GB2514813A

GB2514813A - Fuel cell system and associated method of operation

Info

Publication number: GB2514813A
Application number: GB1310005.2A
Authority: GB
Inventors: Phillip Wardle
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2013-06-05
Filing date: 2013-06-05
Publication date: 2014-12-10
Anticipated expiration: 2033-06-05
Also published as: WO2014195717A1; GB201310005D0; GB2514813B

Abstract

The disclosure relates to a fuel cell system and method of operation, the fuel cell system comprising: a fuel cell outlet line 204; and a containment vessel 202 having a storage module 214 and a plenum 210, wherein the plenum is configured to receive outlet fluid from the fuel cell outlet line, is separate from the storage module and is in thermal contact with the storage module. The fuel cell system may be used in a vehicle, where operating temperatures can be below freezing point. Preferably there is a fluid control system comprising a bypass 224 with two valves 218 & 220 up- and downstream, respectively. Furthermore, there may be a drainage valve 232. The freeze-resistant properties of the containment vessel are due to heat stored in the fluid, the insulating properties of the outer skin 206, the additional heat stored in the thermal mass 216 and the vacuum or reduced pressure formed with the isolated plenum 210.

Description

FUEL CELL SYSTEM AND ASSOCIATED METHOD OF OPERATION

The invention relates to the operation of, and apparatus relating to, a fuel cell system, and in particular though not exclusively to a strategy for conserving heat in a fuel cell system to improve start-up.

Fuel cell cooling water is typically heated to a temperature less than 45 degrees during the operation of a fuel cell system. After the fuel cell is shut down the temperature of the cooling water will fall to that of the ambient environment as heat is lost to the surroundings.

For automotive applications in particular, typical requirements include starting up from below freezing (possibly as low as -20 °C) to replicate environments in which the fuel cell may be used in practice. Since high purity water has a freezing point of 0 °C (at I bar pressure), any water left in the fuel cell system will, given sufficient time, freeze after shut-down of the fuel cell.

For fuel cell systems that inject water into the cathode flow path, it is important that the injected water is of high purity, so as to avoid contamination of the proton exchange membrane (PEM) and consequent degradation of stack performance. This requirement for high purity, however, means that additives cannot be used to lower the freezing point of water.

Power from an external source (which in automotive applications may be a battery of a vehicle) can be used to heat elements within a water tank in order to prevent freezing.

However, maintaining water in a liquid state during prolonged shut-down periods using a heater that draws power from the fuel cell system can cause a substantial drop in the efficiency of the system. This can be the case even where the water storage tank is well insulated.

Intelligent Energy Limited's patent application GB 0718763.6, published as GB 2,453,127, relates to a fuel cell system and a strategy for starting operation of a fuel cell system.

According to a first aspect of the invention there is provided a fuel cell system comprising: a fuel cell outlet line; and a containment vessel having a storage module and a plenum, wherein the plenum is configured to receive outlet fluid from the fuel cell outlet line, is separate from the storage module and is in thermal contact with the storage module.

The outlet fluid from the fuel cell can be hot, such as 90 °C upstream from an exhaust heat exchanger. Heat from the outlet fluid can be used to heat water stored in the storage module, thereby increasing the stored heat in the system without draining its resources. Such additional thermal energy stored in the system can be used to delay or prevent freezing of water within the storage module. The plenum may also provide improved thermal insulation properties for the containment vessel and so improve heat retention within the containment vessel. For these reasons, the fuel cell system may provide a more efficient fuel cell system with improved start-up performance.

The fuel cell system may comprise a fluid control mechanism. The fluid control mechanism may have a flow-through state in which the fluid control mechanism is configured to enable outlet fluid to flow from the fuel cell outlet line through the plenum.

The fluid control mechanism may have an isolated state in which the fluid control mechanism is configured to isolate a volume of outlet fluid within the plenum. In the isolated state, the isolated volume of outlet fluid (which comprises moist hot air) cools, thereby providing at least a partial vacuum in the plenum. In the isolated state, the plenum may be sealed.

The fuel cell system may comprise a bypass conduit. The fluid control mechanism may comprise a first valve. The first valve may be a three way valve provided at a junction between the fuel cell outlet line, a plenum inlet port and an inlet to the bypass conduit.

