Fuel cell system including air inlet baffle and method of operating the same

Abstract

To provide an air inlet baffle intended to heat air using cathode exhaust output from a stack.SOLUTION: A fuel cell system includes a stack 102 of fuel cells, a cathode recuperator 500 configured to heat air using cathode exhaust output from the stack 102, and an air inlet baffle 550 which is disposed between the cathode recuperator 500 and the stack 102, has at least two rows of apertures separated along a vertical direction, and is configured to provide the heated air output from the cathode recuperator to multiple areas of the stack.SELECTED DRAWING: Figure 2A2B

Description

[Technical Field]

[0001] Aspects of the invention relate to electrochemical cell systems, and more particularly to fuel cell systems including an air inlet baffle having openings. [Background technology]

[0002] Fuel cells, such as solid oxide fuel cells, are electrochemical devices that can convert energy stored in a fuel into electrical energy with high efficiency. High-temperature fuel cells include solid oxide fuel cells and molten carbonate fuel cells. These fuel cells can operate using hydrogen and / or hydrocarbon fuels. There are classes of fuel cells, such as solid oxide regenerative fuel cells, that also allow for reverse operation, such that oxidized fuel can be re-reduced to unoxidized fuel using electrical energy as an input. Summary of the Invention

[0003] According to various embodiments, a fuel cell system includes a stack of fuel cells, a cathode recuperator configured to heat air using cathode exhaust gas discharged from the stack, and an air inlet baffle disposed between the cathode recuperator and the stack, the air inlet baffle having at least two rows of openings separated vertically and configured to supply heated air discharged from the cathode recuperator to multiple regions of the stack.

[0004] According to various embodiments, a method of operating a fuel cell system includes supplying heated air and fuel to a stack of fuel cells, operating the stack in a steady-state mode to discharge a fuel exhaust and an air exhaust, supplying the fuel exhaust and the air exhaust to an anode tail gas oxidizer that oxidizes the fuel exhaust, supplying the exhaust from the anode tail gas oxidizer to a cathode recuperator, supplying air to the cathode recuperator, heating the air using the exhaust from the anode tail gas oxidizer and discharging the heated air from the cathode recuperator onto an air inlet baffle positioned between the cathode recuperator and the stack and having at least two rows of openings separated vertically, and supplying the heated air to multiple regions of the stack through the at least two rows of openings.

[0005] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with the general description above and the detailed description below, serve to explain features of the invention. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell system according to various embodiments of the present disclosure. [Figure 2A2B] 2A is a cross-sectional view showing components of a hot box of the system of FIG. 1 according to various embodiments of the present disclosure, and FIG. 2B is a diagram showing an enlarged portion of the system of FIG. 2A according to various embodiments of the present disclosure. [Figure 2C] FIG. 2C is a three-dimensional cutaway view of the central column of the system of FIG. 2A according to various embodiments of the present disclosure. [Figure 2D] FIG. 2D is a perspective view of an anode hub structure positioned below the central column of the system of FIG. 2A according to various embodiments of the present disclosure. [Figure 3A] FIG. 3A is a cross-sectional view illustrating fuel and air flow through the central column of the system of FIG. 2A according to various embodiments of the present disclosure. [Figure 3B]FIG. 3B is a cross-sectional view illustrating fuel and air flow through the central column of the system of FIG. 2A according to various embodiments of the present disclosure. [Figure 3C] FIG. 3C is a cross-sectional view illustrating fuel and air flow through the central column of the system of FIG. 2A according to various embodiments of the present disclosure. [Figure 4A] FIG. 4A is a cross-sectional view showing an air inlet baffle located inside the cathode recuperator. [Figure 4B] FIG. 4B is a simplified partial cross-sectional view showing the air and exhaust flow through the cathode recuperator, stack, and central column. [Figure 4C] FIG. 4C is a side view of the air inlet baffle of FIG. 4A. [Figure 5] FIG. 5 is a simplified cross-sectional view showing the distribution of air from a cathode recuperator 500 to the stack in a comparative fuel cell system without an air inlet baffle. [Figure 6] FIG. 6 is a graph showing the cell fuel distribution (CFD) predicted mean vertical temperature profile of a beginning-of-life (BOL) stack operating at 51 amps in a fuel cell system with the air inlet baffle shown in FIG. 4A. [Figure 7] FIG. 7 is a graph showing the average vertical fuel utilization profile from CFD corresponding to the thermal profile of the stack tested under 51 amp BOL operating conditions. [Figure 8] FIG. 8 is a graph showing the average vertical temperature profile of an early-of-life (BOL) stack operating at 51 amps and a middle-of-life (MOL) stack operating at 66 amps (e.g., with deteriorated cell conditions) in a fuel cell system with the air inlet baffle shown in FIG. 4A. DETAILED DESCRIPTION OF THE INVENTION

[0007] As described herein, various aspects of the present disclosure will be described with reference to exemplary embodiments and / or accompanying drawings in which exemplary embodiments of the invention are shown. However, the present invention may be embodied in many different forms and should not be construed as limited to the exemplary embodiments shown in the drawings or described herein. It will be understood that the various disclosed embodiments may involve specific features, elements, or steps that are described in connection with that particular embodiment. It will also be understood that specific features, elements, or steps, while described in connection with one particular embodiment, may be interchangeable with alternative embodiments or combined with alternative embodiments in various non-illustrated combinations or permutations.

