Project topics and materials
Get Complete Project Material File(s) Now! »
AN ULTRA-CAPACITOR AS ENERGY STORAGE DEVICE FOR POWER CONVERSION APPLICATIONS
The Ultra-Capacitors
An electric capacitor is a passive dynamic one-terminal electric device. In this context, dynamic means the device terminal voltage to current ratio is not a constant and linear. The voltage and current are linked via a differential equation which is in the general case a nonlinear equation. As that, the electric capacitor has capability to store energy as electric charge, more precisely as electric field between the capacitor plates. There are three different types of capacitors, namely electrostatic, electrolytic and electrochemical capacitors. In this dissertation, the electrochemical capacitors, so-called the ultra-capacitors are considered only.
Ultra-capacitors are different from the other type of capacitors mainly because their specific capacitance, [F/dm3] and energy density, [kJ/dm3] are several orders of magnitudes larger than that of electrolytic capacitors. In comparison to electrochemical batteries, the energy density is lower while the power density is larger than that of conventional batteries. Cycling capability is also significantely better compared to batteries. TABLE 2-1 compares the most important properties of the ultra-capacitor versus batteries and other type of capacitors.
Short History of the Ultra-capacitors
The double-layer capacitor effect was discovered and described by Helmholtz in 1879 [69]-[73]. Almost a century after that, a first ultra-capacitor was patented by Standard Oil Company in 1966. A decade after NEC developed and commercialized this device in 1978 [69]-[73]. The first high power ultra-capacitor was developed for military applications by the Pinnacle Research Institute in 1982 [69]-[73]. Ten years after, in 1992, the Maxwell Laboratory had started development of DoE ultra-capacitors for hybrid electric vehicles. Today, the ultra-capacitors are commercially available from a number of manufacturers [74].
Today, the ultra-capacitors are composed of two electrodes separated by a porous membrane, the so-called a separator. The separator and the electrodes are impregnated by a solvent electrolyte. The electrodes are made of porous material such as activated carbon or carbon nano-tubes [69]-[73]. Typical specific surface area of the electrode is about 2000m2/g. Such a large surface area and very thin layer of the charges, in order of nm gives specific capacitance of up to 250F/g [69]-[73]. Rated voltage of the ultra-capacitor cell is determined by the decomposition voltage of the electrolyte. Typical cell voltage is 1 to 2.8V, depending on the electrolyte technology [69]-[73]. To obtain higher working voltage, which is determined by the application, a number of cells must be series-connected into one capacitor module.
Ultra-capacitors as energy storage devices have found very wide application in power conversion due to their advantages over the conventional capacitors and electro-chemical batteries; high energy and power density, high efficiency, high cycling capability and long life.
Overview of Different Technologies
Fig. 2.1 shows taxonomy of the existing types of electrochemical capacitors. Whole family of the ultra-capacitors can be divided into two groups: electric double layer capacitors (EDLC) and pseudo-capacitors. A combination between the EDLC and pseudo-capacitors is group of hybrid capacitors. The EDLC group consists of three-subgroups; activated carbon, carbon nano-tubes and carbon aero gels. In this dissertation, application of activated carbon EDLC is discussed.
Electric Double Layer Capacitors -EDLC
The Ultra-capacitor Structure
In order to increase the capacitance of anultra-capacitor, it is necessary to maximize contact surface are. To achieve this without increaseing in the capacitor volume, one must use a special material for the electrode. This material must have a porous structure and consequently a very high specific surface. The most frequently used material is activated carbon or carbon nano-tubes. In both cases, the specific surface may be as high as 1000m2/g to 3000m2/g. The simplified structure of super-capacitor cell is depicted in Fig. 2.2. The elementary capacitor cell consists of positive and negative current collectors, positive and negative porous electrodes made of activated carbon which are attached on the current collectors, and a separator between the porous electrodes. The separator is material transparent to ions but an insulator for direct contact between the porous electrodes.
Fig. 2.2 Construction of an electrochemical double layer capacitor with porous electrodes (activated carbon).
Since the first development of double layer capacitors, there have been several iterations and models of the basic structure.
The very first work on double layer capacitors was carried out by Helmholtz in 1853. He supposed that the layer in an electrolyte is a single layer of the electrolyte molecules attached to the solid electrode, Fig. 2.3 (a).
where is the solvent electrolyte permittivity and d is thickness of the layer, which equals to the molecule diameter.
The specific capacitance is overestimated compared to the experimentally obtained value. For aqueous electrolyte withR=78 and d=0.2nm, equations (2.1) gives 340 F/cm2, what is much greater than the measured value 10 F/cm2 to 30 F/cm2. Also, the model does not take in account that the capacitance is voltage dependent.
