Bidirectional DC-DC converters for hybrid energy storage systems in electric vehicle applications

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FEDERAL UNIVERSITY OF TECHNOLOGY OF PARANÁ - UTFPR DEPARTMENT OF ELECTRONIC ENGINEERING

GRADUATE PROGRAM IN ELECTRICAL ENGINEERING

GABRIEL RENAN BRODAY

BIDIRECTIONAL DC-DC CONVERTERS FOR HYBRID ENERGY

STORAGE SYSTEMS IN ELECTRIC VEHICLE APPLICATIONS

MASTER’S THESIS

PONTA GROSSA 2016

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GABRIEL RENAN BRODAY

BIDIRECTIONAL DC-DC CONVERTERS FOR HYBRID ENERGY

STORAGE SYSTEMS IN ELECTRIC VEHICLE APPLICATIONS

Master’s Thesis presented as partial requirement for obtaining a Master’s Degree in Electrical Engineering from the Department of Electronic Engineering at Federal University of Technology of Paraná-UTFPR.

Advisor: Prof. Dr. Claudinor Bitencourt Nascimento

Co-Advisor: Prof. Dr. Eloi Agostini Jr.

PONTA GROSSA 2016

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Ficha catalográfica elaborada pelo Departamento de Biblioteca da Universidade Tecnológica Federal do Paraná, Campus Ponta Grossa

n.05/17

B864 Broday, Gabriel Renan

Bidirectional DC-DC converters for hybrid energy storage systems in electric vehicle applications / Gabriel Renan Broday. -- 2017.

267 f. : il. ; 30 cm.

Orientador: Prof. Dr. Claudinor Bitencourt Nascimento Coorientador: Prof. Dr. Eloi Agostini Junior

Dissertação (Mestrado em Engenharia Elétrica) - Programa de Pós-Graduação em Engenharia Elétrica. Universidade Tecnológica Federal do Paraná. Ponta Grossa, 2017.

1. Veículos elétricos. 2. Energia - Armazenamento. 3. Capacitadores. 4. Baterias elétricas. 5. Conversores de corrente elétrica. I. Nascimento, Claudinor Bitencourt. II. Agostini Junior, Eloi. III. Universidade Tecnológica Federal do Paraná. IV. Título.

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Universidade Tecnológica Federal do Paraná Campus de Ponta Grossa

Diretoria de Pesquisa e Pós-Graduação

PROGRAMA DE PÓS-GRADUAÇÃO EM

ENGENHARIA ELÉTRICA UNIVERSIDADE TECNOLÓGICA FEDERAL DO PARANÁ

PR

FOLHA DE APROVAÇÃO

Título de Dissertação Nº 23/2016

BIDIRECTIONAL DC-DC CONVERTERS FOR HYBRID ENERGY STORAGE SYSTEMS IN ELETRIC VEHICLE APPLICATIONS

por

Gabriel Renan Broday

Esta dissertação foi apresentada às 10 horas do dia 15 de dezembro de 2016 como requisito parcial para a obtenção do título de MESTRE EM ENGENHARIA ELÉTRICA, com área de concentração em Controle e Processamento de Energia, Programa de Pós-Graduação em Engenharia Elétrica. O candidato foi arguido pela Banca Examinadora composta pelos professores abaixo assinados. Após deliberação, a Banca Examinadora considerou o trabalho aprovado.

Prof. Dr. Luiz Antonio Correa Lopes (Concordia University)

Prof. Dr. Marcio Mendes Casaro (UTFPR)

Prof. Dr. Claudinor Bitencourt Nascimento

(UTFPR)

Orientador

Prof. Dr. Claudinor Bitencourt Nascimento

Coordenador do PPGEE

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ACKNOWLEDGEMENTS

First, I would like to thank my parents for supporting me in all the way, for sharing my happiness and my fears, for making me who I am. This work would not be possible without you!

To my brothers Geovani and Sérgio for making my life more fun.

To my advisor Prof. Dr. Claudinor Bitencourt Nascimento for putting his trust in me, for believing in me when others did not believe. A person who I hope to take with me for the rest of my life.

To my co-advisor Prof. Dr. Eloi Agostini Jr. for all his contribution in this work. Always punctual in his placements, sharing knowledge in an unique way.

To Prof. PhD Luiz A. C. Lopes for all the moments spend in Montreal, for his technical contribution, experience, good talks and, most important, for his friendship. Also, I would like to extend this acknowledgment to his wife Mylene and his daughter Carol, a family that received me so well in Montreal that made me feel at home.

To my friends Marlon Lessing, Remei Haura Jr. and William Kremes for the technical discussions, friendship and good moments.

To all my friends from the P. D. Ziogas Power Electronics Laboratory at Concordia University, in special to Arvynd Vias, for the good moments and help when I was in Montreal.

To the Brazilian and Canadian governments that through their development agencies could finance this work.

To my better half Lays, for her love and comprehension.

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“Train while they sleep, Study while they have fun, Persist while they rest, And then

Live what they dream”

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RESUMO

BRODAY, G. R. Conversores CC-CC Bidirecionais para Sistemas Híbridos de

Armazenamento de Energia em Aplicações de Veículos Elétricos. 2016. 267 p.

Dissertação (Mestrado em Engenharia Elétrica) - Universidade Tecnológica Federal do Paraná. Ponta Grossa, 2016.

Em um momento em que questões ambientais e a segurança energética estão numa posição de destaque, Veículos Elétricos (VEs) estão no centro das atenções. Entretanto, ainda é difícil para eles substituir os tradicionais veículos de combustão interna e a razão principal para isso é o seu sistema de energia. Normalmente, devido a suas características, baterias são usadas como banco de energia para VEs. No entanto, baterias também apresentam algumas limitações para essa aplicação e o problema no sistema de energia é relacionado a essas limitações. Uma das soluções propostas é se colocar baterias e supercapacitores (SC) em paralelo, resultando em um Sistema Híbrido de Armazenamento de Energia (SHAE). Para fazer essa configuração possível e o fluxo de potência controlável em um SHAE, um conversor CC-CC bidirecional interfaceando a bateria e o SC é necessário. Levando isso em consideração, o estudo de topologias CC-CC bidirecionais é apresentado nessa Dissertação de Mestrado. Primeiro, o estudo de um conversor CC-CC bidirecional com indutor dividido, envolvendo sua análise teórica em regime permanente, análise dinâmica e uma metodologia de projeto com resultados de simulação, é apresentado, resultando na construção de um protótipo experimental com as seguintes especificações de projeto: Fonte de tensão 1 de 300 V, fonte de tensão 2 de 96 V, frequência de comutação de 20 kHz e potência nominal de 1000 W. Então, o estudo de uma segunda topologia, um conversor CC-CC Buck-Boost ZVS bidirecional, envolvendo sua análise em regime permanente e uma metodologia de projeto com resultados de simulação, também é apresentado.

Palavras-Chave: Conversores CC-CC Bidirecionais, Baterias, Supercapacitores,

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ABSTRACT

BRODAY, G. R. Bidirectional DC-DC Converters for Hybrid Energy Storage

Systems in Electric Vehicle Applications. 2016. 267 pp. Master’s Thesis (Master’s

Degree in Electrical Engineering) - Federal University of Technology of Paraná. Ponta Grossa, 2016.

In an era where environmental issues and the energetic safety are in an outstanding position, Electric Vehicles (EVs) are in the spotlight. However, it is difficult for them to replace the ICE vehicles and the main reason for that it is their energy system. Normally, due to some of their characteristics, batteries are used as energy bank in Electric Vehicles. Nevertheless, batteries also present some limitations for this application and the energy system problem is related to these limitations. One of the proposed solutions is to place batteries and Supercapacitors (SC) in parallel, resulting in a Hybrid Energy Storage System (HESS). To make this configuration possible and the power flow controllable in the HESS, a bidirectional DC-DC converter interfacing the battery and the SC is necessary. Taking this into account, the study of bidirectional DC-DC topologies is presented in this Master’s Thesis. First, a study of a bidirectional DC-DC converter with tapped inductor, involving its theoretical steady state analysis, dynamic analysis and design methodology with simulation results, is presented, resulting in the design of an experimental prototype with the following design specifications: Voltage source 1 of 300 V, voltage source 2 of 96 V, switching frequency of 20 kHz and rated power of 1000 W. Then, the study of a second topology, a bidirectional ZVS Buck-Boost DC-DC converter, involving the steady state analysis and a design methodology with simulation results, is also presented.

