This thesis describes a programme of research encompassing the design, optimisation and experimental testing of a brushless doubly fed reluctance machine (BDFRM) for use as an aerospace electrical generator, specifically a direct line connected generator, to widen the input shaft speed beyond the normal constraints imposed by the 360-800Hz specification for a variable-frequency AC aerospace network. BDFRMs offer the functionality of frequency correction, via a control winding, and have the advantage of using a robust reluctance rotor. A partially-rated control winding converter can, in principle, be used to provide the slip power required. A further advantage of the BDFRM is the inherent fail-safe nature of these machines, with the output voltage collapsing as soon as the control winding current has been removed. A synchronous reluctance machine was studied as a means of providing a baseline for the BDFRM performance, including a comparison of the effect of scaling on power density. A large number of time-stepped finite element simulations were undertaken to explore BDFRM performance, in particular, the influence of magnetic saturation in limiting the achievable power density and in compromising power quality. Detailed optimisation of a BDFRM was undertaken, including systematic mechanical design of the rotor for high speed operation. This analysis illustrates the significant compromises in machine electromagnetic performance which result from the need to accommodate mechanical stress. The scope for employing small amounts of permanent magnet material in the rotors of both synchronous reluctance machines and BDFRMs, to improve the machine performance at the lower end of the current density range, was investigated. Following detailed optimisation, a demonstrator machine was manufactured, which includes a skewed rotor. The performance of this machine was measured at a number of test points to verify predictions of output power, voltage and voltage harmonics.

No description available.

The ultimate goal of this thesis was to investigate and produce an MFC with self-sustainable cathode so it could be implemented in real world applications. Using methods previously employed [polarisation curve experiments, power output measurements, chemical assays for determining COD in wastewater and other elements present in anolyte or catholyte, biomass assessments] and with a focus on the cathode, experiments were conducted to compare and contrast different designs, materials and nutrient input to microbial fuel cells with appropriate experimental control systems. Results from these experiments show that: Firstly, the choice of polymeric PEM membrane showed that the most effective materials in terms of power performance were cation exchange membranes. In terms of cost effectiveness the most promising was CM-I, which was the preferred separator for later experiments. Secondly, a completely biotic MFC with the algal cathode was shown to produce higher power output (7.00 mW/m2) than the abiotic control (1.52 mW/m2). At the scale of the experimental system, the reservoir of algal culture produced sufficient dissolved oxygen to serve the MFCs in light or dark conditions. To demonstrate usable power, 16 algal cathode-designed MFCs were used to power a dc pump as a practical application. It has been presented that the more power the MFC generates, the more algal biomass will be harvested in the connected photoreactor. The biomass grown was demonstrated to be a suitable carbon-energy resource for the same MFC units in a closed loop scenario, whereby the only energy into the system was light. In the open to air cathode configuration various modifications to the carbon electrode materials including Microporous Layer (MPL) and Activated Carbon (AC) showed catholyte synthesis directly on the surface of the electrode and elemental extraction such as Na, K, Mg, from wastewater in a power dependent manner. Cathode flooding has been identified as an important and beneficial factor for the first time in MFCs, and has been demonstrated as a carbon capture system through wet scrubbing of carbon dioxide from the atmosphere. The captures carbon dioxide was mineralised into carbonate and bicarbonate of soda (trona). The novel inverted, tubular MFC configuration integrates design and operational simplicity showing significantly improved performance rendering the MFC system feasible for electricity recovery from waste. The improved power (2.58 mW) from an individual MFC was increased by 5-fold compared to the control unit, and 2-fold to similar sized tubular systems reported in the literature; moreover it was able to continuously power a LED light, charge a mobile phone and run a windmill motor, which was not possible before.

