Patent Publication Number: US-2013252116-A1

Title: Model Based Approach For In-Situ WVTD Degradation Detection In Fuel Cell Vehicles

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to monitoring a water vapor transfer (WVT) device used in a fuel cell system, and more particularly to using one or more hydration models to permit in-situ monitoring and evaluation of performance characteristics of the WVT device. 
     Fuel cells, particularly proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cells, require balanced water levels to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the flooding or related blockage of the reactant flowfield channels. On the other hand, too little hydration limits the conductivity of the ion-transmissive membrane that is disposed between catalyzed electrodes; this high ionic resistance can lead to poor electrical performance, as well as premature cell failure. One popular way to promote proper levels of humidification or related water balance within the fuel cell is through one or more WVT units or devices (also referred to as a cathode humidifier unit, membrane humidifier, fuel cell humidifier or the like). In a typical WVT unit configuration, wet-side and dry-side reactant flowpaths (for example, a cathode exhaust and a cathode inlet) are in moisture-exchange communication with one another through a membrane media in the WVT unit such that excess moisture leaving the cathode exhaust may diffuse through the media to the drier flowpath on the cathode inlet. Examples of WVT units may be found in U.S. Pat. Nos. 7,749,661, 7,875,396 and 8,048,585, all of which are assigned to the assignee of the present invention and the entire contents of which are herein incorporated fully by reference. 
     In situations where numerous fuel cells are arranged as part of a module, stack or related larger assembly of fuel cell system components, a good measure of an overall humidification level for the various cell membranes can be derived from a relative humidity sensor placed in the cathode inlet gas stream. This measurement is used in conjunction with other factors, for example, cathode inlet air flowrate, cathode inlet temperature and cathode inlet pressure, to estimate the water transfer rate (WTR) of the WVT unit as one indicia of its performance. 
     There are other ways of acquiring humidity information besides using the aforementioned sensors. One way takes advantage of a fuel cell&#39;s inherent high frequency resistance (HFR), which is a directly-measurable property related to the ability of protons to pass through the cell&#39;s ion-transmissive membrane; this mobility is in turn is a function of the level of humidification of the cell. One approach to using HFR as a way to estimate and control cathode inlet and outlet flow humidities may be found in U.S. application Ser. No. 12/622,212, filed on Nov. 19, 2009 and entitled Online Estimation of Cathode Inlet and Outlet RH from Stack Average HFR, which is owned by the Assignee of the present application and incorporated herein by reference. 
     While determining an HFR between stack terminals may provide a good measure of average stack membrane relative humidity for helping to meet stack efficiency targets, it is not sufficient for identifying issues related to WVT unit degradation or wear. The conventional way of characterizing WVT unit degradation is to perform off-line testing of the unit while on a component test stand. This necessitates removing the WVT unit from the fuel cell system, testing it on the component test stand and reinstalling the unit back in the system; such an approach requires a lot of WVT unit downtime (for example, about 48 hours). Consequently, performing frequent off-line testing of fuel cell systems—such as those contemplated for vehicular applications—as a way to determine unit degradation is not practical. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a method of in-situ WVT unit degradation detection or estimation includes using a combination of a backward-looking (i.e., reverse) model and a forward-looking model. In the present context, in-situ activities are those that are conducted without requiring the WVT unit to be removed from the fuel cell stack or system with which it is operative; as such, measurements and related determinations or predictions may be made while the fuel cell stack or system is operative, or at least without having to remove or otherwise decouple the WVT unit from the remainder of the fuel cell system. Using such models (the first for the unit itself and the second for stack HFR and