U.S. patent application number 10/714232 was filed with the patent office on 2004-05-20 for method and apparatus for controlling a combined heat and power fuel cell system.
Invention is credited to Ballantine, Arne W., Hallum, Ryan, Parks, John W., Skidmore, Dustan L..
Application Number | 20040096713 10/714232 |
Document ID | / |
Family ID | 26855905 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040096713 |
Kind Code |
A1 |
Ballantine, Arne W. ; et
al. |
May 20, 2004 |
Method and apparatus for controlling a combined heat and power fuel
cell system
Abstract
A cogeneration fuel cell system and associated methods of
operation are provided that accommodate a demand for heat as well
as a demand for electric power. The system is operated among
various modes to balance heat and power demand signals. In general,
a fuel cell system is coupled to a power sink and a heat sink, and
a controller is adapted to respond to data signals from the power
sink and the heat sink. As examples, such data signals from the
heat sink may include a temperature indication or a heat demand
signal (such as from a thermostat), and such data signals from the
power sink may include a voltage or current measurement, an
electrical power demand signal, or an electrical load.
Inventors: |
Ballantine, Arne W.; (Round
Lake, NY) ; Hallum, Ryan; (Latham, NY) ;
Parks, John W.; (Loudonville, NY) ; Skidmore, Dustan
L.; (Latham, NY) |
Correspondence
Address: |
Joe Hulett
Wong Cabello PC
Suite 600
20333 SH
Houston
TX
77070
US
|
Family ID: |
26855905 |
Appl. No.: |
10/714232 |
Filed: |
November 14, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10714232 |
Nov 14, 2003 |
|
|
|
10158705 |
May 30, 2002 |
|
|
|
60294776 |
May 31, 2001 |
|
|
|
Current U.S.
Class: |
429/431 ;
429/430; 429/432; 429/437; 429/442; 429/444; 429/483; 429/524 |
Current CPC
Class: |
H01M 8/0432 20130101;
Y02E 20/14 20130101; H01M 8/04007 20130101; H01M 8/04768 20130101;
H01M 8/04589 20130101; H01M 8/04731 20130101; H01M 8/04753
20130101; H01M 8/04373 20130101; H01M 8/04559 20130101; H01M
8/04089 20130101; H01M 8/0612 20130101; H01M 8/04701 20130101; H01M
8/04776 20130101; H01M 8/04738 20130101; H01M 8/04604 20130101;
Y02E 60/50 20130101 |
Class at
Publication: |
429/023 ;
429/024; 429/026; 429/013 |
International
Class: |
H01M 008/04 |
Claims
What is claimed is:
1. A fuel cell system, comprising: a fuel cell, a fuel supply, an
oxidant supply, a power demand sensor, a heat demand sensor, and a
controller; wherein the fuel cell is adapted to receive a fuel flow
from the fuel supply, and an oxidant flow from the oxidant supply;
wherein the controller is connected to each of the fuel supply,
oxidant supply, power demand sensor, and heat demand sensor, and
wherein the controller is further adapted to receive a power demand
signal from the power demand sensor and a heat demand signal from
the heat demand sensor; wherein the controller is adapted to reduce
at least one of the fuel flow and oxidant flow when there is no
heat demand signal and no power demand signal; wherein the
controller is adapted to increase at least one of the fuel flow and
oxidant flow when there is no heat demand signal and there is a
power demand signal; wherein the controller is adapted to increase
at least one of the fuel flow and oxidant flow when there is no
power demand signal and there is a heat demand signal; and wherein
the controller is adapted to increase at least one of the fuel flow
and oxidant flow when there is a power demand and a heat demand
signal.
2. The system of claim 1, wherein the power demand sensor is a fuel
cell voltage sensor that produces a power demand signal when a
voltage of the fuel cell falls below a predetermined level.
3. The system of claim 1, wherein the power demand sensor is a fuel
cell current sensor that produces a power demand signal when an
output current of the fuel cell exceeds a predetermined level.
4. The system of claim 1, wherein the power demand sensor comprises
a fuel cell output current sensor an electrical load sensor,
wherein the power demand sensor produces a power demand signal when
an electrical load on the fuel cell exceeds an output current of
the fuel cell.
5. The system of claim 4, wherein the electrical load on the fuel
cell comprises a parasitic system electrical load and an
application electrical load.
6. The system of claim 1, further comprising a coolant circuit and
a heat sink, wherein the coolant circuit is adapted to transfer
heat from the fuel cell to the heat sink; and wherein the heat
demand sensor is a temperature sensor that produces a heat demand
signal when a temperature of the heat sink is below a predetermined
level.
7. The system of claim 1, further comprising a heat sink, a coolant
circuit, and an oxidizer adapted to oxidize an exhaust gas of the
fuel cell; wherein the coolant circuit is adapted to transfer heat
from the fuel cell to the heat sink; and wherein the heat demand
sensor is a temperature sensor that produces a heat demand signal
when a temperature of the heat sink is below a predetermined
level.
8. The system of claim 1, further comprising a coolant circuit and
a radiator; wherein the coolant circuit is adapted to transfer heat
from the fuel cell to the heat sink; and wherein the radiator is
adapted to remove heat from the coolant circuit.
9. The system of claim 8, wherein the radiator comprises a fan
connected to the controller, and wherein the controller is adapted
to reduce an output of the fan when there is a heat demand signal,
and the controller is further adapted to increase an output of the
fan when there is no heat demand signal.
10. The system of claim 8, wherein the coolant circuit further
comprises a bypass valve and a radiator bypass circuit; wherein the
valve is connected to the controller, and the controller is adapted
to actuate the valve to divert a coolant flow from the radiator to
the radiator bypass circuit when there is a heat demand signal, and
the controller is further adapted to actuate the valve to divert
the coolant flow from the radiator bypass circuit to the radiator
when there is no heat demand signal.
11. The system of claim 6, wherein the heat sink is a water
tank.
12. The system of claim 7, wherein the heat sink is a water
tank.
13. The system of claim 6, wherein the heat sink comprises air
contained in a building.
14. The system of claim 6, wherein the heat sink comprises a
generator portion of an adsorption cooling system.
15. The system of claim 7, wherein the heat sink comprises air
contained in a building.
16. The system of claim 7, wherein the heat sink comprises a
generator portion of an adsorption cooling system.
17. The system of claim 6, wherein the heat sink comprises air
contained in a building and the heat demand sensor is a thermostat
that produces a heat demand signal when a temperature of the air
falls below a predetermined level.
18. The system of claim 7, wherein the heat sink comprises air
contained in a building and the heat demand sensor is a thermostat
that produces a heat demand signal when a temperature of the air
falls below a predetermined level.
19. The system of claim 7, further comprising a valve and a fuel
bypass circuit; wherein the valve is connected to the controller,
and the fuel bypass circuit is adapted to divert a portion of the
fuel flow from an inlet of the fuel cell to the oxidizer; and
wherein the controller is adapted to actuate the valve to divert
the portion of fuel flow from the fuel cell inlet to the oxidizer
when there is a heat demand signal, and the controller is further
adapted to actuate the valve to divert the portion of fuel flow
from the fuel cell inlet to the oxidizer when there is no heat
demand signal.
20. The system of claim 1, wherein the controller comprises a
computer usable medium having computer readable code embodied
thereon.
21. The system of claim 1, wherein the controller is
programmable.
22. A method of operating a fuel cell system, comprising: providing
a fuel flow and an oxidant flow to a fuel cell to produce
electricity; providing the electricity to an electrical load;
transferring heat from the fuel cell to a heat sink by circulating
a first coolant through a first coolant circuit, wherein the first
coolant circuit is adapted to remove heat from the fuel cell and is
further adapted to transfer heat to the heat sink; measuring a
thermal parameter of the heat sink; measuring an electrical
parameter of the electrical load; measuring a performance parameter
of the fuel cell; generating a power demand signal when a power
output of the fuel cell indicated by the performance parameter is
less than a power requirement of the electrical load indicated by
the electrical parameter; generating a heat demand signal when the
thermal parameter of the heat sink is below a predetermined level;
reducing at least one of the fuel flow and oxidant flow when there
is no heat demand signal and no power demand signal; increasing at
least one of the fuel flow and oxidant flow when there is no heat
demand signal and there is a power demand signal; increasing at
least one of the fuel flow and oxidant flow when there is no power
demand signal and there is a heat demand signal; and increasing at
least one of the fuel flow and oxidant flow when there is a power
demand and a heat demand signal.
23. The method of claim 22, further comprising: measuring a voltage
of the fuel cell; and generating the power demand signal when the
voltage of the fuel cell falls below a predetermined level.
24. The method of claim 22, further comprising: measuring an output
current of the fuel cell; and generating the power demand signal
when the output current of the fuel cell exceeds a predetermined
level.
25. The method of claim 22, further comprising: exhausting fuel gas
from the fuel cell to an oxidizer; oxidizing the fuel gas in the
oxidizer to generate heat; and transferring heat from the fuel cell
to the heat sink by circulating a second coolant through a second
coolant circuit, wherein the second coolant circuit is adapted to
remove heat from the oxidizer and is further adapted to transfer
heat to the heat sink.
26. The method of claim 22, wherein the first and second coolant
circuits are in fluid communication and the first and second
coolants are each portions of a common coolant flow.
27. The method of claim 22, further comprising: circulating the
first coolant through a radiator to remove heat from the first
coolant.
28. The method of claim 22, further comprising: circulating the
second coolant through a radiator to remove heat from the first
coolant.
29. The method of claim 22, wherein the heat sink comprises a water
tank, and wherein the thermal parameter is a temperature of water
in the water tank.
30. The method of claim 22, wherein the heat sink comprises air
contained in a building, and wherein the thermal parameter
comprises a temperature of the air contained in the building.
31. The method of claim 22, wherein the heat sink comprises a
generator portion of an adsorption cooling system and wherein the
thermal parameter is a temperature of the generator portion.
32. The system of claim 22, further comprising: diverting a portion
of the fuel flow from an inlet of the fuel cell to the oxidizer in
response to the heat demand signal.
33. A fuel cell system, comprising: a fuel cell, a fuel processor,
an oxidant supply, a power demand sensor, a heat demand sensor, a
controller, and an electrochemical hydrogen separator; wherein the
fuel cell is adapted to receive a fuel flow from the fuel
processor, and an oxidant flow from the oxidant supply; wherein the
controller is connected to each of the fuel supply, oxidant supply,
power demand sensor, and heat demand sensor, and wherein the
controller is further adapted to receive a power demand signal from
the power demand sensor and a heat demand signal from the heat
demand sensor; wherein the hydrogen separator is adapted to receive
the fuel flow from the fuel processor and separate hydrogen from
the fuel flow into a reservoir when the hydrogen separator is
activated; wherein the controller is adapted to reduce at least one
of the fuel flow and oxidant flow when there is no heat demand
signal and no power demand signal; wherein the controller is
adapted to increase at least one of the fuel flow and oxidant flow
when there is no heat demand signal and there is a power demand
signal; wherein the controller is adapted to activate the hydrogen
separator when there is no power demand signal and there is a heat
demand signal; and wherein the controller is adapted to increase at
least one of the fuel flow and oxidant flow when there is a power
demand and a heat demand signal.
34. The system of claim 33, wherein the power demand sensor is a
fuel cell voltage sensor that produces a power demand signal when a
voltage of the fuel cell falls below a predetermined level.
35. The system of claim 33, wherein the power demand sensor is a
fuel cell current sensor that produces a power demand signal when
an output current of the fuel cell exceeds a predetermined
level.
