Optimum Load Of a Proton Exchange Membrane Fuel Cells Based on Hydrogen Flow Control

In the future, proton exchange membrane fuel cells system (PEMFCs) hold promises for clean energy. However, there is a problem that causes the degradation in PEMFC system performance namely the voltage drops due to load fluctuations. The voltage drop is caused by the high load power demand. An important factor to improving PEMFCs performance is the availability of sufficient flow of hydrogen. In this paper optimisation of a PEMFCs load based on the hydrogen flow control is presented. In order to validate this project a model of the PEMFCs is simulated. Then verified by experimental testing using a 2 kW of the PEMFCs. The result shows the hydrogen flow control able to reduce the voltage drop of the PEMFCs during load variation and minimise hydrogen consumption.


Introduction
Technology The PEMFC system is a generator that uses chemical reaction between oxygen and hydrogen to generate electricity [1] [2] [3].Fuel cells is composed of cathode, anode and electrolyte between the anode and Cathode [4].The open cathode type of PEMFCs has shown high efficiency and performance as generator system with simple structure and low parasitic losses [5].Fuel cells need oxygen and hydrogen gases to create electricity [6].Hydrogen is fed into the fuel cell through the anode channels while oxygen flows through the cathode channels [7] [8].PEM fuel cells are a good source of energy for supplying stationary power, however extreme load current variations and load fluctuations cause voltage fluctuations and power problems [7].The availability of hydrogen flow is a very important factor to maintain fuel cell voltage stability during load fluctuations [9].However, the partial pressure of hydrogen must be maintained to avoid damaging the cell membrane [10].Unused hydrogen in the fuel cells system will be discharged along with the inert gas into the atmosphere, therefore this system 1265 (2023) 012011 IOP Publishing doi:10.1088/1755-1315/1265/1/012011 2 must consume as little fuel as possible so that a certain output power becomes efficient and minimizes hydrogen emissions [11].However, hydrogen starvation can also cause a voltage drop [12] [13].Therefore, it is very important to carry out a complete analysis of the voltage drop for fuel cells by experiment and simulation.
Several studies have been carried out to analysis of voltage drops in PEMFC which are caused by operating temperature and the gas pressure [14].In ref [15] an analysis has been carried out for the voltage drop caused by several parameters consisting of hydrogen flow, temperature and current density of platinum catalyst.In ref [16] an analysis of the gas starvation effects in PEMFC to the dynamic response of current and voltage.In ref [17] an analysis of voltage variation under gas starvation for single cell.However, there are very few studies analyzing effect of the load change to voltage drop.Studies on voltage drop due to hydrogen starvation have been carried out but did not analyze it with variations in the load and flow of hydrogen gas.
The focus of this research is to analyze the voltage drop due to load fluctuation with experiment and simulation.The simulation was developed using the Matlab simulink referring to mathematical model of the fuel cell.The fuel cell model was validated by several experiments conducted on an open cathode type of fuel cell with the maximum power of 2 kW.The experiment was carried out with no load connection and with load connection.A complete analysis of the voltage drop was studied using the load and flow parameters of hydrogen with the aim of determining the maximum load and efficiency of the fuel cell.The next section of this paper is organized as follows.In the section methodology, the mathematical model of a proton exchange membrane fuel cell is presented.In the simulation section, the dynamic model of the fuel cell in Simulink Matlab is presented and the parameters used in the simulation.In the experiment section, the fuel cell system setup and the parameters used for the experiment are presented.The Simulation results section presents experimental and simulation results with different hydrogen flow and current load parameters and their effects on voltage drop are discussed.The last section is the conclusion that presents the proposed work.

Methodology 1. Mathematical model of Fuel Cell
A fuel cell is an electrochemical device whose primary function is to convert chemical energy into electric power [14].The principle of the fuel cell process can be described in figure 1.
The fuel cell consists of the main components namely the anode and cathode which are separated by electrolytes, which serves to allow the migration of ions from one electrode to another.
The fuel cell produces an output voltage which is described by the following equation [30]: Figure 1 The Principle of The Fuel Cell rocess [14] Where Ncell is the number of cells.Enerst represents the thermodynamic potential and the reversible voltage of the cell, Vact is the voltage losses due to the activation of the anode and cathode.Vohm is the voltage losses due to internal resistance; Vconc is the voltage drop due to mass transport of the reacting gas [18].Where E 0 is the reference potential (1.229 V), R represents the universal gas constant (8.314J/mole K), T represents the operating temperature of the cell (333 K) and F denotes the Faraday's constant (96,485 C/mole) .Activation voltage losses can be computed by below equations [19].
Where I is current stack.C is voltage activation constant and B is current activation constant [20].The ohmic voltage drop is represented by Ohmic which is affected by contact resistance (Rc) and membrane resistance (RM) with the following mathematical equations [21].
Membrane resistance (RM) can be described with the mathematical equations [22][23] Where ρM (Ω ⋅ cm ) is the membrane specific resistivity can be expressed by suitable equation, another equation can be choose wheather l (cm) is the membrane thickness, A(cm2) is the membrane active area [22] [24] [25].To count a specific coefficient for membrane [26] and Hydrogen valve molar constant (KH2) is calculated by using the following mathematical equation [28].Furthermore, qH2 as hydrogen molar flow and PH2 as hydrogen partial pressure, where MH2 represents the hydrogen molar mass and kan represents the anode valve constant.The reacted hydrogen flow is calculated by the sutable equation [29].

