EXCESS COOLANT FLUID FEED TO FUEL CELL STACKS
The present invention relates to electrochemical fuel cells disposed in a stack formation,
and in particular to cooling systems for such fuel cell stacks.
Conventional electrochemical fuel cells convert fuel and oxidant, generally both in the
form of gaseous streams, into electrical energy and a reaction product. A common type
of electrochemical fuel cell for reacting hydrogen and oxygen comprises a polymeric ion
transfer membrane, also known as a proton exchange membrane (PEM), within a
membrane-electrode assembly (MEA), with fuel and air being passed over respective
sides of the membrane. Protons (i.e. hydrogen ions) are conducted through the
membrane, balanced by electrons conducted through a circuit connecting the anode and
cathode of the fuel cell. To increase the available voltage, a stack is formed comprising
a number of MEAs electrically arranged in series. Each MEA is provided with separate
anode and cathode fluid flow paths. The anode and cathode fluid flow paths
respectively deliver fuel and oxidant to the membrane. The fuel cell stack is typically in
the form of a block comprising numerous individual fuel cell plates held together by end
plates at either end of the stack.
Because the reaction of fuel and oxidant generates heat as well as electrical power, a
fuel cell stack requires cooling once an operating temperature has been reached, to
avoid damage to the fuel cells. Cooling may be achieved at least in part by the delivery
of water to individual cells in the stack in either the anode fluid flow paths (which serves
to hydrate the anode) and/or in the cathode fluid flow path which combines with reactant
water. In each case, evaporative cooling of the fuel cells can occur.
In a typical arrangement, the cooling water is injected into the anode or cathode fluid
flow channels from one or more common manifolds extending down the side of the fuel
cell stack. A potential problem arises from the flow rates of water within such manifolds.
Water may be fed into an inlet at one end of an inlet manifold, from which it is fed into
individual cells in the stack. This results in a reduction in water flow rate along the
manifold away from the inlet. If, for example, a 100 cell stack requires a flow of 100
ml/min delivered at one end of the stack, the flow rate in the manifold at the first cell will
be 100 ml/min; after the 50th cell the flow rate in the manifold may be approximately 50
ml/min, and at the final cell the flow rate in the manifold may be only 1 ml/min. Such
very low flows, e.g. 1 ml/min, in the manifold can lead to reliability problems for a fuel cell
stack. Problems can occur in regions of stagnant or near stagnant flow due to increased
corrosion risk, particularly when using deionised water, and an increased risk of build up
of bacteria.
If deionised water of a high grade (for example, 8 ) is used as cooling fluid, then the
voltage difference between each cell due to the conductivity of the water can be
considered to be sufficiently low so as not to exacerbate corrosion in the stack.
However, if the fuel cell stack is installed such that the deionised water can be recovered
from the coolant outlet of the fuel cell stack for reintroduction to the fuel cell stack, then
the conductivity of the water is likely to increase due to a number of factors, including
C0 2 absorbed from the atmosphere, and washout of ions and impurities or contaminants
from the fuel cell membranes and from metallic components in the fuel cell stack.
Hence, the voltage difference across the injected water will increase, thus providing an
environment where cell corrosion is more likely to occur. At the end of the fuel cell stack
furthest from the cooling water inlet, the flow rate of cooling water in the inlet manifold is
likely to be at a minimum, and ionic deposits from the cooling water are more likely to
form and attack / corrode metal components such as flow plates in the fuel cell stack.
It is an object of the invention to provide a solution to one or more of the above
problems. It is an object of the invention to reduce or eliminate problems that can arise
from very low coolant flows in a coolant distribution manifold of a fuel cell stack. It is an
object of the invention to provide a solution for maintaining appropriate coolant flow
levels within the fuel cell stack.
