Electrochemical cell system having a dual function DC-to-DC converter

A technique includes operating a converter to convert a first voltage produced by an electrochemical cell stack in a power producing mode into a second voltage. The technique includes operating the converter to convert a third voltage into a fourth voltage to drive the electrochemical cell stack in a pumping mode.

BACKGROUND

The invention generally relates to an electrochemical cell system having a dual function DC-to-DC converter.

A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the 50° Celsius (C) to 75° C. temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° temperature range. At the anode, diatomic hydrogen (a fuel) is reacted 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:
Anode: H2→2H++2e−Equation 1
Cathode: O2+4H++4e−→2H2O  Equation 2

The PEM fuel cell is only one type of fuel cell. Other types of fuel cells include direct methanol, alkaline, phosphoric acid, molten carbonate and solid oxide fuel cells.

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 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. Electrically conductive gas diffusion layers (GDLs) may be located on each side of a catalyzed PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from both the anode and cathode flow-fields may diffuse through the GDLs to reach the catalyst layers.

In general, a fuel cell is an electrochemical cell that operates in a forward mode to produce power. However, the electrochemical cell may be operated in a reverse mode in which the cell produces hydrogen and oxygen from electricity and water. More specifically, an electrolyzer splits water into hydrogen and oxygen with the following reactions occurring at the anode and cathode, respectively:
Anode: 2H2O→O2+4H++4e−Equation 3
Cathode: 4H++4e−→2H2Equation 4

An electrochemical cell may also be operated as an electrochemical pump. For example, the electrochemical cell may be operated as a hydrogen pump, a device that produces a relatively pure hydrogen flow at a cathode exhaust of the cell relative to an incoming reformate flow that is received at an anode inlet of the cell. In general, when operated as an electrochemical pump, the cell has the same overall topology of the fuel cell. In this regard, similar to a fuel cell an electrochemical cell that operates as a hydrogen pump may contain a PEM, gas diffusion layers (GDLs) and flow plates that establish plenum passageways and flow fields for communicating reactants to the cell. However, unlike the arrangement for the fuel cell, the electrochemical pump cell receives an applied voltage, and in response to the received current, hydrogen migrates from the anode chamber of the cell to the cathode chamber of the cell to produce hydrogen gas in the cathode chamber. A hydrogen pump may contain several such cells that are arranged in a stack.

SUMMARY

In an embodiment of the invention, a technique includes operating a converter to convert a first voltage produced by an electrochemical cell stack in a power producing mode into a second voltage. The technique includes operating the converter to convert a third voltage into a fourth voltage to drive the electrochemical cell stack in a pumping mode.

In another embodiment of the invention, a system includes an electrochemical cell stack, a converter and a control subsystem. The converter is coupled to the electrochemical cell stack. The control subsystem is adapted to configure the converter to convert a first voltage that is produced by the electrochemical cell stack in a power producing mode into a second voltage. The control subsystem is also adapted to configure the converter to convert a third voltage into a fourth voltage to drive the electrochemical cell stack in a pumping mode.

Advantages and other features of the invention will become apparent from the following drawing, description and claims.

DETAILED DESCRIPTION

Referring toFIG. 1, an electrochemical cell system10(a residential energy station, for example) in accordance with embodiments of the invention includes a dual mode electrochemical cell stack20that functions in one of two modes: a power producing mode in which the stack20functions as a fuel cell stack to produce electrical power; and a pumping mode in which the stack20functions as an electrochemical pump to purify an incoming flow (a reformate flow, for example) to produce a relatively purified fuel flow (a relatively pure hydrogen fuel flow, for example). As described herein, the electrochemical cell system10uses a technique to transition the stack20between the power producing and pumping modes in a significantly short time interval.

