GB2530027A

GB2530027A - Fuel cell system

Info

Publication number: GB2530027A
Application number: GB1415771.3A
Authority: GB
Inventors: Pratap Rama
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2014-09-05
Filing date: 2014-09-05
Publication date: 2016-03-16
Also published as: GB201415771D0; WO2016034853A1

Abstract

A fuel cell system (1, Fig 1) comprises a fuel cell stack (2) and a heat exchanger 23 that receives an exhaust flow 26 from fuel cell stack (2) via inlets 27c-d. Exhaust flow paths 25a- are configured so as to include a central section 28c-d lower than the inlets 27c-d and outlets 29c-d that receive and hold liquid 30 from the fuel cell system when it is shut down. Manifold 26 and heater 31 are also shown and an associated method of operation is described.

Description

FUEL CELL SYSTEM

This invention relates to a fuel cell system. In particular, it relates to a heat exchanger for 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 flow paths. 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 IS stack. The exhaust from the anode and cathode fluid flow paths can be hot and one or more heat exchangers may be used to cool the exhaust fluid leaving the fluid flow paths.

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 fluid flow paths. If the fuel cell is stored or operated in sub-zero conditions, the water in the fuel cell may freeze. Frozen water may cause blockages in the fluid flow paths. The blockages may cause the fuel cell to be starved of fuel or oxidant, which can hinder operation. The water may also cause damage to the fuel cell when it freezes due to expansion. This is a particular problem on shut-down of the fuel cell where water present in the system may pool or collect in parts of the cell and may then freeze, Frozen water may also prevent the fuel cell from being restarted until the water has been thawed. It is known to provide a heater in the fuel cell system, which operates on stored energy, such as from a battery, and maintains the fuel cell system at above-zero temperatures 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 system comprising a fuel cell stack and a heat exchanger arranged to receive an exhaust flow exhausted by the fuel cell stack, the heat exchanger including flow paths therethrough wherein a subset of said flow paths are configured and arranged to receive and hold liquid from the fuel cell system on shut-down of the fuel cell system.

This is advantageous as liquid in the fuel cell system can be stored in the heat exchanger at least on shut-down of the system, which prevents damage to other parts of the fuel cell system if the ambient conditions fall below the freezing point of the liquid. The configuration of the flow paths in the heat exchanger are advantageous as they ensure that a subset of the flow paths receive water, which is allowed to freeze, while the remaining flow paths remain clear of water. The remaining flow paths or clear subset of flow paths ensure that the system can be restarted as flow through the stack and on to the heat exchanger is not hindered or prevented by ice formation in the heat exchanger. Further, given that a heat exchanger is constructed for efficient heat transfer it provides a convenient vessel to store and melt the frozen fluid when the system is restarted.

Optionally, each flow path includes an inlet port and an outlet port and wherein the subset of flow paths within the heat exchanger are configured such that they include a central section that is lower than their inlet port and outlet port. This is advantageous as it allows water to collect in the central section between inlet and outlet ports without draining away to other pans of the system.

The heat exchanger may be arranged relative to the fuel cell stack such that liquid in the system drains to the heat exchanger. Thus, the heat exchanger may be lower than the fuel cell stack.

The fuel cell system may be configured to actively drive fluid to the heat exchanger at least on shut down of the system.

The flow paths not part of the subset may be arranged higher than those in the subset. Thus, liquid will by virtue of gravity collect in the lower flow paths that form part of the subset, leaving the upper flow paths not part of the subset clear. This allows exhaust flow to bypass the flooded (and possibly frozen) zone in heat exchanger and therefore not hinder flow through the fuel cell stack during start up.

The flow paths not part of the subset may be substantially straight and arranged substantially horizontally when the system is in use. This arrangement is advantageous as it ensures a low pressure drop for exhaust flow that bypasses flooded zone during start up.

The heat exchanger may include at least one heater for thawing any ice that forms within the heat exchanger. The heater may be electrically powered. The heater may be arranged to heat coolant that passes through the heat exchanger. The coolant heater may be configured to receive power from the fuel cell stack. The heater may be mounted within the heat exchanger adjacent the subset of flow paths. A plurality of heaters may be provided.

The system may include a pump to blow liquid in the fuel cell stack to the heat exchanger when the fuel cell system is shut-down. The pump may comprise a compressor. The pump may be used to drive a reactant through the fuel cell stack during normal use.

