Abstract:
Systems and methods to control the delivery of coolant to a coolant loop within a vehicular fuel cell system. During periods of low power output from one or more fuel cell stacks, operation of a pump used to circulate coolant through the loop is intermittent, thereby reducing pump usage during such times. The frequency of pump operation, as measured by a pump on/off (i.e., pulsed) cycle, may be adjusted to keep a local temperature rise within the one or more stacks to no more than a small amount over the bulk stack temperature.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to controlling a pump in a fuel cell system, and more particularly to systems and methods for pulsing the flow of coolant to a fuel cell stack in order to reduce parasitic power consumption while limiting stack temperature differential at low stack power levels. 
         [0002]    Fuel cells—as an alternative to using gasoline or related petroleum-based sources as the primary source of energy in vehicular propulsion systems —operate by electrochemically combining reactants. In a representative fuel cell, one of the reactants is typically hydrogen-based and supplied to the anode of the fuel cell, where it is catalytically broken down into electrons and positively charged ions. A proton-conductive electrolyte membrane separates the anode from the cathode and allows the ions to pass to the cathode. The generated electrons form an electric current that is routed around the electrolyte layer through an electrically-conductive circuit that includes a motor or related load such that useful work is produced. The ions, electrons, and supplied oxygen (often in the form of ambient air) are combined at the cathode to produce water and heat. In one automotive form, the motor being powered by the electric current may propel the vehicle, either alone or in conjunction with a petroleum-based combustion engine. Individual fuel cells may be arranged in series or parallel as a fuel cell stack in order to produce a higher voltage or current yield. Furthermore, still higher yields may be achieved by combining more than one stack. 
         [0003]    The heat generated by the reactions in the fuel cell system must be regulated in order to provide efficient system operation, as well as keep the temperature of the system components within their design limits. To accomplish the regulation of heat, coolant flow fields are set up adjacent the reactant flow fields such that a coolant being pumped through the coolant flow fields conveys away excess heat present in the reaction. From there, the coolant is routed to a radiator or other appropriate heat sink to allow the heat to be dissipated. 
         [0004]    It is more challenging to control the speed of the pump used to circulate the coolant during a low power state. For example, continuous pump operation in a low-load stack condition necessitates significant consumption of the electric current produced by the fuel cell, thereby significantly impacting overall system efficiency. The limited ability of the coolant pump to turn down relative to the fuel cell system (where, for example, the fuel cell system will turn down more than 100 to 1 while the pump will only turn down 5 to 1) further hampers the ability of the coolant system to control temperature differences through the stack at such low power levels. In the present context, the ability of equipment to turn down (also referred to herein as “turndown ratio”), is a measure of the pump&#39;s maximum coolant flowrate relative to its minimum coolant flowrate. Similarly, the fuel cell system&#39;s turndown can be defined as its rated maximum power relative to its minimum power. Since the fuel cell stack&#39;s waste heat has a slightly superlinear scale with system power, the fact that the system can turn down beyond the coolant pump means that the coolant pump provides much more coolant flow than is needed to adequately cool the stack and maintain reasonable coolant temperature differences from the inlet and outlet of the stack. Unfortunately, such excess pump capacity leads to operational inefficiencies of the fuel cell system. 
       SUMMARY OF THE INVENTION 
       [0005]    In a first embodiment of the invention, a method of controlling a coolant pump in a fuel cell system is disclosed. In one particular form, the present invention allows effective turn down ratios greater than 5 to 1 to be better responsive to the turn down ratio of the stack or other part of the fuel cell system. While the method is particularly well-suited for use in vehicular applications, it will be appreciated by those skilled in the art that non-vehicular fuel cell applications employing the present invention are also within the scope of the present invention. The method includes determining whether a stack power request for a fuel cell stack is below a first threshold value. As such, the method is particularly configured for low power operational conditions. The method also includes utilizing the stack power request—when it is below the first threshold value —to determine an off time value for a coolant pump that provides coolant to the fuel stack. The method further includes generating, by a processor, a coolant pump control command that causes the coolant pump to selectively provide coolant to the fuel stack such that during the off time, the pump ceases to provide coolant to the fuel stack, while during an on time, the pump is operated to provide coolant. In this way, the delivery of the coolant takes place in a pulsed fashion. Of special significance is that the pump pulsing of the present invention is based on a determination of a pulsing frequency that limits the localized temperature rise of any part within the fuel cell stack to a small amount above the average system temperature within the fuel cell stack. In one form, the maximum permissible local temperature rise is a few degrees, for example, about 3° C. Significantly, during pulsed pump operation, there is a minimum time that the coolant pump must run while in an “on” condition in order to remove the heat produced by the fuel cell stack during the periods where the pump was off. In one form, a typical time is between about 3 and 10 seconds, and is dependent on the thermal mass of the stack and the flowfield design. Likewise, the maximum permissible local temperature rise mentioned above may vary depending on other factors (such as humidification). As such (and depending on variations in such factors), there may be a wider range of acceptable temperatures, for example from 1° C. to 7° C. 
