Patent Application
20080014476
The invention generally relates to a pressure relief system for a pressurized gas, and more specifically, to a pressure relief system for hydrogen that is used as a fuel source in a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. There are many different types of fuel cells, such as a solid oxide fuel cell (SOFC), a molten carbonate fuel cell, a phosphoric acid fuel cell, a methanol fuel cell and a proton exchange membrane (PEM) fuel cell.
As a more specific example, a PEM fuel cell includes a PEM membrane, which permits only protons to pass between an anode and a cathode of the fuel cell. A typical PEM fuel cell may employ polysulfonic-acid-based ionomers and operate in the 50° Celsius (C) to 75° temperature range. Another type of PEM fuel cell may employ a phosphoric-acid-based polybenziamidazole (PBI) membrane that operates in the 150° to 200° temperature range.
At the anode of the PEM fuel cell, diatomic hydrogen (a fuel) ionizes to produce 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 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 Equation 1
O2+4H++4e−→2H2O at the cathode of the cell. Equation 2
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. Catalyzed 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.
The fuel for the fuel cell stack anode is provided by a pressurized flow of hydrogen. As excessive fuel flow pressure can damage the anode and create a hazardous condition within the fuel cell system, care must be taken to ensure that the pressure of the fuel flow that reaches the anode does not exceed a maximum acceptable working pressure for the particular fuel cell configuration.
In an embodiment of the invention, a fuel cell-based system comprises a system controller having a memory, a fuel cell stack to receive a pressurized fuel flow from a fuel source, and a first control system coupled to the system controller and including executable program code stored in the memory. In response to detection of a fuel flow pressure that exceeds a first predefined threshold, the first control system isolates the fuel cell stack from the fuel flow. The first control system includes executable program code stored in a memory in the system controller. The fuel cell system also includes an electromechanical control system that is independent of the first control system. The electromechanical control system isolates the fuel cell stack from the fuel flow and vents the fuel flow based upon detection of a fuel flow pressure that exceeds a second threshold that is greater than the first threshold.
In another embodiment of the invention, a fuel cell system comprises a fuel cell stack, a fuel path to conduct a pressurized fuel flow from a fuel source to the fuel cell stack, and a pressure relief control system connected to the fuel path and a vent path. In response to detection of the fuel flow pressure in the fuel path exceeding a first threshold, the pressure relief control system isolates the fuel cell stack from the fuel path. In response to detection of the fuel flow pressure exceeding a second threshold, the pressure relief control system releases an amount of fuel in the fuel path through the vent path.
In a yet further embodiment of the invention, a fuel cell system comprises a fuel cell stack, a fuel path to conduct a pressurized fuel flow from a fuel source to the fuel cell stack, and a pressure relief control system connected to the fuel path and a vent path. The pressure relief control system is configured to detect a fuel flow pressure in the fuel path and a presence of fuel in the vent path. When the pressure exceeds a threshold, the control system releases an amount of fuel through the vent path. In response to detection of fuel in the vent path, the control system isolates the fuel cell stack from the fuel path.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
Referring to
In general, the fuel cell stack 12 receives an incoming fuel flow at its anode inlet 14 from a fuel source 20. The fuel source 20 may be, as examples, a hydrogen tank, a reformer, etc. As will be explained in more detail below, in one embodiment, the fuel source 20 includes a pressurized flow path including various pressure regulator, supply valve, flow control and venting components to provide a high pressure flow of hydrogen to the anode inlet 14 of fuel cell stack 12. The fuel flow is routed through the anode flow channels of the stack 12 to promote electrochemical reactions inside the stack 12 for purposes of generating electrical power. In some embodiments, the fuel flow produces an exhaust at an anode outlet 17 of the fuel cell stack 12 and may be routed to, as examples, a flare or oxidizer depending on the particular embodiment of the invention.
The fuel cell stack 12 also receives an oxidant flow at its cathode inlet 16 from an oxidant source 24, which may be, for example, an air blower or compressor. The oxidant flow is communicated through the cathode flow channels of the stack 12 for purposes of promoting electrochemical reactions inside the stack 12. In accordance with some embodiments of the invention, the cathode exhaust appears at a cathode outlet 18.
The electrical power that is generated by the fuel cell stack 12 is typically in the form of a DC stack voltage, which is received by power conditioning circuitry 19 and transformed into the appropriate AC or DC voltage for the load 150, depending on the particular application. In this regard, the power conditioning circuitry 30 may include, as examples, one or more switching converter stages, an inverter, etc., as can be appreciated by those skilled in the art.
