Abstract:
The method of operating a passive hybrid power supply in, or near, zero connected load conditions comprises the steps of:
       supplying a stream of substantially pure hydrogen to the anode of the fuel cell;   supplying an stream of substantially pure oxygen to the cathode of the fuel cell;   monitoring an electric current supplied by the storage battery;   monitoring an output voltage shared by the fuel cell and the battery;   evaluating a state of charge (SOC) of the battery based on the electric current and the output voltage;   monitoring a hydrogen pressure in the fuel cell;   monitoring an oxygen pressure in the fuel cell;   limiting the stream of hydrogen and the stream of oxygen and actuating the hydrogen and oxygen recirculating pumps in such a way as to bring and maintain the hydrogen and oxygen pressures below 0.7 bar absolute  while maintaining the hydrogen pressure between 70 and 130% of the oxygen pressure, in such a way as to ensure that the output voltage is maintained at a level corresponding to less than 0.90 volts/cell and does not exceed the maximum voltage limit of the battery.

Description:
[0001]    This application claims priority from European Patent Application No. 09175547.0 filed Nov. 10, 2009, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention concerns a method for limiting the output voltage of a fuel cell/battery passive hybrid power supply operating in, or near, zero load conditions, in such a way as both not to exceed the upper voltage limit of the battery, without it being necessary to stop the fuel cell or to disconnect it from the battery. The invention more particularly concerns such a method wherein the fuel cells of the power supply are of a type designed to use hydrogen as fuel and pure oxygen as oxidizer. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrochemical fuel cells of the above-mentioned type convert reactants, namely a stream of hydrogen and a stream of oxygen, into electric power and water. Proton exchange membrane fuel cells (PEMFC) generally comprise a solid polymer electrolyte membrane disposed between two porous electrically conductive electrode layers so as to form a membrane electrode assembly (MEA). In order to induce the desired electrochemical reaction, the anode electrode and the cathode electrode each comprise one or more catalyst. These catalysts are typically disposed at the membrane/electrode layer interface. 
         [0004]    At the anode, the hydrogen moves through the porous electrode layer and is oxidized by the catalyst to produce protons and electrons. The protons migrate through the solid polymer electrolyte towards the cathode. The oxygen, for its part, moves through the porous cathode and reacts with the protons coming through the membrane at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current. 
         [0005]      FIG. 1  illustrates, in exploded view, a prior art proton exchange membrane fuel cell stack  10 . Stack  10  includes a pair of end plate assemblies  15 ,  20  and a plurality of fuel cell assemblies  25 . In this particular example, electrically insulating tie rods  30  extend between end plate assemblies  15 ,  20  to retain and secure stack assembly  10  in its assembled state with fastening nuts  32 . Springs  34  threaded on tie rods  30  interposed between fastening nuts  32  and end plate  20  apply resilient compressive force to stack  10  in the longitudinal direction. Reactant and coolant fluid streams are supplied to, and exhausted from, internal manifolds and passages in stack  10  via inlet and outlet ports (not shown) in end plate  15 . 
         [0006]    Each fuel cell assembly  25  includes an anode flow field plate  35 , a cathode flow field plate  40  and an MEA  45  interposed between plates  35  and  40 . Anode and cathode flow field plates  35  and  40  are made out of an electrically conductive material and act as current collectors. As the anode flow field plate of one cell sits back to back with the cathode flow field plate of the neighboring cell, electric current can flow from one cell to the other and thus trough the entire stack  10 . Other prior art fuel cell stacks are known in which individual cells are separated by a single bipolar flow field plate instead of by separate anode and cathode flow field plates. 
         [0007]    Flow field plates  35  and  40  further provide a fluid barrier between adjacent fuel cell assemblies so as to keep reactant fluid supplied to the anode of one cell from contaminating reactant fluid supplied to the cathode of another cell. At the interface between MEA  45  and plates  35  and  40 , fluid flow fields  50  direct the reactant fluids to the electrodes. Fluid flow field  50  typically comprises a plurality of fluid flow channels formed in the major surfaces of plates  35  and  40  facing MEA  45 . One purpose of fluid flow field  50  is to distribute the reactant fluid to the entire surface of the respective electrodes, namely the anode on the hydrogen side and the cathode on the oxygen side. 
         [0008]    One known problem with PEMFCs is the progressive degradation of performance over time. Actually, long-term operation of solid polymer fuel cells has been proven, but only under relatively ideal conditions. In contrast, when the fuel cell has to operate in a wide range of conditions, as is the case for automotive applications in particular, the ever-changing conditions (often modeled as load cycling and start-stop cycles), have been shown to reduce durability and lifespan drastically. 
