Patent Publication Number: US-9406966-B2

Title: Fuel cell assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/GB2013/050096, filed on Jan. 17, 2013, which claims priority to Great Britain Patent Application No. 1202591.2, filed on Feb. 15, 2012, the disclosures of which are incorporated by reference in their entirety. 
     The invention relates to fuel cell assemblies, in particular to enclosures for mounting open cathode fuel cell stacks. 
     Conventional electrochemical fuel cells convert fuel and oxidant, generally both in the form of gaseous streams, into electrical energy and a reaction product. A common type of electrochemical fuel cell for reacting hydrogen and oxygen comprises a polymeric ion (proton) transfer membrane, with fuel and air being passed over respective sides of the membrane. Protons (i.e. hydrogen ions) are conducted through the membrane, balanced by electrons conducted through a circuit connecting the anode and cathode of the fuel cell. To increase the available voltage, a stack 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 stack. 
     Because the reaction of fuel and oxidant generates heat as well as electrical power, a fuel cell stack requires cooling once an operating temperature has been reached. Cooling may be achieved by forcing air through the cathode fluid flow paths. In an open cathode stack, the oxidant flow path and the coolant path are the same, i.e. forcing air through the stack both supplies oxidant to the cathodes and cools the stack. 
     Providing uniform air delivery to the cathode electrode surfaces within a fuel cell can be challenging when using compact assemblies. The use of plenum profiles and volumes may not be possible with tight volumetric packaging constraints. 
     According a first aspect of the invention, there is provided a fuel cell assembly comprising:
         an enclosure having a fuel cell stack mounted therein,   an inlet opening into the enclosure,   the fuel cell stack having an inlet face for receiving coolant/oxidant fluid,   a delivery gallery extending from the inlet in the enclosure to the inlet face of the fuel cell stack,   the delivery gallery having a first region and a second region separated by an aperture,   wherein the delivery gallery and aperture are configured such that, in use, coolant/oxidant fluid within the first region of the delivery gallery is turbulent, and coolant/oxidant fluid within the second region of the delivery gallery has a generally uniform pressure.       

     Such a fuel cell assembly can advantageously provide turbulent flow in the first region of the delivery gallery which can be used to cool any components located in the first region, and also provide generally uniform pressure in the second region such that the coolant/oxidant can be uniformly applied to the layers in the fuel cell stack. 
     The aperture may define a restriction to flow between the first and second regions of the delivery gallery. The aperture may represent a reduction in cross-sectional area in the flow path of the coolant/oxidant flow between the first and second regions of the delivery gallery. In this way, a pressure change experienced by the oxidant/coolant as it flows through the aperture can cause the oxidant/coolant to have a generally uniform pressure along the length of the fuel cell stack. 
     The aperture may extend in a longitudinal direction. The aperture may be in the vicinity of an end face of the fuel cell stack or an edge between two faces of the fuel cell stack. The longitudinal direction of the aperture may extend along the edge between two faces of the fuel cell stack. These two faces of the fuel cell stack may be a bottom end face and an inlet face, or a top end face and an outlet face. 
     The width of the aperture may vary in the longitudinal direction, which may assist in providing a uniform air pressure in the second region. The width of the aperture may vary uniformly or non-uniformly in the longitudinal direction. The width of the aperture may be a function of a distance from a fan or air flow generator. The aperture may get wider in the longitudinal direction away from a fan or air flow generator. This can be advantageous in equalising the air pressure in the second region, even though the air pressure in the first region can be lower at positions further away from the fan or air flow generator. 
     The aperture may be defined, at least in part, by an end face of the fuel cell stack and optionally a protrusion extending towards the end face of the fuel cell stack. The end face of the fuel cell stack may be a bottom end face. The protrusion may extend away from an internal wall of the enclosure, and in some examples may comprise part of an internal wall of the enclosure. Providing the aperture in this way can enable the transition between the first region and second region to be located in the vicinity of the end of the stack. The second region may be defined, at least in part, by the inlet face of the fuel cell stack. The first region may be independent of the inlet face of the fuel cell stack. 