Keywords: Bidirectional DC-DC Converters, Batteries, Supercapacitors, Electric

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LIST OF FIGURES

Figure 1.1 – Electric Vehicle by William Morrison...32

Figure 1.2 – Hybrid Vehicle Toyota Prius...34

Figure 1.3 – Inside view of a PHEV...35

Figure 1.4 – Electric Vehicle FIAT/Itaipu Binacional Palio Weekend...37

Figure 1.5 – First electric accumulator...39

Figure 1.6 – Batteries arrangement in EVs...40

Figure 1.7 – Commercial Lead-acid battery...42

Figure 1.8 – Nickel-metal hydride battery bank for EVs...42

Figure 1.9 – Lithium-ion battery module for EVs...43

Figure 1.10 – Commercial Maxwell Supercapacitors...45

Figure 1.11 – Battery/Supercapacitor HESS in EVs applications...47

Figure 2.1 – Traditional DC-DC converters: (a) Buck (b) Boost (c) Buck-Boost...50

Figure 2.2 – Traditional isolated DC-DC converters: (a) Flyback (b) Forward...51

Figure 2.3 – Traditional bidirectional DC-DC converters: (a) Buck/Boost (b) Boost/Buck (c) Buck-Boost...52

Figure 2.4 – Traditional isolated bidirectional DC-DC converters: (a) Flyback (b) Forward...52

Figure 2.5 – Integrated bidirectional Buck/Boost/Buck-Boost DC-DC converter...53

Figure 2.6 – Operating stages of the Forward mode: (a) Buck 1 (b) Buck 2 (c) Boost 1 (d) Boost 2 (e) Buck-Boost 1 (f) Buck-Boost 2………..54

Figure 2.7 – Operating stages of the Reverse mode: (a) Buck 1 (b) Buck 2 (c) Boost 1 (d) Boost 2 (e) Buck-Boost 1 (f) Buck-Boost 2...54

Figure 2.8 – Bidirectional Boost/Buck DC-DC converter...55

Figure 2.9 – Operating stages of the bidirectional Boost/Buck DC-DC converter: (a) Forward Boost 1 (b) Forward Boost 2 (c) Reverse Buck 1 (d) Reverse Buck 2…56 Figure 2.10 – Bidirectional ZVS Boost/Buck DC-DC converter...57

Figure 3.1 – Bidirectional DC-DC converter with tapped inductor...59

Figure 3.2 – Equivalent circuit of the bidirectional DC-DC converter with tapped inductor...60

Figure 3.3 – Forward Buck: Gate signals...61

Figure 3.4 – Forward Buck: Fist operating stage...62

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Figure 3.6 – Forward Buck: Theoretical voltage waveforms in the switches S1 and

S3...64

Figure 3.7 – Forward Buck: Theoretical voltage waveforms in the tapped inductor………....65

Figure 3.8 – Forward Buck: Theoretical waveforms in the magnetizing inductance...65

Figure 3.9 – Forward Buck: Theoretical waveforms of the currents I1 and I2...65

Figure 3.10 – Forward Buck: Theoretical current waveforms in the switches...66

Figure 3.11 – Forward Buck: Voltage conversion characteristic...67

Figure 3.12 – Forward Boost: Gate signals...71

Figure 3.13 – Forward Boost: First operating stage...71

Figure 3.14 – Forward Boost: Second operating stage...73

Figure 3.15 – Forward Boost: Theoretical voltage waveforms in the switches S2 and S3...73

Figure 3.16 – Forward Boost: Theoretical voltage waveforms in the tapped inductor...73

Figure 3.17 – Forward Boost: Theoretical waveforms in the magnetizing inductance...74

Figure 3.18 – Forward Boost: Theoretical waveforms of the currents I1 and I2...74

Figure 3.19 – Forward Boost: Theoretical current waveforms in the switches……....74

Figure 3.20 – Forward Boost: Voltage conversion characteristic………...75

Figure 3.21 – Forward Buck-Boost: Gate signals...78

Figure 3.22 – Forward Buck-Boost: First operating stage...79

Figure 3.23 – Forward Buck-Boost: Second operating stage...79

Figure 3.24 – Forward Buck-Boost: Theoretical voltage waveforms in the switches S1 and S2...80

Figure 3.25 – Forward Buck-Boost: Theoretical voltage waveforms in the tapped inductor...80

Figure 3.26 – Forward Buck-Boost: Theoretical waveforms in the magnetizing inductance………...…80

Figure 3.27 – Forward Buck-Boost: Theoretical waveforms of the currents I1 and I2………81

Figure 3.28 – Forward Buck-Boost: Theoretical current waveforms in the switches...81

Figure 3.29 – Forward Buck-Boost: Voltage conversion characteristic………....82

Figure 3.30 – Reverse Buck: Gate signals...85

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Figure 3.32 – Reverse Buck: Second operating stage...86

Figure 3.33 – Reverse Buck: Theoretical voltage waveforms in the switches S2 and S3...86

Figure 3.34 – Reverse Buck: Theoretical voltage waveforms in the tapped inductor...86

Figure 3.35 – Reverse Buck: Theoretical waveforms in the magnetizing inductance...87

Figure 3.36 – Reverse Buck: Theoretical waveforms of the currents I1 and I2...87

Figure 3.37 – Reverse Buck: Theoretical current waveforms in the switches...87

Figure 3.38 – Reverse Buck: Voltage conversion characteristic...88

Figure 3.39 – Reverse Boost: Gate signals...91

Figure 3.40 – Reverse Boost: First operating stage...92

Figure 3.41 – Reverse Boost: Second operating stage...92

Figure 3.42 – Reverse Boost: Theoretical voltage waveforms in the switches S1 and S3...93

Figure 3.43 – Reverse Boost: Theoretical voltage waveforms in the tapped inductor………....93

Figure 3.44 – Reverse Boost: Theoretical waveforms in the magnetizing inductance...93

Figure 3.45 – Reverse Boost: Theoretical waveforms of the currents I1 and I2...94

Figure 3.46 – Reverse Boost: Theoretical current waveforms in the switches...94

Figure 3.47 – Reverse Boost: Voltage conversion characteristic...95

Figure 3.48 – Reverse Buck-Boost: Gate signals...98

Figure 3.49 – Reverse Buck-Boost: First operating stage...98

Figure 3.50 – Reverse Buck-Boost: Second operating stage...99

Figure 3.51 – Reverse Buck-Boost: Theoretical voltage waveforms in the switches S1 and S2………...99

Figure 3.52 – Reverse Buck-Boost: Theoretical voltage waveforms in the tapped inductor………..100

Figure 3.53 – Reverse Buck-Boost: Theoretical waveforms in the magnetizing inductance...100

Figure 3.54 – Reverse Buck-Boost: Theoretical waveforms of the current I1 and I2...100

Figure 3.55 – Reverse Buck-Boost: Theoretical current waveforms in the switches...101

Figure 3.56 – Reverse Buck-Boost: Voltage conversion characteristic...102

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Figure 4.2 – Equivalent circuit of the bidirectional ZVS Buck-Boost Converter...106

Figure 4.3 – Forward mode: First stage...108

Figure 4.4 – Forward mode: Second stage...110

Figure 4.5 – Forward mode: Theoretical voltage waveforms in the switches S1 and S2………112

Figure 4.6 – Forward Mode: Theoretical voltage waveforms in the transformer...112

