1. Field of the Invention
This invention relates generally to a cascaded fuel cell stack including anode and cathode gas flow and, more particularly, to a cascaded fuel cell stack including anode and cathode gas flow, where bundles of flow tubes are used to control the cathode gas flow provided to each stage in the cascaded stack, and where the flow tubes provide a laminar gas flow and linear pressure changes.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is necessary that a fuel cell operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. The temperature provides the relative humidity within the fuel cells in the stack for a particular stack pressure. Excessive stack temperature above the optimum temperature may damage fuel cell components, reducing the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduces the stack performance. In some fuel cell stack designs, it is necessary to humidify the cathode input gas before it is sent to the fuel cell stack, which increases system complexity and cost.
It is known in the art to direct the anode gas flow and the cathode gas flow through the fuel cell stack in opposite directions (counter flow) to provide an increased humidification of the membranes within the stack. As the anode and the cathode gases flow through the stack, they pick up moisture as a result of the water by-product of the electro-chemical reaction. Particularly, the fuel cells toward the anode gas exhaust end of the fuel cell stack will be humidified mostly by moisture in the anode gas flow and the fuel cells toward the cathode gas exhaust end of the fuel cell stack will be humidified mostly by the anode gas flow.
Further, it is known in the art to cascade the fuel cell stack by separating the fuel cells into multiple stages. In this design, the anode and cathode input flow to one stage comes from the anode and cathode exhaust flow, respectively, from the previous stage. The anode or cathode exhaust flow from one stage is output from the stage at an opposite end of the stage from the input of the anode or cathode flow to the stage.
By separating the fuel cell stack into cascaded stages, the stoichiometry of the stack decreases, where less cathode and anode gas flow is required to achieve the desired stack load. In other words, less anode and cathode gas flow is necessary so that enough of the MEAs in the fuel cells receive enough of the input gas to generate the desired power. The efficiency of the fuel cell system decreases as the stoichiometry of the stack increases because a significant amount of additional hydrogen fuel and cathode air flow may be required to meet the fuel cell stack load. Because hydrogen is combustible and expensive to manufacture, it is desirable to minimize the amount of hydrogen at the anode exhaust of the stack so that collection or treatment of the hydrogen is reduced to minimize system complexity and cost.
Known cascaded fuel cell stack designs typically employ an anode exhaust valve that is generally closed so that the last stage in the stack; operates dead-ended. The anode exhaust valve is periodically opened to purge accumulated gas and water from the last stage of the stack.
U.S. patent application Ser. No. 11/113,574, titled Fuel Cell Operating Method with Improved Hydrogen and Oxygen Utilization, filed Apr. 25, 2005, assigned to the Assignee of this application and herein incorporated by reference, addresses this concern. In that fuel stack design, cathode air flow from the compressor is combined with the cathode exhaust gas from each stage as an input to the following stage. This allows a desired amount of fresh air to be sent to each stage, and still maintain the desired humidity level for the MEAs.
FIG. 1 is a perspective view of a cascaded fuel cell stack 10 of the type disclosed in the '574 application. The fuel stack 10 includes a first fuel cell stack stage 12, a second fuel cell stack stage 14 and a third fuel cell stack stage 16, where the first stage 12 has the most fuel cells and the third stage 16 has the fewest fuel cells. An anode inlet pipe 20 coupled to one end of the first stage 12 receives the hydrogen anode input gas. The hydrogen input gas flows through the anode flow channels in the first stage 12 and is output from the first stage 12 through a flow pipe 22 at an opposite end of the first stage 12. The anode gas flow from the flow pipe 22 enters the second stage 14, and flows through the anode flow channels therein. The anode gas flow is output from the second stage 14 through a flow pipe 24 at an opposite end of the second stage 14. The anode gas flow from the flow pipe 24 enters the third stage 16 and flows through the anode flow channels therein, where it is output from the third stage 16 through a flow pipe 26 at an opposite end of the third stage 16.
Cathode input air enters the third stage 16 through a flow pipe 30. The cathode air flows through the cathode flow channels in the third stage 16, and is output from the third stage 16 through a flow pipe 32 at an opposite end of the third stage 16. Additionally, feed air is provided through a flow pipe 36 into the flow pipe 32 to be combined with the air that has flowed through the third stage 16. The combined cathode air from the flow pipe 32 enters the second stage 14 and flows through the cathode flow channels therein. The cathode air exits the second stage 14 through a flow pipe 40 at an opposite end of the first second 14. Additionally, feed air is provided through a flow pipe 42 that directs the feed air into the flow pipe 40 to be combined with the air that has flowed through the second stage 14. The combined cathode air from the pipe 40 enters the first stage 12 and flows through the cathode flow channels therein to exit the fuel cell stack 12 through a flow pipe 44 as the cathode exhaust at an opposite end of the first stage 12.
The '574 application proposes employing proportional control valves for the cathode air being input to the third stage 16 through the pipe 30, the second stage 14 through the pipe 36, and the first stage 12 through the pipe 42. However, it has been observed that using the proportional valves for this purpose does not provide a linear turn-down ratio. Particularly, using the proportional valves does not provide a linear relationship between the overall cathode input airflow as a result of stack demand and the flow rate of the different cathode input airflows. Because the flow channels within the stages 12, 14 and 16 provide a laminar flow, an increase in the flow rate will have a corresponding increase in the pressure of the flow channels that is linear. However, for a proportional control valve, the orifice used in the valve provides a turbulent flow where the pressure across the valve increases by a power of two with an increased flow rate.
It is necessary that the flow rate through each of the stages 12, 14 and 16 remains constant with the flow rate from the air applied to the input pipes 30, 36 and 42 so that the cathode stoichiometrics for the individual stages remain constant. Therefore, the proportional valves have to be controlled accordingly to provide the desired flow through the stages 12, 14 and 16. Thus, each time the demand on the stack 12 changes, the proper valve position for the proper flow through the proportional valves needs to be recalculated so that the pressure drop remains constant. This requires feedback control, which adds to the cost and complexity of the system. Further, by providing an acceptable turn down ratio, for example 1-100, it is necessary that the proportional valves are also able to operate over this wide of turn-down range.