Abstract:
A power system comprising: a primary power source in electrical communication with a electrolysis cell, wherein the electrolysis cell is in electrical communication with a bus; a secondary power source in electrical communication with the bus, wherein the secondary power source comprises an electrochemical system including a fuel cell. The system further includes: a controller electrically disposed between and in operable communication with the bus and the electrolysis cell, and electrically disposed between and in communication with the bus and the secondary power source. The controller monitors the primary power source, initiates powering by the bridge power source when the primary power source exhibits selected characteristics, initiates the secondary power source when the electrolysis cell is depleted exceeding a first selected threshold, and initiates interruption of powering by the secondary power source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/065,386, filed Oct. 11, 2002, and claims the benefit of U.S. Provisional Application No. 60/410,413, filed Sep. 13, 2002, the contents of both of which are hereby incorporated by reference herein in their entirety. 
     
    
     BACKGROUND  
       [0002]     This disclosure relates generally to electrochemical cell systems, and especially relates to the storage and recovery of energy from a renewable power source and electrochemical cell.  
         [0003]     Geographically remote areas such as islands or mountainous regions are often not connected to main utility electrical grids due to the cost of installing and maintaining the necessary transmission lines to carry the electricity. Even in remote communities where the transmission lines are in place, it is not uncommon for frequent and extended power outages due to weather related faults. In either case, to prevent economic loss in times of an electrical outage, it is often necessary for these communities or industries in these regions to create local “micro” electrical grids to ensure a reliable and uninterruptible power system. This uninterruptible power system may be either a primary system where there is no connection to the main utility grid, or a backup system that activates when an outage occurs.  
         [0004]     Electrical power for the local grids comes from a variety of sources including hydrocarbon based and renewable power sources. Within a particular grid it is not uncommon to have multiple generation sources, such as diesel generators, natural gas generators, photovoltaic arrays, and wind turbines working in combination to meet the needs of the grid.  
         [0005]     Electrical demands placed on the local grid will vary during the course of a day, week, or season. Since it is not often practical or possible to turn generation sources on and off, inevitably excess energy will be created. This excess energy is typically converted into another form of energy such as heat for storage in another medium such as water. In cold climates, the heated water can then be used for other purposes such as heating buildings, cooking or maintaining temperature in equipment. As the load requirements of the grid increase, it is difficult or impossible to recapture the converted energy back into electrical energy for use in the electrical grid. Further complicating matters is that renewable power sources do not typically run continuously at full power and will experience extended periods of low to no energy output (e.g. night time or seasonal low wind periods).  
         [0006]     Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells commonly employed to address back-up power requirement when a grid fails or when a renewable energy source is unavailable. 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 cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.  
         [0007]     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.  
         [0008]     In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems. Regenerative fuel cells may be used in power generation systems as either primary or secondary power sources. However, because regenerative fuel cell systems generally take a certain amount of time from the point of initial activation to delivering full power, there may be a brief delay of power attendant thereto when switching over from a primary power supply to backup power generated by a fuel cell supply. What is needed in the art is a cost effective apparatus and method for bridging short duration power interruptions.  
       SUMMARY OF INVENTION  
       [0009]     Disclosed herein is a power system, comprising: a primary power source in electrical communication with an electrolysis cell, wherein the electrolysis cell source is in electrical communication with a bus; a secondary power source in electrical communication with the bus, wherein the secondary power source comprises an electrochemical system including a fuel cell. The system further includes: a controller electrically disposed between and in operable communication with the bus and the electrolysis cell, and electrically disposed between and in communication with the bus and the secondary power source. The controller monitors the primary power source, initiates powering by the electrolysis cell when the primary power source exhibits selected characteristics, initiates the secondary power source when the electrolysis cell exhibits a selected second characteristic, and initiates interruption of powering by the secondary power source when at least one of the primary power source does not exhibit the selected characteristics and the secondary power source exhibits a selected third characteristic.  
         [0010]     Also disclosed herein is a method for operating a power system comprising: monitoring a primary power source; if the primary power source exhibits selected characteristics: directing power from an electrolysis cell to a bus; and if the electrolysis cell is operated to a first selected threshold, initiating a secondary power source and powering the bus with the secondary power source until at least one of the primary power source does not exhibit the first selected characteristics and the secondary power source exhibits second selected characteristics. The secondary power source comprises a fuel cell.  