The fuel cell system may include a liquid separator downstream of the heat exchanger for separating liquid from the heat exchanger from a gas stream passing through the heat exchanger from the fuel cell stack. Liquid separators may appear anywhere in the system downstream of the fuel cell stack to recover liquid, such as water, The melted liquid water may be supplied to the fuel cell stack for evaporative cooling.

The amount of melted liquid separated by the liquid separator may be used to control operation of the system. A measure of the amount of melted liquid may be used to control heaters associated with the heat exchanger. Thus, a sensor may be provided to measure the amount of liquid extracted by the liquid separator. This may be used by a controller to determine the rate of thawing in the heat exchanger and to control heaters and/or a compressor and/or valves to create a back pressure in the system and/or end plate heaters of the fuel cell stack. These techniques may be used to raise the temperature in the heat exchanger to promote thawing of any ice in the subset of flow paths.

The fuel cell system may include a heater for thawing liquid in the heat exchanger, the heater may be configured to be powered by the energy generated from the fuel cell stack during start-up of the fuel cell system. Thus, power generated by the stack during start up may be used to power an electrical heater which heats up the coolant that runs through a cold side of the heat exchanger to help melt the ice or to power end plate heaters within the fuel cell stack The fuel cell system may be an evaporatively cooled fuel cell system. The heat exchanger may be connected to a cathode exhaust.

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 heat exchanger. This may be achieved using fuel cell stack power to power an electrical air heater at cathode inlet or outlet. Alternatively, valves or restrictions may be provided to cause a back-pressure in the system. This will, in turn, cause the compressor to run less efficiently and output a hotter air stream to the cathode inlet.

According to a further aspect of the invention, we provide a heat exchanger for a fuel cell stack, the heat exchanger configured to receive an exhaust flow exhausted by the fuel cell stack, the heat exchanger including flow paths therethrough wherein a subset of said flow paths are configured and arranged to receive and hold liquid from the fuel cell system on shut-down of the fuel cell system.

The optional features mentioned above apply equally to this further aspect of the invention.

According to a further aspect of the invention, we provide a method of operating a fuel cell system comprising a fuel cell stack and a heat exchanger arranged to receive an exhaust flow exhausted by the fuel cell stack, comprising the steps of; providing a subset of flow paths through the heat exchanger to receive and hold liquid from the fuel cell system on shut-down of the fuel cell system.

The method may comprise providing flow paths not part of the subset configured to be clear of liquid.

The method may include the step of; draining liquid in the fuel cell system to the subset of flow paths in the heat exchanger.

The method may include the step of; applying a blow down gas to the fuel cell system to drive any liquid in the fuel cell system to the heat exchanger for holding in the subset of flow paths in the IC heat exchanger.

The fuel cell system may include a heater or heaters for thawing any frozen liquid in the heat exchanger and the method may include the step of; on start up of the fuel cell system, providing power generated by the fuel cell stack to the heaters to thaw frozen liquid in the heat exchanger.

The method may include the step of; progressively de-energising the heaters in response to increases in power output of the fuel cell stack.

The method may include the step of; controlling a back pressure in the fuel cell system to promote thawing of frozen liquid in the heat exchanger.

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 an example embodiment of a fuel cell system with a heat exchanger; Figure 2 shows a diagrammatical view of the heat exchanger of Figure 1; Figure 3 shows a second example embodiment of a heat exchanger; Figure 4 shows a third example embodiment of a heat exchanger; and Figure 5 shows a flow chart of an example method of operation.

A hydrogen-oxygen fuel cell system is described wherein water is generated as a reaction product and is also introduced into the fuel cell system to hydrate a proton exchange membrane. The fuel cell system is evaporatively cooled. However, 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.

Figure 1 shows a fuel cell system 1 comprising a fuel cell stack 2 and a heat exchanger 3. The flue cell stack 2 includes an anode flow path having an anode flow path inlet 4 and an anode flow path exhaust 5. The fuel cell stack 2 further includes a cathode flow path having a cathode flow path inletS and a cathode flow path exhaust 7. Air or other oxidant maybe supplied by a compressor 9 via the cathode flow path inlet 6. The heat exchanger 3 receives the exhaust flow from the cathode flow path exhaust 7 via an exhaust gas conduitS, The heat exchanger 3 includes a plurality of flow paths 1 Ca, 1Db therethrough which convey the exhaust flow to a heat exchanger exhaust outlet 11. A subset (comprising flow path lob, for example) of the flow paths 1 Ca, 1 Ob is configured and arranged to receive and hold liquid from the fuel cell stack 2 of the fuel cell system 1, particularly on shut-down of the fuel cell system 1. Thus, the subset of flow paths may be arranged such that liquid water drains to the subset of flow paths when exhaust flow ceases.

