This thesis presents a systematic investigation of some novel chalcogenides based on Earth abundant elements in order to test their suitability for sustainable photovoltaic (PV) applications. A comprehensive review of sulfo-salts in the family Cu-Sb-Bi-X (X = S, Se) is presented and indicates that CuSbS2 and Cu3BiS3 have potential as PV absorbers, showing optical band gaps in the desired range and the p-type conductivity required for PV devices. A systematic study about the formation of CuSbS2 and Cu3BiS3 thin films is presented, including of the experimental steps necessary to ensure the formation of stoichiometric films during the sulfurization of metallic precursor layers. Sulfurized CuSbS2 films (~1.2 µm thick) exhibited absorption coefficients of ~105 cm-1 and band gaps of ~1.5 eV. The films were p-type with mobilities of ~10 cm2 V-1 s-1 and resistivities in the range 10 - 1000 kΩ/□. CuSbS2 films were also deposited in a one-stage process by rf sputtering from a ternary target – they were generally more resistive than those deposited by sulfurization. Cu3BiS3 films deposited by sulfurization had band gaps of ~1.4 eV and p-type conductivity, with hole mobilities of ~3 cm2 V-1 s-1. The CuSbS2 films produced by both methods were tested in prototype CuSbS2/CdS heterojunction PV devices, and had efficiencies of ~0.1%. For these devices the dominant transport mechanism was multi-step tunnelling, suggesting that an improvement in the quality of the junction was necessary. Post-growth treatments were trialled, including impurity doping with NaF, Zn and In, and etching the absorber to remove the surface oxides identified by XPS. CuSbS2/CdS devices having efficiencies up to ~1% were achieved by doping the absorber with In and removing unwanted Sb2O3 from the layer surfaces with de-ionized water. Their Vocs and FFs were low and their Jscs were high, as reported by others. Alternative window layer materials were tested, including ZnS and ZnSe, which were expected to have an improved band alignment with CuSbS2 compared to CdS. ZnS gave Voc = 0.56 V, the highest yet reported for CuSbS2. However these alternative window layers had reduced photocurrents and their efficiencies were not greater than with CdS. Overall CuSbS2 and Cu3BiS3 films displayed the optical and electrical properties required for PV, but further developments will be essential in order to achieve high efficiency PV devices using these materials as absorbers.

Population explosion is one of the primary causes for concern in the power sector nowadays because residential buildings consume a high percentage of available electricity in the market. Also, the majority of current power plants use fossil fuel to generate electricity which makes the situation even worse due to the high price of fossil fuel. Consequently, electricity bills have soared dramatically in the last decade. If that was not enough, many countries have a shortage of electricity because they cannot increase their generation capacity to cover electricity demand. Many solutions have been introduced to improve the efficiency of the power grid and reduce electricity price for the users. For instance, Demand Side Management and Demand Response, domestic top-roof renewable micro-plants, and distributed renewable plants are introduced as a part of the solution to improve the situation. However, users are still paying a high percentage of their monthly income to electricity companies, that is because the surplus renewable power is not well utilized. The primary problem here is to find an efficient way to minimize the electricity cost and maximize the utilization of renewable power without using storage systems (batteries). Another issue is to solve the massive power allocation optimization problem in polynomial time. In this thesis, heuristic optimization algorithms are proposed to cope with the complexity of the problem as these kinds of problems are NP-hard. Furthermore, a set of different power allocation problems has been addressed in this thesis. The first one uses an online algorithm to solve power allocation problem that is modeled as a Knapsack problem. Additionally, the thesis has coped with the computational issue of a massive LP-based optimization problem of large buildings. Finally, an MILP-based heuristic algorithm has been used to solve power allocation problem in micro-grids (a set of houses shares renewable power for particulate rate). The empirical experiments and evaluations, in general, show promising results. The findings depict how an appropriate knapsack formulation can be used to address a significant dynamic energy allocation problem in a straightforward and flexible way and how good our heuristic algorithms can solve enormous power optimization problem in polynomial time. Finally, the results prove that our micro-grid model can reduce power bills by using the principle of renewable power sharing for a fair price.