[0008] Various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References to specific examples and implementations are made for illustrative purposes and are not intended to limit the scope of the invention or the claims.

[0009] Ranges may be expressed herein as from "about" one particular value and / or to "about" another particular value. When such ranges are expressed, they include, by way of example, from one particular value and / or to the other particular value. Similarly, when values ​​are expressed as approximations, by use of the prefix "about" or "substantially," it will be understood that the particular value forms another dimension. In some embodiments, a value "about X" may include the value + / - 1% X. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0010] In a solid oxide fuel cell (SOFC) system, air and fuel can be supplied to one or more fuel cell stacks to generate electricity. During operation, the stack may experience vertical temperature gradients due to cell heat generation, convective cooling due to incoming air, and / or radiative coupling between the stack and / or other system components. For example, fuel cells at the top and / or bottom of the stack may be cooler than cells in the middle of the stack, and / or the cells in the middle of the stack may be excessively cooled or heated. Excessive vertical temperature gradients can result in reduced voltage capability, thermal stress, cell degradation, and uneven fuel delivery, which can reduce overall system performance and / or efficiency. This can further lead to fuel starvation and associated failure of some fuel cells in the stack. Accordingly, embodiments of the present disclosure provide a SOFC system including a perforated air inlet baffle that improves stack temperature fluctuations and helps optimize the vertical stack temperature profile for uniform fuel flow to individual cells in the stack.

[0011] FIG. 1 is a schematic diagram of a SOFC system 10 according to various embodiments of the present disclosure. Referring to FIG. 1, the system 10 includes a hot box 100 and various components disposed within or adjacent thereto. The hot box 100 can include a stack 102 containing alternating fuel cells, such as solid oxide fuel cells, and interconnects. Each solid oxide fuel cell in the stack 102 includes a ceramic electrolyte, such as yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), scandia- and ceria-stabilized zirconia, or scandia-, yttria-, and ceria-stabilized zirconia; an anode electrode, such as nickel-YSZ, nickel-SSZ, or nickel-doped ceria cermet; and a cathode electrode, such as lanthanum strontium manganite (LSM). The interconnect can be a metal alloy interconnect, such as a chromium-iron alloy interconnect. The stack 102 can be internally or externally manifolded for fuel.

[0012] Hot box 100 may also include an anode recuperator heat exchanger 110, a cathode recuperator heat exchanger 500, an anode tail gas oxidizer (ATO) 150, an anode exhaust cooling heat exchanger 140, a splitter 158, a vortex generator 159, and a water injector 160. System 10 may also include a catalytic partial oxidation (CPOx) reactor 200, a mixer 210, a CPOx blower 204 (e.g., an air blower), a main air blower 208 (e.g., a system blower), and an anode recycle blower 212, which may be located outside hot box 100. However, the present disclosure does not limit each of the components to a particular location with respect to hot box 100.

[0013] The CPOx reactor 200 receives a fuel inlet stream from fuel inlet 300 through fuel conduit 300A. Fuel inlet 300 may be a fuel tank or a utility natural gas line with a valve controlling the amount of fuel supplied to the CPOx reactor 200. A CPOx blower 204 may supply air to the CPOx reactor 200 during system startup. Fuel and / or air may be supplied to the mixer 210 by fuel conduit 300B. Fuel (e.g., the fuel inlet stream) flows from the mixer 210 through fuel conduit 300C to the anode recuperator 110. The fuel is heated in the anode recuperator 110 by a portion of the fuel exhaust, after which the fuel flows from the anode recuperator 110 through fuel conduit 300D to the stack 102.

[0014] The main air blower 208 can be configured to supply an air flow (e.g., an air inlet flow) to the anode exhaust cooler 140 through air conduit 302A. The air flows from the anode exhaust cooler 140 through air conduit 302B to the cathode recuperator 500. The air is heated by the ATO exhaust in the cathode recuperator 500. The air flows from the cathode recuperator 500 to the stack 102 through air conduit 302C.

