Hybridisation of energy storage for
multiple input DC-DC converters
IN Jiya
orcid.org/0000-0003-2344-3923
Dissertation submitted in fulfilment of the requirements for
the degree
Master of Engineering
in Electrical and
Electronic Engineering
at the North-West University
Supervisor:
Prof R Gouws
Graduation: May 2019
Student number: 29917476
i
One of the biggest challenges towards achieving efficient and effective energy use in electric vehicles is the energy storage systems. Batteries, though being improved with newer technologies, are still not capable of meeting the load requirement while retaining their lifespan. Hybrid energy storage systems have been proposed in literature to solve this problem. It has been proposed previously to combine high power dense energy storage alternatives with batteries which are high energy dense. Supercapacitors (SCs) and hybrid capacitors (HCs) are quite similar energy storage devices as they are both double layer capacitors. However, SCs have a higher power density and a lower energy density than HCs. In battery and SC hybrid system, there has been a reported downside due to the poor energy density of the SCs while in battery and hybrid capacitor system, the low power density of the HCs have been reported to pose a challenge.
The research presented in this work sought to address these shortcomings of both battery-supercapacitor and battery-hybrid capacitor energy storage systems by proposing a hybrid energy storage system that combines both supercapacitors and hybrid capacitors with a battery through a multiple input DC-DC converter. The proposal was verified in simulation and validated by implementing a laboratory prototype. A new hybridisation topology which reduces the amount of resource requirement when compared to the conventional hybridisation topology is introduced. An electric vehicle current profile from previous research was used to test the performance of the proposed topology. A new method of pulse width modulated switching of the gates of the multiple input DC-DC converter power switches using field-programmable gate array technology was also introduced and verified experimentally, this facilitated the switching of the multiple input DC-DC converter in a less complex way when compared to the conventional topologies.
From the results obtained, the hybridisation topology proposed in this research had the lowest cost per unit power at 14.81 $/kW, the lowest cost per unit power to energy ratio at 1:1.3 and also the lowest available power to energy ratio at 1:1.3 thus making it a more attractive hybridisation topology than the two conventional alternatives. The multiple input converter built had efficiency values in excess of 80%. With these results, the objectives of the research were met. The application of the proposed hybrid energy storage system is not limited to only electric vehicles, but is applicable in other renewable energy systems such as photo-voltaic systems, wind turbines and also in applications like electric ships, micro-grids and even electric aircrafts like the more-electric aircraft.
ii
I give profound gratitude to God almighty for the grace, insight and wisdom he granted unto me during the course of this research and even before now. It would have been impossible to make the foot note of life let alone come this far in life.
My gratitude to my supervisor, Prof. Rupert Gouws cannot be over emphasized, you gave me a chance to prove myself and believed in me when it was very difficult, your support academically, morally and financially to mention just a few has contributed immensely to helping me successfully complete this research. I am also very grateful to Dr. Nicoloy Gurusinghe, for taking out time to follow up and contribute to the quality of this research. To my Dad and Mum, Prof. & Mrs. Elisha Z. Jiya, for their love, support and guidiance, I am really grateful. I also appreciate my siblings, Jemimah and Nasara for their support. To my friends, colleagues and mentors, Dave, Ofuafo, Jane, Hannah, Peter, Simon, Wian, Gert, Jeff-Niel and Ellenor, Prue, Pastors Celeste and Willem Nel, Pastor Barend, Dr. A. U. Usman, Prof. Jacob Tsado, Dr. and Dr. (Mrs.) Ogunrombi, Dr. Greg, Dr. Anslem, Dr. Patricia, Nonso, Blessing and Micheal to list just a few, your support is well appreciated.
To the entire faculty and staff of the School of Electrical, Electronic and Computer Engineering, I am grateful for all the support in various ways in which they have contributed to the success of my research. I am also very grateful to the National Research Foundation (NRF) of South Africa for providing me with a bursary to carry out this research and to Eskom for funding other components of the research.
This material is based on research/work supported wholly / in part by the National Research Foundation (NRF) of South Africa (Grant Number: 112236). The research findings are that of the author and not that of the NRF.
iii EXECUTIVE SUMMARY ... i ACKNOWLEDGEMENT ... ii LIST OF FIGURES ... vi LIST OF TABLES ... x NOMENCLATURE ... xi CHAPTER 1: INTRODUCTION ... 1 1.1 Background ... 1 1.2 Problem Statement ... 4
1.3 Objectives of the Research ... 4
1.3.1 Primary Objective ... 4
1.3.2 Secondary Objective ... 5
1.4 Scope of the Study ... 5
1.4.1 Energy Storage ... 5
1.4.2 DC-DC Converters... 6
1.4.3 Intelligent Control System ... 7
1.5 Research Methodology ... 7
1.5.1 Literature Review ... 7
1.5.2 Design and Simulations ... 8
1.5.3 Experimental Tests and Results ... 8
1.5.4 Key Research Questions ... 8
1.5.5 Verification and Validation ... 9
1.6 Publications and Peer Reviews ... 9
1.7 Dissertation Overview ... 12
1.8 Conclusion ... 12
CHAPTER 2: LITERATURE REVIEW ... 14
2.1 Energy Storage ... 16
2.1.1 Double Layer Capacitors ... 16
2.1.2 Batteries ... 18
2.1.3 Fuel Cells ... 21
2.1.4 Flywheel ... 25
2.1.5 Comparison of Energy Storages ... 26
iv
2.2.1 Conventional Capacitor Modelling ... 26
2.2.2 Classical Equivalent Circuit Model ... 30
2.2.3 Ladder Circuit Model ... 32
2.2.4 Transmission Line Model ... 33
2.2.5 Zubieta Model ... 33
2.2.6 Two-branch Model ... 35
2.2.7 Other Unique Models ... 36
2.3 Energy Storage Hybridisation Case Studies ... 37
2.3.1 Battery and SC ... 38
2.3.2 Battery and HC ... 39
2.3.3 Other Hybridisation Topologies ... 39
2.4 DC-DC Converters ... 40
2.4.1 Overview of Basic DC-DC Converter Topologies ... 40
2.4.2 Multiple Input DC-DC Converters... 47
2.4.3 DC-DC Converter Control ... 52
2.5 Advanced Control Topologies ... 53
2.5.1 Fuzzy Logic ... 54
2.5.2 Neural Networks ... 55
2.5.3 Proportional Integral Derivative (PID) Controller ... 56
2.6 Switching Electronics ... 57
2.6.1 Bipolar Junction Transistors (BJTs) ... 57
2.6.2 Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) ... 61
2.6.3 Insulated Gate Bipolar Transistor (IGBT) ... 66
