fuel feed
loop from the condenser of the air feed loop.
Inventors: |
Gurin; Victor; (North Miami
Beach, FL) ; Novak; Peter; (Fort Lauderdale,
FL) |
Correspondence
Address: |
LERNER AND GREENBERG, PA
P O BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Assignee: |
Ener1, Inc.
|
Family ID: |
34739063 |
Appl. No.: |
11/214577 |
Filed: |
August 30, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10746405 |
Dec 24, 2003 |
|
|
|
11214577 |
Aug 30, 2005 |
|
|
|
60519184 |
Nov 12, 2003 |
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Current U.S.
Class: |
429/414 ;
429/415; 429/444; 429/483; 429/492 |
Current CPC
Class: |
H01M 8/1007 20160201;
H01M 8/04097 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/017 ;
429/025; 429/030 |
International
Class: |
H01M 8/04 20060101
H01M008/04; H01M 8/10 20060101 H01M008/10 |
Claims
1. A method of operating a PEM fuel cell, which comprises the
following steps: providing a fuel flow to an anode side of the fuel
cell; providing an air flow to a cathode side of the fuel cell;
recirculating a portion of the air flow, after reaction thereof at
the anode side, from an output to an input of the cathode side; and
selectively pressurizing the fuel flow and the air flow, with a
length of a pulse period and a duty cycle of increased pressure
within the pulse period adjusted to an instantaneous power
requirement of the fuel cell.
2. The method according to claim 1, which comprises recirculating a
portion of the fuel flow, after incomplete reaction at the cathode
side.
3. The method according to claim 2, which further comprises
transferring water generated at the cathode side into the
recirculated portion of the fuel flow to humidify the fuel
flow.
4. The method according to claim 1, which comprises setting the
fuel flow and the air flow as a time-varying mass flow, the mass
flow varying with a load on the fuel cell.
5. The method according to claim 4, wherein the time-varying mass
flow is operative across all loads on the fuel cell.
6. The method according to claim 4, wherein the time-varying mass
flow comprises discrete pulses.
7. The method according to claim 4, which comprises
time-synchronizing the mass flow of the fuel flow with the mass
flow of the air flow.
8. The method according to claim 1, which comprises providing the
air flow with a jet pump, and inducing recirculation in the
recirculation loop with the jet pump.
9. The method according to claim 8, which comprises feeding
recirculated air from the output on the anode side to a suction
input of the jet pump, mixing the recirculated air flow portion
with a fresh air flow portion in the jet pump, and feeding the
mixed air flow to the anode side of the fuel cell.
10. The method according to claim 1, which comprises inducing
pressure variations with a pressure sensor-controlled two-position
pressure regulator having a first, fully open position and a
second, fully closed position.
11. The method according to claim 10, wherein the pressure
regulator is a directly controlled by hydrogen consumption
two-positional pressure regulator in the fuel feed network, and a
slave pressure regulator connected in the air feed network and
controlled by the pressure regulator in the fuel feed network.
12. A method of operating a PEM fuel cell system, which comprises:
providing a membrane/electrode assembly (MEA) including a proton
exchange membrane (polymer electrolyte membrane, PEM) between an
anode chamber with an anode and a cathode chamber with a cathode;
supplying fuel to the anode chamber through a hydrogen supply
network connected to supply hydrogen fuel to the anode; varying a
pressure in a feed portion of the hydrogen supply network, under
control of a fuel pressure regulator, with a duration of a pressure
cycle and a duration of a pressure pulse within the cycle adjusted
in dependence on a magnitude of a fuel cell output requirement;
supplying air to the cathode chamber through an air supply network
connected to supply air to the cathode; varying a pressure in a
feed portion of the air supply network, under control of an air
pressure regulator, and synchronizing the air pressure regulator
with the fuel pressure regulator.
13. The method according to claim 12, which comprises measuring a
pressure in the hydrogen supply network in a master measuring
chamber of a hydrogen supply pressure regulator, communicating via
a feedback line in the hydrogen recirculation loop, and slaving an
air supply regulator to the hydrogen supply pressure regulator, for
synchronizing the pressure cycles and pulses at the anode with the
pressure cycles and pulses at the cathode.
14. The method according to claim 12, which comprises: pumping the
fuel in the hydrogen supply network with a fuel jet pump having an
inducing nozzle and a suction input communicating with an anode
output of the anode chamber; varying the pressure in the feed
portion of the hydrogen supply network with a two-position
pulse-generating hydrogen supply pressure regulator having a
hydrogen input and a hydrogen output communicating with the
inducing nozzle of the fuel jet pump; selectively setting the
regulator to a first, at least substantially closed position and a
second, at least substantially open position for feeding hydrogen
to an input of the anode chamber with pulse-fluctuating pressure;
and pumping the air with an air jet pump having an input receiving
air from an air supply and a suction input communicating with a
cathode output of the cathode chamber; and setting a pressure in
the air supply network with a differential air supply regulator
having an input area communicating with the air supply and an
output area communicating with an inducing nozzle of the air jet
pump.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuing application of copending application
Ser. No. 10/746,405, filed Dec. 24, 2003, which claimed the
benefit, under 35 U.S.C. 119(e), of provisional application No.
