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Available online at www.sciencedirect.com
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Review
A critical review of cooling techniques in proton exchange
membrane fuel cell stacks
Guangsheng Zhang, Satish G. Kandlikar*
Department of Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive, Rochester, NY 14623-5604, USA
article info
abstract
Article history:
Effective cooling is critical for safe and efficient operation of proton exchange membrane
Received 15 July 2011
fuel cell (PEMFC) stacks with high power. The narrow range of operating temperature and
Received in revised form
the small temperature differences between the stack and the ambient introduce significant
14 October 2011
challenges in the design of a cooling system. To promote the development of effective
Accepted 2 November 2011
cooling strategies, cooling techniques reported in technical research publications and
Available online 30 November 2011
patents are reviewed in this paper. Firstly, the characteristics of the heat generation and
cooling requirements in a PEMFC stack are introduced. Then the advantages, challenges
Keywords:
and progress of various cooling techniques, including (i) cooling with heat spreaders (using
Proton exchange membrane fuel cell
high thermal conductivity materials or heat pipes), (ii) cooling with separate air flow, (iii)
PEMFC
cooling with liquid (water or antifreeze coolant), and (iv) cooling with phase change
Stacks
(evaporative cooling and cooling through boiling), are systematically reviewed. Finally,
Heat generation
further research needs in this area are identified.
Cooling
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Review
reserved.
1.
Introduction
With the advantages of high power density, rapid startup and
low operating temperature, proton exchange membrane fuel
cell (PEMFC) is considered to be the most promising candidate
for the next generation power source for transportation,
stationary, auxiliary and portable applications [1e4]. Significant advances have been made in the research and development of PEMFCs, yet barriers to their commercialization still
exist, especially their durability and cost [4,5].
Although PEMFCs have a very high energy conversion
efficiency, there is still a significant amount of heat generated,
comparable to its electric power output which can be up to
100 kW in automotive applications [6]. The heat generated
must be effectively removed to avoid overheating of the
components, especially the membrane. The favorable
working temperature range for the current PEMFCs is usually
from 60 to 80 C. A higher temperature can significantly
exacerbate the degradation of the membrane and the catalyst,
and reduce the stack performance [7,8], while a lower
temperature is not favorable for the reaction kinetics and may
also cause flooding due to lower water saturation pressures at
lower temperatures, which is a major concern from the water
management perspective [9e11].
Effective cooling of PEMFC stacks is very challenging,
especially for automotive applications which require high
power output and high power density. Firstly, the temperature
difference between the PEMFC and the ambient is very small
* Corresponding author. Tel.: þ1 585 475 6728; fax: þ1 585 475 7710.
E-mail address: [email protected] (S.G. Kandlikar).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.11.010
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in comparison with the internal combustion engines. This is
especially true when the ambient temperature is high, such as
in the desert regions. Secondly, the heat removal by the
reactant and the product streams is almost negligible, which
means that most of the waste heat must be removed by the
cooling system [12]. These factors make the heat removal very
challenging and a radiator with very large heat transfer area is
needed [13e15]. To meet the stringent space requirements,
the stack temperature must be allowed to rise to 90e95 C [15]
or even 100 C [16] under difficult heat rejection conditions.
However, it is still difficult for current membrane technology
to meet these high temperature requirements.
In addition to the challenges encountered in heat removal,
cooling methods and cooling system designs also influence
the parasitic power consumption, as well as the volume and
mass of the stack. The cooling system design has a direct
impact in meeting the durability, cost, and performance
targets for commercialization [5].
Some of the cooling strategies employed for PEMFC stacks
include: cooling with increased cathode air supply, cooling
with separate air flow, cooling with heat spreaders (edge
cooling), cooling with liquid (water or antifreeze coolant) and
cooling with phase change [17e21]. Each cooling strategy has
its own advantages, limitations and challenges.
The present work is aimed at promoting the development
of more effective cooling strategies by presenting a critical
review of the reported cooling techniques and systems.
Considering that cooling with an increased cathode air supply
has a limited cooling capability, and may affect PEMFC
performance and durability (membrane dry out) [17,22], this
cooling strategy is not covered in this review. A number of
publications [17,23e25] are recommended for further information on this cooling strategy.
This review is sectioned as follows. Firstly, the characteristics of heat generation and heat transfer in a PEMFC stack
are briefly introduced in Section 2. Then the advantages and
challenges of various cooling techniques and research progress are reviewed in Sections 3e6. Based on the review and
discussion, further research needs in this area are identified in
Section 7.
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converted to electric energy. Ecell and i are the cell operating
voltage and the current density. Note that Etn can be calculated based on the higher heating value (HHV) assuming the
product water to be in the liquid phase, or based on the lower
heating value (LHV) assuming the product water to be in the
gaseous phase. The corresponding values are about 1.48 V
based on the HHV and 1.25 V based on the LHV at standard
conditions (25 C, 1 atm). These values vary little under the
normal PEMFC operating conditions [3]. While it is more
accurate to calculate Etn and q" by considering the liquid water
fraction [22], many numerical models on PEMFC cooling
assumed only gaseous water product and calculated the heat
generation based on the LHV for simplicity [17,30e34].
As indicated in Equation (1), the heat generation rate
increases with increasing current densities and decreasing
cell voltages. It is known that the cell voltage decreases with
increasing current densities [2]. Therefore, the heat generation rate in PEMFC is high at high current densities, exceeding
the electric power output, as shown in Fig. 1. This imposes
a great challenge in the cooling of stacks, especially for automotive applications which require high current density
operation for high power density.
While the calculation for the total heat generation is rather
straightforward, the local heat generation rate, which affects
the performance and the durability of PEMFCs, is not easy to
quantify accurately. The local heat generation rates vary in
both through-plane and in-plane directions. For the throughplane direction, there is still a significant discrepancy in the
understanding of distribution of entropic heat generation
[36e38]. For the in-plane direction, the distribution of the local
heat generation is usually non-uniform due to the interdependence of the local current density, temperature, reactant
concentration and the water content. A large number of
experimental studies have shown the non-uniformity of
current distribution in PEMFCs [39e53], even with small active
areas. Some other experimental studies [54e58] simultaneously measured the distribution of temperature and current
density, showing a strong dependence of the local temperature, which is an indication of the local heat generation, on the
local current density. The non-uniform distributions of the
2.
Heat generation and heat transfer in
PEMFC
It is known that there are basically four sources of heat
generation in a PEMFC, namely the entropic heat of reactions,
the irreversible heat of electrochemical reactions, heat from
the ohmic resistances and heat from the condensation of
water vapor [26e29].