         [0009]    Different types of non-ideal conditions have been identified in the literature. A first of these conditions is referred to as “high cell voltage”; it is known that exposing a fuel cell to low or zero current conditions, leads to higher degradation rates in comparison to operation at an average constant current. A second non-ideal condition is “low cell voltage”; it is further known that drawing a peak current from the fuel cell also leads to increased degradation rates. It follows from the above that, in order to preserve the lifespan of a fuel cell, it is preferable to avoid both “high cell voltage” and “low cell voltage” operating conditions. In the case of commonly known types of PEMFCs, a reasonable upper safety limit for ensuring against the occurrence of high cell voltage should be set no higher than 0.90 volts, preferably no higher than 0.85 volts, and a lower safety limit for ensuring against low cell voltage should be set no lower than 0.65 volts, preferably no lower than 0.70 volts. In other words, the fuel cell should be operated only in the limited voltage range between 0.65 and 0.90 volts, preferably between 0.70 and 0.85 volts. 
         [0010]    Automotive applications are characterized by particularly abrupt changes of load power. For this reason, power supplies designed for automotive applications generally comprise an energy storage battery, such as an electrochemical battery or a super capacitor, associated with the fuel cell system. In this type of power supply (called hereafter a fuel cell/battery hybrid power supply) the battery can work as a buffer: supplying electric power when there is a peak in the load and, conversely, storing excess electric power in case of low or zero load conditions. 
         [0011]      FIGS. 2A and 2B  are two block diagrams respectively showing an active and a passive hybrid power supply. In a fuel cell/battery active hybrid power supply, the fuel cell system is connected to the load circuit through a DC/DC converter, and the battery is connected to the load circuit in parallel with the DC/DC converter as shown in  FIG. 2A . By controlling the gain of the DC/DC converter, it possible to actively adjust the power distribution within the hybrid power supply. A fuel cell/battery passive hybrid power supply is simpler. The fuel cell system and the battery are electrically connected directly in parallel as illustrated in  FIG. 2B . The drawback is that many operating variables of the power supply are uncontrolled. In particular, the current split between the fuel cell system and the battery is imposed by the internal impedance of each device. Furthermore, since the battery and the fuel cell system are directly connected, their voltages are always the same. 
         [0012]    In principle, the use of a fuel cell/battery hybrid power supply allows operating the fuel cells in the desired limited voltage range. However, once the battery is completely charged, it obviously ceases to be available for storing the excess electric power supplied by the fuel cells. Known solutions to this last problem are, disconnecting the fuel cell stack (particularly in the case of a passive hybrid), setting the gain of the DC/DC converter practically to zero (in the case of an active hybrid), or shutting down the fuel cells until the level of charge of the battery reaches a lower threshold. However, start-stop cycles also contribute to the degradation of performance of the fuel cell system, while disconnecting the fuel cell system without shutting it down requires the use of a resistive load to dissipate the energy produced by the stack. This amounts to a considerable waste of energy. 
       SUMMARY OF THE INVENTION 
       [0013]    It is accordingly an object of the present invention to provide a method for limiting the output voltage of a fuel cell/battery passive hybrid power supply operating in, or near, zero load conditions, to a desired limited voltage range adequate for the battery as well as for the fuel cell system, without having to disconnect, or shut down and restart the fuel cell system. 
         [0014]    The method of the present invention is defined by the appended claim  1 . 
         [0015]    According to the present invention, limiting the hydrogen and oxygen streams supplied to the fuel cell while actuating the hydrogen and oxygen recirculating pumps makes it possible to keep the output voltage below a predetermined maximum limit. According to the invention, the maximum limit is the lowest of either the maximum voltage limit of the battery or the maximum voltage limit of the fuel cell system (0.90 volts/cell). 
         [0016]    An advantage of the method of the present invention is that it allows adjusting the power distribution within the passive hybrid power supply, without the need for a variable gain DC/DC converter like the one used in active hybrid power supplies. In particular, the method of the present invention allows maintaining a low output voltage even in near-zero load conditions. More generally, the method of the present invention allows operating a fuel cell/battery passive hybrid power supply with the same overall efficiency as that of a more expensive, more complicated, heavier and larger active hybrid power supply. 
