233_MOHDHAFIZIZZWANBINSAAD2009

7/27/2019 233_MOHDHAFIZIZZWANBINSAAD2009

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A BATTERY CHARGER USING PHOTOVOLTAIC AS A SOURCE

MOHD HAFIEZ IZZWAN BIN SAAD

A thesis submitted in fulfillment of the

requirement for the award of the degree of

Bachelor in Electrical Engineering

Department of Energy Conversion (ENCON)

Faculty of Electrical EngineeringUniversiti Teknologi Malaysia

MAY 2009


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iii

Dedicated, in thankful appreciation for support and encouragement to my

beloved

mother, father, brothers, sisters, friends and beloved supervisor.


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ACKNOWLEDGEMENT

First and foremost, I would like to express my deepest gratitude to my

supervisor, Dr Tan Chee Wei for the encouragement, guidance, comments and

friendship given throughout this project. Without his ideas and advices, this thesis would

not have been the same as presented here.

My special appreciation goes to my family who has been so tolerant and gives

full of supports towards me in order to complete this project. Thanks for their

encouragement, love and emotional supports that they had given to me.

Last but not least, my appreciation goes to all my colleagues and others whom

involve either directly or indirectly in process to finish up this project.

Thank You.


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v

ABSTRACT

This thesis reports the design and simulation of a photovoltaic (PV) charger

system with a specific specifications. The specifications are based on a real datasheet

values. The DC electricity produced by PV module issued to charge a battery using a

DC-DC converter in order to step down the voltage level. A Buck converter has been

chosen in this project. Pulse Width Modulation (PWM) controller is applied to trigger

the power switch (MOSFET) at a desired frequency. In this project, an Integrated Circuit

(IC) SG3524 is used to control the switching of the power MOSFET. However, an

SG3524 alone does not able to trigger the power MOSFET because the amplitude of

PWM is rather small. Therefore, an MOSFET driver circuit using IC MC34151 is added

to amplify the PWM signal. For safety purposes, an opto-isolator, IC 6N137 is added

between PWM controller IC SG3524 and IC MC34151. This opto-isolator protects the

circuit by providing an electrical isolation to the circuit so that the electroniccomponents function under safe operation. The proposed circuit is designed and

simulated using MATLAB/Simulink with calculated parameters, such as, the value of

inductor, output capacitor as well as the load. Finally, a DC-DC buck converter is

implemented and tested in laboratory.


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vi

ABSTRAK

Tesis ini melaporkan simulasi dan rekabentuk sistem cas solar dengan spesifikasi

tertentu. Spesifikasi-spesifikasi adalah berdasarkan nilai-nilai pada helaian data yang

sebenar. Arus terus elektrik yang dihasilkan oleh tenaga solar digunakan untuk

mengecas bateri dengan menggunakan penukar arus terus. Penukar arus terus buck

dipilih untuk menurunkan nilai voltan pada aras tertentu. Kawalan penjana gelombang

denyut (PWM ) digunakan untuk memacu suis kuasa MOSFET pada frekuensi tertentu.

Dalam projek ini, litar bersepadu (IC) SG3524 digunakan untuk mengawal pensuisan

suis kuasa MOSFET. Walaubagaimanapun, litar bersepadu SG3524 yang disambung

terus ke suis kuasa MOSFET tidak dapat memicu suis kuasa tersebut. Ini adalah

disebabkan oleh isyarat penjana gelombang denyut (PWM ) yang terlalu rendah. Dengan

itu, litar pemacu MOSFET digunakan untuk menaikkan isyarat PWM tersebut. Litar

pemacu MOSFET digunakan bersama litar bersepadu IC MC34151. Sebagai langkah

keselamatan, pemisah cahaya (opto-isolator) digunakan sebagai sempadan antara PWM

dan suis kuasa MOSFET. Litar bersepadu IC 6N137 digunakan sebagai pemisah cahaya.

Pemisah cahaya ini akan melindungi litar dari sebarang kerosakan litar pintas. Dengan

ini, litar dapat berfungsi dalam keadaan selamat. Selepas itu, litar yang telah dipersetujui

akan direkabentukkan dan disimulasikan dengan menggunakan MATLAB/Simulink.

Simulasi adalah didasarkan pada nilai-nilai kiraan dan analisis seperti induktor,

kapasitor, dan juga beban. Pada akhir projek ini, penukar arus terus buck dihasilkan

melalui ujian yang dijalankan di makmal.


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vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE PAGE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xiv

LIST OF SYMBOLS xv

LIST OF APPENDICES xvii

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Statement 2

1.3 Objectives 2

1.4 Scope of the Project 3

1.5 Thesis Structure 4

2 LITERATURE REVIEW 6

2.1 Photovoltaic Systems 6

2.1.1 Introduction to Photovoltaic Systems 7


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viii2.1.2 Type of Photovoltaic Systems 10

2.1.3 Photovoltaic As a Generator 13

2.1.4 Solar Cell 14

2.1.5 Solar Cell Model 15

2.1.6 Standard Rating of Photovoltaic Module 18

2.2 DC-DC Converters 20

2.2.1 Introduction 20

2.2.2 DC-DC Converters 21

2.2.2.1 Buck Converter 21

2.2.2.2 Boost Converter 22

2.2.2.3 Buck Boost Converter 22

2.2.3 Buck Converter 23

2.2.3.1 Analysis of Buck Converter 24

2.2.4 Control Principles of Buck Converter 27

2.3 Battery Storage 30

2.3.1 Introduction 30

2.3.2 Battery Storage in Photovoltaic Systems 31

2.3.3 Fundamental Concepts of Battery 32

2.3.4 Lead Acid Battery 35

2.4 Summary 37

3 ANALYSIS AND SIMULATION 38

3.1 Photovoltaic Model 38

3.1.1 Photovoltaic Cell Model Analysis 38

3.1.2 Photovoltaic Cell Model Simulation 39

3.2 Power Converter Stage 41

3.2.1 Power Converter Stage Analysis 41

3.2.2 Power Converter Stage Simulation 44

3.3 Summary 46


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ix4 HARDWARE DEVELOPMENT 47

4.1 Introduction 47

4.2 Power Converter Stage 47

4.2.1 Power Switch 48

4.2.2 Power Diode 48

4.2.3 Inductor 49

4.2.4 Input and Output Capacitor 49

4.2.5 Load 49

4.3 Pulse Width Modulation Controller Stage 50

4.4 MOSFET Driver Circuit 52

4.5 Printed Circuit Board Layout 53

4.6 Summary 55

5 RESULTS AND DISCUSSIONS 56

5.1 Introduction 56

5.2 Simulation Results 56

5.2.1 Photovoltaic Cell Model 56

5.2.2 Buck Converter 58

5.3 Experimental Results 63

5.4 Summary 67

6 CONCLUSION AND RECOMMENDATIONS 68

6.1 Conclusion 68

6.2 Recommendations 69

REFERENCES 70

APPENDIX A 74

APPENDIX B 76

APPENDIX C 78

APPENDIX D 81

APPENDIX E 82


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xAPPENDIX F 88

APPENDIX G 95

APPENDIX H 100

APPENDIX I 102


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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Advantages and Disadvantages of Photovoltaic 9

2.2 Comparison Between Converters 23

3.1 Typical Electrical Characteristic of MSX-60 PV Module 39

3.2 Specification of Buck Converter 41

3.3 Buck Converter Specification 44


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xii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Stand-alone System Equipment and Layout 10

2.2 Grid Connected Equipment and Layout 12

2.3 Hybrid System Equipment and Layout 13

2.4 PV Generator Terms 14

2.5 Operation of a PV Cell 15

2.6 Circuit Diagram of the PV Cell 15

2.7 A typical current-voltage I-V curve for a solar cell 16

2.8 Basic circuit of buck converter 21

2.9 Boost Converter 22

2.10 Buck-Boost Converter 23

2.11 Circuit of Buck Converter 24

2.12 Circuit when switch is closed 24

2.13 Circuit when switch is opened 25

2.14 Voltage Mode Control 28

2.15 Current Mode Control 29

2.16 A Comparison of the different definitions of battery capacity

and the state of charge 33


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xiii3.1 Subsystem block of photo current 40