hydration) as a basis for stack water management is a more accurate way to estimate and control relative humidity for both stack inlet and outlet conditions than through a mere averaging technique. For example, a loss in WVT unit effectiveness at any given vehicle operating condition or time (including, for example, historical operational data) generated by the first model based on WTR feedback coupled with operating condition information can be input into the second model which includes an algorithm to estimate both inlet and outlet relative humidity values of the stack; in one form, the second model may use expected maximum power operations conditions of the fuel cell stack, including temperatures, pressures and flows. Such estimation may form the basis for online control of the fuel cell system, as well as provide indicia of when WVT device service may be warranted. The use of the two models working in conjunction with one another helps compensate for situations where sensed values are prone to inaccuracies, such as due to sensor failure (for example, a humidity sensor is prone to failure when being exposed to liquid water). 
     According to another aspect of the invention, a method of servicing a WVT unit (also referred to as a WVT device) used in a fuel cell system is disclosed. The method includes, in addition to providing in-situ a WVT device water transfer rate and estimating a reduced WVT device effectiveness, estimating the WTR at maximum power conditions at a given vehicle life and comparing the estimated WTR with an initial Beginning of Life (BOL) WTR, and servicing the WVT device when a difference in the values determined by the compared estimations exceeds a predetermined threshold. 
     According to another aspect of the invention, a WVT device for use in a fuel cell system includes one or more dry side flowpaths, one or more wet side flowpaths, a membrane placed relative to the dry and wet side flowpaths such that upon passage of relatively dry and relatively wet fuel cell reactant through the respective flowpaths, an exchange in humidity occurs between the dry and wet reactant streams. The device also includes one or more sensors to measure WTR information, as well as a controller configured to estimate a reduced device effectiveness and estimate a WTR for the device, as well as to estimate a WTR loss in the device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a block diagram of a fuel cell system with a WVT unit; 
         FIG. 2  is a flow diagram showing in-situ modeling of WVT unit degradation according to an aspect of the present invention; 
         FIG. 3  is a graph showing the WVT unit degradation in a representative fuel cell module; 
         FIG. 4  is a graph showing details of the reverse WVT unit model; 
         FIG. 5  shows a vehicle employing a fuel cell system with a WVT unit degradation-detection approach according to an aspect of the present invention; and 
         FIG. 6  is a graph showing the typical relationship between MEA hydration X and cathode RH. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to  FIGS. 1 and 5 , a fuel cell system  10  includes a fuel cell stack  20  made up of numerous individual fuel cells  25 , each of which has an anode  25 A and cathode  25 B separated by an ion-transmissive membrane  25 C, as well as an automobile  1  being powered by fuel cell stack  20  is shown. As will be understood by those skilled in the art, numerous such cells  25  are combined to form the stack  20  such that the power generation is increased. Likewise, numerous such stacks  20  may be used. Referring with particularity to  FIG. 1 , various flowpaths  40 ,  50  are used to convey reactants and their byproducts to and from the stack  20 . A WVT unit  60  is fluidly coupled to either or each of the respective flowpaths  40 ,  50  to promote the balanced humidity levels within one or both of them. As shown with particularity for the cathode-side reactant (i.e., an oxygen-bearing fluid), dry air from a compressor  45  is fed through an inlet flowpath  42  into the WVT unit  60  Likewise, stack cathode exhaust being discharged through an outlet flowpath  44  passes into and through the WVT unit  60 . Inside the WVT unit  60  is a core made up of numerous plates  65  (two of which are shown in more detail as dry side plate  65 A and wet side plate  65 B) that are stacked in an alternating arrangement such that (with the exception of the outermost plates) each plate is sandwiched between plates of the opposing flowpath. A membrane medium  67  is formed between each pair of wet side and dry side plates to allow for selective exchange of humidity between the WVT inlet flowpath  42  and the stack cathode outlet flowpath  44 . 
