Civil and Environmental Engineering - Doctoral Theses

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Logistical considerations in the development of on-farm anaerobic digestion systems in Ireland
(University College Cork, 2023) Ó Céileachair, Dónal; Wall, David; O'Shea, Richard; Murphy, Jerry; Sustainable Energy Authority of Ireland; Gas Networks Ireland
Ireland has 4 million ha of grassland, and over 6.56 million cattle, including over 1.4 million dairy cows. The potential resource of grass silage and animal slurry can be considered significant if utilised in anaerobic digestion for the production of biogas (and biomethane). On-farm anaerobic digestion (AD) can provide a renewable, sustainable fuel (in the form of biogas) that can decarbonise the energy system and reduce emissions from the agricultural sector. However, cattle in Ireland spend the majority of the year outdoors in pasture-based farming systems, leading to intermittent slurry collection by the farmer. Thus, there is an inherent seasonality with regards to feedstock availability. On-farm AD plants may also have no gas grid infrastructure access, and therefore may be required to facilitate a seasonal gas demand of a large energy user in their vicinity. Although the Irish biomethane industry is emerging, there are no significant virtual pipeline or biogas pipeline networks. The work herein assesses these logistical characteristics such as on-farm biomass resources, their location with respect to both gas grid infrastructure and large industry energy users, and how seasonal feedstock availability and seasonal gas demand can affect the operation of an on-farm AD plant. Furthermore, this work investigates the logistical considerations in the development of virtual pipelines and biogas pipelines serving on-farm AD plants, and how to optimise them. The findings from this work indicate that the total on-farm biomethane resource in Ireland is estimated to be 67 PJ a-1. Approximately 17% of this resource is more than 15 km away from gas grid infrastructure whilst also within 15 km of a large industry energy user. Seasonal slurry availability on an Irish dairy farm can lead to a 21% reduction in total biomethane production, whilst increasing the carbon intensity of the biomethane produced (as compared to year-round availability) by 11 g CO2 MJ-1. Seasonal gas demands may be facilitated without significant effect to biomethane sustainability. Liquid digestate must be recirculated during times of no slurry availability to keep the solids content of the digester low and thus ensure sufficient mixing takes place. In the delivery of the energy to the end user, virtual pipelines serving on-farm AD plants may reduce the total routing requirements and emissions by opting for larger biomethane haulage vehicles. The use of mobile-upgrading and compression units may be logistically unfeasible due to the slow upgrading speed of current technologies. Heuristics (practical methods to solve very complex problems) used in GIS software may be employed to quickly find optimal biogas pipeline layouts which either follow the shortest possible path (Steiner minimum spanning tree (MST)) or follow the road network. Commercial software was used to design and cost 20 Steiner MST and 20 road network biogas pipeline layouts. A road network pipeline layout connecting 10 AD plants to the biogas user costs 26% more than a Steiner MST would. Multi-criteria decision analysis revealed that the designed Steiner MSTs always outperformed road network layouts, except when pipeline material cost was of no importance. This work highlights the potential of on-farm anaerobic digestion to provide a source of sustainable, renewable fuel source for decentralised areas and industries in Ireland, whilst highlighting the logistical considerations and challenges associated with feedstock location, seasonal feedstock availability, seasonal gas demand, and the development of gas delivery methods such as virtual biomethane pipelines or biogas pipelines.
Development and implementation of a framework to aid the transition to proactive maintenance approaches on air handling units in the industrial setting
(University College Cork, 2023) Ahern, Michael; Bruton, Ken; O'Sullivan, Dominic; Science Foundation Ireland
This research explores the transition to proactive maintenance of HVAC equipment, specifically AHUs in industrial facilities, to make progress towards climate goals. HVAC systems account for 14% of global energy consumption, with the potential to increase efficiency by 20% by addressing energy-wasting faults according to Roth et al. However, these faults are difficult to detect due to their compensating control logic. This research highlights the potential of digitalisation techniques, particularly AI, to identify and rectify these faults, which would contribute to an approximate 2.8% global efficiency improvement. Notably, the study focuses on the nuances of industrial facilities, which have received limited attention compared to other building types. The research identifies several gaps in existing literature, including the knowledge gap between proactive data analysis and reactive engineering mind-sets, the data gap between high-quality experimental datasets and poor-quality industrial datasets, the operational gap between known baselines in experimental studies and unknown baselines in industrial settings, and the practice-theory gap between data-driven approaches in the literature and rule-based approaches in commercial tools. To address the knowledge gap, this thesis presents the IDAIC framework, a domain knowledge integration-type adaptation of the CRISP-DM process model. The implementation of the framework in an industrial facility to curate a dataset and develop a data assessment decision tree has contributed towards closing the data gap. Additionally, the study proposes extensions to the APAR ruleset and a practical data-driven fault detection method to address the operational gap. The deployment of the IDAIC framework as a tool leverages the UML modelling language to address practical considerations and demonstrate the approach's flexibility. Therefore, the main research outputs include the IDAIC framework, an industrial AHU dataset, a data assessment decision tree, an extension to the APAR ruleset, and a proactive maintenance decision support tool. Notably, this research unveils a fault in which the outside air damper is stuck in the fully open position, leading to estimated annual savings of €60,000. These findings validate the effectiveness of a human-centric, domain expertise-integrated approach that is resilient to industrial challenges, contributing to sustainable energy efficiency improvements and the achievement of climate targets using the best available solutions.
