Experimental investigation and performance analysis on 100 watts air-breathing PEM fuel cell stack.
A fuel cell is an electrochemical device which converts the chemical energy of a fuel and an oxidant (pure oxygen or air) directly into electricity without the intermediate step of classical, chemical combustion used in the normal process of heat extraction from a fuel. It is a direct energy conversion device with high electrical efficiency. One of the most attractive features of these devices, apart from their high efficiency, is that only water, heat and electricity are the products of electrochemical reaction in the cell when pure hydrogen is used as the fuel. It is also a fuel flexible technology which can accept hydrogen obtained from various renewable and fossil energy resources (hydrogen rich gases like natural gas, biogas, and producer gas). This three-in-one (high efficiency, zero/low pollutant emission and fuel flexibility) feature makes the fuel cell an extremely desirable power generation technology with high potential for the future. A simplified working of PEM fuel cell is shown in Fig. 1. Since PEM fuel cell is a low temperature fuel cell, catalyst has to be used on the anode and the cathode side for the electrochemical reactions to happen.
Platinum is used as catalyst both at the anode as well as at the cathode. The electrolyte is a solid polymer membrane which is proton conductive. At the anode the hydrogen gets oxidized to protons releasing electrons. These protons move from anode to cathode through the proton conducting electrolyte and the electrons move through the external circuit to reach the cathode side. At the cathode side the oxygen gets reduced to [O.sup.-.sub.2] and then combines with the proton and the electron producing water on the cathode side of the cell. The anode side, cathode side and overall reaction of a PEM fuel cell are
Anode side: [H.sub.2] => 4[H.sup.+] + [4.sup.-.sub.e] Cathode side: [O.sub.2] + 4[H.sup.+] + [4.sup.-.sub.e] => 2[H.sub.2]O Net reaction: 2[H.sub.2] + [O.sub.2] => 2[H.sub.2]O
The polymer electrolyte membrane (PEM) fuel cell is a promising technology due to its high power density, low operating temperatures, low local emissions, quiet operation, and fast start-up and shutdown. St.Pierre et al  concluded that fuel cells tend to perform differently when arranged in stacks than compared to single cells. Knobbe et al.  reported that proper gas and water management are essential for achieving and maintaining high power output in a PEM fuel cell stack. Yu et al.  observed that the following set of gas feeding conditions (i.e., pressure, temperature & flow rate) and physical conditions (i.e., channel geometry, heat transfer coefficient, and operating conditions) affects the performance of a fuel cell stack. The dynamic behavior of fuel cell stack is of importance to ensure stable performance under varying operating conditions.
Chu et al.  reported that the stack tends to exhibit non uniformity in cell voltages across the stack. Rodatz et al.  reported that the reactant/product concentrations are not even in the flow channels. St.Pierre et al.,  reported that the degradation mechanisms associated with liquid water accumulation also affect stack durability. Zhu et al.  reported that the water and thermal management strategies required for stacks must accommodate the non-uniform distributions of potential, temperature and reactant/product concentrations, which are not necessarily observed at the single cell level. Dhathathreyan et al.  reported that stacks require a gas manifold to facilitate a uniform supply of reactants to all cells, a system for removal of product water and heat, and efficient electrical contacts between cells. Eckl et al.  reported that the parallel feeding of reactants and cooling medium can lead to uneven reactant flow, voltage, and temperature distributions across the stack. Urbani et al.  concluded that the increased electrical resistance due to serial cell connections can cause performance losses. Weng et al.  reported that proper flow distribution, cooling plate design, and end plate design are important for achieving high performance. In this work, the effects of various operating conditions and parameters on the performance of 100 watts air breathing PEM fuel cell stack is analyzed in detail.
II. Experimental Setup:
The air breathing PEM fuel cell stack shown in Fig. 2 is composed of eighteen cells connected in series and assembled using graphite flow field plates and stainless steel end plates. The active area of the cell is 49 [cm.sup.2] (7 cm x 7 cm). The anode plate has single serpentine flow field and cathode plate is ducted type to carry the hydrogen and air respectively.
