U.S. patent application number 11/165620 was filed with the patent office on 2006-12-28 for thermal control of fuel cell for improved cold start.
Invention is credited to Bruce Lin.
Application Number | 20060292406 11/165620 |
Document ID | / |
Family ID | 37451050 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060292406 |
Kind Code |
A1 |
Lin; Bruce |
December 28, 2006 |
Thermal control of fuel cell for improved cold start
Abstract
Improvements in startup time for an electrochemical fuel cell
system from freezing and sub-freezing temperatures are obtained by
utilizing an insulated fuel cell stack in combination with an
thermal control subsystem. Temperature of the insulated
electrochemical fuel cell stack, as well as temperature of the
ambient environment, are monitored and a heating fluid is heated by
thermal transfer with the environment under appropriate thermal
conditions. The heated fluid is then passed to the insulated fuel
cell in order to increase the temperature of the same, typically to
a temperature at or near the temperature of the ambient
environment. In this manner, ambient heat from the environment is
utilized to increase the temperature of the insulated fuel cell
stack, thus improving conditions for subsequent cold start of the
insulated fuel cell stack.
Inventors: |
Lin; Bruce; (Vancouver,
CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 5400
SEATTLE
WA
98104
US
|
Family ID: |
37451050 |
Appl. No.: |
11/165620 |
Filed: |
June 23, 2005 |
Current U.S.
Class: |
429/434 ;
429/442; 429/452 |
Current CPC
Class: |
H01M 8/04365 20130101;
H01M 16/003 20130101; H01M 2008/1095 20130101; H01M 8/04768
20130101; H01M 8/0432 20130101; H01M 8/04268 20130101; H01M 8/04955
20130101; H01M 8/04029 20130101; H01M 8/04007 20130101; Y02E 60/50
20130101 |
Class at
Publication: |
429/013 ;
429/026; 429/024 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Claims
1. A method for thermally controlling an electrochemical fuel cell
system having an insulated electrochemical fuel cell stack, the
method comprising: monitoring the temperature of the insulated
electrochemical fuel cell stack; monitoring the temperature of the
ambient environment; heating a heating fluid by thermal transfer
with the environment when the temperature of the ambient
environment is above the temperature of the insulated fuel cell
stack; and passing the heated fluid to the insulated fuel cell
stack.
2. The method of claim 1 wherein the step of heating the heating
fluid is performed when the temperature of the ambient environment
is above the temperature of the insulated fuel cell stack by a
predetermined temperature offset.
3. The method of claim 2 wherein the predetermined temperature
offset ranges from 1.degree. C. to 15.degree. C.
4. The method of claim 2 wherein the predetermined temperature
offset ranges from 2.degree. C. to 5.degree. C.
5. The method of claim 1 wherein the electrochemical fuel cell
system is a component of an automobile.
6. The method of claim 5 wherein the heating fluid is a coolant in
fluid communication with a radiator.
7. The method of claim 6 wherein the coolant is passed to the
insulated fuel cell stack by a coolant pump.
8. A thermal control subsystem for an electrochemical fuel cell
system having an insulated electrochemical fuel cell stack, the
subsystem comprising: a heating loop fluidly connected to the
insulated electrochemical fuel cell stack, the heating loop capable
of carrying a heating fluid; a heat exchanger fluidly connected to
the heating loop, the heat exchanger capable of transferring
ambient thermal energy from the environment to the heating fluid
such that the temperature of the heating fluid is at or near
ambient temperature; a first temperature sensor capable of
detecting the temperature of the electrochemical fuel cell stack; a
second temperature sensor capable of detecting the temperature of
the ambient environment; a pump for circulating the heating fluid
within the heating loop; a controller in communication with the
first temperature sensor, the second temperature sensor and the
pump, the controller programmed to activate the pump when the
temperature of the ambient environment is greater than the
temperature of the electrochemical fuel cell stack.
9. The thermal control system of claim 8 further comprising a valve
within the heating loop, the valve in a closed position when the
pump is not activated.