     The fuel cell assembly may comprise a diffuser within the first region of the delivery gallery. The diffuser may be configured to impart turbulence on oxidant/coolant received from the inlet of the enclosure. The diffuser may be fixed such that it is not free to move in normal use of the fuel cell assembly. Such a diffuser can provide a convenient means of imparting turbulence that does not require any moving parts or active components that may be more susceptible to damage and may consume power. 
     The diffuser may comprise one or more fuel cell control system components, such as a printed circuit board (PCB), disposed within the delivery gallery. In this way, particularly efficient cooling of the fuel cell control system components can be achieved without requiring any additional components. 
     The fuel cell assembly may further comprise a fan or air flow generator located within the enclosure configured to cause the coolant/oxidant to be transferred from the inlet in the enclosure to the inlet face of the fuel cell stack. The axis of the fan may be directed away from, or otherwise may not point towards, the aperture. This can encourage turbulent flow in the first region of the delivery gallery as the coolant/oxidant encounters one or more components to diffuse the flow as it proceeds to the aperture to the second region of the delivery gallery. 
     The axis of the fan may be perpendicular or transverse to the plane of the layers in the stack. The axis of the fan may be perpendicular to the direction of coolant/oxidant flow through the stack. Aligning the fan and fuel stack in this way can provide a compact fuel cell assembly and encourage turbulent flow of the coolant/oxidant in the first region. 
     The fuel cell stack may comprise an outlet face for expelling said coolant/oxidant fluid,
         the fuel cell stack further including a pair of end faces extending transversely between the inlet face and outlet face,   the enclosure defining a flow path for the coolant/oxidant fluid that is configured to guide the coolant/oxidant fluid to the inlet face, from the outlet face, and over at least one of the end faces.       

     There may be provided a fuel cell assembly comprising:
         an enclosure having a fuel cell stack mounted therein,   the fuel cell stack having an inlet face for receiving coolant/oxidant fluid and an outlet face for expelling said coolant/oxidant fluid,   the fuel cell stack further including a pair of end faces extending transversely between the inlet face and outlet face,   the enclosure defining a flow path for the coolant/oxidant fluid that is configured to guide the coolant/oxidant fluid to the inlet face, from the outlet face, and over at least one of the end faces.       

     Providing the coolant/oxidant flow path in this way can improve the cooling of the fuel cell stack and provide a compact enclosure. 
     The enclosure may define the flow path for the coolant/oxidant fluid to guide the coolant oxidant/fluid over both the end faces. This can improve the cooling effect of the fuel cell stack even further. 
     The enclosure may define an exhaust gallery extending over and adjacent to one end face of the fuel cell stack. The enclosure may further define at least one exhaust port at an edge or corner of the enclosure. An edge can be considered as the meeting of two sides or surfaces and a corner can be considered as the meeting of three sides or surfaces of the enclosure. 
     Providing the at least one exhaust port at an edge or corner of the enclosure can help to prevent the port from being blocked when the assembly is placed next to other objects; the ports can be considered as protected because of their location. 
     The exhaust gallery may be at least partly defined by the outlet face of the fuel cell stack, a top end face of the fuel cell stack, and one or more internal surfaces of the enclosure. In this way, the oxidant/coolant can be exposed to the top end face of the fuel cell stack before it exits the assembly through the exhaust port. 
     The edge or corner of the enclosure defining said at least one exhaust port may be disposed at or beyond a peripheral edge of the fuel cell stack. This can enable the oxidant/coolant to flow over a large proportion of the end face of the fuel cell stack before exiting the exhaust port. 
     The at least one exhaust port may extend around more than one edge of the enclosure. Providing a relatively large exhaust port can enable acceptable performance of the fuel cell stack to be maintained when the exhaust port is partially obscured. 