36. The system of claim 33, wherein the power demand sensor
comprises a fuel cell output current sensor an electrical load
sensor, wherein the power demand sensor produces a power demand
signal when an electrical load on the fuel cell exceeds an output
current of the fuel cell.
37. The system of claim 33, further comprising a coolant circuit
and a heat sink, wherein the coolant circuit is adapted to transfer
heat from the fuel cell to the heat sink; and wherein the heat
demand sensor is a temperature sensor that produces a heat demand
signal when a temperature of the heat sink is below a predetermined
level.
38. The system of claim 33, further comprising a heat sink, a
coolant circuit, and an oxidizer adapted to oxidize an exhaust gas
of the fuel cell; wherein the coolant circuit is adapted to
transfer heat from the fuel cell to the heat sink; and wherein the
heat demand sensor is a temperature sensor that produces a heat
demand signal when a temperature of the heat sink is below a
predetermined level.
39. The system of claim 33, further comprising a coolant circuit
and a radiator; wherein the coolant circuit is adapted to transfer
heat from the fuel cell to the heat sink; and wherein the radiator
is adapted to remove heat from the coolant circuit.
40. The system of claim 39, wherein the radiator comprises a fan
connected to the controller, and wherein the controller is adapted
to reduce an output of the fan when there is a heat demand signal,
and the controller is further adapted to increase an output of the
fan when there is no heat demand signal.
41. The system of claim 39, wherein the coolant circuit further
comprises a bypass valve and a radiator bypass circuit; wherein the
valve is connected to the controller, and the controller is adapted
to actuate the valve to divert a coolant flow from the radiator to
the radiator bypass circuit when there is a heat demand signal, and
the controller is further adapted to actuate the valve to divert
the coolant flow from the radiator bypass circuit to the radiator
when there is no heat demand signal.
42. The system of claim 38, wherein the heat sink is a water
tank.
43. The system of claim 38, wherein the heat sink comprises air
contained in a building.
44. The system of claim 38, wherein the heat sink comprises air
contained in a building and the heat demand sensor is a thermostat
that produces a heat demand signal when a temperature of the air
falls below a predetermined level.
45. The system of claim 33, wherein the reservoir is a pressure
vessel.
46. The system of claim 33, wherein the reservoir comprises a valve
connected to the controller and associated with a connection to the
fuel cell such that the controller is adapted to selectively open
the valve to supply hydrogen to the fuel cell.
47. The system of claim 33, wherein the hydrogen separator
comprises a membrane electrode assembly having an anode side and a
cathode side; the anode side being in fluid connection with the
fuel flow from the fuel processor; the anode side and cathode side
of the membrane electrode assembly each having an electrical
connector; and a power source connected to the anode and cathode
side electrical connectors of the membrane electrode assembly, the
power source providing a potential across the connectors.
48. The system of claim 47, wherein the membrane electrode assembly
comprises a PEM sandwiched on either side by a platinum based
catalyst layer.
49. The system of claim 47, wherein the controller is connected to
the power source and adapted to selectively activate the hydrogen
separator by causing the power source to apply a potential across
the connectors.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 USC 119(e) from
U.S. Provisional Application No. 60/294,776, filed May 31, 2001,
naming Ballantine, Hallum, Parks and Skidmore as inventors, and
titled "METHOD AND APPARATUS FOR CONTROLLING A COMBINED HEAT AND
POWER FUEL CELL SYSTEM" That application is incorporated herein by
reference in its entirety and for all purposes.
BACKGROUND
[0002] The invention generally relates to a combined heat and power
fuel cell system and associated methods of operation.
[0003] A fuel cell is an electrochemical device that converts
chemical energy produced by a reaction directly into electrical
energy. For example, one type of fuel cell includes a polymer
electrolyte membrane (PEM), often called a proton exchange
membrane, that permits only protons to pass between an anode and a
cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel)
is reacted to produce protons that pass through the PEM. The
electrons produced by this reaction travel through circuitry that
is external to the fuel cell to form an electrical current. At the
cathode, oxygen is reduced and reacts with the protons to form
water. The anodic and cathodic reactions are described by the
following equations:
H.sub.2.fwdarw.2H.sup.++2e.sup.- at the anode of the cell, and
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O at the cathode of the
cell.
[0004] A typical fuel cell has a terminal voltage of up to about
one volt DC. For purposes of producing much larger voltages,
multiple fuel cells may be assembled together to form an
arrangement called a fuel cell stack, an arrangement in which the
fuel cells are electrically coupled together in series to form a
larger DC voltage (a voltage near 100 volts DC, for example) and to
provide more power.
[0005] The fuel cell stack may include flow field plates (graphite
composite or metal plates, as examples) that are stacked one on top
of the other. The plates may include various surface flow field
channels and orifices to, as examples, route the reactants and
products through the fuel cell stack. A PEM is sandwiched between
each anode and cathode flow field plate. Electrically conductive
gas diffusion layers (GDLs) may be located on each side of each PEM
to act as a gas diffusion media and in some cases to provide a
support for the fuel cell catalysts. In this manner, reactant gases
from each side of the PEM may pass along the flow field channels
and diffuse through the GDLs to reach the PEM. The PEM and its
adjacent pair of catalyst layers are often referred to as a
membrane electrode assembly (MEA). An MEA sandwiched by adjacent
GDL layers is often referred to as a membrane electrode unit
(MEU).
[0006] A fuel cell system may include a fuel processor that
converts a hydrocarbon (natural gas or propane, as examples) into a
fuel flow for the fuel cell stack. For a given output power of the
fuel cell stack, the fuel flow to the stack must satisfy the
appropriate stoichiometric ratios governed by the equations listed
above. Thus, a controller of the fuel cell system may monitor the
output power of the stack and based on the monitored output power,
estimate the fuel flow to satisfy the appropriate stoichiometric
ratios. In this manner, the controller regulates the fuel processor
to produce this flow, and in response to the controller detecting a
change in the output power, the controller estimates a new rate of
fuel flow and controls the fuel processor accordingly.
[0007] A fuel cell system may include a fuel processor that
converts a hydrocarbon (natural gas or propane, as examples) into a
fuel flow for the fuel cell stack. For a given output power of the
fuel cell stack, the fuel flow to the stack must satisfy the
appropriate stoichiometric ratios governed by the equations listed
above. The amount of a reactant supplied may be referred to in
terms of "stoich". For example, for a given electrical load on a
fuel cell, one stoich of hydrogen and one stoich of air would refer
to the minimum amount of each reactant theoretically required to
produce enough electrons to satisfy the load (assuming all of the
reactants will react). However, in some cases, not all of the
hydrogen or air supplied will actually react, so that it may be
necessary to provide excess fuel and air stoichiometry so that the
amount actually reacted will be appropriate to satisfy a given
power demand.
[0008] Hydrogen that is not reacted in the fuel cell may be vented
to the atmosphere with the fuel cell exhaust, and in some cases may
be oxidized before it is vented. Such exhaust may also contain
small amounts of hydrocarbons that "slip" through the fuel
processor without being reacted. Substantial heat may be generated
as these exhaust components are oxidized, for example by mixing
them with air and passing them through a platinum-coated ceramic
monolith similar to an automotive catalytic converter.
[0009] The fuel cell system may provide power to a load, such as a
load that is formed from residential appliances and electrical
devices that may be selectively turned on and off to vary the power
that is demanded by the load. Thus, the load may not be constant,
but rather the power that is consumed by the load may vary over
time and abruptly change in steps. For example, if the fuel cell
system provides power to a house, different appliances/electrical
devices of the house may be turned on and off at different times to
cause the load to vary in a stepwise fashion over time.
[0010] There is a continuing need for systems and algorithms to
achieve objectives including the foregoing in a robust and cost
effective manner.
SUMMARY
[0011] The invention provides a combined heat and power fuel cell
system and associated methods of operation. Such systems are
commonly referred to as cogeneration systems. In general, the
system and methods of the invention relate to operation of a fuel
cell system among various modes and configurations to balance heat
and power demand signals. The fuel cell system is coupled to both a
power sink and a heat sink. A controller is adapted to coordinate
response to data signals from the power sink and the heat sink. As
examples, such data signals from the heat sink may include a
temperature indication or a heat demand signal (such as from a
thermostat), and such data signals from the power sink may include
a voltage or current measurement, an electrical power demand
signal, or an electrical load.
[0012] In one aspect, a fuel cell system is provided that includes
a fuel cell, a fuel supply, an oxidant supply, a power demand
sensor, a heat demand sensor, and a controller. The fuel cell is
adapted to receive a fuel flow from the fuel supply, and an oxidant
flow from the oxidant supply. The controller is connected to each
of the fuel supply, oxidant supply, power demand sensor, and heat
demand sensor. The controller is further adapted to receive a power
demand signal from the power demand sensor and a heat demand signal
from the heat demand sensor.
[0013] In a first state, the controller is configured to reduce at
least one of the fuel flow and oxidant flow when there is no heat
demand signal and no power demand signal. In a second state, the
controller is configured to increase at least one of the fuel flow
and oxidant flow when there is no heat demand signal and there is a
power demand signal. In a third state, the controller is configured
to increase at least one of the fuel flow and oxidant flow when
there is no power demand signal and there is a heat demand signal.
In a fourth state, the controller is configured to increase at
least one of the fuel flow and oxidant flow when there is a power
demand and a heat demand signal.
[0014] In some embodiments, the power demand sensor is a fuel cell
voltage sensor that produces a power demand signal when a voltage
of the fuel cell falls below a predetermined level. The power
demand sensor can also be a fuel cell current sensor that produces
a power demand signal when an output current of the fuel cell
exceeds a predetermined level. The power demand sensor can also
include a fuel cell output current sensor an electrical load
sensor, wherein the power demand sensor produces a power demand
signal when an electrical load on the fuel cell exceeds an output
current of the fuel cell. It will be appreciated that the
electrical load on the fuel cell can include a parasitic system
electrical load and an application electrical load. For example,
the parasitic load can refer to internal components such as pumps
and blowers that are powered by the fuel cell. The application load
can refer to a residential appliance, as an example.
[0015] The system can further include a coolant circuit and a heat
sink, wherein the coolant circuit is adapted to transfer heat from
the fuel cell to the heat sink. As an example, the heat demand
sensor can be a temperature sensor that produces a heat demand
signal when a temperature of the heat sink is below a predetermined
level.
[0016] In one embodiment, the system can include a heat sink, a
coolant circuit, and an oxidizer adapted to oxidize an exhaust gas
of the fuel cell. The coolant circuit is configured to transfer
heat from the fuel cell to the heat sink, and the heat demand
sensor is a temperature sensor that produces a heat demand signal
when a temperature of the heat sink is below a predetermined level.
In another embodiment, the coolant circuit is adapted to transfer
heat from the fuel cell to the heat sink, and a radiator is
provided to remove heat from the coolant circuit. The radiator can
include a fan connected to the controller, where the controller is
configured to reduce an output of the fan when there is a heat
demand signal. The controller is further configured to increase an
output of the fan when there is no heat demand signal.
[0017] In another embodiment, the coolant circuit further includes
a bypass valve and a radiator bypass circuit. The valve is
connected to the controller, and the controller is adapted to
actuate the valve to divert a coolant flow from the radiator to the
radiator bypass circuit when there is a heat demand signal. The
controller is further adapted to actuate the valve to divert the
coolant flow from the radiator bypass circuit to the radiator when
there is no heat demand signal.
