WO2019120415A1

WO2019120415A1 - Fuel cell assembly with a turbulence inducing device for reduction of a temperature gradient

Info

Publication number: WO2019120415A1
Application number: PCT/DK2018/050368
Authority: WO
Inventors: Torsten Berning
Original assignee: Aalborg Universitet
Priority date: 2017-12-21
Filing date: 2018-12-17
Publication date: 2019-06-27

Abstract

The present invention relates to an air-cooled fuel cell assembly (9) comprising a fuel cell (1) having a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate, said cathode flow plate defining a flow channel for conveying oxidant to the membrane electrode assembly, said flow channel having an inlet and an outlet and said flow channel extending between two opposing perimeters (5a, 5b) of said fuel cell assembly (9), an air pump (2) arranged to generate air flow in said flow channel in a first direction (8), a turbulence inducing device (4) upstream, relative to said first direction (8), of said flow channel, said turbulence inducing device comprising a grid.

Description

FUEL CELL ASSEMBLY WITH A TURBULENCE INDUCING DEVICE FOR REDUCTION

OF A TEMPERATURE GRADIENT

FIELD OF THE INVENTION

The present invention relate

s to a fuel cell assembly comprising a fuel cell having a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate, said cathode flow plate defining a flow channel for conveying oxidant to the membrane electrode assembly, said flow channel having an inlet and an outlet and said flow channel extending between two opposing perimeters of said fuel cell assembly, an air pump arranged to generate air flow in said flow channel in a first direction, a turbulence inducing device upstream, relative to said first direction, of said flow channel, said turbulence inducing device comprising a grid.

BACKGROUND OF THE INVENTION

Air-cooled, low temperature proton exchange membrane fuel cells (PEMFC) are becoming increasingly popular for uninterrupted power supply in remote areas, telecom back-up systems and even as range-extenders in battery electric vehicles. PEMFC are usually running on hydrogen and excessive air to provide oxygen for the electro-chemical reaction to produce electricity. In contrast to liquid-cooled PEMFC which are currently being commercialized by the automotive industry to power the next generation of vehicles, air-cooled fuel cells rely on the removal of waste heat by being provided with an excessive amount of air which absorbs the waste heat and therefore increases its temperature. Consequently, the stoichiometric flow ratio of air in an air-cooled fuel cell is 30-60, whereas in a liquid-cooled fuel cell it is only 1.2-3.0. This means that 30-60 times the amount of oxygen supplied to the fuel cell is participating in the electro-chemical reaction to provide electricity, and the excess air absorbs the waste heat. A typical temperature for the incoming air is 20 °C and for the exit air is 40 - 50 °C.

Advantages of the air-cooled fuel cells include their simplicity because no secondary coolant loop is required. Therefore, an air-cooled fuel cell system is substantially cheaper than a liquid-cooled fuel cell system . Moreover, their design is substantially simpler. A salient disadvantage of air-cooled fuel cells compared to liquid cooled fuel cells is the fact that the maximum current density that can be drawn from such a cell is only in the region of 0.3 - 0.4 A/cm² whereas liquid- cooled fuel cells can deliver up to 1.5 - 2.0 A/cm². This also limits the power density of air-cooled fuel cells to around 0.2— 0.25 W/cm² of active cell area whereas liquid-cooled fuel cells can reach above 1.0 W/cm² active cell area. This means that air-cooled fuel cells do not utilize the expensive platinum catalyst to the same degree as liquid-cooled fuel cells, and they have more severe space- restrictions. It is highly desirable to increase the maximum current that can be drawn from an air-cooled fuel cell and thereby also increase their power density.

From numerical analysis conducted by the inventor it was concluded that the maximum current density that can be drawn from an air-cooled fuel cell is restricted by thermal issues. In particular, the temperature distribution and the distribution of the relative humidity inside the fuel cell was calculated, revealing the following surprising finding : Even at a current density as low as 0.3 A/cm² the predicted temperature in the air-cooled fuel cell can exceed 90 °C, despite the fact that the air provided is just 20 °C and the air leaving the fuel cell is below 50 °C.

