Optimisation of PhotovoltaicPowered Electrolysis for Hydrogen Production for a Remote Area in Libya A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences 2011 Matouk M. Elamari School of Mechanical, Aerospace and Civil Engineering 1 List of Contents List of Figures …………………………………………………...…………………..6 List of Tables………………………………………………………………….……..9 Abstract…………………………………………………………………….……….10 Declaration………………………………………………………………….…...….11 Copyright…………………………………………………………..……….…...….12 Dedication……………………………….…………………..…………………...…13 Acknowledgements………………………………………………………………...14 1 2 CHAPTER 1 INTRODUCTION ..................................................................16 1.1 General Background ................................................................................16 1.2 Project Aim and Scope ............................................................................19 1.3 Objectives of the Research.......................................................................22 1.4 1.5 Major contributions of the thesis ..............................................................23 Overview of the thesis .............................................................................23 CHAPTER 2 HYDROGEN AS A FUTURE ENERGY CARRIER............26 2.1 Energy Sources and Environmental Impacts ............................................26 2.2 Renewable Alternative Energy ................................................................28 2.3 Hydrogen Energy Aspects .......................................................................28 2.4 Hydrogen as an Energy Storage Medium .................................................30 2.5 Major Hydrogen Production Technologies ...............................................32 2.5.1 Steam Reforming .............................................................................32 2.5.2 2.5.3 2.5.4 2.5.5 2.5.6 Partial Oxidation ..............................................................................33 Auto Thermal Reforming .................................................................33 Coal Gasification .............................................................................34 Electrolysis ......................................................................................34 Thermo-Chemical Process ...............................................................35 2.5.7 Photo Processes ...............................................................................36 2.6 Hydrogen Storage ....................................................................................36 2.6.1 Hydrogen Storage in Gaseous Form .................................................36 2.6.2 Hydrogen Storage in Liquid Forms ..................................................37 2.6.3 Hydrogen Storage as Metal Hydrides ...............................................38 2.6.4 Underground (Geological) Storage...................................................38 2.7 Hydrogen Transportation .........................................................................39 2.7.1 Compressed Gas Transport ..............................................................39 2.7.2 Liquid Hydrogen Transport ..............................................................40 2.7.3 Metal Hydride Transport ..................................................................40 2 2.8 2.9 Attractive Advantages for Hydrogen as an Energy Carrier .......................41 Safety Aspects Associated with Hydrogen ...............................................42 3 CHAPTER 3 PHOTOVOLTAIC SOLAR ENERGY AND HYDROGEN PRODUCTION .....................................................................................................45 3.1 Background .............................................................................................45 3.2 Components of Solar Radiation ...............................................................46 3.3 Measurements of Solar Radiation ............................................................48 3.3.1 Pyranometer.....................................................................................48 3.3.2 Pyrheliometer ..................................................................................48 3.4 Harnessing and Using Solar Energy .........................................................50 3.4.1 Thermal Conversion ........................................................................50 3.4.2 Electrical Conversion .......................................................................51 3.5 General Description of PV Cell Technology ............................................51 3.5.1 Silicon Solar Cell Types and Their Efficiencies ...............................52 n-type silicon semiconductor .......................................................................53 p-type silicon semiconductor .......................................................................53 3.6 Photovoltaic Systems ...............................................................................55 3.6.1 Applications of PV Systems .............................................................57 3.6.2 Attractive Features of Photovoltaic System ......................................57 3.7 Solar Hydrogen Production Systems ........................................................58 3.7.1 Solar Photovoltaic-based Electrolysis ..............................................58 3.7.2 Solar Photoelectrolysis .....................................................................58 3.7.3 Hydrogen Production by Concentrated Solar Thermal Energy..........58 3.8 System Components of the PV-Electrolyser Hydrogen Production Process 59 3.8.1 PV Electricity Generation ................................................................59 3.8.2 The Electrolyser ...............................................................................60 3.8.2.1 Alkaline water electrolyser ...........................................................60 3.8.2.2 Proton Exchange Membrane (PEM) Electrolyser .........................61 3.8.2.3 High-Temperature Electrolyser ....................................................63 3.9 PV Electrolyser Coupled with Maximum Power Point Tracking ..............63 3.10 Maximum Power Point Tracking Technologies .......................................65 3.10.1 3.10.2 3.10.3 4 Perturb and Observation (PAO) Method ..........................................65 Incremental Conduction ...................................................................65 Fractional Open Circuit Voltage ......................................................66 CHAPTER 4 A POWER MATCHING SIMULATION OF A SOLAR HYDROGEN PRODUCTION SYSTEM.............................................................68 3 4.1 4.2 PSCAD Software.....................................................................................68 Model Components .................................................................................68 4.3 Input/Output Data ....................................................................................69 4.4 The PV Model .........................................................................................70 4.4.1 PV Equivalent Circuit ......................................................................70 4.4.2 PV PSCAD Model ...........................................................................72 4.4.3 Response of the Model to Changes in Insolation ..............................74 4.4.4 Response of the PV Model to Changes in Temperature ....................75 4.5 DC-DC Buck Converter PSCAD Model ..................................................76 4.6 The PEM Electrolyser PSCAD Model .....................................................77 4.7 PV-PEM Hydrogen Production Power Matching Model ..........................80 4.8 Simulations and Results ...........................................................................82 5 DESIGN DC/DC BUCK ONVERTER FOR PV-PEM HYDROGEN PRODUCTION SYSTEM POWER MATCHING ..............................................89 5.1 Background .............................................................................................89 5.2 Buck Converter Theory and Operation ....................................................90 5.2.1 Purpose of Different Buck Converter Components ...........................90 5.2.1.1 5.2.1.2 5.2.1.3 5.2.1.4 5.2.1.5 Switch..........................................................................................91 Pulse-Width Modulation Circuit...................................................91 Operating Frequency ....................................................................91 Inductor .......................................................................................92 Capacitor .....................................................................................92 5.2.1.6 Free-Wheeling Diode ...................................................................92 5.2.2 Circuit Description and Operation ....................................................92 5.3 PSCAD Simulation of a Buck Converter .................................................96 5.3.1 DC-DC Buck Converter Circuit Using IC TL494 Control Circuit ....98 5.4 Characteristics of the PV-PEM Electrolyser Test Rig ............................ 101 5.4.1 PV Characteristics .........................................................................102 5.4.2 PEM Characteristics ...................................................................... 103 5.4.3 Dependence of Hydrogen Production on the Operating Current of the PEM Electrolyser .......................................................................................... 104 5.5 Design Buck Converter.......................................................................... 105 5.5.1 5.5.2 PWM Circuit ................................................................................. 105 Evaluation of Results ..................................................................... 107 6 REAL TIME EXPERIMENT OF A PV-PEM HYDROGEN PRODUCTION SYSTEM USING A COMMERCIAL SPLIT-PI CONVERTER 112 4 6.1 6.2 Background ........................................................................................... 112 System Components .............................................................................. 112 6.2.1 PV Module .................................................................................... 113 6.3 Measuring Solar Irradiance ....................................................................115 6.4 Split-Pi DC / DC Converter ................................................................... 116 6.4.1 Control of Split-Pi Converter Software .......................................... 118 6.4.2 Maximum Power Point Tracking Algorithm .................................. 120 6.5 6.6 6.7 PEM Electrolyser .................................................................................. 122 Hydrogen Volume Measurement Device ............................................... 125 Results and Discussion .......................................................................... 126 7 CHAPTER 7 DESIGN A PV-HYDROGEN SYSTEM TO POWER A FAMILY HOUSE IN THE SAHARA DESERT IN LIBYA ............................. 131 7.1 Background ........................................................................................... 131 7.2 Solar Energy Sources in Libya and the Hydrogen Option ...................... 131 7.3 Solar Hydrogen System as an Energy Supply for Libyan Remote Areas 132 7.3.1 Design of a Solar Hydrogen Power System for a family House in a Remote Area Located in the Sahara Desert .................................................... 135 7.3.2 7.3.3 7.3.4 7.3.5 7.3.6 Energy Requirement ...................................................................... 137 Fuel Cell Specification ................................................................... 138 Hydrogen Storage .......................................................................... 141 PEM Electrolyser ........................................................................... 141 DC/DC Converter .......................................................................... 142 7.3.7 Size of the PV Array ...................................................................... 143 7.3.8 Monthly Average Energy Supplied and Consumed ........................ 143 7.4 Control and Monitoring of the PV-Hydrogen System ............................ 144 8 CHAPTER 8 CONCLUSIONS ................................................................... 147 8.1 Contributions Made During the Project .................................................. 148 8.2 Suggestions for Future Work ................................................................. 149 9 10 REFERENCES ............................................................................................ 152 CHAPTER 10 APPENDICES ....................................................................159 10.1 Appendix A ........................................................................................... 159 10.2 Appendix B ........................................................................................... 160 5 List of Figures Figure 1.1: Map of Libya [9]. ................................................................................19 Figure 1.2: Electricity in Libya consumption and fuels used in power plants (a) consumption by sector and (b) fuels used in power plants. ......................................20 Figure 1.3: Electric energy consumption per capita for Libya and other countries [8]. ..........................................................................................................................21 Figure 2.1: Primary sources of hydrogen and its applications (The sectors are not scaled) [12]. ............................................................................................................29 Figure 2.2: Solar hydrogen power system for a home [3]........................................31 Figure 2.3: Electrolysis of water.............................................................................35 Figure 2.4: Hydrogen gaseous storage and delivery in the USA [53] ......................37 Figure 2.5: Transmission cost comparison between electricity and hydrogen from [20]. ........................................................................................................................39 Figure 2.6: Heavy duty truck at hydrogen production plant USA [54] ....................40 Figure 3.1: World solar map [26]. ..........................................................................45 Figure 3.2: Interactions of the Earth‟s atmosphere with incoming solar radiation [27] ...............................................................................................................................47 Figure 3.3: Typical pyranometer ............................................................................48 Figure 3.4: pyrheliometer [26]. ..............................................................................49 Figure 3.5: A pyranometer used for the measurement of diffuse radiation [26]. ......49 Figure 3.6: Flat plate solar thermal collector. ........................................................50 Figure 3.7: Schematic diagram of a photovoltaic cell .............................................52 Figure 3.8: Covalent bonds in a silicon atom. .........................................................53 Figure 3.9: Photovoltaic hierarchy [28]. .................................................................55 Figure 3.10: Effects of insolation and temperature on the characteristics of a PV panel . .....................................................................................................................60 Figure 3.11: A schematic construction of alkaline water electrolyser [34]. .............61 Figure 3.12: Schematic diagram of a proton exchange membrane electrolyser [35]. ...............................................................................................................................62 Figure 3.13: PV coupled with an electrolyser using a DC/DC converter for MPPT . ...............................................................................................................................63 Figure 3.14: Photovoltaic coupling with load using a DC/DC Buck converter ........64 Figure 4.1: PV- electrolyser power matching using DC/DC buck converter. ..........69 Figure 4.2: Equivalent circuit of the PV cell. ..........................................................70 Figure 4.3: I-V characteristics of the PV cell. .........................................................71 Figure 4.4: Effect of adding RS on the PV cell‟s I-V curve. ....................................71 Figure 4.5: Effect of adding RSh on the PV cell‟s I-V curve. ...................................72 Figure 4.6: Model of the PV module. .....................................................................72 Figure 4.7: The dependence of I-V characteristics on insolation. ............................75 Figure 4.8: Effect of temperature on the PV model curves. .....................................75 Figure 4.9: PV characteristics used in the PV-PEM PSCAD model. .......................76 Figure 4.10: PWM to produce different duty cycle generator. ................................77 Figure 4.11: PSCAD PEM electrolyser block. ........................................................78 Figure 4.12: I-V and P-V curves for the PEM electrolyser. .....................................79 Figure 4.13: PV-PEM electrolyser PSCAD model .................................................80 6 Figure 4.14: V-P curves for the PV array and the PEM at different duty cycle values (D). .........................................................................................................................82 Figure 4.15: Intersection with the rescaled I-V curve of the PEM. ..........................83 Figure 4.16: Intersection with the rescaled I-V curve of the PEM. ..........................84 Figure 4.17: Relationship between current, voltage, and duty cycle. .......................85 Figure 4.18: Relationship between hydrogen production rate and duty cycle. .........85 Figure 5.1: Buck converter circuit. .........................................................................90 Figure 5.2: Buck converter ON state. .....................................................................93 Figure 5.3: Buck converter OFF state. ....................................................................94 Figure 5.4: (a) CCM and (b) DCM for inductor current. ........................................94 Figure 5.5: ON and OFF waveforms of the buck converter [43]. ............................95 Figure 5.6: PSCAD buck converter simulation. ......................................................96 Figure 5.7: Load voltage and current ripples ..........................................................97 Figure 5.8: Diode voltage wave from (a) Vin = 20 V, D = 0.9 and Vo = 18 V and (b) Vin = 20 V, D = 0.4 and Vo = 8 V. .........................................................................98 Figure 5.9: (a) DC-DC buck converter using IC TL494 and (b) the practical circuit of the circuit shown in (a). ......................................................................................99 Figure 5.10: Diode voltage form (a) V in = 20 V and V out = 5V and (b) V in = 6 V and V out = 5 V. ..................................................................................................... 101 Figure 5.11: PV –PEM test rig. ............................................................................ 102 Figure 5.12: Setup for determining the characteristics of a solar module. ............. 102 Figure 5.13: PV characteristics............................................................................. 103 Figure 5.14: Circuit diagram of the characteristics of the PEM electrolyser. .........103 Figure 5.15: PEM electrolyser characteristics. ...................................................... 104 Figure 5.16: Volume of hydrogen produced as a function of current over a 10minutes operational period. ................................................................................... 104 Figure 5.17: PWM circuit using IC SG3525. ........................................................ 105 Figure 5.18: PV –PEM electrolyser coupled by a buck converter circuit............... 106 Figure 5.19: Buck converter implemented on a PCB. ........................................... 106 Figure 5.20: Oscilloscope images of electrolyser voltage (red) at different duty cycle values (blue). ........................................................................................................107 Figure 5.21: I-V curves for the PV array and the PEM electrolyser. ..................... 108 Figure 5.22: P-V curves of the PV array and the PEM electrolyser. ...................... 109 Figure 5.23: Relationship between the implemented efficiency of the Buck converter and duty cycle. ...................................................................................................... 109 Figure 6.1: Hydrogen production system. ............................................................. 112 Figure 6.2: Power matching photovoltaic-electrolyser system using a Split-Pi converter. .............................................................................................................. 113 Figure 6.3: PV modules facing the sun. ................................................................ 114 Figure 6.4: (a) I-V and (b) P-V curves of a single C21 module under different insolation values. .................................................................................................. 115 Figure 6.5: Split – Pi DC/DC converter. ............................................................... 116 Figure 6.6: Split – Pi converter circuit. ................................................................. 117 Figure 6.7: Flowchart of visual basic control. ....................................................... 119 Figure 6.8: Visual basic software screen outlook. ................................................. 120 Figure 6.9: Hill-climbing MPPT method. ............................................................. 121 Figure 6.10: MPPT programme flow chart. .......................................................... 122 Figure 6.11: The 50-W, h-tec PEM electrolyser. ................................................. 123 7 Figure 6.12: Characteristic curve of the h-tec PEM electrolyser. .......................... 124 Figure 6.13: Resistance-Power PEM electrolyser curve. ....................................... 124 Figure 6.14: Device for measuring the volume of hydrogen produced. ................. 125 Figure 6.15: I-V for PV and PEM electrolyser curves........................................... 126 Figure 6.16: P-V PV and PEM electrolysers power matching. .............................. 127 Figure 6.17: PEM electrolyser input power data during a clear, sunny day. .......... 127 Figure 6.18: PEM electrolyser input power under less favourable insolation conditions. ............................................................................................................ 128 Figure 6.19: Electrolyser input power and PV maximum power relation .............. 128 Figure 6.20: Hydrogen production in relation to changes in insolation. ................ 129 Figure 7.1: Photovoltaic module for a water pump in the Libyan Sahara [51]. ...... 132 Figure 7.2: Daily solar irradiance on horizontal plane through the year in Ghadamis. ............................................................................................................................. 134 Figure 7.3: Complete solar hydrogen power system. ............................................ 136 Figure 7.4: I-V and I-P characteristics for Nexa 1200 from data sheet. ................. 140 Figure 7.5: Estimated amount of energy extracted from the PV system. ............... 144 8 List of Tables Table 2.1: Environmental impacts of conventional energy sources. ........................27 Table 2.2: Choices of hydrogen storage. .................................................................38 Table 2.3: Methods of hydrogen transportation. .....................................................41 Table 3.1: Wave lengths of solar radiation [28]. .....................................................47 Table 3.2: The efficiencies of the threee types of crystalline silicon cells [32]. .......55 Table 3.3: World‟s three largest PV systems as of June 2011 . ...............................56 Table 4.1: Parameters of a crystalline silicon solar cell [41]. ..................................74 Table 4.2: Constant K value of different solar modules tested [5]. ..........................87 Table 5.1: Voltage and current readings at both sides of the Buck converter. ........108 Table 6.1: Specifications of the PV module. ......................................................... 