         [0017]    Furthermore, one will understand that, according to the present invention, by maintaining the hydrogen pressure between 70 and 130% of the oxygen pressure, the method of the invention avoids large pressure differences across the membrane of the fuel cells and, in the particular case of higher hydrogen pressure, avoids fuel starvation at the anode. 
         [0018]    Preferably, the method of the invention maintains the fuel cell voltage in a range corresponding to between 0.70 and 0.85 volts/cell. 
         [0019]    Another advantage of the method of the present invention is that it allows operating the hybrid power supply in zero net output load conditions even when the storage battery is completely charged. Indeed, by reducing the pressure of at least one of the reactants below 0.7 bar absolute  the output power of the hybrid power supply can be reduced to no more than what is necessary to power the auxiliaries (the parasitic load). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Other features and advantages of the present invention will appear upon reading the following description, given solely by way of non-limiting example, and made with reference to the annexed drawings, in which: 
           [0021]      FIG. 1  is an exploded view of a conventional fuel cell stack (prior art); 
           [0022]      FIGS. 2A and 2B  are two block diagrams respectively showing an active and a passive hybrid power supply at a conceptual level (prior art); 
           [0023]      FIG. 3  is a diagram showing a particular embodiment of a fuel cell/battery passive hybrid power supply comprising a fuel cell stack supplied with pure hydrogen and oxygen; 
           [0024]      FIG. 4  is a more detailed functional diagram of the passive hybrid power supply of  FIG. 3 ; 
           [0025]      FIG. 5A  is a diagram showing current/voltage curves for a polymer electrolyte fuel cell at different pressures; 
           [0026]      FIG. 5B  is a diagram showing maximum and minimum battery voltages as a function of the state of charge (SOC) of a storage battery; 
           [0027]      FIG. 6  is a diagram generally showing how the load can be shared between the fuel cell system and the storage battery, and particularly how the passive hybrid power supply makes it possible to dispense with shutting down the fuel cell system in zero connected load operating conditions, even if no energy storage capacity is available in the battery. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The fuel cell stack  1  of the passive hybrid power supply illustrated in  FIG. 3  is of a type designed to use hydrogen as fuel and pure oxygen as oxidizer. It includes end plates  130 ,  140 , a hydrogen inlet port  150  in end plate  130  and an oxygen inlet port  155  in end plate  140 . Stack  1  further includes a hydrogen supply manifold  160  and an oxygen supply manifold  165  respectively for supplying a hydrogen stream and an oxygen stream to a plurality of individual fuel cells. 
         [0029]    Hydrogen and oxygen flow fields associated with each fuel cell are represented by arrows  170  and  175 . A hydrogen exhaust manifold  180  and an oxygen exhaust manifold  185  remove the depleted reactants and the reaction products from the stack through a hydrogen outlet port  190  and an oxygen outlet port  195 . 
         [0030]    As illustrated, the fuel cell system comprises a pressurized hydrogen storage vessel  60  connected the hydrogen inlet  150  of the stack by means of a supply line equipped with a hydrogen supply valve  110  and an ejector pump  113 . A hydrogen pressure sensor  111  is arranged on the supply line near the hydrogen inlet  150  so as to measure the pressure of the hydrogen supplied to the fuel cell stack  1 . A first hydrogen recirculating line  11 R connects outlet port  190  of the stack to the hydrogen supply line, downstream of supply valve  110 . The ejector pump  113  provides for recirculating the leftover hydrogen and for mixing it with fresh hydrogen. 
         [0031]    In a similar way, the fuel cell system comprises a pressurized oxygen storage vessel  65  connected the oxygen inlet  155  of the stack by means of a oxygen supply line equipped with a oxygen supply valve  120  and a vacuum ejector pump  123 . An oxygen pressure sensor  121  is arranged on the supply line near the oxygen inlet  155  so as to measure the pressure of the oxygen supplied to the fuel cell stack  1 . An oxygen recirculating line  12 R connects outlet port  195  of the stack to the oxygen supply line, downstream of supply valve  120 . The ejector pump  123  (or any appropriate type of vacuum pump) provides for recirculating and for mixing the used oxygen with fresh oxygen. 
         [0032]    The stack of the fuel cell system shown in  FIG. 3  further comprises an auxiliary hydrogen inlet  200  and an auxiliary hydrogen outlet  210  connected to each other by a second hydrogen recirculating line  211 R. Line  211 R is equipped with an auxiliary hydrogen pump  213  provided for supplementing ejector pump  113 . The stack  1  also comprises an auxiliary oxygen inlet  205 , an auxiliary oxygen outlet  215  and an auxiliary oxygen pump  223 , arranged on a second oxygen recirculating line  212 R. Auxiliary pump  223  is provided for supplementing ejector pump  123 . 