[0015] The anode exhaust stream (e.g., fuel exhaust stream) generated within the stack 102 is supplied to the anode recuperator 110 through an anode exhaust conduit 308A. The anode exhaust may contain unreacted fuel and may also be referred to herein as fuel exhaust. The anode exhaust may be supplied from the anode recuperator 110 to the splitter 158 via an anode exhaust conduit 308B. A first portion of the anode exhaust may be supplied from the splitter 158 to the water injector 160 and an anode exhaust conduit 308C to the anode exhaust cooler 140. A second portion of the anode exhaust may be supplied from the splitter 158 to the ATO 150 through an anode exhaust conduit 308D. The first portion of the anode exhaust may heat the air inlet stream in the anode exhaust cooler 140 and then be supplied from the anode exhaust cooler 140 to the mixer 210 through an anode exhaust conduit 308E. The anode recirculation blower 212 may be configured to move the anode exhaust through the anode exhaust conduit 308E, as described below.

[0016] Cathode exhaust generated in the stack 102 flows through exhaust conduit 304A to the ATO 150. A vortex generator 159 can be disposed within the exhaust conduit 304A and configured to swirl the cathode exhaust. An anode exhaust conduit 308D can be fluidly connected to the vortex generator 159, or to the cathode exhaust conduit 304A or the ATO 150 downstream of the vortex generator 159. The swirled cathode exhaust can be mixed with a second portion of the anode exhaust provided by the splitter 158 before being provided to the ATO 150. This mixture can be oxidized within the ATO 150 to generate ATO exhaust. The ATO exhaust flows from the ATO 150 to the cathode recuperator 500 through exhaust conduit 304B. The exhaust flows from the cathode recuperator out of the hot box 100 through exhaust conduit 304C.

[0017] Water flows from a water source 206, such as a water tank or water line, through a water conduit 306 to the water injector 160. The water injector 160 injects water directly into a first portion of the anode exhaust, which is fed into the anode exhaust conduit 308C. Heat from the first portion of the anode exhaust (also referred to as the recycled anode exhaust stream) fed into the anode exhaust conduit 308C evaporates the water to produce water vapor. The water vapor mixes with the anode exhaust, and the resulting mixture is fed to the anode exhaust cooler 140. This mixture is then fed from the anode exhaust cooler 140 through the anode exhaust conduit 308E to the mixer 210. The mixer 210 is configured to mix the water vapor and the first portion of the anode exhaust with fresh fuel (i.e., the fuel inlet stream). This humidified fuel mixture can then be heated by the anode exhaust in the anode recuperator 110 before being fed to the stack 102. The system 10 may also include one or more fuel reforming catalysts 112, 114, and 116 located within and / or downstream of the anode recuperator 110. The reforming catalyst(s) reform the humidified fuel mixture before it is supplied to the stack 102.

[0018] The system 10 may further include a system controller 225 configured to control various elements of the system 10. The controller 225 may include a central processing unit configured to execute stored instructions. For example, the controller 225 may be configured to control fuel and / or air flow through the system 10 according to fuel composition data.

[0019] Figure 2A is a cross-sectional view showing components of hot box 100 of system 10 of Figure 1, and Figure 2B shows an enlarged portion of Figure 2A. Figure 2C is a three-dimensional cutaway view of central column 400 of system 10 according to various embodiments of the present disclosure, and Figure 2D is a perspective view of anode hub structure 600 positioned within hot box base 101 in which column 400 can be positioned.

[0020] 2A-2D , the fuel cell stack 102 can be arranged around a central column 400 within the hot box 100. For example, the stack 102 can be arranged in a ring configuration around the central column 400 and positioned on the hot box base 101. The column 400 can include an anode recuperator 110, an ATO 150, and an anode exhaust cooler 140. In particular, the anode recuperator 110 is disposed radially inward of the ATO 150, and the anode exhaust cooler 140 is mounted above the anode recuperator 110 and the ATO 150. In one embodiment, an oxidation catalyst 112 and / or a hydrogenation catalyst 114 can be located within the anode recuperator 110. A reforming catalyst 116 can also be located at the bottom of the anode recuperator 110 as a steam methane reforming (SMR) insert.

[0021] The ATO 150 includes an outer cylinder 152 positioned around the outer wall of the anode recuperator 110. Optionally, an ATO inner cylinder 154 may surround an ATO insulation 156. Thus, the insulation 156 may be located between the anode recuperator 110 and the ATO 150. An ATO oxidation catalyst may be located in the space between the outer cylinder 152 and the ATO insulation 156. A fuel inlet path bellows 854 may be located between the anode exhaust cooler 140 and the ATO inner cylinder 154. An ATO thermocouple feedthrough 1601 extends through the anode exhaust cooler 140 to the top of the ATO 150. The temperature of the ATO 150 may thereby be monitored by inserting one or more thermocouples (not shown) through the feedthrough 1601.

[0022] The anode hub structure 600 may be positioned below the anode recuperator 110 and ATO 150 and above the hot box base 101. The anode hub structure 600 is covered by an ATO skirt 1603. A vortex generator 159 and fuel exhaust splitter 158 are located above the anode recuperator 110 and ATO 150 and below the anode exhaust cooler 140. An ATO glow plug 1602 may be located near the bottom of the ATO 150 to initiate oxidation of the stack fuel exhaust in the ATO during start-up.