2.7 Control Boards ... 69 2.7.1 Arduino ... 69 2.7.2 Neucleo ... 70 2.7.3 Beagle Board ... 71 2.7.4 Raspberry Pi ... 72 2.8 Software ... 73 2.8.1 Matlab® / Simulink® ... 74 2.8.2 LTspice ... 75 2.9 Conclusion ... 76
CHAPTER 3: DESIGN AND SIMULATIONS ... 77
3.1 Concept Design ... 77
v
3.3 Detailed Design and Simulations ... 79
3.3.1 Parameter Identification ... 79
3.3.2 Energy Storage Hybridisation Design ... 81
3.3.3 Multiple Input Converter Design ... 86
3.3.4 Inductor and Capacitor Calculations ... 96
3.3.5 System Control ... 98
3.4 Verification and Validation ... 109
3.5 Conclusion ... 111
CHAPTER 4: IMPLEMENTATION AND RESULTS ... 112
4.1 System Implementation ... 112
4.1.1 Overview of the System ... 112
4.1.2 Multiple Input Converter ... 113
4.1.3 System Control ... 118
4.1.4 Energy Storage Implementation ... 123
4.2 Experimental Results ... 128
4.2.1 Energy Storage ... 129
4.2.2 Multiple Input DC-DC Converter ... 135
4.3 Verification and Validation ... 151
4.4 Conclusion ... 153
CHAPTER 5: CONCLUSION AND RECOMMENDATION ... 155
5.1 Key Research Questions ... 155
5.2 Verification and Validation ... 156
5.3 Summary ... 157
5.4 Future Work and Recommendations ... 160
5.5 Conclusion ... 160
LIST OF REFERENCES ... 161
vi
Figure 1-1 The worldwide harmonized light vehicles test cycle ……….…….2
Figure 1-2 Ragone plot showing various energy storage devices ……….3
Figure 2-1 Overview of the literature review plan ……….….14
Figure 2-2 Case studies from the main topics with major references ………….……15
Figure 2-3 Rudimentary structure of DLCs ……….…16
Figure 2-4 Basic structure of DLCs showing the main components ……….…..17
Figure 2-5 Structure and discharging of Lithium Ion battery ……….…19
Figure 2-6 Schematic of fuel cells ……….22
Figure 2-7 Schematic of transmission line models of DLCs ……….…….27
Figure 2-8 Schematic of the distributed capacitor model ……….……..28
Figure 2-9 Electrical model of electrolytic capacitors ……….29
Figure 2-10 Classical equivalent circuit of a DLC ………….………..30
Figure 2-11 First order circuit model of DLCs ………...31
Figure 2-12 Variations of the classical equivalent circuit model of DLCs …………....31
Figure 2-13 Ladder circuit model of a DLC ………...32
Figure 2-14 Transmission line model of double layer capacitors ……….………..33
Figure 2-15 The Zubieta model of DLCs ……….…………34
Figure 2-16 The two-branch model of DLCs ………...36
Figure 2-17 Schematic of a basic Buck converter ……….……..41
Figure 2-18 Schematic of the dynamic states of a basic Buck converter ……….……41
Figure 2-19 Steady state waveforms for the simple buck converter …………...42
Figure 2-20 Circuit diagram of the basic Boost converter ………...….43
Figure 2-21 Schematic of the dynamic states of the basic boost converter ………....44
Figure 2-22 Steady state waveform for the basic Boost converter ………..…44
Figure 2-23 Schematic of the basic Buck-boost converter ……….…..45
Figure 2-24 Schematic of the dynamic states of the Buck-boost converter …….…...46
Figure 2-25 Steady state waveform of a buck-boost converter ………...46
Figure 2-26 Schematic of four major types of isolated DC-DC converters …………..48
vii
Figure 2-28 Multiple input buck-boost converter ……….………50
Figure 2-29 Non-isolated multiple input converter ……….……….51
Figure 2-30 Multiple input non-isolated bidirectional DC-DC converter ………..……..51
Figure 2-31 Non-isolated multiple input boost converter ……….………….52
Figure 2-32 Voltage and current mode control of DC converters ……….…..53
Figure 2-33 Fuzzy logic controller employed in a DC-DC converter ………...54
Figure 2-34 Online Neural Network Controller for a DC-DC converter ……….….55
Figure 2-35 PID controller regulating a DC-DC boost converter ……….…57
Figure 2-36 Biased junction PNP transistor .………....59
Figure 2-37 PNP and NPN transistors in common base connection ……….60
Figure 2-38 Diode concept and the symbol of an NPN transistor ………..60
Figure 2-39 V-I characteristics of the bipolar junction transistor ……….…..61
Figure 2-40 Enhancement mode MOSFET and Depletion mode-MOSFET ……….…..63
Figure 2-41 Pictorial schematic of LDMOSFET, VMOSFET, and TMOSFET ……….…64
Figure 2-42 E-MOSFET V-I characteristics ………65
Figure 2-43 D-MOSFET V-I characteristics ……….…...65
Figure 2-44 IGBT circuit structure ……….…..67
Figure 2-45 IGBT parasitic element formation ………...67
Figure 2-46 IGBT structure ………..……….….68
Figure 2-47 Symmetric and Asymmetric IGBT structure ..……….…...68
Figure 2-48 V-I characteristics of IGBT ………...69
Figure 3-1 Chapter three layout ……….……….77
Figure 3-2 Conceptual design of hybrid energy storage system ……….…….78
Figure 3-3 Hybridisation of the energy storage devices by direct hard wiring ….…81 Figure 3-4 Results of directly hardwiring the energy storages ……….…82
Figure 3-5 Schematic of hybridizing through DC-DC converters ………..…………...83
Figure 3-6 Schematic of the proposed energy storage hybridisation ……….…84
Figure 3-7 Response of SC and HC when connected directly in parallel ……….…..85
Figure 3-8 Schematic of the multiple input DC-DC converter …..………..….87
Figure 3-9 Steady state waveform of the converter switches in mode A ………..…..88
viii
Figure 3-11 Switching signals of the switches during operation in mode A …….…..89
Figure 3-12 Steady state waveform of the converter switches in mode B …..…….…89
Figure 3-13 Simulation of the converter operation in mode B ……….…90
Figure 3-14 Switching signals of switches during operation in mode B ..……….…...90
Figure 3-15 Steady state waveform of the converter switches in mode C ……….…..91
Figure 3-16 Simulation of the converter operation in mode C ………...……….….91
Figure 3-17 Switching signals of switches during operation in mode C ……..………92
Figure 3-18 Steady state waveform of the converter in modes D, E and F …………...93
Figure 3-19 Multiple input DC converter for the proposed hybridisation ………….…94
Figure 3-20 Steady state waveform of the converter in modes A to D ……….….95
Figure 3-21 Flow chart of the control for the conventional hybridisation ………….100
Figure 3-22 Flow chart of the control for the proposed hybridisation ………..101
Figure 3-23 Block diagram of the control schematic ……..………..102
Figure 3-24 ON-state of the PWM signal of the three-input converter ……….….105
Figure 3-25 OFF-state of the PWM signal for the three-input converter ...106
Figure 3-26 ON-state of the PWM signal for the two input converter …...107
Figure 3-27 OFF-state of the PWM signal for the two input converter ………108
Figure 3-28 Verification of the energy storage devices ..……….…110
Figure 4-1 Layout of chapter 4 ……….112
Figure 4-2 Overview of the concept design ………...113
Figure 4-3 The implementation circuit ……….……..114
Figure 4-4 Flow of control ………...118
Figure 4-5 Logic circuit of the gate switching signals ……….……….120
Figure 4-6 Flowchart of the delay implementation on the FPGA board ……….…..121
Figure 4-7 The VHDL source code of the delay toolbox ………...122