The numerical model also calculates the distribution of the relative humidity inside the fuel cell which is important to understand because in low-temperature fuel cells the conductivity of the membrane depends strongly on its water content. The result of the study showed that the membrane can be expected to be nearly fully hydrated under all operating conditions investigated which is important for an optimum fuel cell performance. Interestingly, the numerical study also revealed that the anode side gas phase is saturated with water vapour under nearly all conditions while in the cathode side liquid water is predicted in the porous gas diffusion layers and even penetrating into the flow channels. Importantly, these calculations confirm the surprising experimental observation made by Dantherm A/S that liquid water is present in the cathode side channels.

As the maximum current density has been found to be limited by the maximum temperature inside the cells, a decrease of the temperature inside the cells would be advantageous. Hence, an improved fuel cell would be advantageous, and in particular a more efficient and/or reliable fuel cell would be advantageous. OBJECT OF THE INVENTION

It is an object of the present invention to provide a fuel cell assembly comprising a turbulence enhancing device suitable for use in an air-cooled fuel cell assembly, wherein the turbulence enhancing device leads to a decrease in the temperature gradient inside the fuel cells.

It is a further object of the present invention to provide an alternative to the prior art.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a fuel cell assembly comprising

a fuel cell having a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate, the cathode flow plate defining a flow channel for conveying oxidant to the membrane electrode assembly, the flow channel extending between two opposing perimeters of the fuel cell;

an air pump arranged to generate air flow in the flow channel in a first direction;

a turbulence inducing device upstream, relative to the first direction, of the flow channel, the turbulence inducing device comprising a grid.

The fuel cell may be an air-cooled open cathode fuel cell.

The air pump may be an air fan or another device suitable for generating a flow of air. The grid is a structure having a plurality of openings through which air may flow. The grid may be shaped in a number of ways. It may be a tiling of one or more geometric shapes such as squares, triangles, honeycombs, rectangles, etc., or it may e.g. be an irregular structure of openings.

By the turbulence inducing device being upstream, relative to the first direction, of the flow channel, is meant that when an air flow is generated in the flow channel by the air pump, the air that flows into the flow channel moves through the turbulence inducing device before flowing into the flow channel. In this way, a turbulent flow is created in part of or the whole of the flow channel.

The air pump may draw air towards itself (suck) or push air away from itself (blow). If the air pump draws air towards itself, the flow channel is located between the grid and the air pump. If the air pump pushes air away from itself, the grid is located between the flow channel and the air pump.

By creating a turbulent flow in the flow channel, a mixing effect is achieved, which reduces the maximum temperature in the flow channel. In addition, a stirring of the gas and liquid mixture in the flow channel would cause some of the liquid water there to evaporate and thereby provide a cooling effect. A reduction in the maximum temperature in the flow channel will increase the maximum current density of the fuel cell, which again will lead to an increase in the maximum power density of the fuel cell.

It is thus an advantage of the present invention that the presence of the turbulence inducing device increases the maximum power density of the fuel cell.

In an embodiment of the invention, the grid may comprise a first set of a plurality of elongate elements being parallel and equidistantly distanced in a single plane with a distance in-between and having a hydraulic diameter, d l.

The hydraulic diameter, d, is a parameter used in calculations involving turbulent flow, defined as four times the cross-sectional area, A, divided by the wetted perimeter P, such that d = 4A/P. The wetted perimeter P is the cross sectional area that is "wet", i.e. the cross sectional area that is in contact with a fluid. In a further embodiment, the elongate elements of the first set of a plurality of elongate elements are not equidistant. In another embodiment of the invention, the grid may comprise a second set of a plurality of elongate elements being parallel and equidistantly distanced in a single plane with a distance in-between and having a hydraulic diameter, d2, wherein the second set of elongate elements are placed at an angle with respect to the first set of elongate elements.

In a further embodiment, the elongate elements of the second set of a plurality of elongate elements are not equidistant.

In a further embodiment of the invention, the plane of the first set of a plurality of elongate elements and the plane of the second set of a plurality of elongate elements are partly or wholly overlapping.

In an embodiment of the invention, the turbulence inducing device is arranged at a distance upstream of the flow channel of at least 2 times of a hydraulic diameter of one or more of the plurality of elongate elements and/or less than 20 times the hydraulic diameter of one or more of the plurality of elongate elements.