[0023] The anode hub structure 600 is used to uniformly distribute fuel from the central column 400 to the fuel cell stacks 102 arranged around the central column 400. The anode flow hub structure 600 comprises a grooved cast base 602 and a "spider" hub with fuel inlet conduits 300D and anode exhaust conduits 308A. Each pair of conduits 300D, 308A connects to a fuel cell stack 102. Additionally, the anode side cylinders (e.g., the anode recuperator 110 inner and outer cylinders and the ATO outer cylinder 152) are welded or brazed into the grooves in the base 602 to provide a uniform volumetric cross-section for flow distribution, as described below.

[0024] 2A and 2B, air enters the top of the hot box 100 and then flows into the cathode recuperator 500, where it is heated by the ATO exhaust discharged from the ATO 150. The heated air then flows through the cathode recuperator 500 and exits the cathode recuperator 500 through air outlets 530. An air inlet baffle 550 can be positioned between the stack 102 and the cathode recuperator 500. As described in more detail below, the air inlet baffle 550 can be configured to control the air flow into the stack 102.

[0025] In the case of a solid oxide fuel cell, the air then flows through the stack 102, causing oxygen ions to diffuse from the cathode electrode through the fuel cell electrolyte to the anode electrode to react with fuel (i.e., the fuel inlet stream) supplied by the anode hub structure 600 at the anode electrode of the fuel cell. The air exhaust flows from the stack 102 and then swirls through the vanes of the vortex generators 159 before entering the ATO 150.

[0026] Splitter 158 can direct a second portion of the fuel exhaust exiting the top of anode recuperator 100 through openings (e.g., slits) in the splitter and into the swirled air exhaust (e.g., into vortex generator 159, or into exhaust conduit 304A downstream of vortex generator 159, or into ATO 150). Thus, the fuel and air exhaust can mix before entering ATO 150.

[0027] 3A and 3B are side cross-sectional views showing the flow distribution through the central column 400, and FIG. 3C is a top cross-sectional view through the anode recuperator 110. Referring to FIGS. 2A, 2B, 3A, and 3C, the anode recuperator 110 comprises an inner cylinder 110A, a corrugated plate 110B, and an outer cylinder 110C, which may be coated with ATO insulation 156. Fuel from a fuel conduit 300C enters the top of the central column 400. The fuel then bypasses the anode exhaust cooler 140 by flowing through the hollow core of the anode exhaust cooler 140, before passing between the outer cylinder 110C and the corrugated plate 110B of the anode recuperator 110. The fuel then flows through the hub base 602 of the anode hub structure 600 and conduit 300D, shown in FIG. 3B, to the stack 102.

[0028] 2A, 2B, 2C, 3A, and 3B, fuel exhaust flows from stack 102 through anode exhaust conduit 308A to hub base 602, from hub base 602 through anode recuperator 110 between inner cylinder 110A and corrugated plate 110B, and through anode exhaust conduit 308B into splitter 158. As shown in FIG. 1, a first portion of the fuel exhaust flows from splitter 158 through anode exhaust conduit 308C to anode exhaust cooler 140, while a second portion flows from splitter 158 through anode exhaust conduit 308D to ATO 150. Anode exhaust cooler inner core insulation 140A can be located between the fuel conduit 300C and the bellows 852 / support cylinder 852A, which is located between the anode exhaust cooler 140 and the vortex generators 159, as shown in FIG. 3A . This insulation minimizes heat transfer and heat loss from the first portion of the anode exhaust stream in the anode exhaust conduit 308C on its way to the anode exhaust cooler 140. Insulation 140A can also be located between the conduit 300C and the anode exhaust cooler 140 to avoid heat transfer between the fuel inlet stream in the fuel conduit 300C and the stream in the anode exhaust cooler 140. In other embodiments, insulation 140A can be omitted from the cylindrical anode exhaust cooler 140.

[0029] 3B also shows that air flows from air conduit 302A to anode exhaust cooler 140 (where the air is heated by a first portion of the anode exhaust), and then from anode exhaust cooler 140 through conduit 302B to cathode recuperator 500. The first portion of the anode exhaust is cooled in anode exhaust cooler 140 by the air flowing through anode exhaust cooler 140. The cooled first portion of the anode exhaust is then supplied from anode exhaust cooler 140 to anode recirculation blower 212 shown in FIG. 1.

[0030] As described in more detail below and shown in Figures 2A and 3B, the anode exhaust exits the anode recuperator 110 and is fed into the splitter 158 through an anode exhaust conduit 308B. The splitter 158 divides the anode exhaust into a first and second anode exhaust portions (i.e., streams). The first stream is fed into the anode exhaust cooler 140 through an anode exhaust conduit 308C. The second stream is fed to the ATO 150 through an anode exhaust conduit 308D.

