Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells

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Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells N. Baharlou Houreh a, M. Ghaedamini b, H. Shokouhmand a,*, E. Afshari b, A.H. Ahmaditaba b a b

School of Mechanical Engineering, College of Engineering, University of Tehran, P. O. Box: 11155-4563, Tehran, Iran Department of Mechanical Engineering, Faculty of Engineering, University of Isfahan, Isfahan, Iran

highlights
Two planar membrane humidifiers for PEM fuel cells are fabricated and tested.
Three configurations including cross flow, parallel flow and counter flow are compared.
Two thermal conditions including isothermal and insulated are implemented for all tests.
Temperature of wet side inlet plays a key role in humidifier performance.

article info

abstract

Article history:

In this study on humidifiers for polymer electrolyte membrane (PEM) fuel cell application,

Received 10 August 2019

the experimental outcome of two air-to-air planar membrane humidifiers with three

Received in revised form

different internal flow patterns including cross, parallel and counter flows are investigated

15 November 2019

under isothermal and insulated boundary conditions. At all temperatures and flow rates,

Accepted 5 December 2019

the conditions of higher performance, corresponding to highest water recovery ratio (WRR)

Available online xxx

and lowest dew point approach temperatures (DPAT), are encountered in the counter flow case, in contrary to the cross flow configuration. The insulation condition with dry inlet

Keywords:

temperature at 30  C and wet inlet temperature at 60  C has a higher WRR index compared

Membrane humidifier

to isothermal condition at 60  C but is lower than isothermal condition at 30  C. The DPAT

PEM fuel cell

in humidifier with insulation condition is approximately equal to that obtained in

Insulation

isothermal condition at 60  C but is much higher than what results in isothermal condition

Isothermal

at 30  C. It can be deduced that the temperature of the wet side inlet plays a key role in the

Flow configuration

humidifier performance. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The balanced performance of the proton exchange membrane (PEM) fuel cell significantly depends on the heat and water management. If the water removal rate in the PEM fuel cell

does not keep up with the water generation rate, it will cause water flooding and thus will hinder the transport of reactant gases by blocking the pores in the porous catalyst and gas diffusion layers, consequently covering up active sites in the catalyst layer and plugging the gas transport channels. Instead, the ionic conductivity of the membrane is strongly

* Corresponding author. E-mail address: [email protected] (H. Shokouhmand). https://doi.org/10.1016/j.ijhydene.2019.12.017 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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dependent on its degree of humidification, culminating at the state of maximum humidification. When the water removal rate exceeds the water generation rate, membrane dehydration occurs which can result in performance degradation due to the significant ohmic losses within the PEM fuel cell [1,2]. There are several methods to manage water inside the PEM fuel cell. These methods include: self-humidifying PEM fuel cells [3,4], use of high air stoichiometry [5], use of passive-type PEM fuel cells (such as air-breathing or air-blowing PEM fuel cells) [6], gas reactants humidification, transient purge [7], integrated electro-osmotic pump [7], use of new designs in flow field structure instead of conventional flow field [8,9] and use of dead-ended channel in PEM fuel cell [10,11]. Nevertheless, generally, a PEM fuel cell uses external humidifier to supply adequate water content because the hydration level of membrane is directly related to the system performance. The use of an external humidifier enables precise control of the humidity and temperature of the fuel cell reactant gases. External humidification methods consist of: enthalpy wheel exchanger [12], bubble humidification [13e15] and membrane humidification. The membrane humidifier has the following advantages in comparison to other practices: i. In comparison to the enthalpy wheel humidifier, the membrane humidifier has no moving parts; therefore, consumes lower energy [16e18]. ii. In comparison to the bubble humidifier, fabricating, testing and data analysing of a laboratory-scale membrane humidifier is easier and more reliable to simulate for different applications and power demands. iii. Similarity between structures of membrane humidifier and PEM fuel cell, especially through using the same membrane (Nafion), simplifies the process of humidifier manufacturing. iv. Membrane humidifier has an exact, easy and direct control over humidity and dew point in comparison to the bubble humidifier [19]. v. Compactness (i.e., needs less space within the fuel cell system) is another advantage in comparison to the bubble humidifier [16,20]. vi. Using membrane humidifier in a gas recirculation system causes a lot of energy saving [21e23]. In this method, the fuel cell cathode inlet is connected to the humidifier dry side outlet and the fuel cell cathode outlet is connected to the humidifier wet side inlet. Therefore, the cathode outlet air with high humidity and temperature is used as the wet gas of humidifier. Membrane humidifier has both dry and wet side channels separated by a polymer porous membrane. The difference in temperature and water concentration between both leads to heat and water transfer from wet to dry side. Water is mainly transferred across the porous membrane through diffusion method. In addition, water can transfer via the porous membrane by hydraulic forces due to the pressure difference between both sides of humidifier. Moreover, temperature difference between both sides may also affect the water transfer [19]. According to the type of fluid entering the wet side, membrane humidifier can be classified as liquid-to-gas or gasto-gas types. According to the flow pattern arrangement,

humidifiers are classified as planar or tubular types. The planar is usually applied for gas-to-gas humidifier and the tubular is usually applied for liquid-to-gas humidifier. In planar humidifiers such as heat exchangers, three kind of flow configurations including counter flow, parallel flow and cross flow can be used. The selection of either configuration should be based on balancing among some attributes. The specifications that seems to be more pronounced in the cross flow configuration are simplicity of assembling, manufacturing cost, relative compactness and less pressure drop at manifolds [24,25]. Concerning the counter flow configuration, more heat generation and water transfer rate, i.e., higher performance is reached [26e28]. Therefore, selecting the most appropriate design in terms of performance or other attributes should be done according to the available possibilities and conditions. In humidifiers, two thermal boundary conditions including isothermal and insulated can be applied. Utilizing the isothermal condition has many advantages, including that the water transfer is not influenced by the heat transfer, preventing condensation within the test module, and an easier establishment of steady state conditions. Using the insulating boundary condition has an important advantage in comparison to the isothermal condition, namely that in addition to mass transfer, heat transfer also occurs through the membrane. Thus, comparing of these two boundary conditions on the humidifier performance necessitates a better analysis. Some experimental studies investigated the performance of the planar gas-to-gas membrane humidifier. Hwang et al. [19] experimentally investigated the performance of a membrane humidifier with cross flow configuration. They revealed that increase of wet side inlet dew point causes decrease of humidifier performance and increase of dry side inlet temperature causes performance improvement at low flow rates. By introducing a dimensionless number that defines the relation between the diffusion time and residence time of the gases, Huizing et al. [29] propounded a method to design a membrane humidifier. The effect of flow rates on the membrane humidifier performance is assessed by Cave and Merida [30] experimentally. They found that the dew point approach temperature (DPAT) decreases with a decrement in flow rate of the dry side. Kadylak and Merida [31] investigated through experiment a counter flow membrane humidifier. They found that increasing the flow rate leads to a decrease of water recovery ratio (WRR). An experimental study of a multi-stage planar membrane humidifier for kW-scale PEMFCs is performed by Yan et al. [32], who found out that increasing the dry side inlet temperature and humidity gives rise to a decrease in both DPAT and WRR, with a slight increase of the pressure drop. Chen et al. [26] conducted an experimental work to investigate the heat and mass transfer of a planar membrane humidifier. According to their results, an increase in air flow rate leads to an increase in water transfer rate. However, there is an optimum air flow rate at which the lowest DPAT and highest WRR occur. At higher than optimal value, WRR decreases and DPAT increases with the increase of air flow rate. An experimental work on analysis of membranes is done by Cahalan et al. [33]. The membranes they tested range across four different classes. The results indicate that the membranes, which transfer the most water, are