In an embodiment of the invention, the turbulence inducing device is arranged at a distance upstream of the flow channel of 0 mm to 100 mm, preferably 0 mm to 20 mm, more preferably 0 mm to 5 mm.

In an embodiment of the invention, the grid is a fractal grid.

A fractal grid is based on a single pattern, which is repeated at different scales, where the number of repetitions at different scales is given by a fractal iteration parameter. The pattern may resemble an "I", be a square, a cross, etc. Thus, the fractal grid may be a so-called fractal I grid, a fractal square grid, a fractal cross grid, etc. In an embodiment of the invention, the grid is not rigid, but is flexible. In another embodiment of the invention, the turbulence inducing device comprises two or more grids. In a further embodiment of the invention, the two or more grids have different structures.

In another embodiment of the invention, the two or more grids are staggered behind one another inside the turbulence inducing device.

By staggered is meant that the two or more grids are placed at a distance to each other. The distance between two adjacent grids need not be the same as the distance between two other adjacent grids. Thus, the two or more grids may be placed at varying distances to each other within the turbulence inducing device.

In an embodiment of the invention, the grid or part of the grid is made of plastic or a metal.

In an embodiment of the invention, the air pump is adapted to provide a Reynolds number of at least 175, preferably more than 500 and even more preferably more than 1000, for the air flow past the grid. The Reynolds number being calculated on the basis of a characteristic length scale of the turbulence inducing device such as e.g. the hydraulic diameter, d, of a set of elongate elements. The Reynolds number is a dimensionless quantity in fluid mechanics describing the ratio of inertial forces to viscous forces within a fluid.

The Reynolds number, R, is defined as the velocity, u, of the fluid times a characteristic linear dimension, L, divided by the kinematic viscosity of the fluid, v, such that R = u · L/ v.

To calculate the Reynolds number for some grids, one can use the freestream velocity, Uo, and the fill factor of the grid, S, such that u = Uo/(l-S). The fill factor S is a dimensionless quantity that characterizes the grid. It is calculated as

S = 1 - Fi/Fo, where Fi is the area unoccupied by the solid part of the grid and Fo is the total area of the grid. For a grid made up of two sets of circular elongate elements at right angles and having the same distance between the elements, S = 1 - (1 - drod/M)² where drod is the diameter of the elongate elements and M is the grid mesh size. The grid mesh size, M, is the distance between the centers of the elongate elements in the case of the square grid.

In an embodiment of the invention, the grid or the two or more grids is made to move up and down or vibrate at regular or irregular intervals.

In another embodiment of the invention, the grid or part of the grid comprises a heat conductive or heat element material and the fuel cell assembly further comprises a heat generating device. If the grid comprises a heat conductive material, the heat generating device is thermally connected to the heat conductive material of the grid. If the grid comprises a heat element material, the heat generating device is electrically connected to the heat element material of the grid.

In an embodiment of the invention, the heat conductive material may be a metal or another material, which has a high heat conductivity.

A heat element material is a material that converts electricity into heat.

In another embodiment of the invention, the heat element material is a material that converts electricity into heat by resistive heating, whereby an electric current passing through the material encounters electrical resistance resulting in generation of heat.

In an embodiment of the invention, the heat generating device, when in use, increases the temperature of the grid or part of the grid by increasing the temperature of the heat conductive or heat element material.

In another embodiment of the invention, the heat generating device is a device that generates an electrical current, which generates heat when being passed through a heat element material, such as e.g. a heat element material in the grid. In a further embodiment of the invention, the heat generating device is a device that generates heat, which is then transferred through a thermal connection to a heat conductive material, such as e.g. a heat conductive material in the grid.

By heating the grid or part of the grid, the air that passes through the grid may be heated, which is suitable when the fuel cell is being used in low temperature conditions, where the air is very cold such as air below 0 °C. If very cold air is sent directly into the fuel cell, the fuel cell will not work. Therefore, the very cold air must be pre-heated before entering the fuel cell.

In a second aspect, the invention relates to a method of operating the fuel assembly according to the first aspect, the method comprising :

identifying a stoichiometric flow ratio from a thermodynamic analysis and fuel cell performance;

calculating an air flow rate;

operating said air pump to provide a flow of air corresponding to the calculated air flow rate.

