Fuel cells (FCs) use hydrogen as their prime fuel source, which promotes them as one of the attractive options for clean energy generators. Though they have been around for some time, their characteristics are not yet fully understood. This study offers a thorough investigation into the characteristics of proton exchange membrane (PEM) type of FCs based power system. This study first presents a concise explanation of the working principles of the PEM electrolyser and FCs supported by novel modelling using MATLAB. The simulation results are then validated by a series of experiments carried out on operational 500 mW FC followed by detailed performance parameters of such type of FCs. Parameters affect the efficiencies of each part of the system are investigated and the total system's efficiency is then calculated. The efficiency of the electrolyser and PEM FC was found to be 85 and 60%, respectively. Polarisation curve has been used in order to evaluate FC's performance. From the polarisation curve, it is noted the efficiency of the FC increases with increasing pressure and temperature. The activation losses are reduced when the temperature increased. Moreover, the mass transfer is enhanced toward reducing the PEMFC's resistance.
1 Introduction
The rising of fossil fuel prices and the growing concerns about the greenhouse gas emissions prompt scientists and researchers to search for other sources of energy that are sustainable with low-cost, high-efficient energy conversion and minimal environmental impact. Hydrogen and bio-fuels can be used as an alternative clean energy source in the near future. Fuel cells (FCs) have promising features as they have high efficiency and low emissions [1]. There are numerous types of FCs that operate in a similar way, but they differ in operating conditions and the type of their materials. Proton exchange membrane FCs (PEMFCs), among the different types of FCs, have attracted the research interest especially for automobile applications. They have high-power density at low operating temperatures (<100°C), quick start-up and zero emissions. However, PEMFC systems still present certain drawbacks and improvements must be made in order to enhance their reliability for industrial applications. Cost reduction, durability, performance evaluation and reliability thus remain strong research issues. Mathematical modelling and simulation are needed to examine the behaviour and characteristics of FCs. These techniques can help in predicting and improving FCs’ performance and reliability. There are several studies that have focused on the study and modelling of FC, among these studies, Hanna et al. [2], which presented the fundamental electrochemical and it also demonstrated the underlying electrochemical and transport mechanisms essential for PEMFC modelling. Chanpeng and Yottana [3] developed a MATLAB–Simulink module to study the operation performance of 1.2 kW PEMFC. Tian [4] proposed PEMFC mechanism modelling based on artificial intelligent to investigate PEMFC mechanism and faults. Tanrioven and Alam [5] presented a methodology for modelling and determining PEMFC's characteristics to enhance its reliability.
This paper offers comprehensive study on the modelling and Simulink the operation mechanism of PEMFCs in order to investigate the reliability and performance of PEMFC under different operating conditions. Mathematical equations and modelling are used to define operating conditions and the performance of PEMFC.
A specific aim of this research is to develop realistic model for investigating the performance of a PEMFC under different operation variables using semi-empirical equations. Subsequently validate the derived model through a series of experiments using purposely designed test setup.
2 FC system
The basic structure of a PEMFC system as depicted in Fig. 1 is composed of the following parts:
(i) Electrolyser: Device that uses to separate the distilled water to hydrogen and oxygen which are used as fuel of FC.
(ii) FC: The main part of this system. FC is an electrochemical device which uses the chemical energy to produce electrical energy.
(iii) DC/DC converter: Device that works to control the output voltage (buck or boost converter).
(iv) DC/AC inverter: The main task of this device is to convert the output voltage from DC signal into AC signal.
Fig. 1 Schematic diagram of FC power system
Hydrogen is one of the most promising alternative sources of energy for the future due to it has the capability of storing energy at high quality. Thus, it has been presented to become the cornerstone of future energy systems associated with other renewable energy sources. Some of researchers have been studied the uses of hydrogen as an energy carrier in storage and transport of energy.
Electrolyser is electrochemical device that uses electricity to decompose water into hydrogen and oxygen. When the voltage across the two electrodes (anode and cathode) exceeds the decomposition voltage of water (which, theoretically, is 1.23 V), pure water will be decomposed into hydrogen and oxygen. Electrolyser's structure and chemical reactions at its anode and cathode are described by Fig. 2 and (1), respectively
(1)
Electrical energy is needed to achieve the previous equation. In addition to the electrolyser device, hydrogen may extract from natural gas as depicted in Fig. 3.