10. The thermal control system of claim 8 wherein the controller is
capable of activating the pump when the temperature of the ambient
environment is greater than the temperature of the electrochemical
fuel cell stack plus a predetermined temperature offset that ranges
from 1.degree. C. to 15.degree. C.
11. The thermal control system of claim 9 wherein the predetermined
temperature offset ranges from 2.degree. C. to 5.degree. C.
12. An automobile comprising a thermal control system of claim 8.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to a thermal control
system for an electrochemical fuel cell, as well as a method for
improving thermal conditions for cold start of an insulated fuel
cell stack.
[0003] 2. Description of the Related Art
[0004] Electrochemical fuel cells convert reactants, namely fuel
and oxidant fluid streams, to generate electric power and reaction
products. Electrochemical fuel cells employ an electrolyte disposed
between two electrodes, namely a cathode and an anode. The
electrodes each comprise an electrocatalyst disposed at the
interface between the electrolyte and the electrodes to induce the
desired electrochemical reactions. The location of the
electrocatalyst generally defines the electrochemically active
area.
[0005] Polymer electrolyte membrane (PEM) fuel cells generally
employ a membrane electrode assembly (MEA) consisting of an
ion-exchange membrane disposed between two electrode layers
comprising porous, electrically conductive sheet material as fluid
diffusion layers, such as carbon fiber paper or carbon cloth. In a
typical MEA, the electrode layers provide structural support to the
ion-exchange membrane, which is typically thin and flexible. The
membrane is ion conductive (typically proton conductive), and also
acts as a barrier for isolating the reactant streams from each
other. Another function of the membrane is to act as an electrical
insulator between the two electrode layers. The electrodes should
be electrically insulated from each other to prevent
short-circuiting. A typical commercial PEM is a sulfonated
perfluorocarbon membrane sold by E.I. Du Pont de Nemours and
Company under the trade designation NAFION.RTM..
[0006] The MEA contains an electrocatalyst, typically comprising
finely comminuted platinum particles disposed in a layer at each
membrane/electrode layer interface, to induce the desired
electrochemical reaction. The electrodes are electrically coupled
to provide a path for conducting electrons between the electrodes
through an external load.
[0007] In a fuel cell stack, the MEA is typically interposed
between two separator plates that are substantially impermeable to
the reactant fluid streams. The plates act as current collectors
and provide support for the electrodes. To control the distribution
of the reactant fluid streams to the electrochemically active area,
the surfaces of the plates that face the MEA may have open-faced
channels formed therein. Such channels define a flow field area
that generally corresponds to the adjacent electrochemically active
area. Such separator plates, which have reactant channels formed
therein are commonly known as flow field plates. In a fuel cell
stack, a plurality of fuel cells are connected together, typically
in series, to increase the overall output power of the assembly. In
such an arrangement, one side of a given plate may serve as an
anode plate for one cell and the other side of the plate may serve
as the cathode plate for the adjacent cell. In this arrangement,
the plates may be referred to as bipolar plates.
[0008] The fuel fluid stream that is supplied to the anode
typically comprises hydrogen. For example, the fuel fluid stream
may be a gas such as substantially pure hydrogen or a reformate
stream containing hydrogen. Alternatively, a liquid fuel stream
such as aqueous methanol may be used. The oxidant fluid stream,
which is supplied to the cathode, typically comprises oxygen, such
as substantially pure oxygen, or a dilute oxygen stream such as
air. In a fuel cell stack, the reactant streams are typically
supplied and exhausted by respective supply and exhaust manifolds.
Manifold ports are provided to fluidly connect the manifolds to the
flow field area and electrodes. Manifolds and corresponding ports
may also be provided for circulating a coolant fluid through
interior passages within the stack to absorb heat generated by the
exothermic fuel cell reactions. The preferred operating temperature
range for PEM fuel cells is typically 50.degree. C. to 120.degree.
C., most typically between 60.degree. C. and 85.degree. C.