3.2 Subsystem block of diode current 40

3.3 PV cell model block 40

3.4 Pulse-width modulator with feedback block system 45

3.5 Buck converter block system 45

4.1 Functional Block Diagram of SG3524 51

4.2 Graph of oscillator frequency vs. timing resistance of SG 3524 51

4.3 Schematic diagram of PWM controller circuit for SG3524 52

4.4 Schematic diagram of MOSFET driver circuit 53

4.5 PCB Layout

(a) Power Stage Buck Converter 54

(b) PWM Controller Circuit 54

(c) MOSFET Driver Circuit 55

5.1 Matlab module I-V Characteristics Curve 57

5.2 Matlab module PV Curve 58

5.3 (a) Sawtooth Waveform 59

(b) PWM Waveform 60

5.4 (a) Input Voltage 61

(b) Inductor Current 61

(c) Output Voltage 62

(d) Output Voltage Ripple 62

5.5 Sawtooth and PWM Waveform 63

5.6 Gate Voltage Output 65

5.7 Inductor Current Output 65

5.8 Input Voltage and Output Voltage Output 66


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xiv

LIST OF ABBREVIATIONS

PV - Photovoltaic

DC - Direct Current

AC - Alternating Current

PWM - Pulse Width Modulation

PCB - Printed Circuit Board

MPPT - Maximum Power Point Tracker

SOC - Standard Operating Conditions (Photovoltaic)

State of Charge (Battery)

STC - Standard Test Conditions

AM - Air Mass

NOCT - Nominal Operating Cell Temperature

S - Switch

D - Duty Cycle

L - Inductance

C - Capacitance

R - Resistance

CCM - Continuous Conduction Mode

DCM - Discontinuous Conduction Mode

DOD - Depth of Discharge

MOSFET - Metal Oxide Semiconductor Field Effect Transistor

IC - Integrated Circuit

ESR - Equivalent Series Resistance


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xv

LIST OF SYMBOLS

W - Watt

IL -

Si - Silicon

Photo Current

Rs -

I

Series Resistance

d

G - Irradiation

- Diode Current

Ga

m - Idealizing Factor

- Ambient Irradiation

k - Boltzmanns Gas Constant

Tc

e - Electronic Charge

- Absolute Temperature of the Cell

Io

I

- Dark Saturation Current

sc -

V

Short circuit current

oc

V

- Open Circuit Voltage

t -

V

Thermal Voltage

max

I

- Maximum Voltage

max

P

- Maximum Current

max

P

- Maximum Power

in

- Efficiency

- Input Power

A - Cell Area

Vs -

f - Frequency

Voltage Source


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xviRt

C

- Timing Resistor

t

V

- Timing Capacitor

gs

Ah - Ampere-hour

- Gate Voltage

Ah - Ampere-hour efficiency


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xvii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A MATLAB SCRIPT OF MSX-60 74

B1 PWM CONTROLLER STAGE 76

B2 DRIVER CIRCUIT 76

B3 BUCK CONVERTER 77

B4 COMPLETE CIRCUIT OF BUCK CONVERTER 77

C DATASHEET OF MSX60 78

D DATASHEET OF VALVE REGULATED LEAD ACID

BATTERIES 81

E DATASHEET OF REGULATED PULSE WIDTH

MODULATORS SG3524 82

F DATASHEET OF POWER MOSFET IRF540N 88

G DATASHEET OF POWER DIODE MUR1520 95

H DATASHEET OF HIGH CURRENT POWER INDUCTOR 100

I DATASHEET OF ALUMINIUM HOUSED

WIREWOUND RESISTORS 102


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CHAPTER 1

INTRODUCTION

1.1 Introduction

Solar energy is also known as photovoltaic (PV). It has been developed since

1970s whereby the human race can get a substantial portion of its electrical power

without burning fossil fuels (coal, oil or natural gas) or creating nuclear fission reactions.

Photovoltaic can bring electricity to the people who live in the rural areas located more

than 100 kilometers from the nearest electric grid connection in their country. For

instance, an ordinary resident who lives in the rural area, the electricity is very important

as it allows her to do housework such as cleaning the whole house, washing clothes and

also cooking. She can also use the electric lamp instead of using kerosene lamps.

Besides, she can earn some money for her family by sewing using the electric sewing

machine. More to the point, photovoltaic can also provide electricity to the remote

transmitter stations in the mountains allowing better communication without building a

road to deliver diesel fuel for the generator.

Nowadays, the applications of photovoltaic are spread widely around the world.

Goes to the fact, these applications help human a lot in their daily life. Much electrical

stuffs also get much easier to use. PV charger system is one of application used to store


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the energy from sun to the battery. Therefore, human can use electricity even though

they are in the jungle, remote area or other places, which are located far away from the

grid-connected system.

1.2 Problem Statement

Nowadays, the usage of renewable energy like photovoltaic becomes the vital

sources. PV electricity is highly appreciated by the public all over the world. It is unique

for many applications of high social value such as providing electricity to people who

need it in remote areas and lives far away from the grid-connected system. In

standalone installations, it must use storage such as battery to provide electricity when

the sunlight is not available.

The growing market for renewable energy technologies has given an impact in a

rapid growth in the need of power electronics. For example, inverter is used to convert

DC to AC and chopper is used to convert DC to DC. Therefore, the technology of

power electronics is crucial in order to design the photovoltaic charger system.

1.3 Objectives

Objectives that need to be met in this project are:

To study the characteristics of photovoltaic, buck converter and battery. To stimulate the photovoltaic charger system by using MATLAB/Simulink. To implement the buck converter into prototype as a converter for the

photovoltaic charger system.


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1.4 Scope of the project

There are three main parts that must go through in this project which are

analysis, simulation and hardware implementation. Before doing any analysis anddesigning the whole project, literature review is needed to provide a good theory and

understanding. The input of knowledge can be obtained in various sources such as

internet, journals, books, magazine, articles and so on.

Analysis and simulation are done by applying PV charger system concept into

the calculation and simulation. Then, the PV cell model and buck converter model was

designed and simulated by using Matlab/Simulink. The purposes are to observe the PV

model output characteristic and output response of buck converter.

After that, the model will be implemented into hardware. The hardware is

divided into three stages which are power stage, pulse-width modulation (PWM)

controller stage, and gate driver stage. Then, the hardware is implemented on the

breadboard to verify the circuits. After ensure that the output voltage is well regulatingat a desired output, printed circuit board (PCB) layout for the hardware is designed by

using PROTEUS 7 PRO. Then, from the designated layout, the circuits are transferred

into the printed circuit board (PCB) separately through the appropriate process. Finally,

the hardware testing and troubleshooting are conducted at the laboratory. After that, the

results from hardware is obtained. Lastly, the simulation result and hardware result are

compared and discussed.


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1.5 Thesis Structure

Chapter 1: Introduction

This chapter describe about background of PV charger systems including PV

history and the importance of photovoltaic for human being. Besides that, problem

statement of this project mainly in the charger system aspect also described as well.

Objectives and the scope of project also can be obtained in this chapter.

Chapter 2: Literature Reviews

Literature reviews contained all the basic information about PV systems, DC-DC

converter, and also battery storage. These include type of PV systems, advantages and

disadvantages of PV, and so forth. The basics operations of solar cell model also

included in this chapter. After that, basic operations of DC-DC converter will be

discussed included the control principle of the converter. Lastly, the information about

the battery storage will be discussed included fundamental concepts of battery, and basic

chemical operation of lead-acid battery.

Chapter 3: Analysis and Simulation

This chapter will present the analysis and simulation of PV cell model and power

stage buck converter. The result of PV cell model and buck converter model are

simulated using MATLAB/Simulink will be discussed in this chapter.


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Chapter 4: Hardware Development

This chapter describe about the components that have been used for power stage

buck converter such as power switch, inductor, capacitor, and so on. Besides, the PWM

controller stage and MOSFET driver circuit also described in this chapter. Lastly, the

design of the power stage buck converter in PCB layout will be discussed.

Chapter 5: Results and Discussions

The simulation results of PV cell model and power stage buck converter using

MATLAB/Simulink will be discussed. After that, the experimental results obtained from

power stage buck converter will be shown and discussed.

Chapter 6: Conclusions and Recommendations

Chapter 6 will concluded the work based on the result and discussion obtained

from this project and suggested some recommendation for future work improvement and

development for this project.


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CHAPTER 2

LITERATURE REVIEW

2.1 Photovoltaic Systems

In 1838, physicist Edmund Becquerel, at the age of nineteen, was became the

first scientist in publish observations about this natural photovoltaic phenomenon of

materials. Edmund's reported observations were considered very interesting yet there

were seemingly no practical applications. This first observation of the photovoltaic

effect in a solid, led to experimentation and speculation in to possible uses of a selenium

solar cell. In 1883, inventor Charles Fritz produced a solar cell with a conversion

efficiency of 1-2 percent. This invention that produced usable electricity from sunlight

caused a considerable amount of excitement for the potential use. However, industrial or

commercial applications did not materialize [1].

