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Polymer electrolyte membrane fuel cells (PEMFCs) have a great potential in becoming a major alternative to fossil fuel combustion technology. One of its major components is the gas diffusion layer (GDL). It is a porous carbon fibre-based material (which is mostly coated with a microporous layer (MPL)) that provides pathways for the transport of reactant gases, excess water, heat, and electrons during PEMFC operation. Hence, there is a need for an effective design of the GDL to optimize the PEMFC performance. One of the efficient and cost-effective tools to optimise fuel cell performance is numerical modelling using for example computational fluid dynamics (CFD) software. However, the anisotropy of the GDL and its anisotropic transport properties are not fully captured in the PEMFC numerical models reported in the literature, leading to unrealistic predictions of the PEMFC performance. In this thesis, a comprehensive 3-D numerical model of a PEMFC, incorporating realistic experimentally measured multidimensional values of the GDL transport properties (gas permeability, diffusivity, thermal conductivity, and electrical conductivity) has been developed to investigate the sensitivity of the fuel cell performance to these anisotropic transport properties. Also, a 2-D model of the transport of reactant species in cathode GDL of the PEMFC was developed. Further, numerical investigations were conducted to study the sensitivity of the PEMFC performance to (i) the interfacial contact resistances between the PEMFC components and (ii) that of a double side MPL-coated GDL. Finally, the impact of a linear porosity gradient distribution in the cathode GDL on the PEMFC global performance and the local distribution of the current density and oxygen concentration in the cathode side of the membrane electrode assembly (MEA) has been investigated. Results from the research show that at low fluid velocities and gas permeability, the transport of the gas reactant species from the channel to the catalyst reaction sites is dominated and controlled by the diffusion mechanism in the GDL. GDL anisotropy has significant impact on the PEMFC performance, overlooking it results in either significant overestimation (if the in-plane values of the transport properties are only considered) or underestimation (if the through-plane values of the transport properties are only considered) of the modelled fuel cell. Also, when designing the carbon fibre paper GDLs, it is recommended that the carbon fibres are more oriented in the through-plane direction than in the through-plane direction to maximise traverse transport properties, in particular electrical conductivity, thermal conductivity, and gas diffusivity. Furthermore, the fuel cell performance is more sensitive to the porosity of the MPL facing the bipolar plate than the MPL facing the catalyst layer. As the porosity of the MPL facing the bipolar plate is predominantly the limiting factor for the distribution of oxygen concentration within the cathode GDL. Also, the grading of the cathode porous transport medium of the PEMFC shows that at optimum design with increasing gradient, the PEMFC global performance as well as the local distribution of the key parameters is improved.

The intensive use of fossil fuels to meet the world energy and water demand has caused several environmental issues, such as global warming, air pollution and ozone depletion. Therefore, the integration of stand-alone decentralised hybrid renewable energy systems is a promising solution to satisfy the global energy-water demands and minimize the effects of fossil fuels utilisation. Among these hybrid technologies, concentrated solar power (CSP) combined with waste-based biogas to power organic Rankine cycle for cogeneration provide the means to generate dispatchable, reliable, renewable electricity and water in high direct normal incidence (DNI) regions around the world. Due to the strong inverse correlation between DNI resources and freshwater availability, most of the best potential CSP regions also lack sufficient freshwater resources. The current study proposes and applies a novel multi-dimensional modelling technique based on artificial neural networks (ANN) for hourly solar radiation and wind speed data forecasting over six locations in Oman. The developed model is the first attempt to integrate two ANN models simultaneously by using enormous meteorological data points for both solar radiation and wind speed prediction. The developed model requires only three parameters as inputs, and it can predict solar radiation and wind speed data simultaneously with high accuracy. As a result, the model provides a user-friendly interface that can be utilised in the energy systems design process. Consequently, this model facilitates the implementation of renewable energy technologies in remote areas in which gathering of weather data is challenging. Meanwhile, the accuracy of the model has been tested by calculating the mean absolute percentage error (MAPE) and the correlation coefficient (R). Therefore, the model developed in this study can provide accurate weather data and inform decision makers for future instalments of energy systems. Furthermore, a novel proposed hybrid solar and biogas system for desalination and electric power generation using advanced modelling techniques to integrate the stand-alone off-grid system has been designed. The novelty emerges from some facts, which are centralised around the use of a hybrid electric generation via Concentrated Solar Power (CSP) and anaerobic digestion biogas to achieve higher stability and profitability. Meanwhile, the cogeneration through the waste heat of the ORC drives the AGMD, which benefits as well from the higher stability due to hybridisation. In addition, an innovative and user-friendly modelling approach has been applied, and this efficiently integrates the individual energy components, i.e. PTC, anaerobic biogas boiler, ORC and AGMD, which fosters the optimisation of the proposed system. The models have been developed in the MATLAB/Simulink® software and have been used to investigate the system area, dimensions, and cost and to ensure that the electrical and water demand of the end-user are met. In addition, a new detailed thermo-economic assessment of the proposed hybrid solar biogas for cogeneration in off-grid applications has been investigated. An energy, exergy, and cost analysis has been performed and to fully utilise this, a sensitivity assessment on the developed model has been analysed to examine the effects of various design parameters on the thermo-economic performance. Finally, implementing an in-depth simulation testing of the system in a rural region in Oman is presented. The novel integrated solar and biogas system that has been designed through advanced modelling in the MATLAB/ Simulink® is integrated with a robust multi-objective optimisation technique to determine the best operating configuration. Three objective functions namely, maximising power and water production, and minimising the unit exergy product costs have been formulated. The turbine efficiency, top ORC vapor temperature and ORC condenser temperature has been selected as the decision variables. The non-dominated sorting genetic algorithm (NSGA-II) has been employed to solve the optimisation problem and produce a Pareto frontier of the optimal solutions. Further, the TOPSIS approach has been used to select the optimal solution from the Pareto set. The study constitutes the first attempt to holistically optimise such a hybrid off-grid cogeneration system in a robust manner.

Biomethanation of carbon dioxide has recently emerged as a competitive technology for upgrading biogas produced from the anaerobic digestion of biowastes. In this configuration, carbon dioxide in biogas is reduced to methane through a biological reaction with hydrogen, resulting in several benefits: increased carbon efficiency, decarbonisation of the natural gas grid and transport, and potentially storage of renewable electricity in the form of high-energy-density fuel. In-situ biomethanation combines conventional biogas production from organic matter with the addition of H2 to produce a higher quality biomethane gas through retrofitting existing anaerobic digestion (AD) infrastructure. However, process engineering challenges remain to the uptake of in-situ biomethanation especially surrounding its continuous operation, control, and economic viability, which are addressed by this research using lab-based and techno-economic studies. An automated rig for the study of continuous in-situ biomethanation, including a monitoring and control system for control of the H2 injection rate to maintain process stability, was designed, commissioned and operated across a series of experiments to study the dynamics and performance of in-situ biomethanation alongside AD of sewage sludge (SS) and food waste (FW). This configuration would correspond to the retrofit of existing industrial-scale AD reactors, maintaining the typical plants' operational strategies and gas storage characteristics (size and pressure). Effects investigated experimentally included: H2 injection systems, gas recirculation and stirring intensity, and variation of the organic loading rate (OLR). All results highlighted the rate-limiting effect of H2 gas-liquid mass transfer, implied by a lack of evidence of biological limitation or inhibition and a relatively high equilibrium H2 content (11-36 % vol.) in the headspace appearing requisite to the gas-liquid transfer. Using a porous sparger as an improved injection system, a higher gas recirculation rate and mechanical stirring intensity improved the H2 conversion and methane evolution rate (MER). In the case of sewage sludge, the highest MER achieved was 0.16 LL-1day-1, with H2 conversion at 75 %, while in the case of FW, the highest MER was 0.23 LL-1day-1 with H2 conversion at 66.3 %. The importance of the OLR on the achievable H2 conversion was also highlighted and rationalised by its relationship with the gas Retention Time (RT). When operating the digesters at a reduced OLR of 1 gVS L-1 day-1, higher H2 conversion was achieved, up to 94 % for SS and 87 % for FW. However, that resulted in a trade-off with lower MER values, at 0.1 LL-1day-1 for SS and 0.16 LL-1day-1 for FW. The techno-economic work modelled the economic viability of in-situ biomethanation in the current market conditions. The analysis considered a hypothetical consumer tariff linked to the wholesale (variable) electricity price, allowing scheduling of hydrogen production from lower-cost electricity. Scenarios were investigated using different electrolyser technologies, varying electrolyser sizes, and biomethane end-use. The economic assessment showed the sale price of biomethane ranging from £94-110 /MWh across all scenarios in current market conditions, with electricity price and electrolyser costs being the most influential parameters on the results.

