The invention relates to operating a fuel cell system during low power demand.
A fuel cell is an electrochemical device that converts chemical energy that is produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several fuel cells may be formed out of a stack of fuel cells in series to produce a higher voltage. The fuel cells may include plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various channels and orifices to, as examples, route the reactants and products through the fuel cell stack. PEMs (each one being associated with a particular fuel cell) may be located throughout the stack between the anodes and cathodes of the different fuel cells.
As an example, a residential fuel cell system may include a fuel processor to convert a hydrocarbon (a natural gas or propane, as examples) into a hydrogen-rich reformat that is consumed by the fuel cell stack pursuant to the above-described reactions. As an example, the fuel system may be used to deliver power to a house, a load that typically varies over the course of each day. In this manner, during the early morning hours, the house typically demands far less power than the house requires during other parts of the day. This variance may affect operation of the fuel processor. As an example, the efficiency of the fuel processor may be optimized to perform over a predefined power range, such as a reformat output sufficient to correspond to a system power output in the range from 2 kilowatts (kW) to 7 kW. When the power demand drops below the 2 kW level, the fuel processor may have difficulty in maintaining a desired quality of the reformat, or the efficiency of the fuel processor may decline.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
In an embodiment of the invention, a fuel cell system includes a fuel cell stack, a fuel processor, a heater and a controller. The fuel cell stack provides an output power that includes a first power that is consumed by a first load. The fuel processor furnishes a fuel flow to the fuel cell stack, and the heater furnishes heat to the fuel processor when enabled. The controller is coupled to the fuel cell stack to monitor the output power and is coupled to the heater to selectively enable the heater to provide heat to the fuel processor when the first power decreases below a predetermined threshold.
Advantages and other features of the invention will become apparent from the following description, from the drawings and from the claims.