Sustainability assessment of the repurposing of wind turbine blades
(University College Cork, 2022) Nagle, Angela J.; Leahy, Paul; Dunphy, Niall; Mullally, Gerard; Science Foundation Ireland
Ireland is regarded as rich in wind resources, with Wind Energy Ireland suggesting that wind generation could make Ireland a renewable superpower. Ireland has chosen to capitalise on this resource by dramatically expanding wind energy generation capacity. However, with this development of wind capacity comes the need to deal with increasing amounts of waste from the wind sector when the turbines reach end-of-life. While much of the turbine is recyclable, the blades, which are made from composite glass fibre and polymer material, is not yet economically viable to recycle. Many blades are being landfilled or incinerated at the end of their working lives. An option that is gaining in popularity is the idea of repurposing blades into useful structures. This thesis considered the question “Can repurposing of end-of-life wind turbine blades from Irish wind turbines offer overall gains in sustainability?” Overall sustainability was by developing a Life Cycle Sustainability Assessment framework, which includes environmental Life Cycle Assessment (LCA), Life Cycle Costing (LCC) and the lesser established social life cycle assessment (s-LCA). LCA was used to first establish co-processing of the discarded blade material in Ireland as the most likely and best case scenario against which to compare repurposing solutions. Three overall scenarios were then developed based on maximizing blade consumption, the substitution of high embodied carbon materials, and serving customer segments who will benefit from circular material use such as the county councils who may be required to adhere to Green Public Procurement requirements. Based on these scenarios and customer segments, an estimated 20% of the blade material estimated to be decommissioned in Ireland could potentially be repurposed, offering reductions of 30,780 kg CO2 equivalents of emissions per year. The concept of using end-of-life blades to make bridges (termed ‘BladeBridge’) was selected from the scenarios for an in depth study. LCA and LCC comparisons against a conventional bridge and the baseline disposal method, showed BladeBridge to be environmentally less impactful than both of these scenarios. From a cost perspective, the first BladeBridge using a specific blade model will be slightly more expensive than a conventional bridge and subsequent blade bridges due to the reverse engineering required to assess the residual structural properties of the blade. However, second and subsequent bridges made from the same blade models would be less expensive. Indicators for s-LCA were developed by considering national shortfalls in Ireland’s performance against the Sustainable Development Goals (SDGs), which can be considered the world’s key performance indicators. In this way, these indicators capture a product’s ability to affect global social indicators in which Ireland most needs to improve upon. Using LCA, LCC and the SDG based s-LCA indicators, a stage/gate screening process was developed which follows the embedded systems model of sustainability. In this framework, early business ideas can be first screened for environmental and social value, and then for cost viability. This framework can help in the development of ‘purpose driven businesses.’ The research presented in this thesis combined life cycle thinking with policy considerations and business value creation, and offers contributions across all of these areas. Some examples of contributions are the positioning of LCA in the repurposing debate; the creation of a framework for life cycle assessments of products made from end-of-life material that can be used to contribute to improving globally defined social and environmental metrics; analytical tools for local authorities to implement green public procurement; and support for Ireland to decouple its renewable energy generation from the production of waste.
