Note: Descriptions are shown in the official language in which they were submitted.
112L.~78 S
This invention relates to electrochemical cells, such
as fuel cells and batteries of type wherein reactant or product
gas is conducted to or from the cells. The invention relates
more particularly to thermal control for such cells.
In the design of fuel cells and like electrical energy
producing devices involving reactant or product gas undergoing
electrochemical reaction (process gas), thermal control is a
dominant para~eter. me electrochemical reactions in such devices
are invariably accompanied by heat generation or heat absorption
because of entropy changes accompanying the reaction and irrever-
sibilities caused by diffusion and activation overpotentials and
ohmic resistance. In the accommodation of thermal control, the
art has looked to various techniques, none of which are entirely
satisfactory.
The thermal control technique seemingly most desirable
takes advantage of the sensible heat of the process gas itself
as a vehicle for thermal control, Thusl if removal o heat from
the cell is desired, the incoming process gas may be supplied
to the cell at a temperature lower than the cell operating tem-
perature such that exiting gas removes heat simply by increase in
temperature thereof in passage through the cell. In this tech-
nique, one adJusts the process gas flow level above the flow
level required for production of preselec~ed measure of electri-
cal energy, such additional process gas serving the heat removal
f~ction. Disadvantages attending this practice include unde-
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178~
sirable pressure drops based on the increased process gas flow,auxiliary power penalty and loss of electrolyte through vapori-
zation or entrainment. By auxiliary power is meant the power
requirements of apparatus aceessory to the fuel cell proper, e.g.,
gas pumps, pressurizing systems and the like. As respects
electrolyte loss, all process gas in this gas sensible heat
technique is in cammunication with the cell electrolyte in its
passage through the cell?and, whére substantial additional gas
is required for thermal, control, a very high electrolyte loss
due to saturation of the gas with electrolyte vapor is observed
~n electrolyte gas resulting in quite high electrolyte loss.
In a sec~nd thermal control technique, the art has
looked to limiting the temperature gradients inside fuel cells
by employment of a bipolar plate having an extended fin disposed
outside the cell proper, as shown in U.S. patent ~o. 3,623,913
~o Adlhart et al. While this technique provides a somewhat more
uniform cell tempe~ature, high gas flow passing directly through
the cell can result in high electrolyte 109s and increased
auxiliary puwer.
A th$rd thermal control technique relies on the
sensible heat of a dielectric liquid. Such sensible-heat liquid
approach requires much lower auxiliary power as compared to
the gaseous heat transfer medium, but requires a separate heat
tran~fer loop and an electrically isolated manifolding system.
To avoid shunt currents between stacked cells, dielectric fluida
1~7~
such as fl~orocarbon or silicon-based oils have been tradi-
tionally used as the heat transfer media. Because the
catalyst material may be poisoned severely by even a trace
amount of these dielectric fluids, a small leak from the
heat transfer loop may be fatal to the cell. Also, the die-
lectric liquids are flammable and have toxic reaction products.
In a forth technique for thermal control, the art
has relied on the latent heat of liquids. Latent heat liquids
(U.S. patents Nos. 3,498,844 and 3,507,702 to Sanderson; U.S.
patent No. 3,761,316 to Stedman; and U.S. patent No. 3,969,145
to Grevstad et al.) can pro~ide heat transfer at nearly
uniform temper~t~re, although there may be some temperature
gradients in the stacking direction if the heat transfer plate
is placed between a group of cells. The auxiliary pawer
requirements are expected to be low. Suitable dielectric fluids
having boiling points in the range of cell operating temperature
can be used, ~ut the disadvantagas of the sensible-heat liquid
approach apply here also. To overcome these disadvantages,
non-dielectric media, such as water, can be used. If water is
used, a suitable quality steam can be genera~ed for use in
other parts of the plant. External heat exchange also is
expected to be efficient because of high heat transfer coeffi-
cients. Unfortuna~elys the use of a non-dielectric liquid neces-
sitate8 elaborate corrosion protection schemes (U.S. patent No.
