PEM fuel-cell stack design for improved fuel utilization

PEM fuel-cell stack design for improved fuel utilization

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PEM fuel-cell stack design for improved fuel utilization In-Su Han*, Jeehoon Jeong, Hyun Khil Shin Research & Development Center, GS Caltex Corp., 359 Expo-ro, Yuseong-gu, Daejeon 305-380, Republic of Korea

article info

abstract

Article history:

We propose a new design for a polymer electrolyte membrane (PEM) fuel-cell stack that can

Received 19 April 2013

achieve higher fuel utilization without using hydrogen recirculation devices such as

Received in revised form

hydrogen pumps or ejectors, which consume parasitic power and/or require additional

27 June 2013

control schemes. The basic concept of the proposed design is to divide the anodic cells of a

Accepted 29 June 2013

stack into several blocks by inserting compartments between the cells, thereby con-

Available online 30 July 2013

structing a multistage anode with a single-stage cathode in a single stack. In this design, a higher gaseous flow rate is maintained at the outlet of the anodic cells, even under dead-

Keywords:

end conditions, and this results in a reduction of purge-gas emissions by hindering the

Fuel cell

accumulation of liquid water and nitrogen in the anodic cells. A 15 kW-class PEM fuel cell

Polymer electrolyte membrane

stack is designed, fabricated, and tested to investigate the effectiveness of the proposed

Fuel utilization

design. The experimental results indicate that the amount of purge gas is significantly

Dead-end operation

reduced, and consequently, a higher fuel utilization of more than 99.6% is achieved.

Stack design

Additionally, the output voltage of the stack fluctuates much less than that of conventional fuel cells owing to the multistage anode design. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

PEM (polymer electrolyte membrane) fuel cells are widely used as the primary power sources in transportation applications such as fuel-cell cars, buses, scooters, boats, underwater vehicles, and spacecrafts, owing to numerous advantages over other power sources including short startup time, compact system volume, low emission of pollutants, and relatively high system efficiency [1e3]. In such PEM fuelcell systems, pure hydrogen is normally used as the fuel, and unused hydrogen is discharged along with inert gases into the atmosphere. For maximum system efficiency and safety, however, these systems should consume as little fuel as possible for a given output power and should minimize the emission of hydrogen to the atmosphere.

The degree to which the hydrogen supplied to a fuel-cell system is consumed to generate thermal and electrical energy is known as the fuel utilization. If a fuel-cell system were to operate with 100% fuel utilization (or in dead-end mode), the amount of hydrogen fed into the anode of a fuel cell would be the same as the stoichiometric flow rate of hydrogen required for the electrochemical reactions. However, in dead-end operation, there is a high risk of fuel starvation at the outlet of the fuel cell, which can result in unstable cell voltages and cell degradation [4e6]. The major causes of fuel starvation are the accumulation of liquid water and the buildup of nitrogen in the anode [6,7]. Liquid water back-diffuses from cathode to the anode if the cathode holds more water, causing so-called flooding [8] at catalytic active sites. Nitrogen gas comes from the cathode air by permeating through the PEM and causes

* Corresponding author. Tel.: þ82 42 866 1732; fax: þ82 42 866 1794. E-mail addresses: [email protected], [email protected] (I.-S. Han). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.136

