Abstract:
Disclosed herein is a method and system for configuring power electronics in an electrochemical cell system. Exemplary embodiments include power electronics having a power converter for an electrochemical cell system. The power converter includes a plurality of interchangeable power converter modules and a motherboard configured to receive the plurality of interchangeable power converter modules. A power rating of the power converter is capable of being changed by adjusting a number of the interchangeable power converter modules attached to the mother-board.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefits of U.S. Provisional Patent Application Serial No. 60/319,927 filed Feb. 6, 2003, the entire contents of which are incorporated herein by reference. 
     
    
     FEDERAL RESEARCH STATEMENT  
       [0002] This invention was made with Government support under contract DE-FC36-98GO10341 awarded by the Department of Energy. The Government has certain rights in this invention. 
     
    
     
       BACKGROUND OF INVENTION  
         [0003]    This disclosure relates generally to power electronics, and especially relates to power electronics associated with the storage and recovery of energy from electrochemical cells.  
           [0004]    Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. An electrolysis cell typically generates hydrogen by the electrolytic decomposition of water to produce hydrogen and oxygen gases, whereas in a fuel cell, hydrogen typically reacts with oxygen to generate electricity. In a typical fuel cell, hydrogen gas and reactant water are introduced to a hydrogen electrode (anode), while oxygen gas is introduced to an oxygen electrode (cathode). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, methanol or other hydrogen source. Hydrogen gas electrochemically reacts at the anode to produce hydrogen ions (protons) and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through a membrane to the cathode. At the cathode, the protons and electrons react with the oxygen gas to form resultant water, which additionally includes any reactant water dragged through the membrane to the cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.  
           [0005]    This same configuration is conventionally employed for electrolysis cells. In a typical anode feed water electrolysis cell, process water is fed into a cell on the side of the oxygen electrode (in an electrolytic cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolytic cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. The hydrogen gas generated may then be stored for later use by an electrochemical cell.  
           [0006]    Electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Electrochemical cell systems performing both functions are commonly referred to as regenerative fuel cell systems. Regenerative fuel cell systems may be used either as a primary power source or a secondary power source to supplement the primary power source. Where the regenerative fuel cell system is used as a secondary power source, an electrochemical cell operates to convert excess electrical energy from the primary power source and/or supplemental energy from another secondary power source (e.g., a solar collector, windmill, etc.) into chemical energy in the form of hydrogen, which can be stored for later use. When the primary source of power is unavailable, the electrochemical cell operates to convert the stored chemical energy into electrical energy.  
           [0007]    The electrical energy input to and/or output from the electrochemical cell typically requires conditioning to ensure its compatibility with the electrical requirements of the load, primary power source, or other secondary power source associated with the electrochemical cell. The devices that perform such conditioning are known as “power electronics”. Power electronics may include, for example, alternating current (AC) to direct current (DC) converters (converters), DC to AC converters (inverters), and DC to DC converters.  
           [0008]    Power electronics play a significant role in the overall electrochemical cell system efficiency. Traditionally, electrochemical cell power electronics efficiencies have been in the 85%-90% range. Power electronics also add significant monetary cost the electrochemical cell system. For example, rectifiers, which are commonly used for AC to DC conversion, represent about 10%-15% of the material cost of the electrochemical cell system. While it is desired to have power electronics that are both efficient and cost effective, these two goals are typically at odds. For example, high frequency switch mode converters are relatively efficient, but the cost of this technology does not readily lend itself to cost reduction. Thus, power electronics that are both efficient and cost effective are desired.  
         SUMMARY OF INVENTION  
         [0009]    Disclosed herein is a method and system for configuring power electronics in an electrochemical cell system. Exemplary embodiments of power electronics for an electrochemical cell system comprise: a first power converter including: a plurality of interchangeable power converter modules, and a first motherboard configured to receive the plurality of interchangeable power converter modules, wherein a power rating of the first power converter may be changed by adjusting a number of interchangeable power converter modules attached to the first motherboard. In one embodiment, a controller is configured to adjust a current output from interchangeable power converter modules attached to the first motherboard. In another embodiment, the power electronics further comprise a second power converter including: a second motherboard configured to receive at least a portion of the plurality of interchangeable power converter modules, wherein a power rating of the second power converter may be adjusted by changing a number of interchangeable power converter modules attached to the second motherboard. In another embodiment, the controller is further configured to adjust a current output from interchangeable power converter modules attached to the second motherboard.  
