A study of ammonia-water desorption in compact counter-flow geometries was conducted. Two novel vapor generation units, comprising integrated desorber, analyzer, and rectifier segments that use microchannel geometries, were conceptualized. The branched-tray concept features a desorber segment that uses predominantly pool-boiling mechanisms for desorption, while the vertical column desorber relies on falling-film evaporation and boiling mechanisms. Both concepts rely on falling-film heat and mass transfer mechanisms in the analyzer and rectifier sections. Segmented heat and mass transfer models, based on available correlations and modeling methodologies, were developed and used for the design of branched tray and vertical column test sections. An experimental facility was designed and constructed to evaluate desorption and rectification heat and mass transfer processes within these components, under realistic operating conditions. Data were analyzed to determine the boiling/evaporation (desorber) and condensation (rectifier) heat transfer coefficients, and to determine values of the desorber liquid and vapor mass transfer coefficients. Additionally, high-speed video and images were used to gain insights into the hydrodynamic phenomena and heat transfer mechanisms in these vapor generation units. Results of the heat and mass transfer analysis were compared with the predictions of correlations and modeling methods in the literature. The vapor generation unit (VGU) test sections were evaluated across a range of concentrated solution mass fractions (0.400 – 0.550), desorber coupling-fluid inlet temperatures (170 – 190ᵒC), and concentrated solution flow rates (0.70 – 1.3 g s-1). Flow rates in this range correspond to desorber liquid Reynolds numbers of approximately 175 to 410 for the branched tray design, and desorber film Reynolds numbers of approximately 90 to 215 for the vertical column. Pressures observed within the VGU test sections ranged from approximately 1620 to 2840 kPa during testing. The novel VGUs were shown to achieve ideal cooling capacities as high as 432 and 323 W for the branched tray and vertical column, respectively. This parameter indicates the cooling capacity that would be achieved by an idealized cooling system using the refrigerant stream produced by the experimental VGU. Ideal COPs of 0.561 and 0.496 were demonstrated for the branched tray and vertical column, respectively. Experimental heat transfer coefficients were found to range from approximately 1860 to 11690 W m-2 K-1 for the pool-boiling desorption of the branched tray VGU. A new correlation was proposed and shown to provide good agreement with the data, achieving average and average absolute deviation of -5.2 and 16.1%, respectively, across the range of conditions tested. Falling-film evaporation/boiling heat transfer coefficients, determined for the desorption process in the vertical column VGU, were found to range from approximately 1290 to 4310 W m-2 K-1. Rectifier condensation heat transfer coefficients ranging from approximately 160 to 250 W m-2 K-1 were observed. Mass transfer coefficients for the desorbers of both concepts were also quantified. These results were used to develop revised heat and mass transfer models of the VGU concepts. The revised models were demonstrated to predict component-level performance with reasonable accuracy, and may be used in the design of future compact VGUs with similar geometries and operating conditions.

More and more attention has been paid to the energy crisis due to the increasing energy demand from industrial and commercial applications. The utilisation of wind power, which is considered as one of the most promising renewable energy sources, has grown rapidly in the last three decades. In recent years, many power converter techniques have been developed to integrate wind power with the electrical grid. The use of power electronic converters allows for variable speed operation of wind turbines, and enhanced power extraction. This work, which is supported by EPSRC and Nheolis under the DHPA scheme, focuses on the design and analysis of control systems for wind power. In this work, two of the most popular AC-DC-AC topologies with permanent magnet synchronous generators (PMSG) have been developed. One consists of an uncontrollable rectifier, a boost converter and an inverter and a current control scheme is proposed to achieve the maximum power point tracking (MPPT). In the control strategy, the output current of the uncontrollable rectifier is controlled by a boost converter according to the current reference, which is determined by a climbing algorithm, to achieve MPPT. The synchronverter technology has been applied to control the inverter for the grid-connection. An experimental setup based on DSP has been designed to implement all the above mentioned experiments. In addition, a synchronverter-based parallel control strategy, which consists of a frequency droop loop and a voltage droop loop to achieve accurate sharing of real power and reactive power respectively, has been further studied. Moreover, a control strategy based on the synchronverter has been presented to force the inverter to have capacitive output impedance, so that the quality of the output voltage is improved. Abstract The other topology consists of a full-scale back-to-back converter, of which the rectifier is controllable. Two control strategies have been proposed to operate a three-phase rectifier to mimic a synchronous motor, following the idea of synchronverters to operate inverters to mimic synchronous generators. In the proposed schemes, the real power extracted from the source and the output voltage are the control variables, respectively, hence they can be employed in different applications. Furthermore, improved control strategies are proposed to self-synchronise with the grid. This does not only improve the performance of the system but also considerably reduces the complexity of the overall controller. All experiments have been implemented on a test rig based on dSPACE to demonstrate the excellent performance of the proposed control strategies with unity power factor, sinusoidal currents and good dynamics. Finally, an original control strategy based on the synchronverter technology has been proposed for back-to-back converters in wind power applications to make the whole system behave as a generator-motor-generator system.