Figure 4.7 – Forward mode: Theoretical waveforms in the magnetizing inductance...113

Figure 4.8 – Forward Mode: Theoretical waveforms in the auxiliary inductance (a) n>1 (b) n<1...113

Figure 4.9 – Forward mode: Theoretical current waveforms in the voltage source V1 (a) n>1 (b) 0<n<=0.5 (c) 0.5<n<1...114

Figure 4.10 – Forward mode: Theoretical current waveforms in the voltage source V2 (a) n>1 (b) 0<n<=0.5 (c) 0.5<n<1...114

Figure 4.11 – Forward mode: Theoretical current waveforms in the switches (a) n>1 (b) n<1...114

Figure 4.12 – Reverse mode: Theoretical voltage waveforms in the switches S1 and S2...119

Figure 4.13 – Reverse mode: Theoretical voltage waveforms in the transformer....119

Figure 4.14 – Reverse mode: Theoretical waveforms in the magnetizing inductance...119

Figure 4.15 – Reverse mode: Theoretical waveforms in the auxiliary inductance (a) n>1 (b) n<1...120

Figure 4.16 – Reverse mode: Theoretical current waveforms in the voltage source V1 (a) n>1 (b) 0<n<=0.5 (c) 0.5<n<1...120

Figure 4.17 – Reverse mode: Theoretical current waveforms in the voltage source V2 (a) n>1 (b) 0<n<=0.5 (c) 0.5<n<1...120

Figure 4.18 – Reverse mode: Theoretical current waveforms in the switches (a) n>1 (b) n<1...121

Figure 6.1 – Block diagram for the control design...139

Figure 6.2 – Bode diagram of the uncompensated system...140

Figure 6.3 – Step response of the compensated system...141

Figure 6.4 – Bode diagram of the compensated system...141

Figure 6.5 – Circuit implemented in PSIM®: Power schematic...142

Figure 6.6 – Circuit implemented in PSIM®: Control schematic...142

Figure 6.7 – Step response: Comparison Converter x Transfer function...142

Figure 6.8 – Forward Buck: Simulated voltage waveforms in the switches S1 and S3………...143

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Figure 6.9 – Forward Buck: Simulated voltage waveforms in the tapped

inductor...144

Figure 6.10 – Forward Buck: Simulated waveforms in the magnetizing inductance LM...144

Figure 6.11 – Forward Buck: Simulated current waveform in switch S1...145

Figure 6.14 – Forward Buck: Simulated waveforms of the currents I1 and I2...146

Figure 6.15 – Forward Buck: Current control...146

Figure 6.16 – Forward Buck-Boost: Simulated voltage waveforms in the switches S1 and S2...148

Figure 6.17 – Forward Buck-Boost: Simulated voltage waveforms in the tapped inductor...148

Figure 6.18 – Forward Buck-Boost: Simulated waveforms in the magnetizing inductance LM………...149

Figure 6.19 – Forward Buck-Boost: Simulated current waveform in switch S1...149

Figure 6.22 – Forward Buck-Boost: Simulated waveforms of the currents I1 and I2...151

Figure 6.23 – Forward Buck-Boost: Current control...151

Figure 6.24 – Reverse Boost: Simulated voltage waveforms in the switches S1 and S3………...153

Figure 6.25 – Reverse Boost: Simulated voltage waveforms in the tapped inductor...153

Figure 6.26 – Reverse Boost: Simulated waveforms in the magnetizing inductance LM...153

Figure 6.27 – Reverse Boost: Simulated current waveform in switch S1...154

Figure 6.30 – Reverse Boost: Simulated waveforms of the currents I1 and I2...155

Figure 6.31 – Reverse Boost: Current control...156

Figure 6.32 – Reverse Buck-Boost: Simulated voltage waveforms in the switches S1 and S2………157

Figure 6.33 – Reverse Buck-Boost: Simulated voltage waveforms in the tapped inductor...157

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Figure 6.34 – Reverse Buck-Boost: Simulated waveforms in the magnetizing

inductance LM...158

Figure 6.35 – Reverse Buck-Boost: Simulated current waveform in switch S1...158

Figure 6.38 – Reverse Buck-Boost: Simulated waveforms of the currents I1 and I2...159

Figure 6.39 – Reverse Buck-Boost: Current control...160

Figure 6.40 – Unified controller: Forward Buck to Reverse Boost...162

Figure 6.41 – Unified controller: Forward Boost to Reverse Buck-Boost...162

Figure 7.1 – RMS current in switch S1 for different values of n...166

Figure 7.2 – RMS current in switch S2 for different values of n...167

Figure 7.3 – Forward mode: Schematic of simulation...168

Figure 7.4 – Forward mode: Voltage across the RC load...169

Figure 7.5 – Forward mode: Simulated Voltage waveform in each turn of the transformer...170

Figure 7.6 – Forward mode: Simulated waveforms in the magnetizing inductance LM...170

Figure 7.7 – Forward mode: Simulated waveforms in the auxiliary inductance LL...171

Figure 7.8 – Forward mode: Simulated waveforms in the switch S1...171

Figure 7.9 – Forward mode: Simulated waveforms in the switch S2...172

Figure 7.10 – Forward mode: Simulated current waveforms in the voltage sources...172

Figure 7.11 – Reverse mode: Schematic of simulation...173

Figure 7.12 – Reverse mode: Voltage across the RC load...174

Figure 7.13 – Reverse mode: Simulated Voltage waveform in each turn of the transformer...175

Figure 7.14 – Reverse mode: Simulated waveforms in the magnetizing inductance LM...175

Figure 7.15 – Reverse mode: Simulated waveforms in the auxiliary inductance LL...176

Figure 7.16 – Reverse mode: Simulated waveforms in the switch S1...176

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Figure 7.18 – Reverse mode: Simulated current waveforms in the voltage

sources...177

Figure 8.1 – Clamping circuits: (a) Passive clamping (b) Active clamping...182

Figure 8.2 – Experimental prototype...184

Figure 8.3 – Tapped inductor……….184

Figure 8.4 – Schematic of the experimental setup...186

Figure 8.5 – Forward Buck: Gate signals (10 V/div)...186

Figure 8.6 – Forward Buck: Voltage (200 V/div) and current (7 A/div) in the switch S1………...187

Figure 8.7 – Forward Buck: Voltage (200 V/div) and current (7 A/div) in the switch S1...187

Figure 8.8 – Forward Buck: Turning-on of the switch S1...188

Figure 8.9 – Forward Buck: Turning-off of the switch S1……...188

Figure 8.12 – Forward Buck: Turning-on of the switch S3……...190

Figure 8.13 – Forward Buck: Turning-off of the switch S3...190

Figure 8.14 – Forward Buck: Voltage (100 V/div) and current (7 A/div) in the voltage source 1...191

Figure 8.15 – Forward Buck: Voltage (30 V/div) and current (7 A/div) in the voltage source 2…...191

Figure 8.16 – Forward Buck: Voltage (100 V/div) and current (10 A/div) in magnetizing inductance...192

Figure 8.17 – Forward Buck: Currents (10 A/div) through each switch...192

Figure 8.18 – Forward Buck: Voltage (100 V/div) and current (7 A/div) in the primary ……...193

Figure 8.19 – Forward Buck: Voltage (100 V/div) and current (7 A/div) in the secondary...193

Figure 8.20 – Forward Buck: Voltage (100 V/div) and current (2 A/div) for the current control of I1...194

Figure 8.21 – Forward Buck: Current I1 in the voltage source ……...194

Figure 8.22 – Forward Buck: Efficiency curve...195

Figure 8.23 – Forward Buck-Boost: Gate signals (10 V/div)...196

Figure 8.24 – Forward Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S1……...197

