Fuel cells can be used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines and as stationary power sources to name a few. The fuel cells receive reactant feed streams and convert the energy in the feed streams into electricity. The reactant feed steams include a fuel feed stream which is passed over an anode in the fuel cell and an oxidant feed stream which is passed over a cathode in the fuel cell to generate electricity.
One type of fuel cell is a proton exchange membrane (PEM) type fuel cell. In a PEM fuel cell hydrogen, in the form of pure H2 from a storage tank or in the form of a reformate flow from a fuel processor, is supplied as the fuel to the anode of the fuel cell and oxygen, in form of pure O2 from a storage tank or in the form of air (O2 admixed with nitrogen (N2)) from the ambient or a storage tank, is supplied as the oxidant to the cathode of the fuel cell. A plurality of individual cells are bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily H2 and CO2, however, CO, N2, and H2O can also be in the reformate gas.
The fuel cell system operates in terms of excess fuel and oxidant feed stream flow rates wherein the energy supplied is greater then the amount of energy required. For example, the anode portion of an individual fuel cell may require approximately 130% of the required energy to generate a given load while the cathode portion may require approximately 200% of the required O2 to complete the reaction of the fuel flow. The excess oxidant and fuel feed stream flows are measured in terms of lambda ‘λ’ whereby the amount of excess H2 required is termed λA and the amount of excess O2 required is termed λC. In the previous example, the excess fuel feed stream λA would be 1.3 and the excess oxidant feed stream λC would be 2.0. This means that 1.0 part hydrogen is converted to electrical energy for every 1.3 parts provided to the anode with the remaining 0.3 part hydrogen exiting the fuel cell stack as anode effluent.
Each PEM fuel cell within a fuel cell stack requires a specific λC and λA to maximize the efficiency of the fuel cell. The specific λC and λA may vary between similar fuel cells. A fuel cell stack having a plurality of fuel cells will have an average λC and λA for the entire stack as a result of the variance between individual fuel cells in the stack. Once the average λC and λA are determined for the fuel cell stack, a sensor is needed to monitor the fuel and oxidant feed stream flows through the fuel cell stack so that the operation of the fuel cell stack can be optimized and so that real-time changes can be made to the reactant feed streams to maintain the efficient operation of the stack.
Therefore, efficient operation of a fuel cell system depends on the ability to effectively control the amount of O2 and H2 provided to the fuel cell stack. Mass flow meters can be used to measure the amount reactant feed streams being provided to the fuel cell stack. However, the mass flow meters cannot measure the specific components of the feed streams. Therefore, if the composition of the feed streams changes, the mass flow meter will not be able to provide a quantitative measure of the amount of O2 and H2 provided to the fuel cell stack. The use of mass flow meters is particularly difficult during transient operation of a vehicular fuel cell system wherein the reformate fuel requirements vary with the changing loads placed on the fuel cell and when the composition of the reformate fuel leaving a fuel processor is also changing.
Thus there is a need for an accurate lambda sensing system which maintains an efficiency of the fuel cell stack but does not add complexity or weight to the fuel cell system. Additionally, there is a need for a lambda sensing system which also corrects inefficient λA and λC values on a real-time basis without the intervention of the user.