Ultracapacitors : future of energy storage 9789383286713, 9383286717

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Ultracapacitors : future of energy storage
 9789383286713, 9383286717

Table of contents :
Title
Contents
1. ELECTROCHEMICAL CAPACITOR
1.1 Introduction
1.2 History
1.3 Basic Principles of Ultracapacitors
1.4 Principle and Energy Storage Mechanism
1.5 EC vs. Aluminum (Al) Electrolytic Capacitors
1.6 Capacitors vs. Batteries
1.7 Benefi ts and Limitations of Ultracapacitors
1.8 Ultracapacitors vs. Flywheels
1.9 Applications
1.10 Cell Voltage
1.11 Comparison with Battery or Other Energy Storage Devices
1.12 Cost of EC Capacitors
1.13 Electrochemical Double Layer Capacitor—Basic Construction
1.14 Components of an Ultracapacitor
1.15 EC Capacitor Sizes
1.16 Future of the Energy Storage Industry
1.17 Uninterruptible Power Supply, Back-Up Power
2. TYPES OF ULTRACAPACITORS
2.1 Basic Construction
2.2 Comparison Chart
2.3 Symmetric and Asymmetric Capacitors
2.4 Types of Capacitors
3. ULTRACAPACITOR CHARACTERISTICS
3.1 Characteristics of Interest
3.2 Equivalent Circuit of Electrical Double-Layer Capacitors
3.3 Equivalent Series Resistance
3.4 Capacitance, Frequency Response and Measurement
3.5 Simultaneous Measurement of Capacitance and ESR Values
3.6 DC Leakage Current (DCL)
3.7 Charge Characteristics
3.8 Self-Discharge
3.9 Factors to Consider in Selecting Optimum Specifi cations
3.10 Ageing
3.11 Cycle Life
3.12 Power versus Energy: Law of Diminishing Returns
3.13 An Ideal Power Buff er
3.14 Low Leakage Current
3.15 Ultracapacitor Performance
3.16 Voltage Drop
3.17 Life of Ultracapacitors
3.18 Comparison with Diff erent Storage Technologies
4. ULTRACAPACITOR CHARGING
4.1 Equivalent Capacitance of an Ultracapacitor Bank
4.2 Size and Number of Cells to be Used in an Application
4.3 Ultracapacitor Charging
4.4 Ultracapacitor Charging-Power Electronics
4.5 Buck Boost Converter
4.6 Supercapacitor Charger Chips
4.7 Special Solutions for Ultracapacitor Charging
5. ULTRACAPACITOR MATERIALS
5.1 Basic Components of Ultracapacitor
5.2 Electrode Materials
5.3 Ultracapacitors of Diff erent Electrode Materials
5.4 Electrolytes in Ultracapacitors
5.5 Separators
5.6 Binders
5.7 Outer Container
5.8 Current Collectors
5.9 Summary
6. CONSTRUCTION OF EC CAPACITORS
6.1 Basic Components
6.2 Preparation of Activated Carbon Electrode
6.3 Coiled Electric Double-Layer Capacitors Manufacturing Process
6.4 Composition with Binders
6.5 Creating the Slurry for Flat Plate Electrode
6.6 Preparation of Electrode from Slurry for Stacked Capacitors
6.7 Cell Preparation
6.8 Activated Carbon
6.9 Technologies for Making Electrodes
6.10 Coin and Multilayer Electric Double-Layer Capacitors
6.11 Flat Cells
6.12 Construction of Prismatic Cell
6.13 Sizes of Ultracapacitors
6.14 Stacking Ultracapacitors
6.15 Summary of Packaging Advantages and Disadvantages
7. ULTRACAPACITOR CELL BALANCING AND MODULES
7.1 EDLC Cells and Modules
7.2 Connecting Cells in Series
7.3 Connecting Cells in Parallel
7.4 Cell Balancing
7.5 Balancing Methods
7.6 Ultracapacitor Modules
7.7 Ultracapacitor System Packaging
7.8 Cell Balancing in Low Duty Cycle Applications
7.9 Safety Provision in Modules
8. HYBRID CAPACITORS
8.1 Hybrid Systems
8.2 Hybrid Capacitor Principle
8.3 Asymmetric Ultracapacitor Systems
8.4 Tantalum Hybrid Capacitors
8.5 Hybrid Electrolytic-Electrochemical Capacitors
8.6 Applications
9. LI-ION CAPACITORS (LIC)
9.1 Lithium Ion Capacitor in Today’s World
9.2 Lithium Capacitor History
9.3 Principle of the Lithium-Ion Capacitor
9.4 Hybrid LIC Construction
9.5 Features of Lithium Ion Capacitor Technology
9.6 Applications
9.7 Nanohybrid-High Performance Li-Ion Capacitor with CNT
9.8 Future Prospects
10. APPLICATIONS IN ELECTRONIC INDUSTRY
10.1 Ultracapacitors in Electronics
10.2 Ultracapacitor as Battery Backup to Alkaline Batteries
10.3 Power Robotic Systems
10.4 Automatic Meter Readers (AMR)
10.5 Consumer Electronics
10.6 High Pulse Power Applications
10.7 Managing Mobile Phone Audio Power
10.8 Wireless Sensor Networks
10.9 Long Life Storage Applications: Personal Emergency Equipment
10.10 Commercial
10.11 Automotive Applications
10.12 Ultracapacitor Prospects in Consumer Electronics
10.13 What Future Holds: Fast Charger Ultracapacitor to Extend Mobile Battery Life
10.14 Guide to Ultracapacitor Design in an Application
11. GRID SYSTEM APPLICATIONS
11.1 Modern Grid Systems
11.2 Energy Storage for the Smart Grid
11.3 Grid Stability and Supply Quality
11.4 Ultracapacitor UPS
11.5 Energy Supply System Considerations
11.6 Ultracapacitors Benefi ts for Grid Storage
11.7 Smart Grid and the “Green” Power Industry
11.8 Cost-Eff ective Regulation
11.9 Renewable Energy
11.10 Renewables Ride-Th rough and Firming
11.11 Wind Energy
11.12 Ultracapacitors Powering New Wind Energy Markets
11.13 Grid Requirements: Low Voltage Ride Th rough
11.14 Power Conditioning
11.15 Passive Devices on Wind Turbine Systems
11.16 Photovoltaic (PV) Power
11.17 Microgrid
11.18 Th e Most Important Facts about Ultracapacitors
12. ULTRACAPACITORS IN VEHICLES
12.1 Ultracapacitors in Vehicles
12.2 Battery Backup
12.3 Power
12.4 Benefi ts and Limitations of Ultracapacitors
12.5 Electric Cars (BEV)
12.6 Power Train Solutions
12.7 Automotive Hybrid Drives
12.8 Boardnet Stabilization, Distributed Power Modules
12.9 Ultracapacitor Modules for Cars
12.10 Fuel Cell Vehicles
12.11 Start-Stop Application
12.12 Formula Zero Go Karts
12.13 Regenerative Braking
12.14 Ultracapacitors for Hybrid Military Vehicles
12.15 Ultracapacitors in Two Wheelers
12.16 Cold Starting / Jump-starting Solutions
12.17 Car Audio Capacitors
12.18 Voltage Stabilizer with 31 F EDLC
13. BUS AND RAIL TRANSPORT
13.1 Ultracapacitors in Transport Industry
13.2 Electric Buses
13.3 Trains
13.4 Energy Storage System (ESS)
13.5 Trams
13.6 Ultracapacitor Regenerative Power Solutions
13.7 Energy Storage for Traction
13.8 Forklifts
13.9 Performance Data
13.10 Fuel Cell Powered Forklifts
13.11 Container Cranes
13.12 Future Scope
14. ULTRABATTERY-ADVANCED BATTERY POWER
14.1 Introduction
14.2 UltraBattery Research
14.3 Charging / Discharging of UltraBattery
14.4 Lower Emissions with Lower Costs
14.5 Applications
14.6 Way Forward
15. MILITARY APPLICATIONS
15.1 Introduction
15.2 Rail Gun
15.3 Coil Gun
15.4 Other Applications
15.5 Space Applications
16. WATER DESALINATION
16.1 Background
16.2 Capacitive Deionization (CDI)
16.3 Membrane Capacitive Deionization (MCDI )
16.4 Capacitive Deionization Process (CDI)
16.5 MCDI Process
16.6 Desalination Performance of Various Carbon Electrodes
16.7 Practical Applications
16.8 Way Forward
17. ULTRACAPACITOR MANUFACTURERS
17.1 Short History of EC Scenario
17.2 Manufacturers Across World Today
17.3 Review of Manufacturers
17.4 Indian Scenario
17.5 Future Prospects
18. PSEUDOCAPACITOR
18.1 Pseudocapacitance
18.2 Pseudocapacitors vs. Other Ultracapacitors
18.3 Pseudocapacitor Materials
18.4 Pseudocapacitor Mechanism
18.5 Advantages and Limitations of Pseudocapacitors
18.6 Metal Oxides
18.7 Conducting Polymers
18.8 Future of Pseudocapacitors
19. NOTES ON USING ULTRACAPACITORS
19.1 Environment
19.2 Storage
19.3 Mounting
19.4 Cleaning
19.5 Mechanical Impact
19.6 Circuit Considerations
19.7 Interconnections
19.8 Safety Issues
19.9 Disposal
19.10 Emergency Response
20. FUTURE SCENARIO
20.1 Modern Day Applications of Ultracapacitors
20.2 Future Application Growth
20.3 New Material Developments
20.4 Cost Reduction
20.5 Ultracapacitors may Overtake Batteries
20.6 Market Trends
20.7 Ultracapacitors in the Grid
20.8 Vehicular Applications
20.9 Ultracapacitors for Wind Turbines
20.10 Water Desalination / Purifi cation
20.11 New Ultracapacitor Work at Constant Voltage
20.12 Electrochemical Flow Ultracapacitor
20.13 Micro Ultracapacitor
20.14 Ultracapacitor for Resistance Welding
20.15 Summary
Appendix 1 List of Ultracapacitor Manufacturers
Appendix 2 Electrochemical Capacitor Selector Worksheet
Appendix 3 Relation of Capacitance to Geometry and Diel. Const.
Appendix 4 Galvanic Series
Appendix 5 UN Document for EDLC Shipping
Appendix 6 UN Document: Asymmetrical Capacitor Defi nition
Appendix 7 Ultracapacitors for Two-Wheelers and Low-Capacity Cars
Glossary
References
Index
Author’s Profi le

Citation preview

ULTRACAPACITORS

‘This book brings together the component technology with system level application information, allowing the technology to be further adopted. Any power engineer should include this book as a must-have reference.’ —Chad Hall, Co-Founder and Vice President of Marketing & Product Management, Ioxus, Inc., New York ‘The book deals with basic principles, manufacturing techniques of ultracapacitors and their applications in variety of fields. This book is the outcome of vast experience and in-depth knowledge of the author in the field of this new type of capacitor. Energy storage is becoming increasingly important for a host of demanding applications. Ultracapacitors are ideal for a variety of new and exciting fields such as electronics, automobiles, and lighting, transmission and distribution systems. The book fills up the need for students, industries and researchers as reference to understand various aspects of ultracapacitors. I strongly recommend the book to practicing engineers, students and researchers. I also hope and wish that the usage of ultracapacitors will spread rapidly in all sectors of industry.’ —Professor (Retired) M.L. Kothari, Department of Electrical Engineering, IIT Delhi ‘Advances in nano technology are enabling capacitor manufacturers to push the envelope in the area of energy storage. Ultracapacitors are beginning to energize all sorts of equipment—from toys and mobile phones to cars and buses. This significant development is aptly captured in R.P. Deshpande’s book. He holistically deals with the subject in his remarkably simple to understand, insightful way.’ —Vinod Sharma, MD, Deki Electronics Limited; Chairman, ESC; Chairman, CII National ICTE Committee

