Browsing by Subject "Energy Transfer"

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    Exciton Transport in Organic Semiconductors
    (2015-06) Menke, Stephen
    Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (LD) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on LD are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors.
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    Low Voltage Ride-Through for Indirect Matrix Converter Based Open-End Winding Drives
    (2017-08) Krishnamoorthi, Santhosh
    Adjustable speed drives (ASD) are one of the major load components in power systems and with the advent of wide band gap devices, which provide efficiencies greater than 95%, variable frequency drives will continue to grow and integrate into the systems. ASDs serve a varied set of processes including HVACs, oilrigs and recently many electric vehicles (EV). The most commonly employed types are the DC-to-AC or AC-to-AC drives with DC/AC drives being more popular in storage and EV applications. AC/AC drives have been dominated by converters using large capacitors with DC bus viz. back-to-back converters. These converters are becoming more reliable and have been tested with new advancements in the industry. In addition, the DC bus capacitor provides an inbuilt energy storage mechanism, which could be used for ride-through operations during fault conditions. In some applications like wind turbines, the presence of large capacitive and reactive components in the drive could be a drawback due to lesser reliability and increased weight. Hence, converters that eliminate the need for large capacitors (viz. cycloconverters and matrix converters) are advantageous in such applications. Matrix converters (MC) have been in research and development for almost three decades, and several topologies and new modulation techniques have been proposed. In addition to elimination of the bulky DC bus capacitor, MCs provide sinusoidal input and output waveforms with lesser harmonics, and have inherent bi-directional power flow capability while offering full input power factor control. In industry, MCs are produced by few manufacturers and is still a niche product. High frequency common mode voltage (CMV) switching is a by-product of the ASDs operating at medium to high frequencies and cause bearing currents to flow, which damage the machine and reduce their lifetime. Elimination or reduction of common mode voltage is a well-researched topic and it has been addressed with plenty of solutions for different kind of drives. One of the recently developed solution is the usage of open-end winding drive modulated using rotating space vectors. Open-end winding machine is constructed by opening the shorted side of an induction machine, which is supplied by another similar converter. Different types of converters including MCs have been used to construct this drive. Matrix converter based open-end winding drive have two types including direct and indirect matrix converter based drives and, this dissertation concentrates on the usage of a three-level indirect matrix converter based open-end winding. It is important that the ASDs are reliable and dependable during fault conditions in the power system. They should be able to ride-through the fault, supply the losses, and maintain the flux in the motor since re-building it could affect the operations. System faults could create over-voltages or voltage sags (sags are more frequent than over-voltages) and many commercial drives address the voltage sag problem with a ride-through solution for up to 30 cycles of interruption. Ride-through solutions include usage of storage devices, modification of the drive or use of inherent kinetic energy. Matrix converters lack an inbuilt storage device and modification of the drive could be expensive. This dissertation proposes a low voltage ride-through method for a three-level indirect matrix converter based open-end winding drive using the input filter capacitor. The three-level indirect MC drive has an advantage over other matrix converter based drives, that it can provide a ride-through solution without the need for modifications or addition of storage devices. The input filter capacitor on the three-level bus between the front-end converter and the two three level inverters is used as the voltage source during the fault while its voltage is maintained by using the kinetic energy from the motor. This is achieved by modification of control loops in a traditional vector control configuration to control the capacitor voltage by drawing power from the motor. In summary, this dissertation describes a three-level indirect matrix converter for an open-end winding drive to eliminate the high frequency common-mode voltage, and proposes a low voltage ride-through method for the operation of the drive during fault conditions using the input filter capacitors as an energy transfer device. The method has been presented with detailed derivations and analyses and been verified using simulations and experimental results using a two-level inverter drive.