According to one aspect, the invention provides a fuel cell stack assembly comprising:
a plurality of fuel cells each having a fluid coolant conduit; and
a coolant feed inlet manifold having a coolant inlet;
the coolant feed inlet manifold coupled to each fluid coolant conduit for
distribution of coolant to each fuel cell; and
the coolant feed inlet manifold further comprising a discharge conduit
located at one end of the coolant feed inlet manifold, the discharge conduit configured to
discharge excess coolant from the coolant feed inlet manifold.
The discharge conduit may comprise an additional plate extending across the stack and
disposed at the one end of the fuel cell stack. The additional plate may comprise a
heater plate, a current collector plate or an insulator plate. The discharge conduit may
comprise a conduit of increased flow impedance compared to the coolant feed inlet
manifold, such that a coolant flow rate from the coolant feed inlet manifold to the
discharge conduit is within a predetermined flow rate range. A second discharge conduit
may be located at an opposite end of the coolant feed inlet manifold to the first discharge
conduit. The second discharge conduit may comprise an additional plate extending
across the stack and disposed at the opposite end of the fuel cell stack. The additional
plate may comprise a heater plate, a current collector plate or an insulator plate. The
discharge conduit may comprise a recirculation path coupled to the coolant inlet for the
recirculation of coolant to the coolant feed inlet manifold. The fuel cell stack assembly
may include a coolant resistivity monitor configured to determine the resistivity of coolant
passing through the recirculation path. The discharge conduit may be coupled to an
external coolant sump or tank. A flow control assembly may be coupled to the discharge
conduit configured to control the flow of coolant fluid from the coolant feed inlet manifold
to the discharge conduit. The flow control assembly may comprise a variable flow
restrictor. The end of the fuel cell stack with the discharge conduit may be an electrically
positive end of the fuel cell stack. An outlet manifold may be coupled to each fluid
coolant conduit of the plurality of fuel cells for receiving coolant from each fuel cell
discharge conduit may form part of the outlet manifold.
Embodiments of the present invention will now be described by way of example and with
reference to the accompanying drawings in which:
Figure 1 is a schematic side view of a fuel cell stack with a coolant feed inlet
manifold and discharge outlet;
Figure 2 is a schematic side view of an alternative fuel cell stack with a coolant
feed inlet manifold and discharge outlet;
Figure 3 is a schematic side view of a fuel cell stack with a coolant feed inlet
manifold and two discharge outlets;
Figure 4 is schematic side view of an alternative fuel cell stack with a coolant
feed inlet manifold and two discharge outlets;
Figure 5 is a schematic view of the fuel cell stack of figure 2 coupled for
recirculating coolant delivery to the coolant feed inlet manifold;
Figure 6 is a schematic view of the fuel cell stack of figure 2 coupled for passage
of discharged coolant to a tank;
Figure 7 is a schematic view of the fuel cell stack of figure 1 comprising a
variable flow controller; and
Figure 8 is a schematic view of the fuel cell stack of figure 2 comprising a
variable flow controller.
The various embodiments described below provide excess coolant injected into a
coolant feed inlet manifold. The coolant may be water, preferably deionised water. A
portion of the injected coolant passes to fluid coolant conduits in the fuel cells in the fuel
cell stack. Another portion, described as the excess coolant, exits the coolant feed inlet
manifold via a discharge conduit without passing through the fuel cells. By providing an
excess of coolant to the coolant feed inlet manifold via the coolant inlet, the flow of
coolant at the end of the manifold furthest from the inlet is sufficient to avoid or reduce
problems arising due to very low coolant flow rates or stagnant coolant in the manifold.
The embodiments described herein do not necessarily require the use of additional
valve, pump and/or controllers in order to achieve the flow rates required to mitigate
coolant stagnation problems. Thus the present invention advantageously provides
improved fuel cell stack assemblies without the logistical considerations, extra
engineering, maintenance considerations and increased cost of including additional
components such as valves. However, the invention allows the incorporation of such
valves if required for further control of the coolant fluid to/from the fuel cells.