     A stack humidity sensor S provides in-situ WTR feedback of WVT unit  60 . Similarly, a resistor R may be connected across the stack  20 . Controller  70  uses values obtained by sensor S and resistor R to measure respectively inlet relative humidity RH in  of stack  20  and HFR. These measurements may form the basis of the two models discussed above. In particular, at least one of such measurements, in conjunction with water specie balance, can be used to estimate a humidity profile that includes outlet relative humidity RH out  of stack  20 . The resistor R may be particularly useful in situations where the sensor S fails to operate correctly, such as due to the presence of liquid water. Such backup measurement is particularly useful because failure circumstances are difficult to diagnose, and often occur during vehicle warm-up and vehicle idle to high power transients. Furthermore, an estimate of RH out  based on water specie balance is very sensitive to temperature and stoichiometry; as such, errors in temperature, air flow or current measurement may limit the ability to provide proper stack humidification control absent a fallback measurement. More particularly, in such situations where the sensor S is not available, the stack HFR measurement from resistor R, which is based on HFR-λ-RH relationships such as described below and in the aforementioned U.S. application Ser. No. 12/622,212, can be used to estimate the in-situ WTR. 
     Referring next to  FIG. 2 , a flow diagram showing the use of the reverse (i.e., inverse) and forward models as a way to predict (among other things) the performance of a WVT unit  60  within fuel cell system  10 , such as when unit  60  may be in need of service or replacement, is shown. The acronym CHU shown in the reverse and forward models  120  and  130  stands for cathode humidification unit and is another name of the WVT unit  60 ; the terms are used interchangeably throughout this disclosure. The in-situ water transfer rate (WTR) feedback  100  of WVT unit  60  that is derived from one of the sensor-based approaches discussed above is used in conjunction with operating condition information  110  (for example, dry inlet and wet inlet stream flow on a dry basis, composition, temperature and pressure) is used as input to a reverse WVT model  120  (also referred to as an effectiveness-based WVT model) to allow the model to provide an on-line estimation of a reduced effectiveness ε t  for the membranes  67  of WVT unit  60  at any vehicle life time. As described below and in U.S. application Ser. No. 12/755,315, filed Apr. 6, 2010 and entitled Using an Effectiveness Approach to Model a Fuel Cell Membrane Humidification Device, which is owned by the Assignee of the present application and incorporated herein by reference, such reduced effectiveness is mostly estimated at low to mid power levels of vehicle or fuel cell system  10  operation, and is based on past (i.e., historical) vehicle or system  10  operating data. 
     In the present context, the reverse nature of WVT model  120  amounts to estimating a loss in WVT unit effectiveness ε t  based on rearward-looking (i.e., past) vehicular data (mostly at low stack power conditions) in the form of the above operating condition information  110  and in-situ WTR feedback information  100  where the effectiveness ε t  is the ratio of the actual mass transfer rate of humidity to the maximum possible mass transfer rate of humidity that would be realized in a counter-flow mass exchanger having an infinite membrane area. Moreover, this measure of effectiveness ε t  depends on the number of mass transfer units, a non-dimensional ratio of the product (also called product value UA) of the mass transfer coefficient U and membrane area A to the minimum mass flow rate on a dry basis of the dry stream and the wet stream flowing through the dry side and the wet side of the WVT unit  60 , respectively. This will be discussed in more detail below. A third non-dimensional parameter employed in the model is a capacity ratio CR, which is the ratio of the minimum mass flow rate on a dry basis of the wet side flow of the outlet flowpath  44  and the dry side flow of the inlet flowpath  42  of the WVT unit  60  to the maximum mass flow rate on a dry basis of the wet side flow of the outlet flowpath  44  and the dry side flow of the inlet flowpath  42  of the WVT unit  60 . The capacity ratio CR may be expressed as: 
     