Modern solar electric power technologies came about in 1954 when Bell

Laboratories experimentation with semiconductors unexpectedly found silicon doped

with certain impurities was very sensitive to light. The final result was the invention of

the first practical solar modules with an energy conversion efficiency of around 6

percent. Over the last few decades, NASA has used photovoltaic cells extensively

proving the technology to be an excellent means to supply electrical power for the


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communications, instruments, and controls in spacecraft. The current space station has a

large solar electric system for generating electricity [1].

Photovoltaic being produced today have greatly improved conversion

efficiencies and much more cost efficient production methods. With today's large scale

production of solar cells the cost of the cells have now become affordable and cost

efficient for many applications requiring electricity. Solar electric systems are now

installed on tens of thousands of homes, businesses, communications stations, and

countless other applications, supplying all or part of their electrical energy needs.

2.1.1 Introduction to Photovoltaic Systems

The fossil fuels such as coal, oil, and natural gas, which maintain our industrial

world, will be surely running out sometime in the twenty-first century. Moreover,

burning such fossil fuels causes the global air pollution, leading to global warming and

acid rain problems. The development of alternative clean energy resources is, therefore,

one of the most urgent subjects with which contemporary scientists have to struggle.

Utilization of solar energy seems to be the most promising and potential, and an

important subject that a number of researchers in the world are now studying. A variety

of ways for utilizing solar energy are known, for example, thermal energy by heat

collectors, electrical energy by silicon solar cells, and chemical energy by

photosynthesis, where the latter is referred to as the conversion of solar energy into

chemical energy. The significant problems commonly are because of the low cost

performance, the low energy conversion efficiency, and the lack of persistence. In order

to replace solar energy for fossil energy on an economical basis, it is necessary to

overcome these problems as soon as possible [2].

The world trend nowadays is to find a non-depletable and clean source of energy.

The most effective and harmless energy source is probably solar energy, which for many


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applications is so technically straight forward to use . Thus modern solar technologies

have been penetrating the market at faster rates. The solar technology that has the

greatest impact on our lives is photovoltaic. Not in terms of the amount of electricity it

produces, but because of the fact that photovoltaic cells work silently, not polluting and

also can generate electricity wherever sun shines, even in places where no other form of

electricity can be obtained [2].

Photovoltaic is a technology that generates direct current (DC) electrical power.

It is measured in Watts (W) or kilowatts (kW) from semiconductors when they are

illuminated by photons. As long as sunlight is shining on the solar cell, it generates

electrical power [3].

Over the years, photovoltaic has emerged to become an application of recognized

potential and has attracted energy and it becomes the most important energy source

among all the sources. The solar energy is different from the other sources as its

available energy is several tens of orders of magnitude greater than our annual

consumption. Therefore, photovoltaic systems that use solar irradiance from the sun

have a high potential to be one of the best renewable energies. There are many PV

applications can be applied, such as PV power stations, building integrated photovoltaic

(BIPV), PV as a source for transportation and so forth.

Typically, the advantages and the disadvantages of photovoltaic are almost

completely opposite of conventional fossil-fuel power plants. Table 2.1 shows the

advantages and disadvantages of photovoltaic systems.


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Table 2.1: Advantages and Disadvantages of Photovoltaic.

Advantages of photovoltaic Disadvantages of photovoltaic

Fuel source is vast and essentially infinite Fuel source is diffuse (sunlight is a

relatively low-density energy)

No emissions, no combustion or radioactive

fuel for disposal (does not contribute

perceptibly to global climate change or

pollution)

Low operating costs (no fuel) High installation costs

No moving parts (no wear)

Ambient temperature operation (no high

temperature corrosion or safety issues)

High reliability in modules (>20 years) Poorer reliability of auxiliary (balance

of system) elements including storage

Modular (small or large increments)

Quick installation

Can be integrated into new or existingbuilding structures

Can be installed at nearly any point-of-use Lack of widespread commercially

available system integration and

installation so far

Daily output peak may match local demand Lack of economical efficient energy

storage

High public acceptance

Excellent safety record


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2.1.2 Type of Photovoltaic Systems

Photovoltaic power systems can be classified as:

i. Stand-alone PV systems.ii. Grid-connected PV systems.

iii. Hybrid PV systems.

i. Stand-alone PV systems

Many photovoltaic systems operate in stand-alone mode. This mode of system

consists of a PV panel as generator, regulator unit, battery as energy storage, inverter

and AC load as shown in Figure 2.1. A stand-alone system involves no interaction with

the utility grid. The battery bank stores energy when the power supplied by the PV

modules exceeds the load demand and releases it when PV supply is insufficient [4].

Figure 2.1: Stand-alone System Equipment and layout [4].

ii. Grid-connected PV systems

Isolated areas are dependent on batteries, whereas places in town have the option

of using a power grid, depending on their power consumption and power suppliers.

Connecting to a power grid allows the power generated from the panels to be back-fed to


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the grid when the sun is out, and to run the structure off the line when the sun is down.

The cost of purchasing a DC to AC converter with a grid controller, compared to using

batteries, varies by the size of the system. Reliance on a grid eliminates the need to

replace faulty batteries that plague the long-term operation of stand-alone systems. The

drawback to grid-connected systems is the number of panels that are needed to provide

enough power for the utility company to consider connecting the system to the grid [5].

A grid-connected system must meet the following criteria to function: voltage

regulation, frequency regulation, power factor control, harmonic distortion controls, and

quick response time. The amount of power a system generates determines if the energy

provided will decrease the amount of the electric bill, or if the excess energy produced

would be sold to the power company [5].

Figure 2.2 represents the system required to connect the panel to the power grid.

A DC to DC converter is needed to hold a near constant output voltage. To maximize

the output of the panel, a maximum power point tracker (MPPT) controller is used. A

MPPT is a boost converter for a single panel or a buck converter when multiple panels

are combined in series. The converters produce a near constant voltage value that

increases the efficiency of the inverter. The capacitor removes any small variations in

the near-constant input voltage to the DC-AC converter. The inverter monitors the

power grid to match the standard voltage and frequency. The controller continuously

compares the frequency of the grid with the inverter, and adjusts the duty ratio to

counter frequency variations [5].


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Solar Panel DC-DC DC-AC Controller

Load

Figure 2.2: Grid Connected Equipment and Layout.

iii. Hybrid connection systems

A system design that combines the advantages of both a stand-alone setup and a

grid-connected setup is deemed a hybrid system. This system relies on the coordination

of multiple controllers to continuously monitor the flow of power from the solar panels,

and regulate the power to fulfill the needs of the structure, replenish the reserve

batteries, and manage the flow of energy to and from the power grid. The basic setup of

a hybrid system is shown in Figure 2.3. The equipment consists of the solar panels, a

MPPT, a charge controller, batteries, and an inverter. The charge controller monitors the

batteries and determines whether to charge them. The high-end inverter matches the

frequency of the power grid and monitors the grid to detect any loss in power. This

system provides an uninterruptible power supply that provides electricity even when the

power grid is offline. This system has the highest cost and requires the replacement and

maintenance of batteries. The use of this type is limited to industrial applications where

backup power may be needed to prevent the stoppage of equipment due to a trip in the

power grid [5].


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Figure 2.3: Hybrid System Equipment and Layout [5].

2.1.3 Photovoltaic As a Generator

A photovoltaic generator is the whole assembly of solar cells, connections,

protective parts, supports, etc. PV generator can contain several arrays. Each array is

composed of several modules, while each module is composed of several cells as shown

in Figure 2.4. Solar cells consist p-n junction fabricated in a thin wafer or layer of

semiconductor (silicon) which are specially treated to form an electric field, positive on

one side (backside) and negative on the other (towards the sun). When solar energy

(photons) hits the solar cell, electrons are knocked loose from the atoms in the

semiconductor material creating electron-hole pairs. If electrical conductors are then

attached to the positive and negative sides, forming an electrical circuit, the electrons are

captured in the form of electric current IL

(photocurrent). When the cell is short-

circuited, this current flows in the external circuit and when open circuit, this current

shunted internally by the intrinsicp-n junction diode. The characteristics of this diode

therefore set the open circuit voltage characteristics of the cell [6].


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Figure 2.4: PV Generator terms [4].

2.1.4 Solar Cell

Solar cells are composed of various semiconductor materials, which become

electrically conductive when supplied by heat or light. The majority of solar cells

produced are composed of Silicon (Si) which exist in sufficient quantities and do not add

any burden on the environment [7].