[0018] The system can also include a fuel bypass circuit associated
with the valve. In such a system, the valve is connected to the
controller, and the fuel bypass circuit is adapted to divert a
portion of the fuel flow from an inlet of the fuel cell to the
oxidizer. The controller is configured to actuate the valve to
divert the portion of fuel flow from the fuel cell inlet to the
oxidizer when there is a heat demand signal. The controller is
further adapted to actuate the valve to divert the portion of fuel
flow from the fuel cell inlet to the oxidizer when there is no heat
demand signal. As an example, the controller can include a computer
usable medium (e.g., memory) having computer readable code embodied
thereon (e.g., firmware or software). Preferably, the controller is
also programmable.
[0019] Embodiments may further include a hydrogen separator, such
as electrochemical hydrogen separator. The hydrogen separator is
adapted to receive the fuel flow from the fuel processor and
separate hydrogen from the fuel flow into a reservoir when the
hydrogen separator is activated. The controller is configured to
activate the hydrogen separator when there is no power demand
signal and there is a heat demand signal.
[0020] As an example, the hydrogen separator can include a membrane
electrode assembly having an anode side and a cathode side. It is
well known in the art that placing an electric potential across an
electrochemical cell, such as a fuel cell, having no electrical
load (as opposed to merely placing an electric load on the fuel
cell as in the case of normal operation) will result in hydrogen
being electrochemically "pumped" from fuel (e.g., reformate) in the
anode to the cathode. This process proceeds essentially according
to the same reactions at the anode and cathode of the fuel cell as
in normal operation.
[0021] For example, such a cell can be placed along the flow path
of the reformate being fed from the fuel processor to the fuel
cell. When there is a heat demand, but no power demand, the
controller reacts enough fuel in the fuel cell to produce the
desired amount of heat. The excess power is sunk to the hydrogen
separator to pressurize a hydrogen tank (e.g., at about two
atmospheres), which will contain essentially pure hydrogen. The
hydrogen tank reservoir can include a valve connected to the
controller and associated with a conduit to the fuel cell such that
the controller can selectively open the valve to supply hydrogen to
the fuel cell (e.g., in response to a sudden load increase).
[0022] The hydrogen separator can be a PEM fuel cell (e.g., a PEM
sandwiched on either side by a platinum based catalyst layer). The
anode side is in fluid connection with the fuel flow from the fuel
processor. The anode side and cathode side of the membrane
electrode assembly each have an electrical connector (e.g., a wire
connected to the each of the anode and cathode flow field plates. A
power source is connected to the anode and cathode electrical
connectors of the membrane electrode assembly and provides an
electric potential across the connectors when the separator is in
an active state. Similarly, the controller can remove the potential
to put the separator in an inactive state. While the separator is
in the inactive state, the reformate simply passes by it on the way
to the fuel cell without effect. In some embodiments, the separator
can also be used, as can the hydrogen reservoir supply to the fuel
cell, when there is a power demand.
[0023] Advantages and other features of the invention will become
apparent from the following description, drawings and claims.
DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic diagram of an integrated fuel cell
system.
[0025] FIG. 2 is a schematic diagram of a control system for an
integrated fuel cell system.
[0026] FIG. 3 is a schematic diagram of an integrated fuel cell
system.
[0027] FIG. 4 is a schematic diagram of a CHP fuel cell system.
[0028] FIG. 5 is a schematic diagram of a CHP fuel cell system.
[0029] FIG. 6 is a flow diagram of a control scheme for a CHP fuel
cell system.
[0030] FIG. 7 is a flow diagram of a control scheme for a CHP fuel
cell system.
[0031] FIG. 8 is a flow diagram of a control scheme for a CHP fuel
cell system.
[0032] FIG. 9 is a flow diagram of a control scheme for a CHP fuel
cell system.
DETAILED DESCRIPTION
[0033] Referring to FIG. 1, an integrated fuel cell system 100 is
shown. Natural gas is injected into the system through conduit 102.
The natural gas flows through desulfurization vessel 104, which
contains a sulfur-adsorbent material such as activated carbon. The
de-sufurized natural gas is then flowed to a conversion reactor 110
via conduit 105. Before being reacted in the conversion reactor
110, the de-sulfurized natural gas is mixed with air 106 and steam
108. It will be appreciated that the conversion reactor 110 is an
autothermal reactor. The converted natural gas, referred to as
reformate, then flows through a series of high temperature shift
reactors 112 and 114, through a low temperature shift reactor 116,
and then through a PROX reactor 118. It will be appreciated that
the primary function of this series of reactors is to maximize
hydrogen production while minimizing carbon monoxide levels in the
reformate. The reformate is then flowed via conduit 120 to the
anode chambers (not shown) of a fuel cell stack 122.
[0034] Air enters the system via conduit 124 and through conduit
106 as previously mentioned. In the present example, the fuel cell
stack 122 uses sulfonated fluourocarbon polymer PEMs that need to
be kept moist during operation to avoid damage. While the reformate
120 tends to be saturated with water, the ambient air 124 tends to
be subsaturated. To prevent the ambient air 124 from drying out the
fuel cells in stack 122, the air 124 is humidified by passing it
through an enthalpy wheel 126, which also serves to preheat the air
124. The theory and operation of enthalpy wheels are described in
U.S. Pat. No. 6,013,385, which is hereby incorporated by reference.
The air 124 passes through the enthalpy wheel 126 through the
cathode chambers (not shown) of the fuel cell stack 122 via conduit
125. The air 124 picks up heat and moisture in the stack 122, and
is exhausted via conduit 128 back through the enthalpy wheel 126.
The enthalpy wheel 126 rotates with respect to the injection points
of these flows such that moisture and heat from the cathode exhaust
128 is continually passed to the cathode inlet air 124 prior to
that stream entering the fuel cell.
[0035] The anode exhaust from the fuel cell is flowed via conduit
130 to an oxidizer 132, sometimes referred to as an "anode tailgas
oxidizer". The cathode exhaust leaves the enthalpy wheel 126 via
conduit 134 and is also fed to the oxidizer 132 to provide oxygen
to promote the oxidation of residual hydrogen and hydrocarbons in
the anode exhaust 130. As examples, the oxidizer 132 can be a
burner or a catalytic burner (similar to automotive catalytic
converters). The exhaust of the oxidizer is vented to ambient via
conduit 136. The heat generated in the oxidizer 132 is used to
convert a water stream 138 into steam 108 that is used in the
system.
[0036] Referring to FIG. 2, a schematic is shown of a control
system for an integrated high temperature PEM fuel cell system.
Such a control system can include the following components, as
examples: (200) an electronic controller, e.g., a programmable
microprocessor; (202) a graphical user interface; (204) software
for instructing the controller; (206) an air blower for providing
the system with air, e.g., the fuel cell cathode and/or the fuel
processor; (208) a fuel blower for driving hydrocarbon into the
fuel processor; (210) a stack voltage scanner for measuring the
stack voltage and/or the individual voltages of fuel cells within
the stack; (212) a coolant pump for circulating a coolant through
the fuel cell stack to maintain a desired stack operating
temperature; (214) a coolant radiator and fan for expelling heat
from the coolant to ambient; (216) a fuel processor inlet air
by-pass valve for controlling the amount of air fed to the fuel
processor; and (218) an oxidizer inlet air control valve.
[0037] Such a control system can operate to control the following
variables, as examples: (220) the fuel processor inlet oxygen to
fuel ratio; (222) the fuel processor inlet water to fuel ratio;
(224) a fuel processor reactor temperature; (226) the voltage of
the fuel cell stack or of individual fuel cells within the stack;
(228) the oxidizer temperature; (230) electrical demand on the fuel
cell system; (232) the cathode air stoich; (234) the anode fuel
stoich; and (236) the system coolant temperature.
[0038] As examples, suitable fuel processor systems are described
in U.S. Pat. Nos. 6,207,122, 6,190,623, and 6,132,689, which are
hereby incorporated by reference. For instance, in the case of a
natural gas fuel processor, the system may include a variable speed
blower for injecting natural gas into the system, and a variable
speed air blower for injecting air into the system. The gas and air
may be mixed in a mixing chamber, humidified to a desired level
(e.g., the system may include some method of steam generation), and
be preheated (e.g., in a gas/gas heat exchanger with heat from
product gas from the fuel processor). The reactant mixture may then
be reacted in an autothermal reactor (ATR) to convert the natural
gas to synthesis gas (H.sub.2O+CH.sub.4-->3H.sub.2+CO;
1/2O.sub.2+CH.sub.4-->2H.sub.2+CO- ). The fuel processor may
also include a shift reactor (CO+H.sub.2O-->H.sub.2+CO.sub.2) to
shift the equilibrium of the synthesis gas toward hydrogen
production to minimize carbon monoxide (CO). The fuel processor may
include multiple shift reactor stages.
[0039] Some fuel processor systems may also include a preferential
oxidation (PROX) stage (CO+1/2O.sub.2-->CO.sub.2) to further
reduce carbon monoxide levels. The PROX reaction is generally
conducted at lower temperatures that the shift reaction, such as
100-200.degree. C. Like the CPO reaction, the PROX reaction can
also be conducted in the presence of an oxidation catalyst such as
platinum. The PROX reaction can typically achieve CO levels less
than 100 ppm. Other non-catalytic CO reduction and reformate
purification methods are also known, such as membrane filtration
and pressure swing adsorption systems.
[0040] In some embodiments, the autothermal reactor can be replaced
by a reforming reactor (e.g., utilizing the endothermic steam
reforming reaction: H.sub.2O+CH.sub.4-->3H.sub.2+CO), or by a
catalytic partial oxidation reactor (CPO reactor:
1/2O.sub.2+CH.sub.4-->2H.sub.2+CO), which is exothermic. These
terms are sometimes used loosely or interchangeably. In general, an
autothermal reactor is a reactor that combines the reforming and
catalytic partial oxidation reactions to achieve a balance between
the respective endothermic and exothermic elements. It should be
noted that fuel processors are sometimes generically referred to as
reformers, and the fuel processor output gas is sometimes
generically referred to as reformate, without respect to the
reaction that is actually employed.
[0041] The ATR catalyst can be a ceramic monolith that has been
wash-coated with a platinum catalyst (as known in the art, e.g.,
operating at over 600.degree. C.). The shift catalyst can also be
platinum wash-coated ceramic monolith (e.g., operating between
300-600.degree. C.). The shift reactor can also include a catalyst
that is operable at lower temperatures. Other suitable catalyst and
reactor systems are known in the art.
[0042] In some embodiments, a desulfurization stage may be placed
upstream from the fuel processor to remove sulfur compounds from
the fuel before it is reacted (e.g., to avoid poisoning the
catalysts of the fuel processor and/or the fuel cell stack). For
example, activated carbon, zeolite, and activated nickel materials
are all known in the art for such application.
[0043] As known in the art, it may be desirable to control the
water to fuel ratio (e.g., steam to carbon ratio) that is fed to
the ATR. For example, it may be desirable to provide on average at
least two water molecules for every carbon atom provided in the
fuel to prevent coking. It may also be desirable in some
embodiments to adjust the air stoich through the fuel cell stack to
control the amount of oxygen that is introduced into the fuel
processor with respect to the amount of fuel that is introduced
(e.g., O.sub.2:CH.sub.4 ratio, which can effect the operation
temperature of the ATR as an example).
