The 5 th TSME International Conference on Mechanical Engineering th December 2014, The Empress, Chiang Mai

Transcription

1 Design and Development of a Small-scale Hot Press Machine for PEFC Membrane Electrode Assembly Application Boonrit Keawprachum 1, Kotchakarn Nantasaksiri 2, Patcharawat Charoen-amornkitt 2, and Nuttapol Limjeerajarus 2, * 1 Production Engineering Program, Faculty of Engineering, Thai Nichi Institute of Technology 2 Research Center for Advanced Energy Technology, Faculty of Engineering, Thai Nichi Institute of Technology 1771/1 Pattanakarn, Suan Luang, Bangkok 10250, THAILAND *Corresponding Author: nuttapol@tni.ac.th, Tel.: Ext. 2922, Fax.: Ext Abstract The hot press method is the most common method used to assemble a polymer electrolyte fuel cell membrane electrode assembly (PEFC MEA). A desired hot press machine for PEFC applications is the one having very smooth contact surfaces of pressing blocks, accurate temperature and force loading control systems. However, a typical hot press machine uses a hydraulic cylinder to generate the load, which is costly as considered a small load for PEFC application. Therefore, in this study, a small-scale hot press machine for PEFC MEA application has been designed and developed using a jackscrew instead of a hydraulic cylinder motor as the load generator. Structural analysis using finite element method on ANSYS software was conducted to check stress occurring due to force and thermal distribution, and safety factor of the hot press machine prior to actual construction. Finally, a small-scale hot press machine specifically designed for PEFC MEA application was successfully developed with a maximum temperature and loading capacity of 130 ºC and 3 kn, respectively, and a decrease in construction cost by 70%. Keywords: Hot press machine, Structural analysis, PEFC, Membrane electrode assembly, Finite element method 1. Introduction Fuel cells are interesting energy converters for future power generation since they have high energy efficiency and release very low (if not zero) pollution emission. One of the most promising fuel cells is polymer electrolyte fuel cell (PEFC) because of its advantages of high efficiency in energy conversion with zero greenhouse gas emission, low temperature operation, low noise and vibration. The most important component of a PEFC is the membrane electrode assembly (MEA). The MEA is, typically, consisted of 3 layers, namely the membrane, the anode and cathode electrode, which are fabricated individually and pressed together by the hot press method at a high temperature and pressure [1]. The appropriate conditions of the hot press method for MEA fabrication are around 4 MPa (2 kn for a 5 cm 2 MEA), ºC, and a load holding time of about 1 min [2,3]. For MEA fabrication in future research of our laboratory, the objective of this study is to design

2 and develop a small-scale hot press machine for assembling single cell 5 cm2 MEAs at the minimum force and temperature of 3 kn (approximately equivalent to 300 kgf or 6 MPa for a 5 cm 2 MEA) and 130 o C, respectively. During the design process, the structural analysis was done by using finite element method (FEM) in ANSYS software to investigate the temperature and load distributions on the machine, stress, thermal stress and safety factor of the structure of the machine, and the high-risk areas to deform plastically. The simulation results would suggest the operating temperature of the load cell, and would confirm that the hot press machine will not deform plastically in the operation. 2. Theory 2.1 Design theory The commonly used transmission systems in a hot press machine can be classified into 3 groups; the pneumatic, the hydraulic and the motor systems. Due to its high stability and precision, the hydraulic system is normally used in the MEA fabrication [4]. However, for such a small load on the MEA application, the hydraulic system is too expensive and overqualified since it can produce a very high force [5,6]. On the other hands, the pneumatic system, a low cost system, can produce a small force which seems suitable for the application. However, a major drawback of the pneumatic is its stability and precision. Therefore, both of them does not respond to the objective of the MEA application. To satisfy those requirements, motor may be the right choice due to its low noise, good precision and stability, and furthermore, it required only a small space. The MEA application, however, requires holding of the force for at least a minute, and thus the rotational force is required to apply continuously without any angular displacement. Hence, a motor alone cannot be used for the MEA fabrication. Meanwhile, a jackscrew, which is commonly used in lifting heavy weights, is a very interesting equipment and can be used with a motor for the hot press method since its major advantage is self-locking by which as the applied load is removed, the contact area will remain still. To achieve the required force, the maximum torque of the motor has to be more than the torque input in this following equation in Fl 2 i (1) Where is the torque input, F is the required force, is the pitch of the jackscrew, is the static efficiency of the jackscrew, and is the gear ratio. Since compressive stiffness of the heating block is very high, the force cannot be controlled. Therefore, to control the force, a spring was used to reduce the stiffness so that the force can be created as a function of distance. Generally, force is measured by a load cell, a device by which deformation is converted into electrical signals by Wheatstone bridge circuit. Since hot press machine is involved with temperature, the load cell can be affected by thermal expansion, and thus deformation occurs. Therefore, thermal management in the hot press machine has to be considered carefully. To reduce such an effect by heat conduction mechanism, as described in Eq. (2), the contact l in as as i

