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Experimental study on temperature characteristics of an air-cooled proton exchange membrane fuel cell stack
Renewable Energy 143 (2019) 1067e1078
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental study on temperature characteristics of an air-cooled proton exchange membrane fuel cell stack Lizhong Luo, Qifei Jian*, Bi Huang, Zipeng Huang, Jing Zhao, Songyang Cao School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, Guangdong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2018 Received in revised form 20 February 2019 Accepted 19 May 2019 Available online 20 May 2019
The aim of this study is to analyze the temperature characteristics of an air-cooled proton exchange membrane fuel cell stack. The temperature information of the stack is obtained by 60 thermocouples and a thermal imaging camera. The experimental results show that the average temperature change rate is only related to the step size of the current change, regardless of the step increases or decreases. And the average temperature of in-planes and through-planes is increased linearly with the increase of current. The temperature distribution is also discussed. As the current increases, the temperature difference on the outer surface of the stack increases from 5.2 C to 12.9 C. The temperature distribution of cells in the stack is affected by the flow of cooling air and uneven water distribution. The law of temperature uniformity with current variation can be described by a quadratic polynomial. And the performance of cells has an important influence on the temperature distribution. This study can provide reference for the development of thermal management strategies for hydrogen-air proton exchange membrane fuel cell stacks in application. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Proton exchange membrane fuel cell stack Dead-end anode Air-cooled stack Temperature variation Temperature distribution
1. Introduction Now proton exchange membrane fuel cells (PEMFCs) receive more and more attention. Temperature plays a larger role in PEMFC performance [1]. Understanding the temperature characteristics of PEMFC stacks is conducive to the thermal management. Temperature can dominate the cell performance [2]. Tiss et al. [3] established a mathematical model to study operating parameters of PEMFCs and demonstrated that the cell temperature strongly controls the dynamic response of PEMFCs. Yu and Jung [4] discussed the relationship between operating temperature and the safety margin during transient operations. They proposed a thermal management strategy based on the operating temperature. Stresses resulting from temperature variations during operation has a non-negligible impact on PEMFCs [5]. Liso et al. [6] studied temperature variations over fast load changes and found that temperature variations in the PEMFC have a negative impact because they generate thermal stresses in the stack and shorten its lifetime. Zhang et al. [7] demonstrated that the local temperature can reflect the local dynamic performance of a PEMFC and the
* Corresponding author. E-mail address: [email protected] (Q. Jian). https://doi.org/10.1016/j.renene.2019.05.085 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
thermal failure mechanism. Meyer et al. [8] proposed that combined current density and temperature gradient can be crucial to predict long-term degradation. Takalloo et al. [9] proved that higher temperature increases the cell performance because the membrane protonic conductivity is increased. It is worth mentioning that the fuel cell with foam metal as the flow field can improve the performance at low temperature condition [10]. These studies shown that temperature variation have a significant impact on PEMFCs. They mainly discussed the effect of temperature on PEMFCs, but less on the influence of other factors on the temperature. Understanding the change in PEMFCs temperature, especially under the condition of load step changes, is helpful for the development of thermal management strategies. Many studies focus on how temperature is distributed and the uniformity of temperature distribution because the temperature distribution affects the performance of PEMFCs. The temperature distribution inside an operating PEMFC is inhomogeneity [11]. Wu et al. [12] investigated an air-cooled PEMFC stack and found a significantly high temperature difference between each cell. Macedo-Valencia et al. [13] used Ansys software to simulate the temperature distribution of an air-cooled PEMFC stack and found that 10 C could be observed from the center of the stack and the top/bottom. Noorkami et al. [14] discussed the temperature distribution of an air-cooled PEMFC stack by thermal imaging and
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observed that the PEMFC stack has a temperature difference of more than 12 C. The temperature difference of an air-cooled PEMFC stack can reach 13.8 C [15]. Wen et al. [16] found that cells at the center of an air-cooled PEMFC stack assisted by pyrolytic graphite have higher temperature. Shimpalee et al. [17] predicted the temperature of an air-cooled PEMFC stack by numerical simulation and observed a symmetrical temperature distribution. The temperature decreased from the inner to outer cells of the PEMFC stack. Factors affecting temperature distribution have also been discussed. Air velocity and bipolar plate in-plane thermal conductivity have an important influence on the temperature distribution of the air-cooled PEMFC stack [18]. The temperature of the aircooled PEMFC stack can be controlled by controlling the inlet air velocity of the cathode air flow manifold [19]. Matian et al. [20] studied a PEMFC stack cooled by a separate air flow and found that increasing the size of the cooling channels can achieve a more uniform temperature distribution. Guo et al. [21,22] found that the non-uniformity of flow distribution cause the non-uniformity of temperature distribution. Metal foams can to be employed in fuel cell to address the problem associated with the temperature gradients [23]. Using the metal foam can produce a more uniform temperature distribution [24]. Afshari et al. [25] shown that the foam metal cooling flow field can reduce the surface temperature difference through simulation. Wan et al. [26] found that the nonuniformity temperature distribution can be improved by increasing operating pressure. Lin et al. [27] found that the temperature distribution of a single cell is related to the initial temperature. As the current density increases, the maximum temperature difference of the air-cooled stack increases [28]. Hakenjos and Hebling [29] observed simultaneously current density, electrical impedance and temperature distribution in a PEMFC. And they found that the temperature distribution is related to local activity. Temperature distribution is also associated with relative humidity distributions along reactant gas channels [30] and the rate of electrochemical reaction in localized regions [31]. Non-uniform temperature distribution causes uneven voltage distribution and current distribution [32]. On the one hand, these researchers have shown the common temperature distribution of PEMFCs and PEMFC stacks under normal operating conditions. When we formulate a thermal management strategy of a PEMFC stack, we must not only know the temperature characteristics of the PEMFC stack under normal conditions, but also the temperature characteristics when the performance difference of cells in the stack is large. The performance difference of cells in a stack can be caused by long-term operation. Due to the cell interaction, the performance of the cells will be affected [33]. In severe cases, the life of the PEMFC stack will be shortened [34]. However, the temperature distribution is rarely shown when the performance difference of cells in a PEMFC stack is relatively large. The temperature characteristics of PEMFC stacks in this case require more research and can provide more reference for the thermal management of PEMFC stacks. On the other hand, most of these studies discuss how temperature is distributed and the factors affecting the temperature distribution. But one problem is that they have less test points for obtaining temperature information. Developing a better thermal management strategy requires more temperature points to get more detailed temperature information. In this paper, the temperature information of an air-cooled PEMFC stack during operation is obtained from a thermal imaging camera and 60 thermocouples. The dynamic process of voltage [35] and the behavior of current [36] have been discussed under the condition of load step changes. Therefore, the temperature variation under different current step sizes is discussed in this study. The temperature distribution of in-planes and through-planes of the stack is displayed and its uniformity is analyzed. Briefly, the