A new and novel GPU accelerated method has been developed for solving the Navier-Stokes equations for bodies of arbitrary geometry in both 2D and 3D. The present method utilises the vortex particles to discretize the governing equations in the Lagrangian frame. Those particles act as vorticity carriers which translate in accordance with the local velocity field. Vorticity information is thus propagated from the vorticity source to the rest of the flow domain in mimicking the advection and diffusion processes of the real flow. In the high-fidelity method, vorticity generation can take place around the bodies. The no-slip condition produces a boundary flux which is subsequently diffused to the neighbouring particles. The new method has been successfully validated by simulating the flow field of an impulsively started cylinder. The calculated drag curve matches well with the theoretical prediction and other numerical results in the literature. To extend the applicability of the code to wind-turbine applications, a simplified re-meshing strategy is adopted which is found to produce small numerical inaccuracies. In the engineering method, a simplified hybrid approach has been developed which decouples the advection and diffusion processes. The viscous effects are ignored on the bodies and are recovered in the wake. For this purpose, the Laplace equation that resulted from the irrotational assumption of the flow has been solved using the boundary element method. The solution produces a dipole distribution that is subsequently converted to viscous particles by employing the Hess' equivalence principle. In addition, an accurate interpolation scheme has been developed to evaluate the dipole gradient across the distorted wake geometry. To reduce the simulation time, the fast multipole method has been implemented on the GPU in 2D and 3D. To parallelize the implementation, a novel data construction algorithm has been proposed. Furthermore, an analytical expression for the velocity strain has been derived. The new developed methods have been applied to problems involving aerofoils and vertical axis wind turbines. Comparisons with experimental data have shown that the new techniques are accurate and can be used with confidence for a wide variety of wind turbine applications.

Coal is going to play an important role in meeting future energy demands but it produces carbon dioxide, which is considered to be a major contributor towards climate change. Oxyfuel technology is a promising carbon capture technology where the N2 in the oxidiser stream is replaced with CO2 to enhance the capture process at the exit of the power plant. The oxyfuel technology results in changing the combustion environment, which may affect the flame characteristics, fuel efficiency, the emissions and overall boiler performance. The aim of this research was to develop an ignition model, which provides further understanding of the flame characteristics due to fuel switching and changes in the combustion environment.\\ An investigation was conducted analysing predictions of different coal devolatilisation models. The three models analysed were CPD, FG-DVC and PC-Coal Lab where the models simulated hot wire mesh experimental conditions. The results showed that the PC-Coal Lab and CPD is able to accurately predict the devolatilisation behaviour for a broad range of coals whereas FG-DVC is less effective. The CPD model is chosen for further investigations over PC-Coal Lab as it entails a licensing cost whereas CPD is open source and proves to be less demanding. An ignition model is developed to understand the fundamental ignition mechanisms of a single particle of a solid fuel, which are categorised as either homogeneous or heterogeneous. The model accurately couples kinetics, heat and mass transport phenomena between the interior and exterior of the particle. The study accounts for variation in ambient conditions (including oxyfuel conditions) and fuel properties where the results are validated against experimental data. On extending the ignition model to different particle size, a correlation is obtained for the ignition mechanism based on particle size and ambient oxygen concentration. The correlation developed is useful in investigating ignition in pilot/full scale boiler assisting in any design and operational changes. Baseline CFD simulations were conducted on an IFRF and Utah furnace replicating their respective experimental flows. The simulations were repeated with integration of the correlation using the methodology developed in previous study. The results are compared against the experimental data suggesting that ignition model improves the overall predictions of the flame lift (i.e ignition zone) and thus the model can be used for further investigating novel combustion environments and fuels. The validated ignition model is applied in investigating the pilot scale burner, which indicated a small improvement in the species prediction but a further development will be required in the turbulent chemistry model.

Vertical Axis Wind Turbines (VAWTs) have potential to provide a greater contribution to the deployment of wind energy around the world. At present however, progress is hindered by a massive shortfall in effectiveness of design methods associated with the complexity of VAWT aerodynamics. Predicting and designing for VAWT flows is not only an interesting technological challenge, but also one of great societal importance. If the performance and efficiency of VAWTs are improved, then their uptake and deployment around the world will be increased. Due to some key advantages of VAWTs, their widespread use in conjunction with Horizontal Axis Wind Turbines (HAWTs) could drastically improve the cost competitiveness of wind energy overall. This thesis describes the development of a new design/optimisation methodology that can be applied to VAWT aerodynamics. It is based on the powerful Adjoint method, which has been applied to other fields with great success. In the application of Adjoint methods to VAWT aerodynamics, this work shows an efficient new way to do VAWT blade shape optimisation. A 'semi-transient' optimisation process is proposed, using Adjoint optimisation data from individual instances in time to improve VAWT performance. Such a method is novel in the field of VAWTs, and the use of Adjoint methods with low cost CFD models provides an efficient optimisation methodology that can be readily adopted by the VAWT design community. Throughout this work the commercial CFD software ANSYS Fluent (18.2) was used, although the proposed methodologies can be reproduced in other CFD codes which contain an Adjoint solver. This thesis opens an exciting new avenue of research departing from conventional parameter-based design methods, and the results show that Adjoint-optimised blades can provide significant increases to a VAWTs power coefficient. Discussion of these novel blade shapes also sheds new and important light on the flow physics and optimal blade geometries throughout the VAWTs revolution. Additionally, the results show that the semi-transient Adjoint method can be used to provide blades which greatly reduce the aerodynamic loading on the VAWT (and thus fatigue damage) whilst improving the average power coefficient.

In light of the continuously increasing level of greenhouse gases, global warming has led to many investigations into the different types of renewable energy technologies, in particular wind turbines. The small-scale H-type Vertical Axis Wind Turbine (H-type VAWT) has been selected as the scope of this thesis due to its several significant advantages over the more commonly adopted Horizontal Axis Wind Turbines (HAWTs). However, the self-starting capability of these types of turbines is still one of the main challenging aspects and this limits their utilisation for the small-scale power generation. Unless the causes that prevent the turbine to self-start are understood sufficiently and designed to overcome the self-starting problem, they may be unsuitable for small-scale power generation. Hence, the present thesis aims to provide a detailed understanding of the self-starting behaviour of the small-scale H-type VAWTs and shed more light on the literature of the self-starting research by conducting a comprehensive analyse considering various design parameters. The Computational Fluid Dynamics (CFD) simulations are utilised as a modelling approach after conducting the accuracy checks in order to obtain high fidelity analysis considering the reasonable accurate and computationally economic aspects. 2D-based turbine dynamic start-up simulations illustrate that not only lift force is required for the turbine to self-start but also the drag force contributes to the turbine torque generation at the critical turbine starting stage where the tip speed ratio (TSR)< 1. This finding encourages enhancing the positive contribution of the drag force by modifying the conventional aerofoil shape. In addition, the effect of the several physical properties, such as the moment of inertia, solidity, and the mechanical resistances on both the overall and the self-starting performance of the turbine is investigated. In contrast with the conventional aerofoils, such as NACA0018, the resistive type, such as the J-shaped aerofoil, is investigated in detail to provide an in-depth understanding of the aerodynamic performance of these kinds of aerofoils considering the oscillating motion and whole turbine aspects. The investigation of the J-shaped aerofoil under the oscillating motion, which is also a novel study for the J-shaped aerofoils, to highlight the advantages and disadvantages of the J-shaped aerofoils under the different operating conditions is found to assist in obtaining a further understanding of the aerodynamic characteristics of the J-shaped aerofoils when they are used in the turbine applications. In addition, the present work presents the first turbine time-varying start-up behaviour investigation for the turbine with the J-shaped aerofoils contrary to other studies in the literature. Even though the J-shaped profile increases the turbine performance in the upstream part of the turbine, increasing the opening length over the aerofoil surface causes a significant aerodynamic efficiency loss in the downstream part of the turbine, in particular when the turbine is rotating faster. Therefore, the utilisation of the optimum J-shaped profile is found to be of utmost importance not only to optimise turbine self-starting performance but also to maintain the performance efficiency at higher rotational speeds. Furthermore, this thesis proposed a novel hybrid blade design for the first time, which combines a conventional aerofoil NACA0018 and its cutting-off counterpart J-shaped aerofoil, to enhance the torque generation at the starting of the turbine with low tip speed ratio TSR values, on the other hand, minimise the efficiency loss at the high TSR values. Although the utilisation of the J-shaped aerofoil in the turbine configurations illustrates an increase in the torque generation at the low TSR values compared to the conventional aerofoils, a hybrid blade configuration that allows using the J-shaped profile in some portion of the blade span is required to reduce the significant performance loss at high TSR values due to the inherent shape of the J-shaped aerofoil. For this purpose, the 3D-based CFD simulations have been conducted for the different configurations of the hybrid blades with the various opening length percentage of the J-shaped aerofoil used. In contrast with the losses due to the J-shaped aerofoils, the hybrid blade designs, especially a design with the closed-tip, is found to be an appropriate selection in terms of a faster start-up time and a higher final rotational speed when the turbine reaches its steady-state conditions.

This thesis aims to substantially contribute to the understanding of the unsteady aerodynamics that remains unclear in the VAWTs. The analysis has been carried out by using computational fluid dynamics techniques very carefully verified with experimental data and using a modified Leishman-Beddoes dynamic stall algorithm. The results of this investigation are based on the analysis of oscillating aerofoils at constant and time-varying Reynolds and VAWTs with one, two and three blades evaluated at the tip speed ratio range of 1.5-5 using four symmetrical aerofoils and two-cambered aerofoils. The results have shown that: (i) the stall-onset angle in the VAWTs is defined by the combined effect of the tip speed ratio, pitch angle, angular frequency and relative velocity in the final value of the non-dimensional pitch rate when the angle of attack approaches the static stall angle; (ii) Under the fully attached regime, the symmetrical aerofoils performed better than the cambered aerofoils especially at the downstream zone of the rotor. This zone has shown to play the most significant role in the reduction of the overall torque coefficient and this reduction increases with the tip speed ratio, thus, a poor lift force coefficient at negative angles of attack can mitigate the advantages of a high lift/drag aerofoil observed at positive angles of attack. (iii) The curvature effect becomes more relevant with the increase of the tip speed ratio but also appears to be affected by the number of blades, therefore, a fast tool to design VAWTs needs to take into account this phenomenon in order to give reliable results. In the methods proposed in this thesis, despite being limited to the cases investigated, they demonstrate to be a potential tool to predict the unsteady loads in the VAWTs. The present investigation gives sufficient aerodynamic information to design strategies that improve VAWT performance.

