ABSTRACT
The development of any nation and its economy at large is centred on reliable and efficient power supply. Many developing nations are yet to actualise this feat, and therefore, there is need to strategize to meet the ever-rising demand for energy. Battery-based inverter systems have over the years provided additional power supply to critical loads, in the absence of public power supply. However, high peak charging/discharging power demands results in the degradation of the battery. Hybrid combination of batteries and other technologies such as super-capacitors can help to provide a single power system with both high energy and power densities. The aim of this project is the design and performance evaluation of a battery/super-capacitor hybrid storage system for renewable energy applications as this can provide an energy source with high power density and high energy density. The paralleling of the battery and the super-capacitor to obtain a single source was established using an Automatic Switching System. Results showed that considerable amount of initial transient current was supplied by the super-capacitor. The peak current drawn from the battery is reduced, which increases the lifetime and performance of the battery. This results in high cost saving as well as improved reliability, which has become increasingly necessary in the alternative power technology.
CONTENTS Page
CONTENTS viii
CHAPTER TWO: LITERATURE REVIEW 7
CHAPTER THREE: MATERIALS AND METHODS 25
CHAPTER FOUR: RESULTS AND DISCUSSION 47
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION 55
LIST OF TABLES
LIST OF FIGURES
| Figure 2.1. | Circuit diagram of LTspice XVII implemented in the design | 9 |
| Figure 2.2. | Comparison of different battery technologies | 11 |
| Figure 2.3. | Hierarchical classification of super-capacitors and capacitors of related types | 12 |
| Figure 2.4. | How Super-capacitors functions | 14 |
| Figure 2.5. | Charge profile of a super-capacitor | 14 |
| Figure 2.6. | Discharge profile of a super-capacitor | 15 |
| Figure 2.7. | Scheme of several rechargeable batteries and SC, and their evaluation with BSCHs. | 19 |
| Figure 2.8. | Circuit of a super-capacitor in parallel with a battery and its Thevenin’s equivalent and frequency domain | 22 |
| Figure 3.1. | Block diagram of the proposed battery/super-capacitor hybrid storage system | 25 |
| Figure 3.2. | A 16.2V/83.33F super-capacitor bank | 26 |
| Figure 3.3. | 12V/200Ah deep cycle battery | 27 |
| Figure 3.4. | Circuit diagram of the automatic switching system | 29 |
| Figure 3.5. | Block diagram of Automatic Switching System | 30 |
| Figure 3.6. | Typical illustration of a protection circuit | 31 |
| Figure 3.7. | Block diagram of a dc power supply | 32 |
| Figure 3.8. | Voltage regulation circuit diagram | 33 |
| Figure 3.9. | Logic Unit | 36 |
| Figure 3.10. | Internal circuitry of the ASS | 37 |
| Figure 3.11. | View of the finished fabricated ASS | 37 |
| Figure 3.12. | Circuit diagram of LTspice XVII implemented in the design | 39 |
| Figure 3.13. | Experimental Setup of the design | 40 |
| Figure 3.14. | Pictorial view of the experimental setup | 40 |
| Figure 4.1. | Voltage profile of the battery/super-capacitor hybrid storage system on no load | 48 |
| Figure 4.2. | Voltage profile of battery/super-capacitor hybrid storage system for inductive load | 49 |
| Figure 4.3. | Waveform of the input voltage | 51 |
| Figure 4.4. | Waveform of the voltage after transformation | 51 |
| Figure 4.5. | Waveform of the output voltage after rectification | 51 |
| Figure 4.6. | Waveform of the output voltage after filtering | 52 |
| Figure 4.7. | Combined waveform of the input, transformed and the filtered voltage | 52 |
LIST OF ABBREVIATIONS
| AC | Alternating Current |
| ASS | Automatic Switching System |
| BSH | Battery/Super-capacitor Hybrid |
| CRO | Cathode Ray Oscilloscope |
| DC | Direct Current |
| EDLC | Electric Double Layer Capacitors |
| EMS | Energy Management Strategy |
| ESS | Energy Storage System |
| EV | Electric Vehicle |
| HESS | Hybrid Energy Storage System |
| HEV | Hybrid Electric Vehicle |
| LIB | Lithium-ion Battery |
| LPF | Low Pass Filter |
| MOV | Metal Oxide Varistor |
| PUS | Power Utility Source |
| SC | Super-capacitor |
| SPICE | Simulation Program with Integrated Circuit Emphasis |
CHAPTER ONE INTRODUCTION
Background to the Study
Backup power systems have become significant in recent technology due to the necessity to provide uninterrupted electric power supply to loads, critical in nature such as security doors, computers, surgical equipment, automated teller machine (ATM), and broadcast and telecommunication equipment (Okundamiya et al., 2014a). Conventionally, standby systems depend on batteries for storage of energy but superior systems may employ generator set. Nonetheless, each of these technologies has drawbacks. In particular, storage batteries offer high discharge proficiency but as the load current increases, the rate capacity outcome significantly degrades their proficiency. Batteries are particularly unreliable (ageing, short service life, low density power, low life cycle and high charge time). Electrochemical storage energy system design and construction with high power and energy densities and long life cycle is of unlimited importance (Zuo et al., 2017).