60/519,184, filed Nov. 12, 2003; the prior applications are
herewith incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to fuel cell assemblies and, more
particularly to fuel cells having integrated feedback for
regulation of water as well as fuel and oxidant supplied
thereto.
[0004] Fuel cells hold great promise for commercial use in mobile
and stationary power supply systems. Fuel cells electrochemically
convert fuels and oxidants to electricity. Fuel cell types include
Alkaline Fuel Cells (AFC), Molten Carbonate Fuel Cells (MCFC),
Phosphoric Acid Fuel Cells (PAFC), Proton Exchange Membrane Fuel
Cells (PEMFC or PEM), Solid Oxide Fuel Cells (SOFC) and Direct
Methanol Fuel Cells.
[0005] There has been significant progress in the development of
fuel cells, including improvements in specific characteristics,
such as increased power density and increased efficiency.
Nonetheless, the wide variations in load demand encountered in most
commercial applications remain a problem for fuel cell based
electrochemical generators, particularly for those that use solid
polymer electrolytes, such as PEMs.
[0006] A PEM fuel cell converts the chemical energy of fuels such
as hydrogen and an oxygen-containing gas (e.g. air) directly into
electrical energy, water and heat. At the heart of a PEM fuel cell
is a membrane electrode assembly (MEA) comprised of a proton
conducting membrane electrolyte sandwiched between two gas
diffusion electrodes. The membrane permits the passage of protons
(H+) generated by the oxidation of hydrogen gas at the anode to
reach the cathode side of the fuel cell and form water, while
preventing passage therethrough of either of the reactant
gases.
[0007] Efficient operation of PEM fuel cells generally requires the
removal of a portion of the water produced. Excess water can feel
up the pores of the gas diffusion layers effectively cutting of the
gases from membrane and stopping the chemical reaction. Load
demands faced by a system in a typical commercial use might vary
from 0 to 1000 mA/cm.sup.2 under a typical load cycle.
[0008] For the optimum operation of such fuel cells, the membrane
should remain sufficiently moist throughout, but not too moist.
Thus, there must be removal of a portion of the water generated at
the cathode, as well as the addition of water at the anode side to
provide sufficient membrane moistness.
[0009] Several characteristics of PEM fuel cells separate them from
other types of fuel cells. For example, in contrast to other fuel
cell types, PEM fuel cells have a narrow range for controlling
optimal concentration of electrolyte in the localized zone of
electrochemical activity comprising the anode, membrane and
cathode. Such membranes have a limited ability for redistribution
of water over the fuel cell working surface area. This performance
characteristic of fuel cells with PEMs is attributed to the reduced
ability of the anode, cathode and membrane (as a group) to
transport water, and to the hydrophobic characteristics of the
materials used.
[0010] These characteristics of solid-polymer membranes become
critical when designing and using fuel cells with large working
surface areas to produce large currents, such as required for
transportation applications (e.g. automobiles, and busses)
especially when a large number of fuel cells are combined in series
to generate high voltage outputs. For example, to build an
electrochemical generator having a capacity of 25 kW at a voltage
of 120 V, a stack comprising 160 fuel cells is required with a
working surface area of approximately 600 cm.sup.2 each. In a
generator with a power rating of 60 kW and a 330 V output, it is
necessary to install 420 fuel cell elements with working surface
area of 740 cm.sup.2 each, connected in series.
[0011] Maintaining the high output characteristics of fuel cells
assembled into stacks to form electrochemical generators is one of
the challenges of electrochemical generator design. In the case of
fuel cells with solid-polymer membranes this task is even more
difficult. The very narrow range over which water concentration
must be controlled imposes strict requirements on the systems that
feed the working gases, as well as on regulation of water
concentration and temperature of each individual fuel cell. In
addition, even at low operating times (1000-2000 hrs),
characteristics of the individual fuel cells in a stack do not
change in a constant or even manner. Progressive and uneven
degradation in performance among the cells demands even more strict
requirements for control of fuel cells assembled into
electrochemical generator systems.
[0012] In high power hydrogen-air electrochemical generators,
hydrogen is supplied from storage tanks with high pressures up to
70 MPa. Systems for supplying gas usually have electric valves on
hydrogen supply and purge lines. A hydrogen pressure regulator is
commonly installed in the gas supply line upstream of the fuel cell
stack. A feedback control pressure regulator is generally provided
which senses variation in pressure at the fuel cell and control
reactants gas flow in a manner proportional to gas usage. Control
of gas flow and pressure (i.e. reduction of pressure from input
pressure to working pressure) is also accomplished using a
regulator.
[0013] For smoother and more precise throttle control, a two-stage
pressure regulator system is usually installed. The pressure
regulator reduces the working pressure of the fuel cell. For
synchronization of hydrogen and air pressures in the fuel cell
stack, a pressure reference line is installed in parallel to
hydrogen supply line to provide a reference pressure to the
regulator.
[0014] This reference line is static and does not consume hydrogen
during fuel cell operation. It is filled with hydrogen during
start-up and emptied (purged) when the fuel cell generator is
stopped or stored. As a rule, a vent valve is installed in the
reference line to restrict pressure, and an electrical valve is
installed for reduction of pressure to atmospheric pressure.
