Energy supply nowadays, being a vital element of a country’s development, has to independently meet diverse, sustainability criteria, be it economic, environmental and social. The main goal of the present research work is to present a methodological framework for the evaluation of alternative energy and fuel Supply Chains (SCs), consisting of a broad topology (representation) suggested, encompassing all the well-known energy and fuel SCs, under a unified scheme, a set of performance measures and indices as well as mathematical model development, formulated as Multi-objective Linear Programming with the extension of incorporating binary decisions as well (Multi-objective Mixed Integer-Linear programming). Basic characteristics of the current modelling approach include the adaptability of the model to be applied at different levels of energy SCs decisions, under different time frames and for multiple stakeholders. Model evaluation is carried for a set of Greek islands, located in the Aegean Archipelagos, examining both the existing energy supply options as well future, more sustainable Energy Supply Chains (ESCs) configurations. Results of the specific research work reveal the social and environmental costs which are underestimated under the traditional energy supply options' evaluation, as well as the benefits that may be produced from renewable energy based applications in terms of social security and employment.

Solution processable organic semiconductors offer a promising route towards low-cost solar photovoltaics. The performance of these devices is critically dependent on the morphology of the thin film active layer and is very sensitive to both the chemical structure and deposition conditions of the materials. In this thesis a range of complementary techniques are used to characterise the morphology, particularly resonant Raman spectroscopy and atomic force microscopy, in addition to analysis of the device performance. By comparing these results we are able to fulfil the aim of this project, which was to elucidate the fundamental relationships between the thin film morphology and photovoltaic performance for a range of organic and hybrid solar cells. For polymer/polymer blends we consider the impacts of nanowire formation, molecular weight, and thermal annealing on the thin film molecular order. By controlling the interactions between the two polymers we are able to increase the charge carrier mobilities by several orders of magnitude, resulting in reduced bimolecular recombination and enhanced device efficiency. For the hybrid polymer/inorganic devices that we consider, we identify an interfacial region of disordered polymer, which can be partly controlled but not fully overcome. We suggest that this represents an intrinsic limitation, which should be addressed by considering alternative routes to interface formation. Donor-acceptor copolymers are an important class of materials showing promising optoelectronic properties for polymer/fullerene solar cells. We consider how various chemical modifications including fluorination, side chain branching, and heavy atom substitution affect the molecular properties and thin film morphology. In particular, we consider the nature of the electronic absorption transitions of diketopyrrolopyrrole-based copolymers and find that the low energy transition is localised on the diketopyrrolopyrrole unit and is very stable to photodegradation, whereas the high energy transition couples more strongly to the donor unit, which is more vulnerable to photooxidation.

Large wind turbines are subjected to the harmful loads that arise from the spatially uneven and temporally unsteady oncoming wind. Such loads are the known sources of fatigue damage that reduce the turbine operational lifetime, ultimately increasing the cost of wind energy to the end users. In recent years, a substantial amount of studies has focused on blade pitch control and the use of real-time wind measurements, with the aim of attenuating the structural loads on the turbine blades and rotor. However, many of the research challenges still remain unsolved. For example, there exist many classes of blade individual pitch control (IPC) techniques but the link between these different but competing IPC strategies was not well investigated. In addition, another example is that many studies employed model predictive control (MPC) for its capability to handle the constraints of the blade pitch actuators and the measurement of the approaching wind, but often, wind turbine control design specifications are provided in frequency-domain that is not well taken into account by the standard MPC. To address the missing links in various classes of the IPCs, this thesis aims to investigate and understand the similarities and differences between each of their performance. The results suggest that the choice of IPC designs rests largely with preferences and implementation simplicity. Based on these insights, a particular class of the IPCs lends itself readily for extracting tower motion from measurements of the blade loads. Thus, this thesis further proposes a tower load reduction control strategy based solely upon the blade load sensors. To tackle the problem of MPC on wind turbines, this thesis presents an MPC layer design upon a pre-determined robust output-feedback controller. The MPC layer handles purely the feed-forward and constraint knowledge, whilst retaining the nominal robustness and frequency-domain properties of the pre-determined closed-loop. Thus, from an industrial perspective, the separate nature of the proposed control structure offers many immediate benefits. Firstly, the MPC control can be implemented without replacing the existing feedback controller. Furthermore, it provides a clear framework to quantify the benefits in the use of advance real-time measurements over the nominal output-feedback strategy.

