Abstract:
An apparatus and method to determine the relative humidity of a fuel cell system. A controller is cooperative with a first device and a second device to receive a valve signal and a high frequency resistance value. The controller controls the relative humidity of a fuel cell stack based on the estimation of the relative humidity of the fuel cell stack based on one or more algorithms. The controller modifies the relative humidity of the fuel cell stack through changes in the position of a valve based on at least one of the valve signal and the high frequency resistance value. In one form, the relative humidity of the fuel cell system is determined without the need of a humidity sensor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to providing an estimation of a fuel cell stack inlet and outlet humidity levels, and more particularly to devices and methods for determining relative humidity during fuel cell operational transients without requiring relative humidity feedback from a sensor. 
     In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode to facilitate the electrochemical conversion of hydrogen and oxygen into electrons and positively charged ions (for the hydrogen) and negatively charge ions (for the oxygen). The electrons flow through an external electrically-conductive circuit (such as a load) to perform useful work, and then on to the cathode. An electrolyte layer separates the anode from the cathode to allow the selective passage of ions to pass from the anode to the cathode. The combination of the positively and negatively charged ions at the cathode results in the production of non-polluting water as a by product of the reaction. In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, the electrolyte layer is in the form of a proton-transmissive membrane; the layered structure formed by this PEM sandwiched between the anode and cathode is commonly referred to as a PEM electrode assembly (MEA). Each MEA forms a single fuel cell, and many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output. The PEM fuel cell has shown particular promise for vehicular and related mobile applications. 
     Balanced moisture or humidity levels are required in the PEM fuel cell to ensure proper operation and durability. For example, it is important to avoid having too much water in the fuel cell, which can result in the blockage of reactants to the porous anode and cathode. Contrarily, too little hydration limits electrical conductivity of the membrane, and in extreme cases can lead to it wearing out prematurely. As such, it is beneficial to have knowledge of the hydration level within a fuel cell, especially PEM fuel cells that frequently operate at elevated temperatures that can impact a cell&#39;s hydration level. 
     High frequency resistance (HFR) is a known diagnostic technique for indirectly measuring MEA hydration. In a typical HFR configuration, sensors use a high-frequency ripple current to measure fuel cell resistance. Although such an approach is particularly sensitive to changes in relative humidity (RH), its sensitivity to other fuel cell conditions can cause erroneous measurements. In other words, the measured fuel cell resistance, or HFR value, is measuring the resistance of the build-up of water in the PEM of the fuel cell and not the wetness of the air. The air has to dry or dampen the PEM for a change to occur in the HFR value. One particular weakness of HFR-based estimation is the inherent lag in HFR, especially at low flow conditions which exhibits a hysteresis response in the HFR value. This hysteresis response means that in situations where rapid inlet humidity changes are present, such changes will not match an average stack HFR value that often lags. This lag may cause a controller to over-dry the stack; such over-drying is particularly prevalent at the cathode inlet, where chemical degradation and consequent PEM thinning may ensue. As such, it remains challenging and difficult to provide accurate estimations of relative humidity levels in a fuel cell system. This is particularly acute in vehicular-based fuel cell systems where reliability, weight and cost further compound the challenges. In conventional configurations, to accomplish monitoring the RH of the stack, a separate humidity sensor is used to measure the RH of the air flowing through the stack. The humidity sensor allows a control system to determine the humidity of the PEM without succumbing to the lag or hysteresis response of the HFR sensors in response to operational transients. Unfortunately, such humidity sensors add cost and complexity to the system. 
     A cathode humidification unit (CHU) model algorithm is used to interpret and modify the RH of the stack. Variations in the effectiveness of the CHU model may be due to part-to-part variation, degradation, or even leaks. Degradation may depend on usage profile and may be different from vehicle-to-vehicle. Furthermore, the CHU model uses an outlet RH value of the stack for the calculations of the RH of the stack. If the outlet RH value has an error, (e.g. due to error in stoichiometry estimation, temperature feedback or even variation in anode water crossover) this would impact the CHU model&#39;s ability to calculate the RH of the stack and cause a circular reference. 