Figure 4-8 SC bank used ……….………..123
Figure 4-9 HC bank used ………..……….123
Figure 4-10 Comparison of the physical sizes of the SC and HC banks ……….……125
Figure 4-11 Picture of the lithium polymer (LiPo) battery used .……….…..125
Figure 4-12 Normalised characteristics of the energy storage regimes ………….…128
ix
Figure 4-14 The validation of the HC bank showing ……….……131
Figure 4-15 The validation of the SC bank ……...……….…..132
Figure 4-16 Transient response of the SC and HC banks …..……….……133
Figure 4-17 Performance of the SC and HC with an EV load profile ...………….……135
Figure 4-18 Laboratory implementation and test setup .……….….136
Figure 4-19 Experimental implementation of the multiple input converter …………137
Figure 4-20 Output of the FPGA board ………...……..138
Figure 4-21 Testing the input and output of the MOSFET driver ……….….…139
Figure 4-22 Overview of the high side and low side switching signal ………….……140
Figure 4-23 Falling edge of the high side switch before delay ………...140
Figure 4-24 Rising edge of the high side switch before the delay ………...141
Figure 4-25 Falling edge of the high side switch after the delay ……….…141
Figure 4-26 Rising edge of the high side switch after the delay ……….…..141
Figure 4-27 Output of the MOSFET drivers ……….143
Figure 4-28 Scope results of the multiple input converter operation in Mode A …..145
Figure 4-29 Scope results of the multiple input converter operation in Mode B …..145
Figure 4-30 Scope results of the multiple input converter operation in Mode C …..146
Figure 4-31 Efficiency results of the converter under varying duty ……….…146
Figure 4-32 Efficiency results of the converter under varying load ………….……....147
Figure 4-33 Transient response of the multiple input converter ………..……....148
Figure 4-34 Transient response of the multiple input converter ……….…..148
Figure 4-35 Testing the conventional hybridisation using an EV profile ……….…...149
Figure 4-36 Testing the proposed hybridisation using an EV profile ………...150
Figure 4-37 Validation of the converter output voltage in mode A ………..….…152
Figure 4-38 Validation of the converter output voltage in mode B ………...152
Figure 4-39 Validation of the converter output voltage in mode C ………...152
x
Table 1-1 Brief comparison of SC, HC and li-ion batteries ……….………….4
Table 2-1 Comparison of batteries for electric vehicle applications …………..……21
Table 3-1 Parameter identification for the simulation studies ……….…80
Table 3-2 Switch conduction of the converter with individual inputs ..……….….…87
Table 3-3 Switch conduction of the two input converter ………..…..95
Table 3-4 Minimum Inductor requirement for operation under CCM …………..……97
Table 3-5 ON-state of the PWM switching of the three-input converter ……….…..103
Table 3-6 OFF-state of the PWM switching of the three-input converter ……..…...103
Table 3-7 ON-state of the PWM switching of the two-input converter …………..…103
Table 3-8 OFF-state of the PWM switching of the two-input converter ……….…...103
Table 4-1 List of parameters and their specifications ………..…....117
Table 4-2 Electrical characteristics of the battery, SC and HC ……….….…124
Table 4-3 Characteristics of the seven different energy storage regimes ….….…126
Table 4-4 Normalised characteristics of the energy storage regimes …….………127
Table 4-5 The input and output voltage and current of the converter ……….…....144
Table 4-6 Comparison of resource use for hybridisation topologies ……….…….150
xi
List of Abbreviations
AC Alternating Current
AFC Alkaline Fuel Cell
ANN Artificial Neural Network
BJT Bipolar Junction Transistor
CAD Computer Aided Design
CADEC Computer Aided Design Environment for Composites
CMC Current Mode Control
CPU Central Processing Unit
CSTBT Carrier Store Trench Gate Bipolar Transistor
DC Direct Current
DLC Double Layer Capacitor
DMFC Direct Methanol Fuel Cell
DMOSFET Depletion Mode MOSFET
DOE Design of Experiments
EDLC Electric Double-Layer Capacitors
EEPROM Electrically Erasable Programmable Read-Only Memory
EMOSFET Enhancement Mode MOSFET
ESL Equivalent Series Inductance
ESR Equivalent Series Resistance
EV Electric Vehicle
FBL Floating Buried Layer
FEA Finite Element Analysis
FET Field Effect Transistor
FLC Fuzzy Logic Controller
FPGA Field-Programmable Gate Array
GPIO General Purpose Input Output
HC Hybrid capacitor
HDMI High Definition Media Interface
I2C Inter-Integrated Circuits
IDE Integrated Development Environment
xii
IEGT Injection-Enhanced Gate Transistor
IGBT Insulated-Gate Bipolar Transistor
IGFET Insulated Gate Field-Effect Transistor
IGR Insulated Gate Rectifier
IOT Internet of Things
JTAG Joint Test Action Group
LCD Liquid-Crystal Display
LDMOSFET Laterally Diffused MOSFET
MCFC Molten Carbonate Fuel Cell
MOS Metal-Oxide-Semiconductor
MOSFET Metal-Oxide-Semiconductor Field-Effect Transistor
NMC Lithium Nickel Manganese Cobalt Oxide
NNC Neural Network Controller
NPT Non-Punch Through
ONNC Online Neural Network Controller
OTG On-The-Go
PAFC Phosphoric Acid Fuel Cell
PCB Printed Circuit Board
PEFC Polymer Electrolyte Fuel Cell
PEMFC Proton-Exchange Membrane Fuel Cell
PID Proportional–Integral–Derivative
PSO Particle Swarm Optimization
PT Punch Through
PV Photovoltaic
PWM Pulse-Width Modulation
RC Resistor Capacitor
RCD Resistor Capacitor Diode
RISC Reduced Instruction Set Computing
RLC Resistor-Inductor-Capacitor
SC Supercapacitor
SD card Secure Digital Card
SOA Safe Operating Area
SOFC Solid Oxide Fuel Cell
xiii
TMOSFET T-groove MOSFET
UART Universal Asynchronous Receiver-Transmitter
ULSI Ultra Large Scale Integration
USB Universal Serial Bus
VHDL VHSIC Hardware Description Language
VHSIC Very High Speed Integrated Circuit
VMC Voltage Mode Control
VMOSFET V-groove MOSFET
WBG Wide Band Gap
WLTC Worldwide Harmonized Light Vehicles Test Cycle
List of Units
$ United States Dollar (USD)
$/kW USD per Kilowatt
$/Wh USD per watt-hour
A Ampere Ah Ampere-hours F Farads GHz Gigahertz Kg Kilogram kHz Kilohertz kW Kilowatt
kW/kg Kilowatt per Kilogram
kWh Kilowatt-hour
kWh/m3 Kilowatt-hour per meter cube
mAh Milliamps-hour
mF Millifarad
MHz Megahertz
MW/m3 Megawatt per Meter Cube
s Second
V Volts
W Watt
Wh/kg Watt-hour per Kilogram
xiv
List of Symbols
µ Micro C Capacitance D Duty Ratio F Frequency I Current k Kilo L Inductance n Nano p Pico Ts Switching time V Voltage Ω Ohm1
This chapter is the backdrop of the whole dissertation. It addresses the vital background information on the need for hybridisation of energy storages and their general applications were highlighted. A section detailing the research problem statement is included, along with the methodology with which the problems stated were addressed. The objectives and research scope are defined; the key research questions are also stated in this chapter. A conclusion is drawn at the end of the chapter, highlighting the key issues addressed in the chapter.
