GB2524313A

GB2524313A - Fuel cell stack

Info

Publication number: GB2524313A
Application number: GB1405017.3A
Authority: GB
Inventors: Pratap Rama; Paul Leonard Adcock
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2014-03-20
Filing date: 2014-03-20
Publication date: 2015-09-23
Also published as: WO2015140529A1; GB201405017D0

Abstract

A fuel cell stack 1 comprising a plurality of fuel cells 2 includes a plurality of plates 8, each plate having delivery channels between an inlet manifold 4 and an outlet manifold 5, the delivery channels configured to deliver a reactant, e.g. air, to an active area of each fuel cell in the fuel cell stack. Each of the plates 8 in the stack 1 is associated with a distinct sump 13, the outlet manifold 5 extending between the delivery channels of the plates 8 and the sumps 13, the sumps arranged, when in use, to be lower than the outlet manifold, such that liquid within the fuel cell stack can collect in the sumps and is compartmentalised therein. Each sump 13 preferably includes a plurality of sump channels (24, figure 2) for holding liquid that drains into the sump, wherein the channels extend between the outlet manifold 5 and a separate secondary outlet manifold (22, figure 2) which receives melted liquid from the channels. This fuel cell stack arrangement is useful in preventing water from gathering and freezing in sub-zero conditions in the flow plates 8, which may cause damage and/or affect the performance of the fuel cell, since the water is collected within the sumps 13.

Description

I

FUEL CELL STACK

This invention relates to a fuel cell stack. In particular, it relates to a sump for a fuel cell stack and a fuel cell system.

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 (proton) transfer membrane, with fuel and air being passed over respective sides of the membrane. Protons (that is, 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 may be formed comprising a number of such membranes arranged with separate anode and cathode fluid delivery channels. Such a 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. The anode delivery channels are formed in flow plates that extend between an anode inlet manifold and an anode outlet manifold. The cathode delivery channels are also formed in flow plates between a cathode inlet manifold and a cathode outlet manifold. The anode inlet manifold delivers fuel to the anode delivery channels for the fuel cells in the stack and any excess fuel leaves the stack by the anode outlet manifold. The cathode inlet manifold delivers the oxidant to the cathode delivery channels for the fuel cells in the stack and any excess oxidant and reaction products leave the stack by the cathode outlet manifold.

In a hydrogen-oxygen fuel cell, one of the reaction products is water. An aqueous solution or water is also used to hydrate the polymeric ion transfer membrane. Thus, liquid and/or gaseous water is present in the anode and cathode delivery channels.

Also, on shut-down of the fuel cell stack, water present in the stack may pool or collect in parts of the cell stack. If the fuel cell is stored or operated in sub-zero conditions, the water in the fuel cell stack may freeze. Frozen water may cause damage to the fuel cell stack. Also, the blockages caused by ice formation in the fuel cell stack may prevent the flow of fuel and/or oxidant, which may stop the fuel cell stack from starting up until the ice blockages have melted. It is known to provide a heater in the fuel cell stack, which operates on stored energy, such as from a battery, and maintains the fuel cell stack at an above-zero temperature to prevent freezing occurring. The battery power is however limited and the fuel cell system may experience freezing if the battery fails or becomes discharged.

According to a first aspect of the invention we provide a fuel cell stack comprising a plurality of fuel cells, the fuel cell stack including a plurality of flow plates, an inlet manifold and an outlet manifold, each flow plate having a delivery channel between the inlet manifold and the outlet manifold configured to deliver a reactant to an active area of an associated fuel cell in the fuel cell stack, wherein each of the plurality of fuel cells is associated with a distinct sump, the outlet manifold extending between the delivery io channels of the plates and the sumps, the sumps arranged, when in use, lower than the outlet manifold such that liquid within the fuel cell stack can collect in the sumps and is compartmentalised therein.

This is advantageous as fuel cell stack includes a specific sump region on an opposite side of the outlet manifold to the fuel cell stack that is configured to receive liquid from the fuel cell stack. Any liquid in the delivery channels can drain to the outlet manifold and enter the sump region. If the ambient conditions are such, the water can freeze in the sump region without causing damage to the active area of the fuel cell stack.

Further, as a dedicated sump region is provided, the outlet manifold remains clear and therefore the fuel cell stack can be restarted even with frozen liquid present in the sump region as fuel and oxidant is free to flow through the inlet manifold, delivery channels and outlet manifold without hindrance from frozen liquid.