Simulation model of fuel cell
The fuel cell model is very important because there is currently no suitable experimental method for observing the voltage drop caused by the hydrogen starvation phenomenon.The fuel cell model was developed using MATLAB simulink based on mathematical equations.The parameters used in the simulation are shown in table 1.

.3 Experimental system and setup
The PEMFC used in this experiment uses an open cathode type with dimensions of cell 34 cm x 7 cm .The cell thickness of 4 mm with the active area cell is 5.5 cm x 29 cm.The test conditions at operating temperature were maintained at 310C.
In this experiment there are 3 inputs of hydrogen flow rate consisting of 3 L/min, 6 L/min and 9 L/min.For the first step, the current load is increased gradually from 1 A to 5 A for each hydrogen flow rate.The purge period for hydrogen gas in the stack is 60 seconds and the purge duration is 1 second.The specific activation procedure is as follows: First, the anode is supplied with dry hydrogen and the cathode is supplied with air using 3 fans.Then the fuel cell is continuously operated without load connection for 30 minutes to make the membrane get good hydration.Each load is applied for 1 second and the average voltage measured over 20 seconds is used to plot the curve.

Results and Discussion
The experimental result with hydrogen flow constant 3 liters per minute ( L/min ) without load current for the first 1 second shows the fuel cell output voltage of 67.501 volts and reaches the maximum voltage of 67.52 volts.The voltage rises to its maximum limit because hydrogen gas has been distributed to all cells.For hydrogen gas flow 9 L/min the fuel cell output voltage is 67.544 volts and reaches the maximum voltage limit at 67.564 volts.The simulation results show that the maximum current load at 3 L/min hydrogen flow is 8 A with a maximum power of 225.68 watts.Different from the experimental results which show maximum power of 211 watts.The difference between the simulation and experimental results for the open cathode fuel cell type is caused by the oxygen supply being taken from the environment using fan so that the oxygen purity is lower and the air pressure decreases depending on the fuel cell insulation.Proper supply of hydrogen flow is also required to produce maximum power.
To validate the experimental results, a simulation model using Matlab Simulink was developed based on the mathematical equations of the fuel cell.This model is simulated to obtain a curve of the relationship between voltage and current, and power in the fuel cell.The results of this simulation are compared with experimental data with the aim of obtaining model accuracy values.
L/min with a peak power of 264 watts.The simulation results show the maximum current load of 8 A with a maximum power of 226 watts.
The experimental results obtained a maximum load of 8 A with a peak power of 233 watts and for simulation results show a maximum load of 8 A with a peak power of 226 watts.
Where R is the percentage of relative error between simulation and experimental data.Vs is the output cell voltage of simulation, Ve is the output cell voltage of experiment.The output performance of the fuel cell by considering the effect of load changes and error analysis with the best results at the input hydrogen flow rate of 9 L/min.The maximum relative error is 3.9%.Based on the experimental results, the maximum current load is 9 A at the input hydrogen flow of 6 In the third experiment and simulation by providing an input hydrogen flow rate of 9 L/min.Experimental and simulation results are shown in Figure 10.
Figure 9 is the relationship curve between the load current and the fuel cell power from the simulation and experiment results at 6 L/min hydrogen flow rate.

Conclusion
A fuel cell with a power rate of 2 kW can be used according to load requirements with hydrogen flow rate control.In this study, an analysis was carried out to determine the appropriate load by providing various current loads with 3 different input hydrogen flows of 3 L/min, 6 L/min and 9 L/min.Experiments were carried out by increasing the load from 1 A to 10 A. The best results based on the verification of simulation data and experimental data were at 9 L/minute of hydrogen flow with the maximum load current of 8 A and the peak power of 233 watts.The average voltage drop is 3.7 volts for every 1 A load increase.

Figure 3 .
Figure 3. Experimental result for 3 L/min hydrogen flow rate without load current

Figure 4 .
Figure 4. Experimental result for 6 L/min hydrogen flow rate without load current.

Figure 5 .
Figure 5. Experimental result for 9 L/min hydrogen flow rate without load current

Figure 6 .
Figure 6.Fuel cell output voltage for various hydrogen flow without load connection

Figure 7 .
Figure 7. Experimental result for 3 L/min hydrogen flow rate with current load at 1.996 A

Figure 8 .
Figure 8. Experimental result for variation of cell performance at different hydrogen flow rates

Figure 9 .
Figure 9.The relationship between current and output power with hydrogen flow input 6 L/min .

Figure 10 .
Figure 10.The relationship between current and output power with hydrogen flow input 9 L/min

Table 1 .
Parameters Used In The Simulation

Table 2 .
Fuel Cell Parameters Used In The Experiment