         [0011]     Also disclosed is a method for operating a power system, comprising: monitoring a primary power source; if the primary power source is insufficient to meet a demand for power: powering a bus with an electrolysis cell and if the bridging power source is depleted to a first selected threshold, initiating a secondary power source and powering with the secondary power source until at least one of the primary power source is sufficient to meet said demand and the secondary power source exhibits second selected characteristics. The secondary power source comprises a fuel cell, and the first selected threshold is that the electrolysis cell comprises sufficient power to power the bus while the secondary power source initiates.  
         [0012]     Further, disclosed herein is a storage medium encoded with a machine-readable computer program code, said code including instructions for causing a computer to implement the abovementioned method for operating a power system.  
         [0013]     Further, disclosed herein is a computer data signal, said computer data signal comprising: instructions for causing a computer to implement the above-mentioned method for operating a power system.  
         [0014]     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  
       [0015]     Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike:  
         [0016]      FIG. 1  is a block diagram of a power system including a secondary power system and a power bridging apparatus;  
         [0017]      FIG. 2  is a state transition diagram depicting an exemplary embodiment for a control methodology of a power system; and  
         [0018]      FIG. 3  is a state transition diagram depicting an alternative embodiment for a control methodology of a power system. 
     
    
     DETAILED DESCRIPTION  
       [0019]     The following description will provide specific examples with respect to the load and power source voltages for example only. It will also be understood that the method and apparatus for bridging short duration power interruptions may be used with different types of primary/secondary sources and/or other operating voltages, and is not limited to the implementations described herein. Various power sources can range from grid power to solar power, hydroelectric power, tidal power, wind power, fuel cell power, and the like, as well as combinations comprising at least one of the foregoing power sources (e.g., via solar panel(s), wind mill(s), dams with turbines, electrochemical cell systems, and the like). It should further be noted that although the disclosed embodiments are described by way of reference to power system with employing a fuel cell as back up power and a capacitor as a bridging power source, it will be appreciated that such references are illustrative only and the disclosed embodiments may be applied to any instance where back up power and/or bridging power sources are desired. Moreover, the references and descriptions herein may apply to many forms of power systems and sources as described above.  
         [0020]      FIG. 1  depicts a block diagram of a portion of power system  10  having a primary power source  32  such as generated grid power or that from a renewable source, a secondary power source  100  and a load  36 , which load  36  is fed from a feeder bus  38 . In the example shown, the primary power source  32  provides power along a primary bus  40 ; e.g., 120/240 volts alternating current (VAC). It will be appreciated that the actual primary supply voltage is based upon the type of power source including, but not limited to other alternating current (AC) voltage sources, direct current (DC) sources renewable sources such as wind, solar, and the like.  
         [0021]     Optionally, a conversion device  42  can be employed to rectify the power type (e.g., alternating current (AC) to direct current (DC), or DC to AC), or to transform the power level (e.g., 48 volts direct current (VDC) to 24 VDC). For example, rectifier  43  can convert 120/240 VAC supply voltage fed from the primary power supply  32  to a 24 VDC supply, fed through feeder bus  38  to load  36 .  
         [0022]     A secondary power source  100  may comprise an electrochemical cell system. The electrochemical cell system may include a fuel cell  34 , or a regenerative fuel cell system comprising a fuel cell  34 , electrolysis cell  62 , an optional power converter  61 , optional associated hardware, optional storage devices  64 , controls, and the like. The size, i.e., the number of cells, of the fuel cell  34  and optional electrolysis cell  62 , and the desired hydrogen production of the electrolysis cell  62  is dependent upon the desired power output of the secondary power source  100  including fuel cell  34 . For example, a secondary power source  100  can include a fuel cell  34  that provides about 50 to about 100 VDC output voltage for use by the load  36 .  
         [0023]     In order to provide backup power for the load  36 , a controller/DC-DC power supply  44  is used to convert the power from the secondary power source  100  to a power receivable by the feeder bus  38 . For example, the input from the fuel cell  34  is converted to an output that is connected to feeder bus  38 , wherein a smooth output is an uninterrupted power that has an average voltage fluctuation of less than about 10% over several seconds. An uninterrupted power is a less than about 0.005 second delay between cease of power supply from primary power source  32  and introduction of power from controller/DC-DC power supply  44 .  
         [0024]     In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the control algorithms for balanced back up power application(s), and the like), controller/DC-DC power supply  44  may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interface(s), and input/output signal interfaces, and the like, as well as combinations comprising at least one of the foregoing. For example, controller/DC-DC power supply  44  may include input signal processing and filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. Additional features of controller/DC-DC power supply  44  and certain processes, functions, and operations therein are thoroughly discussed at a later point herein.  