3,969,145 to Grevsta~u et al; U.S. patent No. 3,923,546 to Xatz
~5
et al.; U.S. patent No. 3,940,285 to Nickols et al.) and!or
the use of an exte~mely low conductivity liquid. During
ope~ation, the conductivity may increase, so means to restore
the low conductivity may also be required. If the cooling
loop is under pressure, good seals are necessary. If a leak
de~elops during the life of the stack because of pinholes
caused by corrosion or deterioration of seals, it could para-
lyze the entire system. Because of the corrosion protection
requirements and intricate manifolding, the cost of the heat
transfer subsystem ope~ating on dielectric coolant could be
substantial.
In applicant's view, the foregoing techniques illustrate
limitations in the state of the art of thermally controlling
uel cells and the like by reliable, simple and cost-effective
practice and a need exists for a fundamentally different
approach to the thermal control problem.
It is an object of the present invention to provlde
a method for operation of electrochemical cells and system
arrangement therefor which enables efficient and simplified
thermal control.
In attaining the ~oregoing and other objects, the in-
vention provides for supplementing the flow of process gas
through an electrochemical cell, in measure required for therm~l
control by sensible heat of process gas, in manner both avoiding
electrolyte loss and pressure ~op increase across the cell. In
impl~menting this process gas sensible-heat technique, the
invention introduces, in addition to the customary process gas
passage in communication with the cell electrolyte through an
electrode a process gas passage in the cell which is isolated
from the cell electrolyte and in thermal communication with a
heat-generating surface of the cell. Such electrolyte-cammunl-
cative and electrolyte-~solated passages are commonly manifolded
to pressurized supply of process gas. The flow levels in the
respective passages are set individually by passage parameters
to provide both for desired level electrical energy cell output
and desired heat removal.
The foregoing and other objects and features of the
inv~ntion will be further understood from the following detailed
discussion thereof and from the drawings wherein like reference
numerals identify like parts throughout.
Fig. 1 is a sectional drawing of an explanatory embodi-
ment of a fuel cell in accordance with the invention, as seen
along plane I-I o Fig. 2.
Fig. 2 is a plan elevation of the Fig. 1 fuel cell,
shown together with accessory process gas supply and treatment
apparatus.
Fig. 3 is a sectional view of Fig. 1 fuel cell as
seen along plane III-III Fig. 1.
Figs. 4 and 5 are perspective illu~;trations of fuel
cell stacke in accordance with the invention.
In Figs. 1 and 3, fuel cell 10 includes anode and
cathode electrodes 12 and 14, of gas diffusion type, and elec-
trolyte matrix or layer 16 therebetween. Separator plates 18 and
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20 are shown in the Qxplanatory Fig. 1 single cell embodiment
as being of unipolar character, defining channel passages
18a, for supplying process gas to anode electrode 12, and
passages 20a, for supplying process gas ~o cathode electrode 14.
Based on the gas diffusion character of electrodes 12 and 14,
passages 18a and 20a constitute electrolyte-communicative passages.
In accordance with the invention, thermal control
plates 22 and 24 are stacked respectively on and under separator
plates 18 and 20. Plate 22 includes condui~ passage 22a
extending in like direction, i e., across the plane of Fig. 1,
with passages 18a and is commonly connected therewith by input
anode gas manifold 26 and output anode gas manifold 28.
Plate 24 includes conduit passage 24a extending in like
die~ction, i.e., into the plane Fig. 1, with passages 20a and
is c~mmonly connected therewith by input cathode gas manifold 30
(Fig. 2I and output cathode gas manifold 32. Since separator
plates 18 and 20 are essentially gas-impermeable, thermal control
plate passages 22a and 24a constitute electrolyte-isolated pas-
sages. Thus, process gases, i.e., anode gas supplied from
manifold 26 and cathode gas supplied from manifold 30 present,
in passages 22a and 24a can be conducted through the ~uel cell
to serve thermal control purposes without contributing to
electrolyte loss. To the contrary, process gases conducted through
channels 18a and 20a give rise to exit gas unavoidably partially
saturated with electrolyte vapor.
As alluded to above, certain electrochemical systems
involve a single gaseous reactantl e.g., zinc-air batteries.