A comparison of two vehicle power-train topologies is performed for a small fuel cell andsupercapacitor powered delivery vehicle to determine the power-train weight and costtrade-off. The topologies use either a single DC-DC converter to interface the fuel cell,with the supercapacitors directly connected to the traction drive, or two convertersseparating both the fuel cell and supercapacitor from the traction drive. Energymanagement strategies are implemented to increase system efficiency at the expense ofincreased supercapacitor storage. The two converter system results in a lower cost and mass design due to a higherutilisation of the supercapacitor buffer. The energy management strategy can increaseefficiency, however, in this study the duty of the vehicle and the cost of hydrogen are lowresulting in a fuel saving that is outweighed by the additional hardware outlay. To interface supercapacitors and fuel cells with traction drives, efficient and compact DCDCconverters are required, therefore a dual-interleaved boost converter with interphasetransformer is designed and a volumetric and mass dense prototype is produced. Theconverter uses an inductor and interphase transformer which are potted in heat sinksbolted to a water cooled cold plate. Thermal finite element analysis and a MATLAB designprogram are used to determine the smallest thermally suitable inductor and interphasetransformer designs. The resulting prototype is superior in mass and volumetric powerdensity to commercial products, achieving 4.3 kW/kg and 7.2 kW/l including casing, usingoff-the-shelf components, and designed with a low part count. The anticipated nextgeneration converter will increase these power densities to 5.2 kW/kg and 9.2 kW/lthrough a modified layout and refined magnetic design.

Electrification has become a pivotal development issue in the developing countries, as it provides a huge range of social and developmental advantages. At the same time it has been realised that delivering electricity in the rural areas, particularly to poor, is a hard task and requires establishment of effective institutions and delivery mechanisms. If not properly planned, highly subsidised rural electrification programmes may end up in drain of resources and damaging impacts on the utilities. These challenges are probably better illustrated in the Indian case, where half of the population still living in dark. In recent years, centralised planning and resource allocation, which used to be the governing principle for development, has been blamed for the failure. As a response to the perceived failure of top-down centralised planning and implementation, bottom-up decentralised participatory models have been proposed by international development organisations. The bottom-up model proposed for electric service delivery seek to involve the users in the delivery process through building micro-institutions and empowering them to plan, manage, monitor, and own the local service delivery mechanism. The proposed model marks the beginning of a new paradigm for electricity service delivery that relies on the users and their democratic capabilities. In this context, this study, drawing on experiences in two cases in Eastern India, analyses the potentials of decentralised participatory model of electricity delivery. It provides an empirical analysis of how and to what extent decentralisation and users’ participation in electricity delivery contributes to efficiency and effectiveness gain in electricity supply system. Moreover, building on participatory democracy, the study analyses the empowering effects of participation in electricity users associations. It concludes that decentralisation and users’ participation has significant contributions to electricity service improvement. Yet, it identifies scope for improvement in the model and suggests some methods and approaches by which the model could be made more efficient and effective, and can produce real gains for the poor.

The concept of harvesting electrical energy from ambient vibration sources has been a popular topic of research in recent years. The motivation behind this research is largely due to recent advancements in microelectromechanical systems (MEMS) technology - specifically the construction of small low powered sensors which are capable of being placed in inaccessible or hostile environments. The main drawback with these devices is that they require an external power source. For example, if one considers large networks of low powered sensors (such as those which may be attached to a bridge as part of a structural health monitoring system) then one can envisage a scenario where energy harvesters are used to transfer the vibration energy of the bridge into electrical energy for the sensors. This would alleviate the need for batteries which, in this scenario, would be difficult to replace. Initial energy harvester designs suffered from a major flaw: they were only able to produce useful amounts of power if they were excited close to their resonant frequency. This narrow bandwidth of operation meant that they were poorly suited to harvesting energy from ambient vibration sources which are often broadband and have time dependent dominant frequencies. This led researchers to consider the concept of nonlinear energy harvesting - the hypothesis that the performance of energy harvesters could be improved via the deliberate introduction of dynamic nonlinearities. This forms the main focus of the work in this thesis. The first major part of this work is concerned with the development of an experimentally validated physical-law based model of an electromagnetic energy harvester with Duffing-type nonlinearities. To this end, a self-adaptive differential evolution vi (SADE) algorithm is used in conjunction with experimental data to estimate the parameters needed to accurately model the behaviour of the device. During this investigation it is found that the response of the energy harvesting device in question is very sensitive to the effects of friction. Consequently, a detailed study is undertaken with the aim of finding whether the model performance could be improved by accounting for this complex nonlinear phenomenon. After investigating several different friction models, a reliable and extensively validated digital model of a nonlinear energy harvesting device is realised. With the appropriate equations of motion identified, analytical approximation methods are used to analyse the response of the device to sinusoidal excitations. The motivation for the second main part of this work arises from the fact that ambient excitations are often stochastic in nature. As a result, much of the work in this section is directed towards gaining an understanding of how nonlinear energy harvesters respond to random excitations. This is an interesting problem because, as a result of the random excitation, it is impossible to say exactly how such a device will respond - the problem must be tackled using a probabilistic approach. To this end, the Fokker-Planck-Kolmogorov (FPK) equation is used to develop probability density functions describing how the nonlinear energy harvester in question responds to Gaussian white noise excitations. By conducting this analysis, previously unrecognised benefits of Duffing-type nonlinearities in energy harvesters are identified along with important findings with regards to device electrical optimisation. As for friction effects, the technique of equivalent linearisation is employed alongside known solutions of the FPK equation to develop expressions approximating the effect of friction on randomly excited energy harvesters. These results are then validated using Monte-Carlo methods thus revealing important results about the interaction between Duffing-type and friction nonlinearities. Having investigated sinusoidal and random excitations, the final part of this work focuses on the application of nonlinear energy harvesting techniques to real energy harvesting scenarios. Excitation data from human walking motion and bridge vibrations is used to excite digital models of a variety of recently proposed nonlinear energy harvesters. This analysis reveals important information with respect to how well energy harvesting solutions developed under the assumption of Gaussian white noise excitations can be extended to real world scenarios.