         [0025]     During operation with a regenerative fuel cell system, the primary power source  32  provides power via optional power converter  61  to an electrolysis cell  62  e.g., an electrolyzer, which generates hydrogen gas. When the optional power converter  61  is not employed, the electrolysis cell  62  may be directly connected to the primary bus  40  via line  65 . The hydrogen generated by the electrolysis cell  62  is stored in an appropriate storage device  64  for later use. At such a point in time as required for operation such as outages of the primary power source  32  and the like, or for a renewable power source, during the day or season where the power generation capability of the renewable power source declines (e.g., night time), the primary power source  32  or secondary power source  100  will need to offset the loss in capacity. The hydrogen previously stored in storage device  64  is supplied to a hydrogen electrochemical device, e.g., fuel cell  34 , which converts the hydrogen into electricity to supply to the load  36 . Power generation will continue until the hydrogen in the storage device  64  is exhausted or the power is no longer required. Reasons for ending power generation may include, for example, the restoration of the grid power, restoration of renewable energy sources (such as solar, wind, wave power, or the like), and/or the determination that peak-shaving is no longer cost effective or no longer required.  
         [0026]     Once the amount of hydrogen in the hydrogen storage device  64  decreases below a pre-determined level, the electrolysis cell  62  engages to replenish the hydrogen supply. Preferably, hydrogen will be replenished whenever the hydrogen storage level in the hydrogen storage device  64  is less than full, and there is power available from the primary power source  32  for the electrolysis operation to ensure the longest possible operational duration capability for the secondary power source  100 , e.g., the fuel cell  34 . Alternatively, hydrogen may be replenished with the addition of hydrogen from another source. For example, another hydrogen generating means, or replacement, replenishment, or supplementation of the existing hydrogen storage device  64 .  
         [0027]     Returning to  FIG. 1  once again for discussion of the secondary power source  100 , because fuel cell systems generally take a certain amount of time from the point of initial activation to delivering full power, there may be a brief delay of power attendant thereto when switching over a primary power source  32  to secondary power source  100  and power generated by a fuel cell  34 . To address this time lapse, a power system may employ a bridging power source  46 . The bridging power source  46  stores electrical energy and temporarily provides power to load  36  in the event of any gap or delay between the transfer of power delivery from the primary power source  32  to the power delivery from the secondary power source  100 , namely, fuel cell  34 . For example, power system  10  may include a monitor of the primary power source  32  (e.g., a grid, solar power, another electrochemical system, and the like); and upon a cease in power from the primary power source  32 , start-up a secondary power system  100  and introduce power from the bridging power source  46  during the time lapse. The bridging power source  46  may comprise a an electrolysis cell, capacitor  48 , and/or battery  49 , and optionally a power converter  50 .  
         [0028]     Charging of the bridging power source may be accomplished in various fashions, depending upon the type of primary power source  32  and the voltage of feeder bus  38  or primary bus  40 , accordingly. The bridging power source can be charged with power from primary bus  40  via optional power converter  50 . Power converter  50  converts the voltage from the bus voltage on feeder bus  38  (or primary  40  depending upon the implementation) to the bridging power source voltage. Meanwhile, a conversion device  42  can be employed, if desired, to adjust the voltage of primary bus  40  to the desired voltage for the feeder bus  38 . Alternatively, the power can pass from primary bus  40  through conversion device  42 , to feeder bus  38 . Power converter  50  may alternatively convert voltages from feeder bus  38  to charge to the bridging power source. Finally, it will be appreciated, that the bridging power source may be operably connected to either primary bus  40  or feeder bus  38  directly. In this embodiment, power from primary bus  40  can be converted from AC to DC, and/or the DC voltage of the feeder bus  38  may be converted to the desired capacitor voltage via power converter  50 . For example, the energy used to charge capacitor  48  or battery  49  can come from the output of rectifier  43  that converts 120 (or 240) VAC on primary bus  40  to 24 VDC on feeder bus  38 . The power converter  50  then converts the low voltage (e.g., 24 VDC) input into an appropriate voltage output, which is then used to charge bridging power source.  