The objective of this research was to investigate the impact of increasing the CO2 concentration in the exhaust flue gas in gas turbines via exhaust gas recirculation (EGR) and selective exhaust gas recirculation study (S-EGR). The gas turbine types explored are the micro gas turbine (MGT) and the combined cycle gas turbine (CCGT). Both gas turbines were integrated with a chemical absorption capture plant operated using MEA, in order to determine the reduction in carbon emissions into the atmosphere. Modelling the CCGT, involved a fuel flexibility study as well as a techno-economic analysis carried out to determine the cost of electricity (COE) and cost of CO2 avoided (COA) to ascertain the economic and energy savings involved as well as the emission reductions. The study on the MGT, entailed the experimental and modelling analysis of S-EGR via CO2 injection into the compressor inlet, with a maximum CO2 injection flowrate of 300 kg/h. The experimental campaign carried out on the MGT and capture plant were based at the LCCC, in Sheffield. The performance of the MGT is investigated over a range of power outputs (100 kWe to 60 kWe) and the emission properties are recorded. It was observed that the CO2 flue gas concentration increased from 1.8 mol% to 9.6 mol% and 1.5 mol% to 11.6 mol% at 100 kWe and 60 kWe respectively, which improved the performance of the integrated capture plant, indicating that S-EGR operation in the MGT could reduce the specific reboiler duty by 15% whilst increasing the liquid-gas ratio, at a constant capture rate of 90%, thus, reducing overall energy costs. The CCGT study entailed the modelling of fuel flexibility via increasing the CO2 content in the fuel from a conventional natural gas composition with 1 mol% CO2 to a maximum CO2 content of 10 mol%. Then, an Exhaust Gas Recirculation (EGR) study, involving a 35% EGR ratio stream from the exhaust to the compressor inlet, in which the CO2 content in both the fuel and the air were increased. In both studies, the CCGT is integrated to a capture plant, to reduce the emissions from the power plant. The results indicate that implementing EGR in a CCGT provides better working conditions for the integrated capture plant, when operating with various fuel compositions. The COE and COA associated with EGR implemented CCGT's are noticeably lower and thus proving its high energy savings and emission reductions.

In recent years, governments around the world have been increasing their attention on energy supply policies. These policies are focused towards three main energy goals that define the energy trilemma: security of supply, affordability and environmental sustainability. In the case of Colombia, the diversification of the energy mix including larger shares of renewable energy sources (RES) is a significant part of the national energy strategy towards a sustainable and more secure energy system. Historically, the country has relied on the intensive use of hydropower and fossil fuels as the main energy sources. Colombia has a huge renewables potential, and therefore the exploration of different pathways for their integration is required. The aim of this study is to assess the integration of variable renewable technologies and flexibility options into national energy systems by analysing future scenarios (towards 2030 and 2040). EnergyPLAN was the modelling tool employed for building the country’s model and simulate the reference year scenario and future alternatives. The study was divided in three research topics for its analysis: initially, the impacts of increasing shares of variable renewable sources in the energy system towards 2030 were analysed using five alternatives scenarios. Subsequently, a techno-economic optimisation was performed in order to assess the combined effects of large-scale energy storage and cross-border interconnections in the power system. Finally, the impact of road transport electrification in supporting the energy transition in the longer term (2040) was evaluated for the national system. The results showed that an increase in the shares of wind, solar and bioenergy combined with energy storage, electric vehicles and a strong interconnected market could achieve significant reductions in CO2 emissions and savings in the total fuel consumption of the country. The results of this work will be of much assistance to policymakers that are developing a roadmap towards low carbon energy systems in Colombia and other countries with similar potential and characteristics.

The Intergovernmental Panel on Climate Change (IPCC) have conveyed in their fifth assessment report that anthropogenic emissions and endeavours are responsible for approximately 100 % of global warming since 1950 [1] and electricity and heat generation account for 41 % of the 32.8 billion tons of global CO2 emission from fossil fuel combustion in 2017 [2]. There is thus a sense of urgency to capture CO2 from large point sources of fossil fuel combustion to limit global temperature rise. Natural gas combustion is envisaged to play a fundamental role towards a zero-carbon economy as opposed to coal and oil due to its low carbon content. However, capturing CO2 from natural gas combustion, which emits about 6.7 billion tonnes of CO2 in 2017 is challenging as it, bestows a parasitic energy penalty on Natural Gas Combined Cycle (NGCC) power plants. This is due to the low partial pressure of CO2 in the flue gas of gas turbines, which necessitate that substantial reboiler heat duty is employed for solvent regeneration. To address the aforementioned impasse, pertinent experimental campaigns at the UKCCSRC- PACT National Core Facility were carried out to simulate Selective-Exhaust Gas Recirculation (S- EGR) under the influence of 40 wt(%) of Monoethanolamine (MEA). This was to enhance the driving force behind CO2 capture and to reduce the Specific Reboiler Duty (SRD), consequently counterweigh against the forfeit on the power plant’s productivity. Furthermore, the impact of varying Pressurized Hot Water (PHW) temperature at the inlet of reboiler was studied. The influence of oxidative degradation of the amine solvent at 15 vol(%) of O2 and 5 vol(%) of CO2 has been experimentally investigated. Results from these studies have demonstrated that Selective Exhaust Gas Recirculation (S-EGR) is favourable in reducing the solvent regeneration energy requirement by about 25 % at CO2 concentration of 6.6 vol(%) prior to flue gas introduction in the Post-combustion Carbon Capture (PCC) system. PHW temperature at 125 °C was identified to give the lowest SRD by 6 % against the baseline SRD. Detection of Dissolved Oxygen (DO) peaks was observed as water from the water-wash column was transferred to the absorber column which may have a possible impact on the oxidative degradation of the amine solvent in the PCC system. The concentration of the Iron in the amine solvent, which is a key indicator of the solvent decay increased by approximately 10 times from 3.68 to 36.20 mg/l over a course of 545 hours of experimental operation. Results and recommendations from these studies will potentially reduce the solvent regeneration energy requirement of the next generation PCC technologies and facilitate the global deployment of such technologies towards decarbonisation of the fossil fuel combustion industries and strengthening the efforts of limiting global temperature increase.

Due to the challenging wind potential and the increasing energy demand, it is important to design the future wind turbines to operate efficiently at low wind speeds. The focus of this thesis is about the small scale straight-bladed Vertical Axis Wind Turbine (VAWT) due to its competitive advantages and promising market. This thesis aims to provide an in-depth understanding of the aerodynamics of small VAWTs and the effect of blade pitch in the turbine performance at low wind speed conditions. Moreover, this thesis proposes a novel procedure for the analysis of turbine performance and the estimation of the Angle of Attack (AOA). This enables the design of the most appropriate fixed pitch and variable pitch configurations. Based on the unsteady Reynolds-Averaged Navier-Stokes (uRANS), the state-of-the-art Computational Fluid Dynamics (CFD) simulation is utilised as a modelling approach and this enables high fidelity analysis with a reasonable computational cost. In order to find the most appropriate CFD methodology for the modelling of a VAWT, detailed verification and validation studies are carried out and this includes the comparisons of the CFD predictions against detailed instantaneous experimental data.

In this research thesis, the modelling of biogas production through the anaerobic digestion (AD) process is investigated, with a key focus on the modelling of low cost small -scale plastic bag types digester, which has a potential application in the rural communities of the developing countries. The present work investigated the thermal modelling of the plastic bag digester and its integration with simplified anaerobic digestion models to assess the feasibility and to develop operating protocols. In order to develop the AD process model, a parameter estimation method was developed, and this was used to describe the methane production of from food waste, green waste and pig manure, using data obtained from batch and semi-continuous experiments. It was found that in the case of a semi-continuous process, the model structure, rather than kinetic parameter values or kinetic equation used, determines the ability of the models to describe the degradation of food waste and green waste. For food waste, inhibition also played an important role in the model's ability to reproduce methane production data obtained experimentally. Further, it was found the kinetic parameter values obtained from the batch and semi-continuous processes are different. The kinetic parameter values or the best kinetic equation obtained from the parameter estimation method were used as input in the biochemical model, which is combined with a thermal model to simulate the performance of different designs of the plastic bag digester in Port Harcourt In Nigeria and Cuzco in Peru. Three designs of the digester were model; CASE 1 simple plastic bag digester without greenhouse cover, CASE 2 plastic bag digester with a greenhouse cover, and CASE 3 plastic bag digester with greenhouse cover and solar heating system. It was found that the addition of external heating sources such as greenhouse and solar heating system in CASE 2 and CASE 3 in Port Harcourt in Nigeria and Cuzco in Peru, has a positive impact on the performance of the digester (improved slurry temperature) compared with CASE 1. However, CASE 3 is not suitable in Port Harcourt, in Nigeria, since the difference in the slurry temperature in CASE 3 and CASE 2 is 1.48 oC, while the model predicts the suitability of CASE 3 in Cuzco, in Peru. The combined thermal model (CASE 2) and the biochemical model (3R) was used to model the operational protocol for small scale digester in different climate of developing countries. The protocol was developed for mono and co-digestion processes. It was found that minimum HRT (Hydraulic retention time) and maximum OLR (Organic loading rate) of a specific feedstock is different given the difference in. For the co-digestion process, the substrate characteristics and the substrate mix ratio have a strong influence on minimum HRT and maximum OLR.