     A reference signal would benefit the CHU model to calculate the RH of the stack without the need for the humidity sensor. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the teachings of the present invention, an apparatus to determine the relative humidity of a fuel cell system. The fuel cell system relies on a relative humidity control apparatus to control the relative humidity of a fuel cell stack. The relative humidity control apparatus uses a controller, a cathode humidification unit, a valve, a first device, and a second device, and a plurality of fuel cells to estimate the relative humidity of the fuel cell stack. A controller is cooperative with the second device and the first device and receives the valve signal and the high frequency resistance value from both devices respectively. The controller controls the relative humidity through changes in the position of the valve fluidly disposed in a plurality of flowpaths of the fuel cell system based on at least one of the valve signal and the high frequency resistance value. In one form, the relative humidity of the fuel cell system is determined without the need of a humidity sensor. 
     In accordance with another aspect of the teachings of the invention a method of estimating the relative humidity during operational transients of a fuel cell system is disclosed. The relative humidity is estimated by receiving a valve signal from a second device and a high frequency resistance value from a first device. The second device is configured to transmit the valve signal where the valve signal corresponds to a position of a valve fluidly disposed in a plurality of flowpaths of the fuel cell system. The first device is configured to transmit the high frequency resistance value from the fuel cell stack. The relative humidity is estimated by executing a plurality of algorithms in a processor of a controller. The first algorithm is a cathode humidification unit (CHU) model and the second is an adaptive algorithm. The adaptive algorithm modifies the CHU model by evaluating the valve signal and high frequency resistance. 
     In accordance with yet another aspect of the teachings of the invention, a method of operating a relative humidity control apparatus and estimating the relative humidity during operational transients of a fuel cell system is disclosed. The relative humidity control apparatus uses a controller, a cathode humidification unit, a valve, a first device, and a second device, and a plurality of fuel cells to estimate the relative humidity of the fuel cell stack. A controller is cooperative with the second device and the first device and receives the valve signal and the high frequency resistance value from both devices respectively. The relative humidity estimates the relative humidity of a plurality of fuel cells by executing a cathode humidification unit model and an adaptive algorithm in a processor of a controller cooperative with the first device and the second device. The adaptive algorithm modifies the cathode humidification unit model by evaluating the valve signal and high frequency resistance value to estimate the relative humidity of the fuel cell stack. The controller controls the relative humidity through changes in the position of the valve fluidly disposed in a plurality of flowpaths of the fuel cell system based on the estimation of the relative humidity of the fuel cell stack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  shows an illustration of a vehicle; 
         FIG. 2  illustrates a flowpath for air through a fuel cell system; 
         FIG. 3  is schematic view of a CHU model; and 
         FIG. 4  illustrates the CHU model modulating a valve to adjust the relative humidity of a stack in the fuel cell system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the invention described in the present disclosure do not rely on a separate and distinct humidity sensor to determine a RH of a fuel cell stack. Instead it utlizes signals from sensors monitoring the stack, or a stack-as-sensor approach, to determine the RH value for the fuel cell stack without the need for the humidity sensor. The stack-as-sensor approach uses a HFR value of the fuel cell stack and together with knowledge of a valve position and an adaptive CHU model, the RH of the fuel cell stack may be determined. 
     The HFR value uses a second device (described below) to measure an amount of water or hydration in a PEM of the fuel cell. The second device may be a resistance sensor used to measure a resistance of an amount of water or hydration of the PEM. The RH of an air flowing through the PEM may be directly determined by the HFR value. A change in the amount of water in the PEM may occur for a change to be reflected in the HFR value. A time it takes for the air to dry the PEM or saturate the PEM with water may appear in the HFR value as a lag of time between the change in the RH of the air and a change in the HFR value. 
