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Applied Thermal Engineering 51 (2013) 1339e1344

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Designing heat exchanger with spatially variable surface area forpassive cooling of PEM fuel cell

Ender Özden a, Ivan Tolj b, Frano Barbir b,*aDepartment of Mechanical Engineering, Middle East Technical University, 06531 Ankara, Turkeyb Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Rudjera Boskovica 32, 21000 Split, Croatia

h i g h l i g h t s

< Variable temperature profile along fuel cell cathode channel was required.< Heat exchanger was designed and modeled that would provide such a temperature profile.< Heat exchanger has variable length fins for passive heat removal.< Relative humidity was close to 100% along most of the fuel cell cathode channel.

a r t i c l e i n f o

Article history:Received 30 August 2012Accepted 21 November 2012Available online 30 November 2012

Keywords:PEM fuel cellsWater and thermal managementPassive coolingCFD

* Corresponding author. Tel.: þ385 21 305 953; faxE-mail address: fbarbir@fesb.hr (F. Barbir).

1359-4311/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2012.11.040

a b s t r a c t

The purpose of this work was to design a heat exchanger for a Polymer Electrolyte Membrane (PEM) fuelcell, which would ensure such a temperature profile along the fuel cell cathode channel resulting in closeto 100% relative humidity along the channel without external humidification. To achieve this, 3Dnumerical simulations of a single PEM fuel cell were performed using commercial Computational FluidDynamics (CFD) software (ANSYS Fluent). Based on the simulation results a variable surface area finnedheat exchanger was designed which allows for passive fuel cell cooling. The results indicate that it ispossible to obtain such temperature and relative humidity conditions inside the fuel cell cathodechannel, using a passive heat exchanger with variable surface area.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

One of the most important aspects of polymer electrolytemembrane (PEM) fuel cells design and operation is water manage-ment. In this type of fuel cells, conductivity of protons througha polymer membrane strongly depends on membrane’s watercontent [1]. The membrane therefore must be well hydratedthroughout a fuel cell. Thismay seemnot to be a problem aswater isproduced in the electrochemical reaction on the cathode. Depend-ing on local conditions in the cathode porous structures, i.e.,temperature, pressure and composition of cathode gas (mixture ofoxygen and nitrogen whose composition is changing as oxygen isbeing consumed), water can be present either as liquid or as vapor.If the cathode gas in immediate vicinity of the polymer membraneis not saturated with water vapor, the membrane can lose itswater, which will affect its protonic conductivity. This situation isfurther complicated bywaterfluxes through themembrane, namely

: þ385 21 305 776.

All rights reserved.

electro-osmotic drag and diffusion. As hydrogen protons travelthrough the membrane each of them carries water molecules fromthe anode to the cathode. However, the resulting water concentra-tion gradient “pushes” some water back from the cathode to theanode (so called backdiffusion). The netflux through themembranedepends on current density,membrane thickness and conditions onboth sides of the membrane. As the state of water and its fluxes infuel cell depend on local temperature, water management is prac-tically inseparable from heat management and it is therefore oftenreferred to as water and heat management. Heat is produced in theelectrochemical reactions, in addition some heat is also generateddue to resistive (ohmic) losses. In order to maintain the desiredtemperature, heat must be removed from a fuel cell. If the excessheat is not being removed at an adequate rate, performance anddurability of the fuel cell would significantly decrease [2].

There are many papers that deal with the problem of water andthermal management in PEM fuel cells.