[0015] The reference line can be filled with inert gas, if
available. The oxidant feed line to the cathode pores in the fuel
cell stack has a filter to remove particles and a compressor to
built up air pressure to a working level. The partial pressure of
oxygen in air is relatively low (about 21%), the largest portion of
air being nitrogen. For the cathode to work effectively, air should
be fed in excess. In this case, the efficiency of oxygen usage is
40%-60% as a rule. At higher rates of oxygen usage, the cathode is
less efficient.
[0016] In current fuel cell stack designs, the air supply system
maintains the design working pressure level on cathode and anode.
For this purpose, the hydrogen pressure regulator has a feedback
connection to the air supply line at the entry point to the fuel
cell. In this case the hydrogen pressure in the anode chamber is
constantly compared with the air pressure in the cathode chamber
and the pressure regulator makes needed adjustments in order to
maintain the correct pressure ratio.
[0017] The system described above for supplying hydrogen and air to
fuel cells with solid-polymer electrolytes is essentially universal
and used in almost all known designs with only minor variations.
However, as explained below, these systems do not provide good
regulation of the water concentration along the cathode and anode
surface of the fuel cell stack, particularly for high and highly
variable load conditions.
[0018] The power output of a hydrogen-air fuel cell mainly depends
on effective performance of the cathodes (oxygen limited
electrodes).
[0019] In this case, there are gas transport restrictions on the
amount of oxygen penetrating through the cathode pores and
available to the cathodes. Drying takes place in some areas of the
cathodes because of low water (vapor) concentration in the air
supplied by the compressor.
[0020] Moreover, compressed feed air at the outlet of the
compressor can be an even higher temperatures (e.g. 110-150.degree.
C.). Thus, there is active removal of water (vapor) by the airflow
which, in turn, leads to drying of the membrane in the air inlet
region. In the air outlet area from the cathodes there occurs the
reverse of this process leading to "flooding" of the cathode
because air flowing in this area is close to saturation by water
vapors and the rate of water uptake (vaporization) is lower.
[0021] Because of low oxygen concentrations in the air after
passing through, most of the cathode chamber and gas flow
restrictions, a large portion of the cathode surface can be in a
condition of "concentration polarization." Concentration
polarization results from restrictions to the transport of the
reactant gases to the reaction sites. This usually occurs at high
current because the forming of product water and excess
humidification blocks the reaction sites. In this situation, there
is increased risk of cross polarization in the area near the gas
outlet from the cathode chamber. This risk becomes much greater
when the fuel cell load is highly variable over short time periods.
Specifically, the risk is greatest when loads are switched from low
to high levels and back in short periods of time, such as tens of
seconds to minutes.
[0022] Such short-term load variations are generally not allowed in
fuel cell operation. Otherwise, non-optimum humidity can lead to
cross polarization. This can cause the cells to operate in an
electrolysis mode, which in turn can lead to direct reaction
between hydrogen and air in the cell resulting in physical damage
to the fuel cell.
[0023] Solving the problem of controlling in fuel cells will
greatly expand potential of their application. However, this does
not solve the problem of the fuel cell's inability to withstand
wide range, short-term variations in load because of high thermal
inertia due to the heat capacity of the fuel cell stack. The
primary unmet requirement for use of hydrogen-air fuel cells in
transportation and many stationary power applications is that fuel
cell generators must be highly reliable in the face of rapid and
wide-range variations in load.
[0024] The above-mentioned issues represent a significant problem
for electrochemical generators with solid polymer fuel cells as
presently installed on electric vehicle prototypes. Currently
available electrochemical generators do not meet consumer
requirements in this regard, and therefore cannot be mass-produced
and marketed for general use. This is not only because of the high
cost and complexity of systems for controlling processes in fuel
cells, but also because a primary application requirement cannot be
met. This requirement is the ability to handle current loads that
vary widely, and sometimes rapidly, for long-term operation (e.g.
more than about 3000 hrs.).
SUMMARY OF THE INVENTION
[0025] It is accordingly an object of the invention to provide a
fuel cell system and method which overcomes the above-mentioned
disadvantages of the heretofore-known devices and methods of this
general type.
[0026] With the foregoing and other objects in view there is
provided, in accordance with the invention, a method of operating a
PEM fuel cell, which comprises the following steps: [0027]
providing a fuel flow to an anode side of the fuel cell; [0028]
providing an air flow to a cathode side of the fuel cell; [0029]
recirculating a portion of the air flow, after reaction thereof at
the anode side, from an output to an input of the cathode side; and
[0030] selectively pressurizing the fuel flow and the air flow,
with a length of a pulse period and a duty cycle of increased
pressure within the pulse period adjusted to an instantaneous power
requirement of the fuel cell.
[0031] A recirculating reagent fuel-cell includes an ion-exchange
membrane interposed between an anode and cathode to form a
membrane/electrode assembly (MEA), the MEA interposed between a
fuel gas diffusion layer and an oxidant gas diffusion layer. An
oxidant and fuel flow network are provided having an input portion
for supplying reagent and an output portion for removing reagent
and reaction products after the electrochemical reaction. At least
one of the oxidant flow network and fuel flow network includes a
recirculation loop formed by a feedback conduit which provides
fluid connection between the input and output portion. The
recirculation loop feeds back a portion of the fuel or oxidant
after electrochemical reaction to their respective input
portion.
