Fuel cell end unit with integrated heat exchanger

[0001] This invention relates to fuel cells and, in particular, to end plates and heat exchangers for fuel cell systems. More specifically, this invention relates to highly integrated, compact heat exchangers for use in superheating fuel gas for high temperature fuel cells.

[0002] A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions.

[0003] In general, a fuel cell, similar to a battery, includes a negative (anode) electrode and a positive (cathode) electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate between each cell.

[0004] In a conventional fuel cell stack for stationary power applications, the active area of the fuel cells is large, typically between {fraction (1/2)} and 1 m². In order to apply a reasonable interface pressure on the cells, a large compressive load must be applied to the cells through the end plates. As the end plates must remain flat to insure intimate contact is maintained with the cells, the end plates are typically thick relative to their length and width. This thickness adds to the overall length of the fuel cell stack and size of the fuel cell power plant.

[0005] In addition, for high temperature fuel cell systems, a heat exchanger is required to heat the fuel gas to near the temperature of the stack prior to delivery to the stack. In one type of fuel cell system, this heat exchanger is placed external to the fuel cell stack as part of the balance of the plant. This requires additional space to accommodate the fairly thick insulation (2-3 inches) used to encase the heat exchanger. Also, in this type of system, process gas must be piped to and from the heat exchanger, adding to both the size and cost of the system.

[0006] As described in U.S. Pat. No. 5,856,034, insulation for the heat exchanger can be eliminated by placing the heat exchanger inside the already insulated fuel cell module enclosing the fuel cell stack. Specifically, the heat exchanger is placed upstream and adjacent the cathode inlet face of the stack, making it necessary to construct the exchanger large enough so as to completely cover the cathode inlet face. Also, in this system, due to the inherent non-uniform temperature distribution at the outlet of the heat exchanger, the stack inlet temperature distribution is also non-uniform. This condition is undesirable as non-uniform cathode inlet temperature not only creates a potential performance variation in the stack but also creates the risk of cell-to-cell wet seal leaks due to thermal expansion differences of the stack face.

[0007] U.S. Pat. No. 5,009,968 describes an end plate structure in which a thin membrane is used to maintain good electrical contact with the end cells of the fuel cell stack. The thin membrane structure is not specifically adapted to uniformly collect electrical current from the stack. U.S. Pat. No. 4,719,157 describes a thin end plate with multiple current collecting terminals used to inhibit deformation of the plate. Again, this arrangement is not specifically adapted to provide uniform collection of electrical current.

[0008] The present invention provides an end unit of a fuel cell stack having an assembly adapted to receive and convey gases in a heat exchange relationship, and/or to restrict electrical current flow from the fuel cell stack to a current collection post.

[0009] The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings in which:

[0010] FIGS. 1 and 2 are isometric views of the end unit in accordance with the principles of the present invention;

[0011] FIG. 3 is a cross-sectional top plan view of the end unit of FIG. 1 taken along the line 3-3 of FIG. 2;

[0012] FIG. 4 is a front cross-sectional view of a fuel cell stack utilizing the end unit of FIG. 1;

[0013] FIG. 5 is a cross-sectional side view of a conventional fuel cell stack; and

[0014] FIG. 6 is a side cross-sectional view of the fuel cell stack of FIG. 4.

[0015] Referring to FIGS. 1, 2 and 3, the illustrative embodiment of the invention has an end unit 1 adapted to be attached to one end of a fuel cell stack. The end unit 1 houses an assembly 2 including first and second units 2A, 2B associated with the flow of a first and second gas, respectively, through the assembly and which act together as a heat exchanger. In the case shown, the first and second gases are fuel cell stack anode or fuel inlet gas and fuel cell stack cathode exhaust gas, respectively.

[0016] Particularly, the first unit 2A has an inlet 3 through which fuel gas passes (depicted by arrow 7) into an inlet chamber 18. Fuel gas collects in inlet chamber 18, flows in a direction 8 through a first set of tubes 19A and is delivered to a turn plenum 20. Fuel gas flows in a direction 9 through the turn plenum 20 and from the plenum 20 flows in a direction 10 through the tubes 19B. The gas is delivered by tubes 19B to an outlet chamber 21 (shown in FIGS. 2 and 3). Fuel gas exits the outlet chamber 21 through an outlet pipe 4 in a direction indicated by arrow 11 and, as described in further detail below with respect to FIG. 6, flows through the fuel cell stack.