objective of the present research is to analyze the temperature characteristics of an air-cooled PEMFC stack. This analysis is potentially attractive in the thermal management of air-cooled PEMFC stacks. 2. Experimental 2.1. Experimental system An air-cooled PEMFC stack was used for tests. This PEMFC stack contains 40 cells and is mainly suitable for portable devices. Five cells were selected from these 40 cells to place thermocouples (see Fig. 1 (a)). These five cells were Cell 2, Cell 11, Cell 20, Cell 29 and Cell 38. Twelve thermocouples were placed on the cathode gas diffusion layer surface of each selected cell. The placement of thermocouples is shown in Fig. 1 (b). All flow channels are straight and gases are fed in cross-flow mode. The detailed parameters of the PEMFC stack are listed in Table 1. Dry, non-heated hydrogen with a purity of 99.999% was supplied to the PEMFC stack. Hydrogen from a gas cylinder was adjusted to 0.05 MPa (Gauge pressure) by a regulator (LEVANWR57, Ningbo Sunrise Electromechanical Co., Ltd. China). A solenoid valve (KSV05B, Koge Micro Tech Co., Ltd. China) was at the exit of the anode. The purge time and purge interval were 0.44 s and 17.86 s, respectively. Ambient air at a temperature of 21.5 C and a relative humidity of 64% was provided by a fan. The air served as oxidant as well as coolant. An electronic load (JT6331A, Nanjing Jartul Electronics Co., Ltd. China) with a data acquisition function was used to measure and record the current and voltage of the PEMFC stack. 60 K-type thermocouples (GG-K-30, Omega Engineering, INC. U.S.A.) and a data acquisition instrument (GM10, Yokogawa Electric Corporation, Japan) with accuracy of ±0.2 C claimed by manufacturer's specifications were used to measure and record temperature data. The acquisition frequency of temperature and voltage data were 2 Hz and 10 Hz, respectively. A thermal imaging camera (TE, Dali Technology Co., Ltd. China) was used to record the temperature information of the outer surface of the PEMFC stack. 2.2. Experimental procedure The experiment is divided into two parts. One is to study the temperature variation under different current step sizes. The different current step sizes are 2.5 A, 5 A, 7.5 A, and 10 A, respectively. The reason for choosing different step sizes is that although there have been discussions about temperature variation under sudden change in current [37], there is little discussion about the effects of different current mutation sizes on temperature. The experimental parameters of this part are listed in Table 2. The reason for choosing this current is that 20 A is the rated current of this PEMFC stack. The other is to study the temperature distribution under different current conditions. These different current conditions are 15 A, 20 A, 25 A and 30 A, respectively. And the reason for choosing these current conditions is that these current conditions are the operating range of this PEMFC stack. The first step in the experiment is to turn on the data acquisition instrument and start measuring and recording the temperature data. Then turn on the fan. The power of the fan was constant during the experiment and the flow rate was 38 m3/h. Next adjust the regulator to provide hydrogen with proper pressure to the PEMFC stack. Then the electronic load is turned on. The PEMFC stack is operated at different current conditions for 10 min. The reason for choosing this operating time is that the temperature of the PEMFC stack can reach a quasi-steady values in 7 min after the current change. The remaining 3 min can be used to analyze the
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Fig. 1. The PEMFC stack used for this study. (a) The PEMFC stack; (b) Cathode flow field and temperature measurement location (red dot) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.). Table 1 Parameters of the FEMFC stack. Characteristic
Value
Thickness of the membrane electrode assembly (m) Thickness of membrane (m) Thickness of catalyst layer (m) Active area (cm2) Loading of Pt electro-catalyst on the anode (mg/cm2) Loading of Pt electro-catalyst on the cathode (mg/cm2) Thickness of gas diffusion layer (m) Width of the anode channel (m) Width of the anode rig (m) Depth of the anode channel (m) Width of the cathode channel (m) Width of the cathode rig (m) Depth of the cathode channel (m) Thickness of graphite bipolar plate (m)
6.5 10-4 1.5 10-5 1.75 10-5 68.5 0.16 0.64 3 10-4 1.2 10-3 2.4 10-3 0.4 10-3 1.3 10-3 1.0 10-3 2.1 10-3 3.0 10-3
Table 2 Experimental parameters for studying temperature variation. Case
I
II
III
Ⅳ
Ⅴ
Ⅵ
Ⅶ
Ⅷ
Initial current (A) Current step size (A) Final current (A)
10 10 20
12.5 7.5
15 5
17.5 2.5
30 10
27.5 7.5
25 5
22.5 2.5
temperature distribution. When the temperature reaches a quasisteady state, the temperature of the outer surface of the stack is recorded by the thermal imaging camera.
3. Results and discussion 3.1. Temperature variation Generally, when the measurement of a PEMFC stack temperature is conducted, a reference point is often chosen in an end plate [27]. In this study, Cell 2 was closest to the end plate. Therefore, the temperature of Cell 2 was chosen to analyze the effect of different current step sizes on temperature variation. Fig. 2 shows the variation of temperature when current is increased to 20 A with different step sizes. Fig. 3 depicts the variation of temperature
when current is decreased to 20 A with different step sizes. In this study, no additional equipment was used to maintain the temperature of the PEMFC stack. The power of the fan was constant, so the amount of air provided to the PEMFC stack was constant. The air can provide enough oxygen to the electrochemical reaction, while taking away part of the generated heat of the electrochemical reaction. In such a situation, the temperature of the PEMFC stack was closely related to the current condition of the PEMFC stack. When the current increases, it means that the electrochemical reaction accelerates, resulting in more heat. This will increase the temperature of the PEMFC stack, as shown in Fig. 2. When the current decreases, the electrochemical reaction slows down. At this time, the heat generated decreases, so the stack temperature decreases, as shown in Fig. 3. Temperature variation is related to the magnitude of the current change. It can be found from Figs. 2 and 3 that when the step size of the current is increased, the change speed of the temperature increases. Average temperature change rate can be used to describe the speed of temperature change. The average temperature change rate T_ is calculated as
T_ ¼ DT=t
(1)
where DT is the magnitude of the temperature change after the current change; t is the time of the temperature change. When the change of the current is increased by step, the average temperature change rate in the first minute are 1.9 C/min, 3.5 C/ min, 5.5 C/min and 7.4 C/min, respectively. When the change of the current is decreased by step, the average temperature change rate in the first minute are 1.9 C/min, 3.6 C/min, 5.5 C/min and 7.2 C/min, respectively. By comparing these data, it can be found that the average temperature change rate is related to the step size of the current. These data also indicate that no matter the current step change is increased or decreased, the average temperature change rate is almost unchanged as long as the change of current is the same. It can be seen from Figs. 2 and 3 that the temperature curve is relatively smooth during the temperature rise or fall, but the curve is no longer smooth after the temperature increases or decreases to
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Fig. 2. Temperature variation under different current step sizes: (a) Step size: 2.5 A; (b) Step size: 5 A; (c) Step size: 7.5 A; (d) Step size: 10 A.
