Model-based Optimisation of Mixed Refrigerant LNG Processes

Natural gas liquefaction processes are energy and cost intensive. This thesis pursues the optimisation of propane precooled mixed refrigerant (C3MR) processes considering variations in upstream gas well conditions, in order to maximise gas well life. Four objective functions were selected for the design optimisation of the C3MR and dual mixed refrigerant (DMR) processes: 1) total shaft work (W), 2) total capital investment, 3) total annualised cost, and 4) total capital cost of both compressors and main cryogenic heat exchanger (MCHE). Optimisation results show that objective function 4 is more suitable than other objective functions for reducing both W and UA (MCHE design parameter). This leads to 15% reduction in specific power for C3MR and 27% for DMR, while achieving lower UA values relative to baseline. The operation optimisation of the C3MR process and its split propane version (C3MR-SP) was performed using four objective functions: 1) total shaft work, 2-3) two different exergy efficiency expressions, and 4) operating expenditure (OPEX). Objective function 3 results in the lowest specific shaft work 1469 MJ/tonne-LNG. For C3MR-SP, however, the lowest specific shaft work is found to be under objective function 1. A comparison of optimisation results across literature studies is impractical due to dissimilar process conditions, feed gas conditions, product quality, and equipment size. A sensitivity analysis highlights the effect of feed gas conditions on performance of the C3MR. For instance, as LNG production decreases from 3 MTPA to 2.4 MTPA over time, the specific OPEX increases from $128/tonne-LNG to$154/tonne-LNG. A subsequent study was conducted focusing on energy benefits of two configurations: integrating natural gas liquids (NGL) recovery unit with C3MR. An integrated NGL recovery within C3MR shows a 0.74% increase in energy consumption as methane concentration of the feed gas decreases, however a frontend NGL recovery unit only has a 0.18% decrease

The application of multi-agent systems to the design of an intelligent geometry compressor

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.In this research, a multi-agent approach was applied to the design of a large axial flow compressor in order to optimise performance and to greatly enlarge the useful operating range of the machine. In this design a number of distributed software/hardware agents co-operate to control the internal geometry of the machine and thereby optimise the compressor characteristics in response to changes in flow conditions. The resulting machine is termed an ‘Intelligent Geometry Compressor’ (IGC). The design of a multi-agent system for the IGC was carried out in three main phases, each supported by computer simulation. In the first phase a steady-state model of the IGC was developed in which global control of the variable geometry is achieved by a single agent. This was used to help identify specific requirements for performance and the underlying parametric relationships. The subsequent phases incorporated additional agents into the machine design to meet these requirements. Initially, agents were deployed to optimise the settings of individual rows of stator vanes. In the final phase, the MAS was extended to incorporate agents into the machine design for the control of individual stator vanes. Simulation results were obtained which demonstrate the effectiveness of the intelligent geometry compressor in achieving delivery pressure regulation over a wide range of steady-state operating conditions whilst optimising overall machine efficiency and avoiding the occurrence of stall. Some of the implications for the physical design of an IGC arising from the MAS concept were briefly considered. The experience of the research supported by the specific results and observations from many simulation trials, led to the conclusion that multi-agent systems can provide an effective and novel alternative approach to the design of an intelligent geometry compressor. By implication, this conclusion may be extended to other intelligent machine applications where similar opportunity to apply a distributed control solution exists

Chemical looping water splitting for hydrogen production, decarbonised steel production and energy storage/generation in-situ with CO2 capture