[0032] The recirculation loop can include a water containing
volume, wherein a portion of the output flow flows through the
water containing volume to generate a humidified flow, the
humidified flow comprising a portion of the oxidant or the fuel
flow supplied to the fuel cell. The volume of the humidified flow
can be adjustable, with the humidified flow volume increasing when
a load on the fuel cell increases.
[0033] At least one of the oxidant and fuel input portions can
include a jet pump therein, where the jet pump induces
recirculation in the recirculation loop. The output flow of the
feedback conduit is preferably used as an input flow to the suction
input of the jet pump. In this embodiment, the jet pump mixes the
portion of the fuel or oxidant flow fed back with externally
supplied fuel or oxidant.
[0034] The water containing volume in the oxidant flow network can
be a condenser for extracting water from the cathode, while the
water containing volume in the fuel flow network can be an
evaporator. In this embodiment, the condenser extracts water from
the cathode in the amount depending on a load on the fuel cell. The
condenser is preferably fluidly connected to the evaporator, with
the condenser supplying the fuel flow network with water.
[0035] The fuel cell can include a fuel flow modulator fluidically
connected with at least one of an input portion of the fuel flow
network and an input portion of the oxidant flow network, wherein
the fuel flow modulator provides a time varying mass flow of fuel
and oxidant. The modulator preferably includes structure for
initiating operation across all fuel cell load conditions. The fuel
flow network can include a fuel flow modulator and the oxidant flow
network can include an oxidant flow modulator, the first modulator
being communicably connected with second modulator and controlling
operations of second modulator. The flow modulator preferably
provides discrete pulses of fuel and oxidant flow, such as through
use in the fuel flow network of a pressure sensor-controlled
two-positional pressure regulator having only two positions, a
first position being a fully open and the other position being
fully closed and through use in oxidant flow network of a
pressure-sensing-two-chambers controlled differential pressure
regulator.
[0036] A method of operating a fuel cell includes the steps of
providing a fuel flow to an anode of the fuel cell and an oxidant
flow to a cathode of the fuel cell, wherein at least one of the
fuel flow and the oxidant flow comprises a recirculated flow
portion. The recirculated flow portion can be a humidified flow.
The fuel flow and the oxidant flow can include a recirculated flow
portion, wherein the method can include the step of transferring
water generated at the cathode into the fuel recirculated portion
to humidify the fuel flow.
[0037] At least one of the fuel flow and the oxidant flow can be a
time varying mass flow, the mass flow varying with a load on the
fuel cell. The time varying mass flow is preferably operative
across all loads on the fuel cell and can comprise discrete
pressure pulses. In a preferred embodiment, both the fuel flow and
the oxidant flow are time varying mass flows, wherein the method
can further comprise the step of synchronizing the time varying
mass flow of the fuel flow with the time varying mass flow of the
oxidant flow.
[0038] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0039] The invention is not intended to be limited to the details
shown, since various modifications and structural changes may be
made therein without departing from the spirit of the invention and
within the scope and range of equivalents of the claims.
[0040] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a schematic of a recirculating reagent fuel cell
system having recirculation loops in both the anode and cathode
side, according to an embodiment of the invention;
[0042] FIG. 2 shows the various components of an exemplary jet
pump;
[0043] FIG. 3 is a schematic model showing elements of an exemplary
regulated gas supply system comprising a closed vessel with
variable gas inflow, consumption and outlet flow;
[0044] FIGS. 4A, 4B, and 4C show examples of gas supply periods,
pauses and cycles of an aperiodic load based reagent flow supply
arrangement under relatively high, intermediate and low external
load conditions, respectively, according to a preferred embodiment
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0045] The invention is an electrochemical generator based on fuel
cells, such as hydrogen-air fuel cells with solid polymer proton
exchange membranes (PEM) that can be used in mobile or stationary
applications. Generators based on the invention provide higher
reliability and higher efficiency as compared to conventional fuel
cells, particularly under rapid and widely varying power demands,
such as those encountered for typical automotive applications.
[0046] A recirculating reagent fuel-cell includes an ion-exchange
membrane interposed between an anode and cathode anode to form a
membrane/electrode assembly (MEA), the MEA interposed between a
fuel gas diffusion layer and an oxidant gas diffusion layer. An
oxidant and fuel flow network are provided having an input portion
for supplying reagent and an output portion for removing excess
reagent and reaction byproducts after electrochemical reaction. At
least one of the oxidant flow network and fuel flow network
includes a feedback conduit to form a recirculation loop, the
recirculation loop feeding back a portion of the fuel and/or
oxidant after electrochemical reaction to their respective input
portion.
[0047] The oxidant flow loop can include a water vapor condenser to
extract water from the cathode chambers, the amount of water being
based on the external load on the fuel cell stack. The fuel flow
network can include an evaporator, where water is fed to the
evaporator in the fuel loop from the condenser in the oxidant loop.
In this embodiment, the portion of the output flow fed back to the
input portion is a humidified flow.