a new quasi-stable value. For most of the time, there are some small fluctuations in temperature. This may be related to the operating condition of the PEMFC stack. The PEMFC stack was working in a dead-end anode mode during all tests. The periodic purge process was used to remove the excess water inside the PEMFC stack to avoid flooding and performance degradation of the PEMFC stack. When the solenoid valve is open, the excess water and some hydrogen inside the PEMFC stack is discharged under the pressure difference between the inside and outside of the anode flow channel. The water and hydrogen can take away some of heat. However, due to the short purge time, only 0.44 s, each purge process to take away heat was very limited. Therefore, the stack temperature dropped only slightly. When the solenoid valve is closed, the temperature was restored. However, in practice, water and hydrogen discharged by each purge process cannot be exactly equal. Especially water, sometimes more water was expelled. This made each purge process to take heat was not the same, so the stack temperature showed fluctuation. 3.2. Temperature distribution of in-planes The output of 60 thermocouples on the cathode gas diffusion layer surface was recorded under different current conditions. The average of the last three minutes of temperature data was used to
analyze the temperature distribution. Under the current condition of 15 A, 20 A, 25 A and 30 A, the temperature distribution of the cathode gas diffusion layer surface of each selected cell is shown in Fig. 4. The temperature distribution of a cell has its own characteristics. As shown in Fig. 4, the lowest temperature of the PEMFC stack appears in the upper left of Cell 2. This is because this position is near the inlet of hydrogen. Hydrogen used in this experiment was not heated, which causes a localized low temperature in the inlet position of hydrogen. After entering the PEMFC stack, hydrogen is heated quickly and only a small area near the inlet shows low temperature. The size of the high temperature area in these different cell planes is different, and the temperature difference between them is larger. The high temperature area of Cell 11 and Cell 38 is relatively large, especially in Fig. 4 (c) and (d). Their temperature is relatively high. In addition, the temperature distribution of Cell 29 is more evenly. The most uniform temperature distribution always appears on Cell 29. The temperature distribution of these different cell planes differs greatly, which reflects the great difference in cell performance of the PEMFC stack used for this study. As shown in Fig. 4, although the current condition of the PEMFC stack are different, the temperature distribution of cathode gas diffusion layer surface has similar characteristics. Along the flow
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Fig. 3. Temperature variation under different current step sizes: (a) Step size: 2.5 A; (b) Step size: 5 A; (c) Step size: 7.5 A; (d) Step size: 10 A.
direction of the air, the temperature of the cathode gas diffusion layer surface gradually rises, and finally presents a high temperature distribution region. This temperature distribution shape is similar to “parabolic” in the downstream region. Actually, this is related to the cooling mode of the PEMFC stack. The air used to cool the stack came from the environment. Along the direction of the cooling air, the temperature gradually increased. The temperature on the inlet side was lower than the temperature on the outlet side. In addition, the stack can also dissipate heat to the environment through the outer surface. In this way, the temperature in the middle position near the air outlet side was higher than the temperature in other positions and finally formed a parabolic shape. As shown in Fig. 4, on the cathode gas diffusion layer surface of a cell, the temperature first increases and then decreases from the top to bottom along the vertical direction except Cell 38. It is more noticeable near the exit side of air. This phenomenon is caused by the uneven distribution of moisture content of the membrane. The temperature gradient is proportional to the water flux in the membrane [38]. And gravity affects the performance of PEMFCs [39] by affecting liquid water in the flow channel [40]. Dry hydrogen entered the PEMFC stack from the top of the PEMFC stack. All the water needed in the membrane mainly came from the water produce by electrochemical reaction. So the upper part of the
membrane was not well wetted. The electrochemical reaction in this area was not intense and produced less heat, so the temperature in this area was lower. All cells were placed vertically, and the water flowed down as a result of gravity. The water concentration increased along the path of the flow channels from top to bottom. This made the water content of the central region of the membrane increase, which is favorable for the electrochemical reaction. Therefore, the middle region temperature of cells was higher. Further, more water accumulated at the bottom of cells. This can worsen the local performance of cells. Too much water clogged the gas channel in the gas diffusion layer and prevented the reaction gas from entering the catalyst layer. In fact, the observation of visible water drops in the transparent tube connected at the anode outlet of the PEMFC stack in the experiment indicated that water flooding have occurred at the bottom of cells. Wen et al. [16] also used this phenomenon to judge the occurrence of flooding. Water flooding made the electrochemical reaction suppressed. Therefore, the reaction produced less heat, and it showed that local temperature drops. When water accumulates at the bottom, it may also cause lower temperature at the bottom. When the stack is subjected to a purge operation, the accumulated water is discharged outside the stack, thereby taking away a portion of heat and lowering the temperature.
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Fig. 4. Temperature distribution of each selected cell under different current conditions. (a) Current 15 A; (b) Current 20 A; (c) Current 25 A; (d) Current 30 A.
The temperature distribution of cells in a stack is affected by the flow of cooling air and uneven water distribution. However, the high temperature area of Cell 38 is in the upper right of the plane, while the high temperature area of other cells is in the middle right part. This shows that the performance of Cell 38 differs greatly from that of other cells. And it indicates that cell performance have an important influence on the temperature distribution of PEMFC stacks. This phenomenon is more obvious under high current condition, as shown in Fig. 4 (c) and (d). And the temperature distribution under high current condition is more likely to highlight this problem.
3.3. Temperature distribution of through-planes Fig. 5 presents the temperature distribution of the air flow crosssection under the different current conditions. As shown in Fig. 5, from the direction of the air flow, the temperature of the air inlet side is lower, and the temperature of the air outlet side is higher. This is directly related to how the PEMFC stack is cooled. The reason has been discussed in section 3.2. As shown in Fig. 5 (a) and (b), when the current of the stack is lower, the temperature distribution of the air flow cross-section presents two bar regions with higher temperature. These two high temperature regions correspond to Cell 11 and Cell 38, respectively. This shows that there is a large amount of heat generated near these two cells in this PEMFC stack. At the distance of 25 mm and 40 mm from the edge of the air inlet side of the stack, the area of high temperature region increases as the PEMFC stack current increases. As shown in Fig. 5 (c) and (d), when the current of the PEMFC stack is higher, these two bars with higher temperature have a tendency to connect together. This means that the local cell has an effect on the nearby cell, and these effects will increase as the current increases.
Thermal image can provide more temperature distribution information. Fig. 6 shows a thermal image of the PEMFC stack seen from the side with air entering under different current conditions. Irregular lines in the thermal image represent thermocouple wires, and three thicker horizontal wires are bolts that secure the PEMFC stack. The thermal image reflects the temperature information of the outer surface of the stack, while Figs. 4 and 5 show the temperature distribution inside the PEMFC stack. So there will be some differences between them. As mentioned in section Introduction, the temperature distribution shown in the existing literature [16,17] is more common: the temperature near the middle is high and the temperature on both sides is low. This represents the temperature distribution of a normal cell or a normal PEMFC stack. It can be found in Fig. 6 that the PEMFC stack has three high temperature regions: in the vicinity of Cell 11, Cell 25 and Cell 38, respectively. Compared with these temperature distribution, an unusual temperature distribution of the PEMFC stack used for this study is observed. As the current increases, the temperature difference on the outer surface of the stack increases from 5.2 C to 12.9 C. This proves that the temperature distribution of the PEMFC stack is inhomogeneous. And it also reflects the large difference in performance of cells in the stack. Heat production in the PEMFC stack is closely related to cell voltage [41]. In the actual case, the voltage generated by each cell in a PEMFC stack is not exactly equal. Although the current is equal, some cell voltage is higher, and some cell voltage is lower. The heat generated by a single cell can be can be expressed as:
Q ¼ ðE UÞI
(2)
where Q is the heat generated by a single cell; E is the reversible voltage; U is the actual voltage of a single cell; I is the current of a single cell. Fig. 7 shows the voltage of cells in the PEMFC stack. As shown in
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Fig. 5. Temperature distribution of the air flow cross-section under different current conditions. (a) Current 15 A; (b) Current 20 A; (c) Current 25 A; (d) Current 30 A.
Fig. 6. Thermal image of the PEMFC stack seen from the side with air entering under different current conditions. (a) Current 15 A; (b) Current 20 A; (c) Current 25 A; (d) Current 30 A.