Doping technique is used to obtain a surplus of positive charge carriers (p-type)

or a surplus of negative carriers (n-type). When two layers of different doping are in

contact, then ap-n junction is formed on the boundary. An internal electric field is built

up which then causes the separation of charge carriers released by light. We all know

that light is composed of small packets called photons. When these photons bombard

our cell, many electrons are freed within the electric field proximity, which then pull the

electrons from the p-side to n-side. Through metal contacts, an electric charge can be

taped. If the outer circuit is closed, then direct current flows as illustrated in Figure 2.5

[7].

PV Cell PV Module PV Panel PV Array


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Figure 2.5: Operation of a PV Cell [7].

2.1.5 Solar Cell Model

Solar cell is not an active device. It works as a diode, i.e. a p-n junction. It

produces neither a current nor a voltage. However, if it is connected to an external

supply (large voltage) it generates a currentId, called diode current or dark current. The

diode determines the I-V characteristics of the cell. The output of the current source is

directly proportional to the light received on the cell (photocurrent,IL

) [8].

A solar cell is usually represented by an electrical equivalent one-diode model as

shown in Figure 2.6.

Figure 2.6: Circuit diagram of the PV cell [6].


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The model contains a current sourceIL, a diode and a series resistanceRS, which

represents the resistance inside each cell and in the connection between the cells. The net

current is the difference between the photocurrent IL and the normal diode currentID :

( )

( 1)s

c

e V IR

mkT

L D L oI I I I I e

+

= = (2-1)

where m is idealizing factor, kis Boltzmanns gas constant, Tc the absolute temperature

of the cell, e electronic charge and V is the voltage imposed across the cell. Io is the

dark saturation current and it is strongly depending on temperature. Figure 2.7 shows the

I-V characteristic of the solar cell for a certain ambient irradiation Ga

and a certain fixedcell temperature Tc [8].

Figure 2.7: A typical current-voltage I-V curve for a solar cell [8].

A real solar cell can be characterized by the following fundamental parameters,

which are also shown in Figure 2.7:

a) Short circuit current


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Short circuit currentIsc =IL

. It is the greatest value of the current generated by a

cell. It is produced under short circuit conditions where Vequals to 0.

b) Open circuit voltage

Open circuit voltage corresponds to the voltage drop across the diode (p-n

junction), when it is traversed by the photocurrent IL (namely Idequals to IL

), namely

when the generated current isIequals to 0. It reflects the voltage of the cell in the night

and it can be mathematically expressed as:

ln( ) ln( )C L LtOC

o o

mkT I IV Ve I I

= = (2-2)

Where ct

mkTV

e= is known as thermal voltage and Tc

is the absolute cell.

c) Maximum power pointMaximum power point is the operating point A (Vmax, Imax

) in Figure 2.7, at

which the power dissipated in the resistive load is maximum:

Pmax = ImaxVmax

(2-3)

d) Maximum efficiency

The PV efficiency, is the ratio between the maximum power and the incident

light power:


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= = (2-4)

Where Ga

is the ambient irradiation and A is the cell area.

e) Fill Factor

Fill Factor (FF) is the ratio of the maximum power that can be delivered to the

load and the product ofIsc and Voc

:

max max max

oc sc oc sc

P V IFF

V I V I = = (2-5)

The fill factor is a measure of the real I-V characteristic. Its value is higher than

0.7 for good cells. The fill factor diminishes as the cell temperature is increased.

2.1.6 Standard Rating of Photovoltaic Module

In comparing different modules, the standard rating system used is a peak power

value given by the manufacturers. This is based on the module maximum power output

at standard test conditions (STC). The current terrestrial standard is an irradiance of

1000 W/m2

at Air Mass AM1.5, and a cell or module temperature of 25 C (Green,

1995) [5]. Generally, the information supplied by PV manufacturers includes the

following parameters:

Pmax

V

: Maximum Power Rating

oc : Open Circuit Voltage


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Isc

V

: Short Circuit Current

mp

I

: Maximum Power Voltage

mp : Maximum Power Current

Another value often supplied by manufacturers is the Nominal Operating Cell

Temperature or NOCT. It is defined as the cell temperature of an open-circuited, rack

mounted module at standard operating conditions (SOC). SOC represents a more

realistic operating condition for a PV module than STC. SOC is defined as an irradiance

of 800 W/m2

, an ambient temperature of 20 C, and a wind speed of 1 m/s. By providing

the NOCT value a user or system designer can calculate a thermal capacitance value for

the module and thereby estimate cell temperatures at other operating conditions (Duffie

& Beckman, 1991) [5].

The size attributed to a PV array is calculated from this STC Wp (peak watt)

rating, even though the standard test conditions described above are rarely experienced

by modules under actual operation. A 20 kW array, for example, consists of an array of

PV modules whose Wp

rating totals 20 kW, though, depending on the location, it is

highly unlikely the array will ever produce a power of 20 kW [5].

The SOC and NOCT values provide a more realistic indication of the output of

modules under actual operation, but, again, these are ideal conditions and not

representative of the full range of operating conditions [5].


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2.2 DC-DC Converters

2.2.1 Introduction

Dc-dc converters are power electronics circuits that convert a dc voltage to a

different dc voltage level [9]. It achieves the voltage regulation by varying the duty ratio

of the switching element. Modern power electronic switches can operate at high

frequencies. The higher the operating frequency, the smaller and lighter the

transformers, filter inductors, and capacitors. In addition, the dynamic characteristics of

converters improve with increasing operating frequencies. The bandwidth of a control

loop is usually determined by the corner frequency of the output filter. Therefore, high

operating frequencies allow for achieving a faster dynamic response to rapid changes in

the load current and the input voltage. High-frequency electronic power processors are

used in dc-dc power conversion [10].

The functions of dc-dc converters are:

to convert a dc input voltage into a dc output voltage

to regulate the dc output voltage against load and line variations to reduce the ac voltage ripple on the dc output voltage below the required level to provide isolation between the input source and the load to protect the supplied system and the input source from electromagnetic

interference

to satisfy various international and national safety standards

The dc-dc converters can be divided into two main types: hard-switching pulse

width modulated (PWM) converters, and soft-switching converters. Advantages of

PWM converters include low component count, high efficiency, constant frequency

operation, relatively simple control and commercial availability of integrated circuit

controllers, and ability to achieve high conversion ratios for both step-down and step-up

application. A disadvantage of PWM dc-dc converters is that PWM rectangular voltage


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and current waveforms cause turn-on and turn-off losses in semiconductor devices,

which limit practical operating frequencies to hundreds of kilohertz [10].

2.2.2 DC-DC Converters

There are three basic types of dcdc converters:

i. Step-down converter (Buck Converter)ii. Step-up converter (Boost Converter)

iii. Step-up-down converter (Buck Boost Converter)

2.2.2.1 Buck Converter

The step-down dcdc converter, commonly known as a buck converter, is shown

in Figure 2.8. It consists of dc input voltage source Vs, controlled switch S, diode D,

filter inductorL, filter capacitorC, and load resistance R. The state of the converter in

which the inductor current is never zero for any period of time is called the continuous

conduction mode (CCM). It can be seen from the circuit that when the switch S iscommanded to the onstate, the diodeD is reverse biased. When the switch Sis off, the

diode conducts to support an uninterrupted current in the inductor [10].

Figure 2.8: Basic circuit of buck converter.


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2.2.2.2 Boost Converter

Figure 2.9 depicts a step-up or a PWM boost converter. It is comprised of dc

input voltage source Vs, boost inductorL, controlled switch S, diode D, filter capacitor

C, and load resistance R. When the switch S is in the onstate, the current in the boost

inductor increases in linear. The diode D is offat the time. When the switch Sis turned

off, the energy stored in the inductor is released through the diode to the input RCcircuit

[10].

Figure 2.9: Boost Converter.

2.2.2.3 Buck Boost Converter

A non-isolated topology of the buckboost converter is shown in Figure 2.10.

The converter consists of dc input voltage source Vs, controlled switch S, inductorL,

diode D, filter capacitor C, and load resistance R. With the switch on, the inductor

current increases while the diode is maintained off. When the switch is turned off, the

diode provides a path for the inductor current. The polarity of the diode which results in

its current being drawn from the output. Table 2.2 shows that comparison betweenconverters [10].


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Table 2.2: Comparison Between Converters.

Buck Converter Boost Converter Buck Boost

Converter

Operation Step down voltage

level

Step up voltage level Step up and down

voltage level or vice

versa.