‘With this book, R.P. Deshpande has written about a critical and pivotal technology. Ultracapacitors will figure prominently in the complex future of energy storage and the author provides  a strong, complete foundation for understanding the technology and its commercial considerations along with a snapshot of the competitive/manufacturing landscape in a dynamic and evolving industry.’ —Michael A. Everett, Chief Technical Officer, Maxwell Technologies, Inc., USA ‘The topic chosen by R.P. Deshpande, whose authority in the world of Capacitors today is unchallenged, is virgin, unexplored and unknown to many experienced engineers. The name “Ultracapacitor” itself is prudently selected to describe the extra benefit this industrial equipment offers in bridging the gaps in smooth availability of the vital power for industrialization of emerging economies and developing nations. The beauty of the contents of this book is that it carries reader from the depth of constructional details without any clutter of complex words. After the successful launching of the first book on Industrial Capacitors which was well-appreciated and consumed by industry owners and users, I strongly believe this is another “jewel in capacitors” that R.P. Deshpande has come up with. The book is a compelling must-have in every industry’s reference for driving energy efficiency and harnessing ecological benefits without any noise and environmental pollution in the industrial domain.’ —Himanshu Dalvi, President and CEO, Lighting Business Group & High Mast, Surya Roshni Ltd, New Delhi

ULTRACAPACITORS FUTURE OF ENERGY STORAGE

R.P. DESHPANDE B. Tech. Hon. Elect. (I.I.T., Bombay); Fellow, The Institution of Engineers (India)

Published by McGraw Hill Education (India) Private Limited, P-24, Green Park Extension, New Delhi 110 016. ULTRACAPACITORS Copyright © 2014 by McGraw Hill Education (India) Private Limited No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers. The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication. This edition can be exported from India only by the publishers, McGraw Hill Education (India) Private Limited. Print Edition: ISBN (13): 978-93-83286-71-3 ISBN (10): 93-83286-71-7 Ebook Edition: E-ISBN (13): 978-93-83286-72-0 E-ISBN (10): 93-83286-72-5

Vice President and Managing Director: Ajay Shukla Publishing Manager—Professional: Mitadru Basu Sr. Production Executive: Rita Sarkar DGM—Sales and Business Development—Professional: S Girish Deputy Marketing Manager—Science, Technology & Computing: Rekha Dhyani General Manager—Production: Rajender P Ghansela Manager—Production: Reji Kumar Information contained in this work has been obtained by McGraw Hill Education (India), from sources believed to be reliable. However, neither McGraw Hill Education (India) nor its authors guarantee the accuracy or completeness of any information published herein, and neither McGraw Hill Education (India) nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that McGraw Hill Education (India) and its authors are supplying information but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appropriate professional should be sought. Typeset at Ninestars Information Technologies Ltd, Chennai and printed at Sanat Printers, Kundli Cover Printer: Sanat Printers, Kundli Cover Design: Kapil Gupta

Foreword Electrical energy storage has become a much talked about subject with regard to power supply utilities, hybrid and battery electric vehicles and several other applications by professionals and also certainly by governments, legislators and policy makers. Even professionals have to re-look into the various options and the distinctions provided between physical and chemical forms of electric energy storage that aptly fits them into the required environment. This book takes a critical look at physical storage of electricity in electrochemical capacitors, ultracapacitors. This book covers various aspects of ultracapacitors and is a genuine effort in itself, useful to both the people from industry as well as academicians, and is a wonderful extended effort following the first book on Capacitors by Shri Deshpande. The book covers all aspects relating to ultracapacitor technology, its materials and applications in various fields. Right from the basics of ultracapacitor and its historical background, the book covers in detail different types of ultracapacitors, their classification based on energy storage mechanism and electrode combinations. Special characteristics of ultracapacitors, their reliability, cycle life, properties and other applications are also discussed in detail. Large scale research and product developments are unfolding in the subject of ultracapacitors and many papers are also being presented at various platforms covering new designs, principles and applications. Stored energy has become a matter of prime importance in power grids, vehicular applications, electronics as well as defence, and the future is galore with changes in technology and associated applications. This book takes you systematically through present technology trends, manufacturing processes and present known applications. Also, it attempts to elaborate on developments taking place, emerging applications and market scenarios in the exciting world of ultracapacitors.

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I am confident that this book will be a useful guide in the usage of ultracapacitors for students, researchers, electrical and electronic industries dealing in energy storage devices, as ultracapacitors are one of the most significant among the new emerging energy storage devices with huge benefits with a bigger role to play in this ever increasing quest for betterment of technologies.

(J S S Rao) Principal Director

Foreword In the context of the variability of the temporal need—the available electrical energy is not sufficient for many applications and the conventional solutions generally have adverse cost benefit ratios. The prospect of a new, viable, and practical short-term energy storage alternative is bound to attract the attention of application designers and energy professionals all over the world. Storage of energy in capacitors for use when and where needed has been a well-known solution practiced in such mundane and diverse devices as photoflash, automobile ignitions, explosive detonators, life-saving defibrillators, etc. In these and many other applications, the energy quantum was limited by the voltage to which the conventional capacitors had to be charged. The new technology of ultracapacitors, however, increases this quantum by several orders of magnitude at lower voltages in view of the large capacitance value possible because of the breakthrough in new dielectric materials as well as in a radically new energy transfer and storage mechanism. These features make it a game changing technology. This book outlines the essentials of the technology of ultracapacitors in a lucid, easy-to-grasp yet comprehensive manner that should satisfy the curiosity of lay readers and energy enthusiasts, as well as meeting the requirements of application design engineers from a wide and diverse range of industries. Certainly, the subject is complex and requires a thorough comprehension of the many nuances and facets of the nature, structure, and mechanisms of capacitors. This includes the concepts of ionic adsorption, charge transfer dynamics without the attendant chemical mass transfer (as in batteries) and issues of voltage limitations, energy densities, specific energy, etc. All radically new technologies require for their acceptance a convincing regime of life-cycle testing, reliability evaluation, and how to withstand wear and tear in the hands of end users. The author of this book has an enviable record of life time involvement in industry and has a successful track record of designing commercially successful applications. His treatment of the topics reflects his mastery of the intricacies involved regarding the principles, processes, and products.

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I welcome the pioneering effort of the author and congratulate the publishers for bringing out this timely, topical, and interesting book, which will contribute substantially in the vital arena of efficient energy storage applications in some of the vitally important sectors of our technological support system. M.U. Deshpande Former Professor, IIT Bombay, Former Director, VNIT, Nagpur

Preface Electrochemical capacitor (EC), invented in the 60s, brought a revolution in capacitor technology. A third type of capacitor came into existence—along with static and electrolytic capacitors commonly known till then. This new capacitor makes extensive use of nanotechnology, a science developed over the past few decades, and has changed the way we perceive and use capacitors. The basic function of electrochemical capacitors is energy storage, and it stores much higher energy than hitherto possible, reaching to levels comparable to battery. Farad was all along considered too large a unit for practical application, and microfarad has been the common unit used for most capacitors. All capacitance meters measure values in microfarads, or a few millifarads. Thanks to EC, Farad is no longer too large, and capacitors of several thousand farads are now available. Although capacitors have been known for energy storage, their energy storage capabilities were extremely limited, and batteries dominated the energy storage function. Though the basic principles were known for a long time, EC had to await the advent and progress of nanotechnology for its proper development. Its economic and commercial usage major development has taken place in the past two decades and new technologies are rapidly evolving. Applications of ECs are today found all over, many times without the knowledge of equipment user. While electrochemical capacitor is a generic term, the trade names “Ultracapacitor” and “Supercapacitor” have become synonymous for these capacitors. I have used the term “Ultracapacitor” (or UC) in most part of this book, while electrochemical capacitor (EC) is also used as a generic name. Several variant of this capacitor are made today, each with its own benefit and special characteristics. Of these, Li-ion capacitors, hybrid capacitors and pseudocapacitors find special place in industry. One of the main uses of ultracapacitors is as battery backups. Batteries have been around for over two centuries, and are found everywhere to store energy. However, batteries have a limited life by nature, and have constraints imposed by virtue of the speed at which they can absorb or discharge energy. The ECs connected across batteries take care of sudden loads and deep discharges, and reduce surge loads on batteries, thereby increasing the battery life several fold.

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In certain emergency equipment and remote locations, ultracapacitors serve as alternative to batteries from the point of reliability and accessibility. We may in near future find them in mobile phones in place of batteries, which may be charged in seconds instead of hours. Toys, medical equipment, screw drivers and other equipment which is only occasionally used, can do with ultracapacitors instead of batteries. These can be charged instantly when required, and the equipment is ready for use. Vehicles are increasingly using ECs for various applications—start-stop, regenerative braking, jump start or cold weather starting, radio or auxiliary equipment, battery backup and so on. In fact, development of ultracapacitors was fast-tracked due to necessity of vehicle industry, mainly to take care of cold start in extreme weather. Today fork lifts, cranes, golf carts and even traction locomotives use them for the benefits they offer. Railways use them for energy recovery with huge savings in operational costs. Cranes in ports recover and reuse the gravitational energy while lifting and lowering of loads. Manufacturers are even dreaming of cars powered solely by ultracapacitors. Already Capabus—a city bus powered solely by ultracapacitors is running on streets of Shanghai and more and more buses are being introduced in U.S. and Europe. Inter-city buses using a combination of Li-ion batteries and ultracapacitors are also making their appearance. Golf carts, cranes and campus transport vehicles are serving at many places in the world, powered by ECs. In addition, hybrid vehicles use them as auxiliary power for several functions, including starting of vehicles from rest. Electric supply systems and utilities find ultracapacitor usage for power quality, grid stability, frequency and voltage regulation and as protection against surges due to sudden load changes. Alternative or renewable energies are being developed and used all over the world, but these need storage when the energy is being generated, to be available for use when needed. Ultracapacitors, along with batteries, make a very useful combination in this respect. Electronic gadgets like computers, laptops, mobile phones, cameras and a host of electronic applications use ECs in large numbers today, and many functions of these devices as also miniaturization of mobiles, camera and some other equipment became possible after ECs appeared on the scene. We may soon have instant remote charging for mobiles, powered by ultracapacitors. Military applications require absolute long term reliability, and ultracapacitor modules come in handy for jump start of vehicles, and also starting them even when the battery may be down. Lighting in remote areas may be provided by ultracapacitors charged by solar power. Submarine and underwater missiles may use ultracapacitors since these do not need replacements like batteries. Even spacecraft uses them for powering on-board equipment because of their long term reliability and dependability. NASA is putting big stake in ultracapacitor

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research so as to improve its performance for rocketry and space missions. Space and military have been among main drivers of initial EC developments for mission critical applications despite prohibitive high cost, where benefits justified the investments. Today the costs have come down by over ninety per cent, and they are now within the reach of most commercial applications. Improvements in production methods, scale of production, new materials and improved technologies will see ultracapacitor market go up by leaps and bounds in near future. Major investments are being made in research and production of these capacitors worldwide, and we may find an explosion of their applications, with costs still heading downwards. China, U.S., U.K., Europe, Russia, Korea and Japan have maximum concentration of ultracapacitor manufacturers as of now, and we will see major investments being made in near future. The lifetime of these capacitors is invariably much more than the equipment they are used in. While batteries have a cycle life of a few hundred to a thousand charge-discharge cycle, ultracapacitors may last even a couple of million cycles. Response time in emergency situations is much faster, and even though ultracapacitors today are not used as independent energy sources, when used with batteries, they extend battery life several folds by avoiding deep discharges. Like most electronic products, even ultracapacitors are getting smaller and thinner, and with higher energy and power densities. These will in turn help miniaturization of electronic equipment. I received requests for writing a book on ultracapacitors from my friends and academicians after the release of my earlier book on capacitors. I have tried to cover maximum aspects of ultracapacitor technology, its materials, as well as applications in various fields. The book starts with the basics of ultracapacitor and its historical background. It then goes to describe different types of ultracapacitors, their classification based on energy storage mechanism and electrode combinations. Special characteristics of ultracapacitors, their reliability and cycle life and other properties are discussed in detail. Different types of ECs, like symmetrical and asymmetrical, Li-ion capacitors, pseudocapacitors, hybrids, etc. are discussed in detail. Applications of EC capacitors are then taken up one by one for different sectors like grid, electronics, vehicles, traction and public transport. Ultracapacitor technology principles are now being applied to batteries called ultrabatteris, which are covered under a separate chapter. One chapter is devoted to water desalination with the help of ultracapacitors (Capacitive Deionization Technology, or CDI), which appears a promising field. Ultracapacitor modules, cell balancing, testing and measurements of ultracapacitors are adequately covered. Since ultracapacitors have large stored energy, and a few may use corrosive