Fig. 2 Functionality of the electrolyser
Fig. 3 Hydrogen energy cycle
There are many methods used for hydrogen energy conversion into electrical energy. FCs are one of those methods and, relatively, they have a number of advantages such as high conversion efficiency.
3 FCs operation principles
Different types of fuels cells and their applications are outlined in Table 1. In this section, operation mechanism of PEMFC will be investigated in order to illustrate its operation principles. As shown in Fig. 4, operation depends on a membrane separator (the electrolyte) and the catalyst which are usually made of platinum powder, carbon paper or cloth with a very thin layer in the range of microns [6, 7].
Fig. 4 Functionality of the PEMFC
Table 1 Types of FCs and their characteristics and usages
FC type
Common electrolyte
Operating temperature
Typical stack size
Efficiency
Applications
PEM
perfluoro sulphonic acid
50–100°C
<1–100 kW
60% transportation
•
backup power
•
portable power
•
distributed generation
•
highway transportation
•
specialty vehicles
typically 80°C
35% stationary
alkaline (AFC)
aqueous solution of potassium hydroxide soaked in a matrix
25–75°C
10–100 kW
60%
•
military
•
space
•
supermarkets
•
hospitals
•
hotels
phosphoric acid (PAFC)
phosphoric acid soaked in a matrix
150–200°C
400 kW
40%
•
distributed generation
100 kW module
molten carbonate (MCFC)
solution of lithium, sodium and/or potassium carbonates, soaked in a matrix
600–700°C
300 kW–3 MW
50%
•
electric utility
•
distributed generation
300 kW module
solid oxide (SOFC)
yttria stabilised zirconia
700–1000°C
1 kW–2 MW
60%
•
auxiliary power
•
electric utility
•
distributed generation
The side of the plate coated by the platinum is placed next to PEM. On entering the hydrogen H2 to the cell, the platinum works on separating it into proton and an electron. The membrane separator allows protons to pass and does not allow electrons. Therefore, electrons have to pass only through the current collectors, and this produces DC electrical current. In the opposite side of the membrane (i.e. at the cathode side), the electron binds with the proton in the presence of a catalyst again and with the presence of oxygen, the water H2O is produced and heat spreads.
4 PEMFC proton exchange membrane
4.1 PEM fuel cells
PEMFCs were invented in 1960s by Niedrach and Grubb invented in General Electric Company. PEMFC is the most commonly used FC type; it is used in various fields due to its small size and its low-temperature operation. PEMFC uses a polymer as membrane (Nafion), which plays the role of mediator in the electrochemical cell, in a solid state and thus it reduces the reaction temperature and increases the efficiency (speed of interaction in the start-up and responding when loading). The polymer separation membrane positioned between two electrodes of perforated platinum and it causes no risk of pollution due to the solid nature. Interaction takes place under a temperature of 80°C, and when the membrane exposed to water it becomes conductive material for ions [8, 9]. The electrodes are made of platinum. The efficiency of such cells is up to 45–50% and their power density is high compared with other types of cells where up to 350–600 mW/cm2 and the reactions are defined in (2)
(2a)
(2b)
(2c)
Another type of polymeric cells runs on methane gas directly, and it differs from the first type in terms of the electrode's materials. These cells have a device for the preparation of fuel, where it works on the saturation of methane with hydrogen. The problem in this type is the crossing of methane from the membrane and there is currently a lot of research to address this issue.
5 Polarisation phenomenon
When the FC is loaded, the voltage goes down by 60 or 70% of the open-circuit voltage due to polarisation as shown in Fig. 5. Both the output voltage and current density determine the characteristic of V–I curve. The output voltage of PEMFC is closely related with thermodynamically predicted in FCs and three major losses that are activation losses, ohmic losses and concentration losses. Activation losses occur due to electrochemical reaction. Ohmic losses occur due to ionic electronic condition. Concentration losses are produced due to mass transport [1, 10].