The heat exchanger 3 also comprises a coolant flow path through which coolant flows to receive IS the thermal energy from the exhaust flow in the flow paths ba, lob. The coolant flow path includes a coolant inlet 12 into the heat exchanger 3 and a coolant outlet 13 from the heat exchanger 3. Further, the heat exchanger 3 is located physically lower than the fuel cell stack 2 such that liquid from the fuel cell stack 2 will drain under gravity to the heat exchanger 3.

The fuel cell system I may include a first water separator 14 upstream of the heat exchanger 3 to remove liquid from the exhaust flow prior to it entering the heat exchanger 3. A second water separator 15 may be provided downstream of the heat exchangerS, which receives exhaust gas from the heat exchanger exhaust outlet 11. The first and second water separators 14, 15 extract water from the exhaust and transfer the extracted water to a water conduit 16. The water conduit 16 is configured to recycle the water and, in this embodiment, returns the water to the fuel cell stack 2 for hydration of the proton exchange membrane in the stack. The recycled water in the conduit 18 may be introduced into the stack directly or into one or both of the anode flow path inlet 4 and cathode flow path inlet 6. It will be appreciated that in other embodiments, zero, one, two, three, four or more or water separator may be provided. In other embodiments, the water conduit 18 may direct the extracted water elsewhere for recycling or for other purposes.

The fuel cell system 1 may include a controller 17, which receives electrical power generated by the fuel cell stack 2 and distributes it to heater system for thawing water that has collected in the heat exchanger 3. The controller may comprise a DC-DC convertor. The heater system may include a coolant heater 16, located in the coolant inlet 12, which is configured to heat the coolant flowing into the heat exchanger 3. Dashed line 20 represents electrical power being supplied from the controller 17 to the coolant heater 18, which may be an electric heater. The heated coolant may be used to assist in thawing any frozen water in the flow path lob. Dashed line 20 also extends into the heat exchanger 3 to provide electrical power to further electrical heaters located in the heat exchanger 3. It will be appreciated that the heater system may have other configurations and may or may not comprise one or both of the coolant heater 18 and further heaters.

Figure 2 shows a diagrammatic view of an embodiment of a plate of the heat exchanger 23. The heat exchanger 23 receives exhaust flow through an exhaust gas conduit and, via an inlet manifold 24, distributes the exhaust flow through a plurality of flow paths 25a-d. An outlet manifold 26 receives the exhaust flow from the flow paths 25a-d and provides a heat exchanger exhaust outlet (not shown in figure 2). The flow paths 25a and 25b are substantially straight and arranged horizontally in use. The flow path 25c comprises an inlet 27c, a central section 28c and an outlet 29c. Likewise, the flow path 25d comprises an inlet 27d, a central section 28d and an outlet 29d. The central sections 28c, 28d are arranged lower than their respective inlets 27c, 27d and outlets 29c, 29d. The flow paths 25c and 25d comprise a subset of the flow paths 25a-d configured and arranged to receive and hold liquid from the exhaust flow of the fuel cell stack 2.

In particular, the central sections 28c and 28d form a well in the flow paths 25c and 25d. Water, by virtue of the shape of the flow paths 25c and 25d and the location of the heat exchanger 3 relative to the fuel cell stack 2, will collect in the central sections 28c, 28d. Figure 2 shows water 3D collected in the flow paths 25c and 25d. The upper flow paths 25a and 25b, being straight and higher than the flow paths 25c and 25d, do not collect water (or collect less water) and therefore remain comparatively clear of water. The coolant also flows over the plate shown in figure 2, although this is not shown for clarity.

In use, the fuel cell stack 2 generates water as a reaction product that is carried in the exhaust flow through the cathode side of the fuel cell stack 2 and out of the cathode flow path exhaust 7.