For general information about the heat generation and
cooling requirements of the PEMFC stacks, the overall heat
generation rate (q", with unit W cm2) in one cell can be easily
calculated using the following equation:
q00 ¼ ðEtn Ecell Þ i
(1)
Etn is known as the thermoneutral voltage [1] or the thermal
voltage [2], which represents the imaginary maximum voltage
of a fuel cell assuming all the enthalpy change of reactions is
Fig. 1 e Heat generation vs. Power generation at different
current density (using experimental data from Fig. 8 of
Ref. [35]; heat generation calculated based on HHV).
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local heat generation and the local temperature further
complicate the cooling of PEMFC stacks.
In addition to the non-uniform distribution of heat
generation, heat transfer in PEMFCs is also very complicated
and can significantly influence the local temperature and
cooling effects. As shown in Fig. 2, Kandlikar and Lu [12]
specified the various heat transfer modes in different
components of a PEMFC and showed a representative
temperature profile across the PEMFC. Note that the values of
the thermal conductivities of the individual components were
based on ex situ measurement [59], which is so far the major
approach in measuring the thermal properties of PEMFC
components [59e70]. It was found that the thermal properties
vary greatly with (a) materials from different manufacturers,
(b) PTFE content (in gas diffusion layer, GDL), (c) water
content, and (d) compression. It is worth noting that the
thermal properties of components and interfaces in PEMFC
can be very different during operation from those measured
in ex situ experiments. On the one hand, the compression of
components in an operating PEMFC is non-uniform under the
lands and the channels [71,72], which can cause varying
thermal properties. On the other hand, heat transfer is
strongly coupled with water distribution and transport in an
operating PEMFC [12], which is another critical issue in PEMFC
development [10,11]. Temperature driven water transport, an
important mechanism of water transport that attracts great
attention recently [73e76], is a very good example demonstrating the strong coupling of heat transfer and water
transport in an operating PEMFC. It includes thermo-osmosis
in the membrane and phase-change-induced flow in porous
diffusion media, both of which strongly depend on the
temperature gradient and average temperature of the
components. In return, such water transport can greatly
influence the heat transfer and temperature distribution, and
therefore effective cooling of PEMFC stacks. Yet thermal
property data from in situ experiments are still very limited
[77e79], and all the reported work used intrusive methods
which caused modification of PEMFC components and
possibly also their thermal properties.
3.
Cooling with heat spreaders
Cooling with heat spreaders, also called edge cooling [20,22] or
passive cooling [80,81], relies on heat conduction in the inplane direction of cooling plates to remove the heat from the
central region to the edges of the PEMFC stack [81]. Heat can be
more easily removed from the edges than from the central
region of a stack. In comparison with the active liquid cooling,
the edge cooling strategy has no coolant circulation inside the
stack, and therefore can eliminate the need for a coolant
pump that must be used in the conventional liquid cooling
systems. It can also reduce the mass and the complexity of the
cooling system, and improve the overall system reliability in
comparison with conventional liquid cooling [81,82], as schematically shown in Fig. 3.
A major challenge in the cooling with heat spreaders is that
the in-plane thermal conductivity of the cooling plates must
be very high to control the temperature variation across the
active area. Using highly thermal conductive materials and
heat pipes are the two main approaches.
3.1.
Using highly thermal conductive material as heat
spreaders
Graphite based materials, e.g. expanded graphite and pyrolytic graphite, are the most widely used as heat spreaders in
PEMFCs due to their high thermal conductivity and low
density. Fluckiger et al. [22] developed a 500 W PEMFC
stack which used separator plates made of expanded graphite
Fig. 2 e A representative temperature profile across a PEMFC along the channel region (Section AA) and land region (Section
BB) with individual layer thermal properties and typical heat generation values presented. Thicknesses and temperature
gradients not to scale [12] (Reproduced from [12] with permission).
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Fig. 3 e Comparison between (a) a conventional thermal management system and (b) a passive (cooling with heat spreaders)
thermal management system of PEMFC in space applications [81] (Reproduced from [81] with permission).
(SGL - SIGRAFLEX with in-plane thermal conductivity of
290 W m1 K1) as heat spreaders, as schematically shown in
Fig. 4. They also developed a thermal model to analyze and
optimize the cell design. It was found that the optimization
requires a trade-off between specific power and specific cost:
a high specific power requires thin separator plates and large
active areas, which, however, are not desirable from the cost
and the heat removal perspectives.
Wen et al. experimentally investigated the effects of
pyrolytic graphite sheets (PGS), which have a very high inplane thermal conductivity (600e800 W m1 K1), as heat
spreaders in a single PEMFC [83] and in a 10-cell stack [84,85].
PGS with thickness of 0.1 mm were cut into the shape of flow
channels and bound with the cathode gas channel plates as
shown in Fig. 5. They measured the temperature distribution
in the plates. It was found that PGS reduced the maximum cell
temperature and improved the temperature uniformity in the
single cell [83]. The results in the stack showed that the PGS
improved the stack performance and significantly alleviated
the flooding problem at low cathode flow rates, thus
Fig. 4 e Top view of an open cell in a PEMFC stack with
edge cooling [22]. (Reproduced from [22] with permission
from Elsevier).
demonstrating the feasibility of using the PGS for thermal
management of small-to-medium-size PEMFC stacks [84].
Cooling PEMFC stacks with heat spreaders was also
investigated to reduce mass and improve reliability of space
fuel cell systems for NASA exploration programs [80e82].
Burke [80] analytically investigated the feasibility of using this
approach to control the temperature variations within 3 C
with heat generation rate 0.3 W cm2. It was found that this
approach requires either low heat conduction distances
(10 cm), very high thermal conductivities (1000 W m1 K1),
or very thick heat spreaders. Materials with high thermal
conductivity, e.g. highly oriented pyrolytic graphite (HOPG)
with thermal conductivity up to 1500e1700 W m1 K1, were
considered as suitable candidates for a heat removal distance
10 cm. While for larger fuel cells (heat conduction distances
Fig. 5 e Precut PGS aligned with the cathode flow field plate
[83]. (Reproduced from [83] with permission from Elsevier).
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10 cm), using heat pipe is probably the only viable approach.