Each sump may include a plurality of sump channels for holding liquid that drains into the sump. Providing a sump in the form of a plurality of sump channels is advantageous as any frozen liquid that forms is split amongst the channels, which makes it easier to thaw.

Also, the sump channels can be shaped to hold the liquid to prevent it spreading over the outlet manifold. Thus, the sump channels may be U-shaped.

so The sump channels may extend between the outlet manifold and a secondary, separate outlet manifold for receiving at least melted liquid from the sump channels. The secondary outlet manifold thus provides a distinct flow path for flow from the sump to leave the fuel cell stack. The provision of two separate outlet manifolds is advantageous as the pressure difference between the manifolds can be controlled to control a liquid level in the sump.

The sump channels may be constructed and arranged such that they include a central portion that is lower than the opening into the outlet manifold and the outlet into the secondary outlet manifold.

The sumps may be each formed by flow paths in an associated flow plate.

The sump channels may include a larger opening at the point they open into the outlet manifold for receiving liquid into the sump.

At least one heater may be provided to heat the sumps. The heater may comprise an end-plate heater for the stack. The heater may be configured and arranged to heat an active area of the cells in the stack, the sumps, the inlet manifold, the outlet manifolds, the incoming reactant or any combination of the above.

The fuel cell stack may be operable to direct air leaving the delivery channels through the sump channels and into the secondary outlet manifold.

Each sump may include an insulator to electrically insulate it from an adjacent sump.

The sumps may be thermally insulated. Thus, the insulator may be electrically insulating and/or thermally insulating. Each sump may include a gasket. The gasket may be resilient to take account of expansion of liquid in the sump if the liquid freezes.

Each sump may be thermally connected to an associated flow plate. This is advantageous as the sumps compartmentalise any frozen liquid into slices' and the flow plate can conduct heat from the active area to each of the slices of frozen liquid in the sump. This provides an efficient way of thawing the liquid.

The fuel cell stack may be configured to raise the temperature of the air introduced into the inlet manifold to aid thawing of any ice that forms in the sumps. This may comprise using power from the stack to power an electrical reactant heater at the inlet.

Alternatively, this may comprise providing a back pressure such that a compressor which drives reactant through the stack outputs a hotter fluid stream to the stack.

There now follows, by way of example only, a detailed description of embodiments of the invention with reference to the following figures, in which: Figure 1 shows a diagram of a fuel cell stack including a sump region; Figure 2 shows a section through A-A of the fuel cell stack of figure 1; Figure 3 shows the fuel cell stack of figure 2 in a first operational state; Figure 4 shows the fuel cell stack of figure 2 in a second operational state; Figure 5 shows the fuel cell stack of figure 2 in a third operational state; and Figure 6 shows a detailed view of one of the cells and its associated sump.

A hydrogen-oxygen fuel cell stack is described wherein water is generated as a reaction product and is also introduced into the fuel cell stack to hydrate a proton exchange membrane of the cells. The fuel cell stack is evaporatively cooled through the introduction of water into the cells to cool them. Managing the water or other liquids in a fuel cell stack is important to ensure efficient operation of the cell. Also, if the fuel cell stack is stored in conditions below the freezing point of water, water management is important to prevent damage to the cell in the event the water freezes. It will be appreciated that this invention has application in other types of fuel cell in terms of liquid management whether or not the liquid is susceptible to freezing in the environmental conditions the cell is operated.

Figure 1 shows a fuel cell stack 1 comprising a plurality of fuel cells 2. The fuel cells comprise an active area located between flow plates 3. The active area comprises the region of each cell in which the reaction takes place and therefore reactants are consumed and reaction products generated. The active area includes an anode electrode and a cathode electrode either side of a proton exchange membrane. The flow plates include channels for conveying reactant over the active area of each of the fuel cells. When fuel cells are arranged in a stack, the flow plates of a fuel cell and an adjacent cell may be integrated to form a bipolar plate. Bipolar plates include, on a first side, channels for a particular fuel cell in the stack and, on an opposed side, channels for an adjacent fuel cell in the stack.

The fuel cell stack I includes a reactant flow path which includes a reactant inlet 3, an inlet manifold 4, an outlet manifold 5 and an exhaust outletS. The reactant flow path in this example comprises a cathode flow path for delivering oxidant, comprising air, to cathodes of the fuel cells in the stack 1. A compressor 7 may be configured to supply the air to the stack 1 via the reactant inlet 3. The inlet manifold 4 receives the air and distributes it to flow plates 8 of the cells in the stack 1. The exhaust outlet 6 receives an exhaust flow gathered by the outlet manifold 5 from the flow plates 8 of the cells 2. The exhaust flow may comprise unused reactant, reaction products and water/water vapour S from evaporative cooling A water separator 10 may be provided to receive the exhaust flow and separate water therefrom. The extracted water may be recycled by way of conduit 11 to the inlet manifold 4 or to any other convenient point for evaporative cooling purposes or to hydrate the membrane of the cells 2. The exhaust flow may be conveyed to a heat exchanger (not shown) by way of exhaust conduit 12.