[0048] The invention provides humidification and resulting membrane
wetness which is based on the fuel cell load. If the load
increases, the fuel cell generates more water, thus more water is
collected in the condenser. Since the output flow portion flowing
through the condenser increases as the load increases, the
humidified flow output by the condenser increases as well based on
the level of the load.
[0049] Although the invention is generally described with respect
to a hydrogen-air electrochemical generator, the invention is in no
way limited to either hydrogen or air. For example, the fuel can
generally-be any oxidizable gas, including mixtures thereof, while
air can more generally be any oxidant gas. Moreover, recirculating
reagent gas flow arrangements according to the invention described
herein can be advantageously used with other types of fuel cells,
particularly for membrane-based fuel cells. In addition, the
aperiodic load based reagent flow supply feature described herein
can be generally used with all fuels cell types, whether membrane
based or not, and more generally, for chemically reactive
systems.
[0050] Referring now to the figures of the drawing in detail and
first, particularly, to FIG. 1 thereof, there is shown a schematic
of a recirculating reagent fuel cell system 100 according to an
embodiment of the invention is shown. System 100 includes fuel cell
5, which includes ion-exchange membrane 29 interposed between an
anode 31 and cathode 27 to form a membrane/electrode assembly
(MEA). The MEA is interposed between porous oxidant gas diffusion
layer 26 and porous fuel diffusion layer 32. Cathode chamber 28 is
bounded by plate 24 which is disposed adjacent to oxidant gas
diffusion layer 26, while anode chamber 38 bounded by flow plate 34
is disposed adjacent to fuel diffusion layer 32. The respective
porous gas diffusion layer/electrode structures typically comprise
a Pt electrocatalyst dispersed on high surface area carbon black,
held together with a binding agents, such as polytetrafluoroethyene
(PTFE, Teflon.RTM.). In most practical electrical chemical
generator applications, system 100 comprises a plurality of fuel
cells 5 hooked in series to form a fuel cell stack. The fuel cell 5
arrangement described herein is not an aspect of the invention.
[0051] The reagent recirculation and control arrangement shown in
both the cathode side 1 and anode side 2 are aspects of the
invention. Cathode side 1 is provided an air supply, preferably
cleaned of particles by suitable filtration, which is fed into a
compressor 10, which provides the necessary flow and pressure of
oxidant (e.g. air) for cathode side 1 of fuel cell 5 to support the
electrochemical reaction. Both an electric motor 12 and an expander
11 are preferably used to drive compressor 10. Expander 11 utilizes
energy from a hot pressurized oxidant output flow after
electrochemical reaction.
[0052] Compressor 10 is in fluid communication with pressure
regulator 25 via line 48. The regulator 25 is preferably of the
type "pressure-sensing-two-chambers-controlled differential
pressure regulator". This preferred type of regulator provides
discrete pressure pulses of gas flow where a timing of these pulses
is synchronized with pulse timing of the regulator 75 and flow
volume through this regulator depends on the external load and the
gas consumption of the electrochemical reaction, which is generally
variable over time, and may be highly variable. Regulator 25 senses
pressure in the output portion of the oxidant flow network and is
communicably connected to regulator 75 on the anode side 2.
[0053] When the fuel cell 5 is operating in an idling mode, with
external loads disconnected, compressor 10 in the cathode side 1
and the compressor (if present) in the anode side 2 is preferably
left running. This condition allows fast re-connection to external
load, because when fuel cells are operated at the lower loads, the
process of hydrogen and oxygen supply does not stop and can be
rapidly increased as needed after re-connecting the external
load.
[0054] To increase the supply of oxidant gas to the cathode side of
fuel cell 5 without the need for additional air intake into system
100, and for extraction of water and depleted oxidant, an oxidant
recirculation feedback loop 15 is provided. Recirculation loop 15
comprises pump 50 which is used to induce oxidant flow through
cathode chamber 28, flow splitter 20, and water vapor condenser 30
and associated connecting lines. In the arrangement shown in FIG.
1, condenser 30 along with its associated lines provides the
feedback conduit between input portion (at pump 50) and the output
portion (at flow splitter 20) of recirculation loop 15. Although
shown in the feedback conduit in FIG. 1, condenser 30 can be
disposed between cathode chamber 28 and flow splitter 20.
[0055] After passing regulator 25, pressurized oxidant comprising
gas is fed into the inducing nozzle 51 of pump 50 at a typical
pressure of 0.2-0.45 MPa. Gas pump 50 is preferably a jet pump. For
recirculation of both fuel in anode side 2 and oxidant in cathode
side 1, jet pumps are preferred because they provide substantially
proportional relation between consumption of recirculation streams
and used gases in the fuel cells during the current production.
Additional positive characteristics of such pumps as compared to
electromechanical pumps include high reliability, and essentially
unlimited time in operation with no need for electrical energy use.
Jet pump 50 can be driven entirely by potential energy of the
compressed oxidant (e.g. stored in reagent tanks). Although jet
pumps are preferred, other pump types may be used with the
invention.