The fluid control mechanism may comprise a second valve. The second valve may be a three way valve provided at a junction between a downstream outlet line, a plenum outlet port and an outlet from the bypass conduit. The first valve may be configured to connect the fuel cell outlet line to either the plenum inlet port or the inlet to the bypass conduit.

The second valve may be configured to connect the downstream outlet line to the plenum outlet pod or the outlet from bypass conduit.

The fluid control mechanism may have drain state in which the fluid control mechanism is configured to allow liquid condensate to drain from the plenum. The fluid control mechanism may comprise a drain valve coupled to the plenum. The fluid control mechanism may be configured to open the drain valve in the drain state.

The storage module may have a water inlet that is connected to the drain valve in order to receive condensed water from the plenum. Water may be received from the drain valve in the drain state. The storage module may have a water inlet that is connected to the second valve in order to receive condensed water from the plenum.

The plenum may be a channel with a serpentine arrangement. The plenum may extend along a side of the storage module. The plenum may surround the storage module. The plenum may comprise a plurality of parallel channels.

The containment vessel may comprise a heat storage mass. The heat storage mass may be in thermal contact with the storage module. The heat storage mass may be distinct from the storage module. The heat storage mass may be configured to retain heat transferred to the containment vessel from the outlet fluid within the plenum.

The heat storage mass may have a specific heat capacity that is larger than a specific heat capacity of a wall of the plenum or storage module. The heat storage mass may have a heat capacity that is larger than a heat capacity of a volume of the storage module when filled with pure water. The heat storage mass may comprise a ceramic powder or block; water; a water and anti-freeze mixture; or any other liquid with good heat retention properties. The storage module may be surrounded by the heat storage mass.

The fuel cell system may comprise a downstream outlet line. The containment vessel may have a plenum inlet port and/or a plenum outlet port. The plenum may be connected to the fuel cell outlet line via the plenum inlet port. The plenum may be connected to the downstream outlet line via the plenum outlet pod.

The fuel cell system may comprise a heat exchanger configured to convey heat from outlet fluid within the plenum to the storage module. A wall of the plenum or storage module may be metallic. The containment vessel may be thermally insulated.

There may be provided a vehicle comprising any fuel cell system disclosed herein.

The fuel cell system may comprise a fuel cell stack. The fuel cell outlet line may be a cathode exhaust line of the fuel cell stack.

According to a further aspect of the invention there is provided a method of operating a fuel cell system that comprises a containment vessel having a storage module and a plenum configured to receive outlet fluid from a fuel cell outlet line, the method comprises, in accordance with a property or state of the fuel cell system: enabling the outlet fluid to flow from the fuel cell outlet line through the plenum in order to provide heat to the storage module; or isolating a volume of fluid within the plenum from the fuel cell outlet line.

The method may further comprise draining a condensate from within the plenum in accordance with a property or state of the fuel cell system.

One or more of the steps of enabling, isolating and draining may comprise changing the operating state cia fluid control mechanism, which may include one or more valves.

The invention will now be described by way of example only, with reference to the appended drawings in which: Figure 1 illustrates a schematic diagram of an arrangement of various components within a fuel cell system disclosed by GB 2,453,127; Figure 2 illustrates a schematic diagram of an alternative arrangement of various components of a fuel cell system including a water containment vessel with a heat exchange plenum configured to receive fluid from a cathode outlet of a fuel cell stack; Figure 3 illustrates a schematic diagram of a simplified arrangement of various components of another fuel cell system including water containment vessel with a heat exchange plenum configured to receive fluid from a cathode outlet of a fuel cell stack; Figure 4 illustrates a schematic diagram of a water containment vessel connected to a fuel cell outlet line by a fluid control mechanism; Figure 5 illustrates a method of operating a fuel cell system; Figure 6 illustrates a schematic diagram of a plenum comprising a serpentine channel; and Figure 7 illustrates a schematic diagram of a plenum comprising a plurality of parallel channels.

