Abstract:
(EN) The inventive system for thermal control of a fuel cell system mounted on a motor vehicle comprises a fluid heat carrier flow loop (BC) for thermally controlling a fuel cell (PAC), means for supplying said fuel cell (PAC) with oxygen (GCA) and hydrogen (R) and an electronic control unit (UCE) which is provided with an estimator (EST) for the temperature (Ts) of the fuel cell solid elements.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to a system and to a method for controlling the temperature of a fuel cell system mounted on board a motor vehicle. 
     II. Description of Related Art 
     Controlling the temperature of the fuel cell is important to prevent the cell from overheating as this could lead to damage to the cell. 
     Standard fuel cells of the PEM (Proton Exchange Membrane) type comprise elemental individual cells which are made up in particular of a bipolar plate and of a Membrane Electrodes Assembly, or MEA for short. These are the solid elements of the fuel cell. 
     A fuel cell also comprises fluid elements including the products obtained by the electrochemical reactions that take place within the fuel cell in order to supply the electrical energy. Such a cell is supplied with hydrogen at the anode, for example by a reformer, and with oxygen at the cathode, generally by an air compressor unit. 
     Systems for controlling the temperature of a fuel cell do exist. 
     Documents U.S. Pat. No. 4,640,873, FR 2 809 535 and JP 9 213357 relate to systems for the temperature control of a fuel cell in which systems the temperature of the cell is assimilated to the temperature of the fluid elements. 
     However, there is a discrepancy between the temperature of the solid elements of the cell and of the fluid elements of the cell, and this leads to temperature control of limited effectiveness because it is based on the temperature of the fluid elements rather than on the temperature of the solid elements. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention allows an estimation of the temperature of the solid elements of the cell with greater precision than assimilating the temperature of the solid elements of the cell to the temperature of the fluid elements of the cell. 
     One aspect of the invention proposes a system for the temperature control of a fuel cell system mounted on board a motor vehicle comprising a flow loop for circulating a heat-transfer fluid for temperature control of the fuel cell, oxygen supply means and hydrogen supply means for supplying the fuel cell, and an electronic control unit. The electronic control unit comprises an estimator for estimating the temperature of the solid elements of the fuel cell. 
     The temperature of the cell is therefore more precise because the temperature of the solid elements is not assimilated to the temperature of the fluid elements. 
     In a preferred embodiment, the electronic control unit further comprises a control module for controlling the operation of the flow loop. 
     For example, the estimator comprises input parameters including the temperature of the heat-transfer fluid upstream of the cell and the temperature of the heat-transfer fluid downstream of the fuel cell. 
     For example the estimator comprises input parameters including the temperature of the fluids leaving the cathode of the fuel cell. 
     For example, the estimator comprises input parameters including the temperature of the fluids leaving the anode of the fuel cell. 
     In one advantageous embodiment, the system comprises a comparator designed to compare the temperature of said solid elements as estimated by the estimator with a desired temperature of said solid elements as stored in memory by the electronic control unit and to transmit said comparison to said control module that controls the operation of the flow loop. 
     The invention also proposes a method for the temperature control of a fuel cell system mounted on board a motor vehicle. The method uses the circulation of a heat-transfer fluid. The temperature of the solid elements of the fuel cell is estimated and the temperature of the cell is controlled on the strength of said estimate. 
     In one advantageous embodiment the circulation of the heat-transfer fluid is altered on the strength of the estimated temperature of the solid elements of the fuel cell and of a desired temperature of the solid elements of the fuel cell. 
     In one advantageous embodiment, said estimate is performed using parameters including the temperature of the heat-transfer fluid upstream of the cell and the temperature of the heat-transfer fluid downstream of the fuel cell, or using parameters including the temperature of the fluids leaving the cathode of the fuel cell or using parameters including the temperature of the fluids leaving the anode of the fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from studying the following detailed description of a number of embodiments taken by way of nonlimiting examples and illustrated by the attached drawings in which: 
         FIG. 1  is a block diagram of a fuel cell device according to the invention; 
         FIG. 2  is a block diagram illustrating a first embodiment of an estimator according to the invention; 
         FIG. 3  is a block diagram illustrating a second embodiment of an estimator according to the invention; 
         FIG. 4  is a block diagram illustrating a third embodiment of an estimator according to the invention; and 
         FIG. 5  is a temperature control system according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a fuel cell temperature control system according to the invention. The system comprises a fuel cell PAC comprising an anode part A and a cathode part C. The anode A is supplied with dihydrogen-rich reformed gas by a reformer X, and the cathode C is supplied with air, and therefore with oxygen, by an air compressor unit GCA. The fluids leaving the anode A pass through an outlet pipe SA and the fluids leaving the cathode C pass through an outlet pipe SC. 
     The system also comprises a flow loop BC for circulating a heat-transfer fluid for controlling the temperature of the fuel cell PAC. A portion of this flow loop BC passing through the fuel cell PAC is depicted in  FIG. 1 . This flow loop BC also comprises, in the conventional way, heat exchangers and condensers. The heat-transfer fluid passing through the cell can be used to cool the cell when its temperature becomes too high, and there is a fear of damage, or to heat the cell during a start-up phase. 
     The temperature control system also comprises an electronic control unit UCE which comprises an estimator EST capable of precisely estimating the temperature of the solid elements of the cell PAC. 
       FIG. 2  depicts a first embodiment of an estimator EST. The estimator EST receives at input the pressure P k , with k=A in the case of anode pressure or k=C in the case of cathode pressure, the current I and the voltage U delivered by the cell, the flow rate Q of the heat-transfer fluid, and the temperatures of the heat-transfer fluid in the flow loop BC upstream T REF   IN  and downstream T REF   OUT  of the cell PAC. 
     In this embodiment, the estimator EST uses the following relationships regarding the temperature of the solid elements of the cell PAC: 
             {               ⅆ     T   s         ⅆ   t       =                 W             ⁢   therm       ⁡     (     I   ,   U     )       -         h             ⁢   A       ·     S             ⁢   A         ⁢     (       T             ⁢   S       -     T             ⁢   A               ⁢   OUT         )       -       h             ⁢   C       ·                     S             ⁢   C       ⁡     (       T             ⁢   S       -     T             ⁢   C               ⁢   OUT         )       -       h   ref     ·       S   ref     ⁡     (       T             ⁢   S       -     T             ⁢   ref               ⁢   OUT         )                     M   PAC     ·     C   p                         ⅆ     T             ⁢   ref               ⁢   OUT           ⅆ   t       =         -     V   ref       ·         T             ⁢   ref               ⁢   OUT       -     T             ⁢   ref               ⁢   IN           L   ref         +           h   ref     ·     S   ref           r   ref     ·     C     p   ,   ref       ·     Vol   ref         ·     (       T             ⁢   S       -     T             ⁢   ref               ⁢   OUT         )                       
in which:
 