[0056] Now referring to FIG. 2, jet pump 50 is shown including
various components designed to control pressure/flow
characteristics. These include the high-speed gas ejection nozzle
51, a stream mixing chamber 52 with diffuser 53 and a receiving
chamber 54 for further gas mixing.
[0057] Gas passing through nozzle 51 forms a high-velocity stream
in the receiving chamber 54. This high-speed stream generates a
lower pressure region at its boundary (according to the Bernoulli
principle) and thereby sucks in gas from receiving chamber 54. The
two streams of air are directed into the mixing chamber 52 where
their speed is equalized due to the mixing. The mixed stream then
passes through a diffuser 53, where the stream is expanded, and the
static pressure increases.
[0058] The coefficient of injection characterizes the ratio between
the mass flow of moistened air fed to the receiving chamber 54 of
the jet pump 50 and the airflow from compressor 10 to nozzle 51.
The degree of compression of the mixed airflow output by pump 50
corresponds to aerodynamic resistance of the recirculation loop
15.
[0059] Throttling of the air stream occurs by passing the oxidant
stream through the valve nozzle 51 of jet pump 50. The pressure
regulator 25 then enables stabilizing amount of oxidant gas going
through the jet pump 50 in the face of arbitrary changes in oxidant
consumption in the fuel cell stack. The optimal upper and lower
levels of oxidant (e.g. air) pressure on the cathode can be
selected for each specific type of porous media.
[0060] Returning again to FIG. 1, after passing pump 50, the
oxidant flow is throttled and the pressure preferably drops to
between about 0.02-0.05 MPa according to the pressure in the
circuit. Heat generated by the fuel cell 100 is shown extracted by
an independent coolant loop designated as 61 in FIG. 1. A portion
of the oxidant, with depleted oxygen concentration after
electrochemical reaction, is directed from an output portion of the
recirculation loop 15 into a flow splitter 20, such as a bleed air
tee. Flow splitter 20 directs a specific portion or amount of bleed
oxidant following electrochemical reaction to expander 11 to use
the energy of this flow to help drive the compressor 10 along with
main drive motor 12, with the remaining depleted oxygen flow going
to condenser 30. Following energy extraction at expander 11, the
depleted oxygen flow can be exhausted to the atmosphere.
[0061] Now turning to anode side 2 of the system 100, anode side 2
provides fuel, such as hydrogen along with humidification to anode
31 of fuel cell 5. Anode side 2 is provided a suitable source of
hydrogen or other fuel, preferably being a filtered source, such as
from a pressure vessel. Hydrogen supplied first reaches solenoid
valve 74 and then pressure regulator 75. Regulator 75 is connected
by piping to a pump 55, such as a jet pump having nozzle 57, which
acts as to induce hydrogen flow in the closed recirculation loop
60. Hydrogen recirculation loop 60 includes pump 55, anode chamber
38, hydrogen evaporator/humidifier 80, and associated tubing. The
hydrogen recirculation loop 60 is a part of the fuel and water
vapor supply system for the anode 31.
[0062] According to a preferred embodiment of the invention, the
anode chamber 38 of fuel cell 30 has channels in the hydrogen feed
stream that direct the hydrogen flow in such way so as to
distribute it uniformly over the anode operating surface. Such
distribution is preferably optimized for different anode sizes and
geometrical forms.
[0063] As noted above, regulator 75 is communicably connected to
regulator 25 cathode side 1. The connection of regulators 75 and 25
can be preferably via a pneumatic line. The controlling set point
of the regulator 75 is used as a reference point for the regulator
25. Such a connection between fuel regulator 75 and air regulator
25 provides synchronization of their operation.
[0064] Two-sided and simultaneous (relative to the polymer membrane
29 in fuel cell 5) control of pressure on anode 31 and cathode 27
is important in the operation of the anode 31, membrane 29, and
cathode 27 as a group. This arrangement improves the dynamic
performance of fuel cell 5 during load variations and also
decreases the degradation rate of volt-ampere characteristics of
the fuel cell stack, due to the active anode and cathode
ventilation to remove inert and contaminating gases and provide for
more uniform distribution of water.
[0065] Pump 55 is shown as a jet pump as well as a pump 50
described with respect to cathode side 1, while regulator 75 is
preferably the "pressure sensor controlled two-positional pressure
regulator" type. Jet pump 55 receives hydrogen supplied via
regulator 75 (when open) which is provided to nozzle 57. Pump 55
mixes hydrogen supplied by regulator 75 (when open) with
recirculated humidified hydrogen flow provided by evaporator 80.
The mixed hydrogen stream emerges from pump 55 and reaches anode 31
of fuel cell 5. Regulator 75 preferably senses pressure along an
output portion 84 of the fuel recirculation loop 60.
[0066] At the hydrogen flow outlet of the fuel cell 5 at T-point
84, a purge line for the anode chamber 38 is preferably connected
with a throttle 87 to restrict hydrogen flow when solenoid valve 88
is fully open.
[0067] FIG. 3 shows a schematic model of elements of an exemplary
regulated gas supply system comprising a closed vessel with
variable gas consumption outflow and compensating inlet flow. As
noted above, pressure regulator 75 is preferably of the type
"pressure sensor-controlled two-positional pressure regulator".