1.1
Background
The effects and threats of global warming to the world due to the use of fossil fuels has driven the increased development of high efficient and environmentally friendly alternative source of energy for use. This has fuelled the researches into renewable energy sources for both domestic and industrial use [1]. The automotive industry is not left out in the mandate for use of green energy as calls for reduced carbon emissions have resulted in the growing number of electric vehicle manufacturers [2]–[4]. Reports, however, indicate that in most of the electric vehicles in use today, the ratio of power to energy (P:E) in the energy storage of these vehicles is approximately 8:1 [5]. This indicates that the storage systems, which is mostly battery cells, are optimized for power dense operations rather than the energy dense operations performed by the vehicle (that is the anticipated peak power rather than for efficient energy use) [6]. This wide ratio is because electric vehicles require high power during acceleration, hence the optimization for power dense operations. To augment for this high power density requirement in electric vehicles, a hybridisation of energy storages is required, since batteries are energy dense.
Repeated acceleration and deceleration of electric vehicles leads to current surges in the batteries resulting from the high crest factors of the load profile, this results in an increase in the internal resistance of the battery [7]. This increased internal resistance will, in turn, reduce the efficiency of the battery and also results in premature failure of the battery [8]. An example of this phenomenon is in figure 1-1 (a) and (b) [9], the worldwide harmonized light vehicles test cycle and its corresponding effect on a battery storage system is presented. The test cycle is divided into two sequences, each sequence lasting for a period of 15 minutes, the first sequence is a period of urban driving while the second is a period of extra-urban or highway driving, from figure 1-1 (b) the corresponding effects of the just described driving cycle on an energy storage system is seen. The effect of the repeated acceleration and deceleration is very detrimental to the efficiency and lifespan of a battery only energy storage system.
2
Energy storage seems to be the biggest challenge in the advancement towards renewable or green energy solutions. Hybridisation of energy storage has been the theme of many researches in this field [10]–[15]; therefore, it is an effective economic solution towards improving the performance of renewable energy systems. The application of hybridised energy storage systems cannot be over emphasized. It finds relevance in a wide area of applications ranging from DC micro grids to electric vehicles of any kind [16]. Generally any electric system with a load curve having high ramp rates (as is seen in figure1-1 (b)); the high crest factors in the load curve of these systems calls for a hybridised storage since it would not be economical to design batteries solely for the peak power application.
Battery
Discharge
(b) S p e e d ( Km /h ) Time (Secs) Time (Secs) C u rr e n t (A ) (a) 140 120 100 80 60 40 20 0 0 200 400 600 800 1000 1200 1400 1600 1800 0 200 400 600 800 1000 1200 1400 1600 1800 30 20 10 0 -10 -20 -30 -40 40 50 Battery Charge Battery Discharge WLTC Urban WLTC ExtraurbanFigure 1-1: The worldwide harmonized light vehicles test cycle (WLTC) (a) Vehicle speed, (b) current drawn from and delivered to the battery by the vehicle / EV motor; for urban
3
The performance characteristics of energy storage devices in automotive power applications can be described using two parameters namely power density (or specific power) and energy density (or specific energy) [17]. The power density is related to the vehicles speed and acceleration; it more or less determines how fast the vehicle can go. The energy density on the other hand relates to the range of the vehicle, i.e. how far the vehicle can go. The relationship of this two performance characteristics when plotted on a graph is called the ragone plot. It can be seen from the ragone plot presented in figure 1-2 [18] that, the batteries especially the Li-ion based batteries lie on the very high energy density axis making them very suitable for high energy density operations in the driving cycle of a vehicle. The hybrid capacitors (Li-ion capacitors) lie on the high energy dense and slightly high power dense axis, comparing them to the supercapacitors (SCs), the SCs have much better power densities making the SC most suitable for high power dense operations in the drive cycle.
HCs are also called lithium ion capacitors (LIC) because they are made by combining the intercalation mechanism of the lithium ion battery with the cathode of the electric double layer capacitor (EDLC), the EDLC is also known as the SC [19]. The HC just came into the market within the few years; it has a much higher energy density than the SC but a lower power density than the SC. They tend to lie in the mid-point between SCs and batteries [4]. As can be seen in table 1-1 [20], this is because they have higher energy densities than SCs, though not as much as batteries, but higher power densities than batteries and lower power densities than SCs [4], [21]. The SCs and HCs especially, pose great advantages in energy recovery or harvesting through regenerative braking because of their ability to accept and dissipate large currents during charging and discharging [4]. Batteries have fallen short in comparison to both
E n e rg y d e n s it y ( Wh /kg ) Power density (W/kg) 100 101 102 103 104 105 106 100 101 102 103 104 10-1 10-2
4
SCs and HCs in the aspect of high power dissipation. Batteries also have a limited charge and recharge cycle; this gives the SCs and the HCs a good advantage in their application for electric vehicles especially since regenerative braking occurs quite frequently in the driving cycle.
Table 1-1: Brief comparison of SC, HC and li-ion batteries (adapted from [20]).
Item SCs HCs Li-ion batteries
Energy Density (Wh/kg) 3 – 6 7 – 14 20 – 100
Power Density (W/kg) 2,000 – 6,000 2,000 – 3,000 100 – 300
1.2
Problem Statement
Much work has been done in literature as regards the hybridisation of batteries and SCs. Although SCs have high power density, they still pose a disadvantage as they limit average autonomy between two recharge points; this is due to their poor energy densities [4], [22]. Less work has been done to combine batteries and hybrid capacitors (HCs) as compared to SCs [23], this most likely is as a result of the high cost of HC devices. All of the work done in the aspect of hybridisation of energy storages as regards to SCs and HCs has been to explore either of the two devices which have varying characteristics on two different extremes, (the SC having a much higher power density than the HC while the HC having a much higher energy density than the SC. This leaves a gap in the mid-point between the high power density of SCs and the high energy density of the HC [24]. This research proposes to fill the gap by hybridising SCs, HCs and batteries to form one energy storage system.
1.3
Objectives of the Research
The objectives of this study were in two categories, the primary objectives and the secondary objectives. The primary objective is the main aim of this study while the secondary objectives were the objectives achieved, in order to achieve the primary objective.
1.3.1 Primary Objective
The primary objective or aim of this research project was to improve the energy management and utilization in electric vehicles by hybridisation of energy storages specifically batteries, SCs and HCs through a multiple-input DC-DC converter, this is in order to close the gap between the high power but low energy densities of SCs and lower power, but higher energy densities of HCs in combination with much higher energy density batteries.