         [0006]    In another embodiment, a controller for a fuel cell system is disclosed. The controller includes one or more processors and a non-transitory memory in communication with the one or more processors. The memory stores instructions that, when executed by the one or more processors, cause the one or more processors to determine whether a stack power request for a fuel cell stack is below a first threshold value. The instructions further cause the one or more processors to utilize the stack power request to determine an off time value for a coolant pump that provides coolant to the fuel stack. The instructions additionally cause the one or more processors to generate a coolant pump control command that causes the coolant pump to stop providing coolant to the fuel stack during the off time and to provide coolant to the fuel stack during an on time, if the stack power request is below the first threshold value. 
         [0007]    In yet another embodiment, a fuel cell system is disclosed that includes a fuel cell stack, a pump for delivery of a coolant through the fuel cell stack and a pump controller comprising one or more processors and a non-transitory memory in communication with the one or more processors. The memory stores instructions that, when executed by the one or more processors, cause the one or more processors to determine whether a stack power request for a fuel cell stack is below a first threshold value. The instructions also cause the one or more processors to utilize the stack power request to determine an off time value for a coolant pump that provides coolant to the fuel stack. The instructions further cause the one or more processors to generate a coolant pump control command that causes the coolant pump to stop providing coolant to the fuel stack during the off time and to provide coolant to the fuel stack during an on time, if the stack power request is below the first threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The following detailed description of specific embodiments can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
           [0009]      FIG. 1  is an illustration of a vehicle having a fuel cell system; 
           [0010]      FIG. 2  is a schematic illustration of the fuel cell system shown in  FIG. 1 ; 
           [0011]      FIG. 3  shows a the pulsation and pulsating frequency of a coolant pump used in the fuel cell system of  FIG. 2 ; and 
           [0012]      FIG. 4  is a flow chart showing the decisions made in order to determine pulsing operation for the coolant pump of  FIG. 2 . 
       
    
    
       [0013]    The embodiments set forth in the drawings are illustrative in nature and are not intended to be limiting of the embodiments defined by the claims. Moreover, individual aspects of the drawings and the embodiments will be more fully apparent and understood in view of the detailed description that follows. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring first to  FIG. 1 , vehicle  10  is shown, according to embodiments shown and described herein. It will be appreciated by those skilled in the art that while vehicle  10  is presently shown configured as a car, it may also include bus, truck, motorcycle or related configurations. Vehicle  10  includes engine  50 , which may be a fully electric or a hybrid electric engine (e.g., an engine that uses both electricity and petroleum-based combustion for propulsion purposes). A fuel cell system  100  that includes at least one stack  105  of individual fuel cells may be used to provide at least a portion of the electric power needs of engine  50 . In a preferred form, the fuel cell system  100  is a hydrogen-based one that may include one or more hydrogen storage tanks (not shown), as well as any number of valves, compressors, tubing, temperature regulators, electrical storage devices (e.g., batteries, ultra-capacitors or the like), and controllers that provide control over its operation. 
         [0015]    Any number of different types of fuel cells may be used to make up the stack  105  of the fuel cell system  100 ; these cells may be of the metal hydride, alkaline, electrogalvanic, or other variants. In one preferred (although not necessary) form, the fuel cells are polymer electrolyte membrane (also called proton exchange membrane, in either event, PEM) fuel cells. Stack  105  includes multiple such fuel cells  105 A-N combined in series and/or parallel in order to produce a higher voltage and/or current yield. The produced electrical power may then be supplied directly to engine  50  or stored within an electrical storage device for later use by vehicle  10 . 
