Theses and Dissertations - Department of Electrical & Computer Engineering (ECE)

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Adaptive control methods for DC-DC switching power converters
(University of Alabama Libraries, 2011) Arikatla, VaraPrasad; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
Tight regulation of the output voltage is often required in many power supply applications, despite the highly dynamic nature of the loads. This is conventionally obtained by the design of high bandwidth feedback loop or recently by using adaptive control methods. The control loop is designed with specified safe bandwidth and gain and phase margins such that it maintains stable operation under variable conditions and parameters. However, this results in a compromise between achievable dynamic performance and robustness of control loop. The large variations in operating points and load makes the system design challenging. The tight regulation requirements, in addition to size and weight requirements, are getting stricter by time, which makes it necessary to investigate new control concepts in order to meet these requirements. Not meeting the tight regulation requirements may result in either the malfunctioning of the device (load) being powered or the destruction of that device. This work focuses on the development and implementation of adaptive control methods that result in the improvement of the dynamic performance of power converter, by utilizing the flexibility of digital controllers to realize advanced control schemes. Four different methods are proposed that improve the dynamic performance of converter without compromising the steady-state performance. A Sensorless Adaptive Voltage Positioning (SLAVP) control scheme is proposed in Chapter 2, in order to realize Adaptive Voltage Positioning (AVP) control without the need for load or inductor current sensing and high-resolution high-speed Analog-to-Digital Converter (ADC) sampling. The SLAVP control law utilizes the readily available error signal of the conventional voltage-mode closed-loop compensated controller, or in other words the duty cycle of a DC-DC buck converter, in order to realize AVP control. The elimination of the need for high-speed and accurate sensing and sampling of currents using the proposed SLAVP control reduces the size and cost of the digital controller, reduces the power losses associated with current sensing and sampling, and simplifies hardware design, apart from improving dynamic performance. In Chapter 3, an Adaptive Digital PID (AD-PID) controller scheme is proposed. The controller adaptively adjusts the integral constant (K_i) and the proportional constant (K_p) of the compensator following a new control law. The control law is a function of the magnitude change in the error signal and its peak value during dynamic transients. The proposed AD-PID controller adaptively detects the peak value of the error signal which is a function of the transient nature and magnitude and utilize it in the control law such that no ocillations are generated as a result of the adaptive operation. As a result, the dynamic output voltage deviation and the settling time of the output voltage are reduced. A novel Compensator Error Observe and Modulate method (CEO&M) for online closed-loop-compensator auto-tuning of digital power controller is proposed in Chapter 4. The proposed method is relatively simple and does not require the knowledge and/or measurement of the power stage or closed-loop frequency response. Moreover, the proposed method does not depend on conventional design methods and the associated rule of thumb design criteria in order to tune closed-loop feedback controllers of power converter for high, and possibly optimum, dynamic performance. Furthermore, two approaches for dynamic variable switching frequency digital control scheme under dynamic transients are proposed in Chapter 5 in order to improve the dynamic performance of the DC-DC switching power converter. The proposed controller varies the switching frequency of the converter, higher or lower than the steady-state frequency, during the transient as a function of peak and magnitude of error signal depending on the amount and type of the transient. Finally, Chapter 6 summarizes this work and provides conclusions before discussing future related research direction.
