Docstoc

Optimization of the H202 –NaBH4 Regenerative Fuel Cell For Space

Document Sample
Optimization of the H202 –NaBH4 Regenerative Fuel Cell For Space Powered By Docstoc
					   Use of COMSOL Multiphysics for
 Optimization of an All Liquid PEM Fuel
               Cell MEA

George H. Miley (Speaker), Nuclear, Plasma
        and Radiological Engineering
E. D. Byrd Electrical & Computer Engineering


University of Illinois at Urbana-Champaign
           Urbana, IL 61801 USA


                   COMSOL USERS CONF. 2006
OCT. 22-24, 2006        BOSTON, MA           1
                                Outline

        NaBH4/H2O2 Fuel Cell
        Description of Model
           Physical Layout
           Electrical Considerations
           Mass/Momentum Balance Considerations
        COMSOL Application Mode coupling
        Pressure Differential Simulations and
         Results
        Land Area vs. Permeability and
         Conductivity Simulation and Results
        Conclusions

                         COMSOL USERS CONF. 2006
OCT. 22-24, 2006              BOSTON, MA           2
                   NaBH4/H2O2 Fuel Cell

  Use in fuel cells is a relatively new development
  H2/H2O2 and NaBH4/H2O2 cells were investigated at
   NPL Associates, Inc., the University of Illinois
   (UIUC), and elsewhere
  Have shown great results, demonstrating the
   general feasibility of a peroxide based cell
  Excellent potential for space applications due to
   high power density and air (oxygen) independence.



                       COMSOL USERS CONF. 2006
OCT. 22-24, 2006            BOSTON, MA            3
       UIUC/NPL Direct Peroxide Fuel Cells


 The sodium borohydride/hydrogen
  peroxide reactions.
 Anode:
                                                    
 NaBH 4  4 H 2O  NaBO2  2 H 2O  8H  8e

Cathode:
                         
 H 2O2  2 H  2e  2 H 2O
                       COMSOL USERS CONF. 2006
OCT. 22-24, 2006            BOSTON, MA               4
   Test Cells - Compact 1-30 W Power Units


  The 15-W cell shown here uses an
  integrated cooling channel to dissipate
  the waste heat generated in the
  relative small 25-cm2 active area.

  An optimized version of this small cell
  generated 36-W at ~ 60ºC,
  representing the highest power
  density reported to date for a small
  fuel cell working at sub-100C.


                                            15-W NaBH4/H2O2 Test Fuel Cell as assembled.
    Flow rate of approximately 200 cm3/min
    Minimal pressure drop even with parallel flow due to low flow rate
    Temperature rise of approximately 15°C
    Heat flux is approximately equal to electrical power (500-W)
                              COMSOL USERS CONF. 2006
OCT. 22-24, 2006                   BOSTON, MA                                       5
            The 500-W UIUC/NPL NaBH4/H2O2 Fuel Cell Stack




The active area per cell was 144 cm2 and 15 cells were employed to provide a total stack active area of
2160 cm2.


                                       COMSOL USERS CONF. 2006
     OCT. 22-24, 2006                       BOSTON, MA                                                    6
       UIUC/NPL Direct Peroxide Fuel Cells




                   COMSOL USERS CONF. 2006
OCT. 22-24, 2006        BOSTON, MA           7
       Objectives for COMSOL modeling

 Gain insight into behaviors governing flow
  and current distributions
 Determine space (diffusion layer parameters,
  conductivity effects, flow channel and land
  dimensions) for detailed optimization physics
 Guide future design improvements


                   COMSOL USERS CONF. 2006
OCT. 22-24, 2006        BOSTON, MA            8
           Model Description- geometry

Physical Layout
 Based on repetitive
  cross section of MEA
  and flow channels.
 Outlined area
  represents the
  physical model.
 Portion of graphite
  plates included to see
  the current density in
  the plate and to be
  able to vary their
  conductivity.


                           COMSOL USERS CONF. 2006
 OCT. 22-24, 2006               BOSTON, MA           9
             Model Description - electrical

   Standard Electrical Model
            DC current conduction - applies to each section with different
             conductivity (graphite, diffusion layers, membrane)
                                       0
            Butler-Volmer Equations
   Anode:                                          Cathode:

 ia  i0,a
             wNaBH 4 0
                       e 
             wNaBH 4  a f a
                               e  a f a       ic  i0,c
                                                               wH 2O2 0
                                                                        e
                                                               wH 2O2  c fc
                                                                               e  c fc   
 a  d 1  m  aeq                            c  d 2  m  ceq

                                  COMSOL USERS CONF. 2006
  OCT. 22-24, 2006                     BOSTON, MA                                                10
                      Modified Bulter-Volmer

 The Butler-Volmer equation was modified to
  obtain an alternative version that is more robust
  when solving numerically in Comsol. In this
  version, the hyperbolic identity of Eq. 2-5 is used
  to form Eq. 2-6.
 cosh(x)  1 2 exp( x)  exp(  x) (2-5)
 i  2  i wNaBH cosh f 
          a          0, a
                                      4(2-6)     a   a
                            wNaBH 4 0
                            wH 2 O 2
         i c  2  i 0,c               cosh c f c 
                            wH 2 O 2 0
                                          COMSOL USERS CONF. 2006
OCT. 22-24, 2006                               BOSTON, MA           11
        Model Description – conservation
                  equations

 Mass Balance

                      Di i   R  u  i

 Momentum Balance – Darcy’s Law
                                    kp
                          v  
                                      
                         COMSOL USERS CONF. 2006
OCT. 22-24, 2006              BOSTON, MA           12
              COMSOL Application Mode
                    Coupling




                   COMSOL USERS CONF. 2006
OCT. 22-24, 2006        BOSTON, MA           13
                   Parameters used

 Necessary parameters (other than exchange
  current and equilibrium potentials, discussed
  next) were acquired through experimental
  means or published values These include the
  conductivities, permeabilities, diffusion
  coefficients, and viscosities given in the
  following table.