Wen et al. [3] studied the thermal management of PEM fuel cellstack with combined passive and active cooling using PyrolyticGraphite Sheets. These sheets were used as heat spreaders and fins,

0

10

20

30

40

50

60

70

80

90

100

020406080100

Cathode channel length (%)

Temperature (°C), isothermal case

Temperature (°C), nonisothermal case

RH (%), isothermal case

RH (%), nonisothermal case

Fig. 1. Temperature and relative humidity profiles along the fuel cell cathode channelfor two cases a) isothermal and b) nonisothermal so that relative humidity is kept closeto 100%. From Ref. [26]

E. Özden et al. / Applied Thermal Engineering 51 (2013) 1339e13441340

and provided passive cooling of the fuel cell stack, along with twosmall fans for forced convection. They concluded that PyrolyticGraphite Sheets were an innovative and economical commercialapplication for the thermalmanagement of small-to-medium-sizedfuel cell stacks.

Sasmito et al. [4] investigated various alternative thermalmanagement strategies for PEM fuel cell stacks; e.g., forced convec-tion in specially design cooling plate/channel. They used a liquid orair as the coolant, edge-air cooling with fins, and combined oxidantand coolant flow with forced and natural convection air cooling.Their results provide useful design guidelines for selection of a suit-able fuel cell thermal management strategy.

Li et al. [5] indicated that thermal management has a significantinfluence on the performance and lifespan of PEM fuel cells. Theypresented a dynamic thermal model and an optimization algorithmfor the temperature control system of PEM fuel cells. The temper-ature of the fuel cell was controlled efficiently, which had greatimpact on performance of PEM fuel cells.

Zong et al. [6] developed a non-isothermal, non-isobaric waterand thermal management model with phase change to simulatethe mass and energy transfer processes inside a PEM fuel cell unitwith a non-uniform stack temperature. Stack temperature, streampressure and temperatures, relative humidity, water vapor molefraction etc. could be predicted based on this model along thechannels on both cathode and anode sides. The results fromuniform stack temperature model and non-uniform stack temper-ature model were compared in this research.

Tohidi et al. [7] investigated the effects of various parameters onPEM fuel cell performance, such as relative humidity, temperature,pressure etc. They indicated that water and thermal management isone of the most critical problems to overcome in the PEM fuel cells.It was concluded that the water content of membrane could beincreased, by increasing the relative humidity of inlet gases.

Jeon et al. [8] studied the effect of relative humidity of thecathode on PEM fuel cells focusing on automotive operation. Theyinvestigated the dependency of current density, membrane watercontents, net water flux on the performance and the uniformity. Itwas concluded that for the automotive operation, the externalhumidifier could be eliminated, but the elimination of the humid-ifier could degrade cell performance and durability due to dehy-dration of the membrane.

Williamson et al. [9] investigated the impact of temperature onair-breathing PEM fuel cells using polarization and impedancespectroscopy. They presented three different active area sizes inboth forced convection and air-breathing cathode configurations. Itwas concluded that temperature plays a larger role in air-breathingfuel cell performance than the actual size of the cell.

Chun et al. [10], investigated fin structures on cathode plate foran air-breathing PEM fuel cell at various temperatures. The finstructures increased air flow at high temperatures leading to anincrease in fuel cell performance. However, the fins were notsufficient to prevent overheating from the electrochemical reaction,so an oversized thermal mass was added to the cell.

Stogiannis et al. [11], studied a micro-structured plate heatexchanger for the thermal management of a fuel cell. Their resultsclearly show that the proposed plate modifications can increasetemperature uniformity across the membrane by more than 60%compared to that of the flat plate.

Reddy and Jayanti [12] investigated three different heat removalstrategies for a 1 kWe fuel cell stack using CFD calculations. Theyconcluded that with proper cooling strategy, the temperaturevariations within the cell may be controlled.

Cozzolino et al. [13] investigated electrical and thermal perfor-mance of a PEM fuel cell stack through numerical model andexperimental data. A one-dimensional electrochemical model had

been integratedwith a thermal model based on a first law approachin order to evaluate the performance of the PEMFC stack. It wasconcluded that the numerical model could support and address theexperimental activity and could allow forecasting the behavior andperformance of the stack.