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dilution of the hydrogen at the anode. In practice, therefore, hydrogen is supplied to a PEM fuel cell in quantities that are much larger than the stoichiometric flow rate in order to provide sufficient forced convection to discharge accumulated water from the anode and to lower the nitrogen concentration. Typically, the fuel utilization of PEM fuel-cell stacks is in the range of 80e95% depending on their design. Therefore, in order to minimize hydrogen loss, unused hydrogen is recirculated into the anode inlet using a hydrogen pump or an ejector [9e11] and the nitrogen gas is purged to the atmosphere. The fuel utilization of a PEM fuel-cell system can be improved by employing a suitable hydrogen recirculation method or by designing a stack that avoids flooding. Several studies have been performed to improve fuel utilization. Nishikawa et al. [12] demonstrated a fuel utilization of 96% for a 5 kW-class PEM fuel cell stack that adopted an internal counter-flow humidification and stack separation method: the stack was divided into two blocks such that the exhaust hydrogen gas exiting the first block was fed into the second one after being separated from liquid water. Uno et al. [13] proposed a pressure swing recirculation system that operated using two check valves and fluid control devices without any recirculation pumps. They then investigated the performance of this system in a single fuel cell; the operation of the single cell was stable over 10 h, but the cell voltage of the single cell fluctuated somewhat with the pressure changes. There have been some experimental and/or simulation studies to investigate PEM fuel cells operating in dead-end mode. Lee et al. [14] carried out an experimental study on water transport in a unit cell under dead-end operation to find cell voltage variations and to visualize water accumulation in the flow-field channels. Mocoteguy et al. [15] investigated the dynamic behavior of a PEM fuel-cell stack operating in deadend mode using both experiments and simulations. They showed that, as the operation time elapsed, the voltage decay trends of a five-cell stack were dependent on water accumulation at the outlet of the cell stack. Choi et al. [16] experimentally investigated the purge characteristics of a single PEM fuel cell operating in anodic dead-end mode. They suggested a hydrogen pulsation method to lower the partial pressure of water vapor in the anode outlet to increase the purge interval and fuel utilization. In this paper, we present a new design for a PEM fuel-cell stack with the aim of improving fuel utilization without using any hydrogen recirculation devices. This design comprises a single-stage cathode and a multistage anode, in which the anodic cells of the stack are divided into several blocks by inserting compartments between the cells. We will first briefly explain how the fuel utilization of a PEM fuel-cell stack is controlled either with or without a hydrogen recirculation device. Then, the proposed stack design is presented together with a method to determine the major design parameters such as the number of blocks and number of anodic cells in each block of the stack. Finally, to investigate the effectiveness of the stack design, we fabricated a 15 kW-class PEM fuelcell stack to verify the improved performance of the stack in terms of fuel utilization, output power, and variations in the average cell voltage.

11997

2. Fuel-cell stack operation with/without hydrogen recirculation devices We briefly explain how the fuel utilization of a PEM fuel-cell stack is controlled either with or without a hydrogen recirculation device in conventional PEM fuel-cell systems. Fig. 1(a) shows a PEM fuel-cell stack operating with a hydrogen recirculation device to improve fuel utilization and Fig. 1(b) describes the operation in dead-end mode without a hydrogen recirculation device. In both figures, the purge valves serve to discharge excess hydrogen or inert gas from the stack and the liquid separators remove liquid water. When the purge valve is closed, the flow rate of fresh hydrogen supplied to the stack is equal to the stoichiometric flow rate. The hydrogen recirculation device recirculates hydrogen back to the inlet of the stack, and thus, the total flow rate of hydrogen circulating through the stack can be much greater than the stoichiometric flow rate and can be controlled by the hydrogen recirculation rate. The purge valve thus opens less frequently than if the stack were operating in deadend mode, because the higher gaseous flow rate increases forced convection and discharges liquid water from the anode, and therefore, higher fuel utilization can be achieved. However, nitrogen, which comes from the cathode air, accumulates at the anode as the operating time proceeds [17,18], and this buildup of nitrogen leads to fuel starvation and voltage decay of the stack. Thus, the purge valve must be periodically opened to lower the nitrogen level at the anode in order to maintain the cell voltage. Some hydrogen will also be lost along with the nitrogen whenever the purge valve is opened, resulting in diminished fuel utilization. Typically, the effect of nitrogen accumulation on voltage decay depends on operating conditions, such as temperature and pressure [19e21]. To recirculate unreacted hydrogen in the stack, it must be equipped with mechanical devices such as hydrogen

a Air

Excess air

Cathode

Purge gas Anode

H

Purge valve Liquid separator Blower / Ejector

Liquid water drain

Hydrogen recirculation

b Air

Excess air

Cathode

Purge gas H

Anode Purge valve Liquid separator Liquid water drain

Fig. 1 e Schematic of a PEM fuel-cell stack: (a) with a hydrogen recirculation device, and (b) without a hydrogen recirculation device (in dead-end operation).

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pumps, blowers, and ejectors, which require additional control schemes and advanced mechanical design and consume parasitic power [11,22]. A PEM fuel-cell stack can be operated in dead-end mode without any hydrogen recirculation device to achieve high degrees of fuel utilization, as shown in Fig. 1(b). The hydrogen supplied to the stack is completely consumed, and consequently, high fuel utilization is achieved while the purge valve is closed. However, the hydrogen flow rate is almost zero near the outlet of the anode, resulting in liquid water not being discharged from the stack owing to a lack of forced convection. Thus, liquid water accumulates at the anode, leading to a drop in the cell voltage over time. To discharge the liquid water before the cell voltage drops below a certain limit, the purge valve must be opened, resulting in the stack losing a large amount of hydrogen and fluctuations in the cell voltage. The purge valve of a stack operating in dead-end mode opens more frequently than that of a stack with a hydrogen recirculation device owing to greater liquid water and nitrogen accumulation at the anode. In addition, its durability tends to be shorter because fuel starvation, which is mainly caused by blocking of the catalytic active sites by liquid water, occurs near the outlet of the anodic cells [5,6]. PEM fuel-cell stacks operating in dead-end mode have few advantages other than a simpler configuration, and thus have rarely been used in commercial applications.