High purity hydrogen (99.99%) and atmospheric air is used as fuel and oxidant respectively. The air is supplied to the cathode side through two 12V DC fan which can be powered from external source or by the stack itself. The fan consumes a power of 2 watts each. Nafion 117 membranes is sandwiched between the gas diffusion layers (GDL) and hot pressed to prepare the membrane electrode assembles (MEAs). The GDL is loaded with 0.5 mg of 40% Pt/C powder per [cm.sup.2]. Scribner 850 e Fuel Cell Test Station is used to determine the stack performance with respect to various operating conditions and parameters. The current scan technique has been employed for obtaining the performance curves.
RESULTS AND DISCUSSIONS
A. Effect of natural and forced convection:
Since this air breathing PEM fuel cell stack have their cathode structure open to the ambient air, the performance is strongly affected by mode of supply of oxidant to the cathode side. In this study, a pair of fan is used to supply the oxidant for reaction and as well as for cooling the stack. Fig. 3 shows the polarization and power density curves for the air breathing PEM fuel cell stack plotted with three different conditions i.e. (i) without fan (ii) with fan powered by the stack & (iii) with fan powered by an external source. The limiting current density of the stack without fan is very less (i.e. 25 mA [cm.sup.-2]) when compared to the stack with fan. The air in the ducts of the cathode side gets heated along with the stack and tends to move upwards due to buoyancy effect and hence fresh air comes in.
This circulation of air in the ducts of the stack without fan is very slow when compared to the air circulation in the stack with fans and results in insufficient oxidant supply Also temperature of the stack rises suddenly which leads to dehydration of the membrane and hence performance drop is observed in the stack without fan. The limiting current densities of the stack with fan powered by the stack and external power source are 160 mA [cm.sup.-2] and 210 mA [cm.sup.-2] respectively. The performance of the stack with fan powered from external source is better than with fan powered by the stack itself. The fan itself will act as an extra load when connected to the stack and due to higher load the stack gets heated faster than the stack with fan powered by the external source. With the fans connected to external power source, the circulation of air in the duct and cooling of the stack is better and hence it is with better performance than the stack without fan and with fan powered by the stack itself.
B. Effect of Forced and Induced Draught:
In this experiment, the polarisation and power density curve is plotted at two modes (i) fan at the top & (ii) fan at the bottom. When fans are placed at the top of the stack, the flow of air will be in induced draught mode and when placed at the bottom it will be in forced draught mode. The fans in both induced and forced draught mode are powered by the stack itself. Fig. 4 shows the polarization and power density curves for comparing the effect of forced and induced draught. The performance in both conditions are almost similar in activation and ohmic region but with slight difference in the mass transport region. The limiting current density of the stack in induced draught mode and forced draught mode is 152 mA [cm.sup.-2] and 160 mA [cm.sup.-2] respectively. A maximum power density of 80 mW [cm.sup.-2] is observed for the stack in forced draught mode and a maximum power density of 76 mW [cm.sup.-2] is observed in induced draught mode. The power density and limiting current density is improved by 5.3% in case of stack operation under forced draught mode comparing to induced draught mode. The flow of oxygen from the duct to the reaction site is by natural convection and the oxygen present in the air, circulating the duct will get consumed by the exothermic reaction. The oxygen deficient air in the duct will tend to move upwards due to buoyancy effect and with forced draught; the oxygen deficient air is forced out by the fresh air entering the cell. The system operates at positive pressure because outside air is drawn through the fan and forced into the system. Whereas with induced draught, the system is under negative pressure i.e. the pressure in the flow area is below atmospheric, because the air in the system is being drawn through the fan. As the circulation of air and heat removal from the system is better in forced draught mode (i.e. when fan is placed at the bottom), a little performance enhancement is found in the mass transport region.