Further, when the fuel cell assembly comprises a heat generating device and the grid or part of the grid is made of a heat conductive material thermally connected to and/or a heat element material electrically connected to the heat generating device, the invention relates to a method of operating the fuel assembly according to the first aspect, wherein the heat generating device causes an increase in the temperature of the grid or part of the grid such that the air flowing through the grid is heated to a temperature of at least 0 °C, preferably more than 5 °C and even more preferably more than 10 °C.

The different aspects of the present invention as described above may each be combined with any of the other aspects as long as it is physically possible. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The present invention and in particular preferred embodiments thereof will now be disclosed in more detail with regard to the accompanying figures. The figures show ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 illustrates schematically a fuel cell assembly according a first

embodiment of the present invention;

Figure 2 illustrates schematically a fuel cell assembly according a second embodiment of the present invention; Figure 3 illustrates schematically a fuel cell assembly according a third

embodiment of the present invention;

Figure 4 illustrates schematically a grid for use in a turbulence inducing device; Figure 5 illustrates schematically a grid for use in a turbulence inducing device;

Figure 6 illustrates schematically a grid for use in a turbulence inducing device;

Figure 7 illustrates schematically a turbulence inducing device comprising two grids;

Figure 8 illustrates schematically a grid for use in a turbulence inducing device;

Figure 9 is a graph showing experimental results of the fuel cell performance obtained with and without a turbulence inducing device;

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows schematically a fuel cell assembly 9 according to an embodiment of the present invention. The fuel cell assembly 9 comprises a fuel cell, which has a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate defining a flow channel 3 for conveying oxidant to the membrane electrode assembly. Such a flow channel 3 extends between two opposing perimeters 5a, 5b of the fuel cell thus allowing for air to flow through the fuel cell.

An air pump 2 is used to create a flow of air into the flow channel 3 in a first direction 8 and acts as a combined cooling and air flow mechanism.

In the embodiment shown in figure 1, the air pump 2, when in use, draws air through a turbulence inducing device 4, which comprises a grid 7. The turbulence inducing device 4 is located upstream of the flow channel 3 relative to the first direction 8 such that air drawn by the air pump 2 passes through the turbulence inducing device 4 before it enters the flow channel 3.

The air pump 2 may be adapted to provide a Reynolds number of at least 175, preferably more than 500 and even more preferably more than 1000, for the air flow past said grid 7. The Reynolds number being calculated using a characteristic length scale of the turbulence inducing device.

The grid 7 or part of the grid 7 may be made of plastic or a metal.

The turbulence inducing device may have two or more grids.

The grid 7 or two or more grids 7 may be made to move up and down or vibrate at regular or irregular intervals.

Figure 2 shows schematically a fuel cell assembly 9 according to a second embodiment of the present invention. The fuel cell assembly 9 comprises a fuel cell, which has a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate defining a flow channel 3 for conveying oxidant to the membrane electrode assembly. Such a flow channel 3 extends between two opposing perimeters 5a, 5b of the fuel cell thus allowing for air to flow through the fuel cell. An air pump 2 is used to create a flow of air into the flow channel 3 in a first direction 8 and acts as a combined cooling and air flow mechanism.

In the embodiment shown in figure 2, the air pump 2, when in use, blows air through a turbulence inducing device 4, which comprises a grid 7. The turbulence inducing device 4 is located upstream of the flow channel 3 relative to the first direction such that air blown by the air pump 2 passes through the turbulence inducing device 4 before it enters the flow channel 3.

The grid 7 or part of the grid 7 may be made of plastic or a metal.

The turbulence inducing device may have two or more grids.

Figure 3 shows schematically a fuel cell assembly 9 according to a third

embodiment of the present invention. The fuel cell assembly 9 comprises a fuel cell, which has a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate defining a flow channel 3 for conveying oxidant to the membrane electrode assembly. Such a flow channel 3 extends between two opposing perimeters 5a, 5b of the fuel cell thus allowing for air to flow through the fuel cell.

In the embodiment shown in figure 3, the air pump 2, when in use, generates a flow of air through the flow channel 3 in a first direction 8 that is either away from the air pump 2 or towards the air pump 2. Before the air enters the flow channel 3 it passes through a turbulence inducing device 4 and after the air leaves the flow channel 3 it passes through another turbulence inducing device 4. The turbulence inducing devices 4 each comprise a grid 7.