The poor lateral conductance of amorphous silicon necessitates the use of a contact covering the entire surface of thin film silicon solar cells to minimise the resistance. This contact must be highly conductive, as well as transparent to allow absorption of light in the inner layers. In recent years this has paved way for transparent conducting oxides (TCOs) to be used in solar technology. However, the absence of a p-type TCO complicates the fabrication of an Ohmic contact to a-Si:H(p). Moreover, the difference in bandgap between the two materials results in a Schottky interface which has not been investigated comprehensively in terms of on-state/optical performance and stability before. This thesis describes the physics of thin film silicon solar cells and their Schottky interface with zinc oxide (ZnO). Here, current-voltage-temperature (I-V-T) measurements and computer simulations are used in order to evaluate the Schottky barrier height of ZnO/a-Si:H(p) and ZnO/μc-Si:H(p) heterojunctions. It is observed that a high doping concentration of the p-layer can reduce the effective Schottky barrier height by increasing the tunnelling transport mechanism at the interface. The same heterojunctions are also tested in ZnO/p-layer/a-Si:H(i)/a-Si:H(n)/ZnO/Ag solar cells. It is found that despite the superior electrical properties of the ZnO/μc-Si:H(p) contact compared to the ZnO/a-Si:H contact, an improved performance is observed in cells using the latter. This contradictory result is explained by a misalignment of the energy bands at the μc-Si:H(p)/a-Si:H(i) interface. This reduces the open circuit voltage (VOC) of the cell in comparison to the a-Si:H(p)/a-Si:H(i) structure. These results lead to the theory behind optimization of a mixed μc-Si:H(p)/a-Si:H(p) window layer that can overcome the Schottky interface without compromising the p-layer/a-Si:H(i) interface. Further, the conventional equivalent electronic circuit of a solar cell is expanded with a Schottky diode in series that represents the non-ideal contact. The analysis of this equivalent circuit shows that non-ideal metal/semiconductor contacts for solar cells can be approximated as Ohmic when they show a Schottky barrier lower than 0.5 eV. Also, the same model allows to distinguish the sections of the cell that degrade during light exposure and current injection. It is observed that all of the solar cells analysed here show a reduction of their VOC, short circuit current (JSC) and fill factor (FF) as a function of time when soaked with 1 Sun light, whereas the fully a-Si:H solar cells show a simultaneous increase of VOC, a decrease of FF, and a minimal decrease of JSC as a function of time when injected with a constant current of 10 mA in the dark. Increase in recombination in the absorption layer of the cell during light exposure can be detected by an increase of the ideality factor (m) of the main junction. On the other hand, the ideality factor (n) of the Schottky junction decreases after current injection. This indicates a detrimental effect on the tunnelling transport mechanism at the contact. Computer simulations reveal that the decrease of n is the result of a change in the a-Si:H(p) hole concentration and doping profile due to the excess of electrons injected during stress. This degradation of the Schottky interface is not observed when μc-Si:H(p) is used as the p-layer. This thesis demonstrates that a complete understanding of degradation of the I-V characteristics of an a-Si:H solar cell can only be achieved when all transport mechanisms of a Schottky contact are considered.

No description available.