Although the fuel cell produces water, and in most cases thisamount of water should be sufficient to keep the reactant gasesfully humidified, most fuel cells need external humidification of thereactant gases. Introduction of ambient air in a fuel cell operating atelevated temperature (60e80 �C) without additional humidifica-tion would result in temperature of the air rising quickly afterentering the stack causing severe drop in relative humidity. Theproduct water in that entrance portion of the fuel cell would not besufficient to prevent severe dehydration. Eventually, down thecathode channel, the product water could be sufficient to humidifyair, but significant portion of the cathode channel would be dry.Operation with dry gases would result in drying out of the polymermembrane, which would adversely affect the fuel cell performanceand durability.

Operation with dry gases, if possible, would eliminate the needfor the devices required for external humidification, and thereforewould result in simplification of the fuel cell system. Operation offuel cell with dry gases has first been proposed by Buchi and Sri-nivasan [14]. Ever since, researchers have been studying thisproblem and attempting to solve it by varying internal configura-tion of flow channels inside the fuel cell [15e18], or by creating self-humidifying membranes by incorporating Pt or metal-oxideparticles into their structures [19e25].

Tolj et al. [26], presented a new concept of not allowing dryconditions inside the fuel cell by controlling the local temperature,i.e., by imposing a temperature profile which would allow main-taining desired relative humidity (close to 100%) utilizing waterproduced in the electrochemical reaction inside the fuel cell. Fig. 1shows the temperature and relative humidity profiles along thecathode channel for a fuel cell operated with untreated ambient air.In case when the fuel cell temperature is maintained constant, thetemperature of air increases sharply as soon as it enters the fuel cell.As a result, relative humidity drops to a very low level (w20%) andthen it gradually increases to 100% at the fuel cell outlet. This meansthat the fuel cell polymer membrane is exposed to dry conditionsthroughout the fuel cell. However, it should be possible to imposea temperature profile along the cathode channel such that relativehumidity quickly reaches saturated or close to saturated conditions,and then maintains such conditions throughout the fuel cell untilthe exit. In both cases, inlet and outlet conditions are identical but

E. Özden et al. / Applied Thermal Engineering 51 (2013) 1339e1344 1341

the temperature and relative humidity profiles are drasticallydifferent, the later resulting in more favorable conditions and thusshould result in better performance and longer life.

Such a temperature profile may be imposed by spatially variableheat transfer rates. Tolj et al. [26], accomplished such a profile byusing a segmented fuel cell, where each segment was kept at thepreviously calculated required temperature by attached Peltierelements. However, although very effective in laboratory testing,the use of Peltier elements would not be practical in real life fuelcells. It is therefore the objective of this work to design a heatexchanger for heat removal from a fuel cell segment which wouldreplace the Peltier element and enable required temperatureprofile along the fuel cell cathode channel.

2. Methodology

In order to achieve the objective, following methodology hasbeen applied: (i) development of a 3D model of a PEM fuel cell byusing ANSYS Fluent; (ii) use of the 3D model to find a temperatureprofile along the cathode channel that will ensure close to satura-tion conditions throughout the channel; (iii) design of a passivecooled heat exchanger to obtain desired temperature profile; and(iv) verification of the designed heat exchanger performance usingthe 3D model.

In the previous study [26], five fuel cell segments were con-nected in series. Each segment was an individual 20 � 1 cm activearea single cell with 5 straight channels for supply and distributionof hydrogen and air on both anode and cathode side, respectively.Hydrogen and air flow in counter-current mode. One of these fuelcell segments designed in previous study [26] is modeled using theANSYS Fluent Fuel Cells Module.

Geometrical details and boundary conditions are presented inTable 1. The actual fuel cell segment and its 3D CFD model areshown in Fig. 2. As seen on the figure, the collector plates aresomewhat larger than the active area of the fuel cell, because of thearea needed to seal the cell. From the heat transfer point of viewthis may be advantageous, because a larger heat transfer area isexposed to passive cooling.