         [0033]    The fuel cell system shown in  FIG. 3  further comprises moisture management means (not shown). As product water is formed on the cathode side of the fuel cells by the combination of hydrogen and oxygen ions, the product water must be drawn away from the cathode side of the fuel cells. In particular, in order to avoid flooding, the moisture management means usually comprise a gas-liquid separator arranged on the oxygen recirculating line  12 R. A second gas-liquid separator is preferably also arranged on the hydrogen recirculating line  11 R. At the same time, moisture must be provided to both the anode and the cathode side of the cells in amounts that will prevent the membrane drying out. 
         [0034]    As can further be seen in  FIG. 3 , the stack  1  is associated with a battery  18  connected in parallel in order to form a fuel cell/battery passive hybrid power supply for delivering electric energy to a load circuit  17 . Preferably, the storage battery  18  is a Li-Ion battery pack. However, according to other embodiments of the invention, any other form of storage battery could be used. Referring now to  FIG. 4 , the operating of the passive hybrid power supply of the present example will be explained in greater detail. As in  FIG. 3 , reference number  1  refers to the fuel cell stack, number  18  refers to the storage battery, and number  17  refers to the load circuit. 
         [0035]    As already explained, the fuel cell stack is part of a fuel cell system  14  comprising an oxygen circuit  52 , a hydrogen circuit  54 , and a cooling circuit  56 . The fuel cell system also includes a fuel cell controller  58  that manages the oxygen, hydrogen and cooling circuits. Apart from the pressure sensors (not shown in  FIG. 4 ) already described in relation to  FIG. 3 , the fuel cell system comprises a stack current sensor  61 , a stack temperature sensor  62  and at least one fuel cell voltage sensor  64 . The fuel cell controller  58  uses the data provided by all the sensors in order to manage the operating of fuel cell system. 
         [0036]    Still referring to  FIG. 4 , number  66  refers to a switch used to disconnect the fuel cell system  14  from the battery  18  and the load circuit  17 , number  67  is a battery current sensor, number  71  is a load current sensor, and number  13  is the battery voltage sensor also shown in  FIG. 3 . According to the present invention, the switch  66  is intended to be used only during start-up and shut-down of the fuel cell system. As previously mentioned, as long as the storage battery  18  and the fuel cell system  14  are connected, their voltages are identical. Therefore the stack voltage measured by the fuel cell voltage sensor  64  and the battery voltage measured by the battery voltage sensor  13  are always the same as long as switch  66  is closed. 
         [0037]    Still referring to  FIG. 4 , one can see that the load circuit  17  is made up of an electric machine  73  that is intended to work as a motor during phases of traction an to work as a generator during phases of regenerative braking. Furthermore, number  75  refers to a motor current sensor, number  77  refers to a motor voltage sensor, number  79  refers to a motor controller, and number  81  refers to a power converter. Depending on the type of electric machine used, the type of converter used for the power converter  81  can vary. If for instance the motor  73  is a DC brushless motor controlled by pulse width modulation, the power converter  81  will be a DC/DC converter supplying a constant output voltage. In contrast, if for example the electric machine  73  is a synchronous motor, the power converter  81  will be a DC/AC converter.  FIG. 4  also shows a power management controller  85  that controls the fuel cell controller  58 , the motor controller  79 , as well as the switch  66 . The power management controller  85  regulates the circulation of power as a function of the position of an accelerator pedal of a vehicle (not shown) and as a function of conditions prevailing in the electricity supply system. 
         [0038]    The fuel cell system  14  is controlled by the fuel cell controller  58 . Controller  58  receives information from the hydrogen pressure sensor  111  ( FIG. 3 ) and the oxygen pressure sensor  121  ( FIG. 3 ), as well as from the fuel cell voltage sensor  64 . According to the illustrated example, the fuel cell voltage sensor measures the output voltage from the fuel cell stack  1  as a whole. Thus the measured output voltage amounts the sum of the contributions from all the individual fuel cells in the stack. As the fuel cells are all subjected to substantially the same operating conditions, they all produce approximately the same output voltage. Therefore, the measured output voltage of the stack can be used to calculate an estimated voltage for an individual fuel cell. However, it is also possible to measure the output voltages of the individual cells separately, or else to divide the individual cells of the stack into several groups, each having an output voltage. 
