Abstract:
The present invention provides a power management and protection system for a hybrid fuel cell system. The hybridization includes a fuel cell stack and an energy storage device (ESD) comprised of either batteries or ultracapacitors or both in parallel with the fuel cell for delivering power to an electrical load. The power management system provides voltage and current protection to the fuel cell stack, the ESD and the load by use of a two stage control system. The first stage limits the current being drawn from the fuel cell stack and the charging rate of the ESD and provides for the voltage output to be within a adjustable predetermined range to prevent an over-voltage condition on the ESD and the load and an under-voltage condition in the fuel cell stack. The second stage limits the current delivered to the load to an adjustable predetermined level and assures that the load will not see an under-voltage condition and in the case of a short circuit prevents rapid discharge of the ESD.

Description:
BACKGROUND  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to energy systems that include fuel cells and other primary power devices.  
         [0003]     2. General Background  
         [0004]     Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy without combustion and without harmful emissions. The basic physical structure, or building block, of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in  FIG. 1 .  
         [0005]     In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode); the electrochemical reactions take place at the electrodes to produce an electric current, water and heat. A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). In a secondary battery, the reactants are regenerated by recharging, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device that theoretically has the capability of producing electrical energy for as long as fuel and oxidant are supplied to the electrodes.  
         [0006]     Individual fuel cells are typically connected in a series arrangement or stack in order to increase the overall potential and power output. The voltage and current output and therefore the power output of a fuel cell system depend on the number of cells in the stack, total active surface area and efficiency  
         [0007]     A common type of fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water, and thus corrosion problems are minimal. Because of the limitation on the operating temperature imposed by the polymer, usually less than 100 C, a H 2 -rich fuel is used. If a fuel cell is compared to an equivalent efficiency heat engine, the fuel cell does not need to achieve the large temperature differential to achieve the same Carnot cycle efficiency as the heat engine. This is because of the added energy gained from Gibbs free energy as opposed to simply the thermal energy. The resulting freedom from large temperature differentials in the fuel cell provides a great benefit because it relaxes material temperature problems when trying to achieve comparable efficiency.  
         [0008]     The actual cell potential is decreased from its ideal equilibrium potential because of irreversible losses, as shown in  FIG. 2 . Multiple phenomena contribute to irreversible losses in an actual fuel cell. The losses, which are called polarization, overpotential, or overvoltage, originate primarily from three sources: 1) activation polarization, 2) ohmic polarization, and 3) concentration polarization. These losses result in a cell voltage (V) that is less than its ideal potential, E (V=E−Losses). The activation polarization loss is dominant at low current density. At this point, electronic barriers must be overcome prior to current and ion flow. Activation losses increase as current increases. Ohmic polarization (loss) varies directly with current, increasing over the entire range of current because cell resistance remains essentially constant. Gas transport losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult to provide enough at high limiting currents where it becomes difficult to provide enough reactant flow to the cell reaction sites.  
         [0009]     The fuel cell will normally operate in the linear portion of the curve shown in  FIG. 2 . Consequently the cell potential varies as the load changes. In the example of  FIG. 2  the linear portion encompasses a potential range of about 0.5 volts to about 0.9 volts and the open circuit potential is even higher. While variation of 0.4 volts may not appear to be a large fluctuation it can have significant impact on the operation of a stack of fuel cells. As an example, assume a stack of 20 cells performing as the curve of  FIG. 2  shows. Accordingly, this stack will have an output potential of 10 to 18 volts and may have an open circuit potential of over 20 volts.  
         [0010]     Much modern electronic equipment that operate at a nominal 12 volts DC will not function when presented with a voltage outside a narrow range, e.g., 11-14 volts. Some equipment, when sensing a high potential at the power input will not turn on for safety and self protection reasons. Similarly, if a low potential is sensed at the power input the equipment will not turn on or shut itself down if the potential drops below the minimum level during operation.  
