Tidal stream turbines have the potential to generate electricity from a renewable source that is highly predictable and non-polluting. The United Kingdom’s geographical location means that it has one of the most significant wave and tidal resources in the world. The implications of controlling horizontal axial flow, variable speed tidal stream turbines for the purpose of optimising energy capture, regulating power and reducing mechanical loads has been investigated. The research was separated into two main areas: control of the turbine in below rated flow speeds where the objective was to maximise energy capture, and control of the turbine in high flow speeds where the aim was to regulate power, shed load and ensure the survivability of the turbine. A control strategy that maximises the power output of a tidal stream turbine in below rated flow speeds, by allowing the rotor speed to be varied, was implemented. The performance of the control strategy was assessed in simulation and then implemented on a hardware in the loop test rig. In both cases the controller performed well and was able to maximise power output from the turbine by varying the rotor speed to maintain the optimum tip speed ratio. Two control strategies for regulating the output power from a tidal stream turbine in high flow speeds were investigated and compared. The first strategy relies on adjusting the pitch angle of the rotor blades to regulate power and the second assumes fixed pitch rotor blades and relies on the stall characteristic of the blades to regulate power. Models of both the pitch and stall regulated turbines were developed and simulated using the commercially available software GH Tidal Bladed®. The control system design behind both strategies was developed in Matlab. Following control implementation a comparison of each model was carried out in terms of performance under turbulent flows, loading and energy yield. Following the comparison it was concluded that the performance of the pitch regulated tidal stream turbine was superior to that of the stall regulated turbine. The research also revealed that the dynamics of each turbine model are significantly different for above rated flow speed operation. This has implications on controller design and loading experienced by each tidal stream turbine.

The use of Combined Heat and Power (CHP) units, heat pumps and electric boilers increases the linkages between electricity and heat networks. In this thesis, a combined analysis was developed to investigate the performance of electricity and heat networks as an integrated whole. This was based on a model of electrical power flow and hydraulic and thermal circuits together with their coupling components (CHP units, heat pumps, electric boilers and circulation pumps). The flows of energy between the electricity and heat networks through the coupling components were taken into account. In the combined analysis, two calculation techniques were developed. These were the decomposed and integrated electrical-hydraulic-thermal calculation techniques in the forms of the power flow and simple optimal dispatch. Using the combined analysis, the variables of the electrical and heat networks were calculated. The results of the decomposed and integrated calculations were very close. The comparison showed that the integrated calculation requires fewer iterations than the decomposed calculation. A case study of Barry Island electricity and district heating networks was conducted. The case study examined how both electrical and heat demands in a self-sufficient system (no interconnection with external systems) were met using CHP units. A solution was demonstrated to deliver the electrical and heat energy from the CHP units to the consumers through electrical and heat networks. The combined analysis can be used for the design and operation of integrated heat and electricity systems for energy supply to buildings. This will increase the flexibility of the electricity and heat supply systems for facilitating the integration of intermittent renewable energy.