                    COMSOL USERS CONF. 2006
OCT. 22-24, 2006         BOSTON, MA           14
                            Parameter set 1
                    Parameter       Value        Parameter      Value
                    σ_Nafion        15 S/m          p_in     1.013e5 Pa
                   σ_Diffusion     2500 S/m        p_diff      500 Pa
                   σ_Graphite     16670 S/m       D_H2O2       3.47e-9
                                                                 m2/s
                       κ           1.22e-11      D_NaBH4      3e-9 m2/s
                                      m2
                       a           1.5 cP       D_NaBO2      1.23e-9
                                                                m2/s
                       c            1 cP             drag       3




                            COMSOL USERS CONF. 2006
OCT. 22-24, 2006                 BOSTON, MA                               15
         Parameter set 2 - determination of the
        exchange current density and reversible
                       potential
 A Hydrogen half-cell was constructed and
  used to determine the exchange current
  density and additional parameters such as the
  Tafel slope in the Butler-Volmer eqns.. The
  reversible potential of each cell half was
  determined using the Gibb’s Free Energies
  applied to the reactants and products in each
  reaction.

                   COMSOL USERS CONF. 2006
OCT. 22-24, 2006        BOSTON, MA            16
          Model verification: I-V Curve calculated for the
       reference case agrees well with corresponding experiment
              – model next used to explore design changes




                      COMSOL USERS CONF. 2006
OCT. 22-24, 2006           BOSTON, MA                         17
       Simulations – Pressure Differential

 Vary the pressure
  differential between the
  two flow channels.
 Reasons
        Different flow
         velocities create
         different pressure
         differences
        Different locations have
         different pressure drops
                        COMSOL USERS CONF. 2006
OCT. 22-24, 2006             BOSTON, MA           18
               Simulations – Pressure
         Differential- Higher values optimal
 Results
        Low pressure drops
         cause less permeation
         in the diffusion layer,
         causing mass transport
         losses.
        High pressure drops
         allow reactants to
         easily reach under the
         land area.
 Reactant permeation
  under flow channel
  depends on fluid
  velocity and location
  along channel.

                             COMSOL USERS CONF. 2006
OCT. 22-24, 2006                  BOSTON, MA           19
         Simulations – Land Area selection

 Current collector land area width to flow
  channel width ratio is varied (collector +
  channel widths = constant).
        Land Area Width varied while also varying
         diffusion layer permeability.
        Land Area Width varied while also varying
         diffusion layer conductivity.

                     COMSOL USERS CONF. 2006
OCT. 22-24, 2006          BOSTON, MA                 20
                Simulations – Land Area – high
              permeability give flexibility in width

 Max Power at different                Permeability
  Permeability with varying
  land areas.
 Low Permeability diffusion
  layers have optimum
  current collector land area
  to flow channel ratio.
 High Permeability
  diffusion layers function
  well with wide current
  collector widths.                      Maximum Power vs.
                                         Land Area Width
                       COMSOL USERS CONF. 2006
OCT. 22-24, 2006           BOSTON, MA                    21
         Simulations – Land Area – optimum with
         high conductivity and equal width design

 Max Power at different              Conductivity
  Conductivity with varying
  land areas.
 High conductivity
  diffusion layers have are
  optimum with equal width
  current collectors and flow
  channels.
 Low conductivity diffusion
  layers function better with
  wider land areas and
  narrower flow channels.
                                       Maximum Power vs.
                    COMSOL USERS CONF. Land Area Width
                                       2006
OCT. 22-24, 2006         BOSTON, MA                    22
                          Conclusions
 Simulations performed of all-liquid PEM fuel cell using COMSOL
  Multiphysics.
 Normalization uses data from half cell for io and Vrev.
 Pressure differentials, conductivities, permeabilities, and current
  collector widths varied in the simulations.
 Cell performance varies with different flow velocities and along the flow
  channels.
 Optimum current collector widths predicted for diffusion layers with
  known conductivities and permeabilities.
 Model is very useful for optimization in region around normalization.
 Simulations narrow region for experimental studies to zone near
  optimum performance. Greatly reduces time and expense of
  experimental studies.


                         COMSOL USERS CONF. 2006
OCT. 22-24, 2006              BOSTON, MA                                 23
                   Acknowledgement

We would like to thank:
        NPL Associates, Inc. for their support with starting
         the project.
        E. Byrd wishes to acknowledge fellow researchers
         N. Luo, J. Mather, G. Hawkins, and L. Guo for their
         help.
        This research was supported by DARPA SB04-
         032.
        Continuing studies are supported by
         DARPA/AFRL.

                       COMSOL USERS CONF. 2006
OCT. 22-24, 2006            BOSTON, MA                      24
                   Thank You

 For more information please contact:
Dr. George. H. Miley    Ethan D. Byrd
UIUC                    UIUC
Phone: (217) 333-3772 email: ebyrd@uiuc.edu
email: ghmiley@uiuc.edu




                   COMSOL USERS CONF. 2006
OCT. 22-24, 2006        BOSTON, MA           25

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:28
posted:7/5/2011
language:English
pages:25