Andreasen, Søren Juhl

Bang, Mads

Korsgaard, Anders

Kær, Søren Knudsen

Nielsen, Mads Pagh

When using pressurized hydrogen to fuel a fuel cell, much space is needed for fuel storage. This is undesirable especially with mobile or portable fuel cell systems, where refuelling also often is inconvenient. Using a reformed liquid carbonhydrate can reduce this fuel volume considerably. Nafion based low temperature PEM (LTPEM) fuel cells are very intolerant to reformate gas because of the presence of CO. PBI based high temperature PEM (HTPEM) fuel cells can operate stable at much higher CO concentrations. This makes the HTPEM very suitable for applications using a reformer, and could simplify reformer design because CO removal is not needed. A fuel like methanol would be a preferable choice for reforming when using HTPEM fuel cells because of its high energy density and low reforming temperatures. The thermal integration and use of HTPEM fuel cells with methanol reformers show promising results. This work demonstrates the use of HTPEM fuel cells (HTPEM) in a 400 W fuel cell power pack. The fuel cell system concept uses a 30 cell HTPEM fuel cell stack designed at the Institute of Energy Technology, Aalborg University. The MEAs employed are Celtec P-series by Pemeas, with an active area of 45cm². The development of the bipolar plates and the stack design itself is an ongoing activity using CFD and optimizing for low pressure drop. Later versions of the stack design is expected to result in a much shorter stack. When using pure H2, the hydrogen circuit is running dead end with occasional purging. Single cell life time tests indicate that very infrequent purging does not accelerate degradation severely. This is a great advantage compared to the LTPEM which often has problems with flooding when not purged. The air supply is realized using a low power axial fan with a high turn down ratio. The stack is placed in an insulation box and the temperature controlled by electrical heating elements. When loading the stack at its nominal load of 400 W it produces enough heat to keep this temperature. For control system design and location of optimal operating conditions, a system model is made, including a HTPEM fuel cell model, expressing the cell voltage as a function of the current density, temperature, CO concentration and stoichiometries. Simulation results from this model are shown. The system model and control design is verified with the actual setup in the lab. For the use of the fuel cell stack in a power pack, a high efficiency DC/DC-converter is designed. The overall control of the power conditioning and the power pack itself is also derived from modelling of the DC/DC converter. Comparing the LTPEM and the HTPEM, the HTPEM fuel cell has a lower cell voltage than the LTPEM, but the developed power pack demonstrates some of the advantages by using a HTPEM fuel cell. This initial system is very simple and there is no need for humidification of the species like in a LTPEM fuel cell system. The use of the HTPEM fuel cell makes it possible to use reformed gas at high CO concentrations without, still with a stable performance.</p