114 Table 6.2: Split – Pi converter switching duty cycle. ............................................ 117 Table 6.3: Specifications of the PEM electrolyser. ............................................... 123 Table 7.1: Climatic conditions of the project site at 30° N and 10° E...................... 134 Table 7.2: Energy requirements for small family house in the Sahara Desert. ....... 137 Table 7.3: Nexa 1200 technical data from datasheet. ............................................ 140 Table 7.4: Specifications for the LM-10000 electrolyser. ..................................... 142 9 Abstract Hydrogen is a potential future energy storage medium to supplement a variety of renewable energy sources. It can be regarded as an environmentally-friendly fuel, especially when it is extracted from water using electricity obtained from solar panels or wind turbines. The focus in this thesis is on solar energy, and the theoretical background (i.e., PSCAD computer simulation) and experimental work related to a water-splitting, hydrogen-production system are presented. The hydrogen production system was powered by a photovoltaic (PV) array using a proton exchange membrane (PEM) electrolyser. The PV array and PEM electrolyser display an inherently non-linear current–voltage relationship that requires optimal matching of maximum operating power. Optimal matching between the PV system and the electrolyser is essential to maximise the transfer of electrical energy and the rate of hydrogen production. A DC/DC converter is used for power matching by shifting the PEM electrolyser I-V curve as closely as possible toward the maximum power the PV can deliver. By taking advantage of the I-V characteristics of the electrolyser (i.e., the DC/DC converter output voltage is essentially constant whereas the current increases dramatically), we demonstrated experimentally and in simulations that the hydrogen production of the PV-electrolyser system can be optimised by adjusting the duty cycle generated by the pulse-width modulation (PWM) circuit. The strategy used was to fix the duty cycle at the ratio of the PV maximum power voltage to the electrolyser operating voltage. A stand-alone PV energy system, using hydrogen as the storage medium, was designed. The system would be suitable for providing power for a family‟s house located in a remote area in the Libyan Sahara. 10 Declaration No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. 11 Copyright Statement The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the „Copyright‟) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the „Intellectual Property‟) and any reproductions of copyright works in the thesis, for example graphs and tables („Reproductions‟), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions can not and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual- property.pdf), in any relevant Thesis restriction declarations deposited in the University Library, The University Library‟s regulations (see http://www.manchester.ac.uk/library/aboutus/regulations) and in The University‟s policy on presentations of Theses. 12 To my beloved family 13 Acknowledgements I wish to express my gratitude to my current supervisor Professor Peter Stansby and my former supervisor, Professor Nick Jenkins, who both gave me valuable advice, and excellent guidance that were essential in the successful completion of the project work. Immense thanks to Dr. Frank Thompson who contributed in the completion of this thesis for his valuable notices and advices throughout the work. My thanks also to the my country Libya for the opportunity they gave me to complete my higher studies by providing the grants for this research. I am grateful also to the Joule centre at the University of Manchester for providing facilities to undertake this work. My sincere acknowledgments and appreciations are to all my friends for their valuable assistance and encouragement. Finally I would like to express my gratitude to my family for their constant encouragement, care, and patience during the production of this work. 14 CHAPTER 1 INTRODUCTION 15 1 CHAPTER 1 INTRODUCTION 1.1 General Background Energy has always been and still is essential for human survival and social development. In recent years, concern about energy resources has become a worldwide issue for the following reasons: Continuing increase in the world‟s population Environmental problems associated with CO2 emissions from the combustion of fossil fuels (The International Energy Agency (IEA) stated that the projected use of fossil fuels will increase CO2 emissions by 57% from 2005 to 2030 [1]). Growing demand for energy to improve living standards. At the present time, a large proportion (about 65%) of worldwide energy demand is met by liquid and gaseous fossil fuels (i.e., petroleum and natural gas) because they are readily available and convenient to use. However, it is expected that the worldwide production of fossil fuels will soon peak and, thereafter, begin to decrease. The need to identify and develop alternative types of clean and sustainable fuel is becoming more and more urgent. The use of hydrogen produced by renewable energy sources (solar, wind, and biomass) as an energy carrier could be the future energy strategy that will replace conventional liquid and gaseous fuels in both stationary and transportation applications. Normally, elemental hydrogen exists as a gas, H2, and molecular hydrogen exists in combination with many other atoms, e.g., H2O and CH4. Molecular hydrogen can be separated from such molecular forms through chemical or physical methods, such as electrolysis of water or by steam reforming of natural gas. Currently, the steam reforming process is the least expensive of the various methods, but it depends on the combustion of a fossil fuel, which produces CO2 emissions. However, the electrolysis process can produce high-purity hydrogen from water without generating 16 CO2. Therefore, hydrogen is an environmentally-attractive fuel, because it burns without producing CO2. It is not a primary energy source, such as oil, coal, and gas, but it can be used effectively as an energy carrier. Energy storage is required with most renewable energy sources. For example, we can consider the use of batteries, which have the following limitations: (1) They are not presently used for seasonal storage (from summer to winter). (2) Their efficiency is low, and it decreases dramatically with time, causing substitutions. (3) Disposal creates an environmental problem. (4) They are expensive, especially if we were to consider the large dimensions required for seasonal energy storage [2]. Most of these limitations could be circumvented by replacing the batteries with a system for using renewable energy sources to generate hydrogen as an energy carrier. One of the most promising ways of hydrogen end use is to produce electricity by recombining hydrogen with oxygen from the air in a fuel cell, which directly converts the chemical energy into electricity. The only by-product of this process is water. With the use of hydrogen in fuel cell systems, there are no harmful emissions, such as CO2, nitrogen oxides, or SO2. Fuel cells are silent electricity generators, so they can be used as auxiliary power generators in hospitals, IT centers, and submarines. Electricity from fuel cells can be used to run appliances and motors, provide light, and to power cars. We can use the energy provided by the sun to generate hydrogen, which, in turn, can be used as an abundant, clean, and efficient source of energy for local use or longerterm, distributed energy supplies. One of the challenges in producing hydrogen by using solar energy (a PV-hydrogen system) is the high cost. Therefore, it is important that such a system be designed and operated in a way that allows it to achieve maximum power output, i.e., by matching the power generated by the PV array and the power used to produce the hydrogen. A large amount of modeling and experimental work on solar-hydrogen production systems has been conducted in the last few decades. Much of the research has 17 focused on optimal coupling between the photovoltaic source and electrolysers based on their current, voltage, and power characteristics. Mismatch problems arise due to variations in the output power from PV systems that are operating under different incident solar radiation (insolation) conditions. Two ways of connecting the PV source with the electrolyser are listed below: 1. Direct coupling between PV array and electrolyser [3][4]. In this method, the key strategy is to find the series-parallel combination of PV modules and electrolyser stacks. This method should meet the following conditions: The PV array should supply a minimum working voltage to the electrolyser in order to split water into hydrogen and oxygen. The PV – electrolyser system should contain the minimum number of electronic devices (power conditioning equipment) to decrease losses. 2. Connection through a DC/DC converter, with maximum power point tracking to ensure optimal power transfer between the PV panels and the electrolyser. Ideally, output from the PV source would be held at the maximum power point in order to achieve the greatest overall efficiency [5]. L. Arriaga and Martinez [3] showed the results obtained by direct coupling of a 2.7kW PV array with 25 cells of PEM electrolyser at different insolation and temperatures. They confirmed that the electrolyser stack was working near the maximum power point at a good range of irradiance (600-800 W/m2). In their experimental work, G. Ahmed and E. El Shenawy [4] compared the hydrogen flow rate obtained by direct coupling of the PV array and electrolyser with the use of a maximum power point tracker. The results showed that greater hydrogen flow rates were achieved by using the maximum power point tracker. Gibson and Nelson [5] developed a comprehensive mathematical model for optimising the efficiency of PV-electrolyses systems. The strategy of the model was to match the maximum power voltage of the PV device and the operating voltage of the proton exchange membrane (PEM) electrolyser. The authors concluded that the optimised PV-electrolyser system increased the hydrogen generation efficiency from 18 2.6% to 12.4%, and this clearly results in minimising the cost of hydrogen production. Sulaiman and Veziroglu [6] developed a solar hydrogen power model for a solarhydrogen energy system in Saudi Arabia. 1.2 Project Aim and Scope Libya is an oil-exporting country located in the middle of North Africa (Figure 1.1), with the Mediterranean Sea as its northern border. It has six million inhabitants and a land area of 1,750,000 km 2 . For the last four decades, Libya has been dependent mainly on fossil fuels (petroleum and natural gas) for its supply of energy; only a very small amount of energy has come from renewable sources. Since Libya has no biomass or geothermal energy sources, it will have to develop wind and solar energy if it is to reduce its dependence on fossil fuels. Figure 1.1: Map of Libya [9]. This thesis is concerned only with electrical energy, fuel use for transportation and domestic heating will not be discussed. 19 The General Electricity Company of Libya (GECOL) is the only Libyan company that produces and distributes electricity in the country. According to GECOL, its installed electricity generating capacity was 6.28 GW in 2008, and this capacity depended entirely on the use of oil and natural gas. The Libyan national grid has an extensive high-voltage network of about 12,000 km spread across the country. In spite of this, Libya, like other North African countries, has remote areas where people live but, as yet, are not connected to the grid. Figure 1.2(a) shows the Libyan consumption of electrical energy by sector during 2008, with the services and residential sectors consuming around 70% of the total. Also, Figure 1.2(b) shows that many power plants have been converted to use natural gas instead of oil so that the export of oil volume (and the associated revenue) could be maximized [7]. Figure 1.2: Electricity in Libya consumption and fuels used in power plants (a) consumption by sector and (b) fuels used in power plants. Libya has a high per capita consumption of electrical energy compared with other North African countries, (UK data included for comparison) [8] as seen in Figure 1.3. According to data from GECOL, the per-capita energy consumption increased from 330 kWh in 1970 to 3920 kWh in 2008, and the peak load of electrical power in Libya has increased continuously at the relatively high rate of approximately 10% per year, while the population growth rate has averaged just over 2% per year. This 20 shows that energy demand is largely controlled by the very rapid improvement of living standards in the country. Figure 1.3: Electric energy consumption per capita for Libya and other countries [8]. Every nation in the world is aware of the two main problems associated with the use of fossil fuels, i.e., 1) they are limited and will be depleted within the next century and 2) using fossil fuels produces severe environmental problems from the effects of CO2 and other emissions. To avoid a future crisis in both of these areas, it is important for Libya to begin the rapid development of renewable energy as a strategic energy policy. Both wind and solar sources can produce significant amounts of electrical energy, and both technologies should be developed as rapidly as possible. This thesis is concerned with one section of solar energy, namely, photovoltaic electrical energy production in which solar radiation is converted into electricity by a silicon p-n junction. This is particularly attractive for Libya, which has abundant sunshine for most of the year. Hydrogen storage offers an alternative to battery storage for solar energy for power systems in remote areas. Hydrogen production via water electrolysis using photovoltaic as a power source and later using the hydrogen produced to generate electricity using fuel cells is an ideal power source for remote areas. 21 The benefits to be obtained when solar hydrogen technology is installed in the remote areas of Libya are: 1- The standard of living will be increased for the people who live in the remote villages. 2- Sufficient electrical energy will be produced to meet local needs. 3- The settlement of people in these areas will be encouraged so they can avoid migration to the crowded cities. 4- CO2 emissions will be reduced, thereby contributing to the solution of the global warming issue. 5- Additional oil can be exported rather than burned in Libya, thereby contributing to the country‟s economic growth. 1.3 Objectives of the Research 1- To review previous work on renewable energy, especially hydrogen systems, with special emphasis on the technology of solar hydrogen production systems with power matching to maximize the efficiency. 2- To use a laboratory-scale, photovoltaic–electrolyser test facility to validate the concept of power matching using a DC/DC converter. 3- To use software engineering, namely PSCAD, to develop a computer code capable of simulating physical processes of solar hydrogen production system components. 4- To experimentally demonstrate a real-time, outdoor, solar hydrogen production system using a computer-controlled DC/DC converter. 5- To design a solar hydrogen power supply system for a Libyan family house in a Saharan remote area. 22 1.4 Major contributions of the thesis Modelling photovoltaic powered electrolyser hydrogen production system using PSCAD software. This developed a system model to incorporate appropriate control strategy for system power matching using DC/DC buck converter. A visual basic software code was developed to controlee Split-PI DC/DC converter. Areal time experiment was conducted to achieve PV-PEM hydrogen production system power matching. The system was examined in real time experiment and the results show the system was working at its maximum power. A stand alone PV energy system using hydrogen as a storage medium was designed. The system provides a power for family house located in a remote area in Libyan Sahara. 1.5 Overview of the thesis The thesis contains eight chapters, and the first chapter provides general background information related to the solar-hydrogen system and its importance, followed by the aim and scope of the thesis. Chapter two provides a summary of the available knowledge that has resulted from the investigation of hydrogen as an attractive energy carrier; hydrogen production methods and technologies; aspects of the use of hydrogen for in electricity generation and transportation; the storage, distribution, and transportation of hydrogen; and the safety features that must accompany the use of hydrogen. Chapter Three illustrates solar irradiance and photovoltaic electricity generation; the main components of the solar hydrogen production system, including the photovoltaic power generator, electrolyser, and DC/DC converter as a power conditioner between the source and the load; the construction of each component; and the operational principles associated with each component. Chapter Four describes the use of PSCAD software engineering methodology to develop a computer model that is capable of exploring the modelling of power 23 matching in a photovoltaic–hydrogen production system. The evaluation shows the different factors that affect the I-V characteristics based on theoretical equations that describe the operation of both the photovoltaic device and the PEM electrolyser. The simulation proved that the value of the duty cycle of the DC/DC converter influences power matching and the hydrogen production rate. Chapter Five describes a number of laboratory experiments with the small photovoltaic-hydrogen production system that were conducted using a test facility. A DC/DC buck converter was designed and implemented on a PCB to match the power between the photovoltaic device and the proton exchange membrane (PEM) electrolyser. The open-loop control method, using analogue IC, was used. The ratio of output to input voltage ratio was given by the duty cycle control in the switching converter. Chapter Six describes the implementation and evaluation of a real-time PV-PEM system scaled up by a factor of 10 from the laboratory unit described in chapter five, using a commercial, computer-controlled, DC/DC converter. Visual basic software code was created to control the converter and to track the maximum power point (MPPT) of the photovoltaic device under different levels of irradiance to maximize the efficiency of the system. The results showed that the readings at the converter terminals followed the maximum power that the photovoltaic device delivered. Chapter Seven includes the design of a photovoltaic–electrolyser system to produce hydrogen that fuels a fuel cell capable of generating the electricity needed for a family‟s house. The location was chosen in Libya, which is characterized by large desert areas, scattered populations, and remote communities. Chapter Eight presents the conclusions that were drawn as a result of the work and suggestions for future work. Appendices of computer codes used in PSCAD and Visual Basic software are included at the end of the thesis. 24 CHAPTER 2 HYDROGEN AS A FUTURE ENERGY CARRIER 25 2 CHAPTER 2 HYDROGEN AS A FUTURE ENERGY CARRIER 2.1 Energy Sources and Environmental Impacts Global demand for energy has grown significantly in the last five or six decades because of industrial development, increases in people‟s living standards, and population growth. Energy sources can be classified into two categories: 1- Renewable energy sources are those that are sustainable and that are renewed by nature, for example solar, wind, tidal, and biomass sources. 2- Conventional or non-renewable energy sources are exhaustible and limited to definite periods of time, depending on the extent of their usage. Fossil fuels, such as oil, natural gas, and coal, have been considered as the main source of world energy due to their convenience and flexibility of use. It is these properties that are responsible for the ever-growing demand for fossil fuels. Unfortunately, non-renewable fuels are always limited, because they were formed many eons ago from the carbon-rich remains of plants and animals, and, at some time in the future, they inevitably will be used up. Another disadvantage of fossil fuel is that, during the process of combustion, significant quantities of many toxic materials are emitted into the air. These emissions pollute the atmosphere, land, and water, and some of them cause global warming, which is becoming a serious, worldwide issue. The environmental impact linked to using conventional energy sources has received widespread attention all over the world in recent years. The global warming effect from using conventional energy sources has been investigated internationally. The result of this investigation is a set of guidelines known as the Kyoto Protocol. This Protocol was adopted initially in December 1997 in Kyoto, Japan, and it was implemented on 16 February 2005. As of 2010, 187 states had signed and approved this Protocol. A part of the agreement requires industrialised countries to reduce their 26 emissions of a "basket of greenhouse gases" by around 5 % between 2008 and 2012, as compared to 1990 levels. To give an overview of the impacts of greenhouse gas emissions, Table 2.1 from [10] provides the environmental impacts of conventional energy sources, including pollution. As can be seen in the table, coal and oil have significant environmental impacts for all of the pollutants presented. It should be noted that about 80% of the world's energy supplies are provided by fossil fuels [10]. (Data for pollution resulting from the use of nuclear energy are included for the sake of completeness). Table 2.1: Environmental impacts of conventional energy sources. To know the problems caused when we are dependent on fossil fuels and how we can find alternative ways of energy production, we must know the ways in which energy is currently being used or consumed. There are four main sectors for using energy in human activities: 1- The private household sector. 2- The transport sector (including private and public transportation). 3- The industrial sector (e.g., manufacturing and agriculture). 4- The commercial and institutional sector (e.g., commercial offices, educational facilities, and health facilities). Thus, the world has to look for new alternative ways of energy production for 27 increasing the use of renewable energy sources to replace conventional energy sources. 2.2 Renewable Alternative Energy Renewable energy is any energy source that is sustainable and doesn't contribute to global warming. Most renewable energy sources are derived from solar radiation, including the direct use of solar energy for heating or photovoltaic electricity generation, and indirect forms, such as wind energy, wave energy, and hydroelectric energy. Tidal sources of energy result from the gravitational pull of the moon and the sun, and geothermal energy comes from the heat available within the Earth. Although the latter is finite on a geological time scale, we can classify geothermal energy as a renewable energy source. The biggest obstacle to renewable energy technology is that these sources of energy are not often convenient or flexible. At the present time, the cost of renewable energy is significantly higher than the cost of energy from fossil fuels. In addition to cost, one of the greatest challenges in utilising renewable energy, particularly solar and wind energy, is the discontinuous or irregular nature of these types of power; the wind does not always blow, and sunlight is not always available. So, it is imperative that energy storage be combined with these renewable energy sources. Renewable technology becomes much more practical when it is used in conjunction with a storage system. Hydrogen could provide energy storage for either wind or solar installations as an alternative to electrical batteries. As a comparison, the best of all possible chemical battery cells, namely a beryllium/air cell, has the capacity to store 24.5 megajoules per kg of reactants, while hydrogen can store about 120 megajoules per kg [11, 12]. Several aspects of hydrogen storage are discussed in the following paragraph. 