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Figure 8.25 – Forward Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S1...197 Figure 8.26 – Forward Buck-Boost: Turning-on of the switch S1...198 Figure 8.27 – Forward Buck-Boost: Turning-off of the switch S1……...198 Figure 8.28 – Forward Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S2...199 Figure 8.29 – Forward Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S2...199 Figure 8.30 – Forward Buck-Boost: Turning-on of the switch S2……...200 Figure 8.31 – Forward Buck-Boost: Turning-off of the switch S2...200 Figure 8.32 – Forward Buck-Boost: Voltage (100 V/div) and current (10 A/div) in

V1...201 Figure 8.33 – Forward Buck-Boost: Voltage (30 V/div) and current (10 A/div) in

V2...201 Figure 8.34 – Forward Buck-Boost: Voltage (200 V/div) and current (10 A/div) in the magnetizing inductance...202 Figure 8.35 – Forward Buck-Boost: Currents (20 A/div) through each switch...202 Figure 8.36 – Forward Buck-Boost: Voltage (200 V/div) and current (10 A/div) in the primary……...203 Figure 8.37 – Forward Buck-Boost: Voltage (200 V/div) and current (10 A/div) in the primary...203 Figure 8.38 – Forward Buck-Boost: Voltage (100 V/div) and current (2 A/div) for the current control of I1...204 Figure 8.39 – Forward Buck-Boost: Efficiency curve……...204 Figure 8.40 – Reverse Boost: Gate signals (10 V/div)...206 Figure 8.41 – Reverse Boost: Voltage (200 V/div) and current (7 A/div) in the switch

S1...207 Figure 8.42 – Reverse Boost: Voltage (200 V/div) and current (7 A/div) in the switch

S1……...207 Figure 8.43 – Reverse Boost: Turning-on of the switch S1...208 Figure 8.44 – Reverse Boost: Turning-off of the switch S1...208 Figure 8.45 – Reverse Boost: Voltage (200 V/div) and current (10 A/div) in the switch

S3…...209 Figure 8.46 – Reverse Boost: Voltage (200 V/div) and current (10 A/div) in the switch

S3...209 Figure 8.47 – Reverse Boost: Turning-on of the switch S3...210 Figure 8.48 – Reverse Boost: Turning-off of the switch S3……...210

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Figure 8.49 – Reverse Boost: Voltage (100 V/div) and current (10 A/div) in the voltage sources V1 and V2...211 Figure 8.50 – Reverse Boost: Voltage (100 V/div) and current (10 A/div) in the magnetizing inductance...211 Figure 8.51 – Reverse Boost: Voltage (100 V/div) and current (10 A/div) in the primary…...212 Figure 8.52 – Reverse Boost: Voltage (100 V/div) and current (10 A/div) in the secondary...212 Figure 8.53 – Reverse Boost: Voltage (100 V/div) and current (5 A/div) for the current control of I1...213 Figure 8.54 – Reverse Boost: Efficiency curve…...214 Figure 8.55 – Reverse Buck-Boost: Gate signals (10 V/div)...215 Figure 8.56 – Reverse Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S1...216 Figure 8.57 – Reverse Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S1…...216 Figure 8.58 – Reverse Buck-Boost: Turning-on of the switch S1...217 Figure 8.59 – Reverse Buck-Boost: Turning-off of the switch S1...217 Figure 8.60 – Reverse Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S2…...218 Figure 8.61 – Reverse Buck-Boost: Voltage (300 V/div) and current (10 A/div) in the switch S2...218 Figure 8.62 – Reverse Buck-Boost: Turning-on of the switch S2...219 Figure 8.63 – Reverse Buck-Boost: Turning-off of the switch S2…...219 Figure 8.64 – Reverse Buck-Boost: Voltage (100 V/div) and current (10 A/div) in

V1...220 Figure 8.65 – Reverse Buck-Boost: Voltage (30 V/div) and current (10 A/div) in

V2...220 Figure 8.66 – Reverse Buck-Boost: Voltage (200 V/div) and current (10 A/div) in the magnetizing inductance……...221 Figure 8.67 – Reverse Buck-Boost: Currents (20 A/div) through each switch...221 Figure 8.68 – Reverse Buck-Boost: Voltage (200 V/div) and current (10 A/div) in the primary...222 Figure 8.69 – Reverse Buck-Boost: Voltage (200 V/div) and current (10 A/div) in the secondary…...222 Figure 8.70 – Reverse Buck-Boost: Voltage (100 V/div) and current (7 A/div) for the current control of I1...223 Figure 8.71 – Reverse Buck-Boost: Efficiency curve...223

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LIST OF TABLES

Table 1.1 – Characteristics of different types of batteries...40

Table 1.2 – Comparison between batteries...44

Table 2.1 – Integrated bidirectional Buck/Boost/Buck-Boost DC-DC converter: Switching Logic...53

Table 2.2 – Bidirectional Boost/Buck DC-DC converter: Switching Logic...56

Table 6.1 – Battery bank in the traction system of commercial EVs and HEVs...136

Table 6.2 – Design specifications for the bidirectional DC-DC converter with tapped inductor...138

Table 6.3 – Components sizing for the bidirectional DC-DC converter with tapped inductor...139

Table 6.4 – Forward Buck: Comparison Theoretical x Simulated...147

Table 6.5 – Forward Buck-Boost: Comparison Theoretical x Simulated...152

Table 6.6 – Reverse Boost: Comparison Theoretical x Simulated...156

Table 6.7 – Reverse Buck-Boost: Comparison Theoretical x Simulated...160

Table 7.1 – Design specifications for the bidirectional ZVS Buck-Boost DC-DC converter...164

Table 7.2 – Components sizing for the bidirectional ZVS Buck-Boost DC-DC converter...168

Table 7.3 – Forward mode: Comparison Theoretical x Simulated...173

Table 7.4 – Reverse mode: Comparison Theoretical x Simulated...178

Table 8.1 – Constructive aspects of the tapped inductor...181

Table 8.2 – Components used in the prototype………..183

Table 8.3 – Forward Buck: Comparison Theoretical x Simulated x Experimental…195 Table 8.4 – Forward Buck-Boost: Comparison Theoretical x Simulated x Experimental………....205

Table 8.5 – Reverse Boost: Comparison Theoretical x Simulated x Experimental..214

Table 8.6 – Reverse Buck-Boost: Comparison Theoretical x Simulated x Experimental……….224

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LIST OF ABBREVIATIONS

B.C. before Christ

CARB California Air Resources Board CrCM Critical Conduction Mode CCM Continuous Conduction Mode

CNPq-BR Brazilian National Council of Technological and Scientific Development DC-DC Direct Current to Direct Current

DCM Discontinuous Conduction Mode DSP Digital Signal Processor

EV Electric Vehicle

Finep Brazilian Financier of Studies and Projects HESS Hybrid Energy Storage System

HEV Hybrid Electric Vehicle ICE Internal Combustion Engine IEA International Energy Agency IGBT Insulated-Gate Bipolar Transistor Li-Ion Lithium-Ion

LiOH Lithium Hydroxide Li3CO3 Lithium Carbonate

LFP Lithium-Iron-Phosphate

MOSFET Metal-Oxide-Field Effect Transistor Ni-Cd Nickel-Cadmium

Ni-Fe Nickel-Iron Ni-Metal Nickel-Metal

NCA Nickel-Cobalt-Aluminum NMC Nickel-Manganese-Cobalt NiMH Nickel-Metal Hydride Pb-Acid Lead-Acid

PCB Printed Circuit Board

PHEV Plug-In Hybrid Electric Vehicle PWM Pulse Width Modulation

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SC Supercapacitor

UPS Uninterruptible Power Supply VRLA Valve-Regulated Lead-Acid ZCS Zero-Current Switching ZEV Zero Emission Vehicle ZVS Zero-Voltage Switching