Duty

Cycyle

D

Ripple

Ratio

Minimum

Inductance

Figure 2.10: Buck-Boost Converter.

2.2.3 Buck Converter

A buck converter is a step-down DC to DC converter where the output voltage is

less than the input voltage. This converter is a simplest power stage topology as shown

in Figure 2.11 [11].In this project, a step down converter is chosen in order to step down

the voltage level.


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Figure 2.11: Circuit of buck converter [11].

The assumptions that are needed for an analysis of the buck converter:

The circuit is operating in the steady state. The inductor current is continuous (always positive). The capacitor is very large and the output voltage is held constant. The switching period is T; the switch is closed for timeDTand open for time (1-

D)T.

The components are ideal.

2.2.3.1 Analysis of Buck Converter

When the switch is closed

Figure 2.12: Circuit when switch is closed [11].


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When the switch is closed, the diode is reversed biased and Figure 2.12 is an

equivalent circuit. From the derivation, we get:

(iL) closed

= (2-6)

When the switch is opened

Figure 2.13: Circuit when switch closed [11].

When the switch is opened, the diode becomes forward biased to carry the

inductor current, and the equivalent circuit of Figure 2.13 applied. From the derivation,

we get:

(iL) open

= (2-7)

The net change in inductor current over one period is zero,

(iL) closed+ (iL) open

= 0 (2-8)


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Using Equation (2.6) and (2.7),

= 0 (2-9)

Solving forVo

,

(2-10)

It can be seen from Equation (2-10) that the output voltage is always smaller than

the input voltage. The buck converter produces an output voltage which is less than or

equal to the input. The output voltage depends only on the input and the duty ratio,D. If

the input voltage fluctuates, the output voltage can be regulated by adjusting the duty

ratio appropriately [10].

The buck converters can operate in two distinct modes with respect to the

inductor currentIL

. When the average value of the input current is low (high R) and orthe switching frequencyfis low, the converter may enter the discontinuous conduction

mode (DCM). In the DCM, the inductor current is zero during a portion of the switching

period. The CCM is preferred for high efficiency and good utilization of semiconductor

switches and passive components. The DCM may be used in applications with special

control requirements, since the dynamic order of the converter is reduced (the energy

stored in the inductor is zero at the beginning and at the end of each switching period).

It is uncommon to mix these two operating modes because of different control

algorithms [10].

For the buck converter, the value of the filter inductance that determines the

boundary between CCM and DCM is given by:


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L min

= (2-11)

ForL > Lmin, the converter operates in the CCM. The filter inductor currentIL

in the CCM consists of a dc component IO with a superimposed triangular ac

component. Almost all of this ac component flows through the filter capacitor as a

currentIc . CurrentIc causes a small voltage ripple across the dc output voltage VO. To

limit the peak-to-peak ripple voltage Vo , the filter capacitance Cmust be greater than

Cmin.

.

Cmin

= (2-12)

Equations (2-11) and (2-12) are the key design equations for the buck converter.

The input and output dc voltages (hence, the duty ratio D), and the range of load

resistanceR are usually determined by preliminary specifications. The value of the filter

inductorL is calculated from the CCM/DCM condition using Equation (2-11). The

value of the filter capacitorCis obtained from the voltage ripple condition Equation (2-

12). Equations (2-11 and 2-12) show that it can be accomplished by using a highswitching frequency f. The switching frequency is limited, however, by the type of

semiconductor switches used and by switching losses [10].

2.2.4 Control Principles of Buck Converter

A buck converter must provide a regulated dc output voltage under varying load

and input voltage conditions. The converter component values are also changing with

time, temperature, pressure, etc. Hence, the control of the output voltage should be

performed in a closed-loop manner using principles of negative feedback. Two most


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common closed loop control methods for PWM buck converters, namely, the voltage-

mode control and the current-mode control [10].

In the voltage-mode control scheme shown in Figure 2.14, the converter output

voltage is sensed and subtracted from an external reference voltage in an error amplifier.

The error amplifier produces a control voltage that is compared to a constant-amplitude

sawtooth waveform. The comparator produces a PWM signal which is fed to drivers of

controllable switches in the buck converter. The duty ratio of the PWM signal depends

on the value of the control voltage. The frequency of the PWM signal is the same as the

frequency of the sawtooth waveform. An important advantage of the voltage-mode

control is its simple hardware implementation and flexibility [10].

Figure 2.14: Voltage mode control [10].

The error amplifier in Figure 2.14 reacts fast to the changes in the converter

output voltage. Thus, the voltage-mode control provides good load regulation, that is,

regulation against variations in the load. Line regulation (regulation against variations inthe input voltage) is, however, delayed because changes in the input voltage must first

manifest themselves in the converter output before they can be corrected. To alleviate

this problem, the voltage-mode control scheme is sometimes augmented by so-called

voltage feedforward path. The feedforward path affects directly the PWM duty ratio

according to variations in the input voltage [10].


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The current-mode control scheme is presented in Figure 2.15. An additional

inner control loop feeds back an inductor current signal. This current signal, converted

into its voltage analog, is compared to the control voltage. This modification of

replacing the sawtooth wavefrom of the voltage-mode control scheme by a converter

current signal significantly alters the dynamic behavior of the converter. The converter

takes on some characteristics of a current source. The output current in PWM buck

converters is either equal to the average value of the output inductor current or is a

product of an average inductor current and a function of the duty ratio. In practical

implementations of the current-mode control, it is feasible to sense the peak inductor

current instead of the average value. Since the peak inductor current is equal to the peak

switch current, the latter can be used in the inner loop which often simplifies the current

sensor. The peak inductor (switch) current is proportional to the input voltage. Hence,

the inner loop of the current-mode control naturally accomplishes the input voltage

feedforward technique [10].

Figure 2.15: Current mode control [10].

Advantages of the current-mode control include: input voltage feedforward, limit

on the peak switch current, equal current sharing in modular converters, and reduction in

the converter dynamic order. The main disadvantage of the current-mode control is its

complicated hardware which includes a need to compensate the control voltage by ramp

signals (to avoid converter instability) [10].


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2.3 Battery Storage

2.3.1 Introduction

In 1791, Luigi Galvani published a report on animal electricity. He created an

electric circuit consisting of two different metals, with one touching a frog's leg and the

other touching both the leg and the first metal, thus closing the circuit. In modern terms,

the frog's leg served as both the electrolyte and the sensor, and the metals served as

electrodes. He noticed that even though the frog was dead, its legs would twitch when

he touched them with the metals. In 1800, Volta invented the battery by placing many

voltaic cells in series, literally piling them one above the other. This Voltaic pile gave a

greatly enhanced net electromotive force (emf) for the combination [12].

Later, starting with the Daniell cell in 1836, batteries provided more reliable

currents and were adopted by industry for use in stationary devices, particularly in

telegraph networks where they were the only practical source of electricity, since

electrical distribution networks did not exist then. Near the end of the 19th

century, the

invention of dry cell batteries, which replaced liquid in electrolyte with a paste, made

portable electrical devices practical. Since then, batteries have gained popularity as they

became portable and useful for a variety of purposes [12].

Batteries are electrochemical devices which are used to supply energy for

electrical and electronic product. Chemical energy stored in a battery is converted into

electric current when battery is discharged. This electric current is produced directly by

chemical reactions which occur within the battery. The quantity of electric energy made

available is a function of the chemical compositions and the amount material present in a

cell. Many sets of different chemicals have been combined, with varying degrees of

success, to make energy storage systems. Each type of battery couple has advantages

and disadvantages with regard to its physical and electrical characteristics. Energy

density, expressed in watt-hours per cubic inch, and power capability in watts per pound

or watts per cubic inch are often used to compare battery system performance [13].


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2.3.2 Battery Storage in Photovoltaic Systems

Stand-alone PV systems require energy storage to compensate for periods

without or within sufficient solar irradiation, such as during the night or during cloudy

weather. In all cases in which electric energy storage is required, the classical

electrochemical accumulator battery is the most convenient form of energy storage for a

PV system [14].

A charge controller is included between the solar generator and the battery. The

charge controller is prevents the battery from being overcharged or deep discharged. The

charge controller usually has a blocking diode, which prevents the battery from

discharging during the night via the solar generator. A good charge controller has very

low internal power consumption and includes a load cut-off switch that protects the

battery against discharge [14].