The energy sector faces a challenge of providing energy with limited carbon dioxide (CO2) emissions due to its contribution to climate change. Currently the energy mix is dominated by fossil fuels and coal is the major contributor of CO2 emissions. Since biomass is classified as a renewable source of energy, its combustion is considered to have a near–zero CO2 emission, and thus it supports the climate change targets. Therefore, to reduce emissions from coal combustion, coal can be depleted by the usage of biomass by either co-firing or full conversion to biomass. Nevertheless, biomass combustion needs more investigations and development in order to increase its efficiency and to reduce the potential operational problems. Computational fluid dynamics (CFD) assists with the predictions of the combustion process and therefore, gives an opportunity to develop and design the efficient biomass conversion and this enables the retrofitting of coal fired boilers to biomass fuel. In this thesis pulverised coal and biomass combustion experiments on the 250 kW UKCCRC PACT Combustion Test Facility have been performed. The experimental data have been used for validation of the CFD model of coal combustion. Further, an extensive sensitivity study of the sub-models for coal combustion has been performed to provide an accurate and reliable CFD model. The validated CFD model has been employed to biomass combustion to investigate the limitation of the CFD predictions. It has been found that irregular shape and large size of the biomass particles causes major impact on the biomass combustion discrepancies in the CFD model. Thus, an in-house code has been developed and the effect of the implementation of irregular shape and large size biomass particles compared with experimental data presented improvement in the CFD predictions.

Post-combustion CO2 capture (PCC) is an important technology for meeting the greenhouse gas control target. Rotating packed beds (RPBs), as a type of process intensification technology, have been proposed as an emerging technology to be used for PCC from the flue gas, and this is because of its high mass transfer coefficient and compact structure which may lead to energy and space savings. The purpose of this thesis is to investigate the hydrodynamics and mass transfer performance of RPBs for CO2 capture through using CFD modelling methods. CFD models with different scales are developed to study the hydrodynamics of RPBs, and finally a multiscale modelling method is proposed to predict the performance of large-scale RPBs. First of all, a 2D VOF-based CFD model with fine grids is built for analysing the characteristics of liquid flow within a laboratory-scale RPB. This model successfully captured the distinct liquid flow patterns in the entrance region and the bulk region of the RPB. The simulation results indicate that increasing the rotational speed dramatically decreases the liquid holdup and increases the degree of the liquid dispersion. Increasing the solvent concentration increases the liquid holdup but the degree of the liquid dispersion decreases. In addition, a 3D representative elementary unit (REU) based mesoscale CFD model is built, which can be used to investigate the hydrodynamics of RPBs in greater detail and accuracy. The REU is used to study the flow at different locations within an RPB, so that the overall flow characteristics within the RPB can be assembled. The proposed approach enables the detailed prediction of the liquid holdup, droplets formation, effective interfacial area, wetted packing area and specific surface area of the liquid within real 3D packing structures throughout the bed. Based on the data from the CFD simulations, new correlations to predict the liquid holdup and gas-liquid interfacial area in the RPB are proposed. Finally, an Eulerian porous media CFD model is developed to analyse the CO2 absorption by MEA solutions in an RPB. The new porous media model, the gas-liquid drag model, the reactive mass transfer model, the heat transfer model and the interfacial area model are integrated into the Eulerian model, and this model successfully simulates the CO2 capture from the flue gas by MEA solutions in the RPB. The results obtained show that KGa increases significantly with the increasing of the liquid flow rate, MEA concentration, and the liquid inlet temperature, while it only increases slightly with the increasing of the rotational speed and the gas flow rate. In addition, the pressure drop significantly increases with the increasing of the rotational speed and the gas flow rate. In this thesis, the CFD modelling is realised though using the ANSYS® Fluent software with user-defined functions (UDFs). For the VOF model, the settings of the inlet boundary conditions and the acquisition of the detailed parameters in the calculation domains are achieved through writing UDFs. For the Eulerian model, the specified submodels for RPBs, such as the porous media model, the gas-liquid drag model, the mass transfer model, the interfacial area model, etc. are implemented in ANSYS® Fluent through writing UDFs.

Accurate predictions of the atmospheric properties are of paramount importance for many engineering applications, such as wind turbine wakes. The simulations of the atmospheric boundary layer (ABL) is a big challenge due to the fact that, with an exception of the ground boundary, it does not have any physical borders (e.g. such as flows in pipes) and consequently reasonable distances from the geometry of interest must be taken. The simulation of the flow near the ground also is challenging because, usually, ground consists of anomalies such as small vegetation, various sized stones etc. which renders impossible the simulation of it. Another big challenge in ABL simulations for both RANS (Reynolds Averaged Navier - Stokes) simulations and LES (Large Eddy Simulations) is the maintenance of inflow conditions throughout the computational domain. The streamwise distance of the computational domain can be of the order of magnitude of many kilometers, especially in wind farm simulations, and consequently huge distortions of the inlet profiles may occur. A fully developed inlet profile must be imposed at the inlet and be maintained throughout the domain, at least for a uniform roughness terrain. While the problem appears to be similar for both RANS and LES, the treatment is far different due to the different nature of RANS and LES approaches. RANS deals with average quantities and the problem with the maintenance of the inlet quantities is a boundary condition and turbulence modeling problem. LES on the other hand, does not include any extra equations (for the resolved part of the flow), consequently the maintenance of the inlet properties appears to be only a boundary condition problem. However, it is not only a boundary condition problem but the way that the turbulence is generated at the inlet plays a vital role, as well as the grid resolution, which makes the situation even more complicated. This thesis is specialized in the simulations of the surface layer of the neutral ABL in both RANS and LES and interactions of the properties of the neutral atmosphere with wind turbine wakes are investigated. Modifications to various RANS models are considered to make the models be mathematically consistent with the properties of neutral atmosphere in order to successfully simulate the neutral ABL by eliminating any streamwise gradients within the empty domain. Then, the adequacy of the modified models is investigated for the simulation of 2 different small wind turbine wakes. Finally, the problem of the inflow generation with synthetic methods for the simulation of the surface layer of the ABL is presented for LES. Periodic boundary conditions are employed to overcome this difficulty along with an extension of a shear stress boundary condition at the upper boundary of the domain in order to maintain the turbulence within the domain. The results of the proposed models show good agreement when compared with available measurements and also improvements when compared with other models found in the literature, especially in the far wake region of the wind turbines as they have the correct recovery of the wake for 2 different wind turbines for various different inlet conditions.

Proton Exchange Membrane (PEM) fuel cells have a great potential to replace conventional fossil fuel dependent power conversion technologies in a wide range of portable, automotive and stationary applications and this is due to their high efficiency, quick start-up and sizing flexibility. However, there are still some technical challenges that hinder the widespread deployment of this clean technology into the marketplace. Two of the key challenges are the water management and thermal management of the fuel cell; any mismanagement of water and/or heat could lead to water flooding or membrane dry-out which are both detrimental to the fuel cell performance and durability. In order to have insights on how to manage water and heat within the fuel cells, a transparent and commercially available PEM fuel cell has been directly visualised using high-resolution digital and thermal cameras at both sides of the fuel cell. With this technique, real-time videos that show how liquid water and heat evolve have been recorded. There has been a particular emphasis on how liquid water forms, accumulates and moves along the flow channels. Further, the sensitivity of the distribution of liquid water and temperature within the fuel cell to the operating conditions has been investigated. For this investigation, a new parameter, termed as the wetted bend area ratio, has been introduced to give an indication on how flooded the flow channels are and subsequently explain the variations in the performance of the PEM fuel cell as the operating conditions change. The main findings are the temperature distribution across the MEA becomes less uniform as the wetted ratio number decreases. Further, the temperature distribution along the MEA at the cathode side becomes less uniform as the air flow rate increases. In addition, there exist optimum values for the operating conditions to increase the fuel cell performance. Since the operation of the PEM fuel cell at high temperatures (i.e. > 100°C) is an increasingly adopted way to resolve water flooding problems, the reliability of the currently used components remain questionable. To partly answer this question, the gas permeability of the diffusion media used in PEM fuel cells has been investigated under higher temperatures for the first time. The results show that the gas permeability increases as the operating temperature increases and this may enhance the reactant transport within the PEM fuel cell.

This thesis uses the Pitzer model to investigate the equilibrium of aqueous species formed after the absorption of CO2 by chemical solvents. Since the parameters in the Pitzer equations are unknown for some species involved in the equilibrium, new expressions that correlate the virial parameters in the Pitzer equations with the ionic radii and charges of the species are derived. Further, since some ionic radii are unavailable in the literature, new expressions are derived to estimate these unknown ionic radii. Furthermore, a case study involving MEA, DEA and NH3 is investigated using the expressions that correlate the parameters in the Pitzer equations with the ionic radii and charges. Finally, a mineral carbonation process involving NH3 as the chemical solvent is simulated in Aspen Plus, and this process aims to investigate the precipitation of calcium carbonate using the fly ash generated in power plants. As a conclusion, the calculated activity and osmotic coefficients of the aqueous species obtained using the equations that correlate the virial coefficients in the Pitzer equations with the ionic radii and charges are in good agreement with the experimental data from the literature up to a temperature 150 °C. Likewise, the expressions derived to estimate the unknown ionic radii produces results that are in good agreement with the literature data. Furthermore, the case study involving MEA, DEA and NH3 shows that the correlating equations accurately predict speciation and partial pressures up to a solvent mass fraction of about 20%. Finally, a comprehensive analysis involving the conversion of the pure CO2 produced in the amine capture plants into a solid is presented, and as a result this new process is capable not only of reducing the electricity consumption of the MEA capture plant by about 18%, but also to produce a product that may be potentially reutilized in industry, namely the PCC.