Fig. 5 Schematic diagram of the FC through the production cycle and three losses
5.1 Activation polarisation ηact
When the cell is loaded, the voltage drops suddenly by (ηact > 50–100 mv) and then remains constant, because of the energy needed to trigger the interaction between gases and oxygen in particular. This drop is linked to the amount of catalysts and density of hydrogen and oxygen on the surface of the catalyst. The polarisation equation is given by the following expressions
(3)
(4)
where i is the density of current and C*O2 is the density of the hydrogen on the catalyst.
5.2 Ohmic polarisation
The ohmic polarisation (η Ohm) is positively proportional to the current. Since the cell's resistance almost constant, the ohmic polarisation changes linearly and this is because of the emergence of resistance while crossing of ions in the electrolyte and ohmic resistance electrodes [11]. The reduction of these resistances by using the appropriate electrolyte and metals in the electrodes can be adopted to overcome this problem, and the equation can be written as follows
(5)
where i represents the current of the cell and Rinternal is the internal resistance of the cell, and is given in the datasheet of the cell or calculated from the following relationship
(6)
5.3 Concentration polarisation
This polarisation occurs at high-current densities due to lack of gases required for the reaction at the electrodes and that causes drop in the cell's voltage, which is calculated from the following relationship
(7)
where β is the cell's constant and (I) is the current density of the cell, Imax is the maximum current density and its value between 1 and 1.5 cm2 and the equation of the final voltage of the cell becomes
(8)
(9)
5.4 Heat and temperature management
When modelling or designing FCs stake, the heat management must be taken into account to ensure that the FC will work at the desired temperature. The management of heat is significantly related to FC's performance. The amount of waste heat in FC equivalent almost its electric power produced leads to reduce the efficiency around 50%.
5.5 Output power of the FC
The capacity of the FC can be obtained from the following equation where I is the current density per centimetre squared in the pole of the cell. A is the electrode surface and V is the voltage of the cell and these values are given in the datasheet of the cell
(10)
6 FC's modelling
To improve the system's performance, design optimisation and analysis of FC systems are important. Modelling and simulation are needed as tools for design optimisation of FCs, stacks and FCs power systems. The performance of FC during operation depending on the final (9), obtained by the integration of (5)–(8). The advantage of the final equation is that it is less influenced by the determinants of mechanical and physical components of the cell and this equation relies on constants, temperature, operating and pressure of the gases involved and the current density and voltage FC. The final equation has been represented in the environment MATLAB to show the electrical performance of the cell and the impact of gases’ pressure and temperature on the cell.
6.1 Voltage/power relationship curve
Fig. 6 shows the relationship between the output voltage of the cell, power density with current density under the operation conditions given in the cell's datasheet such as temperature and pressure (346 K and 3 bar), and the results are similar to the curves in the scientific literature for FCs. This curve is the true measure of the performance of the cell and the conclusion of voltage and current work and the maximum power that can be produced without damaging the cell during the investigation of the effect of pressure and temperature, which will be addressed in the subsequent paragraphs in detail.
Fig. 6 Relationship between voltage, power density and current density
6.2 Power density curve
Fig. 7 shows the power density against the current density. When the power density increased, the current density rises and then the power density goes down because of the increasing polarisation affecting the cell's voltage. Usually the highest point of the curve is not the point of the operation for the cell (i.e. the cell cannot operate at the maximum power because of that cell outcome becomes low). At this point, the water increase and the temperature rises making it difficult for the driving/controlling the cell and shorten its useful life [11, 12].
Fig. 7 Relationship between power and current density
6.3 Effects of pressure and temperature on the polarisation curve and power density
Fig. 8 shows the polarisation curve of the FC for two cases of operation. The first case of works under the conditions of the cell such as temperature and pressure (353 K and 3 bar). The second case of operation is within the statutory requirement (323 K and 1.5 bar), which shows the decline in output cell. Polymeric FCs typically operate at a temperature of 70–90°C and atmospheric pressure (1.5–3.5 bar) as the cell then gives its highest outcome. Therefore, Fig. 8 shows at low temperature and pressure, the voltage declines 0.1 V, and it causes the decline in the outcome by 10% of its nominal output.
