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Research Papers

Equivalent Stiffness Model of a Proton Exchange Membrane Fuel Cell Stack Including Hygrothermal Effects and Dimensional Tolerances

[+] Author and Article Information
Ashley Fly

Department of Aeronautical and
Automotive Engineering,
Loughborough University,
Loughborough LE11 3TU, UK
e-mail: A.Fly@lboro.ac.uk

Rui Chen

Department of Aeronautical and
Automotive Engineering,
Loughborough University,
Loughborough LE11 3TU, UK
e-mail: R.Chen@lboro.ac.uk

Xiaodong Wang

Research Center of Engineering Thermophysics,
North China Electric Power University,
Beijing 102206, China
e-mail: wangxd99@gmail.com

1Corresponding author.

Manuscript received June 29, 2017; final manuscript received December 24, 2017; published online March 13, 2018. Assoc. Editor: Matthew Mench.

J. Electrochem. En. Conv. Stor. 15(3), 031002 (Mar 13, 2018) (10 pages) Paper No: JEECS-17-1077; doi: 10.1115/1.4039141 History: Received June 29, 2017; Revised December 24, 2017

Proton exchange membrane fuel cells (PEMFCs) require mechanical compression to ensure structural integrity, prevent leakage, and to minimize the electrical contact resistance. The mechanical properties and dimensions of the fuel cell vary during assembly due to manufacturing tolerances and during operation due to both temperature and humidity. Variation in stack compression affects the interfacial contact pressures between components and hence fuel cell performance. This paper presents a one-dimensional equivalent stiffness model of a PEMFC stack capable of predicting independent membrane and gasket contact pressures for an applied external load. The model accounts for nonlinear component compression behavior, thickness variation due to manufacturing tolerances, thermal expansion, membrane expansion due to water uptake, and stack dimensional change due to clamping mechanism stiffness. The equivalent stiffness model is compared to a three-dimensional (3D) finite element model, showing good agreement for multicell stacks. Results demonstrate that the correct specification of gasket thickness and stiffness is essential in ensuring a predictable membrane contact pressure, adequate sealing, and avoiding excessive stresses in the bi-polar plate (BPP). Increase in membrane contact pressure due to membrane water uptake is shown to be significantly greater than the increase due to component thermal expansion in the PEMFC operating range. The predicted increase in membrane contact pressure due to thermal and hydration effects is 18% for a stack containing fully hydrated Nafion® 117 membranes at 80 °C, 90% relative humidity (RH) using an eight bolt clamping design and a nominal 1.2 MPa assembly pressure.

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References

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Figures

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Fig. 1

Equivalent stiffness of a solid homogeneous material

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Fig. 2

Compressive stress strain behavior of GDL and gasket, data from Ref. [20]

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Fig. 3

Schematic of the fuel cell stack components

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Fig. 4

Equivalent stiffness representation of a fuel cell stack

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Fig. 5

Different stiffness sections in BPP

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Fig. 6

Graphical illustration of stack displacement and force change under operational conditions

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Fig. 7

1/8th 3D FEA model of a five-cell stack

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Fig. 8

MEA contact pressure distribution (Pa) on 1/4 symmetry of the center cell in a five-cell stack

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Fig. 9

Comparison of equivalent stiffness model and 3D finite element model: (a) gasket and (b) MEA

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Fig. 10

Component compression and pressure distribution for different stack clamping pressures: ((a) and (b)) gasket t=200 μm, E = 50 MPa at 13% strain, ((c) and (d)) gasket t=250 μm, E = 25 MPa at 13% strain, and ((e) and (f)) gasket t=150 μm, E = 100 MPa at 13% strain

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Fig. 11

Change in MEA clamping pressure after operation conditions applied, 1.2 MPa MEA assembly pressure at 20 °C 30% RH

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Fig. 12

Influence of stack compression during assembly on MEA clamping pressure change during operation

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Fig. 13

Variation in MEA clamping pressure during operation with different clamping methods

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Fig. 14

Cell-to-cell contact pressure distribution due to normalized random thickness variation SD = 0.5%: (a) tgasket = 200 μm, Egasket = 50 MPa and (b) tgasket = 300 μm, Egasket = 25 MPa

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