Combustion has played a key role in the development of human society; it has driven the evolution in the manufacturing processes, transportation, and it is used to produce the vast majority of the global energy consumed. The emission of pollutants from the combustion of fossil fuels in power plants lead to the development of advanced clean energy technologies, such as carbon capture and storage. Oxyfuel combustion is part of the carbon capture and storage techniques, and consists in the replacement of the air as oxidiser in the reaction with a mixture of oxygen and recycled flue gas, thus allowing a rich CO2 out-flow stream that can subsequently be compressed, transported and safely stored. The number of phenomena in combustion that are inherently dynamic impede the convention of a unique conception of flame stability. However, the quantification of the flow repeatability can produce insights on the efficiency of the process. This thesis presents the assessment of the stability in swirling flows through the calculation of their spatiotemporal coherence. The experimental data obtained from a 250 kWth combustor allows the assessment of the flame by means of spectral and oscillation severity analyses. A similar methodology is developed to analyse the data from large eddy simulations. The spectral analysis, the proper orthogonal decomposition and the dynamic mode decomposition have been employed to account for the temporal, spatial and spatiotemporal coherence of the flow, respectively. The spatiotemporal coherence is employed as a comprehensive term for the characterisation of the dynamic behaviour in the swirling flows and as a measurable indicator of the stability. This concept can be incorporated into the design of novel combustion technologies that will lead into a sustained reduction in pollutants and to the mitigation of the noxious effects associated to them.

The objective of this research was to investigate the influence of selective exhaust gas recirculation (S-EGR) applied to gas-fired systems and determine if energy and economic savings can be realised. The gas turbine experimental campaigns evaluated the performance of a micro gas turbine (mGT) under S-EGR conditions, simulated by injecting CO2 into the compressor inlet. The mGT performance was affected under these conditions in terms of efficiency, compressor operation and emissions, however, this was more prominent at part loads. The flue gas CO2 concentrations increased by up to 6 times (up to 10.1 vol% at 60 kWe) with respect to the reference case without S-EGR,which can be beneficial to improve the downstream amine capture plant (ACP) performance, as demonstrated by subsequent experimental campaigns. The first assessed ACP performance with flue gas CO2 concentrations of 5.2-9.0 vol%. The results showed that the specific reboiler duty reduced by 21% and the liquid-to-gas ratio increased from 2.59 to 4.22 between the lowest and highest flue gas CO2 concentrations tested. The plant performance was also analysed at varying reboiler temperatures (124-127°C), using a flue gas with 9.0 vol% CO2 concentration. The results indicated that the overall specific reboiler duty and CO2 capture efficiencies reduced at lower reboiler temperatures. Overall, the results indicate that S-EGR operation in the gas turbine could be beneficial since it can lead to reductions in overall energy costs. The economic analysis evaluated the application of two S-EGR configurations (parallel and hybrid) in combined cycle gas turbine (CCGT) power plants, with the cost of electricity ranging from $82-90 and $82-93 per MWhe, respectively. The cost of CO2 avoided for the parallel and hybrid schemes varied from $80-105 and $83- 119 per tonne of CO2 avoided. The sensitivity analysis performed also demonstrated the economic competitiveness of S-EGR against ACP and EGR.

Current extreme weather events are evidence of the global climate change and its effects on the environment. Although natural gas is a "greener" fuel compared to solid fuels, its continuous uncontrolled use will increase further the atmospheric CO2 emissions above sustainable levels. Natural gas fired power plants equipped with a carbon capture plant can serve as a short, and medium, term solution to mitigate global warming. Several research studies have shown that post-combustion capture technology is a readily available option to reduce drastically the CO2 emissions. However, its associated energy demand for separating CO2 from the other exhaust gases is high and needs to be reduced. Therefore, recirculating part of the exhaust gases to the inlet of gas turbine combustors increases significantly the exit CO2 concentration which is the driving force of the capture plant. This fundamental study focuses on the implications of the addition of CO2 on fuel-lean natural gas non-premixed flames. The thermal and chemical effects of adding CO2 in the air stream on the flame chemistry, properties, stability and the formation of pollutant emissions represent the main aim of the present study. Two experimental campaigns were performed in this study and in-flame, post-flame and exit measurements of major and minor combustion species and the flame temperature are performed utilising an in-house built combustion chamber. Experimental results concluded that CO2 has a considerable impact on the combustion process even at relatively low dilution levels. However, the effects of the dilution can be controlled in a beneficial way for the efficiency of the combustion systems. Furthermore, the assessment of the performance of a 1D numerical model on predicting the implications of the addition of CO2 on the flame chemistry was part of this study. The numerical results concluded that robust complex combustion models are needed to examine CO2-diluted combustion systems and it is evident that detailed chemical reaction mechanisms are as necessary as the inclusion of the interaction between turbulence and chemistry.

The awareness of global climate change by emissions of greenhouse gases from fossil fuel combustion is widely known by current society. Polymer Electrolyte Fuel cell (PEFC) technology has been a very promising clean technology with high efficiency that has been used in a wide range of portable, automotive and stationary applications. The fuel cell research has been developing very rapidly and successfully in the last few years. However, some issues remain largely unresolved, namely water management and high cost of the PEFC component. One of the efficient and cost-effective ways to improve the design of the PEFC and consequently resolve the above mentioned issues is through modelling. However, the built PEFC models need to be fed with accurate transport coefficients to enhance their productivity. One of the most important transport coefficients is the gas permeability of the PEFC porous media which highly affects the convective flow. Therefore, in this thesis, thorough experimental studies have been conducted to investigate the gas permeability of gas diffusion media used in PEFCs. The focus has been on the effects of the following on the gas permeability of the gas diffusion layers (GDLs): (i) type of carbon black used in the microporous layers (MPL) attached to the GDL, (ii) carbon and polytetrafluoroethylene (PTFE) loading, and (iii) the thickness of the MPL. Further, a novel method has been proposed to estimate the penetration of the MPL into the carbon substrate (i.e. the GDL before being coated with the MPL ink). Also, the effect of sintering on the gas permeability of the MPL has been investigated for the first time.

Pulverised coal-fired power generation technologies are important for meeting the electricity consumption worldwide, especially for the developing countries. Changing fuels (coal blending, co-combustion, new fuels, etc.) is common practice in the power stations, which may result in the change of ash deposition behaviours. Ash deposition issues can reduce the heat transfer and have a negative effect on the long-term operation of the combustion systems. Therefore, prediction of ash deposition behaviours is significant for the efficient operation of boilers. In this thesis, new ash deposition prediction models based on particle impaction and sticking behaviours, ash melting behaviour and multi-slagging routes have been developed in order to understand ash deposit formation and predict the slagging propensities through using Computational Fluid Dynamics (CFD) methods and ash deposition indices. Regarding the CFD methods, an ash deposition model has been proposed to predict the ash deposit formation on an uncooled probe for the co-combustion of South African coal and palm kernel expeller in an entrained flow reactor. A new revised particle impaction sub-model has been developed in order to minimize the numerical related errors without excessive meshing. The molten fraction model obtained from the chemical equilibrium calculations was employed to predict the particle sticking behaviour. The simulation results show that the revised particle impaction model is suitable to accurately resolve particle impaction without using a prohibitive meshing size. Particle impaction and sticking properties dictate the ash deposit formation. In addition, a CFD-based dynamic ash deposition model has been developed to predict the slagging formation on a cooled probe under high furnace temperatures of Zhundong lignite (rich in alkali and alkaline earth metal elements) combustion in a pilot-scale furnace. The developed model is based on the inertia impaction, the thermophoresis and the direct alkali vapour condensation and incorporates the influence of the heat transfer rate. The results show that particle deposition from the inertia impaction and the thermophoresis dictates ash deposit formation under high furnace temperatures. The deposition caused by the direct alkali vapour condensation is less significant. As deposition time increases, particle impaction efficiency decreases and sticking efficiency increases due to the thermophoresis and the local temperature conditions. In addition, the ash deposition characteristics are influenced under different furnace temperatures, due to the changes in the particle impaction and sticking behaviours. Further, a new method for building the ash deposition indice has been proposed to predict the slagging propensities of coals/blends combustion in utility boilers. The method is based on the initial slagging routes and the sintered/slagging route. Two types of initial slagging routes are considered, namely (i) pyrite-induced initial slagging on the furnace wall, and (ii) fouling caused by the alkaline/alkali components condensing in the convection section. In addition, the sintered/slagging route is considered by the liquids temperature, which represents the melting potential of the main ash composition and is calculated using the chemical equilibrium methods. The partial least square regression (PLSR) technique, coupled with a cross validation method, is employed to obtain the correlation for the ash deposition indice. The results obtained show that the developed indice yields a higher success rate in classifying the overall slagging potential in boilers than some of the typical slagging indices. In addition, both SiO2 and Al2O3 can reduce the slagging potential, but the drop in slagging propensity is more significant by adding Al2O3 compared to SiO2.

