Steady-state multi-energy flow problem in coupled energy networks
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2023-07
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Melbourne Authors
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Electrical and Electronic Engineering
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PhD thesis
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Open Access
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© 2024 Mohammad Mohammadi
Abstract
Energy systems worldwide are experiencing a rapid transition towards a low-carbon future, calling for greater energy efficiency and flexibility in utilising energy networks. This transition is accompanied by the growing adoption of multi-energy technologies like combined heat and power (CHP), power-to-heat, and power-to-gas units, leading to rising interdependencies between different energy networks, particularly in district and energy community applications. Hence, more effective utilisation of coupled energy networks (CEN) can bring several efficiency and flexibility benefits. Developing efficient energy flow models for CEN analysis is thus key to fully unlock these inherent benefits. The steady-state multi-energy flow (MEF) problem is the cornerstone of CEN analysis and lays the foundation for optimal operation and flexibility studies.
This thesis presents a systematic evaluation of the MEF problem for steady-state analysis of CEN, exemplified in the case of coupled electricity, heat, and gas networks, by highlighting three main principles, namely formulations, coupling strategies, and solution techniques. Regarding formulations, a MEF framework is developed to incorporate various formulations for steady-state analysis of CEN, while demonstrating their impact on the convergence and computational properties of MEF models. With respect to coupling strategies, a systematic analysis is conducted on three essential coupling strategies, i.e., Decoupled, Decomposed, and Integrated, which may be associated with parallel, sequential, and simultaneous MEF computations, respectively. A set of fundamental underlying principles that characterise the coupling effectiveness in the MEF problem is introduced, namely, underlying formulations, problem size, interdependencies between networks, and calculation sequence, while highlighting their role in appraising different coupling strategies. In terms of solution techniques, besides classical Newtown-Raphson (NR), the performance of other potentially suitable algorithms, such as Quasi-NR, Levenberg-Marquardt, and Trust-Region, is extensively studied for solving MEF problems. A novel cross-over strategy is then proposed to utilise the synergistic benefits of studied algorithms for improving convergence properties without compromising computational efficiency. Building upon the insights gained from the MEF analysis, a fast decomposed algorithm is introduced for solving large-scale coupled electricity and gas networks with hydrogen injection modelling and gas composition tracking. Finally, this thesis provides novel insights and practical recommendations to identify the most suitable formulations, coupling strategies, and solution techniques for solving the steady-state MEF problem in a robust and computationally efficient way.
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Multi-energy systems, Coupled energy networks, Multi-energy flow, Sector coupling, Fast decomposed algorithm, Hydrogen injection