In practicing the subject invention in such systems, a single
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electrolyte-isolated passage may be commonly manifolded at input
and exit with the single electrolyte-c~mmunicative gas pasa~ge.
Further, as is noted below, plural gaseous reactant cells, e.g.,
fuel cells, may employ thermal control plates for one or the
other of the process gases. Where desired, exit admixing oE
process gas conducted through electrolyte-communicative and
electrolyte-isolated passages may be dispensed with in favor of
common manifolding solely of input process gas supplied to such
diverse character passages. Also5 as discussed below, the
present invention contemplates the introduction of electrolyte-
isolated process gas passages, commonly input manifolded with a
process gas supply, individually per plural cells in a stack of
fuel cells, or either or both of the diverse process gases.
Referring again to Fig. 2, input anode gas manifold
26 is supplied through feed conduit 34, which is in turn fed
from pressurized input anode ga~ supply 36. Process gas from
æupply 36 may be admized with, and thus supplemented by, process
g~s the~etofore conducted through the fuel cell. For this
purpose, output gas from manifold 28 is conducted through
conduit 38 to external heat exchanger unit 40 and thence to a
mixing valve in supply 36. By operation of valve 42, gas msy
be funnel8d to purge conduit 44, ss desired. If removal of heat
from gas conducted through conduit 38 is to be affected prior
to recirculation, as is typical, unit 40 is of heat reducing type
whereb~ gaæ supplied from unit 40 to supply 36 is of temperature
lower than the cell operating te~pera~ure.
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For thermal treatment, purging and recirculation of
cathode process gas, counterpart components include feed conduit
46, pressurized input cathode gas supply 48, output gas conduit
50, purge valve S2, purge conduit 54 and external heat
exchanger unit 56.
In implementation of methods of the invention, process
gas 10w is established at a level or levels, as respects
electrolyte-communicative passages 18a and/or 20a, to attain
predetermined electrical energy to be produced by the electro-
chemical cell. Even assuming reversibility of electrochemical
reactions in fuel cells, a recognized minimum amount of heat is
liberatàd. Also, as alluded to above, irreversibility in fuel
cells, resultant ~rom activation, concentration and ohmic over-
potentials, results in additional heat generation. Typically,
in fuel cells, about fifty per cent of input enthalpy shows
up as heat and the remainder as such predetermined electrical
energy. The heat Qnergy may be ascribed as about one-fifth
reversible heat and four~fifths heat due to irreversibility.
With process gas flow in passages 18a and 20a set in
accordance with such predetermined desired electrical energy call
output, process gas flow in electrolyte-isolated passages, 22a
and/or 24a is now set to obtain a predetermined operating tempe-
rature range for the electrochemical cell. The flow in electrolyte-
isolated passages, is greatly larger than flow in electrolyte-
communicative passages. No completely analytical procedure ap-
plies, since input and exit oriflce geometry, conduit sk~n fric-
tion, c~nduit length and manifold geometries demand empirical
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test. The practice of achieving desired flows in the respective
passages may include variation of size and geometry of the flow
passages and/or the placement of fixed or variably-settable con-
strictions in either or both passages.
Referring to Fig. 4, a preferred embodiment of cell
stack 56 is shown without associated electrical output connections
and encasements. Electrolyte layers and gas difEusion a~Q~des
and cathodes are identified jointl~ as cell assemblies 58a-58j.
The top separator plate 60 is of unipolar type having electrolyte-
co~nunicative channel passages 60a, as in the case of separator
plate 18 ~f Fig. 1, and overlies the anode of top cell assembly
58a. Separator plate 62 is of bipolar type, defining elPctro-
lyte-communicative channel passages 62a, which underlie the
cathode of top cell assembly 58a, and 62b which overlie the anode
of second cell assembly 58b. Bipolar plates 64, 66 and 68 separate
cell assemblies 58b, 58c and 58d, with plate 68 gas passages 68b
overlgin~ the annd~of cell ~ssembly 58e. Separator plate 70 i9
of unipolrr type, having pa6sages 70a underlying the cathode of
cell aseembly 58e. A sub-s~ack of five fuel cells ifi thus pro-
vided. Thermal control plate 72 is disposed beneath such sub-
stack with its conduit passage 72a in communication with heat-
generating surface of the sub-stack, namely, the undersurface of
separator plate 70. A like sub-stack of five fuel cells, inclu-
sive of cell assemblies 58f-58;, is disposed beneath plate 72.