The continuous reduction of the dimensions of CMOS devices has increased the negative bias temperature instability (NBTI) of pMOSFETs to such a level that it is limiting their lifetime. This increase of NBTI is caused mainly by three factors: an increase of nitrogen concentration in gate dielectric, a higher operation electrical field, and a higher temperature. Despite of many years’ research work, there are questions on the correctness of the NBTI lifetime predicted through voltage acceleration and extrapolation. The conventional lifetime prediction technique measures the degradation slowly and it typically takes 10 ms or longer to record one threshold voltage shift. It has been reported that NBTI can recover substantially in this time and the degradation is underestimated. To minimize the recovery, ultra-fast technique has been developed and the measurement time has been reduced to the order of microseconds. Once the recovery is suppressed, however, the degradation no longer follows a power law and there is no industry-wide accepted method for lifetime prediction. The objective of this project is to overcome this challenge and to develop a reliable NBTI lifetime prediction technique after freezing the recovery. To achieve this objective, it is essential to have an in-depth knowledge on the defects responsible for the recovery. It has been generally accepted that the NBTI recovery is dominated by the discharge of trapped holes. For the thin dielectric (e.g. < 3 nm) used by current industry, all hole traps are within direct tunnelling distance from the substrate and their discharging is mainly controlled by their energy levels against the Fermi level at the substrate interface. As a result, it is crucial to have the energy distribution of positive charges (PC) in the gate dielectric, but there is no technique available for probing this energy profile. A major achievement of this project is to develop a new technique that can probe the energy distribution of PCs both within and beyond the silicon energy gap. After charging up the hole traps, they are allowed to discharge progressively by changing the gate bias, Vg, in the positive direction in steps. This allows the Fermi level at the interface to be swept from a level below the valence band edge to a level above the conduction band edge, giving the required energy profile. Results show that PCs can vary by one order of magnitude with energy level. The PCs in different energy regions clearly originate from different defects. The PCs below the valence band edge are as-grown hole traps which are insensitive to stress time and temperature, and substantially higher in thermal SiON. The PCs above the valence band edge are from the created defects. The PCs within bandgap saturate for either longer stress time or higher stress temperature. In contrast, the PCs above conduction band edge, namely the anti-neutralization positive charges, do not saturate and their generation is clearly thermally accelerated. This energy profile technique is applicable to both SiON and high-k/SiON stack. It is found that both of them have a high level of as-grown hole traps below the valence band edge and their main difference is that there is a clear peak in the energy density near to the conduction band edge for the High-k/SiON stack, but not for the SiON. Based on this newly developed energy profile technique and the improved understanding, a new lifetime prediction technique has been proposed. The principle used is that a defect must be chargeable at an operation voltage, if it is to be included in the lifetime prediction. At the stress voltage, some as-grown hole traps further below Ev are charged, but they are neutral under an operation bias and must be excluded in the lifetime prediction. The new technique allows quantitative determination of the correct level of as-grown hole trapping to be included in the lifetime prediction. A main advantage of the proposed technique is that the contribution of as-grown hole traps is experimentally measured, avoiding the use of trap-filling models and the associated fitting parameters. The successful separation of as-grown hole trapping from the total degradation allows the extraction of generated defects and restores the power-law kinetics. Based on this new lifetime prediction technique, it is concluded that the maximum operation voltage for a 10 years lifetime is substantially overestimated by the conventional prediction technique. This new lifetime prediction technique has been accepted for presentation at the 2013 International Electron Devices Meeting (IEDM).