     The fuel cell assembly may further comprise a separation wall configured to separate the delivery gallery from an exhaust gallery. At least part of the separation wall may be movable so as to selectively provide an opening between the delivery gallery and the exhaust gallery. This can enable recirculation of the warm exhaust coolant/oxidant to the delivery gallery, thereby pre-heating the coolant/oxidant that is provided to the inlet face of the fuel cell stack. 
     The at least part of the separation wall may be moveable in accordance with the temperature of the coolant/oxidant in the exhaust gallery and/or delivery gallery. In this way, recirculation can be selectively provided to improve the performance of the fuel cell stack. 
     The fuel cell assembly may further include fuel cell control system components disposed within the exhaust gallery. In this way, the flow path of the oxidant/coolant can be used to cool the fuel cell control system components. 
     The enclosure may define a delivery gallery extending over and adjacent to one end face of the fuel cell stack. The enclosure may further define at least one inlet port at an edge or corner of the enclosure. Providing the at least one inlet port at an edge or corner of the enclosure can help to prevent the port from being blocked when the assembly is placed next to other objects; the ports can be considered as protected because of their location. 
     The delivery gallery may be defined at least in part by the inlet face of the fuel cell stack, a bottom end face of the fuel cell stack, and one or more internal surfaces of the enclosure. In this way, the oxidant/coolant can be exposed to the bottom end face of the fuel cell stack before it enters the fuel cell stack. 
     The edge or corner of the enclosure defining said at least one inlet port may be disposed at or beyond a peripheral edge of the fuel cell stack. This can enable the oxidant/coolant to flow over a large proportion of the end face of the fuel cell stack before entering the fuel cell stack. 
     The at least one inlet port may extend around more than one edge of the enclosure. Providing a relatively large inlet port can enable acceptable performance of the fuel cell stack to be maintained when the inlet port is partially obscured. 
     The fuel cell assembly may further include fuel cell control system components disposed within the delivery gallery. In this way, the flow path of the oxidant/coolant can be used to cool the fuel cell control system components. 
     The walls of the fuel cell stack may be generally parallel with the walls of the enclosure. This can provide a compact assembly. 
     The fuel cell assembly may further comprise a fan or air flow generator located within the enclosure configured to cause the coolant/oxidant to be transferred along the flow path. The axis of the fan may be perpendicular or transverse to the plane of the layers in the stack. The axis of the fan may also be perpendicular to the direction of coolant/oxidant flow through the stack. Aligning the fan and fuel stack in this way can provide a compact fuel cell assembly. 
     The fuel cell assembly may comprise:
         an inlet opening into the enclosure,   a delivery gallery extending from the inlet in the enclosure to the inlet face of the fuel cell stack,   the delivery gallery having a first region and a second region separated by an aperture,   wherein the delivery gallery and aperture are configured such that, in use, coolant/oxidant fluid within the first region of the delivery gallery is turbulent, and coolant/oxidant fluid within the second region of the delivery gallery has a generally uniform pressure.       

     According to a further aspect, there is provided a portable electronic device charging unit comprising any fuel cell assembly disclosed herein. 
    
    
     
       The invention will now be described by way of example, and with reference to the accompanying drawings in which: 
         FIG. 1 a    shows a schematic plan view of a fuel cell assembly according to an embodiment of the invention; 
         FIG. 1 b    shows an end cross-sectional view of the assembly on line b-b of  FIG. 1 a   ; and 
         FIG. 1 c    shows a side cross-sectional view of the assembly on line c-c of  FIG. 1   a.    
     
    
    
     Embodiments disclosed herein relate to a fuel cell assembly comprising an enclosure having a fuel cell stack mounted therein. 
     In some examples, the fuel cell assembly has a delivery gallery extending from an inlet in the enclosure to an inlet face of the fuel cell stack, the delivery gallery having a first region and a second region separated by an aperture. The delivery gallery and aperture are configured such that, in use, air within the first region of the delivery gallery is turbulent, and air within the second region of the delivery gallery has a generally uniform pressure. This can enable effective cooling of electronic components located in the first region due to the turbulent air flow, whilst also providing efficient use of the fuel cell stack as air is distributed evenly between layers in the fuel cell stack. 