In the research described in this thesis, a novel hydrogen production process via integrated chemical looping water splitting technology (ICLWS) was developed. In addition, a state-of-the-art process for production of decarbonised iron through a four-stage chemical looping water splitting technology (CLWSFe) was proposed. Both processes were simulated using the Aspen Plus simulator. Heat integration analysis was applied to the two processes, utilising pinch-point analysis in order to minimise utility usage and optimise their thermodynamic performance. Furthermore, sensitivity analysis was performed for the ICLWS process to detect the optimum operating conditions. Both processes were thermodynamically and economically assessed to determine their viability by comparing them with benchmark processes, i.e. the steam methane reforming process (SMR) that was developed earlier in this work and other competitive chemical looping processes described in the literature. The thermodynamic results showed that the ICLWS and the CLWSFe processes exhibited improved effective efficiency by 12.3% and 20.8% compared with the SMR process. Regarding the efficiency of hydrogen production, the ICLWS process exhibited 11.7% higher efficiency than the SMR process; however, the efficiency of hydrogen production by the CLWSFe process was 1.6% lower than that of the SMR process. For the economic assessment, CAPEX, OPEX and the hydrogen production cost were evaluated for both processes. Results indicated that the hydrogen production cost through the ICLWS process with MgAl2O4 as support material was 17.5% lower than that through the SMR process, and was 26.3% lower through use of the CLWSFe process than through use of the SMR when iron as a saleable product was considered. In addition, a one-dimensional steady-state model was developed to obtain the conversion and temperature profiles for all the reactors involved in the ICLWS and CLWSFe processes. Consequently, the size of each reactor was determined. Furthermore, a system of integrated pumped heat-energy storage (IPHES) was developed by integrating pumped-energy storage with the ICLWS process. Then, an open-cycle gas turbine was merged with this system to form another novel energy-storage system called OIPHES. The transient behaviour of the temperature of the solid inside the storage tanks and the daily energy generation for a selected days in the year in both systems were investigated. Based on that, both systems were assessed thermodynamically and economically. Also, an economic sensitivity analysis was performed for the OIPHES process and a feasibility equation was derived showing the conditions required to enhance the feasibility of the OIPHES system. As a case study, the influence of the hydrogen fuel feed rate on the system’s daily profits was studied. Therefore, the optimum hydrogen fuel feed rate to maximise the daily profits of the OIPHES system was selected.Open Acces