This disclosure relates to a fuel cell system in which heat from a fuel cell outlet line is used to heat water in a water storage module (which may simply be referred to as a orage module"). The fuel cell exhaust line may be a cathode exhaust line for removing exhaust fluid from the fuel cell. The cathode exhaust fluid for an evaporative cooled fuel cell may comprise moist air and typically has a temperature in excess of 80 °C. This temperature is substantially higher than the typical temperature of stored fuel ceU water in conventional fuel cell systems. Heat conduction from the exhaust gases can be used to heat a containment vessel, increasing the stored heat in the system without further draining the resources of the system. The amount of additional external heating required to maintain liquid water in the containment vessel whilst the fuel cell system is shut down is reduced or eliminated, thereby improving the efficiency of the system. Furthermore, the hot fuel cell exhaust fluid may be selectively isolated in a plenum around the storage module so as to provide a vacuum or partial vacuum when the exhaust fluid cools, thereby improving the thermal insulation properties of the containment vessel.

Figure 1 relates to a schematic diagram of a prior art fuel cell system 100 comprising a fuel cell stack 110 and other associated components as described in GB 2,453,127. The fuel cell stack 110 has a cathode flow path and an anode flow path passing through it.

The anode flow path comprises an anode inlet 156 and anode outlet 159 of the fuel cell stack 110 and a number of components external to the fuel cell stack 110 which will not be described further here. The cathode flow path comprises an air inlet 124 leading to an air inlet line 123 and into the stack at the cathode air inlet 126. After passing through an internal cathode volume (not shown) within the fuel cell stack 110, the cathode flow path exits the fuel cell stack 110 into the cathode exit line 121, through the cathode exhaust line 122 and an exhaust shut-off valve 120. During normal operation, the exhaust shut-off valve 120 is partially or fully open. Various components such as a heat exchanger 130, with associated cooling fan 139, and a water separator 131 may be connected to or part of the cathode exit line 121 and exhaust line 122 in the cathode flow path.

A cathode water injection inlet 127 is provided in the fuel cell system 100 and is connected to a cathode water injection line 125. The cathode water injection line 125 may be heated along a part or the whole of its length, and extends between a storage module of a water containment vessel 140 and the cathode water injection inlet 127. A heater 129 may be provided to apply heat to a specific region of the line 125 to heat water passing through the injection line 125 towards the cathode water injection inlet 127. A pressure sensor PX4 may be provided on the cathode water injection line 125 in order to monitor the back-pressure on the line 125 during operation.

Water from the cathode exit line 121 is pumped with a water pump 132, optionally provided with a heater 143, through a water return line 128 towards the storage module

S

* of the water containment vessel 140. Excess water is ejected from the fuel cell system out of the water containment vessel 140 through a water overflow line 144.

Figures 2 and 3 relate to alternative configurations of components that can be associated with a cathode fluid outlet of a fuel cell system, such as the system of figure 1. For clarity, the components attached to the anode inlet, anode outlet and cathode inlet of the fuel cell stack are not illustrated in figures 2 and 3. Components in figures 2 and 3 that are arranged as described above with reference to figure 1 will not necessarily be discussed further below.

Figure 2 illustrates an example of a portion of a fuel cell system bOa that has a water containment vessel 140a that comprises a storage module, in this example a water storage module 114, and a heat exchange plenum 121b (also referred to simply as "the plenum" for brevity). The plenum is separate from the water storage module 114, and is in thermal contact with the water storage module 114.

The water storage module 114 can be used to store water for hydration of the fuel cell stack 110, for example hydration via the cathode inlet line. As will be appreciated by the skilled person, water that is to be provided to the cathode side of a fuel cell must be of high purity in order to avoid poisoning the fuel cell. The water should also have a low or no conductivity, as such conductivity could short circuit the fuel cell stack. As with the fuel cell system of figure 1, the water storage module 114 receives water that has been recycled from the cathode exit line 121c and provides water to the cathode water injection inlet 127 via the cathode water injection line 125.