2.3 Hydrogen Energy Aspects Hydrogen is the simplest element, with an atom consisting of only one proton and one electron. Hydrogen exists as a gas, H2, or in combination with other elements, forming, for instance, water, ammonia, and hydrocarbon compounds. Hydrogen can 28 be separated from hydrocarbon compounds using the heat reforming process, and it can be separated from water by electrolysis. Hydrogen has the highest energy to weight ratio of all fuels; one kilogram of hydrogen contains the same amount of energy as 2.1 kg of natural gas or 2.8 kg of petrol. Hydrogen is not a primary energy source, such as coal and gas, but it can be referred to as an energy carrier. Initially, it has to be produced using existing energy systems based on different conventional primary energy sources. Hydrogen is considered as one of the most promising alternative fuels for the future because of its capability of storing energy of high quality and because of its potential to become an important energy carrier in the future. The ability of hydrogen to improve energy security results from the wide range of options for sources, converters, and applications. Figure 2.1 [12] shows the variety sources for hydrogen production and its utilization aspects. The figure illustrates that hydrogen has a high energy flexibility compared with any other alternative fuel. Figure 2.1: Primary sources of hydrogen and its applications (The sectors are not scaled) [12]. Hydrogen can be produced chemically from hydrocarbons, but the most attractive 29 option is to produce hydrogen by the electrolysis of water, because water covers 2/3 of the Earth and is, therefore, abundant in many parts of the world. The generation of hydrogen is equivalent to the storage of energy in chemical form as a fuel. Ultimately, hydrogen fuel can be used to produce thermal energy by combustion or to produce electricity using fuel cells. If we considered moving towards the large-scale use of hydrogen as fuel, there would have to be a significant investment in infrastructure. All sectors in Figure 2.1, e.g., transport, buildings, and industry would have to be modified extensively [12]. A small-scale development could be established quite simply with the production of hydrogen locally using some renewable energy source and the electrolysis of water. One such example of renewable hydrogen is a solar hydrogen system for a household or small village to supply off-grid applications with electricity for cooking and transportation. Figure 2.2 shows a solar PV–electrolyser fuel cell system to demonstrate the use of hydrogen as an energy carrier in stand-alone applications. Fuel cells operate in a converse manner to an electrolyser; they combine hydrogen with oxygen from the air in an electrochemical process to produce electricity, and the by-product is water. Fuel cells are used to generate power for cars, and they are much more efficient than a car running on petrol. A family car powered by a fuel cell would need around 5 kg of hydrogen to achieve a range of 500 km [14]. 2.4 Hydrogen as an Energy Storage Medium Hydrogen can be attractive electricity storage medium; electricity can be used to produce hydrogen through water electrolysis, while hydrogen can then produce electricity using fuel cells. Therefore, when there is low-demand for energy hydrogen could be stored and later used during high-demand periods. 30 Figure 2.2: Solar hydrogen power system for a home [3]. Such a method is particularly good for an off-grid system in rural regions, where renewable energy is the only energy option. An example of a house powered by hydrogen is given in the following paragraph [15]. A “modular house” has been running on solar power and stored hydrogen in the state of New Jersey in the USA since 2006; the home owner has generated all the power for the home and fuel for his car and other mechanical items used around the house. The system uses solar PV panels to generate electricity using sunlight, and this electricity is used to extract hydrogen from water. Although the system was costly to construct, it avoids all the costs for purchasing electricity, purchasing fuel for heating the house, and purchasing fuel for the car. In addition, no greenhouse gas emissions are produced. For a larger-scale project, one finds that the whole island community at Uist, Shetland Isles, Scotland, relies on hydrogen produced from wind power. This, indeed, demonstrates the flexibility of a hydrogen/renewable source combination. For a solar hydrogen power system, solar panels could be installed in sunny regions, such as desert areas, and the energy produced could be stored and transported as hydrogen over long distances. 31 Hydrogen has a higher energy density versus typical battery materials. Also, the long charging time for rechargeable batteries requires regulators with controls in order to avoid overcharging. In the U.S. and Japan, hydrogen and fuel cells are considered to be core technologies for the 21st century. There is strong investment and industrial activity in the hydrogen and fuel cell arena in these countries, which are driving the transition to hydrogen rapidly with funding for research and development [12]. In many countries, due to the awareness of carbon dioxide emissions, greenhouse gases, and rapidly increasing oil prices, more hydrogen-fuelled automobiles are being produced. 2.5 Major Hydrogen Production Technologies 2.5.1 Steam Reforming Steam reforming of natural gas is currently the least expensive method for hydrogen production, and it is responsible for more than 90% of the worldwide hydrogen production. This is a chemical process in which a mixture of water and natural gas (methane) is used to produce hydrogen from the natural gas. First, the natural gas is cleaned and combined with steam at very high temperature (1100 to 1300 oK). Then, the mixture of gases is passed over a nickel-alumina catalyst, where they are converted to carbon monoxide (CO) and hydrogen (H2). The final step is a catalytic water-gas reaction in which carbon monoxide and water are converted into carbon dioxide (CO2) and hydrogen (H2). The reforming reaction is: CH4 + H2O + 206 kJ/kg => CO + 3 H2……………………………………..2.1 It is then followed by the exothermic shift reaction: CO + H2O => CO2 + H2 + 41 kJ/kg………………………………………....2.2 The overall reaction is: 32 CH4 + 2H2O + 165 kJ/kg => CO2 + 4H2………………………………..….2.3 Unfortunately, production of hydrogen from natural gas has the following disadvantages: 1- It accelerates the depletion of natural gas as a fossil fuel resource. 2- Carbon dioxide is a major byproduct of this process, and, so, in carbon terms, it is only slightly better then burning the original methane. 3- The heating process to form steam creates additional carbon dioxide. Although this process is well established, commercially viable, and is presently being used to meet the demand for hydrogen, it is not sustainable because it is based on the use of fossil fuels. 2.5.2 Partial Oxidation In this process, liquid or gaseous hydrocarbons are mixed with oxygen in a highpressure reactor. The carbon monoxide is reacted with water to form CO2 and H2, and the CO2 is captured and the H2 is purified. The equation for the partial oxidation of natural gas is: CH4+1/2O2 → CO +2H2 …………………………..……………………… 2.4 CO + H2O → CO2 + H2…………………………………………………. .2.5 Hydrogen production by partial oxidation has a theoretical efficiency similar to that of conventional steam reforming, but less water is required. 2.5.3 Auto Thermal Reforming In the auto thermal reforming process, the two previous methods are combined, i.e., natural gas, steam, and oxygen are reacted in a single vessel that consist of two zones, one for combustion and the other for reforming. 33 2.5.4 Coal Gasification Coal gasification is used in large hydrogen production plants and is used commercially for hydrogen production. The coal is partially oxidised with oxygen in a high-pressure reactor. The reason for using oxygen at high pressure and temperature is to reduce the production of nitrogen oxides in the process. The method has advantages in that there are large coal reserves in many parts of the world. The disadvantage of this method is that it produces larger amounts of carbon dioxide than other methods. In addition, slag and ash are the waste products of this process, and these may contain heavy metals that could pollute air, water, and land. 2.5.5 Electrolysis Primary energy sources can be used to produce hydrogen by electrolysis, and the hydrogen can be converted to electricity. Producing hydrogen by water electrolysis has a greater appeal over those processes using hydrocarbons because there are no emissions. Also, electrolysis is preferred for the following reasons: 1. It is a potentially effective way of producing hydrogen locally so it can be located, say, at fuel station. 2. Electrolysis offers a way to use electrical energy generated by renewable sources. 3. Electrolysis, operating in combination with fuel cells, can establish stand-alone energy generator systems. The electrolysis of water is a very simple process in which electricity is used to split water molecules (H2O) into hydrogen (H+) ions and oxygen (O -) ions, as illustrated in Figure 2.3 from [17]. These hydrogen and oxygen ions migrate through the water towards the cathode and the anode, respectively. This process is an efficient method of producing high-purity hydrogen in large quantities with little or no adverse environmental impacts, assuming that the electrical energy required to operate such a process comes from renewable power sources, such as wind, photovoltaic, and hydroelectric systems. 34 Figure 2.3: Electrolysis of water The electrolysis process currently has an energy efficiency of approximately 75%, and, theoretically, the efficiency could be increased to more than 90% in the future [17]. Hydrogen produced by electrolysis results in no greenhouse gas emissions, depending on the source of the electricity used. 2.5.6 Thermo-Chemical Process In this process, water is superheated to a very high temperature (around 2500 oK), whereupon it dissociates into its original components, hydrogen and oxygen. This process has two problems, i.e., 1) a high temperature source is required and 2) the reaction vessel must be made of materials that can withstand such high temperatures. A third problem is the difficulty of separating the hydrogen and oxygen products. Relatively complex chemical methods are used to accomplish this [16]. 35 2.5.7 Photo Processes Sunlight is used in these processes to produce hydrogen from water. The processes can be divided into three main categories, i.e., 1) photo electrochemical, 2) photochemical, and 3) photo biological. This method has low efficiency, so it is used only when small quantities of hydrogen are required [14]. 2.6 Hydrogen Storage There are several different methods and techniques for storing hydrogen. They are dependent on two parameters, i.e., 1) the quantity of gas/liquid and 2) the duration of storage. For example, for long periods (seasonal storage), the hydrogen can be converted to liquid hydrides (e.g., ammonia) or stored as pressurized gas in underground tanks. The underground-tank system has been used since 1971 in the city of Kiel, Germany, where a hydrogen gas tank with a hydrogen content of 65% has been stored in a 32,000-m 3 cavern at a pressure of 80 to 160 bar at a depth of 130 m. Hydrogen must be stored in large quantities for seasonal periods in order to regulate and ensure continuity of supply. In general, there are three well-developed methods for storing hydrogen, i.e., 1) gaseous hydrogen storage, 2) liquid hydrogen storage, and 3) metal hydride storage. 2.6.1 Hydrogen Storage in Gaseous Form Compressed gas storage of hydrogen is the simplest storage solution. The only equipment required is a compressor and a pressure vessel. The only problem with compressed gas storage is low storage density, which depends on the storage pressure. Higher storage pressures result in higher capital and operating costs [21]. Compressed hydrogen gas tanks are the most popular because, unlike liquid hydrogen storage, they do not require refrigeration with the attendant insulation. Since the hydrogen molecule is small, compressed hydrogen systems demand greater care against leakage as compared with pressurized natural gas installations. Also, hydrogen tanks often are made from lighter materials, such as aluminium or carbon/ graphic compounds, than is the case for other gases. 36 There are two categories for pressurized hydrogen storage: Moderately pressurized hydrogen (1-1.5 Mp), which would be used, typically, from underground caverns or large stationary vessels at ground level. Small, high-pressure (20 Mp), cylindrical vessels for industrial applications, where the cylinders are transportable. Figure 2.4: Hydrogen gaseous storage and delivery in the USA [53] 2.6.2 Hydrogen Storage in Liquid Forms In order to reduce the volume required to store a useful amount of hydrogen, particularly for vehicles, liquefaction may be employed. Since hydrogen does not liquefy at temperatures above -253 °C, there is a large amount of energy needed in the liquefaction process [18]. The advantage of liquid hydrogen is that its energy/mass ratio is three times greater than that of gasoline. It has the greatest energy density of any fuel in use (excluding nuclear fuels), and that is why it is employed in all space programs. However, its energy/volume ratio is low. Most liquid hydrogen tanks are spherical, because this shape has the lowest surface area for heat transfer per unit volume. As the diameter of the tank increases, the volume increases faster than the surface area, so a large tank will have proportionally less heat transfer area than a small tank, reducing boil-off. Cylindrical tanks are 37 sometimes used because they are easier and cheaper to construct than spherical tanks and because their volume/surface area ratio is almost the same [18]. The worldwide transport of hydrogen could be conducted in liquid form using ships. Common stationary liquid hydrogen tanks have capacities ranging from 1500 L up to 75,000 L of liquid hydrogen with radial dimensions of 1.4-3.8 m and heights of 314 m. The largest hydrogen tank was announced by NASA; this tank is located at ground level, and it has a hydrogen storage capacity of about 270 tones of liquid H 2. Another liquid hydrogen tank exists at a bus refueling station in London and is operated by British Petroleum. This tank is an underground storage tank [21]. 2.6.3 Hydrogen Storage as Metal Hydrides Metal hydride hydrogen storage uses a specific metallic compound that acts as an absorber and releases hydrogen at constant pressure. The purity of hydrogen used has a direct relationship with the life of the metal hydride storage cylinder. This type of storage is suitable for hydrogen fuel cell cars, where empty cylinders can be exchanged easily for full cylinders [18] . 2.6.4 Underground (Geological) Storage Underground hydrogen storage is one of the most promising technologies for largescale storage of low-pressure gas. Hydrogen can be stored in excavated rock caverns, salt domes, and depleted oil/gas fields. Table 2.2 summarizes the choices available for hydrogen storage with regard to storage times and hydrogen quantities. Method General use Underground Large quantities , long term storage times Liquid Large quantities , long term storage times Compressed Gas Small quantities , short term storage times Metal hydride Small quantity Table 2.2: Choices of hydrogen storage. 38 2.7 Hydrogen Transportation In energy terms, hydrogen has the potential to be a cost-competitive method of transmission over long distances. As shown in Figure 2.5, for any distance greater than 3000 km, the transmission of an equivalent amount of energy using hydrogen transmission would be cheaper than using electricity transmission by wires. There is always a need to have the capability for the long-distance transmission of energy, because the energy sources of the future are likely to be far from the industrial and population centres. This is the case for coal deposits and for nuclear energy, both of pose potential and serious pollution hazards. Similarly, for the case of using solar energy, the areas of maximum solar irradiation in North Africa, Saudi Arabia, Australia, and other tropical areas are generally far removed from populated areas [19, 20]. Figure 2.5: Transmission cost comparison between electricity and hydrogen from [20]. 2.7.1 Compressed Gas Transport Like natural gas, hydrogen can be transported by pipelines. In Germany, a 512-km hydrogen gas pipeline has been operational for several years, providing evidence that transporting hydrogen by pipelines is technologically feasible; the pipeline in 39 Germany varies in diameter from 80 to 150 mm, and the pressure in the pipeline is 150 psi [21, 22]. As an alternative to pipelines, hydrogen can also be transported in various sizes of pressure trucks and vessels, but, for long-distance transport, it is more practical and less expensive to transport hydrogen as a liquid [22]. . 2.7.2 Liquid Hydrogen Transport The liquification of hydrogen is achieved by cooling hydrogen gas below its boiling temperature of -253 °C. When gaseous hydrogen is changed to the liquid form, there is a vast reduction in the volume required to store a useful amount of hydrogen – particularly for vehicles. Figure 2.6 shows a heavy duty truck for a liquid hydrogen transport in a hydrogen production plant in the USA [Ref]. Figure 2.6: Heavy duty truck at hydrogen production plant USA [54] 2.7.3 Metal Hydride Transport Metal hydrides have the advantage of having a low volume/energy density. Hydrides are unique because some can adsorb hydrogen at or below atmospheric pressure and 40 then release the hydrogen at significantly higher pressures when heated. Depending on the alloy chosen, there is a wide range of operating temperatures and pressures for hydrides. This hydrogen transportation process has the advantages that no liquefaction is required and leakage and safety problems are minimised. Hydride transportation has the disadvantage of being limited to very small-scale usage; thus, it has poor mass/energy density values, and the metal alloys that must be used have relatively high costs. Table 2.3 compares the methods for transporting hydrogen and their general use. Method General use Pipeline Large quantities , long distance power transmission Liquid Large distances Compressed Gas Small quantities over short distance Metal hydrides Short distance Table 2.3: Methods of hydrogen transportation. 2.8 Attractive Advantages for Hydrogen as an Energy Carrier There are many advantages of hydrogen as an energy carrier: 1- Hydrogen has the highest energy content per unit weight of any known fuel. 2- When hydrogen is burned in an engine, it produces zero emissions; also, when it combines with oxygen from air in a fuel cell, it produces electric energy and the by products are water and heat. So, no greenhouse gases or other harmful emissions are produced. 3- Hydrogen and electricity are interrelated; they may be substituted for each other fairly easily. This could be valuable in most of our existing equipment and infrastructure if a hydrogen economy were to become commonplace. 4- Hydrogen can be produced locally from numerous sources; it can be produced in domestic places where it is used, or it can be produced at a central location and then distributed in gaseous or liquid form. 41 5- When hydrogen is produced by the electrolysis of water using renewable energy sources, the energy system can be deemed to be sustainable and secure; renewable energy sources, such as solar photovoltaic technology, wind, and hydro, can provide the power needed to produce hydrogen from water. 6- The use of hydrogen requires no new technological breakthroughs. Hydrogen production and its end use as an energy carrier already have been demonstrated; electrolysis and fuel cell technologies are well-established technologies that are currently being used. Hydrogen storage and transportation technologies would be similar to those used in natural gas supply systems. Hydrogen can replace oil and natural gas in most of their end uses, such as for vehicles and electric energy generator systems. 7- Hydrogen can be used in some applications with high efficiency, e.g., in fuel cell cars and in power system applications, in which hydrogen can be converted to electricity and heat at efficiency levels of around 80%. 8- When hydrogen is produced in large quantities and stored in large-scale facilities, it can be transported as an energy carrier over long distances at less cost transmitting electricity by wire. 2.9 Safety Aspects Associated with Hydrogen Hydrogen, like petrol and natural gas, is flammable and can be dangerous under specific conditions. But it can be handled safely if the safety precautions and guidelines are carefully followed. 1- The flammability limits of hydrogen with oxygen are wide. Thus, hydrogen combines explosively with oxygen when the limits are between 4 and 75%, whereas methane combines explosively with oxygen when the limits are between 5 and 15%. 2- Leakage can be a problem in hydrogen systems. The escape velocity of hydrogen is three times that of methane on a volume basis. However, because hydrogen only has about one-third of the energy per unit volume 42 that methane has, leakages of the two gases would amount to approximately the same amount of energy per unit time [20]. 3- Hydrogen is lighter than air and diffuses rapidly. At 25 0C and atmospheric pressure, the density of air is 1.225 kg/m3, while the density of hydrogen is only 0.083 kg/m3. The diffusivity of hydrogen is 3.8 times greater than that of natural gas. Since hydrogen is the lightest element in the universe, it is very difficult to confine. Engineers who design hydrogen systems must ensure that there is adequate ventilation in any installation where hydrogen is being produced, processed, or stored. There are commercially available combustible gas detectors combined with alarms that can sense hydrogen concentrations between 0% and 50% of the lower flammability limit (LFL) [23]. A meter, manufactured by Senko, was purchased for use in the laboratory. The alarm was set at an LFL of 4%, so that safe working conditions could be maintained throughout the experimental work. Summary Hydrogen could be an energy carrier of the future. It is a sustainable fuel option and one of the potential solutions for the current energy and environmental problems. Existing commercial production methods, such as steam methane reformation depends on the combustion of a fossil fuel, which produces CO 2 emissions. The electrolysis process however, can produce high purity hydrogen from water without generating CO2 . 43 CHAPTER 3 PHOTOVOLTAIC SOLAR ENERGY AND HYDROGEN PRODUCTION 44 3 CHAPTER 3 PHOTOVOLTAIC SOLAR ENERGY AND HYDROGEN PRODUCTION 3.1 Background Nuclear fusion is the energy source that heats up the sun. Although the sun‟s interior temperature is in excess of a million degrees Kelvin ( oK), its surface temperature is approximately 5700 oK, and it behaves approximately as a black body radiator with this temperature. It can be termed an inexhaustible energy source since it has been estimated that it will maintain its present stable state for another billion years [24]. The sun is the largest available energy source in our solar system; it supplies the earth with an annual energy of 1.5 x 1018 kWh; the energy received by the Earth from the sun in one hour is adequate for all human energy needs for nearly a year [25][26]. Incident solar radiation (insolation) is fundamental to most other sustainable energy sources, such as wind, waves, biomass, and hydropower. Figure 3.1 [26] shows the average insolation intensity for the entire world with an annual average provided for each location. The regions of maximum insolation are depicted in red, and the areas that receive minimum insolation are shown as blue regions. Both values have units of kWh/m2. Figure 3.1: World solar map [26]. 45 Insolation is attenuated partially as it crosses the atmospheric layers. A considerable portion of solar radiation is reflected back into space, preventing it from reaching the Earth's surface. This happens due to absorption, scattering, and reflection of the incoming radiation in the upper layer of the atmosphere (stratosphere), and the radiation can be attenuated further in the lower layer of the atmosphere (troposphere ), due to clouds and weather conditions. Solar energy can be transformed into a storable chemical fuel in the form of hydrogen. The use of solar energy to drive the electrolysis process in which hydrogen and oxygen are produced from water is a very promising process because it produces no harmful pollutants, it is easy to operate, and maintenance requirements are minimal. Photovoltaic, electrolyser, and fuel cell power systems could be used as an alternative for a photovoltaic system and batteries to provide a supply of power in remote areas. 