3. New stack design for improved fuel utilization 3.1.

Design description

Fig. 2 shows a schematic drawing of the proposed design for a PEM fuel-cell stack and does not include any hydrogen recirculation devices. The design of the cathode is the same as that of a conventional stack in which a sufficient amount of air, typically twice as much as the stoichiometric flow, passes through the entire cathodic cell and is then vented to the atmosphere. However, the anodic cells in the stack are divided into two or more blocks (referred to hereafter as “stages”) by inserting compartments between the cells. Hydrogen flows in a cascade manner from the first stage to the second, the second to the third, until the last stage is reached. Thus, the stack is composed of a multistage anode and a single-stage cathode i-th stage S-th stage

Air

H (F )

1 stage

Cathode

Excess air

2 stage

Purge gas

Purge valve

in a single stack. All of the stages are electrically connected to one another for the transfer of electrons. From the viewpoint of the entire stack, when the purge valve is closed, hydrogen at the inlet of the stack is supplied at exactly the stoichiometric flow, i.e., the hydrogen stoichiometry is one (100% fuel utilization) at the anode. In all of the cascade stages except for the last one, however, the stoichiometry can be much greater than one, depending on how many anodic cells are in each stage. In the last stage, the stoichiometry remains at one. From the first to penultimate stage, therefore, the hydrogen flow rate through the anodic cells is sufficiently high to remove condensed water from the stack. Only the anodic cells in the last stage are under dead-end conditions. The number of anodic cells in the last stage can represent only a small portion of the total number of cells, typically ranging from one to several cells depending on the total number of cells and stages. Since liquid water is removed by a liquid water separator before entering the next stage, water accumulation is significantly reduced in the anode. By applying the proposed design, we can expect a reduction in flooding in the anodic cells of the stack, and thus, we can expect to achieve higher fuel utilization.

3.2.

Design parameters

In this section, we describe how the design parameters of the proposed stack design, such as the number of stages and the number of anodic cells in each stage of a stack, can be determined. We consider that a stack composed of NT cells in total can be divided into S stages and that the ith stage of the stack consists of Ni anodic cells. The hydrogen stoichiometry (li) in the ith stage can be represented as follows: li ¼

Fi ri

where Fi denotes the mass flow rate of hydrogen entering the ith stage in kg s1, and ri is the hydrogen consumption rate in the ith stage in kg s1 described by ri ¼

MH2 I$Ni 2F

1 0 i1 X MH2 @ I NT  Nj A Fi ¼ Fi1  ri ¼ 2F j¼1

Fig. 2 e Schematic drawing of the proposed PEM fuel-cell stack design.

(3)

Note that the hydrogen flow rate to the first stage is equal to the total flow rate of hydrogen entering the stack (F0), which is exactly the stoichiometric flow rate of the stack. Substituting Eqs. (2) and (3) into Eq. (1), we obtain li ¼ @NT 

Liquid water drain

(2)

In the equation above, MH2 is the molar mass of hydrogen (2.02  103 kg mol1), F denotes the Faraday constant (96,485 C mol1), and I is the current in ampere drawn from the stack. The flow rate of the hydrogen entering the ith stage is obtained from the following mass balance:

0 Liquid separator

(1)

i1 X

1, Nj A Ni

(4)

j¼1

For the first stage, the stoichiometry, l, becomes NT/N1 and for the last stage, it becomes 1.0.

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We can rewrite Eq. (4) to express the number of the anodic cells in the ith stage (Ni) as the dependent variable as follows: 20 Ni ¼ [email protected] 

i1 X

0

1, 3 Nj A li 5;

for i ¼ 1; 2; .; until @

j¼1

i X

complexity, in practice, the number of anodic stages would range from two to four. We can reduce the number of stages in the stack by merging the last two or three stages into one if the number of stages calculated from Eq. (5) is more than four, although then the number of anodic cells under dead-end conditions increases: for example, the configuration of the stack with NT ¼ 150 shown in Fig. 3(a) can be reformed into 100 (1st stage), 33 (2nd stage), 11 (3rd stage), and 6 (final stage) from its original configuration of 100 (1st stage), 33 (2nd stage), 11 (3rd stage), 4 (4th stage), 1 (5th stage), and 1 (final stage).

