A resource analysis for Pakistan has demonstrated that abundant crop residues offer a viable, and environmental-friendly alternative to currently inadequate, oil-based power generation. Similarly, in many countries there is legislative pressure and incentives to replace coal with biomass, in electricity production. Efficient and safe exploitation of such biofuels requires data on flame propagation rates and explosibility characteristics. Crop residues (bagasse, rice husk, wheat-straw, corn-cob and peanut-shell) and different raw and thermally treated woods were tested using the modified Hartmann tube and the modified 1 m3 explosion vessel. The modified Hartmann tube was operated for varying ignition delays using a digital timer. A hemispherical disperser with drilled pipe was calibrated for the testing of fibrous and coarse size biomass mixtures. Thermogravimetric analysis data from these materials enabled the application of two different models for the determination of volatile release kinetics. Biomass samples were found to have lower activation energies with higher volatile release rates at low temperatures, compared to coals. Despite their higher ash content, pulverised crop-residues showed leaner minimum explosible concentrations (0.2-0.5 equivalence-ratio) than woods (0.3-0.7) - depending on particle size. Biomass samples were more sensitive to explosion than coal, resulting in flame propagation in coarse-size-range fractions (300-500-μm) that was not experienced with coals. Maximum explosion pressures of near 9 barg were measured for the fine size fraction (less than 63-μm) samples, with no less than 7 barg for coarse size fraction (less than 1 mm). Milling of thermally treated biomass samples resulted in higher fines fraction than untreated biomass, for the same sieve size and this was considered as one of the reasons of the higher reactivity (higher flame speed and higher deflagration index of these samples). The detailed data from this work are usable in the design of safe and efficient combustion systems for power generation from crop residues and other biomass fuels.

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No description available.

Electrical machines may exhibit various types of imbalances and undesirable harmonic distortions. These may increase the torque and flux ripples, acoustic noise, unbalanced three-phase currents, while also reducing efficiency. These types of imbalances and undesirable harmonic distortions cannot be controlled by using the conventional indirect torque control (ITC) and direct torque control (DTC) strategies. For some high-performance motion control, such as precision machine tools, robotics, and servo drives, low torque ripples are, however, obligatory. Nowadays, more studies have been conducted on the ITC strategy to control undesired current harmonics, such as double synchronic reference frames (DSRF), resonant controller, second order generalized integration, and reference current generation. Such strategies, however, can rarely be applied to DTC strategy. In this research, the influence of asymmetric winding impedances, unbalanced back-EMF, and inverter nonlinearity in three-phase surface-mounted PMSMs has been systematically investigated by employing space vector modulations (SVM) based ITC and DTC strategies. This thesis firstly presents a modified ITC strategy by extracting the positive and negative sequence components in the stationary abc frame, and then a coordination transformation is used to control the machine in DSRF. This strategy provides faster dynamic response when compared with the conventional DSRF strategy, since the filters and the decoupling network are not required. Due to the lack of research regarding the DTC strategy under unbalanced conditions, this research investigates and proposes modified cascaded and parallel DTC-SVM strategies. The conventional cascaded DTC strategy is investigated under balanced and unbalanced conditions. Then, a modified control strategy is introduced by adding two compensators (the conventional PI-controller with a resonant controller, and the use of the negative- and positive-sequence voltage vectors) to suppress the 2nd harmonic components in the torque and stator flux linkage. Furthermore, for parallel DTC-SVM, the compensation of the 2nd and 6th harmonic components is investigated by means of either a resonant controller or an adaptive filter. In addition to the simplicity of the proposed strategies, these may also be able to significantly reduce the torque and flux ripples, while maintaining the merit of the fast dynamic response of the conventional DTC strategy even under variable fundamental frequency. Moreover, it has been proven that the compensation from using a resonant controller or an adaptive filter is parameter independent. Thus, regardless of unbalanced conditions, an effective torque ripple minimisation can still be achieved by properly selecting the dominant harmonic compensation.