3.2 Components of Solar Radiation Incoming solar radiation is categorized based on its various modes of interaction with the Earth‟s atmosphere. The four categories are: 1. The radiation passes directly the atmosphere, in which case it is called direct radiation. 2. Be reflected in which solar is reflected after it strikes an atmospheric particle. 3. The radiation can be absorbed by the atmosphere. 4. The radiation can be scattered by the contents of the atmosphere; when this occurs, it is called diffuse radiation, because the small particles and gas molecules it encounters scatter the radiation in random directions [24]. Figure 3.2 illustrates the four possible effects that the Earth‟s atmosphere can exert on incoming radiation from the sun. 46 Figure 3.2: Interactions of the Earth‟s atmosphere with incoming solar radiation [27] The total global radiation received on the Earth is the combination of the direct radiation (also called sunlight) and the diffuse radiation that reaches the ground. Direct radiation comes directly from the sun to the Earth. Diffuse radiation comes to the Earth after being reflected by the ground or by other surfaces it may have encountered. The reflected radiation from the ground is a function of the albedo (or reflectiveness ratio) of the Earth‟s surface [27]. As stated before, the sun has a very high surface temperature and the radiation corresponding to this temperature commonly is divided into various regions or bands on the basis of wavelength, as shown in Table 3.1 [28]. Spectral bands are divided into high frequency, visible frequency, and low frequency bands. High frequency Radiation type Ultra-violet Visible frequency Light 0.38 - 0.78 48 % Low frequency Infra-red > 0.78 45.6 % Frequency Wavelength m < 0.38 Table 3.1: Wave lengths of solar radiation [28]. 47 Fraction energy 6.4 % of 3.3 Measurements of Solar Radiation Insolation depends strongly on the location and local weather. Solar radiation measurements are taken using either a pyranometer (measures global radiation) and/or a pyrheliometer (measures direct radiation). 3.3.1 Pyranometer A pyranometer measures the total radiation arriving from all directions, including both direct and diffuse components. It measures all the radiation that is of potential use in the solar energy system. A typical pyranometer, shown in Figure 3.3 has a thermocouple mounted on a black carbon disc. The amount of voltage generated is related directly to the value of the insolation. Normally, the device is covered by one or two hemispherical glass covers to protect it from sand, rain, and other contaminants, which might affect the radiation measurements. Figure 3.3: Typical pyranometer 3.3.2 Pyrheliometer A pyrheliometer, Figure 3.4 [26], is an instrument that is used to measure insolation 48 resulting from direct solar radiation at a given location through a narrow collimating tube. Since the instruments must be pointed directly at the sun, pyrheliometers are typically mounted on a tracking device that follows the sun‟s movements [26]. Figure 3.4: pyrheliometer [26]. Measurements of diffuse radiation on horizontal surfaces also are made using a pyranometer. This can be achieved by shading the instrument to block the direct beam of sunlight. This is usually done by means of a shading ring as shown in Figure 3.5 [26]. Figure 3.5: A pyranometer used for the measurement of diffuse radiation [26]. 49 An alternative method that can be used to measure insolation has a photodiode sensor that produces current through a calibrated resistance to produce a voltage that is proportional to the insolation. This method is less expensive, but also less accurate, than the method based on thermopiles (devices that convert thermal energy to electrical energy). An example of this technique is the Daystar meter that was used in the present work. 3.4 Harnessing and Using Solar Energy Converting solar energy into useful power is based on capturing solar radiation and preventing it from radiating back into the atmosphere. Two methods are available, i.e., 1) thermal conversion by using solar thermal panels and 2) electrical conversion by using photovoltaic panels. 3.4.1 Thermal Conversion Solar thermal energy refers to technologies that convert radiant energy into usable heat energy to heat up water in a flat plate arrangement. Figure 3.6 illustrates a typical thermal collector. It consists of a large, insulated box with a glass or plastic cover that allows short-wave radiation to pass through and fall onto a dark, heatabsorbing metal plate. A coil of metal tubing through which water is circulating is located on the back of this plate. This solar thermal system could replace other energy sources, such as natural gas and electricity, as a provider of hot water to buildings in sunny regions. Figure 3.6: Flat plate solar thermal collector. 50 Solar thermal systems have four main components: the solar collector panels, water, the storage tank, and the controller. When solar collector exposed to solar radiation it heats up the water passages through the pipes. The water is circulated by pump through the thermal panel to the storage. Solar desalination thermal system can be used to produce drinking water solar by purifying the water in remote areas. It can be used through removing impurities as fluoride and salts to produce drinking water. In this system glass or transparent is used to cover plate of water which is mounted in front of black backing to trap solar energy. When the sun radiation heats the water in the still the water evaporates which then condensed and used as pure water. 3.4.2 Electrical Conversion The second way to use solar radiation is by converting sunlight into electricity. This is achieved by the use of a photovoltaic cell. In 1839, Edmond Becquerel discovered the photovoltaic (PV) effect when he immersed a silver chloride electrode in an electrolytic solution, connected the electrode to a counter metal electrode, and used white light for illumination. Under these conditions, he observed that a voltage and a current were produced. However, the birth of the modern era of PV solar cells occurred in 1954, when D. Chapin, C. Fuller, and G. Pearson at Bell Labs demonstrated solar cells based on p-n junctions in single crystal Si with efficiencies of 5-6% [32]. This original Si solar cell still works today, and single-crystal Si solar cells dominate the commercial PV market [33]. 3.5 General Description of PV Cell Technology Figure 3.7 [27] shows a schematic representation of a photovoltaic cell. The incident photons cause electrons in the photovoltaic material to be freed from atoms, and a current flows from the p-side to the n-side, as explained in the next section. A PV cell is made of semiconductor materials and behaves like other solid-state devices, such as diodes and transistors. The most widely used material for PV cells is silicon. There are two commercially available types of PV cell technologies, i.e., 51 crystalline silicon and thin film. In crystalline-silicon technologies, individual PV cells are cut from large single crystals. In thin-film PV technologies, the PV material is deposited on glass or thin metal that mechanically supports the cell or module. Figure 3.7: Schematic diagram of a photovoltaic cell 3.5.1 Silicon Solar Cell Types and Their Efficiencies Solar cells can be made of many different semiconductors. A crystalline silicon solar cell was used as an example for the theoretical and modelling study in this thesis for two reasons. First, crystalline silicon was the material used in the earliest successful PV devices. Second, and more importantly, crystalline silicon is still the most widely used PV material. Crystalline silicon has band-gap energy of 1.1 electron volts (eV). To produce a solar cell, the semiconductor must be “doped.” Doping is the intentional introduction of chemical elements that can obtain a surplus of either positively charged carriers (p-conducting semiconductor layer) or negatively charged carriers (n-conducting semiconductor layer) from the semiconductor material. A p-n junction results when two differently-doped semiconductor layers are combined and located at the boundary between the two layers. When light is incident on a p-n junction, charge carriers (electrons and holes) are released at its two sides. A key parameter for the charge collection of solar cells is the hole and the mobility of the electrons. The most common solar cell material is silicon. A silicon atom has 52 14 electrons, located in three deferent shells; the outer orbit contains four valence electrons, so it can share with its neighbouring atoms to complete the outer shell with eight electrons. In intrinsic silicon (no impurities added), each atom forms covalent bonds with four adjacent atoms. The results of the covalent bonding are 1) the atoms are held together to form a solid substance and 2) the silicon is a poor conductor of electricity because the four outer electrons are not free to move. n-type silicon semiconductor N (negative)-type semiconductors, Figure 3.8, are formed by conducting the process of doping impurities that have +5 valences, such as phosphorous (P) or arsenic (As), into the silicon semiconductor, with each of the two electrons of silicon and phosphorous atoms forming a covalent bond and one electron remaining free. This process is to increase the numbers of free electrons that have negative charges. The extra electron is only weakly bound to the atom, and it easily can be excited into the conduction band in the p-n junction. p-type silicon semiconductor The purpose of p (positive)-type, Figure 3.8, doping is to create abundances of holes, so impurities, such as boron (B) or aluminium (Al), are added to the silicon, and the result is that one electron is missing from one of four covalent bonds. Thus, the silicon atom can accept an electron from neighbouring atoms to complete the fourth bond. Figure 3.8: Covalent bonds in a silicon atom. 53 These free electrons and holes in the semiconductor are called carriers because they carry a charge from one place to another. When a photon of sunlight enters a semiconductor material, it can free an electron from its position in a p-type semiconductor, and the electron will obtain enough energy from the photon to move freely. The amount of energy required to free an electron is called the band gap of the material, and each material has its own distinctive band gap, which is referred to as the energy difference between the conduction band and valence band. Silicon cells are classified into three categories based on their crystal types, i.e., monocrystalline, polycrystalline, and amorphous. Pure semiconductor material is necessary for the production of monocrystalline cells. Monocrystalline rods are extracted from molten silicon and then cut into thin plates for further processing to form a p-n junction. Polycrystalline cells are produced by pouring liquid silicon into blocks; during this process, crystal structures of varying sizes are formed within a block of material. Again, thin plates are cut from these blocks, and further processing produces a p-n junction. An amorphous or thin-layer cell is formed by depositing a silicon film on glass or another substrate material; the thickness of the layer is less than 1 m. In this case, the p-n junctions are formed during the deposition process. The efficiency of each type of cell is shown in Table 3.3 [32]. This clearly illustrates that monocrystalline cells have a superior performance, but the cost of these cells is higher than that of the other two cells. A typical monocrystalline photovoltaic cell produces less than five amperes per square meter at approximately 0.5 volt DC. Consequently, cells must be connected in series and parallel configurations to produce enough power for any appliance, whether it is to be used for an industrial or a domestic application. 54 Material Approximate level of Monocrystalline Silicon Polycrystalline Silicon Amorphous Silicon efficiency in the laboratory % 24 18 13 Approximate Level of efficiency in Production, % 14 to 17 13 to 15 5 to 7 Table 3.2: The efficiencies of the threee types of crystalline silicon cells [32]. 3.6 Photovoltaic Systems The photovoltaic hierarchy is shown in Figure 3.9 [28]. Electricity is produced by an array of individual PV modules connected in series and parallel to deliver the desired voltage and current. Each PV module, in turn, is constructed of individual solar cells that are connected in the same manner. Photovoltaic (PV) systems are clean, renewable sources of energy that have been used in stand-alone applications for many years. However, with the growing concern over greenhouse gas emissions and other environmental issues, renewable energy sources, such as PV systems, increasingly are being connected to the electricity grid. Europe and Japan are at the forefront of the development of grid-connected PV systems. Figure 3.9: Photovoltaic hierarchy [28]. 55 According to Solarbuzz solar photovoltaic (PV) panels cost an average of $3.05 per watt in Europe, and they have a 20-year lifetime, with an average output of approximately 10.6 W/ft 2 (114 W/m2). The price of the active material and cost of the manufacturing are the main elements that determine the total price of PV technologies. New processes are being researched for producing low-cost wafer silicon (both monocrystalline silicon (sc-Si) and polycrystalline silicon (pc-Si)) so that low-cost materials can be used for thinfilm PV applications. The development of PV systems has been a major focus in Europe and in North America, and highly successful projects have been completed, as shown in Table 3.3[28]. Ontario Canada‟s expanded Sarnia PV plant with a peak power of 97 MW is the largest PV power plant that has been commissioned so far in 2011 [28]. Plant Name Size (MW) Location Sarnia 97 Ontario, Canada Montalto di Castro 84.2 Italy Solarpark 80.7 Germany Table 3.3: World‟s three largest PV systems as of June 2011 . The three most common types of photovoltaic systems are: Photovoltaic systems that feed power directly to into the utility grid. The photovoltaic systems deliver DC power to a power conditioning unit (PCU) that converts the DC to AC and sends the AC power to the load. If the photovoltaic array does not supply enough power to satisfy the demand of the load, the PCU draws additional power from the utility grid. Stand-alone photovoltaic systems, which are effective in remote areas where there is no grid. These systems avoid the need for expensive (and possibly high maintenance) generators. However, such a stand-alone PV system require a means of storing electricity so that power can be maintained during the night when there is no solar insolation. 56 A solar tracker is occasionally used to make a PV array more efficient, but the added cost is often prohibitive. 3.6.1 Applications of PV Systems There are many applications in which PV systems can be used. Some examples of these applications are provided below: Remote site electrification Pumping water and operating treatment systems Healthcare systems Communications Disaster relief applications Security systems Cathodic systems 3.6.2 Attractive Features of Photovoltaic System The photovoltaic systems are known to have many attractive features such as: 1- In many small applications, such as in sunny, rural areas, PV systems are more viable economically than other alternatives. 2- PV systems do not require a fuel to generate electricity. 3- PV systems are more reliable than diesel and wind generators because they do not have moving parts. 4- PV systems consist of individual solar panels and modules. This makes it relatively easy to provide the appropriate size for any particular installation. 5- The expected life of PV cells is about 20 years. 6- No harmful pollutants are created during the operation of PV systems, so there are no harmful emissions. 7- Simple, routine, low-cost cleaning is adequate for maintaining a PV system. 57 3.7 Solar Hydrogen Production Systems Based on the types of energy inputs, hydrogen production using solar energy can be classified into the three types as described below: 3.7.1 Solar Photovoltaic-based Electrolysis This method is based on using electricity produced by PV panels to produce hydrogen by the electrolysis of water. Electrolysis is conducted by passing direct electric current (DC) through the water to generate hydrogen and oxygen. One advantage of PV- electrolyser technology is that it does not emit greenhouse gases. The efficiencies of modern photovoltaic systems and electrolysers are about 20% and 80%, respectively, and the total efficiency of transforming solar radiant energy to energy in the form of hydrogen is nearly 16% [33]. 3.7.2 Solar Photoelectrolysis Photoelectrolysis, which integrates solar photovoltaic energy combined with the electrolysis of water into a single photoelectrode, uses solar energy to extract hydrogen directly from water. Photoelectrolysis uses photoelectrochemical (PEC) light-collecting systems to power the electrolysis of water. When a semiconductor photoelectrode submerged in an electrolyte is exposed to sunlight, the semiconductor generates a voltage that is high enough to extract hydrogen and oxygen from water. 3.7.3 Hydrogen Production by Concentrated Solar Thermal Energy This method uses the thermal energy produced by concentrated solar radiation in heating up a fluid or a chemical source of hydrogen, such as water or fossil fuels, respectively. The most familiar method of hydrogen production is the thermal decomposition of natural gas (NG) in a high-temperature solar chemical reactor. 58 3.8 System Components of the PV-Electrolyser Hydrogen Production Process The energy generated by a photovoltaic array must be combined with a storage system due to the irregular availability of solar radiation. The conventional energy storage method in PV stand-alone systems is batteries. In this thesis, we describe how solar energy can be stored in an alternative energy carrier form by using the electrolysis of water to produce hydrogen. Hydrogen generation and storage is gaining importance in the emerging “hydrogen economy,” and, therefore, it is important to assess the feasibility of a PV - hydrogen electrolysis system. For the particular situation of remote areas in Libya, replacing a diesel generator by a system that consists of a photovoltaic array, an electrolyser, and a hydrogen tank/fuel cell combination could eliminate the difficulties associated with the use of generators , in remote areas, such as the expense and the difficulty of transporting fuel to these areas. If the “hydrogen economy” gains greater acceptance in the future, the face of manufacturing and industry may change significantly. In industrial development, there is a continuing challenge in using fuels and in the demand to lower CO 2 emissions. If hydrogen were to be used on a large scale to replace fossil fuels, many benefits would be realised. 3.8.1 PV Electricity Generation The characteristics of a 60-W, commercial PV panel under different levels of irradiance (kW/m2) are shown in Figure 3.10. The irradiance has a large effect on the short-circuit current (horizontal part of the I–V curves), while its effect on the opencircuit voltage (vertical arm of the curves) is rather weak. The maximum power (Pmax) output of a photovoltaic cell changes with irradiance, i.e., the cell generates more power when the irradiance is higher. Another factor that affects the PV power is temperature, as shown in Figure 3.10. As temperature increases, both voltage and power decrease. This is a particularly severe problem, since the cells are often operated at the maximum power point (mpp). 59 Figure 3.10: Effects of insolation and temperature on the characteristics of a PV panel . 3.8.2 The Electrolyser Water electrolysis is a process in which immersed electrodes are used to pass electricity through to split it into positive hydrogen ions (H +) and negative oxygen ions (O-), which collect at the cathode and anode, respectively. This process is an efficient method for producing high-purity hydrogen without emitting any harmful pollutes or causing any negative environmental impacts. The process must have an electrical input, which can be provided by renewable sources, such as solar, wind, hydroelectric, and geothermal sources. 3.8.2.1 Alkaline water electrolyser Alkaline water electrolysers usually use an electrolyte that contains an aqueous solution of potassium hydroxide (KOH). Figure 3.11 [34] shows a schematic construction of alkaline water electrolyser. Alkaline electrolysers operate at relatively low current densities of <0.4 A/cm2, and their conversion efficiencies range from 60-90%. 60 Figure 3.11: A schematic construction of alkaline water electrolyser [34]. The chemical reactions that occur in an alkaline electrolyser are shown below: The reaction at the anode is: 4H2O + 4e- 2H2 + 4OH- …………………………….................3.1 The reaction at the cathode is: 4OH- O2 + 4e- + 2H2O ………………………………………….3.2 The overall reaction is: 2H2O 2H2 + O2 …………………………………………………3.3 3.8.2.2 Proton Exchange Membrane (PEM) Electrolyser The operation of a PEM electrolyser depends on the use of costly metal catalysts (platinum, platinum/ruthenium) and a solid polymeric electrolyte for transferring protons as schematically shown in Figure 3.12 [35]. The main advantages of a PEM electrolyser over an alkaline electrolyser [12] are: No requirement to pass a liquid electrolyte Much smaller mass and overall dimensions Much lower power consumption The production of higher purity gases 61 The production of compressed gases in the plant with a higher level of safety Operation at a high current density and the built-in ability to operate with transient variations in electrical power input (Hence, it has excellent application flexibility with respect to capturing variable renewable electricity supplies, such as wind and solar power.) Figure 3.12: Schematic diagram of a proton exchange membrane electrolyser [35]. In a PEM electrolyser, the following steps occur to produce hydrogen: Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons). The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode. At the cathode, the hydrogen ions combine with electrons from the external circuit to form hydrogen gas. Anode Reaction: 2H2O → O2 + 4H+ + 4e- ……………………………………………..3.4 Cathode reaction 4H+ + 4e- →2H 2 ………………………………………………………….3.5 62 3.8.2.3 High-Temperature Electrolyser A high-temperature electrolyser is a highly efficient method that is used for massive hydrogen gas production from steam at high temperatures by utilizing both heat and electric power. The advantage of the high-temperature system is its ability to substitute heat for part of electrical energy required to split the water. 3.9 PV Electrolyser Coupled with Maximum Power Point Tracking The I-V photovoltaic characteristics vary considerably with solar insolation and temperature, as shown in Figure 3.13. The operating point at which the PV panel generates its maximum power, Pmax, at a particular voltage, Vmp, and current, Imp, is called the maximum power point (MPP). In some applications, a PV array and load can be coupled directly because they work on the same DC voltage; this method is simple and reliable, but it does not operate at the maximum power point of the PV array. Other applications may require a DC/DC converter to adjust the PV voltage, as shown in Figure 3.13. Figure 3.13: PV coupled with an electrolyser using a DC/DC converter for MPPT . The DC/DC converter may have facilities for computer control, and, then, both voltage adjustment and MPP tracking can be achieved by a software routine. Such a system is described in the following paragraphs. Three different types of DC/DC converters were used in coupling the PV array with 63 the load. The Buck converter, shown in Figure 3.14, is commonly used to step down the voltage from the PV source to the operating load voltage, while a Boost converter is used to step up the voltage. The third type is a combination of the previous two converter types, and it is called the Buck-Boost converter. A Buck-Boost converter is capable of increasing or decreasing the PV voltage to any voltage that is needed by the load. IPV VPV L S1 C1 D iL C2 Vload R load Figure 3.14: Photovoltaic coupling with load using a DC/DC Buck converter A DC/DC converter uses transistor switches as simple on-off switches, which allows the current to pass through the converter circuit or blocks it from doing so. The relationship between the input and output voltages of the converter is controlled by the duty cycle of the switch itself. The duty cycle D (0 < D < 1) is the fraction of the off-time duration of the converter switch cycle, which is referred to as pulsewidth modulation. In the case of a photovoltaic–electrolysis hydrogen production system, the DC/DC converter steps down the PV voltage to the electrolyser operating voltage. As mentioned earlier, the electrolyser cell is capable of producing hydrogen at high efficiency under high current density conditions. The consumption of power is proportional to the instantaneous current density, so the main point to consider is the amount of power or current that can flow to the PEM cell from the DC-DC converter, and this is the task of the control unit of the converter. Since current depends on solar insolation and voltage depends on temperature, the MPP changes as ambient conditions change. The maximum power point tracker is used to make the PV deliver its maximum power. For this purpose, an analog or 64 digital circuit implemented in the DC/DC converter, known as the maximum power point tracker (MPPT), provides an interface between the PV panel and the load. This is usually done by slightly varying the duty cycle of the converter. The current in a PV array varies significantly as insolation varies, whereas the voltage of the PV array varies far less. For a silicon solar cell, the voltage variation at the maximum power point is about 8%, whereas the current at the maximum power point varies by as much as 80% [3]. Moreover, the voltage of the PV array at the maximum power point has a fixed linear relationship with the open circuit voltage, so it is better to choose the voltage as a controlling variable of the duty cycle of the converter. 