The use of nanostructured materials in solar cells enables one to tune their absorption properties leading to a better match to the solar spectrum and subsequently an increased photocurrent through the solar cell. Type II GaSb/GaAs quantum rings (QRs) can significantly extend the spectral response beyond the visible out towards 1.4 µm giving a near optimum band gap for concentrator solar cell applications. Also, in type II band alignment the electrons are weakly localized and the built in electric field drifts the electrons across the depletion region easily. However, the introduction of GaSb QRs in GaAs solar cells degrades the open circuit voltage (Voc) and the incorporation of QRs needs to be optimized to minimize the Voc degradation while maximizing short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The analysis of the photoresponse under the white light illumination has shown that some photogenerated minority holes from the base region can be re-captured by the QRs, which reduces the Jsc and the Voc. Hence, in this thesis, the carrier dynamics and extraction mechanisms occurring in the GaSb QRs is investigated by photoluminescence spectroscopy and current voltage characteristics. The characteristic S-shaped behaviour of the WL peak energy with increasing temperature indicates the prominent carrier trapping in the band tail states leading to potential fluctuations. Systematic measurements of dark current versus voltage characteristics are carried out from 100 to 290 K. Compared with the reference GaAs solar cell, the QRSC exhibits larger dark current, however its ideality factor n is similar at 290 K. QRs are directly probed by using an infrared laser (1064 nm) where the photon energy is conveniently chosen below the bandgap of the GaAs matrix. This enables to investigate the carrier dynamics and extraction mechanisms occurring in the GaSb QRs under a high light concentration. The dependence of the photocurrent on the laser intensity, the bias and the temperature is also discussed. The QR photocurrent exhibits a linear dependence on the excitation intensity over several decades. The thermal activation energy was found to be weakly dependent on the incident light level and increased by only a few meV over several orders of excitation intensity. The magnitude of the relative absorption in QRs when directly probed by using a 1064 nm laser with an incident power density of ~ 2.6 W cm−2 is found to be ~ 1.4 × 10−4 per layer. The thermal escape rate of the holes was calculated and found to be ~ 1011 to 1012 s −1 , which is much faster than the radiative recombination rate 109 s −1 . This behaviour is promising for concentrator solar cell development and has the potential to increase solar cell efficiency under a strong solar concentration. Experiments have shown that QDs embedded in the depletion region could generate both additional photocurrent and dark current. The electron-hole recombination in QDs is the reason for the additional dark current which reduces the open circuit voltage and keeps the conversion efficiency of QD solar cells below the ShockleyQueisser limit. Therefore, the reduction in open circuit voltage and the influence of the location of QR layers and their delta doping within the solar cell is investigated in this work. Devices with 5 layers of delta doped QRs placed in the intrinsic, n and p regions of a GaAs solar cell are experimentally investigated and the deduced values of Jsc, Voc, Fill factor (FF), efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high field region which reduces SRH recombination. The devices with p or n doped QDs placed in the flat band potential (p or n region) show a recovery in Jsc and Voc compared to devices with delta doped QDs placed in the depletion region. However there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n doped QRs placed in the n region shows a slight improvement compared to the control cell. The approach of placing QRs in the n region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QR solar cells. The effect of the introduction of dopants on the morphology of GaSb/GaAs nanostructures is analyzed by HAADF-STEM. The results show the presence of welldeveloped GaSb QRs in both p-doped and n-doped heterostructures. However, in the undoped sample grown under the same conditions such well-developed QRs have not been observed. It is found that p-doping with Be stimulates the formation of QRs, whereas n-doping with Te results in the formation of GaSb nanocups. Therefore, the introduction of dopants in the growth of GaSb nanostructures has a significant effect on their morphology. Bias and temperature dependent EQE measurements are performed to understand the hole extraction from the QRs. In order to study the absorption strength of quantum dots and the various transition states, an approach to derive the below-bandgap absorption in GaSb/GaAs self-assembled quantum ring (QR) devices using room temperature external quantum efficiency measurement results is presented. The importance of incorporating an extended Urbach tail absorption in analyzing QR devices is demonstrated. The theoretically integrated absorbance via QR ground states is calculated as 1.04 ×1015 cm -1 s -1 , which is in a reasonable agreement with the experimental derived value 8.1 ×1015 cm-1 s -1 . The wetting layer and QR absorption contributions are separated from the tail absorption and their transition energies are calculated. Using these transition energies and the GaAs energy gap of 1.42 eV, the heavy hole confinement energies for the QRs (320 meV) and for the WL (120 meV) were estimated.

This thesis researches a novel electricity demand response method and renewable energy management technique. It demonstrated the use of flow batteries and residential hot water heaters to balance wind power deviation from plan. The electricity supply-demand balancing problem becomes increasingly more difficult. A large portion of complexity to this problem comes from the fact that most renewable energy sources are inherently hard to control and intermittent. The increasing amount of renewable energy generation makes scientists research new supply-demand balancing possibilities to adapt for the changes. In this research wind power data was used in most cases to represent the supply side. The focus is on the actual generation deviation from plan, i.e. forecasting error. On the other hand, the methods developed in this thesis are not limited to wind power balancing. Two major approaches were analysed - heating ventilation and air conditioning system control (mainly focused on, but not limited to, residential hot water heaters) and hybrid power system comprising of thermal and hydro power plants together with utility scale flow batteries. These represent the consumption side or the demand response mechanism. The first approach focused on modelling the behaviour of residential end users. Artificial intelligence and machine learning techniques such as neural networks and Box-Jenkins methodology were used to learn and predict energy usage. Both joint and individual dwelling behaviour was considered. Model predictive control techniques were then used to send the exact real-time price and observe the change in electricity consumption. Also, novel individual hot water heater controllers were modelled with the ability to forecast and look ahead the required energy, while responding to electricity grid imbalance. It proved to be possible to balance the generation and increase system efficiency while maintaining user satisfaction. For the second approach, the hybrid multi-power plant system was exploited. Three different power sources were modelled, namely thermal power plant, hydro power pant and flow battery. These sources were ranked by the ability to rapidly change the output of electricity. The power that needs to be balanced was then routed to different power units according to their response times. The calculation of the best power dispatch is proposed using a cost function. The aim of this research was to accommodate for the wind power imbalance without sacrificing the health of the power plants (minimising load variations for sensitive units).