[0044] Suitable fuel cell stack designs are well known. For
example, the fuel cell systems taught in U.S. Pat. Nos. 5,858,569,
5,981,098, 5,998,054, 6,001,502, 6,071,635, 6,174,616, and
09/502,886 are each hereby incorporated by reference. In an
integrated fuel cell system, the fuel cell stack may be associated
with additional components and subsystems. A coolant system may be
used to circulate a liquid coolant through the stack to maintain a
desired operating temperature. A radiator or other heat transfer
device may be placed in the coolant path to provide coolant
temperature control. The coolant may also perform heat transfer in
other areas of the system, such as in the fuel processor, or
cooling reactants exiting the fuel processor to a desired
temperature before entering the fuel cell stack. As an example, the
coolant may be circulated by a variable speed pump.
[0045] The reactant delivery system associated with the fuel cell
stack may include a variable speed air blower or compressor, and
variable position valves and/or orifices to control the amount and
pressure of fuel and air provided to the stack, as well as the
ratio between the two. For a given electrical load, a certain
amount of reactants must be reacted in the fuel cell to provide the
power demanded by the load. In this sense, the amount of air and
fuel supplied to the fuel cell stack may each be referred to in
terms of stoichiometry (i.e., the stoichiometric equations
associated with the fuel cell reactions:
H.sub.2.fwdarw.2H.sup.++2e.sup.-; and
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H- .sub.2O). For example,
supplying 1 "stoich" of reformate means that enough reformate is
supplied to the fuel cell stack to satisfy the power demand of the
load, assuming that all of the hydrogen in the reformate reacts.
However, since not all of the hydrogen in the reformate will
actually react, the fuel may be supplied at an elevated stoich
(e.g., 2 stoich would refer to twice this amount) to ensure that
the amount that actually will react will be enough to meet the
power demand. Similarly, air may also be supplied to the fuel cell
stack in excess of what is theoretically needed (e.g., 2
stoich).
[0046] The reactant plumbing associated with the stack may be
conducted in part by a manifold. For example, the teachings of U.S.
patent Ser. No. 09/703,249 are hereby incorporated by reference.
Such a manifold may be further associated with a water collection
tank that receives condensate from water traps in the system
plumbing. The water tank may include a level sensor. Some fuel cell
systems may require an external source of water during operation,
and may thus include a connection to a municipal water source. A
filter may be associated with the connection from the municipal
water supply, such as a particulate filter, a reverse osmosis
membrane, a deionization bed, etc.
[0047] Some fuel cell membranes, such as those made from sulfonated
flourocarbon polymers, require humidification. For example, it may
be necessary to humidify reactant air before it is sent through the
fuel cell in order to prevent drying of the fuel cell membranes. In
such systems, a reactant humidification system may be required. It
will be appreciated that in systems utilizing reformate, this
generally refers to humidifying only the air fed to the fuel cell
stack and not the fuel stream, since the reformate exiting the fuel
processor is generally saturated. One method of humidification is
to generate steam which is supplied to a reactant stream. Membrane
humidification systems are also known, as well as enthalpy wheel
systems, as taught in U.S. Pat. No. 6,013,385, which is hereby
incorporated by reference.
[0048] The spent fuel exhausted from the fuel cell stack may
contain some amount of unreacted hydrogen or unreacted hydrocarbon
or carbon monoxide from the fuel processor. Before the spent fuel
is vented to the atmosphere, it may be sent through an oxidizer to
reduce or remove such components. Suitable oxidizer designs are
known, such as burner designs, and catalytic oxidizers similar to
automotive catalytic converters. Oxidizers may utilize air
exhausted from the fuel cell stack, and may have an independent air
source, such as from a blower. In some systems, the heat generated
by the oxidizer may be used, for example, to generate steam for use
in the fuel processor or to humidify the fuel cell reactants.
Exemplary oxidizer designs are described in U.S. patent Ser. Nos.
09/727,921 and 09/728,227, which are each incorporated herein by
reference. Fuel cell exhaust oxidizers are sometimes referred to as
"tailgas oxidizers" or "anode tailgas oxidizers" ("ATO").
[0049] Another system that may be associated with the fuel cell
stack is a mechanism for measuring the voltages of the individual
fuel cells within the stack. For example, the teachings of U.S.
Pat. No. 6,140,820, Ser. Nos. 09/379,088, 09/629,548, 09/629,003
are each hereby incorporated by reference. In some systems, the
health of a fuel cell stack may be determined by monitoring the
individual differential terminal voltages (also referred to as cell
voltages) of the fuel cells. Particular cell voltages may
individually vary under load conditions and cell health over a
range from -1 volt to +1 volt, as an example. The fuel cell stack
typically may include a large number of fuel cells (between 50-100,
for example), so that the terminal voltage across the entire stack
is the sum of the individual fuel cell voltages at a given
operating point. As the electrical load on the stack is increased,
some "weak" cells may drop in voltage more quickly than others.
Driving any particular cell to a low enough voltage under an
electrical load can damage the cell, so systems may include a
mechanism for coordinating the cell voltages with the electrical
demand and reactant supply to the fuel cell stack. For example, the
teachings of U.S. patent Ser. Nos. 09/749,261, 09/749,297 are
hereby incorporated by reference.
[0050] A fuel cell stack typically produces direct current at a
voltage which varies according to the number of cells in the stack
and the operating conditions of the cells. Applications for the
power generated by a fuel cell stack may demand constant voltage,
or alternating current at a constant voltage and frequency similar
to a municipal power grid, etc. Integrated fuel cell systems may
therefore include a power conditioning system to accommodate such
demands. Technologies for converting variable direct current
voltages to constant or relatively constant voltages are well
known, as are technologies for inverting direct currents to
alternating currents. Suitable power conditioner topologies for
fuel cells are also well known. For example, the teachings of U.S.
patent Ser. No. 09/749,297 are hereby incorporated by
reference.
[0051] A battery system may also be associated with the power
conditioning system, for example, to protect the fuel cells from
fuel starvation upon sudden electrical load increases on the stack.
A battery system can also be used, as examples, to supplement the
peak output power of the fuel cell system, or to provide continuous
power to an application while the fuel cell system is temporarily
shut down (as for servicing) or removed from the load. The battery
system may also include a system for periodically charging the
batteries when necessary.
[0052] Some fuel cell systems may be operated independently from
the power grid (grid independent systems), while other fuel cell
systems may be operated in conjunction with the power grid (grid
parallel systems). For grid parallel systems, the system may
include a transfer switch to transfer the electrical load between
the fuel cell system and the power grid. For example, in some grid
parallel systems, the electrical load can be switched from the fuel
cell system to the grid when the fuel cell system needs to be shut
down for maintenance. In still other grid parallel systems, the
electrical load can be shared between the fuel cell system and the
grid. The fuel cell can also be used to feed power to the grid (in
this sense, the grid may be referred to as a "sink"), while an
appliance takes its power from the grid. Other arrangements are
possible.
[0053] System controllers may automate the operation of fuel cell
systems to varying degrees, and may have varying capacities for
adjustment and reconfiguration. For example, some controllers may
rely in part on software for instruction sets to provide enhanced
flexibility and adaptability, while other controllers may rely on
hardware to provide enhanced reliability and lower cost. Control
systems may also include combinations of such systems. Controllers
may include an algorithm that coordinates open and closed loop
functions. In this context, an open loop function is one that does
not utilize feedback, such as adjusting a blower according to a
look-up table without verifying the effect of the adjustment or
iterating the adjustment toward a desired effect. A closed loop
function is one that utilizes feedback to iterate adjustments
toward a desired effect.
[0054] In general, the controller circuitry may include data inputs
from system components such as safety sensors and thermocouples
throughout the system. As an example, such data inputs may report
data in the form of variable voltage or current signals, or as
binary on/off signals. The controller circuitry may also include
devices to control the voltage and/or current supplied to various
components in the system, for example to control variable speed
pumps and blowers. The power supplied to system components may be
referred to as the parasitic load.
[0055] Referring to FIG. 3, a schematic diagram is shown of an
integrated fuel cell system 300. A fuel processor 302 receives air
304, steam 306 and natural gas 308. Similar to the fuel processor
discussed with respect to FIG. 1, the fuel processor 302 converts
the reactant streams 304, 306 and 308 into a reformate stream 310
that is flowed through a fuel cell stack 312 where it reacted at
the anode electrodes of the fuel cells in stack 312. The fuel cell
stack 312 also receives a flow of air via conduit 314 that provides
oxygen that is reacted at the cathode electrodes of the fuel cell
stack 312.
[0056] The spent reformate is exhausted from fuel cell 312 via
conduit 316 and is fed to oxidizer 318 to remove any carbon
monoxide, hydrogen, or residual hydrocarbons in the exhaust. The
oxidizer 318 is a catalytic oxidizer similar to an automotive
catalytic converter. The oxidizer receives its oxygen via conduit
320, which channels the air exhausted from the fuel cell stack 312.
In some embodiments, the oxidizer 318 can further receive a
supplemental supply of oxygen to ensure adequate oxygen to oxidize
combustibles in the fuel exhaust 316. In other embodiments, excess
air stoich can be supplied to the fuel cell stack 312 to ensure
that the cathode exhaust 320 has sufficient oxygen.
[0057] The system 300 includes various thermal management aspects.
A coolant is circulated through the fuel processor 302 via inlet
322 and outlet 324. In this example, an outlet of a high
temperature shift reactor is cooled in heat exchanger 326 from
about 600.degree. C. to about 300.degree. C. and the reformate is
then provided to a low temperature shift reactor. The reformate
exits the fuel processor at a temperature of about 200.degree. C.,
and is cooled in heat exchanger 332 to the operating temperature of
the stack 312 (e.g., about 60-80.degree. C.). Heat is generated as
the fuel cell is operated, and the operating temperature of the
stack 312 is maintained by circulating a coolant through the stack
312 via inlet 328 and outlet 330.
[0058] Heat is recovered from the exhaust 334 of the oxidizer 318
in heat exchanger 336. For example, in some embodiments, a coolant
can be circulated through heat exchanger 336 to transfer heat from
the oxidizer to another part of the system 300, such as to preheat
the air 304 and fuel 308 streams fed to the fuel processor 302. In
other embodiments, water can be flowed through heat exchanger 336
to generate steam, which is then used to humidify the air stream
314 that is fed to the fuel cell stack, or to provide the steam
flow 306 that is used in the fuel processor 302.
[0059] Referring to FIG. 4, a schematic diagram is shown of a CHP
fuel cell system 400. A system coolant is circulated through a fuel
cell system 402 to transfer heat from the fuel cell system 402 to a
heat sink 408. In this example, the heat sink 408 is a hot water
tank that would be used to provide hot water via conduit 409 to a
building such as a home or an apartment. The coolant is circulated
out of the fuel cell system 402 via conduit 404, through a heat
exchanger 410, and then back into fuel cell system 402 via conduit
406. A pump inside the fuel cell system drives the coolant
flow.
[0060] A coolant is also circulated from the heat exchanger 410 via
conduit 412 to heat exchange surface 414 in heat sink 408, and then
back to heat exchanger 410 via conduit 416. The coolant flow is
driven by pump 418. In some embodiments, the heat exchanger 410 can
be located in heat sink 408 such that a single coolant loop is
circulated between fuel cell system 402 and heat sink 408. However,
the embodiment shown provides an advantage in that the equipment
associated with the heat sink can be configured independently form
the fuel cell system 402.