[0017] The second unit 2B forms an enclosure for the first unit 2A. In the illustrative embodiment, the second unit 2B has a first (or top) plate 14, opposing side walls 15A and 15B, front and back walls 25A and 25B, and second (or bottom) plate 16 so as to create a generally hollow box structure of the appropriate length and width to match the fuel cell stack and the appropriate depth so as to remain flat within a desired tolerance upon compressive loading of the stack. As shown in FIGS. 1 and 2, the first unit 2A is contained within the hollow interior of the second unit 2B. In addition, an inlet port 5 (shown in FIG. 1) and an outlet port 6 (shown in FIG. 2) are formed in opposing side walls 15A, 15B of the unit 2B. Also shown in the second unit 2B is a plurality of members 17 extending between the first plate 14 and second plate 16 to provide structural support for the second unit 2B. Members 17 will be described in further detail below with respect to the current collection characteristics of the end unit.

[0018] Fuel cell stack cathode gas enters the second unit 2B through inlet port 5 (depicted by arrow 12 in FIG. 1) and flows in a direction substantially transverse to the plurality of tubes 19A, 19B. As described above with respect to the first unit 2A, fuel gas flows along paths 8 and 10 in the first and second sets of the multitude of tubes 19A, 19B. Collectively, the tubes 19A, 19B have the required heat transfer surface area to adequately transfer heat from the hot cathode gas to the fuel gas, thereby raising the temperature of the fuel gas to the desired temperature for delivery to the stack. The cathode outlet gas exits the end plate through opening 6 (as shown by direction 13 in FIG. 2).

[0019] The tubes 19A, 19B of the first unit 2A are designed to be mechanically separated from the first (or top) plate 14 forming the end of the stack, second (bottom) plate 16 and side walls 15A, 15B of the second unit 2B. This configuration prevents both excessive stress on the joints of the unit 2A and thermal distortions from affecting the flatness of the top and bottom plates 14, 16 of the second unit 2B.

[0020] Also depicted in FIGS. 1, 2 and 3 is a separate chamber 23 in the second unit 2B adapted to collect the anode outlet gas from the fuel cell stack by way of an anode outlet manifold (not shown). Fuel cell stack anode outlet gas is delivered to the chamber 23 through an inlet opening 22 formed in a rear wall 25B of the second unit 2B and exits the chamber 23 through an outlet opening 24 formed in a side wall 15A. With the above configuration for the end unit, all gas connections (ducts, pipes and bellows) for delivering and removing process gases to and from the stack are made through the end unit 1 at one end of the stack.

[0021] The path of cathode gas flow through a fuel cell stack employing the end unit 1 of the invention is shown in the fuel cell stack cross-sectional view of FIG. 4. First, cathode inlet gas enters the fuel cell stack 104 along a first face 104A of the stack in a direction depicted by arrows 101. The cathode gas flows through the stack and exits the stack from a second stack face 104B opposite the first (cathode gas inlet) stack face 104A. Attached to the face 104B is a cathode outlet gas manifold 106. Cathode outlet or exhaust gas is collected in the cathode outlet gas manifold 106 and flows through the cathode outlet gas manifold 106 in a direction shown by arrow 102. The cathode outlet gas manifold 106 delivers cathode outlet gas to the end unit 1 through opening 5. Cathode outlet gas then flows through the end unit in a direction represented by arrow 103 and as described above with respect to FIGS. 1-3, and exits the end unit through opening 6. In this configuration, as previously stated, heat is transferred from the fuel cell stack cathode exhaust gas to the anode inlet gas by heat exchange in the end unit 1.

[0022] In a conventional system, as shown in FIG. 5, heat is taken from an inlet stream (depicted by arrows 201) of fuel cell stack cathode gas, requiring an assembly 207 for heat exchange between cathode and anode gases to be disposed along an entire stack face. After flowing through the heat exchanger 207, cathode inlet gas flows through the stack 204 and exits the stack into a cathode outlet gas manifold 206. By providing heat exchange in the end unit attached to the end of a fuel cell stack in accord with the invention, rather than along an entire stack face as in the conventional structure of FIG. 5, large space requirements, non-uniformity in stack inlet temperature distribution and the risk of cell-to-cell wet seal leaks, as discussed above, are obviated.