Continuity, momentum, species, energy, electrochemical andwater transport equations were solved simultaneously by theANSYS Fluent program to obtain the results. Details of the imple-mentation of the equations can be found in Ref. [27]. To ensure thatthe solution was grid-independent, calculations were performedwith different mesh sizes. The reference parameter was the averagecurrent density at a fixed voltage for the grid independence. Thenumber of elements which led to grid-independent solution was

Table 1Values for parameters used in CFD model experimental set-up [26].

Parameters Values

Cathode stream pressure 1.01 barAnode stream pressure 1.01 barCathode stream inlet temperature 30 �CAnode stream inlet temperature 25 �CFuel cell hardware temperature 60 �C or variableRelative humidity of cathode stream 75%Relative humidity of anode stream DryCathode stoichiometry 2.15Current density 0.5 A cm�2

Cathode stream (air) inlet mass flow rate 0.0077 g s�1

Channel length 200 mmChannel width at anode and cathode 1 mmChannel height at anode and cathode 1 mmNumber of channels 5Effective area of the cell 20 cm2

Membrane thickness 0.005 cmMembrane dry density 2 g cm�3

3,752,000. Tetragonal hybrid type elements were used. Thegenerated mesh was not uniform. In the membrane electrodeassembly, because of the thin layers, the mesh was generated withhigher refinement. In the collector plate and flow channelsa coarser mesh was generated.

The temperature profile that was required to ensure 100%relative humidity was obtained by performing CFD simulations.First, 100% relative humidity condition was achieved at the exitsection of the cathode flow channel by applying proper tempera-ture boundary condition at the outside wall of cathode collectorplate. Then, the non-uniform boundary condition, which wasnecessary to give 100% relative humidity along the cathode flowchannel, was obtained by CFD simulations.

The next step was to design a heat exchanger which will give thedesired temperature profile along the cathode collector plate. Anancillary cooling system is needed in PEM fuel cells since the outletstreams contribute little to heat removal [28]. By using heatexchanger, thenecessity of theexternal humidification andanyotherpower consuming devices such as fans or Peltier elements would beeliminated. The designed heat exchanger would be capable ofremoving the generated excess heat through the PEM fuel cell.

Because of a specific required (non-uniform) temperature profilealong the collector plate, the heat exchanger removing theheat fromthe fuel cell should have variable heat transfer area. This could bea simple heat exchanger, consisting of straight fins with variable finheight. Simple heat sink geometries could provide enough passiveheat dissipation for a wide range of applications [29]. For the heatexchanger material aluminum was selected, as it has high thermalconductivity and low cost when compared to other materials.

3. Governing equations and boundary conditions

The governing equations are based onmass, momentum, energyand species conservation including appropriate source terms. Sincethe problem is assumed to be steady, time dependent parametersare dropped from the equations. The resulting equations are:

3.1. Continuity

V$

�rV.�

¼ Smass (1)

where Smass is the source term for continuity equation and isapplicable only for the catalyst region. In the gas channels, gasdiffusion layers and membrane, the source term is Smass is set tozero.

3.2. Momentum

1sð1� sÞV$

�rV.V.�

¼ �Vpþ 1sð1� sÞV$

�mVV

.�þ rg þ Smom

(2)

where Smom is the source term for momentum equation and isapplicable for only porous medium. In flow channels andmembrane, the source term Smom is set to zero.

3.3. Species

V$

�rV.Xi

�¼ �VðrDiVXiÞ þ Si (3)

where the index “i” refers to different species including oxygen,hydrogen and water. Si is the source term for the species and

Fig. 2. PEM fuel cell segment (left); 3D PEM fuel cell model (right).

E. Özden et al. / Applied Thermal Engineering 51 (2013) 1339e13441342

applicable for the catalyst region. In flow channels, gas diffusionlayer and membrane Si is set to zero.