Advanced control and synchronization approaches of voltage source converters for integration of distributed energy resources
(University of Alabama Libraries, 2018) Ramezani, Malek; Li, Shuhui; University of Alabama Tuscaloosa
Voltage source converter (VSC) is an inseparable interfacing fixture for utilizing distributed energy resource (DER) as an AC power supply. This dissertation investigates different control and synchronization techniques for stand-alone and grid-connected DC-AC VSCs. The most common control reference frame for VSCs is the dq reference frame (dq-RF), also known as the synchronous reference frame. The main challenge associated with the VSC control in this reference frame is the strong coupling between d and q axes. In this dissertation, a multi-loop dq-RF control system with a coupling compensation scheme is presented. Then, the droop-based power control technique, which eliminates the need for communication between parallel-connected VSCs and consequently offers a higher reliability, is investigated. A dq-RF-based approach of impedance design, for compensating the inequality of parallel VSCs connecting lines, along with the dq-RF droop control are also proposed. This approach results in an accurate power-sharing among parallel VSCs. The major challenges related to the synchronization unit of a grid-connected VSC control system in the presence of a distorted AC voltage are also briefly investigated in this dissertation. To deal with these challenges, an enhanced complex coefficient filter based PLL is designed and presented. This PLL completely removes the grid voltage imbalance and considerably attenuates the grid voltage dc offset and harmonics while maintaining a fast dynamic response and a simple structure. The VSC-interfaced DER is often required to switch between the islanded and grid-connected operation modes. The VSC integrated into the grid is current-controlled, while in the islanded operation mode is controlled as a voltage source. In the transition between these two modes, first the intended VSC operation mode should be detected, then its control system is reconfigured. To avoid the complexity of the control system and alleviate the drawbacks associated with the control mode transition, a VSC control approach, which mimics the traditional synchronous generator’s universal mode of operation, is studied. A method of power-based active synchronization of the VSC-interfaced DER, with the ability of seamless transition between the islanded mode and connected to the grid, is proposed and integrated with this technique of the VSC control.
Analysis of low voltage regulator efficiency based on ferrite inductor
(University of Alabama Libraries, 2010) Kothakonda, Mridula; Hong, Yang-Ki; University of Alabama Tuscaloosa
Low voltage regulator based on ferrite inductor, using single- and two-phase topologies, were designed and simulated in MATLAB. Simulated values of output voltage and current were used to evaluate the buck converter (i.e., low voltage regulator) for power efficiency and percentage ripple reduction at frequencies between 1 and 10 MHz with variable loads from 0.024 to 4 ohms. The parameters, such as inductance of 20 nH, quality factor of 15 of fabricated ferrite inductor and DC resistance (DCR) of 8.3 milli ohms, were used for efficiency analysis of the converter. High current around 40 A was achieved by the converter at low load values. Low output voltage in the range of 0.8-1.2 V was achieved. The simulated results for the single- and two-phase converter were compared for maximum efficiency and lowest ripple in output voltage and current. The maximum efficiency of 97 % with load of 0.33 ohms and the lowest ripple current of about 2.3 mA were estimated for the two-phase converter at 10 MHz. In summary, the two-phase converter showed higher efficiency and lower ripple voltage and current than those of the single-phase converter. In addition, the efficiency of single- and two-phase converters based on ferrite inductor was compared to single- and two-phase converters based on air-core inductor. It was found that the power efficiency of the two-phase converter using ferrite inductor was 10 % higher than the converter using air-core inductor at 10 MHz with a load of 0.024 ohms.
Battery charge and discharge control for energy management in EDV and utility integration
(University of Alabama Libraries, 2012) Bao, Ke; Li, Shuhui; University of Alabama Tuscaloosa
Electric drive vehicles (EDVs) have many benefits as compared to normal petrol or gas cars. Moreover, the electrification of transportation systems would enable increased electricity generation from carbon-free and renewable energy sources, such as wind, solar, and hydro. However, due to highly distributed and mobile nature as well as high charge and discharge power demand of EDVs, it is important to investigate how to manage EDV charge and discharge to enhance the usage of renewable enough resources in the future smart grid framework. For this purpose, this thesis first investigates typical battery electrochemical properties which are important concerns for the design of EDV charge and discharge. In this section, mathematical and circuit-oriented battery models are investigated to reflect typical battery electrochemical properties. Meanwhile, the relation between mathematical and circuit-oriented battery models is analyzed. Then, this thesis presents an energy control study in a charging station, a typical integrated EDV and utility system. The charging station consists of an AC/DC converter for grid interface and multiple dc/dc converters for EDV battery management. For the grid-side converter, a direct-current control mechanism is employed for reactive power, ac system bus voltage, and DC-link voltage control. For the EDV-side converters, constant-current and constant-voltage control mechanisms are investigated for charging and discharging control. The thesis considers energy management need for charge and discharge of multiple EDVs simultaneously as well as energy transferring from vehicle to grid and grid to vehicle requirements. A real-time simulation model is investigated and the performance of the integrated EDV and utility system is investigated.