To enable the advancement of Si based technology, necessary to increase computing power and the manufacture of more compact circuits, significant changes to the current planar transistor are a necessity. Novel transistor architectures and materials are currently being researched vigorously. This thesis, on the electrical characterisation of non-standard orientated MOSFETs and multi-gate transistors displays detailed insight into the carrier transport and resulting performance limiting mechanisms. The results are composed of three parts. Firstly, the standard method of extracting carrier effective mobility from electrical measurements on MOSFETs is reviewed and the assumptions implicit in this method are discussed. A novel technique is suggested that corrects the difference in drain bias during current-voltage and capacitance-voltage measurements. It is further shown that the lateral field and diffusion corrections, which are commonly neglected, in fact cancel each other. The efficacy of the proposed technique is demonstrated by application to data measured on a quasi-planar SOI finFET at 300 K and 4 K. The second part is based on the electrical characterisation of n+poly-Si/SiO2/Si nand p- MOSFETs fabricated on (100) and (110) substrate orientations with the full range of channel directions. In depth analysis of the electron and hole mobility was performed at 300 K and 4 K. The 4 K mobilities were modelled in terms of ionised dopant impurity, local SiO2/Si interface charge and roughness scattering mechanisms. RMS (root mean squared) roughness values in the range 0.34 − 0.38nm and correlation lengths of 2.0 − 2.3 nm were extracted revealing comparable interface quality between the (100) and (110) surfaces. The third part examines the electrical characterisation of TiN/HfSiO2/Si n- and pfinFETs. Fin top surface and sidewalls are in the (100) and (110) planes respectively. Fins have a height of 65 nm with widths in the range of 1872 nm (quasi-planar) to 12 nm. Detailed analysis revealed vertical compressive strain induced by the gate into the fin sidewalls, which enhanced the electron mobility by 60% above the (110) reference, whilst leaving the hole mobility unaffected. Qualitative analysis of the 4 K mobilities suggests that roughness is higher on the sidewalls than on the top surface. This was attributed to the damage caused by the dry etch, used to pattern the fins. A model for remote charge scattering at the HfSiO2/SiO2 interface was developed. 4 K mobilities from the quasi-planar n- and pfinFETs were then modelled in terms of remote charge, ionised dopant impurity, local SiO2/Si interface charge and roughness scattering mechanisms. Remote charge densities of 8x1012 cm-2 were subsequently extracted. Scattering from these charges was shown to be the dominant scattering mechanism in the quasi-planar n-finFET mobility at 300 K.

This thesis describes the development and characterisation of an airborne ultrasonic communication system using electrostatic transducers. Initially, digital modulation schemes, namely OOK, BFSK and BPSK were used for transmission of a digital binary stream across an air gap of 0 – 1.2 m. BPSK was found to be the most favourable solution for the lowest BER performance. BFSK, being the second best among the three was simpler to implement and was more robust against synchronisation errors. Later, to further improve bandwidth efficiency, several M-nary modulations, namely QPSK, MSK and 8PSK were used. The data rate reached 800 kbps when 8PSK was used. An ultrasonic keyboard using BFSK were developed as a simple application. A real-time communication system was also developed using a PXI system and LabVIEW, with which broadband data communication and real-time performance analysis was realised. The channel response of the ultrasonic communication link was modelled, with the characteristic of ultrasonic absorption in air, transducer response, and spatial field response. The latter was found to be crucial factor when the transreceivers were not aligned in parallel. The results recorded at various distances were in good agreement with those predicted using the system model developed. Several equalisation techniques were looked at in this thesis, to combat the selective attenuation response for longer-range propagation, and its effectiveness has been compared to using multicarrier modulation. This thesis has also looked at multipath interference, which is likely to be effect the performance in future applications. It shows that by using spread spectrum techniques, multipath interference can be tackled in an ultrasonic system.

Spectrum-slicing (SS) is a novel attractive technique for the implementation of optical access network. Its main advantage compared to the conventional optical network is utilizing spectral slices of a broadband source for different data channels. Since the light sources employed are quite cost effective, this technology is economically attractive to the modern communication system. The major objective of this dissertation is to investigate the performance of systems employing spectrum-slicing, for implementing wavelength division multiplexing (WDM) and optical code division multiple access (OCDMA) in optical access systems. The analysis is conducted mainly through the theoretical model based on the statistics, derived from fundamental chi-square and Gaussian distribution. Firstly, the analysis of an SS-WDM system performance when employing an optical preamplifier receiver is carried out to improve understanding of the SS in a realistic situation where dispersion significantly impacts the signal in transmission. It is shown that there exists an optimum optical bandwidth which minimizes the detection sensitivity for a given error probability caused by two competing effects of inherent signal fluctuation and dispersion. The optically preamplified receiver delivers increased transmission capacity and a substantially improved power budget compared to a pin receiver. The results are obtained using the saddle point approximation and compared to the customary Gaussian approximation, which is found to be reasonably accurate in predicting the optimum bandwidth but conservative in sensitivity predictions. The second part of the work investigates the performance of an SS-OCDMA employing and proposes a supporting adaptive coding scheme developed from prime-hop codes. The concomitant higher-order dispersion, beat noise and multiple access interference in incoherent OCDMA systems become limiting factors to the bit error rate. The major thrusts of the new schemes are to alleviate the performance degradation from these impacts especially to reduce the power loss and the bit error rate (BER) degradation due to higher-order dispersion. Performance comparisons between the adaptive PHC and original PHC schemes indicate that the former is more suitable for use in the considered incoherent system in terms of accommodating more users for a given BER. The proposed adaptive method can be universally applied to mitigate dispersion effects in the similar 2-D OCDMA systems. Within the last part of the work, regular low-density parity-check (LDPC) codes are applied for the first time to a SS-WDM system. An adaptive decoding algorithm of low density parity check (LDPC) codes based on the precise SSWDM noise statistics is developed, which outperforms the decoding with conventional Gaussian model. The simulation results for various code rates show that such a forward error correction scheme provides significant coding gain for a dedicated system in terms of improving the overall transmission capacity and available power budget. The adaptive algorithm of LDPC codes can be generically applied to all the asymmetric channels.