The small-scale wind turbine is considered as one of the most effective renewable technologies due to their potential to provide useful amount of electricity, particularly in ‘‘off-grid’’ settings as well as promising future prospects to decarbonise the power sector and ultimately stabilise energy security. Due to the huge potential of the wind resources and financial incentives, the UK is a promising region for small-scale wind energy development but there has been lack of comprehensive assessment of the wind resource for relevant locations. Thus efficient and low cost techniques are urgently needed to assess the resource potential since the long-term measurement techniques usually employed in the large-scale industry are very expensive and often not feasible for small-scale development. The research developed during this thesis focuses on cost effective techniques for predicting the wind resource using two main approaches, namely the boundary layer meteorology and measure-correlate-predict (MCP). These approaches were evaluated using a long-term dataset from the Modern Era Retrospective-Analysis and short-term onsite dataset from meteorological measurement station. To begin with, the performance of a modified methodology based on the boundary layer meteorology was evaluated at four UK sites, and the results were validated using traditional error metrics. Averaged across all sites, the percentage error in the predicted wind power density was found to be about 25% due to the uncertainties associated with the choice of the input parameters. Although the result is very encouraging, it was concluded that such a method is better applied in a ‘‘preliminary’’ analysis to identify viable sites worthy of further investigation. To reduce these uncertainties, an MCP technique was utilised along with onsite measurements over a period of 12 months at a subset of 1 of the 4 UK sites, and the results show a significant improvement on the predicted wind speed and power density. Comparison of both approaches show that the best performing MCP approaches resulted in percentage error in the predicted mean wind speed and power density of 7.2 % and 12.9 % in contrast to the 18.9 % and 17.0% obtained using the boundary layer approach. Seasonal trends, direction behaviours and frequency distribution were analysed and their characteristics reflected the general wind conditions across most UK sites. Based on the output of the wind resource assessment, the potential of a small-scale vertical axis wind turbine (VAWT) was assessed using the double multiple streamtube model. VAWTs based on the Darrieus concept are potentially more efficient and economical, but those with fixed pitch blades are inherently non self-starting and are unsuitable for decentralised application. It is shown that the self-starting problem can be alleviated by a combination of a suitable aerofoil sections, solidity and pitch angles. The thesis provides a technique for inexpensive wind resource assessment where direct long-term measurements are not feasible. In addition, it provides a suitable solution strategy to the problem of self-starting in small-scale fixed pitch wind turbines.

There is a growing need for secure, flexible and inexpensive energy across the world, however there is also a need to simultaneously curb emissions of greenhouse gasses and toxic pollutants. Fossil fuel combustion is expected to meet a significant portion the world's growing energy demand, however CO2 emissions need to be mitigated to avoid potentially catastrophic effects caused by global warming. Carbon capture and storage (CCS) technologies have been developed to permit the use of fossil fuel combustion in a future with strict controls over greenhouse gas emissions. CCS technologies are still yet to be deployed at large industrial scales, and it is necessary to reduce the efficiency overheads associated with CCS before the technology is economically feasible. Computational modelling can play a significant role in designing and optimising CCS technologies for power generation due to its flexibility and comparatively low costs. The work in this thesis develops and validates models for predicting mercury oxidation and thermal radiation under oxyfuel combustion conditions, which is a promising CCS technology that is competitively placed for large-scale implementation. The oxidation of mercury is a key chemical process in mitigating emissions of the toxic metal, and predicting the principal oxidation pathways will improve the design of control technologies. Thermal radiation, which is the most significant mode of heat transfer at combustion temperatures, is a very important physical mechanism for predicting many properties of combustion, such as gas temperatures, chemical reaction rates and heat fluxes, and thermal radiation models must accurately account for changes in the combustion environment. The models developed and validated in this thesis provide new approaches to predict mercury oxidation and thermal radiation under oxyfuel conditions. The results and conclusions from this work offer clear guidance on methods to model thermal radiation in oxyfuel conditions, and provide new insight on the mercury oxidation mechanism.

There is overwhelming evidence for anthropogenically driven climate change and as such it is critical to reduce the emissions of greenhouse gases. A large source of these is combustion for energy. Combined Cycle Gas Turbine (CCGT) power generation is going to be a significant part of the future of base power load generation in the UK and internationally. The CO2 emissions of gas turbines are significantly lower than coal, but must still be addressed. Gas exhaust emissions also present a challenge due to the low volume percentage of CO2 in the exhaust, as the current most mature technology for carbon capture is post combustion capture using solvents, which is significantly more efficient with higher CO2 partial pressures. This thesis looks at potential ways to increase CO2 partial pressure in the gas turbine exhaust for improved capture efficiency. To do this exhaust gas recirculation (EGR), and humidified air (HAT) or a combination could be beneficial. There is a lack of bench and pilot scale experimental research into this area. A Turbec T100 Microturbine was used to investigate the impacts of EGR and HAT, through the addition of CO2 and Steam before the combustor. In addition the turbine particulate emissions were analysed. This is as gas combustion has high emissions of fine particulate matter. Particulates are of increasing consideration as an environmental pollutant, and may contribute to solvent degradation. The investigations found that the performance of the turbine is largely dependant upon ambient temperature. The results matched with the literature showing impact on the combustion process reducing peak temperatures, and an increase in unburned hydrocarbons, and carbon monoxide. This resulted in a slightly reduced generation efficiency. However this loss in turbine efficiency is potentially superseded by a gain in capture efficiency and net CCGT low carbon generation with carbon capture and storage.

With the increasing evidence of the potential disastrous consequences arising from global warming, there is a need to reduce greenhouse gas emissions, and gas-fired power generation represents an attractive option due to its low carbon intensity. Nevertheless, gas is not a zero-emission fuel and therefore it is necessary to control the emissions associated with its usage. Among the carbon capture techniques suitable for gas-fired generation, postcombustion is regarded as the most feasible in the short-term. The additional costs associated with the CO2 capture process can be reduced by employing modified cycle concepts such as EGR and STIG, which are characterised by a diluted combustion environment. The development of accurate numerical models for the combustion process in industrial devices under diluted conditions can be very useful in assessing the impact of dilution on the combustion process, and represents the main goal of the present work. Firstly, the impact of carbon dioxide and steam dilution on natural gas combustion has been assessed by means of detailed simulations of simple unidimensional laminar ames. It has been found that even the relatively low dilutions levels typical of EGR and STIG cycles have a significant impact on the combustion process. Also, the diluting species participate directly in the combustion chemistry, and therefore there is a need to include detailed chemistry and finite rate-effects in a CFD model for realistic configurations. In this respect, the suitability of the RANS and LES FGM/presumed-PDF approach for the modelling of swirling partially-premixed flames has been assessed. The performance of different turbulence models with different levels of mesh refinement have been assessed against in-ame measurements in a lab-scale burner and guidelines for the CFD modelling of industrial devices have been inferred. Finally, the previous findings have been employed to develop a complete CFD model for an industrial MGT combustor, which has been investigated under both air-fired and diluted operation. The numerical results have compared with the available experimental data. It has been concluded that the model is able to predict the impact of dilution on the heat release, flame stabilisation, flow-field and pollutant emissions.

The aim this work was to investigate the effect smart heating control systems have on the energy performance in domestic homes in the UK. An experimental investigation was conducted of three case study buildings which were selected with different constructions and occupancy patterns to represent a cross section of the typical UK housing stock. Temperature data loggers were deployed in each home from 1 February until 31 April 2014 and these logged the internal temperature of the living room, kitchen and master bedroom at 5 minute intervals. A numerical analysis using IES VE (Integrated Environmental Systems Virtual Environment) dynamic thermal modelling software was undertaken of these three case study buildings with the results from the experimental investigation used to provide validation that the thermal performance of the dynamic thermal models was the same as the case study buildings, within experimental tolerances. The thermal performance for each case study building was compared to the CIBSE (Chartered Institute of Building Service Engineers) recommended guidelines for internal thermal comfort temperatures and consistent underheating in each case study building was identified. One dynamic thermal model was selected and taken forward for modifications to be made to the heating control system, with the single zone thermostatic control system being replaced by a 2 zone thermostatic control system and finally a multi zone thermostatic control system. Three thermostat schedules were investigated for the zoned control systems, the first being a 9-5 working schedule assuming the occupants are out of the house between those hours, an always occupied schedule assuming some level of occupation through the day and a nightshift schedule assuming a shift workers pattern not following a traditional Monday to Friday 9 – 5 working week. The study found that increasing the zoning of the control systems did not yield energy savings in every case but did increase the comfort conditions for the occupants and the degree of control the occupants had in their building. The occupancy pattern was found to affect the performance of the zonal heating strategy.