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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sulfonated fluorinated membranes and those transferring the least water, are the non-sulfonated non-fluorinated membranes. In another study [20], they performed a parametric study on a membrane humidifier. According to their results, an increase in temperature, a decrease in absolute pressure and an increase in RH gradient results in an increase in water transfer rate. Ramya et al. [17] fabricated a membrane humidifier and studied the effect of humidification on the fuel cell performance. They concluded that the humidifier is appropriate for both liquid water and exhaust cathode air at the wet side inlet. Tak et al. [34] conducted an experimental analysis of a membrane humidifier with isothermal condition. Their results indicate that the humidifier performs better at lower isothermal bath temperature and higher operating pressure. Experimental studies of a cross flow humidifier with both static and dynamic tests are performed by Ahluwalia et al. [24]. They revealed that an increase in relative humidity of wet side inlet and a decrease in operating pressure leads to an increase in water transfer rate. There are also some analytical and numerical studies investigating the gas-to-gas membrane humidifier performance [27,35]. Bhatia et al. [22] run a performance analysis of the planar and tubular membrane humidifier by applying an analytical model. They revealed that heat transfer is subject to intensive mass transfer. They investigated the interaction between the fuel cell and humidifier on the performance of a recirculation system, where increment of fuel cell current density gives rise to decrement of dry side outlet relative humidity (RH). Park et al. [16] performed a dynamic modelling of a shell-and-tube membrane humidifier. According to their results, heat and mass transfer increase with an increase in flow rate of the dry gas. They showed that the inlet gas RH dose not significantly affect the heat transfer rate. A 2D numerical model of a planar membrane humidifier with cross flow configuration is simulated by Sabharwal et al. [36]. They found that reducing the pressure, dry side temperature and flow rate results in increasing the RH of the dry side outlet. In a previous work [37], the present authors developed a three dimensional numerical model of a planar membrane humidifier. It was found that with an increase in the dry side inlet temperature, the humidifier performs better. However, preheating the inlet dry gas at low flow rates does not have much

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effect on the performance. In a later work [38], a membrane humidifier constituted of a porous metal foam is proposed as a new configuration with the conclusion that using porous foam at both sides and wet side of humidifier leads to dew point at dry side outlet and WRR increment, indicating an improvement in performance. However, humidifier performance does not receive a positive impact by applying porous foam in dry side. In another study [39], for the first time, the effect of obstacle shape and number on a membrane humidifier performance is investigated by the authors through a computational fluid dynamic (CFD) model. The results exhibit that the highest values of water transfer rate, dew point of dry side outlet and pressure drop occur in humidifiers with rectangular obstacles. Furthermore, regarding the pressure drop, using one obstacle does not offer any advantage and at least two obstacles are needed. In this study, two prototypes of air-to-air planar membrane humidifier are fabricated and tested, one with cross flow configuration and another with parallel and counter flow configurations. Comparison of the humidifier performance with three mentioned configurations is performed experimentally for the first time in this study. Another novelty of this study is that all experimental tests are conducted for two thermal boundary conditions: isothermal and insulated. In each of these conditions, the effect of dry side flow rate, wet side flow rate and equal flow rates of both sides on humidifier performance is investigated.

Experimental setup Humidifiers A schematic of membrane humidifier with counter flow configuration is indicated in Fig. 1. Each humidifier contains two stacks of the planar cell. Each stack is made from two plates of 5 mm thickness Polyoxymethylene (POM). POM, sometimes called Polyacetal, is known for its high flexural and tensile strength, stiffness, hardness, low creep under stress, low coefficient of friction, excellent chemical resistance and outstanding fatigue properties [40]. One plate is used as the dry side, and the other is used as the wet side. There are 24

Fig. 1 e A schematic of membrane humidifier with counter flow configuration. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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channels at each side in parallel with the length of 82 mm. The channel ribs are thick enough to endure the compaction strength for bonding. The Nafion 115 membrane is placed in between each plate, with 127 mm thickness. The Nafion is a porous polymer that is pre-treated by boiling in a 3 wt % hydrogen peroxide solution for 2 h, boiling in 0.5 M sulphuric acid for 2 more hours, and finally rinsing and storing in deionized water [30]. Highly reliable sealing is done by O-rings and adhesive. The plates are connected with four screws and fixed together by full sealing. Two aluminium plates on both sides of each humidifier are used as end plates. An image of assembled components of the membrane humidifier is shown in Fig. 2. Geometric specifications and operating conditions of the humidifiers are listed in Table 1.

Test station A schematic and an actual image of the experimental test station are shown in Fig. 3(a) and (b) respectively. In order to supply needed dry air, a compressor with 250-L reservoir is used. The compressor is equipped with a water trap (AF500010) to separate moisture in the output air and two carbon active micro filters (GL100-NSG-W-7012) to separate oil. The exhaust air from the reservoir is divided into two sections and enters dry and wet lines. At the beginning of each line, an ACA02-25 series rotameter with a capacity of 10 m3/h and an accuracy of ±0.2 m3/h is used to measure and adjust the flow rate of the input air and a pressure gauge in order to check the input pressure. On the dry side, after the pressure gauge, an electric heating coil is used to preheat the dry air entering the humidifier to the desired temperature. For an easier and faster temperature adjustment, a bypass flow path is used under the