           [0010]    Exemplary embodiments of an electrochemical cell system comprise a first power source, an electrochemical cell, and a modular power electronics system electrically connected between the first power source and the electrochemical cell. In an embodiment, the modular power electronics system includes: a first power converter for conditioning electrical current flowing between the first power source and the electrochemical cell. The first power converter includes: a plurality of interchangeable power converter modules, and a first motherboard configured to receive the plurality of interchangeable power converter modules, wherein a power rating of the first power converter may be adjusted by changing a number of interchangeable power converter modules attached to the first motherboard. In one embodiment, a controller is configured to adjust a current output from interchangeable power converter modules attached to the first motherboard. In another embodiment, the electrochemical cell system further comprises a second power source, and the modular power electronics system further includes a second power converter for conditioning electrical current flowing between the second power source and the electrochemical cell. The second power converter may include a second motherboard configured to receive at least a portion of the plurality of interchangeable power converter modules, wherein a power rating of the second power converter is adjustable by changing a number of interchangeable power converter modules attached to the second motherboard. In another embodiment, the controller is further configured to adjust a current output from interchangeable power converter modules attached to the second motherboard.  
           [0011]    An exemplary method of configuring power electronics for an electrochemical cell system includes adjusting a power rating of a first power converter by changing a number of interchangeable power converter modules attached to a first motherboard. In one embodiment, the method further includes adjusting a current output from the interchangeable power converter modules attached to the first motherboard using a single controller. In another embodiment, the method further includes adding a second motherboard to a power converter box housing the first motherboard and the single controller, and adjusting a power rating of a second power converter by changing a number of interchangeable power converter modules attached to the second motherboard. In another embodiment, the method further includes adjusting current output from the interchangeable power converter modules attached to the second motherboard using the single controller.  
           [0012]    The above discussed and other features will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike:  
         [0014]    [0014]FIG. 1 is a block diagram of an electrochemical cell system including power electronics;  
         [0015]    [0015]FIG. 2 is a block diagram of a modular power converter providing AC to DC conversion for the electrochemical cell system of FIG. 1;  
         [0016]    [0016]FIG. 3 is a block diagram of a power converter module for a modular power converter;  
         [0017]    [0017]FIG. 4 is a block diagram of a half module for the power converter module of FIG. 3; and  
         [0018]    [0018]FIG. 5 is a block diagram of a modular power converter providing AC to DC and DC to DC conversion. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 1 depicts a block diagram of a power system  10  including a modular power electronics system  11 . In the embodiment shown, modular power electronics system  11  includes an alternating current (AC) to direct current (DC) converter  13 , which is controlled by a controller  15 , to provide power from a primary power source  17 , such as generated grid power or that from a renewable source, and an electrolysis cell  19 . In the example shown, the primary power source  17  provides power along a primary bus  21 ; e.g., 3-phase, 240/480 volts alternating current (VAC). It will be appreciated that the actual primary supply voltage may be based upon the type of power source  17  including, but not limited to, other alternating current (AC) voltage sources, direct current (DC) sources, and renewable sources such as wind, solar and the like.  
         [0020]    During operation of system  10 , the primary power source  17  provides power via power converter  13  to electrolysis cell  19 , e.g., an electrolyzer, which generates hydrogen gas. The hydrogen gas generated by the electrolysis cell  19  is stored in an appropriate storage device  23  for later use by, for example, a hydrogen electrochemical device, e.g., a fuel cell, which converts the hydrogen into electricity.  