C. Effect of Hydrogen Flow Rate:
Fig. 5 shows the polarization and power density curves of the air breathing PEM fuel cell stack at various hydrogen flow rates. The experiments are conducted at 2 lpm and 3 lpm of hydrogen flow rate. All the regions in this polarization curve are similar and no difference is observed with 2 lpm and 3 lpm. As the load applied is same for both the cases, the hydrogen required is also the same, if excess hydrogen is supplied it will be unused and left out.
D. Effect of Hydrogen Inlet Pressure:
Experiments are conducted by varying the hydrogen inlet pressure, since the stack used for the experimental study is of air breathing type, hydrogen inlet pressure will affect the performance of the stack. Fig. 6 shows the effect of hydrogen inlet pressure on the performance of the stack. This experiment is conducted at hydrogen inlet pressure of 2, 3 and 4 kgf/[cm.sup.2]. With 2 kgf/[cm.sup.2], the limiting current density is higher than 3 & 4 kgf/[cm.sup.2].
At higher inlet pressure the water molecules accumulated on the anode side due to back diffusion, are carried away by the gas flowing with high pressure and leads to membrane dehydration. At 2 kgf/[cm.sup.2], the water molecules are balanced and thus membrane is hydrated to show better performance.
E. Effect of Ambient Temperature and Humidity:
The performance of an air-breathing PEM fuel cell stack is strongly affected by the ambient temperature and humidity. Four set ambient temperature and relative humidity (RH) are maintained in a humidity chamber and experiments are conducted inside the humidity chamber. Fig. 7 shows the effect of ambient temperature and humidity of air. When comparing 20[degrees]C & 80% RH and 20[degrees]C & 40% RH, the former showed better performance in the concentration region and almost similar in other two regions.
Although the ambient temperatures in both the cases are same, the performance difference is due to the difference in ambient RH. As the RH increases, the rate of increase of cell temperature will be decreased and prevents membrane dehydration. When comparing 50[degrees]C & 80% RH and 50[degrees]C & 40% RH, though the ambient temperature is same in both the cases, the performance with higher RH is better because with 80 % RH, the rate of increase in cell temperature is less.
F. Effect of Stack Voltage at Various Constant Loads:
The stack is made to run for 3 hours continuously and the stack voltage is recorded with respect to time at constant loads of 3.5A, 4A, 4.5A, 5.5A and 7A. Since it is an air breathing PEM fuel cell stack, continuous operation with lower current density is limited by the liquid water formation and continuous operation with higher current density is affected by dehydration. Fig. 8 shows the variation in stack voltage with time at various constant loads.
At 3.5A constant load, the liquid water formation is very low and it does not affect the stack voltage for 3 hour continuous operation. However the trend is decreasing due to liquid water formation on the cathode side. With 4.0A constant load, the voltage drops near 100th minute due to dehydration. The stack when operated at 4.5 & 5.5A constant load, stack voltage drops at 60th & 40th minute of operation due to dehydration. At higher current densities, the heat generated is higher than at the lower current densities. With 7.0A constant load, the voltage of the stack drops at 20th minutes itself due to more heat generation leading to membrane dehydration.
G. Individual Cell Temperature:
Brief In a fuel cell stack, the performance of each cell will not be the same since it is affected by improper distribution of reactants, uneven heat dissipation, uneven liquid water removal etc., The 18 cell air breathing PEM fuel cell stack is tested for its individual cell temperature at a constant load of 7A and Fig. 9 shows the temperature of individual cells. The temperature of the 10th cell is found to be high and the temperatures of the cells at the ends are found to be lower. The cells in the middle of the stack gets heated quickly than the other cells because of poor heat dissipation. The cells at the ends dissipate the heat to the ambient and hence their temperatures are found to be low. Since the temperatures of the cells in the middle of the stack are higher than the other cells, the cells in the middle of the stack are not affected by flooding but affected by membrane dehydration.