Control and power management schemes for distributed and battery powered systems
(University of Alabama Libraries, 2016) Huang, Wangxin; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
Battery systems are widely used in many applications including portable electronics, EVs/HEVs, and distributed smart power grids. In addition to battery technologies, the battery management system (BMS) plays a critical role in enabling the widespread adoption of battery-powered applications. This dissertation work focuses on addressing several issues and improving performance of several aspects of battery powered applications. These focused topics include online monitoring of battery impedance, charge balancing between battery cells during both discharging and charging operation, and power electronic topologies and control in order to improve reliability, efficiency, and density of the battery-powered applications. In chapter 2, a practical method is presented in order to achieve accurate online battery impedance measurement while maintaining output voltage regulation of the power converter. The proposed method is based on converter duty cycle control and perturbation. As a result, all the external signal injection circuitries are eliminated. In chapter 3 and 4, the charge balancing issue is addressed from the root by automatically adjusting the discharge/charge rate of each cell based on a new distributed battery system architecture with energy sharing control. The proposed energy sharing controller does not require any charge/energy transfer between the cells, thus eliminating the power losses during energy transfer process. To gain insights into the dynamics of the energy sharing controlled distributed battery system, the state-space averaging small-signal modeling and controller design is performed in Chapter 5. Simulation and experimental results are presented for verification. Single-inductor multiple-output DC-DC converter has gained increased popularity in the portable applications where a battery is used to power multiple loads. However, a common issue facing the SIMO converter design is the cross regulation between the multiple outputs during steady-state and dynamic operations. To address this issue, a power-multiplexed controller is presented in Chapter 6 which eliminates the cross regulation between the outputs by multiplexing the conduction of each output channels. Each output is independently regulated under steady-state and dynamic operations regardless of the operating mode, i.e., continuous or discontinuous conduction mode. Chapter 7 summarizes this work and provides conclusions before discussing some possible future research directions related to this dissertation work.
Digital pulse width modulation techniques for power converters
(University of Alabama Libraries, 2010) Arunachalam, Thanukamalam; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
Recently, digital controls are becoming dominant in almost every power electronic application because of the advantage when compared to analog control. This includes the ability of digital controllers to perform more advanced and sophisticated functions that potentially result in improving power conversion efficiency and/or the dynamic performance of the power converter, the ease of digital control function and loop upgradeability (or revision), and reduced sensitivity to component variations. However, there are also some challenges in digital control such as control loop delays that impact the dynamic performance of the power converters and the additional controller power consumption in some digital control implementations. Digital Pulse Width Modulation (DPWM) is one of the most important parts in digital control systems which control the power switch of the power converters. Modulation technique plays a vital role in causing control delays. There are several implementation schemes of digital pulse width modulation such as counter based DPWM, delay line based DPWM, and hybrid based DPWM. The output voltage is required to have little deviation from the reference voltage and fast settling times under transient events. Therefore, in order to maintain a well regulated output voltage, the control signal must instruct the power converter to either turn on (when there is undershoot in output voltage) or turn off (when there is overshoot in output voltage), as fast as possible. The work presented in this thesis suggests a modulation technique that reduces the turn on delay caused by trailing-edge digital modulation and turn off delay caused by leading-edge digital modulation. Reducing the digital pulse width modulation delay reduces the overshoot and undershoot in the output voltage in power converters with digital closed loop control. The proposed modified digital pulse width modulation scheme is verified using computer simulations and experimental results.
High-density high-efficiency power magnetics
(University of Alabama Libraries, 2016) Dang, Zhigang; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
This dissertation presents several concepts and techniques in order to (1) increase the inductance density and power density of power inductors (PIs) with high power efficiency and (2) achieve magnetically coupled wireless power transfer (WPT) systems with higher efficiency and longer transmission distances under varying conditions. Chapter 1 provides an overview and introduction on applications of power magnetic devices and systems along with the challenges facing the state-of-the-art PIs and WPT systems. Chapter 2 develops a concept which results in doubling the saturation current of a high current PI with NdFeB permanent magnet (PMPI). By adding a well-designed small piece of fabricated NdFeB magnet in the air gap of the PI, the saturation current of the PMPI is doubled with the same size and inductance value. Chapter 3 presents a two-phase coupled power inductor (CPI) that utilizes a PM in order to achieve almost doubled saturation current with the same size compared to the CPI and more than 70% core size reduction compared to the single-phase non-coupled PIs. Both the PMPI and PMCI concepts are experimentally verified in DC-DC power converter prototypes. Chapter 4 and 5 present a two-coil and a four-coil reconfigurable WPT system topology, respectively, in order to optimize transmission efficiency under different distance and misalignment conditions. The two-coil reconfigurable WPT system achieves re-configurability by switching between different values of series and shunt capacitors at Tx side and/or Rx side. The four-coil reconfigurable WPT system achieves re-configurability by switching between different sizes of drive loops and load loops. Experimental results verified effectiveness of developed reconfiguration methods. Chapter 6 presents a method to achieve wired power conversion and WPT using a hybrid “Power Converter-WPT system”. By achieving WPT using AC switching ripple of power converter, the system eliminates the need for a transmitter stage of conventional WPT system, which could be beneficial for system size and cost reduction. The method is verified and demonstrated using Buck-WPT system as an example. The last chapter summarizes this work and provides conclusions before discussing some possible future research directions related to the dissertation work.