         [0021]    Operation of the electrolysis cell  19  and its ancillary equipment (e.g., pumps, valves, fans, etc.) is controlled by an electrolyzer control system  25 . For example, once the amount of hydrogen in the hydrogen storage device  23  decreases below a pre-determined level, the electrolyzer control system  25  engages electrolysis cell  19  and its ancillary equipment to replenish the hydrogen supply. Electrolyzer control system  25  also provides control signals to, and receives control signals from, controller  15  of the modular power electronics system  11  via an input/output (I/O) connection  27 .  
         [0022]    Referring to FIG. 2, a schematic block diagram of an embodiment of the modular power electronics system  11  is shown. Modular power electronics system  11  is housed in a single power converter box  51 , which may be rack-mounted. System  11  includes a motherboard  53  having a plurality of power converter modules  55  disposed thereon. Each module is rated for a predetermined power (e.g., 8 kilowatts (kW)), and provides a series/parallel building block for an expandable motherboard. Each converter module  55  is preferably formed on a single circuit board that be coupled to motherboard  53  via a plug-in arrangement, using a card cage for example, so that the converter modules  53  may be easily removed and installed as needed to meet the power requirements of the electrolyzer  19  or as needed for replacement during maintenance. Also connected to motherboard  53  is controller  15  and a system clock  57 , each of which may be mounted directly on, or separated from, motherboard  53 . System clock  57  provides synchronization signals  59  to the modules  55 . Controller  15  may include a microprocessor and associated electronics.  
         [0023]    In the embodiment of FIG. 2, motherboard  53  receives 3-phase AC input and filters the AC input using an arrangement of capacitors  61  or the like. The filtered AC is provided in parallel to modules  55 . Operating power for the motherboard  53 , power converter modules  55 , and controller  15  is provided by a transformer  63  and an AC to DC converter  65 .  
         [0024]    The power converter modules  55  receive a filtered, variable voltage, AC input from the motherboard  53 , and provide a programmable DC output in parallel to the electrolyzer  19 . For example, each module  55  may provide a programmable current output of less than or equal to about 83 amperes DC (ADC), at a voltage of about  10  volts (v) to about 90 V. Controller  15  controls the DC output for each module  55 . Controller  15  senses the voltage at the common DC output of the modules  55  using a voltage monitor line  69 , receives signals  71  indicative of output current at each of the modules  55 , and provides a current program signal  67  to the modules  55  in response to the voltage at voltage monitor line  69  and a signal  73  indicative of a sum of the current signals  71 . In response to the current program signal  67 , the modules  55  adjust the DC output to electrolyzer  19 .  
         [0025]    Controller  15  provides a unique enable signal  75  to each module  55 , which enables and disables individual modules  55 . Signals provided by the modules  55  to the controller  15  include: overtemperature flags  77  indicating a that a temperature associated with a module  55  has reached a predetermined limit, overcurrent flags  79  indicating a current output associated with a module  55  has reached a predetermined limit, open fuse flags  81  indicating that a fuse associated with a module  55  has opened, overvoltage flags  83  indicating an output voltage associated with a module  55  has reached a predetermined limit, and input overvoltage flags  85  indicating an input voltage associated with a module  55  has reached a predetermined limit. Controller  15  also receives a smoke detector signal  87  from a smoke detector located within the power converter box  51 . The smoke detector signal  87  indicates the presence of smoke in the power converter box  51 .  
         [0026]    Controller  15  is coupled to the electrolyzer control system  25  (see FIG. 1) via isolated input/output (I/O) connection  27 . An isolator  89 , used to isolate I/O connection  27 , may include, for example, an optical isolator. Using I/O connection  27 , control signals are provided between the electrolyzer control system  25  and controller  15 . Such signals may include, for example, signals indicating the status of the power converter box  51  (e.g., if smoke has been detected, voltage output, and current output), and signals indicating the status of the modules  55  (e.g., the occurrence of overtemperature, overcurrent, open fuse, overvoltage output, and overvoltage input). These signals may be used by the electrolyzer control system  25 , for example, to modify the operation of the electrolyzer  19  and its ancillary equipment. Such signals may also include signals used by controller  15  to alter the current program signal  67  and, thus, the DC current output to electrolyzer  19 .  