3.10 Maximum Power Point Tracking Technologies In PV systems, significant efficiency gains can be achieved by keeping the load operating point near the knee of the PV panel‟s I-V curve. The three main methods of maximum power point tracking are described below. 3.10.1 Perturb and Observation (PAO) Method In the perturb and observation method, the output current and voltage of the PV array are measured and the resulting power is calculated. The algorithm is based on comparing this calculated power with previous values. A detailed analysis of the PAO method is given in Chapter 6. A drawback of the PAO method is that the operating point oscillates around the MPP. Also, it takes considerable time to track the MPP. In addition, the PAO algorithm can be confused during rapidly changing atmospheric conditions [3]. 3.10.2 Incremental Conduction This method consists of using the slope of the derivative of the power with respect to the voltage in order to reach the maximum power point. The mathematical relationships of power and voltage that are used to track the MPP can be expressed 65 as: dP/dV > O, left of MPP dP/dV = O, at MPP dP/dV < O, right of MPP 3.10.3 Fractional Open Circuit Voltage A PV cell's open circuit voltage will vary under irradiance and temperature conditions with approximate similarity to an array under load. This is the principle behind fractional open circuit voltage (FOCV) and pilot cell methods. It can be represented as simply as: VMPP ≈ k ·VOpen The proportionality constant k depends on the qualities of the particular PV cells being used. In the fractional open-circuit voltage scheme, the array is momentarily disconnected from the converter at regular intervals, and the open circuit voltage is measured. Of course, this results in a temporary loss of power. An alternative is to use one or more pilot cells, which are selected to have the same qualities as the cells in the array. In this case, the main array is always connected to the converter, and the pilot cells are available continuously for voltage circuit optimization. Summary We can conclude that the combination of a photovoltaic array with water electrolysis can transform solar energy into hydrogen. The method is attractive since it requires little maintenance and is environmentally friendly. Furthermore, future cost reduction of PV cells is expected on the industrial scale. As the insolation varies during the day and the year the direct coupled between a photovoltaic source and the electrolyser is unlikely to be at its optimum most of the time. Hence a power conditioning device is needed for the system to maximize hydrogen production. The following chapters will examine DC/DC converters and their role in helping to optimise a photovoltaic – PEM electrolysis hydrogen production system. 66 CHAPTER 4 A POWER MATCHING SIMULATION OF A SOLAR HYDROGEN PRODUCTION SYSTEM 67 4 CHAPTER 4 A POWER MATCHING SIMULATION OF A SOLAR HYDROGEN PRODUCTION SYSTEM 4.1 PSCAD Software PSCAD is a simulation tool for Power System Computer-Aided Design and Electromagnetic Transient for DC. This simulation package has been used often in many renewable power system simulation and design studies. The advancement-oftime simulation has reduced the effort required significantly. One of the advantages of computer simulation is its flexibility and convenience, because parameters of a system model can be easily manipulated; also, computer simulation is essential during the first stages of design to avoid the cost of errors being detected in the later stages of design. In this part of this thesis, the use of PSCAD to develop a simulation model for the solar hydrogen production system is discussed. The system model simulated the performance of power matching between the PV module and the proton exchange membrane (PEM) electrolyser using a DC/DC Buck converter. One of the challenges in producing hydrogen using solar energy (PV-Hydrogen production system) is to keep costs down. Therefore, it is important that the system operate at maximum power. This operation is achieved by matching the power generated by the PV cell with the power required to produce hydrogen. 4.2 Model Components Only the essential components of the solar hydrogen production system were included in this simulation programme. As seen in Figure 4.1 the components included were a photovoltaic module, a DC-DC Buck converter with a duty cycle control circuit, and a PEM electrolyser. 68 Figure 4.1: PV- electrolyser power matching using DC/DC buck converter. 4.3 Input/Output Data The input data to the simulation programme were the solar radiation hitting the photovoltaic module and the ambient temperature. The output results of the simulation are: Characteristics of the photovoltaic current, voltage, and power under standard test conditions (1000 W/m2 and 25 oC). Characteristics of the current and voltage of the electrolyser. Current and voltage readings at the input and output of the DC-DC buck converter under different duty cycle values of the converter switch. Definition of the duty cycle at maximum power matching. To match a PV module with an electrolyser, the first requirement is to know the current (I)–voltage (V) characteristics for both the power source and the load, since each of them is being modelled in PSCAD, and they are coupled with a DC/DC buck converter that acts as the power conditioner of the system. 69 4.4 The PV Model 4.4.1 PV Equivalent Circuit Figure 4.2 shows the equivalent circuit of an ideal PV cell, which is a current generator connected in parallel with a diode. The photocurrent, Iph, represents the current generated by light in proportion to the photons of solar flux hitting the PV cell. In the simplified model, a series resistance was added to represent the voltage losses occurring at the boundary and external contacts. For the practical PV cell model, a shunt resistance Rsh was added to represent the leakage of current that occurred in the cell. In this case, the Iph delivers current to the diode, the parallel resistance R sh, and the load. Figure 4.2: Equivalent circuit of the PV cell. For the actual PV cell and its equivalent circuit, there are two conditions of particular interest, as shown in Figure 4.3, i.e., the current that flowed when the PV terminals were shorted together (Ish short circuit current) and the voltage across the PV terminal when its leads were kept open (Voc open circuit voltage). The I-V characteristics for the PV cell are shown in Figure 4.3. 70 Figure 4.3: I-V characteristics of the PV cell. The working point of the solar cell depends on the load and the solar insolation. The maximum power point (mpp), at which the PV cell delivers its maximum power (Vmp and Imp), occurred near the knee point along the characteristic I-V curve. Adding a series resistance RS to the PV equivalent circuit (Figure 4.4) caused the voltage in the I-V curve to shift by V IRS . For the current leakage, the value of Rsh was generally kept high, and, so, adding the parallel resistance RSh caused the current to decrease by a small amount, V/RSh as seen in Figure 4.5. I Rsh = , Rs 0 V IRs V Figure 4.4: Effect of adding RS on the PV cell‟s I-V curve. 71 I Rsh = , Rs 0 Slope=1/Rsh I V Rsh V Figure 4.5: Effect of adding RSh on the PV cell‟s I-V curve. 4.4.2 PV PSCAD Model Figure 4.6: Model of the PV module. The model takes into consideration the variation in insolation and temperature. Changes in insolation affect the photon-generated current and had a relatively insignificant effect on the open circuit voltage, whereas temperature variations caused the open circuit voltage and the short circuit current to vary only a marginal amount. The following general equation gives the relationship between the current and voltage of a PV cell [38]. The output current (I) is equal to the difference between the photocurrent and the diode current, Id. This output current (I) is given by: qV I I ph I o exp 1 …………………………………..……... 4.1 KT 72 where V is the voltage; Io is the dark saturation current; q is the electron charge (1.602 × 10-19 C); k is Boltzmann‟s constant (1.38 × 10-23 J/K); and T is the temperature. The open circuit voltage was measured at the terminals by setting the output current, I, equal to zero. The open circuit voltage is given by the equation: Voc KT I ph ln 1 ……………………………………………..…...4.2 q Io The current-voltage characteristics of the crystalline silicon cell module can be described by the following formulae [41], which were implemented in FORTRAN codes (appendix A) inside the PSCAD model: T Ta S ( NOCT 20) ..........................................................................4.3 800 E g 1.16 7.02 10 4 T 2 (T 1108)........................................................4.4 I 0 I do ( qE g 1 T 3 1 ) exp( ( ))...........................................................4.5 Tref nK Tref T s J o (T Tref ).......................................................................4.6 1000 qV I d I 0 exp( 1).....................................................................................4.7 nkt I I ph I d ..................................................................................................4.8 I ph I sc where : T = cell temperature in oK T a = ambient temperature, oX I 0 = dark saturation current, A E g = energy gap of cell semiconductor, ? I d = diode current, A 73 I ph = photo current or light-generated current, A V = cell output voltage, V In this PV cell model, a crystalline silicon solar cell was considered, the parameters used in modelling of this type of silicon PV cell are tabulated in Table 4.1 [41]. Symbol/Value q = 1.602 x10 Description 19 Unit Electron charge C k = 1.38x10 23 Boltzmann constant n = 1.792 Non-ideality factor T Input data T ref = 293 Ambient temperature o K Reference temperature o K I sc Short circuit current at reference state A NOCT = 49 Nominal Operating Cell Temperature o J o = 1.6 10 3 Temperature coefficient S Input data I do do = 71.1 10 9 Insolation J/oK C A/oK W/m 2 Diode reversal current A Table 4.1: Parameters of a crystalline silicon solar cell [41]. 4.4.3 Response of the Model to Changes in Insolation The characteristics of the solar cell at different levels of insolation (S) are shown in Fig 4.7. The insolation has a large effect on the short-circuit current (the horizontal part of the I–V curves). If the insolation level increases, the short circuit current increases, keeping the open circuit voltage almost constant (where S is the value of insolation in W/m2). According to the voltage and power curves, the maximum output power of a photovoltaic module changes as insolation changes. When the insolation is greater, the cell generates more power. 74 Figure 4.7: The dependence of I-V characteristics on insolation. 4.4.4 Response of the PV Model to Changes in Temperature The cell temperature varies because of the changes in the ambient temperature and changes in the levels of irradiance. Since only a small fraction of the insolation on a cell is converted to electricity, most of that incident energy is absorbed and converted into heat. Figure 4.8 shows that the cell temperature (in oK) of the PV module increases, while the open circuit voltage decreases, and the short circuit current is almost constant. Figure 4.8: Effect of temperature on the PV model curves. There is a maximum power generated by a PV module occurring at a point called the maximum power point (mpp) with the coordinates V = maximum voltage Vm and 75 maximum current Im. The PV power source characteristics used in this PSCAD simulation are shown in Figure 4.9. Figure 4.9: PV characteristics used in the PV-PEM PSCAD model. 4.5 DC-DC Buck Converter PSCAD Model If the PEM electrolyser is connected directly to the PV module, the operating point is the result of the intersection between the I-V curves of the electrolyser and the PV module. However, if a DC/DC buck converter is placed between the load and the PV module, the operating point depends on the duty cycle of the converter switch. Direct coupling may result in somewhat lower efficiency, due to the losses related to power/voltage matching. A DC-DC buck converter was used for the PV-PEM electrolyser to step down the output voltage of the PV to the level of the PEM operating voltage and to make the PV and PEM electrolyser work at their maximum power. The DC-DC buck converter (described in more detail in Chapter Five) was used in this PSCAD model as a power conditioner between the PV model and the PEM electrolyser model, as shown in Figure 4.13. The converter was built using passive and active elements available in a PSCAD package library. All the elements were considered to be ideal. Also, the converter 76 was designed to operate in continuous conduction mode. A Pulse-Width Modulation (PWM) circuit was used to control the duty cycle D duration (ON and OFF time durations) of the converter switch. The pulse-width modulator was generated by comparing a signal-level DC voltage with a repetitive sawtooth wave form that had a constant peak, as shown in Figure 4.10. Figure 4.10: PWM to produce different duty cycle generator. The duration of the on and off states controls the relationships between the voltage and current on both terminals of the DC-DC buck converter, as the following equation indicates: D(dutycycle ) Ton Vpem Ipv …………………………………. 4.9 Ton Toff Vpv Ipem where ( V pv and I PV ) are the PV voltage and current and ( v pem and I pem ) are the PEM electrolyser voltage and current, respectively . 4.6 The PEM Electrolyser PSCAD Model A PSCAD model of a two-cell PEM electrolyser was developed. This kind of water electrolyser has advantages over the traditional water electrolyser, i.e., it operates at high current density; it avoids using a liquid electrolyte as the alkaline electrolyser does; and it produces a high purity gas that is ideal for use as a fuel for the fuel cell. 77 Figure 4.11: PSCAD PEM electrolyser block. As shown in Figure 4.11, the PEM electrolyser PSCAD model had two inputs and two outputs, i.e., the inputs are current and temperature, and the outputs are voltage and hydrogen volume. As mentioned in Chapter Three, the production rate of hydrogen depends on the current flowing into the electrolyser and the temperature of the water. If the temperature increases, the operating voltage of the electrolyser will decrease; more current flows at a higher temperature. The equations below govern the relationships between the input and output variables. The I-V relationship of a PEM cell at a cell current density of (1 A/m2) is given by the following equations [39][42]: V V0 c a IR , …………..………………………………..4.10 where V is the cell voltage of the PEM, and V 0 is the theoretical dissociation voltage, which depends on absolute temperature T ( oK), as shown: V0 1.5 1.5e 3T 9.5e 5T ln(T ) 9.8e 8T 2 …………………….4.11 The term ηc is an excess voltage on the cathode side with a value varying from 0.05 to 1 V; ηais an excess voltage on the anode side with a maximum value of 0.3 V; R is cell resistance of the PEM electrolyser at the water temperature of 293 OK (25 OC), and its approximate relationship with current [42] is: 78 e ( Ipem1 / 5.7 ) 1 R 0.31 ( Ipem1 / 5.7 ) ……………………..………………………..4.12 1 e where I is the current flowing through the PEM electrolyser. The PEM temperature T was adjusted to room temperature (294 0K). The volume of hydrogen produced was calculated by applying Faraday‟s first law of electrolysis: VH ( m 3 ) R.I .T .t F .P.Z ………………………………………………………..4.13 Where R =8.314 Joule /(mol Kelvin), I = current in A, T is the temperature in K , t = time in sec , F Faraday‟s constant = 96485 C/mol, P ambient pressure in P, Z = number of excess electrons = 2 (for hydrogen) and 4 (for oxygen). Figure 4-12 shows the I-V and P-V characteristics of the PEM cell electrolyser model. The voltage-current graph shows that, for the PEM electrolyser, the current only starts to flow at a certain voltage, after which it rises continuously. The slope of the curve is dependent on the electrolyser equivalent ohmic cell resistance. The applied voltage must be at least above the threshold PEM cell voltage in order for current to flow, which leads to a release of hydrogen at the cathode and oxygen at the anode. Figure 4.12: I-V and P-V curves for the PEM electrolyser. 79 4.7 PV-PEM Hydrogen Production Power Matching Model The PSCAD model of the PV electrolyser hydrogen production system with a buck converter for power matching is shown in Figure 4.13. The power supply of the circuit is a PV module, and its characteristics are shown in Figures 4.9 and 4.12. The short-circuit current was 0.7 A, and the open-circuit voltage was 20 V; the operating voltage of the two-cell PEM electrolyser was approximately 5.3 V. Figure 4.13: PV-PEM electrolyser PSCAD model The component sizes for the DC/DC buck converter were selected as follows [40]: The inductor L value was calculated as: L Vom (1 Dom ) ,………………………………………………………..4.14 f s I LM where: 80 fs = 1 is the switching frequency Ts Dcm is the duty cycle at maximum converter output power ∆ILm is the peak-to-peak ripple of the inductor current Vom is the maximum of the DC component of the output voltage Iom is the DC component of the output current at maximum output power In this simulation, the inductance value L was selected as 0.6 mH. The output capacitor value calculated to give the desired peak-to-peak output voltage ripple was: CO Dcm I om rf sVom , …………………………………………………………4.15 where r is the output voltage ripple defined as r = (∆Vom / Vom), and ∆Vom is the peakto-peak value of the output voltage at maximum power (assumed to be equal to 0.05 V). In this simulation, Co is selected at 500 F. If we need the ripple of the PV output current to be less than 2% of its mean value, then the input capacitor value can be calculated by: Cin (1 Dcm ) I om Dcm ,………………………………………………….4.16 0.02 I pvm R pvm f s where Iom is the converter input current at maximum input power, and Rpvm is the internal resistance of the PV array at the maximum power point, which is defined as: R pvm V pnm I pvm , ………………………………………………………4.17 Cin was selected as 2000 F. 81 where Vpvm is the output voltage of the PV array at the maximum power point. The series resistance Rs was set as 0.08 , and the shunt resistance R sh was set as 200 . To generate different duty cycle values for the DC/DC converter switch, a fixedamplitude, sawtooth signal was compared with a changeable voltage level. A comparator produced pulses with different duty cycles, and the pulses switched the buck converter on and off. 4.8 Simulations and Results The following PSCAD simulation results were obtained for the standard irradiance (1000 W/m2) and the standard temperature (25 0C). The measured values are in volts for voltage readings and in amperes for current values. The DC/DC buck converter model had no losses, and it acted as an ideal step-down DC transformer, making the input and output power of the converter the same value. Figure 4.14 shows the V-P characteristics of both the PV array and the PEM electrolyser. The PV array was able to supply up to 8.3 W. In order for the PEM electrolyser to operate at this power, its voltage had to be 14.7 V. The values of the voltage of the PV array and the PEM electrolyser were not equal to each other. Figure 4.14: V-P curves for the PV array and the PEM at different duty cycle values (D). 82 With a duty cycle value of one, the PEM electrolyser was connected directly to the PV array. The operating point corresponding to this duty cycle was determined by the intersection between the V-I characteristics of both the PV array and the PEM electrolyser, as shown in Figure 4.15. This corresponded to a voltage of 5 V and a current of 0.66 A. The power supplied to the electrolyser in this case was 4 W. This is less than the maximum value of 8.3 W. Figure 4.15: Intersection with the rescaled I-V curve of the PEM. The lossless buck converter modelled in this chapter operated as an ideal transformer for DC power. The voltage at its input terminals was equal to the voltage at its output terminals divided by the value of the duty cycle. On the other hand, the current at its input terminals was equal to the current at its output terminals multiplied by the value of the duty cycle. With the PEM electrolyser supplied from a buck converter, its I-V characteristics, as seen at (referred to) the input terminals of the converter were scaled by the duty cycle of the converter. Each value of a duty cycle would produce a different characteristic at the input terminal of the buck converter. Examples of these characteristics are shown in Figure 4.16 for some possible values of duty cycle. 83 PV maximum power Figure 4.16: Intersection with the rescaled I-V curve of the PEM. Figure 4.17 also shows the I-V characteristics of the PV array superimposed on the set of I-V characteristics of the PEM electrolyser referred to the PV array side of the converter. The intersection between the I-V characteristics of the PV array and those of the PEM electrolyser corresponding to a certain value of duty cycle is the operating point for this duty cycle. As shown by Figure 4.16, reducing the duty cycle will move the operating point to the right. With a duty cycle of D = 0.4, the I-V characteristics of the electrolyser, referred to the PV array side of the converter, intersected the I-V characteristics of the PV array at 14.7 V. This is the voltage at which the PV array supplied its maximum power. With this operating point, the voltage at the electrolyser side of the converter was 5 V, and the current was 1.4 A. Reducing the duty cycle below 0.2 would cause the I-V characteristics of the electrolyser not to intersect those of the PV array. This means that, if the PEM electrolyser is to operate, it would need a voltage on the PV array side of the converter that is higher than the open-circuit voltage of the PV array. The PV array would fail to supply this voltage. Figure 5.18 shows the variation of the current and voltage of both the PV array and 84 the PEM electrolyser with the change of duty cycle. The electrolyser voltage was almost constant, whereas the electrolyser current increased from 0.56 A at a duty cycle of 1.0 to 1.4 A at a duty cycle of 0.4, before it decreased again to 0.005 A at a duty cycle of 0.1. Hydrogen production rate in m3 Figure 4.17: Relationship between current, voltage, and duty cycle. 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Duty cycle D Figure 4.18: Relationship between hydrogen production rate and duty cycle. 85 The relationship between hydrogen production rate and duty cycle value is shown in Figure 4.18. The theoretical hydrogen production volume (m3) was calculated by applying Faraday‟s first law of electrolysis, which showed that, at a D value of 0.4, the maximum hydrogen production rate occurred, while direct connection (where D =1) has less hydrogen production rate. The power matching duty cycle D can be calculated by using voltage and current values, as follows: dutycycleD Vpem (V ) 5.88 Ipv( A) 0.56 0.4 Vpv (V ) 14.71 Ipem( A) 1.40 The duty cycle needed to achieve maximum power point operation is equal to the ratio between the voltage of the PEM electrolyser and the PV array voltage at its maximum power point. Dutycycle VPEM Vmpp ………………………………………….