         [0039]    The fuel cell controller  58  ( FIG. 4 ) controls the pressure of both the hydrogen and the oxygen supplied to the fuel cell stack by adjusting the hydrogen and oxygen supply valves  110 ,  120  and, if necessary, by directly controlling the operation of the auxiliary recirculating pumps  213 ,  223 . The process that allows fuel cell controller  58  to control the reactant pressure in the fuel cells will now be explained in detail. The reactants are consumed in the fuel cells at a rate corresponding to the amount of electric current supplied by the stack  1 . When, in the absence of a change of load, fuel cell controller adjusts one of the supply valves  110 ,  120  towards the open position, the supplied stream of hydrogen or of oxygen increases and exceeds the amount of hydrogen or of oxygen consumed in the fuel cells. This causes the pressure at the anode or the cathode of the fuel cells to increase also. In contrast, when fuel cell controller  58  adjusts one of the supply valves  110 ,  120  towards the closed position, the supplied stream of hydrogen or of oxygen decreases and ceases to be enough to compensate for the amount of hydrogen or of oxygen consumed in the fuel cells. This causes the pressure at the anode or the cathode of the fuel cells to decrease. As previously mentioned, according to the present invention the hydrogen and the oxygen supplied to the fuel cell stack are substantially pure hydrogen and substantially pure oxygen respectively. This feature allows the hydrogen and the oxygen present in the fuel cell to be almost entirely consumed. It is thus possible for the pressure at the cathode and at the anode of the fuel cell to decrease far bellow the external atmospheric pressure, approximately down to the water vapor pressure. Therefore, in the case of a fuel cell stack operating at a temperature of approximately 60° C., the pressure can go as low as 0.2 bar absolute . 
         [0040]    Care is taken that the hydrogen pressure is at least 70% of the oxygen pressure, preferably at least 100% of the oxygen pressure, so as not to induce in the fuel cells the condition known as “fuel starvation”. Fuel starvation, if more than momentary, is known to deteriorate fuel cells. However, other operating conditions wherein the hydrogen pressure is less than 100% of the oxygen pressure can also be advantageous, in particular in the case when it is desirable to increase the water content of the membrane. Furthermore, in order to avoid the appearance of a large pressure difference between the anode and the cathode of the fuel cells, the hydrogen pressure is preferably adjusted to follow the oxygen pressure. At any rate, the hydrogen pressure is confined in a range between +/−30% of the oxygen pressure. 
         [0041]      FIG. 5A  is a diagram showing polarization curves (current/voltage curves referenced  251  to  256 ) for a polymer electrolyte fuel cell operating at a temperature of approximately 60° C. and at six different pressures (2.5 bar abs , 1.5 bar abs , 1 bar abs , 0.62 bar abs , 0.4 bar abs , 0.22 bar abs ).  FIG. 5A  shows that for a constant operating voltage of the fuel cell (or in other words for a constant voltage of the associated storage battery) the current changes considerably with the pressure, thus allowing the power delivered by the fuel cell to be adjusted. In fact, it can be calculated from the curves of  FIG. 5A  that, for a constant operating voltage of 0.85 volts, output power is reduced by almost a factor 10 when the stack is operated at a pressure of 0.4 bar instead of 2.5 bar. This example illustrates one of the advantages of using a fuel cell system supplied with substantially pure oxygen gas instead of air. Indeed, air is a nitrogen rich gas and the presence of the nitrogen makes achieving operating pressures substantially below ambient considerably more difficult. 
         [0042]      FIG. 5B  is a diagram showing the minimum and maximum storage battery voltages as function of the state of charge (SOC) of the battery. As is well known, the closed circuit voltage of a battery in a given SOC is determined both by its open circuit voltage (OCV) and by the voltage loss due to the flow of current through the battery.  FIG. 5B  illustrates that the maximum allowable voltage for a battery in a given SOC is determined by adding the voltage loss associated with the maximum allowable charge current to the OCV. In an equivalent manner, the minimum allowable voltage is determined by deducting the voltage loss associated with the maximum allowable discharge current from the OCV. Naturally, the maximum charge and discharge currents are both a function of the SOC as well. In particular, when the SOC is 100% of the maximum usable charge, the maximum allowable charge current is zero, and when the SOC is 0% of the maximum usable charge, the maximum allowable discharge current is zero. The shaded area in  FIG. 5B  corresponds to the allowable battery operating area. 