3 area between the heating block and the load cell needs to be reduced. mechanism in this model is described by the Laplace equation [7]. Q = ka dt dx (2) where Q is the conduction heat flow, k is the thermal conductivity of the medium, A is the cross-section area of the medium, and dt is the dx temperature gradient. The size of the heaters can be calculated approximately using heat capacity equation divided by a desired heating time as described in Eq. (3) Q = mc T t (3) where Q is the required capacity of the heaters, m is the mass of the heating block, c is the specific heat capacity of the material of the heating block, T is the difference temperature between initial temperature and desired temperature, and t is the desired heating time To ensure that the selected load cell can work properly at the desired conditions, the 3-D simulation was conducted to determine temperature profile that would occur on the load cell so that an appropriate load call can be selected accordingly. Furthermore, the simulation can suggest whether the hot press machine will plastically deform during the critical case of operation. 2.2 Simulation theory Since the highest accumulated heat will occur during the steady-state operation at the desired temperature, the governing equation of temperature distribution by a heat conduction 2 T + 2 T + 2 T = 0 (4) x 2 y 2 z 2 where T is the temperature, and x, y and z are the direction in Cartesian coordinate. The normal strain due to the increased temperature is governed by eq. (5) [8]. ε x th = ε y th = ε z th = α( T) (5) where ε th is the thermal normal strain in the x, y and z direction, α is the coefficient of thermal expansion, and T is the difference between the temperature at the point and strain-free temperature Therefore, the total strain, which includes the effect of elastic strain, is described in Eq. (6) [9], as follows: {ε} = {ε th } + [D] 1 {σ} (6) where {ε} is the total strain vector, [D] is the elastic stiffness matrix, and {σ} is the stress vector. The basic assumptions of this model are listed below: The model was conducted in steady-state condition at the desired temperature (130 o C), which is considered as the critical case. Convection heat transfer coefficient of the surfaces around the heating block were based on our previous study [10].

4 The conduction heat transfer mechanism in the springs was negligible due to their small contact area and coated colour. 3. Research Methodology 3.1 Design of the hot press machine The hot press machine was initially designed by using CATIA software, as seen in Fig.1. As mentioned earlier, the design criteria of the hot press machine is to produce at least a force of 3 kn, a temperature of 130ºC, and a holding time at these conditions for 1 min. The systems of the hot press machine can be divided into 2 sections as follows: 1. Force generation and transmission system 2. Heat generation system For force generation and transmission system, in this study, jackscrew was used for transmitting the generated force from the motor at which the required torque was calculated by Eq. (1). The springs were placed under the heating block to create the force as a function of the displacement. However, the space for placing the springs is limited, therefore the stiffness and the size of the springs had to be selected carefully. Since the heating block was designed to press downward, load cell was placed under the springs and was selected by the maximum desired force. Another advantage of placing the load cell under the heating blocks and the springs is that the heat transfer to the load cell via convection heat transfer can be reduced. The heat was generated by the heaters placed in the slots in both top and bottom heating blocks, as displayed in Fig. 2. The slots were designed to be covered by the contact surfaces, which were machined by a grinding process. The temperatures were measured by using thermocouples. Since the contact surface is required to be smooth as much as possible for preventing the damages on the MEA, therefore the thermocouples were placed at 1-mm beneath the contact surfaces. Fig. 1 The designed hot press machine Fig. 2 Schematic showing the slots in the bottom heating block