[0031] The relative amounts of anode exhaust delivered to ATO 150 and anode exhaust cooler 140 are controlled by anode recirculation blower 212. A faster speed of blower 212 delivers a larger portion of the anode exhaust into anode exhaust conduit 308C and a smaller portion of the anode exhaust to ATO 150 via anode exhaust conduit 308D, and vice versa. The anode exhaust delivered to ATO 150 can be oxidized by the stack's cathode (i.e., air) exhaust and delivered to cathode recuperator 500 via conduit 304B.

[0032] FIG. 4A is a cross-sectional view showing an air inlet baffle 550 positioned inside a cathode recuperator 500, FIG. 4B is a simplified partial cross-sectional view showing the air and exhaust flow through the cathode recuperator 500, stack 102, and central column 400, and FIG. 4C is a side view of the air inlet baffle 550.

[0033] 4A-4C, a cathode recuperator 500 can surround one or more of the stacks 102 and the central column 400 shown in more detail in FIG. 2A. The cathode recuperator 500 can include a cover 510 with an air inlet 510A and an air exhaust outlet 510B, an upper lid 514, an optional fin assembly 518, a lower cylinder 524, and an outer shell 528. The fin assembly 518 can include a cylindrical corrugated plate 520 and an inner wall 522 disposed inside the corrugated plate 520. The corrugated plate 520 can be configured to transfer heat between the incoming air and the outgoing cathode exhaust. The inner surface of the inner wall 522 can be covered with a thermal insulating material.

[0034] An annular air outlet 530 can be formed between the inner wall 522 and the lower cylinder 524. In particular, the air outlet 530 can be formed where the lower cylinder 524 and the inner wall 522 overlap. An annular ATO exhaust inlet 532 can be formed between the outer shell 528 and the lower cylinder 524.

[0035] As shown in FIGS. 4A and 4B , air can enter the cathode recuperator 500 through air inlet 510A, flow under the upper lid 514 along the inner surface of the corrugated plate 520, and exit through air outlet 530. The air then passes through air inlet baffle 550 and can be supplied to the fuel cell stack 102 surrounded by the cathode recuperator 500. Cathode exhaust gas discharged from the stack 102 flows into the central column 400, mixes with anode exhaust gas (not shown) in vortex generator 159, and can then be supplied to the ATO 150. ATO exhaust gas (i.e., oxidized fuel exhaust gas) discharged from the ATO 150 is supplied to the ATO exhaust inlet 532 of the cathode recuperator 500, flows along the outer surface of the corrugated plate 520, over the top surface of the upper lid 514, and then can exit through exhaust outlet 510B. Thus, the air is heated by the heat extracted from the ATO exhaust gas.

[0036] The air inlet baffle 550 may be a cylindrical component disposed within the cathode recuperator 500 and surrounding the fuel cell stack 102. The air inlet baffle may form a barrier between the fuel cell stack 102 and the cathode recuperator 500. The air inlet baffle 550 may be configured to control the flow of heated air to the fuel cell stack 102 to limit vertical temperature variations along the height of the fuel cell stack 102 and optimize for uniform fuel utilization across the cells. For example, during steady-state operation of the system (which occurs after system startup), the air inlet baffle 550 may be configured to provide a vertical cell-to-cell temperature variation in the fuel cell stack 102 of less than about 40°C, e.g., between about 30°C and about 40°C. In some embodiments, during steady-state operation of the system, the air inlet baffle 550 can be configured to control the temperature of the fuel cell stack 102 so that the maximum fuel utilization of the fuel cells in the stack 102 exceeds the average fuel utilization of the entire fuel cell stack 102 by no more than about 1%, e.g., 0.1% to 1% above the average fuel utilization. In some embodiments, the air inlet baffle 550 can be configured to control the temperature of the stack 102 so that the fuel utilization of the fuel cells in the stack 102 is in the range of about 86% to about 91%. Thus, the difference between the minimum and maximum fuel utilization between two different fuel cells in the same stack is 10% or less, e.g., 5% or less, e.g., 4% to 6%.

[0037] The air inlet baffle 550 may include an array of openings 552 configured to direct heated air toward particular portions of the stack 102. For example, in some embodiments, the openings 552 may be positioned on the air inlet baffle 550 such that the air inlet baffle directs more of the air exhausted from the cathode recuperator 500 toward the top of the stack 102 than toward the bottom of the stack 102.

[0038] 4C. However, in other embodiments, the openings 552 can have other shapes, such as horizontal or vertical slits, etc. The location, size, and / or number of the openings 552 can be set according to the desired stack 102 and / or system characteristics that are affected by the air supplied through the air inlet baffle 550.