Super-capacitors also known as ultra-capacitors are comparatively, a fresh technology is preferably suited to the market for backup power systems. Used as a single source or in combination with energy sources such as batteries, super-capacitor systems are showing to be the next trend in high-dependability hold-up power (Okundamiya and Nzeako, 2010). Super- capacitors referred to as electric double layer capacitors (EDLC) store energy without reacting chemically (Nwanya et al., 2016). Effectively super-capacitors link power gaps durably from a few seconds to a few minutes and can be rapidly recharged. They provide numerous advantages over current innovative battery technologies from the perspective of power proficiency, service life, discharge/charge effectiveness and easy assessment of charge state.
A rapidly emerging technology, super-capacitors stores and discharges energy rapidly and effectively. Super-capacitors presently are being used in numerous applications (consumer electronics, energy, automobiles, trains, lifts, aircraft and more) and in future prospect, will be used for several other applications. Super-capacitors supplement a main energy source which cannot constantly offer rapid bursts of power, including sources such as battery or fuel cell, internal combustion engine etc. The future prospect looks bright for super-capacitors which ranks already, a powerful alternative energy resource (Maxwell, 2007).
The maximum current drawn in fuel cell powered electronics, especially with the highly variable loads needs to be reduced. Super-capacitors are extensively utilised to ease load current instabilities in the batteries. The reason is that super-capacitors possess greater cycle proficiency, defined as the ratio of the energy output to energy input. An ultra-capacitor connected in parallel with a battery results in a hybrid energy storage, which can support a discharging current of higher rating, due to the super-capacitors high power density. Hence, decreases the influence of the rate capacity effect. Under pulsated load conditions, super-capacitor acts as a filter that eases maximum stresses on the battery (Shin, 2012).
Hybrid energy storage system (HESS) generally is characterised by a valuable connection of two or more energy storage technologies with additional operating characteristics (such as power and energy density, self-discharging ratio, efficiency and lifespan) (Okundamiya, 2015). Hybridisation allows numerous operation modes of the storage system, thereby integrating the merits of the base technologies and extending their application ranges (Okundamiya et al., 2015; Stavanger, 2009). In a HESS, typically, a single storage is committed to cover “high power” demand, transients and rapid load fluctuations and thus, is characterised by a quick response time, greater efficiency, and high life cycle time. By hybridising diverse energy storage
technologies, one can have all benefits of the energy sources incorporated together, with increase in the systems reliability and optimisation.
A hybrid storage system finds use in special applications such as energy boost in inverters, hence a higher power and energy density storage source is required. This research proposes a hybrid storage system made up of Li-ion battery and super-capacitor bank forming a hybrid storage system. The hybrid storage system is intended to improve the battery life and power capabilities, while providing modularity and low-level access to individual cells. In this study, the parallel connection is established between the super-capacitor and the battery using the Automatic Switching System (ASS).
Statement of the Problem
Reliable and uninterrupted power supply is essential to the development of any nation. However, various shortcomings such as power cuts and line problems in grid supply have necessitated the need to explore alternative power technologies and other sources of energy. To resolve the problem of inadequate electricity, different hybrid energy systems have been designed and analysed in literatures (Han et al., 2017; Kim et al., 2017; Okundamiya et al., 2014b; Okundamiya et al., 2017; Okundamiya and Omorogiuwa, 2015).
Hybrid energy system is proving to be the new wave in power technologies, incorporating various hybrid systems such as battery/super-capacitors hybrid, flywheel/battery hybrid. This study was carried out to provide an improvement to the energy storage system of an inverter, incorporating a battery/super-capacitor hybrid system. Durable energy storage with an acceptable battery energy density and power density of super-capacitor could be achieved by the combination of these two technologies.
Objectives of the Study
The overall aim of the project is the design and performance evaluation of a battery/super- capacitor hybrid storage system for renewable energy applications.
The specific objectives of this study are:
- to design an automatic switching system (ASS) for a battery/super-capacitor hybrid storage system (BSHSS);
- to simulate the developed automatic switching system (ASS) for the designed hybrid energy storage system;
- the realisation of the simulated automatic switching system (ASS); and
- the assessment of the developed hybrid energy storage system.
Scope of the Study
The automatic switching system was designed to operate on 12V energy storage source. This voltage was selected due to cost implication. The inverter employed for this design is also a 12VDC input, 2kVA inverter.
Research Methodology
The following methods will be employed to realise the set objectives:
- design of an automatic switching system for the battery/super-capacitor hybrid storage system will be realised by the use of hardware electronic components like resistors, capacitors and diodes. The parameters of these components will be determined using the basic circuit theorems – Ohms and Kirchhoff’s laws;
- the designed system will be simulated using the LTspice XVII software;
- after simulation, the designed circuit will be implemented using the required components and specifications; and
- performance evaluation of the hybrid energy storage system will be done experimentally
Significance of the Project
Sustainable, reliable and affordable energy supply is essential to the development of any nation and its economy at large. Various sources have proved to be very useful, however, each source has its own pros and cons. The harmful effects of fossil fuels on the environment, high cost of grid power are some of the short comings that have necessitated the need for alternative energy systems. The ever growing demand for improved energy for various applications has also necessitated this project. It also finds significance in areas requiring constant backup power, hence the concept of battery/Super-capacitor hybrid storage system.