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can be confirmed that the performance of cells in the PEMFC stack varies greatly. The performance difference of cells in the PEMFC stack can be caused by long-term operation. Prior to the experiment, this PEMFC stack has been running for more than 2000 min, including 46 start-and-stop tests, and has been close to its operating limit several times. Note that the experiment was not conducted in the optimal condition of the PEMFC stack. Our purpose herein was to introduce the temperature characteristics of the PEMFC stack. In particular, we introduce the temperature characteristics of the PEMFC stack after long-term extreme operating conditions. A conception of Uniformity of Temperature Distribution (UTD) is proposed to evaluate the degree of uniform temperature. A larger UTD means that the difference between most of the temperature and its average value is greater, and the more uneven the temperature distribution; a smaller UTD means that most of the temperature is closer to the average and the temperature distribution is more uniform. It is defined as
Fig. 7. The voltage of cells in the PEMFC stack under current of 15 A.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi #, u" N u X 2 t UTD ¼ N Ti T
(3)
1
where N is the number of temperature measurement points on a plane; T is the average temperature of all temperature measurement points on a plane.
T¼
N X
!, Ti
N
(4)
1
Fig. 8. Flow rate distribution at the air inlet.
Fig. 7, the voltage of Cell 11 and Cell 25 were significantly lower than cells near it. According to Eq. (2), Cell 11 and Cell 25 produced more heat than cells near it. Therefore, Cell 11 and Cell 25 appeared higher temperature, as shown in Fig. 6. Cell 38 had a higher voltage than the adjacent cells. However, Cell 36 and Cell 40 that near Cell 38 had lower voltage. Therefore, the area of the high temperature region near Cell 38 is relatively large, as shown in Figs. 5 and 6. The flow rate distribution may also affect the temperature distribution. A wireless mini hot wire anemometer (testo 405i, Testo SE & Co. KGaA, Germany) was used to measure the air speed. As shown in Fig. 8, in addition to the low air speed at the edge of the air inlet, the air speed in the main area of the section is similar. The average air speed was 1.6 m/s. The flow rate distribution is considered to be uniform. However, the temperature distribution on the outer surface of the stack has three high temperature regions, as shown in Fig. 6. The flow rate distribution and temperature distribution are quite different, indicating that the performance of cells has a great influence on the temperature distribution, while the influence of the flow rate distribution is small. 3.4. Analysis of the inhomogeneity of temperature distribution From the above discussion of the temperature distribution, it
Fig. 9(a) shows the average temperature of different cells under different current conditions. It can be found that the average temperature of cells increases with the increase of current. Cell 11 had the highest average temperature, while Cell 2 had the lowest average temperature. Fig. 9(b) shows the UTD of different cells under different current conditions. It can be found that the temperature uniform decreases with the increase of current. Cell 29 had the best temperature uniformity, and Cell 2 had the worst temperature uniformity. The dashed lines in Fig. 9 are fitted according to data points. As shown in Fig. 9(a), it can be seen that the average temperature of inplanes shows a linear increase with the increase of current. The UTD variation of in-planes in Fig. 9(b) is different. With the increase of current, the UTD variation of Cell 2 is linear, while the UTD variation of Cell 11, Cell 20 and Cell 29 is closer to a quadratic polynomial. The possible reason for this phenomenon is related to the operation of a stack. Cell 2 was near the hydrogen inlet and there was a local low temperature area. The reason has been discussed in section 3.2. The UTD variation of Cell 38 is close to the quadratic polynomial, but it does not fit well. The possible reason for this phenomenon is the large difference in the performance of cells. Fig. 10(a) shows the average temperature of different throughplanes under different current conditions. “Cross-section 10” indicates that the cross section is 10 mm from the air inlet edge. It can be seen that the average temperature of through-planes shows a linear increase with the increase of current. Fig. 10(b) shows the UTD of different through-planes under different current conditions. It can be found that the temperature uniform decreases with the increase of current. In addition, along the direction of air flow, the average temperature and UTD are getting higher and higher. Further, with the increase of current, the law of UTD variation with current changes can be described by a quadratic polynomial. In summary, these phenomena indicate that the cell performance has an important influence on the temperature
Fig. 9. Average temperature and UTD of different in-planes under different current conditions. (a) Average temperature; (b) UTD.
Fig. 10. Average temperature and UTD of different through-planes under different current conditions. (a) Average temperature; (b) UTD.
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characteristics of the PEMFC stack. Although the temperature distributions on in-planes and through-planes are different, the temperature variations on the plane are similar. The average temperature on the plane increases as the current increases. And the relationship between temperature uniformity and current on a plane can be described by a quadratic polynomial. 4. Conclusions An air-cooled proton exchange membrane fuel cell stack is tested in this study. 60 thermocouples and a thermal imaging camera are used to get detailed temperature information about the stack during operations. Its temperature variation and distribution are analyzed. The results of this research can be concluded as follows: (1). No matter the current step change is increased or decreased, the average temperature change rate is almost unchanged as long as the change of current is the same. (2). The average temperature of in-planes and through-planes shows a linear increase with the increase of current. As the current increases, the temperature difference on the outer surface of the stack increases from 5.2 C to 12.9 C. (3). The temperature distribution of cells in a stack is affected by the flow of cooling air and uneven water distribution. The performance of cells has an important influence on the temperature distribution. (4). With the increase of current, the law of temperature uniformity with current change can be described by a quadratic polynomial. (5). Although the temperature distributions on in-planes and through-planes are different, the temperature variations on the plane are similar. This study contributes to the analysis of temperature characteristics of a hydrogen-air proton exchange membrane fuel cell stack in application. It will provide reference for thermal management on air-cooled proton exchange membrane fuel cell stacks. Acknowledgments This research was supported by the National Nature Science Foundation of China (No. 21776095), the Guangzhou Science and Technology Program (No. 201804020048) and Guangdong Key Laboratory of Clean Energy Technology (No. 2008A060301002). Nomenclature E I N PEMFC Q t T_
DT T Ti U UTD
the reversible voltage, V the current of a single cell, A the number of all temperature measurement points on a plane proton exchange membrane fuel cell the heat generated by a single cell, J the time of the temperature change, min the average temperature change rate, C/min magnitude of the temperature change after the current changes, C the average temperature of temperature measurement points on a plane, C the temperature of measurement points on a plane, C the actual voltage of a single cell, V uniformity of temperature distribution
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Rajabian, An investigation of the PEM fuel cells performance with partially restricted cathode flow channels and metal foam as a flow distributor, Energy 118 (2017) 705e715. [25] Ebrahim Afshari, Masoud Ziaei-Rad, Zahra Shariati, A study on using metal foam as coolant fluid distributor in the polymer electrolyte membrane fuel cell, Int. J. Hydrogen Energy 41 (3) (2016) 1902e1912. [26] Zhongmin Wan, Jun Shen, Haining Zhang, et al., In situ temperature measurement in a 5kW-class Proton Exchange Membrane Fuel Cell stack with pure oxygen as the oxidant, Int. J. Heat Mass Transf. 75 (2014) 231e234. [27] Hong Lin, Tao-Fen Cao, Li Chen, et al., In situ measurement of temperature distribution within a single polymer electrolyte membrane fuel cell, Int. J. Hydrogen Energy 37 (16) (2012) 11871e11886.