[0039] In one embodiment, the air inlet baffle 550 may not have openings 552 at the vertical level of the air outlets 530. Therefore, the air exiting the air outlets 530 impinges on the continuous plate portion 553 of the air inlet baffle 550 and is not directly supplied to the fuel cell stack 102. Therefore, the portion of the fuel cell stack 102 located at the vertical level of the air outlets 530 is not overcooled by the direct impingement of the air flow supplied from the air outlets 530. Therefore, the continuous plate portion 553 of the air inlet baffle 550 spreads (i.e., turns) the air exiting the air outlets 530 in the vertical direction (i.e., up and down), and then the air reaches the fuel cell stack 102 through the openings 552. Therefore, the air achieves a more uniform temperature by flowing vertically before reaching the stack 102. Furthermore, because the air impinges on several vertical portions of the stack 102 located at the vertical level of the openings, a single region of the stack 102 is not overcooled by the air flow. This results in a more uniform vertical temperature distribution along the height of the stack 102 .

[0040] Fuel supplied to the fuel cells flows from the bottom of the stack to the top of the stack 102, and fuel exhaust from the fuel cells flows in the opposite direction through the stack 102 through riser pipes or integrated fuel channels within the fuel cell stack 102. Fuel distribution can be affected by the geometry of the stack 102 and variations in fuel properties along the stack 102 due to local temperature variations in the stack 102. For example, higher fuel temperatures can increase fuel flow resistance, thereby reducing fuel flow rate. Vertical stack temperature variations can be caused by variations in fuel cell heat generation, convective cooling of the stack by incoming air, and radiative coupling between the stack and other heat-generating components of the SOFC system.

[0041] For example, Figure 5 is a simplified cross-sectional view illustrating the distribution of air from a cathode recuperator 500 to a stack 102 in a comparative fuel cell system without an air inlet baffle. As shown in Figure 5, air exits the cathode recuperator 500 through air outlet 530 and is first directed to the fuel cells in the center portion CP of the stack 102. As a result, the stack 102 may experience a non-uniform vertical temperature profile due to the incoming air cooling the fuel cells in the center portion CP.

[0042] In contrast, as discussed above with respect to FIGS. 4A-4C , in one embodiment, the portion of the fuel cell stack 102 located at the vertical level of the air outlet 530 is not overcooled by direct impingement of the airflow supplied from the air outlet 530. Thus, the continuous plate portion 553 of the air inlet baffle 550 spreads (i.e., redirects) the air exiting the air outlet 530 vertically (i.e., up and down) before the air reaches the fuel cell stack 102 through the openings 552. Thus, the air reaches a more uniform temperature by flowing vertically before reaching the stack 102. Furthermore, even if the openings 552 are located at the vertical level of the air outlet 530, the air impinges on several vertical portions of the stack 102 located at the vertical level of the openings 552, preventing a single region of the stack 102 from being overcooled by the airflow. This results in a more uniform vertical temperature distribution along the height of the stack 102.

[0043] The inventors have determined that reducing vertical temperature variation within a stack can result in better cell temperature control, voltage performance, and voltage uniformity across a variety of operating conditions, reducing cell degradation and thermal stress. Additionally, an optimal vertical stack temperature profile improves vertical uniformity of fuel flow to the fuel cells within the stack. Fuel cells may degrade and / or fail at excessively high fuel utilization rates due to fuel cell starvation. The maximum fuel utilization rate of the cells within a stack can be a determining factor for overall fuel utilization and system efficiency.

[0044] 4C , for example, the multiple small diameter openings 552 can minimize pressure drop and parasitic losses through the air blower supplying air to the cathode recuperator 500. The multiple small diameter openings 552 can also prevent and / or reduce direct impingement of high velocity air jets on the stack 102, thereby reducing damage to the stack and allowing for smoother distribution of incoming air into the air inlet plenum surrounding the stack 102.

[0045] Thus, the number, diameter, and / or spacing of the openings 552 in each array 554, and / or the arrangement of the arrays 554, can be selected to provide desired stack characteristics, such as a stack temperature profile and corresponding fuel utilization. For example, the openings 552 in the air inlet baffle 550 can be arranged in one or more annular arrays 554 vertically separated from one another by respective continuous plate portions 553 without openings. For example, the openings 552 can be arranged in a first array 554A, a second array 554B, a third array 554C, and a fourth array 554D. However, the present disclosure is not limited to a particular number of arrays 554. For example, each array 554 can include at least one row of openings 552, e.g., 1 to 10 rows of openings 552, 2 to 8 rows of openings 552, or 2 to 5 rows of openings 552.

[0046] In some embodiments, the first array 554A and the second array 554B are positioned below the annular air outlet 530, and the third array 554C and the fourth array 554D are positioned above the annular air outlet 530. In some embodiments, the third array 554C and the fourth array 554D can include more rows of openings 552 than the first array 554A and the second array 554B, and the third array 554C can include more rows of openings 552 than the fourth array 554D. Thus, the air inlet baffle 550 can provide more airflow to the top of the stack than to the bottom.