[0009] Under typical conditions, start-up of the electrochemical
fuel cell stack is under ambient temperatures and the fuel cell
stack can be started in a reasonable amount of time and quickly
brought to the preferred operating temperature. In some fuel cell
applications, it may be necessary or desirable to commence
operation of an electrochemical fuel cell stack when the stack core
temperature is below the freezing temperature of water (commonly
referred to as "freeze-start" conditions), and even at subfreezing
temperatures below -25.degree. C. However, at such low
temperatures, the fuel cell stack does not operate well and rapid
start-up of the fuel cell stack is more difficult. It may thus take
a considerable amount of time and/or energy to bring an
electrochemical fuel cell stack from a starting temperature below
the freezing temperature of water up to an efficient operating
temperature.
[0010] A variety to techniques have been developed and/or proposed
to address this issue, including the addition of various heating
elements and/or heat-exchanging subsystems that are designed to
quickly increase the temperature of the fuel cell stack. Another
technique involves insulation of the fuel cell stack itself. Thus,
if the ambient temperature is at or below the freezing temperature
of water, the stack temperature may stay above freezing for some
extended period of time following shut down, which permits more
favorable starting conditions should the stack be restarted during
this period of time.
[0011] While advances have been made associated with cold start of
fuel cell stacks, there remains a need in the art for improved
and/or more efficient techniques relating to the same. The present
invention fulfills such needs and provides further related
advantages.
BRIEF SUMMARY OF THE INVENTION
[0012] In brief, a thermal control subsystem is disclosed for an
electrochemical fuel cell system, particularly with regard to
improving thermal conditions for cold start of an insulated fuel
cell stack. A disadvantage of insulting the fuel cell stack is that
it is also insulated from increasing temperature as the ambient
temperature rises. Thus, if the insulated fuel cell stack is
exposed to freezing temperatures for a prolonged period of time, it
will take longer for the insulated fuel cell stack to warm as the
ambient temperature increases.
[0013] In one embodiment, a method is disclosed for thermally
controlling an electrochemical fuel cell system having an insulated
electrochemical fuel cell stack. The method comprises monitoring
the temperature of the insulated electrochemical fuel cell stack;
monitoring the temperature of the ambient environment; heating a
heating fluid by thermal transfer with the environment to a
temperature at or near the temperature of the ambient environment;
and passing the heated fluid to the insulated fuel cell until the
temperature of the fuel cell stack is at or near the temperature of
the ambient environment.
[0014] In another embodiment, a thermal control subsystem is
disclosed comprising a heating loop fluidly connected to the
insulated electrochemical fuel cell stack, the heating loop capable
of carrying a heating fluid; a heat exchanger fluidly connected to
the heating loop, the heat exchanger capable of transferring
ambient thermal energy from the environment to the heating fluid; a
first temperature sensor for detecting the temperature of the
electrochemical fuel cell stack (T.sub.s); a second temperature
sensor for detecting the temperature of the ambient environment
(T.sub.a); a pump for circulating the heating fluid within the
heating loop; one or more optional valves within the heating loop;
and a controller in communication with the first temperature
sensor, the second temperature sensor, the optional valve(s) and
the pump, the controller programmed to activate the pump and open
the optional valve(s) when the temperature of the ambient
environment is (T.sub.a) is greater than the temperature of the
electrochemical fuel cell stack (T.sub.s).
[0015] These and other aspects of the invention will be evident
upon reference to the attached figures and following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The provided FIGURES illustrate certain non-optimized
aspects of the invention, but should not be construed as limiting
in any way.
[0017] FIG. 1 is a representative embodiment of a thermal control
subsystem for an electrochemical fuel cell system having an
insulated electrochemical fuel cell stack.
[0018] FIG. 2 is a representative flow chart showing various steps
associated with monitoring and controlling the thermal control
subsystem for an electrochemical fuel cell system having an
insulated electrochemical fuel cell stack.
DETAILED DESCRIPTION OF THE INVENTION
[0019] As mentioned above, a thermal control subsystem for an
electrochemical fuel cell system is disclosed. The electrochemical
fuel cell system comprises an insulated electrochemical fuel cell
stack. The fuel cell stack may be insulated by any of a variety of
suitable techniques known to one skilled in this field, including
(but not limited to) use of insulating materials such as foams, or
by employing appropriate isolation techniques, such as vacuum
gaps.