The aim of this thesis was to develop tools for evaluating the potential sustainability and security of renewable energy systems in the UK, with a long-term view of maximising the potential renewable energy penetration of wind and solar by deployment of electrical energy storage. Using computer modelled renewable energy systems, a number of system variables are considered such as system size, energy sources (solar and/or wind), type of demand load, and capacity and type of storage technology. The results allow for a broad comparison of different types of renewable energy systems, and their optimisation. The optimisation methodology is also critically evaluated with consideration of its robustness and applicability, using two alternative metrics to measure system energy security and two different measurements of energy return on investment (EROI) to measure sustainability. When comparing renewable energy systems, results found that large systems that predominately got their power from wind sources were the most sustainable and secure, using optimisation methods that penalised both their overproduction and underproduction. Nearly all systems benefit from the use of electrical energy storage, without impacting too much on sustainability levels, but larger wind systems used less storage, suffering lower energy security as a result. System performance can best be improved by developing solar power technologies with lower embodied energy costs, followed by a reduction in embodied energy of storage technology. The former will enable more effective use of storage methods, while the latter allows for larger storage capacities with less environmental impact. Sustainability and energy security were given equal priority in the optimisation, however it was found that more sustainable generation technologies were preferable to more secure technologies, as there is more scope to improve energy return on investment than security. Therefore there is a limit, generally around 45-85\% (depending on size of system and choice of technology) to the proportion of time that renewable energy systems using variable energy sources can be autonomous, meaning that energy backup from the grid and/or dispatchable sources is still required.

To meet the need of clean energy, a variety of renewable energy technologies have been developed. Among those, wave energy stands out for its outstanding merits. For instance, wave energy is clean, renewable, has high energy intensity and long resource available time. However, due to the short development history of wave energy technologies, the cost of wave energy is too high compared with the other renewable energy technologies. As well as lack of cost effective Wave Energy Converters (WECs) scenarios, another reason that keeps the cost of wave energy high is that the performance of a WECs may not be accurately assessed during its design and development stage. This leads to error in estimating the cost of energy produced by the full scale device. Therefore, this study aims to drive the cost of WECs down by investigating several major aspects that will bias the assessing of the performance of a WEC during the design and development stage. Literature review suggested the uncertainty in the measurement, the tank width effect (in tank testing), the performance of the simulated simple Power Take Off (PTO) and the scale effect are major aspects that bias the assessment. By tank testing and Computational Fluid Dynamic (CFD) simulation, the above three aspects were investigated. It is found that the uncertainty in the measurement leads to an uncertainty in the power captured at model scale about 5% around the peak output. The appearance of the tank wall will over estimate the performance of a single unit depending on the width of the tank. A 1 : 150th scaled (of the full scale.) device may under estimate the performance of the device by about 34% compared with a 1 : 16.67th scaled device, while a 1 : 50th scaled device under estimate the performance of a 1 : 16.67th scaled device by 6.6%. The CFD simulation demonstrated its advantage over the tank testing when scaling and tank width effect is concerned. Therefore, to better estimate the performance, the assessment shall be carried out by both experiment and numerical simulation. Based on the study carried out, recommendation and guide line for tank testing of a FSCOWC device was given at the end of the thesis.

Electricity networks are becoming more complex due to the introduction of distributed energy sources. The power grid will face fundamental changes in its structure and behaviour. In addition, new technologies will be required in order to maintain the stability of the network. A key enabling technology for the future power networks is autonomous distribution networks which operate in cooperation with distributed grid intelligence and active network management. Local control can be achieved through fast control and communications and needs to be coordinated with the overall system controls. The primary issues for distribution network operation with a high penetration of distributed resources is power flow management as well as voltage control. Active Network Management can be described as the control and management of generation and load in distribution networks. The main concept is to satisfy network limits such as voltage, power and frequency while at the same time increase the connected generation output with the minimum cost. There is a need for coordination between generators to maintain the balance in the network and avoid investment costs. This thesis considers the introduction of Distributed Constraint Optimisation as a way of providing Active Network Management. It is an agent-based coordination method that is able to coordinate generators’ output without violation of network constraints. This method, which is drawn from the Artificial Intelligence community and was previously used for smaller problems such as meeting scheduling, is studied and evaluated for use in power systems networks in order to provide solutions in a decentralised way. Case studies consider both DC and AC power flow management and the solution of the economic dispatch problem. DC power flow management under DCOP provides optimal solutions for radial networks while AC power flow management is examined from a theoretical standpoint until the limitations of distributed constrained optimisation software are addressed.

