Civil and Environmental Engineering - Doctoral Theses

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    Monitoring and modelling the long-term performance of Dublin Port Tunnel
    (University College Cork, 2023) Wang, Chao.; Li, Zili; Friedman, Miles; Science Foundation Ireland; Transport Infrastructure Ireland
    Dublin Port Tunnel, the biggest urban road tunnel in Ireland, functions as a critical part of an arterial road network between Dublin Port and the rest of Dublin city. Since its opening in 2006, the tunnel has been observed with progressively developing deteriorations; onsite observations and maintenance records have identified three main types, i.e., tunnel leakage, lining crack and concrete spalling, as the greatest engineering concerns to asset owners. The increasing structure deformation may disrupt tunnel operation, threaten tunnel serviceability, and/or even endanger tunnel integrity and safety in the long term. To better the understanding of the structural health condition of this critical underground infrastructure, this thesis specifically investigates the long-term ageing performance of a cross passage twin tunnel section of Dublin Port Tunnel subject to various deteriorations through an integrated assessment based on innovative field monitoring (i.e., wireless sensor network) and advanced numerical modelling (i.e., finite element modelling), where previous investigations mainly concentrated on the responses of a single tunnel section to short-term external disturbances (e.g., adjacent construction and surcharge). Characterisation of the selected deteriorated tunnel section on the basis of historical data, maintenance records, geotechnical and hydraulic reports is conducted first. Ground and tunnel parameters are obtained through applying theories and principles of soil and rock mechanics, followed by the determination of the current hydraulic permeability (conductivity) of the deteriorated section on the basis of monitored water flow. A modified analytical model for ground-lining relative permeability is proposed by assuming radial groundwater flow through all three ground layers and the hydraulic health status of the tunnel section is evaluated to be partially permeable. To quantitatively investigate the tunnel structural behaviour, an innovative wireless sensor network field monitoring system is adopted to monitor the long-term structural performance of the select cross-passage twin tunnel section, with the system’s reliability and robustness being tested inside an underground cave in the first place. The trial deployment of the wireless sensor network monitoring system inside the cave proves it functionally effective and environmentally adaptable in harsh underground conditions. In the case study of Dublin Port Tunnel, wireless field measurements of the cross-passage tunnel section show that its deformation is still increasing with time even after more than a decade’s operation. The mode and magnitude of the observed ongoing tunnel deformation are believed to be caused by three effects: twin tunnel interaction effect, cross passage effect and seasonal effect, and the mechanisms behind these observations are deemed to be related to both the surrounding ground and tunnel: ground hydro-geological degradation and tunnel hydro-mechanical deterioration. The seasonal temperature change contributes to the cyclic variation of the elastic and reversible deformation whilst tunnel deteriorations lead to the plastic and irreversible deformation. The field measurements are then compared against the numerical results from a series of soil-fluid coupled three-dimensional finite element analyses where the time-dependent ground and tunnel deteriorations are considered. The three-dimensional finite element (FE) analyses evaluate the individual effect of tunnel hydraulic and mechanical deterioration, ground permeability anisotropy, and ground creep. The numerical rate of tunnel deformation is compared against the field measurements to examine the numerical model. The FE results suggest that limestone rheology is the dominant factor contributing to the ongoing tunnel deformation, despite a slight difference between the deformation modes possibly due to the neglection of localised ground creep in the ground in between the twin tunnels. Parametrically, coupled deterioration leads to greater deformation than that individual deterioration does, and localised deterioration induces more accurate tunnel performance than that uniform deterioration does.