Recently, Burke and his co-workers [81] experimentally
investigated the feasibility of passive cooling with HOPG
composite cooling plates. Thin thermal pyrolytic graphite
(TPG, one type of HOPG) plates with thickness of 0.38 mm were
laminated with even thinner metal foils (stainless steel or
copper foils with thickness of 0.05 mm) to improve the TPG’s
low mechanical strength. Effective thermal conductivity of
those composite cooling plates were measured to be
962 W m1 K1 (316 SS clad TPG) and 1105 W m1 K1 (Copper
clad TPG). The results showed that TPG/metal composite
plates were capable of cooling small fuel cells (heat transmission distance 6 cm). More recently, TPG/copper
composite cooling plates were integrated with different edge
heat exchangers to effectively remove heat from the cooling
plates [82]. It was reported that the cooling plate temperature
would be maintained within the operating range by properly
controlling the flow rate through the heat exchangers.
In comparison with the cooling in conventional low
temperature PEMFCs, cooling with heat spreaders in high
temperature PEMFCs (HT-PEMFC) is easier due to the higher
operating temperature (above 100 C [7,25,86]), which makes
this cooling strategy feasible for PEMFCs with larger active
area. It also offers the advantages of avoiding coolant sealing
problem in active area and more compact cell design over
conventional cooling methods employed in HT-PEMFCs, e.g.
using thermooil, steam or pressurized water [87]. Scholta et al.
[87] developed a 10 cell stack which was externally cooled by
four cooling tubes. The bipolar plates also serve as heat
spreaders. They investigated the temperature distribution in
the bipolar plates by both modeling and experiments. Fig. 6
shows their modeling results of temperature distribution
under typical operating conditions (assuming coolant
temperature fixed at 100 C, q" ¼ 0.28 W cm2 at active area).
The locations of the four cooling tubes (fitting in the semicircular openings on the left and the right) and some thermocouples embedded in the bipolar plate for experimental
validation (indicated by the bracketed numbers) were also
shown. It was reported that temperature variation across the
active area (10.4 cm 10.3 cm) could be controlled below 15 C
under typical operating conditions, which was considered
Fig. 6 e Results from the numerical model for cell
temperature distribution (oC) with edge cooling from four
tubes (positions of temperature sensors also indicated)
[87]. (Reproduced from [87] with permission from Elsevier).
allowable [87] and proved the feasibility of the external cooling design. It is a pity that information of thermal conductivity
and thickness of the bipolar plates, which could influence the
cooling and temperature distribution, were not presented.
It is worth mentioning that there are some designs of
integrated bipolar plates, in which the reactant gas flow fields
and the cooling flow fields are on the same plate surfaces of
bipolar plates [88e90]. The reactant gas flow fields are divided
into multiple sub flow fields, which are surrounded by the
cooling flow fields. Although these cooling techniques use
liquid coolant, they also rely on the in-plane heat transfer
from the reactant gas flow fields to the coolant flow fields.
Therefore, these techniques may be essentially considered as
cooling through the heat spreaders and the bipolar plates
must have a high enough thermal conductivity to avoid large
temperature variation in the in-plane direction. However, the
use of multiple sub flow fields in a bipolar plate reduces the
severity of the heat conduction length requirements in an
edge cooling system.
3.2.
Using heat pipes as heat spreaders
With extremely high effective thermal conductivity, heat
pipes even with a small cross-sectional area can transport
large amounts of heat over a considerable distance with no
additional power input [91]. Therefore, using heat pipes for
cooling of PEMFC stacks has received increasing attention in
recent years [81,91e94]. A great challenge of using heat pipes
as heat spreaders is the design and fabrication of heat pipes
that can be integrated into PEMFC stacks.
Faghri and Guo [91] described two approaches of integrating heat pipes into a PEMFC stack. One approach was to
embed micro heat pipes into bipolar plates with fabricated
holes, as shown schematically in Fig. 7 (a) [95]. The challenge
in this approach is fabricating the bipolar plates with the
requisite holes and sealing the heat pipe within the holes. The
other approach was to integrate flat heat pipes with the
bipolar plates, as shown in Fig. 7 (b) [96]. Fabricating and
sealing such heat pipes could be also challenging.
Vasiliev et al. [92,93] also actively advocated the application of heat pipes in PEMFC thermal management. Different
types of heat pipes were proposed and developed for PEMFC
stacks with different heat dissipation needs, including micro
and miniature heat pipes (1e10 W), loop heat pipes
(10e100 W), pulsating, and sorption heat pipes (100e1000 W).
The heat transfer characteristics of various heat pipes were
also investigated.
Rulliere et al. [94] experimentally and numerically investigated the application of planar heat pipe, which was called
two-phase heat spreader (TPHS), in PEMFC cooling. They
developed a planar TPHS with an evaporating area (cooling
area) of 190 mm 90 mm. The TPHS was made of 109 longitudinal micro-grooves machined in a copper plate. Using
methanol as working fluid, they demonstrated that the
temperature variation over the entire evaporator area was
lower than 1.6 K for a heat generation rate of 0.5 W cm2 at
70 C, which was similar to the working condition of PEMFC.
Other working fluids were also investigated and the results
showed that the choice of working fluid is important. It is
worth noting that the thickness of TPHS was more than
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Fig. 7 e (a) Micro-heat pipe embedded in a bipolar plate [95]
and (b) Integrated bipolar plate heat pipe [96] (Reproduced
with permission).
3.6 mm, which needs to be reduced to be implemented in real
PEMFC stacks for high power density.
Recently, Burke et al. [81] experimentally tested the effective thermal conductivity of planar heat pipes designed for
passive cooling of PEMFC stacks for NASA exploration
programs. A copper planar heat pipe and a titanium planar
heat pipe were tested, and the results were compared with
HOPG composite cooling plates mentioned above. The titanium planar heat pipe was thinner, lighter and more effective
than the copper one. Its thickness was 1.19 mm and the
measured effective thermal conductivity was up to
20,447 W m1 K1, an order of magnitude higher than that of
HOPG composite cooling plates. The very high effective
conductivity of such planar heat pipe made it feasible for large
PEMFC stacks. The low density of titanium also made it lighter
than HOPG composite cooling plates.
4.
Cooling with separate air flow
Considering the fact that increasing the cathode air supply for
cooling can cause dry out of the membrane, it is reasonable to
have separate channels for cooling air to flow through [17].
The cooling channels can be made in the bipolar plates or in
separate cooling plates which are placed between the bipolar
plates. This cooling strategy is usually suitable for PEMFC
stacks in the range of 100 W to 2 kW. For PEMFC stacks with
power output greater than 5 kW, cooling with air may not be
sufficient, or not as advantageous as the liquid cooling [17].