Further, embodiments described herein provide a solution to the problem of low coolant
flow rates and coolant stagnation which can readily be combined with other design
variations for fuel cell stacks, thereby contributing to a modular fuel cell system with
flexibility for tailoring depending on the particular conditions required.
Figure 1 shows a schematic side view of a fuel cell stack 0. The stack 10 comprises a
plurality of fuel cells 1 , each of which has an anode fluid flow path for delivering fuel to
an anode surface of a membrane-electrode assembly and a cathode fluid flow path for
delivering oxidant to a cathode surface of a membrane-electrode assembly. The fuel
cells are held in a stack arrangement by way of end plates 12, 13 in a known manner.
The anode fluid flow paths or the cathode fluid flow paths are provided with coolant
injection for evaporative cooling of the fuel cell stack by way of a coolant feed inlet
manifold 14, which extends down the length of the stack 10 between a coolant inlet 15
and a discharge conduit 6 at opposing ends of the coolant feed inlet manifold 14. The
coolant feed inlet manifold 14 may be described as a coolant / water delivery manifold or
gallery.
As indicated by the arrows in figure 1, coolant flows into the manifold from the coolant
inlet 15, then into each of the fluid flow paths of the separate fuel cells 11. Preferably,
the coolant combines with the fuel or oxidant flow at some point between the coolant
feed inlet manifold 14 and flow channels in the individual fuel cells 11. These flow
channels extend across the active surfaces of the fuel cells 11. The fuel and oxidant may
be introduced into the individual cells 11 using a separate fuel manifold and a separate
oxidant manifold using known techniques. In some embodiments, unused fuel or oxidant
may pass out of the fuel cells into an outlet manifold 17 and, in some embodiments, from
there to one or more exhaust ports / outlets 18, 19. An outlet manifold 17 is not
necessarily required for the anode fluid flow paths if all fuel is consumed at the active
surfaces of the fuel cells 11, particularly if coolant injection is not provided on the anode
sides of the fuel cells 11, although an anode exhaust line may be provided for periodic
purging. In the embodiments described herein, an outlet manifold 17 is shown coupled to
each fluid coolant conduit of the plurality of fuel cells 11 for discharge of at least coolant
from each fuel cell 11.
Also shown in figure 1 is a discharge conduit 16 for excess coolant 20 to pass out of the
coolant feed inlet manifold 14 without passing through the fluid coolant conduits of the
fuel cells 11. The discharge conduit 16 of figure 1 may be described as an external
coolant drain. Figure 1 thus shows a fuel cell stack assembly 10 comprising a plurality of
fuel cells 11 each having a fluid coolant conduit, and a coolant feed inlet manifold 14
having a coolant inlet 15. The coolant feed inlet manifold 14 is coupled to each fluid
coolant conduit of the fuel cells 1 for distribution of coolant to each fuel cell. The coolant
feed inlet manifold 14 further comprises a discharge conduit 16 located at one end of the
coolant feed inlet manifold 14. The discharge conduit 16 is configured to discharge
excess coolant 20 from the coolant feed inlet manifold 4.
By locating the discharge conduit 16 at the opposite end of the coolant feed inlet
manifold 14 to the coolant inlet 15, excess coolant may be injected via the coolant inlet
15 to the fuel cell stack 10 and the portion of the manifold furthest from the coolant inlet
15 need not be subject to very low coolant flow rates. A coolant fluid flow may be
provided at the coolant inlet 15 to the coolant feed inlet manifold 14 such that there is
sufficient flow through the manifold, even at the end of the fuel cell stack 10 furthest from
the coolant inlet 14, to avoid or mitigate coolant fluid stagnation which can lead to
problems as described earlier. Excess coolant 20, which does not pass through the fuel
cells 11, exits the coolant feed inlet manifold 14 by the discharge conduit 6.