         [0043]    As previously stated, since the storage battery and the fuel cell stack are directly connected, their voltages are the same. Therefore, if the output voltage of the fuel-cell stack is above the OCV of the battery and the output voltage further increases, the charge current supplied by the stack to the battery increases as well. Conversely, if the output voltage from the stack is below the OCV of the battery and the output voltage further decreases, the discharge current supplied by the battery is caused to increase. In other words, the storage battery acts as a buffer to limit variations in the total load power connected to the stack. One will understand that, since the storage battery and the stack share the same voltage, the size of the storage battery should be chosen so that its OCV corresponds to an average fuel cell voltage that lies within the interval between the previously mentioned upper and lower safety limits. In the present example, the safety limits for ensuring against high and low cell voltages are 0.90 volts and 0.65 volts respectively. Preferably, the average fuel cell voltage corresponding to the OCV should remain between said upper and lower safety limits for any allowable SOC of the battery; that is for any SOC of the battery in the interval between the SOC corresponding to 0% of the maximum usable charge, to the SOC corresponding to 100% of the maximum usable charge, according to the specifications of the storage battery. 
         [0044]    The fuel cell controller  58  is arranged to reduce the pressure of the reactant gases supplied to the fuel cell stack by partially or completely closing the hydrogen and oxygen supply valves  110 ,  120 . However, if either of the supply valves  110  or  120  is entirely or nearly closed, the corresponding ejector pump  113  or  123  becomes useless, and the flow of used gas trough the recirculating line  11 R or  12 R comes to a standstill. In such a situation, the pressures in the supply ( 160  or  165 ) and the exhaust ( 180  or  185 ) manifolds tend to equalize, and the pressure drop needed for driving reactant gas along the flow fields  170  or  175  disappears. In order to allow the fuel cell stack to continue to operate even when the supply valve  110  or  120  is closed, control unit  15  turns on the corresponding auxiliary pump  213  or  223 . When either of pumps  213  or  223  is operating, it reinjects leftover reactant gas present in the exhaust manifold  180  or  185  into the corresponding supply manifold  160  and  165 . The use of auxiliary pumps  213  and  223  allows to maintain the necessary pressure difference between supply and exhaust manifolds. 
         [0045]    As described above, the fuel cell system of the passive hybrid power supply in which the method of the present invention is implemented, comprises electronic controls, supply valves  110 ,  120  controlled by the fuel cell controller  58 , pumps  213 ,  223 , and a gas-liquid separator. The fuel cell system also comprises a cooling circuit  56  using water pumps, and it can possibly comprise electrical heating means as well. All these elements, and others, form what are called auxiliaries. These auxiliaries need electricity to operate and constitute what is generally referred to as the parasitic load of the fuel cell system. Therefore, when the fuel cell system is working, the power demand is never zero, even when in an idling state (i.e. when in zero connected load operating conditions). In the present example, a realistic number for the parasitic load power is approximately 600 Watts. 
         [0046]      FIG. 6  is a diagram showing how the load can be shared between the fuel cell system and the storage battery. The horizontal lines in the center of the diagram are iso-power lines of a fully charged battery (SOC=100%). The nearly vertical thin lines are iso-power lines of the fuel cell stack. The nearly vertical bold line at the left of the diagram is an iso-power line corresponding to 600 Watts (the parasitic load power).  FIG. 6  shows that, by controlling the reactant pressure in the fuel cells, it is possible to cope with low output load conditions while avoiding high cell voltage, even when the state of charge of the storage battery is already 100%. Indeed, The diagram shows that the OCV for the fully charge battery corresponds to 0.85 volts/cell. If the pressure in the fuel cell is reduced to 0.5 bar while maintaining the fuel cell voltage constant, the amount of power produced by the fuel cell stack is reduced to about 300 Watts. In this case another 300 Watts must be drawn from the storage battery to satisfy the demand of the auxiliaries. Another possibility is to slightly increase the pressure until the stack produces 600 Watts of electric power. 
         [0047]    Still referring to  FIG. 6 , one will notice that the operating point at the intersection of the 2.5 bar line with the 600 Watts iso-power line corresponds to a fuel cell voltage of almost 1 volt. In other words, it is not possible to avoid high cell voltage without lowering the operating pressure in a passive hybrid power supply. In other words,  FIG. 6  illustrates how the present invention makes it possible to dispense with shutting down the fuel cell system in zero connected load operating conditions, even if no energy storage capacity is available in the battery.