With ample radio transmitters scattered throughout urban landscape, RF energy scavenging emerges as a promising approach to extract energy from propagating radio waves in the ambient environment to continuously charge low power electronics. With the ability of generating power from RF energy, the need for batteries could be eliminated. The effective distance of a RF energy scavenging system is highly dependent on its conversion efficiency. This results in significant limitations on the mobility and space requirement of conventional RF energy scavenging systems as they operate only in presence of physically large antennas and conversion circuits to achieve acceptable efficiency. This thesis presents a number of novel design strategies in the antenna and rectifier designs for miniaturized RF energy scavenging system. In the first stage, different energy scavenging systems including solar energy scavenging system, thermoelectric energy scavenging system, wind energy scavenging system, kinetic energy scavenging system, radio frequency energy scavenging system and hybrid energy scavenging system are investigated with regard to their principle and performance. Compared with the other systems, RF energy scavenging system has its advantages on system size and power density with relatively stable energy source. For a typical RF energy scavenging system, antenna and rectifier (AC-DC convertor) are the two essential components to extract RF energy and convert to usable electricity. As the antenna occupies most of the area in the RF energy scavenging system, reduction in antenna size is necessary in order to design a miniaturized system. Several antennas with different characteristics are proposed in the second stage. Firstly, ultra-wideband microstrip antennas printed on a thin substrate with a thickness of 0.2 mm are designed for both half-wave and full-wave wideband RF energy scavenging. Ambient RF power is distributed over a wide range of frequency bands. A wideband RF energy scavenging system can extract power from different frequencies to maximize the input power, hence, generating sufficient output power for charging devices. Wideband operation with 4 GHz bandwidth is obtained by the proposed microstrip antenna. Secondly, multi-band planar inverted-F antennas with low profile are proposed for frequency bands of GSM 900, DCS 1800 and Wi-Fi 2.4 GHz, which are the three most promising frequency bands for RF energy scavenging. Compared with previous designs, the triple band antenna has smaller dimensions with higher antenna gain. Thirdly, a novel miniature inverted-F antenna without empty space covering Wi-Fi 2.4 GHz frequency band is presented dedicated for indoor RF energy scavenging. The antenna has dimensions of only 10 × 5 × 3.5 mm3 with appreciable efficiency across the operating frequency range. In the final stage, a passive CMOS charge pump rectifier in 0.35 μm CMOS technology is proposed for AC to DC conversion. Bootstrapping capacitors are employed to reduce the effective threshold voltage drop of the selected MOS transistors. Transistor sizes are optimized to be 200/0.5 μm. The proposed rectifier achieves improvements in both power conversion efficiency and voltage conversion efficiency compared with conventional designs. The design strategies proposed in this thesis contribute towards the realization of miniaturized RF energy scavenging systems.