System 300 is a model for gas supply using such a regulator to a
fuel cell with variable consumption in response to the speed of the
electrochemical reaction.
[0068] A gas (pressurized air for example) from a source 310 is
modeled as having a mass flow which exceeds a mass flow of the
consumption. For example, the pressure provided P.sub.I=0.5 Mpa can
be introduced into the vessel 390 via pipe 330 which has a
two-position pressure regulator 391 including two solenoids, namely
399 to open and 398 to close. Assume that pressure in the vessel
390 is desired to be maintained at a stable level, such as
P.sub.work=0.3.+-.0.03 MPa.
[0069] A throttle 392 is installed between pressure regulator 391
and the vessel 390 for restriction of gas flow. Gas flows through
pipe 320 which has a throttle 393 to restrict exiting gas flow and
a regulated throttle 394, which reduces gas flow in pipe 320. On
vessel 390, pressure sensors 395 and 397 are installed with
different pressure regulating parameters to operate solenoids 399
and 398, respectively.
[0070] Design of the two-position pressure regulator 391 allows
only two extreme positions of the valve and saddle, "fully open"
and "fully closed." Any intermediate positions of the valve
relative to the saddle are not possible. Throttling of the gas
stream entering the vessel 390 occurs only at the throttle 392. Gas
can exit the vessel only through the pipe 320 at a variable flow
rate controlled by the flow area changing of the regulating
throttle 394. Maximum consumption of the effluent gas through the
pipe 320 is limited by the flow area of the unregulated throttle
393. It is assumed that the maximum gas inflow rate to the vessel
through the pipe 330 is 1.5 times the maximum gas consumption from
the vessel through the pipe 320.
[0071] The object of system 300 is to control of the pressure in
the vessel 390 under conditions of variable gas effluent rates from
the vessel. Two pressure sensors 395 and 397 are installed on the
vessel 390. If the pressure has dropped to the some pre-determined
level (for instance, P.sub.work=0.270 MPa) the first pressure
sensor 395 will command the pressure regulator 391 to open. If the
pressure has reached some pre-determined level (for instance,
P.sub.work=0.330 MPa) the second pressure sensor 397 will command
the pressure regulator 391 to close. As a result, system 300
delivers discrete pulses of gas at a constant pressure to vessel
390.
[0072] In conventional solutions to this problem, a "balanced-type"
pressure regulator controls the gas supply to the fuel cell. The
"balanced-type" pressure regulator in such a circuit has a
measuring space directly after the valve saddle and throttling of
the gas occurs in the gap between the valve and saddle. Such
regulators can replace both pressure sensors 395 and 397 and the
two-position regulator 391.
[0073] FIGS. 4A, 4B, and 4C show exemplary gas supply periods,
pauses and cycles of an aperiodic load based reagent flow supply
system under relatively high, intermediate and low external load
conditions, respectively, according to a preferred embodiment of
the invention for a fixed period of time, T.sub.I. Pop is the
operating pressure, Pmax is the maximum operating pressure, Pmin is
the minimum operating pressure, Pnom is the nominal operating
pressure, T is the time, M.sub.R is the mass circulation flow, Ts
is the hydrogen supply time, Tc is the cycle time and T.sub.p is
the pause time. To implement pauses and cycles of an aperiodic load
based reagent flow a relay-type of pressure regulator can be used.
This preferred regulator has two positions, fully open and fully
closed. In this preferred embodiment, a pressure-sensor controlled
two-positional pressure regulator 75, or arrangement which provides
equivalent flow dynamics responsive to system dynamics.
[0074] FIG. 4A shows the gas supply period, pauses and cycles under
relatively high load conditions. Under the high load conditions,
the cycle time (Tc) which comprises a supply time (Ts) plus the
pause time (Tp) provides a little over two (2) periods in the time
T.sub.I. The supply time (Ts) is nearly equal to the cycle time
(Tc). When the regulator is open the operating pressure (Pop) rises
as a function of time until the time when Pop reaches Pmax, then
the regulator shuts off. While the regulator is off, the operating
pressure decreases until P.sub.Min is reached, and the regulator is
turned on again. FIG. 4B shows the gas supply period, pauses and
cycles under moderate load conditions.
[0075] Compiling the data from FIGS. 4A-4C, the supply time Ts
increases as the load increases. In addition, the mass
recirculation flow M.sub.R increases with increasing load.
[0076] Thus, the preferred pressure-sensor controlled
two-positional pressure regulator" can be characterized as a supply
of gas pressure pulsation and as a supply of a pulsation of
recirculating mass flow where the pulse dynamics change as a
function of load. A difference between the reactant flow
characteristics obtained using the preferred pressure regulator as
disclosed herein as compared to pulsed reactant systems such as
disclosed in U.S. Pat. No. 6,093,502 to Carlstrom, Jr. et al. is
the simultaneous variation of pulse width and pulse period to
extend depending on the external load and gas consumption rate of
the electrochemical reaction provided by the invention. In
addition, Carlstrom's pulsed system is only activated upon
detection of a predetermined high load level, while the pulsed gas
supply of the invention is preferably operable over all load
conditions.