In this example, the plenum 121b is provided in place of the heating elements of the containment vessel of figure 1, although in other examples the plenum 121b can be provided in addition to heating elements. In this example, the plenum 121b of the containment vessel 140a is provided in the cathode fluid flow path between the cathode outlet 133 of the fuel cell stack and a heat exchanger 130. A cathode exit line 121a is an example of a fuel cell outlet line that is connected between the fuel cell stack 110 and an inlet of the heat exchange plenum 121 b. Other examples of a fuel cell outlet line include a coolant outlet of the fuel cell and an anode exit line. Outlet fluid from an anode or cathode exit line may be referred to as exhaust fluid. That is, the terms "outlet fluid" and exhaust fluid" may be used interchangeably.

A downstream exhaust line 121c connects an outlet of the plenum 12Th to the heat exchanger 130. By passing the cathode exhaust fluid through the plenum 121b, heat from the exhaust fluid can be transferred to water within the water storage module 114 in order to raise its temperature. Transferring this additional heat to the containment vessel 140a allows the fuel cell system to reduce the risk of damage to components due to the freezing of water in the containment vessel 140a when the system is not in use.

Additionally, the start up time and/or start-up power consumption of the fuel cell stack can be reduced. Further, the operating efficiency of the fuel cell system bOa can be improved by using heat present in the exhaust to raise the temperature of the water within the water storage module 114 instead of heating the water using a power source, such as a battery or the fuel cell itself It will be appreciated that the heat exchange plenum 121b of the water containment vessel 114 may be provided with exhaust fluid from an anode outlet of the fuel cell stack as an alternative to fluid from the cathode outlet 133. In such examples, any unused hydrogen gas received from the anode outlet may be used to fuel a burner that can be situated within, or associated with, the plenum 121b in order to heat the containment vessel 114.

Figure 3 illustrates an example of a portion of a fuel cell system lOOb that is a further development of the arrangement of the fuel cell system of figure 2. In this system, a number of downstream components may be dispensed with because of the advantages provided by the heat exchange plenum 121b. An outlet of the heat exchange plenum 121b is connected to an external air vent through an exhaust shut-off valve 120 on a cathode exhaust line 122.

A heat exchanger and associated fan are not required in this example because the heat exchange plenum 121b performs the task of extracting heat from the cathode outlet fluid.

Water condenses in the heat exchange plenum 121b due to the loss of heat from the exhaust fluid. The condensed water may be collected in the plenum 121d itself, thereby dispensing with the need for a separate water separator. As such, providing the heat exchange plenum 121b within the containment vessel 14Db enables the provision of a simplified fuel cell system, and so may reduce manufacture and maintenance costs.

The plenum 121d in this example has a drainage port disposed at a position along its length. The drainage port is connected directly to a water pump 132a which is configured to pump water from the drainage port into a water storage module 114 of the water containment vessel 140b. Alternatively, condensed water may be collected from a port or channel in the cathode exhaust line 122 and provided to the water storage module 114.

Again1 excess water is ejected from the fuel cell system 100b out of the water containment vessel 140b through a water overflow line 144. Alternatively, the plenum 121b and storage module 114 of the containment vessel 140b may be arranged such that a drainage valve enables condensate in the plenum 121b to be selectively discharged into the water storage module 114 without the need for the water pump 132a.

Figure 4 illustrates in further detail a water containment vessel 202 connected to a fuel cell cutlet line 204 by a fluid control mechanism.

The water containment vessel 202 is a twin walled vessel that has an outer skin 206 and an inner skin 208. An optional contact portion 209 of the outer skin 206 is in contact with the inner skin 208 in examples where the inner skin 208 completely encloses the inner chamber 212. A heat exchange plenum 210 is defined by, and provided between, a plenum portion of the outer skin 206 (the remainder of the outer skin 206 other than the contact portion 209) and the inner skin 208.

An inner chamber 212 is defined within the inner skin 205. The inner chamber 212 contains a water storage module 214 and, optionally, a heat storage mass 216. The inner skin 208 acts as a barrier configured to separate fluid or solid material within the inner chamber 212 from fluid within the plenum 210.

In the case where there is no contact portion, the plenum 210 may surround the inner chamber 212 or the top surface of the inner chamber 212 may be left open.