     Referring first to  FIG. 1 , a vehicle  10  (e.g., a car, bus, truck, or motorcycle) powered by a fuel cell system is shown. Some components of the fuel cell system may include numerous fuel cells (preferably arranged as one or more stacks  20 ) that convert stored gaseous fuel from a tank  30  into electricity to provide electric power to engine (not shown) that may be a fully electric or a hybrid electric engine (e.g., an engine that uses both electricity and petroleum-based combustion for propulsion power), utilizes the power from the fuel cell system to propel the vehicle  10 . The fuel cell system may also include any number of valves, compressors, tubing, temperature regulators, electrical storage devices (e.g., batteries, ultra-capacitors or the like), and controllers to deliver the fuel from the tank  30  or tanks to the fuel cell system, as well as to provide control over the operation of fuel cell system. Such controllers will be discussed in more detail below. 
     Any number of different types of fuel cells may be used in the fuel cell system (e.g., metal hydride fuel cells, alkaline fuel cells, electrogalvanic fuel cells, or any other type of known fuel cells). Multiple fuel cells may also be combined in series and/or parallel within the fuel cell system as the stack  20  in order to produce a higher voltage and/or current yield by the fuel cell system. The produced electrical power may be supplied directly to an engine (not shown) or stored within an electrical storage device (not shown) for later use by vehicle  10 . 
       FIG. 2  illustrates a flowpath for air  50  through the fuel cell system. The air  50  enters a compressor  40  where the air  50  may be compressed. The air  50  leaves the compressor  40  and enters a heat exchanger  45  where the RH of the air  50  is reduced to produce a dry air  47 . The dry air  47  enters a valve  55  where the dry air  47  may be diverted to enter a tube side  60  of a cathode humidification unit (CHU)  65  or diverted around the tube side  60 . The valve  55  may vary the amount of dry air  47  being diverted around the tube side  60  by a percent opening ranging from 0 to 100 percent. The CHU  65  may be used to increase the RH of the dry air  47  to produce a humid air  70 . The percent opening by the valve  55  is controlled by a controller  110  as shown in  FIG. 3 . At a mixing point  75 , the dry air  47  and the humid air  70  mix to create a mixed air  80  of a specific RH as determined by the control module. 
     The mixed air  80  enters the stack  20  and participates in the energy production process as described in the background section. An exit air  90  leaves the stack  20  either at a higher RH or a lower RH depending on the amount of water in the PEM, an airflow speed over the PEM, and the RH of the mixed air  80 . The exit air  90  may be used by the CHU  65  on a shell side  95  to increase the RH of the air  50  to produce the humid air  70 . The exit air  90  is exhausted from the vehicle as exhaust  100 . 
       FIG. 3  is schematic view of a CHU model  115  used by the controller  110  to determine a hydration of the stack  20  as shown in  FIG. 2 . The HFR value  120  from the first device  220  may be inputted into a HFR-based estimate algorithm  125 . The HFR-based estimate algorithm  125  calculates a stack outlet RH value  130  using air flow  131 , temperature  132 , pressure  133 , and current inputs as described below in greater detail. The stack outlet RH value  130  is inputted into a first temperature correction algorithm  135  which may adjust the HFR RH estimation value  130  to determine a RH shell value  140 . The RH shell value  140  may be the RH of the exit air  90  as it exits the stack  20  corrected for any heat loss in a piping network (not shown) between the stack  20  and the shell side  95  of the CHU  65 . The RH shell value  140  may be used by the CHU model  115  to determine a RH tube value  145 . The RH tube value  145  is an estimate of the RH of the humid air  70  exiting the tube side  60  of the CHU  65 . The RH tube value  145  is calculated based on the CHU model as described below. 