5 1.3.2 Secondary Objective
The aforementioned primary objective or aim was achieved by achieving the following secondary objectives:
1. A combination of batteries, HCs and SCs was used to store up the energy. Hybridisation techniques were examined to determine the best fitting configuration for use for different operational requirements. A number of suitable SCs, HCs and batteries were evaluated using their: sizes, Temperature range, cell voltage, charge / discharge cycles, capacitance, specific energy (Wh/kg), specific power (kW/kg), weight and efficiency to choose the best fitting storage devices to be used.
2. Since the application of DC – DC converters is very pivotal to this research project, several DC – DC converters were researched and investigated to see which of them was most suitable in this application. Numerous DC – DC converters have been developed and are still being developed in research [25], [26]. However, the criteria used to filter the converter to that was used in this research project was their configuration for multiple input, non-isolation and ability for bidirectional operation. A number of converter topologies that fit into this criterion were simulated and the most efficient for the application was implemented.
3. An intelligent controller was developed to provide the PWM signal required by the DC – DC converters that were implemented. This intelligent controller measures vital energy parameters and makes intelligent decisions about which is the most optimal energy source for a particular operation, based on a set of pre-programmed instructions.
1.4
Scope of the Study
This research focussed on improving the energy utilization in electric vehicles by hybridisation of energy storages specifically batteries, SCs and HCs through a multiple-input DC-DC converter. The following subsections briefly highlight the key areas which defined the scope of the study.
1.4.1 Energy Storage
Considering the performance characteristics of SCs, batteries and HCs, the three energy storage devices used in this research is proposed in order to adequately deliver power to the electric vehicle. The battery would be used to cater for the high energy dense operations, the SC and HC to cater for high power dense operations while also providing enough storage space to capture the energy generated during regenerative braking. One interesting thing to note about the ragone plot in figure 1-2 is that fuel cells lie on the high energy density axis
6
even higher than the lithium ion batteries, however this is still a growing technology and is not yet fully mature and so it cannot be used at the time of this research [27]–[29].
SCs were implemented to provide the energy burst required for certain operations like at start and during accelerations or other high power operations since they have the ability to deliver high power over a very short period (that is, they have a high, power density). The available capacitors on the market were evaluated and selection was be based on cost and the best configuration to give optimum power at an adequate voltage level since SCs come in a low voltage level of about 2.7 V.
Hybridisation techniques were examined to determine the best fitting configuration for use for different operational requirements. A number of suitable SCs, HCs and batteries were evaluated using their: sizes, temperature range, cell voltage, charge / discharge cycles, capacitance, specific energy (Wh/kg), specific power (kW/kg), weight and efficiency to choose the best fitting storage devices to be used. Lithium ion capacitors were hybridised with the battery and SCs in this research project to achieve more optimal energy use since they are more energy dense than SCs.
1.4.2 DC-DC Converters
In this research project, bidirectional DC–DC converters were used to charge and discharge the battery, SCs and the HCs. This made the accurate design of highly efficient and fast DC– DC converters very crucial to this research project. The basic buck, boost and buck-boost converters were investigated by means of literature study alongside some unique multiple-input DC-DC converter topologies.
In this research project there was need for multiple-input bidirectional, step up and step down (Buck/Boost) converters, only a few of the existing converters satisfy this criteria. The most suitable converter for this application was selected from the various existing topologies in literature based on the following criteria: they had to be bidirectional, multiple input and non-isolated so that there would be no need to use transformers in the circuit, which would count as weight penalties. According to [30] there are three main topologies of connecting or combining DC–DC converters with multiple inputs, they are magnetically coupling the converters, cascading or connecting them in parallel and connecting the sources in such a way that they share the same inductor and load filter capacitor.
Since the application of DC–DC converters is very pivotal to this research project, several DC– DC converters were researched and investigated to see which of them was most suitable in this application. Very numerous DC–DC converters have been developed and are still being developed in research [25], [26]. However, the criterion used to filter the converter to that was
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used in this research project was their configuration for multiple input, non-isolation and ability for bidirectional operation. A number of converter topologies that fit into this criterion were investigated and the most efficient for the application was implemented.
1.4.3 Intelligent Control System
This research investigated a number of control systems by means of literature study, which could be implemented in the smart controller for the control of the DC–DC converters to achieve an optimal use of energy at all prevailing conditions. Intelligence techniques like Fuzzy logic and Artificial Neural Networks were researched for the purpose of literature study. A number of single board controllers were also researched and the best fitting was implemented to control the DC–DC converters as to how energy is used and stored in the respective energy storage devices during the operation of the load, this single board controller was also used to provide the PWM signal required by the DC–DC converter that was implemented. This single board controller measures vital energy parameters and makes intelligent decisions about which is the most optimal energy source for a particular operation, based on a set of pre-programmed instructions. Further detail about the intelligent control is discussed in the literature review (chapter 2) of this dissertation.
1.5
Research Methodology
The methodology of this research project can be summarised as follows; the literature survey, the laboratory experiment, the software simulation, and then the actual construction of the power supply system with an energy management intelligent control. Laboratory experiments would be carried out to test each component to be used to be sure they are of the required rating. Software simulations were implemented to develop a suitable circuit diagram for the system and to develop the intelligent control software for the system.
This study was divided into several phases to easily achieve the desired outcome. The phases are as follows:
Candidate technology study and literature review Detailed design and simulations
Experimental tests and results Verification and validation 1.5.1 Literature Review
A very comprehensive literature study was done and is reported in this dissertation (chapter 2) to ensure all possible solutions to the problem statement were considered. An in-depth
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study of pre-existing research publications, journals, conference proceedings and textbooks was carried out to address certain key issues that are peculiar to this research project. A further candidate technology study was also carried out to ensure that the latest technology trends were considered and utilised during the design stage. All the components used were evaluated to ensure that the best possible alternative out of the many options was selected for use.
1.5.2 Design and Simulations
Upon completion of the literature and candidate technology study, the formed conceptual design was developed into a theoretical detailed design. During the theoretical design phase, all specifications, standards and integrations between components were determined to ensure sound theoretical design. Using the technology survey as a basis for the theoretical design, the theoretical design was translated into a fully practical detail design that was built within a simulation environment. The hybridisation techniques presented in this research was first simulated using Matlab® Simulink®. The multiple-input DC-DC converter was also simulated using the same software, when the design and simulation results were complete, congruent and satisfactory; a prototype of the converter circuit was built for experimental implementation. 1.5.3 Experimental Tests and Results
The individual components used were tested to be sure they were in good working condition and had not been damaged in transit. Upon completion of any major assembly or construction of components, they were tested to be sure they were still intact and according to specification. The multiple input DC–DC converter was further tested using realistic load profiles obtained from previous research for hybrid electric cars and a solar powered UAV to further validate the prospects of the system.
1.5.4 Key Research Questions
The following questions were investigated by means of literature study, simulation and experimentation, this served as guiding principles in the research.