    Aalborg UniversitetDevelopment of a 400 W High Temperature PEM Fuel Cell Power PackAndreasen, Søren Juhl; Bang, Mads; Korsgaard, Anders; Nielsen, Mads Pagh; Kær, SørenKnudsenPublished in:Proceedings of the Fuel Cell Seminar 2006 ConferencePublication date:2006Document VersionPublisher's PDF, also known as Version of recordLink to publication from Aalborg UniversityCitation for published version (APA):Andreasen, S. J., Bang, M., Korsgaard, A., Nielsen, M. P., & Kær, S. K. (2006). Development of a 400 W HighTemperature PEM Fuel Cell Power Pack: Fuel Cell Stack Test. In Proceedings of the Fuel Cell Seminar 2006Conference Fuel Cell Seminar.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: April 29, 2017Development of a 400 W High Temperature PEM Fuel Cell Power Pack : Fuel Cell Stack Test S. Andreasen, M. Bang, A. Korsgaard, M. Nielsen, S. Kær Aalborg University, Aalborg East, Denmark 1 Introduction  When using pressurized hydrogen to fuel a fuel cell, much space is needed for fuel storage. This is undesirable especially with mobile or portable fuel cell systems, where refueling also often is inconvenient. A reformed liquid hydrocarbon based fuel, like methanol can reduce the storage volume considerably. In tab.1 a comparison of the characteristics of liquid methanol and compressed hydrogen (200 bars) is shown.   Hydrogen (200 bars) Methanol  Density  16,27 kg/m3  793,8 kg/m3  Lower Heating Value  120,1 MJ/kg  19,9 MJ/kg  Energy density (grav.) 33,4 kWh/kg  5,53 kWh/kg  Energy density (vol.)  542,6 kWh/m3  4388 kWh/m3 Table 1: Characteristics of hydrogen and methanol at 25oC. [Engineering Equation Solver]  It is seen that in spite of the high energy content per unit mass of hydrogen; methanol has a clear advantage when it comes to volume, containing about 8 times more energy by volume.  Fuelling NaÞon based low temperature PEM (LTPEM) fuel cells with pure hydrogen shows good performance characteristics, but the fuel cell membranes often show severe intolerance to reformate gas because of the presence of CO, which can result in poor fuel cell performance [1]. PBI (polybenzoemidazole) based high temperature PEM (HTPEM) fuel cells can operate stable at much higher CO concentrations. This makes a HTPEM fuel cell suitable for applications using a fuel reformer, and could simplify reformer design because CO removal is not needed. A fuel like methanol would be a preferable choice for reforming when using HTPEM fuel cells because of its high energy density and low reforming temperatures. The thermal integration and use of HTPEM fuel cells with methanol reformers has already shown promising results [2].  Ina recent publication [3] the authors noted the absence of publications covering HTPEM fuel cell design and HTPEM fuel cell test station design. The work presented in the following seeks to demonstrate some of the issues related to these topics, with the motivations stated above. The use of a 30 cell HTPEM fuel cell stack is demonstrated, with a nominal power of ≈400W at 0,5 A/cm2. This very simple system design demonstrates some of the advantages and disadvantages in using the HTPEM PBI membrane technology in making a reliable simple fuel cell system.   2 Experimental setup  A schematic of the experimental setup can be seen in Þg. 1L.The anode is running on pure hydrogen at a constant pressure and the cathode on atmospheric air.  Andreasen et al. Figure 1: Left: Schematic of the fuel cell system. Right: Picture showing a HTPEM stack prototype.  Pure hydrogen is initially used as anode fuel, to gain performance results and further system control information on an operating HTPEM fuel cell stack. The hydrogen is held at a constant pressure of ≈0,2 bars in the stack, by a pressure reduction valve. An occasional purging (every 5-15 min) is made to remove eventual impurities. When the fuel cell is to be operated on a reformate gas, this strategy is not necessarily the best way to fuel the stack, because of the CO presence in the reformate gas, instead a small overstoichiometric anode supply could be used. A great advantage of the HTPEM fuel cells is that they are much more tolerant to CO poisoning [4]. In Þg. 1R a 30 cell prototype HTPEM stack is shown. The fuel cells used are PEMEAS CELTEC-P with an active area of 45.16cm2. To greatly simplify the fuel cell system, the fuel cell stack is designed with cathode air cooling. This stack design fulÞlls the desire of minimum parasitic losses, by the possibility of using a low power consumption cathode air supply. This is achieved by using a low power blower using ≈3W at nominal capacity, see Þg. 1L. Another advantage of the blower compared to other blowers, is fast response, because of a low moment of inertia. The nominal voltage, current and power of the system is respectively: 17,7V; 22,5A; 400W. The operating temperature is 120-200 oC. CO tolerance is several percent. The reason for the high operating temperatures is to avoid liquid water, when using a PBI fuel cell membrane. Liquid water present in the membrane will ßush the phosphoric acid doped for increased proton conductivity [5], out of the cell.   3 Initial tests  Initially the air side of the stack is connected to a mass ßow controller (MFC) and the stack is characterized with regards to differential pressure as a function of the air ßow, the resultant graph is shown in Þg. 2L. Different air stoichiometry points are marked on the curve, at 0,2 A/cm2 .   Figure 2:Left: Pressure as a function of ßow of the 30 cell HTPEM stack. Right: Cell voltage distribution of HTPEM stack at 0,2 A/cm2 .  Notice the very high stoichiometry of 15, at 0,2A/cm2, because of the cathode cooling. After this initial Andreasen et al.test, the cathode air blower can be mounted and the air ßow can be controlled with a feedback of the differential stack pressure. When applying a load to the stack, the stack temperature will also be used to control the cooling requirement. At low current densities, the heat generated is equal to the heat lost by convection and conduction. For higher current densities, the stack needs cooling to avoid the temperature from rising above the operating temperature limit.   