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LIST OF SYMBOLS

∆IM Magnetizing current ripple

∆t Time interval

C1 Decoupling capacitor in parallel with voltage source 1

C2 Decoupling capacitor in parallel with voltage source 2

CC Clamping capacitor

Cf1 Capacitive filter 1

Cf2 Capacitive filter 2

D Duty cycle

D1 Duty cycle from switch 1

D2 Duty cycle from switch 2

D3 Duty cycle from switch 3

DC Clamping diode

fs Switching frequency

IL Inductor current

ILT1 Current in the primary

ILT2 Current in the secondary

IS1 Current through switch 1

IS1_MIN Minimum value of the current through switch 1

IS1_MAX Maximum value of the current through switch 1

Is1_AVG Average current through switch 1

Is1_RMS RMS current through switch 1

IS2_MIN Minimum value of the current through switch 2

IS2_MAX Maximum value of the current through switch 2

I1 Current in the voltage source 1

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I2 Current in the voltage source 2

I2_AVG Average current in the voltage source 2

ILL Auxiliary inductance current

IM Magnetizing current

IM1 Instant value 1 of the magnetizing current

IM2 Instant value 2 of the magnetizing current

IM_AVG Averagemagnetizing current

LL Auxiliary inductance

LM Magnetizing inductance

LT Tapped inductor

n Turn ration

 Converter efficiency

NP Turns in the primary of the tapped inductor

NS Turns in the secondary of the tapped inductor

PC Rated power

PV1 Power in the voltage source 1

PV2 Power in the voltage source 2

RC Clamping resistor RC Parallel resistor/capacitor S1 Switch 1 S2 Switch 2 S3 Switch 3 S4 Switch 4 to Time interval 0 t1 Time interval 1 t2 Time interval 2

ton Time where the controlled switch is turned-on

toff Time where the controlled switch is turned-off

TS Switching period

V1 Voltage source 1

V2 Voltage source 2

VgS1 Gate signal for switch 1

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VgS3 Gate signal for switch 3

VS1 Voltage across switch 1

VS1_MAX Maximum voltage across switch 1

VLL Voltage across the auxiliary inductance

VLL_1st Voltage across the auxiliary inductance in the first operating stage

VLL_2nd Voltage across the auxiliary inductance in the second operating stage

VLM Voltage across the magnetizing inductance

VLM_1st Voltage across the magnetizing inductance in the first operating stage

VLM_2nd Voltage across the magnetizing inductance in the second operating stage

VLT1 Voltage across the primary

VLT1_1st Voltage across the primary in the first operating stage

VLT1_2nd Voltage across the primary in the second operating stage

VLT2 Voltage across the secondary

VLT2_1st Voltage across the secondary in the first operating stage

VLT2_2nd Voltage across the secondary in the second operating stage

1

ˆd Small signal perturbation in the duty cycle from switch 1

2

ˆ

d Small signal perturbation in the duty cycle from switch 2

3

ˆ

d Small signal perturbation in the duty cycle from switch 3

1

ˆi Small signal perturbation in the current I1 ˆ

M

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SUMMARY

INTRODUCTION...27

THESIS STRUCTURE……...28

1 ELECTRIC VEHICLES AND HYBRID ENERGY STORAGE SYSTEMS: AN OVERVIEW……...30

1.1 CHAPTER INTRODUCTION...30 1.2 ELECTRIC VEHICLES...30 1.2.1 History and Evolution...32 1.2.2 Current Prospects...35 1.2.3 EVs in Brazil...36 1.3 BATTERIES...38 1.3.1 Batteries and EVs...40 1.3.2 Lead-Acid Batteries...41 1.3.3 Nickel-Metal Hydride Batteries...42 1.3.3 Lithium-Ion Batteries...43 1.4 SUPERCAPACITORS...44 1.5 HYBRID ENERGY STORAGE SYSTEMS...46 1.5.1 Battery/Supercapacitor Hybrid Energy Storage System………...47 1.6 CHAPTER CONCLUSION...48

2 BIDIRECTIONAL DC-DC CONVERTERS...49

2.1 CHAPTER INTRODUCTION...49 2.2 DC-DC CONVERTERS...49 2.3 BIDIRECTIONAL DC-DC CONVERTERS...51 2.3.1 Integrated Bidirectional Buck/Boost/Buck-Boost DC-DC Converter...52 2.3.2 Bidirectional Boost/Buck DC-DC Converter...55 2.4 CHAPTER CONCLUSION…...57

3 BIDIRECTIONAL DC-DC CONVERTER WITH TAPPED INDUCTOR: STEADY STATE ANALYSIS...………59

3.1 CHAPTER INTRODUCTION...59 3.2 BIDIRECTIONAL DC-DC CONVERTER WITH TAPPED INDUCTOR...59 3.2.1 Forward Buck...61 3.2.2 Forward Boost...71 3.2.3 Forward Buck-Boost...78 3.2.4 Reverse Buck...84 3.2.5 Reverse Boost...91 3.2.6 Reverse Buck-Boost...98 3.3 CHAPTER CONCLUSION...104

4 BIDIRECTIONAL ZVS BUCK-BOOST DC-DC CONVERTER: STEADY STATE ANALYSIS………105

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4.2 BIDIRECTIONAL ZVS BUCK-BOOST DC-DC CONVERTER...105 4.2.1 Forward Mode...108 4.2.2 Reverse Mode...118 4.3 CHAPTER CONCLUSION...124

5 BIDIRECTIONAL DC-DC BUCK-BOOST DC-DC CONVERTER: DYNAMIC ANALYSIS…..……….125 5.1 CHAPTER INTRODUCTION...125 5.2 SMALL-SIGNAL ANALYSIS...125 5.2.1 Forward Buck...127 5.2.2 Forward Boost...128 5.2.3 Forward Buck-Boost...129 5.2.4 Reverse Buck...131 5.2.5 Reverse Boost...132 5.2.6 Reverse Buck-Boost...133 5.3 CHAPTER CONCLUSION...134

6 BIDIRECTIONAL DC-DC CONVERTER WITH TAPPED INDUCTOR: DESIGN METHODOLOGY AND SIMULATION RESULTS...136

6.1 CHAPTER INTRODUCTION...136 6.2 DESIGN METHODOLOGY...136 6.2.1 Sizing of Components...138 6.2.1.1 Magnetizing inductance LM...138 6.2.1.2 Capacitors C1 and C2...139 6.3 CONTROL DESIGN...139 6.4 SIMULATION RESULTS...141 6.4.1 Forward Buck...143 6.4.2 Forward Buck-Boost...147 6.4.3 Reverse Boost...152 6.4.4 Reverse Buck-Boost...157 6.5 UNIFIED CONTROLLER...161 6.6 CHAPTER CONCLUSION...163

7 BIDIRECTIONAL ZVS BUCK-BOOST DC-DC CONVERTER: DESIGN METHODOLOGY AND SIMULATION RESULTS...164

7.1 CHAPTER INTRODUCTION...164 7.2 DESIGN METHODOLOGY…………...164 7.2.1 Sizing of Components...165 7.2.1.1 Magnetizing inductance LM………...165 7.2.1.2 Auxiliary inductance LL and number of turns ratio n……...………….165 7.2.1.3 Capacitors Cf1 and Cf2…….………..168 7.3 SIMULATION RESULTS...168 7.3.1 Forward Mode...168 7.3.2 Reverse Mode...174 7.4 CHAPTER CONCLUSION...179

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8 BIDIRECTIONAL DC-DC CONVERTER WITH TAPPED INDUCTOR: EXPERIMENTAL RESULTS...180 8.1 CHAPTER INTRODUCTION...180 8.2 EXPERIMENTAL PROTOTYPE...180 8.2.1 Choice of Components……….…180 8.2.2 Tapped Inductor ………181 8.2.3 RCD Clamping………...181 8.3 EXPERIMENTAL SETUP………185 8.4 EXPERIMENTAL RESULTS………...186 8.4.1 Forward Buck……….186 8.4.2 Forward Buck-Boost...196 8.4.3 Reverse Boost...206 8.4.4 Reverse Buck-Boost...215 8.5 CHAPTER CONCLUSION...225 CONCLUSION...226 REFERENCES...228 APENDIX A...234 APENDIX B...239 APENDIX C...245 APENDIX D...250 APENDIX E...255 APENDIX F...258 APENDIX G...261

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INTRODUCTION

Since the past centuries until nowadays, people have the necessity to move from a place to other. Looking for food or a place to live as the first civilizations, or just making the way from home to work every day, people have used different ways over the history to go wherever they want/need.