     A second device  225  produces a valve signal  155  corresponding to the percent opening of the valve  55 . Alternatively, the second device  225  may also produce the valve signal  155  corresponding to a valve command signal  250  as describe below. A mixing algorithm  150  may use the RH tube value  145  and the valve signal  155  to determine a mixed air RH value  160 . The valve signal  155  is a bypass ratio (α) which may be indicative of the percentage of opening of the valve  55 . The mixed air RH value  160  is the RH of the mixed air  80  at the mixing point  75 . A second temperature algorithm  165  may be used to normalize the mixed air RH value  160  to determine a final RH value  170  which is a stack  20  inlet temperature as described below in greater detail. 
     An adaptive algorithm  175  may use the final RH value  170  and a stack inlet RH value  180  to output a parameter  185  that may be used by the CHU model  115  to modify the RH tube value  145 . By modifying the RH tube value  145 , the final RH value  170  may more accurately indicate a RH value close to an actual RH value in the stack  20 . An adapt enable algorithm defines a trigger criteria when the adaptive algorithm  175  may modify the RH tube value  145  based on determining when HFR-based inlet RH estimate is valid. The adapt enable  190  enables the adaptive algorithm  175  via an enable signal  195 . The adapt enable  190  may stop the adaptive algorithm  175  when a load value  200  or the valve signal  155  are changing rapidly. Furthermore, if a stack temperature value  205  is too low or if a stack flooded value  210  indicates the stack is too wet, then the adapt enable  190  may stop the adaptive algorithm  175 . 
     A Water Buffer Model  191  (WBM) is an algorithm which outputs a water value  193  by taking the stack inlet RH value  180  and defining how much water to add to the stack  20  to reach the final RH value  170 . The controller  110  uses the water value  193  to add a quantity of water to the dry air  47  to change the RH of the stack  20 . 
     Lag in the HFR value  120  is a dynamic issue in drive cycles with large transitions between idle and 75% to maximum power. The lag does not significantly impact typical drive cycles, such as Environmental Protection Agency (EPA) city and highway cycles, due to these cycles being low power cycles. The stack  20  operates wet due to a slow thermal response when there is a low power request and less heat is produced. At the end of the drive cycle the operation goes to near idle where the lag in the HFR value  120  is large. Thus the combination of the stack  20  operating wet and the transition to idle makes the lag even worse. The valve  55  is used to estimate the bypass ratio (α). The essence of solution to the lag problem is to factor in the valve  55  upstream that changes the valve signal  155  as the bypass ratio (α) changes to improve the accuracy of the final RH value  170 . For example, if the stack  20  is operating at low air flows (idle) and the stack  20  is indicating that it is hydrated, the HFR value  120  is low. On the contrary, if the valve  55  position is indicating that all the airflow is being diverted around the CHU  65  (i.e. α=0) then the mixed air  80  may be dry and over time the stack  20  inlet will dry out and eventually may show up in a change in the HFR value  120 . 
     The CHU model  115  takes advantage of the bypass ratio (α) to determine the RH of the mixed air  80 . Furthermore the CHU model  115  may have parameters that may be adapted online based on the stack inlet RH value  180  of the stack  20 . The CHU model  115  is utilized that relates the shell side  95  and the tube side  60  RH as an equilibrium relation shown below in equation 1: 
                     RH   in     T   tube_out       =     ɛ   ·     RH   out     T   shell_in                 (   1   )               
where ε is an equilibrium relation that is a function of air flowrate to account for low “effectiveness” of the CHU  65  at high air flow rates. The equilibrium relation may be calibrated for the CHU model  115  and may not degrade over time. The bypass ratio (α) may be calculated based on valve commands and not valve position feedbacks.