The grid 7 or part of the grid 7 may be made of plastic or a metal.

The turbulence inducing device may have two or more grids.

Figure 4 shows schematically a grid 7 for use in a turbulence inducing device 4 and illustrates claim 4. The grid 7 comprises a first set of a plurality of elongate elements 6a, which are parallel and equidistant to each other in a single plane. A distance D1 separates the centres of the elongate elements 6a.

The elongate elements in the first set of elongate elements 6a have a hydraulic diameter, dl.

Figure 5 shows schematically a grid 7 for use in a turbulence inducing device 4 and illustrates claim 5. The grid 7 comprises a first set of a plurality of elongate elements 6a and a second set of a plurality of elongate elements 6b. In the first set of a plurality of elongate elements 6a, the elongate elements are parallel and equidistant to each other in a single plane and in the second set of a plurality of elongate elements 6b, the elongate elements are parallel and equidistant to each other in a single plane. A distance Dl separates the centres of the elongate elements in the first set of elongate elements 6a, while a distance D2 separates the centres of the elongate elements in the second set of elongate elements 6b. The elongate elements in the first set of elongate elements 6a have a hydraulic diameter, d l, while the elongate elements in the second set of elongate elements 6b have a hydraulic diameter, d2.

The elongate elements of the second set of elongate elements 6b are placed at an angle with respect to the elongate elements of the first set of elongate elements 6a.

The plane of the first set of a plurality of elongate elements 6a and the plane of the second set of a plurality of elongate elements 6b may be partly or wholly overlapping.

The grids illustrated in figures 4 and 5 may be arranged at a distance upstream of said flow channel 3 of at least 2 times of a hydraulic diameter of one or more of the plurality of elongate elements 6a, 6b and/or less than 20 times a hydraulic diameter of one or more of the plurality of elongate elements 6a, 6b.

Figure 6 shows schematically a grid 7 for use in a turbulence inducing device 4. The grid 7 has a depth, D3, measured along the first direction 8, where the depth D3 may be 0.1 mm to 10 mm. The grid is further characterized by a grid mesh size M, which may be 0.1 mm to 10 mm, and a bridge thickness D4, may be 0.1 mm to 10 mm.

Figure 7 shows schematically a turbulence inducing device 4 comprising two grids and illustrates claim 7. The turbulence inducing device 4 may have two or more grids 7, which may be similar in structure or different in structure to each other.

The two or more grids 7 may be staggered behind one another inside the turbulence inducing device 4.

Figure 8 shows schematically a grid 7 for use in a turbulence inducing device 4 and illustrates claim 12. The fuel cell assembly 9 comprises a heat generating device 10, and : the grid 7 or part of the grid 7 is made of a heat conductive material 11 and the heat generating device 10 is thermally connected to the heat conductive material 11, and/or

- the grid 7 or part of the grid 7 is made of a heat element material 11 and said heat generating device 10 is electrically connected to said heat element material 11.

The heat conductive material 11 may be a metal or another material, which has a high heat conductivity.

A heat element material 11 is a material that converts electricity into heat. It may be a material that converts electricity into heat by resistive heating, whereby an electric current passing through the material encounters electrical resistance resulting in generation of heat.

The heat generating device 10, when in use, increases the temperature of the grid 7 or part of the grid 7 by increasing the temperature of the heat conductive or heat element material 11.

The heat generating device 10 could be a device that generates an electrical current, which generates heat when being passed through a heat element material 11 in the grid 7.

The heat generating device 10 could be a device that generates heat itself, which is then transferred through a thermal connection to a heat conductive material 11, in the grid 7.

In this way, the heat generating device 10 can cause the grid 7 or part of the grid 7 to increase in temperature by transferring heat to the heat conductive material 11 and/or by causing the heat element material 11 to convert electricity into heat.