Input voltage ripple based sensorless current sharing auto-tuning controller for multiphase DC-DC converters
(University of Alabama Libraries, 2015) Huang, Wangxin; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
Multiphase power converter topology has been widely adopted for Point-of-Load (PoL) converter application. To achieve a high-performance multiphase converter design, several design considerations must be taken into account during the design phase. These factors include but are not limited to high power density (smaller size), fast transient response and equal current sharing which are introduced in Chapter 1. Current sharing, as a key design consideration to ensure reliable operation of multiphase DC-DC converter, is further explored in this work. Equal current sharing control loop is typically required for multiphase converter in order to prevent inductor current saturation and overstressing the devices in certain phases. Several conventional current sharing control schemes are first reviewed in Chapter 2. The major drawback of these schemes is the need for accurate current sensing for each phase, which causes the performance of the current sharing to be highly sensitive to the accuracy of current sensing in addition to increased cost and complexity. To address the need for accurate current sensing, a new digital sensorless current sharing control scheme is proposed and developed in this work. It is based on auto-tuning the duty cycle value for each phase while observing the input capacitor voltage of the multiphase converter. The theoretical basis/observation of the proposed concept is presented and mathematically verified in Chapter 2 followed by an introduction of the operation of the proposed controller. In addition to eliminating all of the current sensors, the proposed controller eliminates the impact of voltage sensing inaccuracies on the performance of current sharing. The simulation verification of the theoretical basis and operation of the proposed controller is performed by using MATLAB®/SIMULINK® software package. The simulation model is introduced and the simulation results are presented in Chapter 3. A proof-of-concept experimental prototype set-up is first introduced in Chapter 4. The design and implementation of the proposed controller are then covered in details. The periodic and continuous auto-tuning operations are demonstrated under both steady-state and load transient conditions. The experimental results are presented and discussed to further validate the proposed controller. Finally, some directions for future work are given.
Magnetic resonance coupled wireless power transfer systems
(University of Alabama Libraries, 2013) Dang, Zhigang; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
Wireless power transfer (WPT) technology has many potential applications such as consumer electronics and electric vehicles (EV). High transmission efficiency with long transmission distance and with large lateral misalignment is desired in WPT systems. Magnetic resonance coupled (MRC) WPT systems are suitable for midrange high efficiency wireless power transfer (WPT). In chapter 2, commonly used four-loop and two-loop MRC-WPT system configurations are analyzed and compared in terms of transmission efficiency and transmission distance first based on the simplified circuit model. An example symmetrical system simulation shows that with the same Tx, Rx, source and load, the four-loop system has longer transmission distance but with relatively lower transmission efficiency compare to the two-loop system. Then, A 3-D physical model of 5-turn, 400mm outer diameter spiral shape four-loop WPT system is developed and simulated by using ANSYS® HFSS® software package. Operation distance of 550mm with nearly constant maximum transmission efficiency of 92.3% is achieved. Laterally misaligned MRC-WPT system is investigated in chapter 3. The TEVD, a region on the transmission efficiency versus Rx lateral misalignment amount curve where the transmission efficiency first sharply drops from high efficiency down to zero and then recovers to a low efficiency value, is identified in this work. The identification of TEVD is verified by simulation results obtained from a developed ANSYS® HFSS® 3-D physical model. Simulation results of the ANSYS® HFSS® 3-D physical model with 5-turn, 60cm outer diameter spiral shape MRC-WPT system show that when the Rx is 30cm vertically away from the Tx, TEVD exists when the lateral misalignment value ranges from 50cm to 70cm. An elimination method for TEVD is proposed in chapter 4. The proposed method utilizes angular rotation of the Rx (or Tx) to eliminate the zero-coupling point which causes the TEVD and boosts the coupling coefficient such that the TEVD is eliminated and the high efficiency region is extended. ANSYS® HFSS® 3-D physical model simulation results show that the proposed method eliminates the TEVD and extends the high efficiency region from 50cm lateral misalignment (83.3% of the Rx diameter) to 70cm lateral misalignment (117% of the Rx diameter). Chapter 5 summarizes the thesis conclusions and sheds the light on future work.