     Alternatively, or additionally, the enclosure defines a flow path for air through the fuel stack that guides the air over at least one end face of the stack. Guiding the air in this way can provide additional cooling to the stack and thermally decouple the stack from the enclosure. The fuel cell assembly may have a delivery gallery such that air can be passed over a bottom end face of the stack before entering an inlet face of the stack and/or an exhaust gallery such that air that leaves the stack through an outlet face can be passed over a top end face of the stack before exiting the assembly. 
       FIGS. 1 a , 1 b  and 1 c    illustrate a fuel cell assembly  100  according to an embodiment of the invention.  FIG. 1 a    shows a top view of the assembly  100 .  FIG. 1 b    shows a cross-sectional view perpendicular to the assembly  100  along the line b-b in  FIG. 1 a   .  FIG. 1 c    shows a cross-sectional view longitudinal to the assembly  100  along the line c-c in  FIG. 1   a.    
     The fuel cell assembly  100  has an enclosure  102  having a fuel cell stack  104  mounted therein. The walls of the fuel cell stack  104  are generally parallel with the walls of the enclosure  102 . The fuel cell stack  104  has an inlet face  106  for receiving fluid, such as coolant or oxidant fluid. The fluid may be air, and will be referred to as air for the rest of the description of  FIGS. 1 a , 1 b  and 1 c   . The fuel cell stack  104  also has an outlet face  108  for expelling the air. The inlet face  106  and outlet face  108  are opposing faces of the fuel cell stack  104 . In the example of  FIG. 1 , the inlet and outlet faces  106 ,  108  can be considered as longitudinal side faces of the fuel cell stack  104 . 
     The fuel cell stack  104  also has end faces extending transversely between the inlet face  106  and the outlet face  108 . The fuel cell stack has a top end face  110  and a bottom end face  112  on opposing sides of the fuel cell stack  104 . The top and bottom end faces  110 ,  112  are located parallel to the planes of the layers that make up the stack  104 . The fuel cell stack  104  also has two side end faces  114 ,  116  on opposing sides of the fuel cell stack  104 . The side end faces  114 ,  116  are transverse to the planes of the layers that make up the fuel cell stack  104 . 
     The enclosure  102 , at least in part, defines a flow path for guiding the air over at least one of the end faces  110 ,  112 ,  114 ,  116  and most preferably at least the top and bottom end faces  110 ,  112 . Arrows indicating the direction of air flow through the assembly  100  are included in  FIGS. 1 a  to 1 c   . In this way, additional cooling can be provided to the fuel cell stack  104 . Also, a compact assembly  100  can be provided. 
     Furthermore, the coupling of heat generated by the fuel cell stack  104  to the enclosure  102  can be reduced as the fuel cell stack is substantially detached from the enclosure  102 . This can be particularly advantageous for examples where the enclosure is made from a heat conducting material such as aluminium. Such enclosures may have a maximum operating temperature. Therefore, detaching the fuel cell stack  104  from the enclosure  102  by providing regions of the delivery gallery  128  and/or exhaust gallery  126  therebetween can help to keep the temperature of the enclosure  102  below its maximum operating temperature. 
     A thermal coating (not shown) may be provided on one or more of the inside surfaces of the enclosure  102  that define the exhaust gallery  126  in order to thermally isolate the heat in the exhaust air from the enclosure  102 . 
     In this example, the top surface of the enclosure  102  comprises a baffle  118  that is spaced apart from the side walls of the enclosure  102  in order to provide an opening into the enclosure  102  around the perimeter of the baffle  118 . As will be discussed below, the opening around the baffle  118  can provide inlet and exhaust ports  120 ,  122  for the air on the edges or corners of the enclosure  102 . An edge of the assembly  100  can be considered as the meeting of two surfaces such as the baffle  118  and a side wall of the enclosure  102 . A corner of the assembly  100  can be considered as the meeting of three surfaces such as the baffle  118  and two side walls of the enclosure  102 . 