In order to describe voltage dependence of the capacitance, Gouy introduced a theory of random thermal motion in 1910, and considered a space distribution of the charge in the electrolyte in proximity of theboundary between the electrolyte and electrode, Fig. 2.3 (b). A few years later, Chapman defined the charge distribution in the electrolyte as a function of linear distance and properties of the electrolyte.
The Ultra-capacitors Macro (Electric Circuit) Model
In this section, the ultra-capacitor macro model is analysed and discussed. The ultra-capacitor macro model is used for the conversion system control analysis and design, as well as evaluation of the ultra-capacitor losses and temperature in different operating modes.
Full Theoretical Model
The traditional model consists of an ideal linear capacitor and equivalent series resistance (ESR). This simple model cannot be used in a super-capacitor model because two phenomena: (1) the capacitance is voltage dependent, and (2) the time/space redistribution of the charge due to porosity of the activated carbon electrodes. The porous electrode structure behaves as a nonlinear transmission line, [75]-[77]. It is known from theory of electric circuit that an electrically short transmission line can be approximated with Nth order RLCG ladder network. At low frequency, below 100 Hz, distributed serial inductance L can be neglected [77]. Distributed conductance G can be neglected too, except if long term steady state analysis is needed. Thus, an approximated model of an ultra-capacitor having porous electrodes is serial connection of two RC leader networks of Nth order, the separator resistance RSP and the current collector resistances RCP and RCN. A schematic diagram of the approximated model is given in Fig. 2.4.
Table of contents :
PART ONE: Introduction and General Considerations of the Dissertation
1. General Introduction
1.1. Background
1.1.1. Short History of Electric Drives
1.1.2. Present
1.1.3. Typical Applications of Controlled Electric Drives
1.1.4. Remaining Technical Issues in Application of Controlled Electric Drives
1.2. Literature Overview
1.2.1. Regenerative Drives Based on Back to Back and Matrix Converter
1.2.2. Regenerative Drives Based on the Energy Storage Concept
1.2.3. The Mains Current Harmonics and Related Issues
1.2.4. Smoothing of the Input Peak Power
1.3. The Dissertation Objective
1.3.1. Parallel Connection of Energy Storage Device and Controlled Electric Drive
1.3.2. The Mains Current Harmonics, DC Bus Voltage Control and Single Phase Supply
1.3.3. Energy Storage and Power Factor Correction Device for Electric Drive Applications
1.4. The Dissertation Organization
1.4.1. Part One: General Introduction
1.4.2. Part Two: Parallel Connected Energy Storage Device for Controlled Electric Drives
1.4.3. Part Three: Three-terminal Power Factor Correction and Voltage Control Device
1.4.4. Part Four: Three-terminal Energy Storage and PFC Device for Controlled Electric Drives
1.4.5. Part Five: Concluding Remarks and Conclusions
PART TWO: Parallel Connected Energy Storage Device for Controlled Electric Drives