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materials, they need care in handling and usage in circuits. Notes on the use of ECs are therefore given to help users and engineers to apprise them on these matters. Appendices at the end give data on several aspects of ultracapacitors, the U.N. safety regulations and tables which will be found useful. Ultracapacitors are subject matter of continued large scale research and developments. Many papers are being presented at various platforms and new designs, principles and applications are unfolding. Stored energy and power availability are on the rise, and we may expect sea changes in the technology associated with applications. This book introduces current technology status, manufacturing process along with applications as of today. An attempt is made to elaborate on developments taking place on new emerging applications and market scenario for this exciting world of ultracapacitors. I trust the book will be helpful in understanding the product, and students of capacitor and energy storage, as well as researchers and electrical and electronic industries will find it useful. Ultracapacitors are one of the most important emerging energy storage devices with immense benefits, and they have much larger role to play in near future. R.P. Deshpande

Acknowledgements While writing this book on ultracapacitors, I received support and guidance from three internationally acclaimed experts. I am grateful to them for their encouraging response, whenever, I approached them for data and technical help. In fact, I was encouraged by each of them to write the book and make it comprehensive enough so that it may be useful to a large section of readers and researchers alike. I would like to thank Maxwell Technologies Inc., San Diego, U.S., for readily accepting my request for assistance. Their Chief Technical Officer, Mr. Michael A. Everett, was an admirable guide and provided me all the data as well as technical guidance that I required. I have extensively drawn material from Maxwell catalogues, papers, and literature for this work. I owe a big debt of gratitude to Maxwell Technologies and Michael for their consistent involvement with this book. Mr. Chad Hall, Co-Founder and VP, Ioxus Inc., N.Y., was also of great help in the shaping of this book. We shared a good rapport, he encouraged me and provided lots of information from the Ioxus literature and papers. Mr. David Evans, Founder and President, Evans Capacitor Company, Rhode Island, U.S., who is among the pioneers of Tantalum-based hybrid capacitors and EDLC, responded to my request with full enthusiasm and provided all the information about his company, the development path of their capacitors, and many other aspects. I would like to acknowledge his words of encouragement and valuable inputs about button type and hybrid capacitors. It would not have been possible to give the book a comprehensive coverage without the help and cooperation of these people. I cherish the relationships that we developed during the course of my work and the bonds of friendship over the time. In addition, I also received valuable support and data from various sources, including Mr. M.H. Lee of Nesscap, Korea, and Mr. John Kim of Vinatech, Korea. Dr. M.L. Kothari, Ex-Professor, IIT Delhi, has vast lifetime academic and professional experience and is currently Emeritus Fellow with IIT Delhi. He critically examined this book and gave suggestions for improvement, just as he did for my earlier book Capacitors: Technology and Trends.

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Acknowledgements

I am grateful to my distinguished friend Dr. M.U. Deshpande, Former Professor, IIT Bombay, Former Director, VNIT, Nagpur, and a well-known personality in engineering faculty, as well as a member of the E-learning Working Group of M. C. & IT, Govt. of India, and Director, Distance Education Council, IGNOU. I acknowledge his valuable suggestions and thank him for writing a foreword to this book. Mr. J.S.S. Rao, Principal Director, National Power Training Institute, has been following my work and was available at every step. He has also contributed a foreword to the book. I received unstinted support from my wife Aruna throughout. My son Abhijeet and daughter-in-law Navita have always stood by me, giving me the strength to write. Friends and well-wishers have helped me by giving their precious time, information, and personal views to improve the book. I am deeply obliged to them all. I would like to thank the entire team of McGraw Hill Education (India), who have published my earlier book also and have worked meticulously towards publishing this book too. R.P. Deshpande

Contents Foreword by J.S.S. Rao v Foreword by M.U. Deshpande Preface ix Acknowledgements xiii

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1.

ELECTROCHEMICAL CAPACITOR 1.1 Introduction 1 1.2 History 3 1.3 Basic Principles of Ultracapacitors 6 1.4 Principle and Energy Storage Mechanism 8 1.5 EC vs. Aluminum (Al) Electrolytic Capacitors 10 1.6 Capacitors vs. Batteries 11 1.7 Benefits and Limitations of Ultracapacitors 12 1.8 Ultracapacitors vs. Flywheels 15 1.9 Applications 16 1.10 Cell Voltage 20 1.11 Comparison with Battery or Other Energy Storage Devices 20 1.12 Cost of EC Capacitors 22 1.13 Electrochemical Double Layer Capacitor—Basic Construction 22 1.14 Components of an Ultracapacitor 24 1.15 EC Capacitor Sizes 25 1.16 Future of the Energy Storage Industry 25 1.17 Uninterruptible Power Supply, Back-Up Power 26

2.

TYPES OF ULTRACAPACITORS 2.1 Basic Construction 28 2.2 Comparison Chart 29 2.3 Symmetric and Asymmetric Capacitors 30 2.4 Types of Capacitors 33

1

28

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3.

ULTRACAPACITOR CHARACTERISTICS 3.1 Characteristics of Interest 51 3.2 Equivalent Circuit of Electrical Double-Layer Capacitors 53 3.3 Equivalent Series Resistance 55 3.4 Capacitance, Frequency Response and Measurement 56 3.5 Simultaneous Measurement of Capacitance and ESR Values 59 3.6 DC Leakage Current (DCL) 60 3.7 Charge Characteristics 61 3.8 Self-Discharge 64 3.9 Factors to Consider in Selecting Optimum Specifications 66 3.10 Ageing 66 3.11 Cycle Life 67 3.12 Power versus Energy: Law of Diminishing Returns 67 3.13 An Ideal Power Buffer 68 3.14 Low Leakage Current 69 3.15 Ultracapacitor Performance 70 3.16 Voltage Drop 70 3.17 Life of Ultracapacitors 72 3.18 Comparison with Different Storage Technologies 73

51

4.

ULTRACAPACITOR CHARGING 4.1 Equivalent Capacitance of an Ultracapacitor Bank 74 4.2 Size and Number of Cells to be Used in an Application 76 4.3 Ultracapacitor Charging 77 4.4 Ultracapacitor Charging-Power Electronics 78 4.5 Buck Boost Converter 80 4.6 Supercapacitor Charger Chips 85 4.7 Special Solutions for Ultracapacitor Charging 86

74

5.

ULTRACAPACITOR MATERIALS 5.1 Basic Components of Ultracapacitor 88 5.2 Electrode Materials 90 5.3 Ultracapacitors of Different Electrode Materials 90 5.4 Electrolytes in Ultracapacitors 108 5.5 Separators 114 5.6 Binders 116 5.7 Outer Container 117 5.8 Current Collectors 117 5.9 Summary 118

88

Contents

xvii

6.

CONSTRUCTION OF EC CAPACITORS 6.1 Basic Components 119 6.2 Preparation of Activated Carbon Electrode 120 6.3 Coiled Electric Double-Layer Capacitors Manufacturing Process 121 6.4 Composition with Binders 123 6.5 Creating the Slurry for Flat Plate Electrode 124 6.6 Preparation of Electrode from Slurry for Stacked Capacitors 124 6.7 Cell Preparation 125 6.8 Activated Carbon 126 6.9 Technologies for Making Electrodes 127 6.10 Coin and Multilayer Electric Double-Layer Capacitors 127 6.11 Flat Cells 129 6.12 Construction of Prismatic Cell 130 6.13 Sizes of Ultracapacitors 131 6.14 Stacking Ultracapacitors 132 6.15 Summary of Packaging Advantages and Disadvantages 133

119

7.

ULTRACAPACITOR CELL BALANCING AND MODULES 7.1 EDLC Cells and Modules 135 7.2 Connecting Cells in Series 136 7.3 Connecting Cells in Parallel 137 7.4 Cell Balancing 138 7.5 Balancing Methods 139 7.6 Ultracapacitor Modules 146 7.7 Ultracapacitor System Packaging 149 7.8 Cell Balancing in Low Duty Cycle Applications 150 7.9 Safety Provision in Modules 151

135

8.

HYBRID CAPACITORS 8.1 Hybrid Systems 152 8.2 Hybrid Capacitor Principle 153 8.3 Asymmetric Ultracapacitor Systems 154 8.4 Tantalum Hybrid Capacitors 163 8.5 Hybrid Electrolytic-Electrochemical Capacitors 168 8.6 Applications 170

152

xviii 9.

Contents LI-ION CAPACITORS (LIC) 9.1 Lithium Ion Capacitor in Today’s World 172 9.2 Lithium Capacitor History 172 9.3 Principle of the Lithium-Ion Capacitor 174 9.4 Hybrid LIC Construction 176 9.5 Features of Lithium Ion Capacitor Technology 177 9.6 Applications 180 9.7 Nanohybrid-High Performance Li-Ion Capacitor with CNT 183 9.8 Future Prospects 187

172

10. APPLICATIONS IN ELECTRONIC INDUSTRY 10.1 Ultracapacitors in Electronics 190 10.2 Ultracapacitor as Battery Backup to Alkaline Batteries 191 10.3 Power Robotic Systems 193 10.4 Automatic Meter Readers (AMR) 193 10.5 Consumer Electronics 195 10.6 High Pulse Power Applications 197 10.7 Managing Mobile Phone Audio Power 204 10.8 Wireless Sensor Networks 206 10.9 Long Life Storage Applications: Personal Emergency Equipment 207 10.10 Commercial 208 10.11 Automotive Applications 209 10.12 Ultracapacitor Prospects in Consumer Electronics 211 10.13 What Future Holds: Fast Charger Ultracapacitor to Extend Mobile Battery Life 212 10.14 Guide to Ultracapacitor Design in an Application 212

190

11. GRID SYSTEM APPLICATIONS 11.1 Modern Grid Systems 214 11.2 Energy Storage for the Smart Grid 215 11.3 Grid Stability and Supply Quality 216 11.4 Ultracapacitor UPS 218 11.5 Energy Supply System Considerations 221 11.6 Ultracapacitors Benefits for Grid Storage 222 11.7 Smart Grid and the “Green” Power Industry 223 11.8 Cost-Effective Regulation 224 11.9 Renewable Energy 224

214

Contents 11.10 11.11 11.12 11.13 11.14 11.15 11.16 11.17 11.18

xix

Renewables Ride-Through and Firming 225 Wind Energy 226 Ultracapacitors Powering New Wind Energy Markets 228 Grid Requirements: Low Voltage Ride Through 229 Power Conditioning 229 Passive Devices on Wind Turbine Systems 230 Photovoltaic (PV) Power 233 Microgrid 236 The Most Important Facts about Ultracapacitors 240

12. ULTRACAPACITORS IN VEHICLES 12.1 Ultracapacitors in Vehicles 241 12.2 Battery Backup 242 12.3 Power 244 12.4 Benefits and Limitations of Ultracapacitors 246 12.5 Electric Cars (BEV) 247 12.6 Power Train Solutions 248 12.7 Automotive Hybrid Drives 249 12.8 Boardnet Stabilization, Distributed Power Modules 253 12.9 Ultracapacitor Modules for Cars 254 12.10 Fuel Cell Vehicles 256 12.11 Start-Stop Application 256 12.12 Formula Zero Go Karts 258 12.13 Regenerative Braking 258 12.14 Ultracapacitors for Hybrid Military Vehicles 259 12.15 Ultracapacitors in Two Wheelers 260 12.16 Cold Starting / Jump-starting Solutions 262 12.17 Car Audio Capacitors 264 12.18 Voltage Stabilizer with 31 F EDLC 265

241

13. BUS AND RAIL TRANSPORT 13.1 Ultracapacitors in Transport Industry 267 13.2 Electric Buses 268 13.3 Trains 271 13.4 Energy Storage System (ESS) 273 13.5 Trams 276 13.6 Ultracapacitor Regenerative Power Solutions 277 13.7 Energy Storage for Traction 279

267

xx

Contents 13.8 13.9 13.10 13.11 13.12

Forklifts 280 Performance Data 281 Fuel Cell Powered Forklifts 282 Container Cranes 284 Future Scope 285

14. ULTRABATTERY-ADVANCED BATTERY POWER 14.1 Introduction 286 14.2 UltraBattery Research 287 14.3 Charging / Discharging of UltraBattery 292 14.4 Lower Emissions with Lower Costs 293 14.5 Applications 295 14.6 Way Forward 296