In the last century, due their easy access and operation, cars with internal combustion engine became the most popular transport mean worldwide. However, with the energy crisis in the world and the environmental issues, some alternatives are being searched.

Considering that, Electric Vehicles (EVs) are being studied and considered a key element against this scenario. However, the fundamental problem in EVs, and what makes difficult for them to replace the traditional vehicles with internal combustion engines, is their energy system. Because their high energy density, batteries are widely used as EVs energy bank, but their low power density, low charge/discharge rates, and the fact of certain loads requires high starting current (which is not good for battery lifetime) represents some limitations for the system.

To deal with this problem, Hybrid Energy Storage Systems (HESS) are implemented. Usually, HESS combines different energy sources, and the main reason to this is to combine benefits and features from different power sources. For those reasons, batteries and Supercapacitors (SC) are combined as HESS in EVs where the SC can act like a buffer against large magnitudes and rapid fluctuations in power, improving the system performance.

There are many advantages over SCs that make them good options for some power applications, like high power density, high charge/discharge rates and extended lifetime. But, in EVs, such as the batteries, they cannot fully supply all the system for two main reasons.

 The energy density in SC is low;

 The price of a SC bank is high.

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 Improvement of the battery lifetime;

 Reduction of the stress on battery;

 Reduction in the battery size and cost;

 Improvement in power management (generation/demand);

 SC can recover more energy from the regenerative braking;

 Battery supports slow transients and the SC fast transients.

To interface the battery and the SC in a HESS, the use of DC-DC converter has shown in the literature to be the best way. This converter must be capable to allow both directions of the power flow and increase or decrease the voltage in each power flow direction. In other words, this converter needs to be a bidirectional converter, and act like a Buck or Boost in both directions.

This way, this Master’s Thesis presents the study of 2 bidirectional DC-DC topologies for HESS. For the first topology, all its theoretical study, involving the steady state and dynamic analyzes, is presented in details. Also, a design methodology subsequently verified by digital simulation is proposed and, finally, an experimental prototype for laboratory implementation is built. Then, for the second topology, just the theoretical analysis and a design methodology is presented and verified by a digital simulation.

THESIS STRUCTURE

This present Master´s Thesis is composed, in addition to the appendices, by a general introduction, eight chapters and a general conclusion, where each chapter presents its own introduction and conclusion.

First, the purpose of the present section, the general introduction, is to place the reader in the context of this work, justifying the motivations about the realization of this research.

In Chapter 1, a brief presentation of the topics that support this work are presented, focusing in EVs and their elements, covering from their historical development to their current stage and discussing the role of the power electronics in this scenario.

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In Chapter 2, a review of some concepts involving DC-DC converters and their applications is presented.

In Chapter 3, the theoretical steady state analysis of the first topology presented in this Master´s Thesis, the bidirectional DC-DC converter with tapped inductor, is performed and presented in details, providing fundamental knowledge for the following chapters.

In Chapter 4, the theoretical steady state analysis of the second topology presented in this Master´s Thesis, the bidirectional ZVS Buck-Boost DC-DC converter, is performed and presented in details.

In Chapter 5, the dynamic analysis of the bidirectional DC-DC converter with tapped inductor is performed, leading to all the equations for the control design of the converter.

In Chapter 6, with the knowledge provided by the theoretical analyses made in the previous chapters, a design methodology for the bidirectional DC-DC converter with tapped inductor is proposed, and, to support the design methodology, simulation results are presented.

In Chapter 7, as well as in Chapter 6, a design methodology and simulation results for the bidirectional ZVS Buck-Boost DC-DC converter is presented.

Then, the experimental results of the bidirectional DC-DC Converter with tapped inductor are presented, analyzed and discussed in Chapter 8.

After completing all the stages of this Master´s Thesis, and after the conclusion of all the chapters, the final conclusions and considerations about this work are summarized in a general conclusion.

Finally, from Appendix A to Appendix F, documents and files that were developed in this work, and which are of interest to the reader, are presented.

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CHAPTER 1 ELECTRIC VEHICLES AND HYBRID ENERGY STORAGE SYSTEMS:

AN OVERVIEW

1.1 CHAPTER INTRODUCTION

In this chapter, the topics that hold the proposal of this work are discussed. First, a brief presentation of Electric Vehicles (EVs), covering from their historical development to their actual stage is presented. Then, some important elements of this technology are presented and discussed.

1.2 ELECTRIC VEHICLES

In an era where the environmental issues and the energetic safety are in an outstanding position, EVs are increasing their popularity. By definition, an EV is a vehicle which is pulled by, at least, one electric motor. In other words, it is a vehicle where the electric motor is directly or indirectly linked to the traction of the vehicle (CASTRO, B. and FERREIRA, T., 2010).

In EVs, there is no Internal Combustion Engines (ICEs) and the vehicle is fully powered by electrical energy. This energy can be provided, among others, by fuel cells and solar panels. However, in most of the cases, it is a battery which makes this function.

When analyzing this rising appeal for EVs, Emadi (2005) and Baran and Legey (2010) attribute that especially to the EVs characteristics, and, when punctual those characteristics, highlight the following points:

 Performance Increasing: Electric Motors are more efficient than ICEs. They show performances in the region of 90% while the ICEs show in a region of 40%;

 Better robustness: Electric Motors are reliable, require less maintenance and work silent and smoothly;

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 Energetic safety: According the International Energy Agency (IEA), from 2007 to 2030, the annual increase of energy demand is 1.5%, whereas the oil offer, at the same period, is 1%. In accumulated terms, the energy demand will increase about 40% and the oil offer just 25%. As electricity is a “home energy” and can be produced independent of the oil, EVs are independent of the oil volatility and scarcity;

 Environmental issues: They are “clean”, with no gas emissions. Even if the electricity for their recharge is generated by fossil fuels, the regulation in the generator sources is easier than in EVs costumers.

Nevertheless, EVs also present some limitations, and are those limitations (normally related to their energy system) that do not allow to them a comprehensive market conquest. Thus, if the main problem of EVs is related to their energy system and the same is basically formed by batteries, it is possible to contend that the most part of the EVs limitations are battery limitations. In summary, those limitations are based in 4 main topics:

 High cost: It is estimated that the battery represents more than 50% of the EV final cost (CASTRO, B. and FERREIRA, T., 2010);

 Battery lifetime: With a lot of charge/discharge cycles, and an inefficient recharging method, the lifetime of a battery can be reduced significantly. Even with the care needed, actually, the batteries available do not present an extended lifetime (BARAN, R. and LEGEY, L., 2010);

 Battery recharging: There is no satisfactory infrastructure for this process, and, depending on the battery type, the recharging process can take a considerable amount of time (EMADI, A., 2005);

 Autonomy: The autonomy of a vehicle is directly related to the energy density of the energy source. To make a comparison, gasoline presents an energy density of 12500 Wh/Kg, whereas the Lead-acid (Pb-acid) battery (commonly used in EVs) presents an energy density of 25 Wh/Kg. That is, to have the same density, it is needed an implementation of an expressive number of batteries, making, from the point of view of weight/volume and cost, impracticable the use of EVs.

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1.2.1 History and Evolution

In spite of being in focus nowadays, EVs are not as new as they seem. The first successful EV is dated by 1891 and was created by William Morrison. This vehicle was equipped with a battery that weighed about 350 kg and could reach 14 km/h. Figure 1.1 presents this vehicle.

Figure 1.1 Electric Vehicle by William Morrison Source: May/Jun IEEE Power & Energy Magazine p.66 (2004)

Analyzing the last years of the 19th century and the beginning of the 20th, EVs were exercising an important role in the American market. To get an idea, in 1899, in USA, were sold 1,575 EVs, 1,681 steam vehicles and 936 gasoline vehicles (also called vehicles with Internal Combustion Engines or ICE vehicles) (BARAN, R., 2010).