         [0011]     While a fuel cell stack can be sized for an expected load range, there can be times in which a load fluctuation can result in a higher current than expected such as start transients. In this situation, as the current increases the stack potential is reduced and may be reduced below what is required for the equipment load to operate. Another circumstance is an extremely high load condition or short circuit. In this case the cell voltage can be driven close to zero and may result in damage to the cell if such a condition persists.  
         [0012]     In the past, efforts have been made to mitigate voltage fluctuations and other variant conditions by using energy storage devices (ESDs), such as batteries or ultracapacitors (also called supercapacitors) across the output of the fuel cell and in parallel with the load, thereby creating a hybrid fuel cell/ESD system. Such hybrid systems can provide greater stability than fuel cell only systems, but the prior art systems have not provided adequate protection to the fuel cell, the ESD, and the load from voltage and current variations in the system. Therefore, there is a need for a hybrid fuel cell/ESD energy system that minimizes the potential for damage to its components from adverse voltage and current conditions.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is a power management system  10  comprising (i) a stack of fuel cells  20 , (ii) an energy storage device  30  (ESD), (iii) first protection circuitry  40 , and (iv) second protection circuitry  50 . The first protection circuitry  40  may act as an input current limiter (by limiting the current drawn into the system from the fuel cell) and a voltage current limiter (by limiting the voltage applied to the ESD and to the load). The second protection circuitry  50  may act as an output current limiter, by limiting the current from the system to the load. The present invention can protect against damage to the load, fuel cell stack, and ESD caused by fluctuations in voltage and current. Also, since it has an ESD, the system according to the present invention extends the optimal operational range for a limited period of time in the case of load spikes. 
     
    
     DESCRIPTION OF THE FIGURES  
       [0014]      FIG. 1  illustrates a typical fuel cell.  
         [0015]      FIG. 2  is a chart showing the relationship between current density and cell voltage for a typical fuel cell.  
         [0016]      FIG. 3  is a block diagram of a power management system according to an embodiment of the present invention.  
         [0017]      FIG. 4  is a schematic of a power management system according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0018]     The power management system  10  of the present invention has (i) a stack of fuel cells  20 , (ii) an energy storage device  30  (ESD), (iii) first protection circuitry  40 , and (iv) second protection circuitry  50 . See  FIG. 3 . The invention provides protection to the fuel cell stack  20 , the ESD  30 , and a load  60 , and also provides improved consistency and robustness of operation over conventional systems.  
         [0000]     Fuel Cell Stack  
         [0019]     The fuel cell stack  20  can be virtually any fuel cell assembly, including but not limited to a PEM fuel cell stack. In one embodiment, the fuel cell stack  20  is capable of continuously delivering a nominal 50 watts of power and operates according to the curve shown in  FIG. 2 , i.e., a single cell potential of 0.5 to 0.9 volts. The fuel cell in this example has an operational range of 10 to 18 volts with a nominal output at 50% efficiency of 12 volts at 4.2 amps. As shown in  FIG. 4 , the fuel cell stack  20  may be coupled to the first protection circuitry  40 , the ESD  30 , and the load  60  through the second protection circuitry  50 . The fuel cell stack  20  produces a stack voltage V stack  across a bus  12   a ,  12   b . The stack current I stack  flows to the load  60  from the fuel cell stack  20  via the bus  12   a ,  12   b . In lieu of the fuel cell stack  20 , other primary power devices, such as batteries, generators, or solar or other alternative power generating devices can be used.  
         [0000]     ESD  
         [0020]     The ESD  30  may be a battery, ultracapacitor (also known as a supercapacitor), or other energy storage device. In one embodiment, the ESD may have a capacity of 14 V. As shown in  FIG. 4 , the ESD  30  may be electrically coupled in parallel with the fuel cell stack  20  across the bus  12   a ,  12   b  to power the load  60 . The ESD is also coupled to the first protection circuitry  40  and the second protection circuitry  50 . The open circuit voltage of the ESD  30  is selected to be similar to the full load voltage of the fuel cell stack  20 .  