Schmidt et al. [97] from Siemens reported the development
of PEMFC stacks cooled by separate air flow for automotive
applications in 2002. They used low cost thin sheet metal as
construction material, in which gas channels and manifolds
were manufactured by stamping and punching processes.
Such processes are suitable for mass production. Gaps
2417
between adjacent cells in the stack were formed for air cooling. A blower was used to provide cooling air under rated
operation, which typically consumed below 3% of the stack
power. Based on this concept, a 35-cell stack with an active
area of 310 cm2 and a 6-cell stack with an active area of
100 cm2 were built, which demonstrated the feasibility of
using low cost air cooled PEMFC stacks for automotive applications. However, it was suggested by the authors that the
water-cooled type stack should be developed for effective
system integration.
Sohn et el. [98] fabricated a 500 W PEMFC stack cooled by
a separate air flow for portable applications. It was found that
the cooling fan consumed less than 2% of the overall power
output with optimized design and operation.
Recently, Matian et al. [99] numerically and experimentally
investigated the design of cooling plates for a typical air-cooled
stack configuration. It was found that more uniform temperature distribution could be achieved by increasing the size of
the cooling channels for a given pressure loss, but thinner
cooling plates need to be used to make the stack compact,
which can result in a higher pressure drop and an increased
parasitic power loss. Therefore, a trade-off between temperature uniformity and pressure drop needs to be considered.
The Nexa power module, introduced by Ballard Power
Systems in 2001, provides power up to 1200 W. It is a representative of PEMFC stacks cooled with a separate air flow [17].
a number of studies have been conducted on this power
module [100e103]. Adzakpa et al. [101] developed a 3D
dynamic thermal model to study the temperature distribution
in a PEMFC stack cooled by air from the bottom to the top. The
model was validated using the experimental data from
a Nexa power module. It was found that the temperature
non-uniformity was high, with temperature at the top part up
to 5 C higher than that at the bottom part. The nonuniformity of the air cooling between the cells of the stack
was also found to lead to large temperature variations, up to
8 C, from one cell to another. Gao et al. [102] developed
a multi-physical dynamic stack model and also validated the
model temporally and spatially against a Nexa power
module. Choi et al. [103] experimentally investigated the
effects of ambient temperature and relative humidity on the
performance of a Nexa power module. They found that the
ambient temperature significantly affected the maximum
power and net efficiency, while the ambient relative humidity
had statistically insignificant effects.
Recently, Okano et al. [100] investigated the effects of a cooling failure by plugging cooling channels on both sides of a unit
cell in a Nexa power module. They found that the temperature
of the plugged cell increased by almost 9 C within 2 h.
The effects of cooling failure were also investigated by Park
and Caton [104], who experimentally compared the performance of an 8-cell stack (100 cm2 active area) with air cooling
and without cooling. It was observed that the stack power
dropped from 150 to 90 W after 40 min of operation without
cooling, while the power was well maintained around 140 W
with normal air cooling during the testing.
In 2006, Ballard introduced the Mark 1020 ACS [105], a new
generation of air cooled PEMFC stacks over the Nexa power
module. It has a power output range of 1e5 kW and provides
several significant improvements over the earlier generation
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by using one third fewer parts and incorporating new, lower
cost materials.
5.
Liquid cooling
It is well known that the heat transfer coefficients with liquid
flow are much higher than that with air flow for the same
pumping power [106]. Therefore, liquid cooling is currently the
most widely used cooling strategy in high power PEMFC stacks
(>5 kW). It is especially suitable for automotive PEMFC stacks
which typically have a power output of 80 kW or higher [6].
The liquid coolant is usually deionized water which has the
advantage of very high heat capacity, or an antifreeze coolant,
e.g. mixture of ethylene glycol and water [2,107], for operation
under subzero conditions. Similar to the cooling with a separate air flow, liquid coolant flows in the cooling channels,
which are usually integrated in the bipolar plates as schematically shown in Fig. 8. Note that it is possible to place more
than one cell between every two liquid cooling layers to
reduce the number of cooling layers. However, it was shown
through numerical modeling that the stack performance
would decrease if the number of cells between the two
successive cooling layers was increased [19,29]. The reason
was attributed to the temperature increase and water content
decrease in the membranes farther away from the cooling
plates. It suggests that simply decreasing the number of
cooling plates without optimization of the coolant channel
configuration may lead to a lower performance and a higher
risk of membrane overheating.
Due to the wide application of liquid cooling, especially for
automotive PEMFC stacks, numerous efforts have been made
in improving the cooling performance, including optimization
of the coolant flow field design, optimization of the cooling
channel geometry, development of alternative coolants and
optimization of the cooling system.
5.1.
Optimization of coolant flow field configuration
Chen et al. [109] conducted a thermal analysis of the coolant
flow field configuration to optimize the cooling flow field
design of a PEMFC stack. A concept of Index of Uniform
Fig. 8 e Schematic of repeat unit of automotive PEMFC
stacks showing coolant channels [108] (Reproduced from
Ref. [108] with permission from Elsevier).
Temperature (IUT), which is the temperature variation over
the entire area of cooling plates, was proposed to evaluate the
degree of uniform temperature profile across the cooling
plates. Six coolant flow field configurations, including three
serpentine-type and three parallel-type, were analyzed and
compared. It was found that serpentine configurations had
lower IUT (better cooling effects) than parallel configurations
because the distribution of coolant flow in parallel configurations was poor. Particularly, the modified serpentine-type
cooling configuration shown schematically in Fig. 9 (b), has
the lowest IUT among all the six configurations. The
improvement was attributed to the alternative distribution of
cold and warm coolant flow passages. The pressure drop of
the cooling passages was also compared and it was found that
parallel-type configuration had a lower pressure drop than
serpentine-type configuration. Therefore, the optimization
between the cooling effects and the pressure drop was suggested for further investigation. A recent numerical investigation by Choi et al. [32] also addressed the effects of the
coolant flow field configuration on the cooling effects and
similar results were obtained.
Interestingly, the convection enhanced serpentine flow
field (CESFF) design developed by Xu and Zhao [110], similar to
the modified serpentine coolant flow field design shown in
Fig. 9 (b), was proved very effective in improving both cell
performance and operating stability as opposed to the
conventional serpentine flow field design. Inspired by the
CESFF design [110] and the modeling results that similar
designs for coolant flow fields could result in a uniform cooling [32,109], a multi-pass serpentine flow field (MPSFF) design
was proposed by Nam et al. [111,112]. Initially the design was
used in reactant flow fields to improve the under-rib convection [111] and then as the coolant flow fields [112]. It was found
through numerical modeling that MPSFF yielded a better
cooling performance than conventional serpentine cooling
flow fields, in terms of both the maximum temperature and
the temperature uniformity [112]. In a recent numerical
investigation [113], parallel MPSFFs as coolant flow field were
also found to have a better performance than conventional
parallel serpentine flow fields with similar pressure drop.