In 1900, considering the cities of Boston, Chicago and New York, 800 of a total of 2.370 vehicles were electric, 1170 were steamers and just 400 were gasoline-powered (SULZBERGER, C., 2004).

According Sulzberger (2004) and Castro and Ferreira (2010), this scenario can be explained by some characteristics of the EVs and, most important, by the disadvantages of the gasoline vehicles at that time. EVs were silent (lower noise levels and absence of vibrations), clean, simple to operate (lack of transmission) and, with the best ways in the urban perimeters, the main problem of EVs (their autonomy) was not a big concern.

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On the other hand, even the gasoline vehicles presenting some advantages (they could travel fast, could be equipped with powerful engines and had a great range due to the easy access to gasoline), they were noisy, smelly and polluting. To start them, they had to be cranked by hand, process that required a strong arm and often resulted in injuries to the handler (SULZBERGER, C., 2004).

However, this scenario changed quickly. From 1899 to 1909, gasoline vehicles sales grew 120 times, whereas the EVs sales just doubled. With that, in 1912 the fleet of gasoline vehicles was already 30 times bigger than the EVs fleet in New York (BARAN, R., 2012).

For Baran (2012), the EVs fast decline occurred, mainly, due to the following factors:

 In 1912, with the invention of the electric starting and, consequently, the abolishing of the manual starting in gasoline vehicles, the starting process on those vehicles was not a problem anymore;

 The discovery of oil reserves dropped the gasoline price;

 In 1920, the roads in USA already interconnected a lot of cities, then, vehicles capable to travel long distances were necessaries;

 The production series system, idealized by Henry Ford, allowed the reduction of the gasoline vehicles price, becoming them very much cheaper than EVs. With the fast technological development of gasoline vehicles and with the EVs still stuck to the slow development of batteries, the industry of gasoline vehicles continued to grow, and EVs were almost forgotten. Their production was reduced drastically and their use was limited just a few applications, such as trash collecting and delivery service (BARAN, R., 2012).

Thus, the EVs remained neglected until the 1970s, when, with the oil crises and the public opinion starting to concern about the environmental and the use of renewable energies, the major automakers looked back to EVs. However, the technological development in EVs was still a big impediment, preventing the developed prototypes to achieve a satisfactory stage and, consequently, the production lines.

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Nevertheless, in the early of the 1990s, with the sustainable development concept even bigger than in the 1970s and with the progress of the batteries development, the attention came back to EVs. In USA, authorities from California decided that the automakers from that state should provide EVs to the costumers and the California Air Resources Board (CARB), government sector responsible for monitoring the air quality, defined a quota of Zero Emission Vehicles (ZEV) sales of 2% in 1998, increasing to 5% in 2001 and 10% in 2003, with bonus to the automakers for achieving this goal (BARAN, R., 2012). Even so, some sectors, specially the major oil companies, were still reluctant to the EVs implementation.

Combining these 2 situations, the Hybrid Electric Vehicles (HEVs) came to the spotlight. Hybrid vehicles combine, at the same time, an electric motor and an ICE. This way, the advantages from each technology can be combined, remediating the previous problems from each one. Then, in 1997, the Toyota launched to the market the HEV Toyota Prius. In 2000, the Toyota Prius arrived to USA, reaching high sales rates, confirming the importance of investments and researches in this area.

Figure 1.2 Hybrid Vehicle Toyota Prius Source: Internet image

Currently, a new approach of HEVs has become more popular, the Plug-In Hybrid Electric Vehicles (PHEVs). With the capability of recharge the battery from external energy sources, even from a regular household wall socket (origin of the term Plug-In), PHEVs combine and optimize the characteristics of EVs and HEVs, improving the battery and the electric motor capability and decreasing the size of the ICE (LAFUENTE, C., 2011).

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Electric Motor ICE Battery Bank Gas Tank Power Electronics

Figure 1.3 Inside view of a PHEV Source: Adapted from Lafuente, C. (2011), p. 6

1.2.2 Current Prospects

Even EVs not being a recent technology, the new generation of costumers sees them as a novelty. However, they still suffer some distrust from costumers and, added to questions like lack of infrastructure and technological development, their insertion in the market is difficult.

Castro and Ferreira (2010) point another factor that has a huge influence in the EVs insertion in the market: the size and the profile of the vehicles fleet from a country is directly related to its economical development. That is, when a country faces low development levels, the vehicular fleet grows slowly and the costumers keep conservatives. Whereas the personal income of this country rises, the fleet will grow significantly and the costumers are more open to new possibilities (change the conventional vehicles for EVs).

Taking these situations into account, to enable the insertion of EVs and consequently their acceptance and success in the market, government actions are essential.

According Castro and Ferreira (2010), countries like USA, Canada, China, Japan and Germany, among others, are investing in five basic incentives to raise the interest for EVs. They are:

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 Bonus to the buyers: The USA, for example, offers a bonus about US$ 7.500,00 in an EV buying, where some regional laws can extend this value. Other European countries offer similar bonus and, in Japan, this bonus can reach US$ 10.000,00 (CASTRO, B. and FERREIRA, T., 2010);

 Discount on taxes to buyers and manufacturers: It is estimated that until 2020, just in USA, the incentive and help to EVs manufacturers and providers can reach about US$ 25 billion. (BARAN, R., 2012) Also, some provinces in Canada offer discounts up to US$ 2.000,00 in taxes for EV buyers and, in the United Kingdom, EVs have a discount on circulating taxes and are free of parking fees in London downtown (CASTRO, B. and FERREIRA, T., 2010);

 Adoption of restrictions to the conventional vehicles: Many countries are adopting stricter parameters in the regulation of gases emission and, to comply these parameters, the development and improvement of the combustion engine is essential.

 Aid to research: In USA, from 2008 to 2013, the government allocated US$ 95 million a year for a formation of a human capital specialist in EVs, and, for researches involving EVs and the development of batteries, this amount reaches approximately US$ 2.4 billion (BARAN, R., 2012) (CASTRO, B. and FERREIRA, T., 2010);

 Implementation of infrastructure: Some countries with a smaller territorial size, like Japan and Israel, are investing in the implementation of fast recharging points all over their territory.

1.2.3 EVs in Brazil

In the global automotive scenario, Brazil has proven to be a leading country. Being one of the top automakers and relying one of the largest fleets, the vehicular electrification is a key factor in the next years to guarantee its energetic safety and the sustainable development.

It is estimated that, in 2030, with the population growth and the economical development, the Brazilian vehicular fleet will reach the mark of 83.7 million of vehicles, being the 5th in the world, just behind China, USA, India and Japan. This

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growth will represent an increase of 127% if compared with the fleet in 2010 (36.9 million of vehicles) (BARAN, R. and LEGEY, L., 2010).

Nevertheless, even with the imminent growth of the fleet, Brazil walks in the opposite way of the world. There are no governmental politics supporting the production and sale of EVs and they do not enjoy advantages in terms of taxes and fees, just in the research field can be seen some public-privates partnerships and investments for EVs, which is very little if considered the importance that EVs are gaining over the years.

Among these partnerships, the development of a Hybrid Electric Bus by the Alberto Luiz Coimbra Institute of Post-Graduate and Engineering Research at the Federal University of Rio de Janeiro (COPEE-UFRJ) in partnership with companies such as Petrobras and Eletra, the partnership between the Brazilian Financier of Studies and Projects (Finep) and Itaipu Binacional for the development of batteries and storage systems and the partnership between Itaipu Binacional, the automaker company FIAT and the Swiss company Kraftwerke Oberhasli (KWO) for the development of a national EV were the most significant in Brazil. It is important to highlight that, in the later case, the main goal of the project was the vehicular technological development in order to make it cheaper and more accessible, and not a serial production (SPERANDIO, M.; SALDANHA, J. and BASSO, C., 2012).