The increasing demand for greater storage densities poses constant challenges for the magnetic recording industry. The move to perpendicular recording occurred recently as this provides one·way of increasing the storage density. For further advance in the techriology it is important to gain an understanding of the micromagnetic and microstructural propertiesof the materials used in the hard drive. The material used in the write head is extremely important as the storage density is increased. CoFe alloys are currently of interest as they have a high magnetic moment and therefore have the potential to write to the high anisotropy materials required for writing at the high densities. Transmission electron microscopy is an ideal tool to gain such information as it provides a way of extracting magnetic information of the nanometre scale and structural information on the atomic scale. Chapter 1 of this thesis provides an introduction to ferromagnetic ma. terials and the magnetic energy contributions which are present. The principles of magnetic recording are then discussed along with the advantages of switching to perpendicular recording. The requirements for the write head material are discussed at the end of chapter 1 and a review of previous work on CoFe materials is given. The main experimental techniques used for the work in this thesis are described in chapter 2. Firstly the sputter deposition of samples is discussed along with the characterisation of the bulk films with a BH looper. A detailed description of transmission electron microscopy (TEM) and the various imaging modes which have been used is given. The Fresnel mode and the differential phase contrast (DPC) mode of Lorentz microscopy have been used to study the magnetisation reversal behaviour of the films and these are described. Electron energy loss spectroscopy (EELS), which was used to investigate the elemental distribution in the films, is also discussed. A dual beam SEM/FIB was used to investigate the surface topology of the films and a brief description of the instrument is given. The final part of chapter 2 deals with the preparation of cross-sectional specimens with the encapsulation method and the patterning of thin films with electron beam lithography for use in the TEM. ' By way of an introduction to the various reversal behaviour which is observed within magnetic films two soft films, NiFeCuMo and CoFeB, have been investigated. The results from these films are discussed in . chapter 3. In chapter 4 results from four CoFe thin films are presented. While all films were of similar total thickness, 50 nm, the differences were the inclusion or otherwise of a seedlayer and the introduction of nonmagnetic spacer layers to form laminate films. The detailed mechanisms for easy and hard axis reversals of the films were investigated. As expected cross-tie walls were observed in the films with thicker CoFe layers and wall displacements were seen with the introduction of the spacer layers. Magnetisation dispersion was reduced as multilayering was introduced and a significant reduction in the grain size from 12.5~2.8 nm to 7.9±1.5 nm was observed. In the laminated films with three spacer layers defect areas where the magnetisation distribution differed markedly from that in the surrounding film were observed and the formation of 360 0 domain walls was noted. Cross-sectional TE11 showed that the layer roughness increased throughout the film thickness and this was thought to be the probable cause of the localised anomalies. Chapter 5 follows on from chapter 4 with an investigation into the origin of the defect areas and 360 0 domain walls. The easy and hard axis magnetisation reversals for four CoFe laminate films were investigated. The main differences between these filIns and those investigated in chapter 4 was the reduction in spacer layer thickness and variation of CoFe layer thickness. In this case the presence of physical defects which significantly influenced the reversal behaviour were noted. A high density of 360 0 domain walls which persisted up to ~elds of a few hundred oersteds were observed. The walls remained throughout reversal and the likely processes which occur during the reversal are presented. A quantitative investigation with DPC revealed that although some of the 360 0 domain walls exist in only one or two layers of the laminate films there are occasions where the walls exist throughout 'the whole thickness of the film. However, the end points of the 360 0 domain walls are still not ·understood. In an attempt to gain an understanding of the physical structure of the defects and some insight into what happened where the 360 0 domain walls ended, SEM, cross-sectional· TEM and EELS have been carried out and the results are presented at the end of chapter 5 and in chapter 6. The defects were found to originate from particles present on the