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[28] Qifei Jian, Bi Huang, Lizhong Luo, et al., Experimental investigation of the thermal response of open-cathode proton exchange membrane fuel cell stack, Int. J. Hydrogen Energy 43 (29) (2018) 13489e13500. [29] Alex Hakenjos, Christopher Hebling, Spatially resolved measurement of PEM fuel cells, J. Power Sources 145 (2) (2005) 307e311. [30] Zeljko Penga, Ivan Tolj, Frano Barbir, Computational fluid dynamics study of PEM fuel cell performance for isothermal and non-uniform temperature boundary conditions, Int. J. Hydrogen Energy 41 (39) (2016) 17585e17594. [31] W.R.W. Daud, R.E. Rosli, E.H. Majlan, et al., PEM fuel cell system control: a review, Renew. Energy 113 (2017) 620e638. [32] Chi-Yuan Lee, Yu-Chun Chiang, Fang-Bor Weng, et al., Flexible micro temperature, voltage and current sensors for local real-time microscopic diagnosis inside high temperature proton exchange membrane fuel cell stack, Renew. Energy 108 (2017) 126e131. [33] Zunyan Hu, Liangfei Xu, Jianqiu Li, et al., A multipoint voltage-monitoring method for fuel cell inconsistency analysis, Energy Convers. Manag. 177 (2018) 572e581. [34] Zunyan Hu, Liangfei Xu, Jianqiu Li, et al., A cell interaction phenomenon in a multi-cell stack under one cell suffering fuel starvation, Energy Convers.
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental study on temperature characteristics of an air-cooled proton exchange membrane fuel cell stack Lizhong Luo, Qifei Jian*, Bi Huang, Zipeng Huang, Jing Zhao, Songyang Cao School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, Guangdong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2018 Received in revised form 20 February 2019 Accepted 19 May 2019 Available online 20 May 2019
The aim of this study is to analyze the temperature characteristics of an air-cooled proton exchange membrane fuel cell stack. The temperature information of the stack is obtained by 60 thermocouples and a thermal imaging camera. The experimental results show that the average temperature change rate is only related to the step size of the current change, regardless of the step increases or decreases. And the average temperature of in-planes and through-planes is increased linearly with the increase of current. The temperature distribution is also discussed. As the current increases, the temperature difference on the outer surface of the stack increases from 5.2 C to 12.9 C. The temperature distribution of cells in the stack is affected by the flow of cooling air and uneven water distribution. The law of temperature uniformity with current variation can be described by a quadratic polynomial. And the performance of cells has an important influence on the temperature distribution. This study can provide reference for the development of thermal management strategies for hydrogen-air proton exchange membrane fuel cell stacks in application. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Proton exchange membrane fuel cell stack Dead-end anode Air-cooled stack Temperature variation Temperature distribution
1. Introduction Now proton exchange membrane fuel cells (PEMFCs) receive more and more attention. Temperature plays a larger role in PEMFC performance [1]. Understanding the temperature characteristics of PEMFC stacks is conducive to the thermal management. Temperature can dominate the cell performance [2]. Tiss et al. [3] established a mathematical model to study operating parameters of PEMFCs and demonstrated that the cell temperature strongly controls the dynamic response of PEMFCs. Yu and Jung [4] discussed the relationship between operating temperature and the safety margin during transient operations. They proposed a thermal management strategy based on the operating temperature. Stresses resulting from temperature variations during operation has a non-negligible impact on PEMFCs [5]. Liso et al. [6] studied temperature variations over fast load changes and found that temperature variations in the PEMFC have a negative impact because they generate thermal stresses in the stack and shorten its lifetime. Zhang et al. [7] demonstrated that the local temperature can reflect the local dynamic performance of a PEMFC and the
* Corresponding author. E-mail address: [email protected] (Q. Jian). https://doi.org/10.1016/j.renene.2019.05.085 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
thermal failure mechanism. Meyer et al. [8] proposed that combined current density and temperature gradient can be crucial to predict long-term degradation. Takalloo et al. [9] proved that higher temperature increases the cell performance because the membrane protonic conductivity is increased. It is worth mentioning that the fuel cell with foam metal as the flow field can improve the performance at low temperature condition [10]. These studies shown that temperature variation have a significant impact on PEMFCs. They mainly discussed the effect of temperature on PEMFCs, but less on the influence of other factors on the temperature. Understanding the change in PEMFCs temperature, especially under the condition of load step changes, is helpful for the development of thermal management strategies. Many studies focus on how temperature is distributed and the uniformity of temperature distribution because the temperature distribution affects the performance of PEMFCs. The temperature distribution inside an operating PEMFC is inhomogeneity [11]. Wu et al. [12] investigated an air-cooled PEMFC stack and found a significantly high temperature difference between each cell. Macedo-Valencia et al. [13] used Ansys software to simulate the temperature distribution of an air-cooled PEMFC stack and found that 10 C could be observed from the center of the stack and the top/bottom. Noorkami et al. [14] discussed the temperature distribution of an air-cooled PEMFC stack by thermal imaging and
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observed that the PEMFC stack has a temperature difference of more than 12 C. The temperature difference of an air-cooled PEMFC stack can reach 13.8 C [15]. Wen et al. [16] found that cells at the center of an air-cooled PEMFC stack assisted by pyrolytic graphite have higher temperature. Shimpalee et al. [17] predicted the temperature of an air-cooled PEMFC stack by numerical simulation and observed a symmetrical temperature distribution. The temperature decreased from the inner to outer cells of the PEMFC stack. Factors affecting temperature distribution have also been discussed. Air velocity and bipolar plate in-plane thermal conductivity have an important influence on the temperature distribution of the air-cooled PEMFC stack [18]. The temperature of the aircooled PEMFC stack can be controlled by controlling the inlet air velocity of the cathode air flow manifold [19]. Matian et al. [20] studied a PEMFC stack cooled by a separate air flow and found that increasing the size of the cooling channels can achieve a more uniform temperature distribution. Guo et al. [21,22] found that the non-uniformity of flow distribution cause the non-uniformity of temperature distribution. Metal foams can to be employed in fuel cell to address the problem associated with the temperature gradients [23]. Using the metal foam can produce a more uniform temperature distribution [24]. Afshari et al. [25] shown that the foam metal cooling flow field can reduce the surface temperature difference through simulation. Wan et al. [26] found that the nonuniformity temperature distribution can be improved by increasing operating pressure. Lin et al. [27] found that the temperature distribution of a single cell is related to the initial temperature. As the current density increases, the maximum temperature difference of the air-cooled stack increases [28]. Hakenjos and Hebling [29] observed simultaneously current density, electrical impedance and temperature distribution in a PEMFC. And they found that the temperature distribution is related to local activity. Temperature distribution is also associated with relative humidity distributions along reactant gas channels [30] and the rate of electrochemical reaction in localized regions [31]. Non-uniform temperature distribution causes uneven voltage distribution and current distribution [32]. On the one hand, these researchers have shown the common temperature distribution of PEMFCs and PEMFC stacks under normal operating conditions. When we formulate a thermal management strategy of a PEMFC stack, we must not only know the temperature characteristics of the PEMFC stack under normal conditions, but also the temperature characteristics when the performance difference of cells in the stack is large. The performance difference of cells in a stack can be caused by long-term operation. Due to the cell interaction, the performance of the cells will be affected [33]. In severe cases, the life of the PEMFC stack will be shortened [34]. However, the temperature distribution is rarely shown when the performance difference of cells in a PEMFC stack is relatively large. The temperature characteristics of PEMFC stacks in this case require more research and can provide more reference for the thermal management of PEMFC stacks. On the other hand, most of these studies discuss how temperature is distributed and the factors affecting the temperature distribution. But one problem is that they have less test points for obtaining temperature information. Developing a better thermal management strategy requires more temperature points to get more detailed temperature information. In this paper, the temperature information of an air-cooled PEMFC stack during operation is obtained from a thermal imaging camera and 60 thermocouples. The dynamic process of voltage [35] and the behavior of current [36] have been discussed under the condition of load step changes. Therefore, the temperature variation under different current step sizes is discussed in this study. The temperature distribution of in-planes and through-planes of the stack is displayed and its uniformity is analyzed. Briefly, the