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    Electrofuels in a circular economy
    (University College Cork, 2023) Rusmanis, Davis; Murphy, Jerry; Wall, David; Science Foundation Ireland
    With the global shift away from traditional dispatchable fossil hydrocarbon fuels, the requirement for energy storage is of increasing importance. Renewable electricity generation is predominately in the form of variable renewable electricity, produced by wind and solar technologies. Intermittent production of electricity leads to an inevitable mismatch in supply and demand between the grid and the consumers. This can lead to periods of surplus power generation – and subsequent dispatch down – during low demand; conversely it can also lead to energy shortages during periods when there is insufficient power generation to match high demand which can lead to grid blackouts. Due to the difficulty in storing significant quantities of electricity via the grid, or batteries, sustainable alternative methods of energy storage must be devised. In recent years, electrofuels have become a centre-stage topic due to the opportunity to store electricity as low-carbon energy vectors which can be utilised where electrification may not be ideal, such as the hard-to-abate sectors of haulage, shipping, agriculture, and industry (iron, steel and chemicals). Further, the generation of these electrofuels could be carried out using electricity which would otherwise be lost, during times of excess production, and low grid demand. Combining technologies such as electrolysers (capable of producing hydrogen and oxygen) and biomethanation (which can combine carbon dioxide with hydrogen to form biomethane), offer a green alternative to fossil natural gas, and can use the existing gas grid as both a distribution system and a sustainable energy storage method. Based on the literature review and previous research, the initial thesis work focused on the design and commission of a prototype three-phase cascading biomethanation system. The fabricated prototype used diffusers as the agitation method and was deemed to be at a technology readiness level (TRL) of 4. The system was designed based on the results of a previously simulated system published by this research group (Voelklein et al., 2019); the results from this study were of similar performance to the previous model. The commissioning process produced carbon conversion rates between 72% and 97% across 3 reactors which can operate in parallel or series. A methane evolution rate of 2.9 L CH4/LVR/d was achieved at medium flow rates. Increasing the flow rates resulted in substantial drops in the conversion efficiency of the system. This limitation was mooted as likely due to the low bubble column height of the system. The design of the system was limited by the dimensions of the system, which were associated with health and safety concerns of the University. Assessing the integration of biomethanation technology into the wider energy, and environment sector, an initial small-scale case study of a circular economy system was carried out, based on local industry. Using a dataset from two local wind turbines to assess available surplus electricity, a small 122 kW electrolyser was proposed to be of a suitable size for the local site. This electrolyser would generate enough by-product oxygen to supply 8.9% of the oxygen demand of the aeration process for the local wastewater treatment plant of approximately 65,000 population equivalent. The product hydrogen could be used directly as a transport fuel, or convert 40% of the CO2 generated by the anaerobic digesters at the wastewater treatment plant into biomethane. This system could reduce the wastewater treatment plant emissions by 3.6% due to the reduced aeration requirement (pumping oxygen instead of air would reduce electricity usage). Should an appropriately sized electrolyser be used, up to a 40% reduction in emissions and energy use could be achieved. However, the small scale of the system was identified as a major barrier to the application of the technology, with the levelised cost of hydrogen evaluated at €8.92/kg H2 (or 27c/kWh). This resulting high cost cannot justify the implementation of a small-scale system to capture intermittent curtailed electricity as initially proposed. Expanding the circular economy system is possible with a carbon-negative emission pathway integrating pyrolysis technology to generate biochar. Anaerobic digesters and pyrolysis systems could potentially reduce greenhouse gas emissions by 42.7 kt CO2 through biomethane production from substrates within a 10km radius and through biochar production from digestate. When considering the sustainability considerations set by the latest version of the EU Renewable Energy Directive, electrofuels may only be certified as a renewable energy supply by affecting a 70% emission saving when compared to the current fossil hydrocarbon fuels. Additionally, the Directive does not allow for emission saving associated with carbon capture and reuse within the biological methanation system despite the added capture and use of otherwise curtailed electricity. Assessing a large-scale circular economy system, once again co-locating electrolysers with anaerobic digesters and biomethanation systems on new or existing large wastewater treatment plants can offer significant benefits. To supply a 10 MW electrolyser operating at an 80% operating capacity, a wind farm of 144MW size (18 x 8 MW turbines) operating at 57 % capacity would be necessary, with an assumed 10% curtailment scenario, reflective of the Irish grid dispatch down of the recent years. The 10 MW electrolyser could supply oxygen for a wastewater treatment plant of 426,000 population equivalent, reducing the plant emissions (and power demand) by 40% when compared to traditional gaseous air aeration. The produced wastewater sludge can be digested to produce biogas. The CO2 component of biogas can be converted to methane using a biomethanation system, requiring 22% of the electrolyser-produced hydrogen. This would result in the capture of 16 ktCO2e per annum while producing enough electrofuels to offset 390 diesel trucks with 94 compressed biomethane trucks and 296 compressed hydrogen trucks.