2. An Ultra-Capacitor as Energy Storage Device for Power Conversion Applications
2.1. The Ultra-Capacitors
2.1.1. Short History of the Ultra-capacitors
2.1.2. Overview of Different Technologies
2.1.3. Electric Double Layer Capacitors -EDLC
2.2. The Ultra-capacitors Macro (Electric Circuit) Model
2.2.1. Full Theoretical Model
2.2.2. Simplified Model
2.2.3. The Ultra-capacitor Energy Capacity
2.3. The Ultra-capacitor Charge/Discharge Methods
2.3.1. Constant Resistive Load
2.3.2. Constant Current
2.3.3. Charging
2.3.4. Constant Power
2.4. Frequency Related Losses
2.4.1. How to Calculate Total Losses in Case that the ESR is a Function of Frequency?
2.4.2. The Current is Periodic Function
2.4.3. The Current is Non-periodic Function
2.5. Trends in the Ultra-capacitors Development
2.6. Short Conclusion
3. Ultra-Capacitor Based Regenerative Electric Drives
3.1. Background
3.2. Operational Modes
3.2.1. Definition of the Reference Voltages
3.2.2. Some Experimental Waveforms
3.3. Ultra-capacitor Selection and Design
3.3.1. Voltage Rating
3.3.2. The Capacitance
3.3.3. Current Stress and Losses
3.3.4. Conversion Efficiency
4. Three-Level Interface DC-DC Converter
4.1. Background and State of the Art
4.2. Three-Level DC-DC Converter
4.2.1. Analysis
4.2.2. Filter Inductor LC0
4.2.3. Filter Capacitors CB1, CB
4.3. Design and Selection of the Active Components
4.3.1. Advanced Semiconductor Switches
4.3.2. Voltage Rating
4.3.3. Conduction and Switching Losses
4.4. The DC-DC Converter Design Example
5. Modeling Aspects and Control Scheme
5.1. Modelling Techniques
5.2. The DC-DC Converter Model
5.2.1. Large Signal Model
5.2.2. Linearization and Small Signal Model
5.3. The DC Bus Circuit Model
5.3.1. A General Case
5.3.2. PWM Inverter fed Variable Speed Drives as DC bus load
5.4. The Entire Conversion System Model
5.4.1. Large Signal Model
5.4.2. Linearization and Small Signal Model
5.4.3. Discussion on the Model
5.5. Control Scheme
5.5.1. The Control Objectives
5.5.2. Control of the Ultra-capacitor Current and Voltage Balancing Error
5.5.3. The Ultra-capacitor and the DC Bus Voltage Control
5.5.4. The Controller(s) Synthesis
5.5.5. Simulation and Experimental Results
5.5.6. Discussion on the Current Controller Response Time and the DC Bus Voltage Control
6. Discussion and Conclusions
6.1. Concept of the Ultra-capacitor Based Controlled Electric Drive
6.1.1. The Drive Immunity on the Mains Power Interruption
6.1.2. The Drive Cost Comparison
6.2. Interface DC-DC Converter
6.2.1. Semiconductors Switches
6.2.2. Passive Components
6.2.3. Conversion Losses
6.2.4. Model and Control Scheme
6.3. Conclusions
PART THREE: Three-Terminal Power Factor Correction Device
7. Background and State of the Art
7.1. Background
7.2. State of the Art
7.3. A Novel Half-DC-Bus-Voltage Rated Boost Rectifier
8. Hybrid Half-DC-Bus-Voltage Rated Boost Rectifier
8.1. The Basic Principle
8.1.1. The LFT Realization
8.1.2. The Mains Current Quality
8.2. The DC-DC1 Converter
8.2.1. Analysis
8.2.2. Design Aspects
8.3. The DC-DC2 Converter
8.3.1. Analysis
8.3.2. Design Aspects
8.4. A Design Example
9. Modeling Aspects and Control Scheme
9.1. Model of Series Resonant Converter
9.2. The Entire Rectifier Model
9.2.1. Large Signal Model
9.2.2. Linearization and Small Signal Model
9.2.3. Matlab/Simulink Model Verification
9.3. Control Scheme
9.3.1. The Control Objective
9.3.2. The DC Bus Voltage Controller
9.3.3. Matlab/Simulink Simulation Results
9.3.4. Experimental Results
10. Single Phase Operation
10.1. Short Introduction
10.2. Single Phase Operation of the Half-DC-Bus-Voltage Rated Boost Rectifier
10.2.1. The Principle
10.2.2. A Short Analysis
10.3. The DC-DC1 Converter under Single Phase Supply
10.3.1. An Ideal Circuit Analysis
10.3.2. The Mains Diode Commutation Effect
10.3.3. The Switch and Boost Diode Stress Analysis
10.3.4. Boost Inductor
10.4. The DC-DC2 Converter Operation under Single Phase Supply
10.5. The DC Bus Capacitor Design
10.5.1. The Voltage Ripple
10.5.2. The Capacitor Current Stress
11. Discussion and Conclusions
11.1. Comparison with State of the Art Solutions
11.1.1. Three Phase Operation
11.1.2. Single Phase Operation
11.2. Three Phase versus Single Phase Supply
11.2.1. The Mains Current
11.2.2. The Switches Losses
11.2.3. The Inductor Size
11.3. Conclusions
PART FOUR: Three-Terminal Energy Storage and PFC Device
12. The Principle
12.1. Introduction
12.2. The Principle
12.2.1. The System Operating Modes
12.2.2. The DC Bus Voltage Reference
12.3. Three-terminal Energy Storage and Power Factor Correction Device
12.3.1. Basic Principle
12.3.2. The DC-DC2 Converter Operating Modes
13. Modeling and Control Scheme
13.1. The System Model
13.1.1. Large Signal Model
13.1.2. Small Signal Model
13.2. Control Aspects
13.2.1. The Control Objectives
13.2.2. Control Scheme
13.2.3. Operational Modes
13.3. Simulation and Experiments
13.3.1. Simulation Results
13.3.2. Experimental Results
14. Conclusion
PART FIVE: Concluding Remarks and Perspectives
15. The Dissertation Contribution
16. Conclusions and Perspectives
16.1. General conclusion
16.1.1. The Ultra-capacitor in Electric Drives and Other Power Conversion Applications
16.1.2. Novel Diode Boost Rectifier
16.1.3. All Together
16.2. Perspectives for Future Work
16.2.1. Commissioning and Self–tuning of the System Controllers
16.2.2. On-line Monitoring and the Ultra-capacitor Life Time Estimation
References
Résume En Françias