1 Nj  NT A

j¼1

(5) In the above equation, the square bracket represents the nearest integer function, which returns the integer closest to the terms inside the brackets, because Ni calculated from the equation must be a positive integer value. As can be seen in the equation, once the stoichiometry has been specified, the number of anodic cells in the ith stage can be determined from the total number of cells in the stack and the summation of cells in the first to the (i  1)th stages. It is useful to examine Eq. (5) graphically to better understand the functional relationship between the number of cells in each stage and the two independent variables, i.e., the total number of cells (NT) and the stoichiometry at each anodic stage. Fig. 3(a) depicts the number of anodic cells at each stage for a fixed stoichiometry of 1.5 for three different stack designs (NT ¼ 50, 100, and 150). As can be seen in the figure, the number of stages increases with the total number of cells. The stacks must be sectioned into 4, 5, and 6 stages for NT ¼ 50, 100, and 150, respectively, to satisfy the stoichiometry of 1.5 in all stages except the last. Fig. 3(b) shows the number of anodic cells in each stage when the total number of cells (NT) has a fixed value of 100 for three different stoichiometries (l ¼ 1.2, 1.5, and 1.8). The figure shows that we must increase the number of stages in order to increase the stoichiometry through the stack, and we have to divide the stack into 3, 5, and 7 stages to obtain l ¼ 1.2, 1.5, and 1.8, respectively, in all stages except for last. If we increase the number of stages, the proportion of anodic cells under dead-end conditions (i.e., the cells in the last stage) will decrease, which could lead to a reduction in gas purges and consequently to higher fuel utilization. However, the number of stages we can use is limited by the maximum pressure drop allowed through a stack with this multistage anode design because hydrogen travels along a pathway (which includes anode channels) that is two or more times longer than in a conventional single-stage design. Considering the pressure drop through the stack and the configurational

100

4.1.

Stack design and fabrication

We used the proposed stack design described in the previous section to fabricate a 15 kW-class PEM fuel-cell stack. The stack was designed to operate properly in the range of 1.5e15 kW, although it can be operated outside of this range. To meet the maximum output power of 15 kW, the stack must consist of 70 cells and the average cell voltage must be greater than 0.7 V at a current of 305 A. The stack was divided into four anodic stages, and then the number of cells in each stage was determined to satisfy the specified hydrogen stoichiometry of each stage as listed in Table 1. The final stage is composed of two anodic cells, which represents only 3% of the total cells in the stack, implying that only two cells are under dead-end conditions during normal operation. Fig. 4 illustrates the hydrogen and air pathways in the stack, in which gassupplying plates are used to direct hydrogen into the intermediate stages in the stack or to extract unreacted hydrogen with liquid water. The stack is equipped with a purge valve at the outlet of the final stage. The following components were prepared: membrane electrode assemblies (MEAs), metallic bipolar-plates, gassupplying plates, insulation plates, current collectors, compression plates, and liquid separators. The MEAs were purchased from Johnson-Matthey Fuel Cell, each of which is a combination of a 30-mm-thick membrane, a Pt/C catalyst with a loading weight of 0.8 mg cm2 in the anode and cathode as a whole, and a gas diffusion layer of SGL 10BC. Flow-fields

a

80

Number of cells

Experimental

100

100

90

70

4.

67

60

b

90

NT = 50

80

N T = 100

70

N T = 150

60

83

λ = 1.2 λ = 1.5 λ = 1.8

67 56

50

50

40

40

33

33

30

30

20

22

10

11

0 0

1

2

11 7 4

24

20 4 3 1

1

22

6

7

11 7 3

0

1

3 4 5 Stage number

14

10 1

5 3

2

1

1

1

0

1

2

3 4 5 Stage number

6

7

Fig. 3 e Number of cells calculated for each stage using Eq. (5): (a) for a total of 50, 100, and 150 cells in the stack (NT), and (b) for hydrogen stoichiometries (l) of 1.2, 1.5, and 1.8.

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Table 1 e Design parameters for the 15 kW-class stack with four anodic stages. 1st stage 2nd stage 3rd stage 4th stage 1.50 47 67

1.45 16 23

1.40 5 7

1.00 2 3

were formed in the metallic bipolar-plates using an etching technique and were coated using an electroplating method to prevent corrosion. The gas-supplying plates were designed and manufactured to introduce hydrogen to the intermediate stages and to serve as an electrical connection between stages. The insulation plates and compression plates were machined from super engineering plastics and stainless steels, respectively. The liquid separators were used to remove liquid water from the hydrogenewater mixture exiting each stage. Fig. 5 shows a photo of the fabricated 15 kWclass PEM fuel-cell stack that was equipped with cell-voltage monitoring cables to measure all of the cell voltages in the stack.