[0020] The thermal control subsystem comprises a heating loop
fluidly connected to the insulated electrochemical fuel cell stack,
the heating loop capable of carrying a heating fluid. The heating
loop is also fluidly connected to a heat exchanger, the heat
exchanger capable of transferring ambient thermal energy from the
environment to the heating fluid. In this way, heat from the
ambient environment may be transferred to the heating fluid within
the heating loop, and then transferred to the insulated fuel cell
stack by circulation of the heating fluid.
[0021] The thermal control subsystem further comprises a first
temperature sensor capable of detecting the temperature of the
electrochemical fuel cell stack (T.sub.s); and a second temperature
sensor capable of detecting the temperature of the ambient
environment (T.sub.a). Suitable temperature sensors in this regard
are well known, and need not be further exemplified.
[0022] A pump is associated with the heating loop such that the
heating fluid can be circulated within the heating loop. The
heating fluid exits the insulated fuel cell stack, enters the heat
exchanger, exits the heat exchanger, and returns to the insulated
fuel cell stack, after which the cycle may be repeated. One or more
optional valves may also be associated with the heating loop such
that the insulated fuel cell stack is isolated from the remainder
of the heating loop. In this manner, thermal transfer from the
insulated fuel cell stack to the heating fluid may be minimized by
preventing convection circulation of the heating fluid.
[0023] A controller is in communication with the first temperature
sensor, the second temperature sensor, the pump and valve(s). The
controller is programmed to activate the pump and open the optional
valve(s) when the temperature of the ambient environment is
(T.sub.a) is greater than the temperature of the electrochemical
fuel cell stack (T.sub.s) as explained in greater detail below.
[0024] During operation of an insulated fuel cell stack, the
temperature of the stack, as well as the associated systems in
direct or fluid communication with the stack, are at an elevated
temperature. As noted previously, the operating temperature for PEM
fuel cells is typically in the range of between 60.degree. C. and
85.degree. C. Thus, the various associated subsystems in direct or
fluid communication with the stack are similarly at an elevated
temperature. For example, the coolant subsystem, including the
radiator, coolant tubing, coolant pump, and the like, are generally
at a temperature on the order of 60.degree. C. at fuel cell
shutdown. Following shut down, and assuming a colder ambient
temperature, the various associated fuel cell subsystems begin to
cool. If, for example, the ambient temperature is at -25.degree.
C., the various associated fuel cell subsystems will cool rapidly.
Under such conditions, and in order to delay cool down of the fuel
cell stack, the stack may be insulated in a manner such that the
stack stays warmer for an extended period of time. Thus, if the
fuel cell is restarted at a later time (e.g., 3 hours later) it may
have only cooled to 20.degree. C., which is still above freezing
and thus permits more favorable restart conditions.
[0025] However, after a sufficiently long period of time, the
insulated fuel cell stack will eventually cool to ambient
temperature, such as -25.degree. C., as discussed above. Restarting
the fuel cell from a temperature below the freezing point of water
is commonly referred to as a "freeze-start", and involves any
number of techniques known to those skilled in this field. If the
ambient temperature increases from, for example, -25.degree. C. to
5.degree. C., the insulation of the fuel cell stack will serve to
delay an increase in temperature of the fuel cell stack. In other
words, the fuel cell stack is insulated from increasing ambient
temperature, such that as the ambient temperature increases, the
insulated fuel cell stack will undesirably remain at a lower
temperature. If this lower temperature is below 0.degree. C.,
freeze-start of the fuel cell is required, even though the ambient
temperature may be above freezing. Further, even if the ambient
temperature remains below freezing, an increase in the temperature
of the fuel cell stack from, for example, -25.degree. C. to
-10.degree. C., is still beneficial by creating more favorable
conditions for freeze-start.
[0026] Accordingly, ambient heat from the environment is utilized
to increase the temperature of an insulated fuel cell stack.