Hybrid nuclear-solar power

Nuclear and solar power, in the form of concentrated solar power (CSP), play a significant role in achieving the ambitious global targets of reducing greenhouse emissions and guaranteeing security of energy supply. However, both power generation technologies still require further development to realise their full potential, especially in terms of attaining economic load following operations and higher thermal efficiencies. Therefore, the aim of this research is to investigate and thermo-economically evaluate the available options of upgrading the flexibility and enhancing the thermal efficiency of nuclear and solar power generation technologies (i.e., through the integration with thermal energy storage (TES) and by hybridising both power generation technologies) while providing reasonable economic returns. The thesis starts with describing the development and validation of several thermodynamic and economic computational models and the formulation of the whole-energy system model. The formulated models are utilised to perform several thermo-economic studies in the field of flexible nuclear and solar power, and to quantify the economic benefits that could result from enhancing the flexibility of nuclear power plants from the whole-energy system perspective. The studies conducted in this research are: (i) a thermo-economic assessment of extending the conventional TES system in direct steam generation (DSG) CSP plants; (ii) a thermo-economic evaluation of upgrading the flexibility of nuclear power plants by the integration with TES and secondary power generation systems; (iii) an investigation of the role of added flexibility in future low-carbon electricity systems; and (iv) a design and operation analysis of a hybrid nuclear-solar power plant. The most common TES option in DSC CSP plants is steam accumulation. This conventional option is constrained by temperature and pressure limits, leading to lower efficiency operations during TES discharging mode. Therefore, the option of integrating steam accumulators with sensible-heat storage in concrete to provide higher-temperature superheated steam is thermo-economically investigated in this research, taking an operational DSG CSP plant as a case study. The results show that the integrated concrete-steam TES (extended) option delivers 58% more electricity with a 13% enhancement in thermal efficiency during TES discharging mode, compared to the conventional steam accumulation (existing) configuration. With an estimated additional investment of $4.2M, the projected levelised cost of electricity (LCOE) and the net present value (NPV) for the considered DSG CSP plant with the extended TES option are respectively 6The option of upgrading the flexibility of nuclear power plants through the integration with TES and secondary power generation systems is investigated for two conventional nuclear reactors, a 670-MWel advanced gas-cooled reactor (AGR) and a 1610-MWel European pressurised reactor (EPR). In both investigated case studies, the reactors are assumed to continuously operate at full rated thermal power, while load following operations are conducted through the integrated TES tanks and secondary power generators. Based on the designed TES and secondary power generation systems, the AGR-based configuration can modulate the power output between 406 MWel and 822 MWel, while the EPR-based configuration can operate flexibly between 806 MWel and 2130 MWel. The economic analysis results demonstrate that the economics of added flexibility are highly dependent on: (i) the size of the TES and the secondary power generation systems; (ii) the number of TES charge/discharge cycles per day; and (iii) the ratio and difference between off-peak and peak electricity prices. Replacing conventional EPR-based nuclear power plants with added flexibility ones is found to generate whole-system cost savings between$30.4M/yr and $111M/yr. At an estimated cost of added flexibility of$53.4M/yr, the proposed flexibility upgrades appear to be economically justified with net system economic benefits ranging from $5.0M/yr and$39.5M/yr for the examined low-carbon scenarios, provided that the number of flexible nuclear plants in the system is small. The concept of hybridising a small modular reactor (SMR) with a solar-tower CSP integrated with two-tank molten salt TES system, with the aim of achieving economically enhanced load following operations and higher thermal efficiency levels, is also thermo-economically investigated in this research. The integration of both technologies is achieved by adding a solar-powered superheater and a reheater to a standalone SMR. The obtained results demonstrate that hybridising nuclear and solar can offer a great amount of flexibility (i.e., between 50% and 100% of nominal load of 131 MWel) with the SMR continuously operated at full rated thermal power output. Furthermore, the designed hybrid power plant is able to operate at higher temperatures due to the addition of the solar superheater, resulting in a 15% increase of thermal efficiency compared to nuclear-only power plant. Moreover, the calculated specific investment cost and the LCOE of the designed hybrid power plant are respectively 5410 $/kWel and 77$/MWhel, which are 2% and 4% lower than those calculated for the nuclear-only power plant.Open Acces

Machine Learning and Artificial Intelligence-Driven Multi-Scale Modeling for High Burnup Accident-Tolerant Fuels for Light Water-Based SMR Applications

The concept of small modular reactor has changed the outlook for tackling future energy crises. This new reactor technology is very promising considering its lower investment requirements, modularity, design simplicity, and enhanced safety features. The application of artificial intelligence-driven multi-scale modeling (neutronics, thermal hydraulics, fuel performance, etc.) incorporating Digital Twin and associated uncertainties in the research of small modular reactors is a recent concept. In this work, a comprehensive study is conducted on the multiscale modeling of accident-tolerant fuels. The application of these fuels in the light water-based small modular reactors is explored. This chapter also focuses on the application of machine learning and artificial intelligence in the design optimization, control, and monitoring of small modular reactors. Finally, a brief assessment of the research gap on the application of artificial intelligence to the development of high burnup composite accident-tolerant fuels is provided. Necessary actions to fulfill these gaps are also discussed

Investigation of the cumulative impact of alkaline electrolysers on electrical power systems