     Providing the inlet and exhaust ports  120 ,  122  in this way can help protect them from becoming blocked, for example if objects are placed on top of, or to the side of, the assembly  100 . 
     As can be seen from  FIG. 1 a   , substantially all of the opening around the baffle  118  can be used as either an inlet port  120  or an exhaust port  122 , although this need not necessarily be the case. An advantage of having relatively large inlet and exhaust ports  120 ,  122  is that the stack  104  can function adequately when either one or both of the ports  120 ,  122  are partially obscured, for example up to 50% obscured. 
     The exhaust port  122  extends around the portion of the baffle  118  that is generally located above the stack  104 . In this way, when the air exits the outlet face  108  of the stack  104  it exits the enclosure  102  through one or more of:
         a region of the exhaust port that is next to the outlet face  108  of the stack—this region of the exhaust port is shown with reference  122   a  in  FIGS. 1 a    and  1   b;      a region of the exhaust port that is next to the inlet face  106  of the stack  104 —this region of the exhaust port is shown with reference  122   b  in  FIGS. 1 a  and 1 b   ; and   a region of the exhaust port that is next to a side end face  116  of the stack  104 —this region of the exhaust port is shown with reference  122   c  in  FIGS. 1 a    and  1   c.          

     The baffle  118  is spaced apart from the top end face  110  of the stack in order to partly define an exhaust gallery  126  therebetween. The exhaust gallery  126  enables air to flow from the outlet face  108  of the stack  104  to any region of the exhaust port  122  over the top end face  110  of the stack  104 . It will be appreciated that any air that exits the enclosure  102  through a region  122   b ,  122   c  of the exhaust port that is not next to the outlet face  108  of the stack  104  will have passed over the top end face  110  of the stack  104 , thereby further cooling the stack  104 . This air flow can be seen from the arrows in  FIG. 1   b.    
     The exhaust gallery  126  is defined by the outlet face  108  of the fuel cell stack  104 , the top end face  110  of the fuel cell stack  106 , a bottom surface of the baffle  118 , one or more internal side surfaces of the enclosure  102 , and first and second separation walls  132 ,  134  that are discussed in more detail below. The exhaust gallery  126  extends between the outlet face  108  of the stack  104  and the exhaust port  122  and is bound, at least in part, by the top end face  110  of the stack  104 . 
     The inlet port  120  is provided by the remainder of the opening around the baffle  118 ; that is, the regions of the opening that are not an exhaust port  122 . 
     A fan  124  is located within the enclosure  102  and sucks air into the enclosure  102  through the inlet port  120 . This builds up the air pressure in the enclosure  102  such that air passes into the inlet face  106  of the stack, out of the outlet face  108  of the stack  104 , and then out of the exhaust port  122 . The combination of the low pressure drop of the settling volume in the delivery gallery  128  and the relatively high restriction offered by the inlet face  106  can promote a uniform longitudinal (relative to the fuel cell stack  104 ) air delivery. Although a fan  124  is described for this embodiment, it will be appreciated that any other generator of air flow could be used. 
     The fan  124  is located in the same plane as the layers of the stack  104 . The axis of the fan  124 , that is the direction of air flow through the fan, is perpendicular or transverse to the plane of the layers in the stack  104 . The axis of the fan  124  is also perpendicular or transverse to the direction of air flow through the stack  104 . Aligning the fan  124  and stack  104  in this way can provide a compact fuel cell assembly. 
     The fan  124  sucks air from outside the enclosure  102  into a delivery gallery  128 , which may also be referred to as an inlet plenum. In this example, fuel cell control system components  130  are located in the delivery gallery  128 . Being able to mount the components  130  in the delivery gallery  128  can make efficient use of space and therefore provide an advantageously small assembly. This can also avoid the need for a separate inlet plenum chamber (as would usually be employed) allowing for a more compact assembly. In addition, passing the air over the components  130  can provide for improved cooling of the components  130  and potentially pre-heating of the oxidant air, which can improve the performance of the fuel cell stack  104 . 