[0077] Again returning to FIG. 1, assuming regulator 75 is the type
"pressure-sensor controlled two-positional solenoid valve," or a
device which provides an equivalent response, which turns on when
the pressure at 84 drops to P.sub.Min, and turns off when the
pressure at 84 reaches Pmax. When regulator 75 is fully open, gas
flows through, such as into the input portion of the recirculation
loop 60 through pump 55, thus raising the operating pressure in
loop 60. When regulator 75 is fully closed, thus pausing the gas
supply provided to loop 60, then pressure in the loop 60 begins
dropping until Pmin is reached, this pressure value is sensed, and
as a result regulator 75 again turns on and a new cycle is
initiated. In its fully closed position, the pressure upstream from
jet pump nozzle 57 is reduced synchronously with the pressure in
the recirculation loop 60, because gas volume between regulator 75
and nozzle 57 is much smaller then gas volume in the recirculation
loop 60 and these two volumes are interconnected. During opening of
the valve in regulator 75 the pressure downstream from it and
before jet pump nozzle 57 is rises rapidly to the regulator's inlet
pressure due to the discrete valve opening and difference (more
then about 10 times) between valve cross section flow versus nozzle
cross section.
[0078] When pressure in the recirculation loop 60 is increased then
Pmax is reached, sensed, and the valve of regulator 75 is also
closed rapidly. To minimize gas flow throttling on the pressure
regulator, its full-open cross section and jet pump nozzle cross
section should be calculated accordingly.
[0079] The invention provides numerous advantages over available
fuel cell systems. For example, advantages of the invention
include: [0080] Increased air feed rate along the cathode working
surface, due to the increasing amount of the air supplied to the
each point provided by recirculation loop 15. This results in
better control of oxidant feed by the air recirculation loop 15 to
the "tri-surface" cathode area. Increased speed leads to increased
active ventilation of cathode pores and surfaces and improved
oxygen supply to the operating cathodes. Implementation of the
oxidant supply design according to the present invention can
increase the rate of oxygen use by the cathode by a factor of 2.5
to 3.5. This increase is equivalent to the increasing the cathode
working pressure by about 1.6-1.9 times. [0081] More uniform water
distribution and efficient water removal from the cathode surface.
Improved humidification of air entering the cathode chamber 28
results in improved water concentration uniformity along the
cathode, especially at the gas inlet and outlet regions. This
advantage is primarily due to the mixing of the air mass flow at
higher temperature and lower humidity from the compressor with the
humidified air mass flow at lower temperature from the
recirculation loop, for example in the proportion of 1:3. [0082]
This advantage results in a significant reduction in the risk of
fire or explosion in the fuel cell due to the decrease in the risk
of "overdrying" at the inlet section of cathode. It should also be
noted that at a certain level of excess air pressure on the cathode
as compared with the hydrogen pressure on the anode can result in
air leaking onto the anode if the hermetic seal of the membrane is
not maintained. When this occurs, a catalytic interaction occurs
resulting in water formation. Such a situation does not increase
the risk of fire however. [0083] More effective water vapor supply
to the entire anode surface is provided. This advantage is due to
the continuous circulation of the humidified hydrogen through the
anode chambers. [0084] Reduced risk of membrane dehydration thus
increasing the electrochemical performance of the membrane assembly
is also provided. This advantage results because of the anode
and/or cathode active surface limitation. [0085] Pulsation of the
working (operating) pressure at the three-phase cathode interface
(gas, catalysts and electrolyte) is a significant advantage, since
active ventilation of the pores occurs and, as a result, nitrogen
(as a passive component of air) is rapidly removed from the active
surface of catalysts. Pressure pulsation in gas-transport pores of
the cathode results in a significant decrease of the "nitrogen
cover" effect. This effect occurs when nitrogen is pressed to the
catalysts surface by the air passing along the three-phase
interface through the gas-transport pores.
[0086] Significant advantages under rapid changes in load over a
wide range are provided by the invention. At the same conditions of
pressure, temperature and air supply from compressor, the magnitude
of the voltage variations during transit to a new steady state load
decreases by a factor of about 1.5 to 2.2.
[0087] The pulsating cathode and anode gas feed system of the
invention also provides significant advantages for preparing a fuel
cell stack for start-up after a period of storage. Upon shut down,
the fuel cell consumes oxygen fully from air before completely
stopping. After long intervals between operation, days or weeks for
example, re-start can be hindered because the active boundary
between cathode and anode is in the state of nitrogen blockade.
That is, access of the components to the three-phase interface is
difficult due to the filling of gas-transport pores (in the
cathode, for example) by nitrogen. The pressure pulsation aspect of
invention addresses this problem by greatly improving the process
of starting electrochemical generator after down-time or
storage.
[0088] The invention thus significantly increases the reliability
and lifetime of the electrochemical generator. The improvements of
this invention enable the use of PEM fuel cell stacks as
electrochemical generators for both mobile and stationary power
units that are able to efficiently respond to rapidly cycling load
conditions.
[0089] While various embodiments of the present invention have been
shown and described, it will be apparent to those skilled in the
art that many changes and modifications may be made without
departing from the invention in its broader aspects. The appended
claims are therefore intended to cover all such changes and
modifications as fall within the true spirit and scope of the
invention.
* * * * *