In figure 1.4, the EV developed by Itaipu, FIAT and KWO is presented.

Figure 1.4 Electric Vehicle FIAT/Itaipu Binacional Palio Weekend Source: Internet Image

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Also, some public calls from the Brazilian National Council of Technological and Scientific Development (CNPq-Brazil) for projects related to the development of EVs technology were relevant in the research field.

Another factor that is directly related to the successful insertion of EVs in the Brazilian market is the support of the National Bank for Economic and Social Development (BNDES), the main long-term credit provider in Brazil. Actions such as a massive marketing for EVs as the technological solution for environmental and transportation issues, support for technological development and production line implementation would be a first step in the EVs expansion (CASTRO, B. and FERREIRA, T., 2010).

Also, the influence of the BNDES in the Flex Fuel Vehicles development can be used as parameter for EVs, where, with the implementation of the actions mentioned above in the beginning of their development, they were able to structure themselves and become the most popular vehicular technology in Brazil.

1.3 BATTERIES

Basically, batteries, also called electric accumulators, are devices that convert chemical energy in electrical energy through a phenomenon known as electrolysis.

The first record of an electric accumulator is dated by 250 B.C., in Syria, where a ceramic container with an iron bar surrounded by a copper cylinder could produce around 1.1 Vdc when full of vinegar (LAFUENTE, C., 2011). Figure 1.5 presents this device.

However, the landmark of the batteries’ history occurred just in 1800, when Alessandro Volta, based on the work of Luigi Galvani about animal electricity, discovered the electrolysis principle and developed the first battery.

In 1859, the French physicist Gastón Plante developed the first Pb-acid rechargeable battery (LAFUENTE, C., 2011). However, according Sulzberger (2004), early Pb-acid batteries were heavy, difficult to recharge, very corrosive, and presented a low power density, around 4 – 6 W/h, requiring approximately between 56 and 80 kg of battery for 0.745 kW/h at the battery terminals. Since then, the

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Pb-acid batteries have passed for constant changes and improvements, both in manufacturing or material means, and today they are the most used battery type in applications requiring energy storage.

Figure 1.5 First electric accumulator Source: Lafuente, C. (2011), p. 9

Nevertheless, the study and use of other materials in batteries development just occurred in the early of 1970s, when Nickel-metal (Ni-metal) and Lithium-ion (Li-ion) batteries were created. According Lafuente (2011), the first Ni-metal batteries were very unstable in the recharging process, problem that was solved with the addition of hydride to the battery composition, resulting then in the Nickel-metal hydride (NiMH) batteries, launched in the market in the early 1990.

Talking about the Li-ion batteries, the first batteries of this type also presented the same problem of the instability in the recharging process, however this problem was solved fast with the exchange of lithium metal by lithium ions in the battery composition and, in 1991, the Sony Corporation started to sell them in Japan (LAFUENTE, C., 2011).

According Castro and Ferreira (2010), the batteries development intensified in the end of 90s and early 2000, when, with the fast advance of sectors such as telecommunications and informatics combined with the spread of mobile devices (cell phones and laptops), the need for smaller devices with more energy storage made the researches in the batteries field grow significantly, resulting in considerable improvements in the battery technology.

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Table 1.1 presents some characteristics of different types of batteries used nowadays.

Table 1.1 Characteristics of different types of batteries

Battery Type Battery Voltage per Cell [V]

Temperature Variation [°C]

Charge/Discharge Rates per Module

Lead-acid 2.1 35-70 600 Nickel-cadmium 1.25 30-50 2000 Nickel-metal hydride 1.4 20-60 600 Nickel-zinc 1.6 40-65 250 Nickel-iron 1.25 40-80 800 Sodium-sulfurous 2.08 300-400 350 Zinc-air 1.62 0-45 70 Lithium-iron 1.66 400-450 500 Lithium-polymer 3.5 0-100 300 Source: Lafuente, C. (2011), pp. 13

1.3.1 Batteries and EVs

Due to their high energy density, batteries are widely used as energy bank in EVs and their main function is the energy storage. Considered the heart of EVs, they are the main element in this technology, making the success of EVs directly proportional to the batteries development.

In EVs, batteries are arranged in modules (more than one battery cell) or in packs (more than one module), as shown in Figure 1.6.

Battery Cell Battery

Module Battery Pack

Figure 1.6 Batteries arrangement in EVs

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According Castro and Ferreira (2010) and Baran (2012), there are some factors that affect the battery choice in EVs. They are:

 Power Capacity: Measured in kW, it is related to the energy transfer. The battery power is a critical factor in EVs, their performance is directly related to how many kW the battery bank can supply;

 Stored Energy: Measured in kWh, it is the parameter that determines the distance to be performed by an EV (the autonomy of the EV) and the weight of the battery system;

 Safety;

 Lifetime: How many charge/discharge cycles and the age of the battery.

 Performance: Performance in different operating temperatures, measurement and thermal management;

 Weight and cost.

Currently, there are 3 types of batteries competing for the establishment of a standard for EVs industry: Pb-acid batteries, NiMH batteries and Li-ion batteries.

1.3.2 Lead-Acid Batteries

The Pb-acid batteries are the most widespread batteries nowadays. Used in applications like Uninterruptible Power Supplies (UPSs), emergency lighting in buildings and ICE vehicles for on-board computer and central locking, among others, those batteries present six cells with 2.1 nominal Volts each, totalizing 12 V in their terminals and, when maintained properly, can present an extended lifetime (LAFUENTE, C., 2011). Figure 1.7 presents a commercial Pb-acid battery.

However, they also present some limitations for EVs applications, such as regular replacement of the electrolyte, mandatory vertical installation and release of hydrogen in the air. Also, for containing dangerous elements (lead and sulfurous acid), some environmental regulations about use, disposal and recycling are applied (CASTRO, B. and FERREIRA, T., 2010).

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Figure 1.7 Commercial Lead-acid battery Source: Internet image

According Lafuente (2011), trying to remedy those problems were created the Valve-Regulated Lead-acid batteries (VRLA), a Pb-acid battery with better performance and capability, using a gel as electrolyte and equipped with a valve to regulate automatically the release of hydrogen.

1.3.3 Nickel-Metal Hydride Batteries

Due to their high energy density, reliability, extended lifetime and allied with the use of the metallic hydride (which does not contaminate the environment), the NiMH batteries are, today, the dominant technology in EVs. Figure 1.8 presents a NiMH battery bank used in EVs.

Figure 1.8 Nickel-metal hydride battery bank for EVs Source: Internet Image

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Nevertheless, limitations such as high cost (due to the elevated use of nickel), weight, heat losses causing a decrement in the efficiency and periodic maintenance are some factors that do not allow these batteries a more significant market conquest.

It is important to highlight that, according Baran (2012), there is no expectation of a growth in the use and technological development of NiMH batteries, while they have practically reached their maximum point of development.

1.3.4 Lithium-Ion Batteries

Nowadays, due to the potential and success already presented in the electronic industry, telecommunication applications and mobile devices, the Li-ion batteries are the biggest bet for the development and future of EVs. Besides, the lithium is not toxic and is a cheap raw material.

The Li-ion batteries are formed, basically, by an anode (negative electrode) usually made of graphite and a cathode (positive electrode) usually derived from Lithium Carbonate (Li3CO3) or Lithium Hydroxide (LiOH). It is estimated that the

cathodes represent 40% of the battery cost. Among the different types of Lithium batteries, some can be highlighted: the Lithium-Nickel-Cobalt-Aluminum batteries (NCA); the Lithium-Nickel-Manganese-Cobalt batteries (NMC) and the Lithium-Iron-Phosphate batteries (LFP) (CASTRO, B. and FERREIRA, T., 2010).

Figure 1.9 presents a Li-ion battery module for EVs.

Figure 1.9 Lithium-ion battery module for EVs Source: Lafuente, C. (2011), pp. 17