         [0027]    Controller  15  may receive an enable signal  91  from the electrolyzer control system  25  via an alternate, isolated connection  93 . In response to receiving the enable signal  91 , the controller  15  would enable or disable one or more modules  55 . Controller  15  may activate a relay  95  to interrupt operating power to the electrolyzer  19  in certain predetermined cases. For example, controller  15  may activate the relay  95  upon detection of smoke in the power converter box  51 .  
         [0028]    Referring to FIG. 3, a power converter module  55  is shown in further detail. Each power converter module  55  includes input isolation, provided by a rectifier  101  and electromagnetic interference (EMI) filter  103 , and a small amount of energy storage on the front end. Within each power converter module  55 , the 3-phase AC input power is converted to DC through rectifier  101 , which comprises six discrete diodes  105  in a bridge configuration. These diodes  105  may have individual heat sinks and may be cooled by forced air. The rectifier  101  feeds EMI filter  103 , which comprises film capacitors  107  and small inductors  109 . The EMI filter  103  provides rectified and filtered DC current to a plurality of half modules  111 . Each half module  111  includes a phase shift bridge converter, output transformer, rectifiers and filtering, current feedback control, and protection circuits, as will be described hereinafter with reference to FIG. 4. Power converter module  55  includes an optional DC input line  112 , which allows the power converter module  55  to be used for either AC to DC conversion or DC to DC conversion, as will be discussed hereinafter with reference to FIGS. 5 and 6.  
         [0029]    In the embodiment shown in FIG. 3, the rectified and filtered DC current is provided in series to the plurality of half modules  111 . A jumper node (not shown) may be provided on each module  55  to allow selection between parallel and series input to the half modules  111  and, thereby, can be used to select an operating voltage for the module  55  (e.g., select between 240 and 480 VAC operation). The DC output of each half module  111  is arranged in parallel, and is provided to motherboard  53 .  
         [0030]    When operated in parallel (e.g., 240 VAC), the two half modules  111  receive the same current program signal  67 , and they both then put out the same current. In series (e.g., 480 VAC), however, active balancing must be done to keep the voltage to each of the two half modules  111  equal. For series operation, input voltage is balanced between the half modules  111  by sensing voltage across capacitors  107 , and proving the sensed voltages to a device  113 . Active balancing is achieved by providing the current program signal  67  to one of the half modules  111 . The half module  111  produces output, but this draws down its input voltage, increasing the voltage across the other half module  111  input. Device  113  senses this imbalance and provides a current program signal  115  to the top converter to command current output. This continues until the input voltages are balanced.  
         [0031]    The voltage sensed across capacitors  107  is also used as an input to overvoltage detection circuitry  117 . Overvoltage detection circuitry  117  compares the voltage input to each half module  111  with a predetermined threshold value. If the voltage input exceeds the threshold, the overvoltage detection circuitry  117  disables one or more half module  111  using enable signals  119 . The overvoltage detection circuitry  117  also provides the input overvoltage flag  85  to controller  15 , and receives the enable signal  75  for the module  55 . In response to receiving the enable signal  75 , the overvoltage detection circuitry  117  provides enable signals  119  to the half modules  111  to enable or disable the half modules  111 .  
         [0032]    Each half module  111  provides various output signals that are used to generate various flags provided to controller  15 . Each half module  111  provides a current flag signal  121  indicating that current output from the half module  111  has exceeded some predetermined threshold. If either half module  111  outputs a current flag signal  121 , the overcurrent flag  79  is provided to controller  15 . Each half module  111  provides a temperature flag  123  indicating that a temperature associated with the half module  111  has exceeded a predetermined threshold. If either half module  111  outputs a temperature flag  123 , the over temperature flag  77  is provided to controller  15 . Each half module  111  also provides an output voltage flag  125  and a fuse flag  127 . The output voltage flag  125  is provided in response to the output voltage from a half module  111  exceeding a predetermined threshold, and fuse flag  127  is provided in response to opening of a fuse associated with a half module. If an output voltage flag  125  or a fuse flag  127  is output by either half module  111 , the overvoltage flag  83  or the open fuse flag  81 , respectively, is provided to controller  15 . Finally, each half module  111  outputs a current signal  129  indicative of output current at each of the half modules  111 . The sum of the current signals  129  is output as current signal  71 .  