Following shut down, the insulated fuel cell stack may be thermally
isolated from the remainder of the heating loop by closing optional
valve(s). In this manner, thermal heat loss via the heating loop
may be minimized (e.g., heat loss via convection circulation of the
heating fluid). The ambient temperature of the environment and the
fuel cell stack temperature may also be measured. Typically, the
fuel cell stack temperature is measured at a location within the
fuel cell stack that is representative of the entire stack (e.g.,
at or near the center of the stack), and is referred to herein as
the "core" temperature of the fuel cell stack. If the core
temperature of the fuel cell stack is higher than the ambient
temperature, the insulated fuel cell stack remains isolated from
the remainder of the heating fluid (e.g., the optional valve(s)
remains closed). On the other hand, if the ambient temperature is
higher than the core temperature of the fuel cell stack, ambient
heat from the environment may be utilized to increase the
temperature of the insulated fuel cell stack.
[0027] As the gradient between the ambient temperature of the
environment and the insulated fuel cell stack increases, the
ability to transfer thermal energy to the insulated fuel cell stack
generally becomes more efficient. For example, a gradient of
10.degree. C. allows more effective thermal transfer than a
gradient of 1.degree. C. Thus, in a more specific embodiment, a
minimum temperature gradient or "offset" is required before
transfer of thermal energy proceeds. This offset may be determined
based on the specific energy requirements of the system at hand,
and should weight the benefits of increasing the temperature of the
insulated fuel cell against the power required of the fuel cell
system to achieve such an increase. For example, a suitable
temperature offset (T.sub.offset) may be in the range of 1.degree.
C. to 15.degree. C., or in the range of from 2.degree. C. to
5.degree. C.
[0028] In addition to determining the temperature of the ambient
environment and the insulated fuel cell stack, the state of charge
(SOC) of the battery associated with the fuel cell system should
also be evaluated. Any load on the battery associated with the
practice of this invention should not deplete the battery to a
level that would be insufficient to power a subsequent start-up of
the fuel cell. In other words, a reserve SOC (SOC.sub.reserve)
should be maintained.
[0029] Provided that the appropriate temperature offset and reserve
SOC are satisfied, the optional valve(s) isolating the insulated
fuel cell stack from the heating loop are opened and the pump turn
on, resulting in the circulation of the heating fluid within the
heating loop. It should be understood that reference herein to the
heating fluid and the heating loop is, in one embodiment,
synonymous with reference to the coolant fluid and the coolant loop
typically used to cool a fuel cell stack. However, in this
embodiment the coolant system is operating as a heating
system--that is, the coolant is operating as a heating fluid, and
the coolant loop is operating as a heating loop. Thus, to clarify
thermal transport is from the ambient environment to the insulated
fuel cell stack (as opposed from the fuel cell stack to the ambient
environment), such elements are referred to as the heating fluid
and the heating loop.
[0030] The heating loop is fluidly connected to one or more exit
ports of the insulated fuel cell stack such that the heating fluid,
after passing through the insulated fuel cell stack, travels
through a heat exchanger in communication with the ambient
environment, thereby heating the heating fluid. The heat exchanger
is, in turn, fluidly connected to one or more input ports of the
insulated fuel cell stack, such that the heating fluid after
exiting the heat exchanger (now heated to a temperature at or near
ambient temperature), passes into the insulated fuel cell stack.
This heated heating fluid transfers thermal energy to the insulated
fuel cell stack, thereby raising the temperature of the insulated
fuel cell stack. The heating fluid then exits the insulated fuel
cell stack for recirculation through the heat exchanger and back
into the insulated fuel cell stack.
[0031] The result of the circulating heating fluid is that the
temperature of the insulated fuel cell stack is raised, typically
to a temperature at or near the ambient environmental temperature.
Once such a temperature is reached, the pump may be stopped and the
optional valve(s) closed in order to stop circulation of the
heating fluid. By this technique, the insulated fuel cell stack can
be freeze-started from a more benign temperature, or may even
result in thawing of the insulated fuel cell stack such that
freeze-start may be avoided. Thus, by insulating the fuel cell
stack, the number of start ups from subzero temperatures may be
reduced (since the insulated fuel cell stack remains at an elevated
temperature for a longer period following shut down), and the
conditions associated with start up, particular freeze-start, may
be rendered more advantageous.