4 Experimental results  The actual operation of the HTPEM stack includes a warm-up mode, a current loading mode and a cool down and shut-down mode. The experimental test results presented here will primarily concern the current loading mode of the system.  The warm-up mode of the system includes raising the temperature of the fuel cell stack, to above 120oC. This is done by external heating sources and monitored with6 thermocouples at different locations in the stack. 2 at the inlet cell, 2 in the middle cell and 2 in the last cell. At each of these three temperature measuring points, one thermocouple is situated in the top of the fuel cell and one is in the bottom in a small recess. The warm-up time of the stack can be quite long, depending on the heat supplied; more improvements are needed if minimization of this time is desired.  When above 120oCa current can be drawn from the fuel cell stack. When loading the stack, heat is generated and the temperatures will start increasing. This results in differences in cell voltages because of different temperatures. In Þg. 2R the cell voltages of the stack are shown at steady-state temperatures at 0,2 A/cm2.  Figure 3: Left: Stack temperature distribution at constant load of 0,2 A/cm2 and λair≈ 15. Right: Polarization curve of 38 cell HTPEM stack.  As seen in Þg. 2R, the highest fuel cell voltage is 0,657 V and the lowest 0,626 V. This is due to the temperature differences in the fuel cell stack. In Þg. 3L, the steady-state fuel cell stack temperature distribution is seen, without external heating, running at a constant air stoichiometry of ≈15 at 0,2 A/cm2 . It is seen that the highest temperatures, as expected, are in the end of the stack, opposite the cathode inlet due to better insulation. At the end of the stack, the local fuel cell temperature is uniform at ≈177oC both in the top and the bottom. In the middle of the fuel cell stack, the top and bottom temperatures differ by a few degrees, 165oC and 172oC respectively. There is a large temperature difference at the inlet fuel cell, the top where the cold cathode air enters being 142oC and the bottom where the hot air exits being 152oC. This is caused by the high air ßow entering the inlet end plate, cooling it, and also the fact that the inlet of the stack is less well insulated.  If all cells of the stack had the same cell voltage as the fuel cell with the highest temperature, the stack voltage would be 19.65V, instead it is ≈18.6 V, a loss of about 1V, which is 9W.  A polarization curve has been made on a stack of the same design but with 38 cells. The curve has Andreasen et al.been made at steady-state temperatures with a manual adjustment of the cooling. This explains the stack voltage jumps seen in the curve in Þg. 3R.  For purposes of later tests on control strategies, a temperature dependent steady-state model of the HTPEM fuel cell voltage is derived from the work of [4], as an input for a dynamic model of the average fuel cell stack temperature. This model is implemented in Simulink and veriÞed against the experimental results.   5 Conclusions  The experiments made on the 30 cell HTPEM stack have resulted in more knowledge on the different issues involved during operation. This includes information on stack and single cell temperature distribution, single cell voltages during operation, the use of overstoichiometric cathode air cooling and dead end hydrogen operation on a HTPEM stack.  The experimental results indicate that the use of cathode air cooling simpliÞes the system greatly, and can result in very simple and robust fuel cell systems. When making a stack with a low pressure drop, it is possible to use fast low power consuming blowers.  The differences in temperature distributions in the fuel cell stack is small except at the inlet fuel cells at 0,2 A/cm2. These temperature differences will be much larger at higher current densities and should be considered when designing the fuel cell stack and determining nominal operating conditions on a system level.  More work must be made on different warm-up strategies, minimizing the startup time.  Future work includes the design of a heat management control system, controlling the startup and operation of the fuel cell stack and monitoring the state of the system. Moreover, general derivation and testing of different control strategies for the fuel cell system both via modeling and experimental veriÞcation should be performed.  The system is to be tested with a DC/DC converter, looking at the load following capabilities of the HTPEM fuel cell system, and the possibilities of using it in a power pack module for a mobile application.  Different load cycle tests are also planned while running the HTPEM stack with a reformate gas. The temperature control will be critical when running with a reformate gas, so a model based control design will be used in order to design the heat management of the system.   References  [1] H. Xu, Y. Song, H. Kunz, J. Fenton, "Operation of PEM fuel cells at 120-150oC to improve CO tolerance", Journal of Power Sources 2006.  [2] C. Pan, R. He1, Q. Li, J. O. Jensen, N.J. Bjerrum, H. A. Hjulmand, and A. B. Jensen, "Integration of high temperature PEM fuel cells with a methanol reformer", Journal of Power Sources 145 (2005), p. 392-298.  [3] J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D. Wilkinson, Z. Liu, and S. Holdcroft, "Review -High Temperature PEM Fuel Cells", Journal of Power Sources 2006.  [4] A. R. Korsgaard, R. Refshauge, M. P. Nielsen, M. Bang, and S. K. Kær, "Experimental Characterization and Modeling of commercial PBI-based MEA Performance", Accepted paper to Journal of Power Sources (2005).  [5] J. S. Wainright, J-T. Wang, D. Weng, R. F. Savinell, and M. Litt, "Acid-Doped Polybenzimidazoles" : A New Polymer Electrolyte, J. Electrochem. Soc. 142 (1995) 121-123.  

Development of a 400 W High Temperature PEM Fuel Cell Power Pack:Fuel Cell Stack Test

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Development of a 400 W High Temperature PEM Fuel Cell Power Pack:Fuel Cell Stack Test

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