The exhaust flow may also include evaporated water vapour that was introduced into the stack to evaporatively cool the stack and/or to hydrate the proton exchange membranes therein. The exhaust flow may pass through the first water separator 14 to remove a proportion of the water therefrom. The exhaust flow then enters the heat exchanger 23 and flows through the flow paths 25a-d. The exhaust flow is cooled in the heat exchanger by heat transfer to the coolant flow. This may cause water vapour in the exhaust flow to condense in the heat exchanger 23.

Any condensed liquid water in channels 25a and 25b is carried through the heat exchanger entrained in the exhaust flow. Any condensed liquid water in subset channels 25c and 25d is also carried through the heat exchanger entrained in the exhaust flow, but some may collect in the central sections 28c and 28d. On shut-down of the fuel cell stack 2 the heat exchanger 23 is configured to receive and retain water in the fuel cell system. The cathode flow path and position of the heat exchanger 23 relative to the stack 2 is such that any water in the cathode flow path drains to the heat exchanger 23. A blow down gas may be passed through the cathode flow path to urge any water through and into the heat exchanger 23. The blow down gas may be urged driven through the system by a compressor and may comprise air, which may be used as the oxidant in normal use. The pressure of the blow down gas may be controlled by valves in the heat exchanger 23 that adjust the back pressure and therefore the cathode flow/exhaust flow through the system 1. The arrangement of the flow paths 25a-d in the heat exchanger 23 results in water entering the heat exchanger 23 being directed to the subset flow paths 25c and 25d rather than the flow paths 25a and 25b. The position of the subset flow paths 25c, 25d relative to the flow paths 25a, 25b and/or the shape of the subset flow paths is such that water is purposively collected in the subset flow paths 25c, 25d where, in freezing conditions, the water is allowed to freeze.

Thus, the heat exchanger 23 includes a subset of water receiving flow paths 25c and 25d and the remaining flow paths 25a and 25b form a further subset configured to remain clear of water. The water can freeze in the subset flow paths 25c and 25d, which will block the flow paths. However, the further subset of flow paths, which are arranged such that water does not couect in them allow exhaust flow therethrough when the system is restarted, as described below. Dividing the water from the system amongst several flow paths in the heat exchanger is advantageous because it makes it easier to thaw compared to a block of ice that may form in other systems.

While the fuel cell system is shut-down external heating power is not required and any water that has collected in the heat exchanger 23 is allowed to freeze. When the fuel cell stack 2 is restarted, oxidant can still flow through the cathode flow path as the flow paths 25a and 25b remain clear of ice, The fuel cell stack may not be able to operate at full power due to the restriction in the heat exchanger (in flow paths 25c and 25d) or due to the reduced cooling performance of the heat exchanger 23 while it contains ice blocking the flow paths 25c and 25d.

In particular, the water that is frozen in the heat exchanger provides evaporative cooling for the stack 2 and therefore the stack 2 may be operated at a power that allows it to operate with the cooling capacity available at that time. The hot cathode exhaust flow in flow paths 25a and 25b may promote thawing of the ice 30 in flow paths 25c and 25d. Alternatively or in addition! the coolant heater 18 may be activated to use the stack's 2 limited power output from controller 18 to help thaw the ice in the flow paths 25c and 25d. Alternatively or in addition a heat exchanger heater 31 may be provided and powered by the controller 17 to heat a region of the heat exchanger containing the subset of flow paths 25c and 25d. Further, the back pressure due to the reduced number of available flow paths 25 through the heat exchanger 23, the compressor 9 may operate less efficiently and thereby heat the cathode flow. This extra heat will be carried through the exhaust to assist melting of the ice. The flow through the flow paths 25a and 25b may be controlled by valves or restrictors to utilise heating of the cathode flow by the compressor 9.

As the water 28c and 28d thaws, further cooling capacity becomes avaflable both due to water being available for evaporatively cooling the stack 2 and increased surface area available in the heat exchanger 23 as the flow paths 25c, 25d become free. With increased cooling capacity, the fuel cell stack 2 can be operated at a higher power output. This increase in power output can be used to power a device to which the system 1 is connected or may be applied to the heaters 18, 31 to accelerate the thawing of any remaining ice. Alternatively, as the available system power increases, the power supplied to the heaters 18, 31 may be reduced and the hot cathode exhaust flow may be relied upon to melt any remaining ice.