         [0016]    Referring now to  FIG. 2 , a schematic illustration of fuel cell system  100  is shown, according to embodiments shown and described herein. The fuel cell system  100  includes a fuel cell stack  105  that includes an inlet cooling fluid manifold  110  and an outlet cooling fluid manifold  115  fluidly coupled to one another by cooling fluid flow channels  120 . Coolant pump  125  circulates a cooling fluid through a substantially closed-circuit coolant loop  130 , where a radiator  135  removes heat from the cooling fluid by exchanging it with a suitable heat sink (indicated by the arrows). Controller  140  regulates the speed of the pump  125 , as well as the opening and closing of one or more valves  145  so that during normal operation of fuel cell stack  105 , it is maintained at a desirable operating temperature (for example, approximately 80° C.). One or more temperature sensors  150  may be used to measure the temperature of the cooling fluid in various locations within the coolant loop  130 . The measured signals may be sent to the controller  140  for subsequent processing or decision-making. The coolant loop  130  uses valve  145  (presently shown as a three-way valve) to include a parallel loop with the radiator  135  such that valve  145  controls what goes into the radiator  135  and what bypasses while never preventing coolant flow into the stack  105 . Significantly, because coolant pump  125  is a variable speed pump, there is no need for a separate valve to control the coolant flowrate. 
         [0017]    Other parts of the fuel cell system  100  include a cathode compressor  155  that is configured to pressurize reactant air and deliver it to the cathode side  160  of stack  105 , while the reactant fuel (such as hydrogen) is delivered to the anode side  165  of stack  105 . Exhaust gases and/or liquids are then removed from stack  105  to be discharged. A number of other valves, such as bypass valve  170 , recirculation valve  175  and backpressure valve  180 , may be included for other system features. For example, bypass valve  170  may be used to dilute the hydrogen left in the cathode of stack  105  that is introduced for catalytic heating. In this way, it is possible to reduce the hydrogen concentration (such as during stack warm-up), as well as for voltage suppression to let compressor  155  sink the stack load. More particularly, the bypass valve  170  can achieve this dilution of the excess hydrogen coming out of the stack  105  by introducing fresh air to the outlet of the cathode side  160  of the stack  105 . As mentioned above, one scenario where such excess hydrogen may be present is that associated with post-shutdown from a previous operation, where the hydrogen that crossed over the various fuel cell membranes remains in the stack until the subsequent start (where the fuel cell system  100  will then open the bypass valve  170  to permit the hydrogen diffusion). The bypass valve  170  may also be used with catalytic heating in case the stack  105  does not convert all the hydrogen to water and the outlet stream needs fresh air to dilute the hydrogen. Likewise, bypass valve  170  may be used by the fuel cell system  100  to bypass air in situations where too much air may otherwise go through the stack  105  that could cause excessive drying out of the fuel cell membranes. For simplicity,  FIG. 2  shows only a cathode and coolant loop, although it will be appreciated by those skilled in the art that a comparable anode loop may also be present that may be configured to operate, mutatis mutandis, in a generally comparable manner. 
         [0018]    Unlike a system where pulsing of coolant pump  125  may be employed to clear gas bubbles in a reactant or coolant flowpath (such as coolant loop  130 ) as a way to prevent localized hot spots, the present invention (in its emphasis on coolant loop rather than reactant loop operation) doesn&#39;t concern itself with the presence of gas bubbles, instead focusing on a control strategy that—through an intentional reduction in coolant flow—produces localized hot spots. More particularly, the control discussed in detail herein determines the coolant pump  125  pulsing frequency f such that intentional localized temperature rises of no greater than a predetermined maximum value are produced. In one even more particular form (and for a given system power level), the localized hot spot temperature rise is kept to within about 3° C. above the average system (i.e., stack  105 ) temperature through a suitable coolant pump  125  pulsing frequency f. In the present context, a local or localized hot spot is one that is of a discrete (rather than systemic) nature. Thus, rather than being indicia of a significant portion (or the substantial entirety) of the fuel cell stack  105  temperature level, a local hot spot would at most cover individual-sized positions in the stack  105  such that a temperature-measuring or related heat-sensing component (if present, such as temperature sensor  150 ) could discern the difference. 