coil. The dry air has been measured to have a RH between 1.1% and 5% through the whole test temperature range. This range of RH is equivalent of a specific humidity ratio of 0.00135 kgvapor/kgair. Because the reservoir size is large and there is a water trap at its outlet, during the entire test period, the dry air inlet of the test station has a constant humidity ratio. Before entering the membrane humidifier, the dry air enters the temperature-humidity sensor (THD-WD1-C) which measures the temperature and humidity. On the wet side, the air is divided into two parts, one part penetrates the bubble humidifier for being humidified, and the other part for preheating enters the electric heating coil through a bypass path. For the bubbling humidifier a 10 L reservoir with insulated walls is used. An element at the bottom of the reservoir is utilized to heat the water inside the reservoir. Dry air enters the bubble humidifier through a steel pipe embedded at the entrance of the reservoir. In order to enhance contact between air and water, the air diffuser is applied at the end of the steel pipe due. The humidified air leaves the reservoir through a gate at the top of the reservoir and is transferred through some hoses to the wet side of the humidifier. The temperature and RH of the wet stream is controlled by the temperature and the height of liquid water inside the bubble humidifier. The exhaust-wet air of the bubble humidifier is mixed with the air out of the coil and adjusted to reach the RH of 99% at the desired temperature. Before entering the membrane humidifier, the wet air enters the temperature-humidity sensor, which measures the temperature and humidity. In both dry and wet sides, the temperature and humidity of the air are measured after leaving the humidifier. The temperature sensor has an accuracy of ±0.1 and the RH sensor has an accuracy of ±0.1%. In these humidifiers, it is

Fig. 2 e Assembled components of the membrane humidifier: (a) end plate with inputs and outputs, (b) end plate with manifolds, (c) plate with manifolds and silicon gaskets, (d) plate with channels and silicon gaskets, (e) Nafion membrane, (f) elastomeric insulation tape. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Pv ¼ Psat 4dry;out

Table 1 e Geometric specifications and operating conditions. Description Geometric specifications Number of channels in each side of each stack Number of stacks in each humidifier Flow channel width, d Flow channel height, h Channel length, L Channel plate width, W Channel shoulder width, R Membrane thickness, t Operating conditions Operating pressure RH of wet side inlet Humidity ratio of dry side inlet Operating temperature (for isothermal condition) Dry side inlet temperature (for insulation condition) Wet side inlet temperature (for insulation condition) Wet side flow rate Dry side flow rate

Unit 24

e

2

mm mm mm mm mm mm

2 2 82 120 2 127

kPa % kgvapor/kgair  C 

C

101.325 99 0.00135 30, 40, 50 and 60 30



C

60

m3/h m3/h


17:27T psat ¼ 0:61078 exp T þ 237:3

WRR ¼

1, 2, 3, 4 and 5 1, 1.5, 2, 2.5 and 3

Analysis indices The performance of the humidifier is analysed in terms of the water vapour transfer rate (WVTR), water recovery ratio (WRR) and dew point approach temperature (DPAT). These parameters are dependent on but complementary to each other [19]. At the insulated condition, the temperature of dry side outlet is another parameter to analysis the humidifier performance. The more the temperature of dry side outlet, the more is the heat transfer rate. WVTR is the water transfer rate from the wet to the dry side through the membrane, determined as follows: (1)

where m_ v;dry;out and m_ v;dry;in are mass flow rates of water at the dry side outlet and inlet, respectively. Mass flow rate of water can be obtained by the experimentally-measured data (RH and temperature). For instance, m_ v;dry;out is obtained as follows: m_ v;dry;out ¼ udry;out m_ a;dry

0:622Pv P  Pv

WVTR m_ v;wet;in

(6)

where m_ v;wet;in is the water mass flow rate at wet side inlet. A higher WRR indicates a better humidifier performance. The humidifier should have a high humidity and temperature at its dry side outlet. Dew point is another evaluation criterion of the humidifier performance, which includes effects of both humidity and temperature. In studies carried out by Refs. [19,26,32,43] the dew point is applied as a criterion for indicating good performance of the humidifier. The closer the dew point at dry side outlet to that of the wet side inlet, the better is the humidifier performance. Therefore, the dew point at dry side outlet, by itself, gives imperfect information regarding the humidifier performance. An evaluation criterion of the humidifier performance should determine how well a humidifier performs compared to how well it could perform ideally. DPAT is used for this purpose and defined as follows [19,32,42]: DPAT ¼ Tdp;wet;in  Tdp;dry;out

(7)

where Tdp;wet;in and Tdp;dry;out are the dew point temperatures at wet side inlet and dry side outlet, respectively. A lower DPAT indicates a better humidifier performance. For the ideal humidifier DPAT is zero. The temperature and RH of the dry side outlet measured in each experiment is converted to the dew point as follows:   Psat T ¼ Tdp ¼ Psat ðTÞ  RH

(8)

Results and discussion

(2)

where m_ a;dry is the mass flow rate of air at dry side and udry;out is the humidity ratio which is calculated as below: udry;out ¼

(5)

where temperature T is in  C (measured in each test) and Psat is in kPa. In most studies, the wet side inlet of the humidifier is set to a constant RH of 99% (fully saturated). Therefore, through tuning the flow rate or temperature of the wet side inlet, the flow rate of water entering the wet side will change. Hence, comparing WVTR at different flow rates alone is not a complete comparison. Therefore, in addition to WVTR, the ratio of the amount of water transferred through the membrane to the maximum amount of water available for transfer should be compared; thus a non-dimensional parameter named water recovery ratio is defined as follows [19,26,32,42]:

necessary to warm the humidifier whole body for 20e30 min before doing the experiments to reach the steady state condition. Due to the high thermal capacity and high density of aluminium, most of this time is used to heat the end plates.

WVTR ¼ m_ v;dry;out  m_ v;dry;in

(4)

where 4dry;out is the relative humidity (RH) at the dry side outlet (measured in each test) and Psat is the saturation pressure obtained by the Tetens equation [41]:

Value

e

5

(3)

where P is operating pressure and Pv is the water vapour pressure:

In this study, two thermal boundary conditions including isothermal and insulated status are applied for all experiments. At all tests three configurations including cross flow, parallel flow and counter flow are compared with each other and effect of dry side flow rate, wet side flow rate and equal flow rates at both sides on humidifier performance is investigated. In all tests, Reynolds number of each channel has

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Fig. 3 e (a) A schematic and (b) an actual image of the experimental test station.

a range between 200 and 1000, indicating laminar flow regime. The air flow rate is one of the important parameters affecting the fuel cell performance. Fuel cell membrane humidity should be of an optimal and controlled amount. Air flow rate has a significant effect on it. Higher air flow rate leads to a faster removal of the generated water from cathode. Faster water removal, in the presence of excess water in the membrane, will prevent flooding, which will improve the performance of the fuel cell [44e46]; However in the presence of water depletion in the membrane, faster water removal leads to membrane hydration, resulting in deterioration of fuel cell performance [47]. On the other hand, lower air flow rates makes consumption of reactants more uniform [48]. Thus, investigation of different air flow rates is necessary to

achieve an efficient fuel cell performance through adjusting the flow rate.