         [0033]    Referring to FIG. 4, a half module  111  is shown in further detail. Each half module  111  includes a chopping circuit  151  to chop the DC input from the module  55  and provide an AC output to transformers  153 . Transformers  153  step the AC either up or down, rectifiers  155  convert the AC to DC, and filter  157  smoothes the resulting DC current. Each half module  111  further includes current feedback control path  159 , and fuse protection  161 .  
         [0034]    In the embodiment shown, chopping circuit  151  comprises a full bridge converter. A full bridge converter is used to for several reasons. Among these are high utilization of the transformer core, good use of semiconductors, and recycling of leakage energy. In this embodiment, a phase shift type of operation is used. This results in soft switching most of the time. Soft switching (or quasiresonant) is when the field effect transistors (FETs)  163  turn on or off into zero voltage, with the voltage transitions following the resonant curve of the transformer and switching capacitors. Low EMI and low losses result.  
         [0035]    Operation of the full bridge converter is achieved by the phase control between the two sets of FETs  163 . Each set of FETs  163  is a series combination, alternatively referred to as a “totem pole”. These are switched alternately on and off with a full square-wave (no pulse width modulation) drive transformer  167  having an input provided by a dual square wave generator  165 . The phasing of each drive transformer  167  ensures that there is no cross conduction. Drive enhancement networks may be used to mitigate the effects of leakage in the drive transformers  167 .  
         [0036]    For example, where the two totem poles both have 100% modulation square-wave drives, the power transformer  153  is connected across the halfway points of the totem poles formed by FETs  163 . When the top and bottom FETs  163  of both totem poles are switched in phase, there is no voltage across the primary winding of transformer  153  and, therefore, no output to the module  55 . When the totem poles are switched completely out of phase, full voltage is applied to the primary winding of transformer  153 .  
         [0037]    The dual square wave generator  165  provides linear control of the phase across the range for full output regulation. The order of switching is such that when a FET  163  turns off, the conduction current commutates the voltage to the opposing FET  163  in the totem pole. Its internal diode then conducts until the FET  163  is turned on. In this manner, very low switching losses are achieved.  
         [0038]    Two transformers  153  are used per full bridge section of FETs  163 . These transformers  153  are connected in series on the input and parallel on the output. Parallel output is used so that more low current rectifiers may be used on the output to increase the current rating. Series input is used to provide current sharing between the output rectifiers  155 . For this reason, current output should be sensed in one leg only.  
         [0039]    The output rectifiers  155  are connected in half-wave center-tap configuration. This gives only one junction drop at a time for higher efficiency. One main inductor  169  is used for both sets of rectifiers  155  to use a common core size with the transformers  153 . A single film capacitor  171  is used for output voltage filtering. The film capacitor  171  provides a fixed impedance for loop gain calculations, and provides a T filter between the inductor  169  and the inductance of the wiring to the electrolyzer  19  (see FIG. 1). Further ripple reduction may be achieved by running the two half modules  111  out of phase (a fixed offset on main clock  57  (see FIG. 5), not to be confused with the phase control regulation).  
         [0040]    Since the output of module  55  is a controlled current, the current feedback control path  159  includes a current sensor  173  (as opposed to voltage sensing), with comparison to the current program signal  67  or  115 . The current sensor  173  includes a low value sense resistor  177  in the output line. The voltage developed across the resistor  177  is amplified by amplifier  179  up to the same level as the current program signal  67  or  115 . The amplified signal is fed to an opamp  175  to generate an error voltage, which controls the dual square wave generator  165 . The amplified signal is also provided as current monitor signal  129  to module  55 . Average current mode control is used, resulting in a circuit having a high bandwidth. A couple of op-amps may be used to condition the current program signal  67  or  115 , a precision clamp may be used to set the maximum current, and a buffer may be used to stiffen the current program signal  67  or  115  after the clamp.  
