Abstract：The incorporation of Power-to-Hydrogen (PtH) units as an energy conversion hub can significantly facilitate the transition toward a low-carbon energy system. However, utilizing existing natural gas infrastructure to transport hydrogen produced by water electrolysis leads to several challenges: inhomogeneous gas composition within the gas network, significant changes in the thermodynamic and hydraulic parameters, and substantial shifts in operational states such as pressure, flow, and linepack, which influence the energy flow of the integrated system. Existing literature presents the steady-state flow model and the equivalent energy flow model. Nevertheless, neither can simultaneously capture the transient transition periods or spatial gas quality differences within the gas network. To address these issues, this paper proposes a dynamic optimal energy flow model for integrated natural gas and electrical power system with hydrogen injection. Firstly, according to the fundamental principles of fluid mechanics, the dynamic gas flow model considering heterogeneous gas injection is derived. It quantitatively describes the relationship between hydrogen mass fraction and flow rate at electrolysis nodes, as well as the differences in gas quality at different positions within the pipeline. Secondly, in conjunction with boundary conditions, the intractable. bilinear terms are equivalently reconstructed into variables with physical significance, namely hydrogen mass flow rate. Assumptions of approximate homogeneous mixing and low hydrogen blending ratios are introduced, transforming the PDE-constrained nonlinear model into a linear difference form. Furthermore, the linearized dynamic gas flow is coupled with the DC power flow. By considering operational constraints and targeting the most economical dispatch of the integrated system, the dynamic optimal energy flow is calculated. The proposed model has been applied to systems of different scales. In the illustrative single pipe case, the transient process of pressure, flow, and mass fraction is analyzed. In the small-scale case, the optimal scheduling of the integrated systems with different models and energy conversion node configurations is comparatively assessed. With an upper limit of 2% for hydrogen blending, the equivalent gas production increases by 53.6%, and total carbon emission is reduced by 0.56% compared to systems with power-to-gas (PtG). It demonstrates that the introduction of PtH can enhance energy conversion efficiency and reduce carbon emissions. Moreover, effects such as a reduction in linepack and pressure in the gas network can impact the operating domain, thereby affecting optimization scheduling results. When solving the optimal dispatch of an integrated system consisting of an IEEE 118 case and a 36-node natural gas system, the computation time was only 20.08 seconds. The following conclusions can be drawn from the simulation analysis: (1) Compared with existing models, the proposed model can reflect the spatial and temporal distribution characteristics of state variables, and reveal the time scale difference of gas network after hydrogen blending. (2) With proper assumptions and simplifications, the dynamic flow model possesses good mathematical properties and superior computational accuracy, which is applicable to large-scale optimization problems. (3) The proposed model can be used in decision support for obtaining the optimal operation strategy of the coordinated systems. In addition, the effects of hydrogen blending on the integrated energy system are analyzed. On one hand, it enhances the efficiency of energy conversion stages and reduces carbon emissions. On the other hand, issues like the adaptability of equipment to hydrogen and a decrease in linepack can affect the operating domain of the system. These factors need to be taken into account during dispatch.
刘文昕, 方家琨, 胡可崴, 钟治垚, 艾小猛. 计及氢气-天然气混输的气电综合能源系统动态最优能流计算[J]. 电工技术学报, 2023, 38(zk1): 1-17.
Liu Wenxin, Fang Jiakun, Hu Kewei, Zhong Zhiyao, Ai Xiaomeng. Dynamic Optimal Energy Flow in the Integrated Natural Gas and Electrical Power Systems Considering Hydrogen-Blended Transient Transportation Process. Transactions of China Electrotechnical Society, 2023, 38(zk1): 1-17.
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2023, Vol. 38 