Pb_0.95 La_0.05 Zr_0.54 Ti_0.46 O_3 thin films for photovoltaic applications
(University of Alabama Libraries, 2012) Vasudevan Nampoori, Harshan; Kotru, Sushma; University of Alabama Tuscaloosa
Ferroelectrics have shown potential as a promising alternative material for future photovoltaic applications. Observance of high open circuit voltages in ferroelectric thin films, have generated considerable interest in the field of ferroelectric photovoltaic in recent years. The field of ferroelectric photovoltaic is evolving and not yet completely understood compared to the semiconductor based photovoltaic technology. This dissertation presents photovoltaic properties of ferroelectric Pb_0.95 La_0.05 Zr_0.54 Ti_0.46 O_3 thin films. The films were fabricated by solution based methods and spin coating technique. The post annealing process on these films was optimized to achieve the desired ferroelectric and dielectric properties. A measurement setup was established to study the PV characteristic of the devices. Dependence of current-voltage (I-V) behavior of the cells on parameters such as electrical poling, annealing temperature, nature of top electrodes, and intensity of illumination, was investigated. The photovoltaic response was shown to improve by using electrodes with low work functions. An electric circuit model was developed to simulate the behavior of a single ferroelectric photovoltaic cell and the dependence of open circuit voltage (Voc) and short circuit current (Isc) on light intensity.
Power electronics architectures and controls for photovoltaic solar energy systems
(University of Alabama Libraries, 2013) Jiang, Yuncong; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
The increasing demand for clean and renewable energy sources utilization in our daily life has placed more challenging requirements on photovoltaic (PV) solar systems power efficiency, tracking speed, system dynamic response, system cost and size. Researchers have investigated various PV solar system architectures and control methods such as maximum power point tracking (MPPT) techniques to improve PV solar system tracking efficiencies under mismatching and partial shading conditions. Improvements in PV system architectures include the development of module integrated converter (MIC) architecture which performs distributed MPPT at panel-level and the development of sub-MIC architecture which is able to track the optimal operating point of PV cell or group of PV cells inside a PV panel. These two architectures improve the tracking efficiencies of the PV system under various weather and load conditions compared to conventional PV system architectures. However, the MIC and sub-MIC architectures all suffer from some common drawbacks: high cost, large size and high power losses due to the increased number of power components and control circuits. This is mainly because such architectures require larger number of power converters, MPPT controllers, and the related parts such as Analog-to-Digital Converters (ADCs) and other conditioning circuits. The target of this dissertation is to develop control schemes and architectures that will result in reduced cost and size and improved MPPT tracking speed. In order to address tracking speed with reduced sensing, this work develops an adaptive step size and adaptive perturbation frequency MPPT control that utilizes a single sensor, which yields improved tracking speed while maintaining reduced cost and size. Then, in order to reduce the cost and size and improve the efficiency of MIC and sub-MIC architectures, this work develops a two-mode single-sensor MPPT control algorithm for multi-channels PV solar systems that requires only one MPPT controller, one sensor, and one ADC and applies this algorithm to parallel and series PV solar systems configurations. However, while the single sensor MPPT controller reduces the cost and size of control part of the system, it still requires multiple power converters, one for each PV solar channel. Therefore, this work progresses to the next step and develops an architecture that only requires a single power converters with a single power inductor for multiple channels in addition