         [0029]     The output of the bridging power source is connected to controller/DC-DC power supply  44 . When bridging power source is used to bridge the gap in power between a switch-over from primary power source  32  to the secondary power source  100 , the controller/DC-DC power supply  44  may be employed to convert the power from the power level of the bridging power source to the power level of the feeder bus  38 . Preferably, power is supplied by the bridging power source for the period of time from a cease in the power supply from primary power source  32  until commencement of power supply from fuel cell  34  (i.e., when the fuel system attains operating conditions and begins to supply a predetermined amount of power).  
         [0030]     In order to determine when, and for what period, to draw power from the bridging power source, sensing lines  52  and  54  are connected from the primary bus  40  and the output of the secondary power source  100  to the controller/DC-DC power supply  44 . In this manner, controller/DC-DC power supply  44  can monitor the status of the primary power source  32  and the secondary power source  100  so that the switching to an appropriate power source may be determined and controlled. It will easily be appreciated that in controller/DC-DC power supply  44 , the DC-DC power supply may optionally be separated from the controller.  
         [0031]     During a normal mode of operation, the power supplied from primary power source  32  (e.g., 120/240 VAC or optionally a DC source) on primary bus  40  is converted (in the depicted configuration) to a DC voltage by rectifier  43  of conversion device  42 . The load  36  draws current from feeder bus  38 , regardless of the source of the power thereto. During the normal mode, bridging power source  46  maintains stored electrical energy in the event of a temporary power interruption.  
         [0032]     In the event of a loss of power from the primary power source  32 , controller/DC-DC power supply  44  senses the loss on the primary bus  40  through sensing line  52 . A signal is then sent by controller/DC-DC power supply  44  to the secondary power source  100  (through line  56 ) to begin generating backup power for feeder bus  38 . Because of the inherent time delay of a fuel cell  34  in producing full power, controller/DC-DC power supply  44  converts the output voltage of the bridging power source to voltage that is directed to feeder bus  38  until the secondary power source  100 , and more specifically the fuel cell  34  is ready to take over so that load  36  sees an uninterrupted supply of power.  
         [0033]     Once controller/DC-DC power supply  44  senses that the fuel cell  34  is generating a desired amount of power, the bridging power source may be disconnected (circuit broken) from feeder bus  38  and backup power is now directed from the secondary power source  100  and more specifically the fuel cell  34 , through controller/DC-DC power supply  44 , and onto feeder bus  38 . Optionally, at the same time, the bridging power source may be recharged through line  58  from feeder bus  38  and power converter  50 . The connecting and disconnecting of the bridging power source to the DC-DC converter within controller/DC-DC power supply  44  may be accomplished with one or more device(s) such as a power field effect transistor(s) (FET; not shown), transistor(s), thyrister(s), relay(s), switching device(s), and the like, as well as combinations including at least one of the foregoing. Optionally, controller/DC-DC power supply  44  may leave the bridging power source in the circuit but draw essentially no power therefrom. If power from the primary power source  32  is subsequently restored, this will be sensed by controller/DC-DC power supply  44 . This time, however, there is no need to discharge the bridging power source, since controller/DC-DC power supply  44  may seamlessly switch from the secondary power source  100  and fuel cell  34  back to primary power source  32  by deactivating the fuel cell  34 .  
         [0034]     Either during operation of the secondary power source  100  (via feeder bus  38 ) and/or after reconnection to primary power source  32  (via primary bus  40 ), the bridging power source  46  (namely the electrolysis cell, capacitor  48 , and/or battery  49 ) may be charged (or recharged, as is appropriate). During charging, current supplied from feeder bus  38  is sent to power converter  50 , which converts the voltage of feeder bus  38  to that appropriate to charge the bridging power source. It should be noted, that once the bridging power source is/are charged, no significant current would be drawn by power converter  50  (if used) from feeder bus  38 . Alternatively, it will be further appreciated that in an implementation where primary power source  32  and primary bus  40  comprise a VDC power source, power may be optionally be drawn directly from the primary bus  40  (or optionally through the power converter  50 ) to charge the bridging power source.  
         [0035]     Moreover, the power converter  50  may, be configured as an AC/DC converter (rectifier) coupled directly to the primary power source  32  and primary bus  40 . In addition, for yet another alternative embodiment, the output voltage of controller/DC-DC power supply  44  may be generated at a slightly lower value than that resultant from the conversion device  42  (e.g., by about 1 to about 3 volts). In so doing, any current flow from controller/DC-DC power supply  44  onto feeder bus  38  would be limited until such time as the primary power source  32  is unavailable.  