……….4.18 In the equation below, it is apparent that the maximum power point voltage (Vmpp) of the PV array has an almost linear relationship with the open-circuit voltage (Voc) of the solar photovoltaic module: Vmpp KVoc …………………………………………………….….4.19 where K is a constant that has different values for different solar panels, and Voc is the open-circuit voltage. The open-circuit voltage (Voc) can be measured by disconnecting the PV at regular intervals. From the practical measurements and characteristics Table 4.2 of different commercial photovoltaic modules undertaken by Thomas and 86 Nelson [6]. The constant factor K of various PV modules were shown to be similar. Model number Model manufacture Type of silicon Voc cells (V) Vmpp (V) K SQ-75 Shell Monocrystalline 21.7 17 0.78 ND-NOECU Sharp Multicrystalline 24.9 20 0.80 Hip-J54BA2 Sanyo Amorphous 66.4 54 0.81 NT-185U1 Sharp Single crystalline 44.9 36.2 0.80 Table 4.2: Constant K value of different solar modules tested [6]. Summary A PSCAD simulation model was developed for photovoltaic (PV) module and proton exchange membrane (PEM) electrolysis hydrogen production system. The system model simulates the performance of power matching between PV and PEM using a DC/DC buck converter. The results show that by adjusting the duty cycle of the buck converter, the system could be optimized and operate at its maximum power. 87 CHAPTER 5 DESIGN DC/DC BUCK CONVERTER FOR PV-PEM HYDROGEN PRODUCTION SYSTEM POWER MATCHING 88 5 DESIGN DC/DC BUCK ONVERTER FOR PVPEM HYDROGEN PRODUCTION SYSTEM POWER MATCHING 5.1 Background A DC/DC converter is an electronic circuit that is used to convert a direct current (DC) source from one voltage to another. Conventionally, linear regulators have been used to regulate voltage; the resistance of the linear regulator varies in accordance with the constant output voltage level. Losing power (i.e., voltage drop across resistance multiplied by the current flow) is one of the main disadvantages of this type of voltage regulation, as the dissipated power is in the form of heat. Also, this type of regulator only can be used for cases in which the required output voltage is lower than the input voltage. It is not possible to have an output voltage that is greater than the input voltage. A switched regulator, which is an electronic circuit that uses a power switch, a diode, a capacitor, and an inductor, is much more versatile than the resistive regulator. The time that the switch remains open during each cycle is varied to maintain a steady output voltage that can be varied as desired. The advantage of a switched regulator is that the inductor stores energy in the “ON” phase and then gives back most of the energy during the “OFF” phase. Also, unlike linear regulators, switched power supplies can step down (buck converter) or step up (boost converter) the input voltage. Step-down buck converters usually are used in solar hydrogen production systems. They are used to step down the voltage of the photovoltaic power source to the lower operational voltage of the electrolyser. A DC/DC buck converter is described in this chapter. Features of the design are discussed, and the circuit is fabricated on a printing circuit board (PCB). The buck converter was designed to match the power for a small laboratory PV-PEM 89 electrolyser test system. 5.2 Buck Converter Theory and Operation The Buck converter circuit components are shown in Figure 5.1, the average load voltage is lower than the input source voltage, and Pulse-width modulation (PWM) is used to control the converter switch. Figure 5.1: Buck converter circuit. 5.2.1 Purpose of Different Buck Converter Components As seen in the buck circuit diagram in Figure 5.1, the buck converter consists of five components: Switch Pulse-width modulation circuit Inductor Capacitor Free-wheel diode We will now provide additional details concerning the function and selection of these components. 90 5.2.1.1 Switch The power switch in a buck converter is for turning the converter on or off, and, in general, this switch must have very fast switching times and be able to withstand the voltage spikes produced by the inductor. Transistors are, typically, used for switching. The signal from the pulse-width modulator (PWM) is connected to the gate of the transistor to determine the on and off time. The load current and the offstate voltage capability decide the size of this power switch. The transistor can be a metal oxide silicon field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT), a bipolar junction transistor (BJT), or a junction field effect transistor (JFET). MOSFETs are used for high frequency power systems. In newer designs, MOSETs have replaced BJTs for use at higher frequencies and lower voltages. The IGBT is also used for power switching. It uses low power to produce the same characteristics as the BJT. The IGBT is highly suited for high power and high voltage applications, because it has a switching speed that is much slower than the switching speed of a MOSFET. Therefore, circuits using an IGBT have lower switching frequencies. 5.2.1.2 Pulse-Width Modulation Circuit The key objective of a pulse-width modulator (PWM) circuit is to obtain a fixed value of the output voltage of the converter. The PWM circuit controls the ON and OFF time durations of the switch. The desired value of the output voltage is achieved by varying the duty cycle of the switch, The pulses of the PWM circuit are generated at a constant switching frequency by comparing a DC voltage level with a repetitive sawtooth wave form that has a constant peak. An error amplifier is used as a comparator, and, when the level of the DC signal is less than the sawtooth waveform, the switch control signal becomes high, causing the switch to turn on; otherwise, the switch is off. 5.2.1.3 Operating Frequency The performance of the buck switch is determined by the operating frequency. The higher the switching frequency, the smaller the physical size of the components 91 becomes. However, there are always losses in each of the components, and these losses tend to increase as frequency increases. Therefore, there will always be some optimum frequency that gives the greatest efficiency of the converter. 5.2.1.4 Inductor The inductor limits the current slew rate through the power switch when the switch is on. When the current through the inductor tends to decrease, the inductor will act as an energy source with negative polarity. Also, the inductor controls the current ripple. The smaller the inductor value, the faster the transient response becomes, and large current ripple is produced, causing higher conduction losses in the switches and in the inductor. Also, smaller inductor values require a larger filter capacitor to minimize ripple in the output voltage. 5.2.1.5 Capacitor The capacitor provides a filtering action for the inductor, and it provides a means of removing the harmonic current from the load. Therefore, the output capacitor minimizes the voltage ripple produced at the output of the buck converter. In addition, voltage ripple is caused to a smaller extent by a high equivalent series resistance in the capacitor. Thus, for maximizing the performance of a step-down converter, a capacitor should be selected that minimises the losses caused by internal series resistance and inductance. 5.2.1.6 Free-Wheeling Diode The free-wheeling diode provides a path for the current from the inductor when the switch is off, so there is always a path for current flow to the load. The diode must be able to turn on and off relatively rapidly to enable the delivery of the energy stored in the inductor to the load. 5.2.2 Circuit Description and Operation The equations that govern the operation of the circuit in the on state and the off state 92 are shown below. ON state: When the switch is ON for time duration DT, the switch conducts the inductor current IL, causing energy stored in it to increase, and the diode becomes reverse biased. This results in a positive voltage, VL = Vin - VO, across the inductor. The VL causes an increase in the inductor current IL, as shown in Figure 5.2. Figure 5.2: Buck converter ON state. diL Vin Vo .....................................................................5.1 dt L OFF state: In the off state ( Figure 5.3) the switch is open and diode D conducts because of the stored inductive energy, which causes the current through the inductor to decrease linearly. At the off state, a negative induced voltage drop across the inductor causes the inductor voltage VL = -VO . V diL O ............................................................................5.2 dt L 93 where Vin and Vo are the converter input and output voltages, respectively; L is inductance; iL is the inductance current; VL is the inductance voltage; t on is the time duration of the ON state; and Ts is the total cycle time. Figure 5.3: Buck converter OFF state. During the ON state and then during the subsequent OFF state, the buck converter can operate in two state modes, i.e., continuous current mode (CCM) and discontinuous current mode (DCM). The difference between the two modes is that the inductor current IL in conduction does not fall to zero in the CCM mode, as is shown in Figure 5.4. The overall performance is usually better in the CCM mode, and the maximum power generated by the source can be obtained. In the DCM mode, the current in the inductor falls to zero for some portion of the switching cycle. This case is used when the maximum load current is fairly low, and, as a result, the overall converter size will be smaller because a smaller inductor can be used. IL IL IL (a) t IL t (b) t Figure 5.4: (a) CCM and (b) DCM for inductor current. 94 t Since, in steady-state operation and in an ideal component, the waveform must repeat from one time period to the next, the integral of the inductor voltage V L over one time period must be zero, where: Ts t on t off DT (1 D)T .........................5.3 Typical waveforms are shown in Figure 5.5 a and b. Figure 5.5: ON and OFF waveforms of the buck converter [43]. Ts ton Ts V dt V dt V dt o...........................................5.4 L 0 L 0 L ton (Vin Vo )ton Vo (Ts ton )...............................................5.5 Vo t on I in D(dutycycle )........................................................5.6 Vin Ts Io 95 In this steady state mode and with ideal components, the output voltage power is equal to the input power, as shown in equation (5.7). I in Vin I out Vout ..................................................................5.7 5.3 PSCAD Simulation of a Buck Converter The PSCAD circuit of a Buck converter is shown in Figure 5.6. The input voltage of the converter is 20 V, and it is connected via the Buck converter to a resistive load of 2 ohms. A sawtooth waveform (minimum of 0 V and maximum of 1 V) is fed to terminal B of a comparator. A steady voltage with a range of 0 V to 1 V is fed to terminal A. By varying this steady voltage, pulses with a variable duty cycle can be generated, and the pulses are then applied to the transistor. The sawtooth frequency was chosen as 5 kHz. This simulation is to show that the buck converter output voltage value is controlled by the duty cycle value of the switch. Figure 5.6: PSCAD buck converter simulation. The inductor and capacitor values for CCM were calculated using the flowing formulae: 96 L min (1 D) R ......................................................5.8 2f C min I L ........................................................................5.9 V0 f where the component values are those illustrated in Figure 5.6. Figure 5.7 shows the simulation results of load voltage ripple and CCM current ripple. It shows the converter output voltage and current ripples, which depend on the values of the inductor, capacitor, and operating converter frequency . Figure 5.7: Load voltage and current ripples It is clear that the waveforms are very similar to those given in Figure 5.5. VD (V) VO ( D=0.9) V 22.0 20.0 18.0 16.0 14.0 y 12.0 10.0 8.0 6.0 4.0 (a) 2.0 0.0 97 (b) Figure 5.8: Diode voltage wave from (a) Vin = 20 V, D = 0.9 and Vo = 18 V and (b) Vin = 20 V, D = 0.4 and Vo = 8 V. 5.3.1 DC-DC Buck Converter Circuit Using IC TL494 Control Circuit The circuit was built as described in the step-down converter Application Note in the IC TL494 data sheet, where the PWM control method was used to control the output current of the converter. Some modifications were made to the original circuit available in the data sheet to increase the maximum output current. The input voltage range was from 10 to 40 V, the output voltage was fixed by the control loop at 5 V, and a maximum current of 1 A was delivered at the output. A Darlington Tip 129 transistor was used as the switching transistor. The values of the inductance and capacitance in the filtering parts were modified from those given on the data sheet, and they are shown in Figure 5.9. . For increased current drive to the Tip129 switching transistor, the internal transistor collectors (pins 8 and 12) were connected in parallel. The TL494 has two error amplifiers; one was used to adjust output voltage, and the other was used to control maximum current. 98 (a) (b) Figure 5.9: (a) DC-DC buck converter using IC TL494 and (b) the practical circuit of the circuit shown in (a). The pulse width was adjusted by the TL494 to maintain the converter output voltage at 5 V. This was achieved by comparing the output voltage to a reference voltage generated inside the IC by simply connecting the output voltage to the (+) input of the error amplifier of the IC through a 5.1-kΩ resistor and the (-) input was connected to the reference voltage pin. Overcurrent protection was achieved by using a second amplifier, so the output 99 current was sensed across the current sensor. In the present circuit, a 0.1-ohm resistor was used, so the voltage across this resistor was Iout 0.1 ohm, which is then compared with the reference voltage. Because of the low value of the sensor voltage, the divider chain of 5.1-kΩ and 150-Ω resistors reduced the reference voltage. Figure 5.10 a and b shows the waveforms of the diode voltage VD and output voltage Vo taken by using a dual-channel, USB picoscope (Pico Technology, Ltd., Country). The input voltage reading is on the left-hand side of the graph (blue), and the output voltage is on the right-hand side (red). It can be seen from the experimental results that the variation of duty cycle was subjected to the changing input voltage. The output voltage was constant and was controlled by the load when the load current was greater than the input current. (a) 100 (b) Figure 5.10: Diode voltage form (a) V in = 20 V and V and V out = 5 V. out = 5V and (b) V in = 6 V 5.4 Characteristics of the PV-PEM Electrolyser Test Rig When a buck converter is used in a PV-PEM hydrogen production system, the input voltage V in , from the PV source, is variable and depends on the temperature and irradiation conditions. The output voltage V o must remain almost constant at the working voltage of the PEM electrolyser. The buck converter duty cycle D controls the input and output voltage values at converter terminals so it can be fixed at the maximum power point voltage of the PV array. It was indicated in Chapter Three that the maximum power matching between the PV and PEM electrolyser was achieved by adjusting the converter duty cycle. A DC/DC buck converter was designed and implemented in PCB to provide practical verification of the power matching between the PV-PEM electrolyser test rig available in our laboratory, which is shown in Figure 5.11. The test rig consisted of a low-power PV panel, Solarex LD 664-431, and a one-cell PEM electrolyser. As the electrolyser is operating at a low voltage, the DC/DC buck converter steps down the PV module voltage to that required by the electrolyser. 101 Figure 5.11: PV –PEM test rig. 5.4.1 PV Characteristics The PV module consisted of two small crystalline solar panels connected in series. The circuit shown in Figure 5.12 was used to obtain the I-V and V-P characteristics. The PV module was illuminated by an artificial light source. Voltage and current readings were obtained as functions of the load resistance value by using two TENMA digital multimeters. Light A V R PV LD 664431 Figure 5.12: Setup for determining the characteristics of a solar module. The experimental I-V and V-P curves, given in Figure 5.13, are for a fixed insolation of 700 W/m2. This light level was achieved with a 500-W halogen lamp at a distance of 40 cm from the PV panel. . 102 0.6 0.4 Power (W) Current (A) 0.5 0.3 0.2 0.1 0 0 1 2 3 4 5 6 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 1 Voltage (V) 2 3 4 5 Voltage (V) Figure 5.13: PV characteristics. 5.4.2 PEM Characteristics This experiment was conducted to obtain the voltage and current requirements to produce hydrogen gas from PEM electrolysis. The circuit is given in Figure 5.14 where the PEM electrolyser is type D–666 484. A V PEM Elec. Figure 5.14: Circuit diagram of the characteristics of the PEM electrolyser. The parameters of the LD-666 484 PEM electrolyser were: Operating voltage: approximately 3 V Operating current: 3 A Maximum current: 4 A Maximum temperature: 80 °C 103 6 Gas generation: approximately 35 ml H2/min at 4 A The purpose of drawing the I-V characteristics of the electrolyser was to determine the operating voltage at which hydrogen production commences. Below this voltage, there is insufficient energy to cause water molecules to dissociate. The input I-V characteristics (response) of the PEM electrolyser in Figure 5.15 were obtained by adjusting the power source. The input currents were defined at different applied voltages (0 - 3.3 V). The current only began to flow at a threshold voltage 14 4 3.5 3 12 POwer (W) Current (A) (Vthreshold ≈ 2 V), and then it increased exponentially. 2.5 2 1.5 1 0.5 0 10 8 6 4 2 0 0 1 2 3 4 5 6 0 1 2 Voltage (V) 3 4 5 Voltage (V) Figure 5.15: PEM electrolyser characteristics. 5.4.3 Dependence of Hydrogen Production on the Operating Current of the PEM Electrolyser To determine the relationship between the volume of hydrogen produced (ml) and the current I (A), the amount of current was varied over a fixed process duration, and the volume of hydrogen produced for that duration was measured. 90 80 Hydrogen (ml) 70 60 50 40 30 20 10 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Current (A) Figure 5.16: Volume of hydrogen produced as a function of current over a 10minutes operational period. 104 6 By plotting the volume of hydrogen gas as a function of current, Figures 5.16, it was apparent that the volume of hydrogen produced had a linear relationship with the current. 5.5 Design Buck Converter 5.5.1 PWM Circuit The use of a pulse-width modulation (PWM) circuit is a way to control the On- OFF time duration of a buck converter switch. The IC SG3525 was used to generate the PWM output. Figure 5.17 shows the PWM circuit used in this converter. The level of an external DC voltage source was compared with the continuous sawtooth signal generated by the IC oscillator. When the sawtooth signal was at a greater voltage than the voltage of the input signal, a comparator was used to produce a pulse for the on duration. When the input signal level was greater than the sawtooth signal, the comparator turned off for the off duration. The duration of the on and off periods can be controlled by varying the level of the input voltage signal by using a voltage source with a variable range from 0 to 5 V. Figure 5.17: PWM circuit using IC SG3525. 105 The buck converter circuit was implemented in PCB combined with the PWM duty cycle circuit as seen in Figure 5.19. The value of the buck converter components were selected in order to have the same values of the Buck converter PSCAD model in Chapter Four, as follows: L = 0.6 mH; Cin = 2000 uF; and Cout = 500 uF, and the main transistor switch, Tip36, was selected, and Darlington Tip 122 transistors were used to increase the current gain. The circuit is shown in Figure 5.18. Figure 5.18: PV –PEM electrolyser coupled by a buck converter circuit. Figure 5.19: Buck converter implemented on a PCB. 106 5.5.2 Evaluation of Results Figures 5.20 (a) and (b) show the oscilloscope images of two duty cycle waveforms (blue) and the operating voltage value of the PEM electrolyser for the designed buck converter. The red line is the DC voltage value (right-hand side) of the PEM electrolyser operating voltage. The results illustrate that, although the duty cycle changed, the voltage of the operating electrolyser remained almost constant. It was noted that the electrolyser current changed, so the electrolyser consumed additional power. V 5 V 5 4 4 3 3 2 2 1 1 0 0 -1 -1 -2 -2 -3 -3 -4 -4 -5 0 20 (a) 40 60 ch A: Frequency(kHz) 80 100 120 140 12.52 160 180 -5 200 µs 09Aug2009 15:48 V 5 V 5 4 4 3 3 2 2 1 1 0 0 -1 -1 -2 -2 -3 -3 -4 -4 -5 0 (b) 20 40 60 ch A: Frequency(kHz) 80 100 120 140 12.52 160 180 -5 200 µs 09Aug2009 15:31 Figure 5.20: Oscilloscope images of electrolyser voltage (red) at different duty cycle values (blue). 107 The following readings Table 5.1 are obtained during the using the Buck converter as power conditioner between the PV module and PEM electrolysers. D Vpv(V) Ipv(A) Vpem(V) Ipem(V) Ppv(W) Ppem(W) 0.05 5.45 0.02 2.4 0.03 0.1 0.07 0.12 5.29 0.03 2.5 0.05 0.15 0.12 0.22 5.19 0.05 2.6 0.08 0.25 0.2 0.31 5.11 0.07 2.7 0.11 0.35 0.29 0.35 5.07 0.08 2.7 0.12 0.4 0.32 0.46 5 0.12 2.8 0.16 0.6 0.44 0.55 4.94 0.14 2.8 0.19 0.69 0.53 0.64 4.81 0.21 2.85 0.27 1.01 0.76 0.72 4.71 0.26 2.85 0.3 1.22 0.85 0.8 4.6 0.31 2.85 0.33 1.42 0.94 0.87 4.49 0.35 2.85 0.36 1.57 1.02 0.93 4 0.46 2.9 0.4 1.84 1.16 1 2.9 0.5 2.8 0.5 1.45 1.4 Table 5.1: Voltage and current readings at both sides of the Buck converter. Figure 5.21 shows the I-V curve obtained at the Buck converter terminals that represents the I-V curves for PV (blue) and PEM electrolyser (red). PV I-V curve PEM I-V curve 0.6 Current (A) 0.5 0.4 0.3 0.2 0.1 0 0 1 2 3 4 5 6 Voltage (V) Figure 5.21: I-V curves for the PV array and the PEM electrolyser. 108 From the experimental curves, it can be seen that each duty cycle D had its corresponding current and voltage values on the I-V curves of the PV array and the PEM electrolyser as seen in Figure 5.21. As the on time duration of the duty cycle increased, the converter output current increased, resulting in the production of Power (W) additional hydrogen. PV P-V curve 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 1 2 PEM P-V curve 3 4 5 6 Voltage (V) Figure 5.22: P-V curves of the PV array and the PEM electrolyser. From the power-voltage curve of Figure 5.22, it can be determined that the direct connection (D = 1) had less power value than the PV module can deliver, which matched up with D = 0.9 at maximum power voltage of nearly 4 V. 0.9 Converter effeciency % 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 Duty cycle D Figure 5.23: Relationship between the implemented efficiency of the Buck converter and duty cycle. 109 The increase in the input power of the PEM electrolyser was determined the electrolyser current and the duty cycle until a maximum power point, after which the input power decreased following the I-V characteristics of the PV array. Although the circuit provided a general idea about the operation of the buck converter, it could be regarded only as an initial design. The converter was found to have efficiencies between 65% and 70% depending in the duty cycle, as shown in Figure 5.23. To increase this efficiency, greater care would be needed in the selection of the switching transistor and the inductance. Summary A DC/DC buck converter was designed and implemented for power matching of a PV-PEM hydrogen production laboratory test rig. Although the circuit gives us the general idea about buck converter operation it could only be regarded as an initial design. The converter was found to have an efficiency between 65% and 70% depending on the duty cycle . To increase this efficiency, greater care would be needed in the selection of the switching transistor and the inductance. Since a commercial Buck-Boost converter, UDIO 60.25.L, was available from Greenenergy Technology [45] it was decided to use this converter for further experiments as described in Chapter 6. 