286

15. MILITARY APPLICATIONS 15.1 Introduction 298 15.2 Rail Gun 300 15.3 Coil Gun 303 15.4 Other Applications 305 15.5 Space Applications 309

298

16. WATER DESALINATION 16.1 Background 311 16.2 Capacitive Deionization (CDI) 312 16.3 Membrane Capacitive Deionization (MCDI ) 313 16.4 Capacitive Deionization Process (CDI) 314 16.5 MCDI Process 316 16.6 Desalination Performance of Various Carbon Electrodes 318 16.7 Practical Applications 319 16.8 Way Forward 320

311

17. ULTRACAPACITOR MANUFACTURERS 17.1 Short History of EC Scenario 321 17.2 Manufacturers Across World Today 323 17.3 Review of Manufacturers 324 17.4 Indian Scenario 336 17.5 Future Prospects 336

321

Contents

xxi

18. PSEUDOCAPACITOR 18.1 Pseudocapacitance 337 18.2 Pseudocapacitors vs. Other Ultracapacitors 338 18.3 Pseudocapacitor Materials 341 18.4 Pseudocapacitor Mechanism 342 18.5 Advantages and Limitations of Pseudocapacitors 344 18.6 Metal Oxides 345 18.7 Conducting Polymers 348 18.8 Future of Pseudocapacitors 348

337

19. NOTES ON USING ULTRACAPACITORS 19.1 Environment 349 19.2 Storage 349 19.3 Mounting 350 19.4 Cleaning 351 19.5 Mechanical Impact 351 19.6 Circuit Considerations 351 19.7 Interconnections 353 19.8 Safety Issues 354 19.9 Disposal 355 19.10 Emergency Response 355

349

20. FUTURE SCENARIO 20.1 Modern Day Applications of Ultracapacitors 357 20.2 Future Application Growth 360 20.3 New Material Developments 361 20.4 Cost Reduction 362 20.5 Ultracapacitors may Overtake Batteries 363 20.6 Market Trends 367 20.7 Ultracapacitors in the Grid 368 20.8 Vehicular Applications 369 20.9 Ultracapacitors for Wind Turbines 371 20.10 Water Desalination / Purification 372 20.11 New Ultracapacitor Work at Constant Voltage 373 20.12 Electrochemical Flow Ultracapacitor 374 20.13 Micro Ultracapacitor 376 20.14 Ultracapacitor for Resistance Welding 378 20.15 Summary 380

357

xxii Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5 Appendix 6 Appendix 7

Contents List of Ultracapacitor Manufacturers 381 Electrochemical Capacitor Selector Worksheet 389 Relation of Capacitance to Geometry and Diel. Const. 390 Galvanic Series 392 UN Document for EDLC Shipping 397 UN Document: Asymmetrical Capacitor Definition 398 Ultracapacitors for Two-Wheelers and Low-Capacity Cars 402

Glossary References Index Author’s Profile

408 415 423 429

Chapter

1

Electrochemical Capacitor 1.1 Introduction Capacitors consist of two metallic electrode surfaces separated by an insulating medium (termed as dielectric). The geometry, as also the properties of the conducting surface and the dielectric, decides the capacitor characteristics such as capacitance, voltage rating, current capacity, etc. Conventionally, capacitors have been categorized as electrostatic and electrolytic capacitors. Electrostatic capacitors use an insulating material in between metallic electrode plates to work as dielectric. They have low capacitance value, do not use electrolytes, and are nonpolar. Plastic, paper, ceramic, Teflon, mica, porcelain, or vacuum types are some examples of electrostatic capacitors. Electrolytic capacitors use solid or liquid electrolyte in their construction, and are characterized by higher capacitance values. Dielectric is an oxide on the metal surface. They are inherently polar in nature (though non-polar back-toback constructions are available for certain applications). Aluminum and tantalum are commonly used electrode metals, while a host of electrolytes are used depending upon the application and type of capacitors. Both the electrostatic and electrolytic capacitors are always limited to microfarads, or sometimes a few millifarads. (Farad has always been considered too large a unit.) Over the past few years, a new type of capacitor has come into existence. This capacitor makes use of nanotechnology scale of highly porous materials for capacitor electrodes, along with an electrolyte. It does not use separate dielectric, but the dielectric layer is naturally formed at the interface of the conductor surface and the electrolyte. The capacitor is known as “electrochemical capacitor”, or EC capacitor for short. The dielectric layer is extremely thin, and depends on the nature of surfaces in contact, as also the electrolyte. However, the energy stored in the capacitor is electrostatic in nature, like any other capacitor, and not electrochemical, as the name might suggest. The high surface area, coupled with the nanometer scale dielectric thickness, gives the capacitor unprecedented capacity to store huge amount of energy. Farad is no longer too large, and capacitance values are measured in Farads, and even Kilo Farads. Instead of using a metal-oxide dielectric, EC capacitors have a dielectric layer that forms naturally with the application of voltage. This dielectric forms in

2

Ultracapacitors

a very thin double layer on the surface of the capacitor’s electrodes. Because of the dielectric double layer, EC capacitors are also known as double layer capacitors (DLC). Electric double layer capacitors (EDLCs) or electrochemical capacitors are electrochemical energy storage devices in which the electric charge is stored in the electrical double layer formed at the interface between electrode and an electrolyte solution. These devices can provide high power capability, excellent reversibility, and long life cycle. Typically, they exhibit up to 200 times or even larger capacitance per unit volume or mass than conventional electrolytic capacitors. Electrochemical capacitors provide a mode of electrical energy storage and delivery, often complementary to batteries. In recent years, a lot of research and investment is being directed to electrochemical capacitors, whose basic function would be storage and supply of electrical energy. Particularly, double layer variant of these, known by alternative names as EDLC (electrochemical double layer capacitors), DESD (digitized energy storage devices), supercapacitors and ultracapacitors have been the subject of intense research and is promising as energy storage devices of future. They are getting all the more importance as these capacitors offer an eco-friendly alternative to battery for energy storage and can work with batteries for backup power peaks. An ultracapacitor is an electrical energy source having virtually unlimited lifetime—longer than any electronic device, and never needs to be replaced. Because of their flexibility, ultracapacitors can be adapted to serve where electrochemical batteries are not well suited. Their intrinsic characteristics make ultracapacitors ideally suited to specialized roles and applications that complement the strengths of batteries. In particular, they have great potential for applications that require a combination of high power, short charging time, high cycling stability, and long shelf life. Ultracapacitors are also gaining importance with the current stress on green technologies. They have, for example, been used in green building, wind turbines, and for frequency regulation in smart grids. Their use is also expanding rapidly in the transportation sector. The Toyota Prius uses them for backup power when braking and some BMW models use them in power assists. Supercapacitorequipped buses have been used in relatively small numbers in California and China is deploying 13,000 electric buses. Ultracapacitors or EDLCs fill an important space in the current set of energystorage devices, bridging the gap between batteries and conventional capacitors. Energy densities greater than electrostatic capacitors make them a better choice for backup applications. They also possess power densities much higher than batteries, allowing them to perform a role in load-leveling of pulsed currents. EDLCs can help to improve battery performance when combined in hybrid

Electrochemical Capacitor

3

power sources, or they can provide an efficient and long-lasting means of energy storage when used on their own. There are two primary uses for ultracapacitors. The first is for temporary backup power and additional short-term emergency power when a primary power source is insufficient. Here ultracapacitors have become an alternative to batteries in applications where the ultracapacitor is charged from the primary power supply, but functions as a backup power source when the primary source fails. The second use for ultracapacitors is in supplying peak power. In these applications, ultracapacitors are used either alone for systems that require peak power delivery or in tandem with batteries for systems that require both constant lowpower discharges for continual function and a pulse of power for peak loads. Here, ultracapacitors relieve batteries of peak power functions, resulting in an extension of battery life and a reduction of overall battery size. Although batteries currently are the most widely used component for primary energy sourcing and energy storage/peak power delivery, ultracapacitors are increasingly being used for energy storage and peak power delivery. Ultracapacitor technologies are evolving to enable a wider range of applications. All have benefited from nanoparticle technology (development of high surface area carbon layers), but one of the most exciting developments in recent years has been the introduction of “proton polymer” technology for the separator system. This technology has the following benefits: High DC capacitance in the 50 mF–1 F range and high capacitance retention at millisecond pulse intervals. A wide range of voltage ratings from 3.6 V to 15 V (and beyond), low ESR (20 mOhms–300 mOhms), low leakage current (2 μA–5 μA), long lifecycle are some of the main benefits. Deep charge–discharge tests of up to 10 million cycles (or 8 months of non-stop testing) do not show any significant effect on these capacitors. Though some applications of ultracapacitors such as memory backup applications are already in widespread use, most applications described above are just in the beginning phase of being adopted. Moreover, ultracapacitors are friendly to environment, help conserve energy, and enhance the performance and portability of consumer devices. Few people are even aware of its existence and it is expected to stay lowly; but the ultracapacitor is a trendy product, right in trend with what people care, what people need, and what people want. The growth potential for ultracapacitor is boundless. 1.2 History The concept of storing electrical energy in the electric double layer that is formed at the interface between an electrolyte and a solid has been known since

4

Ultracapacitors

the late 1800s. Electrochemical capacitors utilize the so-called double-layer capacitance that arises at all electrode interfaces with electrolyte solutions. The concept and model of the double layer appeared in the work of Von Helmholtz (1853) on the interfaces of colloidal suspensions. In 1845, Helmholtz first introduced concepts of the double layer capacitor (DLC) for storing charge on the surface of an electrode, between the electrode and the electrolyte. However, it was only in the early part of last century that a quantum model was developed that could describe the electrochemical processes involved, and the first products based on these concepts were developed only 30 years ago. The first supercapacitors capacitors for commercial applications had capacitance values ranging from a fraction of a Farad (F) to 100 F, but with a low-working voltage range of 2.3 V–2.7 V, and these are routinely used for backup applications today. In most applications, the current requirements are in the micro-amp range, and the ESR values of 5–100 Ω are more than adequate for all these applications. This was subsequently extended to surfaces of metal electrodes by Gouy, Chapman, and Stern, and later in the notable work of Grahame around 1947. However, the first energy storage device was built by Becker of General Electric in 1954 while experimenting with devices using porous carbon electrodes, when they first observed the EDLC effect. They believed that the energy was stored in the carbon pores and the device exhibited “exceptionally high capacitance” although the mechanism was unknown at that time. This first electrochemical capacitor device was patented by Becker (U.S. Patent 2,800,616) in 1957. This was of a crude nature, similar to a flooded battery, both electrodes needed to be immersed in a container of electrolyte, and the device was never commercialized.

FIG. 1.1 SOHIO brochures for “electrokinetic” double layer capacitor.