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    Applications of big data and machine learning in global energy system modelling
    (University College Cork, 2022) Joshi, Siddharth; O'Gallachoir, Brian; Holloway, Paul; Glynn, James; Science Foundation Ireland
    Global efforts to limit atmospheric warming well below 2 degree celcius above pre-industrial levels form the backbone of our response to mitigate the detrimental effects of climate change. The energy sector contributes circa 75% of global GHG emissions, amongst which the Electricity and Heat sectors each contribute ~40%, and the Transport sector contributes ~20% to the total global energy-related GHG emissions. The recent IPCC AR6 report finds that in nearly all possible emission scenarios considered, the world is heading towards a 1.5 degree celcius global temperature rise by the early 2030s. Pursuant to this, Energy Systems Models (ESMs) and Integrated Assessment Models (IAMs) are essential tools that provide energy system pathways to limit global warming below the temperature threshold. Thus, improving the accuracies of ESMs and IAMs will lead to measurable improvement in energy policy formulation and evaluation,thereby increasing the likelihood of meeting the commitments under the Paris Climate Agreement. This thesis develops and applies novel frameworks and methods that use a big data and machine learning driven strategy to improve the technology potential assessment of global decentralised solar PV technology and projection of transport energy service demand. The frameworks and methods developed in this thesis are presented in a format of methodological design principles followed by a case study using them. Specifically, on the supply side, the thesis investigates the global high-resolution spatiotemporal technical potential of rooftop solar PV for 2015 and further growth in the technical potentials from 2020-2050. For this assessment case study, the developed framework utilises a suite of GIS derived geospatial metrics in conjunction with a custom machine learning framework to calculate the global rooftop area at a high spatial resolution. Further using an IAM, the role of decentralised solar PV in global future energy transitions is explored. On the demand side, the thesis introduces a new machine learning model called ‘TrebuNet’ that is capable of high accuracy in estimating future energy service demand in the transport sector. The thesis thus provides the first development of machine learning and GIS based methods to improve the accuracy of global ESMs and IAMs. Particular attention is also paid towards the reproduction and transparency of the methods and the frameworks developed in this thesis for cross- disciplinary research. The thesis contributes to the important task of climate change mitigation by providing a bridge between mature IAM and ESM modelling and emerging machine learning-big data-driven tools. In doing so, this thesis demonstrates how the emerging methods in conjunction with large geospatial open source data, can aid in improving the technology representation of variable renewable energy technology in energy systems. The thesis also lays the foundation for providing solutions to energy system related tasks that are currently limited by high computational costs and data. The datasets and analysis generated by this thesis are presently assisting in unlocking the global role of decentralised renewable energy technologies in future energy systems and are also encouraging shifts in national decarbonisation pathways.