Uncertainty analysis The uncertainty analysis is performed by the method proposed by Moffat [49] in all experiments. If parameter F is calculated from a set of specific variables: F ¼ Fðx1 ; x2 ; x3 ; :::; xN Þ

(9)

The amount of uncertainty in the measurement of F is evaluated as follows: ( dF ¼

N
X vF i¼1

vxi

2 )12 dxi

(10)

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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where dxi is the uncertainty in measuring or calculating variable xi . For parameters that are directly measured, the uncertainty is the accuracy of the measuring instruments. In this study, these parameters are temperature, relative humidity and volumetric flow rate with uncertainties of 0.1 , 0.1% and 0.2 m3/h, respectively. The uncertainties of the main parameters in the humidifier analysis in the range of tested flow rates are given in Table 2.

Isothermal boundary condition In experimental studies conducted by Refs. [31,34,43,50,51], isothermal condition is applied to investigate the humidifier performance. In this study, the isothermal boundary condition is implemented for the whole body of the humidifier by wrapping a heating wire around it. During each test, the temperature of the dry side and wet side inlets and all external walls is measured by the thermometers and adjusted to a desired constant temperature. Utilizing the isothermal condition has many advantages, to mention a few: i. The water transfer characteristics are not influenced by the heat transfer. ii. Isothermal condition can help prevent condensation within the test module, consequently avoiding twophase problems. iii. Easier establishment of steady state condition is obtained by applying isothermal condition. The influence of operating temperature on the humidifier performance is investigated at different temperatures (30, 40, 50 and 60  C). Nevertheless, since the temperature of 60  C is most commonly applied in PEM fuel cell systems, the investigation of flow rates is performed at this operating temperature.

Effect of operating temperature on humidifier performance at three different configurations To study the influence of operating temperature, this latter is varied between 30 and 60  C by keeping the wet side and dry side flow rates constant at 1 m3/h. Other characteristics are kept constant as those listed in Table 1. The influence of operating temperature on the WVTR, WRR, dew point at dry side outlet and DPAT in three flow pattern configurations is shown in Fig. 4(a)-(d), respectively. At all temperatures the highest WVTR and WRR occurs in counter flow humidifier case and the lowest WVTR and WRR occurs in the case of cross flow humidifier. Furthermore, the highest dew point at dry side outlet, consequently the lowest DPAT occurs in counter flow humidifier and the lowest dew point at dry side outlet,

Table 2 e The uncertainties of the main parameters in the range of the tested flow rates. Parameter Water vapour transfer rate Water recovery ratio Dew point Dew point approach temperature

Notation Unit Uncertainty WVTR WRR DP DPAT

mg/s % K K

0.04e0.38 0.13e2.1 0.1e0.44 0.14e0.62

7

consequently the highest DPAT occurs in cross flow humidifier. As depicted in Fig. 4(a) in terms of mass transfer, cross flow configuration has a weaker performance than the other two configurations. In the parallel and counter flow configurations, each channel at the dry side is underneath the channel from the wet side. In other words, each channel of the dry side is directly connected to one channel from the wet side. Direct connection means that water from the wet side channel, after passing through the membrane, reaches directly the dry side channel. However, in cross flow configuration, since the dry side and wet side channels are perpendicular to each other; large portions of each channel at one side are connected to the shoulders of the other side channels. In other words, the useful and optimal area of the membrane for transferring water, in the cross flow configuration, is much less than the other two configurations. Fig. 5 (a) and (b), represent schematics of one square sections (10 mm  10 mm) related to each configuration, cross flow and counter/parallel flow, respectively. Marked areas indicate joint parts between wet and dry sides (the parts of dry and wet side channels in direct connection). Each marked area is a square of side 2 mm. In the cross flow configuration, there are only 4 marked areas, while in the parallel/counter configuration there are 10 marked areas. Nevertheless, in non-marked areas still slight amount of water may be transferred. In this way, the available water in membrane can move toward adjacent channels. Therefore, the amount of water transfer rate in the humidifier with cross flow configuration is less than for two other configurations. As seen in Fig. 4, in all three configurations, increase of temperature leads to increase of WVTR but decrease of WRR. With increasing temperature, the saturation pressure increases. Since the RH of the wet side inlet is set to a constant value of 99% (fully saturated), the increase of temperature results in increase of water vapour pressure, consequently to an increase of water flow rate at the wet side inlet (see Eq. (4)). According to Eq. (6), increasing the WVTR and increasing the flow rate of water at wet side inlet are two opposite factors that affect the WRR. Decreasing WRR with increase of temperature indicates the overcoming of the effect of water flow rate at wet side inlet. For cross flow configuration this is in good agreement with the Kadylak and Merida [31] results. In all three configurations, increase of temperature leads to increase of dry side outlet dew point and increase of DPAT. Since at any temperature, the RH of the wet side inlet is set to a constant value of 99% (fully saturated), the wet side inlet dew point is equal to the wet side inlet temperature, which is the operating temperature of the humidifier. Referring to the Eq. (7), increasing the dew point of dry side outlet and increasing the dew point of wet side inlet are two opposite factors affecting the DPAT. Increasing DPAT with increase of temperature indicates overcoming the influence of the wet side inlet dew point.

Effect of wet side flow rate on the humidifier performance at three different configurations The influence of wet side flow rate on the WVTR, WRR and DPAT at different configurations is demonstrated in Fig. 6(a)(c), respectively. To study the influence of wet side flow rate, this latter is varied between 1 and 5 m3/h by keeping the dry side flow rate constant at 1 m3/h. Operating temperature is

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Fig. 4 e The influence of operating temperature on the (a) WVTR, (b) WRR, (c) dew point of dry side outlet and (d) DPAT at three configurations.

Fig. 5 e Schematics of one square sections (10 mm * 10 mm) of (a) cross flow and (b) counter/parallel flow channels (Marked areas are the joint parts of wet and dry sides). Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Fig. 6 e The influence of wet side flow rate on the (a) WVTR, (b) WRR and (c) DPAT at three configurations.