         [0036]     Employing a system comprising an electrochemical system in conjunction with a high voltage, medium-sized capacitor as part of a power bridging power source, a cost-effective uninterrupted power supply system is realized. This is especially the case when one or more of the sources have an inherent power-up time associated therewith, such as secondary power source  100  including a fuel cell  34 . It should also be noted that the number of components employed may be reduced as disclosed by employing commonality in selected components, e.g., using a common DC-DC power supply  44  connected to the electrolysis cell, capacitor  48 , and/or battery  49 , and the fuel cell  34  instead of multiple power supplies.  
         [0037]     In yet another alternative embodiment in the event of a loss of power from the primary power source  32 , and when the fuel cell  34  is not providing power either because of a fault or because the hydrogen storage device(s)  64  are depleted, the power system  10  may draw power from the bridging apparatus  46  as described earlier. In this instance, the controller/DC-DC power supply  44  converts the output voltage of the bridging power source to voltage that is directed to feeder bus  38  and/or more particularly selected loads to at least facilitate control and diagnostics. Under this conditions, as the stored energy in the bridging power source is depleted, and therefore the voltage drops, additional current may be drawn by the controller/DC-DC power supply  44  to supply a specified load. Ultimately, as additional energy from the bridging power source is expended, the voltage continues to drop and the current will rise, potentially to unacceptable levels. Therefore, in an exemplary embodiment, the charge status of the bridging power source  46  may be monitored. The controller/DC-DC power supply  44  monitors the bridging power source voltage and/or current and the bridging power source may be disconnected (open circuit) from to avoid the controller/DC-DC power supply  44  drawing unacceptably high current levels therefrom. Preferably, the monitoring of the energy stored in the bridging power source and the connection and disconnection thereof will include hysteresis to avoid nuisance connections and reconnections. For example, hysteresis may be employed to ensure that the controller/DC-DC power supply  44  does not reconnect to the bridging power source following a disconnection based upon the voltage rise that may follow a disconnection. Once again, the connecting and disconnecting of the bridging power source to the DC-DC converter within controller/DC-DC power supply  44  may be accomplished with a device such as a power field effect transistor (FET; not shown), or the like.  
         [0038]     In yet another embodiment of power system  10  in the event of a loss of power from the primary source  32 , and either prior to the fuel cell  34  being available or when the fuel cell  34  is not providing power either because of a fault or because the hydrogen storage device(s) are depleted, the power system may draw power from the bridging power source  46  as described earlier. In this embodiment, evaluations of the status of the power system  10  are employed to facilitate establishing a balanced utilization of the available power sources e.g., primary power source  32 , secondary power source  100 , and bridging power source  46  in the power system  10 . For example, for situations involving short duration and/or multiple interruptions less than a selected threshold of the primary power source  32 , it may be advantageous to operate and provide power to the feeder bus  38  from the bridging power source  46  alone, without initiating the secondary power source  100 . This may even be beneficial despite the secondary power source  100  and more specifically the fuel cell  34  having fuel and being available to provide power. Such a configuration avoids nuisance initiations of the fuel cell  34  and thereby provides reduced fuel depletion and enhanced life for the fuel cell  34 . In an exemplary embodiment a four second power interruption of the primary power source  32  is covered by the bridging power source  46  without initiating the fuel cell  34 . Should the interruption of the primary power source  32  exceed the selected threshold e.g., four seconds, the secondary power source  100 , e.g., the fuel cell  34  is initiated to supply power to the load  36  and optionally to recharge the bridging power source  46 . Preferably, the connection and disconnection of the bridging power source  46 , as well as the initiation of the fuel cell  34 , includes hysteresis to avoid nuisance initiations, connections, and reconnections. For example, hysteresis may be employed to ensure that the fuel cell  34  is not, unnecessarily initiated and then shut down. It will be appreciated that with such a configuration, a control system may be implemented, which ensures balance operation utilizing the energy available from the bridging apparatus  46  without unnecessarily expending energy to initiate the fuel cell  34 . Once again, the connecting and disconnecting of the bridging power source to the DC-DC converter within controller/DC-DC power supply  44  may be accomplished with a device such as a power field effect transistor (FET; not shown), or the like.  