Unipolar separator plates 74 and 7~ are endwise of the sub-stack
and bipolar separator plates 78, %0 and 82 are intermediate the
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sub-stack. Thermal control plate 84 is arranged with its conduit
passage 84a in communication with the undersurface of separator
plate 76.
Input anode and cathode gas manifolds 86 and 88 are
shown schematically and separated from stack ~6. Based on the
inclusion of thermal control plates 70 and 84 with anode gas
conduit passages 72a and 84a, manifold 86 supplies process gas
commonly to and through electrolyte~communicatlve and electro-
lyte-~olated passages. Cathode fuel flow from manifold 88 is
limited to elec~rolyte-c~mmunicative passages in this showing.
In the illustrated arrangement, one electrolyte-isolated passage
is associated with each sub-stack of five fuel cells. Where a
thermal control plate is located between sub-stacks, as in the
case of plate 72, it will serve to cool both such sub-stacks.
Other assignment of thermal control plates p~r fuel cells may
be made as desired. Stiffening elements 73 may be introduced
in plate 72, as shown in Fig. 4, to strengthen the stack and
increase heat transfer surface area. Such members are desirably
electrically conductive to further enhance electrical current
passage through plate 72.
In Fig. 5, an arrangement converse to that of Fig. 4
is shown wherein stack 90 is constructed identically with stack
56 of Fig. 4 except for the disposition of the thermal control
plates. Thus, in Fig. 5, thermal control plates 92 and 94 have
~heir conduit passages 92a and 94a disposed for conducting
cathode gas through the stack for heat removal. As will be ap-
preciated, the embodiments of Figs. 4 and 5 may be combined.
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The thermal control method and arrangement of the in-
vention will be seen to provide several important benefits.
Heat transfer is accomplished by using an additional flow of
process gas without req~iring any separate manifolding system,
as is necessary in case of liquid heat transfer medium.
Possibility of corrosion by shunt currents and any ha~mful
effects by leakage are completely eliminated. The system
reliability is, theref~re, much greater than that for liquid
heat transfer media. The electrolyte losses by carry-over or
vaporization to the process gases are minimized because only
a limited amount of process gases contact the electrolyte.
Process gases passing through the thermal control plates do not
contact the electrolyte, so vapor losses due to flow of heat
transfer gases are absent. The thermal control plates can
serve as stiffening members, providing additional strength
to the stack assembly. Further, if it is required to replace
some defective cells during operation, a group of cells between
two thermal control plates can be easily removed and new cells
ca-n be replaced.
The invention may be practiced generally in any
electrochemical cell having a reactant gas and is particularly
adapted to fuel cell usage, such as phosphoric acid fuel cells
wherein the procsss gas used also in thermal control is the cat-
hode gas and/or hydrogenrich anode gas, and molten carbonate
fuel cells wherein the process gas used also in ~hermal control
is air/carbon dioxide cathode gas mixture andlor hydrogen-rich
anode gas mixture.
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1~247~3S
Various changes in the described methods of operation
and in the illustrated systems may be introduced without depart-
ing from the invention. By way of example, one may elect to
supplement process gas furnished by supply 36 and/or supply 48
(Fig. 1) solely with process gas conducted through electrolyte-
isolated passages rather than the described admixture of gases
conducted through both electrolyte-communicative and electrolyte-
isolated passages. To implement this variation, cell output
gases are not manifolded but, rather, are separately issued
with the issuance conduit of the electrolyte-isolated passage
being placed in communication with the input manifold serving
both types of passages. Thus, the particularly disclosed
practices and system embodiments are intended in an illustrative
and not in a limiting sense. The true spirit and scope of the
inven~ion is set forth in the following claims.