Improving the ability of energy systems optimisation modelling to inform national energy policymaking
(University College Cork, 2023-04-27) Aryanpur, Vahid; O'Gallachoir, Brian; Glynn, James; Daly, Hannah E.; Science Foundation Ireland
Energy Systems Optimisation Models (ESOMs) are extensively used to inform energy and environmental policymaking. They generate valuable insights into the possible pathways that reduce our reliance on fossil fuels and achieve ambitious clean energy transition goals. However, the academic literature identifies a number of priority areas for development with ESOMs to improve their ability to generate useful insights applicable to the energy transition. This thesis explores and delivers key developments in several of these dimensions: spatial resolution, energy-economy linkage, significance of model skill, and heterogeneity of consumers. From a policy perspective, this thesis seeks to improve the model-based analysis in the context of national-level energy sector decarbonisation and thus, mitigation policies are critically investigated. Moreover, the impacts of the mitigation actions on local air pollution levels and promoting energy security are also explored. Accordingly, the main contributions of this thesis are improvements to the state-of-the-art energy modelling methods and applications of the enhanced models to answer key policy questions with convincing evidence. The improvements are demonstrated via two well-established energy systems modelling tools in Ireland and Iran. The thesis concludes with several modelling and policy insights and suggestions on interesting areas for further investigation to strengthen the contribution of ESOMs to ensure improved climate mitigation and energy policies. The first weakness is the limited spatial and consumer granularity in ESOMs which constrains their ability to analyse region-specific energy transition pathways. This thesis develops a multi-regional representation of the transport sector within the TIMES-Ireland Model (TIM), an ESOM used to develop ambitious mitigation pathways for Ireland’s energy system. The multi-regional approach captures region-specific characteristics of transport technologies and infrastructures across 26 counties. It also incorporates the heterogeneity of the impact of air pollution in sub-national regions and estimates the ancillary pollution benefits of the mitigation targets in those regions. The spatially explicit modelling approach also reveals higher economic co-benefits than single region modelling. The single-region method masks the higher damage costs in medium and large cities, thus underestimating total benefits. This thesis also develops a multi-consumer approach, more accurately capturing consumer heterogeneity. Having homogeneous consumers in ESOMs tends to oversimplify purchase decisions, especially for capital-intensive technology adoption. TIM simulates vehicle purchase decisions using hurdle rates. This thesis disaggregates consumers into five groups, ranging from low- to high-income families, to incorporate a more realistic representation of their behaviour in vehicle purchasing decisions. The results demonstrates that the model with heterogenous consumers offers higher Electric Vehicle (EV) adoption than a single region model calibrated with average national data and identical consumers. Spatially explicit analysis presents valuable insights into regional EVs diffusion and their electricity consumption at a subnational level which are usually challenging to achieve through an aggregated national model. Secondly, ESOMs often ignore the effects of changes in energy costs on energy service demands, despite their key ability to balance supply and demand. The thesis addresses this by developing a comprehensive representation of the power sector within the MESSAGE model, an ESOM used to explore the impacts of different subsidy reform scenarios in Iran. The thesis develops a soft-linked framework combining MESSAGE with an economic model and analyses both supply and demand sides under harmonised assumptions. The novel soft-linking addresses the structural weakness of ESOMs in capturing the effects of energy price on demand. The hybrid model is used to investigate the impacts of subsidy removal on power demand and the required generation mix. The findings reveal that under an early and steady reform scenario, the system avoids lock-in effect, and thus the development of renewable energy technologies and energy efficiency plans become cost-competitive. By contrast, the late subsidy reform path even with radical removal fails to tackle the lock-in effect’s risk. On the other hand, the long-term energy system transition is deeply uncertain. The hybrid modelling framework in this research is also used to conduct an ex-post analysis exploring the extent to which electricity subsidy reform could have reduced Iran’s energy demand during the last three decades. To minimise the uncertainties, both energy and economic models are calibrated with three decades of historical data. The cost-optimal modelling results are then compared with the real-world transition, revealing a 50% lower cumulative cost in the subsidy removal scenario compared with the real-world transition. This deviation highlights what could have been achieved through the implementation of different policies in the absence of uncertainties, providing valuable insights for informing future policy initiatives. Finally, this hybrid framework is also used to show how synergies and efficiencies from Iran’s energy subsidy reforms and lifting its sanctions could enhance global energy security, with a focus on natural gas. It demonstrates that significant opportunities could be realised through a combination of national energy policy reforms and cross border cooperation in a favourable international environment.