Two major challenges, i.e., bulky electrolytic capacitors and isolation transformers, remain as critical obstacles for further improvement on reliability, power density and efficiency of power electronic converters, which are mainly used to reduce low-frequency voltage ripples and high-frequency common-mode voltage ripples, respectively. In order to overcome the two challenges, the most straightforward way is to simply combine existing solutions developed for each of them. However, this would considerably increase system complexity and cost, which should be avoided if possible. In this thesis, these two challenges are innovatively addressed in a holistic way by using active control techniques. This thesis first focuses on the reduction of low-frequency voltage ripples in conventional half-bridge converters, after adding an actively-controlled neutral leg. As a direct application of this strategy, a single-phase to three-phase conversion is then proposed. After that, a ρ-converter with only four switches is proposed to significantly reduce both low-frequency ripples and high-frequency common-mode ripples in a holistic way. It is found that the total capacitance can be reduced by more than 70 times compared to that in conventional full-bridge converters. As a result, there is no longer a need to use bulky electrolytic capacitors and isolation transformers. Then, the ρ-converter equipped with the synchronverter technology is operated as an inverter for PV applications. Another converter is also proposed for the same purpose but with reduced voltage stress. In order to further reduce the total capacitance and to reduce the neutral inductor in the ρ-converter, a new type of converter, called the θ-converter, is proposed. Finally, two actively-controlled ripple eliminators are proposed to reduce low-frequency ripples in general DC systems while the aforementioned research is focused on some specific topologies. Extensive experimental results are presented to validate most of the developed systems while the rest are validated with simulation results.