[0047] The apertures 552 can have a diameter ranging from about 2 mm to about 20 mm, for example, from about 5 mm to about 15 mm. The apertures 552 in each row can have a center-to-center horizontal spacing ranging from about 2 mm to about 20 mm, for example, from about 5 mm to about 15 mm. The apertures 552 in adjacent rows can have a center-to-center vertical spacing ranging from about 2 mm to about 20 mm, for example, from about 5 mm to about 14 mm.

[0048] In some embodiments, the first array 554A can be positioned about 130 mm to about 150 mm from the bottom of the air inlet baffle plate 550. The second array 554B can be positioned about 240 mm to about 265 mm from the bottom of the air inlet baffle plate 550. The third array 554C can be positioned about 410 mm to about 435 mm from the bottom of the air inlet baffle plate 550. The fourth array 554D can be positioned about 500 mm to about 525 mm from the bottom of the air inlet baffle plate 550.

[0049] Initial column / stack fuel distribution (CFD) modeling predicts that a linear temperature profile with a difference of about 20°C, with higher temperatures at the bottom of the stack or column than at the top, will improve fuel delivery uniformity. To provide high fuel utilization of the fuel cell, low cell-to-cell fuel utilization variation, and high overall system efficiency, the maximum fuel utilization at any cell in the stack should preferably be maintained within about 1% of the average stack fuel utilization. The air inlet baffle can be designed to achieve a stack temperature profile with a slight negative slope in the vertical direction (e.g., from the bottom to the top of the stack), thereby improving vertical fuel distribution along the stack.

[0050] For example, more openings 552 can be located in the upper part of the air inlet baffle 550 than in the lower part thereof, thereby supplying a larger amount of heated air to the upper part of the stack 102 located above the annular air outlet 530 than to the lower part of the stack 102 located below the annular air outlet 530, maintaining the upper part of the stack 102 at a lower temperature than the lower part of the stack 102 and the middle part of the stack 102 located at the level of the annular air outlet 530. Furthermore, the lower part of the stack 102 can be maintained at a lower temperature than the middle part of the stack 102 located at the level of the annular air outlet 530.

[0051] FIG. 6 is a graph showing the average vertical temperature profile of a beginning-of-life (BOL) stack operating at 51 amps when used in a fuel cell system with the air inlet baffle shown in FIG. 4C and the predicted vertical temperature profile based on optimal cell fuel distribution (CFD).

[0052] Referring to FIG. 6, each stack tested included 256 consecutively numbered fuel cells, with cell 1 located at the bottom of the stack and cell 256 located at the top of the stack. The temperatures of cells 1 through approximately cell 32 may be gradually increased, with cell 1 having the lowest temperature and cell 32 having the highest temperature. The temperatures of cells 33 through 224 may be gradually decreased at a relatively constant rate, with cell 33 having the highest temperature and cell 224 having the lowest temperature, such that the vertical temperature gradient of cells 33 through 224 has a substantially linear negative slope. The air inlet baffle plate was shown to produce a vertical stack temperature profile that roughly matched the predicted temperature profile. Therefore, the air inlet baffle plate provides improved fuel distribution.

[0053] Figure 7 is a graph showing the average vertical fuel utilization profile of a stack tested under 51 amp BOL operating conditions. Referring to Figure 7, the graph shows that the maximum fuel utilization of the cells in the stack was 91%, within 1% of the average stack fuel utilization of 90%. Therefore, the air inlet baffle design was shown to closely maintain vertical fuel utilization spread as well as closely maintain heat distribution.

[0054] 8 is a graph showing the average vertical temperature profiles of an early-of-life (BOL) stack operating at 51 amps and a mid-of-life (MOL) stack operating at 66 amps (e.g., with deteriorated cell conditions) when used in a fuel cell system with the air inlet baffle shown in FIG. 4C. As can be seen in FIG. 6, the air inlet baffle closely maintains vertical thermal spread at higher currents and deteriorated cell conditions. Additionally, although the thermal profile shifted to lower temperatures for the 66 amp MOL operation, the slope and nature of the temperature profile roughly matched that of the 51 amp BOL profile, thus indicating improved fuel distribution.

[0055] The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art. Furthermore, the general principles defined herein may be applied to other embodiments without departing from the scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. a fuel cell stack; a cathode recuperator configured to heat air using cathode exhaust gas discharged from the stack; an air inlet baffle disposed between the cathode recuperator and the stack, the air inlet baffle having at least two rows of vertically separated openings, the air inlet baffle configured to supply heated air discharged from the cathode recuperator to multiple regions of the stack; A fuel cell system comprising:

2. The system of claim 1 , wherein during steady-state operation of the system, the air inlet baffle is configured to control a vertical temperature profile of the stack such that the temperature variation between fuel cells is no more than 40° C.