In the power producing mode, the electrochemical cell stack20receives an incoming fuel flow at its anode inlet22. As an example, the incoming fuel flow may be a reformate flow (about fifty percent hydrogen, for example), which is furnished by a fuel processor26. As a more specific example, the fuel processor26may receive an incoming hydrocarbon flow (a liquefied petroleum gas or natural gas flow, as examples), and the fuel source26reforms the hydrocarbon flow to produce an incoming fuel flow (i.e., reformats) to the stack20, which is received at the anode inlet22. In general, the fuel flow is communicated from the anode inlet22through the serpentine flow channels of the anode chamber of the stack20to promote electrochemical reactions pursuant to Eqs. 1 and 2; and the fuel flow produces a corresponding anode exhaust flow at an anode outlet23of the stack20. As examples, the anode exhaust may be partially routed back to the anode inlet22, may be vented to ambient, may be routed to a flare or oxidizer, etc., depending on the particular embodiment of the invention. As another example, the anode chamber may be closed off, or “dead ended” (also called “dead headed”) except for possibly a bleed or purge flow during the power producing mode. Thus, many variations are contemplated and are within the scope of the appended claims.

The stack20also receives an incoming oxidant flow at a cathode inlet28. In this regard, an oxidant source30(an air compressor or blower, as examples) may furnish an air flow that serves as the incoming oxidant flow to the stack20. The incoming oxidant flow is routed through the serpentine flow channels of the cathode chamber of the stack20for purposes of promoting the electrochemical reactions (see Eqs. 1 and 2) inside the stack20to produce electrical power. The oxidant flow through the cathode chamber produces a cathode exhaust flow, which appears at a cathode outlet21of the stack20.

As depicted inFIG. 1, the electrochemical cell system10may include valves24and34, which are operated by a system controller100for purposes of controlling the incoming fuel and oxidant flows, respectively, to the fuel cell stack20. Additionally, the electrochemical cell system10may include valves40and42, which are operated by the controller100for purposes of controlling external communication with the anode outlet23and cathode outlet21, respectively, of the stack20. As further described below, during the transition between the power producing and pumping modes, the controller100operates the valves34and42to isolate the cathode chamber of the stack20from any additional oxidant flow.

During the pumping mode, the controller100closes off the valve34and opens the valves24,40and42for purposes of allowing reformate from the fuel source26to flow through the anode chamber of the stack20. In this mode of operation, the stack20receives electrical power (as further described below) and promotes electrochemical reactions to cause the migration of hydrogen ions across the cell membranes of the stack20to produce purified hydrogen, which appears as an exhaust flow at the cathode outlet21.

The electrochemical cell system10includes a power conditioning subsystem50that, during the power producing mode of the stack20, receives electrical power from the stack20and conditions the power into the appropriate form for the loads of the system10. In this regard, the loads may include auxiliary loads of the electrochemical cell system10, as well as external loads (residential or commercial AC or DC loads, as examples) and possibly an AC power grid.

During the pumping mode, the power conditioning subsystem50provides electrical power to the stack20. The origin of this electrical power may be the AC power grid, energy that is stored in energy storage60(a battery bank, for example) or another source of power.

In accordance with some embodiments of the invention, the power conditioning subsystem50includes a DC-to-DC converter52, which, during the power producing of the stack20, converts the DC stack voltage into a voltage level for a power bus56. The energy storage60is coupled to the bus56, and during the power producing mode of the stack20, power is transferred via the bus56to store energy in the energy storage60. During the pumping mode, the converter52communicates power from the bus56to the stack20by converting the voltage level of the bus56into the appropriate DC stack level for promoting the pumping and achieving the desired stack current.

As also depicted inFIG. 1, the power conditioning subsystem50may include additional components for purposes of conditioning the power from the bus56into the appropriate form for the loads of the system10. More specifically, the power conditioning subsystem50may include another DC-to-DC converter62, which converts the voltage of the bus56into the appropriate voltage or voltages (which appear on output lines64) for auxiliary and external loads of the system10.