60  C and other characteristics are kept constant as those listed in Table 1. At all flow rates, the highest values of WVTR and WRR, also the lowest value of DPAT occurs in counter flow humidifier and the lowest values of WVTR and WRR, also the highest value of DPAT happens in cross flow humidifier. In different wet side flow rates, using counter flow configuration compared to the parallel flow causes a 1%e10% increase in WRR and using parallel flow configuration compared to the cross flow causes a 13%e30% increase in WRR. It can be seen in Fig. 6 that in all three configurations, increasing wet side flow rate leads to increase of WVTR, but to the decrease of WRR. Since the RH of the wet side inlet is set to 99% (fully saturated) for all flow rates, the amount of water flow rate entering the wet side increases with the increase of wet side flow rate. According to Eq. (6), increasing the WVTR and increasing the flow rate of water entering the wet side are two opposite factors that affect the WRR. Decreasing WRR with the increase of wet side flow rate indicates that the effect of increasing water flow rate at the wet side inlet is more prominent than the effect of increasing WVTR. The slope of

WRR variation curve decreases with increasing wet side flow rates. It means that at higher flow rates the effect of flow rate on WRR decreases. For example, in counter flow configuration, when the wet side flow rate varies from 1 to 2 m3/h the WRR is reduced by 8.4% while when the wet side flow rate varies from 4 to 5 m3/h the WRR is reduced by 0.9%. In counter flow configuration, the WRR at the tested wet side flow rates varies from 4.4% to 17.74%. At higher flow rates of wet side, the performance of cross flow configuration approaches to that of the parallel flow. In other words, as the wet side flow rate increase the difference between WRR in parallel flow and cross flow decreases. In parallel channels almost all of the water passes through the area where the two channels are directly connected. In cross-flow arrangement, some of the water passes through an area that one side of the membrane is the channel, and the other side is the shoulder. As the flow rate of wet side increases, the amount of water transfer from these areas increases. This is because more fluid momentum (caused by more flow rate) can overcome membrane resistance against water diffusion. For the wet side flow rate of

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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1 m3/h, the WRR in parallel flow and cross flow configurations are 16.8% and 13.3%, respectively. For the wet side flow rate of 5 m3/h, the WRR in parallel flow and cross flow configurations are 4.3% and 3.8%, respectively. According to Fig. 6(c) at different wet side flow rates, using counter flow configuration compared to the parallel flow causes a 1e8% decrease in DPAT and using parallel flow configuration compared to the cross flow causes a 6e16% decrease in DPAT. In all three configurations, increase of wet side flow rate causes increase of dry side outlet dew point, consequently decrease of DPAT. At a specific constant temperature, referring to Eq. (7) due to the constant dew point of the wet side inlet by changing the wet side flow rate, there is an inverse relation between the DPAT and dew point of the dry side outlet. In counter flow configuration, the DPAT at the tested wet side flow rates varies from 29.5 to 32.89  C.

Effect of dry side flow rate on the humidifier performance at three different configurations The influence of dry side flow rate on the WVTR, WRR and DPAT at different three configurations is illustrated in

Fig. 7(a)e(c), respectively. To study the influence of dry side flow rate, this is varied between 1 and 3 m3/h by keeping the wet side flow rate constant at 4 m3/h. Operating temperature is 60  C and other characteristics are kept constant as those listed in Table 1. At all flow rates, the highest value of WVTR and consequently highest WRR, also the lowest value of DPAT occurs in counter flow humidifier and the lowest value of WVTR, consequently lowest WRR, and also the highest value of DPAT occurs in cross flow humidifier. In different dry side flow rates, using counter flow configuration compared to the parallel flow causes a 2e10% increase in WRR and using parallel flow configuration compared to the cross flow causes a 5e27% increase in WRR. In all three configurations, WVTR and WRR increases with increasing dry side flow rate. Due to the constant water flow rate of the wet side inlet by changing the dry side flow rate, the trend of variations in the WRR will be the same as the WVTR. In fact, referring to Eq. (6), WRR is directly proportional to WVTR. For humidifier with counter flow configuration, this is in good agreement with the experimental results conducted by Cahalan et al. [33] and Chen et al. [42]. In humidifier with counter flow configuration,

Fig. 7 e The influence of dry side flow rate on the (a) WVTR, (b) WRR and (c) DPAT at different configurations. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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the WRR at the tested dry side flow rates varies from 5.3% to 6.1%. According to Fig. 7(c) for different dry side flow rates, using counter flow configuration compared to the parallel flow causes a 1e6% decrease in DPAT and using parallel flow configuration compared to the cross flow causes a 3e15% decrease in DPAT. In all three configurations, increase of dry side flow rate causes the decrease in dew point of dry side outlet, consequently the increase in DPAT. For counter flow configuration this is in good agreement with the experimental results conducted by Cave and Merida [30]. Referring to Eq. (7) at a constant temperature, due to the constant dew point of the wet side inlet by changing the dry side flow rate, an inverse relation between the DPAT and dew point of dry side outlet results. In counter flow configuration, the DPAT at the tested dry side flow rates varies from 30.12 to 44.11  C. At higher flow rates at dry side, the cross flow configuration approaches the parallel flow in terms of performance. In other words, increasing the dry side flow rate leads to a decrease in the difference between DPAT of parallel and cross flows. For a dry side flow rate of 1 m3/h, the DPAT in parallel flow and cross flow configurations are 30.5 and 34.2 , respectively. For dry side flow rate of 3 m3/h, the DPAT in parallel flow and cross flow configurations are 44.7 and 45.6 , respectively.

Effect of equal flow rate at dry and wet sides on the humidifier performance at three different configurations In the previous sections, dry and wet side flow rates were investigated separately. To compare the effect of dry and wet side flow rates on humidifier performance and to see which one is dominant, the effect of equal flow rate at dry and wet side is investigated in this section. In addition, in high stoichiometry of the fuel cell cathode where the air consumption in the fuel cell is low compared to the inlet air, the outlet flow rate of the fuel cell is equal to the inlet flow rate. Under this condition, if we want to use the hot and wet gas of the cathode outlet as the humidifier wet side inlet, it can be assumed that the mass flow rates at humidifier wet side and dry side are nearly equal. The influence of equal flow rate at dry and wet sides on WVTR, WRR and DPAT under different configurations is displayed in Fig. 8(a)-(c), respectively. To study the influence of equal flow rate at dry side and wet side, this is varied between 1 and 3 m3/h. Operating temperature is 60  C and other characteristics are kept constant as those listed in Table 1. At all flow rates, the highest values of WVTR and WRR, also the lowest value of DPAT occur in counter flow humidifier and the lowest values of WVTR and WRR, also the highest value of DPAT are attained in cross flow humidifier. In different equal flow rates of dry and wet side, using counter flow configuration compared to the parallel flow causes a 2e9% increase in WRR and using parallel flow configuration compared to the cross flow causes a 5e31% increase in WRR. In all three configurations, increasing equal flow rate of dry and wet side leads to increase of WVTR and decrease of WRR. Since the RH of the wet side inlet is set to 99% (fully saturated) for all flow rates, the amount of water flow rate entering the wet side increases with the amount of increase of equal wet side and dry side flow rates. According to Eq. (6), increasing the WVTR and increasing the flow rate of water entering the wet