Recently, Asghari et al. [30] numerically investigated the
design of cooling flow fields for a 5 kW PEMFC. A parallel
serpentine flow field design was employed and it was found
that the inlet and outlet manifolds of reactant gases also had
an influence on the temperature distribution in the bipolar
plates. Therefore, they suggested that the uniformity of
temperature distribution should be considered in the design
of reactant manifolds.
In a patent by Lee [114], a coolant flow field design with
regions of varying flow volume, accomplished by controlling
the flow resistance in the channels, was proposed. The regions
with varying flow volume enabled a variable cooling across the
stack, thereby achieving a uniform temperature field.
While most of the works mentioned above aimed at
achieving a uniform temperature distribution in the bipolar
plates, it was pointed out by Wilkinson and St-Pierre [115] and
Mench [2] that the non-uniform cooling could be more beneficial to PEMFC if properly optimized with the reactant gas flow
configuration. If the coolant channel design and the flow rates
are such that the temperature in the gas channels increases
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2419
configurations tested. The findings were consistent with those
by Wilkinson et al. [116]. The performance improvement was
attributed to a better membrane hydration in the reported
configuration.
Different coolant flow field designs, with single or multiple
channels, were also numerically investigated by Hashmi [19].
Rather than the temperature uniformity and the pressure
drop, the total entropy generation was suggested as the
criterion for optimization. It was reported that the conventional single serpentine design was better than the modified
single serpentine design [109] on the basis of the total entropy
generation criterion, whereas the latter was found to be better
on the basis of temperature uniformity.
5.2.
Fig. 9 e Schematic of (a) conventional serpentine flow field
and (b) modified serpentine flow field [109] (Reproduced
from Ref. [109] with permission from John Wiley and Sons).
from the inlet to the outlet, it can be used to effectively mitigate the flooding issue by increasing the saturation pressure
of water along the flow channels. Early in 1998, Wilkinson
et al. [116] patented a stack design with concurrent flow of
coolant and oxidant streams in combination with a countercurrent flow of the fuel and oxidant streams, which had the
features of non-uniform cooling mentioned above. The
experimental results with a 4-cell stack showed that the
design exhibited superior performance at each of the
temperature differentials tested (5, 10, 15 and 20 C) to the
design with concurrent flows of the coolant, and the oxidant
and fuel streams. Recently, Kang et al. [117] numerically
investigated the effects of flow field configuration of the fuel,
air, and cooling water paths on the performance of PEMFCs. It
was found that the configuration with a fuel-air counter flow
(multiple serpentine flow channels) and an air-coolant coflow had the highest performance among five different
Optimization of coolant channel geometry
While significant efforts have been devoted to improving the
liquid cooling performance by optimizing the coolant flow
field design, there are some interesting studies on optimizing
the coolant channel geometry to improve the heat transfer
efficiency between the bipolar plates and the liquid coolant.
To enhance the convective heat transfer between the
bipolar plates and the liquid coolant, Lasbet et al. [118,119]
proposed modifying channel geometries to create chaotic
laminar flow inside the cooling channels. The heat transfer
efficiency, pressure loss and mixing properties of several
chaotic 3D minichannels, namely C-shaped, V-shaped and Bshaped channels, were numerically evaluated and compared
with conventional straight channels. Their modeling results
showed that the 3D chaotic channels could significantly
improve the convective heat transfer performance (Nusselt
number was 4e7 times higher) over that of the conventional
straight channels, outweighing the increase of pressure loss
(1.5e2.6 times higher). While better compromise between the
heat transfer performance and the pressure loss was obtained
with such 3D chaotic channels [118,119], the manufacturing
cost and the cooling plate thickness could be also increased
due to the complex 3D chaotic geometry.
In the development of PEMFC stacks for fuel cell vehicles,
e.g. FCX Clarity, Honda R&D Co. employed a novel V flow stack
structure, in which hydrogen and air flow vertically along the
wavy channels while the coolant flows horizontally across the
vertical gas flow [120e122], as schematically shown in Fig. 10.
With this design, a more uniform cooling was achieved and
the number of cooling layers was reduced by half, which
contributed to a significant reduction in the size and weight of
the PEMFC stack [120].
5.3.
Efforts in maintaining low electrical conductivity of
coolants
Although water and glycol have been widely used as coolants
in PEMFC stacks, there is an issue of the coolant becoming
electrically conductive as it degrades due to ion contamination from the bipolar plates [123] or ionic production from
oxidation of the glycol [107]. Conductive coolant can cause
a current leakage through the coolant loop, which can reduce
stack efficiency and lead to coolant electrolysis, even cause
degradation of the bipolar plates [124,125]. To solve the
problem, the electrical conductivity of the coolant is usually
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Fig. 10 e Detailed view of vertical wavy flow channel separator and coolant flow configuration in Honda’s PEMFC stack
(Reproduced from Ref. [120] with permission).
monitored [123] and a large amount of ion-exchange resin is
used to remove the ionic material from the coolant [107,126].
The approaches of using a filter with activated carbon particles supporting ruthenium to remove the oxidation products
or injecting an inert gas, preferably nitrogen, into the coolant
circulation to purge oxidation products from the coolant were
also proposed [127].
All the approaches mentioned above increase the cost and
complexity of the PEMFC system. Therefore, great efforts have
been made to develop alternative liquid coolants or adding
antioxidants to maintain a low conductivity in the coolant.
Takashiba and Yagawa [107] reported the development of
antioxidant additives to reduce the amount of the ion-exchange
resin in the base coolant of ethylene glycol and water. It was
reported that one of the antioxidants reduced the oxidation
production by 90% and the amount of the ion-exchange resin
was reduced by 40% with the addition of the antioxidant.
Examples of developing alternative coolants or antioxidants also include: (a) using kerosenic hydrocarbon (i.e.
kerosene) or an emulsion of water in kerosene as coolant,
which was dielectric, inexpensive and requires no ionexchange resin at all [125], (b) adding one or more carboxylic
acid salts, preferably the amine or ammonium salts from C5C18 mono- or di-carboxylic acids, to the coolant to maintain
low conductivity [128], and (c) adding specific nanoparticles to
the base coolant (glycol/water mixture) to mitigate the ion
contamination [129,130], which was demonstrated to be
capable of maintaining a low electrical conductivity for
hundreds of hours while exhibiting good thermo-physical
properties without using a de-ionizing filter.
