PhD Oral Exam - Hadis Montazeri Nejad, Building Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

This study aims to identify the most suitable compact-size cogeneration technologies to supply electricity and heat in urban areas with varying energy demands. Renewable fuels, including hydrogen and biogas, are in the focus of the study to assess changes in cogeneration performance compared to fossil fuels such as methane. Following a classification of all available cogeneration technologies, the two most promising technology options for urban energy supply are chosen for detailed analysis, namely a proton exchange membrane fuel cell (PEM fuel cell) and a recuperated micro-gas turbine (MGT). The performance and availability of different cogeneration systems under multiple scenarios, including technology, control, fuel selection, and operational considerations, are modeled. The significance of this study lies in its comprehensive evaluation of the proposed systems' operation under part load conditions and various control strategies. A detailed electrochemical and thermal model is developed to simulate electricity and heat production from the PEM fuel cell. For the recuperated MGT, a comprehensive model has been established to optimize heat recovery from exhaust gases and to find the best control strategy to meet the required heating demand. The developed control strategy involves the precise adjustment of the bypass valve position, which regulates the mass flow rate distribution between the heat recovery unit and the recuperator, where the preheating of the combustion inlet air improves the system's overall efficiency. Additionally, to effectively meet the required heating demands, the positioning of the bypass valve must be carefully coordinated with the charging and discharging cycles of storage tanks, as well as the transition between standby and active modes of auxiliary boilers.

The results show that a PEM fuel cell is a more attractive option for high-efficiency cogeneration because it directly converts chemical energy into electrical energy at a low operational temperature. This eliminates the inefficiencies associated with combustion and mechanical energy conversion in the recuperated MGT. It also reduces thermal dissipation and optimizes hydrogen utilization compared to MGT systems that operate at higher temperatures. At part-loads, when the controllable bypass valve is set at 100% position to meet the peak heating demands, hydrogen yields maximum heat, and electricity generation. Hydrogen combustion, with its high diffusivity, flame speed, and low ignition energy, reaches the highest gas temperatures. Conversely, methane combustion, with its stable combustion characteristics, favorable heat transfer properties, and compatibility with turbine operational conditions at lower power levels, demonstrates the highest part-load efficiency. The performed parametric analysis shows that increasing rotational speed, pressure ratio, and working parameter enhances gas flow rates and gas temperatures, which causes the generated power to peak at the maximum values. Moreover, increasing the cell count and ambient temperature in PEM fuel cells leads to increased hydrogen consumption and enhanced reactive surface area, intensifying the electrochemical reactions and increasing the number of catalytic sites, thereby augmenting the overall efficiency and energy output.

To mitigate emissions from micro-gas turbines, this research focuses on controlling NOx and CO concentrations through advanced design strategies. Based on a comprehensive literature review and input from manufacturers, various combustor designs for micro-gas turbines are compared to identify the most effective technology for reducing emissions. A dual axial swirler combustor is selected because its design features ensure a uniform temperature distribution across the combustion chamber, minimize localized hot spots and significantly enhance fuel-air mixing. These characteristics contribute to improved combustion efficiency and flame stability, even at partial loads, making this type of combustor particularly effective in achieving low NOX and CO emissions. The emission characteristics of the proposed combustion chamber under distinct fuel and operational scenarios are studied using three pollutant level models, including CFD simulations, an Equivalent Chemical Reactor Network (ECRN) model in Ansys Chemkin-Pro, and own mathematical modeling. To assess NOX emissions, a two-step global reaction mechanism developed by Polifke is applied, and a new methodology that integrates a CFD simulation and a temperature-based criterion is proposed to accurately predict CO emissions. Notably, in two distinct control scenarios, including the enriched combustion (ER=1.2) and the highest ambient temperature (33°C), peak NOX emissions are observed during hydrogen combustion, while methane and biogas yield moderate and the lowest NOX concentrations, respectively. Under parametric analysis, the findings reveal that hydrogen and biogas fail to meet pollutant emission limits, regardless of operating conditions. This can be attributed to the high flame temperature of hydrogen and the presence of CO2 in biogas as an inert diluent, which poses critical challenges to combustion efficiency.

In summary, the thesis contributes to comparing compact-size cogeneration technologies fueled by renewable fuels, providing a comprehensive framework for analyzing their performance under various operational conditions. By integrating advanced control strategies and innovative combustion techniques, this study not only identifies key pathways for improving the efficiency of PEM fuel cells and MGTs but also presents novel methodologies for emission reduction, mainly through the deployment of dual axial swirler combustors. The research offers significant insights into the adaptability of cogeneration systems for urban applications, enhancing their environmental sustainability and ability to meet fluctuating energy demands in real-time. This work paves the way for future research on optimizing control strategies for renewable-fueled CHP systems, ultimately contributing to developing more resilient and low-emission energy systems in urban environments.