110 CHAPTER 6 REAL TIME EXPERIMENT OF A PV-PEM HYDROGEN PRODUCTION SYSTEM USING A COMMERCIAL SPLIT-PI CONVERTER 111 6 REAL TIME EXPERIMENT OF A PV-PEM HYDROGEN PRODUCTION SYSTEM USING A COMMERCIAL SPLIT-PI CONVERTER 6.1 Background The PV-hydrogen production system was scaled up by a factor of 10 compared to that of the system described in chapter Five. Two Solar Century PV modules (type C21) were connected in series so that a peak power of 100 W (with 100% insolation) could be delivered to the input terminals of a Split-Pi Buck-Boost converter. A seven-cell, proton-exchange membrane (PEM) electrolyser (h-tec type E107-230) with a power capacity of 50 W was connected to the output terminals of the Split-Pi converter. This latter unit was a commercial DC/DC converter from Green Energy Technologies and was used as a power interface allowing optimal energy transfer from the PV modules to the PEM electrolyser . 6.2 System Components The photovoltaic (PV) powered proton-exchange membrane (PEM) electrolyser hydrogen production system is shown schematically in Figure 6.1. Figure 6.1: Hydrogen production system. The PV-Hydrogen production system consists of two PV modules, connected in series; a computer-controlled DC/DC converter; a PEM electrolyser; and a device to 112 measure the volume of hydrogen produced. Each item is shown in Figure 6.2 and described in more detail below. Figure 6.2: Power matching photovoltaic-electrolyser system using a Split-Pi converter. 6.2.1 PV Module The two solar modules were attached to an aluminium A-frame, as illustrated in Figure 6.3. Castors allowed movement in the horizontal plane, and adjustable support bars allowed the tilt angle of the modules to be changed. Only manual adjustments were made in the present experiments, although, for future work, the support structure could be engineered to provide automated tilt and azimuthal angle variations. . 113 Figure 6.3: PV modules facing the sun. The PV module specifications used in this experimental work are shown in Table 6.1. These readings were taken under standard conditions, i.e., an insolation of 1000 W/m2 and a cell temperature of 25 0C (100% insolation as referred to above). Model type C21-M52D from Solarcentury Silicon crystalline 1240mm 240 mm The sizes Peak power 50 W Peak power voltage Vmax 10 V Peak power current Imax 4:45A Open circuit voltage Voc 11 V Short circuit current Isc 5A Table 6.1: Specifications of the PV module. The measured results of current versus voltage and power versus voltage for each of the PV modules under different values of solar insolation are given in Figure 6.4. 114 (a) (b) Figure 6.4: (a) I-V and (b) P-V curves of a single C21 module under different insolation values. 6.3 Measuring Solar Irradiance Solar insolation was measured using a meter from Daystar, Inc. (type Daystar). This meter provided an insolation scale from 50-1200 W/m2. The sensor responded to a spectral bandwidth of approximately 0.3-1.1 m (i.e., a major part of the solar spectrum), and it had a digital LCD display. 115 6.4 Split-Pi DC / DC Converter The UDIO.60.25.L Split-Pi converter from Green Energy Technologies (Fig. 6.5)is a type of switch-mode power supply. It consists of two MOSFET bridges and two Pi filters, as shown in the circuit diagram below in Figure 6.6. The circuit configuration is similar to the circuit topology for a step-up boost converter followed by step-down buck converter. The magnitude of the output voltage can be greater than or less than the magnitude of the input voltage, as determined by choosing the on and off states of the selecting switches, as shown below. The features of the Split-Pi converter are as follows: 1. 0-60 V input. 2. 0-60 V output. 3. 25 A maximum bi-directional current flow. 4. 1.5 kW maximum common GND non-isolated. 5. Seamless UP/DOWN conversion. 6. Serial digital control. 7. 256-code voltage control. 8. > 98% power efficiency. 9. Exceptionally low voltage ripple (< 10 mV). Figure 6.5: Split – Pi DC/DC converter. The basic circuit (Figure 6.6) consists of four semi-conductor switches and input and output LC filters. The capacitors must be large to control the voltage ripple at the 116 converter terminals. Capacitor C3 is not connected to any external terminal and provides energy storage during switching. Figure 6.6: Split – Pi converter circuit. Only one bridge switches at any time to provide voltage conversion. A straightthrough, 1:1 voltage output was achieved with the top switch of each bridge switched on with the bottom switches off. The output voltage was adjustable on the duty cycle based on the ratio of the switching MOSFET bridge, as shown in Table 6.2 below. Closed-loop control was achieved through an RS 232 input on the Buck-Boost converter. Using visual basic software, a computer was used to specify the input-tooutput voltage ratio, and, by the same serial link, data regarding input and output voltages and currents were fed back to the computer. Code S1 S2 S3 S4 State Comments 0 OFF OFF OFF OFF Open 1 OFF ON PWM* PWM Buck All open circuits no current can flow PWM is linear with code and PWM =code/128 and PWM* is the inverse and = (128- code)/128 LH and RH voltages are equal PWM ratios are set to laniaries the inverse relationship to 127 128 129 to 255 L to R OFF ON PWM* PWM OFF ON Short OFF ON Boost L to R Table 6.2: Split – Pi converter switching duty cycle. 117 By careful selection of components (proprietary Intellectual Property Rights of the company), the Split-Pi technology provides high efficiency (> 98%) direct current (DC-DC) up and down (boost, buck) voltage conversion with the ability to seamlessly sink and source electrical current with identical forward and reverse transfer characteristics. 6.4.1 Control of Split-Pi Converter Software The Split-Pi DC/DC converter from Green Energy Technologies has many advantages over other converters, i.e., users can: -create a software programme to control the converter switches -alter duty cycle pulse-width modulation (PWM) settings by sending a decimal code from 0 to 255 for the voltage ratio control between the converter terminals and record the values of input and output voltage and current. A software computer programme was developed (Appendix B) to control the Split-Pi converter through a serial, digital-control port. This facility was very useful for monitoring and controlling the real-time measurements and for utilizing optimal energy transfer from the PV modules to the PEM electrolyser at all times. The Split-Pi converter uses one side of the device as a reference, which is referred in this work as the left-hand side (LHS), and the PEM electrolyser was connected to this side. The PV power source was connected to the right-hand side (RHS). The RHS voltage can be calculated using the following equation: R VRHS 2VLHS ........................................................................6.1 255 where VRHS is the right-hand side voltage value; VLHS is left-hand side voltage; and R is value of the ratio, which is controlled by the computer. For a simple demonstration of the interfacing procedure, a preliminary Visual Basic 118 program was written. A parameter Rmax (in the range of 0 to 255) was specified, and, then, the program incremented a counter, starting at zero or some other value, Rinitial, until the counter value became greater than R max, at which point the program was terminated. At each incremental point, the current and voltage at the input and output terminals were measured so that input and output power could be calculated. The flow chart for the visual basic programme is shown in Figure 6.7. Start Initialize Ratio initial set Ratio max set Read Iin ,Vin, Iout and Vout For ratio value Store Data Increment Ratio No Is Ratio > Ratio max yes Store Data in file End Figure 6.7: Flowchart of visual basic control. With the screen output, Figure 6.8, displaying current, voltage, and power at both input and output terminals, the procedure can be followed as the ratio, R, is incremented increased from Rinitial to Rmax. The program was initiated by the START 119 button, and it ends when Rmax is reached. At this point there are three options: Press START again to obtain more readings. Press SAVE to save data to file. Press EXIT to leave the routine Figure 6.8: Visual basic software screen outlook. 6.4.2 Maximum Power Point Tracking Algorithm Maximum power point tracking (MPPT) techniques are employed in solar hydrogen production systems to make full utilization of the solar insolation. The main advantage of the maximum power point tracker is to adjust the current and voltage of the photovoltaic module for optimum electrolyser performance. This, in turn, maximizes the hydrogen production rate from the electrolyser. There is an additional influence of the fluctuations of the ambient temperature of the PV modules, but this is generally smaller than the changes in solar insolation. In principle, the temperature changes could be incorporated into the optimization program by having a temperature sensor fixed to the solar modules, but this was not included in the present investigation. A maximum power point tracking strategy, based on the climbing-hill method, was 120 applied in the visual Basic programme code to control the Split-Pi DC/DC converter. The hill-climbing method, Figure 6.9, is widely used in practical PV systems and MPPT controllers due to its simplicity and easy implementation. . Figure 6.9: Hill-climbing MPPT method. In the hill-climbing method, the input power to the converter, P K, is calculated from the input voltage and current values at each PWM duty cycle (ratio). It is then compared with a previous power value, P K-1. If the current power value is greater than the previous power value, then the ratio is incremented, and readings are taken again for a higher ratio. Just after the MPP point, PK is less than P K-1. The “greater than” condition in the flow chart is now false, and the program comes out of the loop and passes to the next stage. At this point, the ratio is decremented to yield a point to the left of the MPP point so that the hill climbing can start over again. The strategy adopted here is that the power will fluctuate closely about an optimum value over and over again until some other condition is set to terminate the program. The flow chart in Figure 6.10 shows that termination occurs when a pre-set “Final Count” is exceeded. This may, typically, be 1000 for a measurement trial of about two hours. In order to start the programme, for example at count 10, we must set an initial power value for PK-1 so that a comparison with the reading P10 can be made. It can 121 simply be set to zero since any reading will be greater than this value. Start Initialize Set ratio initial Set power max Set Final count Read Iin Vin Iout Vout Calculate Pin and Pout Is P in < power max yes no Subtract 2 from Ratio yes Is Count < Final count no Store Data in file End Figure 6.10: MPPT programme flow chart. 6.5 PEM Electrolyser A h-tec PEM electrolyser of Figure 6.11 was used to produce hydrogen from the power generated by the PV module. It consists of seven PEM cells connected in series and is filled with distilled water. This electrolyser was chosen so that the input power requirement would be a reasonable match to the power delivered by the PV module. 122 Figure 6.11: The 50-W, h-tec PEM electrolyser. Table 6.3 shows the PEM electrolyser specifications. Model name E107 (230) (h-tech) Electrode area 7 cells of 16cm2 each Rated power 50W at 14V DC Permissible voltage 10:5 - 14:0V DC Permissible current 0 - 5:0 A DC H2 production 230cm3/min (8.33 x 10-8 kg/min) Table 6.3: Specifications of the PEM electrolyser. An I-V characteristic curve for the electrolyser was plotted to ensure that the specifications provided by the manufacturer were approximately in agreement with measured values, Figure 6.12. 123 4.5 4 Current (A) 3.5 3 2.5 2 1.5 1 0.5 0 0 1 2 3 4 5 6 7 8 9 1011121314151617181920 Voltage (V) Figure 6.12: Characteristic curve of the h-tec PEM electrolyser. From the characteristic curve, it is possible to calculate the internal resistance of the electrolyser. This is shown in Figure 6.13, which shows that the resistance is high at points below cut-off. 300 R (ohm) 250 200 150 100 50 0 0 20 40 Power (W) Figure 6.13: Resistance-Power PEM electrolyser curve. 124 60 6.6 Hydrogen Volume Measurement Device This device is a simple water displacement arrangement similar to the one described in reference [47]. A gas holder with greater capacity than [47] was required for the present system, and this is shown in Figure 6.14. When hydrogen gas enters through the filling tube, the inner cylindrical container is raised, and the level is monitored with an ultrasonic level indicator (Pepperl+Fuchs, model UB 300-18GM-U-V1). This sensor gives an analogue signal that is monitored using a standard multi-meter (e.g., a Thurlby 1905a multi-meter) fitted with a serial port. Figure 6.14: Device for measuring the volume of hydrogen produced. Since a standard RS232 serial interface is fitted to the Split-PI converter and a serial port is also provided by the Thurlby multimetre, provision had to be made for two serial cables to be applied to the computer. The present computer was equipped with only one serial input port, so a four-way RS232 router [48] gave a time-sharing 125 method of interfacing both the Split-PI and the level monitor into the computer. 6.7 Results and Discussion The recorded data points were presented graphically, Figures 6.15 and 6.16. The maximum power output from the PV modules is clearly shown to occur at approximately 15 V. PV I-V curve PEM I-V curve 7 Current (A) 6 5 4 3 2 1 0 0 5 10 15 20 25 Voltage (V) Figure 6.15: I-V for PV and PEM electrolyser curves. Figure 6.16 shows the P-V curve matching of photovoltaic module and electrolyser at all possible code value starting from 0 to 255 during Buck- boost converter operation. 126 PV output power curve PEM input power curve 90 Power (W) 80 70 60 50 40 30 20 10 0 0 50 100 150 200 250 300 Voltage ratio Figure 6.16: P-V PV and PEM electrolysers power matching. Figure 6.17 shows the power delivered by the PV modules to the electrolyser using the Split-Pi converter controlled with the MPPT software. With the exception of a small interval within a two-hour period, the sky was clear, and an irradiation of 850W/m2was occurring. The graph shows that the Split-Pi converter gives good tracking of the maximum power point, and, even when a small cloud covered the sun at a count of 170, maximum power tracking was restored within the next two or three counts. If a smaller “ripple” in the optimum output is required, then the ratio should PEM electrolyser input power (W) be decremented by a smaller amount. 90 80 70 60 50 40 30 20 10 0 0 200 400 600 800 1000 Counter Figure 6.17: PEM electrolyser input power data during a clear, sunny day. 127 Tracking on a cloudier day is given in Figure 6.18, where the solar insolation was in the range 550-800 W/m2. This indicates the performance of the proposed system during more continuous and larger variations in solar irradiance. PEM electrolyser input power (W) 80 70 60 50 40 30 20 10 0 0 200 400 600 800 1000 Counter Figure 6.18: PEM electrolyser input power under less favourable insolation conditions. The results shown below illustrate that the input power for the PEM electrolyser is very near to the path of the maximum power points of the PV module characteristics during the tracking routine. 120 W/m2 220 W/m2 773 W/m2 PEM input power 370 W/m2 80 70 Power (W) 60 50 40 30 20 10 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Voltage (V) Figure 6.19: Electrolyser input power and PV maximum power relation 128 Hydrogen production is given in Figure 6.20. 16 14 Insolation 800 Watts/square metre Volume of gas (litres) 12 10 8 Insolation 500 Watts/square metre 6 4 2 0 0 200 400 600 800 1000 1200 Count Figure 6.20: Hydrogen production in relation to changes in insolation. The gas output of 220 litres per minute over the latter part of the curve (insolation 800 Watts/m2 ) compares well with the manufacturer‟s data sheet for the h-tec 230 electrolyser. Clearly, this insolation was maximised over the two-hour period by manually tracking the sun and ensuring that the PV modules were operating at their maximum power point by the hill-climbing routine in the software. Summary Finally, we can conclude that the system that was composed of two C21 PV Modules, a Split-Pi Buck-Boost converter and an h-tec electrolyser provided a realtime generation unit for converting solar energy into hydrogen. Optimum tracking was achieved using computer control. 129 CHAPTER 7 DESIGN A PV-HYDROGEN SYSTEM TO POWER A FAMILY HOUSE IN THE SAHARA DESERT IN LIBYA 130 7 CHAPTER 7 DESIGN A PV-HYDROGEN SYSTEM TO POWER A FAMILY HOUSE IN THE SAHARA DESERT IN LIBYA 7.1 Background Since electricity plays a substantial role in everyday life, sustainable, rural electrification is needed that can at least be capable of providing necessary light and energy. In this chapter, the design of a solar hydrogen power system for a family house in the remote Sahara Desert is discussed. The design incorporates materials and well-known equipment that are currently commercially available. This system uses relevant technologies to harness the sun‟s radiation and covert it into electricity using photovoltaic panels as the power source. The system uses water electrolysis and fuel cells to use hydrogen as a storage medium to power the house 24 hours per day. 7.2 Solar Energy Sources in Libya and the Hydrogen Option Solar energy is the most abundant renewable natural resource in Libya. The daily average of solar radiation on a horizontal plane is 7.1 kWh/m2/day in the coastal region and 8.1 kWh/m2/day in the southern region, with an average sun duration of more than 3500 hours per year (Saleh Ibrahim, 1993). In 1976, the first photovoltaic system, a cathodic protection station, was established in Libya. This station had a peak power of 650 kW. Since then, photovoltaic systems have been widely used for many applications, such as stand-alone systems to pump water , Figure 7.1, and communication repeater stations in rural areas. 131 Figure 7.1: Photovoltaic module for a water pump in the Libyan Sahara [51]. Since solar energy is only available during the day, it is important to have energy storage facilities. The most common storage facility is batteries, but an alternative is to generate hydrogen gas that can be stored for longer periods of time. Hydrogen has the advantage over batteries as energy storage medium because it is transportable, can be stored indefinitely, and does not create environmental pollution when energy in the hydrogen is converted to electricity or when it is burned to produce heat. Most of southern Libya averages over 6 KWh/m2 per day of global radiation, whereas northern Mediterranean countries receive less than 3 KWh/m2 [51]. This makes hydrogen produced in Libya using solar energy via electrolysis an attractive energy source for domestic use in remote areas as well as for export to Europe in either liquid or gaseous form. Pipelines could be used to transport the hydrogen gas in much the same way as natural gas is presently being transported to Europe. Special refrigerated tanks would be necessary for transporting liquid hydrogen. 7.3 Solar Hydrogen System as an Energy Supply for Libyan Remote Areas There are many villages and remote areas located in Libya and other parts of North Africa as well. These areas are far away from the electricity grid. Economically, the small populations in these remote areas cannot be connected to the grid at 132 competitive costs because of long distances and related line losses. In some Libyan remote areas, diesel engines are used to generate electricity, but some remote areas cannot use diesel engine generators because of difficulties associated with fuel transportation and maintenance. For this reason, the renewable energy sources should be utilized to generate electricity locally for a household and/or water pump applications. Due to the intermittent nature of wind and the low wind speed in the Sahara Desert, photovoltaic systems are the preferred option. They have proven to be more reliable than wind power due to the high solar radiation in these remote desert regions. In this study, the technological focus was on a solar PV-hydrogen power system. This system is based on a PV array, a DC/DC converter, a proton exchange membrane (PEM) electrolyser, and a PEM fuel cell; in combination with a low–tomedium pressure hydrogen storage installation, this would service the power needs of a remote homestead situated in the southwest Libyan desert. For such locations where the transportation of fuel is problematic and costly, it is better to have a stand-alone system using a photovoltaic electricity generator. In this study, a solar-hydrogen system was used to supply the energy needs of a remote household with a daily electrical demand. The system was located in a remote area that has a high level of solar radiation but no access to a central grid that can provide power. A small desert town called Ghadamis was chosen for the study. It is located at latitude of 30° N and a longitude of 10° E. The geographical location is at the intersection of the borders of three countries, namely, Libya, Algeria, and Tunisia. Ghadamis is very old town, and it has unique architectural features; recently, it has become a tourist haven, attracting people from many parts of the world. Table 7.1 shows the climate conditions of Ghadamis. The average global irradiation is 2200 kWhm-2year-1 for 3500 h/year. Hence, the climate condition of this town was considered to be an ideal place for the application of solar hydrogen systems. To optimise the sizes of the different components in the solar hydrogen power system, the weather data and load demand were considered as input data. The sizes of the different components depend on these input data. 133 Sunshine duration 3500h/year Irradiance 2200 kWh/ m 2/year Relative humidity 34 Rainfall 20mm/year Wind speed 8.71 knots Extreme maximum temperature 36.71°C Mean maximum temperature 29.67°C Extreme minimum temperature 8.12°C Mean minimum temperature 14.10°C Table 7.1: Climatic conditions of the project site at 30° N and 10° E. As stated above, the aim of the project was to supply electrical energy to a family house in Ghadamis using three main components, i.e., a photovoltaic source, a DC/DC converter, and an electrolyser-hydrogen storage-fuel cell system. According to data from the Libyan Solar Research Centre (SRC), Ghadamis has an excellent solar profile. Figure 7.2 shows that the daily energy varies from about 4 to 8.5 kWhr for each square metre of photovoltaic cell. Also, Ghadamis has a high number of daylight hours each day; the minimum is 10 hours/day in January and December, and the maximum is 14 hours/day in July. Figure 7.2: Daily solar irradiance on horizontal plane through the year in Ghadamis. 134 Thus, as indicated in Figure 7.2, Ghadamis is an ideal place to utilize solar panels to produce electric energy directly from the sun, and this energy could be stored in the form of hydrogen fuel through water electrolysis. 7.3.1 Design of a Solar Hydrogen Power System for a family House in a Remote Area Located in the Sahara Desert In this thesis, the solar-hydrogen power system to electrify a house in a remote area of the Libyan Desert has the following major components, i.e., PV array, DC/DC converter, PEM electrolyser, hydrogen storage, and PEM fuel cell. Typically, such a house would be occupied by a small family consisting of four members, and the system would provide electrical power for all household applications 24 hr/day. There are two ways to connect the solar hydrogen system to the household load. First, the electricity produced by the PV panels can be used to provide DC power directly for the household applications. Second, the electricity could be used to produce hydrogen (stored energy) for use in running the fuel cell to meet the household demands for electricity day and night. In this project, we used the second type of system due to its advantages over the first type of system, as indicated below: 1. It was a more energy-efficient system, so it required a smaller hydrogen storage capacity. 2. It requires the electrolyser and the fuel cell to work fewer hours, thereby extending their lifetimes. The only disadvantage of the first approach is that it uses more electronic devices rather than the second type. 135 Figure 7.3: Complete solar hydrogen power system. The system, shown in Figure 7.3, demonstrates a means of continuously supplying electrical energy day and night for a family house in the Sahara region. During the daytime, it is estimated that half of the solar electricity will be used directly to provide the energy needs for the house, and the other half will create hydrogen via the PEM electrolyser for storage. At night, the stored hydrogen will power the fuel cell, producing the needed supply of electricity. All the electrical loads in this house were assumed to be DC loads operating with a constant voltage of 24 V. In order for the solar panels to convert sunlight to DC electrical power, a power conditioning and control is required, i.e., a DC/DC converter, and its task is power matching. The PEM electrolyser uses the DC power to produce hydrogen via water electrolysis. Stored hydrogen and oxygen from the air provide the inputs to the fuel cell to generate DC electric power. Provided there was sufficient hydrogen stored, a portion of it could be used as fuel for transportation. The system is required to supply electricity to operate the essential household applications and a water pump, if necessary, as shown in Table 7.2. 