Electrochemical Capacitor

5

General Electric did not follow up on this work. In 1966, researchers at Standard Oil of Ohio developed the modern version of the devices, after they accidentally rediscovered the effect while working on experimental fuel cell designs. Their cell design used two layers of activated charcoal separated by a thin porous insulator, and this basic mechanical design remains the basis of most electric double-layer capacitors. Robert A. Rightmire, a chemist at the Standard Oil Company of Ohio (SOHIO), got a patent (U.S. 3,288,641) in late November 1966. This and a follow-on patent (U.S. Patent 3,536,963) by fellow SOHIO researcher Donald L. Boos in 1970, form the basis for most subsequent patents and journal articles covering all aspects of EC technology. Both of these works described a so-called “electrokinetic capacitor” utilizing porous carbon in a non-aqueous electrolyte which enabled it to be charged up to about 3 V though the operation of the device was not “electrokinetic” in nature (a misnomer). In 1971, Trasatti and Buzzanca recognized that the electrochemical charging behavior of Ruthenium dioxide films was similar to capacitors. Between 1975 and 1980, researchers from Continental Group Inc., carried out extensive fundamental and development work on the Ruthenium oxide type of electrochemical capacitor (Conway, 1997) which behaves as a surface redox pseudocapacitance. Standard Oil did not commercialize their invention, licensing the technology to Nippon Electric Company (NEC) of Japan. NEC, under license, from 1975 carried out further investigations, developed manufacturing capability, and began to market the ‘supercapacitor’ in 1978. These low-voltage devices had a high internal resistance, and were aimed primarily at the emerging CMOS memory backup application and clock chip backup, providing backup power for these devices in VCRs, clock radios, microwave ovens, and similar consumer electronic goods. NEC produced the first commercially successful double-layer capacitors under the name “supercapacitor.” Russian EC manufacturers have been leading developers of EC technology. Their primary motivation for this was the need for cold weather conditions starting of generators and engines. Cold weather conditions starting of vehicles also gave an impetus to the technology. We can characterize the 30-year history of EC technology perhaps best as one of continual breakthrough development, in which initial cost challenges have consistently led to innovations that not only meet cost concerns but open up new avenues of discovery. Energy storage capabilities have steadily gone up, as can be seen from performance improvements of Panasonic capacitors shown in Fig. 1.2. Electrochemical capacitor technology still has miles to go in terms of technical promise and practical applicability. However, majority applications requiring a long duration of discharge as of today are better suited to batteries. If momentary power requirements are found

6

Ultracapacitors 10000

Energy (J)

1000 100 10 1 1980s

1990s

2000s

2010s

Period

FIG. 1.2 Performance improvements in Panasonic Goldcap capacitors. (Source: John Miller)

to cross the border of a battery’s capabilities, a hybrid EDLC/battery configuration may be an optimal solution. Advantage can then be gained from both the power density of the EDLC and the energy storage of the battery. 1.3 Basic Principle of Ultracapacitors Nanotechnology has made possible materials with extremely high porosity, opening huge potential in the development of these capacitors. The two electrodes are usually made of highly porous carbon, separated by a membrane, which allows mobility of charged ions and does not forbid electrical contact. The electrolyte supplies and conducts the ions from one electrode to other. Like capacitors, ultracapacitors store energy in an electric field, which is created between two oppositely charged particles when they are separated. In an ultracapacitor, when voltage is applied across the two metal plates, a charge builds up on the two electrodes—one positive, one negative. This then causes each electrode to attract ions of the opposite charge. The activated carbon, bonded to the plates, is so porous that it offers a surface area of 10,000–100,000 times greater than the linear surface area. In simple terms, the porosity and crevices in the carbon allow more ions to cling to the electrode. In a broad sense, electrochemical capacitors comprise a wide class of electrical energy storage components in which the symmetrical carbon–carbon cell is the most notable and is commonly referred to as an ultracapacitor. The terminology “ultra” comes from the fact that unlike conventional electrostatic field storage components, the ultracapacitor takes the two main contributors to capacity, surface area (S), and charge separation distance (d) to the extreme. Figure 1.3 illustrates how an electronic double layer capacitor (EDLC) is formed

Electrochemical Capacitor

Porous Carbon Electrode

Electrolyte

Porous Carbon Electrode

Electric Double Layer Capacitor (EDLC)

(a)

7

Negative electrode EDLC

........electrolyte........

Positive electrode EDLC

(b)

FIG. 1.3 (a) Double layer formation; (b) Two capacitors are formed in series.

at each electrode (between electrode and electrolyte). Ions in the electrolyte remain in charge balance, but when an external electric field is impressed, will diffuse to the oppositely charged electrode. The electrodes being highly porous (3,000 m2/g), act as very efficient electron and ion accumulators. Electrochemical capacitors (EC capacitors) do not use separate dielectric material. Dielectric is formed as an extremely thin layer at the contact surface of two different electrode materials. One of the electrodes is highly porous solid, and a liquid electrolyte filling it through increases the effective contact area tremendously. This, coupled with extremely low barrier thickness allows very high capacitance values, even a couple of thousand Farads, to be accommodated in capacitors. They combine the high energy potential of batteries with high-energy transfer rate and fast-recharging capabilities of capacitors. The distance between the electronic and ionic charge is very small, roughly 1 nanometer. The ions and electrons shift locations between charging and discharging. In the charged state, a high concentration of ions is located along the electronically charged carbon surface (electrodes). As the electrons flow through an external discharge circuit, slower moving ions shift away from the double layer. During EDLC cycling, electrons and ions constantly move in the capacitor but no chemical reaction occurs. Therefore, electrochemical capacitors can undergo millions of charge and discharge cycles. Upon the application of a voltage between these two electrodes, the ions form a sheet with a thickness of the order of 1 nm at each electrode, thus resulting in the formation of an electrical double layer. This electrical double layer can sustain a very high electric field (>10 MV/cm) without breakdown. The EDLC or supercapacitor is a hybrid between conventional capacitors and the battery. The electrochemical capacitor is based on the double layer phenomenon that occurs between a conductive solid and a solution interphase. The resulting capacitance,

8

Ultracapacitors

FIG. 1.4 65 F, 14 V Ultracapacitor. Ultracapacitor Module

Individual Ultracapacitor Cell Current Collector

Ultracapacitor Module Schematic

Electrolyte Porous Electrode Separator

FIG. 1.5 Ultracapacitor module and cell. (Courtesy: NREL)

the “double layer capacitance”, is a result of charge separation in the interphase. Electronic charge is accumulated on the solid electrode, while counter charge in the form of ionic charge is accumulated in the solution. The EDLC embodies high-power and high-energy density. In response to positive or negative electric polarization of the electrode, relative accumulations of cations or anions develop, respectively, at the solution side of the charged electrode. If the ions of the electrolyte are not faradaically dischargeable (i.e., no electron transfer can occur across the interface, then an electrostatic electrical equilibrium is established at the interface resulting in a “double layer” of separated charges (electrons or electron deficiency at the metal side and cations or anions at the solution side of the interface boundary), negative and positive, across the interface. 1.4 Principle and Energy Storage Mechanism During modern era, greener sources of energy storage are gaining importance, and ultracapacitors have emerged with enormous storage capacity comparable to batteries. Ultracapacitors function similar to capacitors, gaining potential

Electrochemical Capacitor

9

Magnified Carbon Particle (Vast Amount of Surface Area)

Charged Electrodes

Carbon Particles

FIG. 1.6 Vast and dense surface area of the carbon enables large amounts of energy storage in extremely small space. (Source: K.A. Power)

energy from a buildup of opposite charges on the capacitor plates. Capacitance (in turn, the capacity of energy storage) rises in proportion to surface area of plates, so efforts are always on to maximize the surface areas of capacitor plates. For this purpose, metal electrodes are coated with spongy, porous activated carbon, which increases the surface area by 10,000–100,000 times. The plates are then immersed in an electrolyte of positive and negative ions dissolved in a solvent (which works as dielectric), a polarizable insulator which increases the surface area and the capacitance. EDLCs store the electric charge directly across the interface; this is a true capacitance effect. The mechanism of surface charge generation can be enumerated as surface dissociation and ion adsorption from solution. The capacitance arises from an electrochemical double layer (DL). The thickness of the DL is of the order of 5–10 × 10−10 m depending on the concentration of the electrolyte and size of ions. The double layer capacity is about 10–20 μm2 for a smooth electrode in concentrated electrolyte solution. The electrode material is highly porous carbon particles, in the range from 1000 m2 to 2500 m2 per gram. The current collector is generally a metal foil. The porous separator, filled by electrolyte, is employed to physically isolate the carbon electrodes and prevent electrical shorting of the electrodes. The operating principle of the supercapacitor is similar to that of a battery: pairs of electrodes are separated by an ionic conductive, yet electrically insulating, separator. When a supercapacitor is charged, electronic charge accumulates on the electrodes (conductive carbon), and ions of opposite charge from the electrolyte approach the electronic charge. This phenomenon is known as “the double layer phenomenon”.

10

Ultracapacitors

Electrons accumulate at the negative electrode in the porous carbon where they are bound to an electrolyte ion. The reverse process occurs at the anode (right side of graphic) where electron vacancies in the carbon become attached to electrolyte anions. The electrolyte remains conductive so that displacement currents during charging or discharging have a conductive path to follow between the double layer capacitor at each electrode. 1.5 EC vs. Aluminum (Al) Electrolytic Capacitors Aluminum electrolytic capacitors have excellent pulse power characteristics as they can supply power in the microsecond timeframe. However, large Al electrolytic capacitors are reluctantly used by designers because of their poor energy to cost ratio. Aluminum electrolytic capacitors currently cost between $200 and $400 per Farad. EC capacitors, however, cost $2 to less than $0.1 per Farad, giving a new low-cost option for pulse power applications. A comparison of ultracapacitors with other capacitor technologies is given in Table 1.1. EC capacitors offer a higher ESR than Al electrolytic capacitors. While Al electrolytic capacitors can supply power in the microsecond timeframe, EC capacitors show better characteristics in the nanoseconds to seconds timeframe. If power is needed in the microsecond timeframe, one option is to combine a smaller and cheaper Al electrolytic capacitor with an EC capacitor to optimize the power versus cost trade-off. Table 1.1 Comparison of capacitor technologies. Capacitor Type Current (pk/rms) (A)

Voltage Energy Energy Mass Response (V) (Wh/ (Wh/kg) 1 MJ (kg) Time liter) (seconds)

Electrostatic Polymer Film 200,000/300 900 Ceramic MLCC

0.03

0.024

12,000

10−9

Electrolytic Aluminum electrolytic

450

0.087

0.019

15,000

10−4

Electrochemical Carbon 4,800/150 ultracapacitor

2.7

6.4

5.5

51

1

Electrochemical 2,000/150 Lithium-Ion

3.8

286

123

2

103

?/40

Electrochemical Capacitor

11

Comparison with Battery Systems Energy density in Wh/kg 1,000 1,000 s

10,000 s 100 Batteries 10

Pb

100 s

Li-ion Ni-MH

10 s

NiCd 1s

1

Double layer capacitors 0.1 s

0.1

Electrolytic capacitors

0.01 10

100

1,000

10,000

Power density in W/kg Battery type

Energy density Wh/kg

Power density W/kg

Service life in cycles/years

Lead-acid battery

30-50

150-300

300-1,000/3-5

Nickel-metal hydride battery

60-80

200-300

>1,000/>5

90-150

500->2,000

>2,000/5-10

3-5

2,000-10,000

1,000,000/unlimited

Lithium-ion battery Supercaps (double layer capac.)

FIG. 1.7 Comparison with batteries, electrolytics, and double layer capacitors.

1.6 Capacitors vs. Batteries Energy storage in batteries is in the form of chemical energy. Charging and discharging of batteries are electrochemical processes. There are maximum and minimum rates at which they can be charged or discharged. Electrochemical processes are by nature dependent on ambient temperature. These factors necessitate larger batteries to get enough starting current for automobiles, and also cause starting problems in cold weather conditions. Capacitors, when used with batteries in cold atmospheres, provide heavy discharge current for this purpose, and the vehicle can run smoothly in all weathers. Even in normal conditions, ultracapacitors reduce battery size requirements for vehicles. They can be used to give large pulses of energy in most applications. Capacitors may undergo few hundred start-stop cycles every day without any problems. While batteries store and release energy in the form of chemical reactions, which cause them to degrade over time, ultracapacitors work by transferring surface charges. This means they can charge and discharge rapidly, they can be cycled hundreds of thousands of times.

12

Ultracapacitors Charge

U

Discharge Battery

Discharge Charge UltraCap

Q= U*1 dU/dt

FIG. 1.8 Ultracapacitor and battery discharge curves.

