Patent Application
20090117422
1. Field of the Invention
This invention relates generally to a fuel cell system for humidifying the cathode inlet airflow to split fuel cell stacks, where the fuel cell system includes two water vapor transfer (WVT) units that humidify the cathode inlet airflow to the split stacks and, more particularly, to a fuel cell system for humidifying the cathode inlet airflow to split fuel cell stacks, where the fuel cell system includes two water vapor transfer (WVT) units that humidify the cathode inlet airflow to the split stacks and where the cathode outlet gas of one split stack is used to humidify the cathode inlet airflow to the other split stack so as to provide humidity balancing between the split stacks.
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. 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.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The 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.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode inlet gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen 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, where the bipolar plates and the MEAs are positioned between two end plates. 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 reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates 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.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity of the cathode outlet gas from the fuel cell stack is typically controlled to control the relative humidity of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack.
During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, water may accumulate within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the relatively hydrophobic nature of the plate material. The contact angle of the water droplets is generally about 80°-90° in that the droplets form in the flow channels substantially perpendicular to the flow of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds.
Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
Low performing cells, especially at low stack power output, is a problem in fuel cell applications. Low performing cells typically produce more water than other cells, which can lead to flow channel flooding. One flooded cell can start a downward spiral of operation that may ultimately lead to stack failure, especially during low-power operation. As discussed above, the most common cause of low performing cells and fuel cell stack failure is significant cell-to-cell variation as a result of water holdup caused by stochastic variations in gas behavior dynamics.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water vapor in the cathode exhaust gas, and use the water vapor to humidify the cathode inlet airflow. Water in the cathode exhaust gas flowing down the flow channels at one side of the membrane is absorbed by the membrane and transferred to the cathode air stream flowing down the flow channels at the other side of the membrane.
As discussed above, a single compressor provides the cathode inlet airflow to the stacks 12 and 14. The amount of flow sent to the stacks 12 and 14 depends on the flow resistance in the stacks 12 and 14, mostly provided by the cathode flow channels within the stacks 12 and 14 and partly provided by the flow channels within the WVT units 26 and 28. The difference in the flow resistances in the stacks 12 and 14 and the WVT units 26 and 28 is a result of manufacturing tolerances, and typically cannot be improved. Therefore, an unequal flow distribution occurs in the stacks 12 and 14 that provides different humidity levels in the stacks 12 and 14 as a result of the amount of water generated in the stacks 12 and 14. Particularly, for a certain current density produced by the stacks 12 and 14 for a load request, the amount of water generated in the stacks 12 and 14 would be about the same. Because there is less airflow through the cathode side of the stack with the higher resistance, there is less airflow to dry the membranes, which provides more water at the output of the stack with the higher flow resistance. The more water at the cathode output of the stack with the higher flow resistance transfers more water to the WVT unit, which increases the amount of humidity entering the stack. This water cycling could cause the cathode flow channels to be blocked with water that could lead to possible stack failure.
For example, assume a flow resistance in the stack 12 that is larger than the flow resistance in the stack 14. This leads to a lower flow in the cathode flow channels of the stack 12 than the cathode flow channels in the stack 14. The amount of water generated by the stacks 12 and 14 is the same because the currents drawn by both stacks 12 and 14 is the same. This leads to a higher outlet humidity of the cathode exhaust gas from the stack 12 because of the lower airflow therethrough. The more water in the cathode exhaust gas from the stack 12 increases the amount of water transferred in the water vapor transfer unit 26, which results in an increased relative humidity of the cathode inlet air to the stack 12, which again increases the amount of water in the cathode exhaust gas of the stack 12.
This same problem also leads to a lower than desired outlet humidity from the other lower flow resistant stack because there is more airflow which creates a drying effect on the membranes, which leads to less water vapor in the cathode exhaust gas, which provides less water transfer in the WVT unit.
In accordance with the teachings of the present invention, a fuel cell system is disclosed that includes a first fuel cell stack and a second fuel cell stack in a divided stack design. A first water vapor transfer unit is used to humidify the cathode inlet airflow to the first divided stack and a second water vapor transfer unit is used to humidify the cathode inlet airflow to the second divided stack. The cathode exhaust gas from the divided stacks is used to provide the humidification for the water vapor transfer units. In order to provide relative humidity balancing between the first and second fuel cell stacks, the cathode inlet air flowing through one of the WVT units that is sent to one of the fuel cell stacks receives the cathode exhaust gas from the other fuel cell stack and vice versa.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to a fuel cell system including divided fuel cell stacks and separate water vapor transfer units for humidifying the split stacks is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
This configuration of the system 10 provides good relative humidity balancing between the cathode side of the fuel cells in the stacks 12 and 14. For example, assume that the cathode flow resistance in the stack 12 is higher than the cathode flow resistance in the stack 14. As discussed above, this leads to a higher humidity of the cathode exhaust gas from the stack 12 because of the lower airflow through the stack 12. However, because the cathode exhaust gas from the stack 12 is sent to the WVT unit 26, it humidifies the cathode inlet air to the stack 14 so that the amount of water and water vapor transferred to the stack 14 increases. Likewise, a lower humidity of the cathode exhaust gas from the stack 14 occurs because of the higher airflow through the stack 14, which is transferred to the cathode inlet air to the stack 12 through the WVT unit 28.
The fuel cell system 34 shows a crossing of the cathode inlet lines to the stacks 12 and 14. In an alternate embodiment, the cathode exhaust gas lines are crossed to achieve the same effect.
Either embodiment discussed above provides the desired humidity balancing between the stacks 12 and 14. However, the fuel cell system 40 may provide certain advantages in implementation, such as better packaging capabilities.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