A sump region 13 is provided at the base of the outlet manifold 5 comprising a plurality of sumps. Each sump is associated with one of the flow plates 8 of one of the cells 2 in the stack 1. The outlet manifold 5 extends between the channels of the plates 8 and the sump region 13. The sumps are arranged, when in use, lower than the outlet manifold 5 such that liquid within the fuel cell stack 1 can collect in the sumps and be compartmentalised therein.

Figure 2 shows a section through the fuel cell stack 1 through the lines A-A. Figure 2 shows the inlet manifold 4 having, in this example, three manifold channels 20a-c. The manifold channels 20a-c convey air (or other oxidant) to the delivery channels in flow plates 8. The delivery channels of the flow plates S convey the air over the active area of the cells 2 in the stack 1. The delivery channels may be shaped or follow a path that allows any water in the channels to drain to the outlet manifold 5. The flow plate channels are not shown for clarity and instead, an active area 21 of the cell 2 is visible.

The flow plate channels are shown in Figure 6 which will be described below. The stack 1 also includes a secondary outlet manifold 22 which comprises a separate flow path to the outlet manifold 5. The outlet manifold 5 includes the sump region 13 at its base and Figure 2 shows one of the sumps 23 of the sump region 13. The sump 23 comprises a plurality of sump channels 24 (although one sump channel may be provided) which form a flow field structure. The or each sump channel 24 has a sump channel inlet 25 that opens into the outlet manifold 5. The or each sump channel 24 also has, at an opposite end, a sump channel outlet 26 that opens into the secondary outlet manifold 22. Thus, the outlet manifold 5 is in fluid communication with the secondary outlet manifold 22 via the sump channels 24. The or each sump channel 24 also has a central section 27 that is lower than the inlet 25 and outlet 26. The central section 27 may form a well or wells. It will be appreciated that figure 2 shows only some of the sump channels 24 and that the sump channel inlets 25 may extend across the base of the outlet manifold 5 and connect it to the secondary outlet manifold 22. Each sump 23 includes a gasket or is coated to electrically insulate the sump 23 from an adjacent sump 23. Thus, the sump region 13 comprises a plurality of electrically insulated sumps 23 that are each associated with a cell of the stack 1. The gasket or a separate compressible member may be provided to compensate for expansion when the water freezes.

Figures 3 to 6 illustrate how the sump region 13 is used to manage the water that is present in the cell. Figure 3 shows the stack 1 in normal use. Oxidant, comprising air, represented by the shaded arrows 30 enters the flow plate channels through the inlet manifold 4. Coolant, comprising water for evaporative cooling and represented by un-shaded arrows 31, is also supplied to the active area 21 through the inlet manifold 4.

The oxidant is consumed by the active area 21 of the cell 2 and therefore the amount of oxidant leaving the active area 21 and entering the outlet manifold is reduced. This is represented by smaller shaded arrows 32, which shows the oxygen-depleted air. Water is a reaction product of the cell 2 and therefore the amount of water entering the outlet manifold 5 from the active area 21 is greater than the amount entering the active area 21 from the inlet manifold 4. This is represented by larger unshaded arrows 33. Much of the unused oxidant 32 is carried out of the stack via the outlet manifold 5. The unused oxidant also carries with it much of the water vapour and entrained water out through the outlet manifold 5. A proportion of the water, represented by arrows 34, enters the sump 23 through sump inlet channels 25. A proportion of the unused oxidant, represented by arrows 35, also enters the sump 23. The pressure in the outlet manifold 5 will act to drive the water and unused oxidant through the sump channels 24 and into the secondary outlet manifold 22, as shown by arrows 36 and 37. Valves or restrictions may be provided in the outlet manifold 5 and/or secondary outlet manifold 22 to control the pressures in each such that water within the sump region is driven out so through the secondary outlet manifold 22 as required. It will be appreciated that the pressure, reaction rate, and/or amount of coolant will affect the amount of water 38 that sits within the sump region 13. In general, under constant operating conditions the amount of water entering and leaving the sump region will reach an equilibrium.