[0061] For example, the heat sink 408 also includes a level sensor
420 and a temperature sensor 422 (e.g., a thermostat). The level
sensor 420 serves to ensure that the water level in tank 408 stays
above a predetermined threshold. For example, a valve can be
actuated to allow a tap water line to fill the tank 408 to make up
for water flowed out of the tank 408 via conduit 409. The
temperature sensor 422 serves to ensure that the temperature of the
water in tank 408 stays above a desired level.
[0062] The temperature sensor 422 can be connected to a controller
that is further connected to pump 418. As an example, where the
temperature sensor 422 indicates heat is needed to bring the
temperature of the tank 408 to a desired level, the pump 418 can be
turned on to transfer heat from the heat exchanger 410 to the tank
408. Where no heat is needed in the tank, the pump 418 can remain
off such that no heat is transferred from the heat exchanger 410 to
the tank 408. In such a case, the fuel cell system 402 may continue
to circulate coolant through the heat exchanger 410, but may also
operate a radiator in the fuel cell system 402 to expel heat to
ambient to maintain the operating temperatures (e.g., the coolant
temperature) in the fuel cell system 402 at desired levels.
[0063] In some embodiments, the heat exchanger 410 can be located
within the fuel cell system 402. For example, the fuel cell system
402 can include an inlet and an outlet hook-up for a heat sink such
as a water tank. The flow through this circuit can be provided by a
pump in the fuel cell system, or by a pump at the heat sink
location. It will be appreciated that many variations are
possible.
[0064] Referring to FIG. 5, a schematic diagram is shown of another
example of a CHP fuel cell system 500. A coolant is circulated
between a fuel cell system 502 and a radiator 508 to maintain the
operating temperatures (e.g., the coolant temperature) in the fuel
cell system 502 at desired levels. The coolant flows from the fuel
cell system 502 to the radiator 508 via conduit 504 and returns
from the radiator 508 to the fuel cell system 502 via conduit 506.
A blower 509 is associated with the radiator 508 that is actuated
to cool the coolant flowed through the radiator 508 by blowing a
relatively cool fluid across the radiator 508. In this example, the
blower is used to flow cold air from building 514 via conduit 512
across the radiator 508, where the air is heated and is then flowed
back to building 514 via conduit 512. The blower 509 may be
actuated, as an example, by a thermostat located in building 514.
The radiator 508 may also include a second blower to reject
radiator heat to ambient to maintain a desired operating
temperature of the fuel cell system 502 when heat is not required
by the building 514. This example illustrates another means by
which fuel cell system heat can be provided to a heat sink. It will
be appreciated that while the heat sink in this example is
generally the building 514, it could also be defined in terms of
the radiator 508 as a matter of perspective.
[0065] In one aspect of the invention, an integrated fuel cell
system includes a fuel processor, a fuel cell stack, a power
conditioning system, and a control scheme adapted to coordinate the
operation of these systems with at least one heat sink and at least
one power sink. The terms "integrated fuel cell system" and "fuel
cell system" are used interchangeably, and generally refer to a
fuel cell stack that is coupled to components and subsystems that
support the operation of the stack and the application of power
generated by the stack. The term combined heat and power ("CHP")
fuel cell system refers to a fuel cell system that is used to
provide both power and the utilization of waste heat. For example,
a fuel cell can be used to produce electricity, and waste heat from
the fuel cell system can be used for various applications where
heat is needed (e.g., adsorption coolers, water heaters, boilers,
furnaces, etc.) to reduce the fuel or electricity ordinarily
required by such applications. Such systems are also sometimes
referred to as "co-generation" or "co-gen" systems. The utilization
of waste heat can dramatically increase the efficiency of such
systems.
[0066] The fuel processor includes a first coolant system wherein
heat from exothermic reactions within the fuel processor is
transferred to a coolant fluid to maintain a desired temperature in
at least a portion of the fuel processor. The coolant system may be
adapted to simultaneously maintain various different temperatures
within the fuel processor, for example, by varying coolant flow,
heat transfer surface area, and reactant flow within the fuel
processor associated with a given heat exchanger. As an example,
the coolant fluid may be glycol-based, such as propylene glycol.
Exemplary fuel processor systems are described in U.S. Pat. Nos.
6,207,122, 6,190,623, and 6,132,689, the teachings of which are
each hereby incorporated herein by reference.
[0067] Suitable fuel cell stack designs are well known. For
example, the fuel cell systems taught in U.S. Pat. Nos. 5,858,569,
5,981,098, 5,998,054, 6,001,502, 6,071,635, 6,174,616, and
09/502,886 are each hereby incorporated by reference. A fuel cell
stack may also be incorporated that is based on a "high
temperature" PEM, such as the polybenzimidazole ("PBI") fuel cell
membranes manufactured by Celanese. U.S. patents describing this
material include U.S. Pat. Nos. 5,525,436, 6,099,988, 5,599,639,
and 6,124,060, which are each incorporated herein by reference. In
this context, "high temperature" PEM's generally refer to PEM's
that are operated at temperatures over 100.degree. C. (e.g.,
150-200.degree. C.). Stacks based on other high temperature
membrane materials such as polyether ether ketone ("PEEK") may also
be suitable.
[0068] The stack includes a second coolant system that is adapted
to maintain a desired operating temperature of the stack. As an
example, a coolant fluid may be circulated through coolant channels
between each fuel cell in the stack. It is generally desirable that
a coolant flowing through the stack be substantially dielectric to
prevent the coolant from shorting the fuel cells in the stack. This
issue may also be addressed in other ways, for example, by
electrically isolating the coolant as it flows through the stack.
As examples, the coolant can be deionized water or glycol. In some
cases, the first coolant system associated with the fuel processor
may be the same as the second coolant system associated with the
stack. For example, a common fluid may be flowed through both
systems. In other cases, the coolant systems may be distinct, and
may contain different coolant fluids. For example, a coolant such
as glycol with a relatively high boiling point may be used in the
fuel processor, whereas a coolant with a relatively low boiling
point such as water may be used in the fuel cell stack.
[0069] A third coolant system may be associated with an oxidizing
unit that is adapted to oxidize combustible components in the fuel
cell exhaust such as hydrogen and unreacted hydrocarbons before the
exhaust is vented to the atmosphere. As an example, such an
oxidizing unit may resemble an automotive catalytic converter unit
and be maintained at a temperature over 600.degree. C. Maintaining
the oxidizer temperature may require sinking a substantial amount
of heat from the exothermic oxidation into the third coolant
system, depending on factors such as the level of excess hydrogen
that is fed to the fuel cell stack, the amount of residual
hydrocarbons in the fuel cell exhaust and the amount of air that is
supplied to oxidize these components. In some cases, the third
coolant system may be a portion of the first or second coolant
systems.
[0070] A fuel cell stack typically produces direct current at a
voltage which varies according to the number of cells in the stack
and the operating conditions of the cells. Applications for the
power generated by a fuel cell stack may demand constant voltage,
or alternating current at a constant voltage and frequency similar
to a municipal power grid, etc. Integrated fuel cell systems as in
the present invention may therefore include a power conditioning
system to accommodate such demands. Technologies for converting
variable direct current voltages to constant or relatively constant
voltages are well known, as are technologies for inverting direct
currents to alternating currents. Suitable power conditioner
topologies for fuel cells are also well known. For example, the
teachings of U.S. patent Ser. No. 09/471,759 are hereby
incorporated by reference.
[0071] A battery system may also be associated with the power
conditioning system, for example, to protect the fuel cells from
fuel starvation upon sudden electrical load increases on the stack.
A battery system can also be used, as examples, to supplement the
peak output power of the fuel cell system, or to provide continuous
power to an application while the fuel cell system is temporarily
shut down (as for servicing) or removed from the load. The battery
system may also include a system for periodically charging the
batteries when necessary.
[0072] Some fuel cell systems may be operated independently from
the power grid (grid independent systems), while other fuel cell
systems may be operated in conjunction with the power grid (grid
parallel systems). For grid parallel systems, the system may
include a transfer switch to transfer the electrical load between
the fuel cell system and the power grid. For example, in some grid
parallel systems, the electrical load can be switched from the fuel
cell system to the grid when the fuel cell system needs to be shut
down for maintenance. In still other grid parallel systems, the
electrical load can be shared between the fuel cell system and the
grid. Other arrangements are possible.
[0073] The heat sink of the present system is a media to which heat
from any of the above described coolant systems is transferred. In
some systems having multiple distinct coolant loops, heat from one
coolant loop may be transferred to another coolant loop having a
lower temperature by way of a liquid-to-liquid heat exchanger.
Thus, the heat sink may receive heat from throughout the fuel cell
system while directly contacting only one of the coolant loops. In
other embodiments, each coolant loop may be associated with a heat
sink. In still other embodiments, each coolant loop can be
associated with multiple heat sinks.
[0074] In some systems, the coolants are flowed through an
air-cooled radiator, such that heat from the fuel cell system is
transferred to the air around the fuel cell system. In this
example, the radiator is the heat sink. In other embodiments, the
heat sink can be a liquid-to-liquid heat exchanger, for example to
exchange heat from a liquid fuel cell system coolant to a hot water
tank or to an external fluid loop transferring heat to a hot water
tank. The fuel cell system coolant can also be in vapor form as it
contacts a heat sink. In such cases, in some embodiments the heat
sink can serve as a condenser of the vaporized coolant. Also, in
some embodiments, the temperatures of fuel cell system components
can be regulated by direct interaction with heat sinks rather than
by passing heat to a coolant loop and then to the heat sink. The
present invention contemplates that heat from the fuel cell system
can be transferred to a number of applications where heat is
desired, including domestic and commercial hot water tanks, and air
cooled radiators that supply heat to buildings or other
applications.
[0075] The various sources of heat within the fuel cell system may
differ in terms of temperature and amount of heat. For example,
fuel cells are generally operated at temperatures much lower than
fuel processors, such that the temperature of the waste heat from a
fuel cell stack is generally lower than that of the waste heat from
a fuel processor.
[0076] Waste heat at relatively lower temperatures is sometimes
referred to as "lower grade" heat, whereas waste heat at relatively
higher temperatures can be referred to as "higher grade" heat. This
is due to the fact that heat transfer efficiency is generally
greater when heat is transferred across relatively large
temperature differences. Likewise, it is generally less efficient
to transfer heat between masses having a relatively small
temperature difference. Thus, the applicability of a particular
waste heat stream for heat recovery may vary according to the
temperature and mass flow of the waste heat stream, and according
to the temperature and mass (or mass flow) into which the waste
heat is transferred.
[0077] A power sink of the present invention may be any application
to which the power generated by the fuel cell stack is sent. For
example, the fuel cell system may be used to power residential
appliances (e.g., 110 VAC, 60 Hz). The fuel cell system may also be
used to feed power to a utility grid. Other applications may
require direct current power from the fuel cell system. Finally,
the "parasitic load" of any or all of the electric components
within the fuel cell system (e.g., valves, pumps, blowers,
controllers, etc.) can also represent a power sink for the fuel
cell system.
[0078] System controllers may automate the operation of fuel cell
system components to varying degrees, and may have varying
capacities for adjustment and reconfiguration. For example, some
controllers may rely in part on software for instruction sets to
provide enhanced flexibility and adaptability, while other
controllers may rely on hardware to provide enhanced reliability
and lower cost. Control systems may also include combinations of
such systems. Controllers may include an algorithm that coordinates
open and closed loop functions. In this context, an open loop
function is one that does not utilize feedback, such as adjusting a
blower according to a look-up table without verifying the effect of
the adjustment or iterating the adjustment toward a desired effect.
A closed loop function is one that utilizes feedback to iterate
adjustments toward a desired effect.