In some embodiments of the invention, the power conditioning subsystem50may include an inverter66, which converts the DC voltage from the power bus56into one or more AC voltages (that appear on terminals68) for external AC loads, auxiliary AC loads and/or possibly the AC power grid. For the pumping mode, the inverter66may deliver power from the AC grid by communicating an AC signal received at the terminal68into the appropriate voltage level for the bus56.

Among the other features of the electrochemical cell system10, the system10may include a hydrogen storage subsystem135that stores hydrogen that is produced by the stack20during the pumping mode. More specifically, during the pumping mode, the cathode exhaust may be routed through a pressure swing absorber (PSA)130, which removes impurities from the cathode exhaust to further purify the hydrogen stream. The hydrogen storage subsystem135is connected to the outlet of the PSA130.

The system10may also include polarity switches48, which are coupled between the stack20and the power conditioning subsystem50for purposes of ensuring that the appropriate polarity exists between the terminals of the stack20and the power conditioning subsystem50. In this regard, the polarity switches48operate to reverse the polarity of the stack terminals between the power producing and electrochemical cell pumping modes of the stack20.

Among its other features, in accordance with some embodiments of the invention, the electrochemical cell system10may also include a coolant subsystem80, which communicates a coolant through the stack's coolant channels for purposes of regulating the stack temperature.

As also shown inFIG. 1, the system controller100may include a processor106(representative of one or more microprocessors and/or microcontrollers), which executes instructions104that are stored in a memory102for purposes of controlling the various aspects of the system10. In this regard, the controller100may include various output terminals112for purposes of regulating operation of the fuel processor26; opening and closing valves (such as the valves24,28,40and42, as examples); operating various motors (such as a motor of the oxidant source30; for example), controlling the power mode to pumping mode transition, as described in more detail below; regulating operation of the converters52and62; regulating operation of the inverter66; etc., as just a few examples. The controller100also includes various input terminals110for purposes of monitoring sensed conditions, voltages and/or currents of the system10, as well as receiving commands and other information for purposes of controlling operations of the system10.

It is noted that the electrochemical system10depicted inFIG. 1is merely for purposes of an example, as the depicted features of the system have been simplified for purposes of clarifying the certain aspects of the invention, which are described herein. Other variations of the system10are contemplated and are within the scope of the appended claims.

Referring toFIG. 2in conjunction withFIG. 1, in accordance with embodiments of the invention, a technique150may be used for purposes of configuring the powering conditioning subsystem50based on the mode of the stack20. For this example, the technique150is assumed to begin in block154in which the stack20is in the power producing mode. In this mode, power is communicated through the DC-to-DC converter52from the stack20to the bus56, and the converter52operates as a boost converter. When a mode change is to occur (as indicated in diamond156), the controller100reconfigures the load conditioning subsystem50for the pumping mode. More specifically, the controller100operates the polarity switches48to reverse the polarity of the terminals of the stack20; and subsequently, the controller100reconfigures (block164) the DC-to-DC converter52for operation in the reverse direction as a Buck converter. Power is then communicated, pursuant to block168, through the converter52from the bus56to the stack20.

When a mode change is to occur again (as depicted in diamond172), the controller100once again reconfigures the converter52accordingly. In this regard, the controller100reconfigures (block176) the polarity switches48to reverse the polarity of the stack terminals for the power producing mode and reconfigures (block180) the converter52for a power flow from the stack20to the bus56such that the converter52operates as a boost converter. After this reconfiguration, control returns to block154, where power is communicated through the converter52from the stack20to the bus56while the stack20is in the power producing mode.

Referring toFIG. 3, in accordance with some embodiments of the invention, the DC-to-DC converter52may include terminals202that are coupled to the stack20and terminals204that are coupled to the bus56. The converter52may be, as a non-limiting example, a synchronous boost converter200, which is capable of functioning as a boost converter in a power flow direction from the terminals202to the terminals204and is also capable of functioning as a Buck converter in a power flow direction from the terminals204to the terminals202.