substrate before the deposition of the CoFe films. The identity of the particles is still unknown, however, due to the regularity ofthe particles they are thought to be caused from some process after the etching of the substrate and not from handling. Cross-sectional TEM revealed the spacer layers are effective at separating the magnetic layers although there are some instances where the growth in one layer may be affected by that in an underlying layer. In chapter 7 the final experimental work which explores the reversal mechanisms occurring in patterned films is presented. Rather than doing the investigation on the compJex CoFe multilayers, six permalloy shapes . with similar dimensions to a write head were studied. It was found that pole tips with widths of 500 nm and 1 f..Lm were influenced significantly by the reversal occurring within the whole shape. This study revealed the obstacles which the CoFe films with well defined anisotropy have to overcome when patterned down to write head dimensions. Finally, the conclusions from the experimental studies are discussed in chapter 8 along with future work which could be carried out.

There has been an increasing demand for low cost, portable CMOS image sensors because of increased integration, and new applications in the automotive, mobile communication and medical industries, amongst others. Colour reproduction remains imperfect in conventional digital image sensors, due to the limitations of the dye-based filters. Further improvement is required if the full potential of digital imaging is to be realised. In alternative systems, where accurate colour reproduction is a priority, existing equipment is too bulky for anything but specialist use. In this work both these issues are addressed by exploiting nanophotonic techniques to create enhanced trichromatic filters, and multispectral filters, all of which can be fabricated on-chip, i.e. integrated into a conventional digital image sensor, to create compact, low cost, mass produceable imaging systems with accurate colour reproduction. The trichromatic filters are based on plasmonic structures. They exploit the excitation of surface plasmon resonances in arrays of subwavelength holes in metal films to filter light. The currently-known analytical expressions are inadequate for optimising all relevant parameters of a plasmonic structure. In order to obtain arbitrary filter characteristics, an automated design procedure was developed that integrated a genetic algorithm and 3D finite-difference time-domain tool. The optimisation procedure's efficacy is demonstrated by designing a set of plasmonic filters that replicate the CIE (1931) colour matching functions, which themselves mimic the human eye's daytime colour response. The best designs were fabricated and demonstrated a least-mean-square error, in comparison to the desired colour matching functions, of 6.37*10^3, 2.34*10^3 and 11.10*10^3 for the red, green, and blue filters respectively. Notably the spectrum for the red filter contained a double peak, as present in the corresponding colour matching function. Such dual peak behaviour cannot be achieved using a single current dye-based filter. The filters retain the same layer thickness for all structures so they can be defined in a single lithography step. A new approach to enable the fabrication of a multispectral filter array on a CMOS imager is also presented. This combines a Fabry-Perot filter with effective medium theory (EMT) to enable the fabrication of multiple filters in a single cavity length via lithographic tuning of the filter passband. Two approaches are proposed; air-filled nanostructures and dielectric backfilled nanostructures. The air-filled approach is demonstrated experimentally producing three filters with FWHM of 60 - 64 nm. Using the backfilled design, and incorporating a highindex cavity material, a set of twenty three narrowband filters, with a FWHM of 22 - 46nm is demonstrated. A virtual image reproduction process was developed to quantify the image reproduction performance of both the plasmonic and Fabry-Perot filter sets. A typical rgb dye-based filter set used in conventional imagers achieves a mean colour error of 2.711, whereas the experimental data from the plasmonic filters achieves an error of 2.222 which demonstrated a slight improvement in colour reproduction. The multispectral filter set developed in this work performed even better, with 4 filters giving an error of 0.906, 10 filters an error of 0.072 and continued improvement in the colour error reaching 0.047 for 23 filters. All the filter sets proposed are fully compatible with the CMOS process so as to enable direct integration onto CMOS image sensors in industrial foundries in future. The performance of the presented filters also suggest new compact applications in art reproduction, agricultural monitoring and medical imaging.