4.2.

Fig. 5 e Fabricated 15 kW-class stack with liquid separators and cell voltage monitoring cables.

opens by changing its stem position. The purge gas was vented to the atmosphere through a gas meter (Model W-NK-0.5, manufactured by Shinagawa Corp.) that could measure the volumetric flow rate of purge gas within a range of 0.016e5.000 L per minute (LPM) with a maximum error of 0.15% and that displayed the cumulative amount of the purge gas emitted from the anode outlet of the stack. Temperature and pressure sensors monitor the outlet condition of the purge gas at the anode outlet of the stack. A liquid separator was installed at the outlets of each stage along with a liquid level controller (not shown in the figure). The electrical load (Model WCL232, manufactured by TDI Power) automatically controls the amount of current delivered to the stack at a given current with a maximum error of 0.25 A.

Stack testing system

Fig. 6 shows a schematic of the fuel-cell testing system integrated with the 15 kW-class stack. A fuel-cell test station (G500 model) manufactured by Greenlight Innovation was modified to test the stack. Hydrogen and air were fed into bubblingtype humidifiers before entering the stack. The flow rate of the air entering the stack was controlled using an MFC (mass flow controller) manufactured by Bronkhorst High-Tech. The flow rate of hydrogen entering the stack was not controlled, but instead, the inlet pressure of hydrogen is controlled using a pressure regulator. An automatic purge control system was employed to control opening and closing of the purge valve by measuring the voltage of the two cells in the last stage of the stack; the purge valve opens for a short period whenever the voltage of the two cells drops by a specified amount from the average cell voltage of all of the cells in the stack. The manual valve (a needle valve) shown in the figure was used to adjust the amount of purge gas exhausted each time the purge valve

4.3.

Test procedure and conditions

First, the stack was conditioned according to our own conditioning procedure for close to 24 h before measuring its performance. Subsequently, the following experiments were

Current collectors

-

From 1st stage

+

From 2nd stage

Air H2 2nd stage

1st stage

4th stage

Stoichiometry Number of cells Proportion of total cells (%)

3rd stage

Stage

To 2nd stage

From 3rd stage

H2 Purge gas

Excess air To 3rd stage

To 4th stage

Gas supplying plates

Fig. 4 e Hydrogen and air pathways in the 15 kW-class stack.

Purge valve

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12001

Load T

Humidifier

Air

Excess air

MFC

TC

Cooling water

Cooling water supply Manual valve T

T

P

Humidifier

H2

Purge gas Gas meter

Stack VC

PC

Voltage

Pressure regulator

Purge valve control

Fig. 6 e Schematic of the fuel-cell testing system integrated with the 15 kW-class stack.

performed: 1) to find the minimum stem opening of the manual valve (shown in Fig. 6) for which the emission of purge gas is minimized while maintaining stable operation; 2) to test the performance of the stack including fuel utilization and output power; and 3) to evaluate the stability of the output voltage over time for the minimum manual-valve stem opening determined in experiment (1). Table 2 summarizes the operating conditions of the 15 kWclass stack. During the experiments, air was always fed into the stack at the stoichiometry of 2.0 and hydrogen was supplied at an inlet pressure of 0.45 bar (gauge). The inlet temperatures of both hydrogen and air were set at 65  C, and both were 100% humidified before entering the stack. De-ionized water was supplied to the stack as a coolant. The inlet temperature of the cooling water was controlled at 65  C and the outlet temperature was regulated to be below 75  C. The purge valve was set to open for 0.5 s whenever the voltage of the two cells in the last stage dropped 0.1 V below the average cell voltage of the whole stack. The purge gas flow rate was measured and integrated for 30 min using the gas meter to calculate the average purge gas flow rate in LPM. Then, the

Table 2 e Operating conditions of the 15 kW-class stack. Description

Operating variables

Operating conditions

Hydrogen

Stack inlet temperature Stack inlet pressure Inlet relative humidity Stoichiometry

65  C 45 kPa 100% N/A

Air

Stack inlet temperature Stack inlet pressure Stack outlet pressure Inlet relative humidity Stoichiometry

65  C 0e25 kPa (depending on load) 0 kPa 100% 2.0

Stack inlet temperature Stack outlet temperature

65  C 75  C

Coolant

fuel utilization (Uf) was calculated according to the following equation:  Uf ¼

1

 2F PVp $  100% NT I 60RT

(6)

where I (in A) denotes the current drawn from the stack, NT is the total number of cells in the stack, Vp (in LPM) is the volumetric flow rate of purge gas, P (in kPa) is the purge gas pressure, T (in K) is the purge gas temperature, R is the universal gas constant (8.314 L kPa mol1 K1), and F denotes the Faraday constant (96,485 C mol1). Most of the temperatures, pressures, flow rates, and cell voltages were automatically logged in a database every 0.5 s during the experiments using a data acquisition system.