Due to the presence of a fuel (hydrogen, for example) in the fuel cell system 10, the environment in which the system 10 operates may be considered a potentially flammable or hazardous environment. In addition, the fuel is maintained under high pressure, which also can create a hazardous environment and/or damage various components of system 10, such as the fuel cell stack 12 and the inlets of supply valves in the fuel path. Thus, care must be taken to ensure that no hazardous conditions, such as unacceptable concentrations of a flammable fuel or over-pressurization of a particular portion of the system 10, are present either at the startup of system 10 or during operation. To accomplish this, various startup and shutdown protocols may be implemented.
More specifically, in accordance with some embodiments of the invention, upon the startup of the fuel cell system 10, the system 10 controls the communication of electrical power from a power source 26, which, should no unacceptable levels of flammable gas or pressurization be detected, supplies electrical power to the various components of system 10. The power source 26 may be, for example, a battery that is charged during normal operation of the fuel cell system 10 and/or may be a power source (such as a conventional AC source) that is independent of the operation of the fuel cell system 10 altogether. The particular form of the power source 26 is not important to the aspects of the invention that are described herein.
Upon startup of the system 10 from a powered-down state, fault detection circuitry 28 receives power from power supply 26. In accordance with some embodiments of the invention, the fault detection circuitry 28 may include, for instance, a hydrogen detection circuit 30 that responds to detection of hydrogen and an overpressure detection circuit 32 that responds to detection of a high pressure condition within fuel source 20, including anywhere within the fuel path to the stack 12. Detection of hydrogen generally indicates the presence of a potentially flammable environment. Detection of a high pressure condition likewise is indicative of a hazardous environment and/or operating conditions which may result in damage to components of the fuel cell system 10. If a hazard is detected (e.g., flammable gas concentration, overpressure condition, etc.), the fault detection circuitry 28 does not allow power to be communicated from the power source 26 to other components of the system 10. In addition, the fault detection circuitry may also power down the system 10.
If, however, a fault condition is not present at startup, the fault detection circuitry 28 closes a power transfer switch 34 to allow the communication of power from the power source 26 to the power system bus 36, which supplies power to the other components of fuel cell system 10, including a system controller 38.
The system controller 38 generally controls the operations of the fuel cell system 10. In this regard, the system controller 38 includes various input lines 40 and output lines 42 for controlling valves, motors, currents, voltages and sensing various parameters of the fuel cell system 10. Once energized and active, the system controller 38 has the ability to de-energize the entire fuel cell system 10. Likewise, at any time during its operation, should a predetermined fault or hazard level be detected, the fault detection circuit 28 and/or sensor components in the fuel source 20 may also de-energize the entire fuel cell system 10.
Among the other features of the fuel cell system 10, in accordance with some embodiments of the invention, the system 10 may include a cell voltage monitoring circuit 44, which scans the cell voltages of the fuel cell stack 12 to provide statistical information and measured cell voltages to the system controller 38. Additionally, the fuel cell system 10 may include a coolant subsystem 46 to regulate the temperature of stack 12.
It is noted that the fuel cell system 10 is depicted as merely an example of one of many possible implementations of a fuel cell-based system in accordance with embodiments of the invention. Many other variations are contemplated and are within the scope of the invention.
As a more specific example,
As shown in the embodiment depicted in
The refueling path 58 is connected between the supply valve 60 and the fuel orifice 62. The flow orifice 62 may be particularly useful during the refueling process when a high pressure refueling source is connected to fuel source 20. In one embodiment, the flow orifice 62 may limit a potential flow from a high pressure refueling source during a failure of pressure control valve 64 such that the pressure downstream of the pressure control valve 64 does not exceed, for instance, approximately 80 psi, or other acceptable pressure level depending on the particular configuration of the fuel path 52. The tank 50 may be refueled through the refueling path 58 either while the system 10 is powered down or while the system 10 is powered up and operating.
In addition to the fuel path 52 and the refueling path 58, one or more vent paths are provided. For instance, a vent path 54 is provided to vent a portion of the fuel in the fuel path 52 in the event that the pressure in the fuel path 52 upstream of the supply valve 60 exceeds a predefined threshold. Alternatively, the vent path 54 may be configured such that it vents the fuel upstream of the supply valve 60, such as all of the fuel in the tank 50, in the event that the temperature in the vicinity of the tank 50 exceeds a predetermined threshold (e.g., 120° C.). Similarly, a vent path 56 is provided to vent a portion of the fuel in the fuel path in the event that the pressure in the path 52 downstream of the pressure control valve 64 exceeds a predefined threshold.