         [0041]    For fault protection, and possibly transients, current limits are established by a control processor  183 . A current transformer  181  senses the FET  163  bridge current to the transformers  153  and provides a signal indicative of this current to control processor  183 . If the signal indicates that the current has reached a first limit, control processor  183  cuts back the phasing, and if the signal indicates that the current has reached a second limit, control processor  183  resets chopping circuit  151  and initiates a soft start. Control processor  183  may also generate current flag signal  121  in response to the sensed current reaching either of these limits.  
         [0042]    Control processor  183  also implements an over-voltage protection limit by sensing output voltage  185 . If the output voltage  185  exceeds a predetermined limit, control processor  183  may generate an output voltage flag  125 . A temperature sensor  187  provides a signal indicative of a temperature associated with the half module  111  to control processor  183 . If this temperature exceeds a predetermined limit, control processor  183  provides temperature flag  121  as output. Control processor  183  also outputs fuse flag  127  if fuse  161  is opened. Enable signals  119  are received by control processor  183 , and starts or shuts down dual square wave generator  165  in response to the enable signal  119 .  
         [0043]    The modular power electronics system  11  allows a single motherboard  53  to be customized as needed to meet the requirements of the power system  10 . For example, motherboard  53  may be fitted with one or two modules  55  for low power electrolyzers  19 , while the motherboard  53  may be fitted with many (e.g., thirty (30) or more) modules  55  for relatively high powered electrolyzers  19 . By using common (interchangeable) parts, the modular power electronics system  11  takes advantage of volume manufacturing and commonality of parts across a product platform. Also, due to the fact that the modular power electronics system  11  employs circuit board components, it takes advantage of circuit board manufacturing techniques such as pick and place, wave soldering, and surface mount technologies. These technologies help to reduce the price of the modular power electronics system  11 , while providing high efficiency. Indeed, with the modular power electronics system  11 , efficiencies greater than 90% may be achievable.  
         [0044]    [0044]FIG. 5 depicts an alternative embodiment of the modular power electronics system  11 . In this embodiment, an additional motherboard  201  is added to power converter box  51  for providing DC to DC conversion. Motherboard  201  includes a DC input from a DC power source  203 . DC power source  203  may include, for example, an electrochemical cell (e.g., a fuel cell), a capacitor, a battery, a solar collector, or any other DC power source. The DC input is connected in parallel to a plurality of power converter modules  55 , which are mounted to motherboard  201  in a similar manner as that described with reference to motherboard  53 . As shown in FIG. 3, the DC input line  112  may be used for providing the DC input to each module  55  on motherboard  201 . The DC output of motherboard  201  is provided to, for example, electrolysis cell  19 . Control of modules  55  on each motherboard  53  and  201  is provided by controller  15 . It will be appreciated that the number of motherboards added to the system  11  is limited only by the size of the converter box  51  and processing limitations of controller  15 . Thus, the modular power electronics system  11  is highly flexible, providing the ability to add many different converters to a single rack mountable converter box  51 . Alternatively, a single motherboard could be configured to include the circuitry shown on motherboard  53  and motherboard  201 , thus allowing a single motherboard to provide both AC to DC and DC to DC conversion.  
         [0045]    The use of a single controller  15  for all of the converters provides tightly integrated control of the power system  10 . This is especially advantageous for regenerative fuel cell systems, which require power output integration of primary and secondary power sources. The use of a common controller  15  also reduces the cost of the system  11  by eliminating redundant processors. The cost of the system  11  is further reduced by the use of a standard, interchangeable module  55  in both converters and by providing motherboard designs that can be customized by simply adding or removing modules  55 . As previously discussed, by using common parts, the modular power electronics system  11  takes advantage of volume manufacturing and commonality of parts across a product platform.  
         [0046]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.