       
         
           
             CR 
             = 
             
               
                 Min 
                  
                 
                   ( 
                   
                     
                       M 
                       
                         air 
                         , 
                         dry 
                       
                     
                     , 
                     
                       M 
                       
                         wet 
                         , 
                         air 
                       
                     
                   
                   ) 
                 
               
               
                 Max 
                  
                 
                   ( 
                   
                     
                       M 
                       
                         air 
                         , 
                         dry 
                       
                     
                     , 
                     
                       M 
                       
                         wet 
                         , 
                         air 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
     The reverse WVT model  120  cooperates with controller  70  to adjust the position of one or more valves (not shown) that may be used to control the amount of water provided to the cathode inlet flowpath  42  as a way to control the desired amount of water transfer and related fuel cell humidity in the various membranes  67 . 
     In particular, the calculated reduced effectiveness ε t  taken from the reverse WVT model  120  is next used, along with the expected maximum stack power operating conditions, in the forward WVT model  130  to project the WTR at maximum power at the given vehicle life time WTR max     —     pwr   tlife . The forward WVT model  130  is also used to predict BOL WTR at maximum power with known BOL mass transfer coefficient and membrane area and expected maximum power operating conditions, WTR max     —     pwr   BOL . The BOL mass transfer coefficient and membrane area may be based upon known component design value. The predicted BOL WTR can be stored on computer-readable memory that is part of the controller  70 , especially when the controller  70  is configured to include features associated with a traditional Von Neumann (or general-purpose or stored-program) computer that has (among other things) a CPU, input, output and memory, the latter typically in the form of both working (i.e., data-containing or RAM) memory and permanent (i.e., instruction-containing, or ROM, such as system start-up and other features) memory. In one form, the reverse WVT model  120  and the forward WVT model  130  may use the same equations. 
     In one preferred form, the reverse WVT model  120  and the forward WVT model  130  may be implemented on-line in the control software that is loaded into controller  70 . The difference  140  between the predicted BOL water transfer rate WTR max     —     pwr   BOL  and the estimated water transfer rate WTR max     —     pwr   tlife  based on the reduced effectiveness derived from the RH sensor S or the HFR sensor R at maximum power yields the degree of WVT on-line degradation ΔWTR tlife   max     —     pwr . Results associated with this method for one particular membrane module is shown in exemplary fashion in  FIG. 3 , where there was about 17% degradation at 1.0 A/cm 2  after about 238 hr. Comparable results for two other modules (not shown) showed about 14% degradation after 316 at 1.5 A/cm 2  and about 15% degradation at 1.5 A/cm 2  after 120 hr of freeze testing, respectively. 
     Furthermore, the forward WVT model  130  with the reduced mass transfer coefficients estimated real time can be adapted in the stack RH controls via controller  70  to improve stack operating conditions, resulting in enhanced stack performance and durability. For example, in scenarios where stack operates under greater than 100% cathode outlet RH conditions, HFR response does not have enough resolution for stack RH control, such forward WVT model will be used as a primary tool for stack RH control. Improving the WVT model WTR prediction by including WVT membrane material degradation can result in more accurate stack cathode outlet RH prediction and control, thus enhancing stack performance and durability. When the degree of WVT on-line degradation at maximum power ΔWTR tlife   max     pwr   . exceeds a percentage of the BOL WTR at maximum power WTR max     —     pwr   BOL  at any given vehicle life time by a predetermined value (for example 20%), the WVT unit  20  is considered to be at the end of its life and therefore in need of service or replacement in order to regain the desired performance. The model-based approach of the present invention enables the prompt detection of the faster-than-expected WVT degradation rate in WVT unit  60  membranes  67 , so proper actions or plans can be put in place to address the degradation issue at earlier stages. The degraded WVT performance can be used to make an informed decision on stack  20  humidification control for stack  20  durability and freeze purge/start-up development. In addition, the projected WTR at maximum power at the given vehicle life time WVT max     —     pwr   tlife  can also be utilized to improve the stack voltage/power prediction at maximum power by improving ohmic (i.e., IR) loss estimation, thus better projecting stack service time. For example, the stack voltage IR loss is a function of stack inlet and outlet RHs. Including WVT membrane performance degradation enables more accurate stack inlet and outlet RH estimation, thus improving stack IR loss and voltage prediction. 
     For certain operating conditions for a given design of WVT unit  60 , the amount of water transferred can be estimated using the relationships between the number of mass transfer units, the effectiveness, and the mass flow rates of streams established for heat exchanger designs. The well-established relationships between the heat transfer effectiveness and the number of heat transfer units for heat exchanger designs is available for use based on the analogy between heat transfer and mass transfer, as would be readily apparent to those skilled in the art. 
     As discussed with more particularity in the aforementioned U.S. application Ser. No. 12/755,315, the water vapor transfer performance of the WVT unit  60  is modeled using Equations (3) through (8) therein (which form the basis for dependent original claims  5  through  10  of the present application). Referring with particularity to  FIG. 4 , an algorithm depicting how a reverse WVT model  120  is shown. At any given vehicle time, an initial guess of degradation factor K deg,initial  between 0 and 1 is provided. From these equations, the amount of water transferred N w  in gm water/sec can be predicted based on this initial guess. 
     In the reverse WVT model  120 , the degradation factor K deg,t  at any given vehicle life can be obtained by minimizing the difference between the predicted water transfer rate Nw and the measured water transfer rate from a RH sensor based on the past vehicle data. If the WVT water transfer rate from RH sensor (denoted as N sensor,RH ) is not available as an input for the reversed WVT model  120 , the stack HFR measurement (such as depicted in  FIG. 2 ) can be used to estimate the in-situ water transfer rate based on the aforementioned HFR-λ-RH. In the present case,  FIG. 2  is the flow chart which illustrates the model based WVT degradation determination, while  FIG. 4  is the flow chart to illustrate how the reverse WVT model works. As such,  FIG. 4  is a subset of  FIG. 2 . The estimate can be made as follows. 
     HFR based estimation of internal humidification of stack  20  offers a “stack-as-sensor” approach that directly measures the internal state of MEA hydration. HFR is a strong function of MEA hydration λ and a weak function of temperature T, where the equations 1 and 2 below illustrate such relationships: 
     