Providing a subset of flow paths 25c and 25d in which water can be collected in preference of other flaw paths 25a and 25b is advantageous as it allows the fuel cell stack 2 to be restarted (possibly in a reduced power mode) and operated while any frozen water in the subset of flow paths thaws or while the power output of the fuel cell stack 2 is used to thaw any frozen water to unblock the flow paths 25a-d in the heat exchanger 23. Once the flow paths 25c and 25d are clear of ice, the fuel cell stack 2 may be operated at full operating power. Collecting water in the heat exchanger 23 is advantageous as the heat exchanger is inherently designed for efficient heat transfer and therefore provides a convenient vessel for storing water and efficiently thawing it if it freezes.

The further water separator 15 may be used to remove the thawed water from the exhaust flow leaving the heat exchanger 23 along with any other water entrained in the exhaust flow. The water extracted by the further water separator 15 may be supplied to the fuel cell stack 2 via conduit 16 for evaporatively cooling the stack 2. A sensor may be provided in the further water separator 15 to measure the quantity of water being extracted and this may be used by the controller 17 to control the heaters 18, 31 and/or a compressor that provides the cathode flow and/or the power output of the fuel cell system 1.

Figure 3 shows a further embodiment of the heat exchanger 33. This embodiment is substantially similar to the embodiment shown in figure 2. However, in this embodiment, the heat exchanger 33 includes a plurality of serpentine flow paths 35g-j which form a subset 36 of the total number of flow paths 35a-j. The remaining flow paths 35a-f are substantially straight.

Operation of this embodiment is substantially similar to that described above. However, in this embodiment, the heat exchanger 33 includes flow restrictors (not shown) in the flow paths 35a-f not forming part of the subset 38. The flow restrictors restrict flow through the flow paths 35a-35f and thereby encourage flow through the flow paths 35g-j. The flow paths 35g-j follow a more convoluted route through the heat exchanger 33 and therefore present more surface area to the coolant flow. Thus, heat from the exhaust flow is more efficiently transferred to the coolant by passing through the subset of flow paths 35g-j. If the flow paths 35g-j become blocked with ice, the exhaust flow will pass through the free flow paths 35a-f despite the flow restrictors. Thus, the flow restrictors can be considered to promote flow through the subset of flow paths and thereby assist in keeping the remaining flow paths 35a-f clear of water and ice. Ensuring that at least one of the flow paths is kept clear of ice by having designated flow paths to receive excess water ensures that the fuel cell stack can be restarted and oxidant can be flowed through the cathode flow path without any downstream blockages. The flow restrictors can also be configured to provide a back pressure to the exhaust flow when the system 1 is restarted. This will raise the temperature of the exhaust flow which will assist in thawing any ice in the heat exchanger.

Figure 4 shows a further embodiment of the heat exchanger 43. In this embodiment a straight flow path 45a is provided. The remaining flow paths, which form the subset 46of flow paths, each have a progressively lower and therefore larger central section 48b-g or well. The deeper and lower the well in the flow paths 45b-g the more likely water is to collect in the well. For shallow wells, and those located relatively higher in the heat exchanger 43 the exhaust flow is likely to carry any water through the flow path. For flow paths with deeper wells, a higher exhaust flow pressure is required to drive out any water that has accumulated therein. Therefore, in use or on shutdown of the fuel cell stack 2, the flow path 45g will first receive water from the stack 2. If more water is received from the stack, it will accumulate in flow path 45f. Thus, with increasing quantities of water, the flow paths of the subset 46 will be sequentially flooded, This is advantageous as this arrangement keeps a large number of flow paths 45a-g free from water until there is too much water for a particular flooded flow path to hold. There is therefore reduced resistance to flow through the heat exchanger 43 on restart of the fuel cell stack 2 compared to a heat exchanger arrangement in which the flow paths have similar sized wells.

It will be appreciated that the number of flow paths in the heat exchanger in total and/or the number of flow paths comprising the subset of flow paths may be altered. The flow paths iDa, 25a, 25b, 35a-f and 45a arranged to remain clear may not be straight and could have any appropriate shape. Further, they may be arranged in other positions in the heat exchanger. For example, the further subset of flow paths may include valves to prevent flow therethrough during normal use and allow flow when the subset of flow paths are blocked with ice. Flow restrictors may be provided in any of the embodiments to encourage flow through the subset of flow paths.

The flow restrictors may comprise individual restrictions or the manifold 24, 26, may be configured to promote flow through particular flow paths. The subset of flow paths may be reinforced relative to the remaining flow paths.