3.4. Energy

V$

�rV.T�

¼ V�keffVT

�þ ST (4)

where ST is the heat source term for energy equation and is appli-cable at the cathode catalyst layer. For other zones ST is set to zero.

The modeling domain and heat transfer paths included in themodel are presented in Fig. 3. Heat generated in the catalyst layersand the membrane is transferred mainly by conduction to theexterior walls of the fuel cell. From the exterior walls, heat isremoved to the surrounding mostly by convection. The boundaryconditions applied to the model are described below.

3.5. Inlet boundary conditions

For the anode and the cathode inlet mass flow rate boundarycondition is applied. Mass flow rate, fluid temperature and massfraction of the species are specified for the inlet boundary condition.

3.6. Outlet boundary conditions

To the anode and the cathode outlets, pressure outlet boundarycondition is applied. This boundary condition is used at thecontinuum domain exits where pressure values are not knownprior to the solution.

Fig. 3. Modeling domain and heat transfer paths of the model.

3.7. Outer wall boundary conditions

No slip boundary condition is applied at the walls of the model.For the outer walls of the model convective boundary condition isapplied using following equation:

Qw ¼ hAwðTw � T0Þ (5)

For the outer walls heat transfer coefficient h and free streamtemperature T0 are specified. Qw and Tw calculated by Fluent. Aw isthe heat exchange area.

There is no need to define the boundary conditions at the insidecathode channel walls. The program (ANSYS Fluent) calculates thelocal heat transfer coefficient, local wall temperatures and localheat flux between the channel and the wall. The fluid (cathode air)temperature changes along the cathode channel in accordancewiththe previously calculated temperature profile needed to maintain100% (or close to 100%) relative humidity, and this profile is used asan input parameter. Inlet parameters (fluid, mass flow rate,temperature and pressure are listed in Table 1).

4. Results and discussions

Base on the simulation results, confirmed experimentally in theprevious study [26], the heat exchanger area along the cathodechannel required for maintaining close to saturation conditions ofair along the cathode channel was calculated. First, the temperatureprofile that resulted in 100% humidity in the cathode channel wascalculated. Then, according to the resulting temperature profile, therequired heat transfer areawas calculated for different heat transfercoefficients. The results are presented in Fig. 4. Calculations weremade for two different values of heat transfer coefficient i.e.,5.0 W m�2 K�1 for natural convection (still air in laboratory

Fig. 4. Required heat exchanger area along the cathode channel.

Fig. 5. Resulting heat exchanger with variable length fins. Fig. 7. Temperature distribution along the heat exchanger.

E. Özden et al. / Applied Thermal Engineering 51 (2013) 1339e1344 1343

conditions) and 19.0 W m�2 K�1 for forced convection. To achieveheat transfer coefficient values greater than 5.0 W m�2 K�1, a fanwould be required, whichwould require additional power, which inturn would adversely affect the fuel cell efficiency. The value of19.0 Wm�2 K�1 was found to be a limiting heat transfer coefficientvalue. Above this value the PEM fuel cell outer area would have tobe decreased in order to achieve saturation conditions, whichwould be physically impossible.

After examining the CFD simulations obtained for the 100%humidity condition, it was observed that there was no localizedhigh flux heat source present. Therefore, by increasing the surfacearea of the cathode collector plate, it would be possible to have thedesired temperature profile. Also it was observed that radiativeheat transfer through the heat exchanger was very lowwhen it wascompared with the convective heat transfer, because of the lowemissivity and low temperature differences. Therefore, radiativeheat transfer could be neglected.

After necessary heat transfer area for ensuring saturated condi-tion of airwas calculated, afinnedheat exchangerwas designed. Therequiredheat dissipation rate of the entireheat exchanger is 8.5W. Itconsists of 24 ribs, with decreasing rib height in the fluid streamdirection from 62 mm to 16 mm, as shown in Fig. 5. The ribs’thickness is 2 mm and the spacing between the ribs is 8 mm.