Fig. 8 Relationship between current and power density with voltage under different operation conditions
Fig. 9 Losses in FC (activation losses, ohmic losses and concentration losses)
6.4 Polarisation curve of the cell
Fig. 9 shows the voltage of the open-circuit cell E and stages of falling voltage in the FC during operation. Owing to decline resulting from the polarisation of the three (activation ηact and ohmic ηohm and focus ηcon), cell's voltage at start-up lands in sudden, and due to the activation of ηact and then declines until at least this almost disappears. The second decline in the voltage is caused by the internal resistance of the cell ηohm because of the resistance of the ions while crossing in the electrolyte and resistance ohmic electrodes and rise linearly with increasing load current. Third decline in voltage ηcon happens at high-current densities and because of the non-arrival of reaction gases to the electrodes sufficiently, as shown in Fig. 11, the activation losses and the ohmic decline at low-current density is the dominant and most obvious [13–15].
Fig. 10 Influence of gases pressure on performance of FC
Fig. 11 Influence of temperature on performance of FC
Note: in Fig. 12, letter E is used instead of η to indicate polarisation.
Fig. 12 Relationship between power density and voltage
6.5 Impact of pressure on FC voltage
Fig. 10 shows the improvement of the performance of the polymeric FC when the pressure increases. However, this increase must be within the allowable pressure in the datasheet of the cell because the increased pressure on the nominal value of 3 bar will improve performance slightly, where the values of the cell converge when voltage increased pressure more, and this is illustrated by the convergence of the lines in Fig. 10, but this increase leads to higher temperatures and values of the three polarisations where its negative impact will be more positive and this causes the decline in outcome and increase the cost of production.
6.6 Impact of temperature on FC voltage
Fig. 11 shows that the increase in the temperature improves the performance of the cell, causing a decline of the voltage which leads to the reduction of voltage dropping factors and in particular the decline in the activation and increase in the overall outcome of the cell, but under the conditions of operation, in order to prevent the loss of moisture needed for the cell membrane. Usually the temperature of the polymeric FC does not exceed 95°C.
6.7 Operating voltage for FC
Fig. 12 shows the curve of power density against FC voltage. It is possible to obtain higher power at voltage of 0.6–0.8 V and this voltage is the operation voltage of an FC, and as noted earlier, it is important to find a balance between maximum power produced by the cell and the determinants such as operating pressure and heat allowed for the cell in order to not to adversely affect the cell's life and performance over time.
Fig. 13 Polarisation curve obtained from real data
7 Experimental apparatus
To determine the voltage current characteristic of PEM and to validated the modelling results, several experiments have been made in Manchester Metropolitan University's laboratories using PEMFC system. Table 2 listed the values that have been collected form 500 mw FC unit for various load (resistance) values setup. Figs. 13 and 14 show the polarisation curve and output power versus current density, respectively, for real data gathered from experiments.
Fig. 14 Output power against current density
Fig. 15 V–I curve for simulation and experiment results
Table 2 Data collected form 500 mW PEMFC unite
Load, Ω
Voltage, V
Current, A
Power, mW
0
0.24
1.61
386.4
0.1
0.32
1.43
457.6
0.33
0.47
1.05
493.5
1
0.62
0.56
347.2
3.3
0.76
0.22
167.2
10
0.84
0.08
67.2
33
0.90
0.03
27
100
0.94
0.01
9.4
330
0.97
0.01
9.7
∞
1.1
0
0
8 Conclusions
This paper presented a realistic PEMFC dynamic model to allow better understanding of its operating parameters and effects. The impact of gases’ heat and pressure interaction and current density on the output voltage the cell and power density have been analysed to determine the overall efficiency and performance of the PEMFC. Mathematical and simulation model has been developed in order to study the behaviour of the PEMFC under various operations, where it is possible to exploit this model to analyse in detail the impact of the changes on cell behaviour, such as temperature, pressure and load change. Those equations were modelled using MATLAB environment and obtained curves describe the impact of each case. System's curves offer deeper understanding of FCs and enable mitigating factors that degrade PEMFCs performance and reliability. A set of laboratory experiments have been conducted to validate the model results. It is believed the work paves the path toward the implementation of powerful PEMFCs monitoring and control system.
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