The purpose of this work is to evaluate the performance of a specific ac drive topology that is of current interest in industry. With the increasing pressure for compliance with IEEE-519 and other international harmonic standards, many ac drive manufacturing companies are seeking innovative and cost effective solutions for controlling the amount of harmonics produced at the point of common coupling (PCC). The proposed topology is a potential alternative to the three-phase diode bridge which is the conventional rectifier topology for non-regenerative applications. The work of this thesis explains the theory of operation, control algorithms, and potential improvement strategies for the proposed "half-controlled" boost rectifier topology. The entire ac drive system with load is then modeled and the results verified using the Simulink simulation package. It is shown that the proposed topology has several distinct advantages over a traditional diode rectifier such as improved total harmonic distortion (THD) of the current waveforms, dc bus voltage regulation, and power factor control. In addition, these advantages are created at a price point which is significantly lower than that of a conventional fully-controlled pulse-width modulated (PWM) rectifier. The main disadvantage is that the current waveforms in the utility contain even harmonics which may cause significant problems in the power system.

The purpose of this work is to evaluate the performance of a specific ac drive topology that is of current interest in industry. With the increasing pressure for compliance with IEEE–519 and other international harmonic standards, many ac drive manufacturing companies are seeking innovative and cost effective solutions for controlling the amount of harmonics produced at the point of common coupling (PCC). The proposed topology is a potential alternative to the three-phase diode bridge which is the conventional rectifier topology for non-regenerative applications. The work of this thesis explains the theory of operation, control algorithms, and potential improvement strategies for the proposed “half-controlled” boost rectifier topology. The entire ac drive system with load is then modeled and the results verified using the Simulink simulation package. It is shown that the proposed topology has several distinct advantages over a traditional diode rectifier such as improved total harmonic distortion (THD) of the current waveforms, dc bus voltage regulation, and power factor control. In addition, these advantages are created at a price point which is significantly lower than that of a conventional fully-controlled pulse-width modulated (PWM) rectifier. The main disadvantage is that the current waveforms in the utility contain even harmonics which may cause significant problems in the power system.

No description available.

Design, fabrication and characterization of III-Nitride pn junction devices Jae Boum Limb 94 pages Directed by Dr. Russell D. Dupuis This dissertation describes an investigation of three types of III-nitride (AlInGaN) based p-n junction devices that were grown by metalorganic chemical vapor deposition (MOCVD). The three types of devices are Ultra-Violet (UV) avalanche photodiodes (APDs), green light emitting diodes (LEDs), and p-i-n rectifiers. For avalanche photodiodes, a material growth on low-dislocation density GaN substrates, processed with low-damage etching receipes and high quality dielectric passivations, were proposed. Using this technology, GaN APDs with optical gains greater than 3000, and AlGaN APDs showing true avalanche gains have been demonstrated. For green LEDs, the use of InGaN:Mg as the p-layer, rather than employing the conventional GaN:Mg has been proposed. Green LEDs with p-InGaN have shown higher emission intensities and lower diode series resistances compared to LEDs with p-GaN. Using p-InGaN layers, LEDs emitting at green and longer wavelengths have been realized. For p-i-n rectifiers, design, fabrication and characterization of device structures using the conventional mesa-etch configuration, as well as the full-vertical method have been proposed. High breakdown devices with low on-resistances have been achieved. Specific details on device structures, fabrication methods, and characterization results are discussed.

The purpose of this work is to evaluate the performance of a specific ac drive topology that is of current interest in industry. With the increasing pressure for compliance with IEEE–519 and other international harmonic standards, many ac drive manufacturing companies are seeking innovative and cost effective solutions for controlling the amount of harmonics produced at the point of common coupling (PCC). The proposed topology is a potential alternative to the three-phase diode bridge which is the conventional rectifier topology for non-regenerative applications. The work of this thesis explains the theory of operation, control algorithms, and potential improvement strategies for the proposed “half-controlled” boost rectifier topology. The entire ac drive system with load is then modeled and the results verified using the Simulink simulation package. It is shown that the proposed topology has several distinct advantages over a traditional diode rectifier such as improved total harmonic distortion (THD) of the current waveforms, dc bus voltage regulation, and power factor control. In addition, these advantages are created at a price point which is significantly lower than that of a conventional fully-controlled pulse-width modulated (PWM) rectifier. The main disadvantage is that the current waveforms in the utility contain even harmonics which may cause significant problems in the power system.