Hydrogen could be the best candidate fuel for our future, especially in the transportation sector. It could be generated using water electrolysers running with power from carbon-free, renewable resources, since this is zero emission at the point of use, and so can help transition from the energy infrastructure available today into an energy world with a growing renewable electricity supply.This work models a highly distributed electrolyser system e.g. an urban hydrogen filling station network, and explores the Demand Side Management (DSM) potential of these electrolysers to improve the performance of the power system operating under the impact of intermittent renewable power generation.A comprehensive literature review has been carried out on the hydrogen economy, electrolysers and the potential role of storage devices in power systems. Three main areas related to alkaline electrolysers working within power systems were identified for further exploration. - Potential role of electrolysers in the existing distribution networks to increase the integrated wind power capacity - Potential role of electrolysers to stabilise the frequency of the power system - Potential role of electrolysers to absorb any surplus, carbon free, generation within the UK electricity networkThe first item of archival value within this work is the identification, presentation and discussion of electrolyser characteristics which are relevant to the introduction of an acceptable control strategy to integrate such electrolyser loads within the power system and thus provide improved performance of the network when exposed to the highly time variable energy supply from renewable sources. Two types of electrolyser made by NEL Hydrogen are detailed: atmospheric and pressurised. Their characteristics are reported in this thesis using the results from experiments designed by the author. In addition, an experiment has also been carried out on a PEM electrolyser available at Strathclyde University to compare its results with the characteristics of the commercial alkaline units. Second, a novel algorithm for sizing, placing and control of electrolysis based hydrogen filling stations operating within radial distribution networks has been proposed and its performance is assessed using a United Kingdom Generic Distribution System (UKGDS) case study. The controller objective is to dispatch alkaline electrolysers appropriately to increase the amount of integrated wind power capacity and reduce the grid losses within the network while satisfying the network constraints and respecting the electrolyser characteristics.In addition, a MATLAB Simulink model has been developed to investigate the impact of alkaline electrolysers as dynamically controlled loads for the stabilisation of system frequency in the case of a sudden loss of generation and also when the power system has high penetrations of wind power. The electrolysers are controlled according to a droop control strategy. A novel approach to determine the aggregate nominal electrolysis demand for frequency stability purposes has also been proposed in this work, and the financial viability of the proposed strategy to control electrolysers has been assessed.Finally, several scenarios have been modelled to investigate the role of electrolysers to absorb surplus power and produce hydrogen for the fuel cell vehicles in the UK in the year 2050. Different wind, solar and nuclear power generation capacities have been considered. On the demand side, different penetration levels of electric vehicles and hydrogen fuel cell cars have been modelled. The results are discussed and analysed.Keywords: Alkaline electrolysers, Renewable power, Active Network Management, Distribution network, Power system stability, Hydrogen economy, Power system losses, Demand side management, Load Frequency Control, Energy storage.Hydrogen could be the best candidate fuel for our future, especially in the transportation sector. It could be generated using water electrolysers running with power from carbon-free, renewable resources, since this is zero emission at the point of use, and so can help transition from the energy infrastructure available today into an energy world with a growing renewable electricity supply.This work models a highly distributed electrolyser system e.g. an urban hydrogen filling station network, and explores the Demand Side Management (DSM) potential of these electrolysers to improve the performance of the power system operating under the impact of intermittent renewable power generation.A comprehensive literature review has been carried out on the hydrogen economy, electrolysers and the potential role of storage devices in power systems. Three main areas related to alkaline electrolysers working within power systems were identified for further exploration. - Potential role of electrolysers in the existing distribution networks to increase the integrated wind power capacity - Potential role of electrolysers to stabilise the frequency of the power system - Potential role of electrolysers to absorb any surplus, carbon free, generation within the UK electricity networkThe first item of archival value within this work is the identification, presentation and discussion of electrolyser characteristics which are relevant to the introduction of an acceptable control strategy to integrate such electrolyser loads within the power system and thus provide improved performance of the network when exposed to the highly time variable energy supply from renewable sources. Two types of electrolyser made by NEL Hydrogen are detailed: atmospheric and pressurised. Their characteristics are reported in this thesis using the results from experiments designed by the author. In addition, an experiment has also been carried out on a PEM electrolyser available at Strathclyde University to compare its results with the characteristics of the commercial alkaline units. Second, a novel algorithm for sizing, placing and control of electrolysis based hydrogen filling stations operating within radial distribution networks has been proposed and its performance is assessed using a United Kingdom Generic Distribution System (UKGDS) case study. The controller objective is to dispatch alkaline electrolysers appropriately to increase the amount of integrated wind power capacity and reduce the grid losses within the network while satisfying the network constraints and respecting the electrolyser characteristics.In addition, a MATLAB Simulink model has been developed to investigate the impact of alkaline electrolysers as dynamically controlled loads for the stabilisation of system frequency in the case of a sudden loss of generation and also when the power system has high penetrations of wind power. The electrolysers are controlled according to a droop control strategy. A novel approach to determine the aggregate nominal electrolysis demand for frequency stability purposes has also been proposed in this work, and the financial viability of the proposed strategy to control electrolysers has been assessed.Finally, several scenarios have been modelled to investigate the role of electrolysers to absorb surplus power and produce hydrogen for the fuel cell vehicles in the UK in the year 2050. Different wind, solar and nuclear power generation capacities have been considered. On the demand side, different penetration levels of electric vehicles and hydrogen fuel cell cars have been modelled. The results are discussed and analysed.Keywords: Alkaline electrolysers, Renewable power, Active Network Management, Distribution network, Power system stability, Hydrogen economy, Power system losses, Demand side management, Load Frequency Control, Energy storage