5 To reduce the heat flows from the heating blocks to other parts, especially to the load cell, blocks with a small contact area were designed to place between the heating block and the moving plate. 3.2 Model development The purpose of the simulation of this study is to investigate the temperature on the load cell placing area, and plastic deformation on the hot press machine. The geometries were imported from the CATIA software to be simulated in the ANSYS software. For the meshing, a total of 185,062 tetrahedral elements was used in the model. Actually, the heat is generated by the heaters, but for simplicity sake, the model was simplified into constant surface temperature since the desired temperature at the contact surface would be around 130ºC steadily. The convection heat transfer coefficients of the top surface of the top heating block, the 4 side surfaces, and the bottom surface of the bottom heating block were set at 8.081, 8.076, and W/m 2 K, respectively [10]. To reduce simulation time, the heating block was modelled to be close together with a gap of 1 mm, Thus, the convection heat transfer coefficient of the top surface of the bottom heating block and the bottom surface of the top heating block were not set due to the assumption that the medium fluid was not moving, and therefore, the air gap would behave like an insulation. The convection heat transfer coefficient of the other parts was set at 3 W/m 2 K by assuming that the heat hardly transfers out of the machine to the surroundings, which was considered as the critical case. Due to the complexity of modelling, the springs were taken out and replaced by the maximum forces, as displayed in area A and B of Fig. 3. The lead screw, jackscrew housing, and the internal parts were modelled to be one part due to the jackscrew s self-locking. Since the 6 supports of the hot press machine were designed to be a clearance fit, therefore they could expand on both x, y direction but were fixed on z direction. The computational time was about 13 hours (wall clock time), in Intel Xeon E v2 2.1 GHz processor of 32 GB RAM and 4 GB graphic card memory. Z Y X Fig. 3 Schematic of the force inputs represented spring reaction forces 4. Results and discussion 4.1 Simulation results The temperature distribution of the hot press machine during steady-state condition was displayed in Fig. 4. It indicated that the temperature was distributed uniformly in both up and down direction where the maximum temperature occurred around the heating block. The minimum temperature of 42ºC to 52ºC were found around the base of the hot press machine at which the load cell was connected, and the aluminium base, which is connected to the motor

6 ºC MPa Fig. 4 Temperature distribution of the hot press machine during steady-state condition and the jackscrew, as seen in Fig. 4. From this simulation result, it suggested that the operating temperature of the load cell should be around 20-60ºC. After the steady-state thermal analysis was done, the desired maximum load of 3 kn was applied in the model to investigate the stress of the hot press machine. Fig. 5 present the stress distribution on the machine in which the effect of both force loading and thermal distribution were taken into account. As seen in Fig. 5, most parts of the hot press machine were under stress ranging from appoximately 0-16 MPa, contributing to a the safety factor of more than 15. Even the maximum stress occurring at the bottom moving plate resulted in the minimum safety factor of 2, approximately. This indicated that the hot press machine would not deform plastically even it is operated under the critical case. Maximum stress Fig. 5 Stress distribution of the hot press machine during the critical case operation After the structural analysis was completed, the hot press machine was constructed and operated to test that the designed machine could work properly. Finally, the data of the generated force were collected in every 5 seconds for 1 min. to ensure that the force would not change with time (an acceptable range of ±100 N or approximately equivalent to 10 kgf). 4.2 Development of small-scale hot press machine As calculated by Eq. (1), the required torque of the motor was 1.73 N m and thus a motor by which a torque of 3 N m approximately was used due to the factor of safety. The motor transmitted the generated torque through the jackscrew and converted into a usable force by the deformation of the 2 springs (see figure 1). Springs stiffness was 78.5 N/mm (or 8 kgf/mm). These 2 springs were selected and designed by considering the required space in which the spring deformed only