Semiconductor device performance variation due to the granular nature of charge and matter has become a key problem in the semiconductor industry. The main sources of this ‘statistical’ variability include random discrete dopants (RDD), line edge roughness (LER) and metal gate granularity (MGG). These variability sources have been studied extensively, however a methodology has not been developed to accurately represent this variability at a circuit and system level. In order to accurately represent statistical variability in real devices the GSS simulation toolchain was utilised to simulate 10,000 20/22nm n- and p-channel transistors including RDD, LER and MGG variability sources. A statistical compact modelling methodology was developed which accurately captured the behaviour of the simulated transistors, and produced compact model parameter distributions suitable for advanced compact model generation strategies like PCA and NPM. The resultant compact model libraries were then utilised to evaluate the impact of statistical variability on SRAM design, and to quantitatively evaluate the difference between accurate compact model generation using NPM with the Gaussian VT methodology. Over 5 million dynamic write simulations were performed, and showed that at advanced technology nodes, statistical variability cannot be accurately represented using Gaussian VT . The results also show that accurate modelling techniques can help reduced design margins by elimiating some of the pessimism of standard variability modelling approaches.

Concern over global warming and depletion of the ozone layer has stimulated research to develop cooling methods that do not employ environmentally damaging working fluids such as CFCs and HCFCs. Two methods that have been considered are absorption and thermoelectric `Peltier' cooling systems. Absorption systems, using H20/LiBr have the advantage of being able to use low-grade waste heat. However, the large volume, high capital cost and low performance of these systems have inhibited their widespread application. Thermoelectric systems were developed in the 1950s and use of this technology for air-conditioning applications was investigated as early as the 1960s. However, the continued development of thermoelectric systems was slow owing to technical difficulties and the superior performance of vapour-compression systems in terms of coefficient of performance (COP). It is known however that most working fluids employed in vapour-compression systems are damaging to the environment, and as vapour-compression systems contain moving parts, they have the further disadvantage of being noisy and requiring regular maintenance. In recent years therefore, there has been stimulated interest in using thermoelectric "Peltier" cooling systems for domestic refrigerators and air conditioning. Investigation of novel thermoelectric refrigeration systems was carried out in this research. The systems use thermoelectric "peltier" coolers (thermoelectric modules) to produce cooling or heating. Thermoelectric modules are solid state heat pump, which have the advantage of being environmentally friendly, being quiet, have no moving parts and can operate using direct current supplied from photovoltaic solar cells (PVs). This work mainly investigated a passive technology based on integration of a thermal diode and thermoelectric modules for building integrated heat pump. The heat pump uses thermoelectric modules to produce cooling or heating, and the thermal diode to transfer heat in or out of the building, and prevent reverse heat flow in the event of power failure. The heat pump was designed to have the following features: • Very compact and suitable for incorporation within the building structure; • Does not require a plant room and simple to construct; • Easily switched between cooling and heating modes; • Can prevent reverse heat flow in the event of power failure; • Low manufacturing cost; • Environmentally friendly; • Can be driven by solar photovoltaic panels. This work also investigated the potential application of phase change materials (PCMs) in the thermoelectric refrigeration system. The system employs phase change material to improve the performance of a thermoelectric refrigerator as well as the cooling storage capability. The refrigerator employing phase change material was designed to have following features: • Be able to overcome the peak loads and losses during door openings and power-off periods. • Prevent reverse heat flow via thermoelectric modules in the event of the power being turned off by integrating the thermosyphon with the phase change material. • Low manufacturing cost. • Environmentally friendly. • Can be driven by solar photovoltaic panels The research initially involved the investigation of the performance of the components