No description available.

Turbulence enhanced momentum and energy transport is an important mechanism in shaping the flow and temperature field of low current turbulent SF6 (Sulfur-hexafluoride) arcs burning in a nozzle. It is a factor that largely determines the prediction accuracy of the interruption capability of high voltage circuit breakers. Previous study [81] has shown that for SF6 arcs the parameters in the Prandtl mixing length model and the two equation k-ε model must be adjusted to bring agreement between prediction and measurement. The experimental observation [109] of steep temperature or density gradient exists at the arc edge, where turbulence is usually strongest. The density gradient can be regarded as a result of temperature gradient when the pressure difference across the low current arc column is small. In previous studies of turbulence models, the effects of large density or temperature gradient have not been considered. In the present work the k-ε model is modified to take into account the effect of the presence of steep local temperature gradient. The model is first applied to the steady state SF6 nozzle arcs in the current range from 100 A to 1800 A. The performance of the modified turbulence model is assessed by a comparison of the predicted and measured radial temperature profiles at different currents and its behaviours with another two most commonly used turbulence models, the Prandtl mixing length model and the k-ε model. The relevant turbulence parameters of the Prandtl mixing length model are adjusted according to the different nozzle shapes with different values and it has been found that its applicability is limited. The modified k-ε model, which is modified to take into account the effect of large temperature gradient with all default coefficients, can make reasonable prediction for turbulent arcs in the Aachen nozzle [67, 108] under direct current conditions. The model is then applied to the transient nozzle arcs in a GE nozzle [86] and the Campbell nozzle [85]. Finally, a real puffer-type circuit breaker of 252 kV has been used to verify this model. The predicted arc voltage and pressure agree reasonably well with the measurements at both high and low current levels, justifying the applicability of the modified k-ε turbulence model. The auto-expansion circuit breaker is a relatively new interruption technique, which creates the required fast gas flow at current zero using the energy dissipated by the arc itself. A design study has been carried out for a prototype 245 kV auto-expansion circuit breaker at 50 Hz, in order to investigate the influence of key design parameters in terms of arcing conditions at the current zero phase and the critical RRRV (Rate of Rise of Recovery Voltage). Auxiliary nozzle with different lengths, and the severity of leakage from the expansion volume have been implemented for testing the influence of design parameters in an auto-expansion circuit breaker. PC-based arc modelling taking account the effect of nozzle ablation and the mixing process of PTFE vapour with SF6 has been made for auxiliary nozzle investigation. For the effect of leakage, various sizes of a leakage hole are specified on the valve of the expansion volume. The critical RRRV values for the Ref. Case and the its modifications are used for comparison of its interruption capability in order to estimate the influence of these key design parameters. The solution of all of the equations in the arc models is based on a commercial computational fluid dynamics package, PHOENICS. PHOENICS has been extensively used at the University of Liverpool to model the arc behaviours since 1992. Before using PHOENICS to simulate the arc behaviour in this thesis some conditions need to be resolved. These are related with the choice of turbulence model, the computational domain size for electric field, and the influence of the Lorentz force. The differential models for reported works in the thesis is implemented into PHOENICS version 3.6.1. All boundary conditions which are treated as sources terms are discussed in Section 2.3.