The plenum 210 is in thermal contact with the water storage module 214. The plenum 210 is configured to selectively receive outlet fluid from the fuel cell outlet line 204. The fuel cell cutlet line 204 may be a fuel cell exhaust line that is configured to receive exhaust fluid from the fuel cell stack, such as exhaust fluid from a cathode outlet of the fuel cell stack. Such exhaust fluid typically comprises hot air and water vapour. The plenum 210 is therefore configured to transfer heat from the received exhaust fluid to the water storage module 214. The plenum can simultaneously provide two functions: to heat water in the water storage module 214 and to cool and condense any water vapour in, and release latent heat from, the outlet fluid, thus helping to reduce the temperature of fluid in the outlet line.

The inner skin 205 of the containment vessel 202 in this example is a good thermal conductor, such as a metallic material, in order to allow the transfer of heat between the plenum 210 and the inner chamber 212 when the two are not in thermal equilibrium. The inner skin 208 may be considered to be a wall of the plenum 210.

The outer skin 206 of the containment vessel 202 in this example is thermally insulated in order to prevent the loss of heat from the containment vessel. In this way, heat from fluids passing through the plenum 210 is more likely to pass to the inner chamber 212 than through the outer skin 206 to the environment.

The selection of whether the plenum 210 receives outlet fluid (and therefore heat if the outlet fluid is hot) can be achieved by the fluid control mechanism, as discussed in further detail below.

In figure 4 the inner wall 208 partially defines both the plenum 210 and the inner chamber 212. Alternatively, an intermediate heat exchange medium may be provided between the plenum 208 and the inner chamber 212.

Any space within the inner chamber 212 of the containment vessel 202 that is not utilised by water control components or the water storage module 214 may be filled by a heat storage mass 216, which can be a ceramic, liquid or powdered material with good heat retention properties to create an area of thermal mass; the heat storage mass 216 is configured to retain heat transferred from the outlet fluid within the plenum 210. The heat storage mass 216 or the intermediate heat exchange medium may comprise silica sand or a secondary fluid such as mineral oil.

The heat storage mass 216 is distinct from the stored fluid with the water storage module 214 and may be separated from the stored fluid by a container wall or membrane. At least part of the container wall or membrane may also act as a barrier configured to separate fluid within the water storage module 214 from fluid in the plenum 210 and so may be considered to be part of the inner skin 208.

The additional heat stored in the heat storage mass 216 can be used to prevent freezing, or reduce the likelihood of freezing, of water stored in the water storage module 214 and so improve cold starting performance and efficiency. Typically, the heat capacity of the heat storage mass 216 is greater than, for example, 2, 5 or 10 times the heat capacity of the water storage module 214 when it is full of pure water. The heat storage mass 216 increases the amount of heat that can be stored whilst the fuel cell is not in use and so s increases the capability of the system to withstand freezing ambient conditions.

The heat storage mass 216 will typically have a specific heat capacity that is larger than a specific heat capacity of the barrier or inner skin 208. For example, the heat storage mass 216 may comprise a ceramic block; water; a water and anti-freeze mixture, whereas the barrier is typically metallic in order to provide efficient thermal transfer between the plenum 210 and the water storage module 214. The heat storage mass 216 may have a specific heat capacity that is similar to, or larger than, a specific heat capacity of the stored fluid in the water storage module 214, which is typically pure water.

A conventional heater may also be provided within the containment vessel 202 in order to heat the water storage module 214 when a measured temperature falls below a critical temperature level when the fuel cell is shut down. The measured temperature level may be either an ambient temperature of the system or a temperature within the containment vessel 202. The conventional heater may be provided within the water storage module 214 or within the heat storage mass 216 and may be a burner or an electrical heater that is powered by the fuel cell stack or an external power source such as a battery. The burner could take waste purge gas from the fuel cell stack or could take fuel directly from a fuel storage tank of the fuel cell system. Heating the containment vessel using waste anode gas from an evaporatively cooled stack will further improve the overall system efficiency and may be used to heat the system during prolonged storage periods without draining auxiliary and motive batteries of a vehicle.

As an alternative to the arrangement illustrated in Figure 4, the storage module may be surrounded on all sides, including a bottom surface, by the heat storage mass in order to provide further thermal insulation around the storage module. In this case, the storage module may be supported or suspended by structural members or, where the heat storage mass comprises a solid, the storage module may be supported by the heat storage mass.