3. 2. The system of claim 1, wherein during steady-state operation of the system, the air inlet baffle is configured to control the vertical temperature profile of the stack so that the maximum fuel utilization of any of the fuel cells in the stack exceeds the average fuel utilization of the stack of fuel cells by no more than about 1%.

4. The system of claim 3 , wherein during steady state operation of the system, the fuel cell has a fuel utilization rate in the range of 86% to 91%, and the fuel cell comprises a solid oxide fuel cell.

5. the openings being through-holes through the continuous plate portion of the air inlet baffle having diameters in the range of 5 mm to 15 mm; 10. The system of claim 1, wherein adjacent openings in each row are separated by a center-to-center distance in the range of 5 mm to 15 mm.

6. the air inlet baffle is cylindrical; the cathode recuperator comprising an annular air outlet configured to discharge the heated air horizontally toward the stack; The system of claim 1 , wherein the air inlet baffle is configured to vertically turn the heated air discharged from the annular air outlet.

7. the openings are arranged in an array extending around a circumference of the air inlet baffle; The system of claim 6 , wherein each array includes at least two rows of the apertures.

8. the apertures are arranged in a first array, a second array, a third array, and a fourth array; the first array and the second array are positioned below the annular air outlet; The system of claim 7 , wherein the third array and the fourth array are positioned above the annular air outlet.

9. The system of claim 8 , wherein the third array and the fourth array include a greater number of the apertures than the first array and the second array.

10. a continuous plate portion of the air inlet baffle located at the vertical level of the annular air outlet; the air inlet baffle does not have an opening at the vertical level of the annular air outlet; The system of claim 6 , wherein the continuous plate portion of the air inlet baffle is configured to vertically turn the heated air discharged from the annular air outlet.

11. supplying heated air and fuel to a fuel cell stack; operating the stack in a steady state mode to discharge a fuel exhaust and a cathode exhaust; supplying the fuel exhaust and the cathode exhaust to an anode tail gas oxidizer that oxidizes the fuel exhaust; supplying exhaust from the anode tail gas oxidizer to a cathode recuperator; supplying air to the cathode recuperator; heating the air using the exhaust from the anode tail gas oxidizer, the exhaust being comprised of the cathode exhaust from the stack, and discharging the heated air from the cathode recuperator onto an air inlet baffle having at least two vertically separated rows of openings, the air inlet baffle being disposed between the cathode recuperator and the stack; supplying the heated air to a plurality of regions of the stack through the at least two rows of openings; A method of operating a fuel cell system, comprising:

12. 12. The method of claim 11, wherein during the steady state mode, the air inlet baffle controls the vertical temperature profile of the stack such that the temperature variation between fuel cells is no more than 40°C.

13. 12. The method of claim 11, wherein during operation in the steady-state mode, the air inlet baffle controls the vertical temperature profile of the stack such that the maximum fuel utilization of any of the fuel cells in the stack exceeds the average fuel utilization of the stack of fuel cells by no more than about 1%.

14. The method of claim 13, wherein the fuel cell has a fuel utilization rate in the range of 86% to 91% while operating in the steady state mode.

15. the openings being through-holes through the continuous plate portion of the air inlet baffle having diameters in the range of 5 mm to 15 mm; The method of claim 11, wherein adjacent openings in each row are separated by a center-to-center distance in the range of 5 mm to 15 mm.

16. the air inlet baffle is cylindrical; the cathode recuperator includes an annular air outlet that discharges the heated air horizontally toward the stack; The method of claim 11 , wherein the air inlet baffle vertically turns the heated air discharged from the annular air outlet.

17. the openings are arranged in an array extending around a circumference of the air inlet baffle; The method of claim 16 , wherein each array comprises at least two rows of the apertures.

18. the apertures are arranged in a first array, a second array, a third array, and a fourth array; the first array and the second array are positioned below the annular air outlet; the third array and the fourth array are disposed above the annular air outlet; 18. The method of claim 17, wherein the third array and the fourth array include a greater number of the openings than the first array and the second array such that more heated air is supplied to the top of the stack than to the bottom of the stack.

19. a continuous plate portion of the air inlet baffle located at the vertical level of the annular air outlet; the air inlet baffle does not have an opening at the vertical level of the annular air outlet; 17. The method of claim 16, wherein the continuous plate portion of the air inlet baffle vertically turns the heated air discharged from the annular air outlet toward the opening.

20. a greater amount of the heated air is supplied to an upper portion of the stack above the annular air outlet than to a lower portion of the stack below the annular air outlet; 20. The method of claim 19, wherein the upper portion of the stack is maintained at a lower temperature than both the lower portion of the stack and a middle portion of the stack located at the vertical level of the annular air outlet.

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