       
         
           
             
               
                 
                   
                     σ 
                     = 
                     
                       
                         exp 
                          
                         
                           [ 
                           
                             1268 
                              
                             
                               ( 
                               
                                 
                                   1 
                                   303 
                                 
                                 - 
                                 
                                   1 
                                   
                                     273 
                                     + 
                                     T 
                                   
                                 
                               
                               ) 
                             
                           
                           ] 
                         
                       
                       · 
                       
                         [ 
                         
                           
                             0.005139 
                              
                             λ 
                           
                           - 
                           0.00326 
                         
                         ] 
                       
                     
                   
                    
                   
                       
                   
                    
                   
                     
 
                   
                    
                   
                     
                       ( 
                       
                         ohm 
                         - 
                         cm 
                       
                       ) 
                     
                     
                       - 
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     HFR resistance R is calculated as: 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       membrane_thickness 
                       σ 
                     
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         ohm 
                         - 
                         
                           cm 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     From the HFR measurement, stack temperature and stack membrane thickness, the average value of MEA hydration λ can be estimated. The correlation between MEA hydration λ and stack cathode average RH are well-known, as evidenced by the graph in  FIG. 6 , where for example, inputs of operating conditions, such as cathode inlet and outlet pressure, coolant inlet and outlet temperature cathode air flow and stack current permit performing a water specie balance around stack which would in turn yield the stack cathode inlet and outlet RHs and the amount of water in the cathode inlet stream. Subtracting the ambient water flowrate in the cathode air flow (estimated from an ambient RH sensor measurement) from the amount of water flow in the cathode inlet stream would yield the WVT in-situ water transfer rate; the details of such calculations can be found in the aforementioned U.S. application Ser. No. 12/622,212. This WVT in-situ water transfer rate then can be used in the reverse WVT model to estimate K deg,t  at any given vehicle time using the mechanism depicted in  FIG. 4 . With the estimated K deg,t  from the reverse WVT model, along with the expected maximum power operating conditions, the forward WVT is utilized to project the WTR at maximum power at the given vehicle life time WTR max     —     pwr   tlife  using the equations shown in original claims  5  through  10 . From this, the degree of WVT degradation is determined by comparing WTR max     —     pwr   tlife  and the predicted BOL water transfer rate WTR max     —     pwr   BOL . 
     It is noted that terms like “generally”, “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. 
     For the purposes of describing and defining the present invention it is noted that the terms “substantially” and “about” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to 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.