MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel

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1 Journal of Energy and Power Engineering 10 (2016) doi: / / D DAVD PUBLSHNG MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel Mohamed Albarghot 1, Mahmud Sasi 2 and Luc Rolland 1 1. Department of Mechanical Engineering, Memorial University of Newfoundland, St. John s, NL, A1B3X5, Canada 2. Electrical and Computer Engineering, Memorial University of Newfoundland, St. John s, NL, A1B3X5, Canada Received: August 15, 2016 / Accepted: August 29, 2016 / Published: December 31, Abstract: n this paper, the solar panels are used to power an electrolyzer to separate the water into hydrogen and oxygen gas. The electrical equivalent circuit for the proton exchange membrane electrolyzer was developed and implemented in MATLAB/Simulink along with the atmospheric hydrogen storage tank. The voltage (2 volt) and current (1 ampere) were supplied in a similar manner in order to compare the simulated and experimental results. The hydrogen amount is calculated to be (ml/min A) from the model as well as the experimental set-up. The experimental and simulation results were matched. Key words: Photovoltaic, DC/DC buck convertor, electrolyzer, hydrogen tank, MATLAB/Simulink. 1. ntroduction World transport depends heavily on petroleum, as it supplies 95% of total energy and is responsible for almost a quarter of global energy-related emissions. Marine ships around the world that run on fossil fuels are also causing many environmental issues. Over the past decade, transport emissions have increased at a faster rate than those of any other energy sector. The transportation sector accounts for 28% of all US greenhouse gas emissions, 34% of all carbon dioxide emissions, 36-78% of the main components of urban air pollution, and 68% of all oil consumption [1]. Global transport activity will continue to increase along with economic growth. nternational freight has been dominated by ocean shipping, with ships continually increasing in size and number. Fossil fuel usage also raises many important concerns and challenges such as climate change and supply cost increases; for example, in 2002, the use of fossil fuels was responsible for 86% of the world s energy Corresponding author: Mohamed Albarghot, Ph.D. candidate, research field: renewable energy. consumption. n 2003, US electrical energy demands were also reaching a higher value of 24% of total demand around the globe [2]. t has become essential to seek alternative sources of renewable energy that can be easily captured by using waves, sun and wind. Statistics suggest that changing to fuel cell technology could save more than one million per ship per year in fuel costs. The sun, wind and waves provide an unlimited source of renewable energy; solar energy is known as the most sustainable source of the renewable energy. For example, 27.7 GW of PV (photovoltaic) systems were installed worldwide in Compared with 2010, there were also six countries where more than 1 GW of PV has been installed in 2011 [3]. Batteries are not the solution for energy storage since they provide for short term solutions and waist contents comprise significant and dangerous pollutants. Hydrogen remains the only valid source for energy storage. Producing hydrogen can be achieved in large quantities from water electrolysis knowing that water is a clean resource available in large quantities everywhere. Electrolysis will be studied to be brought inboard vehicles such as cargo ships.

2 780 MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel This paper is aimed to improve the results of modeling the electrolysis system and obtaining experimental results. The set-up is powering the electrolyzer with a small PV solar panel that produces 2 V and 1 A. The hydrogen was produced and stocked into the tank. The mathematical model of the electrolysis has been implemented in MATLAB/Simulink. n Section 2, the system component models are detailed. n Section 3, the simulation and experimental set-ups are described. n Section 4, we examine and compare the simulation and experimental results. 2. System Components 2.1 The Photovoltaic Solar Panel The solar energy is transferred directly into electrical energy in the PV panel through a basic physical process. The physical behavior of any solar cell is very similar to the classical p-n junction diode [4]. The relationship between the output current and the output voltage V is found the formula below: where, V* RS / N* Vt ph s * e 1 s2 V* RS / N2* Vt e 1 ( V * R / R. s P * (1) ph is the solar induced current that is equal, where r is the irradiance (light to Ph0 * r / R0 temperature plays important roles because the four parameters ( r, s, R s, and V t ) are function of temperature. t is clearly seen that the lower temperature is the higher power which is obtained from the PV [5]. 2.2 The DC/DC Buck Convertor The buck converter is defined as step-down DC-DC voltage converter. The average output voltage is always less than the input voltage. There are two modes for the buck converter in terms of diode circuits. n the first mode, when the switch is on, the diode becomes reverse biased, so that the supplied energy is stored into an inductor. n the second mode, the diode becomes forward biased when the switch is off, due to the load; it receives the energy from the inductor. The input stays isolated from the output [6]. Fig. 1 Electrical equivalent circuit of the solar cell. intensity ) in W/m 2 falling on the cell. Ph0 is the measured solar generated current for the irradiance R0. s is the saturation current of the first diode. s2 is the saturation current of the second diode. V t is thermal voltage that is equal to KT/q, where K is the Boltzmann constant. T is the device simulation temperature parameter value. Q is the elementary charge on an electron. N and N 2 are the quality factor of the first and second diodes, respectively. V is the voltage across the solar cell [4]. Fig. 1 represents the electrical equivalent circuit of the solar cell [4]. The -V and P-V characteristics curves of the PV module are used in this work under irradiance of 1,000 W/m 2 at 25 C as it is illustrated in Fig. 2. n solar energy, Fig. 2 -V and P-V curves of the PV module.