Figure 2 shows a fuel cell stack assembly 0 comprising a plurality of fuel cells 1 1 each
having a fluid coolant conduit, and a coolant feed inlet manifold 14 having a coolant inlet
15 at one end of the stack. The coolant feed inlet manifold 14 comprises a discharge
conduit 16 located at the other end of the stack, and the discharge conduit 16 is
configured to discharge excess coolant 20 from the coolant feed inlet manifold 14. In this
example, the discharge conduit 16 passes from the end of the coolant feed inlet manifold
14 opposite the coolant inlet 15, across the fuel cell stack 0, parallel to the fuel cell
conduits, to the side of the stack opposite to the coolant feel inlet manifold 14, before
passing out through an outlet manifold 17. In the embodiments described with respect to
figures 2, 4, 5, 6 and 8, the discharge conduit 16 is shown to form part of the outlet
manifold 17, although this need not be the case and the discharge conduit 16 may pass
out from the fuel cell stack via a path separate from the outlet manifold 17.
In some embodiments represented by figure 2, the discharge conduit 16 may comprise
or be formed within an additional plate 2 1 extending across the width of the stack,
parallel to the fuel cells, and disposed at the end of the fuel cell stack 10. This additional
plate 2 1 could be a heater plate or a current collector plate or an insulator plate adjacent
to the end plate 13. The discharge conduit 16 in figure 2 may be described as an
internal coolant drain. The discharge conduit 16 may be formed within the additional
plate 2 1 to allow the passage of excess coolant from the side of the fuel cell stack
opposite the coolant inlet 15. If the discharge conduit 16 is included within a heater plate,
the discharge conduit 16 may preferably be located on the opposite side of the heater
plate to the side incorporating heating elements. Providing the discharge conduit 16
within a heater plate may provide an additional benefit in that, during cold start and
operation, the heater plate may defrost any traces of ice in the discharge conduit 16
allowing improved start up and operation.
The discharge conduit 16 may comprise a conduit of predetermined reduced dimensions
(compared to the dimensions of the manifold 14) to create a back pressure such that a
coolant flow rate from the coolant feed inlet manifold 14 into the discharge conduit 16 is
within a predetermined flow rate range. The discharge conduit 16 may be a length of
pipework having particular dimensions relative to the coolant feed inlet manifold 14 and
may be at least partially serpentine or tortuous in form. The discharge conduit thereby
presents a suitably increased impedance to coolant flow compared to the manifold and
thereby achieves a desired flow rate and back pressure to the manifold. In this way, the
flow parameters for coolant flow within the fuel cells 11 and excess coolant flow out from
the coolant feed inlet manifold 14 may be controlled.
Figure 3 shows a fuel cell stack assembly 10 comprising a plurality of fuel cells 1 each
having a fluid coolant conduit, and a coolant feed inlet manifold 14 having a coolant inlet
15 located towards the centre of the coolant feed inlet manifold 14. The coolant feed
inlet manifold 14 comprises, in this embodiment, a first discharge conduit 16 located at
one end of the coolant feed inlet manifold 14 and a second discharge conduit 22 located
at an opposite end of the coolant feed inlet manifold 14. The discharge conduits 16, 22
are both configured to discharge excess coolant from the coolant feed inlet manifold 14.
Whereas the examples of figures 1 and 2 show the coolant inlet 15 located at one end of
the coolant feed inlet manifold 14, the example of figure 3 shows the coolant inlet 15
located at the centre of the coolant feed inlet manifold 14. The embodiment of figure 3
provides for an excess coolant flow into the coolant feed inlet manifold 15 such that
coolant can flow to each of the fuel cells in the stack 0, including those furthest from the
coolant inlet (those at the two ends of the fuel cell stack), with a sufficiently high flow
so as to mitigate problems from stagnant coolant, or very low coolant flow rates in the
manifold ends remote from the coolant inlet 15. The excess coolant 20 which does not
pass through the flow conduits of the fuel cells 1 exits the coolant feed inlet manifold 14
by the two discharge conduits 16, 22. Figure 3 also shows two exhaust ports / outlets 18,
19 for unused fuel or oxidant to pass out of the outlet manifold 17.