In ultracapacitors, energy is received and stored as electrical energy, and is directly available for use without any time gap. It can work in much wider temperature range than batteries, and even as low as –55°C. The change in capacitance even at –40°C is below 30%, and the capacitor remains functional at both extremes of temperature. The rate at which EDLC can be charged or discharged depends purely on the current that can be permitted, depending upon cables and accessories. Very heavy currents are available for starting an engine or diesel generators. Ultracapacitors are used in a big way as complimentary battery backup, in order to reduce battery size, as also to take over full battery function in engine start applications. (Battery sizes need to be large enough for starting torque in vehicles though smaller sizes are enough while running.) Although batteries excel in energy storage, EC capacitors excel in supplying high power levels. The power density of EC capacitors can reach up to several kW/kg. However, batteries may only reach levels of 0.1 kW/kg–0.5 kW/kg. A comparison of ultracapacitor and battery charge-discharge process is shown in Fig. 1.8. The service life of a battery is rather limited, to a few thousand charge discharge cycles. EDLC by nature, are extremely long lasting, and charge discharge cycles up to 500,000. Thus the life expectancy could go up to 10–15 years and beyond. A battery can be very unreliable, as there is no way to assess how much life has been left in it. When one battery fails, the entire battery bank fails. In contrast, a voltmeter connected across the capacitor gives direct indication of balance charge (or usable energy) remaining on capacitor. 1.7 Benefits and Limitations of Ultracapacitors Ultracapacitors have several disadvantages and advantages relative to batteries as described below. i. Advantages Long life, with little degradation over hundreds of thousands of charge cycles. So they will outlast the entire lifetime of most devices, which makes the

Electrochemical Capacitor

13

device environment friendly. EDLCs can help batteries by acting as a charge conditioner, storing energy from other sources and then using the excess energy to charge batteries at a suitable time. • • • • • • • • • • • • • • •

Low cost per cycle High power output Quick response: fast charge-discharge Energy efficiency: 95%–98% (battery efficiency 70%–80%) (Lead acid battery needs 30% more power than it stores) Good reversibility Very high rates of charge and discharge Extremely low internal resistance (ESR) and consequent high cycle efficiency (95% or more) and extremely low heating levels High specific power. The specific power of electric double-layer capacitors can exceed 6 KW/kg at 95% efficiency Improved safety, no corrosive electrolyte, and low toxicity of materials Simple charging methods—no full-charge detection is needed; no danger of overcharging In some applications, the EDLC can supply energy for a short time, reducing battery cycling duty, and extending life Excellent battery backup. Avoids deep discharge of battery Increases battery life Weighs one-fifth of a battery Can even replace batteries

A comparison of characteristics of batteries and ultracapacitors given in Table 1.2 shows that the two can be excellent complementary to each other, Table 1.2 Performance comparison of ultracapacitors and batteries. Item

Ultracapacitor

Battery

Charging time

1–30 sec.

1–5 hours

Discharge time

0–30 sec.

0.3–3 hours

Energy density (Wh/kg)

5–15

20–150

Power density (W/kg)

2000–10,000

20–150

Charge/discharge efficiency

95%–98%

60%–80%

Recycle life

500,000–1,000,000

500–1000

Temperature range

–50–+70°C

–10–+40°C

14

Ultracapacitors

while ultracapacitors may even replace batteries in some applications requiring short-term performance. ii. Disadvantages The amount of energy stored per unit weight is generally lower than that of an electrochemical battery (3 kW/kg–5 Wh/kg for an ultracapacitor although 85 Wh/kg has been achieved in the lab as of 2010 compared to 30 kW/kg– 40 Wh/kg for a lead acid battery), 100 kW/kg–250 Wh/kg for a lithium-ion battery and about 1/1,000th the volumetric energy density of gasoline. • Highest dielectric absorption of any type of capacitor. • High self-discharge rate – considerably higher than that of an electrochemical battery. • Low-maximum voltage – series connections needed for higher voltages and voltage balancing is required. • Voltage across ultracapacitor drops significantly as it discharges. Effective storage and recovery of energy requires complex electronic control and switching equipment with consequent energy loss. • Very low-internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard similar to any other capacitor of similar voltage and capacitance (generally much higher than electrochemical cells). Even though batteries show poor performance in pulse power applications, they have been extensively used due to their low cost and the lack of any other options. They have high internal resistance (ESR). Thus in order to get desired high power; batteries have to be greatly over-sized. Further, high current levels severely stress batteries and shorten their useful life. Batteries also have a poor life cycle performance (700–1500 cycles) and some battery technologies also require frequent maintenance. For pulse power applications requiring pulse power durations of a few seconds or less, EC capacitors will provide a much smaller and lighter solution. The over 100,000 lifecycle performance and zero maintenance significantly reduce the replacement and maintenance costs. In applications requiring duration greater than a few seconds, EC capacitors should be combined with batteries so that the power and energy needs of the system are optimized. The EC capacitor can supply the initial power pulse, and the batteries take over the long-term energy needs. The result is a much smaller and lighter system, longer battery life, and long-term cost savings. Cost comparison of ultracapacitors and Li-ion battery is shown in Table 1.3. It can be seen that for high power, cost/KW of EDLC is the lowest, whereas batteries are cheapest for total energy output.

Electrochemical Capacitor

15

Table 1.3 Cost of ultracapacitors and batteries. Average Lithium Ion Power Cell1

100 F Ultracapacitor

Li-ion Battery to Ultracap Cost Ratio

Cost per kW

$28.802

$2.96

9.7

Cost per Wh

$0.80

$14.81

0.05

Cost per Wh-Cycle

$0.00083

$0.0000154

54

1.8 Ultracapacitors vs. Flywheels Ultracapacitors can be compared with flywheels as they possess similar response times and power delivery characteristics. A cursory idea of the two technologies can be seen from Table 1.4. Flywheels have security matters associated with them. The flywheel technology is not as mature as ultracapacitors, and ultracapacitors are used more frequently and preferentially for most applications. In practice, with a porous electrode, the situation is much more complex, since the matrix of microscopic pores within the macroscopic electrode structure offers a complex,  series/parallel  equivalent circuit comprised of a distribution of ohmic (resistive) and capacitive elements which leads to the whole electrode having a non-uniform effective resistance and capacitance, dependent on frequency (in a/c modulation) or on the time-scale of pulsed or non-constant rates of charging. As shown in the classical work of de Levie (1963), the equivalent circuit for such electrodes is that of a transmission line. Table 1.4 Ultracapacitor vs. flywheel. Flywheel

Ultracapacitor

System power ratings (kW)

2–500

5–1000

Specific energy (Wh/kg)

15–150

0.5–4

Specific power (W/kg)

900–20000

50–3000

6–100

0.8–4

Power density (MW/m )

0.3–40

0.3–1

Maximum cycles

>100 000

>100 000

Discharge time range

4–60 sec.

1–60 sec.

Life expectancy (hours)

175 000

100 000

Cost ($/kW)

300

500

Efficiency (%)

90–93

90–95

3

Energy density (kWh/m ) 3

16

Ultracapacitors

However, in development of electrochemical capacitors, using aqueoussolution electrolytes, the above distributed–resistance effect has been substantially minimized so that devices having high operating power have been successfully engineered and marketed. Nevertheless, with non-aqueous electrolyte capacitors, which have higher operating voltages up to 3.0 V–3.5 V (hence 9 to 12 times energy density, which depends on the square of maximum operating voltage), the distributed ohmic effects are more significant so that achievable operating power levels are less than those attainable with aqueous electrolyte devices. 1.9 Applications Typical applications in electronics are pagers, personal-data assistance devices, and cell phones. The GSM phone will require a 200-Hz response time to improve the transmit burst in a digital phone system. In these devices, high power is more important than energy density. Therefore, to get the desired frequency response, ultracapacitors use aqueous electrolytes that provide much lower resistance. To attain these frequencies, carbon electrodes need to be thin, with large pores for rapid ion transport through the material. Automotive applications range from hybrid drive trains to power network stabilization to the “electrification” of braking, steering, air conditioning, and other subsystems to improve the fuel efficiency and reliability of the 50–60 million passenger vehicles that roll off assembly lines around the world each year. The ultracapacitor bridges the gap between conventional capacitors and batteries. Though its energy density is 5% or less compared to that of a battery, it is well suited for energy storage applications where high currents are involved and where conventional batteries have deficiencies. The ultracapacitor has a number of advantages over batteries: Table 1.5 Applications of ultracapacitors. Market domain

Application

Automotive

Hybrid drives, board net stabilization, distributed power

Trains, trams

Regenerative braking, voltage stabilization

Buses

Regenerative braking

Aerospace

Electric actuators or latches

Pitch systems

Emergency power for windmill blades

Robotics

Peak and backup power

Telecom

Backup power

Electrochemical Capacitor

17

Performance

The future is in high voltage, cost effective solutions

Cost

Development Focus

2 Volts

Today’s technology is acceptable for low voltage commercial applications 250 Volts

Commercialized

600 Volts

Advanced

1500 Volts

Emerging

State of the Technology

FIG. 1.9 Technology drivers for ultracapacitors. (Courtesy: Ioxus)

– It can be charged and discharged almost indefinitely, whereas few batteries can last 1000 cycles. – It can provide high discharge currents. Batteries experience reduced life if exposed to frequent high-power pulses. – It is made from non-toxic and relatively inexpensive materials. – It can be charged instantaneously, while batteries are damaged by fast charging. – It requires no maintenance and is robust to environmental extremities such as arctic temperatures. An ultracapacitor and battery combination offers the performance of the former with the greater energy storage capability of the latter. It can extend the life of the battery and save on the replacement and maintenance, while battery can be smaller, and enable applications with slimmer profile; and at the same time, increase available energy by providing high-peak power whenever necessary. Figure 1.9 shows applications of ultracapacitors emerging in various fields– electronics, vehicles, and grid systems along with accompanying voltage range of capacitor banks. Various applications of ultracapacitors are listed below: i. All-weather quick start applications The current car battery is geared up to meet peak power needs during engine startup even in coldest weather conditions that impair battery performance.

18

Ultracapacitors

Ultracapacitors can supply the seconds-long peak power unaffected by the weather and permit the battery to be downsized and its useful life extended. The current catalytic converter in cars sends untreated exhaust gas into the environment for a few minutes until it is warmed up and begins functioning. Ultracapacitors can quickly preheat the catalytic converter and enable it to function immediately. ii. Load-leveling and uninterruptible power systems (UPS) Ultracapacitor is not a viable substitute for the battery in UPS as a long-term power source, mainly due to its low-energy storage capacity. However, as a shortterm support for UPS, its rapid response means that it can act as a temporary bridge until an alternative power source comes in. Moreover, the ultracapacitor in the UPS serves a load leveling function by absorbing power surges and spikes, and then releasing clean quality power essential for precision high-tech equipment. iii. No maintenance applications Many buoys in sea lanes emit light during night time using the energy captured from the sun and stored in the battery. The battery needs replacing every couple of years, and the servicing of these widely scattered buoys is an expensive undertaking. The light buoys can be made practically maintenance-free if ultracapacitors are used instead of batteries to store solar energy. Construction sites and road hazards need to be warned of by lighted signs and markers at night time. By using a solar panel and ultracapacitors in the same housing, maintenance-free signs and markers can be produced. These can be quickly set up in field conditions whenever needed without going through expensive and time-consuming wiring process. iv. Peak pulse power applications Unlike analog equipment that draws a steady current, a digital wireless communications device loads the battery with short, heavy current spike during its transmit mode. If an ultracapacitor is added to the system, it can take over the task of providing intermittent pulse power while the battery functions as a supplier of steady current. Users benefit from longer talk time between charges and from the extension of battery life. v. Quick charge applications Ultracapacitors can be charged in seconds, whereas batteries require hours of charging time. Wireless power tools with an ultracapacitor can be charged just

Electrochemical Capacitor

19

before use without any waiting time. Moving toys such as miniature racing cars are also applications that can benefit from quick charge properties of the ultracapacitor. vi. Memory backup applications Already widely used in consumer electronics products, small-size ultracapacitors protect user data and clock information from being lost during short-period power outages or, in case of portable devices, during replacement of batteries. For this use, ultracapacitor is better than battery because it is cheaper and requires no replacing during the lifetime of the application device. vii. Automotive applications and electric vehicles (EV) Use of ultracapacitors for generative braking can greatly improve the fuel efficiency of cars under stop-and-go urban driving conditions. Only ultracapacitors have both storage capacitance and high current handling capability to capture and store large amount of electrical energy generated by braking within a short time and to release it again for reacceleration. The generative braking has the potential to be one of the biggest applications for large-size ultracapacitors. The ultracapacitor can enhance the performance and competitiveness of an EV. It permits faster acceleration, extends the range by generative braking, and extends battery life by freeing it from stressful high-power tasks. viii. Pulse power Applications that can benefit from EC capacitors include medical (x-ray and MRI), welding (spot and contact), audio line stiffening, actuators, large electric motor starting, and power quality such as UPS systems (initial pulse power—no battery replacement). ix. Energy storage Although EC capacitors excel in pulse power situations, EC capacitors are also finding use in energy storage applications. In applications requiring only a few seconds of energy, EC capacitors can provide a smaller and cheaper solution because batteries are usually severely over-sized for this short time span. In addition, some industries such as aerospace and mining have very high maintenance costs and do not find the very small explosion potential of batteries suitable. As the price of EC capacitors continues to decline due to economies of scale, this technology will push into other energy storage areas now serviced by batteries. Critical UPS systems are one possible area as batteries can fail without warning, whereas EC capacitors can be monitored to indicate when they are approaching the end of their life.