In Figure 4 the fluid control mechanism is configured to direct exhaust fluid from the fuel cell stack either through the plenum or via a bypass conduit. The fluid control mechanism comprises a first valve 218 and a second valve 220. The first valve 218 is a three way valve provided at a junction between the fuel cell outlet line 204, a plenum inlet port 222 and an inlet to the bypass conduit 224. The first valve 218 can connect the fuel cell outlet line 204 to either the plenum inlet port 222 or the bypass conduit 224. The second valve is also a three way valve and is provided at a junction between a downstream outlet line 228, a plenum outlet port 226 and an outlet of the bypass conduit 224. The second valve 220 can conned the downstream outlet line 228 to either the plenum outlet port 226 or the bypass conduit 224.

A controller may be provided in order to operate the fluid control mechanism in accordance with a parameter or state of the fuel cells system. For example, the controller may receive a signal indicating that the fuel cell system is in a starting up or shutting down state.

The fuel cell system may comprise a temperature sensor in thermal communication with Is one or more of the fuel cell fuel outlet line 204, the heat storage mass 216 and the water storage module 214. The temperature sensor, or sensors, provides temperature data to may comprise a bi-metallic contact, strip or spring. The controller may set the fluid control mechanism in accordance with the temperature data.

The fluid control mechanism has a flow-through state and an isolated state (or sealed state).

In the flow-through state, the fluid control mechanism is configured to enable fluid to flow from the fuel cell outlet line 204 to the plenum inlet port 222 via the first valve 218, through the plenum 210, out of the plenum 210 through the plenum outlet port 226 and into the downstream outlet line 228 via the second valve 220. In the flow-through state, heat from the outlet fluid in the plenum 210 is introduced into the water storage module 214 of the containment vessel via heat conduction through the barrier. The controller may operate the first and second valves 218, 220 to engage the flow-through state of the fluid control mechanism when the fuel cell system is starting up, or the temperature of the outlet fluid within the fuel cell fuel outlet line 204 is greater than a temperature within the containment vessel 202, for example.

In the isolated state, the fluid control mechanism is configured to isolate a volume of fluid within the plenum 210. The fluid isolation of the fluid is achieved by setting the first and second valves 218, 220 such that the plenum inlet port 222 and plenum outlet port 226 are closed, thereby sealing the plenum. When the plenum 210 is isolated, exhaust fluid can be exhausted from the fuel cell system via the bypass conduit 224. That is, in the isolated state the fluid control mechanism is configured to enable outlet fluid to flow from the fuel cell outlet line 204 to the bypass conduit 224 via the first valve 218, and through the bypass conduit 224 to the downstream outlet line 228 via the second valve 220.

In the isolated state, the temperature and pressure of the fluid within the plenum 210 reduces as heat from the isolated exhaust fluid is transferred to the water storage module 214. Some gas and vapour in the exhaust fluid may condense after sufficient heat has been removed. Such a phase change may further reduce the fluid pressure within the plenum 210. The reduced fluid pressure within the plenum 220 has a similar effect to a vacuum flask as it reduces the conduction of heat between the inner skin 208 and the outer skin and so acts as an additional source of insulation without requiring additional space. This additional source of insulation further assists in reducing heat loss from the water storage module 214 whilst the fuel cell is in a shut down state. The improved heat retention properties of the system reduces the additional heating required whilst the fuel cell is shut down and so further improves the efficiency of the fuel cell system.

The controller may operate the first and second valves 218, 220 to engage the isolated state of the fluid control mechanism when the fuel cell system is shutting down, or when sufficient heat has been extracted from the outlet fluid to put the plenum 210 and the inner chamber 212 in thermal equilibrium, for example. As a further alternative, the controller may engage the isolated state of the fluid control mechanism when the fluid control mechanism has been in the flow-through state for a predetermined period of time.

Drainage of fluid from the plenum 210 can be performed during a drainage state of the fluid control mechanism. During the drainage state, fluid (including liquid condensate) can be removed from the plenum 210 through a drainage port 230 by opening a drainage valve 232, which may also be referred to an ejection valve. The drainage valve 232 may comprise a float that is arranged to automatically release the fluid as the float lifts.