Numerical computations have been performed to evaluate the influence of bedform roughness on turbulent transport of sediments in geophysical flows. Special attention is paid to turbidity currents, which are responsible for the transport of sedimentary rocks far into the deep ocean. It has been suggested that enhanced turbulence mixing in flows over rugose topography contributes to the unexpectedly large runout lengths of naturally occurring turbidity currents. One of the objectives of this study is to provide evidence for against this conjecture. We perform computations over a wide range of periodic arrays of rectangular roughness elements, We find that a strong peak in turbulent mixing occurs when the width-to-height ratio equals a critical value of seven. We also find that a strong peak in resistance to flow occurs at the same critical value. These are competing effects, with the former acting to promote, and the latter acting to diminish runout length. So we are not able to conclude definitively that the enhancement of mixing is responsible for long runout lengths. We continue by considering flows over periodic arrays of shapes which are representative of bedforms that occur in the natural environment. We again find a strong correlation between the optimisation of both turbulence mixing and resistance to the flow. We are unable to distinguish bedform shapes that promote long runout length relative to the flat bed case. However, we are able to distinguish those bedform shapes that have large resistance to flow and large turbulence mixing compared to those that have low resistance and low turbulent mixing, with the latter case occurring for widely spaced asymmetric dunes with a long low angled slope facing the flow. Finally, we develop a model for flow and sediment transport which takes into account erosion and deposition from the bottom boundary. We first apply this model to flow over fixed dune shapes, in order to assess the influence of bedform shape on flow capacity, stratification, and the energy budget. An important result of this study is that flow capacity is optimised for the class of bedform shapes that promote low flow resistance and low turbulent mixing. We conclude by applying the model to the two-way coupled flow of a mobile dune, starting from an initially symmetric inherited dune morphology. We find that, for sufficiently large grain sizes, the dune evolves into a sequence of asymmetric dunes, rather than to a flat bed, and that the long-time evolution tends to be towards those dune shapes that promote large relative flow capacity. However, the model has a discrepancy in that it is unable to prevent the dune shape exceeding the maximum angle of repose. Hence, further work is required before these results can be regarded as reliable.

This PhD aspired to a develop greater insight into the fundamentals of hydrocarbon autoxidation processes in the context of thermal stability of aviation fuels. A number of approaches to develop a better understanding of the observed processes have been considered. This covers establishing the suitability of an automated reaction mechanism generator to develop autoxidation reaction mechanism as well as manipulating the resultant schemes, for example by lumping species and their associated reactions as well as reaction rate based mechanism reduction. Further a number of postulated interactions between contaminants in fuel and oxidised products in thermally stressed hydrocarbons employing ab initio quantum chemistry methods were examined. Finally a set of systematic experiments was carried out to obtain quantitative data of the effect of a number of additives on the stability of thermally stressed hydrocarbons. These tests employed a small scale test device, the PetroOxy which provides reliable and reproducible experimental data under well defined test conditions. The PetroOxy enabled the collection of samples of the deposition products on metal foils for further analysis under a scanning electron microscope with x-ray dispersive spectroscopy for elemental analysis. This provided both the morphology as well as the elemental distribution of the deposits formed on the foils, offering a first hand look at the differences between deposits formed from different additives. This thesis shows that automated mechanism generation is a suitable method when describing the initial steps of autoxidation processes without any additives. It further shows that the PetroOxy is a very useful tool for obtaining systematic, reliable, experimental data for further analysis.

In the energy supply sector, coal will still remain as a dominate role in the foreseeable future because: it is comparatively cheap and widely distributed around the world and more importantly, carbon capture and storage (CCS) technologies make it possible to depend on coal with almost zero emission of carbon dioxide (CO2). CCS involves capturing and purifying CO2 from the emission source and then sequestering it safely and securely to avoid emission to the atmosphere. Both the post-combustion and the oxy-fuel technologies can be applied to existing power plants for CCS retrofit. Accurate prediction of the performance of a CCS plant plays an important role in reducing the technical risk of future integration of CCS with existing power plants. This research combines the fundamental computational fluid dynamics (CFD) and system process simulation technologies so that an efficient co-simulation strategy can be achieved. A 250 kWth coal combustion facility combined with a CO2 post capture plant is taken to test the conception of the CFD and process co-simulation approach. The CFD models are employed to account for the combustion facility and the predicted results on the outlet gas compositions, temperatures and mass flow rates are used to generate reduced order models to linked to the model for the PACT CO2 post capture plant so that a pilot scale whole plant model is achieved and validations have been made where it is possible. Afterwards, the a large scale conventional air-coal firing power plant is taken into investigation: the CFD models for the boiler and the process models for the whole plant have been developed. Further, the potential of retrofitting this power plant to oxy-firing is evaluated using a CFD and process co-simulation approach. The CFD techniques are employed to simulate the coal combustion and heat transfer to the furnace water walls and heat exchangers under air-firing and oxy-firing conditions. A set of reduced order models has been developed to link the CFD predictions to the whole plant process model in order to simulate the performance of the power plant under different load and oxygen enrichment conditions in an efficient manner. Simulation results of this 500 MWe power plant indicate that it is possible to retrofit it to oxy-firing without affecting its overall performance. Further, the feasible range of oxygen enrichment for different power loads is identified to be between 25% and 27%. However, the peak temperature on the superheater platen 2 may increase in the oxy-coal mode at a high power load beyond 450 MWe.

This research is concerned with the application of experimental and numerical methods to the investigation of gas flares. Gas flares are ubiquitous in the oil and gas industry and knowledge of the emissions from flares are of vital importance. Also, flares subjected to a cross-wind behave differently from flares in quiescent atmosphere and the stability of flares are important due to the possibility of blow offs in strong cross-winds. Natural gas flares are common on oil and gas production fields and given that natural gas is predominantly methane, many researchers have carried out numerical studies on methane cross-flow flames. In the present study, a propane cross-flow flame has been numerically investigated in addition to the investigations on methane and the predicted results have been compared against experimental data. The first chapter presents an introduction to the topic of gas flares and also outlines the objectives of the project. The second chapter presents a review of the literature on related topics. The third chapter presents the methods of investigation employed in this thesis. The fourth, fifth and sixth chapters present the results of the investigations carried out while the final chapter gives the general conclusions and suggestions for future investigations. The numerical aspect of the work investigates the capability of the existing codes in the ANSYS Fluent software to predict propane flares both with and without cross-flows. Comparisons have been made between the non-premixed and the partially premixed combustion models in their capability to predict wake-stabilized cross-flow flames. Also, the results obtained using the Reynolds stress turbulence model was compared against the Large Eddy Simulation technique with the latter showing better predictions for the temperature and species. The reaction mechanism of Ranzi et al. (2012) was used to model the kinetics of the propane reactions while the GRI 3.0 was used to model the kinetics of the methane reactions. Soot in the propane flame was modelled with the Moss-Brooke-Hall model. The experimental aspect of the work involved comparing the lift-off behaviour of the methane and the propane flames given the effect of lift-off on the emission from the flames.

This dissertation investigates the implications of the fluid flow on the behaviour of the particle-scale structure of a porous hard rock, based on the Discrete Element Method (DEM). This project is driven by the need to contribute towards a better understanding of the mechanical behaviour of porous rock formations under intense injection conditions and the influence of natural pre-existing rock damage to the hydraulic fracturing mechanism. The proposed numerical scheme incorporates different methods for computing both the solid and co-existing fluid phases. The solid phase (rock sample) has been characterized as a collection of discrete interacting particles, bound by spring-like contacts according to the DEM. Meanwhile, the fluid phase has been modelled by discretising the Navier-Stokes equations for porous media, utilising the fluid coupling algorithm embedded in the Particle Flow Code (PFC3D) software by Itasca. The outcome of this dissertation suggests that the DEM approach is an advanced computational method that can reproduce accurate rock models, adequately describe the inter-particle dynamics and thus contribute towards direct numerical and experimental comparisons, and interpret the geo-mechanical behaviour of the rock materials. Furthermore, this study identifies the importance of shear cracking in the hydraulic fracturing models, whereas conventional theory relates hydraulic fracturing with tensile cracking. Finally, this study focuses on the influences of various parameters, such as the external stress regime, fluid viscosity and pre-existing fractures, on the mechanical behaviour of the rock material in the particle-scale and the hydraulic fracturing process as a whole. This work is in an early stage and it aims to simulate hydraulic fracturing experiments with the use of a 3D modelling and the DEM approach, and to investigate the micromechanical response of the rock. Further research may include areas such as the 3D modelling of pre-cracked rocks using a larger variety of fracture angles.

Achieving the emission reductions that scientists recommend will require the deployment of technologies, such as wind turbines and solar photovoltaics, which have fundamentally different characteristics to the fossil fuel generators that have contributed to the growth and prosperity enjoyed throughout the industrialised world. This research has centred on developing a greater understanding of the technical and economic challenges of increasing variable renewable penetration in electricity systems. Following a review of the literature, three important topics for research are identified and analysed. Initially, the EnergyPLAN tool is used to quantify the benefits of increasing energy storage and interconnection capacity in future British power systems. The findings conclude that increasing the interconnection and storage capacity allows for an increase in the maximum technically feasible wind penetration, this permitting a reduction in system emission intensity. Subsequently, the operational requirements for thermal plants in future power systems are investigated using the PLEXOS Integrated Energy Model. In the scenarios considered, the utilisation of gas plants is relatively low but remains fundamental to security of supply. The findings have important implications for energy policy as government intervention may be required to prevent early decommissioning of gas capacity, should the prevailing market conditions not guarantee revenue adequacy. Finally, using the PLEXOS Integrated Energy Model, a capacity expansion model is developed to understand the long term price implications in systems constrained by emission reduction and system security targets. As the long run costs increase at a greater rate than the short run costs, revenues from the energy market are increasingly insufficient for firm generation capacity to recover costs. The insights highlight the importance of designing power markets that provide incentives to satisfy both emission reduction targets and security constraints, in systems with increasing variable renewable generation.