Production of biofuels in a circular cascading bio-based energy system
(University College Cork, 2022-04-01) Wu, Benteng; Murphy, Jerry; Lin, Richen; Environmental Protection Agency
Anaerobic digestion (AD) systems which incorporate carbon capture, utilization and sequestration technologies may offer a solution to significantly reduce greenhouse gas (GHG) emissions in the heavy duty sectors of transport and industry. However, serious challenges faced by AD hinder its development, and new strategies are needed to achieve sustainable biofuel production. In this thesis bespoke circular cascading bio-based systems, with AD as the core element, integrating electro-fuel production via CO2 biomethanation and value-added pyrochar via pyrolysis of solid digestate, were proposed to produce sustainable biofuels, namely biogas and biomethane. The preliminary energy analysis revealed that the proposed system could increase final net energy output by 70% as compared with a conventional biomethane system if the electricity used to produce hydrogen is assumed to be otherwise curtailed. Pyrochar derived from solid digestate was proposed to be beneficial for both the AD and CO2 biomethanation processes. It is hypothesised that carbonaceous materials could significantly promote direct interspecies electron transfer (DIET) and accordingly enhance and stabilize AD performance. The first experimental work added carbonaceous materials (nanomaterial graphene and more cost-effective pyrochar) to digestion of thin stillage which was under stress due to an acidic shock, to investigate the potential role in improving the stability of the AD process. Results showed that the addition of graphene could help stabilize AD of thin stillage after acidic shock, presumably due to DIET. Graphene amendment (of 1.0 g/L) improved biomethane yield by 11.0% compared with the control group (without material addition) and accelerated the degradation of propionic acid. In comparison, pyrochar addition (both at 1.0 g/L and 10 g/L) shortened lag time but failed to enhance biomethane yield. Modelling of microbial electron transfer estimated that when 50% of the electrons produced from propionate oxidation are transferred through DIET, approximately 85 kJ/mol more energy can be obtained as compared to that of indirect hydrogen transfer. There is a gap in the state of the art in the potential role of carbonaceous materials in ex-situ biomethanation systems (CO2 + 4H2 → CH4 + 2H2O) facing flexible operation conditions due to intermittent gas injection. The variability in gas supply would be caused by green hydrogen production associated with variable renewable electricity. In the second experimental work CO2 biomethanation was examined with graphene amendment (1.0 g/L); the work demonstrated how such amendment could contribute to stabilizing ex-situ biomethanation after repeated periods of intermittent gas supply. The rationale for the stabilizing effect of graphene amendment is suggested as due to the high electrical conductivity and large specific surface of graphene. With the shock of intermittent gas injection, graphene addition enhanced the gas conversion efficiency by 18.2% and production rate by 267% as compared with the control group. However, pyrochar amendment (1.0 g/L) did not lead to promotional effects on the upgrading performance. Extending applications of carbonaceous materials in biofuels to biochemicals (in the third experimental work) the promotional effects of pyrochar amendment were observed in microbial chain elongation for medium chain fatty acid (MCFA, containing 6-12 carbon atoms) production. Optimal pyrochar addition could achieve 115% more MCFA yield than no pyrochar addition; this was demonstrated to be attributed to the high electrical conductivity and surface redox groups of pyrochar. Moreover, the optimal electron donor (ethanol) to electron acceptor (acetate) molar ratio was 2 mol/mol in a pyrochar mediated chain elongation system. Thermodynamic calculations modelled an energy benefit of 93.50 kJ/mol reaction for pyrochar mediated n-caproate production. To assess the economic and environmental benefits of circular cascading bio-based energy systems, process designs for production of gaseous biofuel (biomethane), liquid biofuel (biomethanol), and biofertilizer (digestate) were developed. The minimum marginal abatement cost for the pyrolysis incorporated case was –111.1 €/t CO2-eq when biomethane was sold at 1.03 €/Nm3. The significance of the hydrogen price may be noted as the marginal abatement cost rose to –58.2 €/t CO2-eq when H2 was purchased at €3.40/kg. When methanol was sold at 425 €/t (global weighted average value), the marginal abatement cost for the pyrolysis incorporated case (with H2 at 1€/kg) was 136.5 €/t CO2-eq, which is higher than current carbon credits at 33.5 €/t CO2. Integration of AD, CO2 biomethanation and pyrolysis technologies could be economically and environmentally compelling to produce biomethane, while for biomethanol production, minimisation of methane loss and use of low carbon electricity were necessary to lower the abatement cost.

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