3 MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel 781 The purpose of buck convertor is to regulate DC power supplies and to control DC motor speed. Duty cycle (D) is known as the ratio of the ON time of the switch to the total switching time, and also the ratio of output voltage to the input voltage. The duty cycle is also given by the following: V V o s D (2) s where, V o and V s are output and input voltages, respectively. s and o are input and output current, respectively [6]. The selection parameter of the buck convertor is based on the voltage and current output of the PV. n this work, the PV output is designed to be 5 V and 2 A. Since the electrolyzer load is designed for 2 V and 1 A, it becomes necessary to select a buck convertor in order to regulate the voltage and current. The values of buck convertor parameter such as the inductor, capacitor, and duty cycle were obtained from the website for desired input values for the electrolyzer. Table 1 shows the values of the selected buck convertor. The buck convertor model is generated in MATLAB/Simulink in order to control the output and input between the PV and electrolyzer as shown in Fig. 3. The PD controller is used to check the error value as the difference between the desired set point and the measured variable voltage. o 2.3 Electrolyzer An electrolyzer is defined as an apparatus that separates the water (2H 2 O) into hydrogen (2H 2 ) and oxygen (O 2 ). Water electrolysis may be classified as a reverse process of hydrogen that is fed into a fuel cell. n terms of an electro-chemical reaction happening in the fuel cell to generate DC electricity, it converts DC electrical energy into chemical energy stored in hydrogen. An electrolyzer electrical circuit can be represented as a voltage sensitive nonlinear DC load, so that the higher voltage applied is the higher load current that is circulating and the more H 2 can be generated [7]. There are two types of electrolysis, Alkaline and PEM (Proton exchange membrane), the PEM cells are known to be reversible devices for hydrogen systems when compared to alkaline-based electrolysis. They also have many advantages like Table 1 The values of buck convertor. Tem Value Units Volts in 5 V Volts out 2 V Load current 1 A rms 0.99 A Duty cycle % Frequency 40 KHz L 87e-6 H C 29.17e-6 F Fig. 3 The buck convertor model in MATLAB/Simulink.