1. Are there any benefits to a hybridised energy storage system that has both SCs and HCs? 2. What is the most suitable hybridisation topology of batteries, SCs and HCs for application
in electric vehicles?
3. What is the most suitable DC–DC converter for application in the hybridisation topology proposed?
9 1.5.5 Verification and Validation
Verification and validation was crucial to ensuring that the system proposed, designed and implemented was done right and correctly. Specifically, two key questions were asked at the design and simulation (Chapter 3) and the implementation and results (Chapter 4) stages, 1. “Is the right thing being built?” - Verification and 2. “Is it being built right?” – Validation. Verification is done to check that the approach taken to solve the problem in line with the required specification is right while validation is done to ensure that the right standards in that approach is adhered to.
1.6
Publications and Peer Reviews
Efforts were made to get feedback about the study from industry experts and other professionals in the field of power electronics and other related fields. Particularly, preliminary results were presented at the 2018 5th IEEE PES & IAS PowerAfrica Conference, held in Century City, Cape Town, South Africa. Also four other articles were submitted to Journals of which three have been published with the remaining one undergoing peer review; the citations of these publications are shown below, the full texts are presented in Appendix A for further reading.
I. N. Jiya, N. Gurusinghe, and R. Gouws, “Hybridisation of Battery, Supercapacitor and Hybrid Capacitor for Electric Vehicles,” in 2018 IEEE PES-IAS PowerAfrica Conference, 2018, pp. 351–356, ISBN: 978-1-5386-4163-7.
Article abstract:
This paper proposes a novel topology of hybridizing battery, supercapacitor and hybrid capacitor for optimum utilization of energy in electric vehicles. Hybridization of energy storage has been the theme of much research in the field of power electronics as it is an effective economic solution towards improving the utilization of energy. Batteries have fallen short in comparison to both supercapacitors and hybrid capacitors because of their low power density and limited charge-discharge cycle. Most of the previous research in this field focuses on hybridizing either supercapacitor or hybrid capacitor with the battery but not both. This paper deals with the combination of both supercapacitor and hybrid capacitor with the battery thus addressing the problem of the lack of autonomy between two recharge points in supercapacitors, three hybridization techniques are considered and the balance point of the supercapacitor and hybrid capacitor banks is presented. The prospects of using a multiple-input DC-DC converter is also analyzed. Experimental results are presented to validate the proposed topology. The application of the novel hybridization of the three energy storage devices can be extended to other applications having a load profile with high crest factors.
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I. N. Jiya, N. Gurusinghe, and R. Gouws, “Hybridization of Battery, Supercapacitor and Hybrid Capacitor for Load Applications With High Crest Factors: A Case Study of Electric Vehicles,” International Journal of Engineering & Technology (UAE) ISSN: 2227-524X, Submitted 14th August 2018.
Article abstract:
This paper proposes a novel topology of hybridizing battery, supercapacitor and hybrid capacitor for optimum utilization of energy in electric vehicles. Hybridization of energy storage has been the theme of much research in the field of power electronics as it is an effective economic solution towards improving the utilization of energy. Batteries have fallen short in comparison to both supercapacitors and hybrid capacitors because of their low power density and limited charge-discharge cycle. Most of the previous research in this field focuses on hybridizing either supercapacitor or hybrid capacitor with the battery but not both. This paper deals with the combination of both supercapacitor and hybrid capacitor with the battery thus addressing the problem of the lack of autonomy between two recharge points in supercapacitors, three hybridization techniques are considered and the balance point of the supercapacitor and hybrid capacitor banks is presented. The prospects of using a multiple-input DC-DC converter is also analyzed. An experimental electric vehicle profile was used to verify the proposed topology and the results are presented. The application of the novel hybridization of the three energy storage devices can be extended to other applications having a load profile with high crest factors.
I. N. Jiya, N. Gurusinghe, and R. Gouws, “Electrical circuit modelling of double layer capacitors for power electronics and energy storage applications: A review,” Electronics (ISSN 2079-9292; CODEN: ELECGJ), vol. 7, no. 11, p. 268, Oct. 2018.
Article abstract:
There has been increasing interests in the use of double layer capacitors (DLCs)—most commonly referred to as supercapacitors (SCs), ultra-capacitors (UCs), or hybrid capacitors (HCs)—in the field of power electronics. This increased interest in the hybridization of energy storages for automotive applications over the past few years is because of their advantage of high power density over traditional battery technologies. To facilitate accurate design and simulation of these systems, there is a need to make use of accurate and well validated models. Several models have been postulated in literature, however, these models have various limitations and strengths, ranging from the ease of use down to the complexity of characterization and parameter identification. The aim of this paper is to review and compare these models, specifically focusing on the models that predict the electrical characteristics of DLCs. The uniqueness of this review is that it
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focusses on the electrical circuit models of DLCs, highlighting the strengths and weaknesses of the different available models and the various areas for improvement. I. N. Jiya, W. Snyman, N. Gurusinghe, and R. Gouws, “PWM and Duty Ratio Switching of
Multiple Input Converters Using FPGAs: A Digital Logic Circuit and VHDL Hybrid Approach,” International Journal of Engineering & Technology (UAE) ISSN: 2227-524X, vol. 7, no. 4, pp. 4519–4527, 2018..
Article abstract:
This paper presents work that addresses the need for the simultaneous switching of the gate signals of controllable switches in multiple input DC-DC converters. A hybrid approach of implementing the gate signal switching using a combination of digital logic circuits and the conventional VHDL is used in this research. This approach reduces the complexity of the gate signal switching of multiple input converters when compared to the conventional methods. A new method of designing the digital logic circuits from the steady state wave-forms of the multiple input DC-DC converter is also introduced, the logic circuit was verified in simulation and validated experimentally by implementing it on an FPGA development board. From the experimental results presented, the switching of the multiple input converter was achieved with the possibility of using any type of system controller without affecting the operation of the switching signals. A dead time of up to 400 nanoseconds was achieved between the switching signals and ultimately, the new method of designing the digital logic circuit of the converter operation from the steady state waveform was validated.
I. N. Jiya, N. Gurusinghe, and R. Gouws, “Combination of LiCs and EDLCs with Batteries: A New Paradigm of Hybrid Energy Storage for Application in EVs,” World Electric Vehicle Journal, vol. 9, no. 4, p. 47, Nov. 2018.
Article abstract:
The research presented in this paper proposes a hybrid energy storage system that combines both electrolytic double-layer capacitors (EDLCs) also known as supercapacitors (SCs) and lithium-ion capacitors (LiCs) also known as hybrid capacitors (HCs) with a battery through a multiple input converter. The proposal was verified in simulation and validated by implementing a laboratory prototype. A new hybridisation topology, which reduces the amount of resource requirement when compared to the conventional hybridisation topology, is introduced. An electric vehicle (EV) current profile from previous research was used to test the performance of the proposed topology. From the results obtained, the hybridisation topology proposed in this research had the lowest cost per unit power at 14.81 $/kW, the lowest cost per unit power to energy, and available power to energy ratio, both at 1:1.3, thus making it a more attractive hybridisation topology than the two conventional alternatives. The multiple input converter built had efficiency
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values in excess of 80%. The key take away from this paper is that using the proposed hybridisation topology, the battery is less often required to supply energy to the electric vehicle, and so, its cycle life is preserved. Furthermore, since the battery is not used for the repeated acceleration and deceleration in the entire driving cycle, the battery’s cycle life is further preserved. Furthermore, since the battery is not the only storage device in the energy storage system, it can be further downsized to best fit the required base load; therefore, leading to a more optimized energy storage system by reducing the weight and volume of space occupied by the energy storage system, while also achieving better efficiencies.