Alternatively, the controller may engage the drainage state of the fluid control mechanism when, for example, a predetermined period of the isolated state has elapsed or when a liquid level in the plenum has been exceeded (in examples where the plenum comprises a level sensor).

Optionally, the plenum 210 can be isolated from the fuel cell outlet line 204 by the first valve 218 and from the downstream outlet line 228 by the second valve 220. That is, the first valve 218 and second valve 220 can be set so that the fuel cell outlet line 204 and the downstream outlet line 228 can be coupled by the bypass conduit 224. Alternatively, during the drainage state, the plenum 210 can be isolated from the fuel cell outlet line 204 by the first valve 218 and the second valve can be set so that condensate can drain from the plenum 210 into the downstream outlet line 228 via the second valve 220. Such an implementation does not require a separate drainage valve 232.

The freeze-resistant properties of the containment vessel shown in figure 4 are due to a combination of: 1) the heat stored in the fluid in the water storage module 214; 2) the insulating properties of the material of the outer skin 206 of the containment vessel; 3) the additional heat stored in the thermal mass 216; and 4) the vacuum (or at least reduced pressure) formed with the isolated plenum 210.

Figure 5 illustrates a method 300 of operating a fuel cell system that comprises a containment vessel having a storage module and a plenum configured to receive outlet fluid from a fuel cell outlet line. The method 300 comprises, in accordance with a property or state of the fuel cell system, either: enabling 301 the outlet fluid to flow from the fuel cell outlet line through the plenum in order to provide heat to the storage module; or isolating 302 a volume of fluid within the plenum from the fuel cell outlet line. The isolating step 302 may be performed subsequently to the enabling step 301. States of the fuel cell system include starting up and shutting down and properties may include periods of time, or temperatures or pressures within the fuel cell stack, the plenum or the ambient environment. It will be appreciated that the method 300 may comprise any of the steps performed by the controller described with reference to figure 4. In particular, the method 300 may further comprise draining 303 a condensate from within the plenum in accordance with a property or state of the fuel cell system.

Figure 6 illustrates a schematic diagram of a cross section through a plenum 400 comprising a channel 402 with a serpentine arrangement. A reduction in channel width can increase the effectiveness of the plenum as a heat exchanger. As such, the plenum 400 with a serpentine channel may provide a more efficient means of transferring heat from exhaust fluid to a storage module with which it is in thermal contact than a plenum 400 with a comparatively open structure.

The walls of the plenum 400 may be made of a metallic material n order to enable efficient heat transfer. Suitable'materials for the plenum walls include stainless steels and, more preferably, aluminium alloys, due to their higher thermal conductivity. The plenum wall material may be chosen to be compatible with deionised water within the fuel cell system if the water in the outlet fluid is to be recovered and recycled by the fuel cell system. This compatibility is not required if the fluid within the plenum is purged from the system by, for example, draining into the atmosphere.

Figure 7 illustrates a schematic diagram of a cross section through a plenum 500 comprising a plurality of parallel channels 502. The plenum has an inlet gallery 504 and an outlet gallery 506. The inlet gallery 504 is configured to distribute fluid from a plenum inlet port to the parallel channels 502. The outlet gallery 506 is configured to receive fluid from the parallel channels 502 and provide the fluid to a plenum outlet pod.

The plenum of figure 5 or 6 can be provided in a plane and can co-extend with a side of a storage module. Providing a planar plenum for a planar sided storage module allows for simplification of the design and construction of the fuel cell system.

By providing a plenum comprising one or more channels, or conduits, rather than a comparatively open expanse, fluid flow and pressure drop may be better controlled within the plenum.

The plenum, or a plurality of plenums can be provided surrounding the storage module or an inner portion of a containment vessel in order to increase the heat exchange surface area.

It will be appreciated that features described in regard to one example may be combined with features described with regard to another example, unless an intention to the contrary is apparent.

