|Title of Invention||
A FUEL GAS FLOW FIELD PLATE FOR A FUEL CELL
|Abstract||A fuel gas flow field plate for a fuel cell is characterised by a gradual decrease in the cross section of each of the multiple passes of the flow passage in a finite number of steps, starting from the first pass after the inlet upto the last pass before the outlet. A turning radius which is 1.5 to 9 times the equivalent diameter of the gas flow channels is provided for each channel between the passes. The outlet manifold is smaller than inlet manifold.|
This invention relates to a fuel gas flow field plate for a fuel cell.
Fuel cells are power generation devices that produce an electric current through an electrochemical reaction of a fuel and an oxidant. The most common fuel used in the fuel cell reaction is hydrogen gas, which itself could be derived from a variety of sources. The oxidant could be either pure oxygen or air. Since the theoretical voltage obtained from a hydrogen/oxygen fuel cell is only 1.23 volts per cell, it is common practice to assemble several cells in series to form a fuel cell stack.
Fuel and oxidant gases are supplied to the individual cells through manifolds and are then distributed inside single cells along passages or channels in flow field plates so as to supply reactant gases to the entire area of the electrodes, namely, the cathode and the anode. Fuel gas (a mixture containing hydrogen or pure hydrogen) is fed to the anode and oxidant gas (air or oxygen) is supplied to the cathode via the respective flow field plates. At the cathode, hydrogen ions from the anode and oxygen react electrochemically giving rise to an electric current and forming water as the product. In order to sustain operation of the fuel cell, particularly in the proton exchange membrane fuel cell, it is common practice to humidify the anode inlet gas prior to feeding into the fuel cell.
The fuel gas flow field plate proposed herein helps lower the flow rate of fuel gas to minimize the wastage of unconverted hydrogen, while maintaining adequate supply of reactant to the anode reaction at all current densities; allows operation of the fuel cell at moderate levels of humidification of the inlet gas, thus minimizing the load on the humidification subsystem; and maintains nearly uniform conditions of gas and moisture availability throughout the electrode area, that is, from the inlet to the outlet of the fuel gas in each anode flow field plate.
Various other features and advantages of this invention will be apparent from the following further description thereof
The fuel gas flow field plate for a fuel cell, according to this invention, is characterized by a gradual decrease in the cross section of each of the multiple passes of the flow passage in a finite number of steps, starting from the first pass, after the inlet, up to the last pass, before the outlet; a turning radius which is 1.5 to 9 times the equivalent diameter of the gas flow
channels provided for each channel; and an outlet manifold which is smaller than the inlet manifold.
It may be noted that the equivalent diameter for a non-circular cross section for flow here is defined as four times the hydraulic radius of the channel cross section; the hydraulic radius is in turn defined as the ratio of the area of cross section perpendicular to the direction of flow to the wetted perimeter.
This invention will now be described with reference to the accompanying drawings which illustrate, by way of example, and not by way of limitation, the salient features of one of various possible embodiments of the fuel gas flow field plate proposed herein.
In the embodiment illustrated, it will be seen in Figure 1 that there is a decrease in the cross section CI - C7 of each of the multiple passes PI - P7 of the flow passage F in a finite number of steps, starting from the first pass, after the inlet I, up to the last pass, before the outlet O.
The main benefit accruing from this feature is a reduction in the flow rate of the fuel gas required to maintain a given power output from the fuel cell stack. In the conventional flow field plate, the cross section would be constant fi-om the inlet to the outlet, whereas the flow rate of the fiiel gas stream would be gradually decreasing due to consumption in the fiiel cell reaction. This results in a greater drop in the pressure of the flowing gas than would result due to flow alone, and this reduces effective gas transport from the flowing stream into the electrode. In order to improve the gas transport to the electrode, then, a higher flow rate of the gas is often required. In contrast, a gradual reduction in the cross section as in the present invention, roughly corresponding to the reduction in the flow rate of the gas makes nearly uniform availability of the fiiel gas across the entire area of the electrode while allowing operation of the fiiel cell at lower gas flow rates.
Another benefit arising from this design is operation of the fiiel cell at lower levels of inlet humidification. Since the proton exchange membrane, which is used as the electrolyte, needs to be maintained wet for sustained hydrogen ion transportation, one common approach is to pre-humidify the fiiel gas prior to feeding into the fiiel cell, consuming energy in the form of heat for this purpose. Since the present design of a gradual reduction in the cross section helps in maintaining the flowing gas stream at nearly constant level of saturation from the
inlet to the outlet, it has been found that operation of the fuel cell stack can be carried out at lower level of inlet gas humdification, thus improving the overall system efficiency.
Yet another benefit accruing from the present design is a near-constant velocity of the gas stream. In the conventional flow field plate, the cross section would be constant and the linear velocity (which is defined as the ratio of the volumetric flow rate to the cross sectional area of flow) would decrease continuously fi'om the inlet to the outlet, due to a progressively diminishing volumetric flow rate resulting fi'om gas consumption and a constant area of cross section. In the present design, since, the cross section for flow decreases roughly corresponding to the flow rate of the gas, the linear velocity is maintained nearly constant, which helps in the uniformity of the fuel cell reaction across the entire area of the electrode. This aspect is shown in Figures 2 and 3.
A turning radius RF which is 1.5 to 9 times the equivalent diameter of the gas flow channels is provided for each channel (equivalent diameter as defined earlier). This minimizes the pressure drop within the flow field plate by avoiding sharp turns.
The terms and expressions in this specification are of description and not of limitation, there being no intention in excluding any equivalence of the features illustrated and described, but it is understood that various other embodiments of the gas flow field with a gradually varying cross section and with curved bends between passes as proposed herein are possible without departing from the scope and ambit of this invention.
1. A fuel gas flow field plate for a fuel cell characterized by a gradual decrease in the
cross section of each of the multiple passes of the flow passage in a finite number of
steps, starting from the first pass, after the inlet, up to the last pass, before the outlet;
and a turning radius which is 1.5 to 9 times the equivalent diameter of the gas flow
channels provided for each channel between passes; and an outlet manifold which is
smaller than the inlet manifold.
2. A fuel gas flow field plate for a fuel cell substantially as herein described and
|Indian Patent Application Number||553/MAS/2001|
|PG Journal Number||50/2007|
|Date of Filing||06-Jul-2001|
|Name of Patentee||SPIC SCIENCE FOUNDATION|
|Applicant Address||111 MOUNT ROAD, GUINDY, CHENNAI 600032,|
|PCT International Classification Number||H01 M8/00|
|PCT International Application Number||N/A|
|PCT International Filing date|