1.7
Dissertation Overview
This dissertation has five chapters and three appendices. In Chapter 1, which is this chapter, the background to the research is presented, the problems statement discussed, the key research questions presented, the research methodology and the research objectives have also been discussed. In Chapter 2, a state of the art review of researches in literature was carried out to ensure that an understanding of all the aspects of the research is established, case studies of similar researches was also carried out and areas that presented opportunities for verification and validation in this research were identified.
The design and simulation studies carried out in the course of this research is presented in Chapter three. Specifically, the design and simulation of the hybrid energy storages and the multiple input converter. A section addressing the control of the hybrid energy storage system and the multiple input converter using field-programmable gate arrays (FPGAs) is also presented. In Chapter 4, the experimental implementation of the hybrid energy storages and the multiple input converter system alongside its control is presented. The tests and the results thereof were presented and discussed appropriately, verification and validation was carried out on the experimental system implemented and the results were also discussed and presented as well. In Chapter 5, an overall summary of the entire research is presented, the verification and validation of the entire research is discussed and a conclusion is drawn to bring closure to the research. Recommendations for future research is also made in that chapter.
1.8
Conclusion
This chapter was the introductory chapter of this research project; it presented the background, problem statement and the objectives of this research. It was important to establish the core objective of this research which was based on the problem that was observed when the background study was carried out. The key research questions which were
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necessary for achieving the objective of the research were presented. The abstracts of peer reviewed publications that were generated to present the findings of this research were also presented to give an overview of the research, furthermore, an overview of the entire dissertation has been presented.
The next chapter, the literature review (chapter 2), presents a study of existing research around the major topics as regards this research, this literature study is necessary to achieve an accurate design in order to obtain good results and consequently meet the overall objective of the research.
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This chapter forms the crux of the research that was carried out for this research project. It details all of the technological survey and literature study that was done on each major component and concept that was used in this research project. Figure 2-1 is the literature review plan; it is a broad view of all the topics addressed in this chapter. Every block on the literature review plan is addressed in depth with reference to other published works. In figure 2-2, the different subject areas considered in this Chapter is presented alongside the major references. The relationship between the different sub-topics is highlighted and major references are given as well, these interaction relationships highlight the areas where there is room for possible verification and validation for this research.
Literature Review
2.1 Energy Storage 2.4 DC-DC Conveters 2.5 Switching Electronics 2.7 Control Board 2.6 Advanced Control Topologies 2.1.1 Double Layer Capacitors 2.1.2 Batteries 2.1.3 Fuel Cells 2.1.4 Flywheel 2.4.1 Overview of Basic Topologies 2.4.2 Multiple Input Converters 2.4.3 DC-DC Converter Control 2.5.1 BJT 2.5.2 MOSFET 2.5.3 IGBT 2.6.1 Fuzzy Logic 2.6.2 Neural Networks 2.6.3 PID 2.7.1 Arduino 2.7.2 Neucleo 2.8 Software 2.8.1 Matlab® 2.8.2 LTspice® 2.7.3 Beagle Board 2.7.4 Raspberry Pi 2.3 Energy Storage Hybridisation Case Studies 2.3.1 Battery and supercapacitor 2.3.2 Battery and hybrid capacitor 2.3.3 Other hybridisation topologies 2.2 Double Layer Capacitor Models 2.2.1 Conventional Capacitor 2.2.2 Classical Equivalent Circuit 2.2.3 Ladder Circuit 2.2.4 Transmission Line 2.2.5 Zubieta 2.2.6 Two Branch 2.2.7 Other Unique ModelsFigure 2-1: Overview of the literature review plan showing the main topic areas studied in the literature study.
15 2.1 Energy Storage 2.4 DC-DC Converters 2.5 Switching Electronics 2.7 Control Board 2.6 Advanced Control Topologies 2.1.1 Double Layer Capacitors [35-38] 2.1.2 Batteries [40-42] 2.1.3 Fuel Cells [50-52] 2.1.4 Flywheel [57-61] 2.4.1 Overview of Basic Topologies [117,119] 2.4.2 Multiple Input Converters [120-144] 2.4.3 DC-DC Converter Control [145] 2.5.1 BJT [165-167] 2.5.2 MOSFET [171-175] 2.5.3 IGBT [176-178] 2.6.1 Fuzzy Logic [146-152] 2.6.2 Neural Networks [153-159] 2.6.3 PID [160-164] 2.7.1 Arduino [179-180] 2.7.2 Neucleo [181-183] 2.8 Software 2.8.1 Matlab® [190-191] 2.8.2 LTspice® [192-193] 2.7.3 Beagle Board [184-186] 2.7.4 Raspberry Pi [187-188] 2.3 Energy Storage Hybridisation Case Studies
2.3.1 Battery and supercapacitor [94-104] 2.3.2 Battery and hybrid capacitor [105-108] 2.3.3 Other hybridisation topologies [109-115] 2.2 Double Layer Capacitor Models 2.2.1 Conventional Capacitor [72-73] 2.2.2 Classical Equivalent Circuit [74-76] 2.2.3 Ladder Circuit [74] 2.2.4 Transmission Line [3] 2.2.5 Zubieta [77-81] 2.2.6 Two Branch [83-84] 2.2.7 Other Unique Models [85-93] [94,105,195] [130,149] [82] [172] [144] [145] [150] [157] [113-114] [115] [94-115] [181-188] [82]
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2.1
Energy Storage
Energy storage is at the core of this research, therefore it is important to review in-depth the relevant energy storage devices. This section covers double layer capacitors (DLCs), batteries, fuel cells and the flywheel. These energy storage devices have various types each, but only the few which are relevant for application in electric vehicles are reviewed since the scope of this research is for application in electric vehicles.
2.1.1 Double Layer Capacitors
Double layer capacitors (DLCs) are energy storage devices that use a double layer formed on a large surface of micro porous material such as activated carbon [31]. DLCs currently are of two types, the SC or Ultra-capacitor as is commonly referred as and the HC. SCs are also known as electrolytic double layer capacitor (EDLC) while HCs are often referred to as Lithium-Ion capacitors (LIC) [22]. The distinctive difference between the SC and HC is that the negative and positive electrode (anode and cathode respectively) of the SC are both made from activated carbon. Whereas in the case of the HC, only the positive electrode (cathode) is made from activated carbon, the anode is formed by a lithium doped carbon, this makes the HC have more energy density than the SC [4], [22]. This difference is illustrated in figure 2-3; the HC’s chemistry exhibits a combination of the lithium-ion battery and a SC. But in spite of the differences in their energy and power characteristics, it has been established that they both have the same electrical model and characterization method [22].