objective of the present research is to analyze the temperature characteristics of an air-cooled PEMFC stack. This analysis is potentially attractive in the thermal management of air-cooled PEMFC stacks. 2. Experimental 2.1. Experimental system An air-cooled PEMFC stack was used for tests. This PEMFC stack contains 40 cells and is mainly suitable for portable devices. Five cells were selected from these 40 cells to place thermocouples (see Fig. 1 (a)). These five cells were Cell 2, Cell 11, Cell 20, Cell 29 and Cell 38. Twelve thermocouples were placed on the cathode gas diffusion layer surface of each selected cell. The placement of thermocouples is shown in Fig. 1 (b). All flow channels are straight and gases are fed in cross-flow mode. The detailed parameters of the PEMFC stack are listed in Table 1. Dry, non-heated hydrogen with a purity of 99.999% was supplied to the PEMFC stack. Hydrogen from a gas cylinder was adjusted to 0.05 MPa (Gauge pressure) by a regulator (LEVANWR57, Ningbo Sunrise Electromechanical Co., Ltd. China). A solenoid valve (KSV05B, Koge Micro Tech Co., Ltd. China) was at the exit of the anode. The purge time and purge interval were 0.44 s and 17.86 s, respectively. Ambient air at a temperature of 21.5 C and a relative humidity of 64% was provided by a fan. The air served as oxidant as well as coolant. An electronic load (JT6331A, Nanjing Jartul Electronics Co., Ltd. China) with a data acquisition function was used to measure and record the current and voltage of the PEMFC stack. 60 K-type thermocouples (GG-K-30, Omega Engineering, INC. U.S.A.) and a data acquisition instrument (GM10, Yokogawa Electric Corporation, Japan) with accuracy of ±0.2 C claimed by manufacturer's specifications were used to measure and record temperature data. The acquisition frequency of temperature and voltage data were 2 Hz and 10 Hz, respectively. A thermal imaging camera (TE, Dali Technology Co., Ltd. China) was used to record the temperature information of the outer surface of the PEMFC stack. 2.2. Experimental procedure The experiment is divided into two parts. One is to study the temperature variation under different current step sizes. The different current step sizes are 2.5 A, 5 A, 7.5 A, and 10 A, respectively. The reason for choosing different step sizes is that although there have been discussions about temperature variation under sudden change in current [37], there is little discussion about the effects of different current mutation sizes on temperature. The experimental parameters of this part are listed in Table 2. The reason for choosing this current is that 20 A is the rated current of this PEMFC stack. The other is to study the temperature distribution under different current conditions. These different current conditions are 15 A, 20 A, 25 A and 30 A, respectively. And the reason for choosing these current conditions is that these current conditions are the operating range of this PEMFC stack. The first step in the experiment is to turn on the data acquisition instrument and start measuring and recording the temperature data. Then turn on the fan. The power of the fan was constant during the experiment and the flow rate was 38 m3/h. Next adjust the regulator to provide hydrogen with proper pressure to the PEMFC stack. Then the electronic load is turned on. The PEMFC stack is operated at different current conditions for 10 min. The reason for choosing this operating time is that the temperature of the PEMFC stack can reach a quasi-steady values in 7 min after the current change. The remaining 3 min can be used to analyze the
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Fig. 1. The PEMFC stack used for this study. (a) The PEMFC stack; (b) Cathode flow field and temperature measurement location (red dot) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.). Table 1 Parameters of the FEMFC stack. Characteristic
Value
Thickness of the membrane electrode assembly (m) Thickness of membrane (m) Thickness of catalyst layer (m) Active area (cm2) Loading of Pt electro-catalyst on the anode (mg/cm2) Loading of Pt electro-catalyst on the cathode (mg/cm2) Thickness of gas diffusion layer (m) Width of the anode channel (m) Width of the anode rig (m) Depth of the anode channel (m) Width of the cathode channel (m) Width of the cathode rig (m) Depth of the cathode channel (m) Thickness of graphite bipolar plate (m)
6.5 10-4 1.5 10-5 1.75 10-5 68.5 0.16 0.64 3 10-4 1.2 10-3 2.4 10-3 0.4 10-3 1.3 10-3 1.0 10-3 2.1 10-3 3.0 10-3
Table 2 Experimental parameters for studying temperature variation. Case
I
II
III
Ⅳ
Ⅴ
Ⅵ
Ⅶ
Ⅷ
Initial current (A) Current step size (A) Final current (A)
10 10 20
12.5 7.5
15 5
17.5 2.5
30 10
27.5 7.5
25 5
22.5 2.5
temperature distribution. When the temperature reaches a quasisteady state, the temperature of the outer surface of the stack is recorded by the thermal imaging camera.
3. Results and discussion 3.1. Temperature variation Generally, when the measurement of a PEMFC stack temperature is conducted, a reference point is often chosen in an end plate [27]. In this study, Cell 2 was closest to the end plate. Therefore, the temperature of Cell 2 was chosen to analyze the effect of different current step sizes on temperature variation. Fig. 2 shows the variation of temperature when current is increased to 20 A with different step sizes. Fig. 3 depicts the variation of temperature
when current is decreased to 20 A with different step sizes. In this study, no additional equipment was used to maintain the temperature of the PEMFC stack. The power of the fan was constant, so the amount of air provided to the PEMFC stack was constant. The air can provide enough oxygen to the electrochemical reaction, while taking away part of the generated heat of the electrochemical reaction. In such a situation, the temperature of the PEMFC stack was closely related to the current condition of the PEMFC stack. When the current increases, it means that the electrochemical reaction accelerates, resulting in more heat. This will increase the temperature of the PEMFC stack, as shown in Fig. 2. When the current decreases, the electrochemical reaction slows down. At this time, the heat generated decreases, so the stack temperature decreases, as shown in Fig. 3. Temperature variation is related to the magnitude of the current change. It can be found from Figs. 2 and 3 that when the step size of the current is increased, the change speed of the temperature increases. Average temperature change rate can be used to describe the speed of temperature change. The average temperature change rate T_ is calculated as
T_ ¼ DT=t
(1)
where DT is the magnitude of the temperature change after the current change; t is the time of the temperature change. When the change of the current is increased by step, the average temperature change rate in the first minute are 1.9 C/min, 3.5 C/ min, 5.5 C/min and 7.4 C/min, respectively. When the change of the current is decreased by step, the average temperature change rate in the first minute are 1.9 C/min, 3.6 C/min, 5.5 C/min and 7.2 C/min, respectively. By comparing these data, it can be found that the average temperature change rate is related to the step size of the current. These data also indicate that no matter the current step change is increased or decreased, the average temperature change rate is almost unchanged as long as the change of current is the same. It can be seen from Figs. 2 and 3 that the temperature curve is relatively smooth during the temperature rise or fall, but the curve is no longer smooth after the temperature increases or decreases to
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Fig. 2. Temperature variation under different current step sizes: (a) Step size: 2.5 A; (b) Step size: 5 A; (c) Step size: 7.5 A; (d) Step size: 10 A.