The next step was to couple the designed heat exchanger withcathode side of the fuel cell and to perform final simulation, Fig. 6.

Fig. 6. Heat exchanger coupled with cathode side of the PEM fuel cell segment.

The heat exchanger was mounted to the cathode side of the fuelcell. By performing CFD simulations it was expected to obtainrelative humidity along the cathode channel and to compare withthe previous CFD simulations without the heat exchanger. Also,heat dissipation rate of the heat exchanger could be calculated.

The resulting temperature distribution of the heat exchanger isshown in Fig. 7. The temperature distribution was as expected,keeping the cathode inlet side of the fuel cell cooler than the outletside. This model was developed to support that the heat exchangerwas capable of removing excess heat from the fuel cell. From theCFD simulations, it was observed that the heat removal rate of theheat exchanger was sufficient to cool the fuel cell. Total resultingheat transfer rate of the heat exchanger is 8.78 W, which is 3.3%higher than the calculated/required value of 8.5 W.

Resulting relative humidity of air along the cathode channel isshown in Fig. 8. In this figure, PEM fuel cell with and without heatexchanger is compared. For the case without the heat exchanger,the air stream heats up quickly after entering the cell, which causesdrop in relative humidity. By the end of the channel the air streamgets fully saturated, but throughout the cathode channel the air ispractically dry. For the case with the heat exchanger, relativehumidity only slightly drops below the inlet value and thenincreases to near fully saturated condition and remains at that levelthroughout the channel. This way, a more favorable conditions areachieved, which, as per modeling and experimental results of Toljet al. [26], resulted in better fuel cell performance, i.e., 0.618 V invariable temperature case vs. 0.548 V in isothermal case at 0.5 A/cm2 and ambient pressure. In addition, one may expect thatoperation at close to 100% relative humidity should result in longercell durability as compared to operation with dry conditionsthroughout the channel.

Fig. 8. Relative humidity of air along the cathode channel (air entrance is on the rightside).

E. Özden et al. / Applied Thermal Engineering 51 (2013) 1339e13441344

5. Conclusions

In this paper a 3D CFD model of a PEM fuel cell was applied inorder to find a temperature profile that ensures close to 100%relative humidity along the cathode flow channel. Base on thesimulation results, necessary heat exchanger area was calculatedand the heat exchanger was designed. In order to confirm thatdesigned heat exchanger was capable to ensure desired tempera-ture profile, and in such way achieve close to saturation conditionsalong the cathode channel, the heat exchanger was coupled witha PEM fuel cell. The results show that relative humidity of air wasclose to 100% along the cathode channel.

This method of operating a PEM fuel cell with dry air by main-tain the required temperature profile along the fuel cell cathode,and the resulting simple passive heat exchanger may have appli-cation for relatively low fuel cell power (up to a few hundredwatts).In laboratory fuel cell testing, it will replace the Peltier elementsand eliminate additional power consumed by fans and Peltierelements. Further work will be aimed at investigations and designof variable heat exchange area within the fuel cell stacks.

Acknowledgements

E. Özden gratefully acknowledges Turkish Scientific and Tech-nological Research Council of Turkey (TUBITAK) 2214-ResearchFellowship Programme.

Nomenclature

Aw wall area (m2)Di gas diffusion coefficient (m2 s�1)g gravitational acceleration (m s�2)h convection heat transfer coefficient (W m�2 K�1)keff thermal conductivity (W m�1 K�1)p pressure (N m�2)Qw heat transfer rate (W)s liquid water saturationSi source term for species (kg m�3 s�1)Smass source term for continuity equation (kg m�3 s�1)Smom source term for momentum equation (N m�3)ST source term for energy equation (W m�3)T temperature (K)T0 free stream temperature (room temperature) (K)Tw wall temperature (K)

V.

velocity vectorXi mass fraction of the species

Greek symbolsε porositym dynamic viscosity (kg m�1 s�1)r density (kg m�3)

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