[0023] FIG. 6 shows the path of anode or fuel gas flow through a fuel cell stack employing the end unit 1 of the invention. Fuel gas enters the end unit 1 at inlet 309, fills the inlet plenum 18 (not shown in FIG. 6) and flows through the tubes 19A, turn plenum 20 (not shown in FIG. 6), and tubes 19B in a substantially U-shaped path as depicted by arrows 310. As described above, anode gas is superheated by the transfer of heat from cathode outlet gas flowing transverse to the tubes 19A, 19B. Next, the heated anode gas exits the tubes 19B and flows from outlet plenum 21 (not shown in FIG. 6) of the end unit 1 into a fuel header 308 in a direction depicted by arrow 311. The fuel header 308 is disposed within a fuel gas inlet manifold 307 and extends along the length of the manifold 307. The fuel gas header 308 and manifold 307 permit the heated fuel gas to exit the header and manifold at points along the length of the manifold and flow into the fuel cell stack in a direction depicted by arrows 312. The flow of fuel gas through the fuel cell stack 104 as shown in FIG. 6 is in a direction 312 perpendicular to the direction of the flow of cathode gas through the fuel cell stack, but the anode and cathode gas flow paths do not intersect. After flowing through the stack 104, the fuel gas enters an anode outlet gas manifold 306 and flows in a direction depicted by arrow 313. The anode outlet gas manifold 306 then delivers the gas to the anode outlet gas chamber 23 of the end unit 1 as it flows in a direction shown by arrow 314. In the anode outlet gas chamber 23 of the end unit 1, the anode outlet stream is collected by the necessary ductwork and piping to be delivered to the balance of the fuel cell power plant.

[0024] With the end unit 1 of the invention, any fuel gas leaks that may develop over the life of the unit are immediately swept away from the stack by the cathode outlet gas. This is unlike the case of a heat exchanger placed upstream of the stack, in which a leak must first pass through the stack and cathode outlet manifold before leaving the fuel cell module. The risk of a build-up of the mixture of gases within the fuel cell module is reduced.

[0025] Turning back to FIG. 4, current collection posts 105, 107 are disposed at the positive and negative ends of the fuel cell stack. The current collection post 107 at the positive end of the stack is spaced from the first plate 14 by a plurality of members 17 (also shown in FIGS. 1-3). In the case shown, the members 17 are formed from electrically conductive material and are shaped as cylindrical columns. As shown in detail in FIGS. 1-3, the members 17 extend between the first plate 14 and second plate 16 and are disposed at uniformly spaced intervals among the first and second sets of said tubes 19A, 19B and in the anode outlet gas chamber 23. In the illustrative embodiment, the first plate 14 of the end unit 1 is in electrical contact with the stack, and the second plate 16 (see FIG. 1) is in electrical contact with the current collection post 107. The members 17 thus provide an electrical connection between the first and second plates 14, 16 of the end unit and additionally provide structural support to the end unit, distributing mechanical and thermal stresses in the end unit that develop during operation of the stack.

[0026] FIG. 5 shows a cross-sectional side view of a conventional fuel cell stack. As shown in FIG. 5, current collection posts 205 at each end of the stack are disposed adjacent to the positive and negative ends of the stack and collect current directly from the stack. In the present invention shown in FIG. 4, the separation of the current collection post 107 from the first plate 14 of the end unit 1 by the plurality of members 17 is advantageous in that the members 17 act to restrict electrical current flow slightly, allowing more uniform current collection from the stack through the uniformly spaced members 17.

[0027] In all cases it is understood that the above-described apparatus, method and arrangements are merely illustrative of the many possible specific embodiments that represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and the scope of the invention. For example, while shown in FIG. 4 at the positive end of the stack, the end unit may be disposed at either the positive or negative ends of the fuel cell stack. Also, designs using a plate fin, compact heat exchanger, could also be configured.