Organic solar cells based on P3HT: PCBM bulk heterojunctions show promise for high power conversion efficiencies. The properties of composite donor polymers and acceptor materials play a significant role; hence the need for optimised bulk heterojunctions active layer morphologies is critical for efficient devices. To achieve optimised bulk heterojunctions, compositional factors and processing conditions are two primary aspects to focus on. Thermal annealing has been demonstrated to be one of the most successful processing techniques for morphology optimisation in P3HT – based organic solar cells. However the crucial correlation between composite composition and thermal annealing in P3HT – based devices is not fully understood yet. Combining optical absorption spectroscopy, structural and electrical methods; the properties of P3HT: PCBM blend thin films, with different PCBM percentage weight ratios were studied in this work. Optical absorption spectra results for all three blend ratios, i.e., 1:1, 1:0.8, and 1:0.6, showed that the peak absorption intensity associated with PCBM reduced the most for the 1:1 ratio, after thermal annealing at 175°C. The impact of the correlation between PCBM composition and thermal annealing on photovoltaic performance parameters was demonstrated. For the three different PCBM compositions, the optimum power conversion efficiencies were determined at different optimum thermal annealing conditions. Optimum power conversion efficiency of 3.38% (1:1) was obtained at 175°C, whilst 2.27% (1:0.8) and 1.44% (1:0.6) were demonstrated at 125°C respectively. To further probe the influence of thermally induced PCBM molecular segregation and aggregations, three different thermal annealing strategies were employed; namely, annealing (i) gradually from 50°C – 175°C, in steps of 25°C, 10 minutes each (ii) at high temperature 175°C, for 10 minutes once, and (iii) at 175°C for a longer time, i.e., 60 minutes . Optical absorption spectroscopy results reveal the dependence of PCBM aggregation on different thermal annealing strategies. Employing Raman spectroscopy mapping methods, the surface of thin films were mapped revealing and confirming PCBM rich regions upon thermal annealing. Furthermore exciton generation rate studies proved useful in establishing a good correlation between the estimated excitons generated, with short circuit current densities. The observed increase in excitons generated was also consistent with the photoluminescence spectra results which showed an enhancement in intensities upon thermal annealing. ii Importantly, this work has shown the significance of employing different thermal annealing strategies in nanomorphology control of the bulk of active layers of organic photovoltaic devices. Importantly, it has also been demonstrated in the work of this thesis that gradual thermal annealing, in a controlled manner revealed a more stable and efficient control in tuning the nanomorphologies of P3HT – based solar cells. In addition, impedance spectroscopy and capacitance – voltage measurement techniques have been shown to be very useful tools for characterising organic photovoltaic devices. Herein, it was shown that after thermal annealing at the optimum temperature of 150°C, impedance spectroscopy characterisation revealed extended charge carrier lifetimes in devices. This highlights the significance of having an optimised interpenetrating network within active layers of organic solar cells, as this have a critical impact on charge carrier lifetimes. Capacitance – voltage measurements was used to demonstrate the thermally induced vertical segregation of PCBM molecular aggregates also. The decrease in measured built – on potential from 0.68V (at film/cathode interface) from as cast device to about 0.35V after thermal annealing at 150°C, was shown to be indicative of vertical segregation.

No description available.

Photovoltaic systems provide a solution for harnessing energy from the sun whilst reducing global greenhouse gas emissions. Organic photovoltaics (OPV), based on blends of polymer and fullerene materials, have recently received widespread interest within academia and business due to its potential for low costs, minimum environmental impact and rapid roll-to-roll manufacture. In addition, modules can be customised for flexibility, light-weight and colour. However, OPV modules also show considerably lower efficiencies than mature photovoltaic technologies, as well as much reduced lifetimes. This thesis presents a framework for the analysis of the potential for GHG mitigation by assessing how unique characteristics of the technology may influence its GHG mitigation potential. In the first instance, the extent to which OPV can realise the goal of low costs and minimal environmental impact is assessed. This analysis shows that GHG emissions from OPV manufacture are considerably lower than mature PV technologies. However, the economic cost of OPV, whilst lower on the basis of capital costs, struggles to compete on a levelised basis assuming current projections for lifetime, suggesting that the technology will only have an impact on GHG mitigation outside conventional PV applications. Two potential niche applications are analysed as case-studies. Analysis of using OPV materials as a partially transparent coating in greenhouse structures concludes that currently available OPV materials can provide significant power with minimal impact on crop growth. However, partial coverage with opaque crystalline silicon modules can provide more electricity with less impact on crops. The second case-study looks at the application of OPV in providing off-grid lighting, assessing the emissions and costs associated with this application as well as looking at the long term degradation of OPV in the harsh environments, through a field trial of the technology in Southern Rwanda. Finally, analysis of the transition to solar energy technology shows that OPV can provide much more rapid emission reductions in comparison to mature technology, despite higher levelised emissions. This work concludes that organic photovoltaic technology may be limited to a few niche applications where inferior performance does not present a challenge, or unique properties are advantageous. However, the application case-studies presented in this thesis show a large GHG mitigation potential from such applications, and analysis of the transition to PV demonstrates the added advantage of OPV in realising rapid emission reductions.