Figure 4 illustrates a fuel cell stack assembly 10 comprising a plurality of fuel cells 1
each having a fluid coolant conduit, and a coolant feed inlet manifold 14 having a coolant
inlet 15 located towards the centre of the coolant feed inlet manifold 14, similar to figure
3. The coolant feed inlet manifold 14 comprises, in this embodiment, a first discharge
conduit 16 located at one end of the coolant feed inlet manifold 14 and a second
discharge conduit 22 located at the opposite end of the coolant feed inlet manifold 14.
Both the first discharge conduit 16 and the second discharge conduit 22 comprise an
additional plate 21, 23 disposed at the ends of the fuel cell stack 10. The additional
plates 21, 23 could each be a heater plate or a current collector plate or an insulator
plate adjacent to the respective end plate 12, 13. The discharge conduits 16, 22 are
configured to discharge excess coolant from the coolant feed inlet manifold 14.
The embodiment of figure 4 provides a similar advantage to that of figure 2, in that if the
additional plates 21, 23 are provided as heater plates, any traces of ice present in the
discharge conduits 16, 22 may be defrosted upon cold start of the fuel cell stack 10
without adding complexity to the fuel cell stack 10.
If desired, a fuel cell stack may be formed having a first discharge conduit 16 as shown
in figure 3 exiting the coolant feel inlet manifold 14 directly, and a second discharge
conduit 22 comprised within an additional plate 23 as shown in figure 4. The additional
plates 21, 23 at respective ends may be of different types. The location of the coolant
inlet 5 need not be in the centre of the coolant feel inlet manifold, and may be located
part way along the manifold 14 if desired.
Figure 5 illustrates the fuel cell stack 0 of figure 2 in which the discharge conduit 16
includes a recirculation path 24 coupled to the coolant inlet 15 for the recirculation of
coolant to the plurality of fuel cells 11. A pump, not shown, may be provided within the
recirculation path 24. As shown in figure 5, a coolant resistivity monitor 25 may be
provided to determine the resistivity of coolant passing through the recirculation path 24.
By recirculating the coolant fluid, there is less wastage of coolant fluid. By monitoring the
resistivity of the coolant fluid, it may be determined when the coolant requires
replacement or partial replacement. For example, a resistivity value below a particular
value for deionised water may be used to control replacement of recirculating water or
dilution of recirculating water. An exemplary minimum value could be, for example, 0.1
MOhm.cm. The location of the resistivity monitor 25 may be anywhere along the path of
the recirculated coolant, and may therefore be located, for example, at the coolant inlet
15, in the recirculation path 24, or in the discharge conduit 16 internal to the fuel cell
stack 10. More than one such resistivity monitor 25 may be used if desired, at different
locations along the path of recirculated coolant.
Figure 6 illustrates the fuel cell stack 10 of figure 2 in which the discharge conduit 16 is
coupled to an external coolant sump or tank 26. Thus excess coolant which is no longer
required may be collected for storage, or for dilution and re-use in the fuel cell stack 10.
The embodiments shown in figures 5 and 6 may be combined with the use of a valve
and controller if desired. For example, coolant fluid may be recirculated until a
predetermined value of resistivity of the coolant is reached. Upon reaching the
predetermined value of resistivity, the controller may switch a valve to change the path
of the excess coolant from being recirculated via recirculation path 24 to being
discharged to a tank 26.
Figures 7 and 8 show the embodiments of figures 1 and 2 respectively, further
comprising a flow control assembly 27. The flow control assembly 27 is coupled to the
discharge conduit 16 and is configured to control the back pressure of coolant fluid in the
manifold 14 at the discharge conduit 16, such that the pressure of coolant fluid at the
discharge conduit 16 can be held within a predetermined pressure range. The discharge
conduit 6 may therefore comprise a flow control assembly 27 as a means for varying
flow impedance and thereby back pressure to the manifold 14.