This study focuses on aqueous electrochemical capacitors (ECs) for optimising the cell voltage and understanding the charge storage mechanism of transition metal oxides and hydroxides. The first part discusses how to optimise cell voltage of symmetric and asymmetric ECs. In Chapter 4, a commercial activated carbon (ACS–679) was employed as an electroactive material to demonstrate the necessity of charge–balanced condition of aqueous symmetric ECs in order to improve the utilisation of electrochemically stable potential window of electroactive material. Besides, the effect of stable open–circuit potential on electroactive materials is also addressed here. The concept is on the basis of electrochemical charge–balance principle, therefore, it is believed to be applicable to all the symmetric ECs no matter what kind of material is used as electroactive material. The charge–balanced electric double–layer capacitors (EDLCs) was assembled to evaluate the acceptable cell voltage by means of electrochemical analyses, including cyclic voltammogram, constant current charge–discharge, electrochemical impedance spectroscopy and inductance–capacitance–resistance meter measurements. Moreover, charge–unbalanced EDLCs were used to demonstrate the necessity of charge balance in symmetric ECs since those exhibit lower cell voltage and specific energy compared with the charge–balanced case. Similarly, in Chapter 5, asymmetric ECs consisting of reduce graphene oxide and manganese oxide (RGO//MnOx) were employed as an example to establish criteria for determining the highest acceptable cell voltage of asymmetric ECs with excellent reversibility and capacitor−like behaviour. It is very useful to evaluate the practical specific energy of asymmetric ECs by understanding the highest acceptable cell voltage. Besides, RGO//RuO2 was also demonstrated to verify the validity of the proposed criteria. The second part probes the charge storage mechanism of transition metal oxides and hydroxides in aqueous ECs. In Chapter 6, a wide–ranging study on electrochemical activation from Mn3O4 to MnO2 in 0.5 M Na2SO4 medium were presented, which includes in operando monitoring of the structural evolution during the activation process via in situ Raman microscopy. Other advanced material characterisation techniques, such as powder X–ray diffraction, X–ray photoelectron spectroscopy, scanning electron and transmission electron microscopy, were also applied to analyse the as–prepared and activated MnOx. Due to the structural disordering of electrochemically activated birnessite−MnO2 and residual Mn3O4, the charge storage is attributable to the redox reaction between Mn(III) and Mn(IV) at outer surface active sites, rather than cations and/or protons intercalation into layer structures. In Chapter 7, the redox behaviours of γ−NiOOH/α−Ni(OH)2 in various electrolytes (LiOH, NaOH, KOH, CsOH and NH4OH) are discussed. The charge storage mechanism of γ−NiOOH/α−Ni(OH)2 was studied by means of EQCM and in situ Raman microscopy. Moreover, the computational simulation (DFT+U) based on EQCM results gives a better idea on the compositional changes in the first few potential cycles. The insertion/removal of Li+ does not alter the content of water inside the structure, while the insertion of Na+ and K+ leads to a significant removal of water. The removed water molecules cannot be reversibly re−inserted back into the nickel structure, leading to the loss of water molecules in Ni(OH)2 structure during charge–discharge process. The capability to retain water molecules inside the Ni(OH)2 is crucial for the stability of γ−NiOOH/α−Ni(OH)2 redox reaction.