to a single sensor MPPT controller and single ADC. Following the introduction chapter (Chapter 1), this dissertation is organized as follows: Chapter 2 presents a load-current-based MPPT digital controller with adaptive step size and adaptive perturbation frequency algorithm. By utilizing variable step size algorithm, the speed, accuracy and efficiency of the PV system MPPT are improved when compared to the fixed step size load-current-based algorithm. Furthermore, the proposed adaptive algorithm utilizes a novel variable perturbation frequency scheme which further improves the controller speed. Chapter 3 presents a two-mode single-sensor MPPT control algorithm (SS-MPPT) for N-channel PV solar system with parallel MIC PV solar system architecture. The N-channel SS-MPPT controller is able to track the MPP of each PV solar panel, cell, or groups of cells by using only one current sensor. Moreover, a modified SS-MPPT control strategy is proposed in this chapter that is suitable for parallel MIC PV systems that are connected to the grid through current source inverters. Two advanced MPPT algorithms which can be used to realize the SS-MPPT controller are discussed and compared. The SS-MPPT controllers achieve high tracking efficiency and fast dynamic response at reduced cost and size. Chapter 4 presents a cost-effective series-output-connection MPPT (SOC-MPPT) controller for sub-MIC PV system architecture adopting a single sensor at the output and a single digital MPPT controller. The proposed controller and system architecture is able to reduce the number of sensing circuitry, number of required digital controllers in sub-MIC PV system architecture while achieving high tracking efficiencies under mismatching and partial shading conditions. Chapter 5 presents a PV solar system architecture with a single power converter with a single inductor and single MPPT controller that only requires one sensor. This PV solar system architecture is able to perform maximum power point tracking for N-channel PV solar system at panel-level, cell-group-level and single-cell-level. The low-cost, small size and high-efficiency features of the architecture make it effective and attractive. Chapter 6 concludes the work and gives a brief outlook on possible future directions.
Smart grid power market study in power transmission, distribution and demand systems
(University of Alabama Libraries, 2013) Zhang, Dong; Li, Shuhui; University of Alabama Tuscaloosa
An electric power system is a meshed network which includes three major components: transmission, distribution, and demand systems. The future smart grid will be a highly intelligent electric power system that will have a profoundly impact on all the three areas. This dissertation focuses on 1) the competitive power market study, 2) optimal power management in microgrid, and 3) intelligent demand response strategies of residential system. In the transmission system, generation companies and load serve entities are encouraged to compete for the amount of power generation and load demand. Therefore, utility companies require an efficient and reliable computational tool to analyze the competitive power markets. However, most of existing commercial power system simulators are unable to evaluate a competitive power market directly. This dissertation proposes a method to convert a competitive power market problem in such a way that PowerWorld can be used for broad competitive power market studies, including optimal power flow, unit commitment, and agent-based learning. In the distribution system, it's urgent to develop an efficient computational tool that can be used in the energy management system to control different types of renewable energy in microgrid. This dissertation develops a mechanism to use PowerWorld for comprehensive analysis of microgrid power markets as well as optimal power flow under different power converter operating modes. In the demand system, demand response (DR), a critical component for smart grid, will adjust the load power consumption pattern to achieve high economic efficiency. This dissertation proposes several different DR strategies for home appliance of the residential system.