The generation of electricity from fossil fuels is a major contributor to climate change and cannot be sustained indefinitely. Renewables can provide electricity in a more sustainable manner, however supplies from sources such as wind and solar can be variable and unpredictable. Hydropower and tidal energy offer more predictable generation capacity, making them appealing for a resilient transmission system. This is particularly true in the context of decentralised power grids, which harness smaller amounts of energy from a wide range of sources to improve transmission efficiency and reliability. Yet for tidal power in particular, little work has been done thus far on developing small scale technology capable of working efficiently in low flow speed (< 2 m/s) conditions. This research was conducted to identify, design and develop a device capable of generating pico scale power (< 1 kW) in shallow water, low input tidal and river conditions. The result is the Water Hammer Energy System (WHES), a novel design that makes use of water hammer pressure surges to generate vertical oscillations from a horizontal flow of water. The hydrodynamics of the system are investigated through a combination of experimental and theoretical work, with studies conducted into the e↵ect of input head, flow rate, and device size on performance. A 16 mm diameter experimental device was found to provide a piston with a mean power of 8 mW. Data from this study was used to validate a mathematical model, which predicted a maximum hydrodynamic efficiency of 23.1 % for a 1 m2 device operating at a 0.50 Hz valve closure frequency in 0.4 m/s flow. Assuming a 30 % generator efficiency, such a device operating in the mouth of Poole Harbour (where the peak flow speeds reach 1.69 m/s) could supply an average of 1.14 kW of power. Over the course of a year, this would provide enough electrical energy to supply 2 typical UK houses and o↵set 5.55 tonnes of CO2. 5.85 kW would be generated in a constant flow of 1.69 m/s, sufficient to supply the annual electricity requirements of 11 typical UK households and o↵set nearly 30 tonnes of CO2. With further development, the system may therefore be a viable method of generating pico scale hydropower from shallow water, low input sites.

This study focused on the exploration, assessment and experimental investigation of bio-electrochemical systems (BES) for concurrent phosphorus (P) recovery and energy generation/hydrogen (H2) production. The main aim was to study and understand the parameters for optimisation of the design and the performance of BESs for concurrent phosphorus recovery and energy generation/hydrogen production. In total, four dual chamber bio-electrochemical systems (Microbial Fuel Cells (MFCs) and Microbial Electrolysis Cells (MECs)) were used to investigate the impacts of key design and operational conditions on BES performance. P was precipitated successfully as struvite in both MFCs and MECs. The MFCs and MECs achieved a maximum P precipitation efficiency of 90% and 95% with a maximum columbic efficiency of 10% and 51% respectively. The MFCs and MECs achieved an average of 80 % and 70 % COD removal efficiency respectively, which confirms the ability of these systems to be used in wastewater treatment. Deterioration in both reactors occurred due to P precipitation on the cathode surface and the membrane. The three operational parameters (influent COD, cathode aeration flow rate, and external resistance) were found to have significant impacts on MFC performance and P recovery. In addition, applied voltage and influent COD had significant effects on MEC performance and P recovery. Results were supported through statistical analysis and optimisation modelling using full factorial design, central composite design, and response surface methodology. Generally, results have shown that MFCs and MECs have the potential to concurrently recover P, treat wastewater, and generate electricity/produce H2. Further research is needed to enhance the performance of MFCs for energy generation and MECs for H2 production in addition to P recovery and minimising scaling on electrodes. The results of this study increase the understanding of P recovery mechanisms in MFCs and MECs and can contribute to future BES research. Moreover, the results will help in selecting the optimum operational parameters of BESs depending on the applications and process requirements. Applying BESs in wastewater treatment plants will reduce energy consumption and, at the same time, find an alternative source of P.