Figure 9 is a graph showing experimental results obtained with and without a turbulence inducing device. The filled symbols are measurements of cell voltage and stack power obtained when using a turbulence inducing device with a single grid, while the open symbols are measurements obtained without a turbulence inducing device. In the experiment, a grid similar to that shown in figure 6 was placed 10 mm upstream of the cathode inlet. The grid had a thickness D3 of 1 mm, a grid mesh size M of 1 mm and a bridge thickness D4 of 0.25 mm. The graph shows the surprisingly large effect of a grid as a turbulence inducing device in a fuel cell assembly as the fuel cell efficiency (cell voltage) increased by 6.7% to 22.8 % (red arrows). This is in addition to the increase in maximum power density of the fuel cell of more than 30% (blue arrow). Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

List of reference symbols used

1 Fuel cell

2 Air pump (typically an air fan)

3 Air flow channels

4 Turbulence inducing device

5a, 5b Perimeter of fuel cell stack

6a First set of elongate elements

6b Second set of elongate elements

7 Grid

8 First direction

9 Fuel cell assembly

10 Heat generating device

11 Heat conductive or heat element material

Claims

1. A fuel cell assembly (9) comprising

a fuel cell (1) having a membrane electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate, said cathode flow plate defining a flow channel (3) for conveying oxidant to the membrane electrode assembly, said flow channel (3) extending between two opposing perimeters (5a, 5b) of said fuel cell (1);

an air pump (2) arranged to generate air flow in said flow channel (3) in a first direction (8);

a turbulence inducing device (4) upstream, relative to said first direction (8), of said flow channel (3), said turbulence inducing device (4)

comprising a grid (7).

2. A fuel cell assembly (9) according to claim 1, wherein said grid (7) comprises a first set of a plurality of elongate elements (6a) being parallel and equidistantly distanced in a single plane with a distance (Dl) in between and having a hydraulic diameter (dl).

3. A fuel cell assembly (9) according to claim 2, wherein said grid (7) further comprises a second set of a plurality of elongate elements (6b) being parallel and equidistantly distanced in a single plane with a distance (D2) in between and having a hydraulic diameter (d2) wherein said second set of elongate elements (6b) are placed at an angle with respect to said first set of elongate elements (6a).

4. A fuel cell assembly (9) according to claim 3, wherein said plane of said first set of a plurality of elongate elements (6a) and said plane of said second set of a plurality of elongate elements (6b) are partly or wholly overlapping.

5. A fuel cell assembly (9) according to any of the preceding claims, wherein said turbulence inducing device (4) is arranged at a distance upstream of said flow channel (3) of at least 2 times of a hydraulic diameter of one or more of the plurality of elongate elements (6a, 6b) and/or less than 20 times said hydraulic diameter of one or more of the plurality of elongate elements (6a, 6b).

6. A fuel cell assembly (9) according to claim 1, wherein said grid (7) is a fractal grid.

7. A fuel cell assembly (9) according to claim 1, wherein said turbulence inducing device (4) comprises two or more grids (7).

8. A fuel cell assembly (9) according to claim 7, wherein said two or more grids (7) have different structures.

9. A fuel cell assembly (9) according to claim 7, wherein said two or more grids (7) are staggered behind one another inside said turbulence inducing device (4).

10. A fuel cell assembly (9) according to any of the preceding claims, wherein said air pump (2) is adapted to provide a Reynolds number of at least 175, preferably more than 500 and even more preferably more than 1000, for the air flow past said grid (7).

11. A fuel cell assembly (9) according to any of the preceding claims, wherein said grid (7) or said two or more grids (7) is made to move up and down or vibrate at regular or irregular intervals.

12. A fuel cell assembly (9) according to any of the preceding claims, further comprising a heat generating device (10), and wherein :

- said grid (7) or part of said grid (7) is made of a heat conductive material (11) and said heat generating device (10) is thermally connected to said heat conductive material (11), and/or

- said grid (7) or part of said grid (7) is made of a heat element material (11) and said heat generating device (10) is electrically connected to said heat element material (11).

13. A fuel cell assembly (9) according to any of the preceding claims, wherein said grid(s) (7) or part of said grid(s) (7) is(are) made of plastic or a metal.

14. A method of operating the fuel assembly (9) according to any of the preceding claims, the method comprising:

- calculating an air flow rate;

operating said air pump (2) to provide a flow of air corresponding to said calculated air flow rate.

15. A method of operating the fuel assembly (9) according to claim 12, wherein said heat generating device (10) causes the temperature of said grid (7) or part of said grid (7) to increase such that the air flowing through said grid (7) is heated to a temperature of at least 0 °C, preferably more than 5 °C and even more preferably more than 10 °C.