This dissertation is to investigate the power system service quality of a Mass Rapid Transit (MRT) system and derive the proper transformer planning to enhance the system operation efficiency. The transformer loading factor is improved by proper capacity planning by considering the power demand according to the growth of ridership. To mitigate the harmonic distortion, the installation location and capacity of harmonic filters are designed and verified by computer simulation. In this dissertation, the software programs for the AC/DC load flow study and harmonic analysis have been developed and integrated to simulate the MRT power system. To enhance the accuracy of computer simulation for the system operation with multiple trains on the main lines, the effect of voltage fluctuation to the traction effort of a train set is considered in the AC/DC load flow analysis. The mathematical model of 12-pulse uncontrolled rectifiers without interphase transformers has been derived and implemented in the programs to obtain more accurate simulation results. To achieve better cost effective capacity planning of main transformers, the unit commitment is applied to derive the optimal transformer capacity to meet the annual peak demand and provide reserve for service reliability. The power consumption of an MRT system is varied with the train operation modes and the route gradient, curvature of MRT networks. The motion equation of train sets has been applied to find the dynamic power consumption and travel distance for each time snapshot. The AC/DC load flow analysis is performed to find the annual power loading of traction substations and whole Taipei MRT network. The energy loss, investment cost of main transformers, and the system service reliability are used to define the equivalent cost of all feasible states for each dynamic programming stage. According to the computer simulation, significant cost saving has been obtained by the proposed methodology for transformer capacity planning of Taipei MRT network. Due to the dynamic load behavior of train sets, the stochastic harmonic distortion of an MRT system is simulated. The mathematical model of the 12-pulse uncontrolled rectifiers without interphase transformers is considered in the harmonic load flow analysis to solve the power demand and harmonic injection currents at traction substations for each time snapshot. According to the mean values and standard deviation of injection harmonic currents, the stochastic harmonic load flow analysis is executed to find the average value and the confidential interval of harmonic voltage distortion for all system buses. By this method, the system voltage harmonic distortion can be evaluated more accurately to provide better guidance for the strategy of harmonic mitigation. According to the results of the stochastic harmonic load flow, different strategies of harmonic distortion mitigation are investigated. Both the fixed type and switching type of passive filters are considered to be implemented. The cost function of filter investment and the harmonic distortion are used in the objective function by considering the regulation of harmonic distortion and system voltages as the constraints. By performing the nonlinear programming, the proper capacity of harmonic filters for each harmonic order and the corresponding switching time of unit commitment is determined. To further improve the power quality, the hybrid filter is also proposed for better distortion mitigation. The analysis of distortion mitigation by harmonic filters are performed to demonstrate the effectiveness of the hybrid filters to improve the power quality of MRT systems.

The purpose of this work is to evaluate the performance of a specific ac drive topology that is of current interest in industry. With the increasing pressure for compliance with IEEE-519 and other international harmonic standards, many ac drive manufacturing companies are seeking innovative and cost effective solutions for controlling the amount of harmonics produced at the point of common coupling (PCC). The proposed topology is a potential alternative to the three-phase diode bridge which is the conventional rectifier topology for non-regenerative applications. The work of this thesis explains the theory of operation, control algorithms, and potential improvement strategies for the proposed "half-controlled" boost rectifier topology. The entire ac drive system with load is then modeled and the results verified using the Simulink simulation package. It is shown that the proposed topology has several distinct advantages over a traditional diode rectifier such as improved total harmonic distortion (THD) of the current waveforms, dc bus voltage regulation, and power factor control. In addition, these advantages are created at a price point which is significantly lower than that of a conventional fully-controlled pulse-width modulated (PWM) rectifier. The main disadvantage is that the current waveforms in the utility contain even harmonics which may cause significant problems in the power system.