T REF   IN , T REF   OUT  are the temperatures of the heat-transfer fluid upstream and downstream of the cell, respectively, in K;
 
T S  is the mean temperature of the solid elements of the fuel cell, in K;
 
L ref  is the length of the flow loop BC or cooling circuit, in m;
 
ρ ref  is the density of the heat-transfer fluid, in kg/m 3 ;
 
C p, ref  is the specific heat capacity of the heat-transfer fluid, in J/kg/K;
 
h ref  is the coefficient of convection of the flow loop BC, in W·m −2 ·K −1 ;
 
S ref  is the surface area for exchange of heat between the flow loop and the cell, in m 2 ;
 
V ref  is the volume of heat-transfer fluid from the flow loop in the cell, in m 3 ;
 
W therm (I,U) is the thermal power dissipated by the cell, in W;
 
h A  is the convection coefficient at the inlet side of the anode circuit, in W·m −2 K −1 ;
 
S A  is the surface area for exchange of heat between the anode circuit and the cell, in m 2 ;
 
h C  is the convection coefficient at the inlet side of the cathode circuit, in W·m −2 K −1 ;
 
S C  is the surface area for exchange of heat between the cathode circuit and the cell, in m 2 ;
 
T A   OUT  is the temperature of the fluid elements or gases on the outlet side of the anode, in K;
 
T C   OUT  is the temperature of the fluid elements or gases on the outlet side of the cathode, in K;
 
M PAC  is the mass of the cell, in kg;
 
C p  is the specific heat capacity of the fuel cell in J·kg −1 ·K −1 ;
 
     Using the perfect gas law 
               ρ   k     =       P   k       r   ·     T             ⁢   k               ⁢   OUT                 
in which
 
k denotes A or C;
 