4 782 MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel Fig. 4 Equivalent circuit for single PEM electrolyzer. smaller dimension and mass, lower power consumption, lower operating temperatures [7]. The equivalent circuit of a proton exchange membrane electrolyzer is created in MATLAB/Simulink in order to run the simulation. Fig. 4 shows the equivalent circuit of the single PEM electrolyzer [8, 9]. n order to obtain the -V and hydrogen production characteristics, some equations have been developed for steady state conditions and implemented in MATLAB/Simulink. Eq. (3) models the electrolysis process and it is written as follows: 1 V * R e i rev (3) Eq. (3) indicates a simple equivalent circuit model for the PEM, which has an initial resistance of R i, and reverse potential e rev. The ideal potential V i (electro chemical) is calculated by Eq. (4) [8]: G V i (4) 2F where, ΔG is Gibbs free energy change (J/mol) of hydrogen gas and F is Faraday constant (96,487 C/mol). f the water is in liquid phase, then ΔG for any given temperature T ( C) can be calculated by Eq. (5): G T (5) The value of V i is calculated under nominal operating conditions at room temperature of 20 C and 1 atm pressure. t amounts to V, which is useful for electrolysis and hydrogen production. t is also associated with electro chemistry, so that the one molar volume V m is known from the ideal gas expression in Eq. (6) [8]: R( 273 T ) V m (6) P where, R and P are the ideal gas constant (0.082 l atm k -1 mol -1 ) and pressure, respectively. The hydrogen production rate which is V H (ml/min) with respect to the input current (A) is determined by Eq. (7) [8]: c 3 10 ml 60s s VH Vm ( ) l min 2F( C) 3 Vm (10 )(60) 2F (7) The electro chemical hydrogen energy per second P H2 which is equal to the V H is calculated by Eq. (8) [8]: 3 2FV P V V (8) 2 (10 ) 60 i H 2 m 3 i FVm From the above equations, it can be clearly seen that the useful power which is delivered from the electrolyzer cell relies on the electrolyzer input current and ideal voltage V i. The input electrical power P of PEM electrolyzer cell, which is the function of the V H, can be determined by: 2 P V Ri erev 2 2F 2F Vm R 3 i VH e (9) 3 rev VH10 (60) Vm10 (60) n this work, the reversible potential e rev with respect to the ideal voltage V i is calculated to be V, and the resistance (R i ) of PEM subsystem also equals to Ohm at temperature 20 C and 1 atm. These values are well confirmed by the

5 MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel 783 MATLAB/Simulink module. The input current (A) of the electrolyzer is modeled by Eq. (3), and Eq. (7) is applied for hydrogen production rates V H. For steady state operation, Eq. (10) is used to simplify and determine the input of -V model of the PEM electrolyzer cell as function of pressure and temperature [8]. Fig. 5 illustrates the static model for the PEM electrolyzer in order to compare the experimental results [9]. V T, P R e T P (10) i rev, 2.4 Hydrogen Tank Modeling Compressed hydrogen gas or liquid hydrogen can be stored in tanks by using different techniques like physical hydrogen storage [10]. n order to store the hydrogen gas that is produced by the electrolyzer, a dynamic module for the tank is created in MATLAB/Simulink, and can be expressed as follows, Eq. (11) [10]: P N RT H 2 b b Pbi z (11) M H 2Vb where, P b is the pressure of the tank that is measured in (Pascal), P is the initial pressure of the bi storage tankin (Pascal), R is the universal gas constant (J/kmol K), T is the operating temperature (K), V b is the volume of the tank (m 3 ), T is the temperature, and Z is the compressibility factor as a function of the pressure as shown in Eq. (12): b 3. Simulation & Experimental Set-Up The simulation is done in MATLAB/Simulink environment by creating each component separately, so that the error can be easily controlled and the simulation blocks debugged. Each system block has been implemented and studied to ensure that each one is sufficiently precise to run the simulation and to give adequate results. The modules for PV solar panels, DC/DC buck convertor, electrolyzer, and the hydrogen tank have been created and well matched with each other. The simulation is valid to model different cases. Fig. 7 shows the Simulink model for the whole renewable energy system. Fig. 8 represents Fig. 5 The MATLAB/simulink module for PEM electrolyzer. Z PVm (12) RT where, P and V m are the pressure and molar volume, respectively. t has been noticed that this model determines the tank pressure using the ratio of hydrogen flow rate to the tank [10]. Eq. (11) is implemented in Simulink in order to get the hydrogen stored and delivered to the fuel cell or other applications. Fig. 6 shows the Simulink model of the hydrogen storage system [10]. Fig. 6 Simulink model of the hydrogen storage system.