Power electronic converters (PECs) are used for conditioning the flow of energy between renewable energy applications and grid or stand-alone connected loads. Insulated gate bipolar transistors (IGBTs) are critical components used as switching devices in PECs. IGBTs are multi-layered devices made of different coefficient of thermal expansion (CTE) based materials. In wind and solar energy applications, IGBT’s reliability is highly influenced by the operating conditions such as variable wind speed and solar irradiance. Power losses occur in switching transient of high current/voltage which causes temperature fluctuations among the layers of the IGBTs. This is the main stress mechanism which accelerates deterioration and eventual failures among IGBT layers due to the dissimilar CTEs. Therefore, proper thermal monitoring is essential for accurate estimation of PECs reliability and end lifetime. Several thermal models have been proposed in literature, which are not capable of representing accurate temperature profiles among multichip IGBTs. These models are mostly derived from offline modelling approaches which cannot take operating conditions and control mechanisms of PECs into account and unable to represent actual heat path among each chip. This research offers an accurate and powerful electro thermal and reliability monitoring tool for such devices. Three-dimensional finite element (FE) IGBT models are implemented using COMSOL, by considering complex heat interactions among each layer. Based on the obtained thermal characteristics, electro thermal and thermo mechanical models were developed in SIMULINK to determine the thermal behaviour of each layer and provide total lifetime consumption analysis. The developed models were verified by real-time (RT) experiments using dSPACE environment. New materials, such as silicon carbide (SiC) devices, were found to exhibit approximately 20°C less thermal profile compared to conventional silicon IGBTs. For PECs used within wind energy systems, PECs driving algorithms were derived within the proposed models and by adjusting switching frequency PECs cycling temperatures were reduced by 12°C which led to a significant reduction in thermal stress; approximately 27 MPa. Total life consumption for the proposed method was calculated as 3.26x10-5 which is approximately 1x10-5 less compared to the other both methods. Effects of maximum power tracking algorithms, used in photovoltaic solar systems, on thermal stress were also explored. The converter’s thermal cycling was found approximately 3 °C higher with the IC algorithm. The steady state temperature was 52.7°C for the IC while it was 42.6 °C for P&O. In conclusion, IC algorithm offers more accurate tracking accuracy; however, this is on the expense of harsher thermal stress which has led to approximately 1.4 times of life consumption compared to P&O under specific operating conditions.

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This thesis outlines a detailed study comprising the simulation, design, construction, and experimental validation of two prototypes of looped-tube travelling wave thermoacoustic electricity generators. The prototypes used air at atmospheric pressure as working gas, an audio loudspeaker as linear alternator, while most of acoustic resonator parts were made of PVC components. The hot heat exchangers were externally heated. The first prototype was a small size, single-stage generator powered by combustion from a propane gas burner. The feedback pipe includes a branched alternator and an acoustic impedance matching stub. The effect of the heat input on the generator performance has been investigated. 13W of electrical power, extracted by 12Ω electric load, was achieved at flue gas temperature of 627.4°C and regenerator temperature difference of 430K. The second prototype was a larger sized (resonator inner diameter is 128 mm) two-stage generator. Here, the heat source was an electric air heat “gun” than provides air stream at 650°C. Firstly, the single-stage engine was examined at frequencies of 48.82, 64.45 and 70.31Hz. The experiments show that the streaming can reduce acoustic power from 110.63 to 63.4W at 70.31Hz. Furthermore, the acoustic power increases from 111.45 to 153 W when the frequency is increased from 48.82 to 64.45 Hz. Subsequently, a linear alternator was attached to the engine to convert the produced acoustic power into electricity. At a load of 9Ω the single-stage generator achieved 8.52W of electricity and 1.47% efficiency. The generator was further improved by incorporating an identical second stage. The effects of matching stub length, variable load and heat input were investigated. The two-stage prototype generated 14.18W of electrical power to a load of 9Ω with a thermal-to-electrical efficiency of 2.1%. The result indicated that the generators can supply enough electricity to power LED bulbs, and charge mobile phones and batteries.