Figure 5 shows a method of operating a fuel cell stack. Step 50 comprises providing a subset of flow paths through the heat exchanger to receive and hold liquid from the fuel cell system on shut-down of the fuel cell system. Step 51 comprises draining liquid in the fuel cell system to the subset of flow paths in the heat exchanger and applying a blow down gas to the fuel cell system to drive any liquid in the fuel cell system to the heat exchanger for holding in the subset of flow paths in the heat exchanger. Storing water in the heat exchanger, which is allowed to freeze, is advantageous to protect the reminder of the fuel cell system and provide for easy thawing of the water.

Claims

CLAIMS1. A fuel cell system comprising a fuel cell stack and a heat exchanger arranged to receive an exhaust flow exhausted by the fuel cell stack, the heat exchanger including flow paths therethrough wherein a subset of said flow paths are configured and arranged to receive and hold liquid from the fuel cell system on shut-down of the fuel cell system.
2. A fuel cell system according to claim 1, in which each flow paths includes an inlet port and an outlet port and wherein the subset of flow paths within the heat exchanger are configured such that they include a central section that is lower than their inlet port and outlet port.
3. A fuel cell system according to claim 1 or claim 2, in which the heat exchanger is arranged relative to the fuel cell stack such that liquid in the system drains to the heat exchanger.
4. A fuel cell system according to any preceding claim, in which the flow paths not part of the subset are arranged higher than those in the subset.
5. A fuel cell system according to any preceding claim, in which the flow paths not part of the subset are substantially straighter than those part of the subset and arranged substantially horizontally when the system is in use.
6. A fuel cell system according to any preceding claim, in which the heat exchanger includes at least one heater for thawing any ice that forms within the heat exchanger.
7. A fuel cell system according to any preceding claim, in which the system includes a pump to blow liquid in the fuel cell stack to the heat exchanger when the fuel cell system is shut-down.
8. A fuel cell system according to any preceding claim, in which the fuel cell system includes a liquid separator downstream of the heat exchanger for separating melted liquid from the heat exchanger from a gas stream passing through the heat exchanger from the fuel cell stack.
9, A fuel cell system according to claim 8, in which the amount of melted liquid separated by the separator is used to control operation of the system.
10. A fuel cell system according to claim 9, in which a measure of the amount of melted liquid is used to control heaters associated with the heat exchanger.
11. A fuel cell system according to claim 6, in which the heater is configured to be powered by the energy generated from the fuel cell stack.
12. A fuel cell system according to any preceding claim, in which the fuel cell system is an evaporatively cooled fuel cell system.
13. A fuel cell system 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 flow paths.
14. A fuel cell system according to any preceding claim, in which the fuel cell stack is configured to raise the temperature of a coolant, which is configured to flow through the heat exchanger, to aid thawing of any ice that forms in the flow paths.
15. A fuel cell system according to claim 14! in which a heater is provided to heat the coolant.
16. A fuel cell system according to claim 15, in which the heater is configured to receive power from the fuel cell stack.
17. A method of operating a fuel cell system comprising a fuel cell stack and a heat exchanger arranged to receive an exhaust flow exhausted by the fuel cell stack, comprising the steps of; providing a subset of flow paths through the heat exchanger to receive and hold liquid from the fuel cell system on shut-down of the fuel cell system.
18. A method according to claim 17, in which the method includes the step of; draining liquid in the fuel cell system to the subset of flow paths in the heat exchanger.
19. A method according to claim 17 or claim 18, in which the method includes the step of; applying a blow down gas to the fuel cell system to drive any liquid in the fuel cell system to the heat exchanger for holding in the subset of flow paths in the heat exchanger.
20. A method according to any of claims 17 to 19, in which the fuel cell system includes a heater or heaters for thawing any frozen liquid in the heat exchanger and the method includes the step of; on start up of the fuel cell system, providing power generated by the fuel cell stack to the heaters to thaw frozen liquid in the heat exchanger.
21. A method according to claim 20, in which the method includes the step of; progressively de-energising the heaters in response to increases in power output of the fuel cell stack.
22. A method according to any of claims 17 to 19, in which the method includes the step of; controlling a back pressure in the fuel cell system to promote thawing of frozen liquid in the heat exchanger.
23. A fuel cell system as described herein and illustrated in figures ito 4 of the drawings.