Patent Application
20100209795
The invention generally relates to a power subsystem for a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange 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:
H2→2H++2e− at the anode of the cell, and
O2+4H++4e−→2H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow 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 surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.
A battery may be used in a fuel cell system to supplement power that is provided by the fuel cell stack. In this manner, during times of increased power demand by the fuel cell system's load, the battery may discharge to contribute additional power to the fuel cell system's load to supplement the power produced by the fuel cell stack, and during non-peak times of power demand, the battery may be charged with power produced by the fuel cell stack. This charging and discharging of the battery may be complicated by the ever-changing terminal voltage of the fuel cell stack.
Thus, there exists a continuing need for an arrangement to control the charging and discharging of such a battery in a fuel cell system.
In an embodiment of the invention, a fuel cell system includes a fuel cell stack, a battery system, a power communication line and a power conditioning circuit. The fuel cell stack provides a stack voltage that stays near or below a first maximum voltage. The battery system includes a terminal that is coupled to the power communication line. The power conditioning circuit, in response to the stack voltage, provides a second voltage to the power communication line. The second voltage stays near or below a second maximum voltage, and the second maximum voltage is greater than the first maximum voltage.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
Referring to
More particularly, the fuel cell stack 20 produces a terminal voltage, or stack voltage, (called VTERM) at its output terminal 31. The VTERM voltage may gradually decay over time, as depicted by a waveform of the VTERM voltage in
Due to the battery system's connection to the output terminal of the converter 30, the battery system 41 is charged (if needed) as long as the power that is demanded from the fuel cell stack 20 can be accommodated by the amount of fuel immediately available to the fuel cell stack 20 (i.e., as long as there is enough fuel to maintain a stable VTERM).
The power that is demanded by load the 50 may vary over time, as the load 50 may represent a collection of individual loads (appliances and/or electrical devices that are associated with a house, for example) that may each be turned on and off at various times. As a result, the power that is consumed by the load 50 may rapidly change to produce a transient in power that is demanded from the fuel cell stack 20. This transient may be a significant change in power that deviates from the current steady state level of power present at the time the transient occurs, and the transient may have a time constant that is on the same order or less than the time constant of the fuel processor 22.
Therefore, the fuel processor 22 may not respond quickly enough to increase or decrease its fuel output to respond to a particular transient. As a result, the VTERM voltage (i.e., the terminal voltage of the fuel cell stack 20) may significantly decrease during a rapid increase in power demand, as the fuel processor 22 may respond relatively slowly to the increased demand, and this slow response, in turn, may “starve” cells of the stack 20 for fuel, causing their voltages to decrease. This decrease in the VTERM voltage may cause the converter 30 to be unable to maintain regulation of the VDC voltage if not for the battery system 41. In this manner, for purposes of temporarily meeting the increased power demand until the fuel processor 22 increases its output to the appropriate level, the banks of the battery system 41 stabilize the VDC voltage (and output AC voltage (called VAC)) and provide power to supplement the power that is produced by the fuel cell stack 20.
For times when the load 50 is not demanding such a relatively large power, the VTERM voltage is at a sufficient level to cause the converter 30 to regulate the VDC voltage to a sufficient level for purposes of charging the battery banks of the battery system 41 (if needed) via the power that is provided by the fuel cell stack 20. In some embodiments, the controller 60 may monitor cell voltages within the stack 20, and may remove the load from the stack 20 as necessary to prevent a cell voltage from getting low enough to cause damage to the cell. As an example, the controller 60 can effectively remove the load from the stack 20 by adjusting VDC to be less than the voltage of the power communication line 35. As a further example, in some such embodiments, a diode arrangement may be used to prevent current from sinking to the stack where VDC is less than the voltage of the power communication line 35.
The following discussion assumes steady state operation of the fuel cell system 10 in the absence of power transients. Referring to both
For example, as illustrated in
However, a problem may occur with the above described given system, as depicted in
In contrast to the relationship between the battery and stack voltages that are described in the given system above, for the system 10, the VTERM and VDC voltages are chosen differently, as illustrated in
Due to this relationship, the VDC battery voltage remains above the VTERM stack voltage, regardless of the degree of degradation of the stack 20. As a result, the converter 30 always introduces a gain greater than one to the VTERM voltage to produce the VDC voltage. Therefore, because the converter 30 always has a gain greater than one, the design of the converter 30 may be greatly simplified, as compared to a converter that must have a gain above and below unity for purposes of accommodating a change in the stack voltage.
As an example, in some embodiments of the invention, the converter 30 is preferably a Boost converter with a gain greater than one. As an example, such arrangements may be selected under the invention to reduce the size, weight, complexity, and parasitic losses associated with the system. However, in other embodiments, other types of converter topologies may also be used, such as flyback, Buck, and other switching converters.
It is noted that while it is possible to choose the maximum voltage level of the battery system 41 far below the maximum stack voltage to ensure that the VTERM voltage is always above the VDC voltage, this relationship may not be desirable as the battery system 41 would always be sinking current and charging to some extent. Thus, such an arrangement may be very inefficient in a system whose power efficiency may be a primary design concern.
Referring to
For purposes of powering the load 50, the fuel cell system 10 includes an inverter 33, a component that generates an AC voltage (on output terminals 32) in response to the VDC voltage. Thus, in some embodiments of the invention, the input terminal of the inverter 33 is coupled to the output terminal of the converter 30.
The fuel cell system 10 also includes control valves that may be controlled by the controller 60 to divert some of the fuel flow that is otherwise received by the fuel cell stack 20 to an oxidizer 38 via a flow line 35. The oxidizer 38 may also burn off excess fuel that is not consumed in fuel cell reactions. The control valves 44 may also provide emergency shut off of the oxygen and fuel flows to the fuel cell stack 20. The control valves 44 are coupled between inlet fuel 37 and oxidant 39 lines and the fuel and oxidant manifold inlets, respectively, of the fuel cell stack 20. The inlet fuel line 37 receives a fuel flow from the fuel processor 22, and the inlet oxidant line 39 receives an oxidant flow from the air blower 24. The fuel processor 22 may receive, for example, a hydrocarbon (natural gas or propane, as examples) and convert this hydrocarbon into a fuel flow (a hydrogen flow, for example) that is provided to the inlet fuel line 37.
The fuel cell system 10 may also include water separators, such as water separators 34 and 36, to recover water from the outlet and/or inlet fuel and oxidant ports of the fuel cell stack 20. The water that is collected by the water separators 34 and 36 may be routed to a water tank (not shown) of a coolant subsystem 54 of the fuel cell system. The coolant subsystem 54 circulates (via inlet 56 and outlet 57 coolant lines) a coolant, such as de-ionized water, for example, through the fuel cell stack 20 to regulate the operating temperature of the stack 20.
Other arrangements are possible. For example, referring to
Finally, it will be appreciated that while the above discussion has illustrated the invention with respect to fuel cell systems utilizing fuel processors for fuel delivery, the invention also applies to pure hydrogen systems. For example, the invention may also apply to fuel cell systems using pure hydrogen, including those that are “dead-headed” such that the anode chambers of the fuel cell are exposed to pressurized hydrogen and periodically vented to remove inert materials accumulated in the anode chambers. Such systems may also have fuel delivery response constraints analogous to the lag time issues with hydrogen delivery from fuel processor systems.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.