20

Ultracapacitors

x. Traction One day, material handling vehicles and electric vehicles may use EC capacitors rather than batteries. EC capacitors in traction applications offer the benefits of simple on-board charging, quick opportunity recharging, higher life cycle, maintenance-free 24-hour operation, and superior cold weather performance. However, cost is a main driver for this application. 1.10 Cell Voltage EC capacitors are assembled by combining individual cells in series. The decomposition potential of an EC capacitor’s electrolyte limits its cell voltage. For a capacitor with an aqueous electrolyte, the cell voltage is limited to between 0.8 and 1.6 VDC. The voltage for a cell using a non-aqueous electrolyte can reach as high as 3 or 4 VDC. However, use of a non-aqueous electrolyte does require the use of dry rooms, vacuum chambers, and other expensive processing methods that result in significantly higher production costs. Voltage rating of electrochemical capacitor is solely limited by the decomposition potential of its electrolyte. For aqueous electrolyte like KOH, this limits the usable cell voltage to 0.8 V–1.6 V. Use of nonaqueous electrolyte may increase this to 3 V–5 V. To get higher voltages, series parallel stacks of capacitors are used, each rated about 1.3 V. For example, 11 capacitors in series may be used to get (11 × 1.3 = 14.3 V) 14 Volt capacitor. 1.11 Comparison with Battery or other Energy Storage Devices Energy is stored in an EC capacitor by charge transfer at the boundary between the porous electrode and the electrolyte, and depends on the size of contact area, the size of ions, and the level of electrolyte decomposition voltage. The two electrodes of an EC capacitor are the porous electrode and the electrolyte. The high porosity carbon electrodes provide large area, defining the energy density of the capacitor. An important difference between charging a capacitor and charging a battery is that there is always an increase of voltage on charge (or decrease on discharge) of a capacitor as the charge per cm2 is increased or decreased. An ideal battery has a constant voltage during discharge or recharge except as the state of charge approaches 0% or 100%. Although specific energy by weight is much below that of batteries, the power delivery is very high, and can reach many times that of battery. A comparison of energy and power densities between electrolytic capacitors, batteries, fuel cells, and ultracapacitors in Fig. 1.10 brings this out clearly.

Electrochemical Capacitor

Specific Energy (Wh/kg)

1000 100

10,000s

21

1,000s

100s

Fuel Cells

10s Batteries

10

1s Super Capacitors

0.1s

1 0.01s 0.1 Electrolyte Capacitors 0.0 10

100

1000

10000

100000

Specific Power (W/kg)

FIG. 1.10 Energy and power density comparison of capacitors, batteries, and fuel cells.

While they function formally in storing or delivering electric charge, their mechanisms of charge storage are quite different, in most cases, from those in batteries. Thus, EC capacitors are not substitutes for batteries but are usually complementary to them for charge storage or delivery. They can offer advantage of fast charging or discharging rates over most batteries of comparable volume. Their high-power densities enable them to be employed in interesting complementary ways in hybrid systems with batteries. Ultracapacitors are also durable from the perspective that they do not have any memory effects, or issues with full or partial discharges that effect their overall service lifetime. They are high energy devices that can dump the energy very quickly, allowing them to react to power dips, and other stability phenomenon. Nano Markets/Smart Grid Analysis of the market opportunities in the smart grid space has come to the conclusion that these advantages will bring important new business revenues to the ultracapacitor business. Ultracapacitors are much lighter than batteries of equivalent energy storage capacity. Further, they charge very fast and discharge also very rapidly. Fast discharge means very rapid response, not possible with battery storage. Charge-discharge life cycle is unsurpassed, with a life of over 100,000 cycles, or sometimes even in millions. Storage capacity is independent of number of charge-discharge cycles. Energy density is 10 to 100 times that of conventional capacitors (up to 70 MJ per cu. m.), while charge discharge efficiency can go as high as 98%. Capacitance values range 5 F/cm2, and can reach 650 F/gm. Normal capacitance vales of 0.047 F–2700 F are common, while 5000 F units are also available. Nominal cell voltage of 2.3 V–2.8 V being common, while Li-ion capacitors may go to 3.8 V. Combination and modules are available up to 400 V, and currents may vary from 3 A to 600 A.

22

Ultracapacitors

Most batteries fail to operate at subzero temperatures, whereas EC capacitors can work from −40°C –+50°C, and up to 70°C – 85°C. The leakage rate is extremely low compared to batteries. 1.12 Cost of EC Capacitors Ultracapacitor costs were prohibitively high initially, and their use was restricted to critical uses. However, with developments in materials and increase in production volumes, the costs have come down drastically; and as a result, the usage has increased over a wide range of applications. Table 1.6 shows the reduction levels over the years: Table 1.6 Cost reduction of ECs over time. S. No.

Year

Cost/Farad ($)

Cost/KJ ($)

1

1996

0.75

281.55

2

1998

0.40

151.23

3

2000

0.10

32

4

2002

0.023

7.51

5

2006

0.010

2.85

6

2010

0.005

1.28

1.13 Electrochemical Double Layer Capacitor: Basic Construction The EDLC (ultracapacitor as it is commonly known) construction is based on the principle shown in the Fig. 1.11. Generally two active porous carbon electrodes may be used, with electrolyte filling the remaining space, and a separator used between these two sections. Two layers of dielectric are thus formed, and hence the name “double layer capacitor” or EDLC. Metal can Carbon electrodes

Separator soaked in electrolyte

FIG. 1.11 Electrochemical double layer capacitor.

Electrochemical Capacitor

23

An ultracapacitor or supercapacitor can be viewed as two nonreactive porous plates or electrodes immersed in an electrolyte, with a voltage potential applied across the collectors. A porous dielectric separator between the two electrodes prevents the charge from moving between the two electrodes. Figure 1.12 depicts the charged and uncharged states of ultracapacitor to show its operation. The carbon electrode (porous collector) of EDLC is made from activated carbon particles painted or rolled on a metal foil collector. Energy stored in ultracapacitors is very high. Conventional capacitor has energy density of 0.15 Watt-hour (Wh) per kg, whereas EDLCs may store up to 15 Wh. (1 Wh = 3600 watt-sec = 3600 Joules). Energy storage of these capacitors is their biggest asset, and the stored energy levels are comparable to that of a battery. This fact led Evans Capacitor Company to name one of its ultracapacitor series as “capattery”. Efforts are on to increase the density, when it will store nearly half the energy compared to a similarly sized Li-ion rechargeable battery. Li-ion capacitors already store up to 21 Wh/Kg –25 Wh/Kg. Targets are pointing to the possibilities of approaching 40 Wh/Kg or more in near future. In fact, one manufacturer JOEL, Japan, is hopeful of increasing this level up to a maximum of 75 Wh/kg. By comparison, Ni-MH battery packs around 45 Wh/kg. While present energy densities are much below that of batteries, their power delivering capabilities of up to 2000 W/kg are ten times that of batteries. The basic equations for capacitors are: C = 0 k A /d E = ½ C V2 For higher energy, we need to have: • Higher k • Larger area A • Thinner dielectric In ultracapacitor materials, effective area of the porous electrode is 2000–3000 sq. m./g., while thickness d is in nanometers. The electrolyte gives an extremely thin oxide layer in nanometer thickness at the electrode interface, C activé

Électrolyte

C activé

+ –

(a) Uncharged state FIG. 1.12 Ultracapacitor principle.

(b) Charging

e–

24

Ultracapacitors

and is chosen to have high permittivity. The ratio of surface area to chargeseparation distance (dielectric thickness) in ultracapacitor has gone to 10 raised to 12th power. This is what makes the capacitor “ultracapacitor”. Capacitance of 60 F–360 F per gm is typically available with such materials. Considering the above equation for capacitance now, 0

= 8.8542 ×10−12 F/m (or C2 N−1 m−1)

A is in 2000 sq. m. per g, and d is in nanometers. It is seen that 10−12 in 0 is cancelled out by A and d parameters, and we enter Farad range. Actual capacitance in farads depends upon d, the miniscule separation in nanometers, and k, the dielectric constant of the separation barrier. All efforts of increasing the energy density are directed to these factors – making the charge separation thinnest possible and choice of chemicals to get high k. Of course the third major consideration of increasing effective area/g is of paramount importance. 1.14 Components of an Ultracapacitor The basic components of an EC capacitor consist of: 1. The metallic current collectors (This could also be the outer can). 2. Nanoporous conducting electrodes like carbon composites, Li-ion doped material, etc. 3. Electrolyte, which penetrates the porous electrode and forms the oxide layer. 4. Separator, which does not allow the two electrodes to touch each other. 5. Sealing, which keeps the contents hermetically sealed from atmosphere. Ultracapacitors of all designs or class contain these basic components, and the composition or mix of different constituents create different varieties of capacitors having separate characteristics. These will be discussed in Chapter 2. When the electrolyte comes in contact with the porous electrode, it forms a thin layer of oxide at the interface, the thickness of the layer being of the order of a few nm. The layer has low-breakdown strength of 0.7 V–1.8 V, depending upon the electrolyte and the electrode material. The electrode and each of the two electrodes for a capacitor, and these two capacitors naturally form a series connection. If one capacitor has strength of 1.4 V, the two in series will have combined BDV of 2.8 V, and the resulting total capacitance will have a rating much below this limit.

Electrochemical Capacitor

25

Separator Activated carbon electrode TERMINAL OUTER PP CASE Aluminum case

SEALANT

ANODE PELLET

Seal rubber

SEPARATOR ANODE PELLET

Terminal

(a)

CATHODE

(b)

FIG. 1.13 (a) Wound ultracapacitor construction; (b) Button type EC construction.

The result is most EC capacitors using have rated voltage between 2.3 V and 2.8 V, while some, using different materials for one electrode, can go to 3.6 V–3.8 V. However, the very high area allows very high capacitance, and the energy stored is extremely high compared to conventional capacitors, the stored energy being comparable to that of a battery. The electrode shape, component assembly, etc., are decided on the end use and energy requirement. The electrodes may be in the form of nanocarbon deposited and bonded on a metal foil, and wound on an automatic coil winding machine, with a separator sheet (Fig. 1.13a). Tabs are taken out from the foil for external connections. Alternatively, metal sheet having a deposit of the electrode material is punched into disc shape, and then assembled in a button type container to produce button cells (Fig. 1.13b). A third construction is many times used, called stacked construction, where metal bonded electrodes are arranged alternatively, and connected in parallel to increase surface area of each electrode. These capacitors are usually rectangular shaped. 1.15 EC Capacitor Sizes Ultracapacitors for electronic applications start from low values of 0.047 F button cell type. Flat stackable capacitors are also made in a number of sizes. EC capacitors up to 50,000 F have been made in a single cell, while capacitors up to 3000 F are becoming common. 1.16 Future of Energy Storage Industry The energy storage industry is changing rapidly with increased renewable energy opportunities and advancement of technology. Supporting this, ultracapacitors

26

Ultracapacitors

are primed for a high adoption rate in 2012. Numerous applications can benefit from high power, cost efficient ultracapacitors. According to NanoMarkets/Smart Grid Analysis, the smart grid supercapacitor market will reach $3.8 billion in 2015. Today, however, the market for these systems is worth only about $0.4 billion with by far the biggest chunk of revenues coming from one specialized application, namely, regenerative energy capture with load smoothing for light rail applications. Latest report on the topic, however, suggests that new applications, especially those related to power quality and grid instability applications, are likely to be driven significantly forward by the impressive gains that smart grid supercapacitors have been able to achieve. Compared with conventional capacitors, supercapacitors offer much more charge to be stored per volume. This is achieved through increased electrode surface area and the addition of a liquid electrolyte. Ultracapacitors in the market today use activated carbon as the electrode material. The charge is stored via charge separation and alignment of dipoles in the electrical double layer. The thinness of this layer along with its large electrode surface area allows the super-sized capacity of supercapacitors compared to conventional capacitors. Unlike batteries, charge is separated, but no electrochemical redox reactions occur. Although ultracapacitors have been little more than a niche product for certain high-priced storage applications for a number of years, recent technology and materials improvements suggest that they will have a growing role in practical large-scale storage applications in the smart grid in future. While the 100-Farad (F)-and-below-range of supercapacitors are used in many consumer applications, and are not suitable for large-scale electrical storage, those in the range of 1000 F–5000 F are being used for large-scale grid quality and shortterm UPS applications. 1.17 Uninterruptible Power Supply, Backup Power Backup power systems are now an integral part of most mission-critical installations. Wide range of services such as data centers, communications networks, and plant operations rely on the continuous availability of quality power. Small backup systems traditionally rely on batteries for energy storage, while larger systems may use a generator set or more complex systems such as flywheels, superconducting magnetic energy storage, or fuel cells. Each of these technologies has limitations – batteries are notoriously unreliable, flywheels needed more maintenance than originally thought, and generator sets and fuel cells have poor turn-on response. Ultracapacitors used alone, or with longer term energy sources such as gensets or fuel cells, are the latest in high reliability backup power.