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side are two opposite factors that affect the WRR. Decreasing WRR with the increase of equal flow rate of dry and wet side indicates that the effect of increasing water flow rate of the wet side inlet is more prominent than the effect of increasing WVTR. In addition, it can be found from Fig. 8 that the WRR variation curve has milder slope at higher flow rates. It means that as the equal flow rate of dry and wet side increases, the effect of flow rate on WRR decreases. In counter flow configuration, the WRR at the tested equal flow rates of dry and wet side varies from 8.39% to 17.74%. At higher equal flow rates of dry and wet sides, the performance of cross flow configuration approaches that for parallel flow. In other words, as the equal flow rate increases, the difference between WRR in parallel and cross flow decreases. In equal flow rate of 1 m3/h, the WRR in parallel flow and cross flow configurations are 16.8% and 13.3%, respectively. In equal flow rate of 3 m3/h, the WRR in parallel flow and cross flow configurations are 7.7% and 7.4%, respectively. According to Fig. 8(c) in different equal flow rates of dry and wet side, using counter flow configuration compared to parallel flow results in a 2e5% decrease in DPAT and using parallel flow configuration compared to cross flow leads to a 1e15% decrease in DPAT. In all three configurations, increase of equal flow rates of dry side and wet side leads to a decrease of dry side outlet dew point, consequently an increase of DPAT. At a specific constant temperature, referring to Eq. (7) due to the constant dew point of the wet side inlet by changing the flow rate, there exhibits an inverse relation between the DPAT and dew point of dry side outlet. At higher equal flow rates, the cross flow configuration approaches the parallel flow in terms of performance. In other words, increase of the equal flow rates at dry and wet sides results in a decrease of the difference between DPAT in parallel and cross flows. For equal flow rate of 1 m3/h, the DPAT in parallel and cross flow configurations are 33.8 and 37.1 , respectively. In equal flow rate of 3 m3/h, the DPAT in parallel flow and cross flow configurations are 44.9 and 45.6 , respectively. In counter flow configuration, the DPAT at the tested equal flow rates of dry side and wet side varies from 32.89 to 43.8  C.

Insulation boundary condition In experimental studies conducted by Refs. [19,26,32,33] the insulation condition is applied to investigate the humidifier performance. In this study, the insulation boundary condition is implemented by wrapping humidifier external body with self-adhesive elastomeric insulation tape. (See Fig. 2). The insulation tape is thick enough to ensure that humidifier is well insulated from its surroundings. Using the insulating boundary condition has an important advantage in comparison to the isothermal condition that in addition to the mass transfer, heat transfer through the membrane from wet side to the dry side also occurs. In most PEM fuel cell systems, the temperature of the dry side inlet and wet side inlet of the humidifier is different. Therefore, heat transfer occurs between the dry and wet sides. In order to prevent waste of heat, insulating the humidifier from its surroundings is necessary. With the insulation boundary condition, the wet side and dry side inlet temperatures are assumed 60  C and 30  C, respectively.

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Fig. 8 e The influence of equal flow rate of dry side and wet side on the (a) WVTR, (b) WRR and (c) DPAT at three configurations.

Effect of wet side flow rate on the humidifier performance at three different configurations The influence of wet side flow rate on the WVTR and WRR at different three configurations is demonstrated in Fig. 9(a) and (b), respectively. To study the influence of wet side flow rate, this is varied between 1 and 5 m3/h by keeping the dry side flow rate constant at 1 m3/h. Other characteristics are kept constant as those listed in Table 1. At all flow rates, the highest values of WVTR and WRR is in counter flow humidifier and the lowest values of WVTR and WRR is in cross flow humidifier. In different wet side flow rates, using counter flow configuration compared to the parallel flow causes a 0e5% increase in WRR and using parallel flow configuration compared to the cross flow causes a 0e10% increase in WRR. It can be seen in Fig. 9 that in all three configurations, increasing wet side flow rate leads to increase of WVTR, but decrease in WRR. Since the RH of the wet side inlet is set to 99% (fully saturated) for all flow rates, the amount of water flow rate entering the wet side increases with the increase

of wet side flow rate. According to Eq. (6), increasing the WVTR and increasing the flow rate of water entering the wet side are two opposite factors that affect the WRR. Decreasing WRR with the increase of wet side flow rate indicates that the effect of increasing water flow rate of the wet side inlet is more prominent than the effect of increasing WVTR. The slope of WRR variation curve decreases with increasing wet side flow rates. It means that at higher flow rates the effect of flow rate on WRR decreases. For example, in counter flow configuration, when the wet side flow rate varies from 1 to 2 m3/h, the WRR is reduced by 9.8% while when the wet side flow rate varies from 4 to 5 m3/ h, the WRR is reduced by 1.2%. In counter flow configuration, the WRR at the tested wet side flow rates varies from 5.7% to 22.9%. At higher flow rates at wet side, the performance of cross flow configuration approaches that for parallel flow. In other words, as the wet side flow rate increases, the difference between WRR in parallel flow and cross flow decreases. In parallel channels almost all of the water passes through

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Fig. 9 e The influence of wet side flow rate on the (a) WVTR, (b) WRR, (c) temperature of dry side outlet and (d) DPAT at three configurations.

the area where the two channels are directly connected. In cross-flow arrangement, some of the water passes through an area that one side of the membrane is the channel, and the other side is the shoulder. As the flow rate of air increases, the amount of water transfer from these areas increases. This is because more fluid momentum (caused by more flow rate) can overcome membrane resistance against water diffusion. The influence of wet side flow rate on the temperature of dry side outlet and DPAT at different configurations is displayed in Fig. 9(c) and (d), respectively. At all flow rates, the highest value of dry side outlet temperature occurs for the counter flow humidifier and the lowest value of dry side outlet temperature presents at the parallel flow humidifier. This indicates that the cross flow configuration, although having lower mass transfer rate, has a higher heat transfer rate than the parallel flow. This is because the conduction heat transfer through the channels shoulders of the humidifier completely affects the overall heat transfer. In fact, referring to Fig. 5, in the marked regions, both heat and mass transfer take place

directly through the membrane, while in other regions, the heat transfer happens from the solid parts of the channels via conduction, but mass transfer cannot occur through the solid parts. It can be observed from Fig. 9(c) that as the wet side flow rate increase, the thermal performance of the cross flow humidifier improves. Dry side outlet temperature in cross flow humidifier at lower flow rates is closer to the parallel flow and at higher flow rates is closer to the counter flow humidifier. In all three configurations, increase of wet side flow rate results in increase of dry side outlet temperature. This is due to an increase in the heat transfer rate by increasing the wet side flow rate. At higher flow rates, the effect of flow rate on the temperature of dry side outlet decrease. According to Fig. 9(d), at all flow rates, the highest value of dew point at dry side outlet, consequently the lowest value of DPAT, occurs in counter flow humidifier and the lowest value of dew point at dry side outlet, consequently the highest value of DPAT, happens for cross flow humidifier. At higher wet side flow rates, the performance of cross flow configuration approaches

Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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that for parallel flow. In other words, as the wet side flow rate increases, the difference between DPAT in parallel and cross flows decreases. Referring to Fig. 9(d) in all three configurations, an increase of wet side flow rate causes an increase of the dry side outlet dew point, consequently a decrease of DPAT. At a specific constant temperature, referring to Eq. (7) due to the constant dew point of the wet side inlet, by changing the wet side flow rate, an inverse relation between the DPAT and dew point of dry side outlet arises. The slope of DPAT variation curve decreases with increasing wet side flow rate. It means that at higher flow rates, the effect of flow rate on DPAT decreases. In counter flow configuration, the DPAT at the tested wet side flow rate varies from 27 to 30.4  C.

Effect of dry side flow rate on the humidifier performance at three different configurations The influence of dry side flow rate on the WVTR, WRR at different three configurations is illustrated in Fig. 10(a) and (b), respectively. To study the influence of dry side flow rate,

this is varied between 1 and 3 m3/h by keeping the wet side flow rate constant at 4 m3/h. Other characteristics are kept constant as those listed in Table 1. At all flow rates, the highest value of WVTR, consequently maximum of WRR, occurs for counter flow humidifier and the lowest value of WVTR, consequently lowest WRR, happens for cross flow humidifier. In humidifier with counter flow configuration, the WRR at the tested dry side flow rates varies from 5.3% to 6.1%. It can be seen in Fig. 10 that in humidifier with cross flow and parallel flow configurations, the WVTR and WRR increase with increasing the dry side flow rate but in counter flow humidifier, WVTR and WRR remain almost constant. Thus as the dry side flow rate increases, the performances of parallel and counter flow approach each other. Due to the constant water flow rate of the wet side inlet by changing the dry side flow rate, referring to Eq. (6), WRR becomes directly proportional to WVTR. The influence of dry side flow rate on the dry side outlet temperature and DPAT at different configurations is indicated

Fig. 10 e The influence of dry side flow rate on the (a) WVTR, (b) WRR, (c) temperature of dry side outlet and (d) DPAT at three configurations. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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in Fig. 10(c) and (d), respectively. At all flow rates, the highest value of dry side outlet temperature is for the counter flow humidifier and the lowest value of dry side outlet temperature is for the parallel flow humidifier. In all three configurations, an increase in dry side flow rate results in a decrease in dry side outlet temperature. According to Fig. 10(d) at all flow rates, the highest value of dew point at dry side outlet, consequently the lowest value of DPAT occurs in the counter flow humidifier and the lowest value of dew point at dry side outlet, consequently the highest value of DPAT, in cross flow humidifier. In all three configurations, an increase of dry side flow rate gives rise to a decrease of dry side outlet dew point, consequently an increase of DPAT. For humidifier with counter flow configuration, this is in good agreement with the experimental results conducted by Chen et al. [42]. Referring to Eq. (7), due to the constant dew point of the wet side inlet by changing the dry side flow rate, there is an inverse relation between the DPAT and dew point of dry side outlet. In counter flow configuration, the DPAT at the tested dry side flow rates varies from 27.3 to 43.8  C.

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Effect of equal flow rate at dry and wet sides on the humidifier performance at three different configurations The influence of equal flow rate of dry side and wet side on the WVTR and WRR at different three configurations is displayed in Fig. 11(a) and (b). To study the influence of equal flow rate at dry side and wet side, this is varied between 1 and 3 m3/h. Other characteristics are kept constant as those listed in Table 1. At all flow rates, the highest values of WVTR and WRR is in counter flow humidifier and the lowest values of WVTR and WRR occurs for the cross flow humidifier. In different equal flow rates of dry and wet side, using counter flow configuration compared to the parallel flow causes a 2e4% increase in WRR and using parallel flow configuration compared to the cross flow causes a 0e4% increase in WRR. In all three configurations, increasing equal flow rate of dry side and wet side leads to increase of WVTR and decrease of WRR. Since the RH of the wet side inlet is set to 99% (fully saturated) for all flow rates, the amount of water flow rate entering the wet side increases with the increase of equal wet side and dry side flow rates. According to Eq. (6), increasing the WVTR and

Fig. 11 e The influence of equal flow rate at dry side and wet side on the (a) WVTR, (b) WRR, (c) temperature of dry side outlet and (d) DPAT at three configurations. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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increasing the flow rate of water entering the wet side are two opposite factors that affect the WRR. Decreasing WRR with the increase of equal flow rate of dry and wet side indicates that the effect of increasing water flow rate of the wet side inlet is more prominent than the effect of increasing WVTR. For cross flow configuration, this is in good agreement with the experimental results of Hwang et al. [19]. It can be found from the Fig. 11(b) that in all three configurations the slope of WRR variation curve decreases with increasing the flow rate. It means that at higher equal flow rates of dry and wet side, the effect of flow rate on WRR decreases. For instance, when the equal flow rate varies from 1 to 1.5 m3/h WRR reduces by 6.5%, while when the equal flow rate varies from 2.5 to 3 m3/h, WRR reduces by 1.5%. In counter flow configuration, the WRR at the tested equal flow rates of dry and wet side varies from 9.05% to 22.94%. At higher equal flow rates of dry and wet side, the performance of cross flow configuration approaches that of parallel flow. In other words, an increase in equal flow rate of dry side and wet side leads to a decrease in difference between WRR in parallel and cross flow. The influence of equal flow rate of dry side and wet side on the dry side outlet temperature and DPAT at different three configurations is displayed in Fig. 11(c) and (d). At all flow rates, the highest value of dry side outlet temperature is in counter flow humidifier and the lowest value of dry side outlet temperature is in parallel flow humidifier. At higher equal flow rates of dry and wet side, the thermal performance of the cross flow humidifier will improve. Dry side outlet temperature in cross flow humidifier is closer to the parallel flow at lower flow rates and to the counter flow humidifier at higher flow rates. In all three configurations, an increase of equal flow rate leads to a decrease of dry side outlet temperature. This is due to a decrease in the heat transfer rate by increasing the equal flow rate. At higher flow rates, the effect of flow rate on the temperature of dry side outlet decreases. Referring to Fig. 11(d) at all flow rates, the highest value of dew point at dry side outlet, consequently the lowest value of DPAT, occurs for the counter flow humidifier and the lowest value of dew point at dry side outlet, consequently the highest