The growing demand for energy on the one hand and climate change, which is closely related to increased energy consumption, on the other, have become central issues worldwide. The present investigation aims at identifying potential ways in which renewable energy can become more appealing and efficient by storing the energy produced, for example by a wind turbine or by residual heat, so that energy is readily available for the consumer in both peak and off peak hours. We can look at this both in terms of an energy storage device or as thermal insulation. An ideal thermal energy storage device should be easy and fast to charge and discharge, while sustaining minimal losses when inactive. The first aspect requires a search for materials with high thermal conductivity while in contact with the storage medium (an organic or aqueous-based substance). Amongst the many possibilities, we focus here on nanoparticles within a fluid, i.e. a nanofluid, as a means to store latent heat that is produced for example by a renewable source. Over the past decade nanoparticles have shown the potential to enhance the thermal conductivity of base fluids such as water, ethylene glycol and engine oil, in some cases quite dramatically. The idea here is to embed nanoparticles within a base fluid undergoing a phase change, in order to improve the characteristics of the charge/discharge cycle. However, this phenomenon is poorly understood and the literature is contradictory. Moreover, to avoid the aggregation and improve nanoparticle dispersion, surfactants are added to the nanofluid, thus introducing an additional active element whose role has to be understood. This Thesis focuses on the knowledge gained from simulations and experiments to propose modifications of the nanoparticles-surfactant-base fluid system to improve heat conduction for use in thermal energy storage devices.

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The thesis focuses on the control of asymmetric permanent magnet synchronous generator (PMSG) system, with particular reference to the suppression of its second harmonic (2h) power, DC bus voltage and torque ripples. The asymmetries include the unbalanced resistances, unbalanced inductances, and unbalanced 3-phase back-electromotive forces (EMFs). The mathematical model of the general asymmetries in the PMSG system is firstly presented. The power ripple and torque ripple due to the asymmetries without/with negative-(N-) sequence currents are then analysed in detail. It shows that there are 2h impedances in the synchronous dq-axis frame. Consequently, the N-sequence currents emerge under the conventional current proportional and integral (PI) control, which will result in undesired 2h power, DC bus voltage and torque ripples. To suppress the 2h torque resulted from the N-sequence currents, three typical methods aiming for balanced currents without N-sequence currents are reviewed, evaluated and their relationship is revealed. It shows that all these three methods are capable of suppressing the N-sequence currents as verified by experiments. However, the 2h power and DC bus voltage cannot be suppressed. To suppress the undesired 2h power and DC bus voltage, an improved power control without any sequential component decomposers under general unbalanced conditions is proposed. Its effectiveness is validated by elaborated experiments on a prototype PMSG with inherent asymmetry and deliberately introduced asymmetries. However, the 2h torque is compromised. To solve the 2h torque, power and DC bus voltage simultaneously, the compensation in parallel with the DC bus is investigated in the PMSG system with asymmetric impedances. The undesired 2h power from the PMSG is compensated by the 2h power from the compensation unit. Two topologies of the compensation unit and corresponding control methods are investigated, while the compensation effectiveness is validated by experiments. Furthermore, the compensation unit with external circuits in series with the asymmetric PMSG is investigated. By the compensation in series, the original unbalanced system is modified to a balanced system in theory. Therefore, the N-sequence currents, 2h power, DC bus voltage, and torque ripple can be naturally suppressed. The feasibility of this compensation method is verified by experiments at different speeds and load conditions, although the effectiveness may be slightly affected by the non-linearity of the compensation inductors in practice. Finally, the research of suppressing the 2h DC bus voltage and torque ripple is extended to the dual 3-phase PMSG system with one channel failed. By utilizing the windings, rectifier or inverter in the faulty channel which are still functional, three methods designated as two sets in parallel, two DC buses in parallel and N-sequence currents compensation are investigated, which require minimum extra hardware investment compared with the compensation in parallel and in series.