5.

Results and discussion

In order to measure the performance of the stack accurately, the minimum stem opening of the manual valve on the stack testing system was determined by conducting several preliminary tests. We can approximate that the amount of purge gas produced from the anodic cells is proportional to the stem opening of the manual valve if the stack operates under the same conditions, i.e., a constant current of 305 A was drawn from the stack and all other conditions were the same during the tests. Excessive opening of the manual valve leads to a loss of hydrogen, and therefore, the stem opening of the manual valve should be minimized to reduce the emission of purge gas, while maintaining stable operation of the stack. The minimum stem opening of the manual valve was roughly determined by observing the cell voltages and measuring the purge-gas flow rate for several stem openings (20%, 30%, 40%, and 60%). The amount of purge gas discharged from the anode was measured at various stem openings of the manual valve to find the maximum fuel utilization. Fig. 7 illustrates the purge-gas flow rate and fuel utilization with respect to the stem opening at a maximum power of about 15 kW. As can be seen in the figure, the purge-gas flow rate decreases linearly as the stem opening is made smaller. The

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1.0

100.0

0.8

99.8

0.6

99.6

0.4

99.4

0.2

Fuel utilization / %

Purge gas flowrate / LPM

12002

99.2

Purge gas flowrate Fuel utilization

99.0

0.0 20 30 40 50 60 Stem opening of the manual valve / %

Fig. 7 e Purge-gas flow rates and fuel utilizations measured at various stem openings of the manual valve.

fuel utilization reaches a maximum value of 99.9% with a 20% opening of the manual valve. However, we must investigate whether the stack operation is stable with this stem opening. Fig. 8(a) and (b) shows changes in the cell voltages of the two cells in the last stage of the stack over time when the stem opening was set to 30% and 20%, respectively. Fig. 8(c) and (d) are the subplots of Fig. 8(a) and (b), respectively, which show the cell voltage variations in detail during a 3-min time period. As seen in the figure, the cell voltages increase and decrease as the purge valve opens and closes. For stem openings of more

than 30%, the cell voltages gradually decay when the purge valve is closed but completely recover their original values after it opens. This stable stack operation lasted for more than 110 min. Hence, we conclude that the operation of the stack is stable with these stem openings (more than 30%). However, when the stem opening was further reduced to 20%, the cell voltages exhibited irregular behavior and oscillated more frequently than those with stem openings greater than or equal to 30%. The cell voltage of one of the two cells fluctuated more and sometimes did not recover to its original value (approximately 0.71 V), as can be seen in Fig. 8(b) and (d). Thus, the stack operation can be unstable with such a narrow manual-valve opening. When the stem opening was further reduced to 10%, the stack was shutdown owing to low cell voltages within several minutes after the startup. This can be explained by liquid water and nitrogen, which hinder hydrogen from reaching catalytic active sites, not being completely removed from the anodic cells each time the purge valve opens. Therefore, these preliminary experimental results suggest that the operation of the stack is certainly stable with a stem opening of the manual valve greater than or equal to 30%. The stem opening of the manual valve can be decreased further below 30% if we tune the stem opening more rigorously. However, accounting for safety margins, the minimum stem opening of the manual valve was set to 30% for this stack testing system. The basic performances of the stack including the polarization curve, cell voltage distribution, and ramp-up rate (dynamic load following capability) of the output power were tested at the minimum stem opening (30%) of the manual valve. Fig. 9 shows the polarization and output power curves

0.75

0.75

b

Cell number = 70 Cell number = 69

0.70

Cell voltage / V

Cell voltage / V

a

0.65

0.70

0.65

0.60

0.60 0

30

60

90

0

120

20

40

60

Operation time / min

Operation time / min 0.75

0.75 Cell number = 70 Cell number = 69

c

Cell number = 70 Cell number = 69

d

0.70

Cell voltage / V

Cell voltage / V

Cell number = 70 Cell number = 69

0.65

0.60

0.70

0.65

0.60 110

111

112

Operation time / min

113

60

61

62

63

Operation time / min

Fig. 8 e Changes in the cell voltages of the two cells in the last stage of the stack over time at the following stem openings of the manual valve: (a) 30%, (b) 20%, (c) enlarged sub-plot of (a), and (d) enlarged sub-plot of (b).