The concept of double layer capacitance was first described by Hermann von Helmholtz a
German physicist, in 1853 [32] 104 years later, General Electric received the first patent of electrochemical capacitors based on the structure of double layer capacitance as described by Hermann von Helmholtz; their capacitor used porous carbon electrodes using the double layer mechanism for charging [33]. In the same year, a Japanese company, Nippon Electric, received a license to use a technology that had been initially patented to Standard Oil, this
Activated Carbon (Anode) Activated Carbon (Cathode) Lithium titanate (Anode) Activated Carbon (Cathode) (a) (b)
Figure 2-3: Rudimentary structure of (a) SC and (b) HC showing the different anode and cathode of the two different DLCs (adapted from [20]).
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technology was a device that stored energy in double layer interface. With this technology, the first market-ready electrochemical capacitors were produced for application in memory back-ups of computers in the same year, 1957 [34], [35], however, this was not a successful attempt until in 1971, when the same Japanese company, Nippon Electric produced the SC which then became the first commercially successful double layer capacitor [35], [36]. Since then, the development of double layer capacitors has been quite rapid with improvements as the years went by other companies such as Maxwell technologies, Panasonic, Nesscap, AVX, Cap XX, Taiyo Yuden, Yunasko etc. to mention just a few, these companies have all joined the race to push the frontiers of double layer capacitors further.
The schematic construction of double layer capacitors is shown in figure 2-4. It consists of three basic layers, the electrolyte, the separator and the positive and negative electrodes. “It exploits the double layer of charge formed when a voltage is applied to an electrode immersed in an electrolyte” [37], this is most likely where the name Double Layer Capacitors originate.
The electrolyte and electrodes used in the construction of double layer capacitors have to be selected interdependently as these two largely directly affect the power and energy density of the capacitor cell. The two types of electrolytes currently in use for double layer capacitors are aqueous and organic; however the organic electrolyte is most commonly used in commercial devices. There are various types of materials being used for the electrodes such as metal-oxides, conducting polymer and carbon to mention just a few. But carbon has been more commonly utilized because of its low cost and availability [35]. Different treatments of carbon are being researched for the improvements of the electrode which has a direct impact on the power density of the cell [37], [38]. The separator provides electrical isolation between the two electrodes; however, it is an ion-permeable membrane giving room for ionic charge transfer between the two sections of the electrolyte. Materials used for the separator usually depends on the electrolyte used and they include polymer and paper for organic electrolytes and
+ + + + + + + + + + (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (+) (+) (+) (+) (+) (+) (+) (+) (+) (+) -Anode Cathode Electrolyte Separator
Figure 2-4: Basic structure of double layer capacitors showing the four main components that are utilized in its construction (adapted from [37]).
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ceramic and glass fibre for aqueous electrolytes. More detail on the structure and construction of double layer capacitors can be found in [35], [37], [38].
Currently, the two major double layer capacitors are the Electrolytic double layer capacitor (EDLC) or SCs and the Lithium-Ion capacitors (LIC) or HCs [22]. They are both double layer capacitors but one of the distinctive differences between them is that the anode, the negative electrode, of the HC is formed by a Lithium doped carbon [22] [4]. Therefore, it combines the chemistry of a Lithium battery and that of a SC in its formation. The EDLC often referred to as a SC or Ultra-capacitor, as is seen in figure 2-3, has both its positive and negative electrodes made from carbon, while the HC has its anode made from a different material while the cathode is made from carbon just as in the EDLC. The most common HC is the Lithium ion capacitor, this is formed from the combination of the intercalation mechanism of a lithium ion battery with the cathode of an EDLC, the anode of the lithium ion capacitor is doped with a lithium ion, hence the name lithium ion capacitor.
2.1.2 Batteries
Main components of battery are the materials used for anode, cathode and electrolyte. Electrons released from the oxidation reaction at anode, flow through load connected in the external circuit towards cathode. The cathode in turn accepts the electrons during reduction reaction and the ions flowing through electrolytes complete the cell circuit. Anode and cathode material of the battery determine the cell voltage and the electrolyte determine the chemistry and rate of electrode reactions. Batteries are of two types: primary and secondary. If the anodic and cathodic reactions are reversible, then the battery is electrically rechargeable which is called as secondary type battery. Primary batteries are non-rechargeable. Since secondary batteries are rechargeable, their charge retention capability is lower than that of primary batteries [39]. The following subsections bring to light the major batteries that can be used in electric vehicles as well as highlighting their comparative pros and cons.
2.1.2.1 Lithium Ion
Lithium has high energy density because of its low atomic weight and high ionization potential than the materials used in traditional batteries. It is rechargeable battery. The structure of Lithium Ion battery consists of anode, cathode, porous separator between them and the electrolyte as shown in figure 2-5. The electricity flow is generated by the movement of lithium ions between anode and cathode. While discharging, lithium from the anode which is made up of carbon material gets ionized and emitted in the electrolyte material. Released lithium ions pass through the porous plastic separator between anode and cathode, and then move to the atomic-size holes of cathode made up of lithium metal oxide material. During this process, electrons flow from anode to cathode, supplying the electric current to the load
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connected in the external circuit. As this process is reversible chemical reaction, during battery charging cycle, lithium ions travel from cathode to anode through the porous separator and the battery gets recharged [40].
Anode
Electrolyte Separator M=Mn, Co,Ni
Cathode
Figure 2-5: Structure and discharging of Lithium Ion battery (adapted from [40]). It has applications in portable electronic devices such as laptops, cell phones, etc. Use of inorganic solvents and Solid Electrolyte Interface (SEI) passivating layer affect the primary lithium battery performance. Thermal batteries are primary Lithium Ion batteries used in missile defence applications. Reaction product LiI of Lithium and Iodine as separator and electrolyte is another example of primary Li-ion battery used in medical applications. Main characteristics of secondary Li-ion batteries first introduced by Sony in 1990 are high energy density, no memory effect, high cell voltages (4V) and multiple charging-discharging cycles. 2.1.2.2 Ni-Cad
Nickel-Cadmium was first rechargeable battery with small seal. Electrolyte used is alkaline (KOH). During discharge/charge Cd electrode release the discharge product of Cd(OH)2 and Ni(OH)2 electrode release or insert protons, working reversibly. Key characteristics of Ni-Cd batteries include better low temperature performance, longer cycle life, and constant discharge voltage during high rates of charging and discharging. Disadvantages of Ni-Cd batteries include higher cost of construction, risk of Cd manipulation and memory effects [41]. 2.1.2.3 Ni-MH
Nickel-Metal Hybrid is advanced rechargeable and environment friendly battery available. The Cd electrode in Ni-Cd battery is replaced with hydrogen storage alloy for proton insertion. The electrolyte KOH and positive electrode Ni(OH)2 remain same as in Ni-Cd batteries. With the advantages of high corrosion and oxidation resistance, higher rate of gas recombination, high