Recently, Shyu et al. [131] proposed a design of circulating liquid electrolyte as a coolant for the PEMFC cooling.
With this design, the PEMFC can be operated without
reactant gas humidification and the coolant is nonconductive for electron, reducing the complexity of the
overall system. Unfortunately, the electrical performance of
PEMFC with this design was much lower than that with
conventional design.
5.4.
Optimization of liquid cooling system for
automotive PEMFC stacks
Liquid cooling is widely used in PEMFC stacks with high power
for the automotive applications, where space and weight are
challenging restrictive factors. Fig. 11 shows a reference
automotive fuel cell system configuration [15]. It can be seen
that heat must be transferred out of the vehicle to the ambient
through a radiator. The size of the radiators used in the PEMFC
vehicles is currently very large due to the small temperature
difference between PEMFC stack and the ambient air. It was
shown by Ahluwalia et al. [15] that the size of the radiators is
an extremely important limiting factor. The operating
temperature of the PEMFC stack even has to be increased to
90e95 C under critical driving conditions to accomplish the
heat removal. The thermodynamic analysis by Rogg et al. [13]
and the field tests of Volkswagen’s fuel cell vehicle [16]
showed similar results. However, the PEMFC stacks operating at such a high temperature is still difficult for the
currently used membranes. Therefore, further research on
optimizing the entire cooling system for automotive PEMFC
stacks is warranted.
In order to improve the cooling capability during high heat
generation conditions with a fixed radiator capacity, Kim et al.
[132] proposed using a CO2 air-conditioning unit as a supplementary stack cooling system. Fig. 12 schematically shows the
cooling system layout. The bypass valve 2 is normally closed
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Fig. 11 e A reference automotive fuel cell system configuration [15] (Reproduced from Ref. [15] with permission from
Elsevier).
so that the air conditioner provides the inner cabin cooling.
When there is a higher demand for stack cooling, the bypass
valve 2 will be opened and the inner cooling fan stopped so
that the air-conditioning unit is operated not for cabin cooling
but for stack cooling. It was reported that the heat rejection
rate of the stack cooling system could be increased up to 36%
with the aid of a CO2 air-conditioning unit. In their more
recent study [133], the performance of a CO2 air-conditioning
unit for the PEMFC stack cooling was investigated under
various conditions, and the relationship between cabin cooling and stack cooling was also studied.
Jang et al. [134] developed a simple model to investigate the
dynamic temperature variations of the PEMFC stack, radiator,
cooling water and air at the radiator outlet. Their parametric
studies showed that increasing heat transfer coefficient in the
radiator could be more effective than that in the stack in terms
of complexity and cost. Therefore, it was suggested that more
attention should be given to the design of the radiator for
improving cooling efficiency.
Yu and Jung [135] also numerically investigated the cooling
system including a radiator, a cooling pump and a fan. The
trade-off between the temperature distribution effect and the
Fig. 12 e Design of the stack cooling system using CO2 air conditioner [132] (Reproduced from Ref. [132] with permission
from Elsevier).
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parasitic power loss was investigated. The maximum
temperature of the PEMFC was suggested as the representative operating temperature for feedback control of the cooling
fan while the optimal inlet coolant temperature was suggested as the reference value for the coolant pump control in
minimizing the cooling parasitic losses.
Berger et al. [16] reported their work on optimization of
a cooling system for fuel cell vehicles at Volkswagen using
fluid simulation and wind-tunnel testing. A field test with
demanding uphill driving conditions (ambient temperature
ranged from 50 to 38 C with road inclination of up to 10%) was
also conducted. Fig. 13 shows the actual heat rejection rate
during the field test and the projected heat rejection capabilities by operating PEMFC stack at 85 C and 100 C. It was found
that the PEMFC stack temperature had to be increased to
100 C to reject as much heat as needed during the demanding
driving conditions. It was also reported that the size of the
radiators in PEMFC vehicles could be comparable to those of
internal combustion engine vehicles only when the coolant
temperature was allowed to rise to at least 120 C.
Recently, a parameter, Q/ITD, which is the ratio of rejected
heat (Q) to the initial temperature difference (ITD) between the
PEMFC stack and the ambient air, was proposed as a new
target for the automotive PEMFC stack by some US fuel cell
vehicle developers [136]. This parameter is an indicator of the
radiator size and is considered to be more reasonable than the
stack temperature target in terms of the heat rejection. The
target of Q/ITD for a 80 kW stack at the rated power and a 40 C
ambient temperature was suggested to be lower than 1.35 1.5 kW K1 [136].
6.
Phase change cooling
While liquid cooling have been widely used in large PEMFC
stacks with success, cooling with phase change of the coolant
has a number of attractive advantages over liquid cooling,
e.g., reducing coolant flow rate, simplifying system layout,
Fig. 14 e Evaporative cooling using wick material (a) As
lands or (b) placed around channels of the cathode bipolar
plates [145] (Reproduced from [145] with permission).
and eliminating coolant pumps [137e139]. Unlike liquid
cooling which uses the sensible heat of the coolant, phase
change cooling uses the latent heat of the coolant, which is
usually very high (e.g. the latent heat of water is 2250 kJ/kg at
1 atm, more than 500 times higher than the sensible heat
absorbed by liquid water with a temperature increase of 1 C),
and therefore requires a much lower coolant flow rate. In
addition, the coolant can be circulated through hydrophilic
wicking, pressure difference or density difference, so the
coolant pumps can be eliminated with the phase change
cooling.
There are basically two approaches of phase change cooling: evaporative cooling and cooling through boiling. In
evaporative cooling, the boiling temperature of the coolant is
Fig. 13 e Comparison of heat flow obtained from field test and that can be rejected with stack temperature of 85 C and
100 C for Volkswagen’s fuel cell vehicle [16] (Reproduced from Ref. [16] with permission).
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Fig. 15 e Schematic of PEMFC with WTPs allowing liquid
water exchange [141] (Reproduced from Ref. [141] with
permission from Elsevier).
higher than PEMFC stack temperature, and therefore water is
suitable and typically used. While in cooling through boiling,
the boiling temperature of the coolant must be lower than the
PEMFC stack temperature, which needs selection of suitable
coolants.
