1. Field of the Invention
This invention relates generally to a system and method for determining when to perform an anode exhaust bleed for an anode recirculation loop of a fuel cell system and, more particularly, to a system and method for determining when to perform an anode exhaust bleed for an anode recirculation loop of a fuel cell system, where the method includes using a model to estimate the gas composition in the anode exhaust gas and an inverse valve model to calculate a desired valve flow coefficient to bleed the anode exhaust gas at a flow rate that results in a desired anode stoichiometry.
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 protons and electrons. The protons pass through the electrolyte to the cathode. The 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 input reactant 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 reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of flow field or 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 reactant gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode reactant gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.
It is desirable that the distribution of hydrogen within the anode flow channels in the fuel cell stack be substantially constant for proper fuel cell stack operation. Therefore, it is known in the art to input more hydrogen into the fuel cell stack than is necessary for a certain output load of the stack so that the anode gas distribution is accessible. However, because of this requirement, the amount of hydrogen in the anode exhaust gas is significant, and would lead to low system efficiency if that hydrogen was discarded. Therefore, it is known in the art to recirculate the anode exhaust gas back to the anode input to reuse the discarded hydrogen.
The MEAs are porous and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 80%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode output of the fuel cell stack to remove nitrogen from the anode side of the stack. The bled hydrogen can be sent to any suitable location, such as a combustor or the environment.
In order to operate the fuel cell stack under optimized conditions and to maximize system performance, a large enough amount of hydrogen in the anode recirculation gas and a certain recirculation rate need to be achieved. However, there are currently no hydrogen concentration sensors or flow rate sensors suitable for a fuel cell system. Therefore, a direct controllability of the operation parameter recycle flow and anode hydrogen concentration is not possible.