The flow control assembly 27 may comprise, for example, one or more of a variable flow
restrictor, an orifice plate, a needle valve, tubing of a predetermined length, and tubing of
a predetermined width. For example, if the pressure at the end of the coolant feed inlet
manifold, at the location of the flow control assemblies of figure 7 and 8, is 1 bar, and 50
ml/min of coolant is to be injected via the coolant inlet 15, then a 3 m length of discharge
conduit 16 with a 1 mm diameter may be used between the outlet of the coolant feed
inlet manifold 14 and the coolant inlet 15 to achieve this pressure.
The end of the fuel cell stack 10 where the coolant inlet 15 is located in the
embodiments described with respect to figures 1, 2, 5, 6, 7 and 8 may be selected as a
negative polarity end of the fuel cell stack, and the opposite end of the fuel cell stack
may be selected as a positive end. This could be reversed, if required.
Other embodiments are intentionally within the scope of the accompanying claims.
CLAIMS
1. A fuel cell stack assembly comprising:
a plurality of fuel cells each having a fluid coolant conduit; and
a coolant feed inlet manifold having a coolant inlet;
the coolant feed inlet manifold coupled to each fluid coolant conduit for
distribution of coolant to each fuel cell; and
the coolant feed inlet manifold further comprising a discharge conduit
located at one end of the coolant feed inlet manifold, the discharge conduit configured to
discharge excess coolant from the coolant feed inlet manifold.
2. The fuel cell stack assembly of any preceding claim, wherein the discharge
conduit comprises an additional plate extending across the stack and disposed at the
one end of the fuel cell stack.
3. The fuel cell stack assembly of any preceding claim, wherein the additional plate
comprises a heater plate, a current collector plate or an insulator plate.
4. The fuel cell stack assembly of any preceding claim, wherein the discharge
conduit comprises a conduit of increased flow impedance compared to the coolant feed
inlet manifold, such that a coolant flow rate from the coolant feed inlet manifold to the
discharge conduit is within a predetermined flow rate range.
5. The fuel cell stack assembly of any preceding claim, wherein:
the discharge conduit located at the one end of the coolant feed inlet manifold is
a first discharge conduit; and further comprising:
a second discharge conduit located at an opposite end of the coolant feed inlet
manifold to the first discharge conduit.
6. The fuel cell stack assembly of claim 5, wherein:
the second discharge conduit located at the opposite end of the coolant feed inlet
manifold to the first discharge conduit comprises an additional plate extending across
the stack and disposed at the opposite end of the fuel cell stack.
7. The fuel cell stack assembly of claim 5, wherein:
the additional plate comprises a heater plate, a current collector plate or an
insulator plate.
8. The fuel cell stack assembly of any preceding claim in which the discharge
conduit comprises a recirculation path coupled to the coolant inlet for the recirculation of
coolant to the coolant feed inlet manifold.
9. The fuel cell stack assembly of claim 8, further comprising:
a coolant resistivity monitor configured to determine the resistivity of coolant
passing through the recirculation path.
10. The fuel cell stack assembly of any preceding claim wherein the discharge
conduit is coupled to an external coolant sump or tank.
11. The fuel cell stack assembly of any preceding claim further including a flow
control assembly coupled to the discharge conduit configured to control the flow of
coolant fluid from the coolant feed inlet manifold to the discharge conduit.
12. The fuel cell stack assembly of claim 11 in which the flow control assembly
comprises a variable flow restrictor.
13. The fuel cell stack of any preceding claim in which the end of the fuel cell stack
with the discharge conduit is an electrically positive end of the fuel cell stack.
14. The fuel cell stack assembly of any preceding claim, further comprising
an outlet manifold coupled to each fluid coolant conduit of the plurality of fuel
cells for receiving coolant from each fuel cell.
15. The fuel cell stack assembly of claim 14, wherein the discharge conduit forms
part of the outlet manifold.
16. A fuel cell stack assembly substantially as described herein with reference to the
description and drawings.