Batteries are often used in PV systems for storing energy produced by the PV

during day time and supplying it to electrical loads as needed (during night time or

cloudy weather). Moreover, batteries are also needed in the tracker systems to keep the

operation at MPP in order to provide electrical loads with stable voltages. Nearly, most

of the batteries used in PV systems are deep cycle lead-acid. These batteries have

thicker lead plates that make them tolerate deep discharges. The thicker the lead plates,

the longer the life span. The heavier battery for a given group size, the thicker plates

and the better battery will tolerate deep discharges. All deep cycle batteries are rated in

ampere-hour where Ampere-hour (Ah) capacity is a quantity of the amount of usable

energy it can store at nominal voltage. For example an ampere-hour is one ampere for

one hour or 10 A for one-tenth of an hour and so forth. A good charge rate is

approximately 10 percent of the total capacity of the battery per hour for example 200

ampere-hour battery charged at 20 A [7].


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2.3.3 Fundamental Concepts of Battery

A battery is made from two or more electrochemical cellsconnected in series.

Primary and secondaryelectrochemical cells can be distinguished. Secondary batteries

are called accumulators which have reversible reactions and are rechargeable. An

electrochemical cell consists of two electrodes. Commonly, one is called the positive

electrode and the other, the negative electrode. The positive electrode has a more

positive potential than the negative electrode. Each combination of charged and

discharged active material has a specific electrochemical potential. The potential

difference between the positive and the negative electrode is called the cell potential

[15].

The capacityof a cell is measured typically in ampere-hours (Ah). The capacity

is determined by a constant current discharge down to a defined end-of-discharge

voltage. The capacity depends significantly on the discharge current and the

temperature. Battery manufacturers can define the discharge current and the end-of-

discharge voltage on their own. Therefore, it is very important to check the reference

conditions defined by the manufacturer while comparing the capacity of different

products [15].

Typically, nominal cell voltages are in the range between 1.2 V and 3.6 V.

Therefore, several cells are usually connected in series to build a string of higher

nominal voltage. The nominal voltageof a battery is therefore defined by the number of

cells connected in series times the nominal cell voltage of a single cell [15].

The state of charge (SOC) gives the capacity that can be discharged from a

battery at a certain moment. Hundred percent state of charge means a fully charged

battery, zero percent SOC means that the nominal capacity is discharged. Figure 2.16

shows different definitions of the battery capacity and state of charge. The rated or

nominal capacity is defined as the 10-h discharge capacity C10. This is the basis for the

SOC determination. The rated or nominal capacity does not change during the life of a


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battery whereas the measured capacity changes with time. The state of charge with

respect to the measured capacity is called relative state of charge (SOC r). The practical

capacity is always lesser than the measured capacity. The state of charge definition

related to the practical capacity is the practical state of charge (SOCp

) [15].

Figure 2.16: A Comparison of the different definitions of battery capacity and the state

of charge [15].

The ampere-hour efficiency (Ah) is defined as the ratio of the ampere-hours

discharged from the battery divided by the ampere-hours charged to the battery within a

certain period (typically one month or one year or within a period between two full

charging processes). Often the charge factor is used instead of the amperehour

efficiency. It is defined as 1/Ah. For a sustainable battery operation, charge factors

greater than one are necessary [15].

Instead of SOC the depth of discharge(DOD) is used in the data sheets. DOD is

defined as zero percent when the battery is fully charged and as hundred percent after

the nominal capacity is discharged from the battery (DOD = 100 % SOC). A cycle

refers to a discharge followed by a recharge. Cycles used in datasheets always start

from a fully charged battery up to a certain DOD. A nominal full cycle is a discharge


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down to hundred percent DOD. The cycle lifetimefor a battery is given by the number

of cycles as a function of the DOD [15].

Battery currentsare usually given relative to the battery size. The normalization

of the current to the capacity is an appropriate measure. Therefore, battery currents are

expressed as multiples of the ampere-hour capacity or as multiples of the capacity

defining discharge current. For a battery with a capacity of C= 100 Ah, a current of 10

A is defined as 0.1 C. In the example, 100 A is called the C-rate. I10 is the current

that discharges a fully charged battery within 10 hour down to the defined end-of-

discharge voltage. The typical nomenclature for the capacity is C x

wherex is the time in

which the battery is discharged. For example: C10 = 10 hour I10, or C10 = 100 Ah,

I10 = 10 A = 0.1 C10. Note that 1 I10 is not equivalent to 10 I100 as the C100

capacity is typically larger than the C10 capacity [15].

The end-of-charge voltagedefines an upper voltage limit. Charging of the battery

usually is not stopped on reaching the end-of-charge voltage (other than the end-of-

discharge voltage), but the charge current is reduced accordingly to maintain the end-of-

charge voltage over time [15].

The lifetimeof a battery depends very much on the operating conditions and the

control strategy. Manufacturers usually define two types of lifetime: the float lifetime

(calendar lifetime) gives the lifetime under constant charging conditions without cycling

(typical applications are uninterruptible power supplies), and for continuous cycling

(cycle lifetime, typical applications are fork-lift trucks) [15].

Self-discharge describes the (reversible) loss of capacity on open-circuit

conditions. It depends very much on the temperature [15].

The state of healthis defined as the ratio of the actual measured capacity and the

rated or nominal capacity. The state of health indicates to which extent the battery is still

able to fulfill the requirements. According to the norms, lead acid batteries are at the end


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of their lifetime if the state of health is under 80 percent [15].

2.3.4 Lead Acid Battery

Lead-acid battery is a rechargeable battery invented in 1859 by French physicist

Gaston Plant, Lead-acid batteries are widely used in several applications types. Among

others, these batteries are used in automotive power systems, uninterruptible power

system (UPS) and telecom power supply. Therefore, several battery charger types have

been developed. However, the evaluation in real time of the battery autonomy is an

important part for several application types that uses batteries [16].

The electrochemical storage system is based on the conversion of chemical

energy into electrical energy and vice versa. The amount of energy that can be stored in

a cell is determined by the different energy content of chemical substances that represent

the charged and discharged states. Consequently, the characteristic parameters of the

system are determined by a number of electrochemical reactions and the energetic

changes connected with these reactions. In total, these reactions result in the cell

reactions that characterize the battery itself [15].

Lead acid batteries in the charged state consist of a positive electrode with lead

dioxide (PbO2

) and a negative electrode with lead (Pb) as the active materials. The

following reaction equations describe the main reaction:

Positive electrode PbO2 + 3H+

+ HSO-4 +2e

- PbSO4 + 2H2

Negative electrode Pb + HSO

O (2-13)

-4 PbSO4 + H

++ 2e

-

Cell Reaction Pb + PbO

(2-14)

2 + 2H+

+ 2HSO-

4 2PbSO4 + 2H2

O (2-15)

PbO2 and Pb are both converted to lead sulphate PbSO 4 during discharging.

Sulphuric acid as the electrolyte is used up during the discharging of the battery.

Therefore, the concentration of the sulphuric acid decreases in linear with the state of

charge. This is an important difference with respect to almost all other battery types,


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where the electrolyte has only the function of an ion conductor. In lead acid batteries, it

is in addition the source for the ions to counterbalance the charge dissolved in the

electrolyte from the electrochemical process [15].

The charged electrode consists of lead (Pb) in the solid state. When a discharge

current occurs, two electrons are withdrawn from the metallic lead and dissolution of

Pb2+

ions into the electrolyte occurs. Through diffusion, the charged ions are transported

away from the reaction surface. As the charged ions unbalance the number of positive

and negative ions in the electrolyte, negatively charged ions are necessary to

counterbalance the positive surplus. They are provided as SO42

ions from the sulphuric

acid electrolyte. The SO42

ions are transported by diffusion from the free electrolyte

volume to the reaction site of the electrochemical reaction. From there, the Pb2+

and the

SO42

ions meet and form PbSO4 by a chemical precipitation process. This finally

results in the formation of PbSO4 crystals [15].

During charging, the reverse process takes place. Pb2+

ions are taken from the

electrolyte to form solid Pb during the electrochemical precipitation process. These ions

are transported by diffusion processes to the reaction site. To stabilize the Pb2+

ion

concentration in the electrolyte, a chemical dissolution process of the PbSO4 crystals

takes place. Because the positive ions are removed from the electrolyte through the

electrochemical precipitation process, the SO42

ions need to be transported away from

the reaction site to assure electrical neutrality [15].

The provision of cost effective electrical energy storage remains one of the major

challenges for the development of improved PV systems. Typically, lead acid batteries

are used to guarantee several hours to a few days of energy storage. Their reasonable

cost and general availability has resulted in the widespread application of lead-acid

batteries for remote area power supplies despite their limited lifetime compared to others

[4].