Electrochemical Capacitor

27

For installations that need up to about a minute’s worth of support, ultracapacitors come into their own, whether to buffer poor power transients, allow an orderly shutdown of equipment or transfer to a secondary power source. Considering that the majority of power outages are in the order of seconds, ultracapacitors can relieve the genset or fuel cell from having to come on at all – reducing overall system wear and tear and thus maintenance and costs. As mentioned, ultracapacitors have fast response times and can deliver hundreds of thousands of complete cycles with minimal degradation of performance. Furthermore, cycle depth is not an issue: so ultracapacitors can be microcycled (cycled by less than 5% of their total energy) or full-cycled (by more than 80% of their total energy) with the same long life. Table 1.7 Current capabilities of energy storage technologies. Storage Technology

Pumped Hydro

Compressed Batteries Flywheels SMES Capacitors Air Energy Storage (CAES)

Energy storage 375 K discharge pulses tested

Low-temperature capability

Typically –40°C

Low-temperature capability not affected by aging

High-temperature capability

Typically up to 85°C

Supercapacitors designed to meet 85°C requirement

Low self-discharge

Minimum (averaged over time) self-discharge

Leakage current in μA range from 30 to 65°C

The power supply must effectively run internal system at a relatively constant rate, and also handle the power peaks of wireless connectivity, which sharply reduces the life of most batteries. The power source has also to operate at temperatures ranging from -40°C to +70°C. By using ultracapacitors instead of more traditional energy sources such as Li-ion or NiMH batteries, life expectancy of the power supply in remote transmitting devices is extended to over ten years—representing a 100% to 300% improvement over lead acid batteries. The small cell in each unit is also lighter and smaller, and facilitates a simpler design. Ultracapacitors are slightly more expensive in initial investment, but substantial overall per unit cost savings are achieved through the smaller footprint of the new ultracapacitor-based power supplies. 10.5 Consumer Electronics Portable electronics today contains more features and functions than any time in the past. Present day cameras, video, high fidelity audio, GPS navigation, and wireless communication equipment can be carried in pockets. Despite all the capabilities, these are often limited in use because of the batteries. Large power pulses in moving camera motors, burning images, and bursts of information over wireless systems, all overstress existing batteries, severely limit their practical life. Further, the need for smaller and more lightweight systems is increasing, and innovative approaches are called for to reduce size and heaviness without sacrificing overall performance and reliability of the products. Ultracapacitors are replacing batteries in consumer products where they are charged from the primary power supply, but function as a backup power source when the primary source fails or peak power has to be provided. Consumer appliances are getting smaller, more portable, and have greater functionality than ever before. Aggressive trend toward more features in smaller packages demands lighter, smaller battery power systems which can provide long run times.

196

Ultracapacitors

(a) Notebook PC

(b) Digital music players

(c) e-Book

FIG. 10.2

Ultracapacitors are complementary or standalone solution that surpasses the power and performance limitations of batteries or capacitors. With their ability to store large amounts of energy, release the energy in fast bursts (or pulses), and recharge rapidly, ultracapacitors meet the high power demands of the latest consumer appliances like digital still cameras (DSCs), notebook PCs, digital music players, toys, and e-Books (Fig. 10.2). a) Notebook PCs Short battery life is still one of the limitations of notebook PCs. A laptop is only functional as long as it gets enough energy to power it. Ultracapacitor can reduce excessive power load and thus extend the battery’s run time. It can be used to optimize battery utilization by buffering power demand and smooth fluctuating power loads. It can be charged and discharged thousands of times without degradation, offering extra power during peak load times such as hard drive usage, CD disk writing, or DVD play operations. Ultracapacitors are also used in numerous PCMCIA cards powering notebook PCs today, and enable battery swaps while the notebook is in sleep mode. This complementary system reduces user delays as well as design costs, using existing boost, and buck converters as necessary. b) Digital music players Digital music players record and play music compressed into the MP3 format and can carry thousand or more songs in memory, giving truly portable large music libraries. As with many other mobile appliances, these thin, palm-sized devices are extending the limits of existing battery technology with their increasing power demands and upcoming use of wireless transmission and converged functionality.

Applications in Electronic Industry

197

While DMPs operate on standard rechargeable, AA or AAA batteries, ultracapacitors can provide the power bursts needed for reading a disk drive or enabling wireless transmission. This extends the play time by supporting the battery operations, and enables the design of smaller, lighter devices. c) E-Books E-books enable people on the move to download and store up to 500 books to read anywhere, anytime. Typically powered with four AA batteries, the latest generation of e-Book readers uses new display and electronic ink technology in a compact, lightweight device about the size of a paperback novel. Wireless Internet connectivity gives these appliances the mobility and data access consumers need. Ultracapacitors can enable e-Book battery system to provide the power pulse capabilities to manage peak power demands of specialized display technologies. They also extend usage time and enable the design of smaller, lighter, more cost effective consumer devices. 10.6 High Pulse Power Applications a) Digital cameras Digital cameras have frequent pulse loads—major high-peak demands are observed during the microprocessor activity, writing to disk, and LCD operation. Expensive rechargeable batteries and costly charging circuits are being replaced with embedded ultracapacitors and conventional, disposable alkaline batteries. Alkaline batteries have considerable amount of energy, but normally they alone cannot deliver the power requirements of these cameras. By coupling alkaline batteries with an ultracapacitor, batteries deliver continuous energy and the ultracapacitor delivers pulses of power. Figure 10.3 shows the voltage swing for a defined cycle. It can be seen that the voltage swing is large with rechargeable batteries, but is significantly reduced with the alkaline– ultracapacitor combination. Addition of ultracapacitors to the product allows the use of inexpensive alkaline batteries, which in the long run saves money and allows ease of use. Figure 10.4 shows that the capacitor performance has not changed even after 16 sets of battery changes, giving the same number of cycles for a new battery. Many present day compact digital cameras or smartphones use an LED flash in place of a Xenon flash. The required high current for these cannot be supplied by the battery alone, e.g. if two LEDs provide a flash capability on a mobile phone, they would need about 2– 4 A for a time of about 33– 40 milliseconds. An ultracapacitor comes to the rescue in such situations.

Ultracapacitors 7

7

6

6

5

5 Voltage

Voltage

198

4 3

4 3

2

2

1

1

0

0

200

400 # of Cycles

600

800

0

0

200

400 600 # of Cycles

800

FIG. 10.3 Voltage swing for rechargeable alkaline batteries and alkalineultracapacitor combination. 650

# of Cycles

600 550 500 450 400 350 300

1 2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 battery packs

FIG. 10.4 Capacitor performance unchanged even after 16 sets of battery changes.

b) Digital still cameras (DSCs) Digital still cameras (DSCs) require a substantial amount of energy delivered at very high power for the switch-on, zoom, autofocus, and image capture stages of operation. Additionally, new technologies such as white LED feature lower power consumption but have high-peak power needs. These demands place an extreme load on batteries, quickly lowering the voltage, triggering premature shut-down (even though the battery still has stored energy), and thus restricting the number of images that can be recorded. In a typical digital camera (Fig. 10.5) with an ultracapacitor-enhanced design, one 10F ultracapacitor works with a battery to provide overall system power management. The ultracapacitor drives the initial power and drives functions involved in composing photographs, such as microprocessor, zoom, and flash. Ultracapacitor complements the battery, discharges power during peak loads and recharges between peaks, enabling the camera to operate up to 200% longer and extending the useful life of batteries.

Applications in Electronic Industry

199

FIG. 10.5 Digital cameras.

c) Wireless systems and burst-mode communications Wireless systems using burst-mode communications are used in local areas such as stock and commodity exchange floors, warehouses, and even in restaurants. Order takers and information processors use wireless terminals and modified PDAs to communicate real-time information to a central data centre. Designers are integrating ultracapacitors with battery packs, freeing commodity traders from having to change batteries during the day. Wireless appliances are increasingly becoming feature rich and power hungry. Mobile telephones, personal digital assistants, and wireless network cards all use high-peak power pulses while transmitting, and can overload existing batteries and modem card power sources. The solution is often found in extralarge batteries sized for peak power. Since this option has a size and weight penalty in device design, there is need for more innovative power solutions to satisfy the power pulse demands and size/weight requirements of these devices. These small, thin size, and energy storage ultracapacitors are ideally suited to meet the high-power pulse needs of wireless components and appliances. Their low ESR and high capacitance values extend battery life, reduce design costs through component elimination, and enable a fast marketing. d) Converged handhelds Converged or ‘smart’ handhelds integrate the wireless, voice, and data capabilities of PDAs and mobile phones into a single device. However, other convergent devices merge mobile phone cameras with MP3 players and PDAs, and tools that bring together GPS trackers with personal or emergency alarms. Power demand of these combined devices on the battery system is enormous. Ultracapacitors can meet the power challenges to run extra features, extend battery run time, and improve low-temperature operations, enable memory backup, reduce battery load, and can even replace tantalum capacitors to provide a lighter-weight, thinner device.

200

Ultracapacitors

(b)

(a)

(c) FIG. 10.6 (a) Wireless devices; (b) and (c) PCMCIA cards.

e) PCMCIA and compact flash Laptop computers are very efficient mode of mobile computing today. Many of their communicative networks make them indispensable with their superior processing power. Modern laptops and notebooks often have built-in wireless connectivity, but some who do not have Wi-Fi connectivity need an extra card that plugs into them to give the additional facility. Many Internet providers supply such cards which work efficiently in laptops. This card is the PCMCIA card, known as the PC card. (The PCMCIA is an acronym for personal computer memory card interface adapter). This card depicted in Fig. 10.6(b) and (c) is a credit card-size memory or I/O device that is inserted in a personal computer terminal, usually a notebook or laptop. The most common PCMCIA card is the 28.8Kbps modem for notebook computers. Wireless devices (Fig. 10.6a) capable of delivering 2.5G, 3G, 802.16, and 4G communication services face a critical shortage of power. Pulse transmissions generally exceed the power limitations of wireless modem devices such as PCMCIA, CompactFlash, USB and mini PCI (peripheral component interconnect) cards, leading manufacturers to seek components that consume less power and even add batteries to the products. Compact Flash, which conventionally used Xenon tubes, are being replaced by LED flashes (BrightFlash, for instance). A comparison of the two systems throws the following benefits of ultracapacitor-based LED flash.

Applications in Electronic Industry

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Table 10.3 Comparison of Xenon flash and ultracapacitor-based LED flash. Xenon

BriteFlash LED Flash with Supercapacitor

Bulky: *Large electrolytic storage capacitor *Total volume of xenon solution in Sony *Ericsson K800 ~3.8cc and 7mm thick

Small and thin *Prismatic ultracapacitor and LEDs *Typically 4000 V. Special arcing to other circuits measures and/or clearance is required to prevent arcing to other circuits Mechanical shutter required to prevent overexposure: extra cost, size, and power

Works with a rolling shutter. No mechanical shutter required

High voltage and current pulse for xenon strobe causes electro magnetic interference (EMI)

High current delivered from ultracapacitor, EMI easier to manage

Need a separate LED for video/torch mode

Same LEDs used for flash and video/ torch

Long time to re-charge electrolytic capacitor between photos (~8s for Sony Ericsson K800)

Short time to re-charge ultracapacitor between photos (~2s)

Electrolytic capacitor cannot be used for any other peak power needs

Ultracapacitor can meet all peak power needs in the cell phone including: *Flash pulse *GPS readings *RF transmission for GPRS *Audio

*Very high-powered light delivered in