In the embodiment illustrated in
The fuel source 20 also includes a pressure relief system to prevent the creation of a hazardous environment that may result from an overpressurization event and/or from the accumulation of unacceptable concentrations of hydrogen within the system 10. Overpressurization events may occur as a result of a failure of one or more components of the fuel source 20. For instance, a failure of the pressure control valve 64 may result in an overpressure condition in the portion of the fuel path 52 that is downstream of the control valve 64. As an excessive pressure may result in damage to the fuel cell stack 12, for instance, a pressure relief system may be implemented to detect the pressure in the fuel path 52 and, in response to detection of an overpressure condition, isolate pressure sensitive components of system 10 from the high pressure fuel flow and shutdown the system 10.
In some embodiments of the invention, the fuel pressure relief system may include two independent shutdown and/or pressure relief systems to shutdown system 10 in response to detection of an overpressure condition. The two systems may both be operable in the event of a failure, with the first system shutting down system 10 and the second system relieving a build up of pressure in the fuel path 52. Alternatively, the second pressure relief system may provide a backup relief system that may become operable in the event of a failure of the first system. For instance, in the embodiment shown in
The proper operation of software-based system 70 relies on proper operation of both the pressure detector 72 and the proper execution of the software-based protocol stored in memory 78 in the system controller 38. Should either the detector 72 or execution of the software code fail, the potential exists for the creation of a hazardous environment.
Accordingly, in some embodiments of the invention, a second pressure relief system 82 also is implemented. However, it is contemplated that in other embodiments of the invention, the software-based system 70 may be omitted and only the second pressure relief system 82 is implemented. In a preferred embodiment, the second system 82 is a hardware-based system that operates independently of the software-based system 70. More specifically, the hardware-based system 82 is an electromechanical system that detects an overpressure condition and, in response thereto, controls communication of power to the various components of system 10 without the participation of the software stored in memory 78 of the system controller 38. For instance, in the embodiment shown in
Turning now to
In some embodiments, the pressure set point of switch 90 is less than the maximum inlet pressure rating of the supply valve 66, and the set point of the pressure relief device 92 is greater than the set point of the pressure switch 90. Thus, as an example, in an embodiment in which the maximum inlet pressure rating of the supply valve 66 is 50 psi, the pressure threshold for switch 90 may be approximately 17 psi and the pressure set point for pressure relief device 92 may be 35 psi, although other set points that do not exceed the maximum inlet pressure rating of the supply valve 66 also are contemplated. When the device 92 pressure set point is reached, the pressure relief device 92 opens and relieves the fixed volume of gas via the vent line 56 until the pressure reset point of pressure relief device 92 is reached. The pressure relief device 92 then closes.
In some embodiments of the invention, the pressure reset point of pressure relief device 92 is greater than the pressure threshold of pressure switch 90. For instance, the pressure reset point may be 30 psi in an embodiment in which the pressure threshold of switch 90 is approximately 17 psi. Accordingly, even after device 92 has completed venting a portion of the fuel via the vent 56, the pressure switch 92 will remain in its activated state (e.g., open) and prevent the communication of power to the system controller 38 and other components of system 10. Thus, system 10 cannot be restarted until the initial cause of the overpressurization event is diagnosed and repaired.
In some embodiments, system 10 also may include a hydrogen sensor 94 and a bypass vent 96. In one embodiment, the bypass vent 96 is located in the vent line 56 in close proximity to the hydrogen sensor 94. Accordingly, whenever the pressure relief device 92 is open and relieving pressure from the fuel path 52, a small amount of hydrogen gas will be released through the bypass vent 96 in sufficient amount to be detected by the hydrogen sensor 94. The hydrogen sensor 94 may be coupled to fault detection circuitry, such as circuitry 28, such that if unacceptable hydrogen concentration levels are detected, a signal may be generated which shuts down the system 10, such as, for instance, by removing the power provided to the system controller 38 by providing a signal to the power source 26 that opens switch 34. Thus, in the event of a failure of the pressure relief device 92 in a manner in which it is continually releasing fuel through the vent line 56, the bypass vent 96 and hydrogen sensor 94 may serve to shutdown the system 10, potentially avoiding the creation of a hazardous environment.
In the exemplary embodiment illustrated in
Another exemplary embodiment of a second pressure relief system 82 in accordance with the invention is shown in
In this embodiment shown in
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.
This application claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/806,099, entitled “HYDROGEN SAFETY RELIEF SYSTEM,” which was filed on Jun. 28, 2006, and is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60806099 | Jun 2006 | US |