         [0039]     Continuing with  FIG. 1  and turning now to  FIG. 2 , a state transition diagram depicting an exemplary method of control process  200  for the power system  10  is provided. The process  200  includes numerous modes and the criterion, requirements, events and the like to control changes of state among the various modes. The process  200  initiates with an initialization mode  210  monitoring and evaluation of various sensors ant states to ascertain the status of the power system  10 . Such monitoring may include evaluation of the voltage of the primary power source  32  on the primary bus  40 , e.g., grid power a renewable source, such as wind speed or light level. As disclosed earlier, such a renewable power source includes solar wind, tidal, geothermal resources, and the like, as well as combinations including at least one of the foregoing. Should it be determined that a fault exists, which may be characterized as a shut down event, the process  200  transfers modes to a shut down mode  220  exiting the process  200 . Should it be determined that the power system  10  status check is satisfactory (no fault exists, which may be characterized as a shut down event) the process  200  changes modes to an idle mode  230 .  
         [0040]     In the idle mode  230 , processing is completed to determine the status of selected elements of the power system  10  and query the occurrence of selected commands for mode selection and power system  10  operation. First, from the idle mode  230 , if a manual mode is requested, for example by an external request, e.g. operator, the process  200  changes modes to a manual mode  240 . Conversely, should selected criteria be satisfied for exiting the manual mode, the process  200  reverts to the idle mode  230 . Selected criteria for exiting the manual mode in this instance may include but not be limited to an operator request. Continuing with the idle mode  230 , should the primary power source  32  exhibit satisfactory status, and the hydrogen storage device  64  is empty or lower than a selected level, the process  200  changes state to an electrolyzer mode  250 . In an exemplary embodiment, the evaluation of the status of the hydrogen storage device  64  may include monitoring the pressure of the hydrogen stored in the hydrogen storage device  64 . By monitoring the pressure, of the known storage volume the duration of back up time available from operating the fuel cell  34  of the secondary power source  100  may be deduced. In an exemplary embodiment a hydrogen storage device of a volume of about 200 cubic feet at standard pressure less than or equal to about 200 pounds per square inch (psi) indicates that the hydrogen storage device  64  is considered empty. Should the primary power source  32  exhibit unsatisfactory status and the bridging power source  46  exhibits a satisfactory status, the process  200  changes state to a bridge mode  260 . Conversely, should the primary power source  32  exhibit satisfactory status, and the bridging power source  46  can provide less than a selected amount of power, the process  200  reverts from the bridge mode  260  back to the idle mode  230 . In an exemplary embodiment, when the primary power source  32  is satisfactory and the bridging power source  46  can provide less than about four seconds of power, the process  200  reverts from the bridge mode  260  back to the idle mode  230 . The status of the bridging power source  46  may be ascertained by monitoring the current and/or voltage. For example, for a bridging power source  46  including a capacitor  48 , the status of the power available from the capacitor may readily be determined by well-known means once either the current or the voltage is known. Finally, should the primary power source  32  exhibit unsatisfactory status and the bridging power source  46  also exhibits an unsatisfactory status e.g., has insufficient energy to operate in the bridge mode  260 , the process  200  changes state from the idle mode  230  to the shutdown mode  220 .  
         [0041]     Turning now to the manual mode  240 , consideration may now be given to the criteria for transition thereto and therefrom. As stated earlier, the manual mode  240  may be entered from the idle mode  230 , if a manual mode is requested. Conversely, should selected criteria be satisfied, the process  200  transitions from the manual mode  240  to the idle mode  230 . Continuing with discussion of the manual mode  240  and similar to the idle mode  230 , the manual mode  240  may also be entered from the electrolyzer idle mode  250  and/or the bridge mode  260 , if a manual mode is requested. Finally, upon satisfaction of selected criteria for a manual shutdown, the process  200  changes state to the shut down mode  220 .  
         [0042]     Continuing with  FIG. 2  and the electrolyzer mode  250 , the electrolyzer mode  250  may only be entered as described earlier from the idle mode  230  when the primary power source  32  exhibits satisfactory status, and the hydrogen storage device  64  is empty or lower than a selected level. As stated above, the process  200  transitions from the electrolyzer mode  250  to the manual mode  240  upon a manual mode request. The process also transition from the electrolyzer mode  250  to the bridge mode  260  if primary power source  32  exhibits satisfactory status, and the hydrogen storage device  64  is not empty, nor lower than a selected level. Finally, exiting the electrolyzer mode  250  may be achieved by a transition of the process  200  to the shutdown mode  220  upon the occurrence of a fault exists, which may be characterized as a shut down event and the primary power source  32  exhibits unsatisfactory status, and the bridging power source  46  has insufficient energy.  
