Phd in ENERGY AND ENVIRONMENT

Thesis title: Hybrid energy systems to produce green hydrogen from municipal waste : Techno-economic analysis

This document represents the culmination of a three-year academic investigation into hybrid energy systems, with a focus on optimizing the production of green hydrogen in both an efficient and cost-effective manner. In recent years, the hydrogen production market _that traditionally dominated by grey hydrogen derived from fossil fuels_ has experienced a marked transition towards more sustainable resources. The growing global deployment of electrolysers and the integration of renewable energy sources, such as photovoltaic (PV) systems and wind power, have been key initial efforts in this shift. However, these approaches have led to hydrogen production costs of approximately $5-7/kgH₂, which is significantly higher than the cost of grey hydrogen ($1.5/kgH₂) and blue hydrogen ($2-3/kgH₂). The primary barrier to widespread adoption of green hydrogen at present lies in the cost-efficiency of electrolyser technologies. Consequently, alternative methods are needed to improve hydrogen production's economic and energy performance. The primary focus of this study was the exploration of landfill sites, which store municipal organic solid waste, as a potential resource for hydrogen production. Several landfill sites with energy recovery plants, primarily based on high-temperature gasification of solid waste (1000 to 1200 K), have been identified. The study began by selecting one of these sites as a case study, with the goal of repurposing the facility for hydrogen production. A comprehensive techno-economic analysis was conducted, employing hybrid energy resources to optimize energy performance. The research commenced with a thorough literature review on municipal solid waste (MSW) management and landfill gas (LFG) recovery technologies. MSW landfills are a significant source of LFG, primarily methane (CH₄) and carbon dioxide (CO₂), both of which contribute substantially to greenhouse gas emissions. This thesis investigates various methods for upgrading biogas and treating LFG, emphasizing their potential to generate energy and mitigate emissions. Following the review of advanced technologies in this domain, a mathematical model for each system component was developed using the MATLAB Simulink environment, resulting in the creation of comprehensive energy models. The modelled components included: Water electrolysers: Alkaline, Proton Exchange Membrane (PEM), and Solid Oxide Electrolysis Cells (SOEC) Landfill upgrading systems: Pressure Swing Adsorption (PSA) and Temperature Swing Adsorption (TSA) Methane reforming processes: Conventional Methane Steam Reforming (MSR) and Membrane Reforming Internal heat recovery: Supercritical CO₂ heat pump After evaluating several potential scenarios, three main configurations were selected for detailed analysis: Photovoltaic (PV) Integration with Gasification: This configuration integrates renewable energy sources, such as PV solar panels or wind turbines, with the traditional direct gasification process for hydrogen production. Electrolysers are powered by the generated electricity, and both low- and high-temperature electrolysers are evaluated in different coupling configurations. The system also incorporates a supercritical CO₂ heat pump to supply renewable heat for the high-temperature water electrolysis process. Methane Steam Reforming with Combined Heat and Power (CHP): This layout replaces the existing gasification plant with a methane steam reforming system. Landfill gas is processed in two streams—one directed toward methane purification and the other toward a CHP unit to co-generate heat and power. The configuration explores multiple combinations of upgrading techniques and electrolyser technologies, aiming for optimal energy balance and minimizing the need for external energy inputs. Membrane Reforming: This scenario builds upon the second configuration but replaces conventional methane steam reforming with membrane reforming, which operates at lower temperatures and simplifies the process by eliminating the need for gas purification. While this scenario reduces energy consumption and capital costs, it also results in a slightly lower hydrogen production rate. All scenarios were simulated in Simulink, and their performance compared. In terms of hydrogen production, methane steam reforming configurations yielded the highest output, with SOEC-based systems performing particularly well. Membrane reforming, though less productive, offered the most cost-effective solution at €3.45/kg H₂, while renewable integration scenarios produced the least hydrogen but maintained a fully renewable energy profile. The research also focuses on developing a new model for estimating methane emissions from landfill sites, addressing a critical limitation of the available models. Most existing models, such as LandGEM, rely heavily on specific parameters that must be measured on-site, which makes them applicable only to the particular landfill case for which they were designed. As a result, these models are limited in their flexibility and accuracy when applied to different landfill sites with varying characteristics. To overcome this limitation, a more flexible and adaptable model was developed using artificial intelligence (AI). This AI-based model is designed to be applicable across a wider range of landfill sites, accounting for diverse conditions and site-specific variables. Several machine learning algorithms, including Artificial Neural Networks (ANN), Linear Regression, and Support Vector Regression (SVR), were trained on a comprehensive dataset gathered from Italian waste management reports between 2017 and 2022. By applying various approaches to improve accuracy, the AI-based model was shown to perform well even for new case studies, in some instances surpassing the accuracy of traditional models like LandGEM. This flexibility makes it a valuable tool for broader applications in landfill gas estimation and energy recovery Finally, two analytical reviews were conducted on hydrogen applications. The first reviewed hydrogen blending in natural gas networks, concluding that a 10% blend could be safely integrated into existing infrastructure, with potential increases to 20%. The second review examined the current and future industrial applications of green hydrogen across EU member states, identifying opportunities for substituting grey hydrogen and highlighting technological readiness levels for hydrogen-based industrial processes. An innovative heat pump system, utilizing the reverse Brayton cycle and supercritical CO₂, was also proposed as a sustainable alternative for industrial heat recovery, capable of generating steam for SOEC electrolysis and other industrial applications. Overall, this research contributes to the advancement of sustainable hydrogen production technologies, with a particular focus on integrating renewable energy resources and repurposing landfill sites for green hydrogen production.