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    Sediment transport modelling and geomorphological assessments related to offshore renewable energy developments in the Irish Sea
    (University College Cork, 2022) Creane, Shauna; Murphy, Jimmy; O'Shea, Michael; Coughlan, Mark; Irish Research Council for Science, Engineering and Technology
    A combination of in-situ geophysical, geological and oceanographic datasets, and advanced numerical modelling tools are used to: improve the understanding of hydrodynamics and morphodynamics in the Irish Sea, develop new methods and approaches to investigate hydrodynamics and seabed morphodynamics in an offshore setting, collect and produce novel datasets that will contribute to this scientific field, and facilitate the sustainable growth of anthropogenic activities in the Irish Sea. These new methods and approaches include, using process-based indicators to understand sediment wave development and distribution, utilising ADCP-based suspended solids concentration as a numerical model calibration tool, and the application of a ‘sediment budget’ to an offshore sand bank to understand external influences on the stability of its morphodynamic system. Results provide hydrodynamic proof underpinning the presence of the bed load parting (BLP) in central Irish Sea and associated divergent sediment transport pathways driving sediment dispersal across this tidally-dominated continental shelf sea. Analysis of tidal propagation through the Irish Sea Basin concludes that the origin of the BLP is mainly attributed to the intersection of the north and south tidal fronts at an inclined angle due to Coriolis Forcing and coastline interactions. Minor factors impacting the shape and location of the BLP are the change in tidal character at (a) abrupt bathymetry changes, (b) headlands and intricate coastline topography, and (c) large-scale constrictions. These outcomes set the basis of understanding for the thesis. Building upon this knowledge, analysis of targeted, high resolution, time-lapse bathymetry datasets in the south-western Irish Sea reveals sediment waves in a range of sizes (height = 0.1 to 25.7 m, and wavelength = 17 to 983 m), occurring in water depths of 8.2 to 83 mLAT, and migrating at a rate of 1.1 to 79 m/yr. Combined with numerical modelling outputs, a strong divergence of sediment transport pathways from the previously understood predominantly southward flow in the south Irish Sea is revealed. Furthermore, a new source and sink mechanism are defined for offshore independent sediment wave assemblages, whereby each sediment wave field is supported by circulatory residual current cells originating from offshore sand banks. Reliable sediment transport modelling is required to investigate these physical processes further, therefore, the need for cost-effective sediment validation datasets for 2D sediment transport models is addressed, utilising ADCP-based datasets. A robust spatial timeseries of ADCP-based suspended solids concentration was successfully calculated in an offshore, tidally-dominated setting. Three new validation techniques are deemed advantageous in developing an accurate 2D suspended sediment transport, including i) validation of 2D modelled suspended sediment concentration using water sample-based suspended solids concentration, ii) validation of the flood-ebb characteristics of 2D modelled suspended load transport and suspended sediment concentration using ADCP-based datasets and iii) validation of the 2D modelled peak suspended sediment concentration over a spring-neap cycle using the ADCP-based suspended solids concentration. The robust coupled hydrodynamic and sediment transport model produced from this research is used as a tool of investigation in subsequent chapters. The complex hydrodynamic processes controlling upper slope mobility and long-term base stability of Arklow Bank are determined. Results reveal a flood and ebb tidal current dominance on the west and east side of the bank respectively, ultimately generating a large anticlockwise residual current eddy encompassing the entire bank. The positioning of multiple off-bank anticlockwise residual current eddies on the edge of this cell is shown to both facilitate and inhibit east-west fluctuations of the upper slopes of the bank and control long-term bank base stability. Within Arklow Bank’s morphological cell, eight morphodynamically and hydrodynamically unique bank sections or ‘sub-cells’ are identified, whereby a complex morphodynamic-hydrodynamic feedback loop is present. The local east-west fluctuation of the upper slopes of the bank is driven by migratory on-bank stationary and transient clockwise residual eddies and the development of ‘narrow’ residual current cross-flow zones. Together these processes drive upper slope mobility but maintain long term bank base stability. A sediment budget was successfully estimated for an offshore linear sand bank, Arklow Bank, whereby seven source and nine sink pathways are identified. The restriction of sediment sources off the southern extent of Arklow Bank impact erosion and accretion patterns in the mid and northern sections of the bank after just one lunar month simulation. Where tidal current is the primary driver of sand bank morphodynamics, wind- and wave-induced flow is shown to alter sediment distribution patterns. This advanced body of work forms a robust scientific evidence-base to facilitate the sustainable growth of offshore renewable developments.
