Electrical Characterization and Applications of Supercapacitors A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering By Avinash Lanka B. E., Visvesvaraya Technological University, Belgaum, India, 2012 2016 Wright State University WRIGHT STATE UNIVERSITY GRADUATE SCHOOL July 15, 2016 I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SU- PERVISION BY Avinash Lanka ENTITLED Electrical Characterization and Appli- -cations of Supercapacitors BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science in Electrical Engineering. Marian K. Kazimierczuk, Ph.D. Thesis Director Brian Rigling, Ph.D. Chair Department of Electrical Engineering College of Engineering and Computer Science Committee on Final Examination Marian K. Kazimierczuk, Ph.D. Yan Zhuang, Ph.D. Saiyu Ren, Ph.D. Robert E. W. Fyffe, Ph.D. Vice President for Research and Dean of the Graduate School ABSTRACT Lanka, Avinash. M.S.E.E, Department of Electrical Engineering, Wright State Uni- versity, 2016. Electrical Characterization and Applications of Supercapacitors. Supercapacitors have become reliable energy sources in a wide variety of applica- tions such as automobiles, portable electronics, recreational sports equipment, med- ical equipment, aerospace power supplies, etc. Typically, these are electrochemical capacitors with high values of capacitance (tens or hundreds of farads) and possess the characteristics of both traditional electrolytic capacitors and batteries. They are also known as Ultra-capacitors or Electrical Double Layer Capacitors. They work on the principle of electrical double layer for charge storage. Supercapacitor have a much higher energy and power stored per unit volume over conventional capacitors. They also exhibit longer shelf life and cycle life over lithium ion batteries. Supercapacitors are widely used in applications, which require rapid charge and discharge cycles. In the recent trends in the automotive industry, it has become an imperative tool in boosting the battery performance in electric vehicles. In view of the demand for alternative and greener energy resources, this thesis aims to achieve the following objectives: To perform a rigorous literature survey on the current state-of-the-art technolo- • gies relevant to the supercapacitors and their applications. To study the classification, construction, device operation, and features of the • supercapacitors. To investigate the RC model of supercapacitors. • To perform transient and frequency-domain analyzes for a 470 F and 1500 F • supercapacitors. iii To propose two high-frequency models for a super capacitor namely: • 1. Basic RLC model. 2. Accurate RLZ model. To determine the expressions for the total impedance of the two models. • Validate the theoretically obtained results with SABER circuit simulations and • experiments using a Maxwell 2.7 V 350 F general application supercapacitor. To analyze and design buck-boost and boost pulse-width modulated dc-dc con- • verters supplied by supercapacitors and validate the theoretically predicted re- sults using SABER circuit simulator. iv Acknowledgement Firstly, I would like to express my immense gratitude to my advisor Dr. Marian K. Kazimierczuk, whose support, motivation and wisdom has helped me throughout the intricacies of my research. I would also like to thank my thesis committee members Dr. Yan Zhuang and Dr. Saiyu Ren sincerely for their insightful comments and suggestions. I am grateful to the Department of Electrical Engineering and the De- partment Chair, for giving me this opportunity to obtain my MS degree at Wright State University. An unfeigned thanks also goes to my fellow colleagues Agasthya Ayachit and Dalvir Saini, whose support and experience built the confidence in me. Last but not the least, I would like to thank my mother, who stood by me on every decision I made and who has always positively showed me the right path. The sacrifices she made in life always inspire me to achieve more. v Contents 1 Supercapacitors 1 1.1 Introduction ................................. 1 1.2 History .................................... 1 1.3 ElectricalDoubleLayer ........................... 3 1.4 Construction ................................. 5 1.4.1 Electrode and active materials for electrode construction......... 6 1.4.2Electrolyte .................................. 14 1.4.3Separator................................... 14 1.5 Applications ................................. 15 1.6 BusinessTrends ............................... 18 2 Modeling 20 2.1 EquivalentCircuitModel .......................... 20 2.2 Impedance of the Equivalent Circuit Model . 25 2.3 Equations for charging and discharging . 30 2.4 SaberModel ................................. 