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60

15

65

50

10

40

5

Stack voltage / V

50

Output power / kW

Stack voltage / V

60

30

obtained for the stack plotted against the minimum stem opening of the manual valve. From the experiments, it was found that the stack generates a minimum power of 1.52 kW at 25 A (60.9 V) and a maximum power of 15.2 kW at 305 A (49.8 V), which approximately agrees with the maximum power specified in the design. Fig. 10 illustrates the cell voltage distribution at the maximum power. The voltage distribution is quite even (with a standard deviation of 5.2 mV) and voltage differences between the stages are small. Fig. 11 depicts transient responses of the average cell voltage while the stack power oscillates several times between 1.5 and 15 kW (maximum power) with power ramp-up rates of 50 A s1 and 100 A s1. The stack successfully reached its maximum power with a power ramp-up rate of 50 A s1 and even with a very high power ramp-up rate of 100 A s1 without an emergency stop. This indicates that the stack has a good ramp-up performance for reaching the maximum power from the minimum within 3 s. The stack was tested to find the fuel utilization at various output powers and to verify it during long-term operation.

4th stage 3rd stage 0.9

Cell voltage / V

20 45

2nd stage

1st stage

10

0 0

Fig. 9 e Polarization and output power curves obtained from the 15 kW-class stack.

0.8

50

300

0.7 0.6 0.5

200

400 Time / s

600

800

Fig. 11 e Load-following capability of the stack while the output power oscillates between 1.5 and 15 kW with power ramp-up rates of 50 A sL1 and 100 A sL1.

Fig. 12 shows the fuel utilization as a function of the output power at the minimum manual-valve stem opening (30%). In the figure, each purge-gas flow rate was measured and integrated for 30 min of the stack operation at a given constant current. As can be seen in the figure, the purge-gas flow rate steadily increases with the output power; it is 0.03 LPM at the minimum power and 0.43 LPM at the maximum power. On the other hand, it seems that the fuel utilization has no apparent correlation with the output power, with a minimum of about 99.7% and a maximum of about 99.8% in the operating range of the stack. Fig. 13 shows the fuel utilization calculated from the purge-gas flow rate, which was measured during long-term continuous operation of the stack with a 30% stem opening of the manual valve. The stack had been operated at a constant current of 305 A (maximum power) for more than 70 h without

Purge gas flowrate / LPM

200 Current / A

30

35

0 100

55

40

Voltage Power 0

40

0.7

99.9

0.6

99.8

0.5

99.7

0.4

99.6 Purge gas flowrate Fuel utilization

0.3

99.5

0.2

99.4

0.1

99.3

0.4

0.0

0.3

99.2 0

0.2 1

11

21

31

41

51

61

Cell number

Fig. 10 e Cell voltage distribution of the stack at the maximum power of 15 kW.

Output power / kW

20

3

6 9 Power / kW

12

15

Fig. 12 e Purge-gas flow rate and fuel utilization as functions of the output power when the opening of the manual valve was set to 30%.

Fuel utilization / %

70

12004

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 9 9 6 e1 2 0 0 6

100

1

0.74

a

99.9

0.6 99.8 0.4 99.7

0.2

0

Average cell voltage / V

Purge flow rate Fuel utilization

0.8

Fuel utilization / %

Purge gas flow rate / LPM

2 nd stage

99.6 0

10

20 30 40 50 Operation time / h

60

0.72

0.70 1st stage

0.68

0.66

0.64

70

0

Fig. 13 e Variations in fuel utilization during long-term operation (70 h) of the stack with a 30% stem opening of the manual valve.