7 about 20 mm for generating the required force. This generated force was measured by BOAST SQC-A1T and shown in weighing indicator CM- 013 of PRIMUS. The range of the operating temperature of the load cell was between -30 and 70ºC [11]. This operating temperature was suggested by the simulation, as mentioned earlier. Considering the cost of manufacturing and maintenance, the heaters were designed to be 3 simple, straight rods of 300 W each, and were laid in parallel along a heating block. Therefore, there were 6 heaters in total. The temperature of the contact surfaces was measured by using the inexpensive k-type thermocouple which placed 1 mm beneath the surfaces. The thermocouple indicators were PRIMUS REM-48. was found that the force slightly fluctuated since in a real situation, the jackscrew and the springs may not perfectly self-locking. Nevertheless, the fluctuation was only around ±30 N, as shown in Fig. 7, which is small, as compared with ±100 N of the acceptable range. Fig. 7 The force data collected every 5 seconds for 1 minute In summary, the developed hot press machine can satisfy the desired conditions of PEFC MEA application without any plastic deformation. Moreover, as compared with the cost of a conventional commercial machine [5,6], the cost of this developed machine was 70% lower. Fig. 6 The hot press machine during desired operation The developed hot press machine can generate the desired force of 300 kgf (approximately equivalent to 3 kn) and the temperature of 130ºC, as displayed in Fig. 6. The stability of force loading was shown in Fig. 7. It 5. Conclusion A hot press machine for PEFC MEA application, which can generate the desired force of 3 kn and the temperature and 130ºC, were successfully developed. In the design section, the 3-D model was conducted to investigate the temperature distribution in the load cell placing area so that a proper load cell can be selected. Stress analysis was also carried out to ensure that there will be no plastic deformation of the

8 machine structure. As compared with the cost of a commercial machine, this developed machine with the use of a jack screw can reduce the production cost by 70% (approximately 150,000 Baht). 6. Acknowledgement The authors would like to acknowledge the research fund provided by Thai-Nichi Institute of Technology. 7. References [1] Therdthianwong, A., Manomayidthikarn, P., and Therdthianwong, S. (2007). Investigation of membrane electrode assembly (MEA) hotpressing parameters for proton exchange membrane fuel cell, Energy, vol.32(12), December 2007, pp [2] Hasran, U. A., Kamarudin, S. K., Daud, W. R. W., Majlis B. Y., Mohamad, A. B., Kadhum, A. A. H. and Ahmad M. M. (2013). Optimization of hot pressing parameters in membrane electrode assembly fabrication by response surface method, Hydrogen Energy, vol.38(22), January 2013, pp [3] Larminie, J. and Dicks, A. (2003). Fuel cell systems explained, 2 nd edition, ISBN: , John Wiley & Sons, Chichester, UK. [4] Elias, K. Q. and Kurek, H. K. (2007). Principles of High Performance Membrane Electrode Assembly Fabrication, A major Qualifying Project Report, Worcester Polytechnic Institute. [5] Laboratory Presses and Accessories, Carver, Inc. (2012), Carver 100 th Anniversery [6] Carver Hydraulic Presses & Accessories 2011 Price List, Carver, Inc. (2011), Caver 2011 Price book. [7] Cengel, Y. A., and Ghajar, A. J. (2011). Heat and Mass Transfer: Fundamentals and Applications, 4 th Edition, ISBN: , McGraw - Hill, Singapore. [8] Bundynas, G. R. and Nisbett K. J. (2008). Shigley s Mechanical Engineering Design, 8 th edition, ISBN: , McGraw - Hill, Singapore. [9] ANSYS, Inc. (2009). Theory Reference for the Mechanical APDL and Mechanical Applications, ANSYS, Inc., Southpointe. [10] Chaithanee, N., Charoen-amornkitt, P., Nantasaksiri, K. and Limjeerajarus, N. (2014). A 3-D Numerical Simulation of Temperature Distribution across the Contact Surface of a Small-scale Hot Press Machine for PEFC Applications, paper presented in The 5th TSME International Conference on Mechanical Engineering, Chiang Mai, Thailand. (submitted) [11] Ningbo BOARD Electric co.,ltd. (2014), Boast loadcell Manual.