34 3 Open-Loop Supercapacitor-Powered Buck-Boost Converter 36 3.1 Buck-BoostDesign.............................. 37 3.2 PowerLosses ................................. 40 3.3 Results .................................... 43 4 Open-Loop Supercapacitor-Powered Boost Converter 49 4.1 BoostDesign................................. 50 4.2 PowerLosses ................................. 53 4.3 Results .................................... 55 vi 5 High-Frequency Model of Supercapacitors 58 5.1 RLZModelforSupercapacitors . 60 5.2 ExperimentalSetup ............................. 62 5.3 VoltageRegulation.............................. 68 6 Saber Resuts 71 6.1 Open-Loop Supercapacitor (470 F) Powered Buck-Boost Converter . 71 6.2 Open-Loop Supercapacitor (470 F) Powered Boost Converter ...... 75 7 Conclusions 81 7.1 Summary................................... 81 7.2 Conclusions.................................. 82 7.3 Contributions................................. 83 vii List of Figures 1.1 Electrical Double Layer Capacitor 4 1.2 Maxwell Supercapacitors 5 1.3 Capacitor powered traction vehicle 16 1.4 Supercapacitor capacity forecast 19 2.1 Equivalent model of a supercapacitor [1], [16]. 22 2.2 Equivalent circuit model of a supercapacitor [1]. 25 2.3 Equivalent circuit final impedance. 26 2.4 Output voltage of a 470 F supercapacitor. 27 2.5 Voltage response of immediate branch- 470 F supercapacitor. 27 2.6 Voltage response of Delayed branch- 470 F supercapacitor. 28 2.7 Voltage response of long-term branch- 470 F supercapacitor. 28 2.8 Bode plot for final impedance- 470 F supercapacitor. 29 2.9 Output voltage- 1500 F supercapacitor. 29 2.10 Branch wise voltage response- 1500 F supercapacitor. 30 2.11 Reduced equivalent model of supercapacitor. 31 2.12 Discharging a supercapacitor 32 2.13 Branch-wise voltage responses of 470 F Supercapacitor. 33 viii 2.14 Transmission line model [2]. 33 2.15 Saber RC-model for a supercapacitor. 34 3.1 Equivalent model of a supercapacitor [1]. 36 3.2 Buck-Boost converter powered by a supercapacitor. 38 3.3 Supercapacitor output voltage/ input voltage to the buck-boost converter for CCM. 43 3.4 Duty Cycle of supercapacitor powered buck-boost converter with changing load resistance for CCM. 44 3.5 Duty cycle of the supercapacitor powered buck-boost converter with varying input voltage for CCM. 45 3.6 Duty cycle of the supercapacitor powered buck-boost converter with varying output current. 45 3.7 Efficiency of the supercapacitor powered buck-boost with change in the output current for CCM. 46 3.8 Efficiency of the supercapacitor powered buck-boost with change in the load for CCM. 47 3.9 Efficiency of the supercapacitor powered buck-boost converter with change in the input voltage for CCM operation. 47 3.10 Voltage transfer function of the supercapacitor powered buck-boost converter with change in the input voltage for CCM operation. 48 4.1 Equivalent model of a supercapacitor [1]. 49 ix 4.2 Boost converter powered by a supercapacitor. 51 4.3 Duty cycle of the supercapacitor powered boost converter with varying input voltage for CCM. 55 4.4 Efficiency of the supercapacitor powered boost with change in the output current for CCM. 56 4.5 Efficiency of the supercapacitor powered boost converter with change in the input voltage for CCM operation . 56 4.6 Supercapacitor output voltage/ Input voltage to the boost converter for CCM. 57 5.1 RLC model of a capacitor. 58 5.2 Bode plot of voltage gain of RLC capacitor model. 59 5.3 Improved RLC capacitor model. 59 5.4 Bode plot for impedance of improved RLC capacitor model. 60 5.5 RLZ model for supercapacitor. 61 5.6 Equivalent model of a supercapacitor [1]. 61 5.7 Magnitude and phase plots of impedance of RLZ capacitor model. 62 5.8 350 F 2.7 V maxwell supercapacitor. 62 5.9 Experimental setup 63 5.10 Experimental setup 63 x 5.11 Experimental setup 64 5.12 Magnitude plot of RLZ model-Impedance 64 5.13 Phase plot of RLZ model-Impedance 65 5.14 Comparison of magnitude and phase plots for impedance of RLC and RLZ models. 66 5.15 Resistive part of impedance for RLZ model. 67 5.16 Imaginary part of impedance for RLZ model. 67 5.17 Supercapacitor. 68 5.18 Supercapacitor across a load. 68 5.19 Supercapacitor. 69 6.1 Open-loop supercapacitor powered buck-boost converter. 71 6.2 Input voltage to buck-boost converter. 72 6.3 Output voltage of buck-boost converter. 73 6.4 Inductor current of buck-boost converter. 74 6.5 Supercapacitor current of buck-boost converter. 75 6.6 Open-loop supercapacitor powered boost converter. 76 6.7 Input voltage to boost converter. 77 6.8 Output voltage of boost converter 78 xi 6.9 Supercapacitor current of boost converter 79 6.10 Inductor current of boost converter 80 xii 1 Supercapacitors 1.1 Introduction For many applications in today’s