6 784 MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel Fig. 7 Simulink model for the whole renewable energy system. current (A), and the simulation results are in accordance with the experimental results. Note that the characteristic response is linear [8]. The amount of hydrogen that is obtained from the simulation is ml/min, and the value from the experiments amounts to 7.0 ml/min. The discrepancy is due to the sun variability. Fig. 10 illustrates the input power with the hydrogen production (ml/min). t can be noticed that the hydrogen production increases linearly with the input power; thus the power increases the hydrogen production will increase as well. Table 2 shows the hydrogen production (ml/min) from the experiments, and the time was 2 min. Fig. 11 represents the pressure inside the hydrogen tank model. The hydrogen quantity increased with time. Fig. 8 Experimental set-up for renewable system. the experimental set-up for the solar panel, DC/DC buck convertor, electrolyzer and hydrogen/oxygen tanks. The experiments are produced with a horizon kit. The reading of hydrogen production was reported in (ml/min). Each component configuration was collected from the data sheets [11]. 4. Results and Discussion Fig. 9 Current (A) vs. hydrogen production (ml/min). The results from both simulation and experiments are now compared. The maximum voltage from the PV in experimental set-up is designed to be 2 V and the current is reaching 1 A, however the maximum power of the PV panel is depending on the weather such as cloud and rain can reduce the collected energy. n this work, the maximum voltage and current from the PV panel were obtained and the electrolyzer was able to generate hydrogen with maximum output. Fig. 9 shows the hydrogen production (ml/min) versus the Hydrogen (ml/min) Fig. 10 (ml/min). nput power (W) nput power (W) vs. Hydrogen production

7 MATLAB/Simulink Modeling and Experimental Results of a PEM Electrolyzer Powered by a Solar Panel 785 Table 2 Experimental results. Time (min) H 2 production (ml) Hydrogen pressure Time Fig. 11 The pressure inside the model hydrogen tank. 5. Conclusions Hydrogen pressure vs. time n this work, the renewable energy hydrogen production and storage system has been developed. The PV solar panels were examined in order to capture the sun as long as it is available. The electrolyzer is used to consume the power that is generated from PV panels. The DC/DC buck convertor is used along with the system to regulate and maintain the current values which are fed to the electrolyzer. The PD controller is used to check the error value as the difference between the desired set point and the measured variable voltage. t is assumed that all system components to be steady state and the nonlinearity behavior kept for further studies. The results from both the simulation and experimental trials are corresponding (7.2 ml/min). MATLAB/Simulink provides the simulation environment helping to integrate such system. The experiments confirm the results obtained from the model. Finally, this electolysis unit is environmentally friendly since the waste from it is oxygen only (O 2 ). Acknowledgment The authors would like to thank the Libyan Government for the financial support of this project. References [1] Rolland, L Ship Design Classification for Wind and Solar Energy Capture. n Proceedings of 3rd EC Climate Change Technology Conference, Paper [2] Thomas, C Transportation Options in a Carbon-Constrained World: Hybrids, Plug-in Hybrids, Biofuels, Fuel Cell Electric Vehicles, and Battery Electric Vehicles. nternational Journal of Hydrogen Energy 34 (23): [3] Kerekes, T., Koutroulis, E., Séra, D., Teodorescu, R., and Katsanevakis, M An Optimization Method for Designing Large PV Plants. EEE Journal of Photovoltaics 3 (2): [4] Gow, J. A., and Manning, C. D Development of a Photovoltaic Array Model for Use in Power-Electronics Simulation Studies. EEE Proceedings of Electric Power Applications 146 (2): [5] Wang, C., and Nehrir, H. M Power Management of a Stand-Alone Wind/Photovoltaic/Fuel Cell Energy System. EEE Transactions of Energy Conversion 23 (3): [6] Hussein, K Hybrid Fuzzy PD Controller for Buck Boost Converter in Solar Energy-Battery Systems. EEE nternational Conference on Electro/nformation Technology. [7] Wang, C Modeling and Control of Hybrid Wind/Photovoltaic/Fuel Cell Distributed Generation Systems. Ph.D. thesis, the Montana State University. [8] Atlamand, O., and Kolhe, M Equivalent Electrical Model for a Proton Exchange Membrane (PEM) Electrolyser. Energy Conversion and Management 52 (8): [9] Beainy, A., Karami, N., and Moubayed, N Simulink Model for a PEM Electrolyzer Based on an Equivalent Electrical Circuit. Renewable Energy 978: [10] Al-Refai, M MATLAB/Simulink Simulation of Solar Energy Storage System. nternational Science ndex 8 (2): [11]