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The following factors are considered in the selection of batteries for PV

applications:

Deep discharge (7080% depth of discharge). Low charging/discharging current. Long duration charge (slow) and discharge (long duty cycle). Irregular and varying charge/discharge. Low self discharge. Long life time. Less maintenance requirement. High energy storage efficiency. Low cost.

2.4 Summary

This chapter covers the reviews of PV charger system. A brief discussion on PV

system included basic fundamental operation of PV cell model and also standard rating

of PV module. Besides, the basic operation of DC-DC converter are presented included

the control principles of buck converter. Lastly, this chapter also reviews battery storage

in PV systems, fundamental concepts of battery and basic information regarding lead-

acid battery. All the theory explained in this chapter will be used in order to finish this

project.


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CHAPTER 3

ANALYSIS AND SIMULATION

3.1 Photovoltaic Model

3.1.1 Photovoltaic Cell Model Analysis

The model of the PV module was implemented using a Matlab/Simulink

program. The model parameters are evaluated using the Equations (2-1). The

MATLAB/Simulink program, calculate the current I, using typical electrical parameter

of the module (ISC, VOC

), and the variables voltage, Irradiation (G), and Temperature

(T).

The Solarex MSX60 PV module was chosen for modeling. The MSX 60 module

provides 60 W of nominal maximum power, and has 36 series connected polycrystalline

silicon cells. The key specifications are shown in Table 3.1.

The simulation of PV module considers the series resistance. This series

resistance makes the solution for the net current I(Eqn. 2-1). A Matlab/Simulink script

file was implemented consider the parameters of MSX60 module. All the parameters

and constants of PV model will be applied using Matlab script as shown in Appendix A.


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Table 3.1: Typical Electrical Characteristic of MSX-60 PV Module.

Parameter Variable Value

Maximum Power P 60 Wm

Voltage @P Vm 17.1 Vm

Current @P Im 3.5 Am

Short circuit current I 3.8 Asc

Open circuit voltage V 21.1 Voc

Temperature coefficient of open circuit voltage -(8010) mV/C

Temperature coefficient of short circuit current (0.00650.015) %/C

Temperature coefficient of power -(0.50.05) %/C

NOCT 472 C

3.1.2 Photovoltaic Cell Model Simulation

The simulation of PV cell model is constructed using simulink block. The

subsystem blocks of the PV model are divided into three parts based on Equation (2-1):

Photo current Diode current Photovoltaic current (net current)

Figure 3.1 shows the subsystem simulation of photo current in Matlab/Simulink.

The subsystem simulation of diode current is shown as Figure 3.2.

The net current of the cell is the difference of the photo current, IL and the

normal diode current ID. Figure 3.3 shows the simulation block system of the

photovoltaic current/net current. For this simulation, the inputs of PV cell are irradiance,

G, temperature, T, and photovoltaic voltage, Vpv

.


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Figure 3.1: Subsystem block of photo current.

Figure 3.2: Subsystem block of diode current.

Figure 3.3: PV cell model block.


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3.2 Power Converter Stage

3.2.1 Power Converter Stage Analysis

The design specification of buck converter is shown in Table 3.2. The

specification is for a switching regulator to supply 17.1 V at maximum current 3.8 A

from direct current PV source of maximum voltage 21.1 V.

Table 3.2: Specification of Buck Converter.

Parameter Value

Input voltage 17.1 V

Output voltage 14.5 V

Maximum output power 60 W

Output current 0 - 3.5 A

Switching frequency 50 kHz

Based on the specification for input voltage and output voltage as shown in Table3.1, the required duty cycle can be determined by using Equation (2-10).


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The minimum inductance can be determined by using Equation (2-11).

L min

=

L min

=

=

The buck converter was chosen to operate in CCM mode. Therefore L= 68

was chosen to ensure that inductor current can operate in CCM. After that, the mean,

minimum and maximum inductor current can be determined.

(3-1)

(3-2)


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(3-3)

The capacitance is calculated using Equation (2-12). Let the peak-to-peak ripple

voltage equal to 0.05.

Cmin

= (3-4)

=

= 31.99F

The output filter capacitance Cmust be greater than Cmin.

. Let the output filter

capacitance be 45 percent larger than the minimum capacitance.

Cout= 1.45 x Cmin

= (1.45)(31.99)

(3-5)

= 46.39F


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So, the nearest output filter capacitance was chosen equal to 47F. The

specifications of buck converter as shown in Table 3.3

Table 3.3: Buck Converter Specifications.

Specifications Value

Input Voltage 17.1 V

Output Voltage 14.5 V

Switching Frequency 50 kHz

Output Current 0 3.5 A

Duty Cycle 0.85/85 %

Inductor 68 uH

IL 0.9613 A

ILmax 1.2351 A

ILmin 0.84617 A

Capacitor (Cout 47 uF)

3.2.2 Power Converter Stage Simulation

The simulation of buck converter is constructed using Matlab/Simulink. The

simulation was done in two types of subsystem block, which are pulse width modulation

(PWM) with feedback and power stage buck converter.

In this project, PWM was generated by additional of sawtooth signal and

difference of output voltage compared to the reference voltage. The block for this

system is shown as shown in Figure 3.4.

The power stage block consists DC input voltage, MOSFET, diode, inductor,output capacitor and load resistor. Figure 3.5 shows the power stage block for the buck

converter.


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Figure 3.4: Pulse-width modulator with feedback block system.

Figure 3.5: Buck converter block system.


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3.3 Summary

This chapter presents the simulink model and results of PV cell model and

power converter stage by using MATLAB/Simulink. Besides, this chapter also presents

the calculation analysis of buck converter in determined the specification that used in

hardware implementation.


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CHAPTER 4

HARDWARE DEVELOPMENT

4.1 Introduction

The process of the hardware development can be divided into three parts. The

value of each component was first determined by the calculation. Then, based on the

schematic diagram, each component was placed and connected on the bread board.

Lastly, all the components on breadboard were transferred to the printed circuit board

(PCB). This project consists of three stages of circuit, which are buck converter power

stage circuit, MOSFET drive circuit, and PWM controller stage circuit.

4.2 Power Converter Stage

There are six main components on the power stage, which are power switch,

power diode, power inductor, input filter capacitor, output capacitor and load.


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4.2.1 Power Switch

The power switch of buck converter can be implemented by MOSFET. In this

project, power MOSFET was selected as a power switch because it has higher

breakdown voltage compared to the other power switch. Besides, it can be used in

higher frequency applications where the on-state-resistance has no theoretical limit,

hence switching power loses can be far lower.

Since the P-channel of MOSFET required a complex driver circuit, N-channel

MOSFET is chosen for switching purposes due to low switching losses and simple gate

drive circuit. As a result, MOSFET IRF540N from International Rectifier is chosen. The

value ofRds(on) is small which only 44 . An absolute maximum rating for gate-to-

source voltage Vgs

equal to 20 V. Although the power dissipated by MOSFET is low, a

heat sink is mounted on the MOSFET for heat dissipation and a safety reason.

4.2.2 Power Diode

The important criteria for selecting a power diode are fast switching, high current

rating, reverse voltage and low voltage drop. The current rating must be higher than 0.5

A and the reverse voltage must greater than 25 V for maximum input voltage. Therefore,

MUR1520 diode from International Rectifier was chosen. This power diode is designed

for switching power supplies with ultrafast recovery time up to 35 nanoseconds.

Besides, the peak forward current is 15 A and the peak repetitive reverse voltage, VRRM

is 200 V. The forward voltage drop at 15 A forward current is low which only 1.05 V. A

heat sink was mounted to help the heat dissipation.


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4.2.3 Inductor

In this project, the minimum inductance needed has been calculated in the

chapter three which is 22.5 H. Therefore, a Bourns type 68 H inductor 2120 series

from RoHS Compliant was selected. The current rating is 6.7 A, preventing the

converter from operating at maximum power rating for long duration. This power

inductor operates at very low DC resistance, which is 0.22 .

4.2.4 Input and Output Capacitor

Input filter is necessary to attenuate the switching harmonics of the input current

and protect the converter from the input voltage, thereby increasing the system

reliability. A 10 F miniature aluminum electrolytic capacitor Rubycon was selected as

the input filter. The rating voltage is 25 V, which is greater than input voltage.

From the power stage design, the minimum capacitance for output capacitor is

31.99 F. The output filter capacitance Cmust be greater than Cmin. Therefore, a 47 F

miniature aluminum electrolytic capacitor Rubycon was selected as the output capacitor.