6.1.
Evaporative cooling
Evaporative cooling with water has been studied extensively
[137,140e146] because water is produced in PEMFC and water
is also used for humidification of the reactant gases. Therefore, evaporative cooling system can be integrated with
humidification and water removal [137]. The evaporative
cooling with water can be realized using (a) direct introduction
of the liquid water into the reactant gas channels [146e148],
(b) bipolar plates with in-plane wicking material [145], or (c)
porous water transport bipolar plates [140e142].
Brambilla and Mazzucchelli [146] developed a design of
cooling and humidifying PEMFC through direct injection of
water coming from a single hydraulic circuit. Various types of
reticulated elements with channels for water injection were
incorporated. Demonstration with 15e45 cells with such
a design showed that stacks thus produced are more compact,
less expensive and easier to operate.
Snyder [147] reported a preliminary study on the feasibility
of evaporative cooling by ultrasonic nebulization of water into
the anode gas stream. It was found that the method increased
the cell voltage output and reduced the voltage oscillation as
compared to the conventional water and thermal management methods. However, no conclusion on the feasibility was
reached due to the limitation on the liquid water particle
density in the anode gas stream and high energy consumption
needed to nebulize the water in the study.
Recently, Adcock et al. [148] reported the successful
application of evaporative cooling through direct water
injection in PEMFC stacks ranging from 500 W to 75 kW. It was
shown that the cooling system with this cooling strategy is
much simpler than that with conventional liquid cooling. A
10 kW module with this cooling technique was demonstrated
to successfully start from 25 C.
Goebel [145] from General Motors patented a design of an
evaporatively cooled PEMFC. In this design, a wicking material
is used as the lands (shown in Fig. 14 (a)) or placed around the
2423
channels (shown in Fig. 14 (b)) of the cathode bipolar plates,
while the main part of the bipolar plate is impervious to both
water and gas flow. The wicking material allows in-plane
distribution of the liquid water for evaporative cooling and
internal humidification, thereby simplifying the overall system.
A different approach of evaporative cooling, using porous
water transport bipolar plates, was proposed and continuously
advanced by UTC Power [137,140e144]. The porous bipolar
plates, known as water transport plates (WTPs), allow direct
exchange of liquid water between the gas channels and the
coolant channels through the pores. Fig. 15 shows a schematic
of a PEMFC using the WTPs. With proper design of the size, the
distribution and the surface hydrophobicity of the pores to form
a wet seal to prevent hydrogen or oxygen entering the coolant
stream, simultaneous internal humidification and evaporative
cooling of the PEMFC stacks can be realized. The membrane dry
out or flooding issues can be also prevented [141].
Meyers et al. [137] described a design and a mode of operation for a PEMFC that was evaporatively cooled using WTPs.
The design resulted in a compact, simple system that retained
key advantages of internal humidification (no need for
external humidification) while drastically reducing the water
inventory (important for cold start) and ensuring operability
over a wide range of conditions. Fig. 16 shows the conceptual
layout of a PEMFC system. Coolant pump was eliminated,
which not only reduced the parasitic power consumption, but
also improved the system reliability by avoiding direct contact
of moving mechanical parts with the liquid water. They
developed a 30-cell stack (each cell with an active area of
320 cm2) with this design, which showed very small concentration losses. The ohmic losses were found consistent with
a fully hydrated membrane. It was demonstrated that an
evaporatively cooled PEMFC system can be both durable and
low cost, and can attain a relatively high power density obtained at ambient-pressure.
Fig. 16 e Conceptual layout for evaporatively-cooled PEMFC
system by UTC power [137] (Reproduced from Ref. [137] by
permission of The Electrochemical Society).
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Fig. 17 e Schematic of pumpless cooling of an HT-PEMFC
through water boiling [139] (Reproduced from Ref. [139]
with permission from Elsevier.).
Recently, Darling et al. [142,143] further improved the
evaporative cooling technique by using WTPs only on one side
(anode) while using plate impermeable to gas or water on the
other side (cathode). In comparison with the previous design
that uses WTPs on both sides, this design contains around half
less liquid water when the PEMFC stack is shut down, which
greatly reduces the thermal mass of the stack and thereby
consumes less energy during cold start.
6.2.
Cooling through boiling
Cooling through boiling is a very important cooling technique
due to its very high cooling capability. It has been widely used
in the fields of refrigeration and cooling of high heat flux
devices, such as computer chips, laser diodes, and other
electronics devices and components [149]. This approach can
be also applied in the cooling of PEMFC stacks. Obviously, to
make it applicable, the boiling temperature of the coolant
must be lower than the normal operating temperature of the
PEMFC stack. HFE-7100, a coolant with boiling temperature of
61 C at atmospheric pressure, appears to be a promising
candidate [138,150]. Garrity et al. [138] developed a cooling
plate with HFE-7100, in which the coolant flow was driven by
gravity, thus requiring no coolant pump. They measured the
variation in the mass flow rate, temperature field, and pressure drop at different heat fluxes. The maximum heat flux
before the onset of two-phase flow instability was 3.2 W cm2,
exceeding that in normal PEMFC stacks. The plate wall
temperatures ranged from 66 to 82 C, depending on the heat
flux and the position on the plate. A thermal hydraulic model
was also presented which gave a satisfactory prediction of the
mass flow rate, pressure drop, and temperature field using
standard flow boiling correlations.
Two-phase flow instability is a major concern in cooling
through boiling. Oseen-Senda et al. [151] investigated the
boiling characteristics of pentane (normal boiling temperature
of 36 C) in a vertical circular stainless steel tube using neutron
radiography visualization. It was found that the tube wall
temperature was steady for high flow rates of pentane, but it
became instable as the flow rates decreased. The problem
deteriorated for very low flow rates. Simplified steady and
time-dependent models were developed to explain the
measured wall temperature instabilities. A challenge in their
investigation, however, was that the radiation created by the
neutrons changed the boiling characteristics drastically, so
further study is required before pentane boiling could be used
in PEMFC cooling.
With a boiling temperature of 100 C at 1 atm, water cannot
be used as a boiling coolant for low temperature PEMFCs
unless a vacuum pump and specially designed bipolar plates
Table 1 e Summary of some of the important cooling strategies for PEMFC stacks.