10

20 30 40 Operation time / min

50

60

0.74

b 4th stage

Average cell voltage / V

any shutdown. During the entire operation period, the fuel utilization had been maintained in the range 99.6e99.75%, showing extremely small variations, as can be seen in the figure. These fuel utilizations are notably higher than those previously reported in the literature [12,23,24]. Furthermore, these high fuel utilization values were maintained across the operation range of the stack and during long-term operation without the use of a mechanical device or sophisticated control scheme. These higher fuel utilizations of the stack prove the effectiveness of the proposed design and can be understood by the following reasons: 1) high stoichiometry ranging from 1.4 to 1.5 is maintained in most (97%) of the anodic cells, and the excess hydrogen reduces the accumulation of liquid water and nitrogen at the outlet of the anode; 2) only 3% of the anodic cells in the stack are operating in dead-end mode, which means only this small section experiences water flooding during periods when the purge valve is closed; and 3) liquid water is removed from the intermediate sections of the stack using liquid separators, thus reducing liquid water accumulation through the anodic cells. It is expected that the fuel utilization of the stack can be further improved if we further optimize the channel designs [25e27] or if we apply a more sophisticated purge control strategy [23,28]. We must also consider variations in the output voltage of the stack during the operation of the purge valve because the stability of the output voltage can affect the quality of power delivered. Fig. 14(a) and (b) shows variations in the average cell voltages, obtained by averaging the cell voltages of the cells in each stage of the stack. The voltages were plotted for a given time period (1 h) during long-term operation (more than 70 h) of the stack at a constant current of 305 A and with a 30% stem opening of the manual valve. As shown in the figures, the average cell voltages of the first, second, and third stages show very small variations, while the average cell voltage of the fourth stage fluctuates significantly. These large fluctuations are mainly caused by the pressure swings in the fourth stage arising from the periodic opening of the purge valve. Fig. 15(a)

3rd stage

0.72

0.70

0.68

0.66

0.64 0

10

20

30

40

50

60

Operation time / min Fig. 14 e Variations in the average cell voltages over time at a constant current of 305 A: (a) average cell voltages of the 1st stage, 2nd stage, and 3rd stage, and (b) average cell voltage of the 4th stage.

and (b) depicts variations in the output variables (output power and average cell voltage) of the stack with respect to the anode outlet pressure during the same period as that used for Fig. 14. The anode outlet pressure oscillates between 12 and 21 kPa as the purge valve is opened and closed. Variations in the average cell voltage, however, are very small in spite of the large fluctuations in the anode outlet pressure, and thus, the stack output power is very stable; the smaller the variations in the output voltage are at a constant current, the better the quality of output power is obtained. The average cell voltage varies within a range of 3.3 mV and the output voltage within a range of 0.23 V for a given time period (1 h). These fluctuations are relatively small owing to the small portion (3%) of cells that are affected by pressure swings associated with opening and closing of the purge valve. Therefore, we conclude that the output voltage of the newly designed stack is less sensitive to the opening and closing of the purge valve, in contrast to PEM fuel-cell stacks that employ a dead-end structure or a

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 9 9 6 e1 2 0 0 6

15.3

a

0.718

15.2 Output power

0.716

15.1

Average cell voltage

0.714

15

0.712

14.9

0.710 10

20 30 40 Operation time / min

50

60

Acknowledgments

30

b

This work was financially supported by the Ministry of Trade, Industry & Energy (MOTIE) through Korea Institute for Advancement of Technology (KIAT) and Daegyeong Institute for Regional Program Evaluation (DIRPE).

25 20 15

references 10 Anode outlet pressure 5 0 0

10

20 30 40 Operation time / min

50

60

Fig. 15 e Variations in the stack output variables with respect to the anode outlet pressure at a constant current of 305 A: (a) output power and average cell voltage of the stack, and (b) anode outlet pressure.

recirculation device, which typically show larger fluctuations (within a range of 25e100 mV) in the average cell voltage and output power [16,19,23,29]. However, the cells in the last stage frequently suffer pressure shock and repeatedly operate in dead-end mode as shown in Fig. 15(b). This may lead to a lower durability of the MEAs in the last stage. The durability would be increased by using thicker or reinforced membranes for the MEAs [30,31] in the last stage of the stack if this were to be a concern. The cost increase and overall performance reduction arising from the use of the thicker or reinforced membranes would be considerably small because the MEAs in the last stage represent only a small portion of the total cells.

6.

effectiveness of the stack design, we fabricated and tested a 15 kW-class PEM fuel-cell stack. From the experimental results, we obtained high fuel utilizations of more than 99.6% (corresponding to a stoichiometry of 1.004) at the maximum power while maintaining stable operation. The proposed stack design has the following advantages: 1) lower purge-gas emission and consequently higher fuel utilization without employing a hydrogen recirculation device that consumes parasitic power and/or requires an additional control scheme; and 2) lower fluctuations in the output voltage, which will result in better quality output power.

14.8 0

Anode outlet pressure / kPa

Stack output power / kW

Average cell voltage / V

0.720

12005

Conclusions

A new design for a PEM fuel-cell stack was proposed to improve fuel utilization without the need for hydrogen recirculation devices. The basic idea is based on the division of the anodic cells into several blocks to increase the gaseous flow rate in most of the anodic cells in a stack, even under deadend conditions. Overall, water flooding in the anodic cells could be reduced, resulting in lower purge-gas emission and consequently higher fuel utilization. To verify the

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