4.2.5 Load

A resistive load is needed for the testing purpose. From the power stage design,

15 resistor was chosen in order to provide maximum current 5 A. The power rating of

resistor should be at least 5 W. In this project, a 15 aluminum housed wire wound

high power resistor from Welwyn Components was chosen. The power rating for this

resistor is 25 W. This resistor has high stability with maximum ambient temperature up

to 200 C.


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4.3 Pulse Width Modulation Controller Stage

In this switch mode power supply application which is buck converter and

operating frequency in 50 kHz is needed a controlled circuit to provide a pulse width forMOSFET switching. An integrated circuit (IC) of SG3524 will be introduced here for

the controlled purposes. This type of PWM control IC is optimized for high frequency

up to 300 kHz. Figure 4.1 shows the functional block diagram of SG3524.

The frequency of sawtooth signal is determined by the resistor at pin 5,RT and

capacitor at pin 6, CT. SG3524 can be operating in a frequency range between 100 Hz to

300 kHz and the value ofRTmust be larger than 100 to ensure that charging does not

exceed 5 mA. The operating frequency was set at 50 kHz, the best combination of CT

and RT is 10 nF and 2 k according to the graph of Oscillator Frequency vs. Timing

Resistance in the datasheet. Figure 4.2 shows the graph that determined the value ofCT

andRT

.

In SG3524, pin 12 and pin 13 are the output pins or also known as totem poles

output. Every of this output is added to a resistor so that the output of PWM is wider and

can reach to 95 percent of duty ratio. This PWM need an input voltage which larger than

start threshold voltage in order to operate and can yield an output of pulse width with

amplitude of 3.5 V. Because of the amplitude is low, the MOSFET driver circuit is

added to the gate of MOSFET so that the switching is properly functioning. Figure 4.3

shows that the schematic diagram of PWM controller circuit for SG3524.


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Figure 4.1: Functional Block Diagram of SG3524 [17].

Figure 4.2: Graph of oscillator frequency vs. timing resistance of SG 3524 [17].


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Figure 4.3: Schematic diagram of PWM controller circuit for SG3524.

4.4 MOSFET Driver Circuit

In most power electronic circuits, a difference of potential exists between the

PWM controller circuit and MOSFET. Basically, PWM controller circuit is low voltage

and low-power circuit. The power stage circuit that consists of MOSFET is a high-voltage circuit. Therefore, it becomes necessary for the output channels of the gate-pulse

to be isolated. The isolation function can be provided by using an opto-isolator IC. The

isolation function of this circuit can be seen where the ground of the driver circuit was

different with the ground of PWM controller and power stage. Figure 4.4 shows the

schematic diagram of MOSFET driver circuit.

For safety reason, an opto-isolator was used in the circuit. The opto-isolator

(6N137) allows for DC coupling and generally provides significant protection from

serious over voltage conditions of the driver IC. The driver used in this circuit is

MC34151. The MC34151 is dual inverting high speed drivers specifically designed for

applications that require low current digital circuitry to drive large capacitive loads with


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high slew rates. This device is intended for switching power supplies and dc-dc

converters application due to the high efficiency at high frequency operation. The two

independent high current totem pole outputs ideally suited for driving power MOSFET.

The other advantages of this device are low standby current and enhanced system

performance with common switching regulator control ICs.

Figure 4.4: Schematic diagram of MOSFET driver circuit.

4.5 Printed Circuit Board Layout

In this project, the circuit of power stage, controlled stage and MOSFET driver

will be built on the printed circuit board (PCB) layout. There is no mandatory rule

indicate that the circuits must be built on PCB but this method is very important for


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reducing electromagnetic interference (EMI) effect in the power stage. Besides, by

designing a PCB, it can solve the untidy of connection on the breadboard and can have

higher accuracy of measurements. Figure 5.5 shows the PCB layout of this project.

(a)

(b)


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(c)

Figure 4.5: PCB layout (a) Power Stage Buck Converter (b) PWM Controller Circuit

(c) MOSFET Driver Circuit.

4.6 Summary

This chapter presents the hardware development of power converter stage, PWM

controller stage, MOSFET driver circuit and development of Printed Circuit Board

(PCB) layout. Power converter stage included power switch, power diode, inductor,

capacitor and load. PWM controller stage using IC SG3524 is introduced for the PWM

controlled purposed. Besides, MOSFET driver circuit is presented for isolation and

amplification function of PWM signal. Lastly, further development of the PCB layout is

presented.


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CHAPTER 5

RESULTS AND DISCUSSIONS

5.1 Introduction

In this project, there are two types of result that will be discussed in this chapter

which are simulation results using Matlab/Simulink and experimental results.

5.2 Simulation Results

5.2.1 Photovoltaic Cell Model

The Equations (2-1) from chapter 2 have been implemented in Matlab/Simulink.

The results of I-V and P-V curve characteristics have been produced as shown in Figure

5.1 and Figure 5.2. Based on the Figure 5.1 and Figure 5.2, the short circuit current, the

open circuit voltage, and the maximum power are in very good agreement with the


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MSX-60 datasheet values. These PV simulation results are based on the Matlab PV cell

model block from Figure 3.3.

Figure 5.1: Matlab Module I-V Characteristics Curve.


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Figure 5.2: Matlab Module PV Curve.

5.2.2 Buck Converter

There are two parts of the buck converter simulation results, which are PWM

controller result and power stage result. The results were produced from PWM controller

stage included sawtooth output waveform and PWM output waveform as shown in

Figure 5.3.

The results were produced from the power stage analysis included input voltage,

inductor current and output voltage as shown in Figure 5.4. The simulation results

obtained are based on analysis and calculation in chapter 3. From the calculation, the

desired output voltage is 14.5 V and the ripple peak-to-peak voltage is 0.05 V. Based on


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the simulation result in Figure 5.4 (c) and Figure 5.4 (d), the mean value of output

voltage is 14.5 V and the ripple peak-to-peak voltage is 0.03 V. The difference of ripple

peak-to-peak voltage from the calculation is 0.02 V and the output voltage response

from simulation is slightly different compared to the calculation.

(a)


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(b)

Figure 5.3: (a) Sawtooth Waveform. (b) PWM Waveform.

From the analysis and calculation of inductor current, mean value (ILmean) is

equal to 0.967 A. The maximum (ILmax) and (ILmin) minimum inductor are equal to

1.2865 A and 0.6468 A respectively. From the simulation in Figure 5.4 (b), the mean

value for inductor current is 0.961 A, the maximum inductor current (ILmax) is equal to

1.235 A and the minimum inductor current (ILmin

) is equal to 0.688 A. The response of

the inductor current from the simulation is almost same with the applied analysis.


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(a)

(b)


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(c)

(d)

Figure 5.4: (a) Input Voltage (b) Inductor Current

(c) Output Voltage (d) Output Voltage Ripple.


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5.3 Experimental Results

The hardware experimental was conducted at power electronic laboratory,

Universiti Teknologi Malaysia. During the experiment, there are many types of

equipment used such as digital power supply, oscilloscope, function generator and so on.

Figure 5.5 shows the sawtooth waveform and PWM waveform produced using IC

SG3524. Internal oscillator was produced by connecting the timing resistor and

capacitor resistor (R t& Ct

) to the IC SG3524.

Figure 5.5: Sawtooth and PWM Waveform.


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From the analysis and calculation, the chosen switching frequency is 50 kHz but

from the Figure 5.5, the switching frequency is 61.27 kHz. This is due to the error from

the manufacturer datasheet. Based on Figure 4.2, the oscillator frequency vs. timing

resistance graph from the datasheet given is not relatively accurate value forR t and Ct.

Another factor is the timing capacitor (Ct

). This capacitor has the internal equivalent

series resistance (ESR) that produced electrolyte loss during the experiment. The duty

cycle from Figure 5.5 is 84.44 % is almost the same with the calculated value 85 %.

Figure 5.6 shows the gate voltage output produced by the MOSFET gate drive.

The amplitude of the gate voltage is equal to 16.2 V which is approximately 1 V higher

from the PWM voltage output. The gate voltage is increased by connecting the PWM

circuit to the MOSFET driver circuit. The on state of MOSFET is achieved when the

gate voltage sufficiently exceeds the threshold voltage, Vgs

and forcing the MOSFET

into the ohmic region of operation. Typically, the MOSFET gate voltage is for the on

state is use in the range of ten and twenty. Therefore, the gate voltage 16.2 V is

sufficient enough to force the MOSFET to the ohmic region. The off state is achieved by

lowering the gate voltage below the threshold voltage.

Figure 5.7 shows the output of inductor current (IL). From the figure, the mean

233_MOHDHAFIZIZZWANBINSAAD2009

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