Integration, control and optimization of the solar photovoltaic-battery system in microgrids

This document composes the work realised and the research results developed within the scope of electric energy storage at the Renewable Energy Chair of the University of Évora. The current legal and technological framework of electrochemical energy storage technologies is reported, and its framework is demonstrated in the Portuguese and European contexts. Next, the experimental microgrid that comprises several electric energy storage technologies is described. The lithium-ion and vanadium redox flow technologies were tested and characterized for later validation of the electrical models that describe their performance. A state-of-the-art review allowed the experimentation of energy management strategies that fit the technologies studied, allowing smarter management in residential and services sectors. In this thesis, management algorithms, battery models, and an indication of technical, economic and energy parameters were combined in a tool to study the simulation of the operation of these technologies, allowing to define different operating objectives, fine-tune parameters and even join the operation of different technologies. This work was accompanied by national and international projects, attempting to respond to existing problems in the operation of real systems and gaps identified in the design phase, such as a robust dimensioning tool, with the integration of different battery managing methods; Integração, controlo e otimização do sistema solar fotovoltaico-bateria em microrredes Resumo: Este documento compõe o trabalho realizado e respetivos resultados da investigação desenvolvida no âmbito do armazenamento de energia elétrica na Cátedra Energias Renováveis da Universidade de Évora. Os atuais enquadramentos legais e tecnológicos das tecnologias eletroquímicas de armazenamento de energia são relatados, nos contextos português e europeu. Seguidamente, uma microrrede experimental que inclui diversas tecnologias de armazenamento de energia elétrica é descrita. As tecnologias de fluxo redox de vanádio e de iões de lítio foram objeto de ensaio e caracterização, para posterior validação dos correspondentes modelos que descrevem a sua performance elétrica. A revisão do estado da arte permitiu a experimentação de estratégias de gestão de energia que se adequam às tecnologias estudadas, que permitam a sua gestão inteligente, no contexto residencial e de serviços. Nesta tese, os algoritmos de gestão, os modelos das baterias, a indicação de parâmetros técnicos, económicos e energéticos foram combinados numa ferramenta para estudo da simulação da operação destas tecnologias permitindo definir diferente objetivos, afinar parâmetros e até operar conjuntamente diferentes tecnologias. Este trabalho foi acompanhado pelo paralelismo de projetos nacionais e internacionais, tentado dar resposta a problemas existentes na operação de sistemas reais, e lacunas identificadas na fase de projeto, tal como uma ferramenta de dimensionamento robusto, com a integração de diferentes formas de gerir baterias