Cooling strategy
Heat spreaders/Edge
cooling
Techniques
Advantages
Using highly thermal conductive
material as heat spreaders
- Simple system
- No internal coolant,
- Small parasitic power
Using heat pipes as heat spreaders
- Simple system
- Small parasitic power
- Very high thermal
conductivity
- Simple system
- Small parasitic power
- Strong cooling capability
- Flexible control of cooling
capability
- Simultaneous cooling and
internal humidification
- Simplified system
Cooling with separate
air flow
Liquid cooling
Separate air channels for cooling
Phase change cooling
Evaporative cooling
(Direct water injection, porous
WTPs, wicking lands/channels)
Channels integrated in BPPs
(DI water/antifreeze coolant)
Cooling through boiling
- Elimination of coolant pump
- Simplified system
Disadvantages/Challenges
- Limited heat transfer length
- Non-availability of cost-effective
material with very high thermal
conductivity and good mechanical
properties
- Development of heat pipes with
small thickness and low weight
- Integration of heat pipes with
bipolar plates
- Trade-off between cooling
performance and parasitic power
- Radiator size
- Coolant degradation
- Large parasitic power
- Dynamic control of water
evaporation rate
- Thermal mass of liquid water
on cold startup
- Development of suitable
working media
- Two-phase flow instability
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are used to maintain a low pressure thereby reducing the
boiling temperature of water [152]. However, cooling through
water boiling can be a good option for the HT-PEMFCs which
operate above 100 C. Song et al. [139] demonstrated the
feasibility of this strategy. They developed a 1 kW HT-PEMFC
cooled by boiling water without coolant pump. As schematically shown in Fig. 17, the buoyancy force due to the density
difference between the vapor and the liquid water was
utilized to circulate the coolant between the stack and the
reservoir. It was shown that the stack temperature could be
kept stable at target temperature with an insignificant variation, and that the temperatures were more evenly distributed
from cell to cell. Stable operation during transient condition
was also obtained. These results demonstrated that cooling
through boiling is not only able to eliminate the need of
a coolant pump, but also favor stable and uniform operating
temperature of PEMFC stack.
configurations, and development of alternative coolants to
prevent coolant degradation. Progress has also been made
in optimization of radiator design and cooling system
layout, although the radiator size still remains to be a critical challenge.
For phase change cooling, advantages over conventional
liquid cooling have been demonstrated. Evaporative cooling
with water, which allows internal humidification at the
same time, has been applied in PEMFC stacks by some major
developers, such as UTC Power, greatly simplifying the
design of PEMFC systems. Cooling through boiling has been
demonstrated for HT-PEMFC stacks. For cooling through
boiling for low temperature PEMFC stacks, some promising
working media, e.g. HFE-7100, have been identified and
investigated.
7.2.
7.
Summary and future research needs
7.1.
Summary
Effective cooling is critical for safe and efficient operation of
PEMFC stacks. It also influences the cost and durability as the
membrane safety, water management and reaction kinetics
greatly depend on the operating temperature of PEMFC stack.
Effective and optimized cooling of PEMFC stacks poses
significant challenges, especially for automotive applications
where the power requirement is high while the space is
limited. The problem becomes more severe due to the small
temperature difference between the PEMFC stack and the
ambient air, and an almost negligible heat removal by the
product streams. The heat generation, its removal and the
resulting temperature distribution within the cell components
directly affect the interfacial properties, local heat generation
rate, and water transport.
A number of cooling strategies and techniques have been
developed and applied in practice for PEMFC stacks with
different power ratings. These cooling strategies, except that
with increased cathode air supply, are summarized in Table 1.
As reviewed in this work, significant advancements have
been made in cooling of PEMFC stacks. Some of the highlights
are listed below.
For cooling with heat spreaders, materials with very high
effective thermal conductivity, e.g. HOPG, have been used in
both single cell and PEMFC stacks with small active area and
proved effective. Various heat pipes with extremely high
effective thermal conductivity have also been proposed,
developed or tested as heat spreaders for cooling of PEMFC
stacks with large active area.
For cooling with separate air streams, progress has been
made in improving cooling performance and reducing
parasitic power consumption through optimization of the
design and operation of PEMFC stacks.
The liquid cooling has been widely and successfully applied
in high power PEMFC stacks, especially in automotive
applications. Significant progress has been made through
optimization of coolant flow and reactant gas flow
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Future research needs
Although great progress has been made in developing various
cooling technologies for PEMFC stacks, there are still a number
of outstanding issues. These include the development of a low
cost, high performance, and reliable cooling techniques for
PEMFC stacks to promote their commercialization under wide
range of ambient conditions, from high ambient temperatures
in desert locations to extremely frigid conditions in colder
climates. Some of the important issues are listed below:
Fundamental understanding of the thermal properties and
heat transfer characteristics of PEMFC components through
in situ and ex situ experimental investigations. The importance of such experimental data has been well demonstrated not only on the cooling, but also on the performance
and durability of PEMFC stacks.
Development of high thermal conductivity materials and
heat pipes that can be integrated in bipolar plates. Superior
cooling capability of heat pipes for large area PEMFC stacks
have been demonstrated, while great efforts are still needed
to integrate them effectively at low cost into bipolar plates.
Optimization of the coolant and reactant flow configurations to achieve the desirable distribution characteristics of
local current density, temperature and water content in the
PEMFC at low cost and improved efficiencies. The V flow
stack design with 3D cooling flow by Honda [121] is a very
good example.
Improving heat transfer efficiency of radiators for automotive PEMFC stacks to reduce their sizes. Optimizing the
cooling system design to reduce peak heat rejection load
will also favor the reduction of radiator size. In addition,
research is also warranted in developing a coolant that has
a high ionic resistance, does not degrade over the lifetime of
the stack, and has a low viscosity for reduced pumping
power.
Development of working media suitable for phase change
cooling of PEMFC stacks. Great potentials over single-phase
liquid cooling have been demonstrated. Further investigations of the two-phase flow characteristics of the working
media in multiple parallel coolant channels are warranted.
Development of new membranes than can operate at
temperatures above 100 C with good performance. It has
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been shown in some prototype HT-PEMFC stacks that a high
temperature operation can greatly simplify the cooling of
PEMFC stacks.
It is worth noting that publication of data from field tests of
PEMFC stacks cooling, e.g. those from Volkswagen fuel cell
vehicle field test [16], will promote understanding of the
PEMFC cooling problems in real conditions and thereby
promote the development of more effective and low cost
cooling techniques.
Acknowledgments
This work is conducted in the Thermal Analysis, Microfluidics
and Fuel Cell Laboratory in the Mechanical Engineering
Department at Rochester Institute of Technology and is supported by the US Department of Energy under contract No. DEEE0000470.
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