 
     The adaptive algorithm  175  may estimate the CHU parameter (ε({dot over (m)} air )) for different flow conditions such that the CHU model  115  estimate matches reference inlet RH if the stack  20  when the HFR value  120  may be trusted. In essence the adaptive algorithm (ε)  175  may be solving the following optimization found in equation 2 below: 
     
       
         
           
             
               
                 
                   
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                         RH 
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                           RH 
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                           HFR_CHU 
                         
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     The optimization is broken up into bins spanning low and high flowrates. This optimization problem is solved online via PIA algorithm. An assumption is made on the airflow restrictions (Kv) in the system or by learning valve K v  in anode subsystem and modifying it for CHU adaption. 
     The controller  110  cooperates with the first device  220  and the second device  225  to receive the valve signal  155  and HFR value  120 . The controller  110  cooperates either electrically, optically, wirelessly, or mechanically with the first device  220  and the second device  225 . The controller  110  comprising at least one processor (not shown) and a computer readable medium (not shown) such that instructions stored in the computer readable medium are executed by the at least one processor to control through changes in the position of the valve  55  a RH of a plurality of fuel cells based on at least one of the valve signal  155  and HFR value  120 . 
       FIG. 4  illustrates the CHU model  115  modulating the valve  55  to adjust the RH of the stack  20 . Refer to  FIGS. 2 and 3 . A CHU model plot  252  depicts the CHU model  115  estimating both the stack inlet RH value  180  and the stack outlet RH value  130  purely based on the HFR value  120  only and sending a valve command signal  250  to the valve  55  to adjust the RH of the stack  20 . An adaptive CHU model plot  254  depicts the CHU model  115  estimating both the stack inlet RH value  180  and the stack outlet RH value  130  based on the HFR value  120  and using the adaptive algorithm  175  to determine the RH of the stack  20  and sending a valve command signal  250  to the valve  55  to adjust the RH of the stack  20 . 
       FIG. 4  additionally illustrates a stable valve command signal  250  using the adaptive algorithm  175  than using the CHU model  115  as a standalone algorithm. Furthermore, the adaptive algorithm  175  may reduce the effect of the lag on the valve command signal  250  as shown by the lack of extreme valve command signals  250  as illustrated by a first dip  255 , a second dip  260 , and a crest  265 . The first dip  255  and the second dip  260  illustrate where the CHU model  115  may be catching up the actual RH value of the stack  20  and overshooting the actual value with the crest  265 . 
     The adaptive algorithm  175  may reduce the chance for a dry out of the PEM of the stack and may show an improvement in dry out prevention. The valve command signal  250  for the valve  55  is smooth and may reduce equipment fatigue and promote a longer service life. For example, during a cathode purge, the controller  110  may try to bring the RH of the stack  20  to a RH value of around 35% humidity. Without the adaptive algorithm  175  aiding the CHU model  115 , the stack  20  may reach a RH value of 20% and may severely dry out the stack  20 . With the adaptive algorithm  175  aiding the CHU model  115 , the stack  20  maintains the RH value of 35%. Furthermore, when the fluctuation of the valve command signal  250  may be greatly reduced during the normal drive cycle. 
     The CHU model  115  overshoots the desired control point due to the lag in the HFR value  120 . This occurs due to the fast transients in the drive cycle and the amount of time it takes the HFR value  120  to register a change in the RH of the stack  20  of  FIG. 2 . The CHU model  115  side illustrates in the wet overshoot (0% water vapor transfer unit (WVT) bypass) that occurs at the first dip  255  and the second dip  260  and the dryout (saturated 75% WVT bypass) that occurs at the crest  265 . The adaptive algorithm  175  side of  FIG. 4  illustrates the same drive cycle without the extreme excursion in the valve signal  155 . 
     It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. Likewise, for the purposes of describing and defining the present invention, it is noted that the term “device” is utilized herein to represent a combination of components and individual components, regardless of whether the components are combined with other components. For example, a “device” according to the present invention may comprise an electrochemical conversion assembly or fuel cell, as well as a larger structure (such as a vehicle) that incorporates an electrochemical conversion assembly according to the present invention. Moreover, the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. As such, it may represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
     Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.