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    Integration of anaerobic digestion with bio-electrochemical technologies in cascading circular bioeconomy systems producing biofuel and chemicals
    (University College Cork, 2023) Ning, Xue; Murphy, Jerry; Lin, Richen; O'Shea, Richard; Sustainable Energy Authority of Ireland; Gas Networks Ireland
    Anaerobic digestion is a waste treatment technology, which can alleviate greenhouse gas emissions, reduce environmental pollution whilst simultaneously generating biomethane (a clean energy source which can be used as a direct replacement for natural gas), biofertilizer (which can reduce fossil fertiliser use), and biogenic CO2 (which can be further valorised). However, many anaerobic digestion systems are implemented as standalone systems without optimizing circularity in system design. Conventional anaerobic digestion systems can face challenges such as: low biomethane production rate (in particular from feedstocks with high portions of lignin and cellulose), and inefficient biogas upgrading to biomethane. In an endeavour to simultaneously increase biogas production and upgrading, the potential for integrating anaerobic digestion with three emerging bio-electrochemical technologies in a circular cascading bioeconomy was assessed, including for power to gas, microbial electrolysis cell, and microbial electrosynthesis. An energy balance assessment indicated that these three circular cascading bio-electrochemical systems could display positive energy outputs if the electricity used would have been otherwise curtailed or constrained. This drove the thesis to develop bio-electrochemical cascading anaerobic digestion systems for value-added biofuel and chemical production. Anaerobic digestion is a complex microbial process that involves multiple syntrophic interactions with interspecies electron transfer as a crucial factor influencing digestion efficiency. Biochar has been shown to support direct interspecies electron transfer between fermentative bacteria and methanogenic archaea, thereby increasing biomethane production and reducing reaction times. The first experimental work investigated the biomethane potential in batch two-stage co-digestion of grass silage and cattle slurry, with varying dosages of biochar supplementation. Biochar addition at the optimal dosage of 10 g/L in two-stage digesters led to the highest methane yield of 253 L per kilogram (kg) volatile solid (VS), which was 24% higher than that from two-stage digesters without biochar supplementation. Continuous single-stage and two-stage co-digestion of grass silage and cattle slurry with 10 g/L biochar supplementation were compared in the second experimental work. In continuous trials, operated at an organic loading rate of 4.0 g VS/L/d, the second-stage digester in two-stage digestion produced a methane yield of 237 L/kg VS with 10 g/L biochar addition; this was 7% higher than the second-stage digester without biochar addition. The incorporation of two-stage anaerobic digestion and the addition of biochar was shown to be a promising approach to enhance system stability and improve biomethane production. To expand on the application of conductive materials in improving biogas production in anaerobic digestion, biochar was added to an integrated microbial electrolysis cell and anaerobic digestion system (the MEC-AD system) in the third experimental work. The results demonstrated that the biomethane yield and methane content in biogas in the MEC-AD system (with plain graphite cathode) increased by 68% and 17%, respectively, compared to conventional anaerobic digestion when co-digesting grass silage and cattle slurry. Biochar supplement (10g/L) in the MEC-AD system was shown to further increase biomethane yield by 9% as compared to the MEC-AD system without biochar addition. The combination of an enhanced electric field and biochar addition in the MEC-AD system provides a pathway for effective in-situ bioconversion of carbon dioxide to biomethane and improved substrate utilisation. The overall carbon utilisation of biomass conversion in AD and MEC-AD can be limited by the presence of carbon dioxide (CO2; approximately 30–45%) in the off-gas. This residual CO2 can be upgraded into valuable chemical products (such as acetic acid and ethanol) in microbial electrosynthesis, a process by which microorganisms utilize electrical energy to convert CO2 into value-added compounds. A 3D cobalt and nickel coated carbon felt (CoNi-CF) cathode was developed in this last experimental work and applied in microbial electrosynthesis reactors. The highest acetate concentration obtained from the microbial electrosynthesis reactor was 18.4 mmol/L, with a carbon conversion efficiency (C in acetate) of 75.4%, while the maximum ethanol production achieved was 4.5 mmol/L, with a carbon conversion efficiency (C in ethanol) of 18.6%. This thesis explored the synergistic integration of anaerobic digestion and bio-electrochemical technologies in cascading circular bioeconomy systems, and demonstrated that simultaneously enhanced biogas production and CO2 upgrading can be achieved through efficient direct interspecies electron transfer by adding biochar and/or by imposing an external electricity supply. The results from this thesis can provide guidance on designing future cascading circular bio-systems to produce advanced biofuel and value-added chemicals.