         [0021]     The ESD  30  allows the system to accommodate load fluctuations. The ESD  30  acts as a buffer, absorbing excess current when the fuel cell stack  20  produces more current than the load  60  requires until the ESD is fully charged, and providing current to the load  60  when the fuel cell stack  20  produces less current than the load  60  requires.  
         [0000]     First Protection Circuitry  
         [0022]     In one embodiment, the first protection circuitry  40  may comprise a first series element  42 . This first series element  42  may serve two functions. First, it may act as an input current limiter, by limiting the current I stack  drawn into the the ESD  30  and the second series element  52  from the fuel cell  20 . In one embodiment, it may limit the current so that it is no greater than 5 amperes.  
         [0023]     Second, the first series element  42  may act as a voltage limiter, by limiting and controlling the voltage to the ESD  30 . In one embodiment, when acting as a voltage limiter, the first series element  42  will not deliver power to a load if the fuel cell potential is under 10 volts and does not allow the output voltage to the ESD to exceed 14 volts. In addition, the first series element controls the potential applied V ESD  to the ESD  30  and the second series element  52 .  
         [0024]     These two features—input current limiting and voltage protection—are particularly important when ultracapacitors are used. When such capacitors are completely discharged they initially appear as a short circuit, and can draw large amounts of current. The input current limiter prevents the capacitors from drawing a high current from the fuel cell, and the voltage limiter will not allow the charging of the capacitors until the fuel cell can deliver at least 10 volts. Also, the voltage limiter sets the maximum voltage seen by the capacitors to prevent their overcharging to an excessively high voltage, which can damage the capacitors.  
         [0025]     Although the first series element  42  can act as both the input current limiter and the voltage limiter, the present invention also includes embodiments in which the input current limiting and voltage limiting functions are distributed to more than one component.  
         [0026]     The first series element  42  is electrically connected between the fuel cell stack  20  and ESD  30  and also electrically connected between the fuel cell stack  20  and the second series element  52 .  
         [0027]     The first series element  42  can take the form of a field effect transistor (“FET”); an example being International Rectifier&#39;s IRL7833/S/L family of power metal oxide semiconductor field effect transistors (“MOSFET”) having a drain and source electrically coupled between the fuel cell stack  10  and the ESD  30  and having a gate electrically coupled to an output of the first regulating circuit  44 .  
         [0028]     The first protection circuitry  40  also contains a first regulating circuit  44  coupled to the series element  42  to control the series element  42 , via a control signal, based on various operating parameters of the fuel cell system  10 . See  FIG. 4   
         [0029]     A number of sensors work with the first regulating circuit  44 . A first low threshold voltage sensor  45  is used to sense the output voltage V stack  of the fuel cell stack  20 . If the stack voltage V stack  is below a predetermined error level, the voltage applied to the first protection circuitry  40  through the first threshold voltage sensor  45  will be below the threshold error level of the first regulating circuit  44 , and the first regulating circuit  44  will cause the series element  42  to close.  
         [0030]     Additionally, a stack current sensor  46  is used to sense the current I stack  being delivered by the fuel cell stack  20 . If the stack current I stack  is above an adjustable predetermined error level the control circuit  44  causes a reduction of conduction through the series element  42 .  
         [0031]     Next, at the output of the series element  42  a high voltage error sensor  47  and a second low threshold voltage error sensor  48  are placed. If the high voltage error sensor  47  indicates that the voltage being applied to the ESD V ESD  is in excess of an adjustable predetermined error level, the first regulating circuit  44  will cause the series element  42  to cease conduction. If the second low threshold voltage sensor  48  indicates that the ESD voltage V ESD  is below a predetermined adjustable error level, such as caused by a short circuit, the first regulating circuit  44  will cause the series element  42  to reduce conduction.  
         [0032]     A reverse current blocking diode  49  can be placed between the series element  42  and the ESD  30  to prevent current from flowing from the ESD  30  back through the series element  42  and to the fuel cell stack  20 .  
