Proceedings of EFC2011
European Fuel Cell - Piero Lunghi Conference & Exhibition
December 14-16, 2011, Rome, Italy
EFC11237
THERMAL MANAGEMENT CONSIDERATIONS IN THE DESIGN OF AN
EXPERIMENTAL FUEL CELL WITH MATERIAL EVALUATION
a
a
a
a,b
a
Evan J. See, Rupak Banerjee, Michael M. Daino, Jacqueline M. Sergi, Mustafa Koz,
b
b
a
Jon P. Owejan, Jeffrey J. Gagliargo, and Satish G. Kandlikar
a
Rochester Institute of Technology, Mechanical Engineering Department, Rochester, New York, USA
b
General Motors Electrochemical Energy Research Laboratory, Honeoye Falls, New York, USA
ABSTRACT
In this study, the design of a novel in situ test fuel cell
(FC) to enable investigation of thermal and water
management with commercially equivalent hardware is
presented. Visualization of a multi-instrumented proton
exchange membrane fuel cell (PEMFC) provides a
deeper understanding of water management and
transport processes. Incorporating imaging access leads
to increased constraints within the material selection of
PEMFCs. Material and fabrication implications for optical
access are discussed, including deviations from
commercial materials and production methods. In order to
better assess the design, ex-situ experiments have been
conducted to investigate specific properties of PEMFC
components. Changing manufacturing processes create
variation in flow field geometry, surface roughness, and
hydraulic diameter which has been previously shown to
affect two phase flow and consequently FC performance
and operation. Material properties of the unipolar plate
(UPP) and optical access windows (OAW) are studied
using wettability measurements and their implications on
two phase flow is reviewed. In addition, cooling
requirements change when scaling from stack to a single
cell. A numerical investigation relates the stack material
cooling requirements with the experimental PEMFC.
INTRODUCTION
Thermal and water management is an essential factor
in the performance and operation of a PEMFC. Adequate
hydration of the membrane is required for operation, while
accumulation of product water inhibits reactant transport
within the cell. The coupling of heat and water transport
considerations across all FC components is crucial in
providing the correct balance over necessary operating
ranges. Material properties, such as surface wettability
and thermal conductivity, are vital in the performance and
two phase flow within PEMFCs. Evaluating and validating
material properties is significant to legitimizing the in situ
testing of commercial hardware analogs.
EXPERIMENTAL FUEL CELL DESIGN
The primary objective of the experimental PEMFC
design was to simulate commercial stack performance
through the use of a single visualization cell. The
influence of key components on performance and two
phase flow within the cell can be investigated. To fulfill the
objectives of our investigation into key thermal and water
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interfacial resistances in PEMFCs, a set of 50 cm
PEMFCs was developed to represent scaled current
industry design and performance targets (1). Flow field
design features, such as aspect ratio and channel
geometry, were applied from a previous water
management investigation (2). The incorporation of an
adaptable cooling system with simultaneous visible and
midwave infrared (MWIR) imaging allows detailed insight
into interfacial transport resistances. Additionally, the use
of integrated micro-thermocouples within the UPPs allows
for the characterization of interfacial thermal resistances.
These in situ investigations are designed to provide the
basis for component model development of transport in
channels, manifolds, and associated interfaces; for this
reason, the validation of experimental cell components
and design is a crucial step.
VISUALIZATION WINDOWS
A primary feature of the experimental PEMFC is its
ability to provide simultaneous anode and cathode
visualization in both visible and MWIR wavelengths. Few
materials can provide the necessary transmittance at
visible (~0.4-0.7 um) and MWIR (~3.0-5.0) wavelengths.
Transparent materials were compared to commercial FC
hardware in order to determine optimum OAW material.
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Copyright © 2011
To ensure water flow is unaffected by the OAW material,
the contact angle was measured as an indirect method of
examining surface energy. Graphite plates used in a
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commercial stack and a commercial 50 cm single FC
were utilized, which showed values of 98.3° and 93.4°
respectively. Sapphire was measured to have a contact
angle of 90.2° which represents graphite better than
Lexan, used in previous design iterations. Sapphire also
has a higher thermal conductivity (25 W/m·K) compared
to other transparent materials. This allows its thermal
behavior to be closer to the copper utilized in the UPP of
the FC. Thus, sapphire minimizes alteration to the fluid
behavior, while enabling visible and MWIR visualization.
NUMERICAL ANALYSIS OF THERMAL PROFILES
COMSOL Multiphysics® 4.2 was used to obtain the
steady state temperature profile in the through-plane
direction of a single cell. The bulk fluid flow temperature
o
in the reactant channels was assumed to be 70 C. The
thermal contact resistance was neglected between the
GDL and UPP lands. The commercial and experimental
designs are characterized by extracting temperature
readings from the UPP and GDL interface at the anode
and cathode sides. The commercial hardware’s lands
achieve a uniform temperature distribution on the entire
land region. The experimental PEMFC with a copper UPP
resulted in similar temperature uniformity at land regions.
With graphite UPP, the land temperatures were not
uniform. As shown in Figure 2, the UPP attained higher
land temperatures. Thus, the compensation of a thicker
UPP via higher thermal conductivity was effective.
Figure 1: Channel dimensions of experimental PEMFC
COOLANT CHANNELS
In order to provide precise thermal control, 4
serpentine coolant channels were machined into the
compression plate. Each section allows for independent
control, allowing up to 20°C in variation between cooling
circuits via in line electric heaters regulated with individual
PID controls. This allows for the application of
temperature profiles from PEMFC stack measurements;
allowing it to simulate any location within a FC stack.
BIPOLAR PLATE MATERIAL
Commercially available FC stacks often use
composite graphite bipolar plates due to the low electrical
contact resistance they offer. Coatings of gold and
titanium nitride are used to decrease the contact
resistance for other plate materials, but are prohibitively
expensive for mass production (3). Although composite
graphite is the preferred material, it could not be used due
to its low flexural strength, which causes it to be highly
susceptible to fracture in the 400 micrometer thick lands
under each OAW. Due to thickness constraints of the
OAW, the experimental UPP must be considerably thicker
than commercial hardware. In order to offset the change
in thickness, the thermal conductivity of the experimental
UPP must be higher than that of composite graphite.
Copper provides adequate thermal conductivity, while
also providing sufficient flexural strength. In order to
prevent corrosion of the UPP and maintain low electrical
contact resistance, gold plating was applied to the copper.
Figure 2: Channel temperature variation profile
CONCLUSIONS
An experimental FC was designed for simultaneous
anode and cathode visualization, in both visible and
MWIR wavelengths, providing the basis for in-depth
investigations into interfacial resistances within PEMFCs.
Material deviations from commercial FC stacks were
evaluated and validated.
ACKNOWLEDGMENTS
This work was supported by the US Department of
Energy under contract no. DE-EE0000470.
REFERENCES
(1) US Department of Energy. Hydrogen, fuel cells, and
infrastructure technology programs: multi-year
research, development and demonstration plan.
(2) Owejan JP, Gagliardo JJ, Sergi JM, Kandlikar SG,
Trabold TA. Water management studies in pem fuel
cells, part i: fuel cell design and in situ water
distributions. International Journal of Hydrogen
Energy 2009;34(8):3436-3444.
(3) Hentall PL, Lakeman JB, Mepsted GO, Adcock PL,
Moore JM. New materials for polymer electrolyte
membrane fuel cell current collectors. Journal of
Power Sources 1999;80(1-2):235-241.
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