         [0019]    To the extent that cooling flow pulsing may have been employed in the known art, it is done so with nominal pump operation as a way to produce a concomitant nominal flow of the coolant. Such an approach involves attempting to pulse the flow between two non-zero flow rates (for example, operating at conditions x+y and x−y around a nominal set point x) as a way to create unsteady flow conditions in the respective flowpaths. By contrast, the present invention includes pulsing between the nominal set point and the minimum flow that the pump  125  can provide, which for very low system power levels is zero, thereby minimizing the parasitic power draw of the pump  125 . 
         [0020]    Referring next to  FIGS. 3 and 4  in conjunction with  FIG. 2 , in one form of operation where the power requirements of stack  105  are relatively low (such as during vehicle idle), the need for coolant flow through coolant loop  130  is reduced. In this circumstance, and in a manner unlike that of a conventional approach, the controller  140  can send signals to the pump  125  to have it deliver a pulsed flow of coolant through loop  130 . In operational modes where flow pulsing (rather than continuous flow) is taking place, it is preferable to hold the valve  145  in the same position as it was at the start of the pulsing and keep it constant until the flow pulsing stops, as trying to control the valve during flow pulsing conditions would otherwise add another layer of complexity. In a preferred form, the controller  140  controls an on/off cycle of pump  125  so that periodic bursts of cooling fluid are injected into the inlet manifold  110 . Moreover a pulsed signal sent from controller  140  to pump  125  instructs it on how frequently to turn the pump  125  on and off; this frequency f is at a rate necessary to provide this intermittent cooling fluid flow such that a local temperature rise within stack  105  remains below a threshold difference over that of the remainder (or average) of the stack  105 . Many variables may be used to determine the frequency f (also known as duty cycle) of the on/off (i.e., pulsed) operation, based on operating parameters such as the load on the stack  105 , the volume and temperature of the cooling fluid in coolant loop  130 , the ambient temperature, passenger compartment heating requests, hydrogen bleeding from the anode to the exhaust, or the like. Further, the pump  125  may be left on for a minimum amount of time in order to retrieve original coolant temperatures, as well as remove bubbles from the flowfield. Thus, for example, increasing temperatures of the cooling fluid, as well the amount of coolant being passed through the coolant loop  130  may cause the duty cycle or frequency of the pulsed signal to be increased until the pump  125  is in continuous operation. 
         [0021]    In one form, the time the pump spends in the “off” (i.e., non-operating) condition may be about 3 to 10 seconds, and more particularly, about 5 seconds, while the stack power request that is used to determine the threshold may be about 0.1 amperes per square centimeter. In another form, the time the pump spends in the “off” condition may be about 10 to 30 seconds, and more particularly about 15 seconds if the stack power request is below about 0.05 amperes per square centimeter, while the off the “off” condition time may be about 30 to 80 seconds, and more particularly about 50 seconds if the stack power request is about 0.02 amperes per square centimeter and about 50 to 200 seconds, and more particularly about 100 seconds if the stack power request is about 0.01 amperes per square centimeter. Moreover, even longer “off” times may be permissible at lower current densities because of the lower rate of heat accumulation in the system; it will be appreciated by those skilled in the art from the preceding that the pump duty cycle is subject to system size and configuration, and that these and other particular values are within the scope of the present invention. Likewise, it is preferable to have pump  125  “on” time correspond to a minimum run time to ensure removal of the heat that is still being produced by stack  105  during pump “off” time. In one form, a typical time may be between about 3 and 10 seconds, although such values are dependent on the thermal mass of the stack  105  and the flowfield design. 
         [0022]    In a more detailed form, operating parameters taken into consideration by the algorithm include stack  105  electrical load, cabin heating request, anode bleed and coolant temperature. Other factors, such as non-pulse pump speed requests, may be determined by a different algorithm. When one or more of these parameters crosses a predetermined threshold, the controller  140  generates a signal that can be used to cycle the pump  125  on and off as a way to achieve the necessary coolant flow through loop  130  without pumping too much. It is important to recognize that controlling one device (such as pump  125 ) often impacts other parts of fuel cell system  100 . As such, a formula, algorithm or related strategy used by controller  140  may take advantage of feedback or feedforward terms that take component setpoints, as well as the operational parameters discussed above, into consideration. 