H. Effect of Stack Orientation:
Since the air breathing PEM fuel cell stack is suitable for portable applications; the orientation of the stack should be studied. Fig. 10 shows the effect of vertical and horizontal orientation of the stack at peak load. The stack when oriented vertically performs better than with horizontal orientation.
Though the initial power density in both the cases are same, the vertical orientation stack operations last up to 24 minutes whereas the horizontal orientation stack last only up to 18 minutes. The air in the ducts of the cathode side gets heated along with the cell and due to buoyancy it tends to move upwards, with vertical orientation the consumed air i.e., oxygen deficient air move upwards and fresh oxygen rich air comes into the ducts, where as in horizontal orientation the oxygen deficient air cannot move upwards and circulates in the duct itself, which leads to insufficient oxygen for the electrochemical reaction. Also the by-product water vapor has to diffuse to the atmosphere which also tends to move upwards. In horizontal orientation the water vapor also cannot diffuse and get condensed in the cathode itself.
The performance of the stack with fan powered externally is better in performance than fan powered by the stack itself. The cooling fans placed at the bottom of the stack with forced draught gives the better performance. The hydrogen flow rate does not affect the performance of the stack significantly, since the required amount of gas is consumed by the reaction and excess gas is left out. The hydrogen inlet pressure should be slightly higher than the required amount. The optimum temperature and humidity for the operation of the air breathing PEM fuel cell stack should be with low temperature and high humidity. With continuous operation of the stack, at high current density the performance is affected by dehydration and at low current density over a period of time the performance will be affected by flooding. The cells at the middle of the stack are with high temperature and cooling plates can be used to dissipate the heat to the ambient. Vertical orientation is better for air breathing PEM fuel cell stack as the heated air in the duct tends to move upwards due to buoyancy and allows the fresh air inside the stack for electrochemical reaction.
The authors would like to thank the management of PSG College of Technology, Coimbatore, India for necessary facilities extended to carry out this work.
PEM : Proton Exchange Membrane GDL : Gas Diffusion Layer MEA : Membrane Electrode Assembly Pt/C : Platinum on Carbon DC : Direct Current V : Volt A : Ampere mA : milli Ampere mW : milli Watts RH : Relative Humidity lpm : Litre per minute
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(1) Shanmugasundaram Subramaniam, (2) Gukan Rajaram, (3) Manoj Kumar Panthalingal
(1) Research Scholar, Department of Mechanical Engineering, PSG College of Technology, Coimbatore-641 004, India.
(2) Associate Professor, Department of Mechanical Engineering, PSG College of Technology, Coimbatore-641 004, India.
(3) Associate Professor, Department of Mechanical Engineering, PSG Institute of Technology and Applied Research, Coimbatore-641 062, India.
Received 28 February 2017; Accepted 22 May 2017; Available online 6 June 2017
Address For Correspondence:
Shanmugasundaram Subramaniam, Department of Mechanical Engineering, PSG College of Technology, Coimbatore - 641 004, India,
E-mail: email@example.com; Mobile Number: +919750064158
Caption: Fig. 1: Schematic picture of the operating principle of PEM fuel cell
Caption: Fig. 2: 18 cell air breathing PEM fuel cell stack
Caption: Fig. 3: Effect of natural and forced convection
Caption: Fig. 4: Effect of forced and induced draught
Caption: Fig. 5: Effect of hydrogen flow rate
Caption: Fig. 6: Effect of hydrogen inlet pressure
Caption: Fig. 7: Effect of ambient temperature and humidity
Caption: Fig. 8: Effect of stack voltage at various constant loads
Caption: Fig. 9: Individual cell temperature
Caption: Fig. 10: Effect of stack orientation
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|Author:||Subramaniam, Shanmugasundaram; Rajaram, Gukan; Panthalingal, Manoj Kumar|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Jun 1, 2017|
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