of the thermoelectric refrigeration systems, including thermoelectric modules, heat pipes and heat sinks. The analytical models were developed to evaluate the heat transfer and optimise the design of these components. Correlations between heat transfer and fluid flow inside the heat pipes were explored by computer modelling. The research work further involved the design, modelling, construction and tests of a thermoelectric heat pump prototype. A computer model was developed to evaluate the performance of the heat pump system for two different modes, i. e., cooling and heating, under various operating and ambient temperatures. Laboratory tests were carried out to validate the modelling predictions and experimentally examine the thermal performance of the heat pump. Comparison was made between the modelling and testing results, and the reasons for error formation were analysed and correction was given. Further experimental investigation showed that reducing the temperature of the condenser of the thermal diode could provide a significant improvement of the efficiency of the coefficient of performance (COP) of the system in cooling mode. This can be achieved by using the evaporation of water on the heat sink attached to the condenser. The research work also involved the design, construction and tests of a thermoelectric refrigerator employing phase change material. The work intended to investigate the potential application of phase change materials (PCMs) in the thermoelectric refrigeration system. The system was first fabricated and tested using a conventional heat sink system as the cold heat sink. In order to improve the performance and the storage capability, the system was reconstructed and tested using an encapsulated PCM as a cold heat sink. Results of tests of the latter system showed an improved performance compared with the former system. However, to improve the storage capability, in particular during off-power periods, it was found necessary to integrate the PCM with a thermosyphons, which would allow heat flow in one direction only. Results of tests carried out on the system employing phase change material integrated with thermosyphons showed considerable improvement in the storage capability of the thermoelectric refrigeration system compared with the previous ones. On the basis of the above investigation the further work for improving the performance of the thermoelectric refrigeration system was suggested, which is illustrated in Chapter 7, and its key technical issues are discussed.

This thesis presents an experimental investigation into the feasibility of using a functionally thermal fluid to enhance the performance of a Proton Exchange Membrane (PEM) Fuel Cell. Specifically, a fluid was developed that utilised a liquid-solid phase change to enhance heat transport within the fuel cell. Increasing the convective heat transfer coefficient could permit the use of smaller volumetric flow rates and reduce pumping power. The objective of the thermal fluid was to create isothermal conditions across a fuel cell and to reduce parasitic loadings from pumps and other components to enhance the overall system performance. Additionally, the fluid could reduce the system size and component cost, and stabilise temperature fluctuations within the system. The thermal fluid that was developed constituted a mix of fine, Microencapsulated Phase Change Material (MicroPCM) particles suspended in a single-phase working fluid. For successful integration with the fuel cell, the microPCMs thermal and fluid properties, and their effectiveness in transferring heat, had to be fully characterised and understood. Research consisted of experimental investigations of the fuel cell, followed by microPCM development. Experimentation on the fuel cell stack revealed a requirement for thermal stability and reduction in parasitic load from the pumps. Quantitative characterisation and development of the microPCM properties involved state of the art equipment to measure the latent heat of fusion, melting and freezing points, surface morphology and viscosity of the microPCM slurry. The effects of repeated use of solid to liquid phase change particles upon melting and solidification were studied. This lead to the further development of microPCM particles and experimentally examined in a fuel cell system. The use of MicroPCM developed in this study balanced the improvement in thermal capacity of the fluid with the increase in pumping load, when compared to the use of water alone. The study suggested that with further development of the microPCM slurry, it has the potential to significantly increase the thermal capacity of the fluid and stabilise temperatures across the fuel cell, which in turn would results in improved stack performance and electrical conversion efficiency.