a new quasi-stable value. For most of the time, there are some small fluctuations in temperature. This may be related to the operating condition of the PEMFC stack. The PEMFC stack was working in a dead-end anode mode during all tests. The periodic purge process was used to remove the excess water inside the PEMFC stack to avoid flooding and performance degradation of the PEMFC stack. When the solenoid valve is open, the excess water and some hydrogen inside the PEMFC stack is discharged under the pressure difference between the inside and outside of the anode flow channel. The water and hydrogen can take away some of heat. However, due to the short purge time, only 0.44 s, each purge process to take away heat was very limited. Therefore, the stack temperature dropped only slightly. When the solenoid valve is closed, the temperature was restored. However, in practice, water and hydrogen discharged by each purge process cannot be exactly equal. Especially water, sometimes more water was expelled. This made each purge process to take heat was not the same, so the stack temperature showed fluctuation. 3.2. Temperature distribution of in-planes The output of 60 thermocouples on the cathode gas diffusion layer surface was recorded under different current conditions. The average of the last three minutes of temperature data was used to
analyze the temperature distribution. Under the current condition of 15 A, 20 A, 25 A and 30 A, the temperature distribution of the cathode gas diffusion layer surface of each selected cell is shown in Fig. 4. The temperature distribution of a cell has its own characteristics. As shown in Fig. 4, the lowest temperature of the PEMFC stack appears in the upper left of Cell 2. This is because this position is near the inlet of hydrogen. Hydrogen used in this experiment was not heated, which causes a localized low temperature in the inlet position of hydrogen. After entering the PEMFC stack, hydrogen is heated quickly and only a small area near the inlet shows low temperature. The size of the high temperature area in these different cell planes is different, and the temperature difference between them is larger. The high temperature area of Cell 11 and Cell 38 is relatively large, especially in Fig. 4 (c) and (d). Their temperature is relatively high. In addition, the temperature distribution of Cell 29 is more evenly. The most uniform temperature distribution always appears on Cell 29. The temperature distribution of these different cell planes differs greatly, which reflects the great difference in cell performance of the PEMFC stack used for this study. As shown in Fig. 4, although the current condition of the PEMFC stack are different, the temperature distribution of cathode gas diffusion layer surface has similar characteristics. Along the flow
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Fig. 3. Temperature variation under different current step sizes: (a) Step size: 2.5 A; (b) Step size: 5 A; (c) Step size: 7.5 A; (d) Step size: 10 A.
direction of the air, the temperature of the cathode gas diffusion layer surface gradually rises, and finally presents a high temperature distribution region. This temperature distribution shape is similar to “parabolic” in the downstream region. Actually, this is related to the cooling mode of the PEMFC stack. The air used to cool the stack came from the environment. Along the direction of the cooling air, the temperature gradually increased. The temperature on the inlet side was lower than the temperature on the outlet side. In addition, the stack can also dissipate heat to the environment through the outer surface. In this way, the temperature in the middle position near the air outlet side was higher than the temperature in other positions and finally formed a parabolic shape. As shown in Fig. 4, on the cathode gas diffusion layer surface of a cell, the temperature first increases and then decreases from the top to bottom along the vertical direction except Cell 38. It is more noticeable near the exit side of air. This phenomenon is caused by the uneven distribution of moisture content of the membrane. The temperature gradient is proportional to the water flux in the membrane [38]. And gravity affects the performance of PEMFCs [39] by affecting liquid water in the flow channel [40]. Dry hydrogen entered the PEMFC stack from the top of the PEMFC stack. All the water needed in the membrane mainly came from the water produce by electrochemical reaction. So the upper part of the
membrane was not well wetted. The electrochemical reaction in this area was not intense and produced less heat, so the temperature in this area was lower. All cells were placed vertically, and the water flowed down as a result of gravity. The water concentration increased along the path of the flow channels from top to bottom. This made the water content of the central region of the membrane increase, which is favorable for the electrochemical reaction. Therefore, the middle region temperature of cells was higher. Further, more water accumulated at the bottom of cells. This can worsen the local performance of cells. Too much water clogged the gas channel in the gas diffusion layer and prevented the reaction gas from entering the catalyst layer. In fact, the observation of visible water drops in the transparent tube connected at the anode outlet of the PEMFC stack in the experiment indicated that water flooding have occurred at the bottom of cells. Wen et al. [16] also used this phenomenon to judge the occurrence of flooding. Water flooding made the electrochemical reaction suppressed. Therefore, the reaction produced less heat, and it showed that local temperature drops. When water accumulates at the bottom, it may also cause lower temperature at the bottom. When the stack is subjected to a purge operation, the accumulated water is discharged outside the stack, thereby taking away a portion of heat and lowering the temperature.
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Fig. 4. Temperature distribution of each selected cell under different current conditions. (a) Current 15 A; (b) Current 20 A; (c) Current 25 A; (d) Current 30 A.
The temperature distribution of cells in a stack is affected by the flow of cooling air and uneven water distribution. However, the high temperature area of Cell 38 is in the upper right of the plane, while the high temperature area of other cells is in the middle right part. This shows that the performance of Cell 38 differs greatly from that of other cells. And it indicates that cell performance have an important influence on the temperature distribution of PEMFC stacks. This phenomenon is more obvious under high current condition, as shown in Fig. 4 (c) and (d). And the temperature distribution under high current condition is more likely to highlight this problem.
3.3. Temperature distribution of through-planes Fig. 5 presents the temperature distribution of the air flow crosssection under the different current conditions. As shown in Fig. 5, from the direction of the air flow, the temperature of the air inlet side is lower, and the temperature of the air outlet side is higher. This is directly related to how the PEMFC stack is cooled. The reason has been discussed in section 3.2. As shown in Fig. 5 (a) and (b), when the current of the stack is lower, the temperature distribution of the air flow cross-section presents two bar regions with higher temperature. These two high temperature regions correspond to Cell 11 and Cell 38, respectively. This shows that there is a large amount of heat generated near these two cells in this PEMFC stack. At the distance of 25 mm and 40 mm from the edge of the air inlet side of the stack, the area of high temperature region increases as the PEMFC stack current increases. As shown in Fig. 5 (c) and (d), when the current of the PEMFC stack is higher, these two bars with higher temperature have a tendency to connect together. This means that the local cell has an effect on the nearby cell, and these effects will increase as the current increases.
Thermal image can provide more temperature distribution information. Fig. 6 shows a thermal image of the PEMFC stack seen from the side with air entering under different current conditions. Irregular lines in the thermal image represent thermocouple wires, and three thicker horizontal wires are bolts that secure the PEMFC stack. The thermal image reflects the temperature information of the outer surface of the stack, while Figs. 4 and 5 show the temperature distribution inside the PEMFC stack. So there will be some differences between them. As mentioned in section Introduction, the temperature distribution shown in the existing literature [16,17] is more common: the temperature near the middle is high and the temperature on both sides is low. This represents the temperature distribution of a normal cell or a normal PEMFC stack. It can be found in Fig. 6 that the PEMFC stack has three high temperature regions: in the vicinity of Cell 11, Cell 25 and Cell 38, respectively. Compared with these temperature distribution, an unusual temperature distribution of the PEMFC stack used for this study is observed. As the current increases, the temperature difference on the outer surface of the stack increases from 5.2 C to 12.9 C. This proves that the temperature distribution of the PEMFC stack is inhomogeneous. And it also reflects the large difference in performance of cells in the stack. Heat production in the PEMFC stack is closely related to cell voltage [41]. In the actual case, the voltage generated by each cell in a PEMFC stack is not exactly equal. Although the current is equal, some cell voltage is higher, and some cell voltage is lower. The heat generated by a single cell can be can be expressed as:
Q ¼ ðE UÞI
(2)
where Q is the heat generated by a single cell; E is the reversible voltage; U is the actual voltage of a single cell; I is the current of a single cell. Fig. 7 shows the voltage of cells in the PEMFC stack. As shown in
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Fig. 5. Temperature distribution of the air flow cross-section under different current conditions. (a) Current 15 A; (b) Current 20 A; (c) Current 25 A; (d) Current 30 A.