Solar photovoltaic energy generation and conversion --from devices to grid integration
(University of Alabama Libraries, 2013) Zheng, Huiying; Li, Shuhui; University of Alabama Tuscaloosa
Solar photovoltaic (PV) energy is becoming an increasingly important part of the world's renewable energy. In order for effective energy extraction from a solar PV system, this research investigates solar PV energy generation and conversion from devices to grid integration. First of all, this dissertation focuses on I-V and P-V characteristics of PV modules and arrays, especially under uneven shading conditions, and considers both the physics and electrical characteristics of a solar PV system in the model development. The dissertation examines how different bypass diode arrangements could affect maximum power extraction characteristics of a solar PV module or array. Secondly, in order to develop competent technology for efficient energy extraction from a solar PV system, this research investigates typical maximum power point tracking (MPPT) control strategies used in solar PV industry, and proposes an adaptive and close-loop MPPT strategy for fast and reliable extraction of solar PV power. The research focuses especially on how conventional and proposed MPPT methods behave under highly variable weather conditions in a digital control environment. A computational experiment system is developed by using MatLab SimPowerSystems and Opal-RT (real-time) simulation technology for fast and accurate investigations of the maximum power extraction under high frequency switching conditions of power converters. A hardware experiment system is built to compare and validate the conventional and the proposed MPPT methods in a more practical condition. Advantages, disadvantages and properties of different MPPT techniques are studied,evaluated, and compared. Thirdly, in order to develop efficient and reliable energy conversion technologies, this dissertation compares the energy extraction characteristics of a PV system for different converter configurations. A detailed comparison study is conducted to investigate what enhancements and impacts can be made by using different bypass diode schemes. It is found that compared to micro-converter based PV systems, the central converter scheme with effective bypass diode connections could be a simple and economic solution to significantly enhance PV system efficiency, reliability and performance. Lastly, the development of coordinated control tools for next-generation PV installations, along with energy storage units (ESU), provides flexibility to distribution system operators. The objective of the control of this hybrid PV and energy storage system is to supply the desired active and reactive power to the grid and at the same time to maintain the stability of the dc-link voltage of the PV and energy storage system through coordinated control of power electronic converters. This research investigates three different coordinated control structures and approaches for grid integration of PV array, battery storage, and supercapacitor (SC). In addition, other applications including single-phase Direct- Quadrature (DQ) control and ramp rate limit control are presented in this dissertation. Index Terms - solar photovoltaic, semiconductor physics, I-V characteristics, P-V characteristics, bypassing diodes, uneven shading, power electronic converters, maximum power point tracking, digital control, computational and hardware-based experiments, battery and supercapacitor, control coordination, single-phase DQ control, and ramp rate control.
Temperature gradient approach to grow preferentially-oriented tips pentacene crystals for organic thin film transistors
(University of Alabama Libraries, 2015) Asare-Yeboah, Kyeiwaa; Li, Dawen; University of Alabama Tuscaloosa
As a functionalized pentacene, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is a p-type organic semiconductor with remarkable intrinsic charge carrier transport and stability in ambient conditions. TIPS pentacene is soluble in most organic solvents, making it solution processable. TIPS pentacene, nonetheless, inherently forms acutely anisotropic crystals with large gaps in between the crystals, limiting charge transport and leading to vast variations in organic thin film transistor (OTFT) performance. Described in this dissertation are crystal growth techniques implemented to overcome these challenges. The presented temperature gradient technique, achieves highly aligned crystal arrays with excellent areal coverage which essentially results in an enhanced OTFT performance. The technique is firstly utilized to guide the TIPS pentacene crystal growth. An application of a temperature gradient to a TIPS pentacene solution controls the crystallization process to alleviate the intrinsic crystal misorientation and considerably improve film morphology. Employing this method resulted in TIPS pentacene films with uniform crystal orientations and extensive areal coverage. The favorable crystal morphology gave rise to a significant enhancement in OTFT average mobility compared to OTFTs without the temperature gradient. Employing the temperature gradient technique, however, simultaneously introduced thermal cracks in the films due to the occurrence of thermally induced stress during crystallization, which reduced the device performance of the TIPS pentacene OTFTs. To further improve the performance of TIPS pentacene based OTFTs, TIPS pentacene was blended with polymers to relieve the thermal stress and effectively prevent the generation of thermal cracks. Structural examination of, specifically, TIPS pentacene/Poly(α-methyl styrene) (PαMS) blend films at an optimal weight ratio, revealed a vertical phase segregation with elevated concentrations of TIPS pentacene molecules at the active layer/gate dielectric interface, facilitating charge transport. Thus, OTFTs based on TIPS pentacene/PαMS blends exhibited a dramatic increase in average hole mobility compared to those of pristine TIPS pentacene. In addition, an improved thin film uniformity directly enhanced the device performance consistency. Following the success of employing the temperature gradient technique concurrently with the insulating polymer, PαMS, studies were extended to build OTFTs on flexible substrates, indium tin oxide (ITO) coated polyethylene terephthalate (PET), to dramatically improve TIPS pentacene/PαMS system. Ultimately, TIPS pentacene/PαMS OTFTs on ITO/PET substrates demonstrated the highest achieved mobility from utilizing the temperature gradient system.

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Browsing Theses and Dissertations - Department of Electrical & Computer Engineering (ECE) by Author "Abu Qahouq, Jaber A."

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