As a more specific example,FIG. 4depicts a schematic diagram of the synchronous boost converter200in accordance with some embodiments of the invention. In general, for this example, one of the terminals202is coupled to ground, and similarly, one of the terminals204is coupled to ground. The converter200includes a power stage210, which includes an inductor214that is coupled between the terminal202and a node216. A transistor218has a switched path (a drain-source path, for example) that is coupled between the node216, and ground; and a transistor220(a MOSFET, for example) has a switched path (a metal oxide semiconductor-field-effect-transistor (MOSFET), as an example) that is coupled between the node216and the terminal204. As shown inFIG. 4, capacitors212and224may be coupled between the respective terminals202and204and ground.

The cathode of a diode219may be coupled to the node216, and the anode of the diode219may be coupled to ground; and the cathode of a diode222may be coupled to the terminal204, with the anode of the diode222being coupled to the node216.

The converter200also includes a pulse width modulation (PWM) controller208that provides PWM signals to control the transistors218and224, depending on whether the converter200is operating as a Buck or boost converter. The PWM208controls the switches218and220(i.e., controls the duty cycles of the PWM signals) based on feedback that is obtained either through a feedback network226or a feedback network228, depending on the particular mode of operation of the converter200. The PWM controller208is configured by the controller100to operate either as a Buck converter or boost converter (i.e., to either apply a first Buck control algorithm or a second boost control algorithm) via communication lines that may be coupled to the output terminals of the controller100.

When operating as a boost converter, the PWM controller208controls the transistor218with a PWM signal and leaves the transistor220turned off to configure the power stage210as a boost converter, as depicted inFIG. 5. Thus, in this configuration, power is transferred from the terminal202to the terminal204, and the PWM controller208bases its control on feedback that is indicated by the feedback network226.

For the Buck mode of operation, the PWM controller208operates the transistor220via a PWM signal and turns off the transistor218to achieve a Buck configuration for the power stage200, as depicted inFIG. 6. For this configuration, power flows through the power stage from the terminal204to the terminal202. Furthermore, the PWM controller208uses feedback obtained by the feedback network228for purposes of controlling the PWM signal.

It is noted that the converter200is one out of many possible embodiments of a converter that may step up a voltage in one direction and step down a voltage in another direction. Thus, many other variations are contemplated and are within the scope of the appended claims. As another example,FIG. 7depicts a converter250that may be used in accordance with other embodiments of the invention. The converter250uses a converter circuit251, which is configured for power flow in only a single direction from an input terminal254of the converter circuit251to an output terminal258of the circuit251. The converter circuit251may be, as an example, a Buck-Boost converter, that is capable of operating as a Buck converter in one mode of operation and operating as a boost converter in another mode of operation. The controller100operates the switches280,282,284and286to selectively connect the polarity switches48and bus56to the terminals254and258of the converter circuit251, depending on the mode of operation of the stack20.

As a more specific example, during the power producing mode of the stack20, the controller100closes the switches280and282and opens the switches284and286to couple the polarity switches48to the input terminal254and couple the bus to the input terminal258. During this mode of operation, the controller100configures a controller (not shown) of the converter251to operate as a boost converter (via control lines253, for example). When the stack20is operated in the pumping mode, the controller100closes the switches284and286and opens the switches280and282for purposes of connecting the polarity switches48to the output terminal258and connecting the bus56to the input terminal254. During this mode of operation, the controller100configures the controller of the converter circuit251(via the control lines253, for example) to operate as a Buck converter.

Other variations are contemplated and are within the scope of the appended claims. For example, in accordance with some embodiments of the invention, the DC-to-DC converter52may have a dedicated controller, which is separate from the system controller100and communicates with the system controller100(via a serial communication link, for example) for purposes of controlling specific operations of the converter52, such as controller the converter52to configure the controller52for the correct power flow direction and possibly generating the PWM control signals for the controller52.