[0079] In general, the controller circuitry may include data inputs
from system components such as safety sensors and thermocouples
throughout the system. As an example, such data inputs may report
data in the form of variable voltage or current signals, or as
binary on/off signals. The controller circuitry may also include
devices to control the voltage and/or current supplied to various
components in the system, for example to control variable speed
pumps and blowers. In general, the logic employed by a system
controller may be referred to as a control scheme. In some cases,
fuel cell systems can include multiple independent controllers and
control schemes, that may or may not be coordinated by a common
controller or control scheme.
[0080] In one aspect of the invention, a control scheme is provided
for a combined heat and power fuel cell system that coordinates
control of the system between a heat sink and a power sink. For
example, if the power sink is a set of residential appliances and
the heat sink is a hot water heater for the residence, the demand
for power may be independent from the demand for hot water.
However, the amount of power and waste heat available from the
system are linked because the waste heat is a by-product of
operating the system to produce power. Still, there may be
situations where power is required, but little or no heat is
needed, or vice versa. There is thus a need to efficiently
coordinate and balance the operation of the system between such
demands.
[0081] In one embodiment, the hydrogen stoich is adjusted to meet a
given power demand. The hydrogen stoich may be minimized such that
no more hydrogen is supplied to the fuel cell stack than is
required to meet the electrical load. For example, the teachings of
U.S. patent Ser. No. 09/749,298 are hereby incorporated by
reference. Minimizing the hydrogen stoich in this way increases the
efficiency of the system (without respect to efficiency gains from
waste heat recovery). When hydrogen stoich is supplied in excess of
what is needed to meet the electrical load on the fuel cells, the
excess hydrogen simply passes through the stack unreacted and is
oxidized in the ATO. Thus, under one embodiment of the present
invention, when there is no heat demand (e.g., from a thermostat on
a hot water heater), the system provides only enough hydrogen
stoich to meet the electrical load on the system. When heat is
demanded from the system, the hydrogen stoich is increased so that
excess hydrogen is provided to the ATO, which generates heat that
is transferred to the heat sink. The heat demand signal can be
binary (on/off), or it can be dynamic to increase the heat output
to a desired level.
[0082] As an example, referring to FIG. 6, a flow diagram is shown
of a control scheme 600 for a CHP fuel cell system. In a first step
602, the system determines if there is a power demand signal
indicating an electrical load on the fuel cell system. If there is
no power demand signal, then the system maintains an idle function
604 an continues checking for a power demand signal. In the event
of a power demand signal, the system then determines in a step 606
whether the power output of the fuel cell system is adequate to
meet the electrical load placed on the system. If the power output
is not adequate, then the reactant flow rates are incrementally
increased in step 608 and the system returns to step 606. If the
power output is adequate, the system enters a stoich optimization
mode 610. The stoich optimization mode 610 can include, for
example, reducing the fuel flow to the fuel cell until the voltage
or some other performance parameter of the fuel cell (or of the
weakest cell in a stack) is affected to an unacceptable extent. For
example, to avoid damaging the fuel cell stack from reactant
starvation, it may be desirable to monitor all of the cell voltages
in the stack, and to maintain reactant flow rates high enough to
prevent any of the cells from dropping below 0.4 volts. In some
embodiments, excess air flow is maintained such that only the flow
of the fuel is modulated by the algorithm 600.
[0083] The system also performs a check 612 for a heat demand
signal. For example, the heat demand signal could be a thermostat
indicating that a water tank is below a desired temperature. In the
example shown in FIG. 6, the fuel cell system heat used to supply
the heat in response to a heat demand signal is supplied primarily
from an oxidizer unit (see heat exchanger 336 of FIG. 3). If there
is no heat demand signal, the system continues in optimization mode
610. Where there is a heat demand signal, the system then performs
an increase 614 in the reactant flow rates. For example, in this
example, for a constant power demand, increasing the fuel flow rate
will increase the amount of unreacted fuel in the fuel cell exhaust
that is processed in the oxidizer to generate heat.
[0084] In the next step 616, the system checks whether the heat
being supplied by the system to the heat sink is adequate to meet
the demand for heat. For example, step 616 could include, as
examples, calculating whether enough heat will be made available in
a desired amount of time at a given operating point, or it can
include supplying heat at a given operating point for a period of
time and then checking again whether the desired amount of heat has
been supplied. If not enough heat has been supplied, then the
system further increases the flow rates of the reactants. If the
heat demand has been met, the system returns 618 to its
optimization mode. It will be appreciated that the system is
continually looking for power or heat demand signals by repeatedly
cycling through these determinations. Some embodiments may
eliminate the determination 616 and simply continue producing a
heat demand signal while heat is needed.
[0085] In another embodiment, the hydrogen stoich is similarly
minimized such that no more hydrogen is supplied to the fuel cell
stack than is required to meet the electrical load. When heat is
required from the system, a heat demand signal causes raw
hydrocarbon (e.g., natural gas or propane) to bypass the fuel
processor so that it is oxidized in the ATO to provide the desired
heat. The heat demand signal can be binary (on/off), or it can be
dynamic to increase the heat output to a desired level.
[0086] Referring to FIG. 7, another flow diagram 700 is shown of a
control scheme for a CHP fuel cell system to illustrate various
logical options that may be implemented by a system to balance a
combination of heat and power demand signals. In a first state 702,
there is a power demand, but no heat demand. In response, the
system lowers the reactant flow rates in step 704 to a point where
the power demand can still be met. Step 704 serves to maximize fuel
efficiency. In this mode, the system also exhausts its waste heat
to ambient in a step 706 (e.g., the environment outside the fuel
cell system, or to the atmosphere).
[0087] In a second state 708, there is both a power demand and a
heat demand. In this state 708, the system increases reactant flow
rates in a step 710 until the power and heat demands are both met.
It will be appreciated that the power demand will be met first, and
the heat demand will then be met as the excess reactants reach a
point where the energy produced by oxidizing the excess fuel is
sufficient to meet the heat demand.
[0088] In a third state 712, there Is no power demand, but there is
a heat demand. In this state 712, the system can be configured to
select between two options. In a first response option 714, the
fuel cell system is maintained at a constant power output (or an
output that directly tracks an electrical load), and fuel is
bypassed from the fuel processor directly to the oxidizer to
produce the heat required. In some cases, it may be desirable to
continue operating the fuel processor and to instead bypass the
reformate from the fuel cell stack to the oxidizer. In a second
response option under state 712, the system operates the fuel cell
at a power output sufficient to provide enough waste heat to meet
the heat demand. The excess power produced is then put into a power
sink, such as in charging a battery system or supplying power to a
utility grid.
[0089] In a fourth state 718, there is no power demand and no heat
demand. In this example, the system responds in step 720 by idling,
meaning that the system operates at just a high enough power output
to maintain readiness for general operation. The power produced is
sent to a power sink.
[0090] In another embodiment, the heat sink receives at least a
portion of its heat from the fuel cell stack (e.g., from a coolant
circulated through the stack and contacted with the heat sink). In
this embodiment, the system responds to a heat demand signal (e.g.,
from a thermostat on a hot water heater) by shorting at least one
fuel cell within the fuel cell stack. For example, the teachings of
U.S. patent Ser. No. 09/428,714 are hereby incorporated by
reference. When a fuel cell is shorted, its electrical potential is
driven to zero and all power generated in the cell is in the form
of heat. Essentially, the shorted cell is converted into a
resistive heater. In this way, additional heat can be supplied by
the fuel cell system for a given power output of the fuel cell
stack.
[0091] For example, referring to FIG. 8, a flow diagram 800 is
shown of a control scheme for a CHP fuel cell system. This method
of operation contains the following logical steps: (802) operating
the system at a reactant stoichiometry optimized according to the
power demand; (804) checking for a heat demand signal; (806)
shorting at lest one fuel cell when there is a heat demand signal;
(808) checking whether the heat demand has been met; (810)
deactivating the cell shorting mechanism when the heat demand has
been met; and returning to step (802) to repeat the steps
(802)-(810).
[0092] Such a control scheme may also include a step where the
hydrogen stoich is minimized with respect to the electrical load,
and a step where the reformer output to the fuel cell stack is
increased over what would normally be supplied for a given
electrical load to compensate for the loss of power production of
any cells that are shorted. Similarly, an additional step in the
control logic may be provided where the power conditioning system
compensates for the reduction in voltage from the stack resulting
from having some cells shorted (e.g., a DC to DC conversion
operation is modified to provide a higher voltage to a DC to AC
inverter). In some systems, the power conditioning components may
tolerate a range of input voltages such that such a step is
unnecessary (e.g., a voltage tolerant or multi-voltage
inverter).
[0093] Such a control scheme may also include a step where the
operating conditions of the un-shorted cells are adapted to
optimize the current density and/or voltage of the un-shorted
cells. For example, it is well understood in the art that a fuel
cell voltage and current density can be manipulated according to
the electrical load on the cell and the reactant conditions and
stoichiometry provided. For example, the teachings of U.S. patent
Ser. No. 09/471,759 (referenced above) include the use of a battery
system coupled with a dynamic current limiting device to supply
constant power to an electrical load while controlling the portion
of the load that is placed on the fuel cell stack.
[0094] In another embodiment, the heat sink also receives at least
a portion of its heat from the fuel cell stack. The fuel cells in
the system are divided into at least two sections of cells
connected in series. In a first operating mode, the sections of
cells are connected in series, and in a second operating mode, at
least two sections of cells are operated in parallel. In general,
the first and second operating modes will provide different
operating efficiencies in terms of the amount of heat produced per
unit power. For example, the second operating mode may produce more
heat. In such an embodiment, a heat demand signal may result in the
system switching to the operating mode providing the most heat for
the amount of power produced. Such embodiments may include the use
of a power conditioning system capable of accommodating the
differing voltages associated with each operating mode.
[0095] In another embodiment, the heat sink also receives at least
a portion of its heat from the fuel cell stack. In this embodiment,
to vary the amount of heat that is produced by the fuel cell stack
for a given power output, the reactant stoichiometry is reduced
until the respective voltages of the fuel cells in the stack are
reduced to a point where the stack begins producing more heat with
respect to the amount of power produced. Such embodiments may
include the use of a power conditioning system capable of
accommodating the resulting variation in stack voltage. Also, such
embodiments may include the use of MEA's in the stack that are
tolerant to reactant starvation under load. For example, the
teachings of U.S. patent Ser. No. 09/727,748 are hereby
incorporated by reference.
[0096] Referring to FIG. 9, a flow diagram is shown of a control
scheme for a CHP fuel cell system. This method of operation
contains the following logical steps: (902) operating the system at
a reactant stoichiometry optimized according to the power demand;
(904) checking for a heat demand signal; (906) activating a low
efficiency operating mode where waste heat is increased for a given
power output; (908) checking whether the heat demand has been met;
(910) deactivating the low efficiency operating mode when the heat
demand has been met; and returning to step (902) to repeat the
steps (902)-(910).
[0097] In another aspect, the invention provides a control
apparatus for executing any of the above logic schemes for
coordinating the power and heat output of a fuel cell system.
Techniques for preparing circuitry to provide electronic control
systems are well known in the art, such that a system under the
present invention with the features and aspects described above
could be implemented by one of ordinary skill, for example by
reference in part to the patents mentioned above.