     The delivery gallery  128  is defined by the inlet face  106  of the fuel cell stack  104 , the bottom end face  112  of the fuel cell stack  106 , a number of internal surfaces of the enclosure  102  and first and second separation walls  132 ,  134  that are discussed in more detail below. 
     The delivery gallery  128  extends between the inlet port  120  and the inlet face  106  of the stack  104  and is bound, at least in part, by the bottom end face  112  of the stack  104 . 
     The delivery gallery  128  is separated from the exhaust gallery  126  by one or more of the following:
         1. the fuel cell stack  104 ;   2. a first separation wall  132  that extends between the fuel cell stack  104  and the baffle  118 ; and   3. a second separation wall  134  that extends between the fuel cell stack  104  and the enclosure  102 .       

     Part of the first separation wall  132  is visible in  FIG. 1 c    and prevents air from passing directly to the exhaust gallery  126  over the top of the stack without passing through cathode flow channels in the stack  104 . In this example, the first separation wall  132  is generally vertical and extends across the entire width of the stack  104  (orthogonal to the plane of the drawing of  FIG. 1 c   ). On the side of the stack  104  that includes the outlet face  108 , the left-hand side of the stack  104  shown in  FIG. 1 b   , the first separation  132  wall extends down the side of the stack  104  until it meets the second separation wall  134 . This extension of the first separation wall  132  is not shown in the drawings. 
     The first separation wall  132  also serves to separate the inlet port  120  from the exhaust port  122  around the outside of the baffle  118 . 
     The second separation wall  134  is visible in  FIG. 1 b    and prevents air from passing from the delivery gallery  128  to the exhaust gallery  126  up the side of the stack without passing through cathode flow channels in the stack  104 . In this example, the second separation wall  134  is generally horizontal and extends across the entire length of the stack  104  (orthogonal to the plane of the drawing of  FIG. 1 b   ). The second separation wall abuts the first separation wall  132  at one end and the enclosure  102  at the other end. 
     The second separation wall  134  is this example has an angled upper surface  136  such that the region of the exhaust gallery that is adjacent to the outlet face  108  of the stack  104  is a tapering volume. This can guide air flow and/or permit more uniform air flow into the stack  104 ; the pressure drop across the inlet port  120  to the delivery gallery  128  can equalise the air distribution along the stack  104  whilst the pressure drop across each cell of the stack  104  and the asymmetric (tapering) region of the exhaust gallery  126  next to the outlet face  108  of the stack, can be used to regulate the flow between cells. This can remove the need for an inlet plenum chamber (as would usually be employed) allowing for a more compact assembly. 
     It will be appreciated that the angled upper surface  136  of the second separation wall may be curved, straight, a combination of curved and straight, or any other profile that defines the region of the exhaust gallery that is adjacent to the outlet face  108  of the stack  104  as a tapering volume or provides optimal air flow guiding. 
     Similarly, a portion  138  of the internal surface of the enclosure  102  that is next to the inlet face  106  of the stack  104  may be angled such that the region of the delivery gallery  128  that is adjacent to the inlet face is a tapering volume. In a similar way to that discussed above, the asymmetric (tapering) region of the delivery gallery  128  next to the inlet face  106  of the stack  104  can equalise the compressed air distribution along the stack  104 . 
     The fuel cell assembly  100  of  FIGS. 1 a  to 1 c    will now be described with a focus on the air flow through the delivery gallery  128 , in particular first and second regions  128   a ,  128   b  of the delivery gallery  128 . It will be appreciated that one or more of the features of the fuel cell assembly  100  that are described above may be considered as optional when a fuel cell assembly with first and second regions  128   a ,  128   b  of the delivery gallery  128 , with the associated functionality, is provided. Likewise, the first and second regions  128   a ,  128   b  of the delivery gallery  128 , and the associated functionality, may be considered as optional for fuel cell assemblies that have one or more of the features described above. 