[0032] The energy cost for this process is very low, and quite
manageable for a storage battery to accommodate. Other than minimal
power to open valve(s) and monitor temperatures, the only load
during operation is that of the pump. The pump, however, does not
draw a significant load, and need only be operated for relatively
short period of time. Once the temperature of the insulated fuel
cell stack has reached it desired temperature in relation to the
ambient temperature, the pump is stopped. If a further temperature
offset is satisfied, the process can be repeated. In other words,
the process can be repeated whenever the ambient temperature warms
to a level sufficient to make the thermal transfer of energy to the
insulated fuel cell stack worth the energy cost of running the
pump.
[0033] In the practice of this process, it should be understood
that additional components may be associated with the heating loop
disclosed herein, particularly in the context of automotive
applications. Such additional components include those typically
associated with, for example, an automotive coolant system, and are
well known to one skilled in this field.
[0034] In another embodiment, the heating fluid may be air. As
noted above, the heating fluid serves to transfer thermal energy
from the ambient environment to the insulated fuel cell stack when
a suitable temperature offset has been reached. While a liquid
heating fluid generally has a greater heat capacity than air, air
may also be used as the heating fluid (either alone or in
combination with a liquid heating fluid). In this embodiment, air
is circulated through the heating loop in the manner noted above.
Alternatively, air (at ambient temperature) may be blown into the
insulated fuel cell stack, and exit after have transferred some
portion of its thermal energy to the insulated fuel cell stack. In
this embodiment, the pump may be a blower or compressor, and the
heat exchanger may be omitted, and the heating loop may be closed
or open (with the air, after having passed through the insulated
fuel cell stack, returning to the ambient environment).
[0035] In still a further alternative embodiment, the heating fluid
may be a liquid heating fluid (e.g., coolant), and circulation
through the insulated fuel cell stack is by convection circulation
(as opposed to pump driven). In this manner, the insulated fuel
cell stack is arranged in relationship to the heating loop such
that thermal transfer from the heating fluid to the insulated fuel
cell stack is accomplished by convection circulation of the heating
fluid when a suitable temperature offset has been reached.
[0036] Referring to FIG. 1, a representative thermal control
subsystem for an electrochemical fuel cell system having an
insulated electrochemical fuel cell stack is depicted. Insulated
fuel cell stack 10 is fluidly connected via conduit 20 to radiator
30. Within conduit 20 is a heating fluid (not shown), such as an
antifreeze solution, that exits fuel cell stack outlet port(s) 12
and enters radiator inlet port(s) 32. Within radiator 30, thermal
energy from the ambient environment is transferred to the heating
fluid. Radiator 30 is, in turn, fluidly connected via conduit 25 to
insulated fuel cell stack 10. Heating fluid passing out of radiator
30 via outlet port(s) 34 travels through conduit 25 to fuel cell
stack inlet port(s) 14. Within insulated fuel cell stack 10,
thermal energy from the heating fluid is transferred to the
insulated fuel cell stack.
[0037] Flow of heating fluid from insulated fuel cell stack 10,
through conduit 20 to radiator 30, and then from radiator 30,
through conduit 25 and back to insulated fuel cell stack 10,
constitutes the heating loop. It should be understood that this
heating loop utilizes heating fluid to transfer heat to the
insulated fuel cell stack. Of course, when the insulated fuel cell
is in normal operation, these same components may serve as a
cooling loop, transferring thermal energy from insulated fuel cell
stack 10 to the environment via radiator 30. However, for purposes
of this invention, the coolant serves the opposite purpose--that
is, it carries heat to the insulated fuel cell.
[0038] Circulation of the heating fluid within the heating loop is
accomplished with pump 40. When in operation, valve 28 is opened to
permit circulation of the heating fluid within the heating loop.