Figure 4 shows the fuel cell stack 1 when it has been shut-down. Thus, oxidant 30 and coolant 31 is no longer supplied through the inlet manifold 4. Water that is present in the active area 21 or channels of the flow plates 8 drains into the outlet manifold 5, as represented by arrows 40. The water drains from the outlet manifold 5 into the sump 23 through the sump inlets 25. The water level 41 in the sump 23 will tend to increase. In figure 4, the sump is shown to be almost full. In sub-zero conditions the water that has collected in the sump region 13 is allowed to freeze. Thus, no heaters are provided to maintain a temperature above the freezing point of the water when the cell is not operating.

Figure 5 shows the fuel cell stack I restarting after the water (which is used as coolant for the evaporatively cooled stack) in the sump region 13 has frozen. Oxidant, represented by arrows 40, is provided to the active area 21 through the inlet manifold 4.

Coolant may not be supplied through the inlet manifold on start up as it may be frozen in the sump region 13. Thus, without coolant, the fuel cell stack I may operate at a reduced power output, such as 10% for example, so as not to overheat. In the present embodiment, a greater flow rate of oxidant 50 is supplied during start up to assist in is keeping the fuel cell stack cool while coolant is not available or of limited availability. The oxidant 50 is consumed and unused oxidant (arrows 51) enters the outlet manifold 5.

The reaction product of water (arrows 52) also enters the outlet manifold 5. The unused oxidant 51 and water 52 can leave the fuel cell stack by the outlet manifold 5. The outlet manifold is advantageously clear of ice due to the provision of the sump region. The reaction in the active area generates heat and therefore the unused oxidant 51 and reaction product water 52 has an elevated temperature. The reaction product water 52 will drain to the base of the outlet manifold where it can enter the sump channels 24 (if the sump is not completely full) and transfer thermal energy to the ice in the sump region 13. The unused oxidant 50 will also transfer its thermal energy to the ice as it flows along the outlet manifold 5. Arrows 53 shows the product water interacting with the sump region 13 and arrow 54 shows the unused oxidant interacting with the sump region 13. Thus, the temperature of the sump region will rise and the ice in the sump channels 24 will begin to melt. Further, the heat generated by the stack 2, represented by arrows 55, acts through radiation and/or conduction to melt the ice in the channels 24.

For example, the heat generated in the active area 21 may be conducted to the sump region through the bipolar plates 8, As the ice has melts it is pushed through to the secondary outlet manifold 22 through the sump channels 24 along with the unused oxidant 54. The water from the melted ice travels along the secondary outlet manifold 22 and may be separated from the unused oxidant by water separator 10 for use as coolant for the stack 1. As the ice melts, the ice/water level 56 in the sump region 13 will decrease and reach an equilibrium. The stack I may therefore return to the operational state shown in figure 3. Thus, the stoichiometry may be reduced from the elevated start up period stoichiometry level. The start up period may be sixty seconds or any other time that is required to melt the ice sufficiently to provide sufficient liquid water for cooling, The equilibrium level may be reached based on the stack's operating current, the stoichiometry, the coolant water injection rate and any back pressure applied at the outlet manifolds 5, 26.

Further, with reference to Figure 1, end plate heaters 14, 15 may be provided which uses the reduced electrical power generated by the stack 1 during the start up period to assist in melting the ice. The end plate heaters 14, 15 may extend over only the sump region 13. Other electrical heaters may be provided in the sump region 13 to aid melting of the ice. The other heaters may use the reduced power output of the stack 1 during the start up period. The stack 1 may include backpressure valves (not shown) which may restrict flow out of the outlet manifold 5. Thus, the oxidant flow will be heated to a greater degree by the active area 21 due to the back pressure, which will promote the melting of ice in the sump region 13. Further, the back pressure provided by the backpressure valves may cause the compressor 7 to operate less efficiently and thereby heat the air entering the fuel cell stack 1. This heated air can also aid melting of ice in the sump region as it passes through the outlet manifold 5. Any additional heaters may be controlled based on the amount of water from melted ice received from the fuel cell stack. The additional heaters may be progressively shut down as water coolant becomes available and the stack power can be increased accordingly.

Thus, the provision of the sump region 13 provides a dedicated region for accepting and storing water such that the outlet manifold 5 can remain clear. As the outlet manifold 5 remains clear of ice, restarting of a fuel cell stack I in which the water has frozen is still possible. Further the secondary outlet manifold 22 provides a dedicated route for exhausting melted water from the sump region 13 out of the stack 1. If the stack I is evaporatively cooled and the configuration is such that the water in the sump is the source of coolant, the stack may be started for an initial start-up period at a reduced power and/or with a higher oxidant stoichiometry. This prevents the stack from overheating. The heat generated by the stack 1 in the initial period after start up can be used to melt the ice in the sump region 13. As the ice melts, liquid water for cooling becomes available which can be introduced for evaporative cooling of the stack 1, which, in turn, means that the stack can be operated at higher powers without causing damage to the stack 1. This can accelerate melting of any remaining ice such that the stack 1 can operate at full power.