value of DPAT, for the cross flow humidifier. At higher equal flow rates of dry and wet side, the performance of cross flow configuration approaches that for the parallel flow. In other words, the difference between DPAT in parallel flow and cross flow decreases with the increase in the equal flow rate. In equal flow rate of 1 m3/h, the DPAT in parallel and cross flow configurations are 31 and 32.2 , respectively. In equal flow rate of 3 m3/h, the DPAT in parallel and cross flow configurations are 44.5 and 44.6 , respectively. In all three configurations, increase of equal flow rate of dry and wet side leads to decrease of dry side outlet dew point, consequently to the increase of DPAT. At a specific constant temperature, referring to Eq. (7) due to the constant dew point of the wet side inlet by changing the equal flow rate, there is an inverse relation between the DPAT and dew point of dry side outlet. In counter flow configuration, the DPAT at the tested equal flow rates varies from 30.4 to 44.17  C.

A comparison between isothermal and insulation conditions To compare the isothermal and insulation conditions, the humidifier performance with three thermal conditions including two isothermal conditions with operating temperatures of 30  C and 60  C and one insulation condition with dry side inlet temperature of 30  C and wet side inlet temperature of 60  C is analysed and compared with each other. Other characteristics are kept constant as those listed in Table 1. Fig. 12(a) and (b) indicate the WRR and DPAT variations in terms of equal flow rate of dry side and wet side at three mentioned thermal conditions, respectively. Almost in all flow rates, the highest WRR and the least DPAT (the best performance) is related to the condition of isothermal at 30  C and the lowest WRR and the highest DPAT (the worst performance) occurs in condition of isothermal at 60  C. In other words, the humidifier performance with insulation condition is better than that with isothermal condition at 60  C and is weaker than that with isothermal condition at 30  C. Therefore, it can be concluded that, regardless of the type of thermal boundary condition, the higher average temperature of the humidifier leads to the weaker humidifier performance.

Fig. 12 e (a) WRR and (b) DPAT variations in terms of equal flow rate of dry side and wet side at three thermal conditions. Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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As depicted in Fig. 12 in terms of DPAT, the humidifier performance with insulation condition is very close to that of with isothermal condition of 60  C. Given that at insulation condition the wet side inlet temperature is 60  C, it can be deduced that the temperature of wet side plays a key role in humidifier performance. As the flow rate increases, the humidifier performance with insulation condition becomes closer to that of the isothermal condition of 60  C. Even in flow rate of 2.5 m3/h, the DPAT of humidifier with insulation condition becomes higher than that with isothermal condition of 60  C. In other words at higher flow rates, the humidifier performance with insulation condition becomes weaker. At the insulation condition in comparison to the isothermal condition, the performance of cross flow humidifier is closer to the parallel flow. In other words, with insulation condition, the difference between WRRs in cross and parallel flow humidifiers, also the difference between DPATs are less than those with isothermal condition. When both dry and wet side flow rates are 1 m3/h, the difference between WRRs of parallel and cross flows in humidifier with isothermal condition is 3.35%, while in humidifier with insulation condition is 1.6%. Also the difference between DPATs of parallel and cross flow humidifiers in isothermal and insulation conditions are 3.4 and 1.2 , respectively. When both dry and wet side flow rates are 3 m3/h, the difference between WRRs of parallel and cross flow humidifiers in isothermal and insulation conditions are 0.34% and 0%, respectively. Also the difference between DPATs of parallel flow and cross flow humidifiers in isothermal and insulation conditions are 0.7 and 0.1 , respectively.

PEM fuel cell operating conditions with the fabricated humidifiers Each of the humidifier cells fabricated in this study, with a minimum flow rate on the dry side outlet (Q ¼ 1 m3/h) and RH of 50% entering the fuel cell cathode, can provide the humid air of a commonly used fuel cell of 30 cells with common specifications including cell voltage equal to 0.65 V, a consequent current density of 0.6 A/cm2, an active area equal to the humidifier area (A ¼ 8.2  8.2 ¼ 67 cm2), a cathode stoichiometry of 2 and a power of 26 W per cell. It is obvious that if the maximum flow rate of the dry side outlet (Q ¼ 3 m3/h) is used, the inlet air of a fuel cell with more cells can be humidified. However, if the number of fuel cell cells is increased, the number of humidifier cells can also be increased. The temperature and pressure range for which the above conditions can be reached are 314e328 K and 1e3 bars, respectively.

Conclusion In this work, two air-to-air planar membrane humidifiers with three configurations including cross flow, parallel flow and counter flow and two thermal boundary conditions including isothermal and insulation are tested. In each of these conditions, the effect of dry side flow rate, wet side flow rate and equal flow rates at both sides on the humidifier performance

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is investigated. A higher WRR and a lower DPAT indicate a better humidifier performance. Based on the results, major findings are concluded as follows: 1. At all flow rates and temperatures, among three different configurations, the counter flow humidifier has the highest values of heat and water transfer rates, consequently the best performance. Between the parallel flow and cross flow humidifiers, the higher heat transfer rate occurs in cross flow humidifier and the higher water transfer rate occurs in parallel flow humidifier. The worst performance (lowest WRR and highest DPAT) is for cross flow humidifier. 2. At isothermal condition in all three configurations, increase of dry side flow rate results in increase of WRR and increase of DPAT. At insulation condition, with increase of dry side flow rate the DPAT behaves similar to isothermal condition but WRR behavior is different. 3. The WRR in humidifier with insulation condition and dry side inlet temperature of 30  C and wet side inlet temperature of 60  C is higher than that with isothermal condition of 60  C and lower than that with isothermal condition of 30  C. The DPAT in humidifier with insulation condition is approximately equal to that of with isothermal condition at 60  C and is much higher than that with isothermal condition at 30  C. It can be deduced that the temperature of wet side inlet plays a key role in humidifier performance 4. At the insulation condition in comparison to the isothermal condition, the performance of cross flow humidifier is closer to the parallel flow.

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Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017

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Please cite this article as: Baharlou Houreh N et al., Experimental study on performance of membrane humidifiers with different configurations and operating conditions for PEM fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.017