Claims

Claims I A fuel cell system comprising: a fuel cell outlet line; and a containment vessel having a storage module and a plenum, wherein the plenum is configured to receive outlet fluid from the fuel cell outlet line, is separate from the storage module and is in thermaF contact with the storage module.
2. The fuel cell system of claim 1 comprising a fluid control mechanism having: a flow-through state in which the fluid control mechanism is configured to enable outlet fluid to flow from the fuel cell outlet line through the plenum; and an isolated state in which the fluid control mechanism is configured to isolate a volume of outlet fluid within the plenum.
3. The fuel cell system of claim 2 comprising a bypass conduit, wherein the fluid control mechanism comprises a first valve and a second valve, wherein the first valve is a three way valve provided at a junction between the fuel cell outlet line, a plenum inlet pod and an inlet to the bypass conduit and the second valve is a three way valve provided at a junction between a downstream outlet line, a plenum outlet port and an outlet from the bypass conduit.
4. The fuel cell system of claim 3 wherein the first valve is configured to connect the fuel cell outlet line to either the plenum inlet port or the inlet to the bypass conduit and the second valve is configured to connect the downstream outlet line to either the plenum outlet pod or the outlet from bypass conduit.
5. The fuel cell system of any of claims 2 to 4 wherein the fluid control mechanism further has a drain state in which the fluid control mechanism is configured to allow liquid condensate to drain from the plenum.
6. The fuel cell system of claim 5 wherein the fluid control mechanism comprises a drain valve coupled to the plenum, the fluid control mechanism configured to open the drain valve in the drain state.
7. The fuel cell system of claim 6 wherein the storage module has a water inlet that is connected to the drain valve and/or the second valve in order to receive condensed water from the plenum.s
8. The fuel cell system of any of claims 2 to 7 wherein the plenum is a channel with a serpentine arrangement that extends along a side of the storage module.
9. The fuel cell system of any of claims 2 to B wherein the plenum surrounds the storage module.
10. The fuel cell system of any of claims 2 to 9 wherein the plenum comprises a plurality of parallel channels.
11. The fuel cell system of any preceding claim wherein the containment vessel comprises a heat storage mass that is in thermal contact with the storage module, wherein the heat storage mass is distinct from the storage module and is configured to retain heat transferred to the containment vessel from the outlet fluid within the plenum.
12. The fuel cell system of claim 11 wherein the heat storage mass has a specific heat capacity that is larger than a specific heat capacity of a wall of the plenum or storage module.
13. The fuel cell system of claim 11 or 12 wherein the heat storage mass has a heat capacity that is larger than a heat capacity of a volume of the storage module when filled with pure water.
14. The fuel cell system of any of claims 11 to 13 wherein the heat storage mass is one of: a ceramic powder or block; water; a water and anti-freeze mixture.
15. The fuel cell system of any of claims 11 to 14 wherein the storage module is surrounded by the heat storage mass.
16. The fuel cell system of any of claims 11 to 15 comprising a downstream outlet line and wherein the containment vessel has a plenum inlet port and a plenum outlet port, wherein the plenum is connected to the fuel cell outlet line via the plenum inlet port and the plenum is connected to the downstream outlet line via the plenum outlet port.
17. The fuel cell system of any preceding claim comprising a heat exchanger configured to convey heat from the plenum to the storage module.
18. The fuel cell system of any preceding claim wherein a wall of the plenum or storage module is metallic.
19. The fuel cell system of any preceding claim wherein the containment vessel is thermally insulated.
20. The fuel cell system of any preceding claim comprising a fuel cell stack and wherein the fuel cell outlet line is a cathode exhaust line of the fuel cell stack.
21. A vehicle comprising the fuel cell system of any preceding claim.
22. A method of operating a fuel cell system that comprises a containment vessel having a storage module and a plenum configured to receive outlet fluid from a fuel cell outlet line, the method comprises, in accordance with a property or state of the fuel cell system: enabling the outlet fluid to flow from the fuel cell outlet line through the plenum in order to provide heat to the storage module; or isolating a volume of fluid within the plenum from the fuel cell outlet line.
23. The method of claim 22 further comprising draining a condensate from within the plenum in accordance with a property or state of the fuel cell system.
24. A fuel cell system as described herein with reference to the accompanying figures 2 to 4, 6 and 7.
25. A method of operating a fuel cell system as described herein with reference to the accompanying figure 5.