Fig. 6. Thermal image of the PEMFC stack seen from the side with air entering under different current conditions. (a) Current 15 A; (b) Current 20 A; (c) Current 25 A; (d) Current 30 A.
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can be confirmed that the performance of cells in the PEMFC stack varies greatly. The performance difference of cells in the PEMFC stack can be caused by long-term operation. Prior to the experiment, this PEMFC stack has been running for more than 2000 min, including 46 start-and-stop tests, and has been close to its operating limit several times. Note that the experiment was not conducted in the optimal condition of the PEMFC stack. Our purpose herein was to introduce the temperature characteristics of the PEMFC stack. In particular, we introduce the temperature characteristics of the PEMFC stack after long-term extreme operating conditions. A conception of Uniformity of Temperature Distribution (UTD) is proposed to evaluate the degree of uniform temperature. A larger UTD means that the difference between most of the temperature and its average value is greater, and the more uneven the temperature distribution; a smaller UTD means that most of the temperature is closer to the average and the temperature distribution is more uniform. It is defined as
Fig. 7. The voltage of cells in the PEMFC stack under current of 15 A.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi #, u" N u X 2 t UTD ¼ N Ti T
(3)
1
where N is the number of temperature measurement points on a plane; T is the average temperature of all temperature measurement points on a plane.
T¼
N X
!, Ti
N
(4)
1
Fig. 8. Flow rate distribution at the air inlet.
Fig. 7, the voltage of Cell 11 and Cell 25 were significantly lower than cells near it. According to Eq. (2), Cell 11 and Cell 25 produced more heat than cells near it. Therefore, Cell 11 and Cell 25 appeared higher temperature, as shown in Fig. 6. Cell 38 had a higher voltage than the adjacent cells. However, Cell 36 and Cell 40 that near Cell 38 had lower voltage. Therefore, the area of the high temperature region near Cell 38 is relatively large, as shown in Figs. 5 and 6. The flow rate distribution may also affect the temperature distribution. A wireless mini hot wire anemometer (testo 405i, Testo SE & Co. KGaA, Germany) was used to measure the air speed. As shown in Fig. 8, in addition to the low air speed at the edge of the air inlet, the air speed in the main area of the section is similar. The average air speed was 1.6 m/s. The flow rate distribution is considered to be uniform. However, the temperature distribution on the outer surface of the stack has three high temperature regions, as shown in Fig. 6. The flow rate distribution and temperature distribution are quite different, indicating that the performance of cells has a great influence on the temperature distribution, while the influence of the flow rate distribution is small. 3.4. Analysis of the inhomogeneity of temperature distribution From the above discussion of the temperature distribution, it
Fig. 9(a) shows the average temperature of different cells under different current conditions. It can be found that the average temperature of cells increases with the increase of current. Cell 11 had the highest average temperature, while Cell 2 had the lowest average temperature. Fig. 9(b) shows the UTD of different cells under different current conditions. It can be found that the temperature uniform decreases with the increase of current. Cell 29 had the best temperature uniformity, and Cell 2 had the worst temperature uniformity. The dashed lines in Fig. 9 are fitted according to data points. As shown in Fig. 9(a), it can be seen that the average temperature of inplanes shows a linear increase with the increase of current. The UTD variation of in-planes in Fig. 9(b) is different. With the increase of current, the UTD variation of Cell 2 is linear, while the UTD variation of Cell 11, Cell 20 and Cell 29 is closer to a quadratic polynomial. The possible reason for this phenomenon is related to the operation of a stack. Cell 2 was near the hydrogen inlet and there was a local low temperature area. The reason has been discussed in section 3.2. The UTD variation of Cell 38 is close to the quadratic polynomial, but it does not fit well. The possible reason for this phenomenon is the large difference in the performance of cells. Fig. 10(a) shows the average temperature of different throughplanes under different current conditions. “Cross-section 10” indicates that the cross section is 10 mm from the air inlet edge. It can be seen that the average temperature of through-planes shows a linear increase with the increase of current. Fig. 10(b) shows the UTD of different through-planes under different current conditions. It can be found that the temperature uniform decreases with the increase of current. In addition, along the direction of air flow, the average temperature and UTD are getting higher and higher. Further, with the increase of current, the law of UTD variation with current changes can be described by a quadratic polynomial. In summary, these phenomena indicate that the cell performance has an important influence on the temperature
Fig. 9. Average temperature and UTD of different in-planes under different current conditions. (a) Average temperature; (b) UTD.
Fig. 10. Average temperature and UTD of different through-planes under different current conditions. (a) Average temperature; (b) UTD.
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characteristics of the PEMFC stack. Although the temperature distributions on in-planes and through-planes are different, the temperature variations on the plane are similar. The average temperature on the plane increases as the current increases. And the relationship between temperature uniformity and current on a plane can be described by a quadratic polynomial. 4. Conclusions An air-cooled proton exchange membrane fuel cell stack is tested in this study. 60 thermocouples and a thermal imaging camera are used to get detailed temperature information about the stack during operations. Its temperature variation and distribution are analyzed. The results of this research can be concluded as follows: (1). No matter the current step change is increased or decreased, the average temperature change rate is almost unchanged as long as the change of current is the same. (2). The average temperature of in-planes and through-planes shows a linear increase with the increase of current. As the current increases, the temperature difference on the outer surface of the stack increases from 5.2 C to 12.9 C. (3). The temperature distribution of cells in a stack is affected by the flow of cooling air and uneven water distribution. The performance of cells has an important influence on the temperature distribution. (4). With the increase of current, the law of temperature uniformity with current change can be described by a quadratic polynomial. (5). Although the temperature distributions on in-planes and through-planes are different, the temperature variations on the plane are similar. This study contributes to the analysis of temperature characteristics of a hydrogen-air proton exchange membrane fuel cell stack in application. It will provide reference for thermal management on air-cooled proton exchange membrane fuel cell stacks. Acknowledgments This research was supported by the National Nature Science Foundation of China (No. 21776095), the Guangzhou Science and Technology Program (No. 201804020048) and Guangdong Key Laboratory of Clean Energy Technology (No. 2008A060301002). Nomenclature E I N PEMFC Q t T_
DT T Ti U UTD
the reversible voltage, V the current of a single cell, A the number of all temperature measurement points on a plane proton exchange membrane fuel cell the heat generated by a single cell, J the time of the temperature change, min the average temperature change rate, C/min magnitude of the temperature change after the current changes, C the average temperature of temperature measurement points on a plane, C the temperature of measurement points on a plane, C the actual voltage of a single cell, V uniformity of temperature distribution
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