ρ k  is the density of the anode gas if k=A or of the cathode gas if k=C;
 
P k  is the anode pressure if k=A or cathode pressure if k=C;
 
R is the perfect gas constant; and
 
T k   OUT  is T A   OUT  if k=A, T C   OUT  if k=C
 
and the relationships are simplified by setting the following:
 
             {             α   k     =     1       h   k     ·     S   k     ·   L                       β   k     ⁡     (       P   k     ,     V   k       )       =           P   k     ·     C     p   ,   k       ·       V   k     ⁡     (   t   )         r     ·     α   k                     α   ref     =       C   p         h   ref     ·     S   ref                       
where:
 
C p,k  denotes the specific heat capacity of the anode if k=A or of the cathode if k=C, in J·kg −1 ·K −1 ;
 
V k  represents the velocity of the anode gases if k=A or cathode gases if k=C, in m/s; and
 
L denotes the length of the pipe work, in m.
 
     This then yields the following equations: 
                 T   ^     s     =       T             ⁢   k               ⁢   OUT       +         β   k     ⁡     (       P   k     ,     V   k       )       ·     (     1   -       T             ⁢   k               ⁢   IN         T             ⁢   k               ⁢   OUT           )       -       α   k     ·       L   v     ⁡     (     T             ⁢   k               ⁢   OUT       )       ·     N       k   ·     H   2       ⁢     O   ⁡     (   1   )         OUT               
in which:
 
T k   OUT  is the anode outlet temperature if k=A or cathode outlet temperature if k=C, in K;
 
T k   IN  is the anode inlet temperature if k=A or cathode inlet temperature if k=C, in K;
 
L v  is the latent heat of fusion of waters in J/mol; and
 
N k.H     2     O(l)   OUT  is the flow rate of liquid water leaving the anode if k=A or leaving the cathode if k=C, in mol/s;
 
and
 
{circumflex over (T)} s =α ref ·Q(T ref   OUT −T ref   IN )+T ref   OUT  from the flow loop
 
in which Q is the flow rate of heat-transfer fluid, in mol/s.
 
     The estimator EST can also use linearization of the non-linear system about an operating point. 
       FIG. 3  depicts a second embodiment of an estimator EST 
     This embodiment uses the equation: 
     
       
         
           
             
               
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     The estimator EST receives at input the pressure P k , with k=A in the case of anode pressure or k=C in the case of cathode pressure, used to reset the convection coefficients, the current I and the voltage U delivered by the cell, the flow rate Q of the heat-transfer fluid, and the temperature T A   OUT  of the gases leaving the anode part A of the cell PAC. The temperature T A   IN  at the inlet to the anode part A is deduced from these inputs. 
     The estimator EST can also use linearization of the non-linear system about an operating point. 
       FIG. 4  depicts a third embodiment of an estimator EST. 
     This embodiment uses the equation:
 
 {circumflex over (T)}   s =α ref   ·Q ( T   ref   OUT   −T   ref   IN )+ T   ref   OUT  
 
     The estimator EST receives at input the pressure P k , with k=A in the case of anode pressure or k=C in the case of cathode pressure, used to reset the convection coefficients, the current I and the voltage U delivered by the cell, the flow rate Q of the heat-transfer fluid and the temperature T C   OUT  of the gases leaving the cathode part C of the cell PAC. The temperature T C   IN  of the gases entering the cathode part is deduced from these inputs. 
     The estimator EST can also use linearization of the non-linear system bout an operating point. 
       FIG. 5  depicts one embodiment of a temperature control system according to the invention. The system comprises an estimator as already described, for example that of  FIG. 2 . The system comprises a comparator COMP which makes a comparison between the temperature T s  of the solid elements of the fuel cell PAC as estimated by the estimator EST and a desired temperature T S   DES  as stored in memory by the electronic control unit UCE. The comparison is transmitted to a control module MC capable of controlling the operation of the flow loop BC in which the heat-transfer fluid circulates and of its elements in order to influence the flow rate Q of heat-transfer fluid passing through the cell PAC and the inlet temperature T REF   IN  of the heat-transfer fluid. 
     The invention allows better temperature control of a fuel cell by improving the precision with which the temperature of the solid elements of the fuel cell is estimated.