         [0043]     Turning now to the bridge mode  260 , the bridge mode  260  may be entered from the electrolyzer mode  250  if primary power source  32  exhibits unsatisfactory status, and the hydrogen storage device  64  is not empty, nor lower than a selected level. Additionally, as disclosed earlier, the process  200  transitions from the idle mode  230  to the bridge mode  260  should the primary power source  32  exhibit unsatisfactory status and the bridging power source  46  exhibits a satisfactory status. Conversely, should the primary power source  32  exhibit satisfactory status, and the bridging power source  46  can provide less that a selected amount of energy, the process  200  transfers from the bridge mode  260  to the idle mode  230 . In an exemplary embodiment, when the primary power source  32  is satisfactory and the bridging power source  46  can provide less than about four seconds of power, the process  200  transitions from the bridge mode  260  to the idle mode  230 .  
         [0044]     Continuing with the bridge mode  260 , the bridge mode  260  may also be entered from either the FC started mode  270  or the FC engaged mode  280  if it is determined that the hydrogen storage device(s)  64  are empty (or below a selected level). Similar to the idle mode  230  and electrolyzer mode  250  discussed earlier, a manual mode request results in a transition of the process  200  from the bridge mode  260  to the manual mode  240 . Additionally, exiting the bridge mode  260  may be achieved by a transition of the process  200  to the FC started mode  270  if the primary power source  32  exhibits unsatisfactory status for more than a selected amount of time, and the bridging power source  46  exhibits sufficient power. In an exemplary embodiment, the selected amount of time is a range based upon the selected components of the system. For example, to start the fuel cell  34 . In an exemplary embodiment a range from one to 30 seconds is likely, with greater than or equal to about 4 seconds preferred. Finally, exiting the bridge mode  260  may be achieved by a transition of the process  200  to the shutdown mode  220  upon the occurrence of a fault exists, which may be characterized as a shut down event and the primary power source  32  exhibits unsatisfactory status, and the bridging power source  46  has insufficient energy.  
         [0045]     Continuing with  FIG. 2  and moving now to the FC started mode  270 , it will be evident from the figure and as discussed earlier, that the FC started mode  270  may be entered via the bridge mode  260  if the primary power source  32  exhibits unsatisfactory status for more than a selected amount of time. Once again, in an exemplary embodiment, the time is greater than or equal to about 4 second of load capability. The process  200  transitions from the FC started mode  270  to a FC engaged mode  280  if status indicates that the fuel cell  34  is ready. In an exemplary embodiment, a fuel cell ready indication is provided if the fuel cell  34  is at operating temperature, pressure and the like and prepared to generate power upon the application of a load. Continuing with the FC started mode  270 , another transition out of the mode is if the hydrogen storage device(s)s  64  is empty or below a selected threshold. In this instance, the process  200  transitions to the bridge mode  260  as discussed earlier. Finally, exiting the FC started mode  270  may be achieved by a transition of the process  200  to the shutdown mode  220  upon the occurrence of a fault exists, which may be characterized as a shut down event and the hydrogen storage device(s)  64  is empty or below a selected threshold, and the bridging power source  46  has insufficient energy.  
         [0046]     Turning to the FC engage mode  280 , once again it should be evident that the process  200  transitions to the FC engaged mode  280  from the FC started mode  270  if status indicates that the fuel cell  34  is ready. Additionally, as with the FC started mode  270 , a transition out of the FC engaged mode  280  is if the hydrogen storage device(s)  64  is empty or below a selected threshold. In this instance, the process  200  transitions to the bridge mode  260 . From the FC engaged mode  280 , if primary power source  32  changes status, i.e., exhibits satisfactory status e.g. grid power becomes available, then the FC engaged mode  280  transitions to the FC covering bridge mode  290 . Conversely, if the process  200  is operating from the FC covering bridge mode  290  and the primary power source  32  changes status, i.e., exhibits unsatisfactory status e.g. grid power becomes unavailable then the process  200  transitions back to the FC engaged mode  280 . Unsatisfactory status may include but not be limited to a power source being unavailable inoperable, inadequate to provide power at expected parameters, out of tolerance, and even unfueled and the like. Finally, exiting the FC engaged mode  280  may be achieved by a transition of the process  200  to the shutdown mode  220  upon the occurrence of a fault exists, which may be characterized as a shut down event.  