This thesis is concerned with the modelling of switching arcs in air in high voltage circuit breakers and with a comparative study of interruption capability of air and SF6 switching arcs. Emphasis is given to the identification of dominant energy transport processes for arc interruption and the material properties associated these processes. There have been renewed interests in air arcs because of its possible use in a mixture with other gases as a replacement for SF6 in circuit breakers for environment protection. Computer simulation of the switching air arc has been carried out using arc models based on laminar flow and on turbulent flow for the experimental set up of Fang et al [41] under DC current and that of Frind and Rich [66] for the current zero period. DC arc voltages predicted by arc model assuming laminar flow (LAM) are much lower than those measured. Thus, turbulence is introduced to account for additional power loss mechanism not included in the laminar flow model. Two turbulence models have been used to take into account of turbulence enhanced momentum and energy transport: the Prandtl mixing length model (PML) and the standard k-epsilon model or its modified version. For the DC air nozzle arc of Fang et al [41] the value of the turbulence parameter, c= 0.06, in PML has been chosen to match the predicted arc voltage with that measured at 1 kA DC and a stagnation pressure of 10 bar. PML can give satisfactory agreement with experiments over a DC current range from 250 A to 3 kA. When the standard k-epsilon model is used, the predict arc voltage is much higher than that measured indicating that turbulence cooling is too strong. One of the turbulence parameters of the standard k-epsilon model which controls the dissipation rate of turbulent kinetic energy is adjusted to match the predicted arc voltage with the experimentally measured arc voltage under the same discharge conditions as those for finding the value of c in PML. With this chosen value of 1 = 1.62, the modified k-epsilon model (MKE) gives similar results to those of PML. Three arc models (LAM, PML and MKE) are used to compute the critical rate of rise of recovery voltage (RRRV) for the air nozzle arc of Frind and Rich [66]. The presence of the shock inside the nozzle in the presence of the arc prevents the optimisation of the value of turbulence parameter for PML due to numerical convergence problems. RRRV predicted by PML and LAM are much lower than the experimental value. MKE with 1 = 1.65 is successful in predicting satisfactorily the RRRV at di/dt= 13.5 A/μs for several stagnation pressures. However, it has been found that a single value of 1 chosen for one value of di/dt cannot give satisfactory prediction of RRRV for other values of di/dt. A comparative computational study of SF6 and air switching arcs based on MKE has been carried out for the experimental conditions of Frind and Rich [66] for di/dt= 13.5A/μs at several stagnation pressures. Under the same discharge conditions RRRV of SF6 switching arc is one order of magnitude higher than that of air switching arc. Such large difference in the interruption capabilities of SF6 and air is due to the different dominant energy transport processes responsible for the arc cooling during current zero period. Two material properties of the arc plasma, the product of density and specific heat at constant pressure (ρCP) and that of density and enthalpy (ρh) are responsible for the distinctive arc features for SF6 and air. SF6 switching arc has a distinctive arc core surrounded by a thin region with steep temperature gradient. Under the same discharge conditions as those of SF6 air switching arc has no distinctive core structure. Its radial temperature profile is very broad and arc radius is much bigger than that of SF6. Such broad radial temperature profile of air arc is due to the peaks of turbulent thermal conductivity at 4,000 K and 7,000 K produced by the corresponding peaks of the material property of ρCP of air. For SF6 ρCP has a peak just below 4,000 K, which ensures rapid temperature decay above 4,000 K and a gentle temperature tail below 4,000 K. In comparison with SF6 under the same pressure difference across the nozzle the velocity inside air arc is much higher than that of SF6. With ρh of air being greater than that of SF6 for temperature higher than 7,000 K together with higher velocity enthalpy transport capability of air arc is much higher than that of SF6. Energy balance calculation for the current carrying core indicates that after the breakdown of quasi-steady state turbulent thermal conduction is the dominant energy transport process for SF6 while for air arc axial convection is dominant. As a consequence the rates of decay of arc temperature and arc radius for air arc a few microseconds before current zero are much slower than those of SF6, thus resulting in a large difference between RRRVs for the two gases under the same discharge conditions. To find an alternative arc quenching gas with similar interruption capability to that of SF6 one should aim at ρCP and ρh of the alternative gas with similar features to those of SF6.

Microgeneration technologies like residential solar photovoltaic (PV) systems have been shown to have immense potential for energy security and climate change mitigation. As a way of helping to resolve the decades-long power challenge in Nigeria, this study investigated the barriers to, and motives for, domestic PV adoption in Nigeria. It also assessed whether household PV can lead to increased energy use efficiency and examined the role of Government incentives towards large-scale uptake and diffusion. Adoption and innovation diffusion theories, willingness-to-pay (WTP), coproduction and self-help concepts were employed. Results were analysed using mainly Lagos State household data, collected through questionnaire surveys and interviews. Findings from correlation and logistic regression revealed the major barriers as high capital costs, lack of finance and low awareness. Field survey analysis established the key motives for uptake as power outages, cost-savings, including generator use fuel fraud and access to finance. It also showed that post-PV, adopting households engaged in more energy efficient practices. From this data the PV efficiency cycle was developed to demonstrate how energy conservation occurred. Empirical evidence from the surveys, interviews and LCOE calculations were used to design a verified model for rapid PV diffusion. This decision-making tool can be used by the Government, policymakers, PV designers, SMEs and households for choosing an appropriately-sized module. The results point to the need for regulatory and political intervention. Effective PV awareness creation campaigns and promotional strategies would also be necessary in the changing face of electricity supply in Nigeria.