[0098] In another aspect of the invention, a method is provided for
enabling a fuel cell system to accommodate variable demands for
heat and power output. In one embodiment, the method includes
providing excess fuel to the fuel cell system in response to a
control signal (e.g., a heat demand signal as from a thermostat)
such that the excess unreacted fuel is burned in a fuel cell
exhaust oxidizer to produce heat. In another embodiment, the method
includes shorting at least one fuel cell within the fuel cell stack
in response to a control signal to provide additional heat into a
fuel cell stack coolant fluid. In another embodiment, the method
may include selectively electrically connecting fuel cells in a low
efficiency mode (e.g., some cells in parallel rather than in
series) in response to a control signal (e.g., a heat demand signal
as from a thermostat) to provide additional heat into a fuel cell
stack coolant fluid. In another embodiment, the method may include
selectively fuel starving a fuel cell under load to an operating
point providing a desired balance between power and heat production
of the fuel cell. In each of these embodiments, the method may
further include flowing or selectively flowing a fuel cell system
coolant to a heat sink to transfer fuel cell system waste heat to a
heat sink.
[0099] In another aspect of the invention, an integrated fuel cell
system is provided that is coupled to a power sink and a heat sink.
A controller of the fuel cell system is adapted to respond to data
signals from the power sink and the heat sink. For example, such
data signals from the heat sink may include a temperature
indication or a heat demand signal (such as from a thermostat).
Such data signals from the power sink may include a voltage or
current measurement, an electrical power demand signal, or an
electrical load.
[0100] In one embodiment, the controller is adapted to provide
excess fuel to the fuel cell system in response to a control signal
(e.g., a heat demand signal as from a thermostat) such that the
excess unreacted fuel is burned in a fuel cell exhaust oxidizer to
produce heat. In another embodiment, the controller is adapted to
activate a mechanism to short at least one fuel cell within the
fuel cell stack to provide additional heat into a fuel cell stack
coolant fluid. In another embodiment, the controller is adapted to
selectively electrically connect fuel cells in a low efficiency
mode (e.g., some cells in parallel rather than in series) to
provide additional heat into a fuel cell stack coolant fluid. In
another embodiment, the controller is adapted to selectively fuel
starve a fuel cell under load to an operating point providing a
desired balance between power and heat production of the fuel cell.
In each of these embodiments, the controller may be further adapted
to direct a fuel cell system coolant to a heat sink to transfer
fuel cell system waste heat to a heat sink.
[0101] In another embodiment of the invention, an article of
manufacture is provided that includes at least one computer usable
medium having computer readable code embodied thereon for enabling
the coordination of heat demand and power demand signals in the
operation of an integrated CHP fuel cell system. For example, such
code may implement any of the logic operations and functions
described above, by themselves or in combination.
[0102] In another embodiment, the invention provides at least one
program storage device readable by a machine, tangibly embodying at
least one program of instructions executable by the machine to
perform a method for enabling a fuel cell system to accommodate
simultaneous variable demands for heat and power output, including
any of the features described above, by themselves or in
combination.
[0103] In another embodiment, a fuel cell system is provided that
includes a fuel cell stack and a first coolant circuit. The first
coolant circuit is adapted to circulate a first coolant through the
fuel cell stack and transfer heat from the fuel cell stack to a
heat sink. As previously discussed, the heat sink can be any medium
or object that heat is transferred to. In some embodiments, the
heat sink is a hot water tank. In other embodiments, the heat sink
is a body of air in a building. In still other embodiments, the
heat sink is a generator portion of an adsorption cooling system.
Other heat sink applications are possible.
[0104] A second heat source such as a fuel processor or an exhaust
gas oxidizer is present in the system and a second coolant circuit
is adapted to circulate a second coolant through the second heat
source to transfer heat from the second heat source to the heat
sink. A controller is connected to a first pump and adapted to vary
an output of the first pump, wherein the first pump is located in
the first coolant circuit to drive the first coolant flow. A second
pump is also connected to the controller, which is adapted to vary
an output of the second pump, and wherein the second pump is
located in the second coolant circuit to drive the second coolant
flow. As examples, the controller can be adapted to maintain a
temperature of the fuel cell stack above or below a predetermined
level, or to maintain a temperature of the second heat source above
or below a predetermined level.
[0105] In some embodiments, the heat sink is a heat exchanger
including a first flow path adapted to receive a flow of the first
coolant, a second flow path adapted to receive a flow of the second
coolant; and a third flow path adapted to receive a flow of a third
fluid. For example, the heat exchanger could receive cold water as
the third fluid. In one portion of the heat exchanger, the water
receives heat from the first fluid (e.g., fuel cell coolant) and in
a second portion of the heat exchanger, the water receives
additional heat from the second fluid (e.g., fuel processor or
oxidizer coolant that is at a higher temperature than the first
fluid). The heated water (i.e., heat sink) is then flowed to its
application, in this case a hot water tank.
[0106] Preferably, at least one of the first and second coolant
circuits include a radiator having a variable speed radiator fan.
The radiator allows the system to expel heat to ambient when the
heat is not needed by the heat sink. In some embodiments, a heat
demand sensor is connected to the controller and adapted to vary a
speed of the radiator fan to maintain a temperature of the heat
sink above a predetermined level.
[0107] The system can further include a third heat source and a
third coolant circuit, wherein the third coolant circuit is adapted
to circulate a third coolant through the third heat source to
transfer heat from the second heat source to the heat sink. A third
pump is also connected to the controller, which is adapted to vary
an output of the third pump. As an example, the second heat source
can be a fuel processing reactor, and the third heat source is a
system exhaust gas oxidizer, such that heat is transferred from
both subsystems to the heat sink.
[0108] In another aspect, the invention provides a method of
operating a fuel cell system, including the following steps: (1)
transferring heat from a fuel cell to a first coolant circuit; (2)
transferring heat from a second system heat source to a second
coolant circuit; (3) transferring heat from each of the first and
second coolant circuits to a heat sink; (4) varying a first coolant
flow through the first coolant circuit to maintain a temperature of
the fuel cell below a predetermined level; and (5) varying a second
coolant flow through the second coolant circuit to maintain the
second system heat source below a predetermined level. Such methods
can further comprise selectively flowing at least one of the first
coolant and second coolant through a radiator; and operating a fan
to blow air across the radiator to remove heat from the
radiator.
[0109] In another embodiment, the invention provides a fuel cell
system having a first heat source (e.g., a fuel cell stack) and a
first coolant circuit, wherein the first coolant circuit is adapted
to circulate a first coolant through the fuel cell stack and remove
heat from the first heat source. A second heat source (e.g., a fuel
processing reactor) and a second coolant circuit are also included,
wherein the second coolant circuit is adapted to circulate a second
coolant through the second heat source to remove heat from the
second heat source.
[0110] A first heat exchanger in the system includes a first
coolant flow path and a second coolant flow path, wherein the heat
exchanger is adapted to transfer heat from the first coolant to the
second coolant when a first temperature of the first coolant is
greater than a second temperature of the second coolant. A second
heat exchanger is located along the second coolant circuit
downstream from the first heat exchanger, the second heat exchanger
being adapted to transfer heat from the second coolant circuit to a
heat sink fluid when the second coolant in the second heat
exchanger has a higher temperature than the heat sink fluid. A
radiator system is provided that includes a radiator and a fan, the
radiator system being located along the second coolant circuit
between the first heat exchanger and the second heat exchanger. The
radiator is adapted to remove heat from the second coolant
circuit.
[0111] A controller is connected to a first pump that is adapted to
vary a flow of the first coolant. The controller is also connected
to a second pump that is adapted to vary a flow of the second
coolant. As an example, the controller can be configured to vary a
speed of the pump or radiator fan to maintain the heat sink fluid
above a predetermined temperature.
[0112] In another aspect, the invention provides a fuel cell system
with a system housing and a heat sink vessel. The heat sink vessel
circulates a heat sink fluid. For example, the heat sink vessel can
be a hot water tank that heats water by circulating it within the
vessel, where the water is then flowed to another location for use
outside the system housing. A portion of the heat sink vessel is
contained in an interior of the system housing. It will be
appreciated that in the context of this invention, the term portion
can mean anywhere from less than 1% to 100%. The portion of the
heat sink vessel includes a thermally conductive material. A system
component is fixed onto the portion of the heat sink vessel such
that heat is transferred from the system component to the heat sink
vessel when a temperature of the system component is greater than a
temperature of the portion of the heat sink vessel. The system
component can be any of the following: a pump, a valve, a solenoid,
a fuel cell stack end plate, a water tank, a blower, and a
circuitry housing.
[0113] In another aspect, a fuel cell system is provided that
includes a fuel cell, a fuel supply, an oxidant supply, a power
demand sensor, a heat demand sensor, and a controller. The fuel
cell is adapted to receive a fuel flow from the fuel supply, and an
oxidant flow from the oxidant supply. The controller is connected
to each of the fuel supply, oxidant supply, power demand sensor,
and heat demand sensor. The controller is further adapted to
receive a power demand signal from the power demand sensor and a
heat demand signal from the heat demand sensor.
[0114] In a first state, the controller is configured to reduce at
least one of the fuel flow and oxidant flow when there is no heat
demand signal and no power demand signal. In a second state, the
controller is configured to increase at least one of the fuel flow
and oxidant flow when there is no heat demand signal and there is a
power demand signal. In a third state, the controller is configured
to increase at least one of the fuel flow and oxidant flow when
there is no power demand signal and there is a heat demand signal.
In a fourth state, the controller is configured to increase at
least one of the fuel flow and oxidant flow when there is a power
demand and a heat demand signal.
[0115] In some embodiments, the power demand sensor is a fuel cell
voltage sensor that produces a power demand signal when a voltage
of the fuel cell falls below a predetermined level. The power
demand sensor can also be a fuel cell current sensor that produces
a power demand signal when an output current of the fuel cell
exceeds a predetermined level. The power demand sensor can also
include a fuel cell output current sensor an electrical load
sensor, wherein the power demand sensor produces a power demand
signal when an electrical load on the fuel cell exceeds an output
current of the fuel cell. It will be appreciated that the
electrical load on the fuel cell can include a parasitic system
electrical load and an application electrical load. For example,
the parasitic load can refer to internal components such as pumps
and blowers that are powered by the fuel cell. The application load
can refer to a residential appliance, as an example.
[0116] The system can further include a coolant circuit and a heat
sink, wherein the coolant circuit is adapted to transfer heat from
the fuel cell to the heat sink. As an example, the heat demand
sensor can be a temperature sensor that produces a heat demand
signal when a temperature of the heat sink is below a predetermined
level.
[0117] In one embodiment, the system can include a heat sink, a
coolant circuit, and an oxidizer adapted to oxidize an exhaust gas
of the fuel cell. The coolant circuit is configured to transfer
heat from the fuel cell to the heat sink, and the heat demand
sensor is a temperature sensor that produces a heat demand signal
when a temperature of the heat sink is below a predetermined level.
In another embodiment, the coolant circuit is adapted to transfer
heat from the fuel cell to the heat sink, and a radiator is
provided to remove heat from the coolant circuit. The radiator can
include a fan connected to the controller, where the controller is
configured to reduce an output of the fan when there is a heat
demand signal. The controller is further configured to increase an
output of the fan when there is no heat demand signal.
[0118] In another embodiment, the coolant circuit further includes
a bypass valve and a radiator bypass circuit. The valve is
connected to the controller, and the controller is adapted to
actuate the valve to divert a coolant flow from the radiator to the
radiator bypass circuit when there is a heat demand signal. The
controller is further adapted to actuate the valve to divert the
coolant flow from the radiator bypass circuit to the radiator when
there is no heat demand signal.