When closed, valve 28 prevents circulation of heating fluid within
the heating loop. While valve 28 is depicted in conduit 20, it
should be understood that valve 28 may be at any position along the
heating loop, including (without limitation) immediately adjacent
to inlet/outlet ports 12 and/or 14. Further, multiple valves may be
employed. While the valve may be omitted altogether, closing the
valve(s) provides advantages with regard to retention of heat
within the insulated fuel cell stack.
[0039] Pump 40 and valve 28 are controlled by controller 70. One
skilled in this field will appreciate that any number of
controllers may be used for this purpose, and further disclosure
regarding the same is not necessary herein. Controller 70 is also
in communication with insulated fuel cell stack sensor 50 and
ambient temperature sensor 60. By monitoring the temperature of the
insulated fuel cell stack (e.g., the core temperature) and the
ambient temperature, controller 70 can be programmed to open valve
28 and start pump 40, such that thermal energy from the environment
is transferred to the insulated fuel cell stack by the procedures
disclosed above.
[0040] Power for pump 40 is provided by battery 72, shown in FIG. 1
to be connected to controller 70 (alternatively, the battery may
provide power directly to the pump). To ensure that power to the
pump does not overly deplete the battery, the state of charge (SOC)
of the battery is monitored. As long as the battery has sufficient
charge in excess of some reserve necessary to subsequently restart
the insulated fuel cell stack (SOC.sub.reserve), then the pump can
be run for a period of time sufficient to transfer the desired
level of thermal energy from the environment to the insulated fuel
cell. This may be accomplished by, for example, running the pump
for some fixed period of time, followed by measuring the
temperature of the insulated fuel cell stack and the ambient
temperature. Alternatively, such temperature may be measured while
the pump is running, and the pump shut down after a desired level
of thermal energy has been transferred to the insulated fuel cell
stack.
[0041] In this manner, thermal energy from the ambient environment
serves to heat the heating fluid, which may then transfer such heat
to the insulated fuel cell stack, thereby raising the temperature
of the fuel cell stack to a temperature at or near ambient
temperature. This is particularly useful when, for example, the
ambient temperature is significantly below freezing at night,
followed by a warming trend in the morning hours. An insulated fuel
cell stack, while maintaining heat within the stack following shut
down for a period of time longer than without such insulation, will
also insulated the stack from rising temperatures. If, for example,
the fuel cell stack was at a temperature of -25.degree. C. due to
outdoor storage overnight, in the morning hours the temperature may
increase to, for example, -10.degree. C. Due to insulation of the
fuel cell stack, the increase in ambient temperature will not be
immediately transferred to the fuel cell stack due to insulation of
the stack. The present invention thus provides the transfer of
thermal energy from the ambient environment to an insulated fuel
cell stack via the heating fluid which is in contact with the
ambient environment, thus permitting the use of an insulated fuel
cell stack without the existing drawbacks associated with the
same.
[0042] Referring to FIG. 2, a representative control process is
disclosed. In step 10, the insulated fuel cell stack is shut off
and the coolant valve is closed. In step 20, the ambient
temperature (T.sub.a) and the temperature of the insulated fuel
cell stack (T.sub.s) are measured. If the ambient temperature is
above that of the insulated fuel cell stack plus the desired offset
temperature (i.e., if T.sub.a>T.sub.s+T.sub.offset), the amount
of energy remaining in the battery ("state of charge" or SOC) is
measured, as shown by step 30. In step 40, if the SOC is greater
than the state of charge for a predetermined reserve amount (i.e.,
SOC.sub.reserve), then the coolant valve is opened and the pump run
for a period of time (t.sub.run). On the other hand, if the
SOC.sub.reserve is not satisfied, then in step 50 the pump is not
run in order to prevent depletion of the remaining battery charge.
In step 60, if the ambient temperature is not above that of the
insulated fuel cell stack plus the desired offset, step 20 is
repeated at desired intervals. This entire control process may be
repeated continually or periodically to provide the desired level
of thermal energy to the insulated fuel cell stack.
[0043] From the foregoing, it will be appreciated that, although
specific embodiments of the invention have been described herein
for purposes of illustration, various modifications may be made
without deviating from the spirit and scope of the invention.
Accordingly, the invention is not limited except as by the appended
claims.
* * * * *