     The delivery gallery  128  can be considered as having at least two regions: a first region  128   a  and a second region  128   b . An aperture  140  is located between the first region  128   a  and second region  128   b . This aperture  140  may be referred to as a choke aperture. An inlet port  120  opens into the first region  128   a  in order to provide the air to the delivery gallery. The second region  128   b  is defined, at least in part, by the inlet face  106  of the fuel cell stack. 
     The delivery gallery  128  and/or aperture  140  are configured such that, in use, air within the first region  128   a  of the delivery gallery  128  is turbulent, and air within the second region  128   b  of the delivery gallery  128  has a generally uniform pressure. In this way, the random distribution of the turbulent air flow in the first region  128   a  can be used to cool the fuel cell control system components  130  (or any other electronic components) in the first region. Also, the generally uniform air pressure in the second region is applied to the layers in the fuel cell stack in order to provide efficient and effective operation of the fuel cell stack  104 . 
     The aperture  140  may provide a restriction to air flow between the first  128   a  and second regions  128   b  of the delivery gallery  128 . The aperture  128  may represent a reduction in cross-sectional area in the flow path of the air from the first region  128   a  to the second region  128   b  of the delivery gallery  128 . In this way, the air experiences a pressure change as it flows through the aperture  140  such that a generally uniform pressure is achieved along the length of the fuel cell stack. 
     In this example, the aperture is partly defined by a side surface of the bottom end face  112  of the fuel cell stack and a protrusion  142  from the inside of a side wall of the enclosure  102 . The protrusion  142  extends towards the bottom end face  112  of the fuel cell stack  104 . It will be appreciated that the protrusion  142  need not necessarily be provided as part of the enclosure  102 ; it can be formed by an extension of the bottom end face  112  of the stack  104  or any component or member that provides the necessary restriction for ensuring that the air within the second region  128   b  has generally uniform pressure, in use. In some examples, a protrusion may not be required at all. 
     It can be advantageous to provide the aperture  140  near the bottom end face  112  of the stack  104 . This is because the volume of the first region  128   a  with turbulent flow is maximised for cooling the components  130 , whilst only air with a generally uniform pressure is provided to the inlet face  106  of the stack  104 . 
     Instead of having the protrusion  142  in the vicinity of the bottom end face  112  of the stack  104 , a protrusion  144  may be provided in the vicinity of the top end face  110  of the stack  104 . This may be particularly advantageous if the direction of air flow through the stack  104  is reversed, for example by reversing the direction of the fan  124 , as an aperture defined by the protrusion  144  can provide a region downstream of the aperture that has a generally uniform pressure. In this example, the aperture is partly defined by a side surface of the top end face  110  of the fuel cell stack and the protrusion  144  from the inside of a side wall of the enclosure  102 . The protrusion  144  extends towards the top end face  110  of the fuel cell stack  104 . This protrusion  144  and corresponding aperture may have features and functionality that correspond to the protrusion  142  and corresponding aperture in the vicinity of the bottom end plate  112  that are discussed above. 
     It will be appreciated that when the direction of flow through the stack is reversed, the face of the stack that is labelled with reference  108  will be the inlet face and the ports that are labelled with reference  122  will be inlet openings into the enclosure. Therefore, the plenum extending between the components labelled with references  108  and  122  can be considered as a delivery gallery having a first region upstream of the protrusion  144  and a second region downstream of the protrusion  144 . 
     One or more of the fuel cell control system components  130 , such as a printed circuit board (PCB), may be considered as a diffuser inasmuch as they diffuse air that is provided into the first region  128   a  of the delivery gallery in order to impart turbulence on air flow in the first region  128   a . The fuel cell control system components  130  can be considered as providing for good turbulent flow due to their irregular shape 
     It will appreciated that the fuel cell control system components  130  can be considered as fixed with respect to the enclosure  102 , that is, they are not free to move in normal use of the fuel cell assembly  100 . Therefore, using the components  130  as a diffuser can provide a convenient means of imparting turbulence that does not require any moving parts or active components, which may be more susceptible to damage and may consume power. Furthermore, no additional components are required to achieve the desired turbulence. Embodiments disclosed herein can avoid a need for moving baffles or any additional means for recirculating the air. 