[0119] The system can also include a fuel bypass circuit associated
with the valve. In such a system, the valve is connected to the
controller, and the fuel bypass circuit is adapted to divert a
portion of the fuel flow from an inlet of the fuel cell to the
oxidizer. The controller is configured to actuate the valve to
divert the portion of fuel flow from the fuel cell inlet to the
oxidizer when there is a heat demand signal. The controller is
further adapted to actuate the valve to divert the portion of fuel
flow from the fuel cell inlet to the oxidizer when there is no heat
demand signal. As an example, the controller can include a computer
usable medium (e.g., memory) having computer readable code embodied
thereon (e.g., firmware or software). Preferably, the controller is
also programmable.
[0120] Embodiments may further include a hydrogen separator, such
as electrochemical hydrogen separator. On this subject, the
teachings of U.S. Pat. No. 6,280,865 are hereby incorporated by
reference. The hydrogen separator is adapted to receive the fuel
flow from the fuel processor and separate hydrogen from the fuel
flow into a reservoir when the hydrogen separator is activated. The
controller is configured to activate the hydrogen separator when
there is no power demand signal and there is a heat demand
signal.
[0121] As an example, the hydrogen separator can include a membrane
electrode assembly having an anode side and a cathode side. It is
well known in the art that placing an electric potential across an
electrochemical cell, such as a fuel cell, having no electrical
load (as opposed to merely placing an electric load on the fuel
cell as in the case of normal operation) will result in hydrogen
being electrochemically "pumped" from fuel (e.g., reformate) in the
anode to the cathode. This process proceeds essentially according
to the same reactions at the anode and cathode of the fuel cell as
in normal operation. Depending on the mechanical strength of the
cell used in such a process, the hydrogen output is robust enough
that such a process can be used to pressurize a vessel.
[0122] For example, such a cell can be placed along the flow path
of the reformate being fed from the fuel processor to the fuel
cell. When there is a heat demand, but no power demand, the
controller reacts enough fuel in the fuel cell to produce the
desired amount of heat. The excess power is sunk to the hydrogen
separator to pressurize a hydrogen tank (e.g., at about two
atmospheres), which will contain essentially pure hydrogen. The
hydrogen tank reservoir can include a valve connected to the
controller and associated with a conduit to the fuel cell such that
the controller can selectively open the valve to supply hydrogen to
the fuel cell (e.g., in response to a sudden load increase).
[0123] The hydrogen separator can be a PEM fuel cell (e.g., a PEM
sandwiched on either side by a platinum based catalyst layer). The
anode side is in fluid connection with the fuel flow from the fuel
processor. The anode side and cathode side of the membrane
electrode assembly each have an electrical connector (e.g., a wire
connected to the each of the anode and cathode flow field plates. A
power source is connected to the anode and cathode electrical
connectors of the membrane electrode assembly and provides an
electric potential across the connectors when the separator is in
an active state. Similarly, the controller can remove the potential
to put the separator in an inactive state. While the separator is
in the inactive state, the reformate simply passes by it on the way
to the fuel cell without effect. In some embodiments, the separator
can also be used, as can the hydrogen reservoir supply to the fuel
cell, when there is a power demand. This mode of operation offers
additional flexibility that can be used by the controller to
balance between the dynamic behavior of the heat and power
demands.
[0124] In another aspect, the invention provides a method of
operating a fuel cell system including the following steps: (1)
providing a fuel flow and an oxidant flow to a fuel cell to produce
electricity; (2) providing the electricity to an electrical load;
(3) transferring heat from the fuel cell to a heat sink by
circulating a first coolant through a first coolant circuit,
wherein the first coolant circuit is adapted to remove heat from
the fuel cell and is further adapted to transfer heat to the heat
sink; (4) measuring a thermal parameter of the heat sink; (5)
measuring an electrical parameter of the electrical load; (6)
measuring a performance parameter of the fuel cell; (7) generating
a power demand signal when a power output of the fuel cell
indicated by the performance parameter is less than a power
requirement of the electrical load indicated by the electrical
parameter; (8) generating a heat demand signal when the thermal
parameter of the heat sink is below a predetermined level; (9)
reducing at least one of the fuel flow and oxidant flow when there
is no heat demand signal and no power demand signal; (10)
increasing at least one of the fuel flow and oxidant flow when
there is no heat demand signal and there is a power demand signal;
(11) increasing at least one of the fuel flow and oxidant flow when
there is no power demand signal and there is a heat demand signal;
and (12) increasing at least one of the fuel flow and oxidant flow
when there is a power demand and a heat demand signal.
[0125] In one embodiment, the method may further include measuring
a voltage of the fuel cell; and generating the power demand signal
when the voltage of the fuel cell falls below a predetermined
level. In another embodiment, the method can include measuring an
output current of the fuel cell; and generating the power demand
signal when the output current of the fuel cell exceeds a
predetermined level. Another embodiment can include exhausting fuel
gas from the fuel cell to an oxidizer; oxidizing the fuel gas in
the oxidizer to generate heat; and transferring heat from the fuel
cell to the heat sink by circulating a second coolant through a
second coolant circuit, wherein the second coolant circuit is
adapted to remove heat from the oxidizer and is further adapted to
transfer heat to the heat sink.
[0126] As previously discussed, some embodiments may include a fuel
bypass system to allow fuel to be fed directly to the oxidizer when
heat is demanded. Methods associated with such embodiments may thus
include diverting a portion of the fuel flow from an inlet of the
fuel cell to the oxidizer in response to the heat demand
signal.
[0127] In some cases, the first and second coolant circuits can be
in fluid communication, where the first and second coolants are
each portions of a common coolant flow. In other words, the first
and second coolant circuits are different locations within a single
coolant circuit. In various embodiments, either of the first or
second coolants can be circulated through a radiator to remove heat
from the first coolant. In one embodiment, the second coolant can
be circulated through a radiator to remove heat from the first
coolant. For example, system heat can be absorbed by the first
coolant, transferred to the second coolant in a heat exchanger, and
then expelled to ambient from the second coolant in a radiator. One
advantage provided by such an arrangement is that the first coolant
can be a dielectric stack coolant such as deionized water, whereas
the second material can be an electrically conductive material such
as tap water. The radiator can be made from a less expensive
material since the tap water is flowed through it instead of the
deionized water, which might require more expensive radiator
materials to prevent corrosion.
[0128] As previously discussed, the heat sink can include a water
tank. In such cases, the thermal parameter can be a temperature of
water in the water tank. The heat sink can also be a body of air
contained in a building. In such cases the thermal parameter can be
a temperature of the air contained in the building. The heat sink
can also be a generator portion of an adsorption cooling system. In
such cases, the thermal parameter can be a temperature of the
generator portion.
[0129] In another aspect, the invention provides a method of
operating a fuel cell system including the following steps: (1)
providing a fuel flow and an oxidant flow to a fuel cell stack to
produce electricity; (2) providing the electricity to an electrical
load; (3) transferring heat from the fuel cell stack to a heat sink
by circulating a first coolant through a first coolant circuit,
wherein the first coolant circuit is adapted to remove heat from
the fuel cell stack and is further adapted to transfer heat to the
heat sink; (4) measuring a thermal parameter of the heat sink; (5)
generating a heat demand signal when the thermal parameter of the
heat sink is below a predetermined level; and (6) shorting at least
one fuel cell in the fuel cell stack in response to the heat demand
signal.
[0130] When a fuel cell is electrically shorted (e.g., the anode
electrode is electrically connected to the cathode electrode,
essentially all of the energy produced is converted to heat energy.
While this may harm a fuel cell if it is operated is this manner
too long or if the fuel cell is allowed to overheat, operating the
cell in a shorted mode for controlled periods of time can provide a
desired increase in heat from the fuel cell while not harming its
performance or reliability. For example, a sulphonated fluorocarbon
PEM fuel cell operated in this manner might be kept under
100.degree. C., as an example, either by modulating the coolant
flow through the cell or by removing the short connection at a
desired point. As an example, the short connection can be a jumper
placed between an anode electrode and a cathode electrode, wherein
the jumper has a switch that can be actuated by the system
controller.
[0131] As in previous examples, the heat sink can be a water tank,
a body of air contained in a building, a generator portion of an
adsorption cooling system, etc. The thermal parameter generally
refers to a temperature of the heat sink.
[0132] In another aspect, the invention provides a method of
operating a fuel cell system, including the following steps: (1)
providing a fuel flow and an oxidant flow to a fuel cell stack to
produce electricity; (2) providing the electricity to an electrical
load; (3) transferring heat from the fuel cell stack to a heat sink
by circulating a first coolant through a first coolant circuit,
wherein the first coolant circuit is adapted to remove heat from
the fuel cell stack and is further adapted to transfer heat to the
heat sink; (4) measuring a thermal parameter of the heat sink; (5)
generating a heat demand signal when the thermal parameter of the
heat sink is below a predetermined level; and (6) selectively
connecting at least two fuel cells in the fuel cell stack in
parallel in response to the heat demand signal.
[0133] Without wishing to be bound by theory, a group of fuel cells
generally produce a greater amount of waste heat when they are
connected in parallel rather than in series. One reason is that the
cells generally operate at a lower efficiency in such a
configuration, so that more waste heat is generated. Thus, the
invention provides an embodiment where the balance between the heat
and power demand signals is accommodated by selectively connecting
at least two fuel cells within a group to increase the amount of
heat that is generated for a given amount of power production.
Where a system is adapted to selectively connect one or more cells
in parallel, the cells that are selectively connected are connected
via a switched network, rather than being stack in series as in a
conventional stack. For example, two fuel cells may be connected to
a switch that is connected to two electrical paths. When the system
controller causes the switch to select one of the paths, this
results in the cell being connected in series with another cell.
When the other path is selected, the cell will be connected in
parallel (e.g., connected to a common bus).
[0134] As in previous examples, the heat sink can be a water tank,
a body of air contained in a building, a generator portion of an
adsorption cooling system, etc. The thermal parameter generally
refers to a temperature of the heat sink.
[0135] In another aspect, the invention provides a method of
operating a fuel cell system, including the following steps: (1)
providing a fuel flow and an oxidant flow to a fuel cell stack to
produce electricity; (2) providing the electricity to an electrical
load; (3) transferring heat from the fuel cell stack to a heat sink
by circulating a first coolant through a first coolant circuit,
wherein the first coolant circuit is adapted to remove heat from
the fuel cell stack and is further adapted to transfer heat to the
heat sink; (4) measuring a thermal parameter of the heat sink; (5)
generating a heat demand signal when the thermal parameter of the
heat sink is below a predetermined level; and (6) selectively
operating the fuel cell in a low efficiency mode in response to the
heat demand signal.
[0136] There are various fuel cell operating regimes that result in
relatively low efficiency operation and the production of
relatively high amounts of waste heat. Prior art systems are
generally configured to avoid such regimes as a means of maximizing
system efficiency. However, in systems balancing both heat demand
and power demand signals, it may be desirable to switch between
such modes. Other examples of low efficiency operating modes
include reactant starvation, operation at temperatures outside the
normal operating range of a fuel cell, and producing a given amount
of power at low voltage and high current (e.g., cell voltages less
than 0.4 volts).
[0137] While the invention has been disclosed with respect to a
limited number of embodiments, those skilled in the art, having the
benefit of this disclosure, will appreciate numerous modifications
and variations therefrom. It is intended that the invention covers
all such modifications and variations as fall within the true
spirit and scope of the invention.
* * * * *