         [0023]    Controller  140  includes one or more processors (e.g., a microprocessor, an application specific integrated circuit (ASIC), field programmable gate array or the like) communicatively coupled to memory and interfaces (such as input/output interfaces). These interfaces may receive measurement data, as well as transmit control commands to the various valves (such as valve  145 ), pump  125  and other devices. The interfaces may also include circuitry configured to digitally sample or filter received measurement data, such as temperature data received from temperature sensor  150 ; this data may be configured to be delivered continuously or intermittently at discrete times (e.g., k, k+1, k+2, etc.) to produce discrete temperature values (e.g., T(k), T(k+1), T(k+2), etc.). The memory may be any form capable of storing machine-executable instructions that implement one or more of the functions disclosed herein, when executed by the processor. For example, the memory may be RAM, ROM, flash memory, hard drive, EEPROM, CD-ROM, DVD or other forms of non-transitory devices, as well as any combination of different memory devices. 
         [0024]    Furthermore interfaces and related connections between controller  140  and the various components of fuel cell system  100  may be any combination of hardwired or wireless variety. In some embodiments, the connections may be part of a shared data line that conveys measurement data to controller  140  and control commands to the devices, while in other embodiments, the connections may include one or more intermediary circuits (such as other microcontrollers, signal filters or the like) and provide an indirect connection between the controller  140  and the various system components. In one form, the use of one or more arithmatic unit processors, input, output, memory and control gives controller  140  attributes that allow it to function as a von Neumann computer. 
         [0025]    The memory of controller  140  may be configured to store a program or related algorithm that uses measurement data, operational conditions or related parameters, as well as charts, formulae or lookup tables as a way to provide control over various components, such as pump  125 . The controller  140  may include proportional-integral (PI) or proportional-integral-digital (PID) attributes that utilizes a feedback loop based on operational parameters, such as reactant flow needed by fuel cell stack  105 . Furthermore, controller  140  may utilize a feedforward-based control loop. In either case, controller  140  may generate an algorithmically-based control command that causes the pump  125  to change its operating state, such as its speed or pulsing frequency. It can likewise provide data to control opening and closing of valve  145  (as well as other valves). In one form, the lookup table, formulae or charts may include information derived from a pump or compressor map, as well as information derived from pressure drop models that in turn may utilize setpoint and/or feedback data from the controller  140 . In some embodiments, some or all of the operational parameters may be pre-loaded into memory (such as by the manufacturer of the controller  140 , vehicle  1  or the like). In other cases, some or all of parameters may be provided to controller  140  via the interface devices or other computing systems. Further, some or all of parameters may be updated or deleted via the interface devices or other computing systems. 
         [0026]    Referring with particularity to  FIG. 4  in conjunction with  FIG. 2 , the algorithm embedded in controller  140  includes various decision points that are used to determine whether the coolant pump  125  should be pulsed, and if so, to what pulsing frequency f. Initially, at step  300 , the controller  140  looks at the measured load on the stack  105  as determined by a current sensor (not shown). In step  302 , the controller  140  compares the measured load from step  300  to a threshold value, where such threshold may be stored in a lookup table or other memory device. The controller  140  also checks additional criteria. For example, it verifies or checks on issues related to cabin heating requests, anode bleed and coolant temperature (this last one, for example, pertaining to whether the temperature is below an upper limit). If any of these conditions aren&#39;t true, then normal flow control continues, as shown in step  306 . If on the other hand the conditions for flow pulsing are met, the timer starts at step  304  and the coolant flow pulsing begins at step  308 . In one preferred form, the algorithm uses the measured load on the stack  105  to determine the pulsing frequency to keep the temperature rise around 3° C., and sends a corresponding speed command to the coolant pump  125 . If the stack  105  load is below the lower threshold, then the speed command pulses between 0 revolutions per minute (rpm) and the minimum pump  125  speed (which may typically be around 1800 rpm). If the stack  105  load is between the upper and lower threshold, then the speed command pulses between 1000 rpm and the minimum speed of pump  125 . The enable criteria is continually monitored and if any of the parameters fall out of range, then normal flow control is resumed, as shown in steps  310  and  306 . Otherwise, flow pulsing continues. 
         [0027]    Many modifications and variations of embodiments of the present invention are possible in light of the above description. The above-described embodiments of the various systems and methods may be used alone or in any combination thereof without departing from the scope of the invention. Although the description and figures may show a specific ordering of steps, it is to be understood that different orderings of the steps are also contemplated in the present disclosure. Likewise, one or more steps may be performed concurrently or partially concurrently.