Figure 6 shows a detailed view of one of the cells 2 and outlet manifold 5 and sump region 13 as shown in figure 1. The active region 21 is shown having an anode assembly 60 on one side and a cathode assembly 61 on the other side. Part of a bipolar flow plate 8 is shown providing a delivery channel 62 to the anode assembly 61.

Likewise, part of an adjacent bipolar plate 8 is shown providing a delivery channel 63 to the cathode assembly 61. The delivery channel 62 of the anode assembly 60 does not open into the outlet conduit 5. It can be seen in this figure that the sump 23 is outside of the active area 21, but located between the flow plates 8 at the base of the outlet manifold 5. In this embodiment, the sump region lies wholly within the bounds of the flow plates 8. A single sump channel 24 and its sump channel inlet 25 into the outlet manifold 5 is visible. The sump channel inlet 25 is wider than the remainder of the sump channel 24 provided by a tapering mouth 64. The mouth 64 is more accepting of water into the sump 23. The sump channels themselves 24 are formed between the flow plates 8 below the active area 21. A gasket 65 is provided to electrically insulate the sump 23 from the other sumps of the other cells. The arrow 66 shows the path that water from the delivery channel 63 of the flow plate 8 follows to enter the sump region 13. The provision of a plurality of sumps 23 separated by the flow plates 8 is advantageous as the ice that may form in the sump region is divided into slices by the plates 8 and can therefore be melted effectively using the thermal conductivity of the flow plates and any other heaters.

Claims

CLAIMS1. A fuel cell stack comprising a plurality of fuel cells, the fuel cell stack including a plurality of flow plates, an inlet manifold and an outlet manifold, each flow plate having a delivery channel between the inlet manifold and the outlet manifold configured to deliver a reactant to an active area of an associated fuel cell in the fuel cell stack, wherein each of the plurality of fuel cells is associated with a distinct sump, the outlet manifold extending between the delivery channels of the plates and the sumps, the sumps arranged, when in use, lower than the outlet manifold such that liquid within the fuel cell stack can collect in the surnps and is compartmentalised therein.
2. A fuel cell stack according to claim 1, in which each sump includes a plurality of sump channels for holding liquid that drains into the sump.
3. A fuel cell stack according to claim 2, in which the sump channels extend between the outlet manifold and a secondary, separate outlet manifold configured to receive at least melted liquid from the sump channels.
4. A fuel cell stack according to claim 3, in which the sump channels are constructed and arranged such that they include a central portion that is lower than the opening into the outlet manifold and the outlet into the secondary outlet manifold.
5. A fuel cell stack according to any preceding claim in which the sumps are each formed by flow paths in the associated plate.
6. A fuel cell stack according to any one of claims 2 to 5, in which the sump channels include a larger opening at the point they open into the outlet manifold for receiving liquid into the sump.
7. A fuel cell stack according to any preceding claim, in which at least one heater is provided to heat the sumps.
8. A fuel cell stack according to claim 3, in which the fuel cell stack is operable to direct air leaving the delivery channels through the sump channels and into the as secondary outlet manifold.
9. A fuel cell stack according to any preceding claim, in which each sump includes an insulator to electrically insulate it from an adjacent sump.
10. A fuel cell stack according to any preceding claim, in which each sump is thermally connected to the each plate.
11. A fuel cell stack according to any preceding claim, in which sumps are thermally insulated.
12. A fuel cell stack according to any preceding claim, in which the fuel cell stack is configured to raise the temperature of the air introduced into the inlet manifold to aid thawing of any ice that forms in the sum ps.
13. A fuel cell stack according to any preceding claim, in which the fuel cell stack is configured to control a back pressure in the fuel cell stack to raise its operating temperature to aid thawing of any ice that forms in the sumps.
14. A fuel cell stack according to any preceding claim, in which the fuel cell stack includes at least one heater for thawing any ice that forms within the fuel cell stack.
15. A fuel cell stack according to Claim 14, in which the heater is configured to be powered by energy generated by the fuel cell stack.
16. A fuel cell stack as described herein and illustrated in figures 1 to 6 of the drawings.