This thesis investigates the capture of CO2 from the flue gas of coal-fired power plants using an aqueous solution of MEA, and the main aim of this thesis is the development of an optimized amine-based post-combustion CO2 capture (PCC) process that can be integrated optimally with a pulverized coal-fired power plant. The relevance of this thesis cannot be overemphasised because the reduction of solvent regeneration energy is the focus of most of the solvent-based post-combustion CO2 capture (PCC) research currently being performed globally. From the view point of current research and development (R&D) activities worldwide, three main areas are being investigated in order to reduce the regeneration energy requirement of an amine-based PCC process, namely: (i) development of new solvents with better overall performance than 30 wt% monoethanolamine (MEA) aqueous solution, (ii) PCC plant optimization, and (iii) optimal integration of the PCC Plant, including the associated CO2 compression system, to the upstream power plant. In this thesis, PCC plant optimization and the optimal integration of an optimized PCC Plant, including the associated CO2 compression system, with an upstream coal-fired power plant has been investigated. Thus, an integrated process comprising ~550 MWe (net power after CO2 capture and compression) pulverized coal-fired (PC-fired) supercritical power plant, an MEA-based post-combustion capture (PCC) plant and a CO2 compression system has been modelled, simulated and optimized. The scale-up design of the PCC plant was performed using a novel method based on a rate-based calculation and thus the unnecessary over-design of the PCC plant columns was avoided. Furthermore, because of the importance of the operating pressure of the stripper in a PCC plant integrated to a PC-fired power plant, the impact of the operating pressure of the stripper on the net plant efficiency of the integrated system has been quantified. Also, the impacts of coal type on the overall performance of the integrated process have been quantified.

Experimental and numerical investigations have been conducted in order to evaluate the accuracy of the Mixture Model, a depth-resolved and time-averaged multiphase numerical model, in predicting the behaviour of dilute surge-type turbidity currents. The effects of turbulent dispersion and turbulence modulation upon sediment transport within turbidity currents are directly modelled via their incorporation into the Mixture Model. Modelled predictions of flow front propagation and deposit density are compared against both experimental data and refined two-fluids model from previous studies. When modelled using the formulation of Chen & Wood (1985), turbulence modulation does not affect on the propagation of dilute turbidity currents significantly. Turbulent dispersion can be modelled by incorporating the formulation of Simonin (1991) into the slip equation of the Mixture Model. Its effect is strongest in dilute flows carrying fine particles and diminishes when either grain size or flow concentration increases. Modelled turbulent dispersion effects are too strong in simulations of flows carrying silicon carbide particles; Mixture Model simulations agree poorly with both experimental data or refined two-fluids model results of the deposit mass profile. Yet turbulent dispersion is essential to ensure that model predictions of flows carrying glass beads compare well with experimental data. The reasons for the discrepancy between modelling approaches best suited to each of these flow types remains poorly understood. A new analytical approach is developed to evaluate the effect of the lift force on particles of small, intermediate and large particle Reynolds number immersed in two-dimensional shear flows. The lift force always reduces the magnitude of the particle settling velocity and may push particles forward or backward, depending on the sign of both the lift coefficient and the flow vorticity. Given plausible velocity profiles within natural turbidity currents, the effect of lift force on the sand-like particles immersed in such turbidity currents is negligible. It may become significant when the ratio of the particle density to the flow density approaches unity. New experiments are presented for flows over the flow concentration range 0.25 – 5% and grain size range 58 - 115μm. The data are used to facilitate a more complete validation of the Mixture Model, based on flow front propagation rates, deposit mass density and deposit grain characteristics. Modelling results for first two variables are in good agreement with the experimental data, when turbulent dispersion effects are incorporated. For reasons which remain unclear, the model cannot simulate the unexpected experimental result that deposit grain size is largely unfractionated if the standard deviation of the source material is less than 11 but significantly fractionated if it exceeds 18. This discrepancy requires further work.

Slagging and fouling during the combustion of pulverised coal in boilers is a major problem as power generators strive to improve the efficiency of plants. The coal type has a major influence on the slagging propensity in furnaces. The correlation between predicted results using some of the existing slagging indices and the actual observations made in most conventional boilers has been poor, especially when their use is extended to different coals. In this thesis, a numerical model to predict coal ash particle deposition rate in pulverized coal boilers has been developed. The overall sticking probability of the particle is determined by its viscosity and its tendency to rebound after impaction. The deposition model has been implemented in the Fluent 12.1 software, and the effects of swirling motion ash particle viscosity on deposition rates have been investigated. The predicted results are in good agreement with the reported experimental measurements on the Australian bituminous coals. Also, a novel numerical slagging index (NSI) which is based on ash fusibility, ash viscosity and the content of ash in the coals has been developed. The incoming ash shows significant influence on slag accumulation in boilers. The results of assessment of the slagging potential using the NSI on a wide range of coals and some coal blends correlate very well with the reported field performance of the coals. The NSI has been modified to predict the slagging potential of some coal and biomass blends with < 20% biomass ratio. The results of predictions using the modified index on coals blended with sewage sludge and saw-dust are in good agreement with the experimental data.

This thesis presents some model developments for the simulation and optimization of the design of fuel cells, in particular for the Solid Oxide Fuel Cell (SOFC) and Proton Exchange Membrane Fuel Cell (PEMFC). However, the approaches and models presented can be basically applied to any type of fuel cell. In this study, the multicomponent diffusion processes in the porous medium of a SOFC anode has been investigated through comparison of the Stefan Maxwell Model, Dusty Gas Model and Binary Friction Model in terms of their prediction performance of the concentration polarization of a SOFC anode to mainly investigate the effect of the Knudsen diffusion on the predictions. The model equations are first solved in 1 D using an in-house code developed in MATLAB. Then the diffusion models have been implemented into COMSOL to obtain 2D and 3D solutions. The model predictions have been evaluated for different parameters and operating condi- tions for an isothermal system and assuming that reaction kinetics are not rate limiting. The results show that the predictions of the models are similar and the differences in the predictions of the models reported previously are mainly due to the definition of the effective diffusion coefficient, i.e. the tortuosity parame- ter, and with a tortuosity parameter fitted for each model, the models that take into account the Knudsen diffusion and that do not predict similar concentration polarization. Moreover, in this research, the application of an Adaptive Neuro- Fuzzy Inference System (ANFIS) to predict the performance of an Intermediate Temperature Solid Oxide Fuel Cell and a Proton Exchange Membrane Fuel Cell (PEMFC) have been presented. The results show that a well trained and tested ANFIS model can be used as a viable tool to predict the performance of the fuel cell under different operational conditions to facilitate the understanding of the combined effect of various operational conditions on the performance of the fuel cell and this can assist in reducing the experimentation and associated costs.

In this thesis we investigate a sequence of important inverse problems associated with the bio-heat transient flow equation which models the heat transfer within the human body. Given the physical importance of the blood perfusion coefficient that appears in the bio-heat equation, attention is focused on the inverse problems concerning the accurate recovery of this information when exact and noisy measurements are considered in terms of the mass, flux, or temperature, which we sampled over the specific regions of the media under investigation. Five different cases are considered for the retrieval of the perfusion coefficient, namely when this parameter is assumed to be either constant, or dependent on time, space, temperature, or on both space and time. Theanalytica:l and numerical techniques that arc used to investigate the existence and uniqueness of the solution for this inverse coefficient identification are embedded in an extensiveú computational approach for the retrieval of the perfusion coefficient. Boundary integral methods, for the constant and the time-dependent cases, or Crank-Nicolson-type global schemes or local methods based on solutions of the first-kind integral equations, in the space, temperature, or space and time cases, are used in conjunction either with Gaussian mollification or with Tikhonov regularization methods, which arc coupled with optimization techniques. Analytically, a number of uniqueness and existence criteria and structural results are formulated and proved.

To date the influence of bed roughness onl the propagation and dynamics of gravity currents has been largely neglected. A new physical modelling dataset has been compiled, which details the fundamental affects of several bed roughnesses on lock-release gravity currents. Five bed configurations were chosen encompassing 'grain' and 'form' type elements at a range of spacings. 1%, 5% and 10% initial density excesses were studied and the effect of removing the buoyant ambient fluid between the elements examined. Observations due to changing the current depth relative to the element height were also made. Ultrasonic Doppler velocimetry profiling (UDVP) and video capture techniques were used to analyse stream wise and vertical velocity structures and the affects on the front speed and distance travelled by the current. A 10 depth-averaged model solves modified 2-layer shallow water equations using the method of characteristics to obtain temporal velocity and depth evolution for a current under the influence of a general roughness quantity. 2D and 3D depth-resolved CFD simulations use the commercial software FLUENT to solve the RANS equations and transport of a scalar for the dense current with the RNG k - € turbulence model. The CFD predictions were well validated by the new experimental dataset and provide supplementary predictions of concentration, lateral motion and activity in the vicinity of the roughness elements. Comparison of 20 and 30 models resulted in the conclusion that the 3D model is vital for accurate simulation of internal dynamics of gravity current propagation over beam type bed roughness. In general general, the distance that the front travels decreases with any bed roughness present. This reduction increases with element spacing. The stream wise mean velocity profiles show a reduced velocity maximum further from the bed. Decreased entrainment results from breakdown of larger billows. Also observed is a thicker current, a rounder profile and a shorter, diluted head. Areas of increased vertical motion within the current. associated with decreased horizontal motion are observed, indicative of ejections of ambient fluid from between the elements. The presence of this fluid is found to contribute to ~ 50% of the current retardation. There are also similarities with the effects of bed roughness in open channel and pipe flows, most notably there is a critical element spacing (11'/ kr ~ 7) where the effects of roughness are greatest (where w is element spacing and kr is element height). The experimental and numerical results demonstrate that the application of existing models that rely on experimental validation with smooth beds to situations where a rough boundary is present may lead to significant errors.