     In this example, the axis of the fan  124  is directed towards at least a region of the components  130 . The axis of the fan  124  is not in the direction of the aperture  140  to the second region  128   b . This can discourage laminar flow of air from the fan  124  to the second region  128   b  as the air will be deflected by the components  130  before it reaches the aperture  140 . Therefore, aligning the fan in a specific way can encourage turbulent flow in the first region  128   a . Examples of such specific fan alignments include: not pointing towards the aperture  140 ; transverse to the plane of the layers in the stack  104 ; perpendicular or transverse to the direction of air flow through the stack  104 ; perpendicular or transverse to the principal air flow direction through the first region  128   a ; and toward one or more components located in the first region  128   a  of the delivery gallery. 
     Although the described embodiments show the fan  124  positioned in the inlet flow path providing a somewhat positive air pressure in the delivery gallery  128 , it will be understood that the fan or other air flow generator could be disposed in the exhaust gallery to generate a somewhat negative air pressure therein. 
     In one or more of the embodiments disclosed herein, at least part of the first separation wall  132  may be movable so as to selectively provide an opening between the delivery gallery  128  and the exhaust gallery  126 . For example, at least a portion of the first separation wall  132  may be fixed to the baffle  118  and may be releasable from the top end face  110  of the stack, or at least a portion of the first separation wall  132  may be fixed to the top end face  110  of the stack and may be releasable from the baffle  118 . Providing such an opening can enable warm air to be recirculated from the exhaust gallery  126  to the delivery gallery  128 . This can provide operational advantages as the air flow through the stack  104  is pre-heated. 
     At least a portion of the first separation wall  132  may be provided by a bimetallic strip such that once the temperature of the air in the exhaust gallery  126  or delivery gallery  128  is above or below a threshold value the metallic strip deforms in order to allow or prevent recirculation. In alternative embodiments, the first separation wall  132  may be electrically controlled in accordance with one or more measured operating parameters such as air temperature. The first separation wall  132  may be made from nitinol. 
     As will be appreciated from the description of  FIGS. 1 a  to 1 c   , the aperture  140  in this example extends in a longitudinal direction into the page of  FIG. 1 b   . In some embodiments, the width of the aperture  140  may vary in the longitudinal direction, in order to assist in providing a uniform air pressure in the second region  128   b . The width of the aperture  140  may vary uniformly or non-uniformly in the longitudinal direction. The aperture  140  may get wider in the longitudinal direction away from the fan  124 . This can further assist in equalising the air pressure in the second region  128   b  in spite of the air pressure in the first region  128   a  on the other side of the aperture  140  being lower at positions further away from the fan  124 . 
     It will be appreciated that the width of the aperture  140  can be defined by using a protrusion  142  with the requisite size and shape. 
     It will also be appreciated that the physical properties of any aperture defined by a protrusion  144  provided in the vicinity of the top end face  110  of the stack  104  can have the same characteristics as the aperture  140  in the vicinity of the bottom end face  112  of the stack  104 . Similarly, the physical properties of a protrusion  144  provided in the vicinity of the top end face  110  of the stack  104  can have the same characteristics as the protrusion  142  in the vicinity of the bottom end face  112  of the stack  104 . 
     The dimensions of any aperture or protrusion can be defined so as to tune the pressure response of the aperture as air passes from the first region  128   a  to the second region  128   b.    
     Throughout the present specification, the descriptors relating to relative orientation and position, such as “top”, “bottom” and “side” as well as any adjective and adverb derivatives thereof, are used in the sense of the orientation of the fuel cell assembly as presented in the drawings. However, such descriptors are not intended to be in any way limiting to an intended use of the described or claimed invention. 
     The fuel cell assembly disclosed herein may be suitable for a charger for portable electronic devices such as mobile telephones, personal computing devices and the like.