         [0047]     Continuing once again with  FIG. 2  and turning now to the FC covering bridge mode  290 . The FC covering bridge mode keeps the fuel cell  34  operating to ensure that the bridging power source  46  ( FIG. 1 ) is charged and prepared to address the next power interruption of the primary power source  32 . It will be appreciated that an advantage of the FC covering bridge mode is that it prevents a series of power interruptions from depleting the bridging power source  46 . As discussed for the FC engaged mode, if primary power source  32  changes status, e.g., exhibits unsatisfactory status e.g. grid power becomes unavailable, then the process  200  transitions from the FC covering bridge mode  290  to the FC engaged mode  280 . Conversely, as stated above, if the process  200  is operating from the FC engaged mode  280  and the primary power source  32  changes status, i.e., exhibits satisfactory status e.g. grid power becomes available then the process  200  transitions back to the FC covering bridge mode  290 . Additionally, from the FC covering bridge mode  290 , the process  200  will transition from the FC covering bridge mode  290  to the idle mode  230  if the primary power source  32  exhibits satisfactory status, e.g., grid power is available and in tolerance, and the bridging power source  46  has been charged to a sufficient threshold. In an exemplary embodiment, the bridging power source  46  has been charged to a sufficient threshold if it is substantially fully charged. The FC covering mode  290  also includes a status check and transition to its self in the event the primary power source  32  exhibits satisfactory status, e.g., grid power is available and in tolerance, and the bridging power source  46  has not yet been charged to a sufficient threshold. In an exemplary embodiment, the bridging power source  46  has been charged to a sufficient threshold if it is substantially fully charged. Finally, exiting the FC covering bridge mode  290  may be achieved by a transition of the process  200  to the shutdown mode  220  upon the occurrence of a fault exists, which may be characterized as a shut down event.  
         [0048]     Finally, for consideration of the shut down mode  220 , it will be appreciated, that the state transitions of the process  200  to the shut down mode  220  have each been addressed in explanation of the other modes above. Therefore, additional discussion would be redundant and is avoided for simplicity.  
         [0049]     In an alternative embodiment.  FIG. 3  is a state transition diagram depicting an exemplary method of control process  300  for the power system  10 . The process  300  includes numerous modes and the criterion, requirements, events and the like to control changes of state among the various modes. The process  300  is similar to the process  200  disclosed above yet expanded to include additional modes. The sleep mode and its interfacing criterion provide additional functionality to the power system  10  to facilitate enhanced back up power sourcing in the event of primary power source failure and secondary power source  100  fault or depletion. The process  300 , similar to the process  200 , initiates with an initialization mode  210  monitoring and evaluation of various sensors ant states to ascertain the status of the power system  10 . Such monitoring may include evaluation of the voltage of the grid power  32  on the primary bus  40 , or a renewable primary source  32 , such as wind speed or light level. As disclosed earlier, such a renewable power source  32  includes solar wind, tidal, geothermal resources, and the like, as well as combinations including at least one of the foregoing.  
         [0050]     It will be appreciated that the process  300  includes numerous modes, states, and criteria similar to those disclosed in process  200  above. Therefore, additional discussion of the modes common to process  200  would be redundant and has been omitted. Reference should be made to the description provided herein.  
         [0051]     Continuing with  FIG. 3 , the sleep mode  310  facilitates enhanced back up power sourcing in the event of primary power source  32  failure and secondary power source  100  fault or depletion. The purpose of the sleep mode  310  is to address the condition when there is no other power or power generation capability from the secondary power source  100  available. The sleep mode  310  ensures that minimal control, diagnostic, monitoring and reporting capabilities are kept operating with the power remaining. The sleep mode  310  may be entered with the process  300  transitioning from the bridge mode  260  if the primary power source  32  exhibits unsatisfactory status and the bridging apparatus  46  is substantially depleted and the hydrogen storage device  64  is empty, nor lower than a selected level or another operational fault exists which prevents operation of the secondary power source  100 , and more particularly, the fuel cell  34 . In an exemplary embodiment, the bridging power source  46  may be considered substantially depleted if the bridging apparatus is at a level, which can only support loads such as selected controls, diagnostics, and communication of status. The process  300  transitions from the sleep mode to the initialization mode  210  in the event that the primary power source  32  exhibits satisfactory status e.g., grid available once again.  
         [0052]     The disclosed invention can be embodied in the form of computer or controller implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied in tangible media  70 , such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer or controller, the computer becomes an apparatus for practicing the invention. The present invention may also be embodied in the form of computer program code or signal  72 , for example, whether stored in a storage medium, loaded into and/or executed by a computer or controller, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.  
         [0053]     While the invention has been described with reference to an exemplary 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, but that the invention will include all embodiments falling within the scope of the appended claims.