         [0033]     The first protection circuitry  40  can take the form of Linear Technology Corporation&#39;s LT1641-1/-2 family of controllers along with additional support circuitry.  
         [0000]     Second Protection Circuitry  
         [0034]     Like the first protection circuitry  40 , the second protection circuitry  50  comprises a series element, namely the second series element  52 . The second protection circuitry  50  acts as an output current limiter, by preventing the drawing of excessive current from both the fuel cell  20  and the ESD  30  by a higher than specified load. This could be in the form of an improperly applied load, a load spike, or a shorted output. If the output current limiter detects excessive current being drawn, the limiter close the current source to protect the fuel cell stack  20  and to prevent the ESD  30  from delivering excessive current.  
         [0035]     The second series element  52  is electrically connected between the ESD  30  and the load  60 , and also electrically connected between the first series element  42  and the load  60 . The second series element  52 , acting as the output current limiter controls the flow of current I load  from the ESD  30  and the first series element  42  to the load  60 . See  FIG. 4 . In addition, the second series element controls the potential applied V load  to the load  60 .  
         [0036]     The second series element  52  can take the form of a field effect transistor (“FET”); an example being International Rectifier&#39;s IRL7833/S/L family of power metal oxide semiconductor field effect transistors (“MOSFET”) having a drain and source electrically coupled between the ESD  30  and the load  60  and having a gate electrically coupled to an output of the second regulating circuit  54 .  
         [0037]     The second protection circuitry  50  also contains a second regulating circuit  54  coupled to the series element  52  to control the second series element  52 , via a control signal, based on various operating parameters of the fuel cell system  10 . See  FIG. 4 .  
         [0038]     A number of sensors work with the second regulating circuit  54 . For instance, in one embodiment, a first threshold voltage error sensor  55  is used to sense the voltage V ESD  across the ESD  30 . If the ESD  30  voltage V ESD  is below an adjustable predetermined error level, the voltage applied to the second protection circuitry  50 , will be below the threshold voltage level of the second regulating circuit  54 , and therefore the second regulating circuit  54  will cause the second series element  52  not to conduct.  
         [0039]     Additionally, a load current error sensor  56  may be used to sense the current I load  being drawn by the load  60 . If the load current I load  is above an adjustable predetermined error level the second regulating circuit  54  causes a reduction of conduction through the second series element  52 .  
         [0040]     At the output of the second series element  52  is a second low threshold voltage error sensor  57 . If the load voltage V load  is below an adjustable predetermined error level or if there is a short circuit, the control circuitry will cause a reduction of conduction through the second series element  52 .  
         [0041]     The voltage sensors of the above discussion may be in the form of voltage divider networks or other circuitry well known to those skilled in the art.  
         [0042]     The regulating circuits of the above embodiments may take the form of one or more microprocessors which are programmed to regulate the current through the series element.  
         [0043]     The second stage control circuitry  50  can take the form of Linear Technology Corporation&#39;s LT1641-1/-2 family of controllers along with additional support circuitry.  
         [0000]     Load  
         [0044]     The load  60  may include the work load  62  and other external circuitry  64 , such as an inverter or DC/DC converter or other power conditioning circuitry. See  FIG. 4 .  
         [0000]     Operation and Benefit of Present Invention  
         [0045]     As described above, the present invention can protect against damage to the load, fuel cell stack, and ESD caused by fluctuations in voltage and current. Also, since it has an ESD, the system according to the present invention the present invention extends the optimal operational range for a limited period of time in the case of load spikes.  
         [0046]     Although specific embodiments of, and examples for, the power management system and method are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. For example, the teachings provided herein can be applied to fuel cell systems  10  including other types of fuel cell stacks  20  or fuel cell assemblies, not necessarily the PEM fuel cell assembly generally described above. Additionally, the fuel cell system  10  can make use of digital circuitry such as microprocessors to monitor and control the various fuel cell system parameters. The various embodiments described above can be combined to provide further embodiments.