136 7.3.2 Energy Requirement The load was assumed to be lighting, cooling fans, refrigerator, television, and computer. The load does not include air conditioning, but it could be added by increasing the load‟s power consumption by a factor depending on BTU rating of the air conditioner used. Table 7.2 describes the energy requirements for the household applications. Appliance usage time and the loads were estimated for daytime usage (D) and night-time usage (N). Application No of Power Day(D) Units Per unit (W) Living room light (N) 1 60 2.5 5 300 Dining room light (N) 1 60 2.5 5 300 Kitchen light (N) 1 60 2.5 3 180 Bedrooms light (N) 2 60 5 2 240 Bathroom light (N) 1 40 1.6 2 80 Dining room and living room fans (D and N) 2 60 5 5 1200 Bedrooms Ceiling fans 2 60 5 5 600 Refrigerator (D and N) 1 100 4.16 12 1200 TV and sat.&rece. (D&N) 1 60 2.5 5 300 Computer and accessories (D and N) 1 100 4.16 5 500 60 2.5 2 120 300 8.33 2 600 860 (D) 900 (N) 36 (D) 38 (N) Night(N) Total Operating Energy current hours per consumption (A) day (H) Wh/day (D and N) Small applications (D andN) Water pump (D) Total 1 5620 Table 7.2: Energy requirements for small family house in the Sahara Desert. 137 Based on Table 7.2, during daylight, the PV array must be capable of providing 860 W for household requirements plus the electrolyser load. Fuel cell output power should not be less than 900 W to ensure that the electricity demand of the house can be met at night. To ensure a modest surplus of power, it is suggested that 1 kW of power will feed the home during the day and an additional 1 kW of power will be used provide for the generation of hydrogen. Thus, a 2-kW solar array will be necessary. 7.3.3 Fuel Cell Specification A fuel cell is an electrochemical energy converter that converts chemical energy into electrical current (DC). The present work is concerned with the combination of hydrogen and oxygen to produce electricity. The only product of this reaction is water. In general, fuel cells have many advantages over conventional electrical generators, including a wide range of applications, easy maintenance because there are no moving parts, silent operation, high power density, and clean energy production. Selecting an appropriate fuel cell for a given application must be based on consideration of the I-V curve, rated power, hydrogen consumption rate, and size and volume. Usually, the manufacturer provides the I-V curve in the data sheet. From the chemical equations that describe the operation of a fuel cell, the hydrogen consumption of the fuel cell can be derived. The chemical reactions that occur in a fuel cell are: At the anode: 2H 2 4H 4e ………………………………………………………...7.1 At the cathode: O2 4e 4H 2H 2O …………………………………………………..7.2 In a fuel cell, when one mole of hydrogen is consumed, two electrons are freed from 138 the hydrogen. Thus, the charge involved in the reaction for the total amount of hydrogen consumed will be: Charge = (2 electrons) x (the charge of each electron) x (moles of H 2 consumed) The charge of the electron is equal to 96485, which is the Faraday‟s constant, F. Amount of H 2 consumed = ch arg e 2 xF moles Dividing by time: Amount of H 2 consumed = current moles/sec 2 xF Multiplying by the molar mass of H2 which is 2.02 x 10-3 kg/mole Amount of H 2 consumed = currentx 2.02 x10 3 2 xF kg/sec By substituting the value of F, the equation becomes: Amount of H 2 consumed = 1.05x10 8 I kg/s To get the amount of hydrogen in m 3 /s, we divide by the density of hydrogen, which is 0.084 kg/m 3 . The amount of hydrogen consumed = I x Z x 1.05x10-8 kg/sec, where I is the current withdrawn, and Z is the number of cells in the stack. The commercial Nexa® 1200 fuel cell stack from Ballard, with an output of 1.2 kW, was an ideal fuel cell for producing the electricity required by household application during the night in this project. The technical data shown below from it‟s the company‟s data sheet provide more details. 139 Table 7.3: Fuel cell Nexa 1200 technical data from datasheet. Since the household appliances operate at 24 V, the output voltage of the Nexa 1200 was adjusted to this voltage using a built-in regulator. Figure 7.4: I-V and I-P characteristics for Nexa 1200 from data sheet. Figure 7.4 shows the output voltage, current, and power characteristics for the Nexa 1200 fuel cell. From the graph, we can determine that the 24 V, 52 A, and rated power of 900 W are suitable for meeting the load requirements of the house at night. The main fuel cell reaction is an exothermic reaction, so it produces heat that must be removed to keep the fuel cell at constant temperature. Consequently, a cooling 140 system is required to remove the heat. In the Nexa 1200 fuel cell, cooling achieved by using a fan to blow air across the fuel cell. Delivering a power of 900 W over a 10-hour period with a hydrogen flow of 15 litres per minute (from Table 7.3) will require a total of 10 m3 of hydrogen gas (at 1 bar). Thus, a storage facility for this amount of gas is required, as described in the next paragraph. 7.3.4 Hydrogen Storage Hydrogen that is produced by electrolyser must be stored for later use by the fuel cell to produce electricity. Storage methods and techniques are discussed in Chapter Two. In the solar hydrogen power system, the simplest and most practical way to store the hydrogen gas is by compressing it, so we used cylindrical storage tank for the compressed hydrogen. The main parameters in designing the hydrogen gas storage were volume, pressure, and temperature. The commercial electrolysers available on the market can deliver hydrogen up to a pressure of 30 bar. The electrolyser chosen pumped hydrogen into storage at a pressure of 3 bar. As indicated above, a gas volume of 10 m3 was required, but this can be reduced to 3 m3 if the gas is stored at a pressure of 3 bar. Since an additional quantity of gas will be required for other uses, e.g., cooking or to power vehicles, the total storage is envisaged to be closer to 4 m3 at a gas pressure of 3 bar. A cylindrical hydrogen storage tank was chosen. The storage volume was calculated based on a diameter of 1.5 m and a height of 2.60 m. Such a tank could be situated underground for safety reasons and to avoid excessive temperature fluctuations. 7.3.5 PEM Electrolyser The electrolyser converts the electrical energy produced by the PV array into hydrogen to store in a tank. The electrolyser should be large enough to fill the tank with hydrogen. A PEM electrolyser rated at a pressure of 3 bar was selected from the manufacturer (Hgenerators, type LM-20000) to produce 20,000 ml/min of hydrogen at 1 bar. Thus, the gas output from 10 hours of sunlight will be approximately 12 m 3 141 at 1 bar. This amount of hydrogen will almost fill the storage cylinder (at 3 bar pressure). The specifications for the LM-10000 are shown in Table 7.4. Outflow pressure 3 Bar Hydrogen purity 99.99% DC power 32-36 V and 25 A H2O consumed 1000 ml/h H2 gas production 10,000 ml/min Table 7.4: Specifications for the LM-10000 electrolyser. 7.3.6 DC/DC Converter The DC/DC converter has a varying input voltage from the PV array and the operating voltage of the electrolyser. Two DC/DC converters were used in this system to match the output power of the PV panels to the input power of the load, with one connected to the PV panels to the household load during daylight, and the other connected to the PV with the PEM electrolyser to extract the available maximum power from the PV and supply it to the load. In this work, the Split-pi converter, described in Chapter Six, was an ideal DC/DC converter for use in achieving this purpose due to its ability to match the output characteristics of the PV array to the input characteristics of the PEM electrolyser. This ensured that the maximum power would be transferred from the PV array to the load continuously, thereby maximizing the hydrogen production rate. The specifications of the Split-pi converter (Chapter Six) met our design requirements, and it could be used as a maximum power point tracker and for power matching between the PV array and the load. Since the Split-PI converter was a relatively low-cost item compared to the other items in this system, it may be useful to have two such converters, as shown in Figure 7.3, for added control. 142 7.3.7 Size of the PV Array PV arrays are built up with series-connected and/or parallel-connected combinations of solar cells in order to produce 32 V and 35 A at the input terminal of DC/DC converter. Therefore, for an array of Ns * Np (number of panels in series by the number of panels in parallel), the PV current, voltage, and power can be given, respectively, by the following equations: IPV = Np IPV ………………………………………………………..…….7.3 VPV = Ns VPV ……………………………………………………………7.4 A total power of 2 kW is required, and a suitable solar module would be C21-M52D, manufactured by Solar Century. These modules have a peak power of 50 W, so that 40 modules of 1174 mm x 318 mm (total area of 15 m2) will be required. A line of 10 modules in parallel with a series connection of four lines will give a suitable output to both the electrolyser and the domestic dwelling. 7.3.8 Monthly Average Energy Supplied and Consumed The C21-M52D module has 14.9% efficiency, and the temperature coefficient of open circuit voltage is -0.034V/0C, making it suitable for use in the hot Sahara Desert region. Since we know the average monthly insolation in Ghadamis, Figure 7.2, we can estimate the electric energy extracted from the PV panels and compare it with the load demand. The electricity supplied each month can be estimated as: PV electricity per month = S A 30KWh ………………………….7.5 Where, S = average annual irradiation, kWh/m2-day A = array area, m2 and = module efficiency (14.9%) 143 Figure 7.5 shows the estimated energy extracted from PV panels and energy consumed; the amount of extra energy reaches its maximum value during the summer months, and it can be used to generate more hydrogen for other purposes other than generating electricity or long-term storage. KWh/month extracted from PV KWh/ month household load energy difference KW/h per day 700 600 500 400 300 200 be r be r De ce m No ve m be r Oc to be r Se pt em Au gu st Ju ly Ju ne ay M Ap ril ar sh M Ja nu ar y Fe br ua ry 100 0 Months Figure 7.5: Estimated amount of energy extracted from the PV system. 7.4 Control and Monitoring of the PV-Hydrogen System In the solar hydrogen power system (SHPS), some important parameters to control energy efficiency would be monitored. The parameters include input and output currents and voltages of the DC/DC converters, hydrogen flow rate, and hydrogen pressure in the storage tank. Since the Split–pi converter has the advantages of measuring and controlling input and output electrical parameters using software, a computer could be used to monitor the system. Also, additional sensors could be used to record hydrogen pressure and gas flow rate. Suitable software would be devised to provide both monitoring and control. In case of an emergency, the software would include the capability of shutting the system down. 144 Summary The feasibility of using solar energy and hydrogen production in remote areas in Libya looks to be promising even with existing technology. Future advances in PV cell manufacture and electrolyser design will make this form of energy provision even more attractive and it should be implemented as part of the government‟s energy plans in future years. 145 CHAPTER 8 CONCLUSIONS 146 8 CHAPTER 8 CONCLUSIONS Hydrogen is a clean fuel that produces only water on combustion or when combined with oxygen in fuel cells to produce electrical power. Like electricity, it is an energy carrier, and it has potential for energy storage, transportation, and electricity generation for countless outlets, such as lighting, heating, and powering motor vehicles. This thesis has been concerned with the transformation of solar energy into hydrogen as a storable fuel. The successful production of hydrogen via water electrolysis using photovoltaic electricity as a power source and the subsequent use of stored hydrogen to produce electricity using a fuel cell support the proposal that this technology be used as an ideal stand-alone system, particularly for remote, desert areas in Libya. One of the challenges in producing hydrogen by using solar energy (PV-hydrogen system) is to reduce the overall costs. Therefore, it is important that the system operate at maximum power. This thesis has demonstrated, by mathematical simulation and experimental results, a method of achieving power matching between the photovoltaic array and a proton exchange membrane electrolyser. The use of hydrogen as an energy carrier was thoroughly and critically analysed (Chapter Two). The environmental impacts of non-renewable sources of energy, such as coal, oil, natural gas, and nuclear power, also were presented. Hydrogen production methods and technologies and the aspects of hydrogen transportation and use in electricity generation also were included. Hydrogen storage, distribution, and transportation were discussed in some detail, and, finally, the safety aspects of hydrogen production using relatively small-scale systems were addressed. Aspects of solar irradiance and the basic principles of the photovoltaic process were given in Chapter Three, in which the main components of a solar hydrogen production system were described, along with the general principles of operation for each component. In Chapter Four, a PSCAD software computer model was developed that was 147 capable of exploring modeling for a photovoltaic-hydrogen production system with power matching using a DC/DC Buck converter. The evaluation took into account the different factors that affect the I-V characteristics of a PV array. The simulation proved that the operating voltage of the electrolyser and the PV voltage at maximum power were the key elements in power matching. In Chapter Five, the results of a number of laboratory experiments with a small photovoltaic-hydrogen production test facility were described. A DC/DC Buck converter was designed and implemented on a PCB to match the power between the photovoltaic array and a proton exchange membrane (PEM) electrolyser. In Chapter Six, field trials of a PV-PEM system were conducted. The power capacity of this system was approximately 10 times that of the laboratory unit shown in Chapter Five. In these trials, a commercial, computer-controlled DC/DC converter (Split-pi unit from Green Energy Technologies, Ltd.) was used. Visual Basic software code was developed to control the converter and to track the maximum power point (MPPT) by adjusting the input and output voltage ratio. This allowed us to maximise the efficiency of the system. The results showed that the power output closely follows the maximum output power of the photovoltaic array. A photovoltaic-electrolyser system to produce hydrogen that fuels a fuel cell capable of generating electricity was described in Chapter Seven. Even with components that are currently available, the power would be adequate for the demands of a small family‟s house. The location in Libya that was chosen for the demonstration of the technology was a desert area with scattered populations and remote communities. 8.1 Contributions Made During the Project (1) Optimisation of a photovoltaic-PEM electrolyser hydrogen production system using a DC/DC buck converter. There are non-linearities in both the PV array and the PEM electrolyser in any hydrogen production system. These features were investigated with a PSCAD model and by conducting experimental work in order to determine how to achieve optimal power matching. The key point was to adjust the duty cycle of the DC/DC converter at a value equal to the maximum ratio of PV maximum power point voltage and electrolyser operating voltage. Then, we 148 can be assured that both the PV array and the PEM electrolyser operate at their maximum power when the duty cycle of the converter is set at this value. (2) Design of a stand-alone solar hydrogen power supply system for a family house in a remote area in Libya. The system was designed with currently available components, i.e., solar modules, PEM electrolysers, fuel cells, and the associated electronic control devices. It is understood that many developments are being made in “hydrogen” technology, and, therefore, revisions to this initial design will likely be necessary in the future. 8.2 Suggestions for Future Work The results of solar hydrogen research conducted, as reported in this thesis, have identified some points that must be addressed, and the following recommendations are made for future investigations: (1) Implementing design of the solar hydrogen power system that is described in chapter seven. The components of the system used in this research were selected from available commercial products. The technical and economical issues for a stand-alone solar hydrogen power system for household use should be thoroughly investigated in a house of appropriate size, so, it is proposed that a prototype house be constructed and utilized for future research so that better control of all essential variables can be adequately controlled. (2) Large central solar hydrogen power system to electrify a small, remote community in the Sahara desert and schemes for exporting PV energy. Since the Sahara Desert has a source of clean and inexhaustible solar energy, it is suggested that a large-scale solar hydrogen power system be designed, built, and evaluated in this area. The large-scale system would contain all the components used in the domestic house, but the power ratings would be increased significantly. When proven, such a system could provide electricity from renewable sources for many remote villages and settlements in the Sahara Desert. This type of investigation 149 would require a high capital investment, and it would require government support. At an even higher level, a feasibility study should be conducted to assess the possibility of exporting electricity (stored in the form of hydrogen) from North Africa to Europe. (3) Solar hydrogen fuel station in North Africa. 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Bilgen; "Solar hydrogen from photovoltaic electrolyzer system", Energy conversation and management 42, pp.1047- 1057, 2001. 157 CHAPTER 10 APPENDICES 158 10 CHAPTER 10 APPENDICES 10.1 Appendix A (1) Crystalline solar cell parameters FORTRAN code q = 1.602e-19 ak = 1.38e-23 an = 1.792 Ta = 293.0 Tref = 293.0 aIsco = 2.0 Noct = 49.0 aJo = 1.6e-3 aIdo = 71.1e-9 T = Ta+$S*(aNoct-20)/800 Eg = 1.16-(7.02e-4*(T**2)/(T+1108)) aIo = aIdo*(T/Tref)**3*(exp(q*Eg/an*k)*(1/Tref-1/T)) aIph = aIsco*$S/1000.0+aJo*(T-Tref) aId = aIo*(exp((q*$V)/(n*ak*T))-1) $aI = aIph-aId (2) PEM electrolyser FORTRAN code Vo = 1.1 Ac = 0.05 Aa = 0.05 R = 0.013 $V = Vo+Ac+Aa+$I*R $H = (8.314*$I*($T)*600)/((96485.0*101325.0)*2) 159 10.2 Appendix B Visual Basic Program Code for Split-PI Converter Control Private Sub EXIT_Click() End End Sub Private Sub Command1_Click() Text4.Text = Hex$(CInt(Text5.Text)) End Sub Private Sub ReadData_Click() Dim Res As String Dim Ratio As Integer Dim R As String Dim V_in As Double Dim I_in As Double Dim V_out As Double Dim I_out As Double Dim Result As String Dim Count As Integer Dim Count1 As Integer Dim Count2 As Integer Dim Mass(100) 'Print Hex$(255) With MSComm1 .Handshaking = comXOnXoff .RThreshold = 1 .RTSEnable = False .Settings = "19200,N,8,1" .CommPort = 1 .SThreshold = 1 .InputLen = 50 .InBufferSize = 4096 .PortOpen = True End With 'direct serial with parallel switch 160 'send string message 'counting loop for delay Call vbOut(888, 0) For Count = 1 To 2 Ratio = Text3.Text R = Hex$(Ratio) If Len(R) < 2 Then R = "0" + R End If MSComm1.Output = "NFEV" + R 'MSComm1.Output = "NFEV" + CStr(Hex(Text3.Text)) 'Print Hex$(CInt(Text3.Text)) 'Delay by counting For Count1 = 1 To 900 For Count2 = 1 To 800 Next Count2 Next Count1 ' End delay Text1.Visible = True Text1.Refresh Text1.Text = CStr(Count) Result = MSComm1.Input Text2.Visible = True Text2.Refresh Text2.Text = Result Next Count'' MSComm1.PortOpen = False End Sub Option Explicit' Dim Ratio As Integer Dim R As String Dim V_in(1000) As Double Dim I_in(1000) As Double Dim P_in(1000) As Double Dim V_out(1000) As Double 161 Dim I_out(1000) As Double Dim P_out(1000) As Double Dim Result As String 'Dim Count As Integer Dim Count1 As Integer Dim Count2 As Integer Dim Mass(100) Private Sub Command1_Click() Text4.Text = Hex(Text5.Text) End Sub Private Sub Command2_Click() Text8.Text = Val("&H" + CStr(Text7.Text)) End Sub 'BUCK_boost March 30th 2010 Private Sub EXIT_Click() End End Sub Private Sub ReadData_Click() 'Print Hex$(255) Call vbOut(888, 0) Dim Count As Integer With MSComm1 .Handshaking = comXOnXoff .RThreshold = 1 .RTSEnable = False .Settings = "19200,N,8,1" .CommPort = 1 .SThreshold = 1 .InputLen = 50 .InBufferSize = 4096 .PortOpen = True End With 'send string message 'counting loop for delay 'START LOOP FOR CHANGING RATIO '*************************************** Ratio = 1 162 Do Ratio = Ratio + 1 Text3.Refresh Text3.Text = Ratio R = Hex$(Ratio) If Len(R) < 2 Then R = "0" + R End If MSComm1.Output = "NFEV" + R 'Print Hex$(CInt(Text3.Text)) 'Delay by counting For Count1 = 1 To 1000 For Count2 = 1 To 1000 Next Count2 Next Count1 ' End delay' For Count = 1 To 2' '*****************************LEFT VOLTAGE MSComm1.Output = "NFEI00" 'Print Hex$(CInt(Text3.Text)) 'Delay by counting For Count1 = 1 To 1000 For Count2 = 1 To 1000 Next Count2 Next Count1 ' End delay Text1.Visible = True Text1.Refresh Text1.Text = CStr(Count) Result = MSComm1.Input' If Len(Result) > 10 Then Result = Right$(Result, 10) End If Text2.Visible = True Text2.Refresh Text10.Refresh 'Text2.Text = Mid$(Result, 7, 10) 163 Text10.Text = Int(100 * Val("&H" + Mid$(Result, 8, 10)) * 0.015258)/100 V_in(Ratio) = Val("&H" + Mid$(Result, 8, 10)) * 0.015258 '*********************************RIGHT VOLTAGE MSComm1.Output = "NFEI01" 'Print Hex$(CInt(Text3.Text)) 'Delay by counting For Count1 = 1 To 9000 For Count2 = 1 To 800 Next Count2 Next Count1 ' End delay Text1.Visible = True Text1.Refresh Text1.Text = CStr(Count) Result = MSComm1.Input' Text2.Visible = True Text2.Refresh Text2.Text = Int(100 * Val("&H" + Mid$(Result, 8, 10)) * 0.015258)/100 V_out(Ratio) = Val("&H" + Mid$(Result, 8, 10)) * 0.015258 '****************************LEFT CURRENT MSComm1.Output = "NFEI04" 'Print Hex$(CInt(Text3.Text)) 'Delay by counting For Count1 = 1 To 1000 For Count2 = 1 To 1000 Next Count2 Next Count1 ' End delay Text1.Visible = True Text1.Refresh Text1.Text = CStr(Count) Result = MSComm1.Input' 'Print Len(Result) Text11.Visible = True Text11.Refresh Text11.Text = -Int(100 * (Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) * 164 0.021286)/100 I_in(Ratio) = -(Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) * 0.021286 P_in(Ratio) = Int(100 * V_in(Ratio) * I_in(Ratio))/100 Text12.Visible = True Text12.Refresh Text12.Text = CStr(P_in(Ratio)) '*******************************RIGHT CURRENT MSComm1.Output = "NFEI05" 'Delay by counting For Count1 = 1 To 1000 For Count2 = 1 To 1000 Next Count2 Next Count1 ' End delay Text1.Visible = True Text1.Refresh Text1.Text = CStr(Count) Result = MSComm1.Input ' 'Print Len(Result) Text6.Visible = True Text6.Refresh Text6.Text = Int(100 * (Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) * 0.021286)/100 I_out(Ratio) = (Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) * 0.021286 P_out(Ratio) = Int(100 * V_out(Ratio) * I_out(Ratio))/100 Text9.Visible = True Text9.Refresh Text9.Text = P_out(Ratio) Next Count For Count1 = 1 To 1000 For Count2 = 1 To 1000 Next Count2 Next Count1 Loop While Ratio < 20 Print " " Print " " 165 Print V_in(5)‟ MSComm1.Output = "NFEV00" MSComm1.PortOpen = False End Sub Private Sub Command3_Click() 'Saving data Open "A:\Loop1.csv" For Output As #1 Dim i As Integer For i = 0 To 150 Print #1, Str$(i) + "," + Str$(V_in(i)) + "," + Str$(I_in(i)) + "," + Str$(P_